**Selective Laser Sintering of Nanoparticles**

**Selective Laser Sintering of Nanoparticles**

#### Sukjoon Hong Sukjoon Hong Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.68872

#### **Abstract**

Selective laser sintering of nanoparticles has received much attention recently as it enables rapid fabrication of functional layers including metal conductors and metal‐oxide elec‐ trodes on heat‐sensitive polymer substrate in ambient conditions. Photothermal reactions induced by lasers rapidly increase the local temperature of the target nanoparticle in a highly selective manner, and subsequent sintering steps including melting and coales‐ cence between nanoparticles occur to fabricate interconnected sintered films for various future applications. The mechanism of laser sintering, as well as possible target materials subject to laser sintering, together with experimental schemes developed to improve the process and potential applications, is briefly summarized in this chapter.

DOI: 10.5772/intechopen.68872

**Keywords:** laser, optics, nanoparticle, metal, metal‐oxide, flexible electronics

## **1. Introduction**

In this chapter, we focus on a specific type of sintering that utilizes laser and nanoparticle as its heat source and target material, respectively. Laser and nanoparticle on their own have interesting properties which are advantageous for conventional sintering process. Laser is a tool with a broad range of parameters and enables numerous responses such as remote temperature manipulation and rapid processing speed which cannot be achieved by other mechanical tools. Nanoparticles, having controllable sizes and shapes, find their application in various fields, and melting temperature depression due to their size effect is one of the key properties for sintering as it reduces the temperature required for the sintering process to a great extent. Laser sintering of nanoparticles—combining these two elements—not only possesses both the abovementioned features but also provides additional virtues and allows facile, damage‐free fabrication of functional layers on heat‐sensitive substrate to bring novel applications in the form of flexible electronics. Selective laser sintering of nanoparticles is

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

summarized in this chapter with special emphasis on its mechanism, target material, experi‐ mental schemes, and potential applications.

## **2. Mechanism**

Sintering of nanoparticles with other heat sources such as furnace or convection oven has been long studied. In a conventional process, nanoparticles in the form of ink are coated on a substrate with wet processing or diverse printing techniques, followed by bulk heating of the substrate at an elevated temperature. Thermal sintering usually involves long sintering time (>30 min) at high temperatures (>200°C) which are often not compatible with numerous heat‐sensitive polymer substrates [1]. The main difference of selective laser sintering is that the heat source is substituted with laser. As a laser beam is focused on the designated spot, the optical energy is directly converted into heat through photothermal reaction to control the local temperature with high selectivity and controllability. Laser processing, including selec‐ tive laser sintering of nanoparticles summarized in this chapter, is expected to show constant processing characteristics as far as the laser beam is properly controlled. Laser processing has certain advantages in terms of reproducibility compared to other processing techniques. Laser is an essentially massless and sterile tool so that the problems from mechanical holders or the processing tool itself can be largely prevented. Laser beam is not subject to wear and tear as well. These properties of laser help avoiding any contamination of the material being processed to increase the reproducibility of the proposed process.

Photothermal reactions by laser can be activated through various elementary excitation including interband and intraband transitions, and the detailed reaction depends on the type of absorbing material as well as the laser parameters [2]. We consider a continuous laser as a simple heat source as far as the characteristic time for the initial processing step is consider‐ ably longer than the relaxation time and no phase change occurs during the heating process. For pulsed laser, pulse duration and shape have strong effect on heating characteristics. A single pulse generally brings rapid temperature increase and subsequent cooling. As a result, the temperature swings up and down in case of multiple pulse irradiation, and the average temperature rise is dependent on the repetition rate [3].

(TEM) measurement as in **Figure 1(b)** [8]. The silver nanoparticles show dramatic increase in size when heated at 150°C, while there is no significant change up to 100°C, indicating that the significant melting and coalescence between nanoparticles occur at a temperature as low as 150°C. A more quantitative approach can be found from thermo‐gravimetric analysis (TGA), differential scanning calorimetry (DSC), and electrical resistance measurement according to the temperature as shown in **Figure 1(c)** [9]. A sharp exothermic peak in DSC and a decrease in TGA at ∼150°C indicate that there exists a phase change together with the decomposition of the capping organic molecules at the corresponding temperature. The electrical resistance drops rapidly at the same point as the individual nanoparticles melt and merge to provide a

Reprinted with permission from Ref. [11]. Copyrights 2008 American Institute of Physics.

**Figure 1.** (a) The melting point depression of silver nanoparticles calculated from the Gibbs‐Thomson equation. Reprinted with permission from Ref. [4]. Copyrights 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Observation of sintering behavior of silver nanoparticles at various temperatures by in‐situ probing. Inset: HRTEM image of a silver nanoparticle with self‐assembled monolayer (SAM). Reprinted with permission from Ref. [8]. Copyrights 2012 Elsevier Inc. (c) Melting characteristics of the silver nanoparticles in terms of resistance change (blue), TGA (green), and DSC (purple). Reprinted with permission from Ref. [9]. (d) Ellipsometric measurement of the silver nanoparticle film.

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872 149

Apart from the thermal properties of the target nanoparticles, the optical properties of the material are critical for laser sintering, since photothermal heating characteristics are directly

continuous conducting path.

Thermal properties of the target nanoparticle are important subjects in sintering, and it is still true for the laser sintering. Prior to the application of laser sintering, thermal proper‐ ties of nanoparticle are often investigated in separate steps. The melting temperature of nanoparticle changes abruptly in general due to greatly enhanced surface‐to‐volume ratio at a nanoscale. Previous investigation on the thermal properties of silver nanoparticles is presented in Section 2 as a representative example. The melting point depression of silver nanoparticles at various sizes is calculated from the Gibbs‐Thomson equation as shown in **Figure 1(a)** [4]. It is anticipated from the graph that the silver nanoparticle at ∼5‐nm diameter exhibits significantly lower melting temperature (∼150°C) compared to its bulk counterpart at 960°C. A similar trend can be found in other nanoparticles as well [5–7]. Direct observation of the nanoparticle melting is possible through high‐temperature tunneling electron microscope

summarized in this chapter with special emphasis on its mechanism, target material, experi‐

Sintering of nanoparticles with other heat sources such as furnace or convection oven has been long studied. In a conventional process, nanoparticles in the form of ink are coated on a substrate with wet processing or diverse printing techniques, followed by bulk heating of the substrate at an elevated temperature. Thermal sintering usually involves long sintering time (>30 min) at high temperatures (>200°C) which are often not compatible with numerous heat‐sensitive polymer substrates [1]. The main difference of selective laser sintering is that the heat source is substituted with laser. As a laser beam is focused on the designated spot, the optical energy is directly converted into heat through photothermal reaction to control the local temperature with high selectivity and controllability. Laser processing, including selec‐ tive laser sintering of nanoparticles summarized in this chapter, is expected to show constant processing characteristics as far as the laser beam is properly controlled. Laser processing has certain advantages in terms of reproducibility compared to other processing techniques. Laser is an essentially massless and sterile tool so that the problems from mechanical holders or the processing tool itself can be largely prevented. Laser beam is not subject to wear and tear as well. These properties of laser help avoiding any contamination of the material being

Photothermal reactions by laser can be activated through various elementary excitation including interband and intraband transitions, and the detailed reaction depends on the type of absorbing material as well as the laser parameters [2]. We consider a continuous laser as a simple heat source as far as the characteristic time for the initial processing step is consider‐ ably longer than the relaxation time and no phase change occurs during the heating process. For pulsed laser, pulse duration and shape have strong effect on heating characteristics. A single pulse generally brings rapid temperature increase and subsequent cooling. As a result, the temperature swings up and down in case of multiple pulse irradiation, and the average

Thermal properties of the target nanoparticle are important subjects in sintering, and it is still true for the laser sintering. Prior to the application of laser sintering, thermal proper‐ ties of nanoparticle are often investigated in separate steps. The melting temperature of nanoparticle changes abruptly in general due to greatly enhanced surface‐to‐volume ratio at a nanoscale. Previous investigation on the thermal properties of silver nanoparticles is presented in Section 2 as a representative example. The melting point depression of silver nanoparticles at various sizes is calculated from the Gibbs‐Thomson equation as shown in **Figure 1(a)** [4]. It is anticipated from the graph that the silver nanoparticle at ∼5‐nm diameter exhibits significantly lower melting temperature (∼150°C) compared to its bulk counterpart at 960°C. A similar trend can be found in other nanoparticles as well [5–7]. Direct observation of the nanoparticle melting is possible through high‐temperature tunneling electron microscope

processed to increase the reproducibility of the proposed process.

temperature rise is dependent on the repetition rate [3].

mental schemes, and potential applications.

**2. Mechanism**

148 Sintering of Functional Materials

**Figure 1.** (a) The melting point depression of silver nanoparticles calculated from the Gibbs‐Thomson equation. Reprinted with permission from Ref. [4]. Copyrights 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Observation of sintering behavior of silver nanoparticles at various temperatures by in‐situ probing. Inset: HRTEM image of a silver nanoparticle with self‐assembled monolayer (SAM). Reprinted with permission from Ref. [8]. Copyrights 2012 Elsevier Inc. (c) Melting characteristics of the silver nanoparticles in terms of resistance change (blue), TGA (green), and DSC (purple). Reprinted with permission from Ref. [9]. (d) Ellipsometric measurement of the silver nanoparticle film. Reprinted with permission from Ref. [11]. Copyrights 2008 American Institute of Physics.

(TEM) measurement as in **Figure 1(b)** [8]. The silver nanoparticles show dramatic increase in size when heated at 150°C, while there is no significant change up to 100°C, indicating that the significant melting and coalescence between nanoparticles occur at a temperature as low as 150°C. A more quantitative approach can be found from thermo‐gravimetric analysis (TGA), differential scanning calorimetry (DSC), and electrical resistance measurement according to the temperature as shown in **Figure 1(c)** [9]. A sharp exothermic peak in DSC and a decrease in TGA at ∼150°C indicate that there exists a phase change together with the decomposition of the capping organic molecules at the corresponding temperature. The electrical resistance drops rapidly at the same point as the individual nanoparticles melt and merge to provide a continuous conducting path.

Apart from the thermal properties of the target nanoparticles, the optical properties of the material are critical for laser sintering, since photothermal heating characteristics are directly related to the optical absorption of the nanoparticles. Particles at nanoscale, noble metallic nanoparticles in particular, exhibit unique and tunable optical properties due to their sur‐ face plasmonic resonance, and it has been extensively studied for numerous applications. By tuning the nanoparticle size and morphology, together with strong absorption at Rayleigh scattering regime, efficient and local energy deposition can be achieved by a laser. Since laser sintering is commonly applied to nanoparticle film, optical properties of a nanoparticle film are often more practically meaningful in the laser sintering process. Pan et al. [10] extracted the optical properties of silver nanoparticles from spectroscopic ellipsometry and determined its refractive index (n, k) and thickness from direct fitting of measured data. It was shown that a single Lorentz oscillator well explains the dielectric function of the as‐deposited film, while two oscillators are required for the sintered films. Such tunability in the optical property is a huge benefit in laser processing as it allows strong absorption at a specific wavelength (**Figure 1(d)**) to minimize possible damage at other non‐processed regions [11].

Sintering is often a many‐body problem, yet sintering between two individual nanoparticles has been also investigated through both molecular dynamics simulation and experiments. Necking between two gold nanoparticles can occur even at room temperature (300 K) by local potential gradient, and it is insensitive to laser irradiation as the initial growth occurs very fast (<150 ps) [12]. Molecular simulation shows that major neck growth mechanisms during laser sintering can vary according to the particle sizes, which might include grain‐bound‐ ary sliding/dislocations, surface diffusion, and viscous flow [12]. Laser‐induced nanoweld‐ ing between two metallic nanoparticles has been experimentally confirmed with TEM. Kim et al. [13] reported that gold nanospheres at 13‐nm diameter were successfully welded with picosecond laser pulses with 30‐ps duration. TEM grids loaded with gold nanoparticles were directly irradiated with the laser, and its high‐resolution TEM (HRTEM) image approved that the joint nanoparticles had single‐phase nanocontact to have ohmic electrical connec‐ tion. The time scale required for the absorbed photon energy to be converted into heat is known to be in the picosecond regime [14], however, the nanoparticle coalescence time and dynamics were not well explained earlier. Pan et al. [15] studied laser‐induced coalescence of gold nanoparticles supported on a quartz substrate for the investigation of the coales‐ cence time. The gold nanoparticles were fabricated using e‐beam lithography for a more sys‐ tematic study, and pump‐and‐probe technique has been utilized with 527‐nm pulsed laser and 633‐nm continuous laser as the processing laser and probing laser, respectively. Time‐ resolved transmission traces suggested that the coalescence time for melted nanoparticles is at nanosecond scale as shown in **Figure 2(a)**, and this value is reasonably in agreement to the molecular simulation result.

on silver nanoparticle ink as shown in **Figure 2(b)** [9]. Since overall laser sintering process is a complex multi‐physics problem, which incorporates a number of unexpected issues such as balling defects [17], a parametric study over laser power and scanning speed is often con‐ ducted to find the optimum sintering condition experimentally [9]. A typical example of the parametric study is shown in **Figure 2(c)** with silver nanoparticle film on various substrates. Ag nanoparticle ink was coated on three different substrates—polyimide (PI), polyethylene terephthalate (PET), and glass—and a parametric study in terms of laser power and laser scanning speed was conducted. It is apparent that the status of sintered Ag nanoparticle can be distinguished into three categories, and the area subject to higher laser power and lower scanning speed shows more complete sintering features. Note that there exists damage threshold for plastic substrates such as PI and PET. Due to the monochromaticity of lasers, wavelength is not an easily adjustable parameter in many cases, but the effect of laser wave‐ length in the sintering process has also been investigated for silver nanoparticles by utilizing three different (diode) lasers [18]. Paeng et al. utilized three laser diodes at 405, 514.5, and 817 nm, whose optical penetration depths are 28.3, 43.5, and 171 nm, respectively, for the Ag nanoparticle film. It was found that surface melting morphologies were observed for the laser wavelength with short optical penetration depth due to the strong surface absorption. The threshold laser powers for sintering were different according to the laser wavelength as well.

various substrates. Reprinted with permission from Ref. [9].

**Figure 2.** (a) Snapshots of the dynamic coalescence process between two gold nanoparticles from molecular dynamic simulation. Reprinted with permission from Ref. [15]. Copyrights 2008 American Institute of Physics. (b) Transient resistance change measurement according to the laser irradiation. (c) Parametric study for optimum laser conditions on

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872 151

Practically, single laser sintering involves a large number of nanoparticles, and the overall process should be examined in a more macroscopic perspective. Paeng et al. [16] suggested that silver nanoparticle film under continuous wave‐laser irradiation goes through certain steps of initial contact, neck growth, and coalescence for complete sintering, and the mac‐ roscopic coalescence time measured from pump‐and‐probe technique is around 10 ms. This value is in accordance with the time lag between laser irradiation and conductive metal elec‐ trode formation that was measured by transient resistance change during the laser irradiation

related to the optical absorption of the nanoparticles. Particles at nanoscale, noble metallic nanoparticles in particular, exhibit unique and tunable optical properties due to their sur‐ face plasmonic resonance, and it has been extensively studied for numerous applications. By tuning the nanoparticle size and morphology, together with strong absorption at Rayleigh scattering regime, efficient and local energy deposition can be achieved by a laser. Since laser sintering is commonly applied to nanoparticle film, optical properties of a nanoparticle film are often more practically meaningful in the laser sintering process. Pan et al. [10] extracted the optical properties of silver nanoparticles from spectroscopic ellipsometry and determined its refractive index (n, k) and thickness from direct fitting of measured data. It was shown that a single Lorentz oscillator well explains the dielectric function of the as‐deposited film, while two oscillators are required for the sintered films. Such tunability in the optical property is a huge benefit in laser processing as it allows strong absorption at a specific wavelength

Sintering is often a many‐body problem, yet sintering between two individual nanoparticles has been also investigated through both molecular dynamics simulation and experiments. Necking between two gold nanoparticles can occur even at room temperature (300 K) by local potential gradient, and it is insensitive to laser irradiation as the initial growth occurs very fast (<150 ps) [12]. Molecular simulation shows that major neck growth mechanisms during laser sintering can vary according to the particle sizes, which might include grain‐bound‐ ary sliding/dislocations, surface diffusion, and viscous flow [12]. Laser‐induced nanoweld‐ ing between two metallic nanoparticles has been experimentally confirmed with TEM. Kim et al. [13] reported that gold nanospheres at 13‐nm diameter were successfully welded with picosecond laser pulses with 30‐ps duration. TEM grids loaded with gold nanoparticles were directly irradiated with the laser, and its high‐resolution TEM (HRTEM) image approved that the joint nanoparticles had single‐phase nanocontact to have ohmic electrical connec‐ tion. The time scale required for the absorbed photon energy to be converted into heat is known to be in the picosecond regime [14], however, the nanoparticle coalescence time and dynamics were not well explained earlier. Pan et al. [15] studied laser‐induced coalescence of gold nanoparticles supported on a quartz substrate for the investigation of the coales‐ cence time. The gold nanoparticles were fabricated using e‐beam lithography for a more sys‐ tematic study, and pump‐and‐probe technique has been utilized with 527‐nm pulsed laser and 633‐nm continuous laser as the processing laser and probing laser, respectively. Time‐ resolved transmission traces suggested that the coalescence time for melted nanoparticles is at nanosecond scale as shown in **Figure 2(a)**, and this value is reasonably in agreement to the

Practically, single laser sintering involves a large number of nanoparticles, and the overall process should be examined in a more macroscopic perspective. Paeng et al. [16] suggested that silver nanoparticle film under continuous wave‐laser irradiation goes through certain steps of initial contact, neck growth, and coalescence for complete sintering, and the mac‐ roscopic coalescence time measured from pump‐and‐probe technique is around 10 ms. This value is in accordance with the time lag between laser irradiation and conductive metal elec‐ trode formation that was measured by transient resistance change during the laser irradiation

(**Figure 1(d)**) to minimize possible damage at other non‐processed regions [11].

molecular simulation result.

150 Sintering of Functional Materials

**Figure 2.** (a) Snapshots of the dynamic coalescence process between two gold nanoparticles from molecular dynamic simulation. Reprinted with permission from Ref. [15]. Copyrights 2008 American Institute of Physics. (b) Transient resistance change measurement according to the laser irradiation. (c) Parametric study for optimum laser conditions on various substrates. Reprinted with permission from Ref. [9].

on silver nanoparticle ink as shown in **Figure 2(b)** [9]. Since overall laser sintering process is a complex multi‐physics problem, which incorporates a number of unexpected issues such as balling defects [17], a parametric study over laser power and scanning speed is often con‐ ducted to find the optimum sintering condition experimentally [9]. A typical example of the parametric study is shown in **Figure 2(c)** with silver nanoparticle film on various substrates. Ag nanoparticle ink was coated on three different substrates—polyimide (PI), polyethylene terephthalate (PET), and glass—and a parametric study in terms of laser power and laser scanning speed was conducted. It is apparent that the status of sintered Ag nanoparticle can be distinguished into three categories, and the area subject to higher laser power and lower scanning speed shows more complete sintering features. Note that there exists damage threshold for plastic substrates such as PI and PET. Due to the monochromaticity of lasers, wavelength is not an easily adjustable parameter in many cases, but the effect of laser wave‐ length in the sintering process has also been investigated for silver nanoparticles by utilizing three different (diode) lasers [18]. Paeng et al. utilized three laser diodes at 405, 514.5, and 817 nm, whose optical penetration depths are 28.3, 43.5, and 171 nm, respectively, for the Ag nanoparticle film. It was found that surface melting morphologies were observed for the laser wavelength with short optical penetration depth due to the strong surface absorption. The threshold laser powers for sintering were different according to the laser wavelength as well.

### **3. Materials**

In the early stage, metal nanoparticles, the noble metals in particular, were the main tar‐ get materials for laser sintering due to their low electrical resistance and superior chemical stability. Gold [19–27] and silver [4, 8, 9, 28–35] are the most widely studied materials for laser sintering as conductors at microscale. These nanoparticles are usually prepared in the solution form with a specific solvent at high weight percentage, while small amounts of surfactants are added to prevent unwanted agglomeration or enhance the dispersion of nanoparticles within the solution. For the efficient use of laser, Bieri et al. [19] controlled the diameter of gold nanoparticles so that the absorption depth is minimized at the laser wavelength as confirmed from effective medium theory, Rayleigh scattering, and Mie scattering. Once a focus laser scans the gold nanoparticle film for selective sintering, the remaining nanoparticle solution is washed away with the same solvent used for the nanoparticle ink. It is frequently reported that the topography of the resultant conductor changes significantly with laser parameters [21, 22]. The most obvious trend is that the linewidth widens as the laser power becomes larger, since the area subject to higher intensity than the threshold value for the initiation of the sintering process increases. The morphology of laser‐sintered gold nanoparticle line measured by atomic force microscopy (AFM) often shows a bowl‐shaped geometry which can be attributed to thermocapillary effects that arise from the Gaussian profile of a focused laser beam. This trend can be also found from the sintering of other metal nanoparticles as well [29]. At high incident laser power, the cross‐section morphology of the sintered gold line becomes "sombrero"‐like with the rough surface topography [21], and the possible reason behind such a phenomenon might be the substrate deformation as the maximum temperature at the center region exceeds the softening temperature of the underlying substrate. These morphologies are shown in **Figure 3(a)**. Besides examining the resultant morphology of the sintered metal line, macroscopic electrical conductivity is an important factor to evaluate the performance of the laser as a sintering tool. The resultant electrical conductivity is depen‐ dent on a number of factors such as particle size, irradiated laser power, and translation speed [24], but it can be as low as only two times higher (5.41 μΩ cm) than the bulk value (2.65 μΩ cm) as in **Figure 3(b)**. The difference could be explained by boundary scattering from poly‐ crystalline structures and trapped residual capping agent inside the sintered conductor. The laser power density for the strong coalescence of gold nanoparticles was measured to be in the range of 9000–14,000 W/cm<sup>2</sup> [21].

Silver nanoparticle is a good substitute for gold nanoparticles as it shows comparable elec‐ trical conductivity with high chemical stability at relatively economical price. The resultant minimum resistivity of laser‐sintered silver nanoparticle was as low as 1.59 μΩ cm [9], which is only 130% of the resistivity of its bulk counterpart. One of huge benefits of laser sintering is that the entire process is conducted in ambient conditions at low temperatures which allows using heat‐sensitive, cheap polymer as a substrate. A combinatorial study has been thoroughly done for silver nanoparticle to find the optimum laser condition for various substrates includ‐ ing widely used polymer substrates such as Polyethylene terephthalate (PET) and Polyimide (PI) film. **Figure 3(c)** shows that the resistivity of the silver nanoparticle film on the PET sub‐ strate initially drops and increases dramatically according to the increase in applied laser power due to the thermal damage on the substrate. Minute control of laser power is therefore required for the laser sintering of the metal nanoparticle on flexible substrates. The effect of the underlying substrate has been also confirmed from simulation as well [8]. The mechanical durability of the laser‐annealed silver nanoparticle film on the flexible substrate was further tested with outer/inner bending, stretching, cyclic fatigue, and adhesion tests [32]. The laser‐ sintered silver nanoparticle film shows sufficient mechanical durability, however, void and

**Figure 3.** (a) Near‐field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) of the sintered gold nanoparticle at (upper) 50‐mW and (lower) 300‐mW laser power. Reprinted with permission from Ref. [19]. Copyrights 2003 American Institute of Physics. (b) Resistivity of sintered gold nanoparticle electrode at different laser powers. Reprinted with permission from Ref. [24]. Copyrights 2007 Elsevier Inc. (c) Resistivity of sintered silver nanoparticle

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872 153

Recently, more interest has been focused on the laser sintering of copper nanoparticles [36–38] for the replacement of expensive noble nanoparticles. A huge drawback of copper for the sintering process has been its oxidation at an elevated temperature in the ambient condition. Conventional bulk sintering of Cu nanoparticle in ambient condition is often not effective

porosity within the film should be minimized to improve the overall stability.

electrode at different laser powers. Reprinted with permission from Ref. [9].

**3. Materials**

152 Sintering of Functional Materials

the range of 9000–14,000 W/cm<sup>2</sup>

[21].

Silver nanoparticle is a good substitute for gold nanoparticles as it shows comparable elec‐ trical conductivity with high chemical stability at relatively economical price. The resultant minimum resistivity of laser‐sintered silver nanoparticle was as low as 1.59 μΩ cm [9], which is only 130% of the resistivity of its bulk counterpart. One of huge benefits of laser sintering is that the entire process is conducted in ambient conditions at low temperatures which allows using heat‐sensitive, cheap polymer as a substrate. A combinatorial study has been thoroughly done for silver nanoparticle to find the optimum laser condition for various substrates includ‐ ing widely used polymer substrates such as Polyethylene terephthalate (PET) and Polyimide (PI) film. **Figure 3(c)** shows that the resistivity of the silver nanoparticle film on the PET sub‐ strate initially drops and increases dramatically according to the increase in applied laser

In the early stage, metal nanoparticles, the noble metals in particular, were the main tar‐ get materials for laser sintering due to their low electrical resistance and superior chemical stability. Gold [19–27] and silver [4, 8, 9, 28–35] are the most widely studied materials for laser sintering as conductors at microscale. These nanoparticles are usually prepared in the solution form with a specific solvent at high weight percentage, while small amounts of surfactants are added to prevent unwanted agglomeration or enhance the dispersion of nanoparticles within the solution. For the efficient use of laser, Bieri et al. [19] controlled the diameter of gold nanoparticles so that the absorption depth is minimized at the laser wavelength as confirmed from effective medium theory, Rayleigh scattering, and Mie scattering. Once a focus laser scans the gold nanoparticle film for selective sintering, the remaining nanoparticle solution is washed away with the same solvent used for the nanoparticle ink. It is frequently reported that the topography of the resultant conductor changes significantly with laser parameters [21, 22]. The most obvious trend is that the linewidth widens as the laser power becomes larger, since the area subject to higher intensity than the threshold value for the initiation of the sintering process increases. The morphology of laser‐sintered gold nanoparticle line measured by atomic force microscopy (AFM) often shows a bowl‐shaped geometry which can be attributed to thermocapillary effects that arise from the Gaussian profile of a focused laser beam. This trend can be also found from the sintering of other metal nanoparticles as well [29]. At high incident laser power, the cross‐section morphology of the sintered gold line becomes "sombrero"‐like with the rough surface topography [21], and the possible reason behind such a phenomenon might be the substrate deformation as the maximum temperature at the center region exceeds the softening temperature of the underlying substrate. These morphologies are shown in **Figure 3(a)**. Besides examining the resultant morphology of the sintered metal line, macroscopic electrical conductivity is an important factor to evaluate the performance of the laser as a sintering tool. The resultant electrical conductivity is depen‐ dent on a number of factors such as particle size, irradiated laser power, and translation speed [24], but it can be as low as only two times higher (5.41 μΩ cm) than the bulk value (2.65 μΩ cm) as in **Figure 3(b)**. The difference could be explained by boundary scattering from poly‐ crystalline structures and trapped residual capping agent inside the sintered conductor. The laser power density for the strong coalescence of gold nanoparticles was measured to be in

**Figure 3.** (a) Near‐field scanning optical microscopy (NSOM) and atomic force microscopy (AFM) of the sintered gold nanoparticle at (upper) 50‐mW and (lower) 300‐mW laser power. Reprinted with permission from Ref. [19]. Copyrights 2003 American Institute of Physics. (b) Resistivity of sintered gold nanoparticle electrode at different laser powers. Reprinted with permission from Ref. [24]. Copyrights 2007 Elsevier Inc. (c) Resistivity of sintered silver nanoparticle electrode at different laser powers. Reprinted with permission from Ref. [9].

power due to the thermal damage on the substrate. Minute control of laser power is therefore required for the laser sintering of the metal nanoparticle on flexible substrates. The effect of the underlying substrate has been also confirmed from simulation as well [8]. The mechanical durability of the laser‐annealed silver nanoparticle film on the flexible substrate was further tested with outer/inner bending, stretching, cyclic fatigue, and adhesion tests [32]. The laser‐ sintered silver nanoparticle film shows sufficient mechanical durability, however, void and porosity within the film should be minimized to improve the overall stability.

Recently, more interest has been focused on the laser sintering of copper nanoparticles [36–38] for the replacement of expensive noble nanoparticles. A huge drawback of copper for the sintering process has been its oxidation at an elevated temperature in the ambient condition. Conventional bulk sintering of Cu nanoparticle in ambient condition is often not effective for its application as a conductive layer. Inert gas environment prevents such a problem of Cu nanoparticles during the sintering at high temperature by blocking the oxygen supply required for the oxidation. However, the use of inert gas unavoidably increases the overall production cost as well as the complexity of the manufacturing. Laser sintering provided a possible solution to this problem by reducing the local sintering time. Zenou et al. [36] inves‐ tigated the resistivity of sintered copper nanoparticle line at different laser power and scan velocities as shown in **Figure 4(a)**. The minimum resistivity value of the laser‐sintered copper lines was found to be highly dependent on the sintering time duration, and the lowest resis‐ tivity value was achieved at the shortest local sintering time. At high laser scan velocities, the sintered line resistivity almost approached the resistivity obtained under argon atmosphere, indicating that the oxidation problem is effectively prevented even in ambient conditions by reducing the local sintering time. Kwon et al. [37] characterized the effect of laser sintering of the copper nanoparticle in more detail based on diverse analytic tools including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffractometer (XRD), Fourier transform infrared spectroscopy (FT‐IR), and X‐ray photoelectron spectros‐ copy (XPS) and presented that the laser sintering process offers an enhanced chemical stabil‐ ity together with superior oxide suppression in ambient condition.

sintered TiO<sup>2</sup>

nanoparticle film.

**4. Experimental schemes**

nanoparticles with KrF excimer laser beam at 248‐nm wavelength. Laser irradia‐

to provide

155

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872

tion not only induced melting and coalescence of these nanoparticles but also phase change from the anatase to rutile for different photocatalytic activity (**Figure 4(b)**). ZnO nanoparticles were also sintered with the same optical configuration, and it was shown that they trans‐

enhanced electrical mobility. For the laser sintering of ZnO [40, 41] or TiO [42] nanoparticles, it was found that the environmental gas largely affects the properties of the resultant sin‐ tered film since the surrounding gas changes the amount of oxygen vacancy in the sintered

Most of the laser sintering process is solely based on photothermal reactions, but reductive sintering incorporates change in the chemical composition of the material upon the sinter‐ ing process. Representative examples include reductive sintering of CuO [43] and NiO [44] nanoparticles into copper and nickel films. In the reductive sintering, one of the constituent components such as solvent or capping molecules acts as a reducing agent and converts the metal‐oxide nanoparticle into metal. Besides the as‐prepared nanoparticles, self‐generated nanoparticles from organometallic ink [45], or particle‐free reactive ion ink [46] is also subject

Being a direct writing and non‐contact method, laser sintering contains a number of strengths including high spatial selectivity and mild environmental requirements, yet various experi‐ mental schemes have been developed to solve the remaining weaknesses of the laser sintering process such as inefficient use of material, limited resolution, and throughput. Laser sintering of the nanoparticle is a relatively simple process and typically conducted in three different steps: nanoparticle deposition on the substrate, laser scanning, and removal of the remaining nanoparticle as schematically shown in **Figure 5(a)**. Wet coating process such as spin coating or blade coating is the most direct method to prepare the nanoparticle thin film [9, 29], but a large portion of the nanoparticle is wasted as the remaining nanoparticles are washed away after selective laser sintering process. Various approaches to reuse the nanomaterial are under investigation, and the efforts have been also made in terms of experimental schemes to save the amount of deposited nanoparticles in the first place. Chung et al. [21] integrated a drop‐ on‐demand jetting system in tandem with continuous Gaussian laser irradiation to conduct nanoparticle deposition and sintering in a single platform. As far as jet printing is concerned, background heating was an important factor to evaporate the solvent during or after print‐ ing. Followed by jet printing, focused laser was irradiated to the printed line to further melt and coalesce the nanoparticles selectively. Laser sintering of the jet‐printed nanoparticle pro‐ vided both high uniformity and resolution while saving a large portion of wasted nanopar‐ ticles. In a similar way, micropipette was also used to write gold nanoparticle ink prior to the laser sintering [20]. Meanwhile, deposition of thin nanoparticle ink is not easily applicable to certain types of substrates such as polydimethylsiloxane (PDMS) substrate. Lee et al. [28] modified the original process and proposed capillary‐assisted laser direct writing (CALDW)

form into interconnected porous structures with a single laser pulse of 160 mJ/cm<sup>2</sup>

to the laser sintering process as well, following the procedures as in **Figure 4(c)**.

The application of laser sintering is not limited to metal nanoparticles, and the other class of target nanoparticles include metal‐oxide nanoparticles for their conversion into various func‐ tional layers. Metal‐oxide often requires different laser conditions; for instance, Pan et al. [39]

**Figure 4.** (a) Sintered copper nanoparticle resistivity versus laser irradiance for different laser scan velocities. Reprinted with permission from Ref. [36]. Copyrights 2014 IOP Publishing Ltd. (b) XRD pattern and SEM pictures of TiO2 films on glass after laser annealing with different levels of fluences. Reprinted with permission from Ref. [39]. Copyrights 2012 American Institute of Physics. (c) Process schematics for laser sintering of self‐generated nanoparticles from organometallic ink. Reprinted with permission from Ref. [45]. Copyrights 2010 Optical Society of America.

sintered TiO<sup>2</sup> nanoparticles with KrF excimer laser beam at 248‐nm wavelength. Laser irradia‐ tion not only induced melting and coalescence of these nanoparticles but also phase change from the anatase to rutile for different photocatalytic activity (**Figure 4(b)**). ZnO nanoparticles were also sintered with the same optical configuration, and it was shown that they trans‐ form into interconnected porous structures with a single laser pulse of 160 mJ/cm<sup>2</sup> to provide enhanced electrical mobility. For the laser sintering of ZnO [40, 41] or TiO [42] nanoparticles, it was found that the environmental gas largely affects the properties of the resultant sin‐ tered film since the surrounding gas changes the amount of oxygen vacancy in the sintered nanoparticle film.

Most of the laser sintering process is solely based on photothermal reactions, but reductive sintering incorporates change in the chemical composition of the material upon the sinter‐ ing process. Representative examples include reductive sintering of CuO [43] and NiO [44] nanoparticles into copper and nickel films. In the reductive sintering, one of the constituent components such as solvent or capping molecules acts as a reducing agent and converts the metal‐oxide nanoparticle into metal. Besides the as‐prepared nanoparticles, self‐generated nanoparticles from organometallic ink [45], or particle‐free reactive ion ink [46] is also subject to the laser sintering process as well, following the procedures as in **Figure 4(c)**.

## **4. Experimental schemes**

for its application as a conductive layer. Inert gas environment prevents such a problem of Cu nanoparticles during the sintering at high temperature by blocking the oxygen supply required for the oxidation. However, the use of inert gas unavoidably increases the overall production cost as well as the complexity of the manufacturing. Laser sintering provided a possible solution to this problem by reducing the local sintering time. Zenou et al. [36] inves‐ tigated the resistivity of sintered copper nanoparticle line at different laser power and scan velocities as shown in **Figure 4(a)**. The minimum resistivity value of the laser‐sintered copper lines was found to be highly dependent on the sintering time duration, and the lowest resis‐ tivity value was achieved at the shortest local sintering time. At high laser scan velocities, the sintered line resistivity almost approached the resistivity obtained under argon atmosphere, indicating that the oxidation problem is effectively prevented even in ambient conditions by reducing the local sintering time. Kwon et al. [37] characterized the effect of laser sintering of the copper nanoparticle in more detail based on diverse analytic tools including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffractometer (XRD), Fourier transform infrared spectroscopy (FT‐IR), and X‐ray photoelectron spectros‐ copy (XPS) and presented that the laser sintering process offers an enhanced chemical stabil‐

The application of laser sintering is not limited to metal nanoparticles, and the other class of target nanoparticles include metal‐oxide nanoparticles for their conversion into various func‐ tional layers. Metal‐oxide often requires different laser conditions; for instance, Pan et al. [39]

**Figure 4.** (a) Sintered copper nanoparticle resistivity versus laser irradiance for different laser scan velocities. Reprinted with permission from Ref. [36]. Copyrights 2014 IOP Publishing Ltd. (b) XRD pattern and SEM pictures of TiO2 films on glass after laser annealing with different levels of fluences. Reprinted with permission from Ref. [39]. Copyrights 2012 American Institute of Physics. (c) Process schematics for laser sintering of self‐generated nanoparticles from

organometallic ink. Reprinted with permission from Ref. [45]. Copyrights 2010 Optical Society of America.

ity together with superior oxide suppression in ambient condition.

154 Sintering of Functional Materials

Being a direct writing and non‐contact method, laser sintering contains a number of strengths including high spatial selectivity and mild environmental requirements, yet various experi‐ mental schemes have been developed to solve the remaining weaknesses of the laser sintering process such as inefficient use of material, limited resolution, and throughput. Laser sintering of the nanoparticle is a relatively simple process and typically conducted in three different steps: nanoparticle deposition on the substrate, laser scanning, and removal of the remaining nanoparticle as schematically shown in **Figure 5(a)**. Wet coating process such as spin coating or blade coating is the most direct method to prepare the nanoparticle thin film [9, 29], but a large portion of the nanoparticle is wasted as the remaining nanoparticles are washed away after selective laser sintering process. Various approaches to reuse the nanomaterial are under investigation, and the efforts have been also made in terms of experimental schemes to save the amount of deposited nanoparticles in the first place. Chung et al. [21] integrated a drop‐ on‐demand jetting system in tandem with continuous Gaussian laser irradiation to conduct nanoparticle deposition and sintering in a single platform. As far as jet printing is concerned, background heating was an important factor to evaporate the solvent during or after print‐ ing. Followed by jet printing, focused laser was irradiated to the printed line to further melt and coalesce the nanoparticles selectively. Laser sintering of the jet‐printed nanoparticle pro‐ vided both high uniformity and resolution while saving a large portion of wasted nanopar‐ ticles. In a similar way, micropipette was also used to write gold nanoparticle ink prior to the laser sintering [20]. Meanwhile, deposition of thin nanoparticle ink is not easily applicable to certain types of substrates such as polydimethylsiloxane (PDMS) substrate. Lee et al. [28] modified the original process and proposed capillary‐assisted laser direct writing (CALDW)

heating spot. The amount of heat generation is proportional to the laser intensity in general, and the spatial distribution of the laser intensity at its focus is determined by two factors in principle—wavelength of the laser and the numerical aperture of the lens used for the focus‐ ing, assuming that the optical system is reached at its diffraction limit. In order to push the resolution of laser sintering to nanoscale, ultrashort pulse laser or near‐field optics has been employed. Son et al. [4] utilized an ultrashort pulsed laser with pulse duration of less than 100 fs, together with a 100 × oil immersion objective lens, to minimize the focused laser's spot size as well as the thermal diffusion from the heating spot. As a result, the uniform sintered line at 380 nm was fabricated with 780‐nm wavelength laser. On the other hand, Pan et al. [27] shrank the focused spot size by using near‐field optics. Fluidically assembled microspheres at 3‐μm diameter were employed as microlens array as shown in **Figure 5(b)**, and the normalized intensity distribution underneath the microsphere calculated by discrete dipole approxima‐ tion (DDA) confirming that the full width at half maximum (FWHM) of laser intensity at the designated position can be reduced down to 370 nm. The size of sintered dot diameter was as small as 200 nm, which was further reduced to 50 nm in diameter by the simple post annealing

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872 157

Besides, conventional laser sintering process inevitably suffers from low throughput com‐ pared to other mass production techniques due to its direct writing nature, and diverse experimental schemes have been proposed to solve this problem. Instead of moving the sample stage, Yeo et al. [9] scanned the laser beam using 2D galvanometric scanning mir‐ ror system to increase the sintering speed to a large extent, up to several meters per sec‐ ond. The laser scanner system was controlled by the CAD software to draw arbitrary 2D images, and it successfully fabricated metal micropatterns on a large polymer substrate over a 4‐inch wafer in a fraction of minutes. Pan et al. [42] increased the sintering area per single scanning by using a cylindrical lens that produces the focused spot of ellipti‐ cal cross‐section. A large area sintering was feasible with a relatively small amount of scanning by overlapping the extended focus. On the other hand, An et al. [31] introduced digital micromirror device (DMD) as an optical stamp so that an arbitrary 2D pattern can be sintered at a single exposure without any raster scanning, which is often time consum‐ ing. 10 × 10 star‐shaped silver electrodes with fine‐edge sharpness were fabricated by a step‐and‐repeat scheme to ensure the potential of this process for the large‐area metallic micropattern fabrication. The throughput of laser processing can be further increased as the laser system is compatible to other mass production processes. As a proof of the con‐ cept, Yeo et al. [33] integrated the laser sintering scheme with roll‐to‐roll (R2R) printing system to replace the conventional furnace annealing process and boost the area subject to the laser process. The conceptual image of laser sintering process integrated with the R2R

In the early stage, conducting microlines fabricated by laser sintering of metallic nanopar‐ ticles was a good alternative for photolithographically defined metal patterns, especially

process owing to the densification and beading.

system is illustrated in **Figure 5(c)**.

**5. Applications**

**Figure 5.** (a) Schematics for typical laser sintering of the nanoparticle. Reprinted with permission from Ref. [9]. (b) Near‐field intensity enhancement below the microsphere. Reprinted with permission from Ref. [27]. Copyrights 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (c) A schematic illustration of R2R system integrated with selective laser sintering technique. Reprinted with permission from Ref. [34]. Copyrights 2015 The Royal Society of Chemistry.

of silver nanoparticle for such a substrate that conducts laser sintering of silver nanoparticle in the colloidal environment. On the other hand, some applications require the thickness of the nanoparticle layer to be several microns, which is difficult to anneal with single laser irradia‐ tion due to limited optical penetration depth. For the preparation of such a thick nanoparticle film, Pan et al. [39] combined collision nebulizer with laser sintering to achieve simultaneous deposition and sintering in a single configuration.

Compared to other printing techniques, the minimum feature size achievable by laser sintering process is very small—reaching 2‐μm feature size by tight focusing [9]—but often not as good as state‐of‐the‐art photolithography process. Sintering is a thermally activated process in principle, and the resultant temperature distribution is largely determined by the area subject to the photothermal heat generation as well as the heat diffusion from the heating spot. The amount of heat generation is proportional to the laser intensity in general, and the spatial distribution of the laser intensity at its focus is determined by two factors in principle—wavelength of the laser and the numerical aperture of the lens used for the focus‐ ing, assuming that the optical system is reached at its diffraction limit. In order to push the resolution of laser sintering to nanoscale, ultrashort pulse laser or near‐field optics has been employed. Son et al. [4] utilized an ultrashort pulsed laser with pulse duration of less than 100 fs, together with a 100 × oil immersion objective lens, to minimize the focused laser's spot size as well as the thermal diffusion from the heating spot. As a result, the uniform sintered line at 380 nm was fabricated with 780‐nm wavelength laser. On the other hand, Pan et al. [27] shrank the focused spot size by using near‐field optics. Fluidically assembled microspheres at 3‐μm diameter were employed as microlens array as shown in **Figure 5(b)**, and the normalized intensity distribution underneath the microsphere calculated by discrete dipole approxima‐ tion (DDA) confirming that the full width at half maximum (FWHM) of laser intensity at the designated position can be reduced down to 370 nm. The size of sintered dot diameter was as small as 200 nm, which was further reduced to 50 nm in diameter by the simple post annealing process owing to the densification and beading.

Besides, conventional laser sintering process inevitably suffers from low throughput com‐ pared to other mass production techniques due to its direct writing nature, and diverse experimental schemes have been proposed to solve this problem. Instead of moving the sample stage, Yeo et al. [9] scanned the laser beam using 2D galvanometric scanning mir‐ ror system to increase the sintering speed to a large extent, up to several meters per sec‐ ond. The laser scanner system was controlled by the CAD software to draw arbitrary 2D images, and it successfully fabricated metal micropatterns on a large polymer substrate over a 4‐inch wafer in a fraction of minutes. Pan et al. [42] increased the sintering area per single scanning by using a cylindrical lens that produces the focused spot of ellipti‐ cal cross‐section. A large area sintering was feasible with a relatively small amount of scanning by overlapping the extended focus. On the other hand, An et al. [31] introduced digital micromirror device (DMD) as an optical stamp so that an arbitrary 2D pattern can be sintered at a single exposure without any raster scanning, which is often time consum‐ ing. 10 × 10 star‐shaped silver electrodes with fine‐edge sharpness were fabricated by a step‐and‐repeat scheme to ensure the potential of this process for the large‐area metallic micropattern fabrication. The throughput of laser processing can be further increased as the laser system is compatible to other mass production processes. As a proof of the con‐ cept, Yeo et al. [33] integrated the laser sintering scheme with roll‐to‐roll (R2R) printing system to replace the conventional furnace annealing process and boost the area subject to the laser process. The conceptual image of laser sintering process integrated with the R2R system is illustrated in **Figure 5(c)**.

## **5. Applications**

of silver nanoparticle for such a substrate that conducts laser sintering of silver nanoparticle in the colloidal environment. On the other hand, some applications require the thickness of the nanoparticle layer to be several microns, which is difficult to anneal with single laser irradia‐ tion due to limited optical penetration depth. For the preparation of such a thick nanoparticle film, Pan et al. [39] combined collision nebulizer with laser sintering to achieve simultaneous

**Figure 5.** (a) Schematics for typical laser sintering of the nanoparticle. Reprinted with permission from Ref. [9]. (b) Near‐field intensity enhancement below the microsphere. Reprinted with permission from Ref. [27]. Copyrights 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (c) A schematic illustration of R2R system integrated with selective laser sintering technique. Reprinted with permission from Ref. [34]. Copyrights 2015 The Royal Society of Chemistry.

Compared to other printing techniques, the minimum feature size achievable by laser sintering process is very small—reaching 2‐μm feature size by tight focusing [9]—but often not as good as state‐of‐the‐art photolithography process. Sintering is a thermally activated process in principle, and the resultant temperature distribution is largely determined by the area subject to the photothermal heat generation as well as the heat diffusion from the

deposition and sintering in a single configuration.

156 Sintering of Functional Materials

In the early stage, conducting microlines fabricated by laser sintering of metallic nanopar‐ ticles was a good alternative for photolithographically defined metal patterns, especially

as polymer substrates by applying successive laser sintering for multilayer configuration. The simplest passive electronic component that requires multilayer structure is a crossover capacitor. For its fabrication, a lower level conductor line was firstly ink‐jet printed and processed with laser on a PI film, followed by printing of dielectric layer, and another micro‐conductor line as shown in **Figure 6(a)**. The measured capacitance for ∼200‐nm dielectric layer thickness was 1–10 pF for non‐shorted capacitors [24]. A transistor is a representative example of active electronic components, and it has been proven that the conductor microlines fabricated by laser sintering of metallic nanoparticles can well replace the electrodes including gate, source, and drain. For the fabrication of the organic field effect transistor (OFET), carboxylated polythiophene with increased air stability was used as the semiconducting polymer. The carrier mobility of the resultant OFET on sili‐ con wafer with laser‐sintered source and drain was measured to be around 0.01 cm<sup>2</sup>

and this value is comparable to the lithographically processed OFET fabricated with the same semiconducting polymer [26]. By expanding the idea and incorporating large‐area scanning method, 11,520 OFET arrays were fabricated on a flexible PI substrate with two successive laser sintering processes for source, drain, and gate electrodes [9]. The transistor performance characteristics were measured before and after the 100,000‐times bending cycles and confirmed no significant changes in the OFET performance as shown in **Figure 6(b)**. This result validates that the electrical performance of the devices fabri‐ cated by the current process is acceptable for broad applications in flexible electronics. On the other hand, Lee et al. [41] utilized laser‐sintered ZnO nanoparticle to replace the active

Starting from the simple microconductor lines in minute electronic components, potential applications of laser‐sintered nanoparticles have been extended to a large extent. Hong et al. [29], Suh et al. [38], and Lee et al. [44] applied laser sintering of silver, copper, and nickel nanoparticles, respectively, to fabricate a large‐area flexible transparent conductor based on a metal microgrid structure and utilized it for other optoelectronic devices such as a touch screen panel (**Figure 6(c)**) As the area subject to the laser sintering has increased, laser‐sintered nanoparticles find its application in energy devices as well. R2R‐printed silver nanoparticles followed by rapid laser annealing is proven to be an efficient way of pre‐ paring the current collector for various energy devices including supercapacitors [33, 34] as in **Figure 6(d)**, while sintering of the metal‐oxide nanoparticle can be employed for the active electrode layer of solar cells [39]. Besides, the usage of laser‐sintered nanoparticles is still under investigation as in the example of the nanocomposite of a nanowire‐reinforced

This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government

Ministry of Knowledge Economy. (No. KETEP20174010201310)

channel layer.

nanoparticle matrix film [35].

**Acknowledgements**

/V,

159

Selective Laser Sintering of Nanoparticles http://dx.doi.org/10.5772/intechopen.68872

**Figure 6.** (a) Crossover capacitor schematics on the PI film. Reprinted with permission from Ref. [24]. Copyrights 2007 Elsevier Inc. (b) Output (top graph) and transfer (bottom graph) characteristics of OFET array before and after the bending cycle. Reprinted with permission from Ref. [9]. (c) Demonstration of a metallic grid transparent conductor‐based touch panel fabricated by selective laser sintering process. Reprinted with permission from Ref. [29]. Copyrights 2013 American Chemical Society. (d) Demonstration of a series‐connected supercapacitor based on laser‐sintered nanoparticle current collector to power LED. Reprinted with permission from Ref. [34]. Copyrights 2015 The Royal Society of Chemistry.

for flexible electronics which is not often compatible to the conventional fabrication processes. Laser‐sintered microlines were applied to both passive and active electronics, and Ko et al. [24–26] fabricated various electronic components on rigid substrates as well as polymer substrates by applying successive laser sintering for multilayer configuration. The simplest passive electronic component that requires multilayer structure is a crossover capacitor. For its fabrication, a lower level conductor line was firstly ink‐jet printed and processed with laser on a PI film, followed by printing of dielectric layer, and another micro‐conductor line as shown in **Figure 6(a)**. The measured capacitance for ∼200‐nm dielectric layer thickness was 1–10 pF for non‐shorted capacitors [24]. A transistor is a representative example of active electronic components, and it has been proven that the conductor microlines fabricated by laser sintering of metallic nanoparticles can well replace the electrodes including gate, source, and drain. For the fabrication of the organic field effect transistor (OFET), carboxylated polythiophene with increased air stability was used as the semiconducting polymer. The carrier mobility of the resultant OFET on sili‐ con wafer with laser‐sintered source and drain was measured to be around 0.01 cm<sup>2</sup> /V, and this value is comparable to the lithographically processed OFET fabricated with the same semiconducting polymer [26]. By expanding the idea and incorporating large‐area scanning method, 11,520 OFET arrays were fabricated on a flexible PI substrate with two successive laser sintering processes for source, drain, and gate electrodes [9]. The transistor performance characteristics were measured before and after the 100,000‐times bending cycles and confirmed no significant changes in the OFET performance as shown in **Figure 6(b)**. This result validates that the electrical performance of the devices fabri‐ cated by the current process is acceptable for broad applications in flexible electronics. On the other hand, Lee et al. [41] utilized laser‐sintered ZnO nanoparticle to replace the active channel layer.

Starting from the simple microconductor lines in minute electronic components, potential applications of laser‐sintered nanoparticles have been extended to a large extent. Hong et al. [29], Suh et al. [38], and Lee et al. [44] applied laser sintering of silver, copper, and nickel nanoparticles, respectively, to fabricate a large‐area flexible transparent conductor based on a metal microgrid structure and utilized it for other optoelectronic devices such as a touch screen panel (**Figure 6(c)**) As the area subject to the laser sintering has increased, laser‐sintered nanoparticles find its application in energy devices as well. R2R‐printed silver nanoparticles followed by rapid laser annealing is proven to be an efficient way of pre‐ paring the current collector for various energy devices including supercapacitors [33, 34] as in **Figure 6(d)**, while sintering of the metal‐oxide nanoparticle can be employed for the active electrode layer of solar cells [39]. Besides, the usage of laser‐sintered nanoparticles is still under investigation as in the example of the nanocomposite of a nanowire‐reinforced nanoparticle matrix film [35].

## **Acknowledgements**

**Figure 6.** (a) Crossover capacitor schematics on the PI film. Reprinted with permission from Ref. [24]. Copyrights 2007 Elsevier Inc. (b) Output (top graph) and transfer (bottom graph) characteristics of OFET array before and after the bending cycle. Reprinted with permission from Ref. [9]. (c) Demonstration of a metallic grid transparent conductor‐based touch panel fabricated by selective laser sintering process. Reprinted with permission from Ref. [29]. Copyrights 2013 American Chemical Society. (d) Demonstration of a series‐connected supercapacitor based on laser‐sintered nanoparticle current collector to power LED. Reprinted with permission from Ref. [34]. Copyrights 2015 The Royal Society of Chemistry.

158 Sintering of Functional Materials

for flexible electronics which is not often compatible to the conventional fabrication processes. Laser‐sintered microlines were applied to both passive and active electronics, and Ko et al. [24–26] fabricated various electronic components on rigid substrates as well This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy. (No. KETEP20174010201310)

## **Author details**

#### Sukjoon Hong

Address all correspondence to: sukjoonhong@hangyang.ac.kr

Department of Mechanical Engineering, Hanyang University, Ansan, Gyeonggi‐do, Republic of Korea

## **References**

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Address all correspondence to: sukjoonhong@hangyang.ac.kr

Department of Mechanical Engineering, Hanyang University, Ansan, Gyeonggi‐do,

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[42] Pan H, Lee D, Ko SH, Grigoropoulos CP, Park HK, Hoult T. Fiber laser annealing of indium‐tin‐oxide nanoparticles for large area transparent conductive layers and optical

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**Chapter 8**

**Provisional chapter**

**High-Pressure High-Temperature (HPHT) Synthesis of**

High-pressure techniques have been used extensively in effecting phase changes in materials science for decades. The use of high-pressure high temperature enables changes in material atomic arrangement or structure which in turn brings about changes in functional properties such as magnetism, optical, electrical and thermal conductivity. Highpressure technology is highly specialised and requires understanding to fully utilise its potential as a tool for the development of new and novel functional materials with improved properties. This chapter explores the various high-pressure technologies available and how they have been utilised to obtain a wide range of functional ceramic materi-

**Keywords:** high-pressure high-temperature sintering, functional materials,

**High-Pressure High-Temperature (HPHT) Synthesis of** 

DOI: 10.5772/intechopen.72453

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

superconductors, polymor-

and reproduction in any medium, provided the original work is properly cited.

The discovery of novel properties and quantum states at high pressure has led to a number of new functional material categories. Pressure has long been recognised as a fundamental thermodynamic variable which can be used to manipulate electronic, magnetic, structural and vibrational properties of materials for a wide range of applications. High pressure effectively decreases the atomic volume and increases the electronic density of reactants which results in unusual and interesting properties. There are two basic approaches evident to high-pressure synthesis which involves structural transformation on the one hand and formation of new chemical bonds on the other. Particularly noticeable discoveries in high-pressure physics

**Functional Materials**

**Functional Materials**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72453

als for a wide range of applications.

high-pressure synthesis, phase transition

include metallisation of hydrogen, quantum criticality, high *Tc*

Wallace Matizamhuka

**Abstract**

**1. Introduction**

phism and exotic metals [1].

Wallace Matizamhuka

**Provisional chapter**

## **High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials**

DOI: 10.5772/intechopen.72453

Wallace Matizamhuka Wallace Matizamhuka Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72453

#### **Abstract**

High-pressure techniques have been used extensively in effecting phase changes in materials science for decades. The use of high-pressure high temperature enables changes in material atomic arrangement or structure which in turn brings about changes in functional properties such as magnetism, optical, electrical and thermal conductivity. Highpressure technology is highly specialised and requires understanding to fully utilise its potential as a tool for the development of new and novel functional materials with improved properties. This chapter explores the various high-pressure technologies available and how they have been utilised to obtain a wide range of functional ceramic materials for a wide range of applications.

**Keywords:** high-pressure high-temperature sintering, functional materials, high-pressure synthesis, phase transition

## **1. Introduction**

The discovery of novel properties and quantum states at high pressure has led to a number of new functional material categories. Pressure has long been recognised as a fundamental thermodynamic variable which can be used to manipulate electronic, magnetic, structural and vibrational properties of materials for a wide range of applications. High pressure effectively decreases the atomic volume and increases the electronic density of reactants which results in unusual and interesting properties. There are two basic approaches evident to high-pressure synthesis which involves structural transformation on the one hand and formation of new chemical bonds on the other. Particularly noticeable discoveries in high-pressure physics include metallisation of hydrogen, quantum criticality, high *Tc* superconductors, polymorphism and exotic metals [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The impact of the pressure dimension has not been very conspicuous because a number of substances that exist at high pressure cannot be retained at ambient pressure [2]. Strictly speaking pressure-induced transitions result in *metastable phases* whose properties may or may not change reversibly over a period of an experiment or observation [2]. Such a metastable condition is enabled by the existence of an energy barrier which when surpassed results in a transition to a thermodynamically equilibrium ground state [2]. This explains the crystallisation of amorphous materials upon heating and the conversion of diamond (high-pressure phase) to graphite (ambient pressure phase) at temperatures above 1500 K under normal pressure [2]. Brazhkin in his analysis explains the stability of high-pressure phases on the basis of a simple thermodynamic argument. Simply put, a metastable high-pressure phase possesses Gibb's free energy, G higher than that of a stable phase [2]. He went on to argue that the equilibrium melting temperature of a metastable crystal (\*Tm) is always lower than that of the equilibrium melting temperature of the stable phase (Tm). This basically explains why all metastable highpressure phases transform to more stable phases upon heating at far lower temperatures than the melting temperature of the stable phase (Tm) [2].

a small hole to accommodate the sample. The DAC can reach very high pressures in excess of 150 GPa and can be heated easily using infrared lasers reaching temperatures in excess of

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

http://dx.doi.org/10.5772/intechopen.72453

167

Alternative methods which make use of dynamic pressures generated through the shock wave technique allow for pressures of ~100–1000 GPa over short times of dynamic pressure action (nanoseconds) [3]. The shock wave technique has been used to produce materials in

at pressures of 10–100 GPa [2].

Sintering involves powder compaction at elevated temperatures with or without pressure application to obtain densified solid compacts. Thus the driving force of sintering is a function of surface-free energy, temperature and pressure of the system. At low temperatures and pressures (typically <0.5 Tm), the energy available is insufficient to allow for diffusional mass transport in solid-state sintering of most materials with micron-sized particles [4]. The diffusion coefficient, *D*, is temperature dependent and follows an Arrhenius relationship with the

∆*H*

 is a constant dependent on the atomic planar distance, and the mean frequency of vibration, ∆*H*, represents the enthalpy change associated with overcoming the diffusion energy

The application of pressure during sintering is an effective way of improving the rate of densification as it lower diffusion distances between adjacent particles. This is well articulated by the various creep equations which relate creep rate (linear strain rate) to the densification rate (volumetric strain rate). During hot pressing, the density *D* of the sample increases with

*M* represents the sample mass, *A* the cross-sectional area and the subscripts *0* and *f* refer to

*dt* <sup>+</sup> *<sup>D</sup>* \_\_\_ *dL*

This equation relates the linear strain rate of the body *(dL*/*dt)* to its densification rate *(dD*/*dt).* This is the basis of the Nabarro-Herring creep equation which in essence argues that selfdiffusion within the crystal will cause the solid to deform in an attempt to relieve stress [5].

*L* \_\_\_ *dL dt* <sup>=</sup> \_\_1 *D* \_\_\_ *dD*

*RT* ) (1)

*<sup>A</sup>* = *LD* = *L*<sup>0</sup> *D*<sup>0</sup> = *Lf Df* (2)

*dt* <sup>=</sup> <sup>0</sup> (3)

*dt* (4)

5000 K [3].

*D0*

large volumes typically 1–10 cm<sup>3</sup>

absolute temperature, *T* as follows:

barrier, and *R* is the gas constant.

\_\_

This can be simplified to

initial and final states. Differentiating Eq. (2) yields

*L* \_\_\_ *dD*

−\_\_1

*<sup>D</sup>* <sup>=</sup> *<sup>D</sup>*<sup>0</sup> *exp*(−\_\_\_\_

a decrease in thickness *L*. The variables *D* and *L* are related as follows:

*M*

**2.1. Theory of sintering**

The advancement of high-pressure technology has not been as widespread as other synthesis techniques such as high temperature and catalysis [2]. This emanates from the fact that most high-pressure apparatus are quite complex and costly and the volumes of material obtainable at high pressure are very small [2]. Despite these negative factors, the apparatus that have been developed up to date have given rise to some of the most intriguing material properties ever known to mankind. Lower static pressures in the range 0.1–1 GPa are achievable in fairly large volumes ~0.01–1 m<sup>3</sup> using gas containers, piston-cylinder type and autoclave presses [2]. Pressures in excess of 1 GPa can only be achieved in apparatus which are mechanically operated [2].

## **2. High-pressure technology and apparatus**

High-pressure synthesis can be broadly divided into static and dynamic technologies. Traditionally there are two most widely used static-type apparatus, namely, the piston-cylinder and Bridgeman anvil type. The piston-cylinder pressure zone can accommodate volumes in the range 1–1000 cm<sup>3</sup> for pressures of up to 1 GPa [2]. They can reach maximum pressures of 3 GPa where smaller pressure zone volumes and very large presses are needed [2]. The Bridgeman anvil type can achieve very high pressures depending on the type of anvils: for hard alloy anvils, pressures in the range 15–20 GPa, for SiC anvils 20–70 GPa and for diamond anvils 100–300 GPa are achievable [2]. Commercial presses which incorporate concepts from the Bridgeman anvil and piston-cylinder designs have been developed to synthesise superhard materials obtaining pressures over 5 GPa with volumes of ~1 cm<sup>3</sup> [2]. The most widely used presses in this class are known as the *belt*, multi-anvil apparatus and toroid [2]. Another important laboratory-scale high-pressure tool is the diamond anvil cell (DAC). The DAC is used extensively for exploratory high-pressure synthesis and for characterisation of materials under high pressure. It basically consists of two gem-quality diamonds (~1/3 carat) with flat surfaces (culets) capable of compressing small samples on a metal gasket containing a small hole to accommodate the sample. The DAC can reach very high pressures in excess of 150 GPa and can be heated easily using infrared lasers reaching temperatures in excess of 5000 K [3].

Alternative methods which make use of dynamic pressures generated through the shock wave technique allow for pressures of ~100–1000 GPa over short times of dynamic pressure action (nanoseconds) [3]. The shock wave technique has been used to produce materials in large volumes typically 1–10 cm<sup>3</sup> at pressures of 10–100 GPa [2].

#### **2.1. Theory of sintering**

The impact of the pressure dimension has not been very conspicuous because a number of substances that exist at high pressure cannot be retained at ambient pressure [2]. Strictly speaking pressure-induced transitions result in *metastable phases* whose properties may or may not change reversibly over a period of an experiment or observation [2]. Such a metastable condition is enabled by the existence of an energy barrier which when surpassed results in a transition to a thermodynamically equilibrium ground state [2]. This explains the crystallisation of amorphous materials upon heating and the conversion of diamond (high-pressure phase) to graphite (ambient pressure phase) at temperatures above 1500 K under normal pressure [2]. Brazhkin in his analysis explains the stability of high-pressure phases on the basis of a simple thermodynamic argument. Simply put, a metastable high-pressure phase possesses Gibb's free energy, G higher than that of a stable phase [2]. He went on to argue that the equilibrium melting temperature of a metastable crystal (\*Tm) is always lower than that of the equilibrium melting temperature of the stable phase (Tm). This basically explains why all metastable highpressure phases transform to more stable phases upon heating at far lower temperatures than

The advancement of high-pressure technology has not been as widespread as other synthesis techniques such as high temperature and catalysis [2]. This emanates from the fact that most high-pressure apparatus are quite complex and costly and the volumes of material obtainable at high pressure are very small [2]. Despite these negative factors, the apparatus that have been developed up to date have given rise to some of the most intriguing material properties ever known to mankind. Lower static pressures in the range 0.1–1 GPa are achievable in fairly

[2]. Pressures in excess of 1 GPa can only be achieved in apparatus which are mechanically

High-pressure synthesis can be broadly divided into static and dynamic technologies. Traditionally there are two most widely used static-type apparatus, namely, the piston-cylinder and Bridgeman anvil type. The piston-cylinder pressure zone can accommodate volumes

of 3 GPa where smaller pressure zone volumes and very large presses are needed [2]. The Bridgeman anvil type can achieve very high pressures depending on the type of anvils: for hard alloy anvils, pressures in the range 15–20 GPa, for SiC anvils 20–70 GPa and for diamond anvils 100–300 GPa are achievable [2]. Commercial presses which incorporate concepts from the Bridgeman anvil and piston-cylinder designs have been developed to synthesise

widely used presses in this class are known as the *belt*, multi-anvil apparatus and toroid [2]. Another important laboratory-scale high-pressure tool is the diamond anvil cell (DAC). The DAC is used extensively for exploratory high-pressure synthesis and for characterisation of materials under high pressure. It basically consists of two gem-quality diamonds (~1/3 carat) with flat surfaces (culets) capable of compressing small samples on a metal gasket containing

superhard materials obtaining pressures over 5 GPa with volumes of ~1 cm<sup>3</sup>

using gas containers, piston-cylinder type and autoclave presses

for pressures of up to 1 GPa [2]. They can reach maximum pressures

[2]. The most

the melting temperature of the stable phase (Tm) [2].

**2. High-pressure technology and apparatus**

large volumes ~0.01–1 m<sup>3</sup>

166 Sintering of Functional Materials

in the range 1–1000 cm<sup>3</sup>

operated [2].

Sintering involves powder compaction at elevated temperatures with or without pressure application to obtain densified solid compacts. Thus the driving force of sintering is a function of surface-free energy, temperature and pressure of the system. At low temperatures and pressures (typically <0.5 Tm), the energy available is insufficient to allow for diffusional mass transport in solid-state sintering of most materials with micron-sized particles [4]. The diffusion coefficient, *D*, is temperature dependent and follows an Arrhenius relationship with the absolute temperature, *T* as follows:

$$D = D\_0 \exp\left(-\frac{\Delta H}{RT}\right) \tag{1}$$

*D0* is a constant dependent on the atomic planar distance, and the mean frequency of vibration, ∆*H*, represents the enthalpy change associated with overcoming the diffusion energy barrier, and *R* is the gas constant.

The application of pressure during sintering is an effective way of improving the rate of densification as it lower diffusion distances between adjacent particles. This is well articulated by the various creep equations which relate creep rate (linear strain rate) to the densification rate (volumetric strain rate). During hot pressing, the density *D* of the sample increases with a decrease in thickness *L*. The variables *D* and *L* are related as follows:

$$\frac{M}{A} = LD = L\_0 D\_0 = L\_\uparrow D\_\uparrow \tag{2}$$

*M* represents the sample mass, *A* the cross-sectional area and the subscripts *0* and *f* refer to initial and final states. Differentiating Eq. (2) yields

$$L\frac{d\mathbf{D}}{dt} + \mathbf{D}\frac{d\mathbf{L}}{dt} = \mathbf{0} \tag{3}$$

This can be simplified to

$$-\frac{1}{L}\frac{dL}{dt} = \frac{1}{D}\frac{dD}{dt}\tag{4}$$

This equation relates the linear strain rate of the body *(dL*/*dt)* to its densification rate *(dD*/*dt).* This is the basis of the Nabarro-Herring creep equation which in essence argues that selfdiffusion within the crystal will cause the solid to deform in an attempt to relieve stress [5]. The creep being a result of atoms diffusing from interfaces subjected to a compressive stress (where they have a higher chemical potential) towards those subjected to a tensile stress (lower chemical potential). Herring derived an important relationship between the atomic fluxes *ε* to the grain size, *G*:

$$
\varepsilon\_c^{\prime} = \frac{40}{3} \left( \frac{D\_s \, \Omega \, P\_s}{G^2 kT} \right) \tag{5}
$$

**2.3. Superconducting materials**

for example. Over the years, the *Tc*

term 'high temperature *Tc*

(Bi<sup>2</sup> Sr2 Ca<sup>2</sup> Cu<sup>3</sup> Ox

ductors [7].

and low *Tc*

the c-axis.

The *<sup>Y</sup> Ba*<sup>2</sup> *Cu*<sup>3</sup> *<sup>O</sup>*7−*<sup>δ</sup>*

and Muller discovered high temperature *Tc*

*.* Values such as SrTiO<sup>3</sup>

the late 1990s, Cava et al. discovered a high *Tc*

mentation of a number of copper oxide ceramics with high *Tc*

ducting state (0.5< *δ* <1.0) by adjusting oxygen content.

possess layered perovskite structure with alternate stacks of CuO<sup>2</sup>

However oxygen doping mechanisms are not completely understood.

a *Tc* > 30 K. In 1987, Wu et al. discovered the YBa<sup>2</sup>

) and thallium (TI<sup>2</sup>

becomes zero is known as the critical temperature, *Tc*

In July 1908 a Dutch physicist Heike Kamerlingh Onnes discovered the superconductivity of mercury. He observed that the electrical resistance of mercury dropped down to zero under cryogen conditions (helium cooling at 4.2 K). The temperature at which the resistance

ductors were found to possess such behaviour under liquid helium making the technology complex and costly because helium has limited supply and is costly compared to nitrogen,

steps with the discovery of more and more superconducting materials. In 1986, Bedworz

initiated a rapid development in the research of ceramic oxide superconductors [7]. The

ducting at 93 K which was followed by discovery in 1988 of compounds based on Bismuth

125 K, respectively [8–10]. The most exciting feature of the oxide ceramic superconductors is that superconductivity can be achieved in liquid nitrogen (77 K) which has lower cooling costs than helium [7]. Moreover, the superconductivity of ceramic superconductors can be maintained at considerably high magnetic fields than the conventional metallic supercon-

Another interesting group of superconducting materials is the cubic perovskite and its derivatives. Which have been in existence for over three decades now. They possess a cubic structure

As mentioned earlier, the discovery of La-Ba-Cu-O superconductors triggered the experi-

The metal atom ratio in the structure is well defined as 1:2:3 the oxygen stoichiometry varies widely depending on the synthesis condition [7]. The oxygen content is key in regulating the structures and electrical properties (1.5 in Fei). In the stoichiometry Y-123, *δ* is always positive and can vary from 0.0 to as high as 1.0 [7]. An optimum superconducting property can be obtained by maintaining *δ* at lower value. The YBCO structure undergoes phase transition during heat treatment from a tetragonal-insulating state (*δ* < 0.4) to orthorhombic supercon-

There are two routes to obtaining oxide superconductors, i.e. doping of parent material through cationic substitution or oxygen nonstoichiometry. The more popular method is doping based on oxygen nonstoichiometry compared to cationic (or anion) substitutions.

(YBCO) possesses an oxygen-deficit orthorhombic perovskite structure [7].

(*Tc* = 0.4 K), Ba(Pb,Bi)O<sup>3</sup>

Ba2 Ca<sup>2</sup> Cu<sup>3</sup> Ox . Most conventional metallic supercon-

http://dx.doi.org/10.5772/intechopen.72453

169

(123 phase) which was supercon-

(*Tc* = 40 K) [11].

*.* The copper oxide ceramics

sheets and blocks along

(*Tc* = 0.4 K). In

value of superconducting state has been increasing in

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

' is generally used in literature to denote superconductors with

Cu<sup>3</sup> Oy

perovskite (Ba,K)BiO<sup>3</sup>

at around 35 K in the La-Ba-Cu-O system. This

) with transition temperatures near 110 and

(*Tc* = 12 K) and BaPbO<sup>3</sup>

where *De* is the lattice diffusion, *Ω* atomic volume, *Pa* applied pressure, *G* grain size, *k* Boltzmann constant, *T* absolute temperature and *ε<sup>c</sup>* ′ creep rate.

The value of *ε<sup>c</sup>* ′ is the creep rate equivalent to the linear strain rate (*1*/*L*)*dL*/*dt* in (5) above. It can be deduced that the densification rate can be enhanced by an application of pressure, *Pa* , and a reduction in grain size *G*; the grain size reduction is more enhanced due to the higher power factor. The use of ultrafine nanometric powders has been proven to lower sintering temperatures owing to shorter diffusion distances between particles [6]. Existing experimental evidence has proven that faster densification rate allows full density to be attained at smaller grain size before serious grain growth occurs [6]. In principle, the mechanisms that promote densification (increased temperature) are also responsible for grain growth, and both these mechanisms are proportional to the reciprocal of grain size [6]. Thus the control of the two competing mechanisms presents a challenge in the production of fine−/nano-grained microstructures under conventional sintering.

#### **2.2. Synthesis of functional materials under high pressure**

As discussed earlier, the high-pressure techniques are capable of tuning the structure and properties of materials resulting in the synthesis of novel materials. The most widely used high-pressure techniques consist in the synthesis of equilibrium high-pressure phases that can be maintained after the release of pressure [2]. The low- and high-pressure phases are associated with thermodynamic stability regions in the pressure and temperature (*P*, *T*) diagram of the material [2]. It is noteworthy to mention that in a number of cases under moderate temperatures, pressure transformations result in energy intermediate *kinetic* phases which are associated with lower activation barrier for transformation [2]. Furthermore, high pressure can also modify micro- and macrostructure of a material at nano- and meso-level, i.e. grain size, texture, morphology, defect structure and concentration [2]. That means the morphology and structure of a material can easily be manipulated by varying the P,T conditions. A number of high-pressure materials have shown intriguing characteristics, among them superconducting, semiconducting, optical, electron-transport, thermal and dielectric. Moreover, pressure tuning has proven to be an invaluable tool for obtaining material properties more rapidly and cleanly in comparison to chemical techniques [3]. This is because the entropy changes associated with volumetric transitions are comparatively small which makes pressure much easier to treat than temperature [3]. Pressure studies also provide valuable information on the properties of materials under compression. This information has been used to produce the same materials at ambient pressure by chemical tuning, e.g. doping of host lattice with smaller atoms to induce chemical pressure that produces a high-pressure equivalent with a metastable phase [2, 3].

#### **2.3. Superconducting materials**

The creep being a result of atoms diffusing from interfaces subjected to a compressive stress (where they have a higher chemical potential) towards those subjected to a tensile stress (lower chemical potential). Herring derived an important relationship between the atomic

*De <sup>Ω</sup> <sup>P</sup>* \_\_\_\_\_\_*<sup>a</sup>*

′

creep rate.

is the creep rate equivalent to the linear strain rate (*1*/*L*)*dL*/*dt* in (5) above. It can

*<sup>G</sup>*<sup>2</sup> *kT* ) (5)

applied pressure, *G* grain size, *k*

, and

′ = \_\_<sup>40</sup> <sup>3</sup> (

be deduced that the densification rate can be enhanced by an application of pressure, *Pa*

a reduction in grain size *G*; the grain size reduction is more enhanced due to the higher power factor. The use of ultrafine nanometric powders has been proven to lower sintering temperatures owing to shorter diffusion distances between particles [6]. Existing experimental evidence has proven that faster densification rate allows full density to be attained at smaller grain size before serious grain growth occurs [6]. In principle, the mechanisms that promote densification (increased temperature) are also responsible for grain growth, and both these mechanisms are proportional to the reciprocal of grain size [6]. Thus the control of the two competing mechanisms presents a challenge in the production of fine−/nano-grained micro-

As discussed earlier, the high-pressure techniques are capable of tuning the structure and properties of materials resulting in the synthesis of novel materials. The most widely used high-pressure techniques consist in the synthesis of equilibrium high-pressure phases that can be maintained after the release of pressure [2]. The low- and high-pressure phases are associated with thermodynamic stability regions in the pressure and temperature (*P*, *T*) diagram of the material [2]. It is noteworthy to mention that in a number of cases under moderate temperatures, pressure transformations result in energy intermediate *kinetic* phases which are associated with lower activation barrier for transformation [2]. Furthermore, high pressure can also modify micro- and macrostructure of a material at nano- and meso-level, i.e. grain size, texture, morphology, defect structure and concentration [2]. That means the morphology and structure of a material can easily be manipulated by varying the P,T conditions. A number of high-pressure materials have shown intriguing characteristics, among them superconducting, semiconducting, optical, electron-transport, thermal and dielectric. Moreover, pressure tuning has proven to be an invaluable tool for obtaining material properties more rapidly and cleanly in comparison to chemical techniques [3]. This is because the entropy changes associated with volumetric transitions are comparatively small which makes pressure much easier to treat than temperature [3]. Pressure studies also provide valuable information on the properties of materials under compression. This information has been used to produce the same materials at ambient pressure by chemical tuning, e.g. doping of host lattice with smaller atoms to induce chemical pressure that produces a high-pressure equivalent with a metastable phase [2, 3].

is the lattice diffusion, *Ω* atomic volume, *Pa*

fluxes *ε* to the grain size, *G*:

168 Sintering of Functional Materials

′

where *De*

The value of *ε<sup>c</sup>*

*ε<sup>c</sup>*

structures under conventional sintering.

**2.2. Synthesis of functional materials under high pressure**

Boltzmann constant, *T* absolute temperature and *ε<sup>c</sup>*

In July 1908 a Dutch physicist Heike Kamerlingh Onnes discovered the superconductivity of mercury. He observed that the electrical resistance of mercury dropped down to zero under cryogen conditions (helium cooling at 4.2 K). The temperature at which the resistance becomes zero is known as the critical temperature, *Tc* . Most conventional metallic superconductors were found to possess such behaviour under liquid helium making the technology complex and costly because helium has limited supply and is costly compared to nitrogen, for example. Over the years, the *Tc* value of superconducting state has been increasing in steps with the discovery of more and more superconducting materials. In 1986, Bedworz and Muller discovered high temperature *Tc* at around 35 K in the La-Ba-Cu-O system. This initiated a rapid development in the research of ceramic oxide superconductors [7]. The term 'high temperature *Tc* ' is generally used in literature to denote superconductors with a *Tc* > 30 K. In 1987, Wu et al. discovered the YBa<sup>2</sup> Cu<sup>3</sup> Oy (123 phase) which was superconducting at 93 K which was followed by discovery in 1988 of compounds based on Bismuth (Bi<sup>2</sup> Sr2 Ca<sup>2</sup> Cu<sup>3</sup> Ox ) and thallium (TI<sup>2</sup> Ba2 Ca<sup>2</sup> Cu<sup>3</sup> Ox ) with transition temperatures near 110 and 125 K, respectively [8–10]. The most exciting feature of the oxide ceramic superconductors is that superconductivity can be achieved in liquid nitrogen (77 K) which has lower cooling costs than helium [7]. Moreover, the superconductivity of ceramic superconductors can be maintained at considerably high magnetic fields than the conventional metallic superconductors [7].

Another interesting group of superconducting materials is the cubic perovskite and its derivatives. Which have been in existence for over three decades now. They possess a cubic structure and low *Tc .* Values such as SrTiO<sup>3</sup> (*Tc* = 0.4 K), Ba(Pb,Bi)O<sup>3</sup> (*Tc* = 12 K) and BaPbO<sup>3</sup> (*Tc* = 0.4 K). In the late 1990s, Cava et al. discovered a high *Tc* perovskite (Ba,K)BiO<sup>3</sup> (*Tc* = 40 K) [11].

As mentioned earlier, the discovery of La-Ba-Cu-O superconductors triggered the experimentation of a number of copper oxide ceramics with high *Tc .* The copper oxide ceramics possess layered perovskite structure with alternate stacks of CuO<sup>2</sup> sheets and blocks along the c-axis.

The *<sup>Y</sup> Ba*<sup>2</sup> *Cu*<sup>3</sup> *<sup>O</sup>*7−*<sup>δ</sup>* (YBCO) possesses an oxygen-deficit orthorhombic perovskite structure [7].

The metal atom ratio in the structure is well defined as 1:2:3 the oxygen stoichiometry varies widely depending on the synthesis condition [7]. The oxygen content is key in regulating the structures and electrical properties (1.5 in Fei). In the stoichiometry Y-123, *δ* is always positive and can vary from 0.0 to as high as 1.0 [7]. An optimum superconducting property can be obtained by maintaining *δ* at lower value. The YBCO structure undergoes phase transition during heat treatment from a tetragonal-insulating state (*δ* < 0.4) to orthorhombic superconducting state (0.5< *δ* <1.0) by adjusting oxygen content.

There are two routes to obtaining oxide superconductors, i.e. doping of parent material through cationic substitution or oxygen nonstoichiometry. The more popular method is doping based on oxygen nonstoichiometry compared to cationic (or anion) substitutions. However oxygen doping mechanisms are not completely understood.

The critical magnetic field (*Hc* ) and the critical current density (*J c* ) are the other two important factors that are critical for superconductivity to occur [7, 12]. The *Tc* and *Hc* are intrinsic properties of the material, while *J c* can be varied by the microstructure of the material [7, 12]. It has been observed that the critical value of each parameter (*T*, *H* and *J*) for superconductivity varies with the other two, i.e. the critical current density *J c* will decrease with increasing *H* and *T* [7, 12].

of the highest *Tc*

**ii.** *Tc*

used [13, 21].

PrBa<sup>2</sup> Cu<sup>3</sup> O2

of MgB<sup>2</sup>

sure media [13].

boundaries of MgB<sup>2</sup>

MgB<sup>2</sup>

a separate study, the *Tc*

conducting oxides [13]:

hole carrier concentration in the CuO<sup>2</sup>

**iii.** Defects should be positioned as far from CuO<sup>2</sup>

**iv.** Develop structures with CuO<sup>2</sup>

high-pressure synthesis [17]. The *Tc*

used to search for new high *Tc*

In 2001, magnesium boride (MgB<sup>2</sup>

pressure conditions and possesses a *Tc*

the transition temperature *Tc*

is enhanced by increasing the number of CuO<sup>2</sup>

oxide structure while maintaining optimal doping.

values reaching 130 K for HgBa<sup>2</sup>

produce suites of Hg-1234 and Hg-1223 in the general formula HgBa<sup>2</sup>

researchers have reached consensus on the factors that enhance the value of *Tc*

Ca<sup>2</sup> Cu<sup>3</sup> O8 <sup>+</sup> <sup>x</sup>

value of Hg-1223 was found to increase to 164 K when measured

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

planes as possible.

which depends on a number of factors such as the system

value improved to 97 K under reducing conditions [17].

), a quasi-2D material with strong covalent bonding within

reveal that *Tc*

*c*

of 39 K. Latbalestier et al. in their work observed the

is produced under high-

decreased under high

could be

oxides and the ease of synthesis

*/dP*) of the various samples reported

at high field in MgB<sup>2</sup>

[12, 13, 16–18]. These compounds have been synthesised under high pressure (2–4 GPa) to

in situ under high pressure (30 GPa) [20]. Despite the complexity of these compounds,

**i.** Cationic substitution can be varied until an optimal value is reached beyond which the

planes reduces.

planes that are as flat as possible.

(Pr123) is not superconducting under ambient pressure; however, these materials

. This basically raised interest for the discovery of high-temperature analogues of

It is interesting to note that the most studied superconducting property under pressure is

studied, doping level *n*, type and mobility of defects and sometimes the pressure medium

High-pressure synthesis has also been used to induce superconductivity in compounds containing rare-earth (RE) elements particularly those containing Pr3+ [17]. The parent compound

were found to be superconducting with *Tc* = 52 K under highly oxidising conditions using

The high-pressure route offers additional control over experimental parameters which can be

were attributed to differences in the samples or shear stress effects in the frozen or solid pres-

Magnesium diboride can be synthesised under ambient or elevated pressures [22]. However the superconducting characteristics such as critical current density and irreversible magnetic field are very sensitive to material density, impurity content and structural defects. Serquis

a result of weak connectivity between domains and the presence of impurities in the grain

materials.

the boron layers, was discovered [13]. The superconductive MgB<sup>2</sup>

absence of the problematic weak-link behaviour of the high *Tc*

. Studies carried out soon after the discovery of MgB<sup>2</sup>

et al. reported that the reason for the limited current density *j*

(**Table 1**) [22].

pressure. A variation in the pressure dependences (*dTc*

(Hg-1223) at ambient pressure

[19]. In

171

in the super-

Ca<sup>3</sup> Cu<sup>4</sup> O10 <sup>+</sup> <sup>x</sup>

http://dx.doi.org/10.5772/intechopen.72453

planes which lie close together in the

Superconductors are divided into two categories, type I and type II. Type II superconductors show a more complicated magnetic behaviour but are practically more important [7, 12]. The science behind the operation of these superconductors is found elsewhere [7, 12].

The use of superconducting devices is limited due to the fact that most superconductors must be cooled to low temperatures to be superconducting. A great technological revolution may be triggered by the discovery of a room temperature (RT) superconducting material. It is widely accepted based on scientific literature that the highest *Tc* at 1 atm is 135 K in the Hg-Ba-Ca-Cu-O system which can be increased to about 160 K through the use of high-pressure synthesis.

#### **2.4. High-pressure synthesis of superconductors**

Since the application of high pressure, a number of new superconductors have been discovered which include 22 elemental solids bringing the total number of elemental solid superconductors to 51 [13]. High-pressure studies have provided ideas to enhance *Tc* values through chemical means under ambient pressure. For instance, the substitution of the smaller ion Y3+ for La3+ to generate lattice pressure in La-Ba-Cu-O has led to a large *Tc* enhancement [13]. Strictly speaking, the majority of the high-pressure superconductors entered this state as a result of pressure-induced insulator to metal transition [13]. In his theoretical analysis in 2002, Schilling showed that the *Tc* decreases under pressure for most known superconductors, sometimes rapidly depending on the (*dTc* /*dP*) value. In most simple metal superconductors, *dTc* /*dP* is negative; this arises predominantly from lattice stiffening with increasing pressure [12–14]. On the other hand, transition metals were found not to follow a universal behaviour, thus reflecting complexity and potency of electronic properties in the d-system [13].

High pressure is key in stabilising high-order oxide superconductors and can enhance the *Tc* to values >100 K [15]. The HP synthesis of oxide superconductors is usually conducted at pressures of up to 8 GPa and temperatures up to 1400°C [15]. The energy developed by pressure in solid synthesis processes is small compared to temperature; moreover, the observed kinetic effects are more profound for high-pressure synthesis [15]. For instance, it takes 70 h to obtain a LaFeO<sup>3</sup> perovskite structure at normal pressure and temperature of 1000°C and only 5 min using pressures close to 5 GPa at a constant temperature of 1000°C using Fe<sup>2</sup> O3 and La<sup>2</sup> O3 as precursors [15].

Since the discovery of high-*Tc* superconducting in the cuprate oxide La-Ba-Cu-O at *Tc* = 35 K three decades ago, there is still controversy on the mechanism responsible for the superconducting pairing [13]. In 1993, the Hg-bearing cuprates have been reported to possess one of the highest *Tc* values reaching 130 K for HgBa<sup>2</sup> Ca<sup>2</sup> Cu<sup>3</sup> O8 <sup>+</sup> <sup>x</sup> (Hg-1223) at ambient pressure [12, 13, 16–18]. These compounds have been synthesised under high pressure (2–4 GPa) to produce suites of Hg-1234 and Hg-1223 in the general formula HgBa<sup>2</sup> Ca<sup>3</sup> Cu<sup>4</sup> O10 <sup>+</sup> <sup>x</sup> [19]. In a separate study, the *Tc* value of Hg-1223 was found to increase to 164 K when measured in situ under high pressure (30 GPa) [20]. Despite the complexity of these compounds, researchers have reached consensus on the factors that enhance the value of *Tc* in the superconducting oxides [13]:


The critical magnetic field (*Hc*

ties of the material, while *J*

170 Sintering of Functional Materials

synthesis.

*Tc*

*Tc*

La<sup>2</sup> O3

metal superconductors, *dTc*

in the d-system [13].

to obtain a LaFeO<sup>3</sup>

as precursors [15].

Since the discovery of high-*Tc*

) and the critical current density (*J*

observed that the critical value of each parameter (*T*, *H* and *J*) for superconductivity varies with

Superconductors are divided into two categories, type I and type II. Type II superconductors show a more complicated magnetic behaviour but are practically more important [7, 12]. The

The use of superconducting devices is limited due to the fact that most superconductors must be cooled to low temperatures to be superconducting. A great technological revolution may be triggered by the discovery of a room temperature (RT) superconducting material. It is

Ca-Cu-O system which can be increased to about 160 K through the use of high-pressure

Since the application of high pressure, a number of new superconductors have been discovered which include 22 elemental solids bringing the total number of elemental solid superconductors to 51 [13]. High-pressure studies have provided ideas to enhance *Tc*

ues through chemical means under ambient pressure. For instance, the substitution of the smaller ion Y3+ for La3+ to generate lattice pressure in La-Ba-Cu-O has led to a large

 enhancement [13]. Strictly speaking, the majority of the high-pressure superconductors entered this state as a result of pressure-induced insulator to metal transition [13]. In his

with increasing pressure [12–14]. On the other hand, transition metals were found not to follow a universal behaviour, thus reflecting complexity and potency of electronic properties

High pressure is key in stabilising high-order oxide superconductors and can enhance the

only 5 min using pressures close to 5 GPa at a constant temperature of 1000°C using Fe<sup>2</sup>

three decades ago, there is still controversy on the mechanism responsible for the superconducting pairing [13]. In 1993, the Hg-bearing cuprates have been reported to possess one

 to values >100 K [15]. The HP synthesis of oxide superconductors is usually conducted at pressures of up to 8 GPa and temperatures up to 1400°C [15]. The energy developed by pressure in solid synthesis processes is small compared to temperature; moreover, the observed kinetic effects are more profound for high-pressure synthesis [15]. For instance, it takes 70 h

/*dP* is negative; this arises predominantly from lattice stiffening

perovskite structure at normal pressure and temperature of 1000°C and

superconducting in the cuprate oxide La-Ba-Cu-O at *Tc* = 35 K

*c*

science behind the operation of these superconductors is found elsewhere [7, 12].

factors that are critical for superconductivity to occur [7, 12]. The *Tc*

widely accepted based on scientific literature that the highest *Tc*

**2.4. High-pressure synthesis of superconductors**

theoretical analysis in 2002, Schilling showed that the *Tc*

known superconductors, sometimes rapidly depending on the (*dTc*

*c*

the other two, i.e. the critical current density *J*

*c*

can be varied by the microstructure of the material [7, 12]. It has been

and *Hc*

will decrease with increasing *H* and *T* [7, 12].

) are the other two important

at 1 atm is 135 K in the Hg-Ba-

decreases under pressure for most

/*dP*) value. In most simple

val-

O3 and

are intrinsic proper-

**iv.** Develop structures with CuO<sup>2</sup> planes that are as flat as possible.

It is interesting to note that the most studied superconducting property under pressure is the transition temperature *Tc* which depends on a number of factors such as the system studied, doping level *n*, type and mobility of defects and sometimes the pressure medium used [13, 21].

High-pressure synthesis has also been used to induce superconductivity in compounds containing rare-earth (RE) elements particularly those containing Pr3+ [17]. The parent compound PrBa<sup>2</sup> Cu<sup>3</sup> O2 (Pr123) is not superconducting under ambient pressure; however, these materials were found to be superconducting with *Tc* = 52 K under highly oxidising conditions using high-pressure synthesis [17]. The *Tc* value improved to 97 K under reducing conditions [17]. The high-pressure route offers additional control over experimental parameters which can be used to search for new high *Tc* materials.

In 2001, magnesium boride (MgB<sup>2</sup> ), a quasi-2D material with strong covalent bonding within the boron layers, was discovered [13]. The superconductive MgB<sup>2</sup> is produced under highpressure conditions and possesses a *Tc* of 39 K. Latbalestier et al. in their work observed the absence of the problematic weak-link behaviour of the high *Tc* oxides and the ease of synthesis of MgB<sup>2</sup> . This basically raised interest for the discovery of high-temperature analogues of MgB<sup>2</sup> . Studies carried out soon after the discovery of MgB<sup>2</sup> reveal that *Tc* decreased under high pressure. A variation in the pressure dependences (*dTc /dP*) of the various samples reported were attributed to differences in the samples or shear stress effects in the frozen or solid pressure media [13].

Magnesium diboride can be synthesised under ambient or elevated pressures [22]. However the superconducting characteristics such as critical current density and irreversible magnetic field are very sensitive to material density, impurity content and structural defects. Serquis et al. reported that the reason for the limited current density *j c* at high field in MgB<sup>2</sup> could be a result of weak connectivity between domains and the presence of impurities in the grain boundaries of MgB<sup>2</sup> (**Table 1**) [22].


**Table 1.** A representation of some superconducting materials that have been produced under high-pressure conditions.

#### **2.5. Thermoelectric materials**

Thomas Seebeck's accidental discovery in the 1820s proved that a junction between a bimetallic couple (any two metals) generates a potential as a function of the temperature gradient at that junction. This effect was explained later in the form of an equation below:

$$V = S(T\_h - T\_c) \tag{6}$$

charge carriers in a given material possess kinetic energy which is directly proportional to the temperature. The charge carriers on the hot side possess higher kinetic energy than those on the cold side and will drift towards the cooler side establishing a potential gradient in response to the thermal gradient. Thus to improve the efficiency of thermoelectric generators, it is a common practice to pair an electron conductor (n-type component) with a hole conductor (p-type component) in the thermoelectric circuit. Generally speaking, the Seebeck coefficient is expected to be high to ensure a large steady current, and the thermal conductivity and resistivity must both be low. This essentially explains why thermoelectric research is

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

http://dx.doi.org/10.5772/intechopen.72453

Today, the *Figure of Merit* concept is used to develop thermoelectric devices which convert thermal gradients into useful electric current or convert electric current into a heat source/sink. They can be used in systems where power is in demand and waste heat is in ample supply or conversely where power is readily available and temperature control is critical. However, the wide application of thermoelectric devices has been limited due to their inefficiency (~ 3%) compared to other power sources [25]. The main attraction to the use of thermoelectric devices is that they do not require any refuelling, high reliability, quiet operation, moving parts and low environmental impact [26]. Thus they are often used under extreme environments where access is difficult and reliability is mandatory. The thermocouple remains the most common application of the thermoelectric effect providing an important service to temperature control systems. However, recent improvements in thermoelectric materials will likely push power applications such as thermoelectric generators and heaters into the mainstream. Thermoelectrics (TE) are expected to drive environmentally friendly power systems which produce no pollutants, are

Te3

is used as a refrigerator and possesses a maximum *ZT* value at 300–400 K, whereas PbTe alloys are mainly used in power generation with a maximum *ZT* at 600–900 K [27]. It must be noted that there are a few thermoelectric materials reported to possess high performance

hindered by the toxicity and rarity of these elements [28]. Another group of TE materials is

*Rh and Ir and X* = *P, As and Sb*, and the *V* represents a vacancy inside the relatively large cages

owing to the large Seebeck coefficient (high power factor *S*<sup>2</sup> *δ*) and high hole mobility [30]. However, the lattice thermal conductivity *λ* is very high which limits the TE applications of binary skutterudites. The large vacancies in skutterudite structure form cages into which guest atoms can be introduced without structural distortion of the parent lattice. These vacancies can be partially occupied by rare-earth (RE) ions such as La, Ce and Yb or alkali metal ions such as Ca and Ba [30]. A number of studies have hypothesised that the ions inserted into these cages often rattle around disrupting the phonon modes in the system and reducing the thermal conductivity [25, 30–38]. Furthermore, theoretical calculations in referenced literature

the atom [29]. RE atoms with a charge state of +2, for instance, exhibit relatively high FFLs in


Te3

are of interest for applications

depends on the charge state of

or *VM4*

Te3

173

and PbTe has been

*X12* where *M* = *Co,* 

focused on semiconductors rather than metals or insulators.

compact and are available in a wide temperature range [26].

between 400 and 600 K [27]. However, the widespread application of Bi<sup>2</sup>

the skutterudite compounds which possess a general formula *MX3*

formed by *M* and *X* ions [29]. The binary compounds CoSb<sup>3</sup>

reveal that the filling fraction limit (FFL) of RE atoms in CoSb<sup>3</sup>

comparison to those of a higher charge state of say +3 [29].

The most advanced thermoelectric materials are Bi<sup>2</sup>

where *V* is the applied potential, *S* is a proportionality constant relating the temperature gradient to the potential for the specific bimetallic couple and *Tc* and *Th* are the cold and hot temperatures forming the thermal gradient.

In 1838, Lenz demonstrated that a bimetallic couple was capable of serving a heat sink or source when an appropriate current was passed through it [23]. This concept was coined into an expression referred to as the *Figure of Merit* in 1910 by Altenkitch and was later developed into its modern form by Ioffe in 1949:

$$Z = \frac{S^2 \delta}{\lambda} \tag{7}$$

*S* represents the Seebeck coefficient, *δ* is the electrical conductivity, and *λ* is the thermal conductivity.

The units of Z are inverse of temperature, and it is a common practice to express the figure of merit as a dimensionless value ZT as follows:

$$ZT = \frac{S^2 \delta T}{\lambda} \tag{8}$$

The dimensionless figure of merit *ZT* is the primary tool for comparison of thermoelectric materials. It ranges from zero for poor thermoelectrics to 1.5 for high-performance thermoelectrics [24].

The magnitude of the Seebeck coefficient gives a measure of the ability of a material to develop an electrical potential in response to an applied thermal gradient. Simply put, the charge carriers in a given material possess kinetic energy which is directly proportional to the temperature. The charge carriers on the hot side possess higher kinetic energy than those on the cold side and will drift towards the cooler side establishing a potential gradient in response to the thermal gradient. Thus to improve the efficiency of thermoelectric generators, it is a common practice to pair an electron conductor (n-type component) with a hole conductor (p-type component) in the thermoelectric circuit. Generally speaking, the Seebeck coefficient is expected to be high to ensure a large steady current, and the thermal conductivity and resistivity must both be low. This essentially explains why thermoelectric research is focused on semiconductors rather than metals or insulators.

Today, the *Figure of Merit* concept is used to develop thermoelectric devices which convert thermal gradients into useful electric current or convert electric current into a heat source/sink. They can be used in systems where power is in demand and waste heat is in ample supply or conversely where power is readily available and temperature control is critical. However, the wide application of thermoelectric devices has been limited due to their inefficiency (~ 3%) compared to other power sources [25]. The main attraction to the use of thermoelectric devices is that they do not require any refuelling, high reliability, quiet operation, moving parts and low environmental impact [26]. Thus they are often used under extreme environments where access is difficult and reliability is mandatory. The thermocouple remains the most common application of the thermoelectric effect providing an important service to temperature control systems. However, recent improvements in thermoelectric materials will likely push power applications such as thermoelectric generators and heaters into the mainstream. Thermoelectrics (TE) are expected to drive environmentally friendly power systems which produce no pollutants, are compact and are available in a wide temperature range [26].

**2.5. Thermoelectric materials**

HgBa2 Ca<sup>3</sup> Cu<sup>4</sup> O10 <sup>+</sup> <sup>x</sup>

172 Sintering of Functional Materials

PrBa<sup>2</sup> Cu<sup>3</sup> O2

Thomas Seebeck's accidental discovery in the 1820s proved that a junction between a bimetallic couple (any two metals) generates a potential as a function of the temperature gradient at

**Table 1.** A representation of some superconducting materials that have been produced under high-pressure conditions.

*V* = *S*(*Th* − *Tc*) (6)

where *V* is the applied potential, *S* is a proportionality constant relating the temperature gra-

In 1838, Lenz demonstrated that a bimetallic couple was capable of serving a heat sink or source when an appropriate current was passed through it [23]. This concept was coined into an expression referred to as the *Figure of Merit* in 1910 by Altenkitch and was later developed

*S* represents the Seebeck coefficient, *δ* is the electrical conductivity, and *λ* is the thermal

The units of Z are inverse of temperature, and it is a common practice to express the figure of

The dimensionless figure of merit *ZT* is the primary tool for comparison of thermoelectric materials. It ranges from zero for poor thermoelectrics to 1.5 for high-performance thermo-

The magnitude of the Seebeck coefficient gives a measure of the ability of a material to develop an electrical potential in response to an applied thermal gradient. Simply put, the

and *Th*

*<sup>λ</sup>* (7)

 **value References**

[17]

164 K [20]

39 K —

— [15]

52 and 97 K (under reducing

conditions)

*<sup>λ</sup>* (8)

are the cold and hot tem-

that junction. This effect was explained later in the form of an equation below:

dient to the potential for the specific bimetallic couple and *Tc*

**Sample composition Synthesis conditions** *Tc*

(Pr123) Static high-pressure

MgB<sup>2</sup> Static high-pressure

LaFeO<sup>3</sup> 5 GPa, 5 min static high

(Hg-1223) Static high-pressure

synthesis: 30 GPa

synthesis

synthesis.

pressure

peratures forming the thermal gradient.

into its modern form by Ioffe in 1949:

conductivity.

electrics [24].

*Z* = *<sup>S</sup>*<sup>2</sup> \_\_\_*<sup>δ</sup>*

merit as a dimensionless value ZT as follows:

*ZT* = *<sup>S</sup>*<sup>2</sup> \_\_\_\_*<sup>T</sup>*

The most advanced thermoelectric materials are Bi<sup>2</sup> Te3 - and PbTe-based tellurides [27]. Bi2 Te3 is used as a refrigerator and possesses a maximum *ZT* value at 300–400 K, whereas PbTe alloys are mainly used in power generation with a maximum *ZT* at 600–900 K [27]. It must be noted that there are a few thermoelectric materials reported to possess high performance between 400 and 600 K [27]. However, the widespread application of Bi<sup>2</sup> Te3 and PbTe has been hindered by the toxicity and rarity of these elements [28]. Another group of TE materials is the skutterudite compounds which possess a general formula *MX3* or *VM4 X12* where *M* = *Co, Rh and Ir and X* = *P, As and Sb*, and the *V* represents a vacancy inside the relatively large cages formed by *M* and *X* ions [29]. The binary compounds CoSb<sup>3</sup> are of interest for applications owing to the large Seebeck coefficient (high power factor *S*<sup>2</sup> *δ*) and high hole mobility [30]. However, the lattice thermal conductivity *λ* is very high which limits the TE applications of binary skutterudites. The large vacancies in skutterudite structure form cages into which guest atoms can be introduced without structural distortion of the parent lattice. These vacancies can be partially occupied by rare-earth (RE) ions such as La, Ce and Yb or alkali metal ions such as Ca and Ba [30]. A number of studies have hypothesised that the ions inserted into these cages often rattle around disrupting the phonon modes in the system and reducing the thermal conductivity [25, 30–38]. Furthermore, theoretical calculations in referenced literature reveal that the filling fraction limit (FFL) of RE atoms in CoSb<sup>3</sup> depends on the charge state of the atom [29]. RE atoms with a charge state of +2, for instance, exhibit relatively high FFLs in comparison to those of a higher charge state of say +3 [29].

A practical challenge associated with the fabrication of TE materials with a high *ZT* is the interdependence of *S*, *δ* and *λ* values. The improvement in one parameter usually adversely influences the others [32]. In 1995, Slack proposed a concept based on 'glass-like' thermal conductivity values referred to as the phonon-glass electron crystal (PGEC) [31]. A number of approaches have been adopted to improve the TE performance of skutterudites such a void filling and lattice atom substitution [32–35]. It is noteworthy to mention that in comparison to n-type CoSb<sup>3</sup> -based skutterudites which are well researched, the development of p-type skutterudites still lags behind [32, 35]. Elemental filling is effective in suppressing the thermal conductivity *λ* due to the rattling filler atoms (near unit filling fraction), and a low resistivity is ensured by the high hole concentration (>10<sup>21</sup> cm−3) while maintaining a moderate Seebeck coefficient, *S* [39]. The enhancement of *ZT* by elemental filing is possible through enhancing power factor *PF* (*= <sup>S</sup>*<sup>2</sup> ⁄ *<sup>ρ</sup>*) and supressing thermal conductivity (**Table 2**).

structure [40]. HPS also allows high dopant solubility and affects formation energy of defects both of which can strongly influence electrical transport properties [37, 41]. Yang et al. proved this concept by doping Te with 0.1 mol% Bi under HPS to produce a ZT of 0.72 at 517 K which

duction of thermoelectric materials working in the mid-temperature range (600–900 K) [41].

electrons to improve the power factor and promote rattling inside crystal voids to scatter phonons and reduce lattice thermal conductivity, leading to enhanced thermoelectric per-

prepared through a process melting, quenching, annealing and final consolidation at ambient pressure [41]. This process usually takes several hours; moreover, theoretical investigations suggested the fillable elements, and their filling fraction limits (FFLs) are restricted using such processing methods [41]. This points to the necessity of utilising alternative preparation methods which can broaden the fillable elements and increase FFL [41]. The high-pressure (HP) synthesis method is effective in lowering the reaction temperature and can promote

883 K. The high-pressure synthesis was effective in increasing the filling fraction of Ba into the

The FFL value is the highest reported so far and shows a substantial decrease in conductivity

likely to replace the TE compounds containing rare elements such as Bi, Te, Pb, Co and Sb in the mid-temperature range (600–900 K) application. Thermodynamic studies have shown

unreacted Mg [29]. This is attributed to the Mg boiling point (1363 K) which is very close to

Mg. The high pressure was also attributed to a decrease in synthetic temperature which is favourable for the relaxation of the n-type defects resulting in improved TE properties [29].

A wide range of major families of permanent magnets have been in use over the years which include the low-cost and low energy ferrites and the more expensive and higher-performance rare-earth magnetic materials. Alnico an alloy of aluminium, nickel, and cobalt is one of the first magnetic materials developed in the 1930s for military electronic applications. Alnico magnets are known for their high induction levels with good resistance to demagnetisation and stability; they also possess a high working temperature limit (550°C) at a reasonable cost. It is well suited for automotive and aircraft sensor applications. However in comparison to

melting and boiling point temperatures to obtain a stoichiometric Mg<sup>2</sup>

structure from 0.28 to a maximal of 0.51. In a separate study, the highest doping ratio

Te3


High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials


voids [41]. Kang et al. used HP synthesis method to fab-

S) has been identified as an environmentally friendly TE material

S (1358 K) [29]. The use of high-pressure synthesis can control the

with a stoichiometric ratio at high temperatures without

and PbTe [37].

175

http://dx.doi.org/10.5772/intechopen.72453

crystal which in turn donate


Sb12) with a maximal ZT of 0.99 reached at

S ratio without residual

Sb12 synthesised under a pressure of 2 GPa [25].

is comparable to that of more complex composition alloys such as Bi<sup>2</sup>

The concept involves filling the Sb12-icosahedron voids of CoSb<sup>3</sup>

produced through multiple elemental filling [41]. CoSb<sup>3</sup>

ricate a Ba-filled skutterudite compound (Ba0.51Co<sup>4</sup>

of Yb was reported in the compound Yb029Co<sup>4</sup>

The concept of elemental filling of CoSb<sup>3</sup>

formance [41]. Recently CoSb<sup>3</sup>

increased filling fraction into CoSb<sup>3</sup>

with increasing Yb filling ratio x.

that it is difficult to synthesise MgS<sup>2</sup>

Magnesium sulphide (Mg<sup>2</sup>

the melting point of Mg<sup>2</sup>

**2.7. Magnetic materials**

CoSb<sup>3</sup>

#### **2.6. Development of thermoelectric materials using high-pressure techniques**

The high-pressure technique is one of the modern synthesis methods used to improve the efficiency of TE materials. As mentioned earlier, high pressure enables the synthesis of compounds with a crystal structure or composition which is not achieved at ambient pressure [39]. Bi2 Te3 and its alloys have been fabricated using a variety of methods which include powder metallurgy techniques such as hot pressing, spark plasma sintering (SPS) methods [40], Bridgman and zone melting techniques [40] and ultra-high-pressure sintering (HPS) methods [40]. High-pressure sintering (HPS) is advantageous in that it provides a low-cost route, is suitable for large-scale production and results in more homogeneous nanocrystalline grain and is effective in restraining grain coarsening during sintering [40].

Recent studies have shown that grain refinement of Bi<sup>2</sup> Te3 -based alloys can enhance the thermoelectric performances [40]. In one experiment, a p-type Bi<sup>2</sup> Te3 -based nanomaterial was fabricated using HPS, and a ZT of 1.16 was obtained at room temperature (RT) [27]. The nano-grain structure was attributed to effective reduction in the thermal conductivity. Zou et al. obtained an n-type Bi2 Te2.7Se0.3 compound doped with Gd through HPS at 6.6 GPa followed by annealing. A maximum ZT of 0.74 was obtained at 423 K which was attributed to the nano-grain


**Table 2.** A representation of high-pressure-synthesised thermoelectric materials from selected literature.

structure [40]. HPS also allows high dopant solubility and affects formation energy of defects both of which can strongly influence electrical transport properties [37, 41]. Yang et al. proved this concept by doping Te with 0.1 mol% Bi under HPS to produce a ZT of 0.72 at 517 K which is comparable to that of more complex composition alloys such as Bi<sup>2</sup> Te3 and PbTe [37].

The concept of elemental filling of CoSb<sup>3</sup> -based skutterudites is most promising in the production of thermoelectric materials working in the mid-temperature range (600–900 K) [41]. The concept involves filling the Sb12-icosahedron voids of CoSb<sup>3</sup> crystal which in turn donate electrons to improve the power factor and promote rattling inside crystal voids to scatter phonons and reduce lattice thermal conductivity, leading to enhanced thermoelectric performance [41]. Recently CoSb<sup>3</sup> -based skutterudites with ZT values as high as 2 have been produced through multiple elemental filling [41]. CoSb<sup>3</sup> -based skutterudites are traditionally prepared through a process melting, quenching, annealing and final consolidation at ambient pressure [41]. This process usually takes several hours; moreover, theoretical investigations suggested the fillable elements, and their filling fraction limits (FFLs) are restricted using such processing methods [41]. This points to the necessity of utilising alternative preparation methods which can broaden the fillable elements and increase FFL [41]. The high-pressure (HP) synthesis method is effective in lowering the reaction temperature and can promote increased filling fraction into CoSb<sup>3</sup> voids [41]. Kang et al. used HP synthesis method to fabricate a Ba-filled skutterudite compound (Ba0.51Co<sup>4</sup> Sb12) with a maximal ZT of 0.99 reached at 883 K. The high-pressure synthesis was effective in increasing the filling fraction of Ba into the CoSb<sup>3</sup> structure from 0.28 to a maximal of 0.51. In a separate study, the highest doping ratio of Yb was reported in the compound Yb029Co<sup>4</sup> Sb12 synthesised under a pressure of 2 GPa [25]. The FFL value is the highest reported so far and shows a substantial decrease in conductivity with increasing Yb filling ratio x.

Magnesium sulphide (Mg<sup>2</sup> S) has been identified as an environmentally friendly TE material likely to replace the TE compounds containing rare elements such as Bi, Te, Pb, Co and Sb in the mid-temperature range (600–900 K) application. Thermodynamic studies have shown that it is difficult to synthesise MgS<sup>2</sup> with a stoichiometric ratio at high temperatures without unreacted Mg [29]. This is attributed to the Mg boiling point (1363 K) which is very close to the melting point of Mg<sup>2</sup> S (1358 K) [29]. The use of high-pressure synthesis can control the melting and boiling point temperatures to obtain a stoichiometric Mg<sup>2</sup> S ratio without residual Mg. The high pressure was also attributed to a decrease in synthetic temperature which is favourable for the relaxation of the n-type defects resulting in improved TE properties [29].

#### **2.7. Magnetic materials**

A practical challenge associated with the fabrication of TE materials with a high *ZT* is the interdependence of *S*, *δ* and *λ* values. The improvement in one parameter usually adversely influences the others [32]. In 1995, Slack proposed a concept based on 'glass-like' thermal conductivity values referred to as the phonon-glass electron crystal (PGEC) [31]. A number of approaches have been adopted to improve the TE performance of skutterudites such a void filling and lattice atom substitution [32–35]. It is noteworthy to mention that in comparison

skutterudites still lags behind [32, 35]. Elemental filling is effective in suppressing the thermal conductivity *λ* due to the rattling filler atoms (near unit filling fraction), and a low resistivity is ensured by the high hole concentration (>10<sup>21</sup> cm−3) while maintaining a moderate Seebeck coefficient, *S* [39]. The enhancement of *ZT* by elemental filing is possible through enhancing

*<sup>ρ</sup>*) and supressing thermal conductivity (**Table 2**).

The high-pressure technique is one of the modern synthesis methods used to improve the efficiency of TE materials. As mentioned earlier, high pressure enables the synthesis of compounds with a crystal structure or composition which is not achieved at ambient pressure

der metallurgy techniques such as hot pressing, spark plasma sintering (SPS) methods [40], Bridgman and zone melting techniques [40] and ultra-high-pressure sintering (HPS) methods [40]. High-pressure sintering (HPS) is advantageous in that it provides a low-cost route, is suitable for large-scale production and results in more homogeneous nanocrystalline grain

cated using HPS, and a ZT of 1.16 was obtained at room temperature (RT) [27]. The nano-grain structure was attributed to effective reduction in the thermal conductivity. Zou et al. obtained

ing. A maximum ZT of 0.74 was obtained at 423 K which was attributed to the nano-grain

GPa followed by annealing

**Composition Synthesis conditions ZT value References**

(nano) Static high-pressure synthesis 1.16 at room

Te doped with 0.1% Bi Static high-pressure synthesis 0.72 at 517 K [37]

**Table 2.** A representation of high-pressure-synthesised thermoelectric materials from selected literature.

and its alloys have been fabricated using a variety of methods which include pow-

Te3

Te2.7Se0.3 compound doped with Gd through HPS at 6.6 GPa followed by anneal-

Te3

temperature

423 K

Static high pressure at 2 GPa — [25]

Static high-pressure synthesis 0.95 at 883 K [41]

0.74 maximum at



[27]

[40]

**2.6. Development of thermoelectric materials using high-pressure techniques**

and is effective in restraining grain coarsening during sintering [40].

Te2.7Se0.3 doped with Gd (nano) Static high pressure at 6.6

Sb12 (highest Yb doping

Sb14 (improved filling for Ba

Recent studies have shown that grain refinement of Bi<sup>2</sup>

moelectric performances [40]. In one experiment, a p-type Bi<sup>2</sup>


to n-type CoSb<sup>3</sup>

174 Sintering of Functional Materials

power factor *PF* (*= <sup>S</sup>*<sup>2</sup>

Te3

[39]. Bi2

an n-type Bi2

p-Type Bi2

n-Type Bi2

reported)

Te3

Yb-doped Yb0.29Co<sup>4</sup>

Ba-filled Ba0.51Co<sup>4</sup>

from 0.28 to a maximum 0.51)

⁄

A wide range of major families of permanent magnets have been in use over the years which include the low-cost and low energy ferrites and the more expensive and higher-performance rare-earth magnetic materials. Alnico an alloy of aluminium, nickel, and cobalt is one of the first magnetic materials developed in the 1930s for military electronic applications. Alnico magnets are known for their high induction levels with good resistance to demagnetisation and stability; they also possess a high working temperature limit (550°C) at a reasonable cost. It is well suited for automotive and aircraft sensor applications. However in comparison to newer materials, Alnico possesses lower coercivity which limits its applications. Ferrite magnets referred to as ceramic magnets were commercialised in the mid-1950s and are the least expensive permanent magnets available. The ferrites are produced through sintering of fine iron oxide mixed with either strontium (Sr) or barium (Ba) and a ceramic binder (MM 2018). Ferrites find use in motors in the form of arc-shaped magnets, in magnetic chucks and magnetic tools. Because of their brittle nature, ferrites are not suitable for structural applications, and moreover their thermal stability is limited to 300°C.

**2.8. Development of magnetic materials using high-pressure techniques**

a perovskite structure (ABO<sup>3</sup>

O3

Furthermore, BiFeO<sup>3</sup>

cursors (Bi<sup>2</sup>

BiFeO<sup>3</sup>

BiMnO<sup>3</sup>

MnGe<sup>4</sup>

*Tc*

centre of a cube formed by eight BO<sup>6</sup>

(BFO) is the evaporation of Bi<sup>2</sup>

The use of high-pressure synthesis in magnetic materials has not been fully explored, and there are very limited reports on permanent magnetic alloys consisting of alkali metals (AMs) and 3d metals [43]. With the current research focus towards finding alternatives to traditional highperformance rare-earth magnetic materials, high pressure presents a rare opportunity to be exploited in this area. A majority of materials have been exploited in these groups, the so-called 'multiferroic' (MF) materials which possess ferroelectricity, ferromagnetism and ferroelasticity in a single material [44]. The most desirable property in these materials is the magnetoelectric (ME) coupling which is extremely rare in most compounds [44]. Magneto-electric coupling refers to either induction of magnetisation by an electric field or polarisation by a magnetic field [44]. The scarcity of these materials can be explained from the symmetry and electronic properties point of view details of which is discussed elsewhere [44]. Recent studies have proven that ferromagnetism and ferroelectricity can occur simultaneously in a single phase through an additional electronic or structural driving force which supports ME coupling [44]. High-pressure high-temperature synthesis allows stabilisation of metastable or highly distorted structures which might achieve ME coupling [44]. The majority of MF materials exhibit

), typically consisting of corner sharing BO<sup>6</sup>

melts at 824°C at ambient pressure) [44]. One of the challenges in the synthesis of

is metastable in air and above 675°C; it decomposes into various products,

octahedra [44]. The application of HP has been used

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

http://dx.doi.org/10.5772/intechopen.72453

177

at high temperatures during solid-state synthesis also

Ge<sup>3</sup>

(usually magnetic ions such as Mn or Fe) in the centre of the octahedral site and A ions at the

to induce MF in perovskite compounds by introducing magnetic ions (ferromagnetism) in a

pressure synthesis to obtain a high-temperature solid-state reaction when using oxides as pre-

and below 675°C the density of the sintered product produced by conventional sintering methods is very low which makes evaluation of electrical and magnetic properties impossible [45].

thesis (P = 4 GPa,T = 1273 K) [44]. In 2016, Drygas et al. reported an increase in Mn contents (3–5 at%) in (Ge, Mn)N magnetic materials produced by HP/HT synthesis (7 GPa, 1000°C, 10 min). The Mn solubility in hexagonal GaN is limited to 2.4 at% predicted from a calculated Ga-Mn-N phase diagram [46]. The use of nanomaterials has shown an increase in Mn doping at 7 at% for nano-powders and 10 at% for nanowires [46]. This further proves the advantageous impact of HP/HT synthesis on incorporation of a doping element into a foreign matrix.

The binary Mn-Ge system contains intermetallic phases which exhibit antiferromagnetism

 above room temperature [47]. It is clear that high-pressure synthesis has been the method of choice in the development of intermetallic compounds which would otherwise not react under ambient pressure conditions owing to differences in atomic radii and electronegativities. Under high-pressure conditions, for instance, alkali earth metals are able acquire properties of

(*Tc* = 340 K), are the most interesting as they are known to exhibit ferromagnetism with

possesses a simple perovskite structure and has been obtained under HP/HT syn-

non-centromagnetic structure (distorted). Typical examples include BiFeO<sup>3</sup>

O3

with relatively low magnetic ordering temperatures [47]. The phases, Mn5

under applied fields of about 200 kVcm−1; BFO can decompose leading to hematite Fe<sup>2</sup>

octahedra with B ions

which require high-

O3 [45].

(*Tc* = 304 K) and

Samarium cobalt (Sm-Co) is the first commercially viable rare-earth permanent magnetic material formulated in the 1970s. Their excellent thermal stability, high corrosion resistance and resistance to demagnetisation make them suitable for high-performance application such as most demanding motor applications and medical applications. It is the most expensive magnetic material on a 'per kg' basis; however, this is offset by the low volume required to fulfil a certain task because of its high energy 16MGOe up to 33MGOe. The most powerful commercial permanent magnets are sintered neodymium-iron-boron (Nd-Fe-B) rare-earth magnets with maximum energy product ranging from 26MGOe to 52MGOe. Nd-Fe-B magnets were developed in the 1950s and have found use in applications where higher efficiency and more compact devices are demanded. They are however prone to oxidation and can only be used at temperatures ≤200°C.

The function of permanent magnetic materials in electric machines is to provide magnetic flux [42]. The most important factors required to achieve this function are the saturation magnetisation (*J <sup>s</sup> = μ0 Ms* ) which is required to be as high as possible and affordable; the other factor is high coercivity, *H* [42]. There exists two coercivity parameters used to grade magnetic hardness, i.e. intrinsic coercivity, *<sup>i</sup> Hc* (or j *Hc* ), and technical (or normal) coercivity, *Hc* [42]. Coercivity is basically the ability to resist demagnetisation either from electric or magnetic circuit and thermal demagnetisation from the operating temperature [42]. Soft magnetic materials possess a typical *<sup>i</sup> Hc* < 1 kAm−1, and hard magnetic materials have *<sup>i</sup> Hc* up to approx. 2800 kAm−1 (about 35 kOe) [42]. Permanent magnets suitable for high-temperature applications under strong electric and magnetic circuits must possess high coercivity, ideally *<sup>i</sup> Hc > Hc* . There are only three permanent magnets which show such a characteristic, i.e. hard ferrites and Nd-Fe-B and Sm-Co magnets [42]. The research focus is the development of Dy-free Nd-Fe-B magnets with high *Hc* for high-temperature applications [42]. Dy is a very rare metal, and 97% of the world supply is of Chinese origin [42].

In recent years, there is much interest in high-performance permanent magnets based on rareearth and 3d transition metals (3-TMs) intermetallic compounds [42]. This has been triggered by the need for maximised energy densities at various operating temperatures and to replace the more expensive Dy (dysprosium) in high-performance magnets. Manganese-based magnetic compounds such as Mn-Bi, Mn-Al-C and Mn-Li-N have attracted considerable attention as alternative permanent magnetic materials without critical elements [43]. However, the reaction between alkali earth metals and 3d transition metals to form intermetallic compounds at ambient pressure is not feasible owing to the large difference in atomic radii and electronegativity [2, 43]. Another approach which has been adopted is to incorporate nanoscale soft magnetic phases into a hard magnetic phase matrix to enhance energy density of the composite due to interphase exchange coupling at a reduced cost [42].

#### **2.8. Development of magnetic materials using high-pressure techniques**

newer materials, Alnico possesses lower coercivity which limits its applications. Ferrite magnets referred to as ceramic magnets were commercialised in the mid-1950s and are the least expensive permanent magnets available. The ferrites are produced through sintering of fine iron oxide mixed with either strontium (Sr) or barium (Ba) and a ceramic binder (MM 2018). Ferrites find use in motors in the form of arc-shaped magnets, in magnetic chucks and magnetic tools. Because of their brittle nature, ferrites are not suitable for structural applications,

Samarium cobalt (Sm-Co) is the first commercially viable rare-earth permanent magnetic material formulated in the 1970s. Their excellent thermal stability, high corrosion resistance and resistance to demagnetisation make them suitable for high-performance application such as most demanding motor applications and medical applications. It is the most expensive magnetic material on a 'per kg' basis; however, this is offset by the low volume required to fulfil a certain task because of its high energy 16MGOe up to 33MGOe. The most powerful commercial permanent magnets are sintered neodymium-iron-boron (Nd-Fe-B) rare-earth magnets with maximum energy product ranging from 26MGOe to 52MGOe. Nd-Fe-B magnets were developed in the 1950s and have found use in applications where higher efficiency and more compact devices are demanded. They are however prone to oxidation and can only

The function of permanent magnetic materials in electric machines is to provide magnetic flux [42]. The most important factors required to achieve this function are the saturation mag-

factor is high coercivity, *H* [42]. There exists two coercivity parameters used to grade mag-

[42]. Coercivity is basically the ability to resist demagnetisation either from electric or magnetic circuit and thermal demagnetisation from the operating temperature [42]. Soft mag-

approx. 2800 kAm−1 (about 35 kOe) [42]. Permanent magnets suitable for high-temperature applications under strong electric and magnetic circuits must possess high coercivity, ide-

hard ferrites and Nd-Fe-B and Sm-Co magnets [42]. The research focus is the development of

In recent years, there is much interest in high-performance permanent magnets based on rareearth and 3d transition metals (3-TMs) intermetallic compounds [42]. This has been triggered by the need for maximised energy densities at various operating temperatures and to replace the more expensive Dy (dysprosium) in high-performance magnets. Manganese-based magnetic compounds such as Mn-Bi, Mn-Al-C and Mn-Li-N have attracted considerable attention as alternative permanent magnetic materials without critical elements [43]. However, the reaction between alkali earth metals and 3d transition metals to form intermetallic compounds at ambient pressure is not feasible owing to the large difference in atomic radii and electronegativity [2, 43]. Another approach which has been adopted is to incorporate nanoscale soft magnetic phases into a hard magnetic phase matrix to enhance energy density of the composite

. There are only three permanent magnets which show such a characteristic, i.e.

*Hc* (or j *Hc*

*Hc*

rare metal, and 97% of the world supply is of Chinese origin [42].

due to interphase exchange coupling at a reduced cost [42].

) which is required to be as high as possible and affordable; the other

< 1 kAm−1, and hard magnetic materials have *<sup>i</sup>*

for high-temperature applications [42]. Dy is a very

), and technical (or normal) coercivity, *Hc*

*Hc* up to

and moreover their thermal stability is limited to 300°C.

be used at temperatures ≤200°C.

176 Sintering of Functional Materials

*<sup>s</sup> = μ0 Ms*

netic materials possess a typical *<sup>i</sup>*

netic hardness, i.e. intrinsic coercivity, *<sup>i</sup>*

Dy-free Nd-Fe-B magnets with high *Hc*

netisation (*J*

ally *<sup>i</sup>*

*Hc > Hc*

The use of high-pressure synthesis in magnetic materials has not been fully explored, and there are very limited reports on permanent magnetic alloys consisting of alkali metals (AMs) and 3d metals [43]. With the current research focus towards finding alternatives to traditional highperformance rare-earth magnetic materials, high pressure presents a rare opportunity to be exploited in this area. A majority of materials have been exploited in these groups, the so-called 'multiferroic' (MF) materials which possess ferroelectricity, ferromagnetism and ferroelasticity in a single material [44]. The most desirable property in these materials is the magnetoelectric (ME) coupling which is extremely rare in most compounds [44]. Magneto-electric coupling refers to either induction of magnetisation by an electric field or polarisation by a magnetic field [44]. The scarcity of these materials can be explained from the symmetry and electronic properties point of view details of which is discussed elsewhere [44]. Recent studies have proven that ferromagnetism and ferroelectricity can occur simultaneously in a single phase through an additional electronic or structural driving force which supports ME coupling [44]. High-pressure high-temperature synthesis allows stabilisation of metastable or highly distorted structures which might achieve ME coupling [44]. The majority of MF materials exhibit a perovskite structure (ABO<sup>3</sup> ), typically consisting of corner sharing BO<sup>6</sup> octahedra with B ions (usually magnetic ions such as Mn or Fe) in the centre of the octahedral site and A ions at the centre of a cube formed by eight BO<sup>6</sup> octahedra [44]. The application of HP has been used to induce MF in perovskite compounds by introducing magnetic ions (ferromagnetism) in a non-centromagnetic structure (distorted). Typical examples include BiFeO<sup>3</sup> which require highpressure synthesis to obtain a high-temperature solid-state reaction when using oxides as precursors (Bi<sup>2</sup> O3 melts at 824°C at ambient pressure) [44]. One of the challenges in the synthesis of BiFeO<sup>3</sup> (BFO) is the evaporation of Bi<sup>2</sup> O3 at high temperatures during solid-state synthesis also under applied fields of about 200 kVcm−1; BFO can decompose leading to hematite Fe<sup>2</sup> O3 [45]. Furthermore, BiFeO<sup>3</sup> is metastable in air and above 675°C; it decomposes into various products, and below 675°C the density of the sintered product produced by conventional sintering methods is very low which makes evaluation of electrical and magnetic properties impossible [45].

BiMnO<sup>3</sup> possesses a simple perovskite structure and has been obtained under HP/HT synthesis (P = 4 GPa,T = 1273 K) [44]. In 2016, Drygas et al. reported an increase in Mn contents (3–5 at%) in (Ge, Mn)N magnetic materials produced by HP/HT synthesis (7 GPa, 1000°C, 10 min). The Mn solubility in hexagonal GaN is limited to 2.4 at% predicted from a calculated Ga-Mn-N phase diagram [46]. The use of nanomaterials has shown an increase in Mn doping at 7 at% for nano-powders and 10 at% for nanowires [46]. This further proves the advantageous impact of HP/HT synthesis on incorporation of a doping element into a foreign matrix.

The binary Mn-Ge system contains intermetallic phases which exhibit antiferromagnetism with relatively low magnetic ordering temperatures [47]. The phases, Mn5 Ge<sup>3</sup> (*Tc* = 304 K) and MnGe<sup>4</sup> (*Tc* = 340 K), are the most interesting as they are known to exhibit ferromagnetism with *Tc* above room temperature [47]. It is clear that high-pressure synthesis has been the method of choice in the development of intermetallic compounds which would otherwise not react under ambient pressure conditions owing to differences in atomic radii and electronegativities. Under high-pressure conditions, for instance, alkali earth metals are able acquire properties of


[2] Brazhkin VV. High-pressure synthesized materials: Treasures and hints. High Pressure

High-Pressure High-Temperature (HPHT) Synthesis of Functional Materials

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[3] Badding JV. High-pressure synthesis, characterization, and tuning of solid state materials. Annual Review of Materials Science. 1998;**28**(1):631-658. DOI: 10.1146/annurev.

[4] Pagh HLD, McCormic PG, Rouff AL. Mechanical Behaviour of Materials under Pressure, Creep under High Pressure. Amsterdam: Elsevier Publishing; 1970. pp. 355-356

[5] Rahaman MN Ceramic Processing and Sintering. 2nd ed. NY, USA: Marcel Dekkar;

[6] Matizamhuka WR. Spark plasma sintering (SPS)-an advanced sintering technique for structural nanocomposite materials. Journal of the Southern African Institute of Mining and Metallurgy. Dec 2016;**116**(12):1171-1180. DOI: 10.17159/2411-9717/2016/v116n12a12

[7] Togano K, Superconductive ceramics. In: Somia S et al. editors. Handbook of Advanced

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out a rare earth element. Japanese Journal of Applied Physics. Feb 1988;**27**(2A):L209.

[10] Sheng ZZ, Hermann AM. Bulk superconductivity at 120 K in the Tl–Ca/Ba–Cu–O sys-

[11] Cava RJ, Batlogg B, Krajewski JJ, Farrow R, Rupp LW, White AE, Short K, Peck WF,

[12] Sathish CI. High pressure synthesis and characterization of non-oxide superconductors.

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[14] Eiling A, Schilling JS. Pressure and temperature dependence of electrical resistivity of Pb and Sn from 1-300 K and 0-10 GPa-use as continuous resistive pressure monitor accurate over wide temperature range; superconductivity under pressure in Pb, Sn and In. Journal of Physics F: Metal Physics. Mar 1981;**11**(3):623. DOI: 10.1088/0305-4608/11/3/010/meta.

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Kometani T. Superconductivity near 30 K without copper: The Ba0.6K0.4BiO<sup>3</sup>

tem. Nature. Mar 10, 1988;**332**(6160):138-139. DOI: 10.1038/332138a0

Nature. Apr 28, 1988;**332**(6167):814-816. DOI: 10.1038/332814a0

PhD thesis. Japan: Hokkaido University; 2013

DOI: 10.1007/978-94-010-0520-3\_26

oxide superconductor with-

perovskite.

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DOI: 10.1143/JJAP.27.L209/meta

matsci.28.1.631

1995. pp. 254-270

58.908

**Table 3.** Magnetic properties and corresponding high-pressure synthesis conditions of some representative magnetic materials.

transition metals which enables them to easily interact and react with d-metals to form intermetallic compounds (**Table 3**) [2].

## **3. Outlook**

The above examples are evident of the importance the pressure parameter that plays in the tuning of structural, electronic and magnetic properties of functional materials. The commercial production of functional materials under high-pressure conditions is however still severely restricted owing to the small volume of product obtained at high pressures. The major drawback of high-pressure synthesis is the destabilisation after pressure release. However, high-pressure synthesis still remains a powerful research tool in the discovery of novel materials with unique properties which can be recreated through alternative chemical paths. I agree with the sentiments of most high-pressure specialists that high-pressure research is not yet fully appreciated to its true value.

## **Author details**

Wallace Matizamhuka

Address all correspondence to: wallace@vut.ac.za

Vaal University of Technology, Department of Metallurgical Engineering, Vanderbijlpark, South Africa

### **References**

[1] Mao HK, Chen B, Chen J, Li K, Lin JF, Yang W, Zheng H. Recent advances in high-pressure science and technology. Matter and Radiation at Extremes. 2016;**1**(1):59-75. DOI: 10.1016/j.mre.2016.01.005


transition metals which enables them to easily interact and react with d-metals to form inter-

**Table 3.** Magnetic properties and corresponding high-pressure synthesis conditions of some representative magnetic

**Composition Synthesis conditions Target structure References**

properties

properties

, MnGe<sup>4</sup> — Improved reaction kinetics to form

Perovskite structure with improved magnetic

Improved doping (3–5 at%) to above the limit of (2.4 at%) resulting in superior magnetic

intermetallics exhibiting ferromagnetism

[44]

[46]

[47]

The above examples are evident of the importance the pressure parameter that plays in the tuning of structural, electronic and magnetic properties of functional materials. The commercial production of functional materials under high-pressure conditions is however still severely restricted owing to the small volume of product obtained at high pressures. The major drawback of high-pressure synthesis is the destabilisation after pressure release. However, high-pressure synthesis still remains a powerful research tool in the discovery of novel materials with unique properties which can be recreated through alternative chemical paths. I agree with the sentiments of most high-pressure specialists that high-pressure

Vaal University of Technology, Department of Metallurgical Engineering, Vanderbijlpark,

[1] Mao HK, Chen B, Chen J, Li K, Lin JF, Yang W, Zheng H. Recent advances in high-pressure science and technology. Matter and Radiation at Extremes. 2016;**1**(1):59-75. DOI:

metallic compounds (**Table 3**) [2].

BiMnO<sup>3</sup> 4 GPa, 1273 K under static

high-pressure conditions

7 GPa, 1000°C, 10 min under static high pressure

research is not yet fully appreciated to its true value.

Address all correspondence to: wallace@vut.ac.za

10.1016/j.mre.2016.01.005

**3. Outlook**

Higher Mn doping of

178 Sintering of Functional Materials

(Ge, Mn)N

materials.

Mn5 Ge<sup>3</sup>

**Author details**

South Africa

**References**

Wallace Matizamhuka


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## *Edited by Igor Shishkovsky*

Powder-based materials and treatment technologies rank high in contemporary scientific-technical progress due to their numerous significant technoeconomic qualities. Sintering of such materials allows saving on materials and lowering the cost price of the product, as well as manufacturing complex composite materials with unique combinations of qualities. Materials of record high values of some physicmechanical and also biochemical characteristics can be obtained owing to structural peculiarities of super dispersed condition.

Sintering of functional materials for innovative perspectives in automotive and aeronautical engineering, space technology, lightweight construction, mechanical engineering, modern design, and many other applications requires established relationship in the materials-process-properties system. Therefore, the industry being interested in understanding theoretical modeling, and control over behavior of such powdered materials has promoted the research activities of this manuscript's authors.

Sintering of Functional Materials

Sintering of Functional

Materials

*Edited by Igor Shishkovsky*

Photo by Momolelouch / iStock