**4. Spark plasma sintering technology**

In 1906, Bloxam filed the first patent on the successful consolidation of powder using the SPS technology [20]. Steady progress was made in the mid-1980s into the 1990s. The SPS technology sinters in a conducting die with a simultaneously applied mechanical pressure and DC pulses which allows for simultaneous densification

and sintering process [21]. The SPS set-up consists of a graphite die filled with powder feedstock, uniaxial hydraulic pressing device which achieves 50–250 kN and an electric pulse current of low voltage (<10V) and high currents (1–10 kA) [2]. The system has achieved heating rates of up to 1000°C/min which makes it possible to sinter over very short durations [22]. It can be operated under vacuum or inert gas atmosphere at atmospheric pressure with a maximum temperature of 2400°C.

The sintering mechanisms in SPS are a result of three effects, namely, mechanical, thermal and electrical [2]. The fast heating rates achieved in SPS enables densification while retarding microstructure coarsening owing to the short times required to reach sintering temperature. This allows for the densification of nanopowders with minimal grain coarsening [22]. The SPS system offers a number of advantages over the conventional sintering systems such as hot pressing, hot isostatic pressing (HIP) which include high sintering speeds, high reproducibility, better control of sintering energy and reliability.

The mechanism of sintering is not well understood but several authors have postulated a number of theories. The widely accepted SPS sintering mechanisms involve joule heating, plasma generation and electroplastic effect [2]. The electrical effects are a function of the electrical properties of the powders. For powders that are electrically conducting, current can easily flow through, and heat is generated mainly by joule heating and transferred to the bulk of the powder by conduction (see **Figure 1**) [23]. In the presence of an applied pressure, the electric current through the particles enhances formation of interparticle bonds through localised welding, vaporisation or cleaning of powder surfaces [22]. This ensures a smoother and more favourable path for the current flow. This also promotes the production of high-quality sintered compacts at lower temperatures in a shorter time than conventional sintering methods. The sintering of nonconducting powders, although not well understood, is thought to occur through grain boundary migration and matter transport at higher input voltages.

Although the SPS has the capability to sinter at high heating and cooling rates, the expectation is that the system can sinter without appreciable grain growth. However in reality this is not always the case; a complete avoidance of the grain growth at the sintering temperatures for most nano-grained materials will always promote grain growth. It is therefore imperative to adopt an approach/methodology

**111**

**Figure 2.**

at triple points [11].

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma…*

that is more effective in minimising grain growth using the SPS system. The *twostep* sintering (TSS) approach has been found to be effective in minimising grain growth of ceramic materials during sintering. It is thus important to dedicate the following section to the success studies on the sintering of functional ceramics to

The reliability of ceramic materials is a key function, and it dictates their ultimate

However, the higher-temperature stage if not adequately controlled can lead to some grain growth. In 2000, Chen and Wang proposed a modified TSS methodology which effectively suppresses the accelerated grain growth in the second stage [24, 26]. In the modified TSS approach, a high-temperature heating is performed first for a short duration followed by structural freezing and sintering at a lower temperature. The idea of heating to a higher temperature (T1) followed by fast cooling with no sintering holding time (stage 1 in **Figure 2**) is to eliminate residual porosity at higher temperature and develop a network of grain boundary anchoring

These anchored triple points are thought to have higher activation energy for matter migration than the grain boundaries. The second step effectively proceeds in

performance. A carefully controlled microstructure has a greater impact on the properties and reliability of functional ceramic materials. In the previous section, it has been shown that the development of highly dense nanometric or ultrafine-grainsized ceramics is not easily achievable through conventional sintering. Although the SPS technology has shown great potential in the production of highly dense nanometric materials, it is difficult to maintain microstructural refinement under the high sintering temperatures. It must be underlined that solid-phase sintering requires high temperatures to facilitate diffusion which promotes material densification. However diffusion processes promote not only densification but also grain growth [24]. To achieve grain refinement during sintering, it is therefore imperative to develop a sintering methodology which promotes only densification without stimulating grain growth. This method has been improved over the years to achieve better microstructural refinement. The so-called two-step sintering (TSS) was subsequently introduced in the 1990s by Chu et al. [25]. In essence, the technique consists of two stages of consolidation process, i.e. a first stage performed at relatively low temperature

give an insight and a better understanding of the TSS method.

followed by a higher-temperature stage and subsequent cooling.

*Schematic illustration of the differences between the two TSS approaches [11].*

*DOI: http://dx.doi.org/10.5772/intechopen.86461*

**5. Two-step sintering methodology**

**Figure 1.** *Pulse current flow through the spark plasma sintering technology [22].*

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma… DOI: http://dx.doi.org/10.5772/intechopen.86461*

that is more effective in minimising grain growth using the SPS system. The *twostep* sintering (TSS) approach has been found to be effective in minimising grain growth of ceramic materials during sintering. It is thus important to dedicate the following section to the success studies on the sintering of functional ceramics to give an insight and a better understanding of the TSS method.

#### **5. Two-step sintering methodology**

*Design and Manufacturing*

and sintering process [21]. The SPS set-up consists of a graphite die filled with powder feedstock, uniaxial hydraulic pressing device which achieves 50–250 kN and an electric pulse current of low voltage (<10V) and high currents (1–10 kA) [2]. The system has achieved heating rates of up to 1000°C/min which makes it possible to sinter over very short durations [22]. It can be operated under vacuum or inert gas atmosphere at atmospheric pressure with a maximum temperature of 2400°C. The sintering mechanisms in SPS are a result of three effects, namely, mechani-

cal, thermal and electrical [2]. The fast heating rates achieved in SPS enables densification while retarding microstructure coarsening owing to the short times required to reach sintering temperature. This allows for the densification of nanopowders with minimal grain coarsening [22]. The SPS system offers a number of advantages over the conventional sintering systems such as hot pressing, hot isostatic pressing (HIP) which include high sintering speeds, high reproducibility,

The mechanism of sintering is not well understood but several authors have postulated a number of theories. The widely accepted SPS sintering mechanisms involve joule heating, plasma generation and electroplastic effect [2]. The electrical effects are a function of the electrical properties of the powders. For powders that are electrically conducting, current can easily flow through, and heat is generated mainly by joule heating and transferred to the bulk of the powder by conduction (see **Figure 1**) [23]. In the presence of an applied pressure, the electric current through the particles enhances formation of interparticle bonds through localised welding, vaporisation or cleaning of powder surfaces [22]. This ensures a smoother and more favourable path for the current flow. This also promotes the production of high-quality sintered compacts at lower temperatures in a shorter time than conventional sintering methods. The sintering of nonconducting powders, although not well understood, is thought to occur through grain boundary migration and

Although the SPS has the capability to sinter at high heating and cooling rates, the expectation is that the system can sinter without appreciable grain growth. However in reality this is not always the case; a complete avoidance of the grain growth at the sintering temperatures for most nano-grained materials will always promote grain growth. It is therefore imperative to adopt an approach/methodology

better control of sintering energy and reliability.

matter transport at higher input voltages.

*Pulse current flow through the spark plasma sintering technology [22].*

**110**

**Figure 1.**

The reliability of ceramic materials is a key function, and it dictates their ultimate performance. A carefully controlled microstructure has a greater impact on the properties and reliability of functional ceramic materials. In the previous section, it has been shown that the development of highly dense nanometric or ultrafine-grainsized ceramics is not easily achievable through conventional sintering. Although the SPS technology has shown great potential in the production of highly dense nanometric materials, it is difficult to maintain microstructural refinement under the high sintering temperatures. It must be underlined that solid-phase sintering requires high temperatures to facilitate diffusion which promotes material densification. However diffusion processes promote not only densification but also grain growth [24]. To achieve grain refinement during sintering, it is therefore imperative to develop a sintering methodology which promotes only densification without stimulating grain growth. This method has been improved over the years to achieve better microstructural refinement. The so-called two-step sintering (TSS) was subsequently introduced in the 1990s by Chu et al. [25]. In essence, the technique consists of two stages of consolidation process, i.e. a first stage performed at relatively low temperature followed by a higher-temperature stage and subsequent cooling.

However, the higher-temperature stage if not adequately controlled can lead to some grain growth. In 2000, Chen and Wang proposed a modified TSS methodology which effectively suppresses the accelerated grain growth in the second stage [24, 26]. In the modified TSS approach, a high-temperature heating is performed first for a short duration followed by structural freezing and sintering at a lower temperature. The idea of heating to a higher temperature (T1) followed by fast cooling with no sintering holding time (stage 1 in **Figure 2**) is to eliminate residual porosity at higher temperature and develop a network of grain boundary anchoring at triple points [11].

These anchored triple points are thought to have higher activation energy for matter migration than the grain boundaries. The second step effectively proceeds in

**Figure 2.** *Schematic illustration of the differences between the two TSS approaches [11].*

a *frozen* microstructure due to slower kinetics [27, 28]. The *kinetic window* separates grain boundary diffusion and grain boundary migration. Grain growth experienced at intermediate and high temperatures in fine-grained materials is driven by the significantly higher capillary pressures available in ultrafine grains. For instance, for a grain boundary energy (and surface energy) in ceramics of 1 J/m2 , a capillary pressure of the order 20 MPa at 100 nm, 200 MPa at 10 nm and 2000 MPa at 1 nm grain size. This implies there are significantly higher pressures locked in ultrafine grains to ignite additional kinetic effects at elevated temperatures. Thermodynamically, the temperature (T2) is sufficiently high to allow grain boundary diffusion with minimal grain boundary migration; this promotes densification without significant grain growth. It is however important to select the most suitable T2 temperature. In the case that T2 is too low, sintering proceeds for a prolonged period until it becomes exhausted; on the other hand, if T2 is too high, grain growth is likely to occur.

## **5.1 Two-step sintering as applied to functional ceramic materials**

The use of the TSS methodology to obtain ceramic materials of controlled microstructure has become standard practice. This section gives a detailed discussion on the effects of processing characteristics on grain density and size as well as their contribution to the improvement of mechanical properties of a number of functional ceramic materials investigated in previous studies.

The pioneering work of Cheng and Wang in 2000 serves as the beginning of a new era in TSS methodology. In one of their successful studies, Chen and Wang obtained a density of 99% and a grain size of 123 nm using a T1 temperature of 1250°C and T2 temperature of 1100°C for a pure Y2O3 ceramic material [24, 26]. After several experimental studies, Chen and Wang concluded that the success of grain growth suppression in their work was mainly attributed to triple-point immobility irrespective of whether doping agents were used or not [11]. In a separate study, Mazaheri et al. [29] obtained dense samples of ZnO with limited grain growth under varying conditions. The starting particle size of the ZnO was 31 nm. The most interesting result was obtained with a T1 of 800°C and T2 of 750°C; a relative density and grain size of 98% and ~68 nm were obtained, respectively [29]. The same authors proved that slightly higher temperatures (850°C and 780°C for T1 and T2, respectively) resulted in grain growth and a lower densification of 86% using ZnO material [29]. A further study done at even higher temperatures, with a starting ZnO powder of grain size of 400 nm (0.4 μm) and a T1 of 1100°C and T2 of 1050°C, resulted in a relative density of 95.1% and a grain size of 3.9 μm, signifying the ineffectiveness of the TSS methodology at higher sintering temperatures and larger particle sizes. Several other TSS studies carried out on the ZnO material proved that the use of dopant agents such as Bi2O3, Sb2O3, CoO and MnO assisted in suppressing grain growth [11].

Yttria-stabilised ZrO2 (YSZ) is one of the most important functional ceramics which find its use in a wide range of applications. Several attempts have been focused on attaining nanometric YSZ materials to improve its functional properties. Mazaheri et al. [30] obtained fully densified 3YSZ with an initial grain size of 75 nm and a pressureless sintering regime of T2 (1150°C) and T1 (1300°C) with an isothermal holding time of 30 h at T2 and 1 min holding time at T1. The final grain size achieved was 110 nm. In a separate study, Suarez compared SPS sintering with pressureless TSS methodologies. The starting material was a three Y2O3-stabilised tetragonal ZrO2 (3YTZ) with an initial average particle size of 65 nm. The TSS methodology used a pressureless sintering regime as T1 at 1350°C without holding time and T2 at 1200°C for 15 h, and a final grain size of 125 nm was obtained. On the other hand, the SPS method was carried out at 1150°C with a heating rate of 300°C/min and an isothermal holding time of 30 mins at a pressure of 150 MPa.

**113**

*\**

**Table 1.**

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma…*

of the sintering method in its effectiveness of grain growth control [11].

The grain size obtained was 115 nm. Several other modifications as summarised in **Table 1** were carried out, and an observation that was common to the majority of the studies is that the homogeneity of the green body was very critical to the success

As mentioned earlier, piezoelectric materials' performance is strongly influenced by the grain size of the constituent particles making up the ceramics. High-performing piezoceramics have been obtained using the TSS approach. A number of studies have shown that lower sintering temperatures using the TSS methodology can be utilised to obtain ultrafine grain sizes. BaTiO3 is one of the most popular piezoceramics studied, and a number of studies have been carried out to obtain fine-grained microstructures. In their work, Kim and Han [31] used a 1% dysprosium (Dy)-doped BaTiO3 with a particle size of approximately 17 nm which was compacted at 300 MPa at room temperature. The TSS profile used a T1 of 1300°C and a T2 of 1100°C with a 20 h holding time. A grain size close to 1 μm was obtained at a relative density of 95%. In a separate study, Wang et al. [32] studied two different piezoceramic composites, i.e. pure BaTiO3 with a particle size between 10 nm and 30 nm and a nanometric ferrite of composition Ni0.2Cu0.2Zn0.6Fe2O4 with a starting particle size of 10 nm. The two powders were compacted isostatically at 200 MPa. The two piezoceramics were sintered using two different sintering programmes, i.e. a T1 of 950–1250°C for BaTiO3 and at 850–930°C for ferrite; a cooling rate of 10°C/min was used in both cases. The samples were cooled at 30°C/min to a

*DOI: http://dx.doi.org/10.5772/intechopen.86461*

**Sample Method Starting** 

Pure Y2O3

Pure ZnO

Pure ZnO

**grain size (nm)**

3YSZ\* TSS 0.27 μm — 1500 (5 min)

3YSZ TSS 60–120 — 1300,

3YSZ H-SPS 60–120 100 at

8YSZ SPS 58 50,

*3YSZ, 3 mol% Y2O3-stabilised ZrO2.*

3YSZ SPS 65 150 — 1150,

600°C, 3 min

15 min

*A summary of the two-step methodology used to produce different materials.*

**Pressure (MPa)**

TSS 10–60 — 10°C/min to

**T1 profile (°C)**

1250

10°C/min

3YSZ TSS 65 — 1300 1200, 15 h — — [36]

10°C/m

300 MPa at 1000, 5 min

1150, 200 °C/min, 20 s, 10 MPa

3YSZ TSS 75 150 1300, 1 min 1150, 30 h Density at

TSS 31 — 800 750 98 680 nm [29]

TSS 400 — 1100 1050 95.1 3.9 μm [34]

**T2 profile (°C)**

50°C/min to 1100, 6–30 h dwell

150 MPa, 300°C/min, 30 min

1175, 300 MPa, 30 h

1050, 50 MPa, 2 h

**Relative density (%)**

1300, 10 h — 0.59 μm [35]

1175, 20 h 99.2 184 nm [37]

T1 (83)

**Grain size**

99 123 nm [24, 26]

110 nm [30]

— 115 nm [36]

97.4 173 nm [37]

99.8 190 nm [38]

**Ref**

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma… DOI: http://dx.doi.org/10.5772/intechopen.86461*

The grain size obtained was 115 nm. Several other modifications as summarised in **Table 1** were carried out, and an observation that was common to the majority of the studies is that the homogeneity of the green body was very critical to the success of the sintering method in its effectiveness of grain growth control [11].

As mentioned earlier, piezoelectric materials' performance is strongly influenced by the grain size of the constituent particles making up the ceramics. High-performing piezoceramics have been obtained using the TSS approach. A number of studies have shown that lower sintering temperatures using the TSS methodology can be utilised to obtain ultrafine grain sizes. BaTiO3 is one of the most popular piezoceramics studied, and a number of studies have been carried out to obtain fine-grained microstructures. In their work, Kim and Han [31] used a 1% dysprosium (Dy)-doped BaTiO3 with a particle size of approximately 17 nm which was compacted at 300 MPa at room temperature. The TSS profile used a T1 of 1300°C and a T2 of 1100°C with a 20 h holding time. A grain size close to 1 μm was obtained at a relative density of 95%. In a separate study, Wang et al. [32] studied two different piezoceramic composites, i.e. pure BaTiO3 with a particle size between 10 nm and 30 nm and a nanometric ferrite of composition Ni0.2Cu0.2Zn0.6Fe2O4 with a starting particle size of 10 nm. The two powders were compacted isostatically at 200 MPa. The two piezoceramics were sintered using two different sintering programmes, i.e. a T1 of 950–1250°C for BaTiO3 and at 850–930°C for ferrite; a cooling rate of 10°C/min was used in both cases. The samples were cooled at 30°C/min to a


#### **Table 1.**

*A summary of the two-step methodology used to produce different materials.*

*Design and Manufacturing*

a *frozen* microstructure due to slower kinetics [27, 28]. The *kinetic window* separates grain boundary diffusion and grain boundary migration. Grain growth experienced at intermediate and high temperatures in fine-grained materials is driven by the significantly higher capillary pressures available in ultrafine grains. For instance, for

sure of the order 20 MPa at 100 nm, 200 MPa at 10 nm and 2000 MPa at 1 nm grain size. This implies there are significantly higher pressures locked in ultrafine grains to ignite additional kinetic effects at elevated temperatures. Thermodynamically, the temperature (T2) is sufficiently high to allow grain boundary diffusion with minimal grain boundary migration; this promotes densification without significant grain growth. It is however important to select the most suitable T2 temperature. In the case that T2 is too low, sintering proceeds for a prolonged period until it becomes

exhausted; on the other hand, if T2 is too high, grain growth is likely to occur.

The use of the TSS methodology to obtain ceramic materials of controlled microstructure has become standard practice. This section gives a detailed discussion on the effects of processing characteristics on grain density and size as well as their contribution to the improvement of mechanical properties of a number of

The pioneering work of Cheng and Wang in 2000 serves as the beginning of a new era in TSS methodology. In one of their successful studies, Chen and Wang obtained a density of 99% and a grain size of 123 nm using a T1 temperature of 1250°C and T2 temperature of 1100°C for a pure Y2O3 ceramic material [24, 26]. After several experimental studies, Chen and Wang concluded that the success of grain growth suppression in their work was mainly attributed to triple-point immobility irrespective of whether doping agents were used or not [11]. In a separate study, Mazaheri et al. [29] obtained dense samples of ZnO with limited grain growth under varying conditions. The starting particle size of the ZnO was 31 nm. The most interesting result was obtained with a T1 of 800°C and T2 of 750°C; a relative density and grain size of 98% and ~68 nm were obtained, respectively [29]. The same authors proved that slightly higher temperatures (850°C and 780°C for T1 and T2, respectively) resulted in grain growth and a lower densification of 86% using ZnO material [29]. A further study done at even higher temperatures, with a starting ZnO powder of grain size of 400 nm (0.4 μm) and a T1 of 1100°C and T2 of 1050°C, resulted in a relative density of 95.1% and a grain size of 3.9 μm, signifying the ineffectiveness of the TSS methodology at higher sintering temperatures and larger particle sizes. Several other TSS studies carried out on the ZnO material proved that the use of dopant agents such

**5.1 Two-step sintering as applied to functional ceramic materials**

functional ceramic materials investigated in previous studies.

as Bi2O3, Sb2O3, CoO and MnO assisted in suppressing grain growth [11].

Yttria-stabilised ZrO2 (YSZ) is one of the most important functional ceramics which find its use in a wide range of applications. Several attempts have been focused on attaining nanometric YSZ materials to improve its functional properties. Mazaheri et al. [30] obtained fully densified 3YSZ with an initial grain size of 75 nm and a pressureless sintering regime of T2 (1150°C) and T1 (1300°C) with an isothermal holding time of 30 h at T2 and 1 min holding time at T1. The final grain size achieved was 110 nm. In a separate study, Suarez compared SPS sintering with pressureless TSS methodologies. The starting material was a three Y2O3-stabilised tetragonal ZrO2 (3YTZ) with an initial average particle size of 65 nm. The TSS methodology used a pressureless sintering regime as T1 at 1350°C without holding time and T2 at 1200°C for 15 h, and a final grain size of 125 nm was obtained. On the other hand, the SPS method was carried out at 1150°C with a heating rate of 300°C/min and an isothermal holding time of 30 mins at a pressure of 150 MPa.

, a capillary pres-

a grain boundary energy (and surface energy) in ceramics of 1 J/m2

**112**

T2 range of 1150–850°C for BaTiO3 and 750–870°C for the ferrite with an isothermal holding time of 20 h. For the BaTiO3, the best result achieved showed a grain size of 35 nm and was sintered at 950°C in T1 and at 900°C in T2 for 2 h. On the other hand, a grain size of 200 nm was achieved for the ferrite sintered at 850°C in T1 and 800°C for 6 h in T2. In comparison to the result of Kim and Han discussed earlier, the use of a lower sintering temperature enabled the formation of BaTiO3 with a finer grain size. To elucidate this point, Karaki et al. [33] obtained a mean grain size of 1.6 μm with a relative density of 98.3% starting with a BaTiO3 of average particle size of 100 nm, cold compacted at 200 MPa. A TSS profile almost similar to that of Kim and Han et al. consists of a heating rate of 10°C/min to T1 (1230–1340°C) held for 1 min and a cooling rate of 30°C/min to T2 (1150–1200°C) held for 2, 4, 5 and 20 h. The best result was obtained with a sintering profile consisting of T1 (1320°C) and T2 (1150°C) with a 15 h holding time. There is a clear indication from the above results that BaTiO3 ceramics with high sintered density are obtainable at lower sintering temperature (≤1000°C) for both T1 and T2. Some of the authors observed that the piezoelectric coefficient is strongly influenced by the grain size [11]. Nanometric BaTiO3 ceramics possess superior piezoelectric coefficients in comparison to their micrometric counterparts.

As mentioned earlier, the current work is not meant to be exhaustive but to give a qualitative insight on the research covered so far. **Table 1** gives a summary of some of the successful work carried out and highlights some critical aspects in the TSS methodology as a grain refinement process.

Specific particularities were observed in each of the sintering cycles above. The majority of the authors compared their results with those obtained by conventional sintering and the TSS methodology resulted in superior grain refinement. The initial powder features such as particle size, microstructural homogeneity and green density are quite critical in the success of the TSS process. In the majority of cases, the difference between the T1 and T2 temperatures is <1500°C, and a larger holding time in T2 allowed smaller grain sizes to be obtained. The TSS method has also been shown to improve the material properties (both mechanical and physical).

In some cases the use of dopants was effective in inhibiting grain growth. Although the TSS approach has shown great success in effecting grain refinement, the very long isothermal holding times at T2 might not be suitable for commercial purposes. The SPS technology offers an alternative route for grain refinement, and results in **Table 1** show that it is more effective for materials such as YSZ ceramics. The SPS process generally can achieve grain refinement over shorter time periods.
