**Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings the Surface Properties of Ceramic Coatings**

**Microwave Plasmas as a Processing Tool for Tailoring** 

DOI: 10.5772/intechopen.71686

Emmanuel J. Ekoi, Muhammad Awais and Denis P. Dowling Denis P. Dowling Additional information is available at the end of the chapter

Emmanuel J. Ekoi, Muhammad Awais and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.71686

#### **Abstract**

This chapter reviews the use of low pressure microwave plasmas as a processing technology for both sintering and controlling the surface chemistry of porous ceramic coatings. A particular advantage of microwave processing is its ability to penetrate the surface of the workpiece; enabling rapid volumetric heating and thus reducing the need for external heat sources. The microwave plasma treatments have the ability to sinter materials in minutes rather than the hours taken using conventional furnace processing. This study provides examples of the use of these plasmas to sinter both nickel and titanium nanoparticles. These are used in the fabrication of electrodes for use in dye sensitized solar cells. Further applications of the microwave plasma treatments investigated is for their use in heat treatment to control crystalline phase transitions, as well as a rapid technique to oxidize metal surfaces.

**Keywords:** microwave plasma, porous ceramics, nickel oxide, titanium oxide, coatings

#### **1. Introduction**

Microwaves are electromagnetic waves with wavelengths from 1 mm to 1 m and corresponding frequencies between 0.3 MHz and 300 GHz [1, 2]. The use of microwave treatments (non-plasma), as an energy source for the heat treatment of metals, ceramics and composites has been reported to be more effective than conventional furnace heating methods, due to the improved microstructure and properties, reduction in processing time, etc. [1–3]. These microwave treatments are usually carried out at 0.915 GHz, 2.45 GHz and 20–30 GHz frequencies in agreement with the industrial, scientific and medical (ISM) radio bands set aside for non-communication purposes [2, 4]. During microwave processing, energy is supplied

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 IntechOpen. 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.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

by an electromagnetic field directly to the material by the interaction of the molecules of the material with electromagnetic field. In contrast, conventional thermal processing involves the transfer of energy by conduction, convection and radiation [5].

Clark et al. [1] reviewed the effectiveness of microwave heating and found it to be substrate material dependant. Materials were, thus, grouped into categories (depending on the electromagnetic field-material interaction) as: transparent (low dielectric loss materials); opaque (conductors); absorbing (high dielectric loss materials); and mixed absorbing where there is selective absorption due to differences in dielectric loss in the materials.

Several researchers have demonstrated the capability of using microwave to process metallic and ceramic materials [6–11]. It has been reported that a distinction in the microstructure and porosity distribution can be made between conventional and microwave sintered materials with respect to the pore shape. Microwave sintered samples exhibit pores with more rounded edges than the conventionally sintered materials. The sphericity of pore is reported to be critical in the ductility and strength of the sintered material [11].

In addition to the use of microwaves directly, a plasma discharge can also be formed using the microwave energy, if the appropriate conditions (gas type, pressure, resonator, etc.) are in place. Briefly by way of introduction to this technique, a plasma is described as a collection of unbound charged particles, photons and neutral atomic or molecular species which are electrically neutral on average [12, 13]. Plasma generation arises from the excitation and ionization of gases by energy transfer from various sources to the gases to form excited, charged and neutral species [14]. There are a range of plasma types including high frequency, DC and low frequency discharges [15, 16]. The latter include systems such as glow discharge, corona discharge, hollow cathode discharge [16], while the high frequency consists of radio frequency (RF) and microwave plasma discharges [17].

the power is usually coupled through radiation and thus, bypasses sheath losses [17, 23]. A further advantage of these plasmas is that they can be operated in a wide gas pressure range [24, 25], which combine with the higher electron temperature generally obtained, makes them

Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings

http://dx.doi.org/10.5772/intechopen.71686

111

Metal oxide layers can be obtained using a range of techniques including sol-gel, hydrothermal synthesis, electrophoretic deposition, sputtering, electrochemical treatments (anodic oxidation), chemical vapor deposition (CVD), physical vapor deposition and ion implantation, etc. [26–32]. Most of the techniques (except sputtering and direct metal oxide growth on metal surfaces) require sintering to enhance the packing density, porosity and the adhesion of the deposited coatings. A range of characterisation techniques usually applied includes a FEI Quanta 3D FEG DualBeam scanning electron microscope (SEM) (Morphology); a focused ion beam (FIB)(Thickness); Energy-dispersive X-ray spectroscopy (EDX) (Elemental data); Veeco NT1100 optical profilometer (Surface roughness). Siemens D500 XRD system (Phase identification), a LASCON QP003 and LPC03 ratio pyrometers from Dr. Mergenthaler GmbH & Co. KG. (Temperature measurements) and a J.A. Woollam ellipsometer with a Tauc-Lorentz

Having provided an introduction to microwave plasma treatments along with the associated characterization techniques, the following sections firstly provide an overview of the use of microwave plasmas for the sintering of metal oxide powders. The use of microwave plasma treatments for sintering, has previously been described as rapid discharge sintering (RDS).

capable of providing a higher fraction of ionization and dissociation [24].

**Figure 1.** Schematic representation of CAP microwave plasma system.

fitting via completeEASE® software (Band gap and coating thickness).

Microwave plasma systems are usually electrodeless systems and thus, differ from DC and RF systems which mostly use electrodes to generate plasma. In these systems, microwaves are usually guided using waveguide into the chamber where energy is impacted to the gas to form plasma by partially ionizing the gas [18, 19]. A typical example of microwave plasma system is the circumferential antenna plasma (CAP) microwave reactor, which operates at 2.45 GHz (**Figure 1**). In this CAP system, a cylindrical quartz ring window, 345 mm in diameter, embedded in the wall of the plasma chamber guides the microwaves into the plasma chamber. The microwaves are directed to the plasma chamber through a coaxial waveguide, and expands radially as it moves towards the cylindrical quartz ring window. A perfect rotational symmetry is obtained when the microwave which has uniform amplitude and phase distribution passes through the quartz window [20]. The cylindrical geometry of the configuration ensures that the formation of microwave power density at the window is lower than in the center of the chamber; thus, no plasma is formed adjacent to the window [21].

The use of microwave plasma source for surface and bulk treatment of metal and ceramics has the potential to be more effective than DC and RF plasmas, because of the higher density of active atomic species that can be generated using this type of discharge [22]. Microwave discharges generally have higher electron kinetic temperature and number density because Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings http://dx.doi.org/10.5772/intechopen.71686 111

**Figure 1.** Schematic representation of CAP microwave plasma system.

by an electromagnetic field directly to the material by the interaction of the molecules of the material with electromagnetic field. In contrast, conventional thermal processing involves the

Clark et al. [1] reviewed the effectiveness of microwave heating and found it to be substrate material dependant. Materials were, thus, grouped into categories (depending on the electromagnetic field-material interaction) as: transparent (low dielectric loss materials); opaque (conductors); absorbing (high dielectric loss materials); and mixed absorbing where there is

Several researchers have demonstrated the capability of using microwave to process metallic and ceramic materials [6–11]. It has been reported that a distinction in the microstructure and porosity distribution can be made between conventional and microwave sintered materials with respect to the pore shape. Microwave sintered samples exhibit pores with more rounded edges than the conventionally sintered materials. The sphericity of pore is reported to be criti-

In addition to the use of microwaves directly, a plasma discharge can also be formed using the microwave energy, if the appropriate conditions (gas type, pressure, resonator, etc.) are in place. Briefly by way of introduction to this technique, a plasma is described as a collection of unbound charged particles, photons and neutral atomic or molecular species which are electrically neutral on average [12, 13]. Plasma generation arises from the excitation and ionization of gases by energy transfer from various sources to the gases to form excited, charged and neutral species [14]. There are a range of plasma types including high frequency, DC and low frequency discharges [15, 16]. The latter include systems such as glow discharge, corona discharge, hollow cathode discharge [16], while the high frequency consists of radio frequency

Microwave plasma systems are usually electrodeless systems and thus, differ from DC and RF systems which mostly use electrodes to generate plasma. In these systems, microwaves are usually guided using waveguide into the chamber where energy is impacted to the gas to form plasma by partially ionizing the gas [18, 19]. A typical example of microwave plasma system is the circumferential antenna plasma (CAP) microwave reactor, which operates at 2.45 GHz (**Figure 1**). In this CAP system, a cylindrical quartz ring window, 345 mm in diameter, embedded in the wall of the plasma chamber guides the microwaves into the plasma chamber. The microwaves are directed to the plasma chamber through a coaxial waveguide, and expands radially as it moves towards the cylindrical quartz ring window. A perfect rotational symmetry is obtained when the microwave which has uniform amplitude and phase distribution passes through the quartz window [20]. The cylindrical geometry of the configuration ensures that the formation of microwave power density at the window is lower than in

the center of the chamber; thus, no plasma is formed adjacent to the window [21].

The use of microwave plasma source for surface and bulk treatment of metal and ceramics has the potential to be more effective than DC and RF plasmas, because of the higher density of active atomic species that can be generated using this type of discharge [22]. Microwave discharges generally have higher electron kinetic temperature and number density because

transfer of energy by conduction, convection and radiation [5].

110 Recent Advances in Porous Ceramics

cal in the ductility and strength of the sintered material [11].

(RF) and microwave plasma discharges [17].

selective absorption due to differences in dielectric loss in the materials.

the power is usually coupled through radiation and thus, bypasses sheath losses [17, 23]. A further advantage of these plasmas is that they can be operated in a wide gas pressure range [24, 25], which combine with the higher electron temperature generally obtained, makes them capable of providing a higher fraction of ionization and dissociation [24].

Metal oxide layers can be obtained using a range of techniques including sol-gel, hydrothermal synthesis, electrophoretic deposition, sputtering, electrochemical treatments (anodic oxidation), chemical vapor deposition (CVD), physical vapor deposition and ion implantation, etc. [26–32]. Most of the techniques (except sputtering and direct metal oxide growth on metal surfaces) require sintering to enhance the packing density, porosity and the adhesion of the deposited coatings. A range of characterisation techniques usually applied includes a FEI Quanta 3D FEG DualBeam scanning electron microscope (SEM) (Morphology); a focused ion beam (FIB)(Thickness); Energy-dispersive X-ray spectroscopy (EDX) (Elemental data); Veeco NT1100 optical profilometer (Surface roughness). Siemens D500 XRD system (Phase identification), a LASCON QP003 and LPC03 ratio pyrometers from Dr. Mergenthaler GmbH & Co. KG. (Temperature measurements) and a J.A. Woollam ellipsometer with a Tauc-Lorentz fitting via completeEASE® software (Band gap and coating thickness).

Having provided an introduction to microwave plasma treatments along with the associated characterization techniques, the following sections firstly provide an overview of the use of microwave plasmas for the sintering of metal oxide powders. The use of microwave plasma treatments for sintering, has previously been described as rapid discharge sintering (RDS). The specific focus is on the sintering of nanoparticles of nickel oxide (NiO) and titanium dioxide (TiO<sup>2</sup> ), used as electrodes in dye sensitized solar cells (DSSCs). The effect of the plasma treatments on thermal sensitive crystalline phase changes, is also discussed. A further application reported is that of the use of microwave plasma treatments for the oxidation of metal surfaces, in order to produce porous oxide ceramic (TiO<sup>2</sup> ) layers on the metal surface.
