# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 3 Mechanism of Spark Plasma Sintering  
SPS is a more advanced and efficient sintering technique in terms of its operating parameters compared with the pressureless sintering (PLS), hot press (HP) sintering, and hot isostatic pressing (HIP). SPS makes possible sintering at lower temperatures and in a short period of time by charging the powder particles with electrical energy and effectively applying a high-temperature spark plasma generated at an initial stage of pulse energizing particles in short sintering time. Pulse current energizing provides enhancement of sinterability and densification rate on the material. An electromagnetic field and/or joule heating is applied by continuous ON/OFF DC pulsed high electric current with low voltage. The technique appears like HP heated by a radiative furnace. However, the SPS treated samples can reach relevant qualities like structurally tailoring effect, grain growth control, enhancement of electro-migration, and strong preferential orientation effect [2; 8].  
The SPS process is a dynamical non-equilibrium processing phenomenon related to the passage of plasma from initial to final stages via reacted material characteristics. However, the ON/OFF DC pulse energizing method is one of the best-implemented mechanisms of SPS processing and thus creates (1) spark plasma, (2) spark impact pressure, (3) Joule heating, and (4) an electrical field diffusion effect. In the initial stage of the SPS presented in Figure 2, DC pulsed voltage (ON mode) does apply through die and punches made of a graphite material, powder material, and the subsequent heating (Joule heating) and densification of the powder. An electric noise is heard during the process, attributed to the plasma generation. At the final stage, the formation of the spark discharges is located in the gaps between particle surfaces. At the time of ON mode, the powder particle surfaces are more purified, producing materials with surface-layer composition and microstructure different from that of the core. In the OFF mode, plasma passes throughout the volume of the sintered powder, followed by vaporization, melting of surface, and neck formation. The large pulse energy generates an electromagnetic field effect, such as an electro-migration and preferential orientation of the crystalline phase [8; 38].  
Figure 1: Schematic of spark plasma sintering unit. Reprinted with permission from Advanced Engineering Materials [31].  
The sintering mechanism of nano grained (NG-YAG) ceramics at high pressure was proposed by [PERSON] et al. [39]. According to these authors, the sintering kinetic process can be divided into three stages. In the first step, the sintering pressure promotes a plastic deformation period. At the middle stage, the grain boundary viscous sliding controls the process. The final step, under high pressure, includes the transformation from grain boundary viscous sliding to grain boundary diffusion.  
In the case of GCs, a viscous flow sintering mechanism occurs when the volume fraction of liquid is sufficiently high so that full densification of the compact can be achieved by viscous flow of grain-liquid mixture without having any grain shape change during densification. The viscous flow mechanism, which was first proposed by [PERSON] [40, 41], can be applied in the sintering of viscous materials like glass, which follows the behavior of a Newtonian fluid. [PERSON] [42] suggested that the application of high pressure increases glass viscosity and reduces the crystal growth rate. Crystalline size decrease corresponding to a pressure increase suggests a suppressed or hindered crystal growth. The application of high pressure is thought to increase the viscosity of the glass surrounding the pre-nucleated crystals, reducing the atomic movement and diffusion across the crystal-liquid interface [43].  
From now on, some examples of transparent ceramics and glass-ceramics processed by SPS are discussed.