# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 4 Transparent Ceramics  
ZrO\({}_{2}\) possesses peculiar mechanical and functional properties, especially yttria-stabilized zirconia (YSZ), which has been widely investigated. It has been prepared by glycine-nitrate process and ball milling and the nanocrystalline precursor powders of 8 mol% YSZ ceramics used for SPS. The specimens sintered at 1200 and 1250 \({}^{\circ}\)C were opaque due to high porosity that causes the lowering of transmittance. Yttria-stabilized zirconia transparent ceramics were obtained at 1300 \({}^{\circ}\)C by SPS route and within a short period of time (5 min). With the rise in temperature, direct transmittance decreased due to scattering by grain boundaries and a higher number of oxygen vacancies [44]. Transparent alumina/ceria ceramics were also obtained by SPS [45]. It was found that transparency was enhanced upon the addition of ceria nanoparticles in transparent alumina material. At the initial stage of sintering, ceria nanoparticles contribute to densification, while, at the later stage, they delay alumina grain growth. The best sintering conditions were fixed at 1430 \({}^{\circ}\)C, 80 MPa, and 2 min. SPS transparent YAG ceramics [46] were fabricated at 1400 \({}^{\circ}\)C, the pressure of 50 MPa, and 3 min time, using starting commercial nanocrystalline YAG precursor powders. An increase in the SPS duration and pressure significantly increased the density, especially at temperatures below 1400 \({}^{\circ}\)C. The observed microstructure was in agreement with densification by nano-grain rotation and sliding  
Figure 2: Effects of ON-OFF DC pulse energizing. Reprinted with permission from the InTechopen publisher [38]. at lower densities, followed by curvature-driven grain boundary migration and normal grain growth at higher densities. Residual nanosize pores at the grain boundary junctions were an inherent microstructure feature due to the SPS process. The direct transmission intensity was lower at the lower wavelengths, likely due to the higher Rayleigh scattering in this range, the scattered intensity being inversely proportional to the fourth power of the wavelength. Dy\({}^{3+}\) (3 at%)-doped Y\({}_{2}\)O\({}_{3}\) ceramics were also fabricated by the SPS process [47]. The starting precursor powders were weighed and sieved through 60 mesh screen and used for further SPS measurements under vacuum. The sintering conditions, 600 \({}^{\circ}\)C for 5 min, further increased to 1100 \({}^{\circ}\)C for 3 min and finally to 1400 \({}^{\circ}\)C for 30 min. Similarly, four samples at different final temperatures like 1450, 1500, 1550, and 1600 \({}^{\circ}\)C were also prepared. From XRD profiles, it was observed that no significant changes occurred with the addition of dopant in the ceramic Y\({}_{2}\)O\({}_{3}\) host matrix. A large content of pores in the grain boundary was observed in the sample prepared at 1400 \({}^{\circ}\)C. When increasing the sintering temperature to 1550 \({}^{\circ}\)C, there was a decrease in the pores' size and increase in the grain size together with a uniform distribution of grains. At 1600 \({}^{\circ}\)C, an abnormal increase in grain growth took place, affecting optical transmittance. The maximum direct transmittance (74.5%, at 574 nm) was obtained for the sample sintered at 1550 \({}^{\circ}\)C. The laser parameters were assessed from applying the Judd-Ofelt theory. The prepared transparent Y\({}_{2}\)O\({}_{3}\):Dy ceramic materials were found useful for yellow laser emission corresponding to \({}^{4}\)F\({}_{9/2}\)\(\rightarrow\)\({}^{6}\)H\({}_{13/2}\) transition at 572 nm. [PERSON] et al. [48] reported laser operation in 0.5 at.% Er\({}^{3+}\)-doped YAG polycrystalline ceramics developed by SPS starting from commercial precursor powder size of 271 nm. After obtaining SPS samples, they were treated with hot isostatic pressing (HIP), which helps in obtaining highly transparent ceramics and removing porosity.