# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Silicate GCs
The authors confirmed that the detrimental effect of low-temperature degradation (LTD) of tetragonal ZrO\({}_{2}\) limited the usage as dental materials. TEM images revealed that nano GCs had much less grain boundary, and the silica encapsulating surface layer on nano-sized tetragonal ZrO\({}_{2}\) contributed to the high hydrothermal stability of nano GCs, which was useful for medical applications. The SPS-sintered nano GC had translucency and lower transmittance (20%) under direct transmission measurements. The text under the different samples was observed with the naked eye and was clearly recognized, when the blurry text appeared for 45 Zr, 55 Zr, and 65 Zr samples, respectively. [PERSON] et al. in 2017 [55] studied and prepared phosphor in glass (PIG) using the SPS process (SPS PIG) to be compared in terms of microstructure and optical properties with those of the PIG sintered in an electric furnace. The 20 SiO\({}_{2}\)-30B\({}_{2}\)O\({}_{3}\)-45 ZnO-5 Li\({}_{2}\)O (mol%) glass frit was prepared using an electric furnace at 1200 \({}^{\circ}\)C for 30 min in air. This frit was milled and screened using a 100-\(\upmu\)m-size mesh. Two types of phosphors--yellow phosphor (Y phosphor, Y\({}_{3}\)Al\({}_{5}\)O\({}_{12}\):Ce\({}^{3+}\)) and red phosphor (R phosphor, Ca\({}_{0.2}\)AlSiSr\({}_{0.8}\)N\({}_{3}\):Eu\({}^{2+}\))--were selected and mixed with 5 vol% of the phosphor to convert glass frit into the PIG. These powder samples were SPS-sintered at 520 \({}^{\circ}\)C/40 MPa/10 min conditions at different heating rates and also sintered in an electric furnace at 630 \({}^{\circ}\)C for 30 min at a heating rate of 10 \({}^{\circ}\)C/min in air. The SPS sintered with yellow phosphor PIGs showed a smaller mean pore size and lower porosity than the glass frit plate, and the same composition sintered in an electric furnace. Similarly, the SPS-sintered red PIGs showed a lower porosity and smaller pore size than the glass frit plate and those sintered in an electric furnace. The pore properties were influenced more by the heating rate than by temperature. The porosity of the SPS red PIGs was slightly higher than that of the SPS-sintered yellow PIGs owing to the increased viscosity that prevents the densification based on the mass change and the shape of the phosphor. The SPS-sintered yellow PIGs showed a higher transmittance than the glass without phosphor and those sintered in an electric furnace of yellow PIG. In the case of the red PIGs, the SPS-sintered red PIGs showed a lower transmittance than the glass plate because of the influence of the R phosphors with a high refractive index, but a higher transmittance than red phosphor sintered in an electric furnace. As the porosity of the SPS PIGs decreased, the transmittance increased. Authors remarked that the presence of an appropriate number of small pores with diameters less than 5 \(\upmu\)m in the PIG increased the phosphor intensity and luminous efficiency by increasing the scattering angle of the light that caused the increase in the interaction with the phosphor as compared to the PIGs with no pores.