# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Silicate GCs
The dried 45S5 Bioglass(r) powder was pelletized under uniaxial pressure of 50 kPa. The pellets were subjected to conventional sintering at 700, 800, 900, 950, 1000, 1100, and 1200 \({}^{\circ}\)C and SPS sintering at 600, 750, 800, and 950 \({}^{\circ}\)C. Densities through conventional and SPS sintering were 89% and 92%, achieved at 1200 and 950 \({}^{\circ}\)C, respectively. High densification was obtained at much lower temperatures by the SPS process. No significant changes in density were observed at higher temperatures. Fine and homogenously distributed crystalline particles of the Na\({}_{2}\)Ca\({}_{2}\)Si\({}_{3}\)O\({}_{9}\) phase were present in the glass matrix. The SPS-sintered samples presented fewer microvoids and a higher homogeneity together with fine crystalline particles, which provided enhanced mechanical characteristics. A Young's modulus (~110 GPa) and compressive strength ~110 MPa were achieved higher than those obtained by conventional heating. [PERSON] et al. in 2014 [53] prepared soda-time GCs for the dental application by SPS. The powders were sintered in vacuum at 750 \({}^{\circ}\)C for 3 min and a pressure of 32 MPa. XRD patterns revealed the formation of two crystalline phases: nepheline (Na(AlSiO\({}_{4}\)) and combineite (Na\({}_{4}\)Ca\({}_{4}\)(Si\({}_{6}\)O\({}_{18}\)) with the crystalline size of below 5 \(\upmu\)m. Density was 2.7 g/cm\({}^{3}\) without any porosity. Similarly, they followed cold isostatic pressure (CIP) to obtain soda-time GCs and compared them with SPS-processed ones. In the CIP experiments, the samples were sintered at 750 \({}^{\circ}\)C, a pressure of 300 MPa for a time of 1 h. Density was 2.3 g/cm\({}^{3}\), with 17% of the final porosity size of below 100 \(\upmu\)m. The surface roughness (Sa) was found to be low (0.7 \(\upmu\)m) for the SPS-sintered samples and high (3.2 \(\upmu\)m) for CIP samples. [PERSON] et al. in 2015 [43] discussed and studied the effects of SPS on the microstructure of lithium disilicate (LDS) glass-ceramics and compared with conventional sintering. They compared and analyzed the effects of SPS on two materials--one was IPS e.max CAD, and another one was IPS e.max Press. IPS e.max CAD glass-ceramic samples were processed by spark plasma sintering (SPS) and conventional sintering (CS) for comparison. Samples were sintered at varying temperatures, heating rates, and pressures to analyze their significance on materials. It was noticed that an additional graphite phase was identified in XRD for the SPS-sintered sample owing to contamination from the graphite punch and die used in the SPS set up. To avoid this problem, the authors suggested using molybdenum foils and the possibility of reduction of surface contamination. However, there was a substantial increase in median crystal size between the conventionally-sintered at 840 \({}^{\circ}\)C and the SPS-sintered at 840 \({}^{\circ}\)C. They also noticed that applying higher temperatures was more effective for promoting grain boundary diffusion processes and crystallite growth, also for the IPS e.max CAD glass-ceramic samples. The IPS e.max CAD had a lower crystal fraction of the LDS phase (40%) compared to the IPS e.max Press (70%) after the heat treatment at 840-850 \({}^{\circ}\)C.  
[PERSON] et al. in 2017 [54] investigated on xZrO\({}_{2}\)-(100-x) SiO\({}_{2}\) (x = 45 mol%, 55 mol%, 65 mol%) system, prepared by sol-gel method. Sieved precursor glass powders with 50-100 \(\upmu\)m were further used for SPS methods. The XRD profiles of samples sintered under 1150 \({}^{\circ}\)C/60 MPa/5 min revealed that the only crystal phase in all the samples was tetragonal ZrO\({}_{2}\). The average nano-sized ZrO\({}_{2}\) crystal of the 45 Zr, 55 Zr, and 65 Zr samples was 29.5 nm, 35.1 nm, and 47.5 nm, respectively. The relative density of the 45 Zr, 55 Zr, and 65 Zr glass-ceramics was 3.82 g/cm\({}^{3}\), 4.14 g/cm\({}^{3}\), and 4.53 g/cm\({}^{3}\), respectively. The crystal size of ZrO\({}_{2}\) and the density of samples increased with the concentration of ZrO\({}_{2}\).