# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Chalcogenide GCs
Transmittance at lower dwelling time (2 min) was \(\sim\)55%, and at longer dwelling time was \(\sim\)20% due to the MIE scattering effect of the precipitated nanoparticles. The base glass composition had \(\sim\)70% transmittance. The Vickers hardness was 201, 203, and 167 Hv at a dwelling time of 2, 30, and 60 min, respectively. The Vickers hardness followed a similar trend with a rise in dwelling time up to 30 min time. In 2012, [PERSON] et al. [58] examined the sintering mechanisms in 80 GeSe\({}_{2}\)-20 Ga\({}_{2}\)Se\({}_{3}\) chalcogenide glass composition, as well as crystallization precipitation by SPS. The sintering of glass occurred by viscous flow, reducing surface energy of a porous compact through neck growth and densification involving the deformation of initial particles. Different mechanisms leading to the production of glass-ceramics had been hypothesized. The first one would be the densification of glassy powder through viscous sintering, followed by subsequent devitrification of the matrix. The second mechanism would imply densification and concurrent gradual crystallization of the matrix through the growth of the neck between glassy particles. For one given glass composition, the corresponding crystalline phase had a considerably higher viscosity than the amorphous phase, so the sintering of polycrystalline materials was more difficult than the amorphous phase. This suggested that the first mechanism described above, i.e., achieving full density prior to any significant crystallization, should be favored. To confirm the involved mechanism, the influence of the dwell temperature (250-390 \({}^{\circ}\)C), where the full densification of glass was achieved without crystallization, on the different dwell timings (2, 15, and 60 min) was studied, keeping the pressure constant (50 MPa). At 300 \({}^{\circ}\)C, the shrinkage started intensively. The viscous flow between particles led to the formation of necks until 350 \({}^{\circ}\)C and the plastic deformation of particles. For increasing temperatures, from 250 \({}^{\circ}\)C to 390 \({}^{\circ}\)C, and 2 min dwell time, the particles started to soften and melt together, but a lot of intergranular porosity (27%) was observed. At 330 \({}^{\circ}\)C, viscous flow between particles led to the formation of necks, and, at 390 \({}^{\circ}\)C, the particles were totally melted together, and no porosity was observed. The increase in temperature led to an increase in compactness and residual porosity diminution. Similarly, the compactness increased and induced partial devitrification of Ga\({}_{2}\)Se\({}_{3}\) (or GaGe\({}_{4}\)Se\({}_{8}\)) when the dwell time increased. The authors explained that the densification mechanism occurred prior to devitrification. [PERSON] et al. in 2013 [59] prepared transparent tellurite glasses and glass-ceramics in the 85 TeO\({}_{2}\)-15 WO\({}_{3}\) (mol%.) system by SPS with the main emphasis on understanding and eliminating carbon contamination by studying the effect of initial amorphous particles size, the effect of different carbon diffusion barriers (alumina, tantalum foil, and platinum), as well as the effect of a pressureless sintering step prior to SPS was investigated. They used three sizes of precursor particles, such as coarse, fine, and bulk particles for SPS sintering. SPS measurements at a constant pressure of 50 MPa were sintered. The carbon contamination was higher in the case of the fine particles, leading to dark glass bulks. The use of a diffusion barrier, such as platinum, tantalum foil, or alumina, limited carbon contamination. Pressureless pellets heat-treated at T\({}_{g}\) +30 \({}^{\circ}\)C for 1 h prior to SPS also limited carbon contamination. Yellow glasses with relatively high optical transparency properties were obtained by SPS. By increasing the dwell time at constant pressure and temperature during the SPS experiments, the non-centrosymmetric \(\delta\)-TeO\({}_{2}\) phase crystallized, which generated a second harmonic signal.