# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Chalcogenide GCs  
[PERSON] et al. in 2010 [56] studied the 62.5 GeS\({}_{2}\)-12.5 Sb\({}_{2}\)S\({}_{3}\)-25 CsCl (mol%) composition approached by SPS in order to reduce the synthesis times compared with conventional melting and heat treatment processes. The glass was prepared by melt quenching, milled, and the prepared powder was introduced into the SPS system. SPS processing was conducted in a vacuum atmosphere, a pressure of 100 MPa for all samples, and varying holding times ranging from 2 to 120 min. The transmission spectra confirmed that with increasing holding time, bands were shifted toward longer wavelengths because of MIE scattering, and the absorption bands at 2.9, 4, 6.3, and 7.8 mm due to O-H, S-H, H\({}_{2}\)O, and Ge-O bonding, respectively, were observed. The visible aspects of the as-prepared samples and corresponding XRD spectra are shown in Figure 4. As observed, the density of the SPS-sintered glass-ceramics increased with sintering time from 3.18 g/cm\({}^{3}\) (10 min) to 3.23 g/cm\({}^{3}\) (90 min). XRD and SEM analysis showed an increase of crystal size \(<\) 100 nm with increasing temperature, attributed to the combined effect of pressure applied during SPS and holding time. The intensity of the XRD peaks increased when holding time increased, meaning that the number of nuclei increased, and a fourth peak appeared at 31\({}^{\circ}\) related to CsCl crystals. [PERSON] et al. in 2011 [57] approached a different way for the preparation of glass-ceramics from chalcogenide amorphous powders of composition 80 GeSe\({}_{2}\)-20 Ga\({}_{2}\)Se\({}_{3}\). The glass powders were prepared by mechanical milling with 0 to 80 h and changed their color from gray to red. This was due to a decrease in particle size with an increase in mechanical milling and the chemical reaction that takes place between the raw materials. They introduced 80 h milled glass powders into a carbon mold (diameters from 8 mm to 36 mm) with a graphite/tantalum foil in the inner part of the mold as a carbon diffusion barrier. This powder was sintered under vacuum at 390 \({}^{\circ}\)C, slightly higher than the glass transition temperature (T\({}_{\text{g}}\)), by SPS (pressure \(\sim\)50 MPa) at different well times from 2 to 60 min. The final density was 4.39 g/cm\({}^{3}\) for sintering at 390 \({}^{\circ}\)C under 50 MPa and 2 min of dwell time. The glass prepared by melt quenching showed an equivalent density value. The transmission spectrum of the obtained bulk glassy samples, whatever their dimension was, good in the mid-infrared range as observable in the picture taken by a thermal camera working in the third atmospheric window from 8 \(\upmu\)m to 12 \(\upmu\)m. Those GCs were opaque in the visible region. The incorporation of carbon from graphite mold was observed and confirmed by XRD analysis. They suggested that this contamination could be avoided by pre-compacting the powder (making pellet) at room temperature before introducing in the  
Figure 4: (**a**) Visual aspect of glass-ceramics samples obtained from the 62.5 Ge\({}_{2}\)–12.5 Sb\({}_{2}\)S\({}_{3}\)–25 CsCl-based glass powder for, respectively, spark plasma sintering treatment times of (a) 10 min, (b) 30 min, (c) 60 min,(d) 90 min, and (e) 120 min at 2901 \({}^{\circ}\)C under 100 MPa and (**b**) corresponding XRD patterns. Reprinted with permission from the Journal of the American Ceramic Society [56]. SPS process. For avoiding contamination, in the next experiment, tantalum foil was used as a diffusion barrier. XRD patterns showed no evidence of either carbon or tantalum. In order to promote crystallization inside the glass matrix, they played with dwelling time (2-60 min) of the SPS experiment. The XRD obtained for 30 min time clearly showed that the Ga\({}_{2}\)Se\({}_{3}\) (or Ga\({}_{4}\)GeSe\({}_{8}\)) crystalline phase was nucleated, and a good homogeneity inside the glassy matrix was elucidated. For the treatment time (60 min), at the same temperature and pressure, a second phase (GeSe\({}_{2}\)) was formed, and no cracks were observed.