# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Chalcogenide GCs
The increase in dwell time led to an increment in crystalline fraction, low porosity, and higher density. The optical transmission spectra obtained by SPS was compared with the glass sample prepared by the conventional melt-quenching MQ technique. A lower optical transmission at short wavelengths was noticed for an SPS glass sample due to light scattering by residual porosity, small inclusions/crystals, or residual pollution. However, despite this reduction,the optical transmission was high (up to 70% transmission) for these glasses prepared by SPS. From the graphite mold, the oxygen, water, or carbon dispersion caused impurities on the surface of powders. The surfaces of the sample seemed to be rough and confirmed the formation of the crystalline phase. [PERSON] and co-workers, in 2013 [60], studied GeS\({}_{2}\) glass-ceramics for infrared applications. GeS\({}_{2}\) glasses were obtained by melt quenching and then mechanically ground in the air or in a glove box. Two different ways were used to prepare GeS\({}_{2}\)/\(\beta\)-GeS\({}_{2}\) glass-ceramics. The first way, glass of GeS\({}_{2}\) heated at 490 \({}^{\circ}\)C for some hours to obtain the GC sample. The second way, initially, \(\beta\)-GeS\({}_{2}\) crystals were synthesized by controlled crystallization of GeS\({}_{2}\) glass powders at 493 \({}^{\circ}\)C for 250 h. \(\beta\)-GeS\({}_{2}\) crystals were mixed thoroughly in a planetary grinder with GeS\({}_{2}\) glass powder (85 mol%.) for about 3 h to obtain a homogeneous mixture. These powders were further sintered by SPS. Two samples were prepared, namely, GC1 and GC2. These samples were prepared under the same environmental conditions (450 \({}^{\circ}\)C and 50 MPa) with varying dwell time, i.e., 10 min (GC1) and 15 min (GC2). GC1 was processed under the glove box and GC2 under the air atmosphere. At the dwell timing of 10 min, small crystal fraction (15 mol%) of \(\beta\)-GeS\({}_{2}\) crystal was formed. Further, with the increase of dwell time (GC2), crystal growth was increased, and the crystal looked bigger in size. Some pores at the boundary between glass and crystals led to a lower density (GC2). Higher porosity was observed in GC2 samples compared with the GC1 samples. High porosity derived from the existence of oxide through the air atmosphere on the powder surface. GC1 showed higher transmission (from IR) in comparison with GC2 due to strong scattering caused by pores at the boundary of glass-crystal. The authors proposed that these scattering effects were minimized by the full crystallization of \(\beta\)-GeS\({}_{2}\) of smaller size in base glass. From the transmission curves, the absorption bands at 2.8 \(\upmu\)m and 4 \(\upmu\)m, wavelengths related to the hydroxyl group, were noticed. It was worthy to note that grinding in the glove box was a more efficient approach than air to decrease the contamination by water. Nevertheless, the high transmission window at 10.5 \(\upmu\)m was only about 30%. Strong scattering was mainly due to pores at the boundary glass/crystal. This scattering, still affecting the lower wavelengths, were minimized in the GC1 sample as crystals were homogeneously dispersed. [PERSON] et al. in 2015 [61] prepared tellurium-based GCs by SPS. (Te\({}_{85}\)Se\({}_{15}\))\({}_{60\text{-}60\text{-}6\text{K}}\)As\({}_{40\text{-}0\text{-}0\text{K}}\)Cu\({}_{\text{x}}\) (x = 0, 10, 16.7, 20, 25) glasses were labeled as TEA1, TEA2, TEA3, TEA4, and TEA5, respectively. These tellurites were prepared using the melt quenching method and also the Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) (BST) ceramic system. The powder size (<50 \(\upmu\)m) of both tellurite glass and the BST powders were ball milled and sieved. These powders were sintered at 463 K for 10 min of holding time under a pressure of 40 MPa. Different proportions (--0, 10, 30, and 50%) of BST powders were mixed with tellurite glass powders and named as BST0 (99%), BST10 (99.4%), BST30 (98.1%), and BST50 (97.7%), respectively.