# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### OxofluorideGCs
Similarly, undoped and doped 0.5 Eu\({}^{3+}\) (mol%) GdF\({}_{3}\) transparent GCs were prepared by applied heat treatment at 550 \({}^{\circ}\)C temperature for 1 min to 8 h. The authors noticed that methyl triethoxysilane (MTES)-silicon precursor, in addition to tetraethyl orthosilicate (TEOS), improved the mechanical properties. Dopant Eu\({}^{3+}\) ion incorporated successfully in the crystals, and XRD peaks of the hexagonal and orthorhombic phases of GdF\({}_{3}\) were shifted to larger 20 angles. By careful observation of luminescence data, efficient energy transfer from Gd\({}^{3+}\) to Eu\({}^{3+}\) in the nanocrystals was noticed, as well as an increase in the Eu\({}^{3+}\) reddish visible emission [77]. Different composition of doped 90 SiO\({}_{2}\)-10 NaGdF\({}_{4}\) GCs was also investigated with a similar approach. These GCs were fully transparent and homogeneous when heating the xerogels at 550 and 600 \({}^{\circ}\)C from 1 min to 8 h. Samples treated for \(>\)8 h appeared opaque due to larger NCs size or due to the formation of clusters. The structural analysis--XRD and HRTEM--confirmed the precipitation of NaGdF\({}_{4}\)NCs in two crystalline phases (cubic and/or hexagonal) with a size ranging between 4 and 24 nm, depending on the Na:Gd ratio, heating temperature, and time [78]. This previous extensive work on transparent oxyfluoride GCs by conventional heat treatment was a time-consuming process (from 1 to 192 h), thus constituting its main disadvantage. We realized that it is possible to reduce this time using the SPS technique. Additionally, other properties like full densification, the increment of crystalline fraction, and lowering sintering temperature could also be addressed. Moreover, in the case of sol-gel powders, SPS opened the possibility of getting highly dense and mechanically stable materials that could not be obtained in bulk by the conventional route. Recently, the GlaSS group has been focused on SPS sintering of rare-earth-doped transparent oxyfluoride GCs looking for faster densification in a short sintering time than conventional heat-treatment process. These oxyfluoride GCs have various potential applications, such as host materials for solid-state lasers and optical materials due to low phonon energy of fluoride crystal within the glassy matrix. In order to achieve optical transparency, the platinum foil was used to cover the graphite die mold in order to decrease or even eliminate carbon contamination. The obtained samples are shown in Figure 5. This example corresponded to Nd-doped KLaF\({}_{4}\) glass-ceramics. As can be seen from Figure 5, it is clear that platinum was effective in order to avoid carbon contamination coming from the graphite die. Once reducing the carbon contamination, KLaF\({}_{4}\) transparent GCs were obtained. During the SPS process, the loading pressure was 22 MPa, sintering temperature was 700 \({}^{\circ}\)C, with different holding times (10-20 min) and different particle sizes (>63 \(\upmu\)m and 63-100 \(\upmu\)m). The densified samples had a cylindrical shape, with a diameter of 15 mm and a thickness of about 3 mm. The samples were polished on both sides for optical measurements. Among all GC samples, those doped with 0.5 Nd, a particle size 63-100 \(\upmu\)m, and a treatment time of 20 min had the highest transparency. The increase of the holding time favored the pores' elimination. Crystalline phases were investigated by using X-ray and are shown in Figure 6a. The diffraction peaks were associated with the crystallization of the cubic (\(\alpha\)) and hexagonal (\(\beta\)) polymorph of KLaF\({}_{4}\). The crystal size was nearly between 19-23 nm for cubic phase KLaF\({}_{4}\) and 9-12 nm for hexagonal phase KLaF\({}_{4}\). This size of the KLaF\({}_{4}\) crystalline phase was also confirmed by transmission electron microscopy (TEM), as shown in Figure 6b. Figure 5: Rare earth-doped transparent oxyfluoride GCs prepared by (from left to right) melt quenching, SPS with uncovered graphite die, SPS with platinum foil.