# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### Silicate GCs  
The first report on glass-ceramics prepared by SPS was published by [PERSON] et al. in 2006 [51], dealing with glass-ceramics in the Li\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) (LAS) system doped with erbium ions. Small concentrations of ZrO\({}_{2}\), about 1.02-1.04 mol%, were added as nucleating agent and to promote mechanical strength. The initial bulk glasses were prepared by melt quenching and further heat-treated at 1000 \({}^{\circ}\)C for crystallization to obtain bulk glass-ceramics. Glass powders were also obtained by the sol-gel method and used for SPS sintering. Rapid pulses of 3 ms in length with the pattern of 12:2 = ON:OFF, pressure ranging between 35 and 53 MPa, temperature between 840 and 900 \({}^{\circ}\)C (200 \({}^{\circ}\)C/min), and holding time of 2-5 min were used as sintering parameters for SPS. Samples were heated at 850 \({}^{\circ}\)C, 35 MPa, 2 min; 840 \({}^{\circ}\)C, 53 MPa, 5 min; and 900 \({}^{\circ}\)C, 53 MPa, 5 min and designated as SPS1, SPS2, and SPS3, respectively. The XRD patterns showed that SPS1 and SPS2 were amorphous samples, and SPS3 was crystalline (\(\beta\)-Eucryptite/\(\beta\)-quartz s.s), respectively, after SPS measurements. Only one sample (SPS3-900 \({}^{\circ}\)C/53 MPa/5 min) presented a crystalline fraction of 45% with the size of the crystals of 10 nm. It is interesting to point out that the SPS3 (900 \({}^{\circ}\)C/53 MPa/5 min) sample exhibited a density of 2.48 g/cm\({}^{3}\) very close to that obtained by melt-quenching (2.50 g/cm\({}^{3}\)). The average Vickers mechanical microhardness values were 6.76 and 7.10 GPa for SPS3 and conventional glass-ceramic samples, respectively. The mechanical hardness was also lower for the SPS-sintered GCs sample compared to the melted one. As prepared, all the samples were translucent owing to porosity (less than 100 ppm), as shown in Figure 3a. The maximum size of porosity was found to be 5 \(\upmu\)m, leading to light scattering and reduced transparency. Initially, near infrared NIR emission spectra were measured for all the SPS samples and compared. All the samples showed an emission wavelength of 1530 nm corresponding to the \({}^{4}\)I\({}_{13/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) transition. The emission intensity decreased with the development of crystalline phases (intensity for SPS3 was about six times lower than for SPS1) because of higher erbium concentration (30 wt.%). Er ions relocated within the ZrO\({}_{2}\) phase, and its concentration became high enough to produce concentration quenching on luminescence intensity. Due to higher Er\({}^{3+}\) ion concentration,  
Figure 3: (**a**) SEM micrograph of SPS3, showing the residual porosity and (**b**) IR emission spectra of LAS-Er-Zr samples. Reprinted with permission from the Journal of the European Ceramic Society [51]. SPS, spark plasma sintering. agglomeration predominated, resulting in lower NIR emission intensity for the sample SPS3. SPS1 sample sintered at 850 \({}^{\circ}\)C/35 MPa/2 min conditions showed the highest NIR luminescence profile. NIR emission intensity of the SPS1-sintered sample was compared with bulk glass and glass-ceramic samples (Figure 3b). As observed, the near-infrared (NIR) fluorescence intensity was higher in bulk glass prepared by melt quenching with respect to the other two samples, such as bulk glass-ceramic sample and SPS1. The authors pointed out that porosity must be optimized for use in optical applications. However, the effects of glass powder particle size, preheat treatment of glass powder, pressure, temperature, and holding time were not studied. [PERSON] et al. in 2012 [52] densified for the first time a sol-gel-synthesized 45S5 Bioglass(r)-ceramics by SPS. They found non-transparent GC materials, but the goal was to show the suitability of a method of preparation followed for SPS, following a new way of the sol-gel process rather than the melt quenching method. The sol-gel material was obtained by mixing appropriate precursors.