# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 1 Introduction
The sintering conditions were the same for both--the temperature of 1500-2000 \({}^{\circ}\)C, pressure of 30 MPa, and 5 min of holding time. In the case of HP, temperatures below 1800 \({}^{\circ}\)C provided densification of 90% or less, but at 2000 \({}^{\circ}\)C, densification reached 98% or more. In the case of \begin{table}
\begin{tabular}{l l} \hline Variables related to the starting powders & Powder: shape, size, size distribution, \\ (material variables) & agglomeration, mixture, etc. \\  & Chemistry: composition, impurities, \\  & non-stoichiometry, homogeneity. \\ Parameters related to sintering conditions & Temperature, time, pressure, atmosphere, \\ (process variables) & heating and cooling rate, etc. \\ \hline \end{tabular}
\end{table} 
Table 1: Variables affecting sinterability and microstructure. SPS, sintering temperatures of 1700, 1750, and 1800 \({}^{\circ}\)C allowed densification of 70%, 90%, and 98%, respectively. Sintering temperature then decreased when using SPS. This was due to the self-heat generation between particles through electrical discharge. SiC grain size was found to be similar in both processes. The mechanical fracture toughness of 98% densified SiC sintered by SPS was about 3.4 MPa-m\({}^{1/2}\), which was about 10% higher than that of the HP technique processed sample. In 2007, a Sweden scientist [PERSON] [11] reported a paper on SPS processing of nanostructured oxide ceramics (Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\), BaTiO\({}_{3}\), and SrTiO\({}_{3}\)), especially on the grain growth factor versus grain size of the starting precursor powder. [PERSON] et al. in 2007 [12] prepared tungsten carbide (WC) and tungsten carbide mixed with cobalt (WC-Co) materials by SPS. They prepared dense and hard materials by employing very short holding times (\(<\)2 min) at the sintering temperature. The tungsten carbide (WC) samples densified to 97.6% in a time of 110 s and at a temperature of 1600 \({}^{\circ}\)C. In the case of tungsten carbide mixed with cobalt (WC-Co) materials, the samples densified to 99.2% in only 65 s at 1150 \({}^{\circ}\)C. Cobalt addition caused a significant decrease in the sintering temperature. The authors concluded that the densification mechanism included carbide particle rearrangement and increment of the diffusion due to the viscous flow of the cobalt binder. There was no appreciable variation related to grain growth, the grain size being 380 nm in both materials. [PERSON] and [PERSON] [3] published in 2013 a book chapter 8 on \"Hot Pressing and Spark Plasma Sintering\" in A comparative study of both sintering techniques. They reviewed and outlined the different materials, highlighting the applicability of SPS for different materials and potential industrial applications like ceramic armors, transparent ceramic lenses and windows, thermoelectric, bioactive, and functional graded ceramic materials, and ultra-high temperature materials. However, there was no mention of the applicability of SPS processed materials, such as glasses and transparent glass-ceramics. Since the invention of the glass-ceramics (GCs) first by [PERSON] [13; 14], there was enormous progress on glass-ceramic science and technology. In recent years, many efforts are focused on GCs, especially for optical applications. The control of nucleation and crystallization processes is fundamental for the successful preparation of GCs materials with improved functionalities. The efficiency of GCs depends on the various factors, such as size, shape, surface morphology and physical properties (density and transparency), and precipitation of desired crystalline phases. GCs are normally produced by melt-quenching, followed by suitable thermal treatment to promote nucleation and crystallization of the interested crystalline phases. The preparation of GCs through powder technology implies the combination of viscous flow sintering together with crystallization. Normally, sintering must be completed before the beginning of crystallization for obtaining perfectly dense materials.