# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 1 Introduction  
Sintering through conventional heat treatment in a furnace (free or pressure less) is a high energy and time-consuming method to achieve the desired densification and crystallinity in ceramic and glass-ceramic materials. Pressure-assisted techniques are hot pressing (HP) and spark plasma sintering (SPS). The main drawback of HP is the slow heating rate (5-10 \({}^{\circ}\)C-min\({}^{-1}\)) and the longer times to heat the compact powder through conduction and external source heating element. Researchers are continuously searching for new innovative sintering technologies. Hence, in this context, an alternative process is SPS [1]. SPS is a material processing method, through the production of spark plasma applying pressure, temperature, and current, simultaneously. This method reduces considerably the sintering temperature,provides very fast heating rates, very short holding times, and allows highly transparent materials to be obtained [2]. There is a wide literature available on SPS applied to get ceramic materials with desired properties, as well as other materials like metals, alloys, cernets, intermetallic compounds, composites, and amorphous structures. Some of the advantages of SPS processing [3] related to the supplying of direct electric current through the powder particles are the following:  
* It reduces the sintering temperature and time through applying pressure, also inhibiting crystal growth. * It reduces flaws, voids, and microcracks, and thus favors to improve properties, such as mechanical and optical. * It enhances the homogeneity of material characteristics, which is very important to get high transmittance magnitudes and more luminescence intensity in the case of optical materials. * It's possible to retain the initial shape (near net shape) after SPS. In order to achieve the required microstructure of the sintered body, it is essential to know which variables are involved in the sintering process and how to control them. These variables are mainly divided into two categories: material variables and process parameters [4], as shown in Table 1.  
SPS has been used for the preparation of different types of structural ceramic materials (carbides, oxides, borides, MAX phase, and silicides) and functional ceramic materials [2; 5]. Nanostructured materials show enhanced physical and mechanical properties. Nevertheless, due to interrelations between densification and grain growth, the control of the nanostructure within the bulk material is a difficult task. The above difficulties have been solved by numerous authors approaching SPS sintering in different ways. A two-stage sintering process was adopted by [PERSON] [6] to prepare silicon nitride (Si\({}_{3}\)N\({}_{4}\)) ceramics, obtaining good densification with control of the grain size. Later, [PERSON] and [PERSON] [7] followed a similar two-stage sintering process with a controllable microstructure by studying the effect of several dopants (Mg and Nb) and the initial powder features (particle size, molding processing, and microstructural homogeneity). The high-content Al\({}_{2}\)O\({}_{3}\)-Y\({}_{2}\)O\({}_{3}\)-doped SPS-sintered silicon nitride was investigated by [PERSON] et al. [8]. Fully dense silicon nitride ceramics with the main phase of \(\alpha\)-Si\({}_{3}\)N\({}_{4}\) equiaxed hexagonal grained microstructures were obtained. Calculated grain size from 200 to 530 nm, the high elastic modulus of 288 GPa, and high hardness of 2038 HV were observed from spark plasma sintered at 1550 \({}^{\circ}\)C. Silicon nitride with elongated \(\beta\)-Si\({}_{3}\)N\({}_{4}\) grains, higher hardness of 1800 HV, the density of 3.25 g/cm\({}^{3}\), and [PERSON]'s modulus of 300 GPa at 1650 \({}^{\circ}\)C was observed [9]. [PERSON] et al. [10] in 1995 prepared silicon carbide materials by SPS and characterized their densification and mechanical properties. Silicon carbide, alumina (additive), and yttria (additive) powders were used as precursors. The powders were also used for hot pressing in order to compare both techniques.