# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 6 Conclusions and Future Prospects  
This paper review focuses on the importance of understanding new processing approaches to guide scientists and engineers in the development of alternative and superior materials. Most optical devices are equipped with single crystals, ceramics, glasses, or glass-ceramics owing to their high transmittance. In particular, transparent glass-ceramics have high demand and applications in various fields. Transparent GCs have been produced by melt quenching or sol-gel method, followed by heat treatments in order to precipitate the desired phases and with control of crystalline growth within the glass matrix. The use of an alternative processing method, such as SPS, provides various advantages. It is a time-saving process, and final materials show high mechanical strength compared to the conventional procedure. Although many techniques, such as HP, HIP, and CIP, are available, there is inadequate knowledge of sintering in the development of functional GCs prepared by the SPS method. Many efforts have been made to understand the underlying principle of processing mechanism, problems and their solutions, and possible applications, which still present limitations. The final materials and their properties are influenced by the processing parameters, such as temperature, pressure, time, etc. Several papers have been reviewed to follow a better way to prepare and control crystal growth. Concerning the microstructure, normally a single crystalline phase is preferred rather than mixed or complex phases, grain growth is undesirable, and porosity must be close to zero, as well as carbon contamination. Carbon contamination can be avoided or suppressed by using tantalum/molybdenum/platinum foils as a carbon diffusion barrier and pre-compacting glass powder below the T\({}_{\text{g}}\) of the glass. An increase in the dwelling time normally improves the compactness of GCs via sparks created at the time of sintering and reduces porosity, favoring the growth of necks formed between the particles. Some authors suggest that applying higher temperatures up to certain limits enhance full densification. It is essential to investigate and to find out the dominant densification mechanisms. Transparency can be improved by maximizing the crystallization of nanocrystals in the base glass and thereby eliminating the scattering effects at the boundary between the glass and crystal phase. In the future, it is essential the standardization of the sintering parameters to enable and approach towards easy GCs processing. These materials have great potential due to their optical functionality for photonic applications. Some properties, such as high mechanical strength, laser threshold damage capacity of GCs, and high luminescence efficiency, must be still optimized to promote SPS synthesis as an alternative route. In addition, future research directions regarding optical devices should also include the development of a better and deeper fundamental understanding of the processing-structure-optical property relationship of the materials as well as of the device functioning. The answer is crucial for a \begin{table}
\begin{tabular}{c c c} \hline
**Process/Properties** & **SPS Sintering** & **Conventional Method** \\ \hline Temperature rise rate & Excellent & Difficult \\ Temperature cooling rate & Fast & Slow \\ Homogeneity & Excellent & Fair \\ Temperature rise time & Fast & Slow \\ Holding time & Short & Long \\ Influence of pressure & Excellent & Absence \\ Transparency & Excellent & Good \\ Grain boundary controlled sintering & Excellent & - \\ Control of the nanometric size of the crystals & Excellent & Excellent \\ Carbon contamination & High & None \\ Densification & Excellent & Excellent \\ Mechanical strength & Excellent & Good \\ Processed using & Powder particles & Bulk sample \\ Mass Production & Difficult & Excellent \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of characteristics on spark plasma sintering (SPS) vs. conventional method for the preparation of transparent glass-ceramics.