# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 2 History of Spark Plasma Sintering  
SPS also called pressure-assisted pulse energizing process, pulsed electric current sintering (PECS), electric current-assisted/activated sintering (ECAS), field-assisted sintering technology (FAST), or plasma-activated sintering (PAS) is a promising technology of the 21 st century for the fabrication of novel materials [31].  
Since the last 20 years, the SPS method has reached pronounced importance in the powder metallurgy industry and in powder technology in general for the preparation of advanced materials. As mentioned in the introduction, it is well-known that the SPS is an advanced technique to obtain homogeneous, highly dense nanostructured sintered compacts, fine ceramics, composite materials, new wear-resistant materials, thermoelectric semiconductors, and biomaterials. An important number of SPS technology-based products are available in the market from Japanese industries. The SPS is now touching various action points through the scientific academia -R&D departments and engineered materials-practical industry [2].  
The SPS system was first introduced in Germany around 1910 as an electric source oriented to densify materials. In the USA, [PERSON] patented the first resistance sintering method for sheet metals in 1933 [32]. Thereafter, [PERSON] obtained US patent for the method of sintering copper, brass, or aluminum powder materials [33]. They were considered as the origin of a current hot pressing (HP) technique that commonly applies a high-frequency induction heating method. [PERSON] et al. developed SPS based on the idea of using plasma in an electric discharge machine for sintering metals and ceramics in the early 1960s. Dr. [PERSON] of Japax Inc. originally invented SPS in Japan as \"spark sintering (SS)\" in 1962 and got patent rights in 1966 [34; 35]. Finally, Sumitomo Coal Mining Co., Ltd. (Tokyo, Japan) introduced the present SPS in 1989.  
Figure 1 shows a typical SPS system, which consists of a hydraulic press with a (mostly) vertical single pressurization axis. The pressure is transferred via two steel cylinders (rams); there are two graphite spacers and two graphite punches between upper and lower punch electrodes. The sintered powder is stacked in a cylindrical die and pressed between punches. Everything is built in a water-cooled vacuum chamber. The water-cooled electrodes are connected to an electric power supply. The power supply produces electric current flowing through graphite punches, sintered powder, and particularly through a graphite die. Therefore, the die and the punches are made of an electrically conductive material, which must be able to resist high temperature and pressure. The powder is located in the middle of the die [31; 36].  
In 2009, [PERSON] et al. [37] published a paper on \"Electric current-activated/assisted sintering (ECAS): a review of patents 1906-2008\". In this report, they concluded, \"In the past century, the simultaneous development of basic ECAS apparatuses and peripheral units was fundamental in overcoming intrinsic technological limitations and in optimizing ECAS processes with respect to (a) product size, (b) microstructure homogeneity, particularly for large compacts, (c) process reproducibility and (d) processing parameters\". But, still, to date, these fundamental problems decide the applicability of their use in industry.