Abstract
Spark Plasma Sintering (SPS) involves pressure driven powder consolidation under pulsed direct electric current, usually using graphite dies.. In general, the SPS process results in a finer grain size when high pressures are used for densification. These findings can be rationalised, at least initially, by recognising that while the pressure greatly adds to the driving force for sintering, it does not affect the driving force for grain growth. Therefore, since sintering is achieved more quickly, there is little time for grain growth, which preserves, very nearly, the original grain size in the powder.1,2
The use of electric current discharge to accelerate the densification of metal and ceramic powders has a rich history in the field of materials processing. While patents on this subject date back to even the early 1900s the availability of Japanese commercial machines (e.g., the so called Dr. Sinter by Sumitomo Mining Co.), during the late 1980s stimulated research and development activities in applying pulsed electric fields to powders in order to achieve both very fast consolidation and to minimise grain size in the densified micro/nanostructure. Several related processes have evolved through many embodiments over the past 20 to 25 years.3,4 Some of them include plasma activated sintering (PAS), electric pulse assisted consolidation (EPAC), field-activated sintering technique (FAST), pulsed electric current sintering (PECS), and perhaps the most popular of them all is Spark Plasma Sintering. Some of the earliest work on difficult-to-sinter, additive-free AlN ceramics with atomically clean grain boundaries was done at UC Davis, USA, in the early 1990s by Risbud and his colleagues using a commercial PAS machine.5–7
Over the subsequent 20 years the sintering of numerous materials have been researched with a variety of compositions and structures from hundreds of laboratories around the world using SPS. The National Institute for Materials Science (NIMS) in Japan is a world leader in SPS research led by Dr Sakka, Dr B. N. Kim, Dr Yoshida and Dr Nishimura. Professor Fei Chen from Wuhan University of Technology and Professor Qing Huang at Ningbo Institute for Materials Technology and Engineering are very active in SPS research in China.
This special issue of Advances in Applied Ceramics on SPS contains six papers that describe recent advances in the use of electric fields to assist densification of ceramic powders. SPS has generated much excitement and interest as clear evidence continues to be gathered showing significant effects on properties. The articles in this issue have been organised with an emphasis on characterization on hydroxyapatite, 8 fabrication of ZrO2-Al2O3 (Ref. 9), densification affecting properties in AlN (Ref. 10), electric field on properties of transparent Al2O3 (Ref. 11), densification of SiC by colloidal processing and SPS without sintering additives, 12 and nanostructuring ZnO (Ref. 13). We are thankful to the authors for accepting our invitation to contribute to furthering our knowledge of the rapidly evolving field of SPS sintering.