2nd International Nanotechnology Conference & Expo
April 3-5, 2017 Dubai, UAE
Field-assisted and flash-sintering of yttria ceramics: Grain boundary nanostructure and mass transport phenomena
1National Institute for Materials Science, Japan
2Tokyo University of Science, Japan
3Nagoya University, Japan
Electric field-assisted sintering (FAST) is gaining interest in recent years due to the accelerated consolidation compared to conventional, pressure less sintering. In particular, flash-sintering, where densification occurs almost immediately (typically <5 seconds) under high DC electric field, has attracted extensive attention as an innovative sintering technique. The flash-sintering has been demonstrated in various ceramics, and nearly full density has been achieved at relatively low furnace temperature for very short time.
Y2O3 has special chemical and physical properties such as high resistance to halogen-plasma corrosion and thermal stability, and is therefore known as a promising environment-resistant or optical material. However, Y2O3 is difficult to sinter. Dense, polycrystalline Y2O3 ceramics have been developed by pressure less sintering in vacuum or hydrogen atmosphere at high temperature (typically >1600°C), by hot press sintering, and by hot isostatic pressing process. We have demonstrated that high-purity, undoped Y2O3 can be fully densified by pulsed electric current-assisted sintering; ECAS (or spark plasma sintering; SPS), where a green compact is directly heated by pulsed DC electric current under compressive stress, at a sintering temperature of around 1000°C. Translucent Y2O3 with a relative density of 99% was produced by ECAS at a sintering temperature of 1050°C under a compressive stress of 80MPa. In addition, transparent Y2O3 polycrystals have been obtained by ECAS technique with a combination of sintering temperature and compressive stress of 1050°C-300MPa or 1300°C-100MPa. More recently, almost instantaneous and full densification can be achieved in Y2O3 by flash-sintering, where densification occurs in a few seconds under a threshold condition of temperature and applied field. For instance, full densification is achieved at 1133°C under a field of 500 V/cm. The single-phase nature of ECASed and flash-sintered Y2O3 bodies was confirmed by high-resolution transmission electron microscopy (HRTEM). The FAST and flash-sintering techniques are very effective to produce dense Y2O3 ceramics at relatively low sintering temperatures and short sintering times. It is postulated that densification and grain growth were enhanced by accelerated solid-state diffusion, resulting from both Joule heating and the generation of defects under the applied field. The present paper aims to briefly summarize the recent results on the densification of Y2O3 through the electric current/field-assisted sintering, and to discuss the effect of electric field/current on the grain boundary nanostructure and mass transport in Y2O3.
Hidehiro Yoshida is a principal researcher in the National Institute for Materials Science (NIMS), and also serves as a visiting associate professor at Department of Materials Science and Technology, Tokyo University of Science. He received his doctoral degree in material science in 2001 from The University of Tokyo; the doctoral thesis dealt with high temperature creep resistance of ceramics. His research addresses high temperature mass transport phenomena such as sintering, creep deformation, super plasticity, ionic conduction, and phase-transformation in structural/functional oxide ceramics. Special attention is placed on the relationship between the high temperature mass transport and grain boundary nanostructure. His research achievements cover a broad range of topics, from scholarly research to practical application of engineering ceramics. He has also contributed to the field of geoscience; superplastic flow and microstructural development in the earthʼs mantle.