Spark plasma sintering of nano-crystalline ceramics

Spark Plasma Sintering (SPS) has been successfully applied in the sintering of nanocrystalline BaTiO3 and gamma-Al2O3 in order to reach fully dense nanostructured ceramics at moderate pressure and temperature. BaTiO3 compacts with densities exceeding 97% and gain size of 50 nm have thus been prepared at temperatures not exceeding 850degreesC using a uniaxial pressure of 100MPa. The selection of sintering temperature is critical for the purpose of minimizing grain growth and there is a demarcation temperature (820degreesC) between normal and abnormal grain growth for the nanocrystalline BaTiO3 powder used in this study. Fully dense alpha-Al2O3 compacts containing grains of the size 300nm or less were consolidated from high-purity gamma-Al2O3 powders without the use of additives or seeds. The transition temperature of gamma-Al2O3 to alpha-Al2O3 can be lowered to 960degreesC when gamma-Al2O3 powder is compacted in the SPS apparatus. The possible mechanisms related to this low transition temperature and to the sintering of both BaTiO3 and gamma-Al2O3 are discussed with special reference to the effect of the pulsed electric field generated by the pulsed electrical current used to heat the sample.