The wet grinding of aluminum hydroxide in a tumbling mill was conducted to investigate the effect of abrasive powder from the ball media on the preparation of low temperature sinterable α-alumina powder. The abrasive powder worn from high purity alumina balls reduced the transformation temperature to a-alumina during the calcination process from about 1300°C for the unground sample to about 1000°C for the ground product, with increasing as the amount of the abrasive powder. Only 0.2mass% of contaminant caused a reduction in temperature of about 200°C in comparison with the unground one. The α-alumina powder calcined after grinding is very fine, so the low temperature sintering of such α-alumina calcined powder is easily achieved, forming a dense sintered body. The alumina abrasive powder is considered to play a singnificant role as a seed effect in the formation of α-alumina.

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The authors previously proposed the concepts of powder particles assembly for the creation of multi-functional materials and undertook the development of fundamental techniques for the preparation of complex particles by forced electrification processing. The purpose of the present work is to develop materials with new functions by combining two kinds of reciprocally electrified particles. This paper reports a preparation method and the positive temperature coefficient of resistivity (PTCR) properties of complex particles consisting of semiconductive BaTiO₃ granules and metallic indium powder particles.
The conclusions obtained from the present experiment are as follows:
(1) A vibrating cylindrical electrode can forced-electrify metal, semiconductor and insulator particles positively and negatively.
(2) When the reciprocally electrified particles are mixed in the same region at the same time, complex particles can be created by the electrostatically attractive force working between two kinds of particles.
(3) Indium-semiconductive BaTiO₃ complex particles formed by this process offer a new PTCR material which can be used in arbitrary shapes by filling and packing or drawing and painting.

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