Abstract: In the manufacture of a wiring board, or a heat discharge plates and the like in the field of the electronics, composite materials consisting of substrates and organic polymeric materials and epoxy resin, as well as ceramic materials such as alumina plates, have heretofore been used as substrates of printed wiring boards and heat sinks. These substrate materials, however, are defective in that, because of a low thermal conductivity and an insufficient heat radiation.A highly thermal conductive and electrical insulating substrate of the invention comprises a highly thermal conductive metal plate, such as aluminum, and a film formed thereon, said film being composed of a dispersion of metal oxide particles having a shape factor, of 1 to 1.4, and also, having a polyhedral shape including smooth surfaces in an adhesive organic polymer. The space factor is an average value of the long diameter/short diameter ratio of the metal oxide particles.

Abstract: A method for preparing corundum particles by a hydrothermal process which comprises adding fine particles of corundum to a starting alumina hydrate, and subjecting the resulting mixture to a hydrothermal reaction or treatment, the alumina in the alumina hydrate being deposited on the surfaces of the fine corundum particles while suppressing formation of new corundum crystal nuclei.