窒化ケイ素の2結晶相の生成条件と性質について

Abstract

Silicon nitride, Si3N4, has the two crystal phases, α and β, coexisting with different proportions depending on the nitridation conditions. The effect of nitridation condition on the formation of these two crystal phases and the differences of properties between them have been investigated.The results are summarized as follows:(1) The distribution of different crystal phases in a silicon nitride article obtained by nitridation of metalic silicon powder was such that α-Si3N4 was formed on the surface of the article and β-Si3N4 in the center of it. Also α-Si3N4 was formed in the initial stage of nitridation and β-Si3N4 in the later stage.(2) There existed a tendency that α-Si3N4 was formed at temperature below 1200°C and β-Si3N4 was formed over 1300°C. Further, the finer the granule and the lower the purity, the more α-Si3N4 was formed.(3) α-Si3N4 began to transform to β-Si3N4 at 1400°C by heating in nitrogen and transformed to β-Si3N4 completely at 1800°C.(4) The articles of α-Si3N4 powder and β-Si3N4 powder, which were pressed under the same conditions, were sintered in nitrogen at 1400°∼1800°C for 3 hr. Porosities and mechanical strength were measured in relation to sintering temperature and pressing pressure. The sintered articles of β-Si3N4 powder showed higher densities and lower porosities than those of α-Si3N4 powder. However, in mechanical strength no difference between them was observed.(5) Resistances to various gases of α-Si3N4 and β-Si3N4 at 1200°C have also been measured. β-Si3N4 was better in air, but α-Si3N4 was better in CO2,CO and NH3 gases. In vacuum α-Si3N4 was stable below 1300°C, and β-Si3N4 over 1400°C.