Silicon nitride, Si
3N
4, 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-Si
3N
4 was formed on the surface of the article and β-Si
3N
4 in the center of it. Also α-Si
3N
4 was formed in the initial stage of nitridation and β-Si
3N
4 in the later stage.
(2) There existed a tendency that α-Si
3N
4 was formed at temperature below 1200°C and β-Si
3N
4 was formed over 1300°C. Further, the finer the granule and the lower the purity, the more α-Si
3N
4 was formed.
(3) α-Si
3N
4 began to transform to β-Si
3N
4 at 1400°C by heating in nitrogen and transformed to β-Si
3N
4 completely at 1800°C.
(4) The articles of α-Si
3N
4 powder and β-Si
3N
4 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-Si
3N
4 powder showed higher densities and lower porosities than those of α-Si
3N
4 powder. However, in mechanical strength no difference between them was observed.
(5) Resistances to various gases of α-Si
3N
4 and β-Si
3N
4 at 1200°C have also been measured. β-Si
3N
4 was better in air, but α-Si
3N
4 was better in CO
2,CO and NH
3 gases. In vacuum α-Si
3N
4 was stable below 1300°C, and β-Si
3N
4 over 1400°C.
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