炭化珪素の酸化におよぼす混入不純物の影響:炭化珪素粉末の酸化に関する研究 (第3報)

Abstract

As a part of the studies on the oxidation of silicon-carbide powders the oxidazability of Si and β-SiC, both found in the market products of silican carbide, as well as the influence of Fe2O3, Al2O3 and MgO on the oxidation of pure green silicon carbide were investigated. The results obtained are summarized as follows:(1) Although it was confirmed that metallic silicon was oxidized much easier than α-SiC, the small volume increase accompanied by the reaction would play no more roll than to reduce the SiC content as long as the silicon carbide is used as refractories.(2) β-SiC is fine powder showing a very slow oxidation compared favourably with α-SiC in this respect. Consequently, it showed no trend to become porous, since no volume increase occured even though it was exposed to oxidizing atmosphare for a long time. The contamination of β-SiC in raw material is not only harmless but rather favourable.(3) It was confirmed that Fe2O3 did not give any serious influence on the oxidation of SiC at a temperature lower than 1200°C, and that there was no appreciable volume increase even in contact with water vapour. At about 1400°C the reaction became violent, which however, did not continue long, showing that Fe2O3 is not so harmful an admixture as it is universally believed.(4) At all temperatures Al2O3 held back more or less the oxidation of SiC. It was observed that a violent oxidation occured at 1150°C under the coexistence of water vapour, accompanying large volume and porocity increase. It was thus confirmed that Al2O3 is an impurity to which we should use due precaution against its contamination.(5) MgO is an impurity accerelating the oxidation of SiC at the temperatures higher than 700°C, it is one of the most undesirable impurities.(6) The influence of oxides on the oxidation of SiC is governed:(a) by the stability discriminated by the free energy of the formation of oxides, (b) by the location of the melting points in the binary system [metal oxide]-SiO2, (c) by the reactivity of metal oxide with SiO2, (d) by the properties of the reaction products.Referring to the influence of the addition of Pb3O4, V2O5, and Li2CO3 on the oxidation of SiC such conditions as above were explained satisfactory.(7) The prolonged oxidation of SiC is governed mainly by the nature of the oxide film covering the crystal surface. The fact that even the pure green silicon carbide behaved like an ordinary black silicon carbide when mixed with Al2O3 or MgO may be attributed to the nature of accerelating the crystallization of amorphous silica to quartz and cristobaite.