Synthesis Of Si₃N₄ And Si₃N₄/ALN Powders By Molten Salt Method

Si₃N₄ ceramics are widely used in engines,aerospace and non-ferrous smelting,owning to their excellent mechanical properties,corrosion resistance and chemical stability.Si₃N₄ contains two crystal phases,?and?.And?-Si₃N₄ possesses relative high sintering activity.Therefore,the key to preparing dense and high-performance Si₃N₄ceramics is to obtain Si₃N₄ powders with high?-Si₃N₄ phase content.However,the current methods for preparing Si₃N₄ powders generally suffer from several shortcomings,such as higher synthesis reaction temperature,complex synthesis process and high ? phase content in the prepared products.The molten salt method has the advantages of low reaction temperature,short holding time and high purity of final product.The foam-freeze drying method can prepare porous bodies to avoid the agglomeration of Si during the nitridation process.Based on this,in this thesis,Si₃N₄ and Si₃N₄/AlN powders were prepared by the molten salt method.The effects of the types of molten salt,reaction temperatures,and dwelling times on the preparation of Si₃N₄ and Si₃N₄/AlN powders were investigated.Si₃N₄ powders with high?-Si₃N₄ content were prepared by foam-freeze drying method.The effects of reaction temperatures,solid contents and additive amounts on the preparation of Si₃N₄ powders were studied.The results indicated that:?1?Si₃N₄ powders were prepared by the molten salt method with NaCl-NaF as the molten salt medium,Si powder and C₃H₆N₆ as raw materials.And phase pure?-Si₃N₄powders were obtained at 1200°C for 2 h,which was 200°C lower than the conventional direct nitridation.?-Si₃N₄ nanoplates were synthesized,the diameter of the nanoplates was about 170 nm,and the thickness was about 5.2 nm.The intermediate product of Si nitrided in molten salt was investigated by quenching method with NaCl-NaBr as the molten salt medium,Si powders and C₃H₆N₆ as raw materials.The results showed that the molten salt could etch Si powders to form silicon halides,which further promoted the formation of?-Si₃N₄.?2?The Si₃N₄ powders were prepared by foam-freeze drying with Si powders as starting material,NaCl as additive,cetyltrimethylammonium bromide?CTAB?as blowing agent,ISOBAM-104 and sodium carboxymethyl cellulose?CMC-Na?as binding agents.After 3 h soaking at 1300°C,the content of residual Si in the product was about 3 wt%,the content of?-Si₃N₄ was about 87 wt%,Si₃N₄ granules with diameter of about 1.5?m and whisker with diameter of about 100 nm were synthesized.?3?Si₃N₄/AlN composite powders were prepared by the molten salt method with NaCl-NaBr as the molten salt medium,Si powders,C₃H₆N₆ and AlN powders as raw materials.The nitriding rate of Si increased first and then decreased with the increase of the amount of AlN powders.And the as-prepared Si₃N₄/AlN composite powders had certain anti-hydration performance.