A Study On The Preparation As Well As Their Synthesis Mechanism Of Slicon Nitride Powder And Sialon Powder

Slicon nitride and sialon are high temperaturere strcture ceramics with combinedexcellent properties of high temperature resistance, wear resistance, erosion-resistance,resistance to oxidation and thermal shock, thermal stability, and chemical stability. Inrecent years, they have drawn high attention in some new technology area, and have beenwidely used in scientific and technical areas such as automobile, machinery, metallurgy,aerospace, chemical engineering, ocean exploitation, electronic technology, medical andhealth, broadcast and television and so on.One of important methods of preparing slicon nitride and sialon ceramics is powdersintering. At present, how to balance the cost, quality and scale is crucial to thedevelopment of synthesizing Si₃N₄and sialon powder. Compared with foreign countries, abig gap exists for domestic Si₃N₄and Sialon ceramics is in both theory research andtechnology application. To find a method of synthesizing high quality Si₃N₄and sialonpowder is still theoretical and practical significant.The paper includes preparation of Si₃N₄powder and β-Sialon powder:1) Sodium silicate was chosen as silicon source, and silica precursor pre-prepared withsilic acid gelatination and then mixed with carbon black to synthetize Si₃N₄powder throughcarbon thermal reduction method. The effects of reaction temperature, reaction time, carbonto silicon ratio and N₂flow rate on reaction product were carefully studied. Since elementSi in silicic acid congealed gel existed as SiO₂, in a nanometer size, the synthesistemperature of Si₃N₄powder was reduced. At the same time silicic acid gelatination’s looseand porous structure significantly increased the contacting chance between N₂and siliconas well as its compounds, thus the reaction could be completed in a low nitrogen flow rate.The experimental results showed that: sub-micron alpha Si₃N₄powder could be preparedunder the condition of1400℃and5h with a nitrogen flow rate of1L/min. Si₃N₄compositepowder containing5%β-Si₃N₄was obtained when the temperature was risen to1450℃.Choosing of the right reaction temperature and time is very important. Reactiontemperature decides whether the reaction can be proceeded or not. When the temperature isset the longer the heating time is, the more thoroughly the reaction will be. Carbon tosilicon ratio has an important influence on the morphology of Si₃N₄. A proper increase ofthe carbon to silicon ratio increases the dispersity of the product powder.2) Silica sol, aluminum hydroxide and carbon black were chosen as raw materials, β- sialon powder was synthesised through sol-gel and carbon thermal reduction method withthe aid of Fe₂O₃. The effects of reaction temperature, reaction time and the amount of Fe₂O₃additive on the carbothermal reaction were carefully studied with the help of X-raydiffraction （XRD）, scanning electron microscopy （SEM） and Transmission electronmicroscopy（TEM） analysis. Single phase β-sialon was synthesized at1400℃and4h witha N₂flow rate of1L/min under the aid of1.5%Fe₂O₃. The reaction product’s molecularformula was Si₅AlON₇. Scanning electron microscopy （SEM） showed that the grain israther uniform and long cylindrical, shap with a particle size of0.4μm or so. X-raydiffraction analysis showed that: mullite phase was formed at the beginning with theappearance of O-sialon and X-sialon phases. Those phases were then nitrided and reducedinto β-Sialon at high temperature. Some SiC might apper also at the temperature of1400℃, and would be transformed into β-Sialon with extended time.