Papers by Keyword: α-Si₃N₄

Abstract: α-Si₃N₄ possesses excellent sintering activity, which is used to prepare high performance Si₃N₄-based ceramics and composite refractory. Si₃N₄ powder is always synthesized by nitriding silicon in controlled-atmosphere furnace whose furnace volume is very small(effective volume: 1840×1420×1660mm), the extreme reaction heat is difficult to diffuse, which leads to high reaction temperature and conversion of α-Si₃N₄ to β-Si₃N₄, thus α-Si₃N₄ is difficult to be obtained in controlled-atmosphere furnace. While flame-isolation nitridation shuttle kiln has much larger furnace volume to conduct reaction heat (effective volume: 11500×4190×1684mm), so it owns homogeneous temperature field and stable low-temperature environment which benefits the preparation of α-Si₃N₄. Thermodynamic analysis of Si-N system is shown that Si₃N₄ can be formed by two formats: direct nitridation of Si(s) and indirect nitridation of SiO(g); to ensure completely nitridation, the particle size of silicon powder should be less than 88μm. With reclaimed powder from polysilicon cutting slurry as starting materials, both reactive α-Si₃N₄ and SiC mixed powder were successfully prepared in flame-isolation nitridation shuttle kiln. Because of the gas-gas reaction between SiO(g) and N₂(g), α-Si₃N₄ is fiber-like and in favor of processing high quality Si₃N₄-based materials.

Abstract: In the present study, α-Si3N4 is prepared by using MgO and Al2O3 as the sintering additives and spark plasma sintering (SPS) technique. The SPS sintering mechanism is discussed. The relationship between the content of sintering additives, sintering temperature and relative densities of the samples is analyzed. The results suggest that when the sintering temperature is 1300-1500°C, the content of sintering additives is 6wt.%-10wt.%, the relative density of sintered samples is 64%-96%. When the sintering temperature reaches 1400°C, the content of sintering additives is 10%, the samples can be fully dense sintered and the relative density can be up to 95%. The sintering mechanism is liquid phase sintering. The bending strength of the sintered samples is 50-403MPa and has a close correlation with the relative density.

Abstract: Silicon nitride (Si3N4) is one of the most important advanced ceramics due to its high wear resistance, corrosion resistance, and especially high thermal shock resistance at high temperature. It has been used as cutting-tools, high-temperature bearings and turbochargers for car engines etc. But its applications are still limited by the relatively high cost of Si3N4 powders which are now mostly produced by direct nitridation at high temperature for long time with high energy consumption. Self-propagating high-temperature synthesis (SHS) shows potential as an advanced technology for synthesizing Si3N4. One of the reasons of why SHS Si3N4 powder has not been widely used commercially is that most SHS Si3N4 powder reported in literatures are of high b-phase content and SHS Si3N4 with high a-phase content is very difficult to be made, especially for large samples. Since 1990’s C.C.GE and his group in Laboratory of Special Ceramics and Powder Metallurgy (LSCPM) has been working on the structure formation mechanism of SHS Si3N4 and the nitridation mechanism of large samples of SHS Si3N4. Recently, great effort has been put on the SHS of homogeneous high a-phase Si3N4. In the present work, the effect of nitrogen pressure and purity, the type and amount of diluents on the a-phase content and microstructure of the product synthesized in a reactor of 10 liter capacity are investigated. Through optimization of processing parameters, SHS Si3N4 powder with high a-phase content up to 96% was synthesized.