The Research Of Preparation And Properties Of Si₃N₄ Complex-phase Powders

Si3N4 complex-phase metarials have higher physical and chemical properties than general Si3N4 materials. The preparation of this material had been a main problem in structural materials area. In this paper, Si3N4 complex-phase powders were prepared by direct nitridation by means of doping, controlling the preparation conditions and altering the way of mixing raw materials.The system of Si-N-O and Si-N-O-C were researched by chemical thermodynamics method. The influences of temperature and gas pressure on the phase content of Si3N4 powders were studied. The theory on the phase transformation was provided by phase diagrame.The Si3N4 powders were prepared by direct niitridation of Si doped Fe andα-Si3N4 whiskers on different temperature. The effects of the properties of Si, the content of addition and nitridation system on the properties of Si3N4 powders were studied. The decrease of the partical size of Si and increase of the content of Fe were favourable for improving the properties of Si3N4 powders on low nitridation temperature, but unfavourable on high temperature. The melting of Si was avoided by addingα-Si3N4 whiskers, but its influences on the properties of Si3N4 powders were not so obvious as Fe. The Si3N4 powders which have 95% nitridation, 90%α-Si3N4 content and good shape were prepared by controlling the properties of Si, the content of addition and condition of niridation.The Si2N2O-Si3N4 powders were prepared by adding SiO2 using direct nitidation method. The Si2N2O conent was controlled by altering the conten of SiO2. Then the SiO2 sol was coated on the Si for increasing theα-Si3N4 content and improving the microstructure of powders. As a result, theα-Si3N4 content was increased to 50%, the surface area of powders was increase to 4m2/g, and particle size was decreased more.The C and SiO2 were added into Si for preparing Si3N4-Si2N2O-SiC powders. The phase content was influenced by the content of SiO2 and C/SiO2. Then the C coated by SiO2 sol was added into Si. The contact and reaction surface of SiO2 and C was increased, and the carbothermal reduction was enhanced. Theα-Si3N4 content was increased to 80%, the Si2N2O content was controlled to 10%. A lot of powders were needle-like and rodlike. The particale size was decreased to less-than 1μm,and the dispersity was improved obviously.All the research showed that the Si3N4 complex-phase powders which have controlled phase content, high uniformity, good microstructure can be prepared by means of altering the content of SiO2 and C, improving the method of mixing raw materials and so on. The preparation of this Si3N4 complex-phase powders give a significant way for improving the mechanical and dielectrical properties of Si3N4 materials.