Process And Properties Research Of The SiO_2f/SiO₂-BN Composites By Urea Route

SiO_2f/SiO₂ composites are widely used as a missile radome material in the field of high temperature wavetranspanrent, but its mechanical properties are relatively poor, and the characteristics of ablation and high temperature wavetransparent are not ideal, so its application scope is limited in a certain extent. Although SiO_2f/nitride composites what are made by PIP craft have higher mechanical properties, and the characteristics of ablation and high temperature wavetransparent are better, but the synthesis process of precursor is dangerous, and the cost is higher. Therefore, this experiment prepared SiO_2f/SiO₂-BN composites by urea route and sol-gel craft, so as to improve the deficiencies of SiO_2f/SiO₂ composites and SiO_2f/nitrides composites.In this paper, taken the urea and boric acid as raw materials, the thermodynamics process of BN synthesized by urea route was studied from the thermodynamics angle. Thermodynamic calculations show that, the reaction of BN synthesized by urea route is endothermic reaction; the reaction can be carried forward spontaneously when the temperature is higher than 888K; and the higher the temperature, the more complete the nitridation reaction. In addition, to improve the urea content of reaction materials, the balance yield of BN can be increased obviously. The experimental results also show that, when the ratio of boric acid and urea is 2:4, the balance yield of BN is 88.09% at 850℃.In this paper, the reaction mechanism of urea route was discussed firstly. The results show that, in the reaction process of urea route, boric acid and urea are decomposed at 100℃to 350℃, the BN is synthesized about 780℃by decompositions of ammonia and boron oxide, the nitride reaction basically endes at 800℃.In this paper, the influence laws that the ingredients ratio, water solvent and ethanol solvent effect on the purity of BN synthesized by urea route was studied. The results show that, in a solvent and solvent-free condition, the ideal molar ratio of the boric acid and urea is 2:4, right now BN has higher purity in the reaction products.Based on the reaction thermodynamics process of BN synthesized by urea route, the reaction mechanism and the experimental results, the influence laws that the original SiO_2f/SiO₂ composites density, solvent, technological parameters and other factors effect on the structures and properties of SiO_2f/SiO₂-BN composites were investigated in this article. The results show that: (1) The greater the density of original SiO_2f/SiO₂ composites, the higher the mechanical properties of SiO_2f/SiO₂-BN composites, but the impact of density and porosity is very limited; (2) Ethanol solvent is relatively ideal choice, it can effectively improve the mechanical properties of SiO_2f/SiO₂-BN composites; (3) The ideal preparation temperature of urea route is 850℃, at the same time, the more the cycles of urea route, the greater the density of SiO_2f/SiO₂-BN composites, and the stronger the bending strength. when the fourth time circulation of urea route, SiO_2f/SiO₂-BN composites have optimal mechanical properties, at this time the density of the composites can reach 1.797g·cm^-3, the mass fraction of BN is 7.87%, and the flexural strength increases more 108.29% than original SiO_2f/SiO₂ composites, it reaches 56.24MPa.The results also show that, compared to the original SiO_2f/SiO₂ composites, SiO_2f/SiO₂-BN composites not only have higher mechanical properties, but also have better dielectric and thermal physical properties. The dielectric constant and dielectric loss are respectively low to 3.12 and 0.005; the thermal expansion coefficient is less than 0.5×10^-6/℃from 25℃to 1000℃; and thermal conductivity is about 1.07W/m·K.