【作者】 李成虎；

【导师】 王思青；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2010， 硕士

【摘要】 SiO_2f/SiO₂复合材料是高温透波领域应用较为广泛的一种导弹天线罩材料,但其力学性能相对较差,且烧蚀特性和高温透波不理想,使其应用范围受到限制。而PIP法制备的SiO_2f/Nitride复合材料虽然具有较高的力学性能、较好的烧蚀特性和高温透波性能,但是先驱体合成的工艺较危险,且成本较高。因此,本实验采用尿素法结合溶胶-凝胶工艺制备SiO_2f/SiO₂-BN复合材料,以期改善SiO_2f/SiO₂材料的不足和制备SiO_2f/Nitride复合材料PIP工艺的不足。本文首先从热力学的角度,研究了以尿素和硼酸为原料、采用尿素法合成BN的热力学过程。热力学计算分析表明,尿素法合成BN为吸热反应,当温度高于888K时反应可正向自发进行,且温度越高,氮化反应进行的越彻底;另外,提高反应物料中尿素的含量,可明显增加BN的平衡产率。计算结果也表明,当硼酸和尿素的配比为2:4,反应温度为850℃时,BN的平衡产率为88.09%。本文对尿素法的反应机理进行了初步探讨。研究结果表明,尿素法反应过程中,硼酸和尿素于100℃至350℃发生分解,分解的氨气和氧化硼于780℃左右反应生成BN,至800℃,氮化反应基本结束。本文研究了配料比及水和乙醇做为溶剂对尿素法合成BN纯度的影响规律。研究结果表明:在有溶剂和无溶剂条件下,硼酸和尿素的理想摩尔比均为2:4,此时反应产物中BN具有较高的纯度。本文基于对尿素法合成BN的反应热力学过程和反应机理的分析及实验结果,研究了原始SiO_2f/SiO₂复合材料密度、溶剂及工艺参数等因素对SiO_2f/SiO₂-BN复合材料组成、结构及性能的影响规律。研究结果表明:（1）原始材料SiO_2f/SiO₂材料的密度越大,所获得SiO_2f/SiO₂-BN复合材料的力学性能就越高,但是对密度和孔隙率的影响却非常有限;（2）乙醇溶剂为相对理想的选择,可以有效地改善SiO_2f/SiO₂-BN复合材料的力学性能;（3）尿素法工艺理想的制备温度为850℃,同时随着尿素法工艺次数的增加,SiO_2f/SiO₂-BN复合材料的密度逐渐增加,弯曲强度不断提高,当尿素法工艺循环四次时SiO_2f/SiO₂-BN复合材料获得最优的力学性能,此时复合材料的密度达到1.797g·cm^-3,BN的质量分数为7.87%,弯曲强度更是达到了56.24MPa,比相应原始材料SiO_2f/SiO₂复合材料弯曲强度提高了108.29%。实验结果还表明,实验制备的SiO_2f/SiO₂-BN复合材料相较原始SiO_2f/SiO₂复合材料相比,力学性能有明显改善,同时具有良好的介电性能和热物理性能。其介电常数和介电损耗分别可低至3.12和0.005;室温至1000℃范围内的热膨胀系数小于0.5×10^-6/℃;导热系数约为1.07W/m·K。更多还原

【Abstract】 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.更多还原