| Due to the excellent comprehensive performance, silica fibers hold great potentials for desirable reinforcement materials, which are used to produce radomes of high-Mach number missiles. In this paper, new silica fibers and ordinary silica fibers were investigated. The impact of heat treatment on mechanical properties, composition, structure and morphology of both fibers were compared, and then the mechanism of the enhanced heat resistance of new silica fibers was studied. 2.5D new-SiO2f/SiO2 composites were fabricated via Sol- gel method, and the influence of process parameters on the mechanical properties of composites was studied. The dielectric and thermo-physical properties of the as-received composites were also tested.The mechanical properties of the two kinds of silica fibers, without and with heat treatment under temperature ranging from 200℃ to 1400℃, were explored and compared. When the heat treatment temperature was above 200℃, new silica fibers had greater breaking strength and strength retention ratio than ordinary ones. After heat treatment under 1000℃, the breaking strength and strength retention ratio of the new silica fibers are 100.65 N and 33.8% with only 56.74 N and 19.1% for those of the ordinary silica fibers. When the temperature approached 1200 ℃, the mechanical properties of both kinds of fibers vanished. In a word, ordinary silica fibers are more sensitive to heat treatment.The composition, structure and morphology of both two kinds of silica fibers under different heat treatments were studied and compared. Al existed in new fibers in form of Al-oxide and its amount peaked upon the surface of the fiber body just beneath the sizing agent. The crystallinity degrees of both fibers tended to ascend with the increasing heat treatment temperatures, and both kinds of fibers crystallized completely at temperature 1400℃. The main difference of the morphology between the two kinds of fibers lies in the loose and porous structure upon the surfaces of new fibers, which occurred within the heat treatment temperature from 600℃to 1000℃.The crystallization process of ordinary and new s ilica fibers was investigated. During the heat treatment, the new fibers does not experience two steps of crystallization(surface and internal crystallization). Furthermore, the mechanism of the enhanced heat resistance of new fibers was studied and deduced that the Al-doped layer in the new silica fibers functioned to slow down the crystallization process below a certain temperature.The impacts of process parameters(weft volume fraction of fibers, infiltration-sintering cycle and sintering temperature) on the mechanical properties of the new-SiO2f/SiO2 composites were studied. The results indicated that the weft flexural strength of the composites improved with the increasing weft volume fraction, and the ascent of the infiltration-sintering cycles(varying from 1 to 4) promoted the densification and mechanical properties of the composites. Although the density of the fibers went up with growing temperature(800℃~1000℃), the strength would undergo a significant decrease after it reached a peak. It can be concluded that the optimized sintering temperature to fabricate the new-SiO2f/SiO2 composites can be up to about 900 ℃, and the flexural strength, elastic modulus and density under sintering temperature 900℃, can reach 83.4MPa, 17.2GPa and1.74g/cm3, respectively.The optimized parameters to prepare the new-SiO2f/SiO2 composites was obtained with the weft density of 2.6 root/cm, infiltration-sintering cycles of 4 times and sintering temperature of 800℃~900℃. The composites obtained under the optimized condition proved to have good dielectric and thermo-physical properties, with the dielectric constant, loss tangent, coefficient of thermal conductivit y and coefficient of thermal expansion being 3.19~3.21, 1.9×10-3~2.2×10-3, 0.60~0.62W/(m·K) and 0.52~0.54 ×10-6 K-1, respectively. |
