Processing And Properties Of Three Dimensional SiO_2f/SiO₂ Composite By Sol-gel Method

Fused silica is main candidate material in high temperature wave transmitting field because of it's high temperature resistance,low thermal-conductivity,extremely low thermal expansion and excellent dielectric properties.But the mechanical strength of fuesd silica is poor,and quartz fiber reinforced fused silica is an effective method to improve the mechanical properties.In this paper,we prepared SiO₂f/SiO₂ composite by sol-gel method using quartz fiber needled preform as reinforcement which consist of short quartz fiber,quartz fiber woven cloth and self-made quartz fiber web.The effect of process parameters including the size of fiber,the concentration of silica sol,impregnation method,sintered temperature etc.on the density and mechanical properties of the composite were studied,SiO₂f/SiO₂ composite obtained the optimum properties by adjusting the technological parameters,the main results of this study are as follows:The addition of short fiber into preform can increase the amount of Z-direction,which make the preform getting stronger interlayer bonding force,so the volume fraction of fiber increases with the increase of the weight of short fiber.The volume fraction of fiber in the preform reached to 18.4 Vol% when the mass ration of quartz fiber web to short quartz fiber was 1:1;The volume fraction of fiber in the preform was highest and reached to 35 Vol% when the mass ratio of quartz fiber woven cloth to short quartz fiber was 2:1;the addition of SiO₂ powder can increase the initial density of the preform and shorten the fabrication cycles,the density of the composite reached to 1.6 g/cm3 after only 6-cycles when the mass ratio of SiO₂ powder to quartz fiber web was 1:1.The impregnation method combined with vacuum and pressure is beneficial to improve the impregnation efficiency,the composite got higher density and less volume of closed hole when the impregnation of silica sol with gradient concentration from high to low.The flexural strength of the composite increased by 100% after 8-cycles co MPared to 12-cycles because of the degradation of fiber during the process of impregnation and drying cycles;the flexural strength of the composites decreased with the increasing of sintering temperature,the bending strength increased by 67% when the composite sintered at 450 oC co MPared to 800 oC,the degradation of the fiber and the crystallization of matrix were the main reason for the poor performance.The addition of short fiber into preform make the process of impregnation easier,so the density of the composite reached to 1.51g/cm3 after 5-cycles when the mass ratio of quartz fiber web to short quartz fiber was 1:1,but the short quartz fiber is unfavorable to the mechanical properties of the composite;the composite shows a high level of isotropy and the average flexural strength at four direction of the same plane was 35 MPa;the bending strength of the composite can be improved remarkably by adding the quartz fiber woven cloth to the quartz fiber web,and the composite achieved highest flexural strength was 55 MPa and excellent interlaminar shear strength;The silica powder added to the quartz fiber web can improve the volume fraction of the fiber in the preform,thereby improving the mechanical properties of the composite,and bending strength of the composites up to 53 MPa.