Properties And Preparation Research Of The SiO_2f/SiO₂ Composites With Stitched Fiber Cloth Reinforcement

SiO_2f/SiO₂ composites have been widely used in high temperature wavetransparent field for its excellent thermal,mechanical and dielectric performances.The SiO 2f/SiO2 composites are commonly prepared by sol-gel method.In terms of fiber preform,3D and 2.5D braided performs are technically complex and economical-costly,which constrain them from broad applications.Meanwhile,2D cloth perform demonstrates insufficient inter-laminar shear strength.In this study,stitch technique was employed to improve the integrity of the 2D cloth.The effects of stitch parameters on the mechanical properties of the composites were detailed studied.A rapid gel curing process was put forward to promote the homogenous d istribution of the matrix.The effects of sintering temperature and holding time on the microstructure of the reinforcing fiber and the composites were investigated,and the thermal-physic and dielectric properties of the composites were tested.The effects of stitch intensity and fiber volume fraction?Vf?on the densification and final density of the composites were explored.The results showed that the stitch fibers were in favor of the infiltration of silica sol in the Z direction.Thus,by increasing the stitch intensity,the infiltration efficiency was largely improved in first three cycles of densification.However,the infiltration of silica sol through the inter-laminar voids became suppressed as the stitch intensity increased,the increase of the composite density slowed down in the following 4-6 cycles of densification.Vf determined the gaps between fiber cloth layers.The inter-laminar infiltration efficiency increased as Vf decreased,but more inter-layer defects were generated and the final density of the composite was decreased.The optimized stitch intensity and fiber volume fraction were found to be 5mm×5mm¹0mm×m0mm and 40%⁴7% respectively,which led to a composite density beyond 1.7g/cm3.The effects of composite density variation?from external to internal of the composite?,fiber volume fraction and stitch intensity on the mechanical properties of the composites were studied.It was found that the density of the composite decreased gradually from external to internal of the material,which led to the decreases of both the tensile modulus and inter-laminar shear strength.As the density decreased from 1.76 to 1.63g/cm3,the tensile strength of the composites decreased from 37.4 to 26.5MPa.When the density of the composite was higher than 1.7 g/cm3,the tensile modulus of the composite increased as the fiber volume fraction increased from 40 to 47%.Meanwhile,the corresponded tensile strength,flexural strength and inter-laminar shear strength of the composites increased initially and the maximum values were obtained at a fiber volume fraction of 43%,which then declined as the fiber volume fraction further increased.The decrease of the strengths at higher fiber volume fraction was probably due to the mechanical degradation of the reinforcing fibers induced during the mould pressing of the fiber cloth.With a specific fiber volume fraction of 43% and a density between 1.75-1.77 g/cm3,both the tensile strength and flexural strength of the composites decreased as the stitch intensity increased.This was because the increased stitch intensity generated higher density of defects within the fiber performs,which promoted the extension of the cracks accelerating the failure of the composites.The effect of stitch density on the inter-laminar shear strength was not evident at current density range.The shear strength remained at around 12 MPa.Based on the gel analysis of colloidal silica sol,a rapid gel curing was developed.Compared with the conventional drying curing process,rapid gel curing can ensure an uniformly distributed matrix.However,the derived matrix contained a large number of micro-pores,the overall composite density was as low as 1.47g/cm3.The mechanical properties of composites made by rapid-gel way was investigated.Compared to composites prepared through conventional drying process,its density,elastic modulus,Flexure strength and inter-laminar shear strength was lower with tensile strength a little higher,however,the fracture toughness properties was better.To improve ablation resistance properties,TEOS was employed to further densify the porous matrix,which resulted in a dense matrix microstructure.The effects of heat treatment conditions on the structure of the fiber and the composites were studied.Fiber microcrystalline was observed above 847.69?,which led to the dramatic decline of the fiber strength.When heat treated at 800?,the fiber strength suffered from only slight degradation and remained stable as the treating time extended from 1h to 4h.The prepared co mposite?800?,1h?displayed moderate bonded fiber-matrix interface as well as dense matrix.Fiber-matix debonding and fiber pull-out were observed at the fracture surface.The thermal physical and dielectric properties of the SiO 2f/SiO2 composites were tested.The specific heat,coefficient of thermal conductivity,CTE,dielectric constant and loss tangent of the composites increased with increasing temperature.The specific heat remained stable at 0.8-1·kJ· kg-1·K-1 in the temperature range of RT⁸00?.The coefficient of thermal conductivity was about 0.6 W·m-1·K-1 at RT³00?.The average CTE was around at 0.6×10^-6·K-1 during RT-700?.The dielectric constant and loss tangent of the composites at room temperature and 1000?were 3.13 and 0.72×10^-6,and 3.18 and 1.96×10^-6,respectively.