| In this paper,short quartz fiber reinforced silicon oxide based(Si O2f/Si O2)thermal insulation composites were prepared by casting molding using short cut quartz fibers as reinforcements and silica sol,silica powder,and fused quartz powder as matrix materials.The effects of matrix powder ratio and fiber volume fraction on the bulk density,apparent porosity,mechanical properties,coefficient of thermal expansion,and thermal cycling of Si O2f/Si O2composites were studied.The microstructure of the composite was characterized using field emission scanning electron microscopy（SEM）.The Si O2f/Si O2composite material with the optimal matrix powder ratio and fiber volume fraction was prepared.The thermal cycle of the composite material in the service environment was simulated,and the mechanical properties of the composite material after thermal cycle were tested.The phase composition of the composite material at different thermal cycle temperatures was characterized by X-ray diffraction（XRD）.The results are as follows:（1）With the increase of the amount of fused quartz powder added,the internal pores of Si O2f/Si O2composites are gradually filled with silica powder.The apparent porosity,flexural strength,and compressive strength all showed a trend of first increasing and then decreasing,and the average thermal expansion coefficient showed a trend of decreasing in the range of 300℃to 900℃.When the mass fraction of fused quartz powder is 21.3%,i.e.the mass ratio of silica powder to fused quartz powder is65:35,the comprehensive performance of Si O2f/Si O2composite material prepared is the best:The bulk density is 1.43 g·cm-3,the apparent porosity is 27.29%,the flexural strength is 14.0 MPa,the compressive strength is 19.1 MPa,and the average coefficient of thermal expansion between 300℃and 900℃is 0.77×10-6/℃.Due to the existence of a small amount of pores inside the composite,which provides space for the volume expansion of the composite during thermal cycling,the compressive strength improvement rate after 20 thermal cycles at 900℃is 43.74%.（2）With the increase of fiber volume fraction,the mechanical properties of the composite gradually improve,and the average thermal expansion coefficient gradually decreases in the range of 300℃to 900℃.The comprehensive performance of Si O2f/Si O2composites prepared when the fiber volume fraction is 7.0%is the best:The apparent porosity is 31.37%,the bulk density is 1.49 g·cm-3,the flexural strength is 13.0MPa,the compressive strength is 20.8 MPa,the thermal cycle compressive strength increase rate is 44.7%,and the average thermal expansion coefficient between 300℃and 900℃is 0.93×10-6/℃。（3）The composite material was prepared using the optimal matrix powder ratio and fiber content,and thermal cycling was performed on the prepared composite material by simulating the actual service environment.The experimental results show that the composite material prepared in this paper undergoes partial crystal transformation only at temperatures above 1200℃,and amorphous silicon dioxide undergoes partial transformation toα-Cristobalite.The defects in the composite material can provide space for the volume expansion effect that occurs during thermal cycling.After thermal cycling at lower temperatures,the mechanical properties are improved,and the average thermal expansion coefficient tends to stabilize between 300℃and900℃.When the thermal cycling temperature is 900℃and the number of thermal cycles is 20,the porosity of the composite material is 28.52%,the bulk density is1.55g·cm-3,the bending strength is 9.5 MPa,and the compressive strength is 40.1MPa.After 20 thermal cycles,the increase rate of compressive strength is 91.1%,and the average coefficient of thermal expansion between 300℃and 900℃is 1.11×10-6/℃,the average thermal expansion coefficient increased by 6.1%after 20 thermal cycles,and the thermal conductivity was 0.898W/m·K at 800℃.After 20 thermal cycles,the thermal conductivity was 1.026W/m·K,increasing by 14.25%. |
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