采用薄膜沸腾CVI以双热源加热的方法在900~1200℃下热解二甲苯前驱体增密二维针刺炭毡预制体,30~35 h内制备出密度1.70 g/cm3~1.73 g/cm3的C/C复合材料。研究致密化过程中热解炭基体的沉积速率变化规律,应用排水法和偏光显微镜分别测试材料的密度及热解炭层的厚度。结果表明,当沉积温度由900~1000℃升高至1100~1200℃时,沉积前沿的厚度拓宽,热解炭的初始沉积速率增大,但高沉积温度下预制体边缘将优先完成致密化,导致材料的平均密度由1.72~1.73 g/cm3降低至1.70,致密化均匀性变差,材料轴向和径向方向的密度偏差高于0.04 g/cm3。上热源开多个轴向通孔可使沉积前沿的厚度减小,前驱体在预制体内的传输效率提高,进而改善较高沉积温度下材料的致密化效果。
Two-dimensional needle-punched carbon fiber felts were densified using film boiling chemical vapor infiltration with heat-ers above and below the felt and using xylene pyrolysis at 900-1 200℃. The pyrocarbon deposition rate was calculated from the mass gain of the composites. The density of the composites and the thickness of the pyrocarbon were measured by the Archimedes method and polarized light microscopy, respectively. The effects of deposition temperature and heater configuration on the densification behav-ior of the composites were investigated. Results showed that C/C composites with a density of 1. 70-1. 73 g/cm3 were produced after 30-35 h densification. The deposition front thickness and initial deposition rate of the pyrocarbon increased with deposition temperature from 900-1 000 ℃ to 1 100-1 200 ℃. However, the difficulty of precursor transfer increased at higher deposition temperatures due to a more rapid densification at the preform edge, leading to a density decrease of the composites from 1. 72-1. 73 g/cm3 at 900-1 000℃and 1 000-1100 ℃ to 1. 70 g/cm3 at 1 100-1 200℃ if upper heater was not used. Moreover, the densification uniformity of the com-posites decreased, and their density gradients along both the axial and radial directions were larger than 0. 04 g/cm3 . When the upper heater with many holes along its axial direction was used, the thickness of the deposition front decreased whereas the mass transfer effi-ciency of precursor into the preform increased, which had the advantage of improving the density and uniformity of the composites for the higher deposition temperature of 1 100-1 200 ℃.
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