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采用体积含量分别为7.1%和14.2%的两种碳纤维各向同性针刺毡为预制体,研究了预制体内部初始碳纤维表面积与自由体积比值(A/V值)对热解碳的等温等压化学气相渗透动力学以及热解碳织态结构变化的影响。实验研究以22.5 kPa纯甲烷为碳源气体,沉积温度为1 095℃,气体平均停留时间0.1 s,沉积时间从20 h到120 h不等。热解碳沉积后测量了C/C复合材料的表观密度、开气孔率、热解碳的真密度及其内部分布。利用正交偏光显微镜测试了热解碳的织态结构。研究结果表明,热解碳的初始沉积过程主要受气相-表面形核机制控制,随着致密化的进行,逐渐转变为表面生长机制。这种从形核到表面生长机制的转化,导致了热解碳的织态结构相应地从低织构向中织构、最后到高织构的转化。上述沉积机制和结构的变化,都是由于碳纤维编制体初始A/V值的不同以及由于热解碳不断沉积导致的A/V值的不断增大引起的。增大碳纤维编制体的初始A/V值,可以导致这些转化在致密化过程中提前发生。上述结果能够很好地证明以前相关研究的结论。
Two kinds of carbon fiber isotropic needle felts with volume contents of 7.1% and 14.2% were used as preform. The isothermal isobaric pressure of pyrolytic carbon was studied on the ratio of surface area to free volume (A / V) Chemical vapor infiltration kinetics and the effect of pyrolysis carbon structure change. The experimental study used 22.5 kPa pure methane as carbon source gas, the deposition temperature was 1 095 ℃, the average residence time of gas was 0.1 s, and the deposition time ranged from 20 h to 120 h. The apparent density, open porosity, true density of pyrolytic carbon and its internal distribution of C / C composites were measured after pyrolytic carbon deposition. Orthogonal polarization microscope was used to test the texture structure of pyrolytic carbon. The results show that the initial deposition of pyrolytic carbon is mainly controlled by the gas-surface nucleation mechanism, and gradually changes to the surface growth mechanism as the densification progresses. This conversion from nucleation to surface growth mechanism results in a corresponding shift of the texture structure of pyrolytic carbon from low texture to medium texture and finally to high texture. The above deposition mechanism and structure changes are caused by the difference of the initial A / V value of the carbon fiber preparation body and the continuous increase of A / V value due to the continuous deposition of pyrolytic carbon. Increasing the initial A / V value of the carbon fiber braid can result in these transitions occurring earlier in the densification process. The above results can well prove the conclusions of the previous related research.