Chemical vapor infiltration of carbon fiber felts with uniform initial bulk density of 0.47 g·cm-3 was investigated at the ethanol partial pressures of 5-20 kPa,as well as the temperatures of 1050,1100,1150 and 1200°C.Ethanol,diluted by nitrogen,was employed as the precursor of pyrolytic carbon.Polarized light microscopy(PLM),scanning electron microscopy and X-ray diffraction were adopted to study the texture of pyrolytic carbon deposited at various temperatures.A change from medium-to high-textured pyrolytic carbon was observed in the sample infiltrated at 1050°C.Whereas,homogeneous high-textured pyrolytic carbons were deposited at the temperatures of 1100,1150 and 1200°C.Extinction angles of 19°-21° were determined for different regions in the samples densified at the temperatures ranging from 1100 to 1200°C.Scanning electron microscopy of the fracture surface after bending test indicated that the prepared carbon/carbon composite samples exhibited a pseudo-plastic fracture behavior.In addition,fracture behavior of the carbon/carbon samples was obviously effected by their infiltration temperature.The fracture mode of C/C composites was transformed from shearing failure to tensile breakage with increasing infiltration temperature. Results of this study show that ethanol is a promising carbon source to synthesize carbon/carbon composites with homogeneously high-textured pyrolytic carbon over a wide range of temperatures(from 1100 to 1200°C).
LI Wei,ZHANG ShouYang ,YAN XiaFeng,LI HeJun&LI KeZhi State Key Laboratory of Solidification Processing,Northwestern Polytechnical University,Xi’an 710072,China
Varying the flow rate of natural gas from 50 to 80 to 120 l/h, isotropic pyrocarbon produced by hot wall chemical vapor deposition at 1000 ℃ were examined by X-ray diffraction and Raman spectroscopy. The X-ray data were evaluated by Scherrer equation, and the intensity ratio of D to G band derived from Raman data was used to evaluate the lateral extension of isotropic pyrocarbon. The experimental results show that the d002-spacing of isotropic pyrocarbon decreases from 0.3499 nm to 0.3451 nm, while the stack height increases from 6.5 to 8.4 nm with the increase of flow rate of natural gas. The intensity ratio of D to G band and lateral extension of isotropic pyrocarbon increases with natural gas flow rate increasing. After heat treatment, all the crystallite parameters (stack height, lateral extension, and d002-spacing) decrease, indicating the improvement of the arrangement of the basic structural units of isotropic pyrocarbon.
