| A low-cost continuous powder coating process has been developed to produce a flexible pre-impregnated pitch-based towpreg. The process was used to produce towpreg from Mitsubishi AR mesophase pitch powder and three different carbon fibers: T300 PAN-based fiber, P55 pitch-based fiber, and an experimental high thermal conductivity pitch-based ribbon fiber. The towpreg then was hot pressed into unidirectional composites, carbonized at {dollar}1100spcircrm C,{dollar} oxidized and then graphitized at {dollar}2400spcircrm C.{dollar}; It was observed that the PAN-based fibers developed strong fiber/matrix bonding and the pitch-based fibers developed poor fiber/matrix bonding. This resulted in high flexural strengths in the graphitized composites reinforced with the T300 fibers (841 MPa) and low flexural strengths in the graphitized composites reinforced with the P55 fibers (196 MPa). In addition, it was found that during consolidation the ribbon fibers oriented normal to the pressing direction.; The thermal conductivity (parallel to the fibers) of the graphitized T300/AR-120 and P55/AR-120 composites was 80.5 and 135.5 {dollar}rm W/m{lcub}cdot{rcub}K,{dollar} respectively. These results, along with x-ray analysis, indicated a significant development of preferred crystalline order (parallel to the fibers) upon graphitization at {dollar}2400spcircrm C.{dollar} The composites reinforced with ribbon fibers exhibited 3-D anisotropy, with a thermal conductivity (transverse to the fibers) of 213.5 {dollar}rm W/m{lcub}cdot{rcub}K,{dollar} higher than that parallel to the fibers (145 {dollar}rm W/m{lcub}cdot{rcub}K).{dollar} These results indicated that fiber shape can affect matrix properties in carbon/carbon composites.; A finite element model was developed to predict the thermal conductivity of carbon/carbon composites, both parallel and transverse to the fibers. This model accounts not only for the anisotropic nature of the fibers and matrix, but also for random porosity and different types of fiber/matrix bonding. The model was able to accurately predict the average thermal conductivity of the composites produced in this study. |
