Forced-flow Thermal-gradient Chemical Vapor Research Articles

Texturing of polycrystalline SiC films on graphitic carbon in laminated composites grown by forced-flow chemical vapor infiltration

The growth and orientation of SiC on pyrocarbon using forced-flow thermal gradient chemical vapor infiltration to deposit alternate layers of these materials has been examined by transmission electron microscopy. The SiC layers were determined to be predominantly of the cubic (β) polytype, and the close-packed {111} SiC planes were found to be mostly oriented parallel and next to the highly dense {0001} C basal planes of the pyrocarbon layers at the interface. A model involving close-packed plane sequencing is proposed for the C/SiC interface.

Model for prediction of matrix microstructure for carbon/carbon composites prepared by forced flow-thermal gradient CVI

A model has been developed to predict the microstructure of the carbon matrix deposited during forced flow-thermal gradient chemical vapor infiltration (FCVI). This method employs previous thermodynamic-microstructure and densification models to determine conditions throughout the preform as a function of time. The model was verified by comparison with samples prepared over a range of deposition temperatures, reagent concentrations and flow rates. The model also showed that it should be possible to deposit a matrix of uniformly high thermal conductivity onto conventional size carbon fibers, as well as small diameter, low cost, high thermal conductivity carbon whiskers.

Carbon/carbon processing by forced flow-thermal gradient chemical vapor infiltration (FCVI) using propane

A 2 3 factorial statistically designed experiment was used to study fabrication of carbon/carbon composites using the forced flow-thermal gradient chemical vapor infiltration process. Propane, diluted with hydrogen, was used as the feed. The independent variables were the temperature of the bottom of the preform holder, the concentration of the reagent, and the total flow rate. The response variables were infiltration time, final porosity, rate of weight gain, and uniformity of infiltration. It was found that the infiltration time was affected only by the temperature of the preform and the concentration of the reagent. Infiltration times as low as 7 hours were achieved. The bulk density of the final composite was found to be independent of the operating conditions, provided the process was continued until a fixed pressure drop was achieved. Density variations with position within a given composite were no more than ± 0.08 g cm −3. Deposition efficiency ranged from 3 to 14% and increased with temperature while decreasing with flow rate. The values of the coating thickness near the hot and cold sides of the composite were used to calculate the rate constant for carbon deposition from propane. The activation energy was found to be from ~23.6 Kcal mol −1 and the rate constant was given by ln k s(cm s −1) = 2.2–23610/RT.

Carbon/carbon processing by forced flow-thermal gradient chemical vapor infiltration using propylene

The fabrication of carbon/carbon disks using the forced flow-thermal gradient chemical vapor infiltration process was studied using a 2 3 factorial statistical design. Propylene, diluted with hydrogen, was used as the reagent. The independent variables were the temperature of the bottom of the fibrous preform, concentration of the reagent, and total flow rate. The response variables were infiltration time, final porosity, rate of weight gain, and uniformity of densification. It was found that the infiltration time and rate of weight gain were affected by each of the three independent variables. It was also observed that the densification of the composites could be monitored by the in situ measurement of the back pressure. The composites were cut into 12 slices 1 cm long, 4 mm wide and 2 mm deep to measure the uniformity of densification. It was found that the density of the slices varied very little with position within an individual composite disk. Also, coating thickness as a function of position was measured for different composites. In most of the runs the coating thickness increased exponentially from the cold side to the hot side of the disk. The coating thicknesses near the cold and hot sides were used to calculate the rate constant for the pyrolysis of propylene in the preform. The activation energy was found to be ~21.0 kcal mol −1 and the rate constant is given by ln k s = 1.5–21034/RT.

1-D model for forced flow-thermal gradient chemical vapor infiltration process for carbon/carbon composites

A one-dimensional model was developed for the forced flow-thermal gradient chemical vapor infiltration of carbon/carbon composites. The infiltration time predicted by the model agreed very well with experiments, where propylene and propane were used as the hydrocarbon source. The model was also validated by interrupting the infiltration and comparing predicted with observed densities.

Fabrication of carbon-carbon composites by forced flow-thermal gradient chemical vapor infiltration

Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. The preforms for the infiltration were prepared by stacking 40 layers of carbon cloth in a graphite holder. The preforms were infiltrated with carbon using propylene or methane as a reactant, with hydrogen as a diluent. Composites with porosities as low as 7% have been processed within 8-12 h. The highest deposition rate obtained in the present study was ∼3 μm/h, which is more than an order of magnitude faster than the typical value of 0.1-0.25 μm/h for the isothermal infiltration process.