Pulse CVI of porous carbon

Abstract

A unique process of pulse CVD was proposed by Bryant [1] about ten years ago. This method is characteristic of the sequential steps in a cycle (gas feed in a moment, holding and evacuation), which improve markedly the uniformity of thickness, composition and structure of the coated films along the long substrate zone. Another CVD method, chemical vapour infiltration (CVI), has been attempted by Hannache et al. [2] and Rossignol et al. [3] to deposit BN or TiC in the carbon fibre preform; however, it takes a long time to infiltrate a variety of pore sizes because of the slow diffusion of reactants against the evolution of product gases (the molar quantity of gaseous component in the product is larger than that in the reactants), such as HF in the reaction of BF3 + NH3 BN + 3HF, and HC1 in the reaction of TIC14 + C H 4 ~ TiC + 4HC1. The combination of pulse CVD and CVI is considered to be extremely effective in infiltrating fibre preforms, porous materials and tentatively shaped powder. In this letter titanium nitride, as a model compound, was infiltrated into porous carbon (Poco Co. CZR-1, specific gravity 1.59, porosity 29%) from a gas mixture of TiC14, N2 and H2 at 850 to 1050 ° C. The pore size of the carbon ranges between 0.2 and 0.5 pm. A p l a t e l e t o f l 5 x 10 × 0 .7mm3was cut from the carbon block, polished with abrasive paper up to no. 800, and washed in acetone using an ultrasonic wave. The reaction chamber was narrowed as shown in Fig. 1 to about 50 cm 3 in order to facilitate quick filling and evacuation. A source gas reservoir of 700 cm 3 was positioned before the inlet valve, and the electromagnetic valves for inlet and outlet were controlled by a microcomputer. Fig. 2 shows the relation between the weight gain per cycle and the hold time (Th) at three levels of temperature. In our apparatus it took 0.10 to 0.14 sec to build up the pressure of the reaction chamber from 1 to 5 torr to 600 torr, and took 0.2 sec to evacuate from 600 to 30 torr, and 5 to 7 sec to 1 to 5 tort. The source gas composition was H2 66%, N 2 33% and TIC14 0.9%. At 950 ° C, the weight gain changes slightly with increasing hold time, which suggests that deposition took place unexpectedly fast, and the differences in the weight gains at 950 and 1050°C show the deposition from the flow-through gas to the bottom dead-space during the build-up time is much faster at 1050°C than that at 950 ° C. In contrast, the weight gain at 850 ° C increases with holding time up to 10 sec and exceeds that at 1050 ° C, which means that deposition on the inner wall of the reaction vessel reduced the amount of reactants with which the pore of the substrate is filled at 950 and 1050 ° C. At 850 ° C, the reactants in the filled pore react to deposit slowly on