Determining the shortest production time for glassy carbon ware

Because of the high production rate of gaseous reaction products in critical temperature ranges where out-diffusion is relatively slow, glassy carbon ware is difficult to make in thick section by pyrolysis of phenolic resin, without causing kilning faults. Using wedge shapes of cured phenolic resin we found the greatest thickness possible for a fixed heating rate during postcuring (400-500 K) and precarbonization (500-875 K), the stages in which failures occur. In postcuring, the critical heating rate varies inversely as the fifth power of critical thickness; in precarbonization, it varies inversely as the third power. Heating rate can be raised much faster at other stages of pyrolysis, leading to fully carbonized ware at 1500 K. Mass spectrometry shows the main gas product is steam; carboniferous gases are also evolved during precarbonization. We discuss diffusion models applicable to any firing process in which volatiles need to diffuse from solids. (C) 1997 Elsevier Science Ltd. All rights reserved.