DETERMINING THE SHORTEST PRODUCTION TIME FOR GLASSY-CARBON WARE

Because of the high production rate of gaseous reaction products in cr itical temperature ranges where out-diffusion is relatively slow, glas sy carbon ware is difficult to make in thick section by pyrolysis of p henolic resin, without causing kilning faults. Using wedge shapes of c ured phenolic resin we found the greatest thickness possible for a fix ed heating rate during postcuring (400-500 K) and precarbonization (50 0-875 K), the stages in which failures occur. In postcuring, the criti cal heating rate varies inversely as the fifth power of critical thick ness; in precarbonization, it varies inversely as the third power. Hea ting rate can be raised much faster at other stages of pyrolysis, lead ing to fully carbonized ware at 1500 K. Mass spectrometry shows the ma in gas product is steam; carboniferous gases are also evolved during p recarbonization. We discuss diffusion models applicable to any firing process in which volatiles need to diffuse from solids. (C) 1997 Elsev ier Science Ltd. All rights reserved.