Isochronal and Isothermal Studies of Density Decrease in Heat Treated Glass-Like Carbon

摘要

The density decrease in heat treated glassy carbon GC has been studied both as a function of heat treatment temperature (HTT) and time (HTt). Isochronal studies were done in the temperature range of 1000 to 2700 deg C for 3 h, while isothermal studies were done in the temperature range of 1200 to 2500 deg C for 0 to 156 h. The density loss in heat treated GC is found to be mainly due to volume expansion and very little due to weight loss. Two different mechanisms of density loss are found to be operative, one in the temperature region of 1000 to 1600 deg C followed by a transition region of 1600 to 2000 deg C and another one in the temperature region of greater than 2000 deg C. The first mechanism is shown to be gas pressure caused by the evolution of mainly hydrogen from the pores. This is the source of both the volume expansion and weight loss that are temperature dependent but not time dependent. In these higher temperature regions, the values of all three parameters are found to be time dependent, and the volume expansion is shown to be due to relaxation of internal stresses present in GC that are due to anisotropic thermal expansion in GC. Analysis of the time dependent weight loss data gives an activation energy of about 98 +- 12 kcal/mole, suggesting the migration of carbon diinterstitials as the responsible process for weight loss. The weight loss in heat treated GC for the first time has been associated with the removal of carbon interstititals. Since it is known that this is the source of graphitization in soft carbons, it is suggested that in all carbons the structural changes can be correlated with changes in weight. In particular the mechanism of graphitization is suggested to be simply the exsolution of carbon interstitials. (ERA citation 11:036358)