HIGH-TEMPERATURE STUDIES:I. Reactions of the Refractory Silicides with Carbon and with Nitrogen II. Thermodynamic Properties of the Carbides III.Heat of Formation of the 3πu State of C2 from Graphite

摘要

InThe reactions of silicides of Ti, Zr, Ce, Nb, Ta, Mo, and W with carbon and with nitrogen were studied at temperatures around 2000°K. The data were used to establish ternary phase diagrams and, to set upper and lower limits on the heats of formation of the silicides. The heats. of formation of the silicides are compared with those for borides, car¬bides, nitrides, oxides, sulfides, and halides.nIInMethods are suggested for estimating absolute entropies of carbides. Available data on the free energies, heats, and entropies of formation of the carbides have been summarized. Many of the older data have been re¬calculated using a third-law method for treating the data. Many of the thermodynamic quantities are estimated for carbides where no data exist. Limits of reliability are given for the data.nIIInThe heat of formation of the 3πu state of C2 has been determined-as 191.4±5 kcal at 0°K. The technique used is to observe the difference in heats of formation of 3πu C2 and Zr gas originating from ZrC in a carbon tube furnace, by measuring relative emission intensities of the features as a function of temperature.