U.S. BoM, Albany Research Center, Albany, Oregon, 1987; :1-17
The vaporization thermodynamics of the aluminum-silicon carbides Al4SiC4 and Al8SiC7 were studied by the Bureau of Mines over the temperature range of approximately 1,500 to 1,700 k. Vapor pressures were measured by a knudsen effusion-mass spectrometry technique in which powdered samples were effused from graphite crucibles through small orifices. For each compound, a total of six orifice diameters was tested to verify equilibrium within the cell. Experimental results are based on the following equations for incongruent decomposition: al4sic4(cr) = 4al(g) + sic(a,hex) + 3c(gr); al8sic7(cr) = 8al(g) + sic(a,hex) + 6c(gr). The observed vapor pressure-temperature relationships determined with a cell orifice area of 2.0 X 10-3 cm2 for al4sic4 and 1.81 X 10-3 cm2 for al8sic7 are log p = (-18,682/t) + 12.476, for al4sic4 (1,550 to 1,643 k) and log p = (-19,308/t) + 12.421, for al8sic7 (1,526 to 1.784 K), with p in pascals and t in kelvins. Third-law analysis of the data provided the following heats of formation: al4sic4, ah at 298.15 K = -50.0 Kcal/mol; and al8sic7, ah at 298.15 K = -92.1 Kcal/mol.