| Abstract |
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. Aluminum, mass number 27, was the only vapor species detected over the range 4 to 200 amu. Experimental results are based on the following equations for incongruent decomposition: Al4SiC4(cr)=4Al(g) + SiC(alpha,hex) + 3C(gr); Al8Sic7(cr) = 8Al(g) + SiC(alpha,hex) + 6C(gr). The observed vapor pressure-temperature relationships determined with a cell orifice area of 2.0 X 10(-3) cm(2) for Al4SiC4 and 1.81 X 10(-3) cm(2) for Al8SiC7 are the following: 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 at 298.15K: Al4SiC4 -50.0 kcal/mol and Al8SiC7 -92.1 kcal/mol. |
