No abstract

No abstract

No abstract

The TcC(g) molecule is observed above a liquid technetium carbide phase of unknown composition by using Knudsen effusion mass spectrometry. Intensity data for Tc(g) between 2229 and 2560 K and for TcC(g) between 2393 and 2595 K give a third-law bond energy of D°0 = (134 ± 7) kcal moT1 for TcC(g). A value of D°0 = (140 ± 2) kcal moT1 is obtained by the second-law method.

The vaporization thermodynamics of aluminum silicon carbide was investigated using Knudsen effusion mass spectrometry. Vaporization occurred incongruently to give A]@), Sic@), and graphite as reaction products. The vapor pressure of aluminum above (A14SiC4 + Sic + C) was measured using graphite effusion cells with orifice areas between 1.1 x lo-' and 3.9 x 10-4cm2. The vapor pressure of aluminum obtained between 1427 and 1784 K using an effusion cell with the smallest orifice area, 3.9 X log p (Pa) = -(18567 & 86) (K/T) + (12.143 f 0.054) The third-law calculation of the enthalpy change for the reaction A14SiC4(s) = 4Al(g) + SiC(hex) + 3C(s) using the present aluminum pressures gives AH"(298.15 K) = (1455 f 79) kJ emol-'. The corresponding second-law result is AbH"(298.15 K) = (1456 f 47) kJ*mol-'. The standard enthalpy of formation of A14SiC4(s) from the elements calculated from the present vaporization enthalpy (third-law calculation) and the enthalpies of formation of A]@) and hexagonal SIC is AH/"(298.15 K) = -(221 2 85) kJ-mol-'. The standard enthalpy of formation of A14SiC4(s) from its constituent carbides Al4C3(s) and SiC(c, hex) is calculated to be AH"(298.15 K) = (38 & 92) KJ-mol-'. cm', is expressed as