Vaporization Thermodynamics and Enthalpy of Formation of Aluminum Silicon Carbide

Abstract

The vaporization thermodynamics of aluminum silicon carbide was investigated using Knudsen effusion mass spectrometry. Vaporization occurred incongruently to give Al(g), SiC(s), and graphite as reaction products. The vapor pressure of aluminum above (Al4SiC4+ SiC + C) was measured using graphite effusion cells with orifice areas between 1.1 × 10−2and 3.9X10−4 cm2. The vapor pressure of aluminum obtained between 1427 and 1784 K using an effusion cell with the smallest orifice area, 3.9X10−4 cm2, is expressed aslog p (Pa) =−(18567 ± 86) (K/T) + (12.143 ± 0.054)The third‐law calculation of the enthalpy change for the reaction Al4SiC4(s) = 4Al(g) + SiC(hex) + 3C(s) using the present aluminum pressures gives ΔH°(298.15 K) = (1455 ± 79) kJ·mol−1. The corresponding second‐law result is ΔH°(298.15 K) = (1456 ± 47) kJ·mol−1. The standard enthalpy of formation of Al4SiC4(s) from the elements calculated from the present vaporization enthalpy (third‐law calculation) and the enthalpies of formation of Al(g) and hexagonal SiC is ΔH°f= ‐(221 ± 85) kJ·mol−1. The standard enthalpy of formation of Al4SiC4(s) from its constituent carbides Al4C3(s) and SiC(c, hex) is calculated to be ΔH°(298.15 K) = (38 ± 92) KJ·mol−1.