Measurement of the Free Energy of Formation of Silicon Carbide Using Liquid Gold as a Silicon Potentiometer

The free energies of formation of 3C‐ and 6H‐silicon carbide were determined between 1623 and 1898 K by measuring the silicon solubility in liquid gold at equilibrium with silicon carbide and graphite. The solution behavior of the Au–Si system provides a wide compositional range for the small difference in the activity of silicon exhibited by the two polytypes, thus offering greater experimental precision over previous similar methods. For 6H‐SiC, ΔG (J/mol) = ‐116900(±7.2) + 38.2(±4.1)T (where T is in K) relative to liquid silicon and graphite standard states is in excellent agreement with other investigations, indicating that the method is capable of determining the free energy of formation with a precision of about 0.7 kJ/mol. Our measurements on 3C‐SiC exhibited greater experimental scatter, which may have been due to variations in the density of non‐equilibrium defects of the crystals used, although experimental problems encountered with the complex crystal shapes may have also contributed. Nevertheless, the precision of the technique is sufficient to address some issues of polytype stability and perhaps free energy contributions of nonequilibrium defects.