Experimental measurements on configurational free energy change of the graphite-glassy carbon equilibrium

Abstract

A precise thermodynamic measurement of the equilibrium: Cgraphite = Cglassy carbon employing a solid oxide electrolyte in the temperature range 650[deg]-1150[deg]C was carried out to gain structural information on glassy carbons. The measured Gibbs free energy change yields configurational (residual quantities at zero Kelvin) enthalpy and entropy differences, which represent a two parameter measure of the structural differences between a given glassy carbon and graphite, since the above reaction involves vanishingly small differences in composition, specific heat, and hence vibrational contributions. Many of the experimental and commercially available samples of glassy carbons studied exhibited at elevated temperature thermodynamic carbon activities lower than graphite. For various samples studied, the zero point entropy varied from 3.1 to 11.3cal/g-mole-[deg]K. The thermodynamic parameters are correlated with X-ray and density measurements. The configurational enthalpy and entropy changes are interpreted in terms of strain related and atomic disorder parameters, respectively.