Fictive Temperature, Structural Relaxation, and Reality of Residual Entropy

Abstract

By determining the fictive temperature, Tf, in two ways from the same Cp data, we investigate whether the residual entropy, Sres, of a glass could be an artifact of using the Cp d ln(T) integral in the glass-liquid temperature range. Although the integral gives only the upper and lower limits of the real entropy change, it is still useful and is distinguished as Delta(sigma). We determine Tf(sigma) from Delta(sigma) and the usual TfH from the Cp dT integral for two metal alloy glasses, a basalt composition glass and a spray-quenched propylene glycol glass from the available data, and find that Tf9sigma is about the same as TfH within errors. To substantiate it, we report a differential scanning calorimetry study performed during cooling of the Mg65Cu25Tb10 and Pd40Ni10Cu30P20 melts and on heating their glassy states at the same rates. In addition, we simulate Cp-T plots from a known model for nonexponential, nonlinear relaxation and analyze the data. The quantity Delta(sigma) on cooling the liquid and heating the glass differs negligibly; that is, net change in a temperature cycle between glass and its melt is close to zero, a characteristic of a nearly reversible change. We conclude that spontaneous enthalpy release has little effect on the entropy change determined from the Cp d ln(T) integral and, contrary to recent suggestions, Sres is real.