Fictive Temperature, Structural Relaxation, and Reality of Residual Entropy

By determining the fictive temperature, T-f, in two ways from the same C-p data, we investigate whether the residual entropy, S-res, of a glass could be an artifact of using the C-p 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 T-f(sigma) from Delta sigma and the usual T-f(H) from the C-p 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 T-f(sigma) is about the same as T-f(H) 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 C-p-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 C-p d ln(T) integral and, contrary to recent suggestions, Sres is real.