Structural changes ina-Se near the glass transition by thermal relaxation kinetics

Abstract

A multiple-relaxation-rate approach is used to model the enthalpic relaxation behavior of amorphous selenium ($a$-Se) near its glass-transition temperature ($T_g$). The model parameters are completely determined by matching the behavior predicted by the model to that of the real material as shown in viscosity and heat-capacity measurements. The family of structure-dependent relaxation curves, $R_m\mathrm{}\left(T\right)\mathrm{}$, derived from this model, provides a new insight on the structural dynamics of $a$-Se. Two distinct regimes are observed, with a transition near the normally observed $T_g$. For $T>\mathrm{}{T}_g$, the relaxation curves are of the form $R_m^H\mathrm{}\left(T\right)=A_m\exp [a+\frac{b}{(T-T_0)}]$, where the range of $A_m$ is relatively small, and $T_0\lesssim T_g$. For $T<\mathrm{}{T}_g$. the curves are of the form $R_m^L\mathrm{}\left(T\right)=B_m(c_m+\frac{d_m}{T})$, where the slowest relaxation rate curve is 4—5 orders of magnitude slower and has an activation energy, $d_m$ about 50% larger than the fastest curve. The fastest curve is a smooth continuation of its hightemperature counterpart and approximately corresponds in rate and activation energy to viscous relaxation. The local structural inhomogeneity for $T<\mathrm{}{T}_g$ implied by the extremely wide spectrum of relaxation times is exactly the behavior expected from the communal entropy model of the glass transition of Cohen and Grest. Previous investigators had not seen this effect so clearly; it was not noticeable in network glasses, and was only of minor importance in polymers. Its importance in $a$-Se is a consequence of its mutable chain structure. In addition, the observation that the relaxation time measured by the viscosity is faster than the thermal relaxation rates implies that the viscosity is not determined by the slippage of entangled chains, as one might imagine by analogy to the sulfur system, but rather by shear in low-density regions; i.e., selenium should be considered to be more like wet sand than like seaweed.