Thermodynamic parameters of bonds in glassy materials from viscosity temperature relationships
Abstract
Doremus's model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The nonlinear nature of the equation obtained means that the fitting process is nontrivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO_{2}, GeO_{2}, B_{2}O_{3}, anorthite, diopside, xNa_{2}O(1x)SiO_{2}, xPbO(1x)SiO_{2}, sodalimesilica glasses, salol, and αphenylocresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anticorrelation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found.
 Publication:

Journal of Physics Condensed Matter
 Pub Date:
 October 2007
 DOI:
 10.1088/09538984/19/41/415107
 Bibcode:
 2007JPCM...19O5107O