Relaxation time spectra and the peculiarities of the process of boron anhydride glass transition

Abstract

For a boron anhydride glass, the pre-exponential coefficient, B α , and the activation energy, u α , of the Boltzmann-Arrhenius equation are estimated by temperature-frequency dependence of the α -process. The temperature dependence of activation energy obeys the well-known Vogel-Fulcher-Tammann equation. A standard glass transition temperature, T g , is equal to 220°C, and the corresponding activation energy is equal to 110 kJ/mol. From these data, it can be concluded that glassy B 2 O 3 , by contrast with fused silica, is a linear polymer. Its viscosity and the process of glass transition, as in organic linear polymers, is determined with the overcoming of intermolecular bonds and the mobility of boron-oxygen chains rather than the rupture of the chemical B O bonds. A method of calculation of the continuous relaxation time spectra is proposed based on the Kohlrausch function, commonly used to describe the relaxation characteristics in the glass transition region. The parameter b , characterizing a relaxation time spectrum width at temperatures lower than a critical one, T c = 300°C, is b = 0.5. At higher temperatures, b increases, approaching a maximum possible value b = 1, and the relaxation time spectrum transfers to a δ -function and degenerates into the spectrum with a single relaxation time. This change is explained by the fact that structural micro-inhomogeneity of the B 2 O 3 liquid gradually disappears as the temperature increases. A method of evaluation of the degree of micro-inhomogeneity of inorganic glasses by b is proposed.