The glass-transition, structural relaxation and shear viscosity of silicate melts

The enthalpy, volume and shear relaxation for a wide range of silicate melt compositions is discussed; especially the relationship between the glass transition temperature Tg, viscosity and the structural parameters NBO/T and fragility. For a number of silicate melt compositions ranging from 50 to 85 mole% SiO2 and from 0 to 2 NBO/T and from “strong” to “fragile”; it is shown for each melt composition that the glass transition temperatures obtained from enthalpy, volume and shear relaxation are the same (within error) for a fixed observation rate, and therefore for each composition, the viscosities at the calorimetric and volumetric glass transition temperatures are the same; and for each melt composition the energy terms required to describe shear, volume and enthalpy relaxation are the same. The average shear viscosity of the present silicate melts at Tg (defined to be the temperature at the peak of the heat capacity and thermal expansion curves) for a thermal history of 5°C min−1 cooling- and heating-rates is 1011.22(σ = 0.33) Pa s. ass transition temperatures discussed here are obtained by scanning calorimetry and dilatometry, and shear viscosity measurements. The relaxation of the physical properties volume and enthalpy is therefore being observed as a function of temperature in melts whose structure is not in equilibrium with the ambient temperature. With variations of more than 30°C in determinations of the calorimetric Tg from different laboratories, it has been difficult to resolve a possible variation in viscosity at Tg as a function of melt composition (at high viscosities, a ∼ 20°C change in temperature is generally equivalent to ∼ 0.5 log10 unit in viscosity for silicate melt compositions). For the present very accurate measurements in the same laboratory with melts of the same thermal history, and a consistent definition of Tg, one order of magnitude range in viscosity as a function of melt composition has been observed at Tg. This range in viscosity at Tg is due to a combination of the composition dependence of the common energy terms for enthalpy, volume and shear relaxation; the distribution of relaxation times for each of these processes; and the differencesin elastic shear modulus as a function of melt composition.