Viscous flow mechanisms of fluoride glasses in wide temperature range.

Fluoride glasses have attracted much attention because of infrared optical properties.The chemical bonds in fluoride glasses are much ionic compared with those in oxide glasses.Therefore，it is expected that many properties are different between fluoride glasses and oxideglasses。In the present study，the viscosity of fluoride glasses was measured in the wide temperature range.In the low temperature range，the viscosity in10^6-10^&lt；12&gt；Pa·s was measured by the penetration method by using TMA(Thermomechnical Analyzer)。In the limited range，the viscosity was found to be expressed by the Fulcher@equation。The glass transition temperature.T_g，was determined by DSC and thermal expansion measurements as a function of the heating rate.The glass transition temperature at zero heating rate，T_&lt；go&gt；，was determined by exporatolation。It was found that the viscosity at T_&lt；go&gt；is always10^&lt；12&gt；Pa·s irrespective of chemical composition of glass.For the measurement of high temperature viscosity，the rotational viscometer was constructed in which the atmosphere can be controlled。The vicosity in10^&lt；-1&gt；-10^2Pa·s can be measured by this viscometer。It was found that at high temperature，the viscositu is very low and does not change with temperature so much whereas in the low temperature，the viscosity is very high and changes extraordinarily with temperature.The viscosity in the range between glass transition and 1200°C cannot be expressed by a simple Fulcher@equation and can be expressed by the Macedo-Litovitz@equation，log eta=A+B/T+C/(T-To)，with the assumption of B=BoT^&lt；-2&gt；。

Fluoride glasses have attracted much attention because of infrared optical properties. The chemical bonds in fluoride glasses are much ionic compared with those in oxide glasses. Therefore, it is expected that many properties are different between fluoride glasses and oxideglasses. In the present study, the viscosity of fluoride glasses was measured in the wide temperature range.In the low temperature range, the viscosity in 10^6 - 10^<12> Pa・s was measured by the penetration method by using TMA (Thermomechnical Analyzer). In the limited range, the viscosity was found to be expressed by the Fulcher equation. The glass transition temperature. T_g, was determined by DSC and thermal expansion measurements as a function of the heating rate. The glass transition temperature at zero heating rate, T_<go>, was determined by exporatolation. It was found that the viscosity at T_<go> is always 10^<12>Pa・s irrespective of chemical composition of glass.For the measurement of high temperature viscosity, the rotational viscometer was constructed in which the atmosphere can be controlled. The vicosity in 10^<-1> - 10^2 Pa・s can be measured by this viscometer. It was found that at high temperature, the viscositu is very low and does not change with temperature so much whereas in the low temperature, the viscosity is very high and changes extraordinarily with temperature. The viscosity in the range between glass transition and 1200ﾟC cannot be expressed by a simple Fulcher equation and can be expressed by the Macedo-Litovitz equation, log eta = A + B/T + C/(T-To), with the assumption of B=BoT^<-2>.