Viscosity of glasses

Glasses, like metals, are formed by deformation. Liquid metals have a low viscosity (about the same as that of water), and transform discontinuously to a solid when they are cast and cooled. The viscosity of glasses falls slowly and continuously as they are heated. Viscosity is defined in the way shown in Fig. 19.7. If a shear stress is applied to the hot glass, it shears at a shear strain rate 7. Then the viscosity, ij, is defined by... 

On studying the temperature dependence of the kinematic viscosity of glasses it appears that the formula... 

The relationship between the A, B and x values of glasses and their composition is not yet clear, in contrast to the relationship between x and the composition of saturated mineral oil fractions. The lack of accuracy of x apparently hampers the establishment of this relationship for glasses. This can only be an incitement to determine the viscosity of glasses with greater precision than has been possible so far, thus enabling the x values in the case of the glasses to be calculated more precisely. 

The viscosities of glasses increase with the fraction of bridging oxygens. Figure 15.9 shows the increase of rj with Y. 

Greenspan, H., and E. Fischer Viscosity of Glass-forming Solvent Mixtures at Low Temperatures. J. phys. Chem. 69, 2466 (1965). 

Rheology, the melting, forming, annealing procedures, and limitations of use at high temperature is determined by the viscosity of the glass. Viscosity is measured between 10 and 10 Pa s[10 " and 10 p]. Also, viscosity of glasses is compared qualitatively. 

Considerable attention has been paid in the literature to the viscosity of glass in the transformation range, in particular in connection with annealing during which internal stresses are to be eliminated. In the transformatin range, the viscosity of glass is a function of temperature as well as time. Viscosity changes as a function of time... 

Even at room temperature, long-term measurements have proved the existence of slow irreversible deformations in glasses which, however, are accompanied by delayed elastic effects on loading. The viscosity of glass and its viscous flow is usually conside-... 

The commercial viscosimeter was completed by the high-temperature vertical tube furnace with a control device and evaluation system of the firm NETZSCH, enabling to measure viscosity of glasses up to 1600°C. The viscosimeter and the furnace were joined by means of a pedestal, enabling the adjustment of the system to one common axis. 

Fulcher. G. S., Analysis of recent measurements of the viscosity of glasses. 7. Am. Ceram. Soc. 

In equation (3.01), A and B are empirical constants, Vocc is the volume occupied by the constituent particles and v/ is the free volume. In equation (3.02), r]a, C and To are constants. VTF equation implies that viscosities of glass forming supercooled liquids are non-Arrhenius and To is the temperature which linearizes the data of the non-Arrhenius plot. Cohen and Turnbull (Cohen and Turnbull, 1959 Turnbull and Cohen, 1961,... 

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