Glass transformation temperature

Although widely accepted, this model has a number of flaws. Shelby, for example, notes that the traditional model predicts a continuous increase in the connectivity of the network as Qg units replace Q4 units. The glass transformation temperatures of alkali germanate glasses. 

Viscosity data are usually presented in one of two forms. The first form of presentation, which is termed the isothermal viscosity, reports the viscosity at specified temperatures. The second form of presentation reports the temperature at which specified viscosities occur, e.g., the values of the Littleton softening temperature or the glass transformation temperature. In general, temperatures referring to a specified viscosity are termed isokom temperatures for that viscosity. If a series of curves showing the isokom temperatures are presented on a figure, the individual curves are termed isokoms (lines of constant viscosity). 

Most dilatometers used in studies of glasses are constructed from vitreous silica. Since the average linear thermal expansion coefficient of vitreous silica is only about 0.55 ppm over typical temperature ranges covered in these measurements, the correction factor for the apparatus expansion is quite small. Furthermore, since virtually all glasses have glass transformation temperatures less than that of vitreous silica, the upper use temperature of = 1000 °C imposed by the viscosity of this material is rarely of concern. 

Addition of alkali oxides to germania initially reduces the thermal expansion coefficient, which passes through a minimum at 2 to 5 mol% alkali oxide. Further additions of alkali oxides result in a continuous increase in the thermal expansion coefficient out to the limit of glass formation. The position of the minimum in thermal expansion coefficient is near the low alkali germanate anomaly in viscosity and glass transformation temperature, which occurs at 2 mol% alkali oxide. No unusual behavior in the thermal expansion coefficient is found in the 15 to 20 mol% alkali oxide region where the traditional germanate anomaly in density and refractive index occurs. Replacement of alkali oxides by alumina reduces the thermal expansion coefficient, but has little effect on the shape of the thermal expansion coefficient versus composition curve, which still displays a minimum at 2 to 5 mol% alkali oxide. 

Explain how and why the thermal expansion coefficient and glass transformation temperature of each of the following types of glasses vary with glass composition. 

These optical absorptions can be bleached, or thermally annealed, by heating to sufficiently high temperatures. The thermal stability of the defects differs widely, so that the elimination of one defect may occur at room temperature, while the elimination of another requires heating to near the glass transformation temperature of the glass. 

Figure 11.5. Effect of water concentration on the glass transformation temperature of oxide glasses...

All oxide glasses contain chemically bound water, usually in the form of hydroxyl. The effects of water content on the infrared spectrum, glass transformation temperature, and melt viscosity are very important in a number of commercial applications. Variations in reported values for Tg of glasses of supposedly the same composition are probably due to variations in water content, which should be specified in any work dealing with the properties which are particularly sensitive to water concentration. 

The Kissinger method can also be used to determine the activation energy for viscous flow, by replacing the exothermic peak maximum temperature with the glass transformation temperature in equation 12.8. This method is particularly useful for glasses where it is difficult to form the sample needed for application of the beam-bending or fiber elongation methods discussed in Chapter 6. 

The most important properties of commercial glasses are the density, refractive index, thermal expansion coefficient, glass transformation temperature, strength and elastic modulus, and chemical durability. These properties will be reviewed for each of the major categories of commercial glasses. 

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