Unit 15 Glass Properties
Density
The thermal history of a glass determines its properties. For example, the density of most glasses is dependent upon its thermal history and decreases with temperature. The density of glasses covers a range from 2.2 to 8.0 g/cm3. Glasses cooled at various rates from above the annealing point will differ in density with the more rapidly cooled glasses having a lower density. As the thermal conductivity and the specific heat can be calculated from the oxide composition, so can the density according to HUGGINS and SUN.
Glass technologists have determined empirically for a certain composition field how additions or substitutions affect the different properties of a glass. This method of calculating a property of a glass on the basis of additivity cannot be applied to glasses which contain boric oxide. The density of glasses was among the first properties that was calculated on the basis of composition. Density measurements are widely used for quality control.
Thermal Expansion
The thermal expansion of glasses from room temperature into the softening range has been used for many years in order to determine the transformation temperature, the temperature at which the supercooled liquid changes into a glass. Even in a fully annealed glass the thermal expansion is not uniform over a large temperature range. The coefficient alpha represents the change in length per unit of length per degree rise in temperature within a given temperature region. The increase in thermal expansion of well annealed glasses in their softening range is attributed to the formation of defects such as vacant anion sites of incomplete coordination. Defects introduce asymmetries into the short range order of glasses and increase the thermal expansion by raising the thermal vibrations. This explanation applies equally well to silicate glasses with their ionic networks and to organic or molecular glasses.
The mean coefficient of linear thermal expansion generally is expressed in units of 10[-7]cm[2]/K. For catalogue purposes, glass manufacturers quote the value for the range 0C° to 300C°, but other ranges, such as 20C° to 100C°, are not uncommon.
Since thermal expansion is an important consideration in sealing glass to another material, another useful coefficient is from 25°C to the setting point of the glass. The setting point is that temperature at which the glass on cooling becomes a rigid elastic body, and numerically is defined as 5°C above the strain point. The largest changes of thermal expansion occur in the transformation range. This range generally is about 100°C in width and is centered near the strain point of the glass. The expansion coefficient of glass also depends on its composition. Alkalies raise the coefficient markedly, silica or boric oxide generally lower the coefficient, the other oxides act intermediate. The expansion coefficient of glass can be calculated with fair accuracy from the composition by use of appropriate factors. The most generally applicable are those of ENGLISH and TURNER.
The weight percentage of each component of the glass is multiplied by the corresponding factor for the component. The sum of the products is the linear expansion coefficient per °C for the glass. As an example, the glass with the composition SiO2 76%, Na2O 12%, CaO 12% shows the following calculation:
76×0.05×
=3.80×
12×4.32×
=51.84×
12×1.63×
=
This calculated expansion coefficient refers to the temperature range of 25°C to 90°C.
Thermal Conductvity
The thermal conductivity of a glass is the rate of heat flow per unit area under existence of a temperature gradient in the glass. Glass is a poor conductor of heat and this property is inherent in the random structure of glass. The thermal conductivity of glasses ranges from 0.0042 to 0.0126J/cm·s °C. They can be calculated according to the RATCLIFFE oxide factors from the chemical analysis of the glass.
The thermal conductivity of glasses decreases with decreasing temperature in constrast to crystals. The thermal conductivity is more influenced by the structure of a glass than by its chemical nature. It responds strongly to defects and heterogeneities in the structure. The free path of a phonon as an unit of vibrational thermal energy in a glass is much smaller than that in a crystal, because of the lack of long range order in the glass. Glasses have much lower thermal conductivities than crystals.
Selected from "Process Mineralogy of Ceramic Materials", W. Baumgart, A. C. Dunham, G. C. Amstutz, Heidelberg and Hull, 1984
Words and Expressions
thermal conductivity 导热率
additivity 加和性
be attributed to 归因于
asymmetry 不对称
quote 引用
sealing glass 封装玻璃
setting point 固化点
strain point 应变点
inherent 内在的
heterogeneity 异质
phonon 声子
in contrast to 与…比较
