TABLE OF CONTENTS Trends in the density, thermal expansion coefficient, refractive index, and viscosity of glasses, as a function of bulk composition serve as the basis for many of the common structural models used today. These models were generated long before Raman, NMR, and other modern spectral techniques were developed. While details of these models have been refined using more sophisticated methods, the basic concepts of network structures, bridging and non-bridging oxygen formation, and changes in coordination number with changes in composition were originally proposed in an attempt to explain trends in property behavior. This approach to glass structure remains common even today, with many structural models proposed on the basis of property studies and later confirmed by the results of spectral studies.
The density of a material is defined as the mass of the substance per unit of volume:
| (7.1) |  |
where ? is the density, M is the mass, and V is the volume of the sample. If the sample is free of bubbles, voids, or other defects, the calculated density is the true density of the material. If, however, the sample contains bubbles, which is occasionally the case for glasses, the calculated density will be less than that of the true density and is termed the apparent density. Inclusions with higher densities than the true density, which might, for example, be due to particles of unmelted batch or crystals formed during cooling, will cause the apparent density to be greater than the true...
