Model Supplies Reliable Results

Dr. Boris A. Shakhmatkin, Dr. Natalia M. Vedishcheva, Institute of Silicate Chemistry of the Russian Academy of Sciences St. Petersburg, Russia

Glass is one of the oldest man-made materials, already known in 3000 B.C. to the ancient Egyptians. Over the millennia, man has experimented with glass to achieve different properties and effects. Today a broad palette of glass art, consumer glassware and industrial glasses is available, as well as a number of methods and processes for the analysis and modification of the physical and chemical properties of glasses.

Attractive Alternatives

Researchers today have an extensive amount of experimental data on the properties of oxide glass-forming systems, upon which many of our industrial glasses are based. Sophisticated new techniques such as MAS-NMR, EPR and EXAFS enable a deeper insight into the glass structure. Traditionally changes in the structure were believed to cause changes in the glass properties. However, neither a better understanding of the structure/property relationship in a given system nor detailed knowledge of its properties as such, can help to predict a priori how a system will react upon the introduction of new components. To answer this question, an entirely new study would have to be conducted, a time and labor intensive undertaking, especially when multi-component glasses are involved. Therefore the possibility of modeling the behavior of glass-forming systems is a very attractive alternative.

Thermodynamic modeling based on the model of associated solutions enables a large variety of properties of glasses to be predicted on a unified basis, over extremely wide concentration and temperature regions, without use of adjustable parameters and with a high degree of accuracy. The approach applies to systems formed from any number of components with different chemical natures (basic and acidic oxides). The formalism of the model of associated solutions enables the equilibrium concentrations of all the species (the salt-like products and the unreacted oxides) present in a glass to be determined as a function of glass composition, temperature and pressure. With this information the following properties of glass can be modeled: density, refractive index, heat capacity, electrical conductivity, diffusion coefficient of ions as well as the red-ox equilibria in glasses, their chemical durability and the ability to crystallize.

Successful Application

However in theory, the range of physical properties, which can be calculated using the model of associated solutions is actually considerably broader. Analytical expressions have been derived therefore for the isothermal compressibility, the coefficient of thermal expansion, the magnetic and mechanical properties of glasses.

In addition to glass properties, structural characteristics can also be modeled, for example the distribution of the basic structural units as a function of the glass composition and temperature. A good agreement between the model and experimental dependencies points to the fact that the thermodynamic approach can successfully be used for estimating the influence of temperature on the glass structure.