Glass - Back to the Future!

Presenting Author:
Natalia M. Vedishcheva

article posted 22 June 2016

Dr. Natalia M. Vedishcheva is Leading Scientist at the Institute of Silicate Chemistry of the Russian Academy of Sciences, where she has been working since graduating from Leningrad State University (Chemistry) and has gained her Ph.D. degree in Physical Chemistry. Her research interests include calorimetric studies of the thermodynamic properties of oxide systems (glasses and crystals), and thermodynamic modelling of the structure-property relationships for a variety of oxide glasses. At present, her attention is mainly focused on thermodynamic modelling of the intermediate-range order in borate and borosilicate glasses.

In 2001, Dr. Vedishcheva shared the Otto Schott Research Award for her contributions to the field of thermodynamic modelling of the physical properties of oxide glasses, and in 2004 she was elected to the Fellowship of the Society of Glass Technology. She has subsequently served the Society as a member of its Board of Fellows, and is an active member of Technical Committee TC03 (Glass Structure) of the International Commission on Glass. She represents Russia on the International Advisory Boards of the series of conferences on Borate Glasses, Crystals & Melts and on the Structure of Non-Crystalline Materials.

The Thermodynamic Origin of Compositional Nanoheterogeneity in Glasses

1Natalia M. Vedishcheva, 2Adrian C. Wright*
1Institute of Silicate Chemistry of the Russian Academy of Sciences, Nab. Makarova 2, St. Petersburg, 199034, Russia.
2J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG6 6AF, U.K.

In studies of the vitreous state, it is important to note that glasses are products of chemical interactions between the substances forming the batch, rather than the result of direct melting. Experimentally, this is confirmed by the large negative values of the Gibbs free energies and enthalpies of formation of glasses. Note that this view is in line with the concepts of the vitreous state, as put forward by Faraday, Mendeleev, Turner, Porai-Koshits and Krogh-Moe. In the present paper, it will be shown that the structural changes observed experimentally during the formation of glass networks, viz. B3 → B4-→ BO2- or Si4 → SiO3- → SiO22- etc., where represents a bridging oxygen atom, are inseparable from the transformations involving the (network-modifying) cation-oxygen polyhedra, MOm→MOn (n>m), as network-modifying oxides interact with the network-formers. Hence, the corresponding binary and multicomponent glasses should be considered as systems formed from a combination of borate and/or silicate basic structural units, and the oxygen polyhedra surrounding network-modifying cations. This way of approaching the vitreous state is entirely consistent with the concept of cybotactic groupings in both silicate (Porai-Koshits) and borate glasses (Krogh-Moe), and became the basis for the concept of the chemical structure of glasses developed by Shakhmatkin for any system with chemical interaction between its components. Here, glasses are considered as solutions formed from nano-scale chemical groupings, whose stoichiometry and structure are closely related to those of the crystalline compounds existing in the given system. It will be shown that information about the equilibrium concentrations of these chemical groupings enables a wide range of glass properties to be calculated, and yields important information concerning the structural short- and intermediate-range order. In particular, quantitative structural criteria are developed for estimating the tendency of sodium borosilicate glasses towards phase-separation.