Glasses - Theoretical aspects

Glasses have been known and used for a very long time. Over the centuries, their composition has been improved, making them an everyday, even indispensable, item. Successive improvements have been the result of an empirical approach based on macroscopic observations.

The formation of glass structure is still imperfectly understood, and analysis remains a delicate task. Recent spectroscopic techniques provide local information on selected glass elements. Taken together, this information leads us to propose the existence of local order at first neighbors and long-range disorder.

However, this knowledge is insufficient to account for the properties of glasses, and ongoing efforts are being made to obtain information on the medium-range structure. It is in this range, between one and a few nanometers, that we should find the explanation for the structural arrangement of glasses.

In a disordered material, experimental techniques for accessing the radial distribution function provide one-dimensional representations of the three-dimensional network. This information is necessarily averaged, and does not allow us to distinguish specific details of the structure.

Many of the hopes for a better understanding of glass structure lie in modelling. This science is making remarkable progress every year, and its impact on knowledge of glass structure should be essential insofar as it becomes possible to distinguish original patterns that do not correspond to the global averaged analysis derived from experimental techniques.

It is likely that certain phenomena or properties are due to the presence of particular patterns or structural entities in the glass network. These structural entities may be present in very low concentrations.

To add to the difficulty of understanding glass, the closely related phenomenon of glass transition is still poorly understood. In recent years, many different approaches have been developed to elucidate this phenomenon. However, it remains difficult to account for both the thermodynamic and relaxation aspects of this transition. There is no single theory that accounts for all the various physically observed aspects associated with this transition.

So, in many respects, glass remains a complex and little-understood material. This makes it all the more interesting for the scientific community.

The "Glasses" article is the subject of two booklets:

AF 3 600 Theoretical aspects

AF 3 601 Properties and applications

The subjects are not independent of each other.

Readers will need to refer to the other booklet often...