Glass - Back to the Future!

Presenting Author:
Dr Reinhard Conradt

article posted 22 Jul 2016

Dr Reinhard Conradt Reinhard Conradt has been serving as professor at RWTH Aachen University for the past 20 years and is now an emeritus. He studied physics and obtained a doctoral degree in physical chemistry in 1981. His engagement with the world of glass started at the Fraunhofer Institute of Silicate Science in Würzburg (1980 - 1986). From 1987 to 1996, he had a position of lecturer at Chulalongkorn University, Thailand. During this time, he completed his habilitation, which was accepted by RWTH Aachen University. Presently, he is president of the German Society of Glass Technology DGG, and chairman of TC23 'education' of ICG. The focus of his research has been on thermodynamics of glasses, especially of industrial multicomponent systems, as well as of the melting process.

Energetic Implications of Chemical Nanoheterogeneity in Mineral Glasses

Reinhard Conradt
RWTH Aachen University, GERMANY

In the introduction, the formation of nano-heterogeneities in glasses is interpreted in terms of the "strategy" of a material to bypass crystallization on the way to minimizing its volume and energy, e.g., during cooling from a liquid to a rigid condition. This results in a distinct energetic and entropic difference between a glass and its corresponding isochemical (poly)crystalline state. Results obtained from quantum mechanical ab initio calculations suggest that the local structures of one-component glasses develop towards the structures of low-density polymorphs of the same stoichiometry. These polymorphs reflect distinct energetic minima. They may, but not always do exist as real crystalline phases.

In a second part, heat capacities and elastic moduli of glasses and crystalline polymorphs are investigated. These quantities reflect randomly distributed and distinctly directed phononic states, respectively, and thus are excellent probes to reveal structural features of materials. At temperatures well above Debye's temperature, heat capacities probe the short-range order, i.e., the nature of cation polyhedra, whereas the elastic properties probe the nature of linkage among such polyhedra, which may be viewed as the shortest level of intermediate-range order. It extends over 1-3 nm only. The phonon-based macroscopic properties of glasses and crystals alike are determined by these entities. This is in agreement with Turner's and Porai Koshits' visionary statements on glass structure. Any randomness at extended length scales has little or no effect on the resulting macroscopic properties. Random mixing of such entities, however, results in enthalpies and entropies of mixing which are negligibly small as compared to the formation terms. This, in turn, leads to very simple superposition principles by which the properties of multi-component glasses (with typically 5-10 components) can be predicted in a fully quantitative way.

In a third part, the approach outlined above is demonstrated by experimental results (heat capacities and elastic moduli) of two industrial glasses, i.e., a float glass and an ECR fibre glass.