Anisotropie glasses and glass melts - a survey

Abstract

Α general introduction describes intentional and unintentional anisotropic phenomena resulting from chemical and thermal inhomogeneities (internal stresses) as well as anisotropies induced by external elastic stresses and strains in homogeneous glasses for special opdcal purposes. Further the paper will point out the possibilities and various phenomena of producing defmed anisotropies by means of flow stresses and strain rates in glass mehs which may be frozen-in by cooling under load. It will be distinguished between Single - and double-phase glass melts and glasses which show very different kinds and effects of anisotropies. In particular, optical anisotropies (birefringence, specific birefringence) and structural-mechanical anisotropies (densities, shrinkages and expansions during annealing) of single-phase glasses and glass melts will be investigated under defined flow condidons (in-situ measurements directly in glass melts by the classical flow birefringence) and geometries (e.g. cylindric bulk glass samples, glass fibres, glass rods). In double-phase glass Systems the following anisotropic properties will be treated: the form birefringence and the distribution birefringence of decomposed glasses, the orientation of melts with suspended anisometric crystals and birefringence effects of chemically leached microporous glasses. Finally, a new phenomenon will be reported where the frozen-in birefringence of a glass shows an other sign after deformation above Tg than the stress optical constant of the same glass at room temperature before this treatment above Tg.