Structure-sensitive investigations on glass fibers from the system SiO2-Al2O3-CaO

Glass fibers of the system SiO2-Al2O3-CaO were prepared with respect to def ined drawing conditions and investigated with structure-sensitive methods ( birefringence, radial and axial alterations by thermal treatment at T-g). T hese glass fibers, which lie in a compositional sense roughly between the d isilicate and silica glasses concerning the ratio network modifiers to netw ork formers, exhibit similar optical anisotropies. On the other hand, they show remarkably lower axial and radial alterations which are exclusively co ntractions on annealing for all applied drawing conditions in contrast to t he silica or the alkali disilicate glass fibers. Obviously, the filling of free volume hollows with and the reinforcement of them by the Ca2+ ions and the smaller polarizability of the latter as compared to alkali ions are re sponsible for the relatively low structural anisotropy. An increase of the optical anisotropy with increasing CaO concentration is observed at compara ble viscosities. This is a consequence of the increasing incorporation of t he network modifier oxide which produces a weaker network strength and a la rger polarizability.