Redrawn phase-separated borosilicate glasses: A TEM investigation

Two alkali aluminoborosilicate glasses containing oriented crystalline particles have been investigated by Transmission Electron Microscopy (TEM). Both materials have been thermally treated in order to phase-separate crystalline droplets and have been then redrawn at a temperature above their softening point to elongate and orient them. In one sample the crystalline phase is a Ag(Cl, Br) solid solution, which gives the material birefringence properties, while in the other sample the particles on the surface have been chemically reduced to Ag, giving the material polarizing properties (Polarcor (TM)). Most of the observed needle-like particles are built up by a small number of well grown crystallites often separated by voids. Both the crystallites and the voids occupy the whole width of the particles and can assume very different lengths. Crystallites inside a particle have different orientations.