Institution: | Institut für Nichtmetallische Werkstoffe - Anorganische Werkstoffe - Technische Universität Berlin, Englische Str. 20, 1000, Berlin 10, FRG |
Abstract: | The ESR spectra of Cr3+ and Cr5+ in soda-lime-silicate glass fibres were investigated with respect to defined parameters of the fibre drawing process from a nozzle. The following results concerning the structure of the fibres were obtained: Although the short-range order of the Cr3+ and Cr5+ ions is identical in the structure of the bulk glass as well as in the fibre glass, changes of the covalent portion of the bonds between the Cr5+ ion and its oxygen ligands were observed. The fibre glass structure is a frozen-in deformed or orientated (this depends on definition) silicate glass network, which influences the bonds in the square bipyramidal Cr5+O6] 7? complexes in the following manner: the σ-bonds between Cr5+ and the four oxygens in the square plane of the bipyramid are more covalent in the fibre than in the bulk glass. They are also orientation-dependent: more covalent for those of the randomly distributed complexes with their c-axes perpendicular to the fibre axis than parallel to it. The π-bond of the CrO3+ oxygen is less covalent in the fibre than in the bulk glass and is also orientation-dependent: more covalent for those complexes with their c-axes parallel to the fibre axis than perpendicular to it. The ability of the glass network to form Cr5+O6] 7? or CrO3+ complexes increases with the following increasing parameters of the fibre drawing process: melting temperature, oxygen partial pressure, mean cooling rate, and tensile stress. This fact is primarily a consequence of the shift of the redox-equilibrium and of the difficulty for the structural change from an octahedral symmetry of the Cr3+ and Cr5+ to a tetrahedral symmetry of the Cr6+ and vice versa during the fibre drawing process. The Cr5+O6] 7? complex is highly unstable and is an intermediate state. |