The role of Titania in the formation of MgO - A1(_2)O(_3)-SiO(_2) glass ceramics

Glasses and ceramics of the MgO-A1(_2)O(_3)-SiO(_2) system have been founded and characterised by X-Ray Diffraction and optical and electron microscopy (Chapter 3). The hole and electron trapping centres generated by γ- irradiation of the specimens have been studied by electron spin resonance, (x-band), and optical absorption spectroscopy, (Chapters 4 and 5) . Computer simulations of the e . s . r . spectra have been used to deduce the Hamiltonian parameters and distributions of each 'defect' site. We conclude that the traps responsible for the hole resonances are non-bonding oxygen ions singly bonded to an SiO(_4) tetrahedra drain the glass specimens, and oxygen ions bridging between SiO(_4) and A10(_4) tetrahedra in the ceramics. I.R. absorption and Raman scattering studies are reported in Chapter 6. A phase separated glass structure is indicated consisting of a high silica phase with a framework silicate structure, and a high modifier (Mg) phase which degrades from a cyclosilicate structure to a chain silicate structure as TiO(_2) is added to the base glass. Corresponding changes in the hole resonance line shape are also interpreted in terms of structural changes within the high modifier glass phase and found to be consistent with 'sheet' and 'chain' silicate structures. TiO(_2) appears to promote the phase separation by the formation of Al-Ti complexes. Analysis of the e . s . r . and optical absorption spectra from the radio chemically reducted Ti(^3+) ions suggests titanium sites with the essential symmetry of tetragonally compressed octahedron (D(_4h)). We are unable to suggest a specific A1-Ti complex. The e.s.r. and vibrational spectra are invariant with heat treatments of the glasses at temperatures below that at which the first crystalline phase appears, and we find no evidence therefore of structural changes associated with such heat treatments. In Chapter 7 we consider a model for the promotion of phase separation in the MgO-A1(_2)O(_3)-SiO(_2) system by TiO(_2), and also present evidence which suggests that the first metastable crystalline phase to form originates from the TiO(_2) rich glass phase. Thereby the secondary role of the TiO(_2) in the production of fine grain glass ceramics is indicated