No description available.

No description available.

The electrical behaviour of a series of lithium silicate glasses has been characterised by the versatile a.c. impedance technique. The advantage of using a combination of complex plane and spectroscopic plots in the data analysis is shown. The compositional dependence of the conductivities of the phase separated glasses, which exhibit complex two-phase spinodal decomposition or nucleation and growth textures, is related to the volume fraction of the phases present in the conduction pathway and the tortuosity of the effective medium. The compositional dependence of the conductivities of homogeneous lithium silicate glasses is accounted for by postulating a glass structure composed of silicate anion clusters which are surrounded by a lithia-rich phase which constitutes the preferred conduction pathway. Annealing effects are also reported. The effect of surface roughness on the response of the electrode/electrolyte interface, a phenomenon originally reported by de Levie, and contact problems between the metal electrode and the glass electrolyte are discussed. Novel results regarding the effect of gold electrode recrystallisation on the a.c. response of glass electrolytes are reported. The a.c. impedance technique is shown to be a very useful, surface sensitive tool for monitoring interfacial phenomena such as atmospheric corrosion and surface ion-exchange. The technique is also successfully applied to studies of the mechanism of glass-ceramic formation, where the identification of surface crystallisation products and residual glass, by electrical measurement, is possible. Conclusive evidence for the presence of an effective medium conduction mechanism (percolation theory) in the inhomogeneous glass-ceramic, is given. The electrical data are corroborated by electron microscopy, x-ray diffractometry, energy dispersive x-ray analysis and Fourier Transform Infrared Spectroscopy.

Refractory materials must, in their everyday environment, withstand high stress levels which are a result of mechanical and thermal loadings. Any failure which results from these applied stresses can have serious financial and human consequences and therefore should be avoided. One key aspect to understanding the thermal shock behaviour of refractories is the mechanical behaviour at low temperatures. In this thesis the mechanical behaviour of a small range of multiphase refractories is explored. In particular the stress-strain response and its influence on the fracture behaviour is investigated. Experiments, performed on magnesia and magnesia spinel composites, indicate that non-linear stress-strain behaviour accompanied by permanent deformation upon unloading is a result of the release of microscale residual stresses by microcracking. A micromechanical constitutive model combining these features was developed using linear elastic composite theory and isotropic continuum damage mechanics. This non-linear stress-strain behaviour also gives rise to increasing toughness as crack propagation occurs. This increase in toughness results from an expansion which occurs when microscale residual stresses in front of the crack tip are relaxed by microcracking. A micromechanical model has been developed based upon the specifically developed constitutive model and previous work on transformation toughening. These models are capable not only of simulating experimental results, but can also indicate the microstructures which are most likely to exhibit extensive non-linear stress-strain behaviour and strongly rising toughness curves.

Binary series CoO-P205 and ternary series CoO-NiO-P205 glasses are prepared. The density, d.c. conductivity, highfield I-V characteristicsr memory switching actiong a.c. electrical conductivity, infra-red absorption and optical absorption edge of the glasses are presented. The historical background and the formation and theory of the glassy state are reviewed. Previous works on the electrical conductivity of phosphate glasses are reviewed in the framework of Mott's theory. In the binary glasses, the glass acquires a more compact structure with increased CoO content whereas for the ternary series the results show otherwise. A polaronic model is shown to be generally applicable to explaining the results of electrical conduction measurements. The pre-exponential factor containing the term exp (-2aR) arising from electron tunnelling should not be ignored; thus the theory of the small polaron hopping is in the non-adiabatic regime. The high-field ohmic behaviour is observed up to a field of about 3x 104 V-cm-l'. Apparentlyt the field dependence is a property of the bulk material. Thin films of glass with 30 Mol % CoO show memory switching phenomena. The results of a.c. measurements characterize hopping conduction as a dominant process in cobalt-phosphate glasses. Results of the infra-red spectra measured in the range 400 cm-1 to 4000 cm-1 show that the phosphate polyhedra dominate the structure of the glasses. Measurements of the optical gap in the binary series show decreasing values with increased CoO content. It is believed that the fundamental absorption arises from indirect transitions.

The aims of the project were to investigate the fundamental processes involved when a glass is exposed to sodium vapour, and to examine the different ways of improving the resistance towards sodium attack. With regard to the interaction of glasses with sodium, the silicates were studied in most detail. The discolouration of the silicate glasses by sodium was treated as a surface phenomenon and techniques such as ESCA, MRS and ESR were used to investigate the interaction. The results from, these various techniques all point to the same conclusion, that the mechanism of sodium attack is the progressive breaking of Si-O bonds to form Si-0- bonds. The reaction between borate glasses and sodium showed certain similarities with the silicates. This led to the suggestion that the same mechanism of attack might also apply to the borates. In attempting to improve the resistance of transparent materials towards sodium attack, two methods were considered. The first approach was to change the chemical composition by selecting components of a glass which are inert to sodium. The chemical compatibilities of the components were assessed using an equilibrium thermodynamic argument to consider the possible reactions with sodium, and a number of materials have been suggested to be inert in this way. The second method of improving the resistance was to rearrange the structure of a glass by crystallisation. The crystallisation behaviour and the microstructures of two glass compositions - MgBAl (3-2) and CAl (6) were studied in detail, and transparent glass-ceramics were derived from them. It was found that the improvement in resistance towards sodium attack was only marginal in the case of crystallising the glasses in bulk. However, substantial improvements were observed in the case of surface crystallisation.

The roles of tin in two silicate based glass systems have been investigated by NMR and Môssbauer spectroscopies and by physical property measurements of the glasses. The first glass system investigated was the stannous silicate (binary SnO-Si02) glass. Glasses with SnO contents ranging from 17 to 72 mol.% have been made by melting pelleted powder in an alumina crucible. It was found that alumina crucibles are unsuitable for making glass with <20 mol.% SnO because of attack on the crucible at the high melting temperature (_>_1600°C). Silica crucibles will not withstand such high temperature and tin will attack a platinum crucible. The ability of this system to form glass past the orthosilicate composition has been discussed in terms of the polarizing power of Sn2+ and the structure of SnO. The 119Sn NMR results did not give much structural information due to the high chemical shift anisotropy of Sn 2+ site but they showed that the glass also contains trace amounts of Sn4+species. The 29Si MAS NMR results showed that SnO does not depolymerise the silicate network to the same extent as Na20 or even Pb0. Computer simulations of the 29 Si MAS NMR spectra showed that, for SnO <-30 mol.%, the disposition of Qn species is consistent with the binary model, which means that SnO is acting the role of modifier. For compositions > 30 mol.% SnO, the Qn distribution follows the statistical model and this has been interpreted as SnO now acting as an intermediate. The 119Sn MOssbauer results confirmed this interpretation. The Sn2+ isomer shift decreases with increase of SnO which is indicative of increasing covalent character of the Sn—O bonds while the larger quadrupole splitting suggests distortion of the SnO polyhedral structure in the glass. The relation of the Sn 2+ isomer shift to the quadrupole splitting and the temperature dependence of the isomer shift of Sn2+ indicate the formation of Si—O—Sn linkages at high SnO contents. The decrease of the viscosity of the glass with increasing SnO is small when compared to the decrease of the viscosity in alkali metal and alkaline-earth oxides silicates when the respective modifier oxide is increased in those glasses. The variation of the density, thermal expansion and refractive index with SnO content showed discontinuities in the region of 30-45 mol.% SnO. This has been interpreted as being the point where SnO changes its role from that of modifier to intermediate. The results of differential thermal analysis and devitrification of SnO-Si02 glasses showed that glass with 40 mol.% SnO can be heat treated in the temperature range of 570° to 680 °C to produce metastable SnSiO3 crystals. SnSiO 3 decomposed to SnO + Si02 at temperatures above —700°C and, at temperatures greater than 720°C, oxidation of SnO to Sn02 and Si02(glass) to Si02 (cristobalite) took place. The second glass system is tin-doped float glass. This is glass of the float composition remelted with tin(II) oxalate in silica crucibles under normal atmosphere conditions. In this way it has been demonstrated that we can mimic the tin oxide distribution found within the tin diffusion region in float glass. Synthesis of the glass has shown that both Sn2+ and SO+ can be assimilated simultaneously in the glass but there is a solubility limit for SO+. The 1195n Mbssbauer results showed that Sn2+ and SO+ played different structural roles in the glass. The environment of Sn2+ in glass is similar to that in amorphous SnO while the SO + structure in glass does not change significantly compared to crystalline Sn02. The Debye temperatures and recoil free fractions showed that Sn2+ is less rigidly bound to the network modifier site while SO + is rigidly bound at network former sites in the glass. The different structural roles of 5n 2+ and SO+ in the glass were reflected in the some of the physical properties of the glasses.

The use of sintered cordierite/enstatite glass-ceramics as interlayers for joining silicon nitride to itself and to metals has been investigated. The role of the additives B203 and P205, which control the dynamics of sintering and crystallisation, has been studied using SEM, XRD and non-isothermal DTA-based measurements of activation energy. The measured activation energies for the crystallisation of μ-cordierite, for compositions with no additives, with B203 only, and with P205 only, did not differ significantly and were in the range 415-460 kJ mol-1. When both B203 and P205 were present this was increased to 503-524 kJ mol-1. The activation energy for α-cordierite formation when no additives were present was 952 ± 57 kJ mol-1. This was substantially reduced by the presence of B203 (540 ± 27 kJ mol-1), P205 (668 ± 41 kJ mol-1) and when both were present (352 ± 26 kJ mol-1). Cordierite/enstatite glass-ceramics have been successfully used to join silicon nitride to itself. Joining at 1050-1100'C in N2 with an applied load of ~ 2.5 MPa, resulted in joint strengths, measured in 4-pt bending, of 110-170 MPa. This is comparable to the intrinsic strength of the glass-ceramic and sufficient for practical applications. These strengths were obtained using an interlayer with a TCE (5.7 MK-1) greater than that of the silicon nitride (3.0 MK-1). Suggestions for further improvements to the joint strength are discussed. The use of a glass-ceramic joint with graded thermal expansion to bridge a TCE mismatch is discussed, and the geometrical restrictions on the joint, which limit possible practical applications, are outlined. The concept of a ceramic/metal compression joint with a glass-ceramic interlayer has been demonstrated for joining silicon nitride to both Nimonic alloy 80A and Ti. The requirements for continuity of electronic structure at the Nimonic 80A/glass-ceramic and the Ti/glass-ceramic interfaces are satisfied by reaction between the glass/glass-ceramic and, the pre-oxidised surface of the Nimonic alloy to form a MgTi205-Al2TiO5 solid solution phase, and the Ti to form Ti5Si3. For the lower WE mismatch (Ti-silicon nitride) the residual joining stresses generated on cooling were marginally too high and need to be further reduced, either by a slight alteration to the joint geometry and/or a smaller WE mismatch.

Fibres were produced from glasses with the following compositions: A= 25% Li20 74% SiO2 1% P205 B= 29% LiO2 68% SiO2 1% P205 1% ZnO 1% K20 by drawing the fibres from a single tip platinum bushing. The fibres, in the form of tows, were then subjected to a series of heat treatments in which the nucleation and crystallization temperatures and times were varied. The tensile strength and elastic modulus was determined for each sample of fibres and use was made of microscopy and X-ray diffraction to determine the microstructure resulting from the subsequent heat treatments. The data obtained from the tests performed in this investigation required a statistical analysis because of the variance in any one measured parameter. This is often found to be the case when investigating small volumes of material such as fibres. A model was proposed, for each composition, relating the strength to crytallization time and an attempt was made to justify each model on the basis of the empirical data obtained and indirect evidence arising from the statistical interpretation of this data. The strength of crystallized fibres produced from composition A. decreased with crystallization time. This was explained by assuming that the strength controlling flaws were introduced by the appearance of a surface crystal layer and that the flaw size was proportional to the thickness of the layer. Evidence was produced to reinforce this argument. Two proposals regarding the elastic modulus, were tested and it was found that observed moduli could beat be described by considering the fibre to be a composite cylinder with an outer annulus of lithium disilicate surrounding a core of glass. Glass B. fibres did not behave in the same way as A. after crystallization except at high crystallization temperature. In general the strength would decrease thenincrease again slightly before finally decreasing. This behaviour was explained by assuming that the mean intercrystal spacing controlled flaw size until the depth of the surface crystal layer became large enough to dominate. Some research is described in which an attempt is made to inhibit surface nucleation/crystallization by using a vapour phase ion-exchange treatment. This work remains incomplete but there was some indication that it would be successful in allowing higher strengths to be achieved in glass-ceramic fibres after prolonged crystallization.

No description available.