Dielectric properties of some oxide and oxynitride glasses
The main part of this thesis describes dielectric studies on a series of oxynitride glasses prepared at Wolfson Centre for High Strength Materials, University of Newcastle upon Tyne. Bridge techniques were used to determine the room temperature values of permittivity (ε') and dielectric loss (ε") for some Y-Al-Si and Nd-Al-Si oxynitride glasses and for further compositions in the Mg-Al-Sl and Ca-Al-Si systems. Over the range 500 Hz to 10 KHz the frequency dependencies of ε' and conductivity (σ) were found to be in good agreement with the Universal law of dielectric response i.e. (ε'- ε (_∞)) oc ω((^n-1) and σ (ω) oc ω(^n) giving n = 1.0 ± 0.1 for all compositions examined. In all four systems the addition of nitrogen increased the permittivity while, at each concentration of nitrogen (including the oxide glasses) the permittivity increased with cation type in the order magnesium, yttrium, calcium, neodymlum. As regards the higher frequency regime coaxial line techniques were used to determine, at room temperature, both ε' and ε" for some Mg-Al-Si, Ca-Al-Si and Y-Al-Si oxynitride glasses over the frequency range 500 MHz to 5 GHz. Here the frequency dependencies of ε' and ε" are also consistent with the Universal Law of dielectric response in that (ε'- ε(_∞)) oc ω((^n-1)) and ε" oc ω((^n-1)) for all glass compositions. The high experimental value of the exponent (n = 1.0 ± 0.1) suggested the limiting form of lattice loss situation. In this frequency range (as also at the lower frequencies) the addition of nitrogen increased the permittivity and it was confirmed that in both the oxide and oxynitride glasses ε' is influenced by the cation, being increased with cation type in the order magnesium, yttrium, calcium as at lower frequencies. At still higher frequencies, in the microwave region, special cavity perturbation methods of measurement were developed and used. Preliminary studies are also reported on the temperature dependencies of ε' and ε" for a similar range of glass compositions and on the optical behaviour of the oxynitride glasses both in the visible and infra-red regions of the spectrum. In the Appendices summaries are given of published papers on oxynitride glasses (Appendix I) and also on single crystal MgO, Fe/MgO, Cr/MgO and Co/MgO (Appendix II) a study which formed an addendum to the main work.