GaN films were deposited by cyclic pulsed laser deposition (cyclic-PLD) at different substrate temperatures between 400 and 600◦C. Alignment of the films along the c-axis, grain size, roughness and conductivity increased with temperature. Surface elemental ratio N/Ga was determined by X-ray photoelectron spectroscopy (XPS) and was smaller for films deposited at higher temperature. A decrease of the resistivity of the films agreed with higher metallic (Ga) surface concentration. Bulk elemental ratio N/Ga, determined by Rutherford backscattering spectroscopy (RBS), was higher than the XPS ratio and showed a very small tendency to decrease with deposition temperature. At 600 ◦C, there was evidence of contamination of the films by oxygen probably resulting from diffusion from the sapphire (Al2O3) substrate. These results suggest that adjustments in the deposition conditions are needed in order to have high crystal alignment, large grain size and good N/Ga ratio.

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An understanding of space charge build-up in the polymeric insulation of power cables is important in determining how aging occurs and progresses and, also in predicting cable lifetime. In this investigation electric-field induced space charge in peelings from XLPE (cross-linked polyethylene) cables was measured using two different methods: the pulsed electro-acoustic technique (PEA) and the combined procedure of isothermal and non-isothermal charging/discharging currents (FTSDC). These two methods allow the study of space charge in highly insulating materials. Also, since electric fields of different orders of magnitude are applied to the sample in the two methods, it is possible to analyze different characteristics of the space charge traps. Prior to the measurements the samples were subjected to conditioning to remove volatiles. Cable peelings from various brands aged under different conditions (including field aged and thermally aged samples) were studied as received from the manufacturers. Some of the samples have undergone further ageing in AC electric field (50Hz) for 1000h to see the influence of further ageing on space charge build-up. The results for the different types of samples are compared in an attempt to correlate different ageing parameters.

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCHNH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCHNH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.

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Mixtures of fly ash, calcite and soda ash were prepared by keeping the amount of fly ash at 80 wt% and adding the other raw materials in order to obtain Na2O/CaO ratios of 1/3, one and three. On melting all mixtures led to black glassy materials. Several heat treatments were performed on these glasses at temperatures suggested by DTA. The crystalline phases precipitated during the heat treatments were identified by XRD on powdered samples and the microstructures of the obtained glass ceramics were observed by SEM. The formulations tended to precipitate pyroxene solid solutions and aluminium silicates of calcium and/or sodium, during the heat treatments. The glass ceramics mainly contained fine crystals. However some of the materials obtained from glasses with Na2O/CaO ratios of 1/3 and three had non-uniform microstructures. It is generally concluded that the microstructure of these crystallised glasses is critically dependent on both composition and heat treatment.

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