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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Crystal Nucleation and Growth in Baria-Silica Glasses

Ramsden, Anthony Hugh January 1978 (has links)
The kinetics of crystal nucleation and growth of barium disilicate were studied in baria-silica glasses containing 25 to 35 mole% baria. In this composition range, liquid-liquid immiscibility exerted a pronounced influence on crystal nucleation kinetics. The progressive shift in composition of the baria-rich matrix phase with time caused changes in both the thermodynamic driving force and in the kinetic barrier to nucleation which in turn caused a marked increase of nucleation rate. Study of the nucleation kinetics in relation to quantitative data of the morphology of the two liquid phases showed no evidence of heterogeneous nucleation at the liquid-liquid interfaces. Crystal growth measurements at lower temperatures showed appreciable induction times which were caused by a change in growth morphology from spheres to rapidly growing needles nucleated at the sphere-glass interfaces. The induction time decreased with rise in temperature. Comparison of crystal growth rates in phase separating and non-phase separating glasses showed that phase separation increased the growth rates due to the accompanying shift in composition of the baria-rich phase during heat treatment. This composition shift also caused an apparent reduction in the measured activation enthalpies for growth in the phase separated glasses. The morphology of the two liquid phases had no influence on crystal growth rates. All the glasses gave constant growth rates, except at high temperatures where growth rates increased with time. Reasons for this behaviour are discussed. No effects on the kinetics of nucleation and growth in a baria-silica based glass were observed on application of electric fields of 4 kV cm 1. A theoretical calculation showed that the field necessary to observe an effect would be much larger than is possible to achieve in practice, due to joule heating and electrical breakdown.

The environment of iron in silicate glasses

Bingham, Paul A. January 2001 (has links)
Iron species in glass are vital to applications such as solar control, and greater understanding of these species is required. Redox, coordination, distribution and environment of iron in alkali-alkaline earth-silica glasses containing 0-5 molar % Fe203 have been studied using a multi-technique approach. Wet chemical analysis provided values of the Fee+/EFe redox ratio and these are in agreement with those obtained from optical and Mössbauer techniques over a wide range of glass compositions and Fe203 contents. The Fee+/EFe ratio is independent of iron content in the glasses studied, provided that melting conditions allow sufficient equilibration with the furnace atmosphere. As iron content increases, Fe-Fe near-neighbour interactions have an increasing influence on measured properties. Mössbauer parameters indicate a wider range of site occupancies by Fe 2+ and Fe 3+ at low iron contents. Electron spin resonance suggests some clustering of Fe ions even at low iron concentrations. The amount of clustering is proportional to the square of the molar Fe203 content. Increasing alkali / alkaline earth ionic radius ratio promotes clustering at all Fe203 contents. The effects of alkali and alkaline earth ions on the redox, distribution, coordination and environment of Fe ions in these glasses generally fall into two behavioural categories which have been termed "collective" and "selective". Collective behaviour occurs when alkali and alkaline earth ions have similar effects on a property and the overall effect is cumulative. This is characterised by proportionality with theoretical optical basicity of the glass. Many parameters associated with Fe 2+ ions fall into this category. Selective behaviour occurs when alkali and alkaline earth ions have opposing effects on a property, suggesting competition or selectivity between ion types. This is characterised by proportionality with the alkali / alkaline earth ionic radius ratio. The redox ratio and several parameters associated with the clustering, coordination and environment of Fe 3+ ions fall into this category. Glasses containing MgO can exhibit behaviour dissimilar to the other alkaline earth oxides; molar volume, redox, coordination and clustering are affected. These phenomena may be due to some Mg 2+ ions occupying tetrahedral sites.

Microwave assisted processing of Nanocrystalline Barium Titanate based capacitor devices

Venkatachalam, Vinothini January 2011 (has links)
Interest towards fabrication of nanostructured electro ceramic devices has witnessed exponential growth in recent years, owing to the requirements of miniaturization, multifunctionality and improved reliability. The major hurdle in realising the full potential of nano ceramics is preventing the unwanted grain growth whilst achieving high densification during conventional high temperature processing. In this project, a detailed study was performed on the fabrication of nanostructured barium titanate based X7R multilayer ceramic capacitors (MLCCs) using microwave assisted heating. The main processing stages involved in MLCC manufacture were; (i) nano BT powder synthesis, (ii) making nano BT ink formulations suitable for screen printing, (iii) sintering of components using conventional, microwave, hybrid heating methods and (iv) performance evaluation of the end products.

Memory switching processes in chalcogenide glasses

El-Khishin, Ahmed Tawfik January 1979 (has links)
The aim of this work was to investigate the processes iInvolved in the memory switching phenomenon in chalcogenide glasses. The experimental study was performed using the stable memory switching composition Ge15Te815b252 although some trials were made on compositions from the pseudo-binary As 2T83 + GeTe. The experimental study included characterization of the materials by measurement of their electrical conductivity as a function of temperature and electric field. The lock-on time to establish a memory on-state was measured as a function of pulse parameters, ambient temperature, device geometry, parallel capacitance and substrate thermal conductivity. The results were consistent with a high temperature filament leading to the growth of a crystalline filament. The capacitive discharge energy was found to play an essential role in the memory switching process indicating the thermal nature of the on-state even during the first nanoseconds of the switching event. The measurements of the lock-on time as a function of device diameter and substrate thermal conductivity indicated that the filament temperature may not be critically dependent on the substrate thermal properties while the device diameter had a substantial effect on the filament temperature. The results of the study of the effect of illumination on lock-on time (to check non-thermal mechanisms) were not conclusive. However, It was found that the effect of the electric field is to inhibit nucleation. Thus the decrease of the lock-on time with the amplitude of the applied pulse was explained in terms of the effect of the capacitive discharge energy and not an electric field enhanced nucleation rate. The memory performance of the different compositions from the pseudo binary As2Te3 + GeTe was correlated to the glass transition temperature, crystallization temperature and the composition of the crystalline filament. The discussed processes were related to the device failure mechanisms and operating conditions in order to improve the device lifetime and its reliability.

Aqueous injection moulding of ceramics

Huzzard, Roy January 2002 (has links)
No description available.

The development and utilization of the ceramic shell mould technique and mould surface release agents in the design and forming of small scale studio hot glass

McCartney, Aron January 2001 (has links)
No description available.

Reactions of carbon and graphite with potassium vapour and carbon dioxide

Josling, J. K. January 1981 (has links)
No description available.

Design and processing of high-strength anorthite/mullite porcelains

Batista, Sofia Alexandra Ferreira January 2001 (has links)
No description available.

Strain in Portlandite crystals

Barker, A. P. January 1981 (has links)
No description available.

Laboratory melting of glasses to commercial standards of homogeneity measured by the Christiansen filter

Aylward, N. H. January 1981 (has links)
No description available.

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