Several techniques have been developed for the encapsulation, and subsequent hotisostatic pressing (HIPing), of silicon nitride (Si3N4) based ceramics. Green-state and densified billets of silicon nitride were vacuum encapsulated in either glass tube or powder. Glass powder encapsulation allows complex shaped ceramic pieces to be HIPed. A selection of silicon nitride compositions were HIPed to near-theoretical density (>97% T. D. ) after encapsulation in either Pyrex glass tube or powder. The silicon nitride compositions studied included single yttria (Y203) additive materials that cannot be densified by conventional pressureless sintering, hence the requirement for pressurised sintering. Similar ceramic compositions were also densified using the commercial ASEA HIP process for comparison. The bulk ceramic microstructure was generally similar to pressureless sintered type materials, with a complete a- to ß- Si3N4 transformation, although a finer microstructure and lower matrix phase volume were apparent. The ceramic/encapsulant interaction during HIP was generally assessed using a boron nitride (BN) interlayer. When a thick layer (> 50 μm thickness) was retained after HIP negligible interaction was apparent. A thin silicon oxynitride (Si2N2O) surface layer was observed to form with thinner interlayers. Penetration of the molten encapsulant glass through the porous BN layer occurs during HIP, leading to an increase in the Si4+ and 02- concentration at the ceramic surface and the subsequent re-precipitation of Si2N2O in preference to ß- Si3N4. Direct penetration of the encapsulant glass into the porous ceramic occurs in the absence of a BN barrier layer and a similar encapsulant dependent compositional modification was observed. Sub-surface ceramic contamination by boron was apparent in isolated samples HIPed in a Pyrex-type glass at ASEA. Interaction between the encapsulant and ceramic did not significantly affect the post-HIP surface oxidation rate, when compared with the bulk material.

The short-range structural order, in glasses from the technologically important Na20-LhO-B203-Si02 system, is investigated using NMR and thermophysical techniques. The change in the populations of tetrahedrally coordinated borate units and various trigonal borate units is deduced from llB NMR and compared with the most widely accepted, NMR based, structural model of Dell, Bray and Xiao 1. Differences from the model are discussed and substantiated with results obtained from 29Si NMR. Structure sensitive properties such as density, glass transition temperature and thermal expansion coefficient are reported across a wide range of modifier concentrations. This information is then related to and compared with the structural arrangements obtained from NMR studies of less complex systems. An empirical density modee·3 designed for the single-modifier borosilicate systems has been used to gain information regarding the structural arrangements in the current system. Investigation of specific, carefully controlled, compositional changes to the current HL W vitrification glass, MW, has been carried out. The effect of 8 20 3 concentration on the chemical durability of glasses from this system has been studied using dynamic leach tests. In addition, the role of La203, a waste simulant, has been investigated using similar analytical methods, and suggestions as to its primary function in the glass have been discussed. Finally, a preliminary study of the interaction of several compositions, related to MW, with high-energy a-particles and UV -radiation, has been pursued. The presence of radiation induced structural and physical changes has been probed using ESR and 'Y-ray spectroscopy, enabling suggestions for the types of defects produced and nuclear reactions taking place to be made.

The objectives for the thesis were to generate tough ceramics utiising the toughening mechanisms inherent to zirconia materials. The aims have been realised with the successful fabrication of hot pressed silicon nitride / zirconia composite ceramics. The zirconia was prestabilised with two different types of dopant additives, yttria and ceria, with the intention of understanding the chemical compatibility with the silicon nitride matrix and the overall effect on the subsequent mechanical properties. The volume fraction of added zirconia was also varied. The increased toughness over silicon nitride materials alone was attributed to the toughening agents inherent to zirconia which existed either in the form of the tetragonal polymorph or the monoclinic variant. The toughening modes were dependent on initial chemistry of the composite system. When the zirconia was prestabilised with yttria the tetragonal polymorph was retained within the composite. The enhanced toughness was attributed to a transformation toughening mechanism. However, when the zirconia was prestabiised with ceria the depletion of Ce from solid solution with the zirconia during processing resulted in the formation of the unstabiised monoclinic variant. The enhanced toughness was attributed, in this case, to a microcrack type energy absorption mechanism, similar to several ZTA composite ceramics. Additionally, an experiment using ultrasound non-destructive testing, indicated that Tetragonal Zirconia Polycrystals (TZP) is ferroelastic and, as such, can provide a significant contribution to enhanced levels of fracture toughness in these materials or composites containing the same. Further work has been conducted to actually observe, as a function of applied unia.xial stress, the crystallographic changes occurring within the bulk of a 3Y-TZP ceramic via neutron elastic scattering at the ILL, Grenoble, France. This experiment has provided clear direct proof of the ferroelastic nature of zirconia. A similar experiment will be carried out at the Rutherford Laboratory, though with significantly improved statistics. An approach to improve the high temperature properties of TZP via the chemical alteration of the grain boundary phase was also considered. As a preliminary step the grain boundary volume was increased through controlled additions of the grain boundary composition in the form of both a premilled and a premelted glass. Poor fired densities were attained, however, due to the solute additive partitioning from the generation of an enhanced grain boundary phase to overstabilisation of the zirconia resulting in the formation of cubic stabilised zirconia. Furthermore, the incorporation of nitrogen within the grain boundary phase, via sintering TZP with sole additions of A1N, resulted in the attainment of poor fired densities and hence was not considered a suitable method for grain boundary modification.

No description available.

A critical review of phase equilibria studies on the edge binary and face ternary sections of the quaternary system Na₂O-CaO-B₂O₃-SiO₂ is presented. Phase equilibria has been studied by means of the static quenching technique in the binary subsection CaO,B₂O₃-B₂O₃ and the ternary system Na₂O-CaO-B₂O₃. Phase diagrams are presented for the stable and (general) metastable equilibria in the subsection, along with a discussion on the stability and range of the phases CaO, 2B₂O₃ and 'CBx '. Two previously unreported compounds: Na₂O,CaO,4B₂O₃ and 3Na₂O,6CaO,11B₂O₃ were found in the boron-rich part of the ternary system. Unit cell dimensions and X-ray powder patterns are given. A liquidus diagram and a subsolidus compatibility diagram were determined. There is no evidence of solid solution in the system. The general reaction kinetics in the ternary, the distribution of the major compatibility volumes in the quaternary and the application of borate fluxes in bone china are discussed.

An introduction to the phenomena of pressure crack figures is given with special reference to static-impact figures produced on glass and diamond. The optical techniques and the apparatus used are described, including the Inter-ferometric technique used for the detection and measurement of the surface distortions for crack figures obtained first on glass, and then on diamond. The spherical impactors used were of steel (for glass), tungsten carbide and diamond (mainly for diamond). A study is made of the fracture strength properties of ten different types of optical glass and the results are compared with other properties of the glasses, including measurements of their surface hardnesses obtained by an abrading method described in the Appendix. Orientated crack figures have been produced on three different faces of diamond, the shapes being hexagonal on the octahedral and the dodecahedral faces, and square on the cubic face. For the octahedral and a cubic face, the development of these figures is studied as the load is gradually increased; and the mechanism of the crack formation and the accompanying cracking effects within the body of the crystal are discussed in terms of easy cleavage and shattering, respectively. It is found that the octahedral face is definitely the least resistant to fracture by this method, the cubic face appearing to be most resistant: also the critical stresses involved are found to be considerably less than a theoretically calculated value which indicates some form of flaw distribution over the surface analogous to the Griffith cracks for glass. It has also been seen that the octahedral face of diamond can be cracked in this way by using Tungsten Carbide or Sapphire balls, although in the latter case the tip of the ball was shattered by the occurrence of multiple slip. Both for glass and diamond, the observations obtained from the interferograms of the surface distortions is considered to offer strong evidence for the existence of micro plastic flow. No signs of micro slip could be detected in the studies on diamond except possibly in connection with the crystallo-graphic shattering which occurred internally for one test on the octahedral stone.

The durability in the lung of inorganic fibrous materials, such as asbestos and man-made mineral fibres, appears to be a major determinant of their pathogenic potential. However, studies have been inadequate in explaining differences in the physiological durability of such inorganic fibres. This study used an iterative approach to determine key factors affecting physiological durability of a soda-lime silicate bulk glass, A-glass, E-glass, Lead-glass, Cemfil and alumino-silicate ceramic fibres. The aims were to develop a) the current theoretical understanding of durability and b) a suitable in-vitro screening test for durability. Materials were exposed to simulations of the lung environment, which included a) exposure to Gamble's fluid, water, serum and other simulated fluids, b) long-term exposure to the intra-macrophage environment and c) exposure to rat lung. Durability was characterised by scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray microanalysis (EDXA). The use of secondary ion mass spectrometry (SIMS) was also explored, though further development was required in this area. Fibre behaviour depended on fibre composition and thenature of the exposure environment. The ceramic was durable in all environments, whilst A-glass, Lead-glass and the soda-lime silicate were prone to nucleophilic attack and leaching. The effects of in-vivo exposure were consistent with the response in-vitro. However, exposure to the intramacrophage environment in-vitro did not affect fibre durability and this surprising result should be investigated. Physiological durability was related to the ability of the fibre to resist nucleophilic attack and a hybridization bonding model was examined in order to explain the behaviour of some silicate glasses. It was recommended that models based on the molecular bonding were developed to encompass a wider range of materials. Occupational exposure and inhalation of asbestos fibre can cause lung disease and although the mechanisms of asbestos pathogenicity remain uncertain, attention has also focussed on the potential effects of other inorganic fibres. Comparative studies on behaviour of these materials in the lung have strongly implicated the durability and hence lifetime of the fibre to be a major determinant of the pathogenic potential. However, durability studies have generally been inadequate in explaining differences in physiological durability of inorganic fibres and hence provision of theoretical models. This study used a novel iterative approach to determine key factors affecting physiological durability of a range of glass and ceramic materials. The objective was to develop the theoretical understanding of the durability of inorganic fibrous materials in the lung and appraise in-vitro methods for determination of fibre durability to validate a suitable screening test. The durability of a range of glass and ceramic materials has been characterised using in-vitro and in-vivo simulations of the human lung environment; novel exposure systems have been developed and durability behaviour has been characterised by application of traditional analytical methods and by development and application of secondary ion mass spectrometry (SIMS) techniques. Appraisal of in-vitro simulations revealed that fibre behaviour depended upon fibre composition and exposure conditions' durability of the fibres in-vitro was related to model fluid pH. Fibre response in-vivo was rationalised by assuming localised pH variation. This work supports the use of a range of in-vitro exposure conditions to identify key determinants of fibre durability and to characterise chemical behaviour, and is critical of the use of single in-vitro screening tests which will reflect fibre behaviour under specific conditions. Resistance of the inorganic fibre network to hydrolytic attack was suggested as a key determinant of durability and a theoretical model was developed to predict this.

The alkaline-earth silicate fibres are a new generation of man-made insulation materials. The materials are amorphous on manufacture and have been shown to be soluble in physiological solutions and to be cleared from the lung in animal exposure trials. This reported study provides a thorough investigations of the devitrification behaviour of Superwool X-607 (Morgan Materials Technology), and two further compositions, code names A2 and B3.Thermal exposures were made within the region extending from 700 to 1250°C and 10 minutes to 3240 hours in clean furnace environments. The devitrified microstructures and products were identified using X-ray powder diffraction and analytical electron microscopy. Details are provided of the development of specimen preparation techniques to enable fibre cross-sections to be analyzed in the TEM.The devitrification products are presented as a function of exposure temperature and time for all 3 compositions. The amorphous glass separated into a silica-rich phase and an alkaline-earth silicate rich phase and the development of these amorphous phases is presented and discussed. The subsequent devitrification of these separated phases into associated silica crystalline phases and alkaline-earth silicate crystalline phases, and, in each case, the subsequent phases and transformations with increasing thermal exposure are also presented and discussed. For the crystalline silica phases, the following unusual transformation situation was identified in all 3 compositions: amorphous silica → alpha-quartz → alpha-cristobalite → tridymite. In comparison with established understanding of silica phase transformations, the following anomalies were identified and explained: a) the formation of quartz as the primary crystalline silica phase at temperatures ≥1000°C, and b) the subsequent formation of alpha-cristobalite, the low-temperature form, other than B-cristobalite, the high-temperature form, which was the only silica phase identified in the devitrified aluminosilicate fibres. For the alkaline-earth silicate phases, two forms of wollastonite solid solution were characterised. The low-temperature form, containing more Mg[2+], transforms to the high-temperature form and diopside at temperatures above 900°C. The low-temperature anomaly, ie the formation of the immature high-temperature form below 900°C is possibly due to a secondary phase separation. Pseudowollastonite was found to be metastable in the investigated temperature region. It is suggested that the detailed transformation process among these alkaline-earth silicate phases has been identified for the first time. This thesis also includes a detailed review of published studies concerning this materials systems as well as recommendations for further work.

An investigation has been carried out into the effects of three common oxide impurities, TiO2, A12O3, and SiO2/ on the properties and behaviour of Y-T.Z.P. These impurities are present in varying amounts in almost all commercially available Y-T.Z.P. materials, and substantial costs are incurred in removing them in the purest systems. However, the effects of these impurities, both individually and in combination have received relatively little study in the published literature, and it has not been made clear to what degree these impurities influence the properties and behaviour of the material. To carry out the investigation it has been necessary to develop a novel technique for introducing the impurities as dopants into a high purity, commercially available Y-T.Z.P., whilst retaining a high degree of chemical homogeneity in the material. The technique developed uses a variant on the alkoxide sol-gel process to coat the individual powder particles with a thin layer of dopant atoms and offers a number of advantadges over other doping techniques. The process could be exploited to solve a variety of ceramic processing problems. The results obtained from impurity doped materials showed that alumina and silica reduced the sintering temperature and promoted enhanced densification at lower sintering temperatures, whilst titania impaired the sintering at lower temperatures. Alumina additions resulted in pronounced grain growth and associated destabilisation of the tetragonal phase of zirconia, particularly for higher sintering temperatures. A factorial experiment was carried out to obtain additional, and previously unreported information. This showed that there were significant interactions occurring between all of the additives investigated some of which appeared to be beneficial. An investigation into the effect of the additives on the mechanical properties (hardness and fracture toughness) was carried out for a range of sintering temperatures. The results of these experiments suggested that the impurities had very limited direct effects on the transformation toughening mechanism, although there were differences in properties associated with the effects of the impurity additions on the microstructures of the sintered materials.

The Problem: Given a set of glass furnaces, producing 3 colours of glass and grouped together in factories, to find a method of calculating a stable pattern of timing of normal rebuilds over a seven year period. Objective: To minimize the total cost of production and repairs Constraints (1) No two furnaces in the same factory can be rebuilt in the same quarter* (2) No two amber or green furnaces to be rebuilt in the same quarter; no more than two white furnaces to be rebuilt in the same quarter. (5) Given sales demands for each colour glass in each quarter. The sum of the production over 3 consecutive quarters must be greater than or equal to the sum of the demand over these 3 consecutive quarters. Initially an analytic approach of monitoring the variables that effect cost and production was tried. It became apparent that some essential data on furnace behaviour was either not in existence or at a very experimental stage - in particular data on furnace life expectancy and ageing effects on costs and production. In its place the following method evolved. I would be given a number of optional schedules for each furnace along with data on production and costs. Out of these one has to choose one schedule for each furnace in such a way that we have a minimum cost combination that satisfies all the constraints. We now have a different problem to solve. The simplified problem: Given a set of glass furnaces, producing 3 colours of glass and grouped together in factories, and given one set of possible repair schedules for each furnace. To find a method of choosing one repair schedule for each furnace to give a stable pattern of timing of normal rebuilds over a seven year period. Objective and Constraints are the same as before.