A busca pelo prolongamento da qualidade da vida tem incentivado o aparecimento de novos materiais. Vidros que possuem fosfato em sua composição têm encontrado um grande espaço em aplicações como biomateriais, e sua utilização como scaffolds pode ser promissora, pois além de sua estrutura ser mais próxima da parte mineral dos tecidos ósseos, eles são materiais com alta solubilidade em meios aquosos. Vidros niobofosfato foram estudados em trabalhos anteriores e provou-se que a adição de óxido de nióbio em vidros fosfato melhora sua durabilidade química. Há também estudos que revelam que o óxido de nióbio pode ser utilizado como biomaterial devido a sua biocompatibilidade. Neste trabalho determinou-se a taxa de dissolução de uma ampla faixa de composições de vidros niobofosfato em soluções aquosas, e em soluções simuladoras de fluído corpóreo (SBF), com diferentes condições de ensaio. Os vidros estudados neste trabalho com composições (32 - 37)P2O5 (2- 15)Nb2O5 (6-7)Na2O (46-53)CaO, % em mol, foram produzidos a partir da mistura e fusão dos precursores em forno elétrico na faixa de temperatura de 1300ºC - 1400ºC por 0,3h. Foram produzidos sete tipos de vidros com composições diferentes, tendo como base a variação do teor de óxido de nióbio. Os vidros foram submetidos a análises de citotoxicidade e foram considerados não citotóxicos. Análises por difração de raios X mostraram que os materiais são amorfos e não se observou a presença de fases cristalinas após imersão tanto em solução aquosa, quanto em SBF. A densidade dos vidros variou de 2,84 a 3,14 g/cm3 em função do teor de óxido de nióbio. Observou-se a perda de massa dos vidros imersos em água e em SBF a 37ºC por 21 dias, sendo que a taxa de dissolução é maior quanto menor o teor de nióbio. As taxas de dissolução também variam de acordo com as condições de imersão das amostras. Micrografias eletrônicas de varredura mostraram a superfície das amostras antes e após imersão. Observou-se a formação de trincas nas superfícies de algumas amostras, resultado da liberação de matéria para o meio lixiviante. Houve aparente formação de uma camada superficial em uma determinada composição. Análises por espectrometria de fluorescência de raios X por energia dispersiva (EDX) determinaram os teores dos elementos presentes nas composições dos vidros, antes e após as imersões. Observou-se a presença de Al2O3, resultado da contaminação do vidro pelo cadinho, e também uma variação significativa no teor de P2O5 causado pela volatilização do mesmo no processo de obtenção do vidro. A variação dos teores dos elementos na superfície de alguns vidros após os períodos de imersão é causada principalmente pela liberação preferencial de P, Na, Al. Os teores dos elementos P, Na, Nb e Ca foram determinados por EDX nas soluções aquosas utilizadas para os testes de lixiviação. / The need to extend and improve the quality of life is leading to the search of new materials. Glasses containing phosphorous in their compositions are considered biomaterials and can be used as scaffolds because they have a structure close to the mineral component of bone tissues, and present relatively high solubility in water. The chemical composition, morphology, and mechanical properties of bioactive glasses can be properly fit to allow bonding with bones. Niobium phosphate glasses have been previously investigated. The addition of niobium oxide improves the chemical durability of phosphate glasses, and they are considered biocompatible. In the present work the dissolution rate of a wide range of niobium phosphate glass compositions were determined in water and Simulated Body Fluid (SBF). The glass compositions were in the range (32 - 37)P2O5 (2-15)Nb2O5 (6-7)Na2O (46-53)CaO mol%, and they were prepared by melting reagent grade chemical compounds in the temperature range 1300ºC - 1400ºC for 0.3h in an electrical furnace. Seven different glasses were prepared. The glasses were considered non-toxic after performing cytotoxicity tests. No evidence of crystalline phases was observed on X-rays diffraction patterns before and after the corrosion tests. The glass densities are in the range of 2.84 - 3.14 g/cm3. The density increases as the amount of niobium oxide increases. A mass loss was determined after immersing glass samples in water and in SBF at 37ºC during 21 days. The dissolution rate decreases as the amount of niobium oxide increases. The dissolution rate depends on the immersion medium and conditions. Cracks on the glass surfaces were observed by scanning electron microscopy after the corrosion tests which are related to the material release from the glass surface to the liquid. There is also evidence of formation of a layer on the surface of a specific glass composition. The glass compositions were determined before and after the corrosion tests by X-ray Fluorescence Spectrometry (EDS). Aluminum was detected in the glass composition. This element is related to the contamination of the glass by the crucible used in the glass melting. A significant variation of the amount of P2O5 compared to the nominal composition was also noticed, due to the volatilization of phosphorous during the glass melting. The surface glass composition changes during the leaching tests in water or SBF because of the preferential release of P, Na, and Al. EDS was also used to determine the concentration of P, Na, Nb, and Ca in the leaching solution by using the dry drop method.

The mechanical behaviour of woven fabric-based continuous silicon carbide fibre reinforced Pyrex (SiC/Pyrex) and calcium alminosilicate (SiC/CAS) matrix composites under quasi-static and cyclic tensile loading has been investigated. Both a plain weave and a satin weave architecture were examined for each material type. Under quasi-static loading for all systems except the low temperature processed Pyrex system (which failed prematurely) a linear elastic region was observed up to an applied strain of 0.04-0.06%. Above this strain (the matrix microcracking threshold) a reduction in the composite modulus was seen. The reduction in composite stiffness is attributed to matrix microcracking, and the morphology of matrix microcracking was examined and quantified using an edge replication technique. In all systems the matrix microcrack density was seen to increase approximately linearly with increasing strain up to failure. The corresponding reduction in the composite modulus at failure was 40-50%. Associated with the damage there is hysteretic behaviour and an increasing residual strain. The strain to failure of the satin weave composites was higher than the plain weave composites. In the cyclic fatigue tests the number of cycles to failure decreased with increasing peak stress level. A progressive reduction in the composite modulus was seen with cycles even when the applied strain was below the matrix microcracking strain threshold. It is likely that at strains below this threshold there is non-interacting matrix microcracking which does not initially affect the composite modulus. However, on continued tensile fatigue cycling these microcracks grow through a possible sub-critical crack growth mechanism reducing the laminate modulus. A modified shear lag model was used to model the reduction in composite stiffness as a function of the measured matrix crack density. The woven composite was converted to an equivalent cross-ply sub-laminate on to which the matrix microcracks were superimposed. A model allowing for the presence of microcracks in both the matrix and transverse plies gave the best agreement between the experimental and predicted reduction in modulus.

This Thesis is primarily concerned with the effects of abrasive machining (diamond grinding) and diamond indentation on the fracture properties of a range of silicon nitride materials. Test specimens machined to surface finishes representative of those found on Aero Gas Turbine components were produced for Modulus of Rupture (MOR) testing, and variations in the fracture strengths were assessed. Optical and Scanning Electron Microscopy (SEM) were performed as a means of identifying the nature of the defects found within these materials. Having determined the dependence of strength and reliability on the machined surface finish, attempts were made to palliate the machining damage by thermal annealing and Nitrogen Ion Implantation. X-ray diffraction residual stress measurements were performed in order to quantify the magnitude of the near surface stresses in both the "as machined" and annealed conditions.* Diamond indentation techniques (Vickers and Knoop) were employed in order to determine the hardness of the materials studied and to quantify the extent of the Indentation Size Effect (ISE). These studies were then extended to the point of indentation fracture as a means of assessing the materials fracture toughness (KIC) and the nature of the crack systems beneath the indentation. *Residual stress measurements were carried out on a sub contract basis at the CEGB Central Laboratories by P E J Flewitt and D Lonsdale, their help throughout this work is gratefully acknowledged.

No description available.

This Thesis describes a study of the compaction and the sintering of ceramic powders within the context of their "near net shape forming" into dense bodies. The operation of near net shape forming is the manufacture of ceramic parts with a required external dimensional tolerance combined with a defect free internal microstructure. The current study considers these requirements by focusing upon the fundamental facets of near net shape forming operations. The effects of the processing and material parameters on the compaction behaviour of agglomerated alumina powders have been investigated experimentally. It has been shown that the green compacts, formed from the agglomerated alumina powders by the uniaxial die pressing technique, possess certain density distribution patterns due to the frictional forces during the compaction process, and these density distributions depend primarily upon the properties of the agglomerates and the lubrication state of the die wall. The density distributions in the green compacts have been determined experimentally using the coloured layer technique and predicted using a first order model. Due to the inhomogeneous density distributions in the green compacts, the shrinkage of the compacts was not homogeneous. As a result, the shapes of the sintered compacts were not geometrically linear scaled replicas of the green compacts and the shape distortions in the sintered compacts have been characterised. It has been shown that there is a relationship between the extent of the shape distortions and the density distributions in the green compacts; that is, the more inhomogeneous green compacts the more is the shape distortions in the resulting sintered compacts. Sintering and grain growth equations, modified from established relationships, have been used for the sintering simulations in order to predict both the progression of the density and grain size of the sintered compacts. In order to predict the overall shape of the sintered compacts, a first order model has been developed by combining the predicted density distributions in the green compacts and the predicted density of the sintered compacts. The extent of the shape distortions have been reduced by optimising the properties of the agglomerates and the state of the die wall lubrication condition.

No description available.

Transparent glasses embedded with ferroelectric/nonlinear optic crystallites have been in increasing demand as these exhibit promising physical properties. These could be fabricated in large sizes and shapes with high optical homogeneity accompanied by high degree of transparency over a wide range of wavelengths of light. Amongst a variety of glasses that are known, borate-based glasses are of particular interest owing to their greater transparency, good chemical and mechanical stability, low materials cost, and useful electrical and dielectric properties. Keeping the potential multifarious applications of transparent glass-microcrystal composites in view, BaO-0.5Na2O-4.5B2O3, BaO-0.5Li2O-4.5B2O3, SrO-0.5Li2O-4.5B2O3, 3BaO-3TiO2-B2O3 and Li2O-3B2O3 glasses and glass-microcrystal composites were fabricated. These glasses on controlled heat treatment at appropriate temperatures yielded BaNaB9O15, BaLiB9O15, SrLiB9O15, Ba3Ti3B2O12 and LiB3O5 crystalline phases, respectively. Further transparent surface crystallized BaO-0.5Na2O-4.5B2O3 glasses were fabricated using ultrasonic treatment and their thermal properties have been investigated in detail using differential scanning calorimetry. It is observed that these glasses were homogeneously crystallizing on the surfaces after Ultrasonic treatment which can be exploited for planner wave-guide applications. Glass forming ability, thermal stability, glass-transition behavior, crystallization kinetics and viscosity of these glasses were studied extensively using various methods and rationalized by invoking various models. The above glasses have been characterized for their dielectric and electrical relaxation properties (as these properties are related to their electro-optic and non-linear optical properties) over 30- 600oC temperature range and frequencies (100 Hz -10 MHz) that are normally of interest in the applications of these materials. Several interesting features such as high ionic conductivity, marginally low dielectric loss and high dielectric constant behavior along with low thermal coefficient of dielectric constant were observed in these glasses and were rationalized using various models. The combination of these dielectric characteristics suggests that these are potential candidates for electrical energy storage device applications.

No description available.

No description available.

No description available.