Porous ceramics have been widely used in many fields. Among the fabrication techniques for porous ceramic, freeze casting has recently attracted much attention as being a versatile, low cost and environmental friendly process. In this study, alumina and Al\(_2\)O\(_3\)-ZrO\(_2\) were utilized as model materials to investigate the preparation technique of freeze casting. Basic factors such as initial solids loading, cooling rate, and sintering temperature that would affect the final morphologies and the effect of additives have been studied. Porous alumina ceramics with lamellar microstructure exhibited compressive strengths up to 123 MPa for 33% porosity and 55 MPa for 42% porosity, making them suitable to be considered for potential load-bearing applications. In a two-phase system (Al\(_2\)O\(_3\)-ZrO\(_2\)), the choice of particle size of the ceramic powder was of great significance, which can lead to engulfment and phase segregation. BaTiO\(_3\) and Lead zirconate titanate (PZT) were employed to demonstrate the application of the freeze casting technique. BaTiO\(_3\)-epoxy composites exhibited modest piezoelectric constant but the dielectric constant was 1 order of magnitude higher than conventional composites with randomly distributed ceramic particles. For the 2-2 PZT-epoxy composites, with an increase of initial solids loading from 11 vol.% to 25 vol.%, the volume of ceramic phase in the composite gradually increased from about 30 vol.% to 50 vol.%, the piezoelectric constant \(d_{33}\) increased from about 103 pC/N to 203 pC/N, demonstrating the potential applications of this technique for the fabrication of 2-2 piezocomposites.

Ceramic cores are utilised during turbine blade manufacture to develop cooling channels within the finished component. These cores, fabricated by ceramic injection moulding, must possess an array of properties to ensure minimal defect development during casting. The carrier system, powder interactions and component forming, critical in ceramic injection moulding, have been investigated within this project. The binder systems were developed using paraffin wax, ethylene vinyl acetate and polyethylene with stearic acid incorporated as a surfactant. Binder system development illustrated that a wide range of melting, rheological and mechanical properties could be produced by binder blending. The moulding success was improved by feedstock optimisation, with optimal surfactant levels determined between five and ten ‘layers’ upon the powder surface. Optimisation maximised the achievable ceramic solids loading, measured and estimated by density and rheological methods. Binder systems illustrating a good resistance to fracture with adequate flow characteristics have been shown to successfully form complex cores to high yields. Success in component forming was limited by feedstock fluidity, final components were more prone to crack development when the binder system had reduced crack resistance and success of the wicking process was limited in some systems by formulation with proposed mechanisms explained.

Structural ceramics such as zirconia and alumina are widely used in the materials industry owing to their high hardness, chemical inertness and electrical insulation properties. However, they bear the disadvantage of low fracture toughness that has limited their further applications. As such, ceramics with improved fracture toughness are desired in many engineering fields. As for silicones, they are extensively used in current micro-electromechanical system (MEMS) components such as fuel cells and combustion engines. However, in hot and aggressive environments, silicon reduces functionality and efficiency of the components. Besides, low hardness and toughness of silicone material is undesirable in MEMS components. The ideal MEMS components require that the material has excellent structural strength at high temperature, exceptional thermal shock resistance and resistant to chemical corrosion. In view of these limitations, ceramics are currently being used to fabricate MEMS components. This PhD project sets out to tackle the disadvantages of ceramics such as brittleness and low electrical conductivity by developing ceramic nanocomposites using nanostructured fillers. Graphene nanoplatelets (GNPs) - a newly emerging carbon nanomaterial and alumina were chosen as the reinforcing fillers. Two types of composites namely alumina toughened zirconia (ATZ) and yttria stabilized zirconia- graphene (YSZ-Gr) composites were fabricated and their properties were investigated. These composites were produced by gel-casting route on PDMS soft molds. There were two main problems in fabrication of YSZ-Gr composites: (i) achieving homogenous dispersion of graphene in ceramic matrix and (ii) production of good quality graphene. The first problem was solved by dispersing commercially available graphene platelets in a surfactant;cetyltrimethylammonium bromide (CTAB). Scanning electron microscopy (SEM) ascertains homogenously dispersion of graphene in the ceramic matrix. In order to solve the second problem, processing parameters such as stirring speed and duration were optimised before fabricating the composites. Nearly dense ATZ and YSZ-Gr ceramic composites were obtained after sintering and infiltration with PDC resin (RD-212a). The prepared ceramic composites were characterized and their properties were studied. Analysis shows that hardness and fracture toughness of composites increased with the addition of fillers. For ATZ composites, there was an improvement of ≈25.26 % in fracture toughness and ≈16.29 % in hardness with 20 wt. % alumina. For YSZ-Gr composites, there was an improvement of ≈20.31 % in fracture toughness and ≈25.78 % in hardness with 1 wt. % GNP. Electrical conductivity of YSZ-Gr composites was increased by ≈9 orders of magnitude with 2 wt. % GNP compared to pure YSZ. The idea of this research is to use the two materials (ATZ and YSZ-Gr) to fabricate a MEMS scale chemical microthruster for space applications. Graphene provides conductive paths to decompose propellant (hydroxylammonium nitrate, HAN) and hence produces thrust. The characterization and testing of microthruster revealed that the proposed design is a success where; optimum thrust of 180.5 mN was achieved at a propellant flow rate of 60 µl/min. Future work should focus on optimization of chamber geometry, nozzle geometry and fuel choices to enable thruster to produce larger forces while decreasing the power consumption. Modelling and simulation of MEMS microthrusters can be used to verify the experimental results obtained.

A study of the acid leaching of a series of glasses has been made. This was undertaken to provide information concerning the production of microporous glass membranes with reverse osmosis desalination potential. The chemical compositions of the glasses investigated were developed from a commercial glass composition known as 'E-glass'. This glass~phase separates into mutually interconnecting acid soluble and acid insoluble phases, during cooling from the molten state. Treating the glass with mineral acid solutions removes the soluble phase, yielding a porous glass skeleton. The amount of Si02 in the original composition was varied, and it was found possible to produce microporous glass skeletons from glasses having a range of Si02 contents. These glasses also phase separated into mutually interconnecting phases during cooling. The parent glasses were studied by viscosity and electrical conductivity measurements. The results of those investigations were consistent with the rapid phase separation of the glasses. The leaching rates of the glasses were studied by weight loss measurements and those measurements indicated that the leaching rate was controlled by the durability of the acid soluble phase. The leached glasses were characterised by some electron microscopy, but primarily by inert gas adsorption measurements. The porous glasses contained pores of about 2 nm radius although there was also evidence of the presence of even finer pores. The reverse osmosis performance of several of the glasses was investigated using single hollow fibre membranes and salt rejections of up to 50% were recorded.

A diverse range of microscopy techniques and mechanical testing methods have been used to characterize glass and glass-ceramic composites. The focus of the work has been a commercially available Barium Magnesium Aluminosilicate matrix reinforced by Tyranno SiC based yam type fibres. The mechanical behaviour has been related to the microstructure through use of models from the literature. The temperature range of study has been from room temperature to 1300°C in air. The microstructure of the BMAS(fyranno was a diphasic mixture of celsian and indialite/cordierite although the manufacturers intention was a monophasic bariumosumilite. The carbon rich interface was found to be thin (l0-15nm) but the composite displayed impressive strength when compared to similar glass-ceramic composites reported in the literature. The matrix could be converted to the equilibrium bariumosumilite phase by heating in an inert atmosphere at 1370°C (or possibly lower) but matrix elemental diffusion into the fibres is likely to impair fibre strength. Tensile failure was by conventional matrix microcracking with load transfer to the in line fibres. However the composite strength was found to be dependent upon the strain rate as was the microcracking threshold associated with cracking of the 0° plies. Failure of the UD BMAS(fyranno was by longitudinal splitting before the expected ultimate strength (from the 0,90° results) was reached. This was due to an apparent notch sensitivity in this fibre architecture, a trait not observed in the 2-D composite. Direct measurement methods were used to establish the interfacial shear strength and these were compared to various models. These were based on matrix cracking thresholds, matrix crack spacing and a relatively new method where an 'inelastic strain index' was found from loading and unloading curves or hysteresis loop widths. Greatest fidelity with the direct methods was found with the last of these models. As with all composites with carbon enriched interfaces oxidation of the interface and fibres was found to impair strength when tested in air at temperatures as low as 600°C and possibly below this when testing at lower strain rates. At high strain rates, near room-temperature-strengths were achieved, even at 1l00°C, as the degrading effects of the oxidizing environment had less time to act. Long term exposure at high temperatures (1200°C) was responsible for formation of an embrittled surface layer up to 70J.lm thick. Within this layer the fibres were severely degraded and strong bonding prevailed at the interface. At temperatures in excess of the expected fibre pyrolysis temperature, (l100°C), the composite was seen to shrink along the length of the fibre axis and dilate normal to it which was attributed to fibre instability. Stabilising the fibres by heat treatments at 1200°C for 24 hours was seen to improve the creep performance in terms of the total strain accumulated within the 100 hours of the creep tests. The creep was comparable to other commercial glass ceramics (CAS/Nicalon and BMAS/BN/SiC/Nicalon) indicating the dominance of fibre creep properties on those of the composite. Cycling of the creep load seemed to result in a greater embrittled depth from the surface but failure at 100MPa and 1200°C was not observed within 240 hours of testing. Other systems were investigated such as the CAS/Nicalon, MAS/Nicalon and AS/Nicalon. Of these the AS/Nicalon was used for modelling the creep behaviour since it represented a simple system where matrix creep was accompanied by elastic deformation of the fibres. A model from the literature was used to explain an apparent increase in the elastic modulus during load cycling at high temperature and also the lower strain accumulation seen during load cycling compared to conventional creep tests.

Ferroelectric glass-ceramics have been investigated from the Bi203-Ge02, BiOl.s-Ge02-B0I.5, Bi0I.5-Ge02-Te02, 5PbO-3Ge02, PbO-Ge02-NbzOs, and PbsGe30u-PbNbz06-Si02+15%Ah03 systems. DTA, XRD and SEM analysis were used to obtain crystallographic and microstructural information. The dielectric properties and ferroelectric hysteresis loop behaviour of selected samples were determined. The stoichiometric Bjz03:Ge02 (BjzGeOs) composition devitrified on cooling, giving rise to the investigation of new systems BiO\.S-Ge02-B0I.5 and Bi0I.5-Ge02-Te0 2. It was found that the glass-forming region in Bi0I.5-Ge02-Te02 is narrow and good parent glasses for precipitating BjzGeOs crystals were not obtained. However, pure BizGeOs based glass-ceramic can be successfully formed from BiOl.s-Ge02-BOI.5. SEM backscatter imaging of these glass-ceramics showed surface crystallisation and XRD analysis confirmed that the preferred orientation is perpendicular to (311) planes. The dielectric behaviour and ferroelectric hysteresis loop study of the Bi2GeOs based glass-ceramic heat treated at 475°C for 12 hours, showed that this material can be ferroelectric at room temperature with Ps = 14 flC/cm2 and has Curie temperature at about 407°C. Glasses of compositions PG(PbSGe30I J)-xPN(PbNhz06) (x = 0.5, I, 2 3) were investigated from the PbO-Ge02-NbzOs system. Most of the samples devitrified on cooling and have poor mechanical strength except the sample PG-O.S PN sample which also contains interesting phases: ferroelectric PbSGe3011 and dielectric pyroniobate PhzNhz07. The surface crystallisation of PbSGe3011 with a-axis orientation and the bulk crystallisation of PhzNbz07 phase in this sample could be observed using SEM and XRD analysis. By applying heat treatment at 667°C for 48 hours to this sample, surface crystallisation along the a-axis can be enhanced. The Curie temperature of this heat treated sample is about 166 °C with Ps = 1 flC/cm2 from dielectric measurement and ferroelectric hysteresis loop behaviour. More samples were also investigated but it was difficult to form glass-ceramics containing both PbSGe3011 and PbNbz06 crystals from this PbO-Ge02-NbzOs system. In order to obtain the multiple ferroelectric PbSGe3011 and PbNbz06 based-glass ceramics, six glasses along the tie line from 62.5 mol%PbO: 25 mol%Ge02: 12.5 mol%Si02 to 40 mol%PbNbz06: 60 mol%Si02 were investigated from the PbSGe3011: PbNhz06: Si02+ 15%Ah03 system. Most of the glasses exhibited glass-in glass phase separation. From DT A analysis and subsequent crystallisation information, the most likely possible parameters, which control the glass-in glass phase separation, may be the NbzOslSi02 ratio for the glasses near the PbSGe3011 rich composition and Ah03 for the glasses near the PhzNhz06 rich composition. This system offered many interesting materials such as cubic pyrocWore PhzNbz07 based glass-ceramics and the orthorhombic PbNbz06 based glass-ceramics, and they are also mechanically robust.

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The work presented in this thesis is aimed at evaluating the potential for using supercritical carbonation (SCC) in conjunction with novel processing techniques, to fabricate new blended calcareous matrix composites with superior engineering properties and lower environmental impacts than conventional cement-based materials. Taking combinations of waste materials such as steel slag (SS) and fuel ash (PFA), binders such as hydrated and cement and various aggregate types to manufacture green forms and exposing them to supercritical carbon dioxide has produced a number of promising ceramic materials. The project looked at novel ways to process the ‘green forms’ from these composites, such as dry- and wet-compression moulding, 3-D printing and hand lay up technique that was adapted from the fibre-reinforced polymer industry. Work concentrated on optimising mix designs, green processing and SCC conditions to produce the highest strength materials. Three main avenues were explored. The effects of mix design, different curing regimes and SCC treatment, on the microstructure and chemistry of the composites was investigated using SEM, TSP, DTA, XRD, helium pycnometry and other techniques. Investigation showed that SCC process significantly enhances the mechanical and microstructural properties of carbonated products. It was shown that SCC treatment activates materials such as steel slag, that in the unground state are not activated by high temperature curing, to form useful composites. It was revealed that the relationship between the ‘degree of carbonation’ and strength is not straightforward and the order in which the various phases in the concrete react is important. Microstructural investigations hinted that the bond between carbonate limestone aggregate and the carbonated matrix was much stronger and more intimate (less porous) than for other aggregates. Chemical analysis also determined how much carbon dioxide could be ‘locked-up’ in the samples and this data was then used in the life-cycle assessment (LCA) of potential products. LCA was used to assess the green credentials of the SCC process and results were encouraging; a net reduction in CO2 emission of around 50% can potentially be achieved. Overall, the project has made many significant advances both in the practical application of SCC to ceramic composite manufacture and in the science of the reaction between sc-CO2 and cementitious phases. The technology could now be exploited by the manufacturing industry as a lowtemperature, rapid, low raw material cost and a sustainable route for manufacture of a wide range of ceramics.

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