No description available.

No description available.

The thesis studies the sintering behaviours and mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) ceramics. The sintering behaviours of BSCF are studied by sintering BSCF powder using a series of sintering temperatures and dwell times. Under all circumstances, only a cubic perovskite structure is identified in as-sintered samples. The relative density of BSCF increases with increasing sintering temperature and dwell time, but shows a more significant increase with increasing temperature. While the grain size increases with increasing sintering temperature and dwell time, it is found that the increasing temperature contributes much more significantly than increasing dwell time in grain growth. The shape of grain size distribution profile is independent of sintering temperature and dwell time, but the profile shifts with different sintering conditions. The grain maintains an aspect ratio of 1.8 irrespective of sintering conditions. Similar findings are also made on the Ni-doped BSCF, but it is found that Ni doping inhibits the grain growth and retards the densification of BSCF while it has little influence on the grain size distributions and grain aspect ratio distributions. The grain growth exponent (n) and apparent activation energy (Q) are also systematically studied. It is found that grain boundary diffusion is the dominant controlling mechanism for BSCF while both grain boundary and lattice diffusions are the equally dominant controlling mechanisms for BSCF-Ni8. The fracture stress of BSCF is measured by both three-point and ring-on-ring bending tests at room and high temperatures. The fracture stress determined by three-point bending tests is consistently higher than that value measured by ring-on-ring tests for a given temperature. By utilising Weibull statistics a close prediction is made of the three-point values from the ring-on-ring values. Compared with the Young’s modulus of BSCF obtained from three-point bending tests between RT and 800 °C, the values determined from ring-on-ring tests shows a fairly good agreement. However, the Young’s modulus measured by both bending tests is lower than that value determined by micro-indentation tests. Hardness and fracture toughness are independent of grain size and grain orientation. Porosity is the dominant factor in Young’s modulus, hardness and fracture toughness of BSCF. The intrinsic hardness, intrinsic Young’s modulus and intrinsic fracture toughness of BSCF are also determined. The subcritical crack growth (SCG) of BSCF is also studied using constant load method at RT and constant stress rate method at 800 °C. It is found that that BSCF is not susceptible to SCG at RT but becomes relatively sensitive to SCG at 800 °C. The results are subsequently used as a basis for a strength–probability–time (SPT) lifetime prediction. Ni doping increases the Young’s modulus, hardness and fracture toughness of BSCF determined micro-indentation tests at RT. Both hardness and Young’s modulus show a non-monotonic trend with Ni doping content, which is attributed to the porosity and secondary phase. The intrinsic hardness, intrinsic Young’s modulus and intrinsic fracture toughness of 8 mol% Ni-doped BSCF are determined. Dopants have little influence on grain orientation and the distribution of grain boundary misorientation angles of BSCF.

No description available.

An electrochemical technique has been developed for the production of precursors to ceramic films on hydrogen sorbing metal substrates. It involves the electrolysis of aqueous metal salt solutions which yields hydrogen at the cathode, resulting in local generation of base (hydroxide ions) around this electrode. Such conditions promote the precipitation of metallic hydroxides from a suitable electrolyte. If the local alkaline environment is not disrupted by convective or other forces, then a solid phase accumulates near the cathode, and forms an adherent gel-like structure on its surface. In order to maintain deposition, it is essential that gaseous hydrogen evolution is minimised, and preferably eliminated. This can be achieved by use of a hydrogen sorbing cathode material, such as palladium. The electrode, and adherent film (or, in appropriate circumstances, the deposit alone) can then undergo a subsequent calcination treatment to yield the ceramic layer. It is possible to generate both porous and compact structures by this method, depending on the potential programme employed during deposition. Research has been conducted into the understanding of mechanisms involved in porosity control of films deposited during different potential regimes, with view to establishing routes to layers of predetermined physical structure. In-situ optical methods were employed to complement the electrochemical techniques, providing valuable insight into the initial mechanisms of film formation and the subsequent thickening processes. The utility of the precipitation process was illustrated by the fabrication of films which demonstrated a variable conductivity over a range of humidities appropriate to sensing application. Investigation into the use of a bipolar palladium electrode as an aid to generating thick film deposits was carried out. The device comprised a palladium plate, operated as a bipolar electrode in aqueous electrolyte. Under suitable conditions, the negative face of this electrode can be made to generate and absorb hydrogen, whilst simultaneously, the positive face oxidises hydrogen transported across the bipolar substrate by diffusion. Thus the cathode face is a non-gassing electrode on which thick deposits of metal hydroxide can be grown. This line of research lead to the realisation of a self-feeding hydrogen anode at the electrode's positive face. Further research was undertaken to assess the electrochemical properties of this anode. The effective operating window for hydrogen oxidation was investigated, and the effect of prolonged potential cycling, elevated temperature and bipolar plate thickness on this region was also considered.

Titanium and boron nitride and carbide, titanium diboride were synthesized by carbothermic reduction as single phase as well as mixtures intended to form composite materials. The aim of the project is to study the physical chemistry of carbothermic reduction for the production of pure nonoxide ceramic powders and also for the in-situ formation of ceramic/ceramic partially-densified composites. The thermodynamic and kinetic factors that govern the phase constituents are discussed and the effect of processing parameters on the morphology and extent of reduction are also established. The first part of the present investigation is aimed at the production of titanium nitride, carbonitride and carbide powders and the in-situ formation of TiN/TiC partially-densified composites by the carbothermic reduction of titania in suitable nitriding atmospheres. The investigation includes the aspects of the thermodynamics and kinetics of the nitriding reaction and points out the reaction mechanism by identifying the phase formed after the nitridation process. The microstructures produced after the reduction-nitridation process have been correlated with the thermodynamic and kinetic parameters. The synthesized titanium nitride powder was identified as the carbonitride phase, Ti(CxN1_x), having a range of composition. The rate of reduction of TbO2 was found to be determined by the rate of oxygen diffusion in the sub-oxide lattice and the derived value of activation energy in the temperature range 1473K to 1773K from the Arrhenius plot is 120 kJ-mole-1 of T102. TI305 was found as a high temperature precursor phase for the formation of titanium nitride. The use of iron chloride as catalyst and activated charcoal in the mixtures of oxide increased the yield of titanium nitride phase by enhancing the rate of reduction of titanium oxides. The morphology of titanium carbonitride particles was dependent upon the reactivity of carbon and the temperature. The calculated equilibrium phase fields were found to be in agreement with the experimental data and provide a means to select the variables for the reduction condition for designing a required ceramic microstructure. The microstructure of boron nitrides is closely related to the structural chemistry of carbon and nitriding agent. The main aim of the second part of the project was to synthesize boron nitride and carbide powders and whiskers by carbothermic reduction of boric anhydride (6203) in nitrogen atmosphere and also to understand a relation between the processing parameters and the phases produced. The effect of processing conditions such as the gas composition, reactivity of carbon, reaction temperature and time as well as the composition of starting materials on the synthesis of boron nitride and carbide phases were studied. The reactivity of carbon, B/C ratio and gas composition were the most important variables that determined the formation, structure and morphology of the nitride. During the nitridation process, boron carbide phase also formed and played a significant role. The investigation also reports the evidence for the formation of metastable forms of BN i. e wurtzite and cubic BN. We also report the results of the solubility of nitrogen in C-saturated B4C structure. The third part of the present work is aimed at the production of TiB2 powders. Aspects of the formation of two or three ceramic phase mixtures were also examined together with the relative stability of the single phase mixed diborides with respect to pure diboride phase. The central aim of this part is to establish the mechanism of the synthesis reaction leading to the formation of uniform size of titanium diboride crystals. Titanium boride (TiB2) powder was produced in the powder form by the reduction of ingredient oxides with carbon via a gas-solid phase reaction. For the production of the composite microstructure, the nitrogen partial pressure was found to be the most critical factor. In the composite microstructure, the titanium nitride particles have a submicrometer size whereas the boride particle size is only a few micrometers with predominantly hexagonal morphology. Some calculated equilibrium phase fields have been experimentally verified. The empirical verification is a useful tool to establish the correctness of the calculated phase diagram. The theoretical approach therefore enables to identify the condition for the formation of phase mixtures. The constituent phases depend on the reduction conditions. For example, nitrides in equilibrium with Ti62 can only form above a critical nitrogen partial pressure whereas TiC or B4C form in the inert atmospheres. This result is applicable to all other ceramics. The investigation also shows the viability of production of the composite powder mixture via the oxide co-reduction technique. The synthesis of TIB2/TiN, TiB2ýC, TB2/TN/BN and mixed diboride composites is possible by employing the reduction route.

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.

See Coversheet for system requirements. The aim of the research is to integrate computer technologies and environmentally - sensitive materials and processes into the practice of the ceramic designer-maker, in order to assess the impact of new technologies on practice. A critical contextual review (including analysis of visual material) revealed a developing interest in environmental issues and computer technologies in designer-maker practice. A shift away from a philosophy which historically has been anti-industrial, towards a wider spectrum of craft production was noted. This diversity was shown in the types of production and the scale of manufacture - from ‘one-offs’ to industrial manufacture. New technologies were acknowledged by critics, commentators and practitioners as facilitating this development, although concerns about the potentially detrimental affect that computer technology could have on craft skills was voiced. A link between a pragmatic philosophy of ‘craft’ practice and new approaches to computer systems design highlighted a perception of the validity of ‘craft’ as a contemporary skill. The lack of established methodologies for practice-based ceramic design research has led to the development of a naturalistic approach within this work which is both holistic and emergent. By necessity this methodology places the design researcher at the centre of the inquiry, and uses practice as the main research vehicle. Selected research outcomes were peer reviewed through two significant international touring exhibitions: ‘Hot Off the Press: Ceramics and Print’ and ‘Objects of Our Time’. Initial investigations concentrated on the development of environmentally-sensitive lustre glazes [lead and cadmium free] incorporating an innovative ‘safe’ reduction firing system. Subsequently, ceramic surface designs and three dimensional forms were developed through the integration of: computer assisted design work (CAD); computer assisted manufacture (CAM); colours and glazes; and environmentally-sensitive screen printing, and existing making methods. The outcome was new aesthetic qualities and an extension of practical capabilities. A critical framework for the analysis of research outcomes was developed in order to make explicit and transferable some of the tacit knowledge embodied in research investigations. The analysis was developed through the use of a computer database system from which an electronic document was developed, allowing the integration of a large amount of visual material into the thesis. The research demonstrated that the integration of new technologies into the holistic and emergent practice of the ceramic designer-maker was appropriate. Many advantages of computer technologies for the ceramic practitioner are identified as transferable to the wider field of designer-maker practice and embody the potential to enhance future developments in this field.

No description available.