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No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Carbon-carbon composite materials have been identified as one of the most potential materials for light weight and high temperature applications. Mechanical properties of carbon-carbon composites do not degrade even at temperature as high as 2000°C. However the main problem in their use in high temperature oxidizing environment is their tendency to oxidize at temperatures of 400°C and above. Therefore some oxidation protection mechanism is mandatory to make these materials available for high temperature applications. It is the purpose of the current research to develop a viable high temperature oxidation protection system for carbon-carbon composites. It has been shown that such a coating system must have at least two layers; a gradient porous SiC layer aimed to redistribute the stress produced due to CTE (coefficient of thermal expansion) mismatch and a dense top layer of a suitable material meant to protect carbon-carbon composite substrate from oxidation. Materials for the top layer experimented during the current research were SiC, ShN4 and HfC. Pack cementation technique was used to develop the gradient SiC layer while top dense layers were deposited by using the reactive sputtering technique. To improve the oxidation protection and crack resistance of the top dense coating multilayering approach was adopted. During the current research basically four different coating systems were produced, characterized and then tested at high temperature for their oxidation performance. These coating systems were, gradient SiC layer plus dense sputtered SiC layer, gradient SiC layer plus dense sputtered ShN 4 layer, gradient SiC layer plus dense sputtered SiC/ShN 4 layers, gradient SiC layer plus dense sputtered SiClHfC layers. Oxidation testing of these coatings in atmosphere showed that these coatings are thermodynamically stable at all test temperatures studied (1300-1575°C), except coatings with a ShN 4 layer. ShN4 becomes thermodynamically unstable at 1575°C. These coatings remained mechanically stable (no spallation) except the coatings with HfC layers. Coatings with HfC layers spalled off at all temperatures. Investigation into the causes of spallation indicated that the thickness (20-25 flm) of the converted SiC gradient layer was insufficient, plus the processing conditions during the deposition of HfC were the main causes for their failure.

In this thesis a complete study of silicon carbide (SiC) foam is presented. The aim of which is to show that this material is sufficiently understood that it could be used as a support material in the vertex detector at the International Linear Collider (ILC). Through this research a deep understanding of SiC foams has been acquired. This knowledge has come from a combination of machining and material tests, along with detailed analyses of SiC foams at the level of its microstructure. The later was carried out by modelling a foam as a set of tetrakaidecahedra unit cells. These were in turn analysed using Finite Element Analyses. The elastic moduli from these studies were within 16% of the manufacturer's values for SiC foams with relative densities 8%, 12% and 16%. The ability to machine the material was vital if it were to be used to make a support structure. Studies presented here showed that this was indeed possible and intricate components were machined, which led the way for the first all SiC foam vertex detector prototype. This structure was temperature cycled to check its stability. The best obtained flatness relative to room temperature was 8 ± 6 um: in the x - y plane of the ladder, over the temperature range -20°C < T < -lOoC. To ensure that an all SiC foam detector was competitive with the current baselines, a series of performance tests were carried out. One of the benchmark tests was the vertex finding capability at the Z-pole energy. This test showed that all the concepts were within 5% of each other. Also carried out were tests on the impact parameter resolutions and the purity and efficiency of heavy jet favour tagging. The research presented here shows that SiC foam is a credible choice and should be seriously considered in 'any future R&D into vertex detector support structures.

The principle objective of this project was to optimise extraction of cellulose nanofibres from banana tree and rye grass feedstocks and to investigate the potential use of these products as high value-reinforcing agents in thermoplastic polymers, particularly in polyvinyl alcohol (PV A) and polyethylene (PE). To this end, in association with the Agri-Food and Biosciences Institute (AFBr) in Belfast, the following extraction techniques were investigated to obtain cellulose nanofibres from these natural fibre sources: (i) chemical modification, involving mercerisation, acid hydrolysis and chemical bleaching; (ii) mechanical treatment, using a high speed (Turrax) mixer and high pressure microfluidisation and (iii) chemical and mechanical (chemo-mechanical) processes, by combining TEMPO-oxidation and high pressure microfluidisation. The nanofibres produced were characterised using scanning electron and transmission electron microscopy, particle size measurement (static image analysis and laser diffraction), chemical analysis (zeta potential analysis, fourier transform infrared spectroscopy and x-ray diffraction), and thermogravimetric analysis. It was especially evident that the chemo-mechanical procedure yielded higher aspect ratio nanofibrils, a greater yield and higher crystallinity, than nanofibrils made by solely chemical or mechanical treatments. The cellulose nanofibres obtained were subsequently incorporated into PVA by a solution casting technique. The effect of different treated nanofibres on the mechanical, structural and thermal properties of these composites was determined. It was notable that banana nanofibres made by TEMPO-oxidation and high pressure microfluidisation showed phenomenal reinforcing effects in the PVA. Furthermore, ryegrass nanofibre, derived from the high speed Turrax mixer, was , incorporated into PV A and PE by using more commercially acceptable melt processing procedures, involving surface treatment of the nanofibres using silane coupling agents, their pre-dispersion in a PVA carrier, twin-screw extrusion compounding and compression lamination methodologies. Varying degrees of success were seen, from poor dispersion using cryogenically milled nanofibres, yielding little effect on mechanical properties, to very significant enhancement with melt processable PVA, being of a similar order to solution cast nanofibre-reinforced PVA To assess the relative reinforcing efficiency of cellulose nanofibres, conventional PE composites were also made using macro-scale banana and sisal fibres. To aid compatibility and enhance interfacial bonding between fibre and matrix in this system, maleic anhydride modified polyethylene was applied with both nano- and macro- fibre variants. By way of example, there was a 100% improvement in tensile modulus of conventional banana fibre-reinforced PE composite with a 30 wt% loading of micron-sized banana fibres, whereas a 300% improvement was recorded in tensile modulus for cellulose nanofibre-reinforced PVA with only 5 wt% of cellulose nanofibres.