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Abstracts (International Journals and Books)

[5]   I. J. Davies, B. Djuricic, S. Pickering, and J. Th. M. De Hosson, “HRTEM investigation of silicon nitride powder coated with yttrium oxide-precursor”, J. Surf. Anal., 3(2) pp. 394-400 (1997).

Abstract: Ceramic matrix composites based on silicon nitride (Si₃N₄) are potential candidates for use as high-temperature structural materials. However, the high melting point and low self-diffusion coefficient of Si₃N₄ makes fabrication difficult without the addition of a sintering aid such as yttrium oxide (Y₂O₃) or aluminium oxide (Al₂O₃). Addition of sintering aid through mechanical mixing does not produce homogeneous distribution on the microscale and results in poor control of the sintered Si₃N₄ intergranular phases which decreases the oxidation resistance and mechanical properties of such ceramics. An improved method for homogeneous distribution of sintering aid might involve the coating of each ceramic powder particle with a small amount of sintering aid - techniques previously proposed include precipitation from metal salts or metal hydroxides and electrostatic deposition from a colloidal suspension. In this study, Si₃N₄ powders were coated with 5 wt.% Y₂O₃-precursor using a chemical precipitation technique and calcined up to 1400 ^oC in nitrogen or argon. Investigations using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were carried out in order to determine the dependence of coating morphology and atomic structure on calcination conditions. Energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were also utilised to determine the dependence of the amount of yttrium and the phases present on processing conditions. It was concluded from EDS observations that Y₂O₃-precursor was successfully deposited onto the surface of Si₃N₄ whilst direct observation from HRTEM indicated the precursor to consist initially of discrete hemispherical nanocrystalline particles (with typical dimensions 10-100 nm) which were unaffected by calcination at 600 ^oC in nitrogen. However, calcination at 1400 ^oC in nitrogen or argon resulted in partial sintering of the coated powder together with crystallisation of the Y₂O₃-precursor.

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[4]   B. Djuricic, I. J. Davies, S. Pickering, D. McGarry, E. Bullock, M. Verwerft, P. M. Bronsveld, and J. Th. M. De Hosson, “Study of particle coatings for the design of intergranular phases in engineering ceramics”, Silicates Industriels, 60(7/8) pp. 203-210 (1995).

Abstract: The deposition of nano-dimension coatings of Y₂O₃ and/or Y₂O₃/Al₂O₃ precursor material onto Si₃N₄ and SiC particles provides a methodology for the uniform dispersion of sintering aid and the compositional tailoring of intergranular phases in engineering ceramics. Coatings were precipitated from aqueous solution containing the appropriate metal nitrates and urea, onto the dispersed ceramic particles, at a rate determined by the in-situ hydrolysis of urea. Under the same processing conditions different coatings were obtained. The morphology of the deposited coatings was shown to be sensitive to the coating chemistry and the ceramic particle surface conditions. Both discrete (Y- and Y+Al-containing) and continuous (Y+Al-containing) coatings were deposited on the surfaces of Si₃N₄ and SiC core particles. The coated powders were characterised using scanning, transmission, and high resolution electron microscopy. Possible mechanisms for the formation of the coatings are discussed. Si₃N₄ powder coated with Y+Al-containing sintering aid was sintered to near full density. Microscopic examination reveals an “in-situ composite” microstructure.

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[3]   I. J. Davies and R. D. Rawlings, “Mechanical properties in compression of low density carbon/carbon composites”, Composites, 25(3) pp. 229-235 (1994).

Abstract: Compressive properties of a porous 2-D planar-random carbon/carbon (C/C) composite preform densified by chemical vapour infiltration (CVI) of pyrolytic carbon were determined and compared to those obtained previously for similar C/C composite densified using a non-CVI technique. Stress/strain curves and failure modes of the CVI composite were similar to those seen in the non-CVI C/C composite. For specimens of equivalent density, the CVI technique resulted in improved in-plane properties, but inferior out-of-plane properties, when compared to the non-CVI composite. Compressive property anisotropy ratios for the CVI composite were slightly reduced compared to the uninfiltrated preform state whilst comparison between compressive and (previously determined) flexural properties showed general agreement. A microstructural model of the composite developed by the authors was used to explain the results.

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[2]   I. J. Davies and R. D. Rawlings, “Mechanical properties in flexure and tension of low density carbon-carbon composites”, Carbon, 32(8) pp. 1449-1456 (1994).

Abstract: The mechanical properties of low density 2-D planar-random carbon-carbon composites with densities in the range 0.17-0.40 Mg·m^-3 were determined by 3-point bend and tensile testing and correlated with the microstructure. Three-point bend and tensile specimens fractured in a brittle manner, with the former failing through a flexural failure for span-to-depth ratios from two to eight. Tensile strength and modulus values were lower than the respective flexural values. This was thought to be due to the increased volume under stress, and to the derivation of the flexural modulus equation not taking into account the shear components of the beam deflection, respectively. Mechanical properties were isotropic within the planes, and greater than the respective out-of-plane values, and mechanical property values increased with increasing density, which was accounted for by the reduction in microstructural anisotropy, in particular, associated with the increasing proportion of recycled material.

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[1]   I. J. Davies and R. D. Rawlings, “Microstructural investigation of low-density carbon-carbon composites”, J. Maters. Sci., 29(2) pp. 338-344 (1994).

Abstract: The microstructure of low density (0.13-0.64 Mg·m^-3) carbon-carbon composites was investigated using optical microscopy, scanning electron microscopy, and image analysis. All samples initially contained varying proportions of rayon precursor carbon fibres, recycled fibrous material and phenolic resin precursor matrix, and were manufactured utilizing a vacuum moulding technique. Some of the composites were densified using the chemical vapour deposition (CVD) of pyrolytic carbon. All of the composites were shown to have a two-dimensional planar random microstructure, with a distinct layering effect being seen on the microscopic (and sometimes macroscopic) level. The degree of laying in the composites was quantified utilizing image analysis and was found to be most pronounced in samples containing no recycled material, and least pronounced in samples containing all of its fibrous constituent as recycled material. The composites were found to be very porous, the pores consisting of mainly interconnecting open pores (typically 65-85% of the sample volume). In non-CVD samples the fibrous material was held together by thin (<5 mm) discrete “matrix bonds”, with a few large (typically 100 x 200 x 800 mm) fibre bundles also existing within the structure. In the CVD-processed material the deposit coat on the fibres was of even thickness throughout the composite and joined together fibrous material that was not previously in contact.

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