Use of sol chemistry and fine grained precursors in the production of controlled microstructural polycrystalline continuous oxide fibres
AbstractNovel mullite fibres have been produced on a laboratory scale by pyrolysis of sol-gel spun precursors, having nanosized grains or discrete atomic dispersion, stabilised by in-situ precipitation of a minor phase addition of zirconia. A complex blend of chloride free precursors in aqueous media enabled good extrusion and drying characteristics, exhibiting Newtonian flow behaviour and draw down to fired fibre diameters, typically in the range of 10 - 20~m, the finest being almost defect free. The inorganic salt/alkoxide precursor ratio was optimised to 55:45 weight % addition after firstly using commercial grade precursors and then further optimised using lab produced aluminium acetotartrate (AA T) in order to remove unwanted alkali and alkaline earth oxide impurities. Various colloidal silca sols were also evaluated, the finest particle size (7nm) preferred for extrusion and sintering properties. Flow characteristics (rheology) and ageing with time were determined by cone and plate rheometry on three different sol concentrations, the highest concentration was found to age (thicken) faster but was easily extruded to make fibre over a 5 week period due to its shear thinning Newtonian flow behaviour. Various spectroscopic and microscopic techniques including rcp, XRF, XRD, solid state NMR, SEM and TEM were employed to determine the purity, oxide phase evolution and microstructural stability with temperature and time. During conversion of the sol-gel fibres to the polycrystalline fibre SEM imaging and elemental analysis showed that careful heat treatment was necessary to remove volatile components such as sulphate in order to avoid large residual porosity and week fibres. Densification of fibres between 900 - 950°C was critical, as up to 26% linear shrinkage would result. The formation of nano porous y-alumina was apparent from low angle XRD scans and concurred from Al27 solid state NMR analysis which manifested itself as deformations in the fibre longitudinally resulting in "kinks" further exacerbated during sintering and mullite formation above 1200°C. Si29 NMR confirmed that tetrahedral peaks at 11 Oppm between 990 - 11 OO°C were due to heterogeneous colloidal silica, which subsequently reacted to form a fine stable orthorhombic mullite (3AI 20 3.2Si02) above 1200°C in and around which the zirconia existed in the tetragonal form, as defined by XRD analysis. TEM imaging demonstrated that the mullite microstructure had been stabilised and porosity removed due to in-situ precipitation of zirconia and subsequent sintering. The microstructure was compared to 3M Nextel 720 mullite/alumina fibre and found to be of similar dimensions. Optimisation of the zirconia addition was found to be 5% by weight, which also allowed the fine microstructure to be maintained without severe grain growth up to 1400°C. A relatively slow firing rate was shown to almost half the size of the mullite crystals due to controlled sintering and densification, although commercially firing rates of several hundred degrees per hour are more desirable. Such fibres exhibited an average tensile strength of 3.4GPa after heat treatment to 1250°C and superior Dicarlo ratio creep rate properties at and above this temperature compared to the 3M Nextel 720, the best commercial fibres that were currently available on the market. Discussions with QinetiQ (Famborough) are being held with the aim of exploiting the sol chemistry within a development project with the ambition of scale up from lab to production scale in order to supply fibres for fabrication of ceramic matrix composites.
PublisherUniversity of Wolverhampton
TypeThesis or dissertation
DescriptionA thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy
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