Browsing by Author "Gillen, AL"

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    Colloidal processing of zirconium diboride ultra-high temperature ceramics
    (John Wiley and Sons, 2013-05-21) Tallon, C; Chavara, DT; Gillen, AL; Riley, D; Edwards, L; Moricca, SA; Franks, GV
    Colloidal processing of the Ultra-High Temperature Ceramic (UHTC) zirconium diboride (ZrB2) to develop near−net-shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB2 suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex-shaped components that can be densified at lower sintering temperatures without pressure. © 2013, The American Ceramic Society.
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    The effect of extreme temperature in an oxidising atmosphere on dense tantalum carbide (TaC)
    (Springer, 2013-01-01) Lashtabeg, A; Smart, M; Riley, D; Gillen, AL; Drennan, J
    This study describes the microstructure development as dense tantalum carbide (TaC), which is subjected to extreme temperature environments (3,000 °C) in the presence of oxygen. These are conditions that structural materials are expected to experience in hypersonic aero-propulsion applications. The conditions produce molten oxide which may provide a temporary resistance to rapid oxidation and may go some way to repair thermal shock cracks, however, at the same time the liquid is observed to attack the dense ceramic both chemically and mechanically. A reaction mechanism is suggested which involves dissolution of TaC in the oxide melt and a two step oxidation; first the reaction of TaC with oxygen to form Ta(O,C) and TaO x , resulting in dissolved dissociated carbon, followed by the reaction of dissolved carbon with oxygen to produce gas. This microstructural analysis of one of the candidate ultra-high temperature ceramic materials for hypersonic flight provides new insight into the mechanism of TaC oxidation and the role of the liquid oxide layer in acting not only as a protective layer to further oxidation, as is commonly reported, but also as a dynamic component that promotes erosion of the TaC surface and is a source of further oxygenation of the TaC surface. If the formation of the liquid phase can be better controlled and the reaction of the liquid phase with the matrix be slowed and stabilised, then the formation of a liquid phase at the surface of TaC may provide a key to designing materials that can withstand the rigours of hypersonic flight. © 2012, Springer.
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    HIPed tailored hollandite waste forms for the immobilization of radioactive Cs and Sr.
    (Wiley-Blackwell, 2009-05) Carter, ML; Gillen, AL; Olufson, KP; Vance, ER
    Hot isostatically pressed tailored hollandite waste forms were used to demonstrate the immobilization of Cs and Sr are separable from spent nuclear fuel, as well as Ba and Rb. Four hollandite formulations were investigated, two samples with ∼12 wt% waste loading (on an oxide basis) and two with ∼18 wt% waste loading. Two of the samples were Al-substituted and the other two contained Mg. The hollandite in the Al-substituted samples contained all the waste cations, as designed, but this was not the case in the Mg-substituted samples. The hollandite in the Mg-substituted samples did not contain all the waste cations, with ∼50% of the Sr forming SrTiO3 as a secondary phase. This resulted in waste forms that were not as durable, with respect to Cs, as their Al counterparts. The formation of SrTiO3 had little effect on the Sr release rates and was not detrimental to the Mg-substituted hollandite waste form. For the Al-substituted samples, the MCC-1 normalized release rates were <0.06 g·(m2·day)−1 at 0–28 days for all elements, while the Cs release rates remained at 2.0 g·(m2·day)−1 at 0–28 days for the Mg-substituted samples. © 2009, Wiley-Blackwell