Morphological and phase composition changes during sintering of ultralight Al 2 O 3 TiO 2 foams
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The morphological and phase composition changes during sintering of ultralight pure Al2O3 and Al2O3/TiO2 foams were investigated in the temperature range of 900–1600 °C in air. The titania (10 wt%) was incorporated during a nonhydrolytic foaming procedure to facilitate its uniform dispersion in the alumina matrix. The changes of linear shrinkage and related changes in effective density and porosity demonstrate a higher reactivity in the Al2O3/TiO2 foam. The formation of tialite phase in the Al2O3/TiO2 foams occurred below 1300 °C and remained stable during cooling under 1200 °C without stabilizers.
I. INTRODUCTION
Ceramic foams can be classified into open-cell and closed-cell materials.1 This classification is based on the state of membranes covering the face of the cells. In closed-cell foams an entire wall separates each cell from its neighbor, while in open-cell foams the walls are perforated. In reticulated foams the walls have all been completely removed leaving only the struts that form the interface between three or more cells. In reality, foam structures include a combination of these cell types, which determine the properties and applications of the foam. For example, reticulated ceramic foams, which have high permeability, are excellent high-temperature filters and catalyst supports, while closed-cell and partially open-cell foams are appropriate for thermal insulation.2,3 One of the critical stages in any ceramic technology is the sintering stage, which includes various processing parameters, such as heating profile and atmosphere. During sintering, two main phenomena occur simultaneously; the first is pore shrinkage and densification, which leads to dimensional shrinkage, and the second is grain growth.4 The evolution of microstructure during sintering of alumina and alumina/titania foams has not been previously studied. Knowing the sintering behavior of such foams is esential to their eventual application. One of our goals in the direction of new-generation ultralight ceramic foam development is to prepare a multicomponent foams with required improved properties. In this work, titania was chosen as the secondary component (dopping agent) due to its special chemical (catalytical) and thermal properties5 and compatibility with alumina in a wide range of concentrations and temperatures. Titanium dioxide exists in three crystalline J. Mater. Res., Vol. 17, No. 4, Apr 2002
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modifications: anatase, brookite, and rutile. The hightemperature rutile phase is used as a white pigment for paints on account of its high refractive index, relative chemical inertness, and abundance. The anatase phase is used in photocatalitic applications because of its photochemical activity. In addition, titania was chosen for dopping the alumina foam since we expected to form the tialite phase at a relatively low temperature, due to the high TiO2 dispersion and homogeneity in the foam. Aluminium titanate (Al2TiO5) is unique in its outstanding thermal shock resistance because
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