Microstructural variations in mullite ceramics derived from diphasic sols using different forming techniques
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A.R. Boccaccini Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, United Kingdom
Mullite ceramics with controlled microstructure in terms of residual glassy phase and density were produced from diphasic silica/boehmite sols using pressure filtration, die-pressing, and extrusion techniques. The effect of the compaction process on the green and sintered densities and on the final mullite microstructure was investigated. It is shown that glassy-phase-free mullite with high green and sintered densities can be produced from engineered diphasic colloidal suspensions using pressure filtration. The results show that the green body formation technique strongly determines the final sintered density, but it has no effect on the final phase composition of diphasic sol-derived sintered mullites.
Mullite (3Al2O3 ⭈ 2SiO2) ceramics have a number of desirable properties for high-temperature applications, such as very good chemical and thermal stability, resistance to thermal shock and creep, low thermal expansion coefficient (4-5 × 10−6 C−1), and good high temperature strength.1–2 One of the widely used techniques in the fabrication of mullite ceramics is the colloidal processing route or socalled “diphasic sol” technique, which usually involves a suspension of boehmite (or alumina) and amorphous silica particles as starting materials.3–7 When the colloidal approach is used to prepare diphasic gels, orthorombic mullite can be formed at relative low temperatures, in the range of 1150–1350 °C via direct reaction of the alumina-rich and silica-rich components of the gel.3–7 Colloidal processing of diphasic mullite reduces the sintering temperature as a result of increased chemical homogenity, absence of large particle agglomerates, development of uniform green microstructures, and exploitation of viscous flow of the amorphous silica phase. The mullitization reaction rate is increased by using nano-sized alumina in the form of boehmite and amorphous silica particles, leading to the formation of shorter mass transport distances.
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 13 Mar 2015
Although significant progress has been made in the fabrication of mullite components derived from colloidal techniques for use at medium range temperatures (up to 800 °C), the most critical issue about the elimination of a residual glassy phase within the final sintered mullite suitable for use at high temperatures (>1200 °C) is still open to research. The fundamental question in diphasic colloidal processing of mullite remains therefore, “Which are the factors determining the formation of aluminosilicate glassy phase in mullite derived from diphasic sols comprising boehmite (␥–AlOOH), as a precursor for alumina, and amorphous silica particles?” To clarify this point, a better understanding of the interactions between nanoparticles having different surface charges with
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