Comparison of powder synthesis routes for fabricating (Ba 0.65 Sr 0.35 )TiO 3 ceramics
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Powders of Ba0.65Sr0.35TiO3 have been prepared by solution-based and conventional mixed-oxide routes. Powders made by freeze-drying a precursor solution of mixed catecholate complexes of BaTiO3 and SrTiO3 had the smallest particle size, but secondary grain growth during sintering negated the anticipated benefits of the nano-sized powder in relation to ceramic densification and microstructural control. Addition of manganese oxide suppressed secondary grain growth for catecholate powders, allowing 97% dense ceramics to be produced at a sintering temperature of 1300 °C, with grain sizes of 艋5 m. Doped mixed-oxide samples continued to show secondary grain growth, leading to coarse microstructures with grain sizes 艋100 m after sintering at 1400 °C. Curie peaks for catecholate samples were sharper than those for mixed-oxide samples, suggesting a more uniform distribution of Ba and Sr ions in the powders. Difficulties were encountered in controlling the (Ba + Sr)/Ti ratio of powders made by an oxalate solution-precipitation route.
I. INTRODUCTION
Ceramics in the BaTiO3–SrTiO3 solid solution system (BST) exhibit favorable dielectric properties at microwave frequencies making them candidate materials for use in wireless communications systems. The drive toward miniaturization has also created an upsurge in thinfilm BST applications, for example tunable filters. Pulsed laser deposition (PLD) has, in recent years, become a very popular means of depositing BST thin-films, and for this there is a requirement to fabricate dense high-quality BST pellets for use as targets. The traditional mixed-oxide means of ceramic fabrication employs starting powders of barium carbonate, strontium carbonate, and titanium dioxide, which are mixed and milled, followed by calcination at temperatures Ⰷ1000 °C to form the desired single phase by solid-state reaction. Alternative powder synthesis techniques, based for example on solution precursors, can in principle attain smaller particle sizes and better control over chemical homogeneity. Interest in wet-chemical processes for producing electroceramic powders dates back to the 1950s with the publication of a method for BaTiO3 powder based on co-precipitation from oxalic acid solutions.1 It has developed to become a very successful commercial route for the production of high-purity BaTiO3 powders. The principle of the oxalate route is common to many subsequent a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0175 1390
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 6, Jun 2006 Downloaded: 16 Mar 2015
methods, namely an aqueous solution of the requisite ions is prepared, from which an intermediate solid phase is isolated by means of a chemical reaction, e.g., complexation or hydrolysis, and then thermally decomposed to form the desired oxide powder.2 Other examples of wet-chemical routes for titanate powders include those based on peroxy complexes,3 carboxylic acid-dihydroxy alchohol gels,4 alkoxide complexes,6,7 and dihydroxy pheno
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