Chemical processing of (Ba, Sr)(Ti, Zr)O3 perovskite ceramics

Abstract

Stoichiometric high-purity submicrometre-size perovskite powders have long been of interest for the preparation of dense ferroelectric bodies, thin film electronic components piezoelectrics and electro-optic materials. In recent years, numerous methods of preparing fine perovskite powders have been developed, such as the thermal decomposition of oxalates, citrates and carbonates [1-3], the organic sol-gel technique from metal alkoxides [4-6] and more recently the metallo-organic decomposition [7, 8]. The organic techniques which lead to homogeneity, stoichiometry and high purity in fine particles, offer a great flexibility because of the large variety of organic precursors that are available. However, concerning the very interesting barium or strontium perovskites, the use of the expensive alkaline earth alkoxides is generally very inconvenient because of their high reactivity to moisture and of the high molecular complexity which decreases their solubility in alcohols. We have studied the synthesis of alkaline earth perovskite ceramics from a sol-gel route, previously proposed by Kiss et al. [9] for barium titanate, in which titanium and zirconium alkoxides are hydrolysed by an aqueous solution of barium and/or strontium hydroxides. The method first makes possible the synthesis of a cubic perovskite phase from 100 ° C [10, 11], second, a low sintering temperature (1150°C in the barium titanate) of the powder compact to near theoretical density, and finally the preparation of uniform grain-size bodies. Fig. 1 shows the diagram of preparation of perovskites by the alkoxide-hydroxide method. The organic mixture containing titanium and/or zirconium normal butoxides and 2-butanol in the equimolar proportions is hydrolysed, at about 80 ° C, in a carbon dioxide-free atmosphere, by an aqueous solution containing barium and/or strontium hydroxides with a concentration of about 2.5moll-L For all the compositions, doping elements (Nb, Sb, La, etc.) can be easily introduced by mixing alkoxides in inorganic salts with the organic solution of butoxides. Reactions which take place during this method have been previously described by Flashen for the barium titanate [10]. When a pH of 11 to 14 is maintained, in the presence of hydroxyl ions, four molecules of water and two hydroxyl groups attach through their oxygen atoms to the titanium of Ti(OR)4 in a nucleophilic process and, following an electronic rearrangement, four molecules of alcohol are expelled (Fig. 2). The result of the initial step is an anion of the formula Ti(OH)~ or Zr(OH)~ which is neutralized by the Ba 2+ or Sr 2+ ions in the solution following Equation 1.