Morphological evolution of barium titanate synthesized in water in the presence of polymeric species
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Morphological evolution of barium titanate synthesized in water in the presence of polymeric species Roger B. Bagwell, J¨urgen Sindel, and Wolfgang Sigmund Max-Planck-Institut f¨ur Metallforschung and Institut f¨ur Nichtmetallische Anorganische Materialien, Universit¨at Stuttgart, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, 70569 Stuttgart, Germany (Received 30 June 1998; accepted 12 November 1998)
The synthesis of barium titanate (BaTiO3 ) was investigated in water at 90 ±C in the presence of polymeric additives. Homopolymers (polyacrylic acid) and block copolymers (polyethylene oxide-block-polymethacrylic acid) were added during synthesis to influence particle morphology and size distribution. The polymers affected the morphological evolution of the forming powder by adsorbing preferentially on specific planes. The polymeric species additionally slowed the formation of barium titanate. The barium concentration also changed the morphology, particle size, and other powder characteristics.
I. INTRODUCTION A. Barium titanate
Barium titanate is an important material in the electronic ceramics industry due to its high dielectric constant and ferroelectric properties.1 Barium titanate synthesis is possible by mixed oxide calcination, solution synthesis, and a number of other techniques.2–12 Producing homogeneous, easily dispersible powder is critical, leading to a greater interest in the hydrothermal synthesis of barium titanate and the corresponding potential for better control of particle size, distribution, morphology, and degree of aggregation. Solution synthesis techniques have also been shown to be effective for production of barium titanate thin films.2,3 Control of solution conditions is necessary for the formation of phase pure materials in solution and is particularly important for barium titanate. Thermodynamic calculations10,11 have shown that the formation of barium titanate in water is favorable in regions of barium concentrations .1024 M and pH . 12, with barium titanate powder yields greater than 99%. The solution conditions are critical to stay in the region where barium titanate is thermodynamically stable and also to ensure titanium solubility in the form of Ti(OH)x 42x species. Temperature is another important variable, particularly concerning the kinetics of the reaction.2 A carbon dioxide-free environment is also necessary to avoid barium carbonate precipitation.10,11 Many authors agree that the formation of barium titanate from solid titania precursors proceeds by the dissolution of the titanium as hydroxylated species followed by precipitation of the barium titanate.2–4 The source for the titanium in the reaction can be amorphous gels, solid titania powder, or titanium organometallics,2–12 with the less crystalline materials typically reacting more rapidly. The solid titania particles also act as heterogeneous nucleation sites in the case of powder precursors.3,4 As the 1844
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J. Mater. Res., Vol. 14, No. 5, M
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