Hydrothermal Processing of BaTiO 3 /Polymer Films
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ABSTRACT Hydrothermally derived films of BaTiO 3 were fabricated by reacting thin layers of titanium organometallic liquid precursors in aqueous solutions containing Ba(OH) 2 and having a high pH. Cubic submicron polycrystalline films of BaTiO 3 (thickness = 1 jim) were formed at 70'C. Low concentrations of block copolymers of polybutadiene and polystyrene were incorporated into the liquid precursor to prevent precursor film cracking. Higher polymer concentrations allowed fabrication of polymer/ceramic composite films by virtue of the low temperature used in hydrothermal processing.
BACKGROUND Over the last decade efforts have increased to develop practical analogs to organic-polymer/ceramic composites found in nature. 1 Composites of polymers and ceramics are used in a wide variety of applications. Ceramic particles are introduced into elastomers to enhance the polymer's mechanical properties, 2 while composites of polymers with SiO 2 or TiO 2 are being evaluated as waveguides, 3 and for other optical applications. 4 Below we discuss how one may use hydrothermal processing not only to fabricate ceramics, but also to form thin film composites containing polymers and ceramics. Small additions of Kraton D1102*, a block copolymer of polystyrene and polybutadiene (70% polybutadiene by weight, MW 30,000) were used to maintain the mechanical integrity of low molecular weight precursor films during drying, while larger additions led to the formation of a colloidal dispersion of BaTiO 3 particles in a polymeric matrix after hydrothermal processing. Hydrothermal processing is a route to fabricate ceramics below 100'C. It involves the formation of crystalline materials from reactants in an aqueous medium under strongly alkaline conditions. Hydrothermal processing is used to fabricate large single crystals of quartz via seeding, and in the formation of zeolites from aluminosilicate gels. 5 In 1988, Lilley and Wusirika fabricated monosized powders of BaTiO 3 by dispersing TiO 2 powders in a concentrated solution of Ba(OH) 2 .6 Dogan et al. used transmission electron microscopy (TEM) to observe the formation of hydrothermally derived BaTiO3 powders in a solution of Ba(OH) 2 with TiO 2 particles. 7 Their observations showed that the TiO 2 initially dissolved in the aqueous Ba(OH) 2 , and led to the nucleation of nanometer-sized BaTiO3 particles in cubic phase.
Shell Chemical Co., Belpre, OH 45714 63 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
Calculations defining the thermodynamics of hydrothermal processing were performed by Lencka and Riman. 8 They derived stability diagrams outlining the requirements to synthesize BaTiO 3 among other materials. Their results showed that successful fabrication of BaTiO 3 required a pH > 12, and a barium concentration on the order of 10-4 molal or higher. The modelling showed that it is possible to fabricate BaTiO 3 at room temperature, although this has yet to be done experimentally. Finally, they demonstrated the importance of eliminating CO 2 from the reaction
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