Preparation of (Pb,Ba)TiO 3 powders and highly oriented thin films by a sol-gel process
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Solid solution Pb1-xBaxTiO3, with particular emphasis on Pb0.5Ba0.5TiO3, was prepared using a sol-gel process incorporating lead acetate trihydrate, barium acetate, and titanium isopropoxide as precursors, acetylacetone (2,4 pentanedione) as a chelating agent, and ethylene glycol as a solvent. The synthesis procedure was optimized by systematically varying acetylacetone: Ti and H2O:Ti molar ratios and calcination temperature. The resulting effects on sol and powder properties were studied using thermogravimetric analysis/differential scanning calorimetry, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller analysis, and x-ray diffraction (XRD). Crystallization of the perovskite structure occurred at a temperature as low as 450 °C. Thin films were prepared by spin coating on (100) MgO. Pyrolysis temperature and heating rate were varied, and the resultant film properties investigated using field-emission scanning electron microscopy, atomic force microscopy, and XRD. Under optimized conditions, highly oriented films were obtained at a crystallization temperature of 600 °C.
I. INTRODUCTION
It has recently been shown that the ferroelectric tetragonal distortion of BaTiO3, in which the a and c lattice parameters differ by 1.1%, can be used for actuation.1 Strains of approximately 0.9% have been demonstrated. Even greater strain is expected for PbTiO3, which exhibits a tetragonality of 6.3%. However, this material requires a large coercive field to induce domain wall motion and is prone to brittle fracture. Solid solutions of Pb1-xBaxTiO3, or PBT, with intermediate composition will presumably present a compromise between large actuation and probability of mechanical failure. In order for PBT to be successfully used in microactuation applications and also be integrated into silicon device technology, it is necessary to synthesize high-quality epitaxial thin films at low temperatures. Furthermore, if crystallization is carried out below the Curie temperature, structural and microstructural changes associated with the cubic to tetragonal phase transition can be eliminated. Though there are several widely used techniques for fabricating oxide thin films, including physical vapor deposition, ion beam sputtering, electron beam evaporation, and pulsed laser ablation, the sol-gel method has been selected here because of its potential for low-temperature crystallization and the possibility of low-cost fabrication. In the sol-gel process, controlled hydrolysis of dissolved a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0199 1492
J. Mater. Res., Vol. 19, No. 5, May 2004
metalorganic precursors followed by a condensation reaction results in the formation of a three-dimensional network of particles.2 Key challenges in the sol-gel synthesis of PBT are the identification of a solvent system in which multiple metalorganic precursors are mutually compatible and the preparation of a sol stable against uncontrolled hydrolysis. Moreover, though sol-gel and other solution tech
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