Electrochemical synthesis of barium titanate thin films
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Electrochemical synthesis of barium titanate thin films R. R. Bacsa,a),b) G. Rutsch,c) and J. P. Dougherty Center for Dielectric Studies, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 19 January 1995; accepted 22 September 1995)
Polycrystalline films of barium titanate (BaTiO3 ) have been synthesized on titanium (Ti) substrates by the galvanostatic anodization of Ti in a solution of 0.4 M Ba(OH)2 . Crystalline films are formed at temperatures under 100 ±C within 10 min at a current density of 25 mAycm2 at atmospheric pressure. Crystallinity of the films is found to increase with both current density and time of reaction. At 90 ±C, a film of 1 mm thickness is formed after 10 min; grain sizes up to 0.5 mm are obtained. Microstructure of the films is found to be critically dependent on the pretreatment of the titanium anode. Capacitance measurements on the film yield a dielectric constant of 200 with a minimum tan delta of 0.09 at 10 kHz. On the basis of the voltage-time curves, it is interpreted that an amorphous intermediate is formed which crystallizes to form perovskite BaTiO3 .
I. INTRODUCTION
The electrochemical-hydrothermal method has been employed by several workers in the past for the synthesis of crystalline barium titanate thin films at temperatures much lower than other methods of thin film preparation, such as sputtering and sol-gel which require a temperature of 400–500 ±C for crystallization of the films.1–5 In an earlier communication6 we have reported the formation of barium titanate thin films by the electrolysis of a titanium anode at temperatures under 100 ±C. The formation of barium titanate required that the pH of the solution was over 13. Following this, Bendgale et al.7 have reported the electrothermal synthesis of barium titanate at temperatures as low as 55 ±C with the addition of NaOH at low current densities for long time (maximum 2.5 mAycm2 for 24 h). They have also found that the cell voltage remained less than or equal to 3 V during the reaction. However, our experiments have shown that at increased current densities (25 mAycm2), highly crystalline barium titanate films with dielectric constants comparable to those obtained by hydrothermal methods8,9 are formed within 10 min. The voltage-time curves show a steep increase in voltage up to 30 V in contrast to the low current density results. Crystalline barium titanate is formed both with and without the addition of NaOH. The voltage-time curves have been traced at different current densities and temperatures from 75–90 ±C, indicating the formation of an insulating oxide within 5 s. In this article, the preparation and dielectric properties of electrochemically formed barium
titanate films have been reported. The voltage-time curves have been discussed in light of the existing models for anodization of titanium metal in basic solutions. II. EXPERIMENTAL A. Sample preparation
Titanium foils of 0.5 mm thickness and 99.6% purity
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