Electrochemical, hydrothermal, and electrochemical-hydrothermal synthesis of barium titanate thin films on titanium subs
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Well-crystallized polycrystalline films of cubic BaTiO3 have been synthesized on Ti metal substrates by electrochemical, hydrothermal, and electrochemical-hydrothermal reaction of a Ti metal with a saturated solution of Ba(OH)2 in the temperature range 80 °C-200 °C. The films can be produced in the electrochemical process by the anodization of a Ti substrate in Ba(OH)2 solution at pH = 13 and current densities of the order of 10-50 mA/cm 2 . The films can also be synthesized by the hydrothermal reaction of Ba(OH)2 and a Ti substrate without electrical bias. A combination of these two techniques reduces the reaction time to 30-45 min. The films were found to have good adherence on the substrate and films of thickness up to 2 /urn could be formed within a reaction time of 45 min. Capacitance measurements have yielded a dielectric constant of 300. The films prepared by the three different techniques have been compared for crystallinity and microstructure. The thickness of the films increased with temperature and time of reaction. The crystallinity decreased with extended time of reaction for a given temperature. It is likely that a reactive intermediate such as Ti(OH)4 (aq.) is formed in all the three cases which leads to the formation of crystalline BaTiO3.
I. INTRODUCTION Ferroelectric BaTiO3 thin films have gained importance in the electronics industry because of their application as capacitors and thermistors. A number of techniques such as sputtering1^1 and vacuum evaporation5'6 have been used to prepare BaTiO3 films. Most of these methods need high temperature to form ferroelectric BaTiO3 films. The high temperature treatments cause the reaction of the film with the substrate and sometimes lead to loss of adherence. Recently, Xu, Shaikh, and Vest have reported a metallo-organic decomposition method to prepare BaTiO3 thin films.7 This method also used high temperature (greater than 500 °C) to remove the organics. Sol-gel methods have also been reported for the preparation of BaTiO3 thin films.8 On the other hand, electrochemical methods are increasingly being used for the preparation of ceramic thin films.9-10 These methods offer the advantage of simple instrumentation, control of thickness, low preparation temperature, and high purity. Thin films of barium and strontium titanates were synthesized using the electrochemical-hydrothermal technique.11-12 In this technique, a titanium substrate was electrolyzed in a solution of Ba salt electrolyte and the electrolysis was carried out under hydrothermal conditions at temperatures ranging from 100-250 °C. In another experiment, it was also found that at elevated temperatures (400-600 °C), it was possible to produce tetragonal BaTiO3 films by hydrothermal reaction of Ti with a saturated solution of Ba salts.13 ForJ. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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mation of BaTiO3 films on polymer and glass substrates also has been reported recently.14'15 In this context, we report studies on the various conditions under whic
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