Single-step fabrication of potassium tantalate films by hydrothermal-electrochemical method at lower temperatures
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Potassium tantalate thin films (KT) were hydrothermal-electrochemically and electrochemically synthesized on tantalum substrates under galvanostatic conditions in KOH solutions at temperatures from 50 to 150 °C. The pyrochlore structures were characterized by x-ray diffraction and scanning electron microscopy for the films formed under a variety of conditions. It was found that the reaction temperature, potassium hydroxide concentration, and current density significantly affect the formation and the morphology of the KT films. When the reaction temperatures were higher than 80 °C and the KOH concentrations were greater than 2.0 M, very crystalline films with excellent film flatness and good adherence on the substrate were obtained. Based on the experimental results, it was confirmed that the formation and growth of KT films by the hydrothermal-electrochemical and electrochemical method follow a dissolution-crystallization mechanism.
I. INTRODUCTION
The hydrothermal-electrochemical method offers an inexpensive and environmentally friendly route to synthesize thin (or thick), shaped, sized, and oriented ceramic films and coatings in aqueous solution in one step for a variety of applications.1–4 This method benefits the electrochemical anodic oxidization of the metal substrate in an aqueous alkaline solution under hydrothermal conditions. Some materials could be prepared simply by electrochemical methods at room temperature or nearroom-temperature without using sealed pressure vessels. Various double oxides films or powders, such as BaTiO3, SrTiO3, PbTiO3, CaTiO3, LiNbO3, BaWO4, LiCoO2, and LiNiO2, etc.,1–7 have been fabricated by this technique by our group. Recently, we also developed a flow cell system for the hydrothermal-electrochemical fabrication of ceramic films.8–10 This technique uses the solution flow under the hydrothermal-electrochemical conditions and makes it possible to make multilayered structure of films by only changing the channel of the solution flow. Potassium tantalate (KT) has been extensively investigated because of its technological importance and fundamental interest.11–21 Known as incipient ferroelectric materials, KT films have been used in electric capacitors for dynamic random-access memories.22 KT films also exhibit high-temperature protonic conductive properties
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J. Mater. Res., Vol. 15, No. 5, May 2000 Downloaded: 05 Apr 2015
when they are doped with a rare-earth element, which attracts considerable interest for possible applications to fuel cells, steam electrolyzes, and humidity sensors.23–25 The KT film and related compounds have previously been prepared using pulsed-laser deposition,26 liquidphase epitaxy,27 and the sol-gel process.28,29 However, these techniques are all capital and energy intensive and require either high vacuum or high temperature. Materials with the pyrochlore structure type, existing over a wide range of natural occurrences and chemistry,
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