Low temperature/low pressure hydrothermal synthesis of barium titanate: Powder and heteroepitaxial thin films
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Barium titanate powder and heteroepitaxial thin films were successfully produced by hydrothermal routes at ambient pressure and temperatures less than 100 °C. This processing method provides a simple low temperature route for producing epitaxied barium titanate thin films on single-crystal SrTiO3 substrates and powders which could also be extended to other systems. A dissolution/reprecipitation growth mechanism also was proposed for the formation of barium titanate by this route using previously published aqueous stability diagrams. Repeated hydrothermal treatments improved film thickness and surface coverage at the expense of increased surface roughness.
I. INTRODUCTION Barium titanate is an important material in the electronics industry, particularly because of its high dielectric constant and ferroelectric properties, and thus, its application as a capacitor material and a piezoelectric transducer. !~3 It has also generated interested for applications such as chemical sensors and thermistors.4"6 Barium titanate powder has been traditionally prepared by either reacting powders at high temperatures or chemical methods (sol-gel), and formed as bulk polycrystalline materials from these powders by conventional powder consolidation and densification methods. 78 For applications requiring thin films, vapor deposition techniques (e.g., metalorganic deposition) have been employed. 910 All of these conventional methods synthesize and form BaTiO3 materials at high temperatures relative to the hydrothermal approaches that have recently been introduced. There is current interest in using hydrothermal routes to synthesize materials at relatively low temperatures." Much of the current work is based on earlier investigations in geological systems.12 Hydrothermal synthesis offers the advantages of simplicity, low cost, high product purity, and the ability to control particle size.13 Large quartz crystals have been grown by the hydrothermal method for more than 50 years.14-15 Barium titanate can be directly synthesized from an aqueous solution as powders13-16"17 and as polycrystalline thin films.18'19 Lilley and Wusirika16 patented the production of submicron BaTiO3 powder using the hydrothermal method by reacting TiO2 powder in a boiling, aqueous solution of barium hydroxide for ~48 h. Others have used titanium sources such as titanium metal, TiCU, and TiO2 gels at temperatures ranging form 100 °C to 400 °c. 1317 ' 20 Yoshimura et a/.18 reported forming dense, polycrystalline BaTiO3 thin films by electrochemically reacting a 1784 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 7, Jul 1995 Downloaded: 29 Jan 2015
titanium foil in a Ba(OH)2 solution at temperatures above 100 °C. Others have grown polycrystalline thin films on a range of substrates including glass, silicon, and polyphenylen sulfide (PPS).19'21'22 Hydrothermal heteroepitaxial growth of BaTiO3 on SrTiO3 single-crystal substrates was demonstrated by Kajyoshi et al.23 at temperatures between 400 and 800 °C and a pressure of 10 MPa. In their case the SrTiO
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