Work by previous investigators has shown that BaTiO, films can be synthesized from solution over temperature ranges from 80°C to greater than 200°C. In the present work, electrically insulating crystalline films of BaTiO, have been electrochemically deposited on titanium substrates at temperatures as low as 55°C. Auger spectroscopic analyses with depth profiling indicate that a titanium oxide layer whose thickness is governed by current density acts as a precursor to BaTiO,. Formation of BaTiO, is found to be favored only in highly alkaline solutions. This is consistent with the phase stability reported for the Ba-Ti-C0,-H,O system at 25°C. Lower processing temperatures (55°C) favor the formation of thick, electrically resistive, and wellcrystallized BaTiO, films, apparently due to increased oxygen solubility in the electrolyte solution. Films produced at 100°C are much thinner and are electrically conductive due to fissures and pores in their microstructure. Initial studies on the effect of current density indicate the formation of thinner and porous films with thicker titanium oxide intermediate layers.
Polycrystalline BaTiQ thin films of approximately 1 ~m thickness have been synthesized on Ti substrates by an electrochemical process using nonalkali electrolytes, at temperatures as low as 55~The effect of various processing parameters, such as solution chemistry, atmosphere, quality of substrate surface, applied voltage, temperature, and reaction time have been discussed. Film thickness and uniformity increase with reaction time up to 24 h. The quantity of total electric charge passed through the electrolytic cell is found to govern the film thickness and uniformity. Dielectric characteristics of the as-prepared and heat-treated films have also been reported. Use of tetraethylammonium hydroxide, a nonalkali base reagent to adjust solution pH, has resulted in films having improved dielectric properties as opposed to the films prepared with alkali metal bases such as NaOH. Heat-treatment further improves the dielectric properties through a phase transition to the thermodynamically stable tetragonal phase from the as-prepared pseudo-cubic phase as confirmed by the x-ray diffraction data.
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Preparation of BaTiO3 thin films on titanium substrates by hydrothermal-electrochemical methods at temperatures ranging from 80 to 200°C have recently been reported in literature. In the present work, polycrystalline, homogeneous, and electrically insulating films of BaTiO3 have been electrochemically synthesized at temperatures as low as 55°C. Effects of various electrochemical and thermodynamic parameters such as temperature, solution pH, electrolyte composition, current density, and atmosphere on phase stability and film characteristics have been investigated. Formation of BaTiO3 is favored only under highly alkaline conditions. This finding is consistent with the reported phase stability criteria for BaTiO3 in Ba-Ti-CO2-H2O system. Auger spectroscopic analyses with depth profiling indicate a titanium oxide layer, whose thickness depends on current density, acts as a precursor to BaTiO3. Use of highly oxidizing atmospheres promotes the formation of thick, well-crystallized and electrically insulating films of BaTiO3 at low temperatures. Films synthesized at 55'C exhibit a characteristic bilayer structure consisting of a large grained porous top layer above a fine grained and dense bottom layer. In contrast, the films prepared at higher temperatures are uniform, fine grained but porous. Preliminary experiments indicate the feasibility to electrochemically synthesize BaTiO3 at temperatures near 25°C.
The environmental degradation of materials poses a serious limitation in the utility of engineering materials. The corrosion of metals, for example, has been estimated to represent a 4–5% decrease in the Gross National Product each year. To this, add losses involved in the replacement or restoration of ceramic structures such as buildings and transportation systems, i.e., the “infrastructure,” and the result is a significant sacrifice of economic strength.Most of us are familiar with the consequences of exposing materials to environments in which the materials are chemically unstable and convert into substances that are unable to perform the function for which the original material was selected. The corrosion of metals into soluble or insoluble oxidation reaction products, chain scission or molecular mutation in polymers, even hydrolysis and leaching of silicious ceramic compounds represent behavior which ultimately limits the service applications of most engineering materials. For example, aluminum and its alloys are unsuitable for use in environments where oxide formation rates are high enough to represent a problem with respect to useful service life.
Low Temperature Electrochemical Synthesis and Dielectric Characterization of Barium Titanate Films Using Nonalkali Electrolytes.-Phase pure polycrystalline BaTiO3 films of ∼ = 1 µm thickness are obtained by electrochemical deposition on a Ti substrate at 55 . degree.C using alkali-free electrolytes containing Et4NOH as base reagent. Phase stability, film uniformity, microstructure, and thickness depend on various electrochemical parameters. The film thickness, which increases with the reaction time, is determined by the total electric charge passed through the electrolysis cell. The improvement of the dielectric properties, due to the use of Et4NOH instead of NaOH, is further raised by heat treatment at 200 • C in air which induces phase transition from pseudo-cubic to tetragonal BaTiO3. -(VENIGALLA, S.; BENDALE, P.; ADAIR, J. H.; J.
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