Deposition of inorganic bronze coatings over ceramic foams
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A method for applying conductive inorganic tungsten bronze (NaxWO3) coatings over ceramic foams is presented. The general basis of bronze coating involves an intermediate layer between the ceramic surface and the redox external agent (liquid or gas phase). The intermediate layer is a multifunctional element and its function depends on the agents involved in redox reaction. The bronze film is formed either on the intermediate layer or in place of it. The initial melt included Na2WO4, WO3, and NaCl. The reticulated ceramic foams, made from pure alumina (Al2O3), were prepared by a standard impregnation method. In all depositions, the temperature was above the melting point of the Na2WO4/WO3 system. The effect of NaCl content on the final coating properties was investigated in the 10–40 mol% range. The best results were obtained with hydrogen reduction of 40% NaCl in the melt. The bronze crystals size in the coating was 0.8–1.6 m. The bronze layer was continuous throughout the foam. The obtained coating has a bright yellow-gold color, consistent with a stoichiometric parameter value (x) of 0.85–0.95. The resistivity of coated foams decreased from above 12 ⍀ mm to below 1 ⍀ mm when the NaCl concentration in the melts increased from 10% to 40%, respectively, due to improved continuity of the bronze crystals. These highly conductive, bronze-coated alumina foams, can potentially serve as high-temperature electrodes in aggressive conditions.
I. INTRODUCTION
Ceramic foams are being used in numerous industrial applications such as filtration molten metals at elevated temperatures, light thermal insulation, catalytic supports with high surface area, etc. These applications are based on the specific foam properties that include low density, low (or high) thermal conductivity, high permeability, thermal stability and high resistance to chemical attack. Often, the ceramic foams are produced from silica, cordierite, alumino-silicates, i.e., mullite.1–4 Recently, we focused on the preparation of light alumina foams5–7 and modification of their surface properties.8–10 The typical ceramic foams mentioned above are dielectric, which prevents their usage as conductors. To facilitate this application, the foams have to be made either from conductive ceramics or to be coated with stable conductive layers. The main thrust of this work is the coating of ceramic foams with a conductive layer, which is stable at high temperatures and aggressive environments.11,12 The coating material was selected to be an inorganic tungsten bronze with unique electrical conductivity, thermal and chemical stabilities. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0148 J. Mater. Res., Vol. 20, No. 5, May 2005
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The bronzes are nonstoichiometric compounds with an empirical composition of MxTO3; where M is the alkali metal, x has some value between 0 and 1, and T is tungsten, molybdenum, vanadium, etc. The tungsten bronzes have been extensively studied
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