Deposition of inorganic bronze coatings over ceramic foams

摘要

A method for applying conductive inorganic tungsten bronze (Na_xWO_3) 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 Na_2WO_4, WO_3, and NaCl. The reticulated ceramic foams, made from pure alumina (A1_2O_3), were prepared by a standard impregnation method. In all depositions, the temperature was above the melting point of the Na_2WO_4/WO_3 system. The effect of NaCl content on the final coating properties was investigated in the 10-40 mol percent range. The best results were obtained with hydrogen reduction of 40 percent NaCl in the melt. The bronze crystals size in the coating was 0.8-1.6 |xm. 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 ft mm to below 1 OMEGA mm when the NaCl concentration in the melts increased from 10 percent to 40 percent, 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.