Development of Zirconia Electrolyte Films on Porous Doped Lanthanum Manganite Cathodes by Electrophoretic Deposition
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allows the zirconia electrolyte to be deposited in only a single step. Furthermore, the electrolyte film thus produced is of comparable quality to that obtained by the conventional, but more expensive, EVD process. EXPERIMENTAL PROCEDURE Two inch sections (22mm dia) of AES porous (30-35%) LDM cathode tubes were obtained from Siemens-Westinghouse, USA. After washing with ethanol, a fugitive graphite interlayer coating was applied by carbon sputtering on the outer surface of the porous ceramic tube, using an ordinary carbon coater. Onto this interlayer, a particulate zirconia film was later deposited by EPD. For EPD, a 5wt% stable suspension was obtained after ball milling and ultrasonication of washed 8mol% yttria stabilized zirconia powders (TZ-8Y brand, Tosoh) in pure glacial acetic acid (99.9% purity, J.T. Baker). Deposition was performed with a special electrophoretic cell designed and fabricated to allow film deposition on the outside of the tube but prevent simultaneous deposition in the inside of the tube. In the present investigation, EPD was performed in two types of samples - one set with, and one set without, graphite interlayer coatings on the LDM tube substrates. Hereafter, the LDM tubes without any interlayer (as received sample surface) will be referred to as WILDM, and samples where fugitive graphite interlayers are used will be denoted as GILDM. Electrophoretic deposition was performed on these two types of substrates in the above-described suspension using an applied field (Acopian high voltage power supply) within the range of 50-150V/cm for different times (30s-600s). Details of the suspension preparation, application of the fugitive interlayer onto ceramic substrates, and descriptions of the electrophoretic cell and electrophoretic deposition kinetics are reported in full elsewhere [6-10]. After electrophoretic deposition, the zirconia films were dried in an air oven at 60 0C and sintered in the temperature range between 1200 and 13000 C. Microstructures of both the fired and unfired coatings were observed under a scanning electron microscope (Philips XL 20) and an optical microscope. The zeta potential of the suspension was measured by a Coulter Delsa 400. RESULTS AND DISCUSSION Removal of unwanted surface ions present in the Tosoh brand powder was the first step in the preparation of a stable suspension with reproducible deposition characteristics. It was found that at least eight washing cycles were necessary to remove all the chloride ions present in the commercial precursors [6,8]. The same washed powder was used for all the deposition experiments. The zeta potential of the acetic acid + zirconia powder suspension was A4OmV, with the particles being positively charged. The positive charge on the particles' surface then allowed them to deposit on the outer surface of porous ceramic tube (cathode) when the latter was negatively charged. The purpose of applying the fugitive graphite interlayer was not to make the porous ceramic cathode (LDM) conducting. The cathode already had sufficient condu
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