Characterization of Vapor Deposited, NanoStructured Membranes
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Characterization of Vapor Deposited, NanoStructured Membranes Alan F. Jankowski1, Nerine J. Cherepy1, James. L. Ferreira1, and Jeffrey P. Hayes2 Lawrence Livermore National Laboratory 1 Chemistry & Materials Science, and 2Mechanical Engineering Livermore, CA 94551-9900, U.S.A. ABSTRACT The vapor deposition methods of planar magnetron sputtering and electron-beam evaporation are used to synthesize materials with nanostructured morphological features that have ultra-high surface areas with continuous open porosity at the nanoscale. These nanostructured membranes are used in a variety of fuel cells to provide electrode and catalytic functions. Specifically, stand alone and composite nickel electrodes for use in thin film solidoxide, and molten carbonate fuel cells are formed by sputter deposition and electron beam evaporation, respectively. Also, a potentially high-performance catalyst material for the direct reformation of hydrocarbon fuels at low temperatures is deposited as a nanostructure by the reactive sputtering of a copper-zinc alloy using a partial pressure of oxygen at an elevated substrate temperature. INTRODUCTION The use of vapor deposition technologies improves the performance and figure of small solid-oxide fuel cells.[1,2] The development of porous electrodes using micro electronic fabrication methods [3-6] enabled the development of MEMS-based [6-8] thin-film fuel cells. For example, a thin-film solid-oxide fuel cell was micro-fabricated with a porous nickel anode and a zirconia electrolyte.[7] Testing was conducted with air for the oxidant and a 2-6 cc·min-1 flow of 4% H2 for the fuel. A maximum power output of 145 mW·cm-2 at 0.4 Volts was produced from a 2 mm sq test cell at 873 K. The sputter deposition process can be used to create nanostructured membranes that possess continuous open porosity. The deposition parameters needed to control the porous nanostructure are found to be tractable for many materials. In general, structural morphologies found for conventionally sputtered coatings can range from porous columnar to dense polycrystalline. The transition in morphology through four zones of growth type occurs with increasing substrate temperature and sputter gas pressure. “Zone 1” has a structure consisting of tapered crystallites separated by voids. A transition “Zone T” has a structure consisting of densely packed fibrous grains and a smooth surface. “Zone 2” features continuous columns from the substrate to a surface characterized by crystalline facets. Lastly, “Zone 3” represents the recrystallized grain structure. The primary effect of increased temperature is an enhancement of surface and bulk diffusion. The new growth zone found for the stabilization of a porous nanostructure appears to be a variant of “Zones 1-2”. A three-dimensional polycrystalline deposit with continuous open porosity was produced under the general conditions of an increased working gas pressure and a substrate temperature approximately half the absolute melting point. The objective to further integrate hydrocarb
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