ADVANCED CORROSION RESISTANT MATERIALS FOR ELECTROCHEMICAL APPLICATIONS

DEPOSITION OF THIN DIAMOND FILMS BY PLASMA ASSISTED CHEMICALVAPOR DEPOSITION (PACVD) IS A RAPIDLY EMERGING AREA OF TECHNOLOGY BEING PURSUED IN THE UNITED STATES, JAPAN AND THE SOVIET UNION. DIAMOND FILMS HAVE UNIQUE CHEMICAL AND PHYSICAL PROPERTIES SUCH AS OXIDATION RESISTANCE AND HARDNESS. BORON-DOPED DIAMOND FILMS GROWN BY PACVD CAN HAVEELECTRICAL RESISTIVITIES APPROACHING 1 OHM-CM. PORE-FREE, ADHERENT FILMS OF LESS THAN 1-MICRON THICKNESS HAVE ALREADY BEEN DEPOSITED ON TUNGSTEN SUBSTRATES. WE PROPOSE TO EVALUATE THESE FILMS AS AN ELECTRICALLY CONDUCTIVE, CORROSION-RESISTANT COATING FOR BIPOLAR SEPARATOR PLATES (CARBON-CARBON COMPOSITES) IN PHOSPHORIC ACID FUEL CELLS (PAFC). PRESENT PLATE DESIGNS FAIL BECAUSE OF CORROSION OF THE CARBON FILLER AND ACID ABSORPTION BY THEPLATE WHICH LEADS TO EVENTUAL SHORTING OF THE CELL. THIN DIAMOND FILMS DEPOSITED ON THE BIPOLAR PLATES, IN ADDITION TO BEING CORROSION RESISTANT, CAN ALSO ACT AS A BARRIER LAYER, THUS PREVENTING ACID ABSORPTION. DURING PHASE I, PACVD REACTOR PROCESS CONDITIONS AND SUBSTRATE PRETREATMENTSWILL BE OPTIMIZED TO DEPOSIT PORE-FREE, ADHERENT, CONDUCTIVEFILMS ON THE CARBON-COMPOSITE PLATES. THE FILMS WILL BE CHARACTERIZED AFTER DEPOSITION AND THE CORROSION PROPERTIES AND ACID ABSORPTION OF THESE COATED PLATES IN HOT CONCENTRATED PHOSPHORIC ACID WILL BE DETERMINED. THE FILMS WILL ALSO BE CHARACTERIZED AFTER CORROSION TESTING FOR ANY CHANGES IN ELECTRICAL CONDUCTIVITY OR MORPHOLOGY.