RHODIUM-TUNGSTEN ALLOYS FOR HIGH source. 1 he ratio of rhodium to tungsten was TEMPERATURE ELECTRODES DEPOSITED BY controlled by changing the temperature of the PHO_f OLYl IC CHEMICAL VAPOR DEPOSITION ctmmbcr holding the organometallics, to control ttm relative vapor pressures of the two materials. Rhodium and tungsten were both deposited from M.A. Ryan, A, Kisor, R.M. Williams, carbonyls, RhCOCl ancl W(CO)6. It has been reported B. Jcffries-Nakamura, and D. O’Connor that deposition from WF6 results in purer tungsten deposits than deposition from the carbonyl [1 O, 121; Jcl Propulsion Laboratory wc found cicposits from carbonyls to have inclusions California Institute of Technology of carbon, oxygen and CO, but annealing the Pasadena, CA 91109 deposited material under vacuum in the presence of a zirconium getter was sufficient to chive off inclusions. I.lcctrocfcs used in solid electrolyte devices such as high temperature fuel cells and direct conversion devices such as the alkali metal thermal-to-electric References converter (AMT EC) must meet several criteria for effective operation of the device. At the same time 1. R.M. Williams, El. Jeffries-Nakan-lura, Ml. thal. electrodes must be electrically conducting, they Underwood, 8.L, Wheeler, M. F. Loveland, S.J. must be thin or porous enough to allow fuel or Kikkert, J.1. lamb, ? .Cole, J.?, Kumn]crr and C.P. E\ankston, J. fkctrochern. Sot., 136, 893 (1989). operating fluid to pass through them. In addition, ttlc:jc clcctroc{cs are often operated at high ?, M.A. F{yan, B. Jeffries-Nakamura, R,M. Williams, temperatures in corrosive atmospheres, so they must M.L, Underwood, D. O’Connor and S. Kikkert, Proc. bc sufficiently refractory to allow operation over long .? 7th htcrsociet y Energy Conversion Engineering periods. Conference, SAE, 3, 7 (1992). 3. M,A. Ryan, B. Jeffries-Nakamura, R.M. Willianw, Previous work in this laboratory has shown that M,L. Underwood, D, O’Connor and S. Kikkert; Proc. alloys of rhodium and tungsten, FthzW and Rh~W, 2Gth Irrtersocieiy Energy Conversion Engineering make excellent electrodes for use in AMTEC cells, Conference, ANS, 5, 463 (1991). where the electrodes arc operated at temperatures 4. P, M, George, F%. D. Dissertation, California Institute up to 1200 K in sodium atmosphere [1 ,21. In of Technology, 1971. addition, we have shown that the efficiency of such 5, J.V, Armstrong, A.A. Burk, J,M. D. Coey and K. eloctrodcs can be increased by depositing grids of Moorjan, App/. Phys. Lett., 50, 1231 (1986). molybdenum metal on the solid electrolyte before sputter depositing alloy electrodes [3]. Investigatic)n 6. D.K. Liu, R.J. Chin and A.L. lai, Chenv’sfry of of the properties of these electrodes led to research Materials, 3, 13, (1 991). into the possibility of making integral grids plus 7. Y.-G. Kim, D. Byun, C. }Iutcliings, P.A. Dowben, H. electrodes of rhodium-tungsten alloy, to eliminate \&l: and K. Schroeder, J. App/. Phys., 70, 6062 contact resistance between the grid lines and the elcctrodo. Molybdenum metal makes a good current 8. R. F<. Krchnavek, H.H. Gilgen, J.C. Chen, P.S. Shaw, carrying grid, as the conductivity is high and it can T .J, Licata and fl.M. Osgood Jr., J. Vat. Sci. be photoclcpositcd in domains from an organo- 7echno/. f?, 5, 2’0 (1 987). rnctallic vapor by foc:ussing UV light into the shape 9. D,J. Ehrlich and J.Y. Tsao, J. Vat. Sci. 7echnol. B, desired. }Iowevcr, the grain growth clf Mo metal is 1, 969 (1983). relatively rapic{ at temperatures above 1100 K and the operating lifetime of a device may be significantly 10. G.A. Kovall, J.C. Matthews, and R. Solanki, J. Vat. shortened if Mo gricis are used [2]. Grain growth in Sci. 7echnol. A, 6, ?353 (1988). rhodium-tungsten alloys is significantly slower than 11. E.B, Flint, J, Messclhaeuser and Il. Suhr, A@. Surf. in molybdenum. While the conductivity of FU~XW may Sci., 54, 56 (1992) be lower than that of Mo, the possibility of improving 12. J,S, Cohan, }1.Yuan, R.S. Williams and J.1. %ink, current collection, eliminating contact resistance Appl. Phys. Lett., 60, 1402 (1992]. bctwcon the current collection grid and the electrode, and of maintaining long operating lifetimes of 13. S. M. Shin, C. W. Draper, M,E. Mochel and J.M. electrodes in high temperature devices makes Rigsbee, Mal. Letf., 3(7,8), ?65, (1985). investigation c)f the conditions for photocfeposition of 14. J. Ctlaiken, M.J. Casey and M. Villarica, J. Phys. F{I]XW alloys interesting. Chem., 96, 3183 (1991).

Very little work has been done previously on ptlotodcposilion of alloys [4-7], although there arc scwcral research grolJps looking into photodeposition of pure metals [8-1 2] and there has been research into fabricatic)n of metal alloy clusters using laser ctlcmistry [13, 141. [n our laboratory, rhodium- tungsten alloys were made by photolytically co- dopositing rhodium and tungsten fronl their respective organo-metallic vapors, using a single ~JV