SURFACE SCIENCE STUDIES OF SN AND LI FILMS ON REFRACTORY METAL SUBSTRATES FOR FUSION APPLICATIONS

Abstract

Liquid metal plasma facing components (LM-PFCs) such as lithium (Li) and tin (Sn) have been proposed as potential solutions to first wall and divertor challenges in tokamak fusion reactors. These liquid metals are of interest due to their regenerative and heat transfer properties which would allow them to withstand high heat and particle fluxes emanating from the plasma. Fundamental investigations of these metals using a surface science approach will yield insights into how they can be optimized for use in the various engineering configurations proposed for a fully functional reactor.

Ultrathin (up to 10 monolayers, ML) pure Li and Sn films in the solid and liquid state were deposited on polycrystalline substrates of molybdenum (Mo), molybdenum alloy (titanium zirconium molybdenum, TZM), tungsten (W), and single-crystal Mo(100). The thermal behavior, film structure, composition, oxidation characteristics and deuterium uptake capabilities of these deposited films were studied under controlled ultrahigh vacuum (UHV) conditions with Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and temperature programmed desorption (TPD).

Generally, for Li films deposited on Mo, TZM and W, the monolayer of Li in contact with the substrate is bound much stronger than in bulk Li films, and thermally desorbs at much higher temperatures. Interfacial Li on Mo(poly) has a higher thermal stability than that on TZM(poly), where the limiting values for the desorption activation energies, Ed, are 3.56 and 2.84 eV, respectively, in the low coverage, high temperature desorption tail. LEIS indicates some clustering or interdiffusion of the Li films on the TZM substrate at 500 K. No appreciable irreversible absorption of Li occurs on Mo, TZM or W under the conditions of these experiments. The Li films grown on the TZM substrate showed non-ideal layering at the monolayer level.

For post-oxidized Li films on TZM, no Li desorption occurred until temperatures above 620 K, and then Li desorbed from the surface via three desorption peaks at temperatures of 860, 990 and 1220 K. The formation of lithium oxide (Li2O) and peroxide (Li2O2), respectively was observed after post-oxidation. The peroxide converts to oxide after heating to 680 K with no Li desorption, and then this film decomposes to liberate Li into the gas phase while leaving oxygen at the TZM surface. Heating the LiOx films to 1070 K caused oxidation of the substrate to form MoO2 and MoO3, and also a condensed binary lithium molybdenum oxide (LixMoOy) phase. The LiOx film at 310 K initially wetted the TZM substrate well and no de-wetting of the LiOx film occurred prior to evaporation of Li above 680 K. Li deposition on an oxygen-containing TZM surface formed a Li-O-Mo interfacial oxide. This was most clearly seen for the thinnest, sub-monolayer Li films studied. Li desorption from multilayer Li films on oxygen-containing TZM surfaces occurred in a metallic Li multilayer peak and three other oxide-derived peaks at 812, 934 and 1157 K.

Sn films deposited on Mo, TZM and W demonstrated stability up to temperatures of 900 K, and thereafter begin to desorb. Multilayer desorption of these films was observed at temperatures of 1187-1270 K. The Sn monolayer films are stable until very high temperatures of about 1800-1900 K on these substrates. These films formed islands after film deposition at 310 K for all substrates studied, with agglomeration of liquid droplets to form larger clusters occurring after annealing to temperatures greater than 500 K. A complex clustering behavior was seen for the Sn films studied on TZM. The electronic properties of sub-monolayer Sn on Mo was modified as demonstrated by a -0.4 eV binding energy shift for the Sn 3d core level indicative of strong Sn-Mo electronic interactions. Sn films were found to oxidize rapidly at 800 K to form SnO2 due to facile O and Sn interdiffusion. Deuterium uptake on Sn films at 310-750 K from irradiation using 700 eV D2+ ions showed lower uptake by liquid Sn compared to solid Sn films. Irradiation of oxidized Sn films by a low flux of 700 eV D2+ caused reduction of the film to metallic Sn.