Calcium Oxide Coating on Vanadium Alloys in Liquid Lithium
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Calcium Oxide Coating on Vanadium Alloys in Liquid Lithium
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J.-H. Park, K. Natesan, and D. L. Smith Energy Technology Division and Technology Development Division** Argonne National Laboratory, Argonne, IL 60439 USA ABSTRACT We have fabricated CaO coatings on vanadium metal and various vanadium alloys (V-10Cr, V-1Ti, V-4Cr4Ti, and V-5Cr-5Ti), for use in liquid metal blankets of fusion reactors. Initially, by monitoring weight change as a function of time, oxygen was charged into the vanadium alloys (rectangular sample, 2 x 1 x 0.1 cm) at 710°C by flowing Ar-O2 gas. Oxygen-charged samples were exposed in 2.8 at.% Ca-Li at temperatures of 600 and 700°C for times between 50 and 120 h. The thickness of the in-situ-formed CaO was 13 to 30 µm in the experimental conditions. Thicker CaO films were formed at higher oxygen contents and 700°C, while thinner CaO films were formed at lower oxygen contents and 600°C. The data show promise for in-situ formation of CaO layers on V alloys for the fusion blanket application. INTRODUCTION Corrosion resistance of structural materials and magnetohydrodynamic (MHD) forces and their influence on thermal hydraulics and corrosion are major concerns in the design of liquid-metal blankets for magnetic fusion reactors (MFRs). The objective of this study is to develop in-situ coatings at the liquid-metal/structuralmaterial interface, with emphasis on coatings that can be converted to an electrically insulating film to prevent adverse currents generated by MHD forces from passing through the structural walls [1]. Several electrically insulating oxides such as binary oxides (CaO, BeO, MgO and Y2O3), ternary perovskite (CaZrO3), spinel (MgAl2O4), and nitrides (e.g., AlN ) can be applied as coatings to V/Li MFRblanket systems. We have chosen to pursue development of in-situ CaO film deposition on the vanadium alloys because CaO is thermodynamically stable in liquid Li, has the highest electrical resistivity desirable for a thin film, forms in-situ, and is greatly beneficial for healing of defects, such as microcracks and open pores [2,3]. Based on its high thermodynamic stability CaO can be formed in-situ to heal most of the defects on insulator coatings. Therefore, this study of the in-situ CaO-forming process is very important in developing electrical insulator coatings for MFR applications in the V/Li system. The main process can be stated as Ca + O = CaO, where Ca dissolves in liquid Li and then reacts with O near the surface of V alloys to form CaO. The CaO layer grows based on the inward ambipolar diffusion of 2+ Ca ion and the two electrons through the CaO layer to react with the O in the V alloys at the CaO/V 22+ interface. The O accepts two electrons to form O ions, and reacts with Ca ions to form CaO. As a result, the CaO layer is dynamically adhesive, and O levels decrease in the V alloys. It is expected that the rate of the process will increase under netron fluency. Additionally, CaO has low activation with neutron bombardment. Both Li and Ca-Li are liquid metals that do not in
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