Electrodeposition of Re-Mo Alloy
Authors: Liu Quanhong , Huang Qiang 
Published: December 2024

Rhenium is a 5d transition metal in the VIIB group well-known for its high density, high melting temperature, and wide variety of oxidation states1. It has been used in applications such as high temperature machineries, aeronautics, as well as catalysis. It is also a type I superconductor of interest for cryogenic electronic applications. On the other hand, molybdenum is a 4d group VIB transition metal also known for its exceptional mechanical properties, including strength, resilience, and high-temperature resistance, rendering Mo a fundamental alloying element in aerospace and heating equipment. In addition, Mo exhibits a high melting point and superior electrical conductivity in small dimensions due to a short mean free path, and self-limiting formation of oxide, making it potentially useful in the fabrication of electrical contacts and interconnects2,
3,4. The incorporation of Re into Mo significantly lowers the plastic-brittle transition temperature. Furthermore, it raises the recrystallization temperature of Mo and enhances the high-temperature performance5. More importantly an enhanced Tc, the superconducting transition temperature, has been reported for ReMo alloys and makes this alloy of more interest for cryogenic electronics6. Although ReMo alloys have been successfully prepared using vacuum deposition methods such as magnetron sputtering7, to the best of our knowledge there have not been any reports on the electrodeposition of such alloys. In our previous work, electrodeposition of metallic Mo using concentrated acetate solutions has been systematically studied8. The present work continues from previous study, reports a new electrodeposition route for ReMo alloys, and presents a systematic study on the chemistry and process.

ReMo electrodeposition is carried out on rotating Cu disk electrodes. The electrolytes contain ammonia perrhenate (NH4ReO4), sodium molybdate (Na2MoO4), citric acid, and highly concentrated acetates such as lithium acetate or ammonia acetate. The effects of the concentrations of metal precursors (ReO4
-, MoO4
2-), citric acid, and acetates, as well as the deposition potential and time are systematically studied. Alloy film composition and thickness are measured using x-ray fluorescence spectroscopy. The faradic efficiency and the partial current densities of Re and Mo are calculated and compared. The surface morphology of films is characterized by SEM.

Figure 1 shows the partial current densities of Re and Mo at various citric acidconcentrations. It is worth noting that Mo and Re salts cannot be co-dissolved into a clear solution at the concentrations used without the addition of citric acid. Thus, a minimum concentration of citric acid of 0.1 M is used in this study. As shown in Figure 1, the Re deposition rate increases with citric acid concentration when the concentration is low, reaching a maximum partial current density at 0.2 M before it decreases gradually. A similar trend is also observed for the Mo deposition rate. However, the partial current density of Mo drops quickly to zero when the concentration of citric acid reaches 0.7 M. This suggests that Mo deposition is completely inhibited in highly concentrated citric acid solutions. On the other hand, the acetic solutions consisting of ammonium acetate and lithium acetate is however found necessary for Mo deposition. Details will be further discussed in the poster presentation.

1 Woolf, A. A., "An outline of rhenium chemistry," Quarterly Reviews, Chemical Society
15, 372 (1961).

2 Gardner, D. S., Onuki, J., Kudoo, K., Misawa, Y. & Vu, Q. T., "Encapsulated copper interconnection devices using sidewall barriers," Thin Solid Films
262, 104-119 (1995).

3 Gall, D., "Electron mean free path in elemental metals," Journal of Applied Physics
119, 085101 (2016).

4 Founta, V., Soulié, J.-P., Sankaran, K., Vanstreels, K., Opsomer, K., Morin, P., Lagrain, P., Franquet, A., Vanhaeren, D., Conard, T., Meersschaut, J., Detavernier, C., Van de Vondel, J., De Wolf, I., Pourtois, G., Tőkei, Z., Swerts, J. & Adelmann, C., "Properties of ultrathin molybdenum films for interconnect applications," Materialia
24, 101511 (2022).

5 Leonhardt, T., Carlén, J.-C., Buck, M., Brinkman, C. R., Ren, W. & Stevens, C. O., "Investigation of mechanical properties and microstructure of various molybdenum-rhenium alloys," AIP Conference Proceedings
458, 685-690 (1999).

6 Singh, V., Schneider, B. H., Bosman, S. J., Merkx, E. P. J. & Steele, G. A., "Molybdenum-rhenium alloy based high-Q superconducting microwave resonators," Applied Physics Letters
105 (2014).

7 Andreone, A., Barone, A., Dichiara, A., Mascolo, G., Palmieri, V., Peluso, G. & Diuccio, U. S., "MO-RE SUPERCONDUCTING THIN-FILMS BY SINGLE TARGET MAGNETRON SPUTTERING," Ieee Transactions on Magnetics
25, 1972-1975 (1989).

8 Liu, Q. & Huang, Q.," Electrodeposition of molybdenum from water-in-acetate electrolytes", manuscript submitted for publication

Figure 1