Review of vanadium purification on an element-by-element basis for use in a superconducting heat switch

Certain superconducting devices can be made more thermodynamically efficient by maximizing control of electron conduction while inhibiting phonon transport. The application that motivated this work is the superconducting heat switch (SHS) used in adiabatic demagnetization refrigerators. Superconducting metals such as vanadium with a high Debye temperature demonstrate strong potential for high switching ratios in these applications, but only if proper purification efforts are undertaken. Each impurity in the superconductor's bulk has different impacts on its lattice structure and transport properties. Therefore, understanding how to remove specific elements while leaving others unaffected can translate to independent modulation of electron and phonon conduction. For a highly efficient SHS, removal of silicon and magnetic impurities (e.g., iron, nickel, cobalt) is paramount. This review examines the ability of six purification methods to remove impurities from vanadium on an element-by-element basis. Electrorefining was shown to be the best-suited purification method for removal of the key impurities of interest for an SHS, but this technique must be used in conjunction with other purification methods such as electrotransport to produce SHS-quality vanadium.