Scientific interests

Fabrication of niobium-based materials by the Direct Electrochemical Reduction of Oxides for superconducting applications

The direct electrochemical reduction of oxides, or electrodeoxidation, is performed in a sealed reactor under flowing argon, at a slight positive pressure to prevent oxygen ingress. An Inconel reactor is contained within an Instron furnace, and a seal is established with a rubber O-ring between the reactor and its lid. This lid contains several holes which are blocked with bungs through which mechanical and electrical connections are made to the furnace components. The reactor is air-cooled. The complete system is as illustrated below.

In my current experiments, oxide pellets are prepared by uniaxial pressing, and these green pellets are sintered to improve their strength and resilience whilst maintaining high surface-connected porosity. These pellets are contained in a niobium mesh basket construction, which is supported on a rod with a purpose-designed clamp to form the cathode. The anode is graphite supported on a metal rod (which screws into it). The salt, currently CaCl2 and NaCl at the eutectic composition, is contained in a crucible (Inconel or alumina) at the bottom of the reactor.

Attention is currently focussed on the formation of niobium-tin intermetallics by the reduction of pellets containing Nb₂O₅ and SnO₂. The superconducting Nb₃Sn phase (of the A15 structure) is of industrial importance, and this phase is the simplest of the niobium-tin intermetallics to produce by this process. Its high melting point and brittleness make it difficult to assemble useful superconducting wires directly from a powder of Nb₃Sn. For this reason it is usually produced by the heat treatment of an assembled conductor. This is the case in the bronze and internal tin processes, in which niobium is reacted with tin supplied through a copper-tin matrix; however, this also applies to a variety of powder-metallurgical routes (often referred to as powder-in-tube) in which niobium-tin intermetallic powders are reacted with a niobium tube to form the A15 phase. The other intermetallic phases, produced electrochemically, could therefore also be useful as the starting materials for this process.

It has been confirmed that all three intermetallic phases (Nb₃Sn, Nb₆Sn₅ and NbSn₂) can be produced by this method. Work is currently underway to obtain phase-pure samples of each intermetallic phase and identify the optimum conditions by which these can be prepared. Efforts to develop further understanding of the mechanism and sequence of phase formation, with a view to optimising the process, are also being made. This system is particularly complex because the reduction of SnO₂ produces liquid tin at the reduction temperatures.