Direct Electrochemical Reduction of Oxides

A process for the Direct Electrochemical Reduction of Oxides was developed and patented by Fray, Farthing and Chen (the Cambridge FFC process) in this department, and they have applied it to (amongst other materials) niobium, titanium and their alloys. Many oxides, when made the cathode of an electrochemical cell with a calcium chloride electrolyte, can be reduced to their metals. For elements like titanium and niobium, this eliminates the complex and costly sequence of processes applied in the current industrial route. Additionally, by using a suitable mixture of oxides, alloys and intermetallic compounds may be directly produced. Attempts have been made in this group to produce niobium-tin intermetallic compounds by this route, which could permit the development of novel low cost, high critical current superconducting wires.

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.

Schematic illustration of the equipment used for electrodeoxidation

Photograph of the equipment used for electrodeoxidation

Applications

Niobium-tin intermetallics

In our Nb-Sn electrodeoxidation experiments, pellets of Nb₂O₅ and/or SnO₂ were prepared by uniaxial pressing, and these green pellets were sintered to improve their strength and resilience whilst maintaining high surface-connected porosity. These pellets were contained in a niobium mesh basket construction, and supported on a rod with a purpose-designed clamp to form the cathode. A graphite rod formed the anode. The salt, CaCl₂ and NaCl at the eutectic composition and a temperature typically in the 700-900°C range, was contained in a crucible (Inconel or alumina) at the bottom of the reactor.

The superconducting Nb₃Sn phase (of the A15 structure) is of most 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 most of the powder-metallurgical routes (often referred to as powder-in-tube) in which niobium-tin intermetallic powders are reacted with a niobium tube (or another intermetallic) to form the A15 phase. The other intermetallic phases, produced electrochemically, could therefore also be useful as the starting materials for this process.

Work in this group has confirmed that all three intermetallic phases (Nb₃Sn, Nb₆Sn₅ and NbSn₂) can be produced by this method - it has been possible to avoid tin loss from the system despite it forming a liquid at the operating temperature. Forming a single, phase-pure intermetallic over than the A15 phase has, however, proved rather difficult for thermodynamic and kinetic reasons, combined with the difficulty of confining the liquid tin phase.

Related publications

Links

• Materials Chemistry Group and its FFC spin-off companies, Metalysis and British Titanium
• FFC Process Overview and metals produced by the process (on the Metalysis website)
• Inventors: Prof. Derek Fray, Dr. Tom Farthing and Dr. George Chen