In-situ Resistometry for Heat Treatment Monitoring and Control

Motivation

Superconducting wires containing a brittle A15 phase, for example Nb₃Sn, must first be processed as a composite of more ductile components, the A15 phase being subsequently formed during heat treatment once the conductor had been deformed to its final dimensions. The heat treatment conditions and the detailed design of the wire cross-section strongly influence the resulting superconducting properties, but discovering the optimum wire design and heat treatment schedule for a given application is generally a matter of time-consuming and costly trial and error. The optimisation process also needs to be repeated when the requirements change - for example, a different operating temperature or a larger magnetic field - or to take account of processing restrictions (the maximum attainable heating rate, for example). Our work on interpreting resistance measurements, modelling reaction-diffusion and calculating resistances has the potential to significantly simplify the optimisation process.

Concept

The electrical resistivity of a composite material depends sensitively on the dimensions and distribution of each phase, and on the properties of each phase - their chemical compositions, crystal structures, defect populations and strain state. For a sample in which these properties are changing, for example a composite wire during heat treatment, this can provide useful information about the changes occurring inside the sample without disrupting processing. It is not possible to directly extract the structural changes of interest from the measured resistance, as this resistance depends on so many factors. However, careful modelling of the design of a composite wire and how that structure evolves during heat treatment can be used to calculate the resistance, and comparison of measurements with calculations (in the full knowledge of the heat treatment history) allows useful information to be extracted.

For bronze-route Nb₃Sn conductors, modelling the evolution of the structure during heat-treatment requires a good understanding of the reaction-diffusion processes (informed by experimental measurements), as well as the thermomechanical behaviour of the composite. This modelling in itself allows for optimisation of wire designs and heat treatments without performing resistance measurements. Combining this with resistometry could provide a powerful technique for monitoring the formation of the superconducting phase non-invasively during reaction, for both research and production purposes. Additionally, a computer-controlled furnace system could be developed to adjust the heat treatment parameters directly on the basis of the measured resistivity, to optimise the properties of the wire for a given application. That is the ultimate goal of our research in this field.

In addition to the calculation and modelling requirements, we have needed to develop a procedure for the in-situ resistance measurements at high temperature. To ensure that we can measure small changes in the resistivity, we use a four-terminal alternating current (AC) technique, with a lock-in amplifier. Measurements are conducted in vacuum to eliminate oxidation effects and isolate the sample from external disturbances. The temperature is monitored using a type K thermocouple next to the sample, and both temperature and the potential difference across a measured length of the sample are continuously recorded with a computer. The temperature is controlled using a Eurotherm controller and a tube furnace. This can also be connected to the computer, allowing us to use this system not just for resistometric monitoring of reaction processes, but also for true computer-controlled reaction.

More details

Further details are available - please refer to the links below, and the list of publications at the bottom of the page.

• Resistance measurement techniques
• Resistance calculations
• Reaction-diffusion calculations

Related materials

• Nb₃Sn
• Nb₃Al

Related publications