The metal alloy that does not expand under heating 

Breakthrough in materials research: an alloy of several metals has been developed that shows practically no thermal expansion over an extremely large range

Most metals expand as their temperature increases. For example, Paris’ Eiffel Tower is up to 15 centimetres taller in summer than in winter due to its thermal expansion. However, this effect is extremely undesirable for many technical applications. For this reason, the search has long been on for materials that always have the same length regardless of the temperature. Invar, for example, an alloy of iron and nickel, is known for its extremely low thermal expansion. Explaining this property physically was not clear… until now.

A collaboration between theoretical physicists at the Technical University of Vienna (TU Wien) and researchers at University of Science and Technology, Beijing, has used computer simulations to understand the ‘Invar effect’ in detail and develop a so-called pyrochlore magnet, an alloy with even better thermal expansion properties than Invar.

Over an extremely wide temperature range of more than 400 Kelvin, the experimental pyrochlore magnet’s length only changed by around one ten-thousandth of one per cent per degree Kelvin.

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“[In general], the higher the temperature in a material, the more the atoms tend to move – and when the atoms move more, they need more space. And so, the average distance between them increases,” according to Dr Sergii Khmelevskyi from the Vienna Scientific Cluster (VSC) Research Centre at TU Wien.

“This effect is the basis of thermal expansion and cannot be prevented. But it is possible to produce materials in which it is almost exactly balanced out by another, compensating effect.”

Khmelevskyi and his team developed computer simulations used to analyse the behaviour of the magnetic materials at finite temperatures on the atomic level.

“This enabled us to better understand the reason why Invar hardly expands at all,’ said Khmelevskyi.

“The effect is due to certain electrons changing their state as the temperature rises. The magnetic order in the material decreases, causing the material to contract. This effect almost exactly cancels out the usual thermal expansion.”

It had already been known that the magnetic order in the material is responsible for the Invar effect. But the computer simulations have made it possible to understand the details of this process so precisely that predictions for other materials could be made.

“For the first time, a theory is available that can make concrete predictions for the development of new materials with vanishing thermal expansion,” said Khmelevskyi.

In order to test these predictions in practice, Khmelevskyi worked together with the experimental team of Professor Xianran Xing and Assistant Professor Yili Cao from the Institute of the Solid State Chemistry, University of Science and Technology, Beijing to produce the so-called ‘pyrochlore magnet’.

In contrast to previous Invar alloys, which only consist of two different metals, the pyrochlore magnet has four components: Zirconium, niobium, iron and cobalt.

“It is a material with an extremely low coefficient of thermal expansion over an unprecedentedly wide temperature range,” says Cao.

This remarkable temperature behaviour has to do with the fact that the pyrochlore magnet does not have a perfect lattice structure that always repeats itself in exactly the same way. The composition of the material is not the same at every point, it is heterogeneous. Some areas contain a little more cobalt, some a little less.

Both subsystems react differently to temperature changes which allow the details of the material composition to be balanced point-by-point in such a way that the overall temperature expansion is almost exactly zero.

The material could be of particular interest in applications with extreme temperature fluctuations or precise measuring techniques, such as in aviation, aerospace or high-precision electronic components.

For further reading please visit: 10.1093/nsr/nwae462