Scientists Develop New Alloy That Breaks the Rules of Thermal Expansion

By Vienna University of Technology February 15, 2025

Collected at: https://scitechdaily.com/scientists-develop-new-alloy-that-breaks-the-rules-of-thermal-expansion/

Scientists have developed a new alloy composed of multiple metals that exhibits nearly zero thermal expansion across an exceptionally wide temperature range.

Most metals expand as their temperature rises. The Eiffel Tower, for example, stands about 10 to 15 centimeters taller in summer than in winter due to thermal expansion. However, this effect is highly undesirable for many technical applications. As a result, researchers have long sought materials that maintain a constant length regardless of temperature. One such material is Invar, an iron-nickel alloy known for its extremely low thermal expansion. The physical explanation for this property, however, remained unclear until recently.

Now, a collaboration between theoretical researchers at the Vienna University of Technology (TU Wien) and experimentalists at the University of Science and Technology Beijing has led to a significant breakthrough. Using complex computer simulations, they have unraveled the invar effect in detail and developed a so-called pyrochlore magnet—an alloy with even better thermal expansion properties than Invar. Over an exceptionally wide temperature range of more than 400 Kelvins, its length changes by only about one ten-thousandth of one percent per Kelvin.

Thermal expansion and its antagonist

“The higher the temperature in a material, the more the atoms tend to move – and when the atoms move more, they need more space. The average distance between them increases,” explains 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.”

Segii Khmelevskyi and his team developed complex computer simulations that can be used to analyze the behavior 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,” says 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 the usual thermal expansion.”

It had already been known that the magnetic order in the material is responsible for the invar effect. But only with the computer simulations from Vienna, it became 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,” says Sergii Khmelevskyi.

The pyrochlore magnet with Kagome planes

In order to test these predictions in practice, Sergii Khmelevskyi worked together with the experimental team of Prof. Xianran Xing and Ass. Prof. Yili Cao from the Institute of the Solid State Chemistry of the University of Science and Technology Beijing. The result of this cooperation has now been presented: 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 Yili Cao.

This remarkable temperature behavior 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. This allows 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.

Reference: “Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets” by Yanming Sun, Ruohan Yu, Sergii Khmelevskyi, Kenichi Kato, Yili Cao, Shixin Hu, Maxim Avdeev, Chin-Wei Wang, Chengyi Yu, Qiang Li, Kun Lin, Xiaojun Kuang and Xianran Xing, 17 December 2024, National Science Review.
DOI: 10.1093/nsr/nwae462