This metal alloy does not expand when heated

A new metal alloy only changes its length by around one ten-thousandth of one percent per degree Celsius, and this over a temperature range of 400 degrees. It is a so-called pyrochlore magnet, which is a non-uniform alloy of zirconium, niobium, iron and cobalt. The material was discovered through the cooperation of researchers from the Technical Universities of Vienna and Beijing.

The theoretical basis came from Austria, from Sergii Khmelevskyi and Soner Steiner. The two studied a metal alloy of iron and nickel called Invar, which is known for its low coefficient of thermal expansion. Invar was discovered in 1896 by Swiss physicist Charles Édouard Guillaume, who was awarded the Nobel Prize for Physics in 1920 (one year before Albert Einstein). The material is used in many areas where high length stability is required in the event of temperature fluctuations, from measuring devices to laser housings and baseplates for computer chips.

Atoms move faster when heated, so they need more space, which is why materials expand. This is also fundamentally the case with invar, but there is an opposing magnetic effect that partially offsets the heat-induced expansion: Individual atoms repel each other magnetically, which increases the atomic spacing and thus the crystal lattice in the metal. As the temperature rises, the magnetic effect decreases, so that the two effects balance each other out to a considerable degree.

Computer simulation reveals details

Khmelevskyi and Steiner have developed an elaborate computer simulation that can be used to analyze the behaviour of materials at an atomic level. “This enabled us to better understand the cause of the invar effect,” reports Khmelevskyi. “It's because certain electrons change their state as the temperature rises.”

Only the Vienna computer simulation makes it possible to understand the processes so precisely that predictions for other materials are possible: “For the first time, a theory is available that can make concrete predictions for the development of new materials with vanishing thermal expansion,” says Khmelevskyi happily.

Pyrochlore magnet is the practical proof

To prove the theory in practice, the Viennese worked together with Xianran Xing and Yili Cao from Beijing University of Technology. The result of this cooperation has now been presented: the pyrochlore magnet. Its temperature behavior is due to the fact that the alloy is not perfectly balanced, but the distribution of the various components is heterogeneous. Some areas contain slightly more cobalt than others. The sub-areas react differently to temperature changes. By precisely balancing the material composition in the laboratory, the temperature-induced expansion can be reduced to almost exactly zero.

Khmelevskyi and Steiner published their findings at the end of 2023 in the Journal of Physical Chemistry, C 128/1: Predictive Theory of Anomalous Volume Magnetostriction in Fe-Ni Alloys: Bond Repopulation Mechanism of the Invar Effect. This was followed a year later by the publication of the results of the Chinese experiments in the National Science Review, nwae462, by Sun et al: Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets.

(ds)