Scientists have developed a new alloy composed of multiple metals that exhibits almost no thermal expansion over an extremely wide temperature range. Most metals expand as temperature increases. For example, the Eiffel Tower is 10 to 15 centimeters taller in the summer than in the winter due to thermal expansion. However, this effect is highly undesirable for many technical applications.

Most metals expand when heated, but materials like Inkar resist this expansion due to changes in magnetic sequence. Researchers at the University of Technology Vienna and the University of Science and Technology Beijing used simulations to understand this effect, leading to the development of a thermogreen stone magnet with better thermal stability over a wide temperature range. Source: Technical University of Vienna

Therefore, researchers have long searched for materials that maintain a constant length regardless of temperature. One such material is Invar (unchanged steel), an iron-nickel alloy known for its extremely low thermal expansion rate. However, until recently, the physical explanation for this property remained unclear.

Now, theoretical researchers at the Technical University of Vienna (TUWien) in collaboration with experimentalists at the University of Science and Technology Beijing have achieved a major breakthrough. Using sophisticated computer simulations, they revealed the Invar effect in detail and developed a so-called pyrochlore magnet - an alloy with even better thermal expansion properties than Invar. Over an extremely wide temperature range of over 400 Kelvin, its length changes by only about one ten thousandth per Kelvin.

Thermal expansion and its opposite

"The higher the temperature of the material, the greater the tendency of the atoms to move, and the greater the movement of the atoms, the more space is required. The average distance between them increases," explains Dr. Sergii Khmelevskyi from the Vienna Science Cluster (VSC) research center at TU Vienna. "This effect is the basis of thermal expansion and cannot be avoided. However, we can create materials in which it is almost completely offset by another compensating effect."

Segii Khmelevskyi and his team developed sophisticated computer simulations that can be used to analyze the atomic-level behavior of magnetic materials at finite temperatures. "This allows us to better understand why Invar barely expands," says Khmelevskyi. "This effect is due to certain electrons changing their state as the temperature increases. The magnetic order in the material is reduced, causing the material to shrink. This effect almost completely cancels out the usual thermal expansion."

It is already known that magnetic order in materials is responsible for the Invar effect. But only through computer simulations in Vienna has it been possible to understand the details of this process so precisely that predictions can be made for other materials. "This is the first time that a theory can concretely predict the development of new materials in which thermal expansion disappears," Sergey-Khmelevsky said.

Pyrochlore magnet with Kagome flat surface

To test these predictions in practice, Sergii Khmelevskyi collaborated with the experimental team of Professor Xianran Xing and Associate Professor Yili Cao at the Institute of Solid State Chemistry at the University of Science and Technology Beijing. The result of this collaboration is now available: so-called pyrochlore magnets.

Unlike previous Invar alloys, which were composed of only two different metals, pyrochlore magnets have four components: zirconium, niobium, iron and cobalt. "This is a material with an extremely low thermal expansion coefficient over an unprecedented wide temperature range," said YiliCao.

This extraordinary temperature behavior is related to the fact that pyrochlore magnets do not have a perfect lattice structure, which does not always repeat in exactly the same way. The composition of the material is not the same at every point, it is heterogeneous. Some areas have slightly more cobalt and some areas have slightly less cobalt. The two subsystems react differently to temperature changes. This allows the details of the material composition to be balanced point by point so that the overall temperature expansion is almost exactly zero.

This material could be particularly useful in applications where temperature fluctuations are extreme or measurement techniques are precise, such as in aviation, aerospace or high-precision electronic components.

Compiled from /ScitechDaily