Most metals increase in size when heated. This property can be problematic for various industrial uses. Recently, researchers have developed a new material that remains nearly the same size across a broad temperature range.
Most metals swell when their temperature goes up. For instance, during summer, the Eiffel Tower can be about 10 to 15 centimeters taller than it is in winter because of this thermal expansion. However, this behavior is highly undesirable in many technical fields. Therefore, there has been an ongoing search for materials that maintain a consistent length regardless of temperature changes. One notable material is Invar, a blend of iron and nickel, which is recognized for its very minimal thermal expansion. Yet, until now, the underlying reasons for this property were not entirely understood.
Recently, a partnership between theoretical scientists at TU Wien in Vienna and experimental researchers at the University of Science and Technology in Beijing has led to a significant advancement: Through advanced computer simulations, they have gained a detailed understanding of the invar effect, leading to the creation of a new type of pyrochlore magnet—an alloy that outperforms Invar in terms of thermal stability. Over a wide temperature range exceeding 400 Kelvins, its dimensional change is just about one ten-thousandth of one percent per Kelvin.
Understanding Thermal Expansion
“As the temperature rises in a material, the atoms start to move more around – and as they move more, they require additional space. This increases the average distance between atoms,” explains Dr. Sergii Khmelevskyi from the Vienna Scientific Cluster (VSC) Research Centre at TU Wien. “This movement underpins thermal expansion, which is an unavoidable effect. However, it is possible to engineer materials where this expansion is countered by another balancing effect.”
Dr. Khmelevskyi and his team utilized intricate computer simulations to observe how magnetic materials behave at elevated temperatures on an atomic scale. “This helped us gain insights into why invar hardly expands at all,” states Khmelevskyi. “The phenomenon is linked to certain electrons altering their states with temperature increases. This leads to a decrease in the material’s magnetic order, resulting in contraction. This contraction nearly perfectly offsets the typical thermal expansion.”
While it was already acknowledged that magnetic order influenced the invar effect, the Vienna-based simulations allowed for a detailed examination of this mechanism, enabling predictions for other materials. “For the first time, we have a theoretical framework that can accurately forecast the development of new materials exhibiting virtually no thermal expansion,” says Sergii Khmelevskyi.
The Innovative Pyrochlore Magnet
To validate these theoretical findings, Sergii Khmelevskyi collaborated with a research group led by Prof. Xianran Xing and Assoc. Prof. Yili Cao from the Institute of Solid State Chemistry at the University of Science and Technology Beijing. Their joint effort has resulted in the creation of the pyrochlore magnet.
Unlike earlier invar alloys that comprised only two metals, the pyrochlore magnet consists of four elements: Zirconium, niobium, iron, and cobalt. “This material displays an exceptionally low thermal expansion coefficient over a remarkably broad temperature spectrum,” says Yili Cao.
This exceptional thermal performance is attributed to the pyrochlore magnet’s non-uniform lattice structure, which doesn’t replicate itself identically throughout. The material’s composition varies at different locations, creating a heterogeneous mixture. Some regions might contain slightly more cobalt, while others have less. These differing subsystems respond uniquely to temperature variations, which allows for a balanced adjustment of the material’s composition at various points, leading to an almost complete negation of thermal expansion.
This innovative material could prove particularly valuable in scenarios involving drastic temperature variations or exact measurement processes, like those seen in aviation, aerospace, or high-precision electronic devices.
