Towards tailor-made heat expansion-free materials for precision technology

Tokyo, Japan – Scientists from Tokyo Metropolitan University have discovered that a hydrogen-absorbing material shrinks in one direction upon heating, so-called negative thermal expansion (NTE). They found that this NTE is driven by a phase transition in the alignment of magnetic moments, an entirely different mechanism from its hydrogen-free counterpart. Since hydrogenation can be tuned, their findings promise customized high-precision ingredients in materials which don’t change in volume on heating, for next-generation precision nanotechnology.

Most materials tend to expand when heated. This can be problematic: glass containers often break when hot liquids are suddenly poured in, while buildings, bridges, and rail tracks need spacing and flexible joints to counter expansion on hot days. The same applies at the nanoscale, where minute changes in the size of components can jeopardize vital connections in circuitry, or induce damaging stresses when building blocks expand by different amounts.

This has led scientists on a journey to find materials which don’t change in volume under heating. A promising avenue is the field of negative thermal expansion (NTE) materials, which shrink instead of expand when heated. Engineering the right composite with both positive and negative thermal expansion at the atomic level might reduce or ultimately negate any changes in volume, a game changer at the bleeding edge of nanotechnology. Unfortunately, scientists don’t fully understand how NTEs work.

A team led by Associate Professor Yoshikazu Mizuguchi from Tokyo Metropolitan University has been exploring transition metal zirconides, a class of crystalline materials made up of a transition metal and zirconium. In recent work, they had found that cobalt zirconide showed NTE properties in a specific direction relative to its atomic structure, so-called uniaxial NTE, largely driven by changes in the vibrational properties of the atomic structure.

Interestingly, cobalt zirconide also happens to be a hydrogen-absorbing substance. As they studied its hydrogen-storing properties, the team discovered that hydrogenated cobalt zirconide also showed uniaxial NTE, but not in the same way as the original material. Below the Curie temperature, when magnetic moments line up to form a ferromagnetic phase, they found that heating causes the material to shrink along a specific axis, while expanding in another. Importantly, this form of NTE was clearly underpinned by the transition of the material to a ferromagnetic state. Since cobalt zirconide is also known to have superconducting properties, the material presents a rare insight into the interplay between three different physical phenomena: ferromagnetism, superconductivity, and NTE.

The team note that the amount of hydrogen in the cobalt zirconide structure can be tuned. This means that the degree by which NTE induces volume change might also be controlled. This is a radical new paradigm for designing custom compounds which do not change volume under thermal expansion, a material for the next generation of nano-engineered device components.

This work was supported by a JST-ERATO Grant (Number JPMJER2201), JSPS KAKENHI Grant Numbers JP25KJ1992 and JP23KK0088, the TMU Research Fund for Young Scientists, and a Special Award for Science Tokyo Advanced Researchers (STAR) funded by the Institute of Science Tokyo.

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