Jul 28, 2023 20:00:00

The mystery of over 100 years of the metal `` Invar '' that does not cause thermal expansion is elucidated

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Thermal expansion ” is the property of expanding when the temperature rises and shrinking when the temperature drops. Solids such as metals and wood, liquids, and gases basically have this property. However, it was known that an alloy called ' Invar ', which is a combination of iron and nickel in a certain ratio, hardly causes thermal expansion, but the principle was not clarified. A research team led by Brent Fultz , a materials scientist at the California Institute of Technology, has elucidated the principle that Invar does not cause thermal expansion.

A thermodynamic explanation of the Invar effect | Nature Physics
https://doi.org/10.1038/s41567-023-02142-z

Some Alloys Don't Change Size When Heated. We Now Know Why. | www.caltech.edu

https://www.caltech.edu/about/news/some-alloys-dont-change-size-when-heated-we-now-know-why

Some alloys don't change size when heated, and we now know why
https://phys.org/news/2023-07-alloys-dont-size.html

Thermal expansion is a phenomenon that occurs when the atoms of a material vibrate violently as the temperature increases. The more strongly the atoms vibrate, the more they are pushed away from their neighbors, increasing the space between them. As a result, the material becomes less dense and its overall size increases.

Thermal expansion occurs on hot days, and there is a danger that buildings will bend and be damaged. is taking place.

On the other hand, invar, which was discovered in 1895, changes little in size and density at all temperatures. ``The discovery of invar with no thermal expansion was probably unprecedented,'' said Stephen Rohaus of the University of California.

Invar, which has little thermal expansion properties, is used in applications such as clocks, telescopes, and liquefied natural gas tanks. However, it has not been clarified until now why Invar does not have the property of thermal expansion.

Previous research has revealed that thermal expansion is related to the central concept of thermodynamics, '

entropy '. Entropy is a measure of the disorder due to the difference in temperature of the positions of atoms in a system . It is believed that as the temperature increases, so does the entropy of the system.

Since the increase in entropy with increasing temperature is considered to be fundamentally universal, in order to explain the lack of thermal expansion of Invar, the factor to counteract the thermal expansion due to the increase in entropy is had to explain.

``So far, only certain alloys that are ferromagnetic have been treated as invar, so it has been suspected for many years that there is some relationship between thermal expansion and ferromagnetism,'' Rohaus said. says.

Fultz et al.'s research team used an experimental device that can measure both magnetism and atomic vibrations to observe atomic vibrations in Invar and the spin state of electrons at elevated temperatures.

When the temperature is low, the number of electrons sharing the same spin state increases in Invar, and the repulsion acts to push the atoms outward. On the other hand, when the temperature was increased, the spin states of some electrons began to flip. As a result, the electrons no longer repel each other, and the electrons huddle together and contract. However, as the temperature rises, the atoms of Invar begin to vibrate and expand. The contraction and expansion of the invar occur in a delicate balance, resulting in no change in the size of the invar.

The research team's findings reveal how this balance is aided by the interplay between vibration and magnetism, such as where magnetism changes the frequency at which atoms vibrate. This discovery is also expected to lead to an understanding of thermal expansion in other magnetic materials and the development of magnetism-based refrigeration materials.

``Our study is an important discovery that unlocks a scientific mystery that spans more than 100 years,'' Rohaus said. 'Thousands of studies have been reported, but none have explained why thermal expansion does not occur in Invar.'