2023-08-06 04:00:15
Virtually all materials, whether solid, liquid, or gas, expand when heated and contract when cooled. This is called thermal expansion. However, a class of metal alloys, called Invars, seem to deviate from this rule. In a new study, researchers from the team of Brent Fultz, professor of science and physique applied, have tried to unravel the mystery of this constancy thermal of the Invars.
Thermal expansion occurs when atoms of a material vibrate more as its temperature increases, thus repelling their neighboring atoms. However, Invars, alloys of iron and nickel, resist this change in size and density over a wide range of temperatures.
Stefan Lohaus, a materials science student and the study’s first author, points out that it’s almost inconceivable to find metals that don’t expand. This exceptional property of Invars makes them valuable materials in applications requiring extreme precision, such as the manufacture of timepieces, telescopes and other precision instruments.
In this study, the researchers used “Maxwell’s relations” to independently measure the thermal expansion caused by the magnetism and by atomic vibrations. They did this by measuring the vibration spectra and magnetism of small Invar samples under pression in an anvil cell of diamond.
Density of phononic states of the Invar at different pressures. The experimental 57Fe partial DOS curves measured by NRIXS (black curves) are compared to the calculated DOS (orange curves), both normalized to 1. The error bars show thestandard deviation sequential NRIXS scans. Under ambient conditions, the NRIXS measurement is in agreement with the DOS measured by INS (blue markers).
Credit: Nature Physics (2023).
The measurements showed a cancellation of thermal expansion due to atomic vibrations and magnetism. The two phenomena varied with temperature and pressure, but in such a way as to maintain their equilibrium. Using a new, precise theoretical approach, the researchers demonstrated how this balance was promoted by the interactions between vibration and magnetism.
The experimental device consisted of a cell with a diamond anvil, which makes it possible to strongly compress samples of material. In this case, a small piece of Invar alloy was compressed to a pressure of 200,000 atmospheres. The researchers sent a powerful beam of X-rays through the alloy, reacting with the vibrations (phonons) of its atoms. This interaction changed the quantity d’energy carried by X-rays, allowing researchers to measure the magnitude of the vibrations of atoms.
“It’s exciting because this has been a problem in science for over a hundred years,” says Lohaus. “There are literally thousands of publications trying to show how magnetism causes contraction, but there was no comprehensive explanation of the Invar effect.”
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