Some Alloys Don’t Change Size When Heated. We Now Know Why

Metals and alloys commonly expand when heated, a principle known as thermal expansion, crucial to the operation of hot air balloons and thermostats. This expansion is due to atoms vibrating more intensely at higher temperatures, increasing the space between them, and thus the material’s size. However, a unique class of metal alloys, known as Invars, defy this rule, maintaining their size across a vast temperature range.

Discovered over a century ago, Invars’ non-expansive properties have been a mystery. Now, researchers from Caltech, led by Stefan Lohaus and under the guidance of Professor Brent Fultz, reveal the secret behind one Invar’s stability. Their study, published in Nature Physics, connects Invars’ behavior with entropy—a measure of disorder—and magnetism.

Previously, it was recognized that entropy increases with temperature, leading to thermal expansion. The team suspected that magnetism, particularly in ferromagnetic materials, was key to Invars’ unusual behavior. Utilizing specialized equipment capable of measuring both magnetism and atomic vibrations, they examined how Invars’ electrons, through their spin states, counteract expected expansion.

The experiments, conducted at Argonne National Laboratory’s Advanced Photon Source, used X-rays to observe vibrations in the Invar atoms, alongside sensors to detect electron spin interference patterns. As Invars were heated, the changing spin states allowed electrons to cozy up, which would typically lead to contraction. However, the increase in atomic vibrations balanced this effect, leaving the Invar’s size unchanged.

This discovery is a leap in materials science, offering insights into the development of other materials and potential applications in magnetic refrigeration. Understanding the Invar effect could lead to advances in precision engineering, where materials that don’t expand with heat are invaluable.

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