Material with zero thermal expansion

It’s a basic rule of chemistry and physics: when you heat things up, they get bigger. While there are exceptions (like water and ice), it’s difficult to find a material with zero thermal expansion.

But new research from the University of New South Wales and the Australian Nuclear Science and Technology Organisation has found a compound that doesn’t thermally expand – at least, not between -269°C and 1126°C.

The researchers examined a substance made from scandium, aluminium, tungsten and oxygen (Sc1.5Al0.5W3O12), bonded together in a crystalline structure.

“We were conducting experiments with these materials in association with our batteries-based research, for unrelated purposes, and fortuitously came across this singular property of this particular composition,” Neeraj Sharma, an associate professor at UNSW, and senior author on a paper describing the substance, published in Chemistry of Materials.

It’s a relatively simple material to make, and aluminium, tungsten and oxygen are all readily available elements (in the forms of alumina and tungsten oxide), so it should be possible to manufacture at a large scale.

“The scandium is rarer and more costly, but we are experimenting with other elements that might be substituted, and the stability retained,” says Sharma.

Materials with zero thermal expansion are in high demand in a range of engineering disciplines. They’re required for high-precision mechanics, aerospace components, and medical implants. This material is stable across the widest range of temperature discovered so far, from 4 Kelvin (nearly absolute zero) to 1400K.

The researchers still aren’t certain how the material remains stable for such a large range, but they have some clues from crystallographic data gathered at ANSTO’s Centre for Neutron Scattering. There are small distortions in bonds between the material’s oxygen atoms that may allow it to absorb temperature changes.

“Is it the bond lengths that are expanding? Is it the displacement of the oxygen atoms? Or, is the whole polyhedral rotating? We have three factors that are correlating.

“At this point, it is not clear if one or all of these contributing factors are responsible for the stability over a range of temperatures and we are investigating further to try and isolate the mechanism,” says Sharma.

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