Zero thermal expansion over a wide temperature range
Researchers from UNSW, led by Associate Professor Neeraj Sharma, have discovered a material that does not expand or contract over an extremely wide temperature range and may be one of the most stable materials known.
Using instruments at ANSTO’s Australian Synchrotron and Australian Centre for Neutron Scattering, as well as other techniques, the team demonstrated that the material made of scandium, aluminium, tungsten and oxygen did not change in volume from 4 to 1400 K (-269 to 1126°C). This makes it suitable for potential use in high-precision mechanical instruments, control mechanisms, aerospace components and medical implants, in which stability at varying temperatures is critical.
“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,” said Assoc Prof Sharma, an ARC Future Fellow and former employee of ANSTO.
Comprehensive neutron scattering measurements were conducted at the Australian Centre for Neutron Scattering. The results, published in the journal Chemistry of Materials, confirmed the structural stability of Sc1.5 Al0.5W3O12 with only minute changes to the bonds, position of oxygen atoms and rotations of the atom arrangements. Investigations of other forms of the material were undertaken on the powder diffraction beamline at the Australian Synchrotron, but slightly different ratios of the elements did not show the zero thermal expansion.
“Curiously, the experiments suggest these minute atomic displacements and adjustment appear to be undertaken cooperatively,” said Senior Instrument Scientist Dr Helen Maynard-Casely, who assisted with the measurements on the high-resolution powder diffractometer Echidna.
“Movements and rotations of atoms and radii are quite ordinary, but this correlated behaviour was quite unexpected.”
The crystallographic data from the diffraction experiments reflects the combination of subtle but observable distortions of the polyhedral units, bond lengths, angles and oxygen atoms that allow the material 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,” Assoc Prof Sharma said.
“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,” she added. The researchers noted, however, that because this specific material composition demonstrated this property, factors other than atomic radii could be at play, such as more complex crystallographic or dynamic behaviour.
Because of the relatively simple synthesis of the materials and the good availability of alumina and tungsten oxide, large-scale manufacture is a possibility. “The scandium is rarer and more costly,” Assoc Prof Sharma noted, “but we are experimenting with other elements that might be substituted, and the stability retained.”
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