Most materials expand when heated, but a few materials do the opposite and contract. Researchers at Nagoya University are now adding to this curious class of materials.
A layered ruthenate Ca2RuO4-y exhibits NTE; the sintered-body structure shows colossal NTE when extremely anisotropic thermal expansion of the crystal grains produces deformation, consuming open spaces (voids) on heating; the total volume change related to NTE reaches 6.7% at most, the largest reported so far. Image credit: Koshi Takenaka et al.
In a paper published in the journal Nature Communications, Nagoya University researcher Koshi Takenaka and co-authors describe a reduced ruthenate ceramic material — made up of calcium, ruthenium and oxygen atoms — that shrinks by a record-breaking 6.7% when heated.
“We report the discovery of giant negative thermal expansion (NTE) for reduced layered ruthenate (Ca2RuO4-y),” the researchers said.
“The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volume change reported to date.”
“We observed a giant negative coefficient of linear thermal expansion α=-115*10-6 K-1 over 200 K interval below 345 K.”
The results may provide industrial engineers with a new class of composite materials that can be used to increase the accuracy of processes and measurements, to improve the stability of device performance, and to prolong device lifetimes.
“The size of the volume change, as well as the operating temperatures for negative thermal expansion can be controlled by changing the composition of the material,” the authors said.
“When the ruthenium atoms are partially replaced by iron atoms, the temperature window for negative thermal expansion gets much larger.”
“This window extends to above 473 K for the iron-containing material, which makes it particularly promising for industrial use.”
Noting that the volume changes were triggered at the same temperature that the reduced ruthenate material changed from a metallic to a non-metallic state, Dr. Takenaka and colleagues looked at changes in the arrangement of the atoms using X-ray techniques.
They saw dramatic changes on heating, with the internal atomic structure expanding in some directions but contracting in others.
Although the internal structure showed a net contraction, the crystallographic changes were not big enough to explain the giant volume changes in the bulk material.
Instead, the scientists turned their attention to the material’s overall structure, and found empty voids around the ceramic grains.
“The non-uniform changes in the atomic structure seem to deform the microstructure of the material, which means that the voids collapse and the material shrinks,” Dr. Takenaka said.
“This is a new way of achieving NTE, and it will allow us to develop new materials to compensate for thermal expansion.”
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Koshi Takenaka et al. 2017. Colossal negative thermal expansion in reduced layered ruthenate. Nature Communications 8, article number: 14102; doi: 10.1038/ncomms14102
