The layered ruthenate ceramic material contracts on heating, or exhibits negative thermal expansion (NTE). The sintered-body structure shows colossal NTE when extremely anisotropic thermal expansion of the crystal grains produces deformation, consuming open spaces (voids). The total volume change related to NTE reaches 6.7% at most, the largest reported so far.
The layered ruthenate ceramic material contracts on heating, or exhibits negative thermal expansion (NTE). The sintered-body structure shows colossal NTE when extremely anisotropic thermal expansion of the crystal grains produces deformation, consuming open spaces (voids). The total volume change related to NTE reaches 6.7% at most, the largest reported so far.

Researchers based at Nagoya University in Japan have discovered a ceramic material that contracts on heating by more than twice the previous record-holding material.

The machines and devices used in modern industry are often required to withstand harsh conditions. When the environmental temperature changes, the volume of the materials used to make these devices usually changes slightly, typically by less than 0.01%. Although this may seem like a trivial change, over time this thermal expansion can seriously degrade the performance of industrial systems and equipment.

Materials that contract on heating, or negative thermal expansion materials, are therefore of great interest to industrial engineers, as these materials can be mixed with normal materials that expand on heating. The aim is to produce a composite material with a desired thermal value, typically zero, which is maintained even at the extremely low operating temperatures used in cryogenic and aerospace engineering.

In a study published in Nature Communications, the researchers report a reduced ruthenate ceramic material, composed of calcium, ruthenium and oxygen (Ca2RuO4-y), that shrinks by a record-breaking 6.7% when heated. This is more than double the current record for a negative thermal expansion material, and the bulk material expands again when it is cooled. This discovery may lead to a new class of composite materials that could help improve the stability of device performance, prolong device lifetimes, and increase the accuracy of processes and measurements.

The size of the volume change, as well as the operating temperatures that trigger negative thermal expansion, can be controlled by changing the composition of the material. 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 200°C for the iron-containing material, making it particularly promising for industrial use.

After noticing that the volume changes were triggered at the same temperature as caused the reduced ruthenate material to switch from a metallic to a non-metallic state, the researchers used X-ray techniques to investigate changes in the arrangement of the atoms. 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 researchers 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,” explains corresponding author Koshi Takenaka. “This is a new way of achieving negative thermal expansion, and it will allow us to develop new materials to compensate for thermal expansion.”

This story is adapted from material from Nagoya University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.