In a lattice-based material with negative thermal expansion properties, a group of nuclear chemists from the Chinese Academy of Sciences and a colleague from Changzhou University have discovered a novel relaxation mechanism. The group constructed lattice-based materials and observed their properties as they were heated in their study, which was published in Chemical Science.
When heated, most materials expand, which can be problematic in numerous applications. For instance, product failure can result from product expansion in electronic devices. On the other hand, some materials actually expand when heated. Such materials have what are known as bad warm extension (NTE) properties.
A material that has both positive and negative developments might possibly bring about net zero change, in this way protecting the uprightness of its current circumstances. In this new endeavor, the exploration group set off on a mission to make a NTE material with a controllable degree of shrinkage during warming for use with controllable positive warm extension materials.
The team created the material by constructing tiny, cage-like structures that shrink to a certain temperature when exposed to it. The cages’ spring-like properties cause the shrinking. They were made by inserting organic linkers into a metal-organic rotaxane skeleton. The resulting material is given the team’s name, U3(bcbpy)3(CB8). They point out that their method differs from traditional hinging models, which shrink due to atom vibration.
The material performed in a stepwise manner, as demonstrated by testing: heating caused some shrinkage that remained constant. At that point, more intensity prompted one more degree of contraction, showing a level of command over the cycle.
However, the group also discovered a novel relaxation mechanism that involved a contraction of the negative thermal expansion in accordance with a particular material recovery stage. The researchers suggest that the material could be used in applications requiring heat-sensitive size and shape control. However, they point out that the use of uranium may limit such use.
More information: Qiu-Yan Jin et al, Colossal negative thermal expansion in a cucurbit[8]uril-enabled uranyl-organic polythreading framework via thermally induced relaxation, Chemical Science (2023). DOI: 10.1039/D3SC01343J
