A research group led by Prof. TONG Peng from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has significantly improved the negative thermal expansion (NTE) effect of Cu₂P₂O₇, a new but excellent NTE material, and prepared a zero thermal expansion (ZTE) Cu₂P₂O₇/2024Al composite with high specific thermal conductivity and good machinability.

With the progress of high-tech fields, it is not possible to adjust the dimensions of precision equipment. However, thermal expansion with temperature is a fundamental property of many commonly used materials that is difficult to control. Combining NTE materials with ordinary positive thermal expansion materials is an efficient way to produce ZTE materials.

In this study, the researchers discovered that increasing the sintering temperature improves the NTE effect of Cu₂P₂O₇ (from -13.88 ppm/K to -36.6 ppm/K at 250-350 K). The vibration of the bridging oxygen atom causes the NTE effect in Cu₂P₂O₇. The results of temperature dependent X-ray powder diffraction, temperature dependent neutron powder diffraction, and X-ray absorption near edge structure showed that Cu₂P₂O₇ produced at lower temperatures had more oxygen vacancies, resulting in a weaker phase transition and NTE effect.

A series of Cu₂P₂O₇/2024Al composites were prepared by the powder metallurgy method in a fast-hot-pressing sintering apparatus using enhanced Cu₂P₂O₇ as reinforcement. The composite exhibited ZTE (-0.014 ppm/K at 273-333 K) with up to 50% Cu₂P₂O₇ volume addition.

Compared with other reported isotropic ZTE materials, this ZTE composite had a low density of 3.41g/cm^-3 and a high thermal conductivity of 49.7 WK^-1m^-1, resulting in a high specific thermal conductivity.

In addition, its bending strength could reach 205 MPa, and this type of composite had good machinability due to the matching moduli of Cu₂P₂O₇ and 2024Al, as well as a well-bonded interface.

"This study provides a good candidate NTE material for compositing and a kind of ZTE composite with excellent comprehensive properties, which is expected to be used in high-tech fields with high demand for dimensional stability." said XIE Lulu, first author of the study.

The enhanced negative thermal expansion of Cu₂P₂O₇ associated with oxygen vacancy. (Image by XIE Lulu) 

The excellent comprehensive properties of zero thermal expansion Cu₂P₂O₇/2024Al composite. (Image by XIE Lulu)