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News | Press release | Focus | Chemistry | Materials | Quantum Physics
A study involving researchers from the CEA, Shimane University in Japan, and the Culham Science Centre for Fusion Energy in the UK has revealed limitations of the Arrhenius law, an empirical law of chemical kinetics widely used for more than a century to describe the variation in chemical reaction rates with changes in temperature. The study shows that quantum mechanics is essential for understanding the evolution of materials and their microstructures. Based on a study of defect motion in a tungsten sample at cryogenic temperature, these results are published in the journal Nature Materials on January 27, 2020.
The Arrhenius law is widely used to describe and calculate the variation in the rate of a chemical reaction as a function of temperature. According to the law, in a given material at an ultra-low temperature, the motion of any atoms heavier than hydrogen is substantially slowed or even "frozen": it describes the material and its defects as motionless. A study published in Nature Materials on January 27, 2020, now identifies limitations of the Arrhenius law on this point.
Motion of dislocation loops in low-temperature tungsten
Researchers looked into the motion of defects in a tungsten sample (1) at cryogenic temperature. In a material of this kind, clusters of interstitial atoms (2) form at nanometer scale and are trapped by impurity atoms (see illustration). By subjecting this material to electron irradiation, the researchers observed that a cluster can leave the perimeter of one impurity before being trapped by another.
To interpret their experimental measurements, they had to use quantum mechanics, a theory which explains the phenomena observed when the behavior of particles of very low mass, such as electrons, muons or even hydrogen atoms, is studied. Quantum mechanics is therefore essential to understand, calculate and simulate the evolution of materials and their microstructures.
These results suggest that all the classic experiments conducted at cryogenic temperatures need to be revisited (e.g. annealing experiments to measure variations in material resistivity or internal friction), where quantitative interpretation has to date been based solely on the Arrhenius law.
(1) The mass of a tungsten (W) atom is equivalent to that of 183 hydrogen (H) atoms.
(2) These defects consist of "interstitial" atoms. These are badly positioned extra atoms in the regular stack of atoms constituting the crystal lattice of a metal or mineral.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.