A five-year joint study by Chinese and Canadian researchers has formed a theoretical model via computer simulation to estimate properties of hydrogen nanobubbles in metal. The international team included Chinese researchers from the Institute of Solid State Physics of the Hefei Institute of Physical Science together with their Canadian partners from McGill University. The results were reported in Nature Materials on July 15th, 2019.
The scientists believe their research may allow quantitative understanding and assessment of hydrogen-induced damage in hydrogen-rich settings such as fusion reactor cores.
Hydrogen, the most copious element in the known universe, is a highly anticipated fuel for fusion reactions and therefore, a vital focus of the research. In some hydrogen-enriched settings, for example, tungsten armor in the center of a fusion reactor, the metallic material may be completely and irreparably damaged by prolonged exposure to hydrogen.
As the smallest element, hydrogen can easily enter metal surfaces via gaps between metal atoms. These hydrogen atoms can be easily ensnared inside nanoscale voids ("nanovoids") in metals formed either during manufacturing or by neutron irradiation in the fusion reactor. These nanobubbles become larger and larger under internal hydrogen pressure and finally result in metal failure.
Not unexpectedly, the interplay between hydrogen and nanovoids that boost the development and growth of bubbles is said to be the key to such failure. Yet, the regular properties of hydrogen nanobubbles, such as their number and the strength of the hydrogen captured in the bubbles, has mostly been unidentified.
Moreover, available experimental methods make it virtually impossible to directly monitor nanoscale hydrogen bubbles. To handle this issue, the researchers suggested using computer simulations based on fundamental quantum mechanics instead. However, the structural complications of hydrogen nanobubbles rendered numerical simulation very difficult. Therefore, the team needed five years to create sufficient computer simulations to answer their questions.
Eventually, however, they learned that hydrogen trapping behavior in nanovoids—although seemingly complex—really follows basic rules.
Primarily, individual hydrogen atoms are captured, in a mutually exclusive manner, by the inner surface of nanovoids with distinct levels of energy. Next, after a period of surface adsorption, hydrogen is pushed—because of limited space—to the nanovoid core where molecular hydrogen gas then collects.
Following these procedures, the researchers built a model that precisely predicts properties of hydrogen nanobubbles and accords well with the latest experimental interpretations.
In the same manner, as hydrogen filling the nanovoids in metals, this study fills an enduring void in understanding how hydrogen nanobubbles develop in metals. The model offers a robust tool for assessing hydrogen-induced damage in fusion reactors, thus opening the door to collecting fusion energy in the future.
The research was supported by the National Magnetic Confinement Fusion Energy Research Project, the National Natural Science Foundation of China, a Discovery Grant by the Natural Sciences and Engineering Research Council of Canada, and the China Scholarship Council.