Scientists create a predictive model for the hydrogen-nanovoid interaction in metals

A collaborative study by Canadian and Canadian scientists over a five-year period produced a theoretical model, based on computer simulation, to predict the properties of hydrogen nanobubbles in metals.

The international team consisted of Chinese scientists from the Institute of Solid State Physics of Hefei Institute of Physical Science and their Canadian partners from McGill University. The results will be published in Nature's materials the 15th of July.

The researchers believe that their study could provide a quantitative understanding and assessment of hydrogen-induced damage in hydrogen-rich environments such as fusion reactor cores.

Hydrogen, the most abundant element in the known universe, is a highly anticipated fuel for fusion reactions and therefore constitutes a major focus of study.

In some hydrogen-enriched environments, for example a tungsten shield in the core of a fusion reactor, a metallic material can be seriously and irreparably damaged by significant exposure to hydrogen.

Being the smallest element, hydrogen can easily penetrate the metal surfaces through the spaces between the metal atoms. These hydrogen atoms can be easily trapped in nanoscale voids ("nanovoids") in metals created during fabrication or by irradiation of neutrons in the fusion reactor. These nanobulles get bigger and bigger under pressure of internal hydrogen and eventually lead to metal failure.

Unsurprisingly, the interaction between hydrogen and nanowires promoting the formation and growth of bubbles is considered the key to this failure. Yet the fundamental properties of hydrogen nanobubbles, such as their number and the strength of hydrogen trapped in bubbles, are largely unknown.

In addition, the available experimental techniques make it virtually impossible to directly observe hydrogen bubbles at the nanoscale.

To tackle this problem, the research team proposed instead to use computer simulations based on fundamental quantum mechanics. However, the structural complexity of hydrogen nanobulles has made numerical simulation extremely complex. It took researchers five years to produce enough computer simulations to answer their questions.

However, they eventually discovered that the behavior of trapping hydrogen in nanovids, although apparently complicated, obeyed simple rules.

First, individual hydrogen atoms are adsorbed, mutually exclusive, by the inner surface of nanovolides having distinct energy levels. Secondly, after a period of surface adsorption, hydrogen is pushed – due to limited space – to the nanovoid nucleus where the molecular hydrogen gas accumulates.

By following these rules, the team has created a model that accurately predicts the properties of hydrogen nanobubbles and fits well with recent experimental observations.

Just as hydrogen fills nanovolids in metals, this research fills a long time vacuum in the understanding of the formation of hydrogen nanobubbles in metals. The model provides a powerful tool for assessing the damage caused by hydrogen in fusion reactors, thus paving the way for future recovery of fusion energy.