New model could help make hydrogen-powered flight safer

Researchers at the University of Oxford have developed a new model to help predict how hydrogen moves through metallic components used in hydrogen-powered aircraft.

The study, ‘Thermodynamic model for thermomigration in metals’, published in Physical Review Letters, addresses a long-standing challenge in understanding how hydrogen behaves inside metals when they are exposed to temperature gradients.

Hydrogen is a potential alternative to conventional aviation fuels which could reduce carbon dioxide emissions. But using hydrogen in aircraft presents new engineering challenges, including the risk of hydrogen embrittlement – when metals can become damaged and lose mechanical strength when exposed to hydrogen.

“The theoretical work represents an important step forward. At the same time, we are also working towards generating new experimental measurements of thermomigration in structural metals to further validate and refine these models.”

Professor Felix Hofmann, Principal Investigator

At a metal surface, hydrogen molecules can break apart and enter the material as individual atoms. These atoms can then alter the material's mechanical behaviour, even at very low concentrations. Being able to predict where hydrogen will accumulate is therefore important for assessing the long-term reliability of components.

“While hydrogen embrittlement of metallic alloys was first reported by Johnson in 1875, over 150 years ago, it remains extremely difficult to predict”, says lead researcher Dr Daniel Long (University of Oxford). “A key challenge is to understand how different driving forces control the evolution of hydrogen concentration inside components.”

One of these forces is thermomigration: the movement of hydrogen in response to a temperature gradient.

This is particularly important for hydrogen-powered aircraft because hydrogen fuel systems are expected to operate across very large temperature ranges. Liquid hydrogen is stored at temperatures below −250°C and must be converted into gas before entering the engine. Heat exchangers used during this process can create significant temperature differences within metallic components.

Despite the potential importance of thermomigration, experimental measurements have been scarce and its underlying mechanisms in metals have remained poorly understood. The Oxford research provides a new mechanistic framework that could enable engineers to model embrittlement under realistic operating conditions.

The researchers found that thermomigration is driven in part by changes in the electrostatic fields and vibrational energy of hydrogen along a temperature gradient. However, the study shows that these effects alone cannot explain the phenomenon.

Because thermomigration involves systems that are out of equilibrium, heat transfer and hydrogen diffusion occur at the same time and interact with one another. The researchers identified an ‘electron-wind’ effect as an important contributor. Heat-carrying electrons moving down a temperature gradient interact with mobile hydrogen atoms, influencing the direction in which the hydrogen atoms move through the metal lattice.

The researchers have incorporated the model into a numerical framework for predicting hydrogen uptake and retention in metal components. The work was carried out in collaboration with Rolls-Royce, which is exploring hydrogen as a potential fuel for aviation. Rolls-Royce materials specialist Louise Gale says that understanding hydrogen embrittlement will be an important part of developing hydrogen-powered flight.

“The integration of hydrogen embrittlement risk into component integrity assessments is going to be a key enabler for hydrogen powered flight,’ she says. ‘It will not be possible to do this with empirical testing alone – accurate modelling of hydrogen transport and embrittlement will be essential.”

Dr Long adds: “It’s a privilege to collaborate with Rolls-Royce’s technical specialists on this advanced engineering challenge, and to contribute towards addressing a significant gap in scientific understanding. It was remarkable to find there has been little progress in this area since the experimental campaigns of the Space Race era.”

The research was supported by the Rolls-Royce-led HYEST programme and the EPSRC programme grant ‘Making Hydrogen Work in Zero Carbon Jet Engines’.