High Pressure Hydrogen and Helium

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Physics and Astronomy

Abstract

My PhD will be focused on using novel and traditional techniques to investigate Helium and Hydrogen at high pressures. This includes examining the structure of Phases II, III and IV of Hydrogen as part of the Hecate group at Edinburgh. Carrying on work from previous PhD projects I will attempt to gain insight in to these Phases by developing simplified models of their molecular interactions and look at developing novel techniques for predicting their Raman spectrum. Both high pressure Helium and Helium-Hydrogen mixtures will be investigated using density functional theory. The study of Helium will be focused on explaining the difference between the EOS and phase diagrams of its different isotopes. Studies of potential mixtures will look at both the stability of structures as well as the solubility of Helium in Hydrogen.

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509644/1 01/10/2016 30/09/2021
1861538 Studentship EP/N509644/1 01/10/2016 31/03/2020 Samuel Ramsey
 
Description We have explored the low-pressure equation of state of helium isotopes from first principle quantum mechanical methods. We have highlighted the main difficulties involved in studying this regime, i.e. the effects of zero-point motion on low mass atoms. These quantum nuclear effects give rise to the isotope effect seen in the helium equation of state with the lighter helium-3 having a higher zero-point pressure effect than the heavier isotope. We investigated multiple techniques for using density functional theory in this regime. In doing this we found the range of pressures over which each technique was applicable and why each broke down when it did.

This allowed us to continue with an investigation into hydrogen and helium mixtures. Raman experiments that measure the hydrogen vibration produce observe an unexplained shift when helium is introduced. There have been multiple explanations proposed but as of yet none of them could explain the change in the hydrogens change in behaviour. We have shown through first principle calculations that this change can be explained due to coupling effects between the hydrogen molecules. As the helium density increases the coupling between hydrogen molecules decreases as they are spread further apart. As the coupling has the effect of lowering the frequency of the vibration in pure hydrogen and so as helium is introduced the frequency of the hydrogen vibration increases.
Exploitation Route This large zero-point pressure effect seen in the helium equation of state is not taken in to account for in literature around helium defects in iron lattices. This could potentially affect both the formation of helium atoms in the iron lattice and the formation of the hydrogen bubbles which weaken the metal. Helium bubbles are usually found in steels used near nuclear reactors as alpha particles spontaneously decay into helium atoms within the metal. We hope to investigate the magnitude of effect in the time we have remaining, however, if this effect is significantly large enough a full investigation into the zero-point effects of helium in iron should be carried out by others.

Having mapped the costs and benefits of various techniques of modelling helium in the low-pressure regime others can begin to use first principle methods to study systems in which these large quantum nuclear effects are important. Now that a proper account of the effects of helium mixtures on hydrogen it opens up the possibility of directly comparing experimental measurements with calculations of an isolated hydrogen molecule under pressure. This could lead to the development of better models of hydrogen molecules under pressure.
Sectors Energy