TOTAL SCATTERING TECHNIQUES FOR INVESTIGATIONS OF DISORDER IN THE SOLID STATE

Hydrogen will be an important energy carrier in the future energy chain based on renewables. Hydrogen storage in solids, so-called metal hydrides, has several advantages compared to compressed hydrogen gas. Most notably they can store hydrogen in a much denser and safer way. Ti-V-M alloys (M = Cr, Mn, Fe, Co or Ni), so-called bcc alloys, have decent hydrogen capacities (2-3 wt.% H) as well as suitable thermodynamics and kinetics for hydrogen storage purposes. Vanadium (V) is 40 times more expensive titanium (Ti), which is a severe limitation for utilization of the Ti-V-M alloys for large-scale applications. Much cheaper ferrovanadium alloys (FeV = Fe~0.2,V~0.8) can be used instead, although the reduction in price comes at the cost of lower hydrogen capacity. A Ti-V-Fe alloy with 10% Fe is for instance able to absorb 1.75 hydrogen (H) atoms pr metal (M) atom, while Fe free alloys can store 2 H/M. The difference is surprisingly large, since H easily can be accommodated in interstitial sites surrounded by up to 50% Fe in other alloys. We wanted to see if the reduced capacity could be explained from the atomic structure of the material. A total neutron scattering study was therefore conducted on the Ti-V-Fe hydride at the neutron source ISIS in UK. Total scattering is sensitive to the short-ranged features in the atomic structure and is therefore needed to fully understand the structure of disordered materials, such as bcc alloys and their hydrides. The total neutron scattering measurement, combined with Reverse Monte Carlo (RMC) modelling, revealed that Fe tend to form clusters with sizes of a few nanometers in the alloy. The interstitial sites in these clusters are not favorable for hydrogen since iron one its one does not form metal hydrides. The RMC models indeed showed that almost all empty intestinal sites where found in the Fe clusters. The study thus offers a clear explanation of the H capacity loss induced by the Fe.

The proposed project aims to establish national competence on investigations of structural disorder using total scattering techniques with neutrons and synchrotron radiation. Important objectives are to establish good routines for total scattering measure ments at the national JEEP II reactor at IFE and at the Swiss Norwegian Beam Lines (SNBL) at ESRF, France. This will give great added value for many present users of powder diffraction techniques at the two facilities since total scattering offer highly c omplementary information to regular diffraction experiments. It will also open up for new science e.g. investigations of nanoparticles, glasses and liquids which are not approachable by diffraction. The project will employ a postdoc fellow for three year s. The methodological competence will be partly built at IFE by investigations of materials of increasing complexity; from very simple model systems to materials of real scientific interest. In addition, a research visit of 4 months to the world leading g roup of Dr. David Keen at ISIS, UK is planned, which is foreseen to be highly stimulating for the research fellow. A 3 month stay at SNBL is also planned.