A review of uranium-based thin films

Abstract

Thin films based on silicon and transition-metal elements dominate the semi-conducting industry and are ubiquitous in all modern devices. Films have also been produced in the rare-earth series of elements for both research and specialised applications. Thin films of uranium and uranium dioxide were fabricated in the 1960s and 1970s, but there was little sustained effort until the early 2000s. Significant programmes started at Oxford University (transferring to Bristol University in 2011), and Los Alamos National Laboratory (LANL) in New Mexico, USA. In this review we cover the work that has been published over the last ∼20 years with these materials. Important breakthroughs occurred with the fabrication of epitaxial thin films of initially uranium metal and UO${}_2$, but more recently of many other uranium compounds and alloys. These have led to a number of different experiments that are reviewed, as well as some important trends. The interaction with the substrate leads to differing strain and hence changes in properties. An important advantage is that epitaxial films can often be made of materials that are impossible to produce as bulk single crystals. Examples are U${}_3$O${}_8$, U${}_2$N${}_3$ and alloys of U-Mo, which form in a modified bcc structure. Epitaxial films may also be used in applied research. They represent excellent surfaces, and it is at the surfaces that most of the important reactions occur in the nuclear fuel cycle. For example, the fuel-cladding interactions, and the dissolution of fuel by water in the long-term storage of spent fuel. To conclude, we discuss possible future prospects, examples include bilayers containing uranium for spintronics, and superlattices that could be used in heterostructures. Such applications will require a more detailed knowledge of the interface interactions in these systems, and this is an important direction for future research.

Keywords:

• uranium
• actinides
• thin films
• epitaxy

Acknowledgments

We would like to thank our many industry supporters, particularly Dave Goddard and Rob Burrows of the National Nuclear Laboratory, and Dave Geeson and Norman Godfrey of the AWE for provision of advice and materials over the years. We would also like to acknowledge our international collaborators at the JAEA, INL, CEA and ESRF who have helped enrich and expand the scope of this growing field of research.

We would like to give particular credit to Bill Stirling, Mike Wells, Stan Zochowski, Mike Thomas, Sean Langridge, and the late Roger Cowley for their interest and support over many years. Early funding for this program was obtained from the European Commission's Joint Research Center, Karlsruhe, Germany, and we thank Jean Rebizant for organising this valuable assistance. Constructive comments on the manuscript were made by Ladia Havela from Prague, and we thank him for these. We also acknowledge direct and indirect input from several generations of PhD students, in particular Rebecca Nicholls.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

We would like to acknowledge the funding and support from the Engineering and Physical Sciences Research Council (EPSRC), UK. Recent grants in advanced fuels (ATLANTIC, EP/S011935/1) and nuclear waste and decommissioning (TRANSCEND, EP/S01019X/1) have opened up new research avenues and provided PDRA and PhD student support, which has been invaluable to the continuation of this field. More directly, we thank the EPSRC for the recent award of a new deposition and surface characterisation facility (EP/V035495/1), becoming a national nuclear user facility (FaRMS: https://www.nnuf.ac.uk/farms).