Multiscale modelling of radiation effects in insulators

Completed

Nuclear Fission and Fusion(Nuclear Fusion)
Nuclear Fission and Fusion(Nuclear Fission, Nuclear supporting technologies)
Not Energy Related

Basic and strategic applied research

PHYSICAL SCIENCES AND MATHEMATICS (Physics)

Not Cross-cutting

Dr DM Duffy
Physics and Astronomy
University College London

Standard

EPSRC

01 October 2009

30 September 2012

36 months

£1

No relevance to Underpinning Sectors

London

Physical Sciences

Dr DM Duffy, Physics and Astronomy, University College London

The modification of materials by various types of radiation has wide ranging applications and implications. In some situations materials modification is desirable, such as the controlled nanostructuring of materials by lasers beams and ions. In other cases materials modification is strongly undesirable, such as the damage to structural materials caused by radiation in fission and fusion power plants or the degradation of semiconductor devices in radiation environments. In any case, a fundamental understanding of the processes involved is essential for the control and prediction of the changes to structure and properties.Modelling has played a vital role in the understanding of radiation effects in materials. The information of about defect distribution and mobility provided by atomistic simulations gives a level of detail not possible from experiment alone. Molecular dynamics (MD), is widely used model processes, such as radiation damage, that occur on nanosecond timescales and nanometre length scales. It is impossible to treat such large systems using quantum mechanical techniques, such as density functional theory, therefore the interactions between the atoms must be represented by relatively simple functional forms which effectively integrate out the electronic degrees of freedom. Nevertheless much information can be gained from classical MD simulations.There are, however, situations in which the electrons play a more active role and in these cases a classical MD model neglects important effects. Some types of radiation damage fall in to this category. Some types of radiation, such as low energy ions, interact with materials by direct collisions with atoms. Atoms are knocked out of their lattice sites and displacement damage occurs. Electromagnetic irradiation, on the other hand, excites electrons and the decay of the excited states generally deposits some energy in the lattice, resulting in heating and possibly defects. Very energetic heavy ion irradiation also results in strong electronic excitation along the path of the ion. It is energy transfer from such electronic excitations to the lattice that we propose to include in classical MD simulations.We have recently developed a methodology that includes electronic energy transfer in classical simulations of metallic materials. In this proposal we outline a scheme for extending this methodology to insulating materials, where the band gap results in interesting and complex behaviour not observed in metals. In some insulating materials the decay of an excited electron can lead directly to the formation of a defect pair and such materials are particularly sensitive to ionising radiation. Ions and defects in ionic crystals can exist in more than one charge state, which can result in trapping of electronic defects. The synergy between displacement damage and electronic excitations is a particular issue for many applications. Excited electrons and holes may be trapped at defects caused by displacement events, resulting in modified diffusion rates and subsequent clustering. Such effects are responsible for radiation enhanced diffusion.The new methodology we propose will improve the predictive power of models of radiation effects in insulating materials, which will help in the selection of materials for nuclear technology and nuclear waste disposal, and in the design of techniques for nanostructuring materials

14/11/11