Contact: Prof JB Malherbe
Radiation damage: Prof E Friedland
Diffusion: Dr NG van der Berg
PBMR fuel development
The interaction of neutrons with materials that are used in nuclear power reactors, and the subsequent behaviour of the materials have been studied extensively in the past. These studies concentrated on the low-temperature (i.e. below 600 ºC) regime. The South African Pebble-Bed-Modular-Reactor (PBMR) Generation IV reactor was designed to operate at higher temperatures. There was, therefore, a need to study the properties of the materials at the high operating temperatures and under irradiation conditions. The following effects were studied both experimentally and theoretically:
- Diffusion and radiation-enhanced diffusion through multi-layer systems
- Neutron-induced radiation damage and its thermal annealing
- Interaction between nuclides and the coating layers
The nuclear materials research stemmed from expertise in the study of ion solid interactions and expertise in surface physics.
Irradiation induced effects
Irradiation of materials with ions, neutrons, electrons or photons can alter their mechanical, electrical and optical properties significantly. Basic mechanisms involved in such phenomena are not only of academic interest but also of considerable importance for modern industry, which relies heavily on materials with special properties. The aim of this program is to systematically investigate the change of structural, compositional, electrical and optical properties due to irradiation. These studies are undertaken with the following objectives in mind:
- Enhancement of the general knowledge on the physical processes involved in radiation effects.
- Identification of new possibilities for industrial and scientific application of irradiated materials and irradiation techniques.
- Development of the materials characterisation capability to support the University’s proposed materials research programme.
- High-level training of students and development of expertise in this field. Special emphasis will be given to corrective action.
- Extension of collaboration with other researchers/groups both locally and internationally.
Mechanisms responsible for damage in crystalline solids after heavy ion implantations are rather complex and depend to a large extent on the crystal structure and on the forces between lattice atoms. Total amorphisation is normally found after high-dose irradiation of covalent materials, while such total destruction is not found in metals where displaced atoms rearrange themselves into extended defects. Mixed defect structures are observed in some ionic crystals.
According to simple theoretical considerations, the thickness of the damaged layer is expected to be related to the implanted ion distribution. This is normally found in covalent materials, while largely enhanced damage depths are observed in most metals. Abnormal saturation effects of the tail region are observed in some ionic crystals at high fluences.
Energy loss and range studies of ions in solids
Energy loss processes of ions in solids are not only of academic interest but of considerable importance for the high-tech industry, which is increasingly making use of ion implantation techniques for the modification of surface properties in metallurgical and electronic applications. An accurate knowledge of energy-loss cross sections is vital for the controlled implantation and characterization of impurity depth profiles in material surfaces.
Low-energy bombardment by ions leads to the sputtering of the substrate. Sputtering usually causes a modification of several surface properties. Our studies concentrate on compositional changes due to preferential sputtering and other secondary radiation-induced processes, and on topography development. These surface modifications are not only of fundamental interest in the study of ion-solid interactions, but are also of great importance for ion implantation techniques used in large scale in the semiconductor industry. Topography development due to doping by ion implantation, plasma etching, and various methods of ion beam-assisted metallization can severely affect packing densities of micro-electronic structures. The proper functioning of metal-semiconductor contacts is critically dependent on the interfacial disorder and on compositional changes after ion bombardment processes.
Noble gas sputtering is ubiquitously used in surface analytical techniques for cleaning the surface in situ and for depth profiling. The above discussion shows that sputtering can cause many detrimental affects to surfaces. The results of the above phenomena are employed to develop algorithms for quantitative Auger electron spectroscopy and X-ray photoelectron spectroscopy in the case of sputtering. These algorithms correct some of the detrimental effects of low-energy ion bombardment.
Nitrogen implantation into materials has many technological applications. It is known that nitrogen bombardment of some metals leads to an increase in wear resistance in hardness of these metals. Recently, our group also started working on nitrogen implantation of GaAs, to determine the range profiles of low-energy nitrogen and to determine the composition of the implanted layer. Recently it was discovered that a blue laser can be manufactured from GaN.
Our group has done extensive investigations concerning the effect of ion implantation parameters on topography development, surface disorder and composition. A key aspect of this study has been the close collaboration with a large number of other groups. These included the Chemistry and Geology departments and the Electron Microscope Unit of our Faculty, the Physics department of the University of the Transkei, the research laboratory of TELEKOM at Darmstadt, the Universität Braunschweig and the Max-Planck-Institut für Plasmaforschung in Garching. Results for InP and GaAs have been published in review papers and were reported at several international conferences. In addition, Prof Malherbe had been invited to write several review papers in the field.