T. Diaz de la Rubia

RADIATION EFFECTS IN CRYSTALLINE CERAMICS FOR THE IMMOBILIZATION OF HIGH-LEVEL NUCLEAR WASTE AND PLUTONIUM

This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion. {copyright} {ital 1998 Materials Research Society.}

Ion beams in silicon processing and characterization

General trends in integrated circuit technology toward smaller device dimensions, lower thermal budgets, and simplified processing steps present severe physical and engineering challenges to ion implantation. These challenges, together with the need for physically based models at exceedingly small dimensions, are leading to a new level of understanding of fundamental defect science in Si. In this article, we review the current status and future trends in ion implantation of Si at low and high energies with particular emphasis on areas where recent advances have been made and where further understanding is needed. Particularly interesting are the emerging approaches to defect and dopant distribution modeling, transient enhanced diffusion, high energy implantation and defect accumulation, and metal impurity gettering. Developments in the use of ion beams for analysis indicate much progress has been made in one-dimensional analysis, but that severe challenges for two-dimensional characterization remain. The ...

Defect production, annealing kinetics and damage evolution in α-Fe: An atomic-scale computer simulation

Abstract Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small ...

Displacement damage in irradiated metals and semiconductors

This chapter discusses displacement processes in energetic cascades. It describes the fundamental mechanisms of producing atomic rearrangements and point defects in cascades, the configurations of defects in their primary state of damage, and the fates of these defects as they migrate away from their nascent locations. When energetic particles penetrate solids, they lose their energy through a series of elastic two-body nuclear collisions with target atoms and through excitation of the electronic system. It is the elastic collisions that are of primary interest for damage creation in metals and most semiconductors because they lead to the production of Frenkel pairs, which are vacancies and self-interstitial atoms, and to rearrangements of atoms on their lattice sites. The atomic displacement process begins with the creation of a primary knock on atom (PKA), which is any target atom struck by the irradiation particle. Displacements in energetic cascades are a part of a complex dynamics that involves both energetic recoils, which are characterized by binary collisions, and intense local heating.

Thermal stability of helium-vacancy clusters in iron

Molecular dynamics calculations were performed to evaluate the thermal stability of helium–vacancy clusters (HenVm) in Fe using the Ackland Finnis–Sinclair potential, the Wilson–Johnson potential and the Ziegler–Biersack–Littmark–Beck potential for describing the interactions of Fe–Fe, Fe–He and He–He, respectively. Both the calculated numbers of helium atoms, n, and vacancies, m, in clusters ranged from 0 to 20. The binding energies of an interstitial helium atom, an isolated vacancy and a self-interstitial iron atom to a helium–vacancy cluster were obtained from the calculated formation energies of clusters. All the binding energies do not depend much on cluster size, but they primarily depend on the helium-to-vacancy ratio (n/m) of clusters. The binding energy of a vacancy to a helium–vacancy cluster increases with the ratio, showing that helium increases cluster lifetime by dramatically reducing thermal vacancy emission. On the other hand, both the binding energies of a helium atom and an iron atom to a helium–vacancy cluster decrease with increasing the ratio, indicating that thermal emission of self-interstitial atoms (SIAs) (i.e. Frenkel-pair production), as well as thermal helium emission, may take place from the cluster of higher helium-to-vacancy ratios. The thermal stability of clusters is decided by the competitive processes among thermal emission of vacancies, SIAs and helium, depending on the helium-to-vacancy ratio of clusters. The calculated thermal stability of clusters is consistent with the experimental observations of thermal helium desorption from α-Fe during post-He-implantation annealing.

Comparative study of radiation damage accumulation in Cu and Fe

Bcc and fcc metals exhibit significant differences in behavior when exposed to neutron or heavy ion irradiation. Transmission electron microscopy (TEM) observations reveal that damage in the form of stacking fault tetrahedra (SFT) is visible in copper irradiated to very low doses, but that no damage is visible in iron irradiated to the same total dose. In order to understand and quantify this difference in behavior, we have simulated damage production and accumulation in fcc Cu and bcc Fe. We use 20 keV primary knock-on atoms (PKAs) at a homologous temperature of 0.25 of the melting point. The primary damage state was calculated using molecular dynamics (MD) with empirical, embedded-atom interatomic potentials. Damage accumulation was modeled using a kinetic Monte Carlo (kMC) algorithm to follow the evolution of all defects produced in the cascades. The diffusivities and binding energies of defects are input data for this simulation and were either extracted from experiments, the literature, or calculated using MD. MD simulations reveal that vacancy clusters are produced within the cascade core in the case of copper. In iron, most of the vacancies do not cluster during cooling of the cascade core and are available for diffusion. In addition, self-interstitial atom (SIA) clusters are produced in copper cascades but those observed in iron are smaller in number and size. The combined MD/kMC simulations reveal that the visible cluster densities obtained as a function of dose are at least one order of magnitude lower in Fe than in Cu. We compare the results with experimental measurements of cluster density and find excellent agreement between the simulations and experiments when small interstitial clusters are considered to be mobile as suggested by recent MD simulations.

Progress in the development of a molecular dynamics code for high-energy cascade studies

Abstract We discuss recent progress in the development of a new molecular dynamics program for studies of high-energy displacement cascades. The new code, termed MOLDY-CASK, implements a vectorized algorithm to calculate the forces between atoms. Timing runs show a large increase in efficiency when compared to other, not fully vectorized MD codes. The code also implements several types of isotropic many-body interatomic potentials as well as three-body potentials for semiconductors. We present preliminary results of a 25 keV cascade in Cu where the calculation has been carried out in a computational cell containing 500000 atoms.

Simulating materials failure by using up to one billion atoms and the world's fastest computer: Brittle fracture.

We describe the first of two large-scale atomic simulation projects on materials failure performed on the 12-teraflop ASCI (Accelerated Strategic Computing Initiative) White computer at Lawrence Livermore National Laboratory. This is a multimillion-atom simulation study of crack propagation in rapid brittle fracture where the cracks travel faster than the speed of sound. Our finding centers on a bilayer solid that behaves under large strain like an interface crack between a soft (linear) material and a stiff (nonlinear) material. We verify that the crack behavior is dominated by the local (nonlinear) wave speeds, which can be in excess of the conventional sound speeds of a solid.

Diffusion and interactions of point defects in silicon: molecular dynamics simulations

Abstract We have simulated the motion and clustering of vacancies and interstitials in silicon using molecular dynamics methods. The diffusion coefficients of isolated defects were calculated from atomic displacements in simulations performed over a wide range of temperatures. The results give an apparent migration energy barrier of E(M)V=0.43 eV for vacancies and E(M)1=0.9 eV for interstitials. The diffusion coefficients are between 10−6 and 10−5 cm2/s at 800°C, and are in approximate agreement with recent first-principles calculations, although they are many orders of magnitude larger than the most direct experimental measurements. Simulations with high concentrations of defects show that like defects aggregate into stable clusters, and that individual defects are bound to these clusters with energies in the range of 0.6–2.3 eV. Defect clusters have mobilities which can differ substantially from those of the individual defects. The di-interstitial has a dramatically smaller diffusion barrier, E(M)21 ≈ 0.2 eV, whereas the tri-interstitial has a mobility which is so small that it is difficult to measure accurately by molecular dynamics simulations. We discuss some of the implications of these simulations for diffusion under silicon device-processing conditions.

Simulating materials failure by using up to one billion atoms and the world's fastest computer: Work-hardening

We describe the second of two large-scale atomic simulation projects on materials failure performed on the 12-teraflop ASCI (Accelerated Strategic Computing Initiative) White computer at the Lawrence Livermore National Laboratory. This investigation simulates ductile failure by using more than one billion atoms where the true complexity of the creation and interaction of hundreds of dislocations are revealed.