The structure of irradiated material near a primary knock on atom shortly after impact is largely unknown. Molecular dynamics simulations with classical force fields provide the foundation for our current understanding of the resulting cascade. Atomic level structural characterization is often in terms defects within the context of a perfect bulk, however, the choice of the best representation is complicated because the density of defects is high, the material is inhomogeneous and it is not in equilibrium. Here we explore the adaptation of tools typically employed to characterize homogeneous equilibrium liquids to the highly defected region of the cascade. The cascade structure shows some resemblance to that of the liquid or glass phase. The local temperature temporarily exceeds the melting temperature and the free energies of the liquid and defected crystal are comparable. Analysis of cascade structure will be important to the interpretation of first principles calculations of the electronic and magnetic states in cascade structures.

A computational technique is developed to predict the statistics of internal elastic fields of three-dimensional dislocation systems in deforming crystals. The internal elastic fields are computed based on 3D dislocation realizations generated by the method of dislocation dynamics simulation. Preliminary results are presented for the statistical characteristics of the elastic strain, lattice rotation and dislocation density tensor fields. The importance of the current analysis is discussed in the context of direct comparison of simulations with spatially resolved 3D X-ray microscopy measurements of lattice rotation and the dislocation density tensor.

The present work reports surface segregation in polycrystalline yttria-stabilised zirconia (cubic) including 10 mol% Y2O3 (10YSZ). The 10YSZ specimen was annealed in the range 1073 K - 1673 K in the gas phase of controlled oxygen activity. The segregation-induced intensity profiles of 89Y, 40Ca, 28Si, 27Al, 133Cs, 197Au and 90Zr was measured using secondary ion mass spectrometry (SIMS). The data obtained show that (i) annealing of 10YSZ results in the formation of segregation-induced concentration gradients of 89Y, 40Ca, 28Si, 27Al and (ii) segregation-induced profiles depend on oxygen activity.

Temperature dependent, dark current-voltage characteristics (I-V-T) of dye sensitized solar cells (DSSCs) were used to study the TiO2 surface defects on photovoltaic performance. Three kinds of DSSCs were used for this study. (1) DSSC with no surface treatment, (2) DSSCs with Al2O3 and (3) HfO2 treatments. Activation energy of charge transport, obtained from the I-V-T of the three DSSCs, suggest that Al2O2 and HfO2 surface treatment on the nanoporous TiO2 effectively suppressed the electron capture by the surface states from the conduction band of TiO2 and the consecutive electron transfer to the electrolyte in which the TiO2 is permeated. The reference DSSC showed activation energy of 1.03eV while the DSSCs with Al2O3 and HfO2 surface treatment showed 1.27 and 1.31eV respectively. The higher activation energy, in case of HfO2 surface treatment, suggest that electronically active TiO2 surface states present near to the redox potential of the electrolyte was reduced and hence resulted in improved photovoltaic performance than Al2O3 treated DSSC and the reference DSSC.

Thin films are the building blocks of small devices technology. The mechanical characterization of layers is a central step for their integration in industrial process. Instrumented indentation is an experimental measurement technique well suited to small scales.

In the film on substrate geometry, the deformation pattern during indentation is modified as compared to semi infinite homogenous solid. In this work, the effect of the constrained geometry on the indentation test is investigated on model material: Cu single crystal. The constitutive laws for the materials are based on a crystal plasticity model. This is not a strain gradient model as in [1] since no material length scale is introduced. The approach is similar to [2], except that the hardening is physically based, using dislocations densities on the 12 slip systems of the FCC crystal as internal variables. This modelling strategy gave good quantitative agreements with experiments in the case of various bulk Cu single crystals. It is used here in order to explore the geometry effect due to the finite thickness of elastic-plastic films deposited on elastic substrates. The criteria of comparison between the finite thickness films and the bulk samples are curves of indentation forces and stiffness versus indentation depth on the one hand, surface deformation on the other hand; it is straightforward to get these data from the finite elements simulations and from the atomic force microscopy (AFM). The simulations are compared to experimental data obtained on Cu films deposited on Si and Cu single crystals.

The effects of ion irradiation damage on dislocation generation and propagation in austenitic stainless steels were studied by means of in situ transmission electron microscopy and electron tomography. Tensile samples were irradiated in situ to a dose on the order of 1017 ions/m2 with 1MeV Kr+ and strained at 300 K as well as 673 K. Dislocation motion through the irradiation-obstacle field was jerky and discontinuous, dislocation pile-ups formed in grain interiors and at boundaries, long straight dislocations were generated decorating the channel-matrix walls, and dislocation cross-slip within the channel created debris along the channel leading to channel widening. Electron tomography was applied for the first time to reveal new detail about the dislocation reactions in the channel wall.

In order to investigate the formation of precipitates such as MC carbides and intermetallic compounds in the friction stir welded and post-heat-treated Inconel 718 alloy, this work was carried out. Furthermore, the microstructural and mechanical properties of welds and post-heat-treated material were evaluated to identify the effect on precipitates formed during post-heat-treatment. Friction stir welding (FSW) was performed at a rotation speed of 200 rpm and welding speed of 150 mm/min; heat treatment was performed after welding at 720 °C for 8 hours in vacuum. As a result, the grain size due to FSW was notably refined from 5–20 μm in the base material to 1–3 μm in the stir zone; this was accompanied by dynamic recrystallization, which resulted in enhancements in the mechanical properties as compared to the base material. In particular, applying heat treatment after FSW led to improvements in the mechanical properties of the welds—the microhardness and tensile strength increased by more than 50% and 40% in fraction, respectively, as compared to FSW alone.

Here we study the effect of radiation-induced point defect distributions on the optical reflectivity signal in GaAs using coherent acoustic phonon spectroscopy. We demonstrate that the presence of point defects significantly modifies the optical response, allowing estimation of the depth-dependent defect distribution in a nondestructive and noninvasive manner. We show that the observed changes are dependent on defect-induced changes to the electronic structure, namely defect-induced band tailing of the direct 1.43eV band edge. This provides a method for subsurface investigations on the complex interaction between different defects species and optoelectronic structure.

First principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculations are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structure of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic (001) double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. Another plausible reference state is the antiferromagnetic (001) single layer (AFM-1). We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that the calculated defect formation energies have fairly high dependence on the reference state chosen: in some cases this is due to instability of the reference state, a problem which does not seem to apply to AFM-d and AFM-1. Furthermore, there is a correlation between local free volume magnetism and energetics. Despite this, a general picture emerge that point defects in austenitic iron have geometries similar to those in simpler, non-magnetic, thermodynamically stable FCC metals. The defect energies are similar to those in BCC iron. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures.

We present a general method for investigating the energetics of small impurity-vacancy clusters in crystalline materials. We use a combination of molecular dynamics and Monte Carlo methods to locate low energy configurations of the bubbles, from which the binding energies of various point defects can be determined. This method is applied to case of hydrogen bubbles in alpha-iron. Clusters with ratios of up to 10 hydrogen atoms to a vacancy are studied. We find that hydrogen does help to stabilize voids in alpha-iron, but that hydrogen is quite weakly bound to these voids. Ratios of up to approximately 4 can be supported at low temperatures.

A fundamental understanding of the radiation damage effects in solids is of great importance in assisting the development of improved materials with ultra-high strength, toughness, and radiation resistance for nuclear energy applications. In this presentation, we show our recent theoretical investigation on the magnetic structure evolution of bulk iron in the region surrounding the radiation defects. We applied the locally self-consistent multiple scattering method (LSMS), a linear scaling ab-initio method based on density functional theory with local spin density approximation, to the study of the magnetic structure in a low energy cascade in a 10,000-atom sample for a series of time steps for the evolution of the defects. The primary damage state and the evolution of all defects in the sample were simulated using molecular dynamics with empirical, embedded-atom inter-atomic potentials. We also discuss the importance of thermal effect on the magnetic structure evolution.

The need for the development and deployment of reliable and efficient energy storage devices, such as lithium-ion rechargeable batteries, is becoming increasingly important due to the scarcity of petroleum. In this work, we provide an overview of commercially available cathode materials for Li-ion rechargeable batteries and focus on characteristics that give rise to optimal energy storage systems for future transportation modes. The study shows that the development of lithium-iron-phosphate (LiFePO4) batteries promises an alternative to conventional lithiumion batteries, with their potential for high energy capacity and power density, improved safety, and reduced cost. This work contributes to the fundamental knowledge of lithium-ion battery cathode materials and helps with the design of better rechargeable batteries, and thus leads to economic and environmental benefits.

The study of dislocation nucleation has gained increasing attentions recently primarily due to the advancement of small scale mechanical testing methods. Based on the classic Rice model of dislocation nucleation from a crack tip in which the dislocation core is modeled by a continuous slip field, a nonlinear finite element method can be formulated with the interplanar potential as the input, and the development of interplanar slip field can be solved from the resulting boundary value problems. The effects of geometric boundary conditions, loading patterns, etc. can be conveniently determined, as opposed to the time consuming molecular simulations. To validate the method, we compare the simulations results of homogeneous dislocation nucleation and heterogeneous dislocation nucleation from a two-dimensional crack tip to the literature results. As proposed by Rice and Beltz (J. Mech. Phys. Solids, 1994), the activation energy for dislocation nucleation from a three-dimensional crack tip depends on the finite thickness in the direction parallel to the crack tip, which has been successfully reproduced in the finite element simulation results reported here.

Nuclear fuel pins exhibit distortions in the UO2 lattice in response to temperature gradients, defects and the introduction of fission product (FP) gases. These distortions can have a significant influence on the activation barriers associated with fission product gas diffusion. A predictive understanding of this relationship is particularly relevant to anticipating the evolution of fission product gases during rapid temperature transients. Density Functional Theory (DFT) has the capacity to provide a relationship between lattice distortion and FP gas diffusivity by generating estimates of dilation dependent activation energies.

As a first step in this direction, the relation between lattice dilation and activation energy for isolated vacancies within an otherwise pristine block of alpha-quartz is quantified, where precise experimental data is readily available. The results lend confidence to the basic approach which is based on a one-shot transition state method, developed to lessen the computational resources required by the full transition state method. This technique is first applied to α-quartz for O vacancy hopping and diamond-Si for Si vacancy hopping. The method is subsequently extended to consider isolated uranium vacancies in UO2. This in turn is further generalized to estimate the activation volume for Kr atoms in UO2. Thus two different types of defects are considered; those of species native to the material and, in the case of UO2, FP gases introduced through the fission process.

Void formation in irradiated materials is an intriguing and technologically important physical process associated with radiation damage. In this communication, we present a diffuse interface model for simulating void formation in materials under irradiation. Voids are treated as aggregates of vacancies left from the cascade damage. The emergence of the void ensembles in the irradiated material is modeled by an Allen-Cahn equation coupled with two Cahn-Hilliard equations governing the space and time evolution of vacancies and interstitials. The governing system of equations includes stochastic generation of point defects representing the cascade process, reaction of vacancies and interstitials, interaction of point defects with extended defects (viz., void surface and grain boundaries) and thermal fluctuations in defects. Numerical simulations demonstrating the model capabilities with respect to nucleation and growth of voids and swelling of the irradiated material are presented.