Study on the dissolution of ß-precipitates in the Zr-1Nb alloy under the influence of Ne ion irradiation.

The stability of ß-precipitates in the Zr-1Nb alloy has been studied under Ne ion irradiation of energy 250 keV by insitu transmission electron microscope as a function of irradiation dose. The irradiation was carried out up to ∼136 dpa at 573 K. Microstructural investigations have shown that up to ∼38 dpa, precipitates showed an increase in size, and for irradiation doses >38 dpa, the size of the precipitates was noticed to reduce. Post-irradiation energy-dispersive spectrometry of the specimens revealed the Nb concentration throughout the matrix to be ∼0.8-1.5%. Three-dimensional atom probe tomography was also carried out for irradiated specimens to look for the presence of any nanoclusters. However, Nb clustering was not observed in the specimens. It is proposed that the dissolution of the precipitates may be facilitated by an increase in the solubility limit of Nb in Zr caused by irradiation. The solubility limit may increase by the introduction of defects generated by irradiation and by the destabilization of the ß-phase. This may result in back-diffusion of Nb atoms to the matrix by radiation-enhanced diffusion to lower the strain produced by the defects, resulting in the dissolution of the precipitates.

Understanding amorphization mechanisms using ion irradiation in situ a TEM and 3D damage reconstruction.

In this work, ion irradiations in-situ of a transmission electron microscope are performed on single-crystal germanium specimens with either xenon, krypton, argon, neon or helium. Using analysis of selected area diffraction patterns and a custom implementation of the Stopping and Range of Ions in Matter (SRIM) within MATLAB (which allows both the 3D reconstruction of the collision cascades and the calculation of the density of vacancies) the mechanisms behind amorphization are revealed. An intriguing finding regarding the threshold displacements per atom (dpa) required for amorphization results from this study: even though the heavier ions generate more displacements than lighter ions, it is observed that the threshold dpa for amorphization is lower for the krypton-irradiated specimens than for the xenon-irradiated ones. The 3D reconstructions of the collision cascades show that this counter-intuitive observation is the consequence of a heterogeneous amorphization mechanism. Furthermore, it is also shown that such a heterogeneous process occurs even for helium ions, which, on average induce only three recoils per ion in the specimen. It is revealed that at relatively high dpa, the stochastic nature of the collision cascade ensures complete amorphization via the accumulation of large clusters of defects and even amorphous zones generated by single-helium-ion strikes.

Effect of density and Z-contrast on the visibility of noble gas precipitates and voids with insights from Monte-Carlo simulations.

In this work, a detailed analysis of He, Ne, Ar, Kr and Xe precipitates in a complex borosilicate glass using transmission electron microscopy (TEM) with in-situ ion implantation is presented. With in-situ monitoring, the real-time dynamics of precipitate and void evolution under ion implantation was followed. Using appropriate equations of state and, Monte-Carlo simulations to supplement the TEM images, we then discuss in detail the possibility and ways of differentiating the precipitates of various noble gases from empty voids. It is shown that all the noble gases precipitate as inclusions of supercritical fluid. With the aid of the simulations, the crucial role played by the size and density of the precipitates and atomic number of the gas atoms in affecting the visibility of the precipitates is highlighted. The results show that the precipitates and voids can be unambiguously differentiated in the case of Xe and Kr whereas the precipitates of other lighter noble gases cannot be differentiated from the voids. However, the precipitate and void evolution under ion irradiation follow different dynamics, knowledge of which allows one to differentiate between the precipitates and voids even for lighter noble gases. Besides shedding light on the subject of noble gas precipitation and identification of the precipitates and voids, the study highlights the complexity in dissociating the behaviour of voids from the process of precipitate re-solution. This type of knowledge is pivotal in developing models describing the evolution of precipitates, voids and macroscopic porosity in a number of materials.

Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy.

Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium ß-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, bubble diameter remained constant throughout the implantation, but bubble density increased with implantation time. ß-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the bubble nucleation and growth behavior.

Ion-beam-induced bending of semiconductor nanowires.

The miniaturisation of technology increasingly requires the development of both new structures as well as novel techniques for their manufacture and modification. Semiconductor nanowires (NWs) are a prime example of this and as such have been the subject of intense scientific research for applications ranging from microelectronics to nano-electromechanical devices. Ion irradiation has long been a key processing step for semiconductors and the natural extension of this technique to the modification of semiconductor NWs has led to the discovery of ion beam-induced deformation effects. In this work, transmission electron microscopy with in situ ion bombardment has been used to directly observe the evolution of individual silicon and germanium NWs under irradiation. Silicon NWs were irradiated with either 6 keV neon ions or xenon ions at 5, 7 or 9.5 keV with a flux of 3 × 1013 ions cm-2 s-1. Germanium NWs were irradiated with 30 or 70 keV xenon ions with a flux of 1013 ions cm-2 s-1. These new results are combined with those reported in the literature in a systematic analysis using a custom implementation of the transport of ions in matter Monte Carlo computer code to facilitate a direct comparison with experimental results taking into account the wide range of experimental conditions. Across the various studies this has revealed underlying trends and forms the basis of a critical review of the various mechanisms which have been proposed to explain the deformation of semiconductor NWs under ion irradiation.

Rapid and damage-free outgassing of implanted helium from amorphous silicon oxycarbide.

Damage caused by implanted helium (He) is a major concern for material performance in future nuclear reactors. We use a combination of experiments and modeling to demonstrate that amorphous silicon oxycarbide (SiOC) is immune to He-induced damage. By contrast with other solids, where implanted He becomes immobilized in nanometer-scale precipitates, He in SiOC remains in solution and outgasses from the material via atomic-scale diffusion without damaging its free surfaces. Furthermore, the behavior of He in SiOC is not sensitive to the exact concentration of carbon and hydrogen in this material, indicating that the composition of SiOC may be tuned to optimize other properties without compromising resistance to implanted He.

Ion implantation in nanodiamonds: size effect and energy dependence.

Nanoparticles are ubiquitous in nature and are increasingly important for technology. They are subject to bombardment by ionizing radiation in a diverse range of environments. In particular, nanodiamonds represent a variety of nanoparticles of significant fundamental and applied interest. Here we present a combined experimental and computational study of the behaviour of nanodiamonds under irradiation by xenon ions. Unexpectedly, we observed a pronounced size effect on the radiation resistance of the nanodiamonds: particles larger than 8 nm behave similarly to macroscopic diamond (i.e. characterized by high radiation resistance) whereas smaller particles can be completely destroyed by a single impact from an ion in a defined energy range. This latter observation is explained by extreme heating of the nanodiamonds by the penetrating ion. The obtained results are not limited to nanodiamonds, making them of interest for several fields, putting constraints on processes for the controlled modification of nanodiamonds, on the survival of dust in astrophysical environments, and on the behaviour of actinides released from nuclear waste into the environment.

Reversible loss of Bernal stacking during the deformation of few-layer graphene in nanocomposites.

The deformation of nanocomposites containing graphene flakes with different numbers of layers has been investigated with the use of Raman spectroscopy. It has been found that there is a shift of the 2D band to lower wavenumber and that the rate of band shift per unit strain tends to decrease as the number of graphene layers increases. It has been demonstrated that band broadening takes place during tensile deformation for mono- and bilayer graphene but that band narrowing occurs when the number of graphene layers is more than two. It is also found that the characteristic asymmetric shape of the 2D Raman band for the graphene with three or more layers changes to a symmetrical shape above about 0.4% strain and that it reverts to an asymmetric shape on unloading. This change in Raman band shape and width has been interpreted as being due to a reversible loss of Bernal stacking in the few-layer graphene during deformation. It has been shown that the elastic strain energy released from the unloading of the inner graphene layers in the few-layer material (~0.2 meV/atom) is similar to the accepted value of the stacking fault energies of graphite and few layer graphene. It is further shown that this loss of Bernal stacking can be accommodated by the formation of arrays of partial dislocations and stacking faults on the basal plane. The effect of the reversible loss of Bernal stacking upon the electronic structure of few-layer graphene and the possibility of using it to modify the electronic structure of few-layer graphene are discussed.