Publications

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Localized corrosion of Cu and Al thin films exposed to aqueous NaCl solutions was studied using liquid cell transmission electron microscopy (LCTEM). We demonstrate that potentiostatic control can be used to initiate pitting and that local compositional changes, due to focused ion beam implantation of Au+ ions, can modify the corrosion susceptibility of Al films. This journal is

The progressive build up of fission products inside different nuclear reactor components can lead to significant damage of the constituent materials. We demonstrate the use of time-domain thermoreflectance (TDTR), a nondestructive thermal measurement technique, to study the effects of radiation damage on material properties. We use TDTR to report on the thermal conductivity of optimized ZIRLO, a material used as fuel cladding in nuclear reactors. We find that the thermal conductivity of optimized ZIRLO is 10.7 ± 1.8 W m^-1 K^-1 at room temperature. Furthermore, we find that the thermal conductivities of copper-niobium nanostructured multilayers do not change with helium ion irradiation doses of 10¹⁵ cm^-2 and ion energy of 200 keV, demonstrating the potential of heterogeneous multilayer materials for radiation tolerant coatings. Finally, we compare the effect of ion doses and ion beam energies on the measured thermal conductivity of bulk silicon. Our results demonstrate that TDTR can be used to quantify depth dependent damage.

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Transmission electron microscopy (TEM) is a valuable methodology for investigating radiation-induced microstructural changes and elucidating the underlying mechanisms involved in the aging and degradation of nuclear reactor materials. However, the use of electrons for imaging may result in several inadvertent effects that can potentially change the microstructure and mechanisms active in the material being investigated. In this study, in situ TEM characterization is performed on nanocrystalline nickel samples under self-ion irradiation and post irradiation annealing. During annealing, voids are formed around 200 °C only in the area illuminated by the electron beam. Based on diffraction patterns analyses, it is hypothesized that the electron beam enhanced the growth of a NiO layer resulting in a decrease of vacancy mobility during annealing. The electron beam used to investigate self-ion irradiation ultimately significantly affected the type of defects formed and the final defect microstructure.

A new open-source project evaluation tool, entitle the Materials Engineering Tetrahedron (MET), has been developed to determine the economic viability of materials design, selection, processing, and validation costs associated with any infrastructure-based project. MET improves project design by providing an economic perspective to the traditional materials science tetrahedron, relating microstructure, processing, property, and performance through the introduction of value-based economic costs for each side of the tetrahedron. The resulting size and distortion, from a regular tetrahedron illustrates the balance between the system, component, or material fabrication project detailed. Furthermore, the MET model also allows for increased budget efficiency and the potential for improved identification of cost saving mechanisms.

We investigated the effects of helium ion irradiation energy and sample temperature on the performance of grain boundaries as helium sinks in ultrafine grained and nanocrystalline tungsten. Irradiations were performed at displacement and non-displacement energies and at temperatures above and below that required for vacancy migration. Microstructural investigations were performed using Transmission Electron Microscopy (TEM) combined with either in-situ or ex-situ ion irradiation. Under helium irradiation at an energy which does not cause atomic displacements in tungsten (70 eV), regardless of temperature and thus vacancy migration conditions, bubbles were uniformly distributed with no preferential bubble formation on grain boundaries. Moreover, at energies that can cause displacements, bubbles were observed to be preferentially formed on the grain boundaries only at high temperatures where vacancy migration occurs. Under these conditions, the decoration of grain boundaries with large facetted bubbles occurred on nanocrystalline grains with dimensions less than 60 nm. Finally, we discuss the importance of vacancy supply and the formation and migration of radiation-induced defects on the performance of grain boundaries as helium sinks and the resulting irradiation tolerance of ultrafine grained and nanocrystalline tungsten to bubble formation.

In this study, we demonstrate chemical doping of a topological insulator Bi₂Se₃ using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi₂Se₃ thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi₂Se₃, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.

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The reliability of nanomaterials depends on maintaining their specific sizes and structures. However, the stability of many nanomaterials in radiation environments remains uncertain due to the lack of a fully developed fundamental understanding of the radiation response on the nanoscale. To provide an insight into the dynamic aspects of single ion effects in nanomaterials, gold nanoparticles (NPs) with nominal diameters of 5, 20, and 60 nm were subjected to self-ion irradiation at energies of 46 keV, 2.8 MeV, and 10 MeV in situ inside of a transmission electron microscope. Ion interactions created a variety of far-from-equilibrium structures including small (∼1 nm) sputtered nanoclusters from the parent NPs of all sizes. Single ions created surface bumps and elongated nanofilaments in the 60 nm NPs. Similar shape changes were observed in the 20 nm NPs, while the 5 nm NPs were transiently melted or explosively broken apart.

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Here we investigated the microstructural response of various Pd physically vapor deposited films and Er and ErD₂ samples prepared from neutron Tube targets to implanted He via in situ ion irradiation transmission electron microscopy and subsequent in situ annealing experiments. Small bubbles formed in both systems during implantation, but did not grow with increasing fluence or a short duration room temperature aging (weeks). Annealing produced large cavities with different densities in the two systems. The ErD₂ showed increased cavity nucleation compared to Er. The spherical bubbles formed from high fluence implantation and rapid annealing in both Er and ErD₂ cases differed from microstructures of naturally aged tritiated samples. Further work is still underway to determine the transition in bubble shape in the Er samples, as well as the mechanism for evolution in Pd films.