Janelle Wharry

Effects of proton irradiation on structural and electrochemical charge storage properties of TiO2 nanotube electrodes for lithium-ion batteries

The effects of proton irradiation on nanostructured metal oxides have been investigated. Recent studies suggest that the presence of structural defects (e.g. vacancies and interstitials) in metal oxides may enhance the materials electrochemical charge storage capacity. A new approach to introduce defects in electrode materials is to use ion irradiation as it can produce a supersaturation of point defects in the target material. In this work we report the effect of low-energy proton irradiation on amorphous TiO2 nanotube electrodes at both room temperature and high temperature (250 °C). Upon room temperature irradiation the nanotubes demonstrate an irradiation-induced phase transformation to a mixture of amorphous, anatase, and rutile domains while showing a 35% reduction in capacity compared to anatase TiO2. On the other hand, the high temperature proton irradiation induced a disordered rutile phase within the nanotubes as characterized by Raman spectroscopy and transmission electron microscopy, which displays an improved capacity by 20% at ∼240 mA h g−1 as well as improved rate capability compared to an unirradiated anatase sample. Voltammetric sweep data were used to determine the contributions from diffusion-limited intercalation and capacitive processes and it was found that the electrodes after irradiation had more contributions from diffusion in lithium charge storage. Our work suggests that tailoring the defect generation through ion irradiation within metal oxide electrodes could present a new avenue for designing advanced electrode materials.

EBSD and TEM Analysis of the Heat Affected Zone of Laser Welded AISI 304/308 Stainless Steel

The objective of this study is to utilize a range of advanced microscopy techniques to characterize the heat affected zone (HAZ) of an AISI 304 stainless steel (SS) laser weld. Laser weldments of 304 SS, made with Type 308 SS as a filler material, are of interest to the nuclear power industry because of its low energy input as compared to conventional welding techniques. Laser welds are especially relevant for mid-life weld repairs of light water reactor (LWR) internals, which contain helium, irradiationinduced voids, and other irradiated microstructural phenomena [1]. Conventional welding processes such as gas tungsten arc welding (GTAW) introduce thermal stresses, which leads to the formation of helium bubbles on the grain boundaries [2]. This phenomenon directly results in disastrous heliuminduced cracking on the weld boundary. Laser welding, however, is hypothesized to reduce the helium coalescence and cracking at the weld boundary, due to the reduced heat input that inhibits surface cracking and subsurface defects [3]. Nevertheless, no studies have characterized the laser welded microstructures at resolutions greater than scanning electron microscopy (SEM) level resolution. In this work, we utilize a combination of SEM and transmission electron microscopy (TEM) to understand the laser welded microstructure of a laser weld on unirradiated and irradiated 304 SS.

In situ TEM Fracture Testing for Shallow Ion Irradiated Layers

The objective of this work is to demonstrate in situ transmission electron microscopic (TEM) bend tests for evaluating the fracture toughness of materials. Conventional fracture toughness tests, such as Charpy V-notch or compact tension (CT) configurations, require large specimen volumes having homogeneous properties and microstructures throughout that volume in order to obtain meaningful quantitative results [1]. But these test geometries are unfeasible for volume-limited materials such as thin films or ion implanted or irradiated layers, and for hazardous specimens that are difficult to handle in large quantities, such as radioactive materials. Hence, in situ TEM nanomechanical testing may offer a transformative method to evaluate the fracture properties of such materials. In situ TEM mechanical testing also offers the distinct advantage of enabling concurrent TEM-resolution imaging/video with quantitative mechanical testing of sub-micron-sized electron-transparent specimens, enabling direct linkage of plastic phenomena to mechanical behavior.

In situ TEM Mechanical Testing: An Emerging Approach for Characterization of Polycrystalline, Irradiated Alloys

In situ transmission electron microscopic (TEM) mechanical testing techniques enable concurrent TEMresolution imaging/video and mechanical testing of sub-micron-sized electron-transparent specimens. Because nuclear materials are often volume-limited, due to constraints imposed either by radioactivity levels or near-surface ion irradiation damage layers, in situ TEM mechanical testing presents great potential for analyzing these small specimen volumes. But thus far, only a few studies have conducted in situ TEM mechanical tests on irradiated or engineering alloys. Irradiated alloy work [1] focused on single-crystal Cu that was irradiated after the TEM specimen was fabricated, while the oxide dispersion strengthened (ODS) alloy tested [2] was unirradiated. The objective of the present study, then, is to extend the use of in situ TEM mechanical tests to ODS alloys that have previously been irradiated in bulk form, which is a conventional specimen configuration amongst the nuclear materials community.

Collected Data Set Size Considerations for Atom Probe Cluster Analysis

Atom probe tomography is increasingly being used to complement transmission electron microscopy (TEM) to characterize microstructures, particularly for nano-featured materials containing phases that are below TEM resolution limits. Local electrode atom probe (LEAP) tomographic cluster analysis algorithms provide an objective means to identify and measure the size and number density of these nano-scale phases. However, there is a lack of standardized methodology for quantifying average cluster size, which presents ambiguities and challenges when attempting to compare nanocluster morphology between different specimens. A critical consideration in LEAP data analysis is the number of ions collected from each needle. Thus, the objective of this study is to consider the effect of LEAP collected sample size on the measured cluster size, and to suggest methods to improve the fidelity of comparing average cluster sizes between larger and smaller data sets.