Alistair Garner

Identifying suboxide grains at the metal-oxide interface of a corroded Zr-1.0%Nb alloy using (S)TEM, transmission-EBSD and EELS.

Here we report a methodology combining TEM, STEM, Transmission-EBSD and EELS to analyse the structural and chemical properties of the metal-oxide interface of corroded Zr alloys in unprecedented detail. TEM, STEM and diffraction results revealed the complexity of the distribution of suboxide grains at the metal-oxide interface. EELS provided accurate quantitative analysis of the oxygen concentration across the interface, identifying the existence of local regions of stoichiometric ZrO and Zr3O2 with varying thickness. Transmission-EBSD confirmed that the suboxide grains can be indexed with the hexagonal ZrO structure predicted with ab initio by Nicholls et al. (2014). The t-EBSD analysis has also allowed for the mapping of a relatively large region of the metal-oxide interface, revealing the location and size distribution of the suboxide grains.

A method for accurate texture determination of thin oxide films by glancing-angle laboratory X-ray diffraction

The present article describes a modification to the standard method of glancing- angle X-ray diffraction for accurate measurement of the texture of thin oxide films. The technique resolves the problems caused by overlapping diffraction peaks originating from multiphase materials with asymmetric unit cells and the peak broadening associated with sample tilt during glancing-angle texture measurement. The entire 2? range of interest is recorded as a function of sample orientation, and the integrated intensities from different crystallographic planes are extracted from fitted diffraction profiles. The technique allows for pole figures to be plotted from diffraction peaks that could otherwise not be resolved and separates contributions from neighbouring peaks, leading to a more accurate representation of the existing oxide texture. The proposed method has been used for determining texture in a 3 mm layer of monoclinic/tetragonal zirconium oxide grown during aqueous corrosion testing and has been verified by additional synchrotron X-ray diffraction measurements.

Investigating the effect of zirconium oxide microstructure on corrosion performance: a comparison between neutron, proton and non-irradiated oxides

18th International Symposium on Zirconium in the Nuclear Industry 1Materials Performance Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK 2Studsvik Nuclear AB, 611 82 Nykoping, Sweden 3Westinghouse Electric Company, Hopkins, SC 29061, USA 4Rolls Royce, Derby DE21 7XX, UK 5Amec Foster Wheeler, Walton House, Faraday Street, Birchwood Park, Risley, Warrington WA3 6GA, UK

Developments in Large Volume 3D Analysis via P-FIB: EBSD & EDS

The integration of electron backscattered diffraction (EBSD) and energy dispersive spectroscopy (EDS) on scanning electron microscopes (SEM) is an increasingly common method of characterising materials. EDS offers chemical quantification and spatial distribution of the elements whilst EBSD enables microstructural characterisation. The integration of these two techniques with simultaneous acquisition, as in the AZtec platform, enables full material characterisation and data correlation within a single user interface.

Understanding Corrosion and Hydrogen Pickup of Zirconium Fuel Cladding Alloys: The Role of Oxide Microstructure, Porosity, Suboxides, and Second-Phase Particles

We have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a range of zirconium alloys at different stages of oxidation. Samples from both autoclave and in-reactor conditions were available to compare, including ZIRLO, Zr-1.0Nb and Zr-2.5Nb samples with different heat-treatments. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM 2,3 were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave and in-reactor samples, with shorter, less well-aligned monoclinic grains and more tetragonal grains seen in the neutron irradiated samples. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Atom Probe Tomography (APT) analysis of SPPs can reveal the main and the minor element distributions respectively. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of β-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in β-Nb SPPs in the oxide reveal the fully oxidised Nb state in the SPPs deep into the oxide, but Nb in the crystalline SPPs near the metaloxide interface. EELS analysis and automated crystal orientation mapping with TEM have also revealed Widmanstatten-type suboxide layers in some samples with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen rich zone is part of the protective oxide that is rate limiting in the key in the transport processes involved in oxidation. Porosity in the oxide has a major influence on the overall rate of oxidation, and there is much more porosity in the rapidly oxidising annealed Zr-1.0Nb alloy than found in either the recrystallised alloy or the similar alloy exposed to neutron irradiation.

Scalable Patterning of Encapsulated Black Phosphorus

Atomically thin black phosphorus (BP) has attracted considerable interest due to its unique properties, such as an infrared band gap that depends on the number of layers and excellent electronic transport characteristics. This material is known to be sensitive to light and oxygen and degrades in air unless protected with an encapsulation barrier, limiting its exploitation in electrical devices. We present a new scalable technique for nanopatterning few layered BP by direct electron beam exposure of encapsulated crystals, achieving a spatial resolution down to 6 nm. By encapsulating the BP with single layer graphene or hexagonal boron nitride (hBN), we show that a focused electron probe can be used to produce controllable local oxidation of BP through nanometre size defects created in the encapsulation layer by the electron impact. We have tested the approach in the scanning transmission electron microscope (STEM) and using industry standard electron beam lithography (EBL). Etched regions of the BP are stabilized by a thin passivation layer and demonstrate typical insulating behavior as measured at 300 and 4.3 K. This new scalable approach to nanopatterning of thin air sensitive crystals has the potential to facilitate their wider use for a variety of sensing and electronics applications.