Utilising DualEELS to probe the nanoscale mechanisms of the corrosion of Zircaloy-4 in 350 °C pressurised water

Characterisation of materials utilised for fuel cladding in nuclear reactors prior to service is integral in order to understand corrosion mechanisms which would take place in reactor. Zircaloy-4 is one such material of choice for nuclear fuel containment in Pressurised Water Reactors (PWRs). In particular, the metal-oxide interface has been a predominant focus of previous research, however, due to the complex oxidation process of zirconium cladding, there is still no clear understanding of what is present at the interface. Using Scanning Transmission Electron Microscopy (STEM) and Dual Electron Energy Loss Spectroscopy (DualEELS), we have studied the corrosion of this material under conditions similar to those that could be encountered in service. It is shown that under all conditions, whether during faster oxidation in the early stages, slow growth just prior to the transition to a new growth regime, or in the faster growth that happens after this transition, the surface of the metal below the scale is loaded with oxygen up to around 33 at%. Approaching transition, in conditions of slow growth and slow oxygen supply, an additional metastable suboxide is apparent with a thickness of tens of nm. By studying changes in both chemical composition and dielectric function of the material at the oxide scale – metal interface with nanometre resolution, quantitative mapping could be achieved, clearly showing that this is a suboxide composition of ZrO and a Zr oxidation state close to +2

Low-loss electron energy loss spectroscopy in a scanning transmission electron microscope of GaInNAs

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The corrosion of Zr(Fe, Cr)₂ and Zr₂Fe secondary phase particles in Zircaloy-4 under 350ºC pressurised water conditions

Using Scanning Transmission Electron Microscopy (STEM) coupled with Dual Electron Energy Loss Spectroscopy (DualEELS) and scanned diffraction, the corrosion and incorporation of Secondary Phase Particles (SPPs) in the oxide layer of Zircaloy-4 material has been investigated. This study focuses on mapping the corrosion of Zr₂Fe and Zr(Fe, Cr)₂ precipitates during the oxidation process and depicting their morphology as the oxidation front advances through the material. It has been found that Zr₂Fe SPPs retain the same general shape as in their pre oxidation stage, and transform to a nanocrystalline homogeneous mixed oxide, with a strong crystallographic texture, but hitherto unknown structure. The Zr(Fe, Cr)₂ Laves-phase SPPs however, oxidise in a notably more complicated manner. As the α-Zr around an SPP begins to oxidise, the SPP is completely encapsulated by the ZrO₂ whilst much of the SPP remains initially unoxidised. But, on oxidation, significant elemental segregation takes place, usually leaving a Cr₂O3-rich cap, a nanocrystalline Zr,Cr mixed oxide body and veins of well-crystallised metallic iron. Both forms of SPP have a different expansion on oxidation compared to the Zr, resulting in cracking of the ZrO₂

A transmission electron microscopy study of fe-Co alloy nanoparticles in silica aerogel matrix using HREM, EDX, and EELS

Magnetic nanocomposite materials consisting of 5.5 wt% Fe-Co alloy nanoparticles in a silica aerogel matrix, with compositions FexCo1x of x = 0.50 and 0.67, have been synthesized by the sol-gel method. The high-resolution transmission electron microscopy images show nanoparticles consisting of single crystal grains of body-centered cubic Fe-Co alloy, with typical crystal grain diameters of approximately 4 and 7 nm for Fe0.5Co0.5 and Fe0.67Co0.33 samples, respectively. The energy dispersive X-ray (EDX) spectra summed over areas of the samples gave compositions FexCo1x with x = 0.48 0.06 and 0.68 0.05. The EDX spectra obtained with the 1.5 nm probe positioned at the centers of 20 nanoparticles gave slightly lower concentrations of Fe, with means of x = 0.43 0.01 and x = 0.64 0.02, respectively. The Fe0.5Co0.5 sample was studied using electron energy loss spectroscopy (EELS), and EELS spectra summed over whole nanoparticles gave x = 0.47 0.06. The EELS spectra from analysis profiles of nanoparticles show a distribution of Fe and Co that is homogeneous, i.e., x = 0.5, within a precision of at best 0.05 in x and 0.4 nm in position. The present microscopy results have not shown the presence of a thin layer of iron oxide, but this might be at the limit of detectability of the methods