EELS Spectrum Imaging and Tomography Studies of Simulated Nuclear Waste Glasses
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0985-NN06-01
EELS Spectrum Imaging and Tomography Studies of Simulated Nuclear Waste Glasses Guang Yang, Zineb Saghi, Xiaojing Xu, Russell Hand, and Günter Möbus Engineering Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, United Kingdom
ABSTRACT Electron energy loss spectroscopy (EELS) fine structure is a powerful technique for analyzing oxidation levels of rare-earth oxides and coordination numbers in glasses and ceramics, especially for boron. To exploit the unique advantage of EELS over x-ray absorption spectroscopy (XAS)/x-ray absorption near edge structure (XANES), namely nm-scale spatial resolution, EELS spectrum imaging across precipitates in glasses has been employed to detect lateral changes of EELS fine structure. Alkali borosilicate (ABS) glasses doped with Cr2O3, CeO2 and ZrO2 or Fe2O3 were melted to simulate high level radionuclide immobilization glasses. Precipitates with diameter in the range of ~20 nm to ~500 nm were found homogeneously distributed in the glasses. Ce valence was found to be mainly +3 in the glass matrix, and +4 in crystalline precipitates, while some amorphous particles show +3 as well. Another powerful TEM technique for the analysis of glass-nanocomposites is electron tomography, as it is up to now the only technique for the three-dimensional reconstruction of nanoparticles. A 3D reconstructed nuclear waste glass is presented in this paper by using a tilt series of ADF STEM images covering a glass fragment of ~ 3µm field of view containing several tens of nanoparticles distributed throughout its volume. INTRODUCTION Multi-component borosilicate glass systems have an ability to accommodate a wide range of cations, thus they have been used for the immobilization of high level nuclear waste (HLW) for several decades [1, 2]. Several techniques have been developed to analyze the chemical/ physical information in glass, such as nuclear magnetic resonance (NMR) [3, 4], x-ray absorption spectroscopy (XAS) [5], extended x-ray absorption fine structure (EXAFS) [6], Raman [7] etc. However, the application of these techniques is restricted to bulk specimens or to the micron scale features and none of them can clarify the structure changes due to precipitation or phase separation in the original glasses. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscopy (STEM) allows the usage of probe sizes down to 1nm enabling analysis of the glass composition, atomic coordination and elemental oxidation states with nanoscale spatial resolution. Ce, as one of the most important surrogate elements for Pu, has been studied widely, e.g. [8]. Ce can adopt two oxidation states in glass, +3 and +4, which have different solubilities in alkali borosilicate glasses. The white lines in its M4,5 edge region of the EEL spectrum enable quantification of its valence as demonstrated for glasses [9], crystalline minerals [10] and even to evaluate the electron irradiation damage on ceria [11]. This paper reports the extension of s
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