Analytical STEM of Borosilicate Glasses Containing Molybdates.
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Analytical STEM of Borosilicate Glasses Containing Molybdates. Rick J. Short, Günter Möbus, Guang Yang, Russell J. Hand, Neil Hyatt, William E. Lee. Dept of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK ABSTRACT Borosilicate nuclear waste glasses with various amounts of simulated waste elements have been examined by analytical TEM. This preliminary Scanning Transmission Electron Microscopy (STEM) study evaluates the capability of EELS for mapping coordination parameters of the base glass structure, to apply EDX-mapping and HAADF Z-contrast mapping for the characterisation and shape-mapping of common precipitates in nuclear glasses, such as molybdates, and to study irradiation effects.
INTRODUCTION Borate, borosilicate, and alkali-borosilicate glasses have been the subject of a large number of studies applying a multitude of characterisation techniques [1-5], such as NMR, X-ray absorption spectroscopy, Raman spectroscopy, XPS, ESR, TEM, and others. The aims of these spectroscopies are to determine the glass structure (radial distribution function), short and medium range order, and local coordination, e.g. of boron as a function of alkali content. Solute elements or precipitates have been analysed for valence and/or coordination [6-7]. However, it is still not clear to what extent the basic glass structure examinations on binary / ternary systems can be extrapolated to complex compositions of nuclear waste glasses with more than 20 elements present. Borosilicates as the basis for nuclear waste glasses and current research needs are e.g. summarised in [8-9]. A promising technique which can be used simultaneously for the nanoscale imaging of inhomogeneous or phase separated glasses and for local structural analysis of glass coordination is scanning transmission electron microscopy (STEM), using three signals on three detectors: (i) EDX-spectroscopy in STEM leading to 2D chemical maps of glass/precipitate systems, (ii) High-angle annular dark field (HAADF)-STEM leading to Z-contrast images with intensity roughly proportional to t*Z2, where t is thickness and Z atomic number, and (iii) EELS (electron energy loss spectroscopy) with a focused beam, especially near-edge-fine-structure (ELNES), aiming at chemical and coordination information with resolution down to nm-sized volumes. The glass precipitation studies in this work are aimed at establishing the dependency of crystal formation in nuclear borosilicate waste glasses on the conditions of glass melting (oxidising vs reducing) and on post-melt heat treatments. Molybdates are an important model system of both practical and scientific relevance due to the varying Mo valence leading to different solubility limits and different overall glass durability values [10].
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EELS ANALYSIS OF GLASS STRUCTURE Of the six borosilicate glasses A-F (table 1), three TEM samples I-III have been analysed by EELS to test the sensitivity and reliability of coordination measurements (see also [1113]) against established techniques, such as NMR. The
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