Electron Irradiation and Electron Tomography Studies of Glasses and Glass Nanocomposites
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Electron Irradiation and Electron Tomography Studies of Glasses and Glass Nanocomposites G. Möbus, G. Yang, Z. Saghi, X. Xu, R.J. Hand, A. Pankov, M.I. Ojovan Immobilisation Science Laboratory, Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK ABSTRACT Characterization of glasses and glass nanocomposites using modern transmission electron microscopy techniques is demonstrated. Techniques used include: (i) high-angle-annular dark field STEM for imaging of nanocomposites, (ii) electron tomography for 3D reconstruction and quantification of nanoparticle volume fractions, and (iii) fine structure electron energy loss spectroscopy for evaluation of boron coordination. Precipitation of CeO2 nanoparticles in borosilicate glasses is examined as a function of glass composition and redox partner elements. A large increase in the solubility of Ce is found for compositions where Ce retains +IV valence in the glass. Irradiation experiments with electrons and γ-rays are summarized and the degree of damage is compared by using changes in the boron K-edge fine structure, which allows the gradual transition from BO4 to BO3 coordination to be followed. INTRODUCTION Some of the most important benefits of transmission electron microscopy for the study of nuclear waste materials lie in the high-spatial resolution imaging of inhomogeneities, such as nanoscale precipitates in glass composite materials. Another important application is in high spatial resolution analysis, e.g. to track glass coordination parameters as a function of temperature and/or irradiation. In this paper we present examples of each of these fields, namely 2D and 3D mapping of nanocomposites by STEM in part 1, and irradiation induced transformation of boron coordination in glass in part 2. Borosilicate glass chemistry is diverse with respect to phase separation phenomena and crystallization behavior and has therefore attracted major interest in both the glass community as well as in nuclear and hazardous waste immobilization community. Understanding the chemistry of precipitation aids design of glass compositions that avoid unwanted precipitates, but also enables the design of glass composite materials, that can achieve high waste loadings, by deliberately developing selected precipitates [1-3]. The glasses considered in this paper are summarized in tables 1 and 2. PART I: GLASS COMPOSITE MATERIALS: IMAGING & VALENCE ESTIMATION The solubility and redox chemistry of Ce (a common Pu surrogate) was studied by adding a constant fraction of 4mol% CeO2 to 5 borosilicate glasses, varying in glass composition or other waste loading elements, so as to provide a variety of redox active partner elements for Ce, as detailed in Table 1. The microstructure of glass fragments was examined by dark-field STEM
(Fig. 1 a-d), where heavy particles appear bright (Z-contrast). Glasses (i) and (ii) differ in alkali choice, where Na/Li is more industrially relevant, while K has been chosen for its higher electron beam resistance. In previous work we have i
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