Quantification of Displacement Fields from Lattice Images (HREM) and Electron Exit Waves in Nanostructured Composite Oxi
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Quantification of Displacement Fields from Lattice Images (HREM) and Electron Exit Waves in Nanostructured Composite Oxides H. A. Calderon, A. Huerta, C. Kisielowski1, R. Kilaas1. Dept. Ciencia de Materiales, ESFM-IPN, Apdo. Postal 75-707, Mexico D.F., 1 NCEM-LBNL, Berkeley CA, USA. ABSTRACT Oxide composites have been produced by means of mechanical milling and sintering. Such materials consist of magnetic particles embedded in an insulating oxide matrix, which can have application as magnetic wave absorbers. Different systems have been investigated including MgO-MgFe2O4, FeO-Fe3O4 and other more complex ferrites. Milling induces the formation of a solid solution of cations and oxygen. Depending on temperature and system, sintering can produce a particle dispersion or simply the starting of the corresponding phase transformation. Spark plasma sintering was performed between 773 and 1373 K in order to reduce both processing time and temperature. High resolution (HREM) and conventional transmission electron microscopy show that an spinodal decomposition takes place in the system FeO-Fe3O4 during transformation from the solid solution. Generally the oxygen lattice is unique throughout the material but there is a distinctive distribution of cations giving rise to a particle dispersion having an spinel structure in a insulating matrix with a NaCl cubic structure. INTRODUCTION Materials for absorption of electromagnetic waves can be made of pure materials (ferrites) [1,2] or by using a polymer to support a dispersion of magnetic particles [3]. Ferrites can be used as electromagnetic wave absorbers since their physical properties such as permittivity and permeability depend upon chemical composition, density (postsintering density in the case of compacted poeders), grain size and porosity [4]. Some efforts have been made also to evaluate the effect of a nanostructure (including grain size and scale of the magnetic centers) on the wave absorbing properties of this type of materials. The present investigation deals with the production of a wave absorber by using mechanical milling. Such an experimental method produces nanostructured materials usually in a non stable condition but proper heat treatment can be used to achieve the desired dispersion of nanomagnetic particles in the insulating matrix [5]. During production of such materials, sintering of powders was applied at different temperatures and the resulting nanostructures have been characterized by means of Mossbauer spectroscopy, X-ray diffraction, electron microscopy and measurement of magnetic properties. Most of such techniques can be combined to follow the development of a nanostructure. However, the required characterization at a nanometric scale could only be offered by High Resolution Electron Microscopy (HREM). This technique has been traditionally used to obtain a detailed view of the microstructure but it is difficult to perform precise measurements due to restrictions imposed by the image formation process. A HREM image is produced by interference o
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