Processing and Magnetic Properties of Ni-Fe Alloy Dispersed Alumina Nanocomposites
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Processing and Magnetic Properties of Ni-Fe Alloy Dispersed Alumina Nanocomposites Jai-Sung Lee, Sung-Tag Oh and Seok Namkung Dept of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, KOREA ABSTRACT An optimum route to fabricate the Al2O3/Ni-Fe alloy nanocomposites with sound microstructure and magnetic properties was investigated. The composites were fabricated by the hot pressing of powder mixtures of Al2O3 and nano-sized Ni-Fe alloy, in which the alloy was prepared by solution-chemistry routes using Ni- and Fe-nitrate powders and a subsequent hydrogen reduction process. Microstructural observation of reduced powder mixture revealed that the Ni-Fe alloy particles of about 20 nm in size were homogeneously surrounded Al2O3, forming nanocomposite powder. The hot-pressed composite showed improved mechanical properties and magnetic response. The properties are discussed in terms of microstructural characteristics such as the distribution and size of Ni-Fe dispersion.
INTRODUCTION Ceramic matrix metal nanocomposites have been of interest because of the possibility of superior mechanical and functional properties [1-3]. For example, fracture strength of Al2O3 could be increased three times by the dispersion of nano-sized Ni particles [4]. Besides the improved mechanical properties, Al2O3/Ni nanocomposite also exhibits the ferromagnetic properties such as high coercive force, because of the fine magnetic dispersions, which indicates a functional value of structural composites. However, for a wide application of this novel concept to fabricating ceramic-based materials having a synergetic combination of mechanical and functional properties, an elaborate approach to microstructure control and the optimal fabrication process is required, especially in the composites system with multicomponent dispersion. One of the objectives of this study was, therefore, to prepare homogeneous powder mixtures of Al2O3, which has been well examined as a structural ceramic, and nano-sized Fe-50 wt% Ni alloy, as a material having ferromagnetic property. The other purpose of the study was to evaluate the mechanical and magnetic properties of the hot-pressed composites and to analyze the relationship between the microstructure and those properties.
EXPERIMENTAL PROCEDURE A mixture of metallic nitrates of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O, of which composition corresponded to the Ni-50 wt% Fe in the final composite, were initially dissolved in ethyl alcohol. Subsequently, high-purity α-Al2O3 powder with particle size of 0.2 µm (AKP-53, Sumitomo Chemical Co., Japan) was mixed with the above-mentioned solution and ball-milled for 24 h with high-purity Al2O3 balls. The powder mixture would result in Al2O3 /10 wt% Ni-Fe alloy after reduction in a reducing atmosphere. Dried mixture was calcined at 400ºC for 2 h in air to remove organic and nitrate residuals. Soft agglomerates of the calcined powder were crushed Y8.40.1
by wet and dry ball-milling for 24 h. The powder mixture was kept in a graphite die and reduced by using H2 gas a
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