Annealing effect on microstructure and magnetic properties of flake-shaped agglomerates of Ni–20wt%Fe nanopowder
Introduction
The rapid growth and development of communication devices using the electromagnetic wave of GHz range has created more serious electro-magnetic interference problems [1]. As the best solution for this issue, electromagnetic wave absorbing materials have been developed and applied to industrial products. These materials are classified into three types according to their absorption mechanism: dielectric loss, conductive loss and magnetic loss. The latter magnetic-type absorbers which are composites made of soft magnetic powder embedded in polymer offer interesting features according to their complex permeability and permittivity [2], [3].
In general, metal oxides as ferrite powders are used for the application of electromagnetic absorption media in the MHz range of the electromagnetic wave. These ferrite materials, however, are rarely applied in the GHz range due to the sharp decrease of magnetic loss caused by their Snoek’s limit [4], [5], [6]. On contrary, the magnetic-type composite absorber containing soft magnetic metallic materials such as Fe–Si–Al or Ni–Fe alloy powders are suitable for electromagnetic wave absorbers in the GHz range in that in that they still possess high magnetization values even in a high frequency range and their Snoek’s limit shifts to a higher frequency than that of ferrites [1], [2], [7], [8].
As far as the optimal powder structure for electromagnetic absorption media in a high frequency range is concerned, a flake-shaped thin structure is known to be advantageous owing to the following reasons. One thing is due to the fact that the flakes are thin enough to overcome the skin effect in GHz frequencies [7], [8], [9], [10]. The other is based upon the fact that the flake-shape can remarkably increase the powder packing density of the powder-binder composite sheet materials, which improves the electromagnetic absorption property in the GHz range [10]. Currently flake-shaped magnetic alloy powders are produced by forging the atomized alloy powders using high energy ball-milling technology. Such a forging process results in storing high stress in the powders which deteriorates the soft magnetic properties, with the consequence of lowering the magnetic loss property [8], [11], [12].
In order to solve these problems described, we develop a new conceptual design of flake-shaped magnetic metal powders based upon a bottom-up process. It is basically distinguished from a top-down process for the forged powders as described. The key-idea of this technology is to fabricate flake-shaped nanopowder agglomerates with much less strain energy compared to the forged atomized powders. The nanopowder agglomerates are porous, so they can easily deform into flake shape by particle rearrangement. Also, such microstructures are presumably expected to bring unique EMI absorbing performance. As the first step to verify this idea, the present study has focused on understanding the relationship between microstructure and the magnetic properties of flake-shaped metal magnetic nanopowder agglomerates. As the material system for experiment, a Ni–Fe permalloy with high permeability was chosen. Based on our previous work [13], [14], microporous Ni–Fe agglomerate powders were produced by hydrogen reduction of ball-milled oxide mixture of NiO and Fe2O3. And then these agglomerate powders underwent flake-milling and subsequently annealing treatment. Microstructural developments occurred during annealing and related magnetic properties were examined and their relationship was discussed.
Section snippets
Experimental
The flake-shaped Ni–20wt%Fe nanopowder agglomerates were prepared by hydrogen reduction of ball-milled oxide powders and subsequent flake-milling processes [13], [14], [15]. Briefly, a powder mixture of NiO–Fe2O3 with a final composition of Ni–20wt%Fe was prepared by blending NiO (4 μm, 99.99%) and α-Fe2O3 (30 μm, 99.99%) powders. Ball-milling of oxide powders was performed in zirconia bead-mill at a speed of 2400 rpm for 10 h in which methyl alcohol was used as a process control agent (PCA). After
Effect of annealing on the microstructure development in the flake-milled nanopowders
Fig. 1 shows SEM micrographs of (a) as-reduced- and (b) as-flake-milled Ni–Fe nanopowder agglomerates. The as-reduced agglomerates consisted of nanoparticles of about 30 nm in size including micro-pores. The micro-pores in the agglomerates enhance particle rearrangement during milling so as to easily form flake-shaped agglomerates. As a result, through only low energy milling process, the spherical agglomerates with 10 μm in diameter could be deformed into thin plate-like agglomerate powders in
Conclusion
The microstructural and magnetic properties of flake-shaped Ni–20wt%Fe magnetic nanopowder agglomerates were investigated for the purpose of developing electromagnetic absorption material in a high frequency range based upon a bottom-up process. It was found that the spherical agglomerates which consisted of nanoparticles of about 30 nm in size were deformed into thin plate-like agglomerate powders of 1 μm in thickness by only low energy milling process. The XRD analysis revealed that the
Acknowledgements
We would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and ISTK (Korea Research Council for Industrial Science and Technology) of Republic or Korea (Grant B551179-12-02-00). This work is also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant 2013R1A1A2008874). The authors would like to thank Dr.
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