Synthesis of boron nitride nanolayers encapsulating iron fine particles and boron nitride nanotubes
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Synthesis of boron nitride nanolayers encapsulating iron fine particles and boron nitride nanotubes Hisato Tokoro1, Shigeo Fujii1, Takeo Oku2 and Shunsuke Muto3 1
Hitachi Metals, Ltd., Advanced Electronics Research Laboratory, Mikajiri 5200, Kumagaya, Saitama, 360-0843, Japan 2 Osaka University, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research, Mihogaoka 8-1, Ibaraki, Osaka, 567-0047, Japan 3 Nagoya University, Department of Nuclear Engineering Graduate School of Engineering, Furo-Cho, Chikusa-ku, Nagoya, 464-8603, Japan
ABSTRACT Boron nitride (BN) nanolayers encapsulating iron (Fe) fine particles have been synthesized by annealing mixtures of hematite (α-Fe2O3) and boron powders at 1373 K for 2 hours in nitrogen atmosphere. The Fe particles had an average diameter of ~300 nm with BN nanolayers coating of ~10 nm. The α-Fe2O3 was transformed into Fe and then Fe-B on a process of annealing. The Fe-B was decomposed into Fe and BN, and consequently Fe particles coated with BN nanolayers were synthesized. They showed soft magnetic properties with coercivity of 1.5 kA/m. The BN nanolayers encapsulation was effective on improving oxidation resistance. BN nanotubes with diameter of ~100 nm were also synthesized as a resultant product by this method.
INTRODUCTION Magnetic metal nanoparticles are suitable for potential applications like high-density magnetic recording media [1], magnetic fluids [2], magnetic carrier in clinical cure [3] and other novel magnetic devices. Iron (Fe) or Fe-based alloy nanoparticles have an advantage of high saturation magnetization for these applications, whereas deterioration by oxidation has kept them away from practical usage. To overcome this problem, nanocoating techniques for the metal nanoparticles have been reported. An arc discharge method has been proposed to coat metal nanoparticles with graphite carbon [4 – 6] or boron nitride (BN) [7, 8]. Surface oxidization of Fe and Co nanoparticles has also been studied [9, 10]. However, the arc discharge method is not considered to be suitable for a mass production because of low yield. The surface oxidation takes long times and causes decrease of saturation magnetization. We have discovered a simple method that can synthesize BN nanolayers encapsulating Fe fine particles massively for industry by employing α-Fe2O3 and B powders as starting materials. A reaction process of this method, morphology of resultant products and magnetic properties are discussed in this paper.
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EXPERIMENTAL Commercially available α-Fe2O3 (hematite, 99.7%) and crystalline boron (99%) powders were used as starting materials. Their particle sizes were 30 nm and 20 µm (325 mesh under), respectively. Ratio in weight of α-Fe2O3 to B was 1:1. A differential thermal analysis (DTA-TG) was carried out from 300 K to 1773 K at temperature rise of 10 K/min in N2 atmosphere in order to study reaction process between Fe2O3 and B from a thermodynamic point of view. Samples were prepared by annealing mixtures of the α-Fe2O3 and
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