Particle size dependence in low temperature nitridation reaction for Fe16N2

doi:10.1016/j.jallcom.2005.12.134

Journal of Alloys and Compounds

Volume 449, Issues 1–2, 31 January 2008, Pages 7-10

https://doi.org/10.1016/j.jallcom.2005.12.134 Get rights and content

Abstract

Low temperature nitridation of α-Fe powder was studied by reduction of α-Fe₂O₃ with three particle sizes using flow hydrogen. Most suitable α-Fe₂O₃ powder had an average particle size of 60 nm. After hydrogen reduction, the α-Fe showed average particle size of about 120 nm. The yield of ferromagnetic Fe₁₆N₂ obtained at 140 °C for 100 h was the highest of its nitrided product among the products nitrided in a temperature range of 130–170 °C. Thermal decomposition of iron pentacarbonyl was also applied to obtain a narrow particle size distribution of monodispersed α-Fe spherical particles of 30 nm. It was performed in the presence of oleic acid stabilizer in octyl ether solvent.

Introduction

Fe₁₆N₂ has attracted much attention due to its possible gigantic magnetization [1]. Most of the investigations have been performed on its thin films prepared by applying various kinds of deposition methods such as MBE and sputter deposition [2], [3], [4]. The values expected for the magnetic moments have been scattered over a range between 2.4 and 3.9 μ_B.

Ferromagnetic Fe₁₆N₂ fine powder was recently prepared by low temperature nitridation of α-Fe of about 20 nm crystallite size obtained from γ-Fe₂O₃ [5]. Its saturation magnetization was 162 emu/g at room temperature. The value was too small compared to the average magnetic moment of 2.52 μ_B estimated from the hyperfine fields in its Mössbauer spectrum. A saturation magnetization value of 225 emu/g was recently observed on the nitrided product at 130 °C for 100 h from the α-Fe fine powder with about 100 nm particle size, which was obtained by the reduction of γ-Fe₂O₃ in hydrogen flow at 500 °C for 8 h [6]. An additional paramagnetic component was present in its Mössbauer spectrum with an area ratio of 19%. A maximum saturation magnetization of 278 emu/g could be expected for a sample entirely formed of the ferromagnetic phase. This value was about 30% larger than that of α-Fe. There was a wide particle size distribution of γ-Fe₂O₃ in the range of 5–100 nm. Its small size region might lead to a formation of either superparamagnetic Fe₁₆N₂ or higher nitrides such as Fe₄N with a smaller magnetization.

The low temperature nitridation reaction should strongly depend on the starting powder, especially on its particle size, size distribution, surface morphology and the preparation method. In the present study, particle size dependence of the low temperature nitridation was studied on α-Fe fine powders prepared from α-Fe₂O₃ with narrow particle size distribution.

Section snippets

Experimental

Three kinds of α-Fe₂O₃ fine powders with narrow particle size distributions at 30 nm (A), 60 nm (B) and 200 nm (C) supplied by Sakai Chemicals Co. Ltd. (FRO3, 6, 20) were used as starting materials. They were reduced to α-Fe in hydrogen stream at 500 °C for 1 h. They were directly nitrided in ammonia flow for 100 h at a temperature range between 130 and 170 °C without air exposure.

Another preparation method was also applied to obtain monodispersed α-Fe fine particles smaller than 30 nm with a narrow

Results and discussion

Morphology of the three kinds of α-Fe₂O₃ particles was hexagonal plate-like with a narrow particle size distribution as shown in Fig. 1. After their reduction at 500 °C, the α-Fe particles were slightly sintered together with each other at their grain boundary forming porous mass. Primary particle sizes of the α-Fe powders were larger than those of the original α-Fe₂O₃ particles. They were 60, 120 and 300 nm for A, B and C, respectively.

Fe₁₆N₂ was formed as a mixture with the unreacted α-Fe after

Conclusion

In summary, particle size dependence of low temperature nitridation to prepare Fe₁₆N₂ with a gigantic magnetization was studied starting from α-Fe₂O₃ powder with a narrow particle size distribution. The α-Fe powder with 120 nm in particle size obtained by hydrogen reduction of α-Fe₂O₃ powder with a particle size of 60 nm was the most suitable to prepare Fe₁₆N₂. The final product was a mixture with α-Fe possessing a saturation magnetization of 207 emu/g. Thermal decomposition of iron pentacarbonyl

Acknowledgement

This research was partly supported by a Grant-in-Aid for Exploratory Research from JSPS (#17655090).

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The estimated mole fraction of α″-Fe16N2 was about 0.30–0.40. A different approach by using a two-step route where iron oxides are reduced to α-Fe in hydrogen atmosphere followed by a low-temperature nitrogenation has proven to be more effective, yielding high-purity α″-Fe16N2 [5,22–27]. However, different studies conducted on iron oxide reduction with hydrogen have shown that at atmospheric pressures a temperature of at least 663 K is necessary for the complete reduction to α-Fe [5,23–25,27],which leads to particle coalescence.
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as well as including an oxidation step before the reduction step [29]. Besides the nitridation conditions applied during synthesis, the NP size is known to influence the formation of the α″-Fe16N2 phase [28,30]. As reported, NPs with a large diameter have a reduced tendency to react with core α-Fe under nitridation conditions [28,29].
α″-Fe₁₆N₂ nanoparticles (NPs) have been identified as one of the most promising rare-earth-free magnetic materials and synthesized with high purity in laboratory investigation recently. This study reports characteristics and magnetic performance of spindle shaped core–shell α″-Fe₁₆N₂/Al₂O₃ NPs successfully synthesized from hematite NPs. A two-step reaction process, i.e., hydrogen reduction and ammonia nitridation treatments, was employed to produce the α″-Fe₁₆N₂ phase (making up to ∼99 wt.% of the core content). The high-purity NPs exhibited saturation magnetization and coercivity of 186 emu/g and 2.2 kOe, respectively, at 300 K. The obtained α″-Fe₁₆N₂/Al₂O₃ NPs possessed a spindle shape with an internal pore structure. Their porosity increased from ∼10% to 50–60% during the two-step synthesis process, however, at the expense of crystallite size. The combined use of the present synthesis method and hematite NP enables reduction in the synthesis reaction time, as well as the fabrication of NPs with excellent magnetic performance. Furthermore, the present results suggest the potential of rare-earth-free magnetic core–shell α″-Fe₁₆N₂/Al₂O₃ NPs in industrial applications.
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α″-Fe16N2 material has been known as a ferromagnetic material with the highest saturation magnetization [1–4]. Excellent magnetic properties make this material as a prospective candidate for rare-earth-free magnetic material applications [5–8]. Although many investigations into the synthesis and evaluation of α″-Fe16N2 materials have been reported, most of these studies have focused on the formation of bare-typed particles [9–11].
The introduction of an oxidation treatment to the synthesis of spherical and core–shell α″-Fe₁₆N₂/Al₂O₃ nanoparticles (~62 nm) from plasma-synthesized core–shell α-Fe/Al₂O₃ nanoparticles has been found to result in a high yield of α″-Fe₁₆N₂ phase of up to 98%. The oxidation treatment leads the formation of a maghemite phase with open channeled structures along the c-axis, facilitating penetration of H₂ and NH₃ gases during the hydrogen reduction and nitridation steps. The saturation magnetization and magnetic coercivity of the core–shell α″-Fe₁₆N₂/Al₂O₃ magnetic nanoparticles were found to be 156 emu/g and 1450 Oe, respectively. The detailed effects of the oxidation on the formation of α″-Fe₁₆N₂ phase were investigated by characterizing the morphology (SEM, TEM and BET), elemental composition (EDX, EELS, and XAFS) and magnetic properties (Mössbauer and MSPS) of the prepared particles. The good magnetic properties obtained have the potential for future applications such as rare-earth-free magnetic materials.
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The saturation magnetization was 225 emu g−1 at room temperature, but an additional paramagnetic component was observed in the Mössbauer spectrum with an area ratio of 19%. It was found out that the α-Fe powder around 100 nm in size obtained by the hydrogen reduction of starting iron oxides was suitable for the successive low temperature nitridation, achieving the higher yield of α″-Fe16N2.14 Iron oxides prepared in aqueous solution failed in its preparation even in a similar particle size to the previous studies.15
Fine powder of iron oxide has been required in the preparation of ferromagnetic α″-Fe₁₆N₂ powder in low temperature nitridation. Magnetite and maghemite mixture was obtained by the reaction of iron acetylacetonate Fe(acac)₃ in benzyl alcohol. It was reduced to α-Fe in hydrogen at 400 °C. Particle size of the α-Fe was 300 nm in the reduction of iron oxides obtained from 2 g of Fe(acac)₃. It decreased to 100 nm in the preparation using 8 g of Fe(acac)₃. After the successive nitridation under ammonia flow at 160 °C, the highest yield of α″-Fe₁₆N₂ in 66 wt.% was observed on the nitrided product from the latter α-Fe, because of the smallest α-Fe particle size after the reduction in the present study. The yield and reproducibility of α″-Fe₁₆N₂ formation in low temperature nitridation was improved using the iron oxide prepared in non-aqueous benzyl alcohol compared to the use of magnetite obtained from aqueous solution.

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