Improvement of magnetization of submicron-sized high coercivity Sm₂Fe₁₇N₃ powder by using hydrothermally synthesized sintering-tolerant cubic hematite Open Access

Sm₂Fe₁₇N₃ is a promising candidate for high performance magnets for use in motors exposed to high temperatures since it has a large anisotropy field and a high Curie temperature with high saturation magnetization.^1,2 Although bulk Sm₂Fe₁₇N₃ magnets were not prepared because of their decomposition around 873 K,³ we developed a low-thermal-load process combining a high-pressure compaction and a current sintering and demonstrated an isotropic Sm₂Fe₁₇N₃ bulk magnet.⁴ Since no anisotropic Sm₂Fe₁₇N₃ powder which have a high coercivity expected to be able to use in the motors has been reported so far, the next challenge is its development.

Sm₂Fe₁₇N₃ powders having high coercivity, over 20 kOe, have been developed by decreasing the particle size to the submicron scale by Hirayama et al. and us, separately.^5,6 While both the powders with submicron-sized particles (denoted hereafter as “submicron-sized powder”) exhibited high coercivity, considerable aggregation of particles was observed, which made the remanent magnetization low. Although pulverization treatment of a milling with media increased remanence magnetization, the coercivity was decreased as shown in Fig. 1. Also in previous studies, the powders prepared by a pulverization process exhibited lower coercivity than the powders prepared by a non-pulverization process because of defects, such as edges, strains, oxide and etc., which formed by the pulverization treatment and decreased local anisotropy field of particle.^5–8 Thus, a method for preparing a well-dispersed submicron-sized Sm₂Fe₁₇N₃ powder without pulverization is needed to produce an anisotropic powder having both high remanent magnetization and high coercivity.

In this study, we investigated the reason for the aggregation and developed a precursor to produce a well-dispersed powder. As a result, the degree of aggregation decreased significantly and the remanence improved along with high coercivity.

The procedure using a co-precipitation technique was described in our previous report.^6,9 Cubic hematite powder of 150 nm average diameter was newly developed by a hydrothermal reaction using Ca(NO₃)₂ as a structure directing agent. KOH aqueous solution (2 mol/L) was added dropwise to a mixture of Fe(NO₃)₃⋅9H₂O and Ca(NO₃)₂ aqueous solution in which the Fe/Ca and OH^-/NO₃^- molar ratio was adjusted to 3.0 and 1.0, respectively. Then, the solution was poured into a Teflon lined stainless cylindrical container, heated at 453 K for 12 h, washed with water, and dried in an oven. The obtained hematite powder was impregnated with Sm(NO₃)₃⋅6H₂O. The Fe/Sm molar charge ratio was adjusted to 5.5.

The precursor was heated at 473 K for 1 h under vacuum to remove the adsorbed moisture, and then, reduced at 973 K for 6 h in a H₂ atmosphere.

One gram of the hydrogen-reduced sample and 0.5 g of granular metallic Ca were placed in an iron crucible. The sample was heated at 1173 K for 1 h in an Ar atmosphere.

The samples after reduction-diffusion were nitrided in an NH₃-H₂ (1:2 vol/vol) atmosphere at 693 K for 1 h, then annealed in a H₂ atmosphere at 693 K for 1 h to adjust their N content to an appropriate level, and annealed in Ar for 0.5 h to remove the H₂ adsorbed in Sm₂Fe₁₇N_3.¹ The sample was washed with water several times and then with a diluted acetic acid aqueous solution to remove the impurities, washed again with water, and dried in vacuo. The obtained powders were further annealed in vacuo at 473 K for 3 h to remove the hydrogen in the Sm₂Fe₁₇N₃ powder, which was absorbed during the washing of Ca residuals.⁶ 

Powder X-ray diffraction (XRD) patterns were recorded using CoKα radiation (45 kV, 40 mA). The average sizes of the sample particles were determined from scanning electron microscopy (SEM) images (more than 200 particles were measured). The magnetic properties of the samples were determined using vibrating sample magnetometry (VSM) at 300 K in vacuo with a maximum magnetic field of 9 T. For the VSM measurements, the powders were oriented in the resin under a static magnetic field of 2 T and measured along the easy magnetization direction. The O and N contents in the samples were determined by an inert gas fusion method.

To understand the cause of particle aggregation, the powders were microscopically observed at each treatment step, as shown in Fig. 2. The aggregates, which consist of Sm-Fe oxide particles of several tens of nanometer in size, shown in Fig. 2 (a), were reduced with H₂ at 823 K to α-Fe and SmFeO₃ particles with an average diameter of 0.4 $μm$ and around 60 nm, respectively. The phases were determined by XRD and EDX analyses. The α-Fe and SmFeO₃ particles grew to 1.0 $μm$ and 130 nm in size, respectively, upon increasing the H₂ reduction temperature to 973 K. After all the treatments with reduction-diffusion at 1173 K were conducted using the precursor reduced by H₂ at 973 K, aggregates of Sm₂Fe₁₇N₃ particles, where the average particle size and the average aggregate size were 0.67 $μm$ and 1.5 $μm$⁠, respectively, were formed, as shown in Fig. 2 (d). The Sm₂Fe₁₇N₃ aggregates grew and became one particle at higher temperatures of reduction-diffusion, as shown in Fig. 2 (e) and (f), in which the average diameter of the Sm₂Fe₁₇N₃ particles after reduction-diffusion at 1253 K was about 1.3 $μm$⁠. It is known that the mechanism for the formation of Sm₂Fe₁₇ particles in reduction-diffusion is the diffusion of Sm species, which were formed by reducing Sm oxide with Ca, to the Fe particles.¹⁰ Since the particle size of Sm₂Fe₁₇N₃ was smaller and the aggregates were bigger than the size of the Fe particle, a Sm₂Fe₁₇N₃ aggregate seemed to be formed by multinucleation and growth of Sm₂Fe₁₇N₃ particles in an α-Fe particle. Thus, to synthesize about 0.7 $μm$ sized Sm₂Fe₁₇N₃ particles without aggregation, by calculating the ratio of unit cell volume of α-Fe and Sm₂Fe₁₇N₃ and the above experimental result, it was thought that α-Fe powder with particles of less than 0.5 $μm$ in size was required.

Although the size of α-Fe particles could be decreased by lowering the H₂ reduction temperature, the α-Fe particles provided had a wide size distribution as shown in Fig. 2 (b). The particle growth mechanism of α-Fe particles in the H₂ reduction of the Sm-Fe oxide was considered to be diffusion-limited growth, determined from the results shown in Fig. 2 (b), (c), and it was difficult to control the growth. A preferable way is using a uniform iron oxide powder, which is tolerant to sintering of particles and growth in H₂ reduction.¹¹ 

We developed submicron to micron-sized platelet and acicular α-Fe particles by H₂ reduction of platelet hematite particles and acicular goethite particles, which were tolerant to sintering in the H₂ reduction treatment.^12,13 In that method, Sr(NO₃)₂ or Ba(NO₃)₂ was used as a structure directing agent, and SrCO₃ or BaCO₃, which was formed during the synthesis process, acted as an inhibitor to the sintering and particle growth. Although Sr and Ba are considered unreactive to metallic Fe and Sm, Ca(NO₃)₂ was more suitable as a structure directing agent because Ca is used in the reduction-diffusion process as a reducing agent and does not react with Fe and Sm. Thus, first, the method for preparing a small-sized iron oxide particle tolerant to sintering and growth in the H₂ reduction using Ca(NO₃)₂ was examined.

A synthesis condition was developed where Ca(NO₃)₂ was found to act as a structure directing agent under the hydrothermal condition to produce cubic hematite of a few hundred nanometers in diameter, which was bigger than the cubic hematite produced without using Ca(NO₃)₂ (Fig. 3 (a), (b)). The formation of CaCO₃ was confirmed by XRD (Fig. 3 (c)), similar to using Sr(NO₃)₂ and Ba(NO₃)₂. The amount of Ca was determined to be around 2–3 at% for Fe by EDX analysis. The particle size of cubic hematite was also controllable by changing the amount of KOH (OH^-/NO₃^- = 1.0 for Fig. 3 (b) and 1.6 for Fig. 3 (d)).

The hematite powders with/without Ca(NO₃)₂ were reduced by H₂ at 773 K for 6 h. While the cubic hematite particles without Ca(NO₃)₂ sintered during H₂ reduction, the cubic hematite with Ca(NO₃)₂ maintained their size and dispersity after H₂ reduction, as shown in Fig. 3 (e) and (g), respectively. The CaCO₃ was also considered to inhibit the sintering and particle growth in the H₂ reduction.

The developed cubic hematite shown in Fig. 3 (b) was impregnated with Sm(NO₃)₃⋅6H₂O and subjected to H₂ reduction. From the SEM observation (Fig. 4 (a)), it was confirmed that the growth of α-Fe particles was considerably suppressed compared to the conventional precursor prepared by a co-precipitation technique as shown in Fig. 2(a). The reduced powder was subjected to reduction-diffusion, nitridation, washing, and dehydrogenation treatment.⁶ Fig. 4 (c) and (d) show the SEM images of submicron-sized Sm₂Fe₁₇N₃ powders prepared from the developed cubic hematite and a conventional co-precipitates precursor, respectively. A comparison of the magnetic performance between the two precursors is shown in Fig. 4 (e) and (f). As can be seen, the aggregates were significantly reduced by using the developed cubic hematite powder, and the refined submicron-sized Sm₂Fe₁₇N₃ powder exhibited both higher remanent magnetization and coercivity. Because the particle size and N and O content of the samples were similar, the improvement in the magnetic properties was considered to be derived from an improvement in the particle dispersity. And, the increase in the degree of particle orientation improved remanent magnetization, as in the case of pulverization shown in Fig. 1. Note that in contrast to the pulverization process, which decreased the coercivity, using the developed precursor improved the coercivity rather than just preventing the decrease. Although the reason for the increase in the coercivity is not clear yet, a difference in the amount of the impurity phase was thought to be a possible reason. Because the reduction-diffusion in this study was conducted at a low temperature of 1173 K to prepare a finer powder, Sm-rich phases, such as SmFe₃ and SmFe₂, which would transform to the soft magnetic phase of α-Fe or iron nitride in the nitridation process,^14,15 were formed with Sm₂Fe₁₇N₃. Almost all the impurity phases were removed by washing with diluted acetic acid, but the impurities present in the grain boundary of the aggregates were difficult to remove. Better dispersity of the powder would help removing the impurity phase, resulting in enhanced coercivity.

Submicron-sized Sm₂Fe₁₇N₃ powders prepared by the reduction-diffusion process using conventional precursors exhibited very high coercivity but poor remanence because of the aggregation of Sm₂Fe₁₇N₃ particles. In this study, microscopic observation indicated that a reason for the aggregation of submicron-sized Sm₂Fe₁₇N₃ powder was the growth of α-Fe particles in the precursor. Hence, cubic hematite, about 100 nm in diameter, tolerant to sintering and particle growth in H₂ reduction, was newly developed as a precursor. Aggregates of submicron-sized Sm₂Fe₁₇N₃ powder were significantly reduced and it improved both the remanent magnetization and coercivity. Although the remanent magnetization is not yet at a sufficient level to use because the particles were still agglomerated, this study demonstrated that it was possible to control the degree of particle aggregation by designing a precursor. Further improvement in the production process will provide a high performance anisotropic Sm₂Fe₁₇N₃ powder with high coercivity to achieve a Sm₂Fe₁₇N₃ bulk magnet for high performance motors.