Sm2Fe17N3 Powder Research Articles

Structures and magnetic properties of Sm2Fe17N3 bulk magnets prepared by spark plasma sintering with dynamic compression

The spark plasma sintering (SPS) method can consolidate powders at low temperatures over a short period. The addition of dynamic compression (DC) can produce bulk materials at lower temperatures. To demonstrate the effect, the study used SPS-DC to consolidate Sm2Fe17N3 powder, which decomposes into α−Fe and SmN phases above 873 K. The SPS-DC method was able to consolidate Sm2Fe17N3 powder into bulk material at 473 K. The Sm2Fe17N3 magnet produced at 473 K by the SPS-DC method showed a relative density of 88 %, comparable to that of the Sm2Fe17N3 magnet produced at 673 K by the conventional SPS method. The Sm2Fe17N3 bulk magnet produced at 473 K by the SPS-DC method exhibited a high coercivity of 9.45 kOe.

Production of Sm2Fe17N3 Bulk Magnets

Sm2Fe17N3 powder exhibits excellent magnetic properties but is unstable and decomposes into α-Fe and SmN phases at high temperatures. Therefore, the key to producing Sm2Fe17N3 bulk magnets is to reduce the deterioration of Sm2Fe17N3 powder during sintering. Herein, Sm2Fe17N3 bulk magnets were made using the spark plasma sintering (SPS) method with the addition of zinc stearate powder and zinc powder. Adding small amounts of zinc stearate powder and zinc powder improved the magnetic anisotropy and the coercivity of the magnets, respectively. The magnets produced by the SPS method using zinc stearate powder and zinc powder exhibited enhanced magnetic properties almost comparable to those of Sm2Fe17N3 powder.

Exploiting mechanochemical activation and molten-salt-assisted reduction-diffusion approach in bottom-up synthesis of Sm2Fe17N3

Sm2Fe17N3 powders were prepared by a novel molten-salt mechanochemically assisted reduction-diffusion (RD) approach. CaCl2, plays a critical role during mechanochemical processing as a dispersant, facilitating the RD process by dissolving the precursors in a molten flux and further assisting during the washing step by efficient removal of by-products thus minimizing the surface oxidation of Sm2Fe17N3 powder particles. Various milling times and RD temperatures were examined, and synthesis conditions were optimized to achieve pure-phase Sm2Fe17N3 powders with low aggregation of magnetic particles. Powders synthesized at 950 °C RD exhibited the highest hard-magnetic properties: coercivity Hc of 13.5 kOe, and maximum energy product (BH)max of 19.4 MGOe. This was attributed to the formation of fine single-phase Sm2Fe17N3 particles as well as minimal oxidation of particle surface. Furthermore, by densifying the Sm2Fe17N3 powders with high-pressure spark plasma sintering, a bulk magnet with (BH)max of 16.5 MGOe and a relative density of 86 % was produced indicating that the obtained Sm2Fe17N3 powders had low oxygen content.

Synthesis of ultra-high coercivity Sm2Fe17N3 powder by homogeneous reduction-diffusion with rotary furnace

In a reduction-diffusion (R-D) process using Ca, the uneven distribution of Ca in the reaction vessel was found to have a significant effect on the size uniformity of the Sm2Fe17 particles. It was considered that lumps of Sm-Ca-Fe formed due to the uneven distribution of Ca, and coarse Sm2Fe17 particles formed in the lumps. The R-D process using a rotary furnace was developed to solve the problem of uneven distribution of Ca in the R-D reaction, and drastically reduced the formation of Sm-Ca-Fe lumps and reduced the number of coarse Sm2Fe17 particles. As a result, Sm2Fe17N3 powders with even higher coercivity could be obtained by the R-D process. In particular, an ultra-high coercivity of over 30 kOe was achieved by reducing the average particle size to 0.5 µm or less. The highest coercivity in this study was 31.7 kOe, which is the highest value ever achieved with a Sm2Fe17N3 single-crystalline powder. The coercivity coefficient β of the synthesized powder from 303 K to 493 K was − 0.36 %/K, and the powder maintained coercivity of more than 10 kOe at 473 K.

The Effect of Milling with Combined Surfactants on the Magnetic Properties and Microstructure of Submicron Sm2Fe17N3 Powders

The results of the application of a combination of several surfactants in preparing submicron hard magnetic Sm2Fe17N3 powders by milling in a centrifugal mill are reported. Along with methyl caproate, to protect the powder against oxidation, the efficiency of siloxane has been studied. The application of the combined surfactants allowed us to increase both the (BH)max of the Sm2Fe17N3 powder to 23.6 MG Oe and its corrosion resistance. The effect of milling kinetics on the angular dependence of the coercive force Hc has been determined. Information on the primary mechanism of magnetization reversal of the powders has beenobtained.

Study of the anisotropic Sm2Fe17N3 powders with high performance

The strip casting technique was first used in this work to prepare the Sm2Fe17 parent alloys. Next, the Sm2Fe17N3 powders were synthesized using a gas–solid reaction of Sm2Fe17 powders with N2. Then, the anisotropic Sm2Fe17N3 powders with high performance were prepared by a surfactant-assisted grinding method in gasoline solvent. It was found that the coercivity (iHc) of ground powders increases with decreasing the average particle size (Dm), while the saturation magnetization (Ms) decreases. The maximum value of iHc of 17.5 kOe was obtained at the Dm = 1.3 µm. The ground powders with Dm of 3.1 µm show a remanence ratio (Mr/Ms) over 95% and a maximum energy product [(BH)max] of 35 MGOe. The Sm2Fe17N3 powder exhibited a very high iHc of 36.1 kOe at 10 K. Above 300 K, Sm2Fe17N3 powders mixed with epoxy resin showed a low remanence temperature coefficient of α(RT-100°C) = −0.077% °C−1.

Preparation of submicron-sized Sm2Fe17N3 fine powder by ultrasonic spray pyrolysis-hydrogen reduction (USP-HR) and subsequent reduction–diffusion process

A new reduction–diffusion method for preparing submicron-sized Sm2Fe17N3 was developed. First, submicron-sized α-Fe/Sm2O3 composite precursors with good dispersibility were prepared directly by ultrasonic spray pyrolysis and hydrogen reduction (USP-HR). Then, the precursor was successfully converted into Sm2Fe17N3 powder by Ca reduction–diffusion and nitridation. The results displayed that the precursor prepared by USP-HR was a monodisperse homogeneous composite of spherical particles of α-Fe and Sm2O3. α-Fe and Sm2O3 exist in an intercalated structure in the composite spherical particles. This creates conditions for the preparation of ultrafine Sm2Fe17N3 particles with uniform sizes and dispersity. The results show that Sm2Fe17N3 is a single-phase, composed of submicron-sized particles with no other impurity phases. The physical property measurement system shows that the coercivity of 0.616 ± 0.347 µm particles reaches 14.7 kOe.

Possibility of high-performance Sm2Fe17N3 sintered magnets by low-oxygen powder metallurgy process

Inspired by the previous result that serious coercivity deterioration of Sm2Fe17N3 powder during sintering can be overcame by suppressing surface oxidation of the powder, this study aimed to explore the possibility of high-performance Sm2Fe17N3 sintered magnets by a low-oxygen powder metallurgy process. Jet-milling under a low oxygen atmosphere was used to prepare Sm2Fe17N3 fine powders with a low oxygen content as well as high (BH)max reaching 42.8 MGOe (341 kJ/m3). All low-oxidized powders were clearly confirmed to be sintered without coercivity deterioration, and powders with appropriate particle sizes were densified to a relative density exceeding 90% by high-pressure current sintering, despite use of a low sintering temperature below the thermal decomposition temperature. However, the sintered magnets showed an unexpected decline in remanence from the powder, which was caused by a decline in saturation magnetization, and as a result, the (BH)max of the obtained sintered magnets was 24 MGOe (191 kJ/m3).

Metal-coated Sm2Fe17N3 magnet powders with an oxide-free direct metal-metal interface

We successfully prepared Sm2Fe17N3 magnet powders uniformly coated with non-magnetic metals without oxides in the interface between the magnet phase and the coating layer. The whole process from powder preparation by jet milling to coating was undertaken in a low-oxygen atmosphere. Powder coating was investigated using two different dry thin-film preparation methods, arc-plasma deposition (APD) and DC magnetron sputtering. The powder was continuously stirred during coating in order to ensure the uniformity of the coating layer. Scanning electron microscopy with energy dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy demonstrated that an arc-plasma deposited Zn layer on Sm2Fe17N3 powder had nearly 100% coverage and an oxide-free direct metal-metal interface with the underlying magnet phase. The effects of metal-coatings and coating methods on the coercivity of the magnet powder were evaluated with Zn, Ti, and Al as the coating materials and with APD and DC magnetron sputtering as the coating methods. With all three coating metals, the sputter-coated Sm2Fe17N3 powders showed higher coercivity than the raw powders. The APD powders had lower coercivity than the sputter-coated powders, and in the case of Ti, coercivity was even lower than that of the raw powder. Transmission electron microscopy with EDS and nano-beam electron diffraction suggested that the high energy particles deposited by APD damaged the magnetic phase. This study demonstrated that nanometer-thick coatings of non-magnetic metals without oxides in the interface with the magnet phase have the effect of enhancing the coercivity of Sm2Fe17N3 powders, provided the proper coating method is selected.

Synthesis of Sm2Fe17N3 powder having a new level of high coercivity by preventing decrease of coercivity in washing step of reduction-diffusion process

Synthesis of Sm2Fe17N3 powder by a reduction-diffusion technique requires a washing process to remove CaO and Ca residues, but it is typically associated with a decrease of coercivity. Synchrotron radiation X-ray diffraction study revealed forming of a SmFe5 poor hard magnetic phase in a water washing treatment. Cross-sectional electron microscopic analysis of particles revealed an acetic acid washing treatment to dissolve the impurity phases accompanied surface damages on Sm2Fe17N3 particles. A new washing process to avoid forming of the factors which decrease coercivity was examined, and Sm2Fe17N3 powders having about 20% higher coercivity than that in the previous reports, achieving up to 28.1 kOe, were successfully demonstrated.

Progress with insulating nanocomposites based on ferrite plating of Sm2Fe17N3 micropowders

Abstract Typical exchange spring magnets are composed of two phases, a rare-earth hard magnetic material and a metallic soft magnetic material, whose magnetization value is higher than those of hard magnetic phases, such as α-Fe, Fe-B, or Fe-Co. A novel high-electrical-resistance composite magnet was fabricated by consolidating micron-sized ferromagnetic nitride Sm2Fe17N3 powder particles coated with a continuous nano-sized soft magnetic ferrite oxide layer, which suppressed intergrain conductivity but sustained the magnetic exchange interactions between grains [21] . A non-magnetic resin or ceramic insulator may suffice for the coated layer but gives rise to high electrical resistance of the magnet with large deterioration of magnetization. The soft magnetic ferrite oxide layer not only suppressed intergrain conductivity, but also only slightly reduced the magnetization of the magnet. At present, the only exchange spring magnet having a combination of a soft magnetic oxide and hard magnetic nitride is the ferrite/Sm2Fe17N3 composite magnet. Sm2Fe17N3 powder particles with a size of 2 μm were coated with an “iron ferrite” layer with a grain size of about 10 nm by ferrite plating, which is an aqueous process, following which the samples were consolidated by the explosive consolidation (EC) technique. The coercivity and rectangularity of the ferrite/Sm2Fe17N3 composite magnet decreased slightly compared to those of a Sm2Fe17N3 magnet. The fully dense ferrite/Sm2Fe17N3 magnet exhibited a resistivity of about 4000 μΩ cm, which is ten times larger than that of a fully dense Sm2Fe17N3 magnet. Thus, the soft magnetic ferrite layer in the composite magnet maintained the magnetic exchange coupling between ferromagnetic Sm2Fe17N3 grains and simultaneously suppressed intergrain electrical coupling to increase the resistivity. This decreased the eddy current loss and improved the high-frequency characteristics of the composite magnets. Therefore, ferrite/Sm2Fe17N3 composites are promising materials for magnets that are operated at high frequencies in advanced applications, such as electric vehicle magnets.

A method to improving the coercivity of sintered anisotropic Sm-Fe-N magnets

In this study, bulk anisotropic sintered Sm2Fe17N3 magnets were prepared from submicron-sized Sm2Fe17N3 powder by spark plasma sintering. The pure sintered Sm2Fe17N3 magnets contain high α-Fe content and have a very low coercivity of 4.1 kOe and poor squareness of 7.1%. The SmCu nanoflakes were added as oxygen absorber to provide a low-oxygen sintering condition. The resultant sintered magnets contain low α-Fe content and show a high coercivity of 10.3 kOe. In this SmCu added sintered magnets, the decomposition of Sm2Fe17N3 phase was suppressed and SmCu powder was oxidized instead. The experimental results of this study suggest that an addition of suitable high activity oxygen absorber may be effective in preventing the formation of α-Fe precipitates and improving the coercivity in the fabrication of R-TM compounds based nanostructured bulk magnets.

Consolidation of Sm2Fe17N3 magnets with Sm-based eutectic alloy binder

Consolidation of Sm2Fe17N3 powder with a new Sm-based alloy as a metal binder was examined. Investigation of various Sm-based alloys led to the discovery of a Sm-Fe-Cu-Al alloy having a low melting point of 495 °C, which is more than 100 °C lower than the thermal decomposition temperature of Sm2Fe17N3. The metallographic structure of the Sm-Fe-Cu-Al alloy ingot was a SmCu/Sm eutectic structure containing solid-solution Fe, Cu and Al. Pulverized Sm-Fe-Cu-Al powder as a binder was mixed with Sm2Fe17N3 powder, and the mixture was hot-pressed by a current sintering apparatus. The Sm-Fe-Cu-Al alloy binder did not affect the magnetization of Sm2Fe17N3 adversely after consolidation. The binder-added Sm2Fe17N3 hot-pressed compacts successfully maintained the coercivity of the raw powder over a wide range of hot-pressing temperatures, whereas additive-free Sm2Fe17N3 hot-pressed compacts showed a significant decrease in coercivity. Therefore, the Sm-Fe-Cu-Al alloy binder overcame the weakest point of Sm2Fe17N3 magnets. XRD analysis suggested that the Sm-Fe-Cu-Al binder restrained precipitation of the α-Fe phase, which is thought to be the probable cause the decrease of coercivity.

Thermal stability of Sm2Fe17N3 magnet powders

The time dependence of coercivity of fine Sm2Fe17N3 powder compacts exposed in air and in He gas (oxygen-free atmosphere) was investigated at 110°C and 150°C. At 110°C in air, coercivity rapidly decreased within 5 h, reached a minimum at around 5h, and then gradually increased over 1000 h. Similar behavior was observed in the He gas atmosphere, although the minimum coercivity value was higher than that in air exposure. Since the oxygen content was confirmed to be constant, the coercivity minimum behavior is independent of oxidation. The temperature dependence of the anisotropy field, coercivity and saturation magnetization of fine Sm2Fe17N3 powder compacts was measured in air at 110°C and analyzed by using Kronmüller’s formula.

Development of Methods to Improve Corrosion Resistance of Hard-Magnetic Sm2Fe17N3 Materials

Corrosion resistance of hard magnetic Sm2Fe17N3 materials is studied. It is shown that addition of cobalt (up to 8 wt.%) makes it possible to improve corrosion resistance of these materials by 20–25% without reducing their magnetic properties. It is established that additional treatment of Sm2Fe17N3 powder with organosilicon liquid GKZh 136-41 makes it possible to improve corrosion resistance by a factor of 2–3.

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

We investigated the cause of forming of aggregates which decreases magnetization of a submicron-sized Sm2Fe17N3 powder. The aggregation was considered to be caused through sintering and growth of α-Fe particles in a hydrogen reduction treatment. We newly synthesized cubic hematite powder which has tolerance for the sintering of particles in a hydrogen reduction treatment. The aggregation of submicron-sized Sm2Fe17N3 particles was significantly reduced by using the cubic hematite as a precursor, and the magnetization was improved without decreasing the high coercivity.

High coercivity Sm2Fe17N3 submicron size powder prepared by polymerized-complex and reduction–diffusion process

High coercivity Sm2Fe17N3 submicron size powder was synthesized by the polymerized-complex (PC), hydrogen reduction (HR) and reduction–diffusion (RD) processes. A mixture of nano size SmOx, FeOx and SmFeO3 particles was first synthesized by the polymerized-complex method, which was then reduced to α-Fe and SmOx by hydrogen. Subsequent reduction diffusion using Ca metal led to the formation of submicron size Sm2Fe17 powder with a mixture of CaO. After nitriding and rinsing, Sm2Fe17N3 powder with the average diameter of 0.69±0.17μm was obtained, which exhibited the coercivity of 2.32T, the highest ever reported for ternary Sm2Fe17N3 anisotropic powder.

518 低融点金属を添加したSm_2Fe_<17>N_3粉末の低温固化成型プロセス

518 低融点金属を添加したSm_2Fe_<17>N_3粉末の低温固化成型プロセス

Microstructural behavior on particle surfaces and interfaces in Sm2Fe17N3 powder compacts during low-temperature sintering

Sm2Fe17N3 sintered compacts were prepared below 450°C by a high-pressure current sintering technique. The coercivity of the sintered compacts decreased linearly as the sintering temperature increased. Transmission electron microscopic analyses indicated that thin Fe-rich layers containing α-Fe phases were formed just inside the initial oxide layer on the particle surfaces and interfaces in the sintered samples. The generation of α-Fe phases was supposed to cause the coercivity decrease. In addition, X-ray photoelectron spectroscopy analysis revealed that Fe2O3 and FeO contained in the oxide layer of the raw powder disappeared subsequent to heat treatment. These results suggested that the α-Fe phases were generated by the oxidation–reduction reaction between the initial iron oxides and the primary Sm2Fe17N3 phase but not by thermal decomposition or exogenous oxidation during sintering. This mechanism was supported by the fact that extending the sintering time did not result in any further decrease in the coercivity.

Correlation between Microstructure and Magnetic Properties in Sm<sub>2</sub>Fe<sub>17</sub>N<sub>3</sub> Magnet Prepared by Pulsed Current Sintering

The consolidation of Sm2Fe17N3 powder was examined by pulsed current sintering below its decomposition temperature. Although decomposition of the powder was not observed, its coercivity decreased dramatically. Annealed powder was observed by transmission electron microscopy in order to clarify the mechanism for the reduction in the coercivity. In powder annealed at 673K, a thin nanostructured layer was formed around the Sm2Fe17N3 particles. The crystal structure of this thin layer had a crystallographic symmetry higher than that of the Sm2Fe17N3 structure. This higher symmetry layer appeared to have a smaller magnetic anisotropic energy, allowing it to act as a nucleation site for a reverse magnetic domain, when a reverse magnetic field is applied to the magnetized powder. Therefore, the coercivity of the powder decreased below its decomposition temperature. Irregular growth of the thin layer was observed in powder annealed above the decomposition temperature, and the grown region was confirmed to be an iron phase. [doi:10.2320/matertrans.M2012169]