2019 Volume 60 Issue 10 Pages 2179-2182
An in-field heat treatment furnace utilized for a magnet with a 50 mm room-temperature bore was made to study magnetic field effect on gas-solid reaction. Using this furnace, nitridation of Sm2Fe17 powder was performed under a 5-T magnetic field and a 0.1-MPa nitrogen gas pressure for the temperature range 623 to 743 K. Applying a magnetic field of 5 T promoted nitridation by approximately 0.6 nitrogen atoms per formula unit compared with zero-field nitridation, and almost fully nitride Sm2Fe17N2.9 was obtained at 743 K.
Fig. 3 Temperature dependence of the magnetization for Sm2Fe17Nx prepared by the zero-field heat treatment (ZFHT) and 5 T in-field heat treatment (IFHT-5T) at the nitriding temperature Tn of 623 K (a) and 743 K (b). For comparison, the data of Sm2Fe17 and Sm2Fe17N3 are also shown.
In 1990, nitride Sm2Fe17Nx with a rhombohedral Th2Zn17-type structure was discovered using the N2 gas-solid reaction technique on host Sm2Fe17.1) The host’s cell volume expands by 6%–7% to accommodate three nitrogen atoms at the 9e-interstitial sites.2) The fully-nitride (FN) Sm2Fe17N3 has high Curie Temperature TC of 746 K, high saturation magnetization μ0MS of 1.51 T and large anisotropy field of 21 T at room temperature.3) Sm2Fe17N3 is a suitable candidate for high-performance permanent magnets.2,4) However, Sm2Fe17N3 alloy is mainly used in bond magnets, despite its excellent intrinsic magnetic properties. Sm2Fe17N3 decomposes to α-Fe and SmN above 720 K.4) Therefore, it is difficult to prepare sintered or high dense Sm2Fe17N3 magnets without thermal decomposition.5)
Magnetic field H stabilizes the ferromagnetic phase by gaining large magnetic free energy GM. For example, ferromagnetic phases of LaCo5Hx,6,7) MnBi8–12) and MnAl13,14) were induced by in-field heat treatment (IFHT). Therefore, it is expected that IFHT is one of the useful methods to prepare sintered or high dense Sm2Fe17N3 with suppressing thermal decomposition. However, to the best of our knowledge, the IFHT effect on nitridation of the Sm2Fe17 system has not been reported in detail. In this paper, we report our IFHT furnace for gas-solid reaction under magnetic fields up to 5 T and the initial results of the IFHT experiment on nitridation of Sm2Fe17.
First, we show in Fig. 1 the schematic diagrams of our IFHT furnace system for gas-solid reaction under reaction field of μ0Ha ≤ 5 T. This system is mainly constructed from the gas controlling system (I), the IFHT system with a cryocooled superconducting magnet (CSM) (II) and a high-pressure heat treatment (HPHT) system (III). The gas controlling system consists of a vacuum pump (a), gas cylinders (g1–g3), pressure gauge (b) and vacuum gauge (c). The IFHT system consists of a CSM with a 50 mm room-temperature bore (Tamakawa Co., Ltd) (d), a power supply for the magnet (e), a quartz cell (diameter d = 14 mm) (f), a Pt–PtRh thermocouple (d = 1 mm) (h), a non-inductive Nichrome heater (d = 24 mm) (i), a water-cooled jacket (j) and a temperature controller (k). The powder sample in a quartz boat (l) was set with the thermocouple (h) in the quartz cell (f), which was placed in the center of magnetic field. The IFHT was carried out in the gas pressures Pn up to 0.1 MPa. The HPHT system consists of a stainless steel reaction cell (m), an electric furnace (n), a Pt–PtRh thermocouple (d = 1 mm) (o) and the temperature controller (k), which was utilized for synthesizing nitride by zero-field heat treatment (ZFHT) under Pn ≤ 0.7 MPa.
Schematic diagrams of the in-field heat treatment furnace system for gas-solid reaction under magnetic field. (a) vacuum pump, (b) pressure gauge, (c) vacuum gauge, (d) cryocooled superconducting magnet, (e) power supply, (f) quartz cell, (g1–g3) gas cylinders, (h) thermocouple, (i) non-inductive Nichrome heater, (j) water-cooled jacket, (k) temperature controller, (l) quartz boat, (m) stainless steel reaction cell, (n) electric furnace, (o) thermocouple.
The host Sm2Fe17 compound was prepared by induction melting of the constituent elements under an argon atmosphere and then annealed at 1423 K for 24 h in an argon atmosphere for homogeneity treatment, which was provided by Prof. H. Fujii (Hiroshima University).15) The Sm2Fe17 ingot was crushed and pulverized into powder with particle size less than 53 µm in diameter. The ZFHT and IFHT of μ0Ha = 5 T (IFHT-5T) was performed by the following procedure. The powder sample in the quartz boat (l) placed in the quartz cell (f) was evacuated by vacuum pump (a) below 1 Pa at 303 K, and then thermally activated at 473 K for 2 hours. Nitridation treatment was performed using nitrogen gas of Pn = 0.1 MPa at nitriding temperature Tn of 623 and 743 K for 24 hours in the CSM (d). The nitrogen content x in the nitrided powders was estimated from the increased sample mass after nitridation. The nitridation condition of ZFHT and IFHT-5T are listed in Table 1. In this study, the high-pressure nitridation treatment15) was performed by heating Sm2Fe17 powder at 743 K for 24 h under Pn of 0.7 MPa for preparing FN Sm2Fe17N3 as a standard nitride.
To examine the phases of ZFHT and IFHT-5T samples, X-ray powder diffraction (XRD) measurements were performed using Cu-Kα radiation at room temperature. Magnetization M data were corrected using a superconducting quantum interference device magnetometer (SQUIDM) for temperature T range of 10–310 K and a vibrating sample magnetometer (VSM) for T range of 300–830 K. The M data was measured using magnetically oriented powder to easy axis for 10 ≤ T ≤ 310 K. The data measured by VSM was normalized by the data detected by SQUIDM at 300 K. TC of the sample was determined by differential scanning calorimetry under μ0H = 0 T.
The estimated x in the Sm2Fe17Nx powder prepared by ZFHT and IFHT-5T are shown in Table 1. In this table, the data for the host Sm2Fe17 and FN Sm2Fe17N3 prepared by high-pressure nitridation treatment are also shown. It is clear that x of IFHT-5T is larger than that of ZFHT for all Tn. By applying μ0Ha = 5 T, x increased by 0.4–0.6 nitrogen atoms per formula unit. For condition of Tn = 743 K, almost FN Sm2Fe17N2.9 was obtained using IFHT-5T, while x was only 2.3 using ZFHT.
Figure 2 shows the XRD patterns of the ZFHT and IFHT-5T samples at Tn = 623 K (a) and 743 K (b). The XRD patterns of the host Sm2Fe17 and FN Sm2Fe17N3 are shown in Fig. 2(a) and (b), respectively. The broad XRD profile appeared at 2θ = 44.2°, which was due to α-Fe phase precipitation (solid triangles in Fig. 2) during ZFHT and IFHT-5T. As seen in Fig. 2(a), the separation of 300, 024, 220, 303 and other reflection peaks was detected, whereas the separation became broad and unclear with higher 2θ.
X-ray powder diffraction patterns of the zero-field heat treatment (ZFHT) and 5 T in-field heat treatment (IFHT-5T) samples at Tn = 623 K (a) and 743 K (b). The solid triangles indicate the precipitation of α-Fe phase during the nitridation treatment. Broken lines are guides for the reader. The patterns for host Sm2Fe17 and fully nitride Sm2Fe17N3 are included in this figure.
Figure 3 shows the T dependence of the M (M-T curve) in μ0H = 1 T for Sm2Fe17Nx prepared by ZFHT and IFHT-5T at Tn = 623 K (a) and 743 K (b). For comparison, the M-T curves of Sm2Fe17 and FN Sm2Fe17N3 are also shown in Fig. 3(a). For 0.6 ≤ x ≤ 2.3, the M-T curves of ZFHT and IFHT-5T did not show the Brillouin-function like curve but a superposition of two M-T curves. The character of the M-T curves for 0.6 ≤ x ≤ 2.3 is consistent with the previous reports.3,16) M of IFHT-5T was larger than that of ZFHT. The M-T curve of IFHT-5T (x = 2.9) at Tn = 743 K was almost the same as that of FN Sm2Fe17N3.
Temperature dependence of the magnetization for Sm2Fe17Nx prepared by the zero-field heat treatment (ZFHT) and 5 T in-field heat treatment (IFHT-5T) at the nitriding temperature Tn of 623 K (a) and 743 K (b). For comparison, the data of Sm2Fe17 and Sm2Fe17N3 are also shown.
Figure 4 shows the magnetization curves (M-H curves) of Sm2Fe17Nx prepared by ZFHT and IFHT-5T at Tn = 623 (a) and 743 K (b). The M-H curves were measured at 10 K. For comparison, the M-H curves of Sm2Fe17 (broken curves) and FN Sm2Fe17N3 (solid curves) are also shown. MS of Sm2Fe17, Sm2Fe17N2.9 and FN Sm2Fe17N3 at 10 K was determined from the extrapolation of the M vs. 1/H2 curve to infinite field.17) The determined MS and TC are listed in Table 1. MS and TC of Sm2Fe17N2.9 prepared by IFHT-5T are almost same values of FN Sm2Fe17N3.
Magnetization curves at 10 K of Sm2Fe17Nx prepared by the zero-field heat treatment (ZFHT) and 5 T in-field heat treatment (IFHT-5T) at the nitriding temperature Tn of 623 K (a) and 743 K (b). The open and closed circles indicated the ZFHT and IFHT-5T, respectively. The data of Sm2Fe17 (broken curves) and Sm2Fe17N3 (solid curves) are also shown.
Here, we briefly discuss present results from the viewpoint of the gain of GM, ΔGM. Figures 2 and 3 indicates that the samples for 0.6 ≤ x ≤ 2.3 were not a single phase of Sm2Fe17Nx. Kobayashi et al. reported that homogeneous nitrogen distribution of intermediate nitrogen content (0 < x < 3) is almost impossible.16) Fujii et al. reported that grain growth of the FN phase from the nitrogen-poor (NP) phase became more dominant than the diffusion process during nitridation for Pn ≥ 0.1 MPa.15) Considering these reports, it is considered that the NP and FN phases coexisted in the samples of 0.6 ≤ x ≤ 2.3, and Ha promoted FN (high M) phase grain growth in Sm2Fe17 powder under present IFHT conditions.
The IFHT effect on gas-solid,6,7) liquid-solid,8–12) and solid-solid13,14) reactions for ferromagnetic materials has previously been reported. These effects occurred by the large ΔGM. In our case, ΔGM under H can be written as
| \begin{equation} \Delta G_{\text{M}} = \mu_{0}H\Delta M, \end{equation} | (1) |
| \begin{equation} \Delta M = M_{\text{SF}}-M_{\text{SFN}}, \end{equation} | (2) |
Comparing x of ZFHT at Tn = 623 and 743 K, x increases by one nitrogen when the increase of Tn is about 70.5 K. This suggests that the thermal energy gain of 43 K is required for increasing x of 0.6 by IFHT-5T (x = 2.9) from ZFHT (x = 2.3) at Tn = 743 K. This energy gain is comparable to magnetic energy gain: ΔGM = 35 K. Therefore, the field-induced phase transition (grain growth) by ΔGM probably make a contribution to the observed IFHT effect on nitridation of Sm2Fe17. For more discussing this effect, other magnetic field effects such as magnetostriction, nitrogen-diffusion process and so on for T range of 600–750 K may be examined. In this study, however, we found that nitrogen-gas IFHT helps to synthesize Sm2Fe17N3 at lower temperature or for shorter time.
The IFHT furnace for gas-solid reaction used magnetic fields up to 5 T was developed. ZFHT and IFHT-5T for Sm2Fe17 powder was conducted under nitrogen gas pressure of 0.1 MPa at 623 and 743 K for 24 h to investigate the IFHT effect on the nitridation to Sm2Fe17Nx. Our results clearly show the promotion of nitridation of the Sm2Fe17 magnet by applying H. By IFHT-5T, the nitrogen content x increased by approximately 0.6 nitrogen atoms per formula unit, compared with ZFHT, and almost FN Sm2Fe17N2.9 was obtained at 743 K.
The authors are grateful to Prof. H. Fujii of Hiroshima University for providing the Sm2Fe17 host compound. This work was partly supported by KAKENHI 17H00289 and Iketani Science and Technology Foundation 0301019-A. Magnetization measurements were performed at the Institute for Materials Research, Tohoku University and the Institute for Solid State Physics, The University of Tokyo.
