Microstructure and magnetic properties of Sm2Fe17Nx compound and their magnets prepared by field-activated combustion

Journal of Alloys and Compounds 428 (2007) 350–354

Microstructure and magnetic properties of Sm2Fe17Nx compound and their magnets prepared by field-activated combustion Jinwen Ye, Ying Liu ∗ , Guoli Zhu, Mei Chen, Shengji Gao, Mingjing Tu Department of Metal Materials, Sichuan University, Chengdu City 610065, China Received 8 March 2006; received in revised form 22 March 2006; accepted 22 March 2006 Available online 3 May 2006

Abstract The microstructure and properties of isotropic Sm2 Fe17 Nx powder and their magnets prepared by field-activated combustion at 400–500 ◦ C were investigated by XRD, SEM and EDXS methods. The results show that there is a small fraction volume of H atom left in the sample after the HDDR process at 775 ◦ C, and the powder nitridinged at 500 ◦ C for 5 h is nearly composed of single Sm2 Fe17 Nx phase. During the field-activated pressureassisted synthesis (FAPAS) process, the Sm2 Fe17 Nx compounds partly decomposed into SmN and ␣-Fe phases. With increase of temperature, the effect of sintering gets better and the obtained density is larger, but the magnetic property is decreased. The best magnetic property of isotropic Sm2 Fe17 Nx magnet is obtain at 450 ◦ C, which is Br = 0.660 T, Hcj = 1006 kA/m, (BH)max = 72 kJ/m3 . © 2006 Elsevier B.V. All rights reserved. Keywords: Metal materials; Rare-earths; HDDR; Hydrogenation; Nitride

1. Introduction Since Coey and Sun [1] discovered the interstitial compound Sm2 Fe17 Nx , it has attracted many researchers’ attention. As a kind of permanent magnetic material, Sm2 Fe17 Nx exhibits fairly high saturation magnetization (1.54 T) [2], high anisotropy field (14 T) under room temperature and high Curie temperature (476 ◦ C) [3]. These intrinsic magnetic properties are comparable with or better than those of Nd2 Fe14 B compound. It shows to be a candidate for intending permanent magnets. Unfortunately, because the Sm2 Fe17 Nx compounds decomposed into SmN and ␣-Fe phases at 650 ◦ C or above [4], the conventional sintering route is not fit for processing of compacted magnets. Hence the present applications of the Sm2 Fe17 Nx compounds are limited to be bonded magnets. But due to the additional polymer [5,6] or metal [7,8], the magnetic properties of Sm2 Fe17 Nx magnets will reduce. In order to obtain high performance Sm2 Fe17 Nx magnets, recently some researchers had reported sintering Sm2 Fe17 Nx magnets made by special approach such as spark plasma sintering (SPS) [9] and explosive sintering [10]. In despite of the properties of their magnets

are low, they do a salutary research on Sm2 Fe17 Nx sintering magnets. A few years ago, the simultaneous application of an electrical field combined with an applied pressure during combustion synthesis [referred to as the field-activated pressureassisted synthesis (FAPAS) process] was found to produce good quality dense intermetallic compounds in one step [11]. The aim of this work is to extend the simultaneous combustion synthesis and consolidation of nanomaterials and low temperature decomposable materials through a combination of mechanical and field activation. Indeed, this process was used successfully to form nanocrystalline bulk FeAl intermetallic compound with a relative density close to 99%[12]. In the present paper we propose to apply the FAPAS technique to isotropic Sm2 Fe17 Nx powders made by hydrogenation– disproportion–desorption–recombination (HDDR) process in order to investigate a new route (hereafter called FAPAS process) to prepare a dense Sm2 Fe17 Nx magnets. 2. Experimental 2.1. Preparation of isotropic Sm2 Fe17 Nx powder

∗

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0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.063

Sm2 Fe17 alloys were produced by industrial melting method with initial materials whose purities were 99.9% or above in argon atmosphere. The ingot

J. Ye et al. / Journal of Alloys and Compounds 428 (2007) 350–354

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simultaneously applying a uniaxial pressure of 20 Mpa. At last the sample was cooled while the pressure was maintained.

2.3. Characterization of isotropic Sm2 Fe17 Nx powder and magnets by field-activated combustion The phases of isotropic Sm2 Fe17 Nx powders during the preparation process and magnets made by field-activated combustion were measured by DX-2000 X-ray diffraction with Cu K␣ radiation. The Jada6.5 software was used to determine the lattice constants. The microstructures of the burnished samples (Sm–Fe alloy in different stages and the isotropic Sm2 Fe17 Nx magnets by field-activated combustion in different temperatures) were observed by JSM-5600LA scanning electron microscope (SEM) and local phase composition was determined by energy dispersive X-ray spectrometry (EDXS). The apparent density of the end-product was evaluated by a liquid displacement method. The properties of isotropic Sm2 Fe17 Nx magnet by field-activated combustion was measured by LakeShore7410 VSM with a field of 2.88 T. Fig. 1. Schematic diagram for the field-activated combustion process for Sm2 Fe17 Nx magnets. was annealed in argon atmosphere at 1050 ◦ C for 12 h. Then the homogenized ingot was crushed into powders of less than 10 ␮m in size. The HDDR process was conducted as following: the crushed powders were placed in a furnace and heated to 775 ◦ C at a rate of 5 ◦ C/min in a high purity hydrogen atmosphere of 0.1 MPa, held for an hour, subsequently the furnace was vacuumized to ×10−4 Pa, held for 2 h. After cooled to room temperature, the powders were nitrified at 500 ◦ C for 5 h in pure nitrogen atmosphere of 0.3 MPa.

2.2. Preparation of isotropic Sm2 Fe17 Nx magnets by field-activated combustion The Sm2 Fe17 Nx powders without any additive were first cold compacted into a horny alloy die using a uniaxial pressure of 20 MPa. The density of green bulk (Ø 10 × 10 mm) resulting from this, ranged from 3.5 to 4.5 g/cm3 . The sintering was preformed using a field-activated pressure-assisted synthesize apparatus (FAPASA). This apparatus consists of a uniaxial 110 MPa press combined with a 10 V, 1750 A ac power supply, to simultaneously provide current and pressure to a conductive die sample. A schematic of the experimental set up is shown in Fig. 1. The green bulk of Sm2 Fe17 Nx compound was placed into a dimensionadjustable die inside FAPASA reaction chamber which was then at ×10−3 Pa and backfilled with N2 (industry N2 , 99.999% pure). Heating the die results in the combustion of the Sm2 Fe17 Nx sample while the applied pressure densities the sample. The detail process is as follows: the cold compacted Sm2 Fe17 Nx sample was progressively held at 1200 A (corresponding to the sample temperature of 400–500 ◦ C) for 0.17–0.25 s, then keep the temperature for 300 s (Fig. 2) and

3. Results and discussion 3.1. Characterization of isotropic Sm2 Fe17 Nx powders by HDDR process X-ray diffraction patterns of Sm2 Fe17 alloy and their nitrides are shown in Fig. 3. Lattice parameters and structures of the powders are represented in Table 1. The reflections in the ascast ingot (Fig. 3(a)) can be attributed to the Sm2 Fe17 major phase, an SmFe3 /SmFe2 minor phase and lots of ␣-Fe phase. As can be seen from, the SEM image of the as-cast Sm2 Fe17 alloy [Fig. 4(A)], the black is extensive dendrites of free Fe (a), surrounded by the gray Sm2 Fe17 phase (c) and about 25 vol.%

Fig. 3. X-ray diffraction patterns of Sm2 Fe17 alloy and their nitrides: (a) as-cast, (b) as-homogenized, (c) Sm2 Fe17 powder after HDDR process and (d) nitride.

Table 1 Phase and lattice parameters under different estate

Fig. 2. Typical curve of temperature for isotropic Sm2 Fe17 Nx magnet during field-activated combustion process.

Sample

Phases

a (nm)

c (nm)

V (nm3 )

x

As-cast

Sm2 Fe17 /␣-Fe/ SmFe2–3 Sm2 Fe17 /(␣-Fe) Sm2 Fe17 (Hx ) Sm2 Fe17 Nx

0.85852

1.24694

0.79593

–

0.85683 0.86236 0.87410

1.23892 1.24723 1.26520

0.78770 0.80326 0.83720

– 0.5 2.8

As-homogenized HDDR Nitride

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Fig. 5. X-ray diffraction patterns of Sm2 Fe17 Nx magnets by field-activated combustion (a) nitride (after HDDR); (b) 400 ◦ C; (c) 450 ◦ C; (d) 500 ◦ C.

3.2. Effect of field-activated combustion temperature on microstructure of isotropic Sm2 Fe17 Nx magnets

Fig. 4. (A) SEM image of as-cast Sm2 Fe17 showing ␣-Fe: (a), SmFe2 /SmFe3 (b), Sm2 Fe17 (c); (B) SEM image of as-homogenized Sm2 Fe17 showing ␣-Fe (a), SmFe2 /SmFe3 (b), Sm2 Fe17 (c) and distributing of Fe (uppermost line), distributing of Sm (middle line).

samarium-rich phase (b) are at the edge of Sm2 Fe17 phase (c), which are white (EDXS detect). After annealed at 1050 ◦ C for 12 h, the unfinished reaction would hold on between the Sm2 Fe17 matrix grains and samarium-rich phase. At last, the main phase in as-homogenized ingot is Sm2 Fe17 with rhombohedral Th2 Zn17 type structure, co-existing with neglectable ␣-Fe phases, and samarium-rich phase is undetectable [see Fig. 3(b)], which are according with the result of SEM [Fig. 4(B)]. The result of EDXS test by a line suggests that samarium and iron elements is homogeneous in the alloy. It can be seen from Fig. 3(c) that the major phase of the alloy still exhibits rhombohedral Th2 Zn17 -type structure after HDDR process. But compared to the as-homogenized Sm2 Fe17 alloys, the characteristic peaks of recombination sample are shifted to a little lower diffraction angles, which confirms that the lattice of Sm2 Fe17 alloys are expansible. According to the reference [13], there is a small fraction volume of H atom left in the sample. As a consequence of being nitridinged at 500 ◦ C for 5 h [Fig. 3(d)], a new phase was formed with the same structure as Sm2 Fe17 alloy with the Bragg peaks shifted towards lower diffraction angles about 0.8◦ . It is indicated an expansion of the unit cell. The volume of expanding reaches at 6.28%. A conclusion can be drawn from the X-ray diffraction pattern of the nitride that the powder is nearly composed of single Sm2 Fe17 Nx phase.

X-ray diffraction patterns of Sm2 Fe17 Nx magnets by fieldactivated combustion at different temperature are shown in Fig. 5. Compared to the nitrogenous powders [Fig. 5(a)], the peaks of Sm2 Fe17 Nx phase in magnets by field-activated combustion at 400 ◦ C are shifted towards lower diffraction angles and the ␣-Fe phase in the magnets is detected, which suggested that the following reactions take place during the field-activated combustion process: Sm2 Fe17 Nx + N2 → Sm2 Fe17 N3

(1)

Sm2 Fe17 Nx → SmN + α-Fe

(2)

The unsaturated nitride was nitridinged to x = 3, at the same time the decomposed reaction (2) had happened. X-ray intensity peaks diffracted from ␣-Fe (1 1 0) and Sm2 Fe17 Nx (3 0 3) were used to estimate the relative volume fraction of the disproportionate mixture phases. With the field-activated combustion temperature increasing the relative intensity of ␣-Fe (1 1 0) peak keeps increasing and the peaks of Sm2 Fe17 Nx phase are slight shifted towards higher diffraction angles [see Fig. 5(c) and (d)]. It is indicated that the decomposed reaction is dominant gradually. Fig. 6 shows the SEM photograph of Sm2 Fe17 Nx magnets by field-activated combustion at different temperature. Table 2 lists their magnetic properties. Compare to the SEM photograph of green bulk (Fig. 6A), raw powders of Sm2 Fe17 Nx magnets are consolidated precisely or partly consolidated after fieldactivated sintered. EDXS result shows that both the black area Table 2 Magnetic property of Sm2 Fe17 Nx magnets by field-activated combustion in different temperature Temperature (◦ C)

Density of green bulk (g/cm3 )

Density of end-product (g/cm3 )

Br (T)

400 450 500

4.5 4.4 4.2

6.1 6.5 6.6

0.598 0.660 0.434

Hcj (kA/m) 1030 1006 1008

(BH)max (kJ/m3 ) 65.4 72.0 48.6

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Fig. 6. Photograph of Sm2 Fe17 Nx magnets by different field-activated combustion temperature. (A) Green bulk; (B) 500 ◦ C; (C) 450 ◦ C; (D) 400 ◦ C.

and the bright area are Sm2 Fe17 Nx phase. The black area is just sintered and the bright area is partly consolidated. With the increase of field-activated combustion temperature, the consolidated area (black area in the photograph) was enlarged (see Fig. 6B–D), and the enhancement of density (see Table 2) was also increased. But as can be seen from Table 2, the magnetic property was decreased. Fig. 7 shows the demagnetic curve of isotropic Sm2 Fe17 Nx magnets at 450 ◦ C. The best magnetic property is achieved as follows: Br = 0.660 T, Hcj = 1006 kA/m, (BH)max = 72 kJ/m3 .

4. Conclusions 1. The isotropic Sm–Fe–N permanent magnetic powders were prepared from the as-homogenized Sm2 Fe17 alloy by hydrogenation–disproportion–desorption–recombination (HDDR) and nitrogen processes. There is a small fraction volume of H atom left in the sample after HDDR process at 775. As a consequence of being nitridinged at 500 ◦ C for 5 h, the powder is nearly composed of single Sm2 Fe17 Nx phase. 2. The isotropic Sm2 Fe17 Nx magnets were successfully prepared by field-activated pressure-assisted synthesis (FAPAS) process. During the field-activated combustion process, the Sm2 Fe17 Nx compounds were partly decomposed into SmN and ␣-Fe phases. With the increase of field-activated combustion temperature, the effect of sintering is better and density is larger, but the magnetic property is decreased. The best results for the magnetic properties of isotropic Sm2 Fe17 Nx magnets are achieved as follows: Br = 0.660 T, Hcj = 1006 kA/m, (BH)max = 72 kJ/m3 . References

Fig. 7. Demagnetic curve of isotropic Sm2 Fe17 Nx magnet by field-activated combustion at 450 ◦ C.

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