- Email: [email protected]
Use of block copolymer templates for chemical synthesis of Nd2Fe14B nanocomposites with controlled magnetic properties
Materials Chemistry and Physics 227 (2019) 265–268
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Use of block copolymer templates for chemical synthesis of Nd2Fe14B nanocomposites with controlled magnetic properties
T
Hiroaki Wakayama∗, Hirotaka Yonekura Toyota Central R&D Laboratories, Inc., 41-1, Yokomichi, Nagakute, Aichi, 480-1192, Japan
H I GH L IG H T S
Fe B nanocomposites were prepared via self-assembled block copolymer templates. • Nd percentages of Nd, Fe, and B in the raw materials were varied. • The Fe B nanocomposites with large magnetization or large coercivity were obtained. • Nd • Obtained Nd Fe B nanocomposites contained few oxides or residues of reducing agent. 2
14
2
14
2
14
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite Polymer Metallic composite Magnetic material
The purpose of this study was to construct structurally controlled Nd2Fe14B nanocomposite magnets with controlled magnetic properties. Block copolymer templates were used to chemically synthesize Nd2Fe14B nanocomposites. The Fe, Nd, and B precursors were introduced into a polystyrene-b-poly(2-vinylpyridine) template. After the block copolymer was oxidatively removed, the oxidized magnetic material was reduced with CaH2 and washed with water to remove residual Ca species. The nanostructures and magnetic properties of the resulting materials could be controlled by adjusting the atom percentages of Nd, Fe, and B in the raw materials. Neodymium-rich raw materials afforded a Nd2Fe14B/Nd nanocomposite that had a large coercivity and was composed of Nd2Fe14B surrounded by a nonmagnetic Nd phase, which weakened the magnetic coupling of Nd2Fe14B and impeded magnetic field inversion. In contrast, when the atom percentages of the three elements in the raw materials were nearly stoichiometric, a nanocomposite consisting mainly of Nd2Fe14B was obtained, and its saturation magnetization was 158 emu/g (94% of the theoretical value for Nd2Fe14B [169 emu/g]).
1. Introduction Dysprosium (Dy) is used in the preparation of highly heat resistant Nd2Fe14B magnets, but because Dy is scarce and expensive, alternative methods to produce such magnets are in demand. One method is to reduce the Nd2Fe14B grain size to the nanometer scale [1]. Chemical synthesis is used to produce nanoparticles with a wide variety of constituents, including metals and oxides [2,3], but chemical synthesis of Nd2Fe14B nanoparticles is difficult because of the instability of rare earth metals [4,5]. Only a few studies of the chemical synthesis of Nd2Fe14B nanoparticles have been reported [6–8]. In addition, the saturation magnetization of the synthesized nanoparticles was < 105 emu/g, which is much lower than the theoretical saturation magnetization of Nd2Fe14B (169 emu/g) [7,8], because of their low purity. Synthesizing highly pure Nd2Fe14B nanoparticles remains a challenge. Recently, we developed a process for chemically synthesizing metal ∗
[9] and metal oxide nanostructures [10] by using block copolymer templates. In this process, the precursors of the metals or metal oxides are introduced into separate blocks of the block copolymer templates. Condensation of the precursors in their respective polymer blocks at scales on the order of several tens of nanometers leads to easy diffusion and crystallization of metals or metal oxides at low temperatures by means of reactions between adjacent precursors within each block. In this process, the crystallization temperature is much lower than that in other methods, such as those involving solid-state reactions. The lower temperature prevents grain growth and produces a very pure product with little byproducts. Recently achieved breakthroughs in nanocomposite magnets have yielded bulk nanocomposites stronger than the corresponding singlephase, rare-earth magnets [11–13]. However, constructing structurally controlled rare-earth nanocomposite magnets with high magnetic performance is challenging.
Corresponding author. E-mail address: [email protected] (H. Wakayama).
https://doi.org/10.1016/j.matchemphys.2019.01.073 Received 10 May 2018; Received in revised form 28 January 2019; Accepted 30 January 2019 Available online 02 February 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
When Nd2Fe14B magnetic nanoparticles are fabricated into bulk magnets, the formation of a grain boundary phase between the magnetic nanoparticles is necessary. Mixing and sintering the nanoparticles with particles of Nd or Nd-Cu is effective for this purpose, but making rare-earth-rich fine powders is difficult. The goal of this research was to construct structurally controlled Nd2Fe14B nanocomposite magnets with controlled magnetic properties. We speculated that by using our templating method and adjusting the percentages of Nd, Fe, and B in the raw materials, we could generate a material consisting of a Nd phase around a Nd2Fe14B fine powder. If the Nd2Fe14B fine powder could be compounded with the Nd phase on a nanometer scale, the result might be an increase in coercivity because the magnetic Nd2Fe14B would be surrounded by the nonmagnetic phase Nd, which could be expected to weaken the magnetic coupling of Nd2Fe14B and thus impede magnetic field inversion. In this study, we used our block copolymer template method for the chemical synthesis of Nd2Fe14B nanocomposites and investigated the influence of the percentages of Nd, Fe, and B in the raw materials on the nanostructures and magnetic properties of the Nd2Fe14B nanocomposites.
Table 1 Elemental compositions of raw materials and samples after heat treatment and washing. Sample
1 2
Elemental composition of raw materials (at%)
Elemental composition after heat treatment and washing (at%)
Nd
Fe
B
Nd
Fe
B
20.3 14.3
73.6 80.0
6.1 5.7
19.0 15.3
75.0 79.5
6.0 5.2
2. Materials and methods Neodymium(III) tris(acetylacetonate) hydrate (96%, SigmaAldrich), iron(III) tris(acetylacetonate) (99%, Sigma-Aldrich), and 1,1bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ferrocene (> 98%, Tokyo Chemical Industry) were dissolved in a 0.5 wt% solution of polystyrene-b-poly(2-vinylpyridine) (number average molecular weight ratio [MnPS:MnP2VP], 102,000:97,000; polydispersity index, 1.12; PolymerSource) in toluene (99.5%, Wako Chemical). After being stirred for 3 h at 90 °C, the solution was transferred to a Petri dish and calcined at 180 °C for 3 h and 350 °C for 6 h under a flow of N2. The calcined sample was oxidized at 800 °C for 6 h in air. Finally, 0.15 g of the oxidized sample with 0.13 g of CaH2 (Nacalai Tesque), was heated at 800 °C for 6 h under a vacuum. The major byproduct of this reduction process was calcium oxide containing residual calcium metal; these Ca species were removed by washing the reaction mixture with deionized water in a glove box and then evaporating the water under a vacuum. Scanning transmission electron microscopy (STEM) and energydispersive spectrometry (EDS) were carried out with a transmission electron microscope (JEM-2010FEF(HR), JEOL) operated at 200 keV. Xray diffraction (XRD) patterns were collected with Cu Kα radiation on a diffractometer (RINT-TTR, Rigaku) operated at 40 kV and 50 mA. Magnetic properties were measured at 300 K with a vibrating sample magnetometer (VSM-3S-15, Toei Industry Co.). Before the magnetic properties were measured, the samples were saturated in a 100-kOe pulsed magnetic field generated with a pulsed high-field magnetometer (TPM-1-15, Toei Industry Co.). Elemental analysis was carried out by means of inductively coupled plasma atomic emission spectrometry (CIRIOS-120EOP, Rigaku).
Fig. 1. XRD profiles after washing of (a) sample 1 and (b) sample 2.
3. Results and discussion One nanocomposite sample was prepared from raw materials enriched in Nd (sample 1), and another was prepared from raw materials with nearly stoichiometric percentages of Nd, Fe, and B (sample 2). The elemental compositions of the raw materials were compared with those of the samples after heat treatment and washing (Table 1). The XRD patterns of both samples after washing showed diffraction peaks attributable to a Nd2Fe14B hard magnetic phase (Fig. 1). The XRD pattern of sample 1 also showed peaks attributable to Nd (Fig. 1a), and the pattern for sample 2 showed small peaks for Nd2Fe17, suggesting that sample 2 contained a small amount of Nd2Fe17. Comparison of the XRD patterns of both samples before (Fig. 2) and after washing (Fig. 1) revealed that little CaO residue from the reducing agent remained in either sample after washing. No peaks due to oxides derived from
Fig. 2. XRD profiles before washing of (a) sample 1 and (b) sample 2.
oxidation of Nd and Fe by water during the washing step were detected. The atom percentages of Nd, Fe, and B in samples 1 and 2 (Table 1) were plotted on a Nd-Fe-B phase diagram (Fig. S1) [14]; the data point for sample 1 fell into the area of the diagram corresponding to Nd2Fe14B and Nd, and the data point for sample 2 fell in the area corresponding to Nd2Fe14B and Nd2Fe17. These results are consistent with the XRD results. In the STEM image of sample 1 (Fig. 3a), nanoparticles with 266
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
Fig. 3. (a) Scanning transmission electron microscopy image of sample 1, along with corresponding distributions of (b) Fe, (c) Nd, and (d) both elements. (e) Scanning transmission electron microscopy image of sample 2, along with corresponding distributions of (f) Fe, (g) Nd, and (h) both elements.
the Nd2Fe14B and impeded magnetic field inversion. The saturation magnetization of sample 2, consisted mostly of Nd2Fe14B, was 158 emu/g, or 94% of the theoretical value for Nd2Fe14B (169 emu/g). This value is much higher than previously reported values for chemically synthesized Nd2Fe14B [7,8] because of the high purity of the Nd2Fe14B nanoparticles in the sample.
diameters of less than several hundred nanometers were observed. The distributions of Nd and Fe (as determined by EDS) for the same field of view showed regions where Nd and Fe overlapped and regions where only Nd could be detected (Fig. 3b–d); we considered these regions to be Nd2Fe14B and Nd phases, respectively. These results suggest that Nd formed around Nd2Fe14B magnetic nanoparticles, and many nanoparticles with composite structures were observed (Fig. 3d). Nanoparticles with diameters of less than several hundred nanometers were also observed in the STEM image of sample 2 (Fig. 3e). The distributions of Nd and Fe (determined by EDS) in the same field of view showed regions where Nd and Fe overlapped (Fig. 3f–h), which suggests that the nanoparticles in sample 2 consisted mainly of a Nd2Fe14B hard magnetic phase with only very small amounts of other phases. We compared the magnetization curves of samples 1 and 2 before and after they were washed with water (Fig. 4). Both samples showed hard magnetism with coercivity. In both cases, the fact that the magnetization was substantially higher after washing than before suggested that washing had removed residual Ca species, as shown in the XRD results, and thus had increased the volume fraction of the Nd2Fe14B phase. In contrast, the coercivity of both samples was lower after washing than before washing. The unwashed samples contained large amounts of Ca species that served as pinning sites, that is, nonmagnetic barriers that impeded magnetic field inversion. After washing, the coercivity of sample 1 was larger than that of sample 2. The STEM image of sample 1 indicated that Nd2Fe14B was surrounded by a nonmagnetic Nd phase. This Nd phase weakened the magnetic coupling of
4. Conclusions The atom percentages of Nd, Fe, and B in the raw materials used for block-copolymer-templated chemical synthesis of Nd2Fe14B magnetic materials could be adjusted to control the nanostructures and magnetic properties of the obtained materials. Specifically, we were able to synthesize materials with large magnetization or large coercivity by varying the chemical composition of the raw materials. Conflicts of interest The author declares that there is no interest regarding the publication of this paper. The research was performed as part of the employment of the authors. employer: TOYOTA CENTRAL R & D LABS., INC. Notes All the data used to support the findings of this study are available
Fig. 4. Magnetization curves of (a) sample 1 and (b) sample 2 before (red) and after (green) washing. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 267
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
from the corresponding author upon request.
(FexNd1-x)0.6B0.4 nanoparticles, Physica B 354 (2004) 117–120. [7] V. Swaminathan, P.K. Deheri, S.D. Bhame, R.V. Ramanujan, Novel microwave assisted chemical synthesis of Nd2Fe14B hard magnetic nanoparticles, Nanoscale 5 (2013) 2718–2725. [8] P.K. Deheri, V. Swaminathan, S.D. Bhame, Z. Liu, R.V. Ramanujan, Sol-gel based chemical synthesis of Nd2Fe14B hard magnetic nanoparticles, Chem. Mater. 22 (2010) 6509–6517. [9] H. Wakayama, H. Yonekura, Y. Kawai, Three-dimensional periodically ordered nanohetero metallic materials from self-assembled block copolymer composites, ACS Macro Lett. 2 (2013) 284–287. [10] H. Wakayama, Y. Kawai, H. Yonekura, Three-dimensional bicontinuous nanocomposite from a self-assembled block copolymer for a high-capacity all-solid-state lithium battery cathode, Chem. Mater. 28 (2016) 4453–4459. [11] X. Li, L. Lou, W. Song, G. Huang, F. Hou, Q. Zhang, H.-T. Zhang, J. Xiao, B. Wen, X. Zhang, Novel bimorphological anisotropic bulk nanocomposite materials with high energy products, Adv. Mater. 29 (2017) 1606430. [12] X.H. Li, L. Lou, W.P. Song, Q. Zhang, G.W. Huang, Y.X. Hua, H.-T. Zhang, J.W. Xiao, B. Wen, X.Y. Zhang, Controllably manipulating three-dimensional hybrid nanostructures for bulk nanocomposites with large energy products, Nano Lett. 17 (2017) 2985–2993. [13] G. Huang, X. Li, L. Lou, Y. Hua, G. Zhu, M. Li, H.-T. Zhang, J. Xiao, B. Wen, M. Yue, X. Zhang, Engineering bulk, layered, multicomponent nanostructures with high energy density, Small 14 (2018) 1800619. [14] B. Hallemans, P. Wollants, J.R. Roos, Thermodynamic assessment of the Fe-Nd-B phase diagram, J. Phase Equil. 16 (1995) 137–149.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchemphys.2019.01.073. References [1] M. Nakamura, M. Matsuura, N. Tezuka, S. Sugimoto, Y. Une, H. Kubo, M. Sagawa, Preparation of ultrafine jet-milled powders for Nd-Fe-B sintered magnets using hydrogenation–disproportionation–desorption–recombination and hydrogen decrepitation processes, Appl. Phys. Lett. 103 (2013) 022404. [2] M. Kim, S.A. Hong, N. Shin, Y.H. Lee, Y. Shin, Synthesis of strontium titanate nanoparticles using supercritical water, Ceram. Int. 42 (2016) 17853–17857. [3] S. Sun, Recent advances in chemical synthesis, self-assembly, and applications of FePt nanoparticles, Adv. Mater. 18 (2006) 393–403. [4] X. Wang, X.M. Sun, D. Yu, B.S. Zou, Y. Li, Rare earth compound nanotubes, Adv. Mater. 15 (2003) 1442–1445. [5] M. Yada, M. Mihara, S. Mouri, M. Kuroki, T. Kijima, Rare earth (Er, Tm, Yb, Lu) oxide nanotubes templated by dodecylsulfate assemblies, Adv. Mater. 14 (2002) 309–313. [6] M. Tortarolo, R.D. Zysler, H. Troiani, H. Romero, Magnetic order in amorphous
268
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Use of block copolymer templates for chemical synthesis of Nd2Fe14B nanocomposites with controlled magnetic properties
T
Hiroaki Wakayama∗, Hirotaka Yonekura Toyota Central R&D Laboratories, Inc., 41-1, Yokomichi, Nagakute, Aichi, 480-1192, Japan
H I GH L IG H T S
Fe B nanocomposites were prepared via self-assembled block copolymer templates. • Nd percentages of Nd, Fe, and B in the raw materials were varied. • The Fe B nanocomposites with large magnetization or large coercivity were obtained. • Nd • Obtained Nd Fe B nanocomposites contained few oxides or residues of reducing agent. 2
14
2
14
2
14
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocomposite Polymer Metallic composite Magnetic material
The purpose of this study was to construct structurally controlled Nd2Fe14B nanocomposite magnets with controlled magnetic properties. Block copolymer templates were used to chemically synthesize Nd2Fe14B nanocomposites. The Fe, Nd, and B precursors were introduced into a polystyrene-b-poly(2-vinylpyridine) template. After the block copolymer was oxidatively removed, the oxidized magnetic material was reduced with CaH2 and washed with water to remove residual Ca species. The nanostructures and magnetic properties of the resulting materials could be controlled by adjusting the atom percentages of Nd, Fe, and B in the raw materials. Neodymium-rich raw materials afforded a Nd2Fe14B/Nd nanocomposite that had a large coercivity and was composed of Nd2Fe14B surrounded by a nonmagnetic Nd phase, which weakened the magnetic coupling of Nd2Fe14B and impeded magnetic field inversion. In contrast, when the atom percentages of the three elements in the raw materials were nearly stoichiometric, a nanocomposite consisting mainly of Nd2Fe14B was obtained, and its saturation magnetization was 158 emu/g (94% of the theoretical value for Nd2Fe14B [169 emu/g]).
1. Introduction Dysprosium (Dy) is used in the preparation of highly heat resistant Nd2Fe14B magnets, but because Dy is scarce and expensive, alternative methods to produce such magnets are in demand. One method is to reduce the Nd2Fe14B grain size to the nanometer scale [1]. Chemical synthesis is used to produce nanoparticles with a wide variety of constituents, including metals and oxides [2,3], but chemical synthesis of Nd2Fe14B nanoparticles is difficult because of the instability of rare earth metals [4,5]. Only a few studies of the chemical synthesis of Nd2Fe14B nanoparticles have been reported [6–8]. In addition, the saturation magnetization of the synthesized nanoparticles was < 105 emu/g, which is much lower than the theoretical saturation magnetization of Nd2Fe14B (169 emu/g) [7,8], because of their low purity. Synthesizing highly pure Nd2Fe14B nanoparticles remains a challenge. Recently, we developed a process for chemically synthesizing metal ∗
[9] and metal oxide nanostructures [10] by using block copolymer templates. In this process, the precursors of the metals or metal oxides are introduced into separate blocks of the block copolymer templates. Condensation of the precursors in their respective polymer blocks at scales on the order of several tens of nanometers leads to easy diffusion and crystallization of metals or metal oxides at low temperatures by means of reactions between adjacent precursors within each block. In this process, the crystallization temperature is much lower than that in other methods, such as those involving solid-state reactions. The lower temperature prevents grain growth and produces a very pure product with little byproducts. Recently achieved breakthroughs in nanocomposite magnets have yielded bulk nanocomposites stronger than the corresponding singlephase, rare-earth magnets [11–13]. However, constructing structurally controlled rare-earth nanocomposite magnets with high magnetic performance is challenging.
Corresponding author. E-mail address: [email protected] (H. Wakayama).
https://doi.org/10.1016/j.matchemphys.2019.01.073 Received 10 May 2018; Received in revised form 28 January 2019; Accepted 30 January 2019 Available online 02 February 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
When Nd2Fe14B magnetic nanoparticles are fabricated into bulk magnets, the formation of a grain boundary phase between the magnetic nanoparticles is necessary. Mixing and sintering the nanoparticles with particles of Nd or Nd-Cu is effective for this purpose, but making rare-earth-rich fine powders is difficult. The goal of this research was to construct structurally controlled Nd2Fe14B nanocomposite magnets with controlled magnetic properties. We speculated that by using our templating method and adjusting the percentages of Nd, Fe, and B in the raw materials, we could generate a material consisting of a Nd phase around a Nd2Fe14B fine powder. If the Nd2Fe14B fine powder could be compounded with the Nd phase on a nanometer scale, the result might be an increase in coercivity because the magnetic Nd2Fe14B would be surrounded by the nonmagnetic phase Nd, which could be expected to weaken the magnetic coupling of Nd2Fe14B and thus impede magnetic field inversion. In this study, we used our block copolymer template method for the chemical synthesis of Nd2Fe14B nanocomposites and investigated the influence of the percentages of Nd, Fe, and B in the raw materials on the nanostructures and magnetic properties of the Nd2Fe14B nanocomposites.
Table 1 Elemental compositions of raw materials and samples after heat treatment and washing. Sample
1 2
Elemental composition of raw materials (at%)
Elemental composition after heat treatment and washing (at%)
Nd
Fe
B
Nd
Fe
B
20.3 14.3
73.6 80.0
6.1 5.7
19.0 15.3
75.0 79.5
6.0 5.2
2. Materials and methods Neodymium(III) tris(acetylacetonate) hydrate (96%, SigmaAldrich), iron(III) tris(acetylacetonate) (99%, Sigma-Aldrich), and 1,1bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ferrocene (> 98%, Tokyo Chemical Industry) were dissolved in a 0.5 wt% solution of polystyrene-b-poly(2-vinylpyridine) (number average molecular weight ratio [MnPS:MnP2VP], 102,000:97,000; polydispersity index, 1.12; PolymerSource) in toluene (99.5%, Wako Chemical). After being stirred for 3 h at 90 °C, the solution was transferred to a Petri dish and calcined at 180 °C for 3 h and 350 °C for 6 h under a flow of N2. The calcined sample was oxidized at 800 °C for 6 h in air. Finally, 0.15 g of the oxidized sample with 0.13 g of CaH2 (Nacalai Tesque), was heated at 800 °C for 6 h under a vacuum. The major byproduct of this reduction process was calcium oxide containing residual calcium metal; these Ca species were removed by washing the reaction mixture with deionized water in a glove box and then evaporating the water under a vacuum. Scanning transmission electron microscopy (STEM) and energydispersive spectrometry (EDS) were carried out with a transmission electron microscope (JEM-2010FEF(HR), JEOL) operated at 200 keV. Xray diffraction (XRD) patterns were collected with Cu Kα radiation on a diffractometer (RINT-TTR, Rigaku) operated at 40 kV and 50 mA. Magnetic properties were measured at 300 K with a vibrating sample magnetometer (VSM-3S-15, Toei Industry Co.). Before the magnetic properties were measured, the samples were saturated in a 100-kOe pulsed magnetic field generated with a pulsed high-field magnetometer (TPM-1-15, Toei Industry Co.). Elemental analysis was carried out by means of inductively coupled plasma atomic emission spectrometry (CIRIOS-120EOP, Rigaku).
Fig. 1. XRD profiles after washing of (a) sample 1 and (b) sample 2.
3. Results and discussion One nanocomposite sample was prepared from raw materials enriched in Nd (sample 1), and another was prepared from raw materials with nearly stoichiometric percentages of Nd, Fe, and B (sample 2). The elemental compositions of the raw materials were compared with those of the samples after heat treatment and washing (Table 1). The XRD patterns of both samples after washing showed diffraction peaks attributable to a Nd2Fe14B hard magnetic phase (Fig. 1). The XRD pattern of sample 1 also showed peaks attributable to Nd (Fig. 1a), and the pattern for sample 2 showed small peaks for Nd2Fe17, suggesting that sample 2 contained a small amount of Nd2Fe17. Comparison of the XRD patterns of both samples before (Fig. 2) and after washing (Fig. 1) revealed that little CaO residue from the reducing agent remained in either sample after washing. No peaks due to oxides derived from
Fig. 2. XRD profiles before washing of (a) sample 1 and (b) sample 2.
oxidation of Nd and Fe by water during the washing step were detected. The atom percentages of Nd, Fe, and B in samples 1 and 2 (Table 1) were plotted on a Nd-Fe-B phase diagram (Fig. S1) [14]; the data point for sample 1 fell into the area of the diagram corresponding to Nd2Fe14B and Nd, and the data point for sample 2 fell in the area corresponding to Nd2Fe14B and Nd2Fe17. These results are consistent with the XRD results. In the STEM image of sample 1 (Fig. 3a), nanoparticles with 266
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
Fig. 3. (a) Scanning transmission electron microscopy image of sample 1, along with corresponding distributions of (b) Fe, (c) Nd, and (d) both elements. (e) Scanning transmission electron microscopy image of sample 2, along with corresponding distributions of (f) Fe, (g) Nd, and (h) both elements.
the Nd2Fe14B and impeded magnetic field inversion. The saturation magnetization of sample 2, consisted mostly of Nd2Fe14B, was 158 emu/g, or 94% of the theoretical value for Nd2Fe14B (169 emu/g). This value is much higher than previously reported values for chemically synthesized Nd2Fe14B [7,8] because of the high purity of the Nd2Fe14B nanoparticles in the sample.
diameters of less than several hundred nanometers were observed. The distributions of Nd and Fe (as determined by EDS) for the same field of view showed regions where Nd and Fe overlapped and regions where only Nd could be detected (Fig. 3b–d); we considered these regions to be Nd2Fe14B and Nd phases, respectively. These results suggest that Nd formed around Nd2Fe14B magnetic nanoparticles, and many nanoparticles with composite structures were observed (Fig. 3d). Nanoparticles with diameters of less than several hundred nanometers were also observed in the STEM image of sample 2 (Fig. 3e). The distributions of Nd and Fe (determined by EDS) in the same field of view showed regions where Nd and Fe overlapped (Fig. 3f–h), which suggests that the nanoparticles in sample 2 consisted mainly of a Nd2Fe14B hard magnetic phase with only very small amounts of other phases. We compared the magnetization curves of samples 1 and 2 before and after they were washed with water (Fig. 4). Both samples showed hard magnetism with coercivity. In both cases, the fact that the magnetization was substantially higher after washing than before suggested that washing had removed residual Ca species, as shown in the XRD results, and thus had increased the volume fraction of the Nd2Fe14B phase. In contrast, the coercivity of both samples was lower after washing than before washing. The unwashed samples contained large amounts of Ca species that served as pinning sites, that is, nonmagnetic barriers that impeded magnetic field inversion. After washing, the coercivity of sample 1 was larger than that of sample 2. The STEM image of sample 1 indicated that Nd2Fe14B was surrounded by a nonmagnetic Nd phase. This Nd phase weakened the magnetic coupling of
4. Conclusions The atom percentages of Nd, Fe, and B in the raw materials used for block-copolymer-templated chemical synthesis of Nd2Fe14B magnetic materials could be adjusted to control the nanostructures and magnetic properties of the obtained materials. Specifically, we were able to synthesize materials with large magnetization or large coercivity by varying the chemical composition of the raw materials. Conflicts of interest The author declares that there is no interest regarding the publication of this paper. The research was performed as part of the employment of the authors. employer: TOYOTA CENTRAL R & D LABS., INC. Notes All the data used to support the findings of this study are available
Fig. 4. Magnetization curves of (a) sample 1 and (b) sample 2 before (red) and after (green) washing. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 267
Materials Chemistry and Physics 227 (2019) 265–268
H. Wakayama and H. Yonekura
from the corresponding author upon request.
(FexNd1-x)0.6B0.4 nanoparticles, Physica B 354 (2004) 117–120. [7] V. Swaminathan, P.K. Deheri, S.D. Bhame, R.V. Ramanujan, Novel microwave assisted chemical synthesis of Nd2Fe14B hard magnetic nanoparticles, Nanoscale 5 (2013) 2718–2725. [8] P.K. Deheri, V. Swaminathan, S.D. Bhame, Z. Liu, R.V. Ramanujan, Sol-gel based chemical synthesis of Nd2Fe14B hard magnetic nanoparticles, Chem. Mater. 22 (2010) 6509–6517. [9] H. Wakayama, H. Yonekura, Y. Kawai, Three-dimensional periodically ordered nanohetero metallic materials from self-assembled block copolymer composites, ACS Macro Lett. 2 (2013) 284–287. [10] H. Wakayama, Y. Kawai, H. Yonekura, Three-dimensional bicontinuous nanocomposite from a self-assembled block copolymer for a high-capacity all-solid-state lithium battery cathode, Chem. Mater. 28 (2016) 4453–4459. [11] X. Li, L. Lou, W. Song, G. Huang, F. Hou, Q. Zhang, H.-T. Zhang, J. Xiao, B. Wen, X. Zhang, Novel bimorphological anisotropic bulk nanocomposite materials with high energy products, Adv. Mater. 29 (2017) 1606430. [12] X.H. Li, L. Lou, W.P. Song, Q. Zhang, G.W. Huang, Y.X. Hua, H.-T. Zhang, J.W. Xiao, B. Wen, X.Y. Zhang, Controllably manipulating three-dimensional hybrid nanostructures for bulk nanocomposites with large energy products, Nano Lett. 17 (2017) 2985–2993. [13] G. Huang, X. Li, L. Lou, Y. Hua, G. Zhu, M. Li, H.-T. Zhang, J. Xiao, B. Wen, M. Yue, X. Zhang, Engineering bulk, layered, multicomponent nanostructures with high energy density, Small 14 (2018) 1800619. [14] B. Hallemans, P. Wollants, J.R. Roos, Thermodynamic assessment of the Fe-Nd-B phase diagram, J. Phase Equil. 16 (1995) 137–149.
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