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A facile synthesis of anisotropic SmCo5 nanochips with high magnetic performance
Chemical Engineering Journal 343 (2018) 1–7
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A facile synthesis of anisotropic SmCo5 nanochips with high magnetic performance
T
⁎
Ming Yuea, , Chenglin Lia, Qiong Wua, Zhenhui Maa,b, Huanhuan Xua, Subhashini Palakaa a b
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China School of Material Science and Engineering, Shaanxi University of Science & Technology, Xi’ an 710021, China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
Highly anisotropic SmCo single crys• talline nanochips have been synthe5
sized.
nanochips exhibit coercivity • The 19.3 kOe and energy product of
of
14.4 MGOe.
Co(OH) nanoflakes facilitate the for• mation of SmCo nanochips as a tem2
5
plate.
A R T I C L E I N F O
A B S T R A C T
Keywords: SmCo5 Nanochips Magnetic anisotropy
Highly anisotropic SmCo5 single crystal particles are promising candidates for high-density data storage and building nanocomposite permanent magnets with large maximum energy product. Previously, nanocrystalline SmCo5 particles obtained by calciothermic reduction exhibit low magnetic energy product due to magnetic isotropy caused by severely agglomerated equiaxial particles. In this study, we report a new strategy to fabricate highly anisotropic SmCo5 nanochips by reductive annealing of precursors with a special designed morphology. The precursors containing single crystal Co(OH)2 nanoflakes and crystalline Sm(OH)3 nanorods were synthesized by a hydrothermal process, and converted into SmCo5 nanochips by subsequent thermal reduction, during which the Co(OH)2 nanoflakes facilitate the formation of SmCo5 nanochips as a template and Sm(OH)3 nanorods help to maintain the hexagonal morphology and single crystal nature. The SmCo5 nanochips exhibit strong magnetic anisotropy and excellent room temperature magnetic properties with a remanence of 7.7 kG, coercivity of 19.3 kOe, and maximum energy product of 14.4 MGOe, which is the highest recorded value for chemical synthesized nanostructured SmCo5 particles.
1. Introduction
[1–3] has recently drawn renewed interests in high-density data storage and nanocomposite permanent magnet with large maximum energy product [4–25]. SmCo5 is a promising candidate as the hard-phase in nanocomposite magnets maintaining their high coercivities [26,27]. Especially, in a recent developed bottom-up approach [6] pursuing
As the first generation of rare earth permanent magnetic materials, SmCo5 compound bearing large magnetocrystalline anisotropy (Ku = 2 × 108 erg/cm3) and high Curie temperature (Tc = 1020 K)
⁎
Corresponding author. E-mail address: [email protected] (M. Yue).
https://doi.org/10.1016/j.cej.2018.02.060 Received 19 November 2017; Received in revised form 9 February 2018; Accepted 12 February 2018 Available online 22 February 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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M. Yue et al.
controllable microstructure and desirable magnetic properties in nanocomposite magnets, SmCo5 particles with high coercivity are frequently chosen as the hard phase for building the magnets with soft iron-based phase [28–34]. To achieve nanostructured SmCo5 particles with good magnetic performance, a novel approach combining solution-phase chemical synthesis with reduction annealing has been established [35–41]. In this technique, Sm-containing and Co-containing precursors were fabricated first by the solution-phase method, then undergoing a reductive annealing at elevated temperature to form the final products. By this approach, numerous investigations have reported nanostructured SmCo5 particles with reasonable coercivity. For instance, Ma et al. [42] reported synthesis of 10–40 nm SmCo5 nanoparticles with coercivity of 20 kOe by reductive annealing of polycrystalline Co(OH)2 and amorphous Sm(OH)3 precursors. However, most previously reported SmCo5 particles exhibit equiaxial morphology due to the high temperature annealing [39,41,42]. As a result, the particles are magnetically isotropic and exhibit low remanence and energy product, undermining their good potential in building nanocomposite magnets. Therefore, it is crucial to obtain high performance single crystal SmCo5 particles with new strategy. In the present study, we use a new strategy to synthesize anisotropic SmCo5 nanochips by employing the hydroxide with specially designed morphology as precursors. Firstly, single crystal Co(OH)2 nanoflakes and crystalline Sm(OH)3 nanorods were fabricated by a hydrothermal route. Such hydroxide precursor was further reduced to generate anisotropic SmCo5 nanochips, where the original morphology of the Co (OH)2 nanoflakes was well preserved with the assistance of the Sm (OH)3 nanorods during the annealing process. The prepared SmCo5 nanochips exhibit good magnetic performance. Moreover, the reaction mechanism of the new route was investigated.
Fig. 1. The XRD pattern of as-prepared precursors with peaks indexed as Sm(OH)3 (JCPDS No. 83-2036) and Co(OH)2 (JCPDS No. 30-0443), (a) Sm(OH)3 and Co(OH)2 synthesized simultaneously and TEM images of the precursors, Co(OH)2 (b) and Sm(OH)3 (c) synthesized separately.
(SEM, ZEISS-SUPRA55) and transmission electron microscopy (TEM, Tecnai-F20). For TEM observations, the samples were dispersed in hexane. The drops of the well dispersed nanoparticles were placed over the carbon coated microscopic copper grids (200 mesh size) and were subsequently dried. The magnetic properties of the nanochips were measured at room temperature (300 K) using a physical property measurement system (PPMS, Quantum Design) under a maximum applied field of 100 kOe.
2. Materials and methods 2.1. Synthesis of precursors
3. Results and discussion The precursors were prepared via a hydrothermal process. First, 0.73 g (2 mmol) SmCl3·6H2O and 2.02 g (8.5 mmol) CoCl2·6H2O were dissolved together in 40 ml deionized water. A 20 ml of 1.38 M aqueous NaOH solution was added drop-wise into the above solution with stirring at room temperature for 30 min. The solution was then transferred into a Teflon vessel (100 ml) for hydrothermal reaction at 453 K for 12 h using an autoclave reactor. After cooling to room temperature, the resultant was centrifuged at 6000 rpm for 3 min to obtain brown powder. The powder was further washed three times with deionized water and dried at 333 K for 12 h.
3.1. Synthesis of the precursors Fig. 1 shows the X-ray pattern of the as-prepared precursors. The diffraction peaks are indexed as Sm(OH)3 and Co(OH)2 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 83-2036 and No. 30-0443). Note that the sharp peaks also suggest good crystallization of Sm(OH)3 and Co(OH)2, which may be attributed to the hydrothermal process. A minor Co3O4 impurity phase is also found in the as-prepared precursor as shown in the X-ray patterns due to the existence of a little oxygen during hydrothermal process, which would take part in a subsequent reductive reaction similar to that of Co(OH)2. The precursors have been further investigated using SEM and TEM, as shown in Fig. 2. The SEM and TEM images in Fig. 2a-b show that the precursors contain two types of particles with remarkably different morphology, i. e., nanoflakes and nanorods. The hexagonal nanoflakes have widths in the range of 250–400 nm with thicknesses of 30–40 nm, while the nanorods have diameters of 15–20 nm with lengths of 150–250 nm. In addition, quite a few nanorods are distributed on the surface of the nanoflakes. Fig. 2c and 2d display the TEM images of unmixed Sm(OH)3 nanorods and Co(OH)2 nanoflakes, which were prepared in absence of another raw material using a same process, respectively. Furthermore, the hexagonal morphology suggests that single crystalline Co(OH)2 nanoflakes have been obtained. Different from the precursors of polycrystalline Co(OH)2 and amorphous Sm(OH)3 synthesized by co-precipitation strategy, the single crystal structure may lead to final products with various morphologies [39].
2.2. Synthesis of SmCo5 nanoflakes The prepared precursor powder was mixed with 6.0 g of Ca powder, 3.0 g of KCl and 2.0 g of CaO. The mixture was transferred to a tungsten crucible which was inserted into a steel tube. The tube was degassed for 3 times to remove air and moisture. Subsequently, the tube was flushed with forming gas (93% Ar + 7% H2) and heated to 1198 K at a rate of 8 K/min. The reaction was kept for 90 min before quickly cooling to room temperature. The product was washed with deionized water and 5% acetic acid to dissolve CaO, KCl, and extra Ca. A black powder was obtained by centrifuging at 8000 rpm for 3 min. The powder was finally washed with deionized water and ethanol and dried under vacuum. 2.3. Characterization of the precursors and final products The crystallographic structure of the precursors and final products was identified by X-ray diffraction (XRD, Rigaku Ultima Ⅳ) with a CuKα wavelength X-ray source. The microstructure and morphology of above samples were investigated using scanning electron microscopy 2
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Fig. 2. The SEM image (a) and TEM image (b) of the precursors, Sm(OH)3 and Co(OH)2 synthesized simultaneously. TEM images of Sm(OH)3 (c) and Co(OH)2 (d) synthesized separately. The inset is the FFT pattern of a Co(OH)2 hexagonal nanoflake.
3.2. Synthesis of anisotropic SmCo nanochips Fig. 3 shows the XRD patterns of SmCo particles prepared by reductive annealing of Sm(OH)3 and Co(OH)2 with KCl, CaO, and Ca powders. It can be seen from Fig. 3a that the diffraction peaks of randomly oriented SmCo nanochips match well with the standard SmCo5 pattern (JCPDS No. 65-5599), indicating that precursor has converted into hexagonal SmCo5 after undergoing high temperature reduction. For magnetic alignment, a cylindrical sample was prepared by mixing the as-prepared SmCo5 powders with epoxy, which solidified slowly, in a static magnetic field of 2.2 T. The diffraction peaks of magnetically aligned SmCo5 particles exhibit enhanced (0 0 l) peaks as shown in Fig. 3b. The intensity ratio of (0 0 2) to (1 1 1) reflection peaks, I(002)/ I(111), changes from 0.32 into 11.22 after magnetic alignment, revealing that the high anisotropic SmCo5 particles have been obtained by reductive annealing of single crystal Co(OH)2 and crystalline Sm(OH)3. Fig. 4a shows the SEM image of the synthetic SmCo5 particles. It can be observed that the particles exhibit plate-like morphology with diameters ranging from submicron to several microns and thicknesses from tens to hundreds of nanometers, hereafter named “nanochips”. Considering the dimension of the precursors of Co(OH)2 nanoflakes, the nanoflake precursors should be aggregated under high temperature and be subsequently sintered to form the nanochips with larger size. Some nanochips inevitably aggregated and grew during the high temperature
Fig. 3. XRD patterns of the randomly oriented sample (a) and magnetically aligned sample (b) of SmCo5 nanochips compared with the standard diffraction pattern of hexagonal SmCo5 (JCPDS No. 65-5599).
3
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Fig. 4. (a) The SEM image of as-synthesized SmCo5 flakes; (b) TEM, (c) HRTEM and (d) SAED of a single SmCo5 nanochip.
magnetization (Ms) of 8.2 kG. The large difference between the two loops indicates the strong magnetic anisotropy of the sample, which leads to a high remanence ratio Mr/Ms of 0.94. Fig. 5b shows the demagnetization curves of the sample. The high remanence (Mr) of 7.7 kG, large coercivity (Hci) of 19.3 kOe, and the good squareness results in a maximum energy product of 14.4 MGOe, which is the highest reported value for chemically synthetized SmCo particles [23,29,35,42–45].
heat treatment. However, the adhesion strength between the chips is quite weak and they can be easily ground and dispersed without surfactants in a hexane ultrasonic bath to produce SmCo5 nanochips. Fig. 4b–d show the TEM image, HRTEM lattice image, and corresponding SAED of individual SmCo5 nanochip after grinding, respectively. The diameter of the chip is about 1.3 μm. HRTEM observation (Fig. 4c) displays a interplanar distance in the hexagonal SmCo5 structure of 0.433 nm, corresponding to the (1 0 1 0) plane in SmCo5. Fig. 4d displays the SAED pattern of this nanochip. The periodic diffraction dots present a hexagonal array, suggesting the single crystal nature of the SmCo5 nanochip. Furthermore, the three dots corresponding to (1 0 1 0), (1 1 0 0) and (0 1 1 0) lattice planes, confirm that caxis (easy axis) of SmCo5 is perpendicular to the surface of the nanochip. The HRTEM and SAED images in Fig. 4c and Fig. 4d derive from a same area in Fig. 4b with zone axis [0 0 0 1]. Thus, stacked nanochips morphology, similar to that shown in Fig. 4a, is easy to achieve with good orientation. The magnetization as a function of the magnetic field of the SmCo5 nanochips has been measured by PPMS at a maximum applied magnetic field of 100 kOe, as shown in Fig. 5. Fig. 5a presents the hysteresis loops of magnetically aligned SmCo5 nanochips [along the magnetically easy (parallel to the direction of the magnetic field applied in the alignment procedure) and hard (perpendicular to the direction of the magnetic field applied in the alignment procedure) directions]. They exhibit ferromagnetic behavior at room temperature (300 K) with a saturation
3.3. Reaction mechanism for SmCo5 nanochips The synthetic process can be described by Eqs. (1)–(8). Eq. (1) and Eq. (2) are the precipitation reactions of Sm(OH)3 and Co(OH)2, respectively. The TEM observation in our previous report [39] confirmed that the precipitation reaction at room temperature would yield amorphous Sm(OH)3 and polycrystalline Co(OH)2 particles. In this work, during the subsequent hydrothermal reaction process, amorphous Sm(OH)3 grew into crystalline nanorods and polycrystalline Co (OH)2 transferred into single crystalline hexagonal nanoflakes at 453 K as shown in Eq. (3) and Eq. (4). Dehydration may occur during the temperature-rise period, as shown in Eq. (5) and Eq. (6). With the increase of temperature, CoO was reduced first [Eq. (7)], and then SmCo5 was formed with further reduction of Sm2O3 when the temperature increased to 1198 K [Eq. (8)], which is consistent with the results of previous studies [35,46]. 4
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Fig. 5. (a) Magnetization hysteresis loops of the magnetically aligned SmCo5 nanochips (along the magnetically easy (parallel to the direction of the magnetic field applied in the alignment procedure) and hard (perpendicular to the direction of the magnetic field applied in the alignment procedure) directions) and (b) second quadrant M−H curves for the aligned SmCo5 nanochips at 300 K.
into CoO completely while only part of Sm(OH)3 has been decomposed at 573 K. As the annealing temperature increases to 773 K and 1073 K, the diffraction peaks of Sm2O3, CoO and Co are observed in Fig. 6b and 6c, suggesting that part of CoO has been reduced by H2. The TEM images of these samples in Fig. 7 a-b indicate that the morphology and size of Sm2O3 nanorods are similar to those of Sm(OH)3 nanorods annealed at 573 K and 773 K, and single crystal Co(OH)2 nanoflakes converted into polycrystal CoO/Co nanoflakes with much defects or cavities on their surface. Although the decomposition and reduction process make the CoO/Co nanoflakes broken and incomplete, the frame of the Co(OH)2 nanoflakes is preserved due to the support of Sm2O3 nanorods. When the temperature reaches 1073 K, the mutual integration of Sm2O3 nanorods and CoO/Co nanoflakes as well as the neighboring nanoflakes may form larger hexagonal nanoflakes as shown in Fig. 7c. As a contrast, we also heated the unmixed Co(OH)2 precursor to 1073 K in forming gas (93% Ar + 7% H2). The XRD data (Fig. 6d) shows that pure Co phase is formed, and TEM result (Fig. 7d) illustrates that the Co(OH)2 nanoflakes without Sm(OH)3 nanorods have shrunk and agglomerated. The hexagonal morphology completely disappeared at 1073 K. Therefore, the Sm2O3 nanorods on CoO/Co surface are expected to facilitate maintaining the hexagonal morphology of nanoflakes during the dehydration and heating process. Based upon the results, the morphology evolution process of the SmCo5 nanochips can be explained schematically as depicted in Fig. 8. The crystalline Sm(OH)3 nanorods and single crystal Co(OH)2 nanoflakes can convert into Sm2O3 nanorods and CoO nanoflakes with the morphology almost unchanged during lower temperature heating period. Then CoO is reduced to Co, and finally the Co and Sm2O3 particles are further reduced to SmCo5 particles with Ca as reductant at higher temperature. In this process, the mutual integration of neighboring particles convert small nanoflakes into large nanoflakes, where the Sm2O3 nanorods play a key role in maintaining the morphology and keeping the single crystal nature of the final SmCo5 nanochips.
Fig. 6. The XRD patterns of the Sm(OH)3 and Co(OH)2 mixture precursors heated to 573 K (a), 773 K (b), 1073 K (c) and the Co(OH)2 precursor heated to 1073 K (d).
SmCl3 + NaOH → Sm(OH)3 (amorphous) + NaCl
(1)
CoCl2 + NaOH → Co(OH)2 (polycrystalline) + NaCl
(2)
Sm(OH)3 (amorphous) → Sm(OH)3 (crystalline)
(3)
Co(OH)2 (polycrystalline) → Co(OH)2 (single crystalline)
(4)
Sm(OH)3 → Sm2 O3 + H2 O
(5)
Co(OH)2 → CoO + H2 O
(6)
CoO + H2 → Co + H2 O
(7)
Co + Sm2 O3 + Ca → SmCo5 + CaO
(8)
4. Conclusions
To clarify the specific mechanism of the reaction kinetics, the precursors containing Sm(OH)3 and Co(OH)2 were annealed without other additives in forming gas (93% Ar + 7% H2) at 573 K, 773 K, and 1073 K. The products after annealing were characterized using XRD and TEM. The XRD pattern of the sample annealed at 573 K in Fig. 6a is indexed as Sm(OH)3, Sm2O3 and CoO (JCPDS No. 83-2036, No. 421461 and No. 43-1004), implying that Co(OH)2 has been decomposed
In summary, highly anisotropic SmCo5 single crystalline nanochips have been synthesized by reductive annealing of crystalline Sm(OH)3 nanorods and single crystalline Co(OH)2 nanoflakes. The as-prepared SmCo5 nanochips exhibit sheet-like morphology with their diameters in the range of submicron to several microns and the thicknesses from tens to hundreds of nanometers. The nanochips exhibit high coercivity of 19.3 kOe and large maximum energy product of 14.4 MGOe, which is 5
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Fig. 7. TEM images of the mixture precursor heated to 573 K (a), 773 K (b), 1073 K (c) and the Co(OH)2 precursor heated to 1073 K (d).
Fig. 8. Schematic diagram of the morphology evolution in SmCo5 nanochips during the process of reductive annealing of the precursors of Sm(OH)3 nanorods and Co(OH)2 nanoflakes.
very helpful discussion.
attributed to the strong anisotropy of SmCo5 nanochips as a result of the specially designed morphology of precursor, in which the Co(OH)2 nanoflakes facilitate the formation of SmCo5 nanochips as a template and Sm(OH)3 nanorods help to maintain the hexagonal morphology and single crystal nature. This work presents a novel method for preparing high performance anisotropic SmCo5 particles.
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Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos. 51401001, 51331003, 51701109) and the International S&T Cooperation Program of China (No. 2015DFG52020). The authors are indebted to Prof. Z. Altounian for 6
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A facile synthesis of anisotropic SmCo5 nanochips with high magnetic performance
T
⁎
Ming Yuea, , Chenglin Lia, Qiong Wua, Zhenhui Maa,b, Huanhuan Xua, Subhashini Palakaa a b
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China School of Material Science and Engineering, Shaanxi University of Science & Technology, Xi’ an 710021, China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
Highly anisotropic SmCo single crys• talline nanochips have been synthe5
sized.
nanochips exhibit coercivity • The 19.3 kOe and energy product of
of
14.4 MGOe.
Co(OH) nanoflakes facilitate the for• mation of SmCo nanochips as a tem2
5
plate.
A R T I C L E I N F O
A B S T R A C T
Keywords: SmCo5 Nanochips Magnetic anisotropy
Highly anisotropic SmCo5 single crystal particles are promising candidates for high-density data storage and building nanocomposite permanent magnets with large maximum energy product. Previously, nanocrystalline SmCo5 particles obtained by calciothermic reduction exhibit low magnetic energy product due to magnetic isotropy caused by severely agglomerated equiaxial particles. In this study, we report a new strategy to fabricate highly anisotropic SmCo5 nanochips by reductive annealing of precursors with a special designed morphology. The precursors containing single crystal Co(OH)2 nanoflakes and crystalline Sm(OH)3 nanorods were synthesized by a hydrothermal process, and converted into SmCo5 nanochips by subsequent thermal reduction, during which the Co(OH)2 nanoflakes facilitate the formation of SmCo5 nanochips as a template and Sm(OH)3 nanorods help to maintain the hexagonal morphology and single crystal nature. The SmCo5 nanochips exhibit strong magnetic anisotropy and excellent room temperature magnetic properties with a remanence of 7.7 kG, coercivity of 19.3 kOe, and maximum energy product of 14.4 MGOe, which is the highest recorded value for chemical synthesized nanostructured SmCo5 particles.
1. Introduction
[1–3] has recently drawn renewed interests in high-density data storage and nanocomposite permanent magnet with large maximum energy product [4–25]. SmCo5 is a promising candidate as the hard-phase in nanocomposite magnets maintaining their high coercivities [26,27]. Especially, in a recent developed bottom-up approach [6] pursuing
As the first generation of rare earth permanent magnetic materials, SmCo5 compound bearing large magnetocrystalline anisotropy (Ku = 2 × 108 erg/cm3) and high Curie temperature (Tc = 1020 K)
⁎
Corresponding author. E-mail address: [email protected] (M. Yue).
https://doi.org/10.1016/j.cej.2018.02.060 Received 19 November 2017; Received in revised form 9 February 2018; Accepted 12 February 2018 Available online 22 February 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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controllable microstructure and desirable magnetic properties in nanocomposite magnets, SmCo5 particles with high coercivity are frequently chosen as the hard phase for building the magnets with soft iron-based phase [28–34]. To achieve nanostructured SmCo5 particles with good magnetic performance, a novel approach combining solution-phase chemical synthesis with reduction annealing has been established [35–41]. In this technique, Sm-containing and Co-containing precursors were fabricated first by the solution-phase method, then undergoing a reductive annealing at elevated temperature to form the final products. By this approach, numerous investigations have reported nanostructured SmCo5 particles with reasonable coercivity. For instance, Ma et al. [42] reported synthesis of 10–40 nm SmCo5 nanoparticles with coercivity of 20 kOe by reductive annealing of polycrystalline Co(OH)2 and amorphous Sm(OH)3 precursors. However, most previously reported SmCo5 particles exhibit equiaxial morphology due to the high temperature annealing [39,41,42]. As a result, the particles are magnetically isotropic and exhibit low remanence and energy product, undermining their good potential in building nanocomposite magnets. Therefore, it is crucial to obtain high performance single crystal SmCo5 particles with new strategy. In the present study, we use a new strategy to synthesize anisotropic SmCo5 nanochips by employing the hydroxide with specially designed morphology as precursors. Firstly, single crystal Co(OH)2 nanoflakes and crystalline Sm(OH)3 nanorods were fabricated by a hydrothermal route. Such hydroxide precursor was further reduced to generate anisotropic SmCo5 nanochips, where the original morphology of the Co (OH)2 nanoflakes was well preserved with the assistance of the Sm (OH)3 nanorods during the annealing process. The prepared SmCo5 nanochips exhibit good magnetic performance. Moreover, the reaction mechanism of the new route was investigated.
Fig. 1. The XRD pattern of as-prepared precursors with peaks indexed as Sm(OH)3 (JCPDS No. 83-2036) and Co(OH)2 (JCPDS No. 30-0443), (a) Sm(OH)3 and Co(OH)2 synthesized simultaneously and TEM images of the precursors, Co(OH)2 (b) and Sm(OH)3 (c) synthesized separately.
(SEM, ZEISS-SUPRA55) and transmission electron microscopy (TEM, Tecnai-F20). For TEM observations, the samples were dispersed in hexane. The drops of the well dispersed nanoparticles were placed over the carbon coated microscopic copper grids (200 mesh size) and were subsequently dried. The magnetic properties of the nanochips were measured at room temperature (300 K) using a physical property measurement system (PPMS, Quantum Design) under a maximum applied field of 100 kOe.
2. Materials and methods 2.1. Synthesis of precursors
3. Results and discussion The precursors were prepared via a hydrothermal process. First, 0.73 g (2 mmol) SmCl3·6H2O and 2.02 g (8.5 mmol) CoCl2·6H2O were dissolved together in 40 ml deionized water. A 20 ml of 1.38 M aqueous NaOH solution was added drop-wise into the above solution with stirring at room temperature for 30 min. The solution was then transferred into a Teflon vessel (100 ml) for hydrothermal reaction at 453 K for 12 h using an autoclave reactor. After cooling to room temperature, the resultant was centrifuged at 6000 rpm for 3 min to obtain brown powder. The powder was further washed three times with deionized water and dried at 333 K for 12 h.
3.1. Synthesis of the precursors Fig. 1 shows the X-ray pattern of the as-prepared precursors. The diffraction peaks are indexed as Sm(OH)3 and Co(OH)2 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 83-2036 and No. 30-0443). Note that the sharp peaks also suggest good crystallization of Sm(OH)3 and Co(OH)2, which may be attributed to the hydrothermal process. A minor Co3O4 impurity phase is also found in the as-prepared precursor as shown in the X-ray patterns due to the existence of a little oxygen during hydrothermal process, which would take part in a subsequent reductive reaction similar to that of Co(OH)2. The precursors have been further investigated using SEM and TEM, as shown in Fig. 2. The SEM and TEM images in Fig. 2a-b show that the precursors contain two types of particles with remarkably different morphology, i. e., nanoflakes and nanorods. The hexagonal nanoflakes have widths in the range of 250–400 nm with thicknesses of 30–40 nm, while the nanorods have diameters of 15–20 nm with lengths of 150–250 nm. In addition, quite a few nanorods are distributed on the surface of the nanoflakes. Fig. 2c and 2d display the TEM images of unmixed Sm(OH)3 nanorods and Co(OH)2 nanoflakes, which were prepared in absence of another raw material using a same process, respectively. Furthermore, the hexagonal morphology suggests that single crystalline Co(OH)2 nanoflakes have been obtained. Different from the precursors of polycrystalline Co(OH)2 and amorphous Sm(OH)3 synthesized by co-precipitation strategy, the single crystal structure may lead to final products with various morphologies [39].
2.2. Synthesis of SmCo5 nanoflakes The prepared precursor powder was mixed with 6.0 g of Ca powder, 3.0 g of KCl and 2.0 g of CaO. The mixture was transferred to a tungsten crucible which was inserted into a steel tube. The tube was degassed for 3 times to remove air and moisture. Subsequently, the tube was flushed with forming gas (93% Ar + 7% H2) and heated to 1198 K at a rate of 8 K/min. The reaction was kept for 90 min before quickly cooling to room temperature. The product was washed with deionized water and 5% acetic acid to dissolve CaO, KCl, and extra Ca. A black powder was obtained by centrifuging at 8000 rpm for 3 min. The powder was finally washed with deionized water and ethanol and dried under vacuum. 2.3. Characterization of the precursors and final products The crystallographic structure of the precursors and final products was identified by X-ray diffraction (XRD, Rigaku Ultima Ⅳ) with a CuKα wavelength X-ray source. The microstructure and morphology of above samples were investigated using scanning electron microscopy 2
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Fig. 2. The SEM image (a) and TEM image (b) of the precursors, Sm(OH)3 and Co(OH)2 synthesized simultaneously. TEM images of Sm(OH)3 (c) and Co(OH)2 (d) synthesized separately. The inset is the FFT pattern of a Co(OH)2 hexagonal nanoflake.
3.2. Synthesis of anisotropic SmCo nanochips Fig. 3 shows the XRD patterns of SmCo particles prepared by reductive annealing of Sm(OH)3 and Co(OH)2 with KCl, CaO, and Ca powders. It can be seen from Fig. 3a that the diffraction peaks of randomly oriented SmCo nanochips match well with the standard SmCo5 pattern (JCPDS No. 65-5599), indicating that precursor has converted into hexagonal SmCo5 after undergoing high temperature reduction. For magnetic alignment, a cylindrical sample was prepared by mixing the as-prepared SmCo5 powders with epoxy, which solidified slowly, in a static magnetic field of 2.2 T. The diffraction peaks of magnetically aligned SmCo5 particles exhibit enhanced (0 0 l) peaks as shown in Fig. 3b. The intensity ratio of (0 0 2) to (1 1 1) reflection peaks, I(002)/ I(111), changes from 0.32 into 11.22 after magnetic alignment, revealing that the high anisotropic SmCo5 particles have been obtained by reductive annealing of single crystal Co(OH)2 and crystalline Sm(OH)3. Fig. 4a shows the SEM image of the synthetic SmCo5 particles. It can be observed that the particles exhibit plate-like morphology with diameters ranging from submicron to several microns and thicknesses from tens to hundreds of nanometers, hereafter named “nanochips”. Considering the dimension of the precursors of Co(OH)2 nanoflakes, the nanoflake precursors should be aggregated under high temperature and be subsequently sintered to form the nanochips with larger size. Some nanochips inevitably aggregated and grew during the high temperature
Fig. 3. XRD patterns of the randomly oriented sample (a) and magnetically aligned sample (b) of SmCo5 nanochips compared with the standard diffraction pattern of hexagonal SmCo5 (JCPDS No. 65-5599).
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Fig. 4. (a) The SEM image of as-synthesized SmCo5 flakes; (b) TEM, (c) HRTEM and (d) SAED of a single SmCo5 nanochip.
magnetization (Ms) of 8.2 kG. The large difference between the two loops indicates the strong magnetic anisotropy of the sample, which leads to a high remanence ratio Mr/Ms of 0.94. Fig. 5b shows the demagnetization curves of the sample. The high remanence (Mr) of 7.7 kG, large coercivity (Hci) of 19.3 kOe, and the good squareness results in a maximum energy product of 14.4 MGOe, which is the highest reported value for chemically synthetized SmCo particles [23,29,35,42–45].
heat treatment. However, the adhesion strength between the chips is quite weak and they can be easily ground and dispersed without surfactants in a hexane ultrasonic bath to produce SmCo5 nanochips. Fig. 4b–d show the TEM image, HRTEM lattice image, and corresponding SAED of individual SmCo5 nanochip after grinding, respectively. The diameter of the chip is about 1.3 μm. HRTEM observation (Fig. 4c) displays a interplanar distance in the hexagonal SmCo5 structure of 0.433 nm, corresponding to the (1 0 1 0) plane in SmCo5. Fig. 4d displays the SAED pattern of this nanochip. The periodic diffraction dots present a hexagonal array, suggesting the single crystal nature of the SmCo5 nanochip. Furthermore, the three dots corresponding to (1 0 1 0), (1 1 0 0) and (0 1 1 0) lattice planes, confirm that caxis (easy axis) of SmCo5 is perpendicular to the surface of the nanochip. The HRTEM and SAED images in Fig. 4c and Fig. 4d derive from a same area in Fig. 4b with zone axis [0 0 0 1]. Thus, stacked nanochips morphology, similar to that shown in Fig. 4a, is easy to achieve with good orientation. The magnetization as a function of the magnetic field of the SmCo5 nanochips has been measured by PPMS at a maximum applied magnetic field of 100 kOe, as shown in Fig. 5. Fig. 5a presents the hysteresis loops of magnetically aligned SmCo5 nanochips [along the magnetically easy (parallel to the direction of the magnetic field applied in the alignment procedure) and hard (perpendicular to the direction of the magnetic field applied in the alignment procedure) directions]. They exhibit ferromagnetic behavior at room temperature (300 K) with a saturation
3.3. Reaction mechanism for SmCo5 nanochips The synthetic process can be described by Eqs. (1)–(8). Eq. (1) and Eq. (2) are the precipitation reactions of Sm(OH)3 and Co(OH)2, respectively. The TEM observation in our previous report [39] confirmed that the precipitation reaction at room temperature would yield amorphous Sm(OH)3 and polycrystalline Co(OH)2 particles. In this work, during the subsequent hydrothermal reaction process, amorphous Sm(OH)3 grew into crystalline nanorods and polycrystalline Co (OH)2 transferred into single crystalline hexagonal nanoflakes at 453 K as shown in Eq. (3) and Eq. (4). Dehydration may occur during the temperature-rise period, as shown in Eq. (5) and Eq. (6). With the increase of temperature, CoO was reduced first [Eq. (7)], and then SmCo5 was formed with further reduction of Sm2O3 when the temperature increased to 1198 K [Eq. (8)], which is consistent with the results of previous studies [35,46]. 4
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Fig. 5. (a) Magnetization hysteresis loops of the magnetically aligned SmCo5 nanochips (along the magnetically easy (parallel to the direction of the magnetic field applied in the alignment procedure) and hard (perpendicular to the direction of the magnetic field applied in the alignment procedure) directions) and (b) second quadrant M−H curves for the aligned SmCo5 nanochips at 300 K.
into CoO completely while only part of Sm(OH)3 has been decomposed at 573 K. As the annealing temperature increases to 773 K and 1073 K, the diffraction peaks of Sm2O3, CoO and Co are observed in Fig. 6b and 6c, suggesting that part of CoO has been reduced by H2. The TEM images of these samples in Fig. 7 a-b indicate that the morphology and size of Sm2O3 nanorods are similar to those of Sm(OH)3 nanorods annealed at 573 K and 773 K, and single crystal Co(OH)2 nanoflakes converted into polycrystal CoO/Co nanoflakes with much defects or cavities on their surface. Although the decomposition and reduction process make the CoO/Co nanoflakes broken and incomplete, the frame of the Co(OH)2 nanoflakes is preserved due to the support of Sm2O3 nanorods. When the temperature reaches 1073 K, the mutual integration of Sm2O3 nanorods and CoO/Co nanoflakes as well as the neighboring nanoflakes may form larger hexagonal nanoflakes as shown in Fig. 7c. As a contrast, we also heated the unmixed Co(OH)2 precursor to 1073 K in forming gas (93% Ar + 7% H2). The XRD data (Fig. 6d) shows that pure Co phase is formed, and TEM result (Fig. 7d) illustrates that the Co(OH)2 nanoflakes without Sm(OH)3 nanorods have shrunk and agglomerated. The hexagonal morphology completely disappeared at 1073 K. Therefore, the Sm2O3 nanorods on CoO/Co surface are expected to facilitate maintaining the hexagonal morphology of nanoflakes during the dehydration and heating process. Based upon the results, the morphology evolution process of the SmCo5 nanochips can be explained schematically as depicted in Fig. 8. The crystalline Sm(OH)3 nanorods and single crystal Co(OH)2 nanoflakes can convert into Sm2O3 nanorods and CoO nanoflakes with the morphology almost unchanged during lower temperature heating period. Then CoO is reduced to Co, and finally the Co and Sm2O3 particles are further reduced to SmCo5 particles with Ca as reductant at higher temperature. In this process, the mutual integration of neighboring particles convert small nanoflakes into large nanoflakes, where the Sm2O3 nanorods play a key role in maintaining the morphology and keeping the single crystal nature of the final SmCo5 nanochips.
Fig. 6. The XRD patterns of the Sm(OH)3 and Co(OH)2 mixture precursors heated to 573 K (a), 773 K (b), 1073 K (c) and the Co(OH)2 precursor heated to 1073 K (d).
SmCl3 + NaOH → Sm(OH)3 (amorphous) + NaCl
(1)
CoCl2 + NaOH → Co(OH)2 (polycrystalline) + NaCl
(2)
Sm(OH)3 (amorphous) → Sm(OH)3 (crystalline)
(3)
Co(OH)2 (polycrystalline) → Co(OH)2 (single crystalline)
(4)
Sm(OH)3 → Sm2 O3 + H2 O
(5)
Co(OH)2 → CoO + H2 O
(6)
CoO + H2 → Co + H2 O
(7)
Co + Sm2 O3 + Ca → SmCo5 + CaO
(8)
4. Conclusions
To clarify the specific mechanism of the reaction kinetics, the precursors containing Sm(OH)3 and Co(OH)2 were annealed without other additives in forming gas (93% Ar + 7% H2) at 573 K, 773 K, and 1073 K. The products after annealing were characterized using XRD and TEM. The XRD pattern of the sample annealed at 573 K in Fig. 6a is indexed as Sm(OH)3, Sm2O3 and CoO (JCPDS No. 83-2036, No. 421461 and No. 43-1004), implying that Co(OH)2 has been decomposed
In summary, highly anisotropic SmCo5 single crystalline nanochips have been synthesized by reductive annealing of crystalline Sm(OH)3 nanorods and single crystalline Co(OH)2 nanoflakes. The as-prepared SmCo5 nanochips exhibit sheet-like morphology with their diameters in the range of submicron to several microns and the thicknesses from tens to hundreds of nanometers. The nanochips exhibit high coercivity of 19.3 kOe and large maximum energy product of 14.4 MGOe, which is 5
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Fig. 7. TEM images of the mixture precursor heated to 573 K (a), 773 K (b), 1073 K (c) and the Co(OH)2 precursor heated to 1073 K (d).
Fig. 8. Schematic diagram of the morphology evolution in SmCo5 nanochips during the process of reductive annealing of the precursors of Sm(OH)3 nanorods and Co(OH)2 nanoflakes.
very helpful discussion.
attributed to the strong anisotropy of SmCo5 nanochips as a result of the specially designed morphology of precursor, in which the Co(OH)2 nanoflakes facilitate the formation of SmCo5 nanochips as a template and Sm(OH)3 nanorods help to maintain the hexagonal morphology and single crystal nature. This work presents a novel method for preparing high performance anisotropic SmCo5 particles.
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Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Nos. 51401001, 51331003, 51701109) and the International S&T Cooperation Program of China (No. 2015DFG52020). The authors are indebted to Prof. Z. Altounian for 6
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