- Email: [email protected]
shell composite particles through spray pyrolysis
Accepted Manuscript Synthesis of magnetically exchange coupled CoFe2O4/CoFe2 core/shell composite particles through spray pyrolysis Guangqian Du, Shijie Wang PII:
S0925-8388(17)30806-X
DOI:
10.1016/j.jallcom.2017.03.037
Reference:
JALCOM 41075
To appear in:
Journal of Alloys and Compounds
Received Date: 29 November 2016 Revised Date:
23 January 2017
Accepted Date: 4 March 2017
Please cite this article as: G. Du, S. Wang, Synthesis of magnetically exchange coupled CoFe2O4/ CoFe2 core/shell composite particles through spray pyrolysis, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Synthesis of Magnetically Exchange Coupled CoFe2O4/CoFe2 Core/shell Composite Particles through Spray Pyrolysis
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Guangqian Du1, Shijie Wang1*
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Corresponding author could be contacted by: [email protected] (S. Wang). 1
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Institute of Urban and Rural Construction, Hebei Agricultural University, Baoding, Hebei
Province 071001 China Abstract:
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Magnetically exchange coupled CoFe2O4/CoFe2 composite particles were synthesized through spray pyrolysis, followed by H2 reduction. The H2 concentration was controlled to be 5, 8 and 10 vol.%, respectively, in three parallel experiments. The products show a core/shell structure, with CoFe2O4 as the core, and CoFe2 as the shell. The structure is confirmed by TEM-EDX and
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STEM analysis. The XRD patterns indicate the existence of both CoFe2O4 and CoFe2 phases in the composites. The magnetic hysteresis loop of CoFe2O4/CoFe2 composite particles reduced by 5 vol.% H2 is smooth and kink-free, indicating an effective exchange coupling between CoFe2O4
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and CoFe2. Henkel Plots are further used to investigate the magnetic exchange coupling in the composites. This paper presented a convenient, environmentally friendly and effective method to synthetize magnetically exchange coupled composite particles.
Keywords: nanostructured materials; permanent magnets; magnetic measurements; oxide
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materials; composite materials.
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1. Introduction Permanent magnet plays an important role in modern society, as it is related to both power generation, and power consumption [1-4]. Magnetically exchange coupled hard/soft magnetic materials are predicted to be the next generation of permanent magnets, because it makes use of the high magnetization of the
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soft phase, and high coercivity of the hard phase, achieving an enhanced maximum energy product ((BH)max) over a single hard phase [5-8]. According to Skomski’s exchange coupling theory, to achieve an effective exchange coupling, the size of the hard phase is preferred to be in single domain size (usually hundreds of nanometers) to maximize its coercivity, the size of the soft phase (usually less than 20 nm) is
to be well distributed with each other at nanoscale [5-7].
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required to be less than twice domain wall thickness of the hard phase, and both hard and soft phases need
Since the experimental discovery of enhanced (BH)max in Nd2Fe14B/Fe3B composites over single phase
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Nd2Fe14B by Coehoorn and his coworkers in 1989 [9], the synthesis of magnetically exchange coupled hard/soft materials has been extensively explored and investigated. Generally, the specific synthesis methods include ball milling [10], wet chemistry synthesis of core/shell structure [11, 12], and magnetic self-assembly [13-15]. Ball milling method is to mechanically mill hard and soft phases to reach the critical size for exchange coupling. This method is very popular because of its easy operation and large scale production. However, in the ball milling process, when the size of the soft phase is reduced to the
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correct size, the size of the hard phase is usually over reduced (much less than single domain size), leading to a low (BH)max. Wet chemistry synthesis of hard/soft core/shell structured magnetic composites has been used to synthetize FePt/Fe3O4 [16], FePt/Co [11], FePt3/CoFe2O4 [17] and etc. Compared to the ball milling method, the advantage this one is that the size of the hard and soft phase could be controlled,
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respectively. However, wet chemistry process generates pollutions, and it is only applicable for some of the hard/soft materials. Furthermore, a high temperature is usually necessary to initiate and maintain the chemical reaction, which could possible result in the mass diffusion between core and shell, and destroy
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the hard and soft phases due to the low thermal stability of the materials at nano scale. Magnetic selfassembly is a new concept proposed by Xu and his co-workers [18]. In this method, the hard/soft composites were synthesized by making use of the remanent magnetization of the hard phase, which could magnetically attract the soft phase, forming a core/shell structure. The magnetic exchange coupling is demonstrated between hard and soft phases in the products. However, this method also requires a complicated experimental process. It is a great desire for magnetic materials research to find an easy, green and effective method to synthesize magnetically exchange coupled hard/soft materials. Cobalt ferrite (CoFe2O4) is a very popular hard magnetic material due to its large magnetocrystaline anisotropy, low cost and high chemical stability [19]. Its (BH)max could be further improved by exchange 3
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coupling with a soft magnetic phase, such as FeCo. In recent years, the synthesis and magnetic properties of CoFe2O4/CoFe2 have been widely investigated [20-23]. Generally, the materials was fabricated by partially reducing CoFe2O4 [24-26]. However, these methods all require many steps in the synthesis process. This paper proposed a convenient and environmentally friendly method to synthesize
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magnetically exchange coupled CoFe2O4/CoFe2 composite particles. 1. Experimental
Two parts are included in the CoFe2O4/CoFe2 composites synthesis system. The first is a spray pyrolysis process to generate CoFe2O4 particles, and second is a reduction process to partially reduce CoFe2O4
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particles into CoFe2O4/CoFe2 composites. The whole synthesis system is simplified and shown in Fig. 1.
Fig. 1. A schematic of experimental system for CoFe2O4/CoFe2 core/shell particle synthesis. The synthesis of CoFe2O4 particles is based on Xu’s previous report with little modification [27, 28].
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Specifically, Fe(NO3)3·9H2O (Sigma Aldrich, 97%, 12 mmol) and CoCl2·6H2O (Sigma Aldrich, >98%, 6 mmol) were dissolved in 100 mL of DI H2O, and then ultrasonicated for 10 mins to solubilize the precursors. The solution was then transferred to an ultrasonic nebulizer (2.5 MHz) to generated liquid
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droplets, which were carried by Ar gas to go through the first furnace (length: 1.4 m; inner diameter: 0.1 m; 800 oC), with carrier gas flow rate of 5 L/min. After the first furnace, another carrier gas of Ar/H2 carried the products from the first furnace to go through a second surface (length: 1.4 m; inner diameter: 0.1 m). After the second furnace, the particles were cooled down by a heat exchanger, and collected by a fiber filter. To control the reduction level of CoFe2O4 particles, the reducing gas (Ar/H2) with different volume percentage of H2 is used. The reduction gas of Ar/H2 ((95/5 vol.%, 92/8 vol.% and 90/10 vol.%) is used in three parallel experiments. The measurement of X-ray diffraction (XRD) patterns was carried out by Bruker D8 diffractometer using Cu-Kα radiation (λ=1.5406 Å). Transmission Electron Microscopy (TEM) equipped with an Energy 4
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Dispersive X-ray (EDX) spectrometer, and Scanning Electron Microscopy (SEM) were used to analyze the particle size, morphology and elemental distribution. Magnetic properties were checked at room temperature with a vibrating sample magnetometer (VSM) in an applied field up to 1 T.
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2. Discussion
Fig. 2. (a). TEM image of CoFe2O4 particles (reduced by Ar/H2 (95/5 vol.%)); (b). TEM image of a single CoFe2O4 particle (reduced by Ar/H2 (95/5 vol.%)); (c) TEM image of a single CoFe2O4/CoFe2 core/shell particle ; (d). TEM image of CoFe2O4/CoFe2 core/shell particles; (e). EDX line scan from Fig. 2b; (f).
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EDX line scan from Fig. 2c.
The low magnification TEM images of both CoFe2O4 (first furnace) and CoFe2O4/CoFe2 particles (the
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second furnace, reduced by Ar/H2 (95/5 vol.%)) are shown in Figs. 2a and d. It can been seen that both particles are spherical, and the different contrast on CoFe2O4/CoFe2 particles indicates their core/shell structure. High magnification TEM images of CoFe2O4 and CoFe2O4/CoFe2 particles are displayed in Figs. 2b and c, and the corresponding TEM-EDX line scanning results across a single particle were displayed in Figs. 2e and f. The O and Fe signals are constant across the particle in Fig. 2e, indicating these two elements distribution is homogenous in a CoFe2O4 particle. In Fig. 2f, the O signal is very low at both ends of the line scan, indicating that the surface of CoFe2O4 is mainly reduced into CoFe2. The characterization results in Fig. 2 clearly demonstrate that the surface of CoFe2O4 is reduced into CoFe2
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after the H2 reduction, forming a core/shell structure of CoFe2O4/CoFe2, and the shell thickness is around
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20 nm when the H2 concentration is 5 vol.%.
Fig. 3. (a) SEM image of CoFe2O4/CoFe2 composite particles; (b) A high resolution TEM image
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indicating the boundary of CoFe2O4 and CoFe2; (c). STEM image of CoFe2O4/CoFe2 particles; (d). EDX line scanning result in Fig. 3c (with molar percentage).
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The SEM image of CoFe2O4/CoFe2 particles reduced by Ar/H2 (95/5 vol.%) is shown in Fig. 3a. It can be seen that the particles have a relatively large size distribution. Based on the calculation of 100 particles, the particle size is 832 nm. Previous research results have shown that the average particle size could be regulated by changing the concentration of the precursor solution in the spray pyrolysis process [27]. A high resolution TEM image of CoFe2O4/CoFe2 particle is shown in Fig. 3b. Due to the difference in electron density, different contrast represents different materials. The dark area represents CoFe2O4, while the bright area represents CoFe2. The high resolution TEM (HRTEM) image in Fig. 3b indicates the lattice constant of 2.5 Å in CoFe2 area, corresponding to the crystal plane of (110) in body centered cubic (BCC) Fe-Co alloy. A Scanning Transmission Electron Microscopy (STEM) image of CoFe2O4/CoFe2 particles is show in Fig. 3c, and elemental analysis was conducted on the edge of a particle. The 6
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corresponding result is shown in Fig. 3d. The low level of O indicates that the surface of the particle is
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mainly reduced, and the molar ratio of Fe to Co is close to 2 to 1.
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Fig. 4. XRD patterns of CoFe2O4 particles and CoFe2O4/CoFe2 composites. The XRD patterns of both CoFe2O4 and CoFe2O4/CoFe2 particles are shown in Fig. 4. The XRD patters of CoFe2O4 particles show an inverse spinel structure, and match well with corresponding JCPDS card (NO. 00-022-1086). After reduction, all CoFe2O4 peaks are maintained, and an extra peak (110) peak at ~45o indicates the existence of Fe-Co phase. Higher concentration of H2 resulted in a stronger peak of Fe-Co,
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indicating a higher percentage of CoFe2 reduced from CoFe2O4.
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Fig. 5. CoFe2O4 particles and CoFe2O4/CoFe2 composites: (a). Magnetic hysteresis loops; (b). BH(max)
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The magnetic hysteresis loops of CoFe2O4 particles and CoFe2O4/CoFe2 core/shell particles are displayed in Fig. 5a. The magnetization and coercitity of pure CoFe2O4 are 62.3 emu/g and 2518 Oe, respectively. After partial reduction, CoFe2O4/CoFe2 core/shell particles are formed. With the increase of H2 concentration, the coercivity is decreasing and the magnetization is increasing. Furthermore, compared with the smooth and kink-free magnetic hysteresis loop of CoFe2O4/CoFe2 (Ar/H2:95/5 vol.%), the
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hysteresis loops of CoFe2O4/CoFe2 (Ar/H2:92/8 vol.%) and CoFe2O4/CoFe2 (Ar/H2:90/10 vol.%) presented obvious kinks. This could be attributed to the fact that the CoFe2O4 is over reduced, and CoFe2 can not be fully magnetically exchange coupled with CoFe2O4, resulting in a magnetic decoupling. The maximum energy product ((BH)max) is an important parameter to evaluate the strength of a permanent
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magnet. The (BH)max of CoFe2O4 particles and CoFe2O4/CoFe2 composites is presented in Fig. 5b. The change of (BH)max could be explained with the help of the magnetic hysteresis loops in Fig. 5a. When the H2 concentration is 5 vol.%, the reduced CoFe2 could be fully magnetically exchange coupled with
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CoFe2O4, presenting a smooth hysteresis loop, as shown in Fig. 5a. Therefore, its (BH)max is increased over pure CoFe2O4, reaching at 0.78 MGOe, higher than the best value of CoFe2O4 particles (0.73 MGOe) reported by Cabral etc., but lower than the CoFe2O4/CoFe2 composites (1.22 MGOe) reported by Leite etc. [24]. However, when the H2 concentration is increased to 8 and 10 vol.%, the CoFe2O4 is over reduced, and the reduced CoFe2 could only partially magnetically exchange coupled with CoFe2O4, resulting unsmooth hysteresis loops in Fig. 5a. Therefore, their (BH)max is decreased from pure CoFe2O4.
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Fig. 6. Henkel plots of CoFe2O4 particles and CoFe2O4/CoFe2 composites.
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The magnetic exchange coupling between hard and soft phases could be further investigated by Henkel Plot, which could be defined as:
δM = Md(H) – [1-2Mr(H)],
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where Md is the normalized demagnetization remanence, Mr is the normalized isothermal magnetization
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remanence, and H is the applied magnetic field [13, 26]. A positive peak in Henkel Plot indicates a magnetic exchange coupling between hard and soft phases, while a negative peak indicates a
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magnetostatic interactions among particles. It can be seen that in Fig. 6, pure CoFe2O4 particles present a negative peak, indicating magnetostatic interactions. When CoFe2O4 particles are reduced with H2 (5 vol.%), its Henkel Plot is dominated by a positive peak, indicating that magnetic exchange coupling is occurring between CoFe2O4 and CoFe2. When the H2 concentration is further increased to 8 and 10 vol.%, then Henkel Plot is dominated by a negative peak again, indicating the disappearance of magnetic exchange coupling between CoFe2O4 and CoFe2. 3. Conclusions CoFe2O4/CoFe2 composite particles were synthesized in a two-step process, including spray pyrolysis and H2 reduction. The core/shell structure of CoFe2O4/CoFe2 composite particles were confirmed by TEM, 9
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STEM and EDX, and shell thickness is around 20 nm. The crystal phases of CoFe2O4 and CoFe2 were confirmed by XRD patterns. The magnetic exchange coupling between CoFe2O4 and CoFe2 in the composites was confirmed by the smooth magnetic hysteresis loop, which indicates one-phase magnetic behavior. Henkel Plot is further used to demonstrate the existence of magnetic exchange coupling
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between CoFe2 and CoFe2O4 in CoFe2O4/CoFe2 composites reduced by Ar/H2:95/5 vol.%. This paper presented an easy method to synthesize magnetically exchange coupled CoFe2O4/CoFe2 composites. References
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[3] Y. Geng, T. Ablekim, P. Mukherjee, M. Weber, K. Lynn, J.E. Shield, High-energy mechanical milling-induced crystallization in Fe32Ni52Zr3B13, J. Non-Cryst. Solids. 404 (2014) 140-144. [4] C. Hu, A. Yen, N. Joshi, R.L. Hartman, Packed-bed microreactors for understanding of the dissolution kinetics and mechanisms of asphaltenes in xylenes, Chem. Eng. Sci. 140 (2016) 144-152. [5] R. Skomski, J.M.D. Coey, Giant Energy Product in Nanostructured 2-Phase Magnets, Phys. Rev. B. 48(21) (1993) 15812-15816. [6] R. Skomski, J.M.D. Coey, Exchange Coupling and Energy Product in Random 2-Phase Aligned Magnets, IEEE. T. Magn. 30(2) (1994) 607-609.
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[7] R. Skomski, Aligned 2-Phase Magnets - Permanent Magnetism of the Future (Invited), J. Appl. Phys. 76(10) (1994) 7059-7064. [8] R. Skomski, J.M.D. Coey, Permanent magnetism, Institute of Physics Publishing, Bristol, 1999. [9] R. Coehoorn, D.B. Demooij, C. Dewaard, Meltspun Permanent-Magnet Materials Containing Fe3B as the Main Phase, J. Magn. Magn. Mater. 80(1) (1989) 101-104.
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[10] N. Poudyal, C. Rong, V.V. Nguyen, J.P. Liu, Hard-phase engineering in hard/soft nanocomposite magnets, Mater. Res. Exp. 1(1) (2014) 016103.
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[11] F. Liu, J.H. Zhu, W.L. Yang, Y.H. Dong, Y.L. Hou, C.Z. Zhang, H. Yin, S.H. Sun, Building Nanocomposite Magnets by Coating a Hard Magnetic Core with a Soft Magnetic Shell, Angew. Chem. Int. Edit. 53(8) (2014) 2176-2180. [12] J.M. Soares, V.B. Galdino, F.L.A. Machado, Exchange-bias and exchange-spring coupling in magnetic core-shell nanoparticles, J. Magn. Magn. Mater. 350 (2014) 69-72. [13] X. Xu, Y.-K. Hong, J. Park, W. Lee, A.M. Lane, J. Cui, Magnetic self-assembly for the synthesis of magnetically exchange coupled MnBi/FeCo composites, J. Solid. State. Chem. 231 (2015) 108-113. [14] X. Xu, Y.-K. Hong, J. Park, W. Lee, A.M. Lane, Exchange coupled SrFe12O19/FeCo core/shell particles with different shell thickness, Electron. Mater. Lett. 11(6) (2015) 1021-1027. [15] X. Xu, Y.-K. Hong, J. Park, W. Lee, A.M. Lane, Ex situ synthesis of magnetically exchange coupled SrFe12O19/FeCo composites, AIP. Adv. 6(5) (2016) 056026. [16] H. Zeng, S.H. Sun, J. Li, Z.L. Wang, J.P. Liu, Tailoring magnetic properties of core/shell nanoparticles, Appl. Phys. Lett. 85(5) (2004) 792-794. 10
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[17] V. Urbanova, M. Magro, A. Gedanken, D. Baratella, F. Vianello, R. Zboril, Nanocrystalline Iron Oxides, Composites, and Related Materials as a Platform for Electrochemical, Magnetic, and Chemical Biosensors, Chem. Mater. 26(23) (2014) 6653-6673. [18] X. Xu, J. Park, Y.-K. Hong, A.M. Lane, Magnetically self-assembled SrFe12O19/Fe–Co core/shell particles, Mater. Chem. Phys. 152(0) (2015) 9-12.
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[19] E. Manova, B. Kunev, D. Paneva, I. Mitov, L. Petrov, C. Estournes, C. D'Orleans, J.L. Rehspringer, M. Kurmoo, Mechano-synthesis, characterization, and magnetic properties of nanoparticles of cobalt ferrite, CoFe2O4, Chem. Mater. 16(26) (2004) 5689-5696. [20] F.C.M. Filho, L.L. Oliveira, S.S. Pedrosa, G.O.G. Rebouças, A.S. Carriço, A.L. Dantas, Impact of core-shell dipolar interaction on magnetic phases of spherical core-shell nanoparticles, Phys. Rev. B. 92(6) (2015).
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[21] Y. Zhang, Z. Yang, B. Zhu, S. Chen, X. Yang, R. Xiong, Y. Liu, Exchange-spring effect in CoFe2O4/CoFe2 composite nano-particles, J. Alloy. Compd. 567 (2013) 73-76. [22] Y. Li, Y. Hu, J. Huo, H. Jiang, C. Li, G. Huang, Stable Core Shell Co3Fe7–CoFe2O4Nanoparticles Synthesized via Flame Spray Pyrolysis Approach, Ind. Eng. Chem. Res. 51(34) (2012) 11157-11162.
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[23] C. Hu, N.C. Garcia, R. Xu, T. Cao, A. Yen, S.A. Garner, J.M. Macias, N. Joshi, R.L. Hartman, Interfacial Properties of Asphaltenes at the Heptol–Brine Interface, Energy Fuels 30(1) (2016) 80-87. [24] G.C.P. Leite, E.F. Chagas, R. Pereira, R.J. Prado, A.J. Terezo, M. Alzamora, E. Baggio-Saitovitch, Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite, J. Magn. Magn. Mater. 324(18) (2012) 2711-2716. [25] L. Guo, X. Shen, F. Song, M. Liu, Y. Zhu, Characterization and magnetic exchange observation for CoFe2O4–CoFe2 nanocomposite microfibers, J. Sol-Gel Sci. Technol. 58(2) (2011) 524-529.
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[26] J.M. Soares, F.A.O. Cabral, J.H. de Araújo, F.L.A. Machado, Exchange-spring behavior in nanopowders of CoFe2O4–CoFe2, Appl. Phys. Lett. 98(7) (2011) 072502. [27] X. Xu, J. Park, Y.K. Hong, A.M. Lane, Ethylene glycol assisted spray pyrolysis for the synthesis of hollow BaFe12O19 spheres, Mater. Lett. 144 (2015) 119-122.
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[28] X. Xu, J. Park, Y.K. Hong, A.M. Lane, Synthesis and characterization of hollow mesoporous BaFe12O19 spheres, J. Solid. State. Chem. 222 (2015) 84-89.
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Highlight: •
CoFe2O4/CoFe2 core/shell composite particles were synthesized through a convenient method. The core/shell structure of the composites were determined by TEM, EDX and STEM.
•
Magnetic exchange coupling was achieved between CoFe2O4 and CoFe2 in CoFe2O4/CoFe2
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•
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core/shell composite particles.
S0925-8388(17)30806-X
DOI:
10.1016/j.jallcom.2017.03.037
Reference:
JALCOM 41075
To appear in:
Journal of Alloys and Compounds
Received Date: 29 November 2016 Revised Date:
23 January 2017
Accepted Date: 4 March 2017
Please cite this article as: G. Du, S. Wang, Synthesis of magnetically exchange coupled CoFe2O4/ CoFe2 core/shell composite particles through spray pyrolysis, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Synthesis of Magnetically Exchange Coupled CoFe2O4/CoFe2 Core/shell Composite Particles through Spray Pyrolysis
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Guangqian Du1, Shijie Wang1*
*
Corresponding author could be contacted by: [email protected] (S. Wang). 1
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1
Institute of Urban and Rural Construction, Hebei Agricultural University, Baoding, Hebei
Province 071001 China Abstract:
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Magnetically exchange coupled CoFe2O4/CoFe2 composite particles were synthesized through spray pyrolysis, followed by H2 reduction. The H2 concentration was controlled to be 5, 8 and 10 vol.%, respectively, in three parallel experiments. The products show a core/shell structure, with CoFe2O4 as the core, and CoFe2 as the shell. The structure is confirmed by TEM-EDX and
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STEM analysis. The XRD patterns indicate the existence of both CoFe2O4 and CoFe2 phases in the composites. The magnetic hysteresis loop of CoFe2O4/CoFe2 composite particles reduced by 5 vol.% H2 is smooth and kink-free, indicating an effective exchange coupling between CoFe2O4
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and CoFe2. Henkel Plots are further used to investigate the magnetic exchange coupling in the composites. This paper presented a convenient, environmentally friendly and effective method to synthetize magnetically exchange coupled composite particles.
Keywords: nanostructured materials; permanent magnets; magnetic measurements; oxide
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materials; composite materials.
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1. Introduction Permanent magnet plays an important role in modern society, as it is related to both power generation, and power consumption [1-4]. Magnetically exchange coupled hard/soft magnetic materials are predicted to be the next generation of permanent magnets, because it makes use of the high magnetization of the
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soft phase, and high coercivity of the hard phase, achieving an enhanced maximum energy product ((BH)max) over a single hard phase [5-8]. According to Skomski’s exchange coupling theory, to achieve an effective exchange coupling, the size of the hard phase is preferred to be in single domain size (usually hundreds of nanometers) to maximize its coercivity, the size of the soft phase (usually less than 20 nm) is
to be well distributed with each other at nanoscale [5-7].
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required to be less than twice domain wall thickness of the hard phase, and both hard and soft phases need
Since the experimental discovery of enhanced (BH)max in Nd2Fe14B/Fe3B composites over single phase
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Nd2Fe14B by Coehoorn and his coworkers in 1989 [9], the synthesis of magnetically exchange coupled hard/soft materials has been extensively explored and investigated. Generally, the specific synthesis methods include ball milling [10], wet chemistry synthesis of core/shell structure [11, 12], and magnetic self-assembly [13-15]. Ball milling method is to mechanically mill hard and soft phases to reach the critical size for exchange coupling. This method is very popular because of its easy operation and large scale production. However, in the ball milling process, when the size of the soft phase is reduced to the
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correct size, the size of the hard phase is usually over reduced (much less than single domain size), leading to a low (BH)max. Wet chemistry synthesis of hard/soft core/shell structured magnetic composites has been used to synthetize FePt/Fe3O4 [16], FePt/Co [11], FePt3/CoFe2O4 [17] and etc. Compared to the ball milling method, the advantage this one is that the size of the hard and soft phase could be controlled,
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respectively. However, wet chemistry process generates pollutions, and it is only applicable for some of the hard/soft materials. Furthermore, a high temperature is usually necessary to initiate and maintain the chemical reaction, which could possible result in the mass diffusion between core and shell, and destroy
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the hard and soft phases due to the low thermal stability of the materials at nano scale. Magnetic selfassembly is a new concept proposed by Xu and his co-workers [18]. In this method, the hard/soft composites were synthesized by making use of the remanent magnetization of the hard phase, which could magnetically attract the soft phase, forming a core/shell structure. The magnetic exchange coupling is demonstrated between hard and soft phases in the products. However, this method also requires a complicated experimental process. It is a great desire for magnetic materials research to find an easy, green and effective method to synthesize magnetically exchange coupled hard/soft materials. Cobalt ferrite (CoFe2O4) is a very popular hard magnetic material due to its large magnetocrystaline anisotropy, low cost and high chemical stability [19]. Its (BH)max could be further improved by exchange 3
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coupling with a soft magnetic phase, such as FeCo. In recent years, the synthesis and magnetic properties of CoFe2O4/CoFe2 have been widely investigated [20-23]. Generally, the materials was fabricated by partially reducing CoFe2O4 [24-26]. However, these methods all require many steps in the synthesis process. This paper proposed a convenient and environmentally friendly method to synthesize
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magnetically exchange coupled CoFe2O4/CoFe2 composite particles. 1. Experimental
Two parts are included in the CoFe2O4/CoFe2 composites synthesis system. The first is a spray pyrolysis process to generate CoFe2O4 particles, and second is a reduction process to partially reduce CoFe2O4
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particles into CoFe2O4/CoFe2 composites. The whole synthesis system is simplified and shown in Fig. 1.
Fig. 1. A schematic of experimental system for CoFe2O4/CoFe2 core/shell particle synthesis. The synthesis of CoFe2O4 particles is based on Xu’s previous report with little modification [27, 28].
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Specifically, Fe(NO3)3·9H2O (Sigma Aldrich, 97%, 12 mmol) and CoCl2·6H2O (Sigma Aldrich, >98%, 6 mmol) were dissolved in 100 mL of DI H2O, and then ultrasonicated for 10 mins to solubilize the precursors. The solution was then transferred to an ultrasonic nebulizer (2.5 MHz) to generated liquid
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droplets, which were carried by Ar gas to go through the first furnace (length: 1.4 m; inner diameter: 0.1 m; 800 oC), with carrier gas flow rate of 5 L/min. After the first furnace, another carrier gas of Ar/H2 carried the products from the first furnace to go through a second surface (length: 1.4 m; inner diameter: 0.1 m). After the second furnace, the particles were cooled down by a heat exchanger, and collected by a fiber filter. To control the reduction level of CoFe2O4 particles, the reducing gas (Ar/H2) with different volume percentage of H2 is used. The reduction gas of Ar/H2 ((95/5 vol.%, 92/8 vol.% and 90/10 vol.%) is used in three parallel experiments. The measurement of X-ray diffraction (XRD) patterns was carried out by Bruker D8 diffractometer using Cu-Kα radiation (λ=1.5406 Å). Transmission Electron Microscopy (TEM) equipped with an Energy 4
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Dispersive X-ray (EDX) spectrometer, and Scanning Electron Microscopy (SEM) were used to analyze the particle size, morphology and elemental distribution. Magnetic properties were checked at room temperature with a vibrating sample magnetometer (VSM) in an applied field up to 1 T.
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2. Discussion
Fig. 2. (a). TEM image of CoFe2O4 particles (reduced by Ar/H2 (95/5 vol.%)); (b). TEM image of a single CoFe2O4 particle (reduced by Ar/H2 (95/5 vol.%)); (c) TEM image of a single CoFe2O4/CoFe2 core/shell particle ; (d). TEM image of CoFe2O4/CoFe2 core/shell particles; (e). EDX line scan from Fig. 2b; (f).
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EDX line scan from Fig. 2c.
The low magnification TEM images of both CoFe2O4 (first furnace) and CoFe2O4/CoFe2 particles (the
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second furnace, reduced by Ar/H2 (95/5 vol.%)) are shown in Figs. 2a and d. It can been seen that both particles are spherical, and the different contrast on CoFe2O4/CoFe2 particles indicates their core/shell structure. High magnification TEM images of CoFe2O4 and CoFe2O4/CoFe2 particles are displayed in Figs. 2b and c, and the corresponding TEM-EDX line scanning results across a single particle were displayed in Figs. 2e and f. The O and Fe signals are constant across the particle in Fig. 2e, indicating these two elements distribution is homogenous in a CoFe2O4 particle. In Fig. 2f, the O signal is very low at both ends of the line scan, indicating that the surface of CoFe2O4 is mainly reduced into CoFe2. The characterization results in Fig. 2 clearly demonstrate that the surface of CoFe2O4 is reduced into CoFe2
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after the H2 reduction, forming a core/shell structure of CoFe2O4/CoFe2, and the shell thickness is around
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20 nm when the H2 concentration is 5 vol.%.
Fig. 3. (a) SEM image of CoFe2O4/CoFe2 composite particles; (b) A high resolution TEM image
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indicating the boundary of CoFe2O4 and CoFe2; (c). STEM image of CoFe2O4/CoFe2 particles; (d). EDX line scanning result in Fig. 3c (with molar percentage).
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The SEM image of CoFe2O4/CoFe2 particles reduced by Ar/H2 (95/5 vol.%) is shown in Fig. 3a. It can be seen that the particles have a relatively large size distribution. Based on the calculation of 100 particles, the particle size is 832 nm. Previous research results have shown that the average particle size could be regulated by changing the concentration of the precursor solution in the spray pyrolysis process [27]. A high resolution TEM image of CoFe2O4/CoFe2 particle is shown in Fig. 3b. Due to the difference in electron density, different contrast represents different materials. The dark area represents CoFe2O4, while the bright area represents CoFe2. The high resolution TEM (HRTEM) image in Fig. 3b indicates the lattice constant of 2.5 Å in CoFe2 area, corresponding to the crystal plane of (110) in body centered cubic (BCC) Fe-Co alloy. A Scanning Transmission Electron Microscopy (STEM) image of CoFe2O4/CoFe2 particles is show in Fig. 3c, and elemental analysis was conducted on the edge of a particle. The 6
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corresponding result is shown in Fig. 3d. The low level of O indicates that the surface of the particle is
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mainly reduced, and the molar ratio of Fe to Co is close to 2 to 1.
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Fig. 4. XRD patterns of CoFe2O4 particles and CoFe2O4/CoFe2 composites. The XRD patterns of both CoFe2O4 and CoFe2O4/CoFe2 particles are shown in Fig. 4. The XRD patters of CoFe2O4 particles show an inverse spinel structure, and match well with corresponding JCPDS card (NO. 00-022-1086). After reduction, all CoFe2O4 peaks are maintained, and an extra peak (110) peak at ~45o indicates the existence of Fe-Co phase. Higher concentration of H2 resulted in a stronger peak of Fe-Co,
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indicating a higher percentage of CoFe2 reduced from CoFe2O4.
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Fig. 5. CoFe2O4 particles and CoFe2O4/CoFe2 composites: (a). Magnetic hysteresis loops; (b). BH(max)
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values.
The magnetic hysteresis loops of CoFe2O4 particles and CoFe2O4/CoFe2 core/shell particles are displayed in Fig. 5a. The magnetization and coercitity of pure CoFe2O4 are 62.3 emu/g and 2518 Oe, respectively. After partial reduction, CoFe2O4/CoFe2 core/shell particles are formed. With the increase of H2 concentration, the coercivity is decreasing and the magnetization is increasing. Furthermore, compared with the smooth and kink-free magnetic hysteresis loop of CoFe2O4/CoFe2 (Ar/H2:95/5 vol.%), the
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hysteresis loops of CoFe2O4/CoFe2 (Ar/H2:92/8 vol.%) and CoFe2O4/CoFe2 (Ar/H2:90/10 vol.%) presented obvious kinks. This could be attributed to the fact that the CoFe2O4 is over reduced, and CoFe2 can not be fully magnetically exchange coupled with CoFe2O4, resulting in a magnetic decoupling. The maximum energy product ((BH)max) is an important parameter to evaluate the strength of a permanent
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magnet. The (BH)max of CoFe2O4 particles and CoFe2O4/CoFe2 composites is presented in Fig. 5b. The change of (BH)max could be explained with the help of the magnetic hysteresis loops in Fig. 5a. When the H2 concentration is 5 vol.%, the reduced CoFe2 could be fully magnetically exchange coupled with
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CoFe2O4, presenting a smooth hysteresis loop, as shown in Fig. 5a. Therefore, its (BH)max is increased over pure CoFe2O4, reaching at 0.78 MGOe, higher than the best value of CoFe2O4 particles (0.73 MGOe) reported by Cabral etc., but lower than the CoFe2O4/CoFe2 composites (1.22 MGOe) reported by Leite etc. [24]. However, when the H2 concentration is increased to 8 and 10 vol.%, the CoFe2O4 is over reduced, and the reduced CoFe2 could only partially magnetically exchange coupled with CoFe2O4, resulting unsmooth hysteresis loops in Fig. 5a. Therefore, their (BH)max is decreased from pure CoFe2O4.
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Fig. 6. Henkel plots of CoFe2O4 particles and CoFe2O4/CoFe2 composites.
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The magnetic exchange coupling between hard and soft phases could be further investigated by Henkel Plot, which could be defined as:
δM = Md(H) – [1-2Mr(H)],
[1]
where Md is the normalized demagnetization remanence, Mr is the normalized isothermal magnetization
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remanence, and H is the applied magnetic field [13, 26]. A positive peak in Henkel Plot indicates a magnetic exchange coupling between hard and soft phases, while a negative peak indicates a
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magnetostatic interactions among particles. It can be seen that in Fig. 6, pure CoFe2O4 particles present a negative peak, indicating magnetostatic interactions. When CoFe2O4 particles are reduced with H2 (5 vol.%), its Henkel Plot is dominated by a positive peak, indicating that magnetic exchange coupling is occurring between CoFe2O4 and CoFe2. When the H2 concentration is further increased to 8 and 10 vol.%, then Henkel Plot is dominated by a negative peak again, indicating the disappearance of magnetic exchange coupling between CoFe2O4 and CoFe2. 3. Conclusions CoFe2O4/CoFe2 composite particles were synthesized in a two-step process, including spray pyrolysis and H2 reduction. The core/shell structure of CoFe2O4/CoFe2 composite particles were confirmed by TEM, 9
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STEM and EDX, and shell thickness is around 20 nm. The crystal phases of CoFe2O4 and CoFe2 were confirmed by XRD patterns. The magnetic exchange coupling between CoFe2O4 and CoFe2 in the composites was confirmed by the smooth magnetic hysteresis loop, which indicates one-phase magnetic behavior. Henkel Plot is further used to demonstrate the existence of magnetic exchange coupling
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between CoFe2 and CoFe2O4 in CoFe2O4/CoFe2 composites reduced by Ar/H2:95/5 vol.%. This paper presented an easy method to synthesize magnetically exchange coupled CoFe2O4/CoFe2 composites. References
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Highlight: •
CoFe2O4/CoFe2 core/shell composite particles were synthesized through a convenient method. The core/shell structure of the composites were determined by TEM, EDX and STEM.
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Magnetic exchange coupling was achieved between CoFe2O4 and CoFe2 in CoFe2O4/CoFe2
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•
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core/shell composite particles.