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Structural-controlled chemical synthesis of nanosized amorphous Fe particles and their improved performances
Journal of Alloys and Compounds 651 (2015) 551e556
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural-controlled chemical synthesis of nanosized amorphous Fe particles and their improved performances X.Y. Yang, B. Yang*, X.P. Li, Y. Cao, R.H. Yu School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2015 Received in revised form 31 July 2015 Accepted 19 August 2015 Available online 20 August 2015
High-performance amorphous Fe nanoparticles with controlled structure and good thermal stability have been fabricated using an optimized chemical method by a direct reduction reaction of Fe2þ ions with NaBH4 as reducing agent. The addition of polyvinylpyrrolidone (PVP) as a surfactant, the properties of solution acidalcaline and the variation of reaction solvent compositions have shown great influences on the morphologies, surface compositions and magnetic properties of the products. The addition of surfactant PVP leads to the formation of fully amorphous Fe nanoparticles. Partial crystallization will occur in the samples prepared without PVP addition and result in their high saturation magnetization up to 168.0 A m2/Kg. The alkali reaction solution with NaOH addition can facilitate the formation of amorphous FeeB phases in the products instead of the surface B concentration in neutral solution, which promotes their good thermal stability. Reactions in the ethanol-water solvent favor the formation of smaller amorphous iron nanoparticles with about 20 nm in size. The relatively simple chemical synthesis and improved performances including controlled structure, good thermal stability and excellent intrinsic magnetic properties for these amorphous iron nanoparticles promise their potential applications in highperformance multifunctional magnetic devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Amorphous iron nanoparticles Reduction reaction Controlled structure Surface compositions Improved performances
1. Introduction Fe-based soft magnetic materials play an important role in many technical applications such as magnetic sensors, data recording and electric motors because of their overall high performances such as high saturation magnetization, low coercivity and good thermal stability [1e4]. As is known, Fe-based metallic glasses show much better mechanical and magnetic properties than those of their crystalline counterparts owning to the absence of grain boundaries and crystal magnetic anisotropy [5,6]. The traditional bulk Fe-based amorphous alloys have been usually prepared by melt-spinning technology or copper mold casting methods [7e9]. Nevertheless, their poor glass-forming ability and high cost restrict the actual applications for these Fe-based amorphous alloys in highperformance magnetic devices. It has been demonstrated that the nanosized magnetic materials exhibit better intrinsic mechanical and magnetic properties compared with their corresponding bulk alloys fabricated by traditional physical methods [10,11]. Various
* Corresponding author. E-mail address: [email protected] (B. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.08.156 0925-8388/© 2015 Elsevier B.V. All rights reserved.
controllable chemical methods have been widely used to fabricate magnetic metallic nanostructures with controlled morphology and particle size [12e15]. For further applications, these chemically synthesized magnetic particles with ideal structure can be densified via powder compacting molding process to full-density bulk soft magnetic composite materials. In our previous work, nearlyspherical FeCo particles with particle size ranging from 100 nm to 5 mm have been prepared by a direct chemical method and can be easily compacted into full-density magnetic devices [16,17]. Recently, much attention has been focused on amorphous Febased nanomaterials and their effective chemical fabrication due to their good soft magnetic properties, mechanical properties and low preparation cost. Many researches have been performed to synthesize varieties of amorphous magnetic nanoparticles by chemical method using borohydride (NaBH4) as the reducing agent for compensating the limitation of traditional physical preparation methods [18e20]. In our recent work, nanosized amorphous Fe particles with narrow size distribution and good magnetic properties were prepared by a direct chemical reduction of Fe2þ ions with NaBH4 as reducing agent at an optimal NaBH4 addition rate of 4.0 mL/min [21]. In this work, high-performance amorphous Fe nanoparticles with combined ideal structure, high saturation
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magnetization and good thermal stability have been fabricated using an optimized chemical method. It has been found that synthesis processes such as the addition of PVP as a surfactant, the properties of solution acidalcaline and the variation of reaction solvent compositions have shown great influences on the phase formation of these chemically synthesized nanosized iron particles. The influences of the preparation conditions on the morphologies, surface compositions and magnetic properties of these amorphous nanosized iron particles have also been systematically studied. 2. Experimental In our previous work, we have reported a direct chemical method for preparing fully amorphous iron nanoparticles at room temperature by the reduction of Fe2þ with NaBH4 as reducing agent in FeCl2$4H2O aqueous solution at an optimal NaBH4 addition rate of 4.0 mL/min [21]. In the present work, amorphous iron nanoparticles with controlled morphologies and modulated surface compositions have been prepared by the optimized chemical method according to a procedure previously reported [21] through adjusting the key reaction parameters including the addition of surfactant PVP, change of solution acidalcaline and the variation of reaction solvent compositions. In the following typical procedures, four samples have been obtained by the modified chemical methods in four different reaction systems, respectively: (a) the only addition of PVP (0.5e1 g) in ferrous aqueous solution, (b) no addition of PVP in ferrous aqueous solution, (c) the combined addition of PVP (0.5e1 g) in ferrous aqueous solution and NaOH (0.06e0.1 g) in NaBH4 solution and (d) the only addition of PVP (0.5e1 g) in the mixed ethanol-water solvent. All the samples in this work are fabricated at a fixed NaBH4 addition rate of 4.0 mL/ min and collected after drying the reaction products in a vacuum oven at room temperature for 6e8 h. Some of them were annealed at 475e600 C for 10 min under high-purity Ar atmosphere to further investigate their crystallization characteristics. The phase structure of the samples was characterized by X-ray diffraction (XRD) using a D/max 2500PC X-ray diffractometer with Cu Ka radiation. The microstructure of the samples was identified by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) using a JEOL JEM-2100 transmission electron microscope operated at 200 kV. The precise compositions of the samples were determined using an inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkineElmer Optima 7000 DV). The thermal properties were analyzed by differential scanning calorimetry (DSC, STA 449 F3, NETZSCH) under an Ar atmosphere at a heating rate of 10 C/min. The surface states of the samples were investigated by the X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) with an Al Ka excitation source. Magnetic properties for the samples were measured with a vibrating sample magnetometer (VSM, Lakeshore 7307) under a maximum magnetic field of 10 kOe at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns for the four samples prepared under different reaction conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1. It can be seen from Fig. 1b that three broad peaks are observed with 2q values of 44.8 , 65.2 , and 82.6 , which are in good agreement with characteristic peaks of the bcc-Fe phase. These broad peaks in the diffraction patterns also indicate partially crystallized states for the sample b prepared with no PVP addition,
Fig. 1. XRD patterns of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanolwater solvent with the volume ratio of 1:1.
which can be confirmed by the SAED patterns of mixture phases and a relatively low crystallization transition temperature in the DSC curves discussed below. However, the other three samples with the addition of PVP exhibit the same diffraction characteristics with a broad peak around 2q of 45 , which indicates the same fully amorphous states in these samples [22]. The further results show that the addition of PVP in any amount of 0.5e1 g with other reaction conditions fixed can facilitate the formation of fully amorphous samples, which clearly suggests that the surfactant PVP promotes the formation of amorphous nanosized iron particles fabricated by chemical reduction. Fig. 2 shows the representative TEM micrographs and the corresponding SAED patterns of the four samples prepared under different conditions. It can be seen from the SAED patterns in Fig. 2b that besides the diffuse halo, there are a few single crystal diffraction spots, which indicates a mixture of crystalline and amorphous phases in the sample prepared with no PVP addition. The SAED patterns for the other three samples prepared with the addition of PVP (sample a, sample c and sample d) only show the diffuse scattering halos, which further demonstrates the fully amorphous phases in these samples. The reduction reactions of Fe2þ ions with NaBH4 as strong reducing agent will have run quickly to generate Fe nucleuses, which may simultaneously form amorphous Fe nanograins with short-range order under room temperature and then grow into amorphous Fe nanoparticles. It has been reported that the short-range order of the nanograins can be observed in the boron-rich interfacial regions of the Fe50B50 nanoglass, which promotes the formation of their amorphous state [18]. The average particle sizes and their distribution of these samples estimated from more than 100 particles by selecting several TEM images for each sample are listed in Table 1. As can be seen from Fig. 2 and Table 1, the addition of PVP, the properties of solution acidalcaline and the variation of reaction solvent compositions show an obvious effect on the morphologies of these products. The only addition of PVP in the reaction systems lead to the formation of uniform amorphous Fe nanoparticles with an average particle size of 90 nm (seen in Fig. 2a). While the sample prepared without any addition of PVP (sample b) exhibits a chain-like morphology with smaller particles and broader size distribution. It can be concluded that the
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Fig. 2. The TEM micrographs of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
Table 1 The Fe/B ratio, average particle sizes and the values of saturation magnetization Ms and coercivity iHc of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1. Samples prepared at different reaction conditions
The Fe/B ratio (at%)
Average particle sizes (nm)
Ms (A m2/kg)
iHc
(a) 1 g PVP þ aqueous solution (b) aqueous solution (c) 1 g PVP þ 0.06 g NaOH þ aqueous solution (d) 1 g PVP þ ethanol-water solution
68.32:31.68 81.53:18.47 64.28:35.72 68.15:31.85
80 90 150 20
140.4 168.0 138.6 127.5
121.5 315.6 72.5 146.3
addition of surfactant PVP favors the formation of fully amorphous Fe nanoparticles with large particle sizes and narrow size distribution. It has been reported that PVP has often been used to regulate nucleation and growth of the nucleuses of nanoparticles so as to adjust their crystalline state and compositions in chemical process [23,24]. In this work, much more B element detected by ICP-OES below are concentrated in nanosized amorphous iron particles prepared with the addition of PVP. However, lower B element content is detected for the sample prepared with no PVP addition. The B concentration has been proved to affect the formation of amorphous and crystallized iron phases [25]. It can be seen that the addition of PVP influences the crystalline state of the product in this work. Moreover, PVP can influence the particle sizes and surface compositions by wrapping the nucleuses rapidly after their formation and further control their growth [23]. Compared with Fig. 2aec, it can be seen that another addition of NaOH into the NaBH4 solution can result in much larger particle size increasing from 90 to 150 nm. Our further experiments show that the increased particle sizes of the products with addition of NaOH (0.06e0.1 g) have little relation to their addition amount. This may be ascribed to the alkaline environment due to the addition of NaOH into the reaction system, which is helpful to accelerate the reduction reaction of Fe2þ ions and promotes the growth of amorphous Fe nanoparticles. It has been reported that during the chemical reduction of Fe2þ ions to pure Fe, the PH values of reaction solution will be decreased from near neutral PH value to about 4.26
(Oe)
due to the release of Hþ ions [26,27], and the addition of NaOH with appropriate amount can facilitate the reduction reaction. It can be found from Fig. 2d that the average particle size (20 nm) for the sample produced in the mixed ethanol-water solution is much smaller than that of the sample prepared in water solvent (sample a), which may be attributed to the accelerated reduction reaction rate in the mixed solution. The further work shows that the average particle size of the product can be increased to 40 nm with decreasing the ethanol/water volume ratio to 2:3. As a result, the particle sizes of the products tend to decrease with the increasing amount of the ethanol in solvent, which indicates that ethanol solvent may promote the nucleation rate of Fe atoms but inhibit their growth so as to result in smaller particle sizes and narrow distribution. This can be ascribed to the relatively high dielectric constant of ethanol-water solvent, which can prevent the nucleus of amorphous Fe nanoparticles from growing by coagulation [28]. The compositions of the four samples determined by ICP-OES are listed in Table 1. It can be found that both Fe and B element are detected in all the samples. The lower B element content is detected for the sample prepared with no PVP addition, compared with those of the samples prepared with PVP addition. However, the concentration of B for the sample prepared with NaOH addition increases significantly. It can be concluded that the addition of surfactant PVP and NaOH promote the accumulation of B element, which may be ascribed to their influences on the reduction reaction rate. The significantly different B concentration results in the
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different element states such as surface B concentration and formation of FeeB phases, which can be confirmed by the followed DSC and XPS analysis. The crystallization characteristics of these amorphous iron nanoparticles have been studied by DSC measurement. As shown in Fig. 3, the three curves for the samples prepared without NaOH addition show similar thermal behavior with only one distinct exothermic peak at temperature ranging from 462 to 485 C. These single exothermic peaks may be associated with the transformation from amorphous Fe phases to the crystallized a-Fe phases. The much higher crystallization temperature than that of pure amorphous Fe (about 310 C [29]) is believed to result from their slight surface B concentration [21]. Compared with the DSC curves for sample a and sample b in Fig. 3, the transition peak shifts to higher temperature for the sample prepared with PVP addition (sample a), which may be due to their different surface B compositions. It can be concluded that the PVP addition facilitates the formation of amorphous structure and enhances the thermal stability of these amorphous iron nanoparticles. The much lower crystallization temperature for the sample prepared at the mixture solvent (sample d) can be attributed to their lower crystallization activation energy due to their much smaller particle sizes [30]. It can be found that the DSC curve for the sample prepared with the addition of NaOH exhibits two exothermic peaks at about 472 and 512 C. The two exothermic peaks indicate a two-stage crystallization process, which may be correlated with two different amorphous phases in this sample. This amorphous sample with two exothermic peaks was annealed at temperatures a little higher than the transition points in DSC curves to investigate its detailed crystallization process. Our previous study has shown that the annealed amorphous Fe nanoparticles prepared without NaOH addition only reveal a bcc-Fe phase in their XRD patterns [21]. Fig. 4 shows the XRD patterns of this amorphous sample with two exothermic peaks annealed under an Ar gas atmosphere at 475 C and 600 C for 10 min. It can be obviously found that besides a-Fe phase, other crystallized FeeB phases including Fe2B and Fe3B are precipitated in the annealed samples at two transition temperature points. The simultaneous precipitation of a-Fe and FeeB phases in the annealed samples initially prepared with NaOH addition further
Fig. 3. DSC curves of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanolwater solvent with the volume ratio of 1:1.
Fig. 4. XRD patterns of the annealed samples initially prepared at the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution and then being annealed under Ar atmosphere at 475 C and 600 C for 10 min.
confirm that there are two amorphous phases in this sample. Furthermore, it can be seen that the XRD peak intensities corresponding to the Fe2B and Fe3B phases in the annealed samples at 600 C is much stronger than those at 475 C. It can be concluded that the first transition temperature at 472 C in DSC curve corresponds to the process of the nucleation and growth of a-Fe and FeeB primary crystalline phases while the other transition temperature at 512 C can be related with the complete transformation to a-Fe and FeeB alloys. The crystallization temperatures of two phases observed in this work are in good agreement with those of Fe-based amorphous materials reported [5,6,31]. According to the above analysis, the addition of surfactant PVP and NaOH in the reaction system exert different influences on the compositions of the amorphous samples prepared under corresponding reaction conditions. The surface element states of Fe, B and O atoms in the four samples prepared under different reaction conditions have been analyzed by XPS narrow-scan spectrums shown in Fig. 5. The obviously different surface compositions and distribution of B element of the four samples are observed due to different reaction conditions. It can be seen from Fig. 5.1 that B 1s spectrums for the three samples prepared without NaOH addition exhibit the very similar curves and peak locations, which indicates the same B element state in these samples and has been proved to be the surface B concentration [21]. It is noteworthy that there is obvious positive shift of XPS peak locations by 0.8e1 ev for the sample prepared with NaOH addition, which demonstrates the presence of B element in an alloyed state in this sample [32]. It can be found from Fig. 5.2 that the Fe 2p spectrums for the four samples exhibit similar curve shapes but with different bump locations, which illustrates the different Fe element states in these samples. The two peaks with Fe 2p1/2 at about 719.6 eV and Fe 2p3/2 at about 706.9 eV in all the spectrums indicate a metallic state of Fe atoms. The other two characteristic peaks observed with Fe 2p1/2 at about 723.4 eV and Fe 2p3/2 at about 710.0 eV for the samples prepared in the aqueous solution without NaOH addition demonstrate the formation iron oxides of Fe3O4 [33]. While for the samples prepared with NaOH (0.06 g) addition in NaBH4 solution and the only addition of PVP (1 g) in ethanol-water solvent, the peak intensity corresponding to metallic Fe is much stronger, indicating that slighter oxidization occurs on the surface of the samples. Furthermore,
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Fig. 5. XPS narrow-scan spectrums of (1) B 1s, (2) Fe 2p and (3) O 1s for the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
another broad bump at about 715.3 eV between the Fe 2p1/2 at about 723.4 eV and Fe 2p3/2 at about 710.0 eV is observed. The two peaks along with this bump are consistent with Fe2þ ions [34]. The surface oxidation state of the four samples can be also confirmed by XPS spectrums of O 1s in Fig. 5.3 with slight shift of peak locations. It can be concluded that the surface oxidation of the amorphous Fe nanoparticles can be controlled effectively by adjusting the reaction conditions. Fig. 6 shows the room-temperature hysteresis loops of the four samples prepared at different reaction conditions. It can be found from Fig. 6 that all the hysteresis loops exhibits typical ferromagnetic characteristics with good intrinsic magnetic properties. The values of saturation magnetization (Ms) and coercivity (iHc) for
these samples are also listed in Table 1. The variations of saturation magnetization for these samples can be mainly ascribed to their different surface oxidation and B concentration. The Ms value of 168.0 A m2/Kg for the sample prepared with no PVP addition is much higher than those of the three samples prepared with PVP addition, which can be resulted from the high concentration of Fe and the formation of ferrimagnetic Fe3O4 in the surface of this sample. The amorphous sample prepared with no PVP addition in ferrous aqueous solution show a relative higher iHc value of 315.6 Oe among the four samples, which can be mainly ascribed to magnetic shape anisotropy caused by their chain-like morphologies. As can be seen, due to higher concentration of B atoms and the formation of amorphous FeeB phases in the sample prepared with NaOH addition, much lower Ms value is obtained in this samples. Furthermore, the value of Ms decreases significantly for the samples prepared in mixed ethanol-water solvent, which can be resulted from their much smaller particle sizes. As discussed above, high-performance nanosized amorphous Fe particles with controlled structure, good thermal stability and improved intrinsic magnetic properties can be prepared by a controllable chemical reaction. The relatively simple chemical method, low cost and enhanced performances for these amorphous Fe nanoparticles show great potential applications for multifunctional magnetic devices. 4. Conclusions
Fig. 6. Magnetic hysteresis loops of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
The nanosized amorphous Fe particles with controlled structure and improved performances have been prepared by a controllable chemical reduction reaction through adjusting the addition of surfactant PVP, the properties of solution acidalcaline and reaction solvent compositions. The addition of surfactant PVP promotes the formation of fully amorphous large-grain iron nanoparticles with narrow size distribution and surface B concentration. The sample
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prepared with NaOH addition exhibits a two-stage crystallization process and results in the formation of amorphous FeeB phases in the samples instead of surface concentration, which proves their good thermal stability. Amorphous iron nanoparticles with diameter of about 20 nm and slighter surface oxidization are obtained in ethanol-water solvent. High values of saturation magnetization up to138e140 A m2/Kg due to the high concentration of Fe atoms and slight surface oxidation are obtained in the samples prepared with PVP addition in the aqueous solution. The as-synthesized sample prepared without PVP addition reaches higher saturation magnetization of 168.0 A m2/Kg because of their partial crystallization and higher Fe concentration. Meanwhile, the relatively higher iHc value of 315.6 Oe is also observed in this sample due to the magnetic shape anisotropy caused by their chain-like morphologies. Acknowledgments This work was supported by Beijing Natural Science Foundation under Grant No. 2132039 and the National Nature Science Foundation of China under Grant No. (51101007, 51171007 and 51271009). References [1] A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222e1244. [2] D.L. Huber, Synthesis, properties, and application of iron nanoparticles, Small 1 (2005) 482e501. [3] D.C. Jiles, Recent advances and future directions in magnetic materials, Acta Mater. 51 (2003) 5907e5939. [4] T. Hyeon, Chemical synthesis of magnetic nanoparticles, Chem. Commun. (2003) 927e934. [5] F.L. Kong, C.T. Chang, A. Inoue, E. Shalaan, F. Al-Marzouki, Fe-based amorphous soft magnetic alloys with high saturation magnetization and good bending ductility, J. Alloy. Compd. 615 (2014) 163e166. [6] J.H. Zhang, C.T. Chang, A. Wang, B.L. Shen, Development of quaternary Febased bulk metallic glasses with high saturation magnetization above 1.6 T, J. Non Cryst. Solids 358 (2012) 1443e1446. [7] C. Suryanarayana, A. Inoue, Iron-based bulk metallic glasses, Int. Mater. Rev. 58 (2013) 131e166. [8] Y.R. Zhang, R.V. Ramanujan, A study of the crystallization behavior of an amorphous Fe77.5Si13.5B9 alloy, Mater. Sci. Eng. A 416 (2006) 161e168. [9] J.H. Yao, J.Q. Wang, Y. Li, Ductile Fe-Nb-B bulk metallic glass with ultrahigh strength, Appl. Phys. Lett. 92 (2008) 251906-1e251906-3. [10] J.X. Fang, U. Vainio, W. Puff, R. Würschum, X.L. Wang, D. Wang, M. Ghafari, F. Jiang, J. Sun, H. Hahn, H. Gleiter, Atomic structure and structural stability of Sc75Fe25 nanoglasses, Nano Lett. 12 (2012) 458e463. [11] M.E. Mchenry, D.E. Laughlin, Nano-scale materials development for future magnetic applications, Acta Mater. 48 (2000) 223e238. [12] T. Fan, D. Pan, H. Zhang, Study on Formation mechanism by monitoring the morphology and structure evolution of nearly monodispersed Fe3O4 submicroparticles with controlled particle sizes, Ind. Eng. Chem. Res. 50 (2011) 9009e9018. [13] O. Crisan, K. von Haeften, A.M. Ellis, C. Binns, Structure and magnetic
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural-controlled chemical synthesis of nanosized amorphous Fe particles and their improved performances X.Y. Yang, B. Yang*, X.P. Li, Y. Cao, R.H. Yu School of Materials Science and Engineering, Beihang University, Beijing, 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2015 Received in revised form 31 July 2015 Accepted 19 August 2015 Available online 20 August 2015
High-performance amorphous Fe nanoparticles with controlled structure and good thermal stability have been fabricated using an optimized chemical method by a direct reduction reaction of Fe2þ ions with NaBH4 as reducing agent. The addition of polyvinylpyrrolidone (PVP) as a surfactant, the properties of solution acidalcaline and the variation of reaction solvent compositions have shown great influences on the morphologies, surface compositions and magnetic properties of the products. The addition of surfactant PVP leads to the formation of fully amorphous Fe nanoparticles. Partial crystallization will occur in the samples prepared without PVP addition and result in their high saturation magnetization up to 168.0 A m2/Kg. The alkali reaction solution with NaOH addition can facilitate the formation of amorphous FeeB phases in the products instead of the surface B concentration in neutral solution, which promotes their good thermal stability. Reactions in the ethanol-water solvent favor the formation of smaller amorphous iron nanoparticles with about 20 nm in size. The relatively simple chemical synthesis and improved performances including controlled structure, good thermal stability and excellent intrinsic magnetic properties for these amorphous iron nanoparticles promise their potential applications in highperformance multifunctional magnetic devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Amorphous iron nanoparticles Reduction reaction Controlled structure Surface compositions Improved performances
1. Introduction Fe-based soft magnetic materials play an important role in many technical applications such as magnetic sensors, data recording and electric motors because of their overall high performances such as high saturation magnetization, low coercivity and good thermal stability [1e4]. As is known, Fe-based metallic glasses show much better mechanical and magnetic properties than those of their crystalline counterparts owning to the absence of grain boundaries and crystal magnetic anisotropy [5,6]. The traditional bulk Fe-based amorphous alloys have been usually prepared by melt-spinning technology or copper mold casting methods [7e9]. Nevertheless, their poor glass-forming ability and high cost restrict the actual applications for these Fe-based amorphous alloys in highperformance magnetic devices. It has been demonstrated that the nanosized magnetic materials exhibit better intrinsic mechanical and magnetic properties compared with their corresponding bulk alloys fabricated by traditional physical methods [10,11]. Various
* Corresponding author. E-mail address: [email protected] (B. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.08.156 0925-8388/© 2015 Elsevier B.V. All rights reserved.
controllable chemical methods have been widely used to fabricate magnetic metallic nanostructures with controlled morphology and particle size [12e15]. For further applications, these chemically synthesized magnetic particles with ideal structure can be densified via powder compacting molding process to full-density bulk soft magnetic composite materials. In our previous work, nearlyspherical FeCo particles with particle size ranging from 100 nm to 5 mm have been prepared by a direct chemical method and can be easily compacted into full-density magnetic devices [16,17]. Recently, much attention has been focused on amorphous Febased nanomaterials and their effective chemical fabrication due to their good soft magnetic properties, mechanical properties and low preparation cost. Many researches have been performed to synthesize varieties of amorphous magnetic nanoparticles by chemical method using borohydride (NaBH4) as the reducing agent for compensating the limitation of traditional physical preparation methods [18e20]. In our recent work, nanosized amorphous Fe particles with narrow size distribution and good magnetic properties were prepared by a direct chemical reduction of Fe2þ ions with NaBH4 as reducing agent at an optimal NaBH4 addition rate of 4.0 mL/min [21]. In this work, high-performance amorphous Fe nanoparticles with combined ideal structure, high saturation
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magnetization and good thermal stability have been fabricated using an optimized chemical method. It has been found that synthesis processes such as the addition of PVP as a surfactant, the properties of solution acidalcaline and the variation of reaction solvent compositions have shown great influences on the phase formation of these chemically synthesized nanosized iron particles. The influences of the preparation conditions on the morphologies, surface compositions and magnetic properties of these amorphous nanosized iron particles have also been systematically studied. 2. Experimental In our previous work, we have reported a direct chemical method for preparing fully amorphous iron nanoparticles at room temperature by the reduction of Fe2þ with NaBH4 as reducing agent in FeCl2$4H2O aqueous solution at an optimal NaBH4 addition rate of 4.0 mL/min [21]. In the present work, amorphous iron nanoparticles with controlled morphologies and modulated surface compositions have been prepared by the optimized chemical method according to a procedure previously reported [21] through adjusting the key reaction parameters including the addition of surfactant PVP, change of solution acidalcaline and the variation of reaction solvent compositions. In the following typical procedures, four samples have been obtained by the modified chemical methods in four different reaction systems, respectively: (a) the only addition of PVP (0.5e1 g) in ferrous aqueous solution, (b) no addition of PVP in ferrous aqueous solution, (c) the combined addition of PVP (0.5e1 g) in ferrous aqueous solution and NaOH (0.06e0.1 g) in NaBH4 solution and (d) the only addition of PVP (0.5e1 g) in the mixed ethanol-water solvent. All the samples in this work are fabricated at a fixed NaBH4 addition rate of 4.0 mL/ min and collected after drying the reaction products in a vacuum oven at room temperature for 6e8 h. Some of them were annealed at 475e600 C for 10 min under high-purity Ar atmosphere to further investigate their crystallization characteristics. The phase structure of the samples was characterized by X-ray diffraction (XRD) using a D/max 2500PC X-ray diffractometer with Cu Ka radiation. The microstructure of the samples was identified by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) using a JEOL JEM-2100 transmission electron microscope operated at 200 kV. The precise compositions of the samples were determined using an inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkineElmer Optima 7000 DV). The thermal properties were analyzed by differential scanning calorimetry (DSC, STA 449 F3, NETZSCH) under an Ar atmosphere at a heating rate of 10 C/min. The surface states of the samples were investigated by the X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) with an Al Ka excitation source. Magnetic properties for the samples were measured with a vibrating sample magnetometer (VSM, Lakeshore 7307) under a maximum magnetic field of 10 kOe at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns for the four samples prepared under different reaction conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1. It can be seen from Fig. 1b that three broad peaks are observed with 2q values of 44.8 , 65.2 , and 82.6 , which are in good agreement with characteristic peaks of the bcc-Fe phase. These broad peaks in the diffraction patterns also indicate partially crystallized states for the sample b prepared with no PVP addition,
Fig. 1. XRD patterns of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanolwater solvent with the volume ratio of 1:1.
which can be confirmed by the SAED patterns of mixture phases and a relatively low crystallization transition temperature in the DSC curves discussed below. However, the other three samples with the addition of PVP exhibit the same diffraction characteristics with a broad peak around 2q of 45 , which indicates the same fully amorphous states in these samples [22]. The further results show that the addition of PVP in any amount of 0.5e1 g with other reaction conditions fixed can facilitate the formation of fully amorphous samples, which clearly suggests that the surfactant PVP promotes the formation of amorphous nanosized iron particles fabricated by chemical reduction. Fig. 2 shows the representative TEM micrographs and the corresponding SAED patterns of the four samples prepared under different conditions. It can be seen from the SAED patterns in Fig. 2b that besides the diffuse halo, there are a few single crystal diffraction spots, which indicates a mixture of crystalline and amorphous phases in the sample prepared with no PVP addition. The SAED patterns for the other three samples prepared with the addition of PVP (sample a, sample c and sample d) only show the diffuse scattering halos, which further demonstrates the fully amorphous phases in these samples. The reduction reactions of Fe2þ ions with NaBH4 as strong reducing agent will have run quickly to generate Fe nucleuses, which may simultaneously form amorphous Fe nanograins with short-range order under room temperature and then grow into amorphous Fe nanoparticles. It has been reported that the short-range order of the nanograins can be observed in the boron-rich interfacial regions of the Fe50B50 nanoglass, which promotes the formation of their amorphous state [18]. The average particle sizes and their distribution of these samples estimated from more than 100 particles by selecting several TEM images for each sample are listed in Table 1. As can be seen from Fig. 2 and Table 1, the addition of PVP, the properties of solution acidalcaline and the variation of reaction solvent compositions show an obvious effect on the morphologies of these products. The only addition of PVP in the reaction systems lead to the formation of uniform amorphous Fe nanoparticles with an average particle size of 90 nm (seen in Fig. 2a). While the sample prepared without any addition of PVP (sample b) exhibits a chain-like morphology with smaller particles and broader size distribution. It can be concluded that the
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Fig. 2. The TEM micrographs of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
Table 1 The Fe/B ratio, average particle sizes and the values of saturation magnetization Ms and coercivity iHc of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1. Samples prepared at different reaction conditions
The Fe/B ratio (at%)
Average particle sizes (nm)
Ms (A m2/kg)
iHc
(a) 1 g PVP þ aqueous solution (b) aqueous solution (c) 1 g PVP þ 0.06 g NaOH þ aqueous solution (d) 1 g PVP þ ethanol-water solution
68.32:31.68 81.53:18.47 64.28:35.72 68.15:31.85
80 90 150 20
140.4 168.0 138.6 127.5
121.5 315.6 72.5 146.3
addition of surfactant PVP favors the formation of fully amorphous Fe nanoparticles with large particle sizes and narrow size distribution. It has been reported that PVP has often been used to regulate nucleation and growth of the nucleuses of nanoparticles so as to adjust their crystalline state and compositions in chemical process [23,24]. In this work, much more B element detected by ICP-OES below are concentrated in nanosized amorphous iron particles prepared with the addition of PVP. However, lower B element content is detected for the sample prepared with no PVP addition. The B concentration has been proved to affect the formation of amorphous and crystallized iron phases [25]. It can be seen that the addition of PVP influences the crystalline state of the product in this work. Moreover, PVP can influence the particle sizes and surface compositions by wrapping the nucleuses rapidly after their formation and further control their growth [23]. Compared with Fig. 2aec, it can be seen that another addition of NaOH into the NaBH4 solution can result in much larger particle size increasing from 90 to 150 nm. Our further experiments show that the increased particle sizes of the products with addition of NaOH (0.06e0.1 g) have little relation to their addition amount. This may be ascribed to the alkaline environment due to the addition of NaOH into the reaction system, which is helpful to accelerate the reduction reaction of Fe2þ ions and promotes the growth of amorphous Fe nanoparticles. It has been reported that during the chemical reduction of Fe2þ ions to pure Fe, the PH values of reaction solution will be decreased from near neutral PH value to about 4.26
(Oe)
due to the release of Hþ ions [26,27], and the addition of NaOH with appropriate amount can facilitate the reduction reaction. It can be found from Fig. 2d that the average particle size (20 nm) for the sample produced in the mixed ethanol-water solution is much smaller than that of the sample prepared in water solvent (sample a), which may be attributed to the accelerated reduction reaction rate in the mixed solution. The further work shows that the average particle size of the product can be increased to 40 nm with decreasing the ethanol/water volume ratio to 2:3. As a result, the particle sizes of the products tend to decrease with the increasing amount of the ethanol in solvent, which indicates that ethanol solvent may promote the nucleation rate of Fe atoms but inhibit their growth so as to result in smaller particle sizes and narrow distribution. This can be ascribed to the relatively high dielectric constant of ethanol-water solvent, which can prevent the nucleus of amorphous Fe nanoparticles from growing by coagulation [28]. The compositions of the four samples determined by ICP-OES are listed in Table 1. It can be found that both Fe and B element are detected in all the samples. The lower B element content is detected for the sample prepared with no PVP addition, compared with those of the samples prepared with PVP addition. However, the concentration of B for the sample prepared with NaOH addition increases significantly. It can be concluded that the addition of surfactant PVP and NaOH promote the accumulation of B element, which may be ascribed to their influences on the reduction reaction rate. The significantly different B concentration results in the
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different element states such as surface B concentration and formation of FeeB phases, which can be confirmed by the followed DSC and XPS analysis. The crystallization characteristics of these amorphous iron nanoparticles have been studied by DSC measurement. As shown in Fig. 3, the three curves for the samples prepared without NaOH addition show similar thermal behavior with only one distinct exothermic peak at temperature ranging from 462 to 485 C. These single exothermic peaks may be associated with the transformation from amorphous Fe phases to the crystallized a-Fe phases. The much higher crystallization temperature than that of pure amorphous Fe (about 310 C [29]) is believed to result from their slight surface B concentration [21]. Compared with the DSC curves for sample a and sample b in Fig. 3, the transition peak shifts to higher temperature for the sample prepared with PVP addition (sample a), which may be due to their different surface B compositions. It can be concluded that the PVP addition facilitates the formation of amorphous structure and enhances the thermal stability of these amorphous iron nanoparticles. The much lower crystallization temperature for the sample prepared at the mixture solvent (sample d) can be attributed to their lower crystallization activation energy due to their much smaller particle sizes [30]. It can be found that the DSC curve for the sample prepared with the addition of NaOH exhibits two exothermic peaks at about 472 and 512 C. The two exothermic peaks indicate a two-stage crystallization process, which may be correlated with two different amorphous phases in this sample. This amorphous sample with two exothermic peaks was annealed at temperatures a little higher than the transition points in DSC curves to investigate its detailed crystallization process. Our previous study has shown that the annealed amorphous Fe nanoparticles prepared without NaOH addition only reveal a bcc-Fe phase in their XRD patterns [21]. Fig. 4 shows the XRD patterns of this amorphous sample with two exothermic peaks annealed under an Ar gas atmosphere at 475 C and 600 C for 10 min. It can be obviously found that besides a-Fe phase, other crystallized FeeB phases including Fe2B and Fe3B are precipitated in the annealed samples at two transition temperature points. The simultaneous precipitation of a-Fe and FeeB phases in the annealed samples initially prepared with NaOH addition further
Fig. 3. DSC curves of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanolwater solvent with the volume ratio of 1:1.
Fig. 4. XRD patterns of the annealed samples initially prepared at the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution and then being annealed under Ar atmosphere at 475 C and 600 C for 10 min.
confirm that there are two amorphous phases in this sample. Furthermore, it can be seen that the XRD peak intensities corresponding to the Fe2B and Fe3B phases in the annealed samples at 600 C is much stronger than those at 475 C. It can be concluded that the first transition temperature at 472 C in DSC curve corresponds to the process of the nucleation and growth of a-Fe and FeeB primary crystalline phases while the other transition temperature at 512 C can be related with the complete transformation to a-Fe and FeeB alloys. The crystallization temperatures of two phases observed in this work are in good agreement with those of Fe-based amorphous materials reported [5,6,31]. According to the above analysis, the addition of surfactant PVP and NaOH in the reaction system exert different influences on the compositions of the amorphous samples prepared under corresponding reaction conditions. The surface element states of Fe, B and O atoms in the four samples prepared under different reaction conditions have been analyzed by XPS narrow-scan spectrums shown in Fig. 5. The obviously different surface compositions and distribution of B element of the four samples are observed due to different reaction conditions. It can be seen from Fig. 5.1 that B 1s spectrums for the three samples prepared without NaOH addition exhibit the very similar curves and peak locations, which indicates the same B element state in these samples and has been proved to be the surface B concentration [21]. It is noteworthy that there is obvious positive shift of XPS peak locations by 0.8e1 ev for the sample prepared with NaOH addition, which demonstrates the presence of B element in an alloyed state in this sample [32]. It can be found from Fig. 5.2 that the Fe 2p spectrums for the four samples exhibit similar curve shapes but with different bump locations, which illustrates the different Fe element states in these samples. The two peaks with Fe 2p1/2 at about 719.6 eV and Fe 2p3/2 at about 706.9 eV in all the spectrums indicate a metallic state of Fe atoms. The other two characteristic peaks observed with Fe 2p1/2 at about 723.4 eV and Fe 2p3/2 at about 710.0 eV for the samples prepared in the aqueous solution without NaOH addition demonstrate the formation iron oxides of Fe3O4 [33]. While for the samples prepared with NaOH (0.06 g) addition in NaBH4 solution and the only addition of PVP (1 g) in ethanol-water solvent, the peak intensity corresponding to metallic Fe is much stronger, indicating that slighter oxidization occurs on the surface of the samples. Furthermore,
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Fig. 5. XPS narrow-scan spectrums of (1) B 1s, (2) Fe 2p and (3) O 1s for the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
another broad bump at about 715.3 eV between the Fe 2p1/2 at about 723.4 eV and Fe 2p3/2 at about 710.0 eV is observed. The two peaks along with this bump are consistent with Fe2þ ions [34]. The surface oxidation state of the four samples can be also confirmed by XPS spectrums of O 1s in Fig. 5.3 with slight shift of peak locations. It can be concluded that the surface oxidation of the amorphous Fe nanoparticles can be controlled effectively by adjusting the reaction conditions. Fig. 6 shows the room-temperature hysteresis loops of the four samples prepared at different reaction conditions. It can be found from Fig. 6 that all the hysteresis loops exhibits typical ferromagnetic characteristics with good intrinsic magnetic properties. The values of saturation magnetization (Ms) and coercivity (iHc) for
these samples are also listed in Table 1. The variations of saturation magnetization for these samples can be mainly ascribed to their different surface oxidation and B concentration. The Ms value of 168.0 A m2/Kg for the sample prepared with no PVP addition is much higher than those of the three samples prepared with PVP addition, which can be resulted from the high concentration of Fe and the formation of ferrimagnetic Fe3O4 in the surface of this sample. The amorphous sample prepared with no PVP addition in ferrous aqueous solution show a relative higher iHc value of 315.6 Oe among the four samples, which can be mainly ascribed to magnetic shape anisotropy caused by their chain-like morphologies. As can be seen, due to higher concentration of B atoms and the formation of amorphous FeeB phases in the sample prepared with NaOH addition, much lower Ms value is obtained in this samples. Furthermore, the value of Ms decreases significantly for the samples prepared in mixed ethanol-water solvent, which can be resulted from their much smaller particle sizes. As discussed above, high-performance nanosized amorphous Fe particles with controlled structure, good thermal stability and improved intrinsic magnetic properties can be prepared by a controllable chemical reaction. The relatively simple chemical method, low cost and enhanced performances for these amorphous Fe nanoparticles show great potential applications for multifunctional magnetic devices. 4. Conclusions
Fig. 6. Magnetic hysteresis loops of the samples prepared under different conditions: (a) the only addition of PVP (1 g) in ferrous aqueous solution; (b) no PVP addition in ferrous aqueous solution; (c) the combined addition of PVP (1 g) in ferrous aqueous solution and NaOH (0.06 g) in NaBH4 solution; (d) the only addition of PVP (1 g) in ethanol-water solvent with the volume ratio of 1:1.
The nanosized amorphous Fe particles with controlled structure and improved performances have been prepared by a controllable chemical reduction reaction through adjusting the addition of surfactant PVP, the properties of solution acidalcaline and reaction solvent compositions. The addition of surfactant PVP promotes the formation of fully amorphous large-grain iron nanoparticles with narrow size distribution and surface B concentration. The sample
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prepared with NaOH addition exhibits a two-stage crystallization process and results in the formation of amorphous FeeB phases in the samples instead of surface concentration, which proves their good thermal stability. Amorphous iron nanoparticles with diameter of about 20 nm and slighter surface oxidization are obtained in ethanol-water solvent. High values of saturation magnetization up to138e140 A m2/Kg due to the high concentration of Fe atoms and slight surface oxidation are obtained in the samples prepared with PVP addition in the aqueous solution. The as-synthesized sample prepared without PVP addition reaches higher saturation magnetization of 168.0 A m2/Kg because of their partial crystallization and higher Fe concentration. Meanwhile, the relatively higher iHc value of 315.6 Oe is also observed in this sample due to the magnetic shape anisotropy caused by their chain-like morphologies. Acknowledgments This work was supported by Beijing Natural Science Foundation under Grant No. 2132039 and the National Nature Science Foundation of China under Grant No. (51101007, 51171007 and 51271009). References [1] A.H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222e1244. [2] D.L. Huber, Synthesis, properties, and application of iron nanoparticles, Small 1 (2005) 482e501. [3] D.C. Jiles, Recent advances and future directions in magnetic materials, Acta Mater. 51 (2003) 5907e5939. [4] T. Hyeon, Chemical synthesis of magnetic nanoparticles, Chem. Commun. (2003) 927e934. [5] F.L. Kong, C.T. Chang, A. Inoue, E. Shalaan, F. Al-Marzouki, Fe-based amorphous soft magnetic alloys with high saturation magnetization and good bending ductility, J. Alloy. Compd. 615 (2014) 163e166. [6] J.H. Zhang, C.T. Chang, A. Wang, B.L. Shen, Development of quaternary Febased bulk metallic glasses with high saturation magnetization above 1.6 T, J. Non Cryst. Solids 358 (2012) 1443e1446. [7] C. Suryanarayana, A. Inoue, Iron-based bulk metallic glasses, Int. Mater. Rev. 58 (2013) 131e166. [8] Y.R. Zhang, R.V. Ramanujan, A study of the crystallization behavior of an amorphous Fe77.5Si13.5B9 alloy, Mater. Sci. Eng. A 416 (2006) 161e168. [9] J.H. Yao, J.Q. Wang, Y. Li, Ductile Fe-Nb-B bulk metallic glass with ultrahigh strength, Appl. Phys. Lett. 92 (2008) 251906-1e251906-3. [10] J.X. Fang, U. Vainio, W. Puff, R. Würschum, X.L. Wang, D. Wang, M. Ghafari, F. Jiang, J. Sun, H. Hahn, H. Gleiter, Atomic structure and structural stability of Sc75Fe25 nanoglasses, Nano Lett. 12 (2012) 458e463. [11] M.E. Mchenry, D.E. Laughlin, Nano-scale materials development for future magnetic applications, Acta Mater. 48 (2000) 223e238. [12] T. Fan, D. Pan, H. Zhang, Study on Formation mechanism by monitoring the morphology and structure evolution of nearly monodispersed Fe3O4 submicroparticles with controlled particle sizes, Ind. Eng. Chem. Res. 50 (2011) 9009e9018. [13] O. Crisan, K. von Haeften, A.M. Ellis, C. Binns, Structure and magnetic
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