Research articlesMagnetic-field-enhanced reactive synthesis of MnBi from Mn nanoparticles
Introduction
The Mn-Bi alloys have attracted continuous theoretical and practical research interests because of their large coercivity (Hc), unusual positive temperature coefficient of coercivity, and great potential as rare-earth free magnets since the systematic studies made by Thielmann and Guillaud in the 1940’s [1], [2]. The bulk MnBi magnets were usually prepared by sintering the compacted MnBi powders at certain temperatures while the MnBi micro-powders were usually prepared by melting Bi and Mn metals followed by grinding, annealing, and ball milling [3], [4], [5]. The MnBi powders showing large coercivity usually have fine grains, which can be obtained by melt spinning and/or ball milling [6], [7]. Several attempts to prepare MnBi nanoparticles (NPs) have also been made. The MnBi NPs embedded in Bi matrix were prepared by melt-spinning of Bi rich Mn-Bi alloys [8]. Anisotropic MnBi NPs with size in the range 100–300 nm had been fabricated from a mixture of Bi2O3 and Mn in the presence of Ca as reducing element by a mechanochemical process [9]. MnBi particles were also prepared by annealing the mixture of Mn and Bi NPs synthesized using a wet chemical reduction process, in which MnCl2, Bi(NO3)3, and some other solvents and reducing agents were used [10]. A metal-Redox method using a number of chemicals, including Mn2(CO)10, bismuth neodecanoate, 1-octadecene, toluene, etc., was employed to synthesize colloidal MnBi NPs [11]. In this work, the Mn NPs prepared using a simple physical arc discharge process were used as precursors for the preparation of MnBi NPs in large scale. Bulk MnBi was prepared by reactive sintering of the high pressure compacted mixture of the Mn NPs and Bi.
The magnetic field has substantial effects on the microstructure and magnetic properties of MnBi during growth [12], [13], [14], [15], [16]. The MnBi crystals are usually grown preferentially along the direction of field when solidified under a magnetic field [17]. Grain alignment due to magnetic-field annealing in MnBi:Bi nanocomposites has been observed also [18]. The magnetic field effects on reactive sintering of MnBi at temperatures between 523 K and 553 K have been studied systematically by Mitsui and co-authors. [14], [15], [16]. The size of the Mn powders used for reactive sintering of MnBi ranges from several micro-meters to larger than 75 μm [14], [15], [16]. It was found that high magnetic fields dramatically enhanced the formation of MnBi phase [14]. The low sintering temperature employed to keep the low temperature phase MnBi stable is not favorable for atomic diffusion and the reactions between Mn and Bi. The large particle size of the precursor powders may also lower the reaction rate of Mn and Bi. A well mixing of Mn and Bi in nanoscale can to some extent improve the reaction kinetics [10]. Moreover, the grain size of the MnBi powders formed after sintering is dependent on the size of the precursor powders, and this may affect the magnetic properties of the products. In this work, Mn NPs prepared by arc discharge method were employed for reactive sintering of MnBi NPs and bulk.
Section snippets
Experiments
The precursor Mn nanoparticles were prepared by the traditional arc discharge method, in which high purity Mn metal served as the anode and a tungsten needle served as the cathode [19]. After the arc discharge process, the product in the chamber was passivated for 12 h. The Mn powders deposited on the water-cooled chamber were collected in air and used for subsequent reactive sintering. The commercially bought Bi powders were mixed with the Mn NPs in a nominal composition of Mn55Bi45. The
Results and discussion
Fig. 1 shows the XRD patterns of the powders obtained by sintering the mixture of Mn NPs and Bi powders under zero field for 8 days at 573 K and 606 K, respectively. The XRD patterns of both samples could be indexed with MnBi and Bi. The diffraction intensity of Bi at 2θ ∼ 27° is comparable to that of the MnBi at 2θ ∼ 28° for samples annealed at 573 K. However, the diffraction intensity of Bi is lower than that of the MnBi obtained at 606 K, indicating a higher fraction of MnBi in the sample
Conclusions
The isotropic MnBi NPs were prepared by using reactive sintering of Mn NPs and Bi under zero-field, while the anisotropic bulk MnBi were prepared from Mn NPs by using field-assisted sintering. The fraction of MnBi in the zero-field sintered NPs is higher than 68.6 wt%. The large surface area and small size of the Mn NPs enhanced the formation of ferromagnetic phase during sintering and the magnetic performance of the MnBi NPs, which show a coercivity (Hc) up to 0.8 T and a remanent
Acknowledgements
We thank the National Natural Science Foundation of China (Nos. 11074227, 51671177), China and the Future Materials Discovery Program through the National Research Foundation of Korea (No. 2016M3D1A1027835), South Korea funded by the Ministry of Science and ICT (2016M3D1A1027835).
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