Nd 2 Fe 14 B/FeCo Core-Shell Nanoparticle Synthesis Using Galvanic Substitution Based Electroless Plating

: Core-shell structured magnetic nanoparticles combine hard and a soft phases to improve energy efficiency. The mutual interaction of the two phases can lead to the exchange spring effect leading to higher magnetic energy. In this regard, synthesis of Nd 2 Fe 14 B based core-shell structured powders have proven to be elusive due to the relatively reactive nature of this phase. In this study, a process has been established for successfully coating the surface of Nd 2 Fe 14 B powders with FeCo layer using galvanic displacement method. Initially, a binary phase magnetic powder was synthesized containing Nd 2 Fe 14 B and Nd 2 Fe 17 phase. Subsequently, the powders were coated using a Co precursor at 303 K. During coating the metastable Nd 2 Fe 17 phase was dissolved and the Fe ions were released into the solution. Subsequently, the Fe ions deposited together with the Co ions on the surface of Nd 2 Fe 14 B powder to form a FeCo shell. The deposited layer thickness and composition was confirmed using TEM analysis.


Introduction
Core-shell structured powders are a special class of materials having multifunctional properties [1][2][3][4][5][6]. This unique powder structure allows a convenient superimposition of the core and shell phase properties [5,6]. Owing to their high functionality, core-shell structured powders have been developed for a wide range of applications including magnets [7][8][9][10][11], semiconductors [12], organic/inorganic composites [13,14], and metal matrix composites [5,15]. These specialized materials have allowed the researchers to go beyond the property limits of traditional single phase materials [5]. In particular, core-shell structured magnetic nanoparticles have the potential to achieve enhanced magnetic energy product [8]. This enhancement effect, known as the exchange-spring magnetic effect, is caused by the mutual interaction of hard and soft magnetic phase [16]. Exchange-spring effect has been successfully demonstrated on several hard magnetic systems including Nd-Fe-B [7], Sm-Co [8], Fe-Pt [10] and various ferrites [11]. These researches give ample evidence for the immense potential of spring magnets as highly energy efficient materials.
Theoretically, the Nd2Fe14B based magnets are the most suitable material system for obtaining spring magnets. Atomistic studies and finite element simulations have revealed the superior potential of the Nd2Fe14B based core-shell powders in realizing the maximum magnetic energy [17,18]. However, given the complexity of the Nd2Fe14B crystal structure, the fabrication of such powder with a uniform soft magnetic coating is challenging. Therefore, despite the encouraging results in other magnetic systems, no such successful attempt obtaining core-shell structured Nd2Fe14B powder has been reported. In this regard, there is an imminent need for developing a coating technique for obtaining an Nd2Fe14B powder coated with a soft magnetic shell.
To that end, here we report the development of a novel process for synthesizing Nd2Fe14B powder coated with a FeCo layer. Initially a nanostructured Nd2Fe14B powder containing a metastable Nd2Fe17 phase was fabricated by electrospinning, calcination and reduction of Nd, Fe and B precursors. Afterwards the electroless plating of the prepared powder was done in a Cobalt (Co) bath. During the plating process, the galvanic substitution of the metastable Nd2Fe17 phase was done to ionize the Fe ions while preserving the Nd2Fe14B phase. Concomitantly, the Fe ions were co-deposited along with Co on the surface of Nd2Fe14B. Finally, the formation of the FeCo layer was confirmed.

Materials and Methods
The starting Nd2Fe14B powder was synthesized via electrospinning, calcination, reduction and washing process. Initially fibers were obtained by electrospinning of a PVP solution containing Nd, Fe and B precursors in an electrospinning machine (NSLAB, Elmarco). The resultant fibers were calcined in air to obtain Nd-Fe-B oxide fibers and remove the PVP. Afterwards the oxide fibers were reduced in an argon atmosphere using Calcium (Ca) granules at 1123 K. Finally, the reduced fibers were broken down into powder during washing. The detailed description of each step can be found elsewhere [19].
The phase change, microstructure and composition of the powders were analyzed using X-ray diffraction analyzer (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscope (TEM). The shell formation mechanism was analyzed through primary particle analysis. The average particle size and deviation were calculated by analyzing 50 particles using the ImageJ program.

Results
The morphological changes during each step of Nd2Fe14B powder synthesis are shown in Fig. 1. After the electrospinning step, the obtained polymeric fiber had a smooth surface with a diameter of around 750 ± 50 nm ( Fig. 1 (a)). Upon calcination, the diameter of the fibers reduced to 550 ± 100 nm as the water and organic constituents were removed ( Fig. 1(b)). A high degree of porosity is visible in the calcined oxide fibers. The polymer to precursor ratio used for electrospinning was kept high to induce porosity within the fibers after calcination [20]. As a result, these fibers can be easily converted into nanosized powder during the subsequent processing. Figure 1 (c) shows the fiber morphology after reduction treatment in the presence of Ca granules. The Ca reacts with the oxide fibers and deposits on the fiber surface and within the pores in the form of CaO. The diameter at this stage increased to 900 ± 200 nm. Finally, the reduced sample was washed in an NH4Cl solution to remove the CaO from the sample. The fibrous structure broke down during the washing step and nanosized powder was obtained. Fig. 1 (d) shows the resultant powders obtained by using ultrasonication of the washed sample. The finally obtained powder had an average particle size of 252 ± 38 nm. However, the nanosized powder particles were present in the form of agglomerates. Table 1 shows the fiber shape diameter information and the particle size analysis results of the nanocomposite powder.  The powder obtained after washing and ultrasonication was pretreated in Sn and Pd solutions for sensitization and activation respectively. This treatment is necessary for enhancing the deposition rate by activating the powder surface [21]. Figure 2 (a) shows the SEM and EDS mapping of the pretreated powder. The powder size and morphology of the pretreated powder remained similar to the as synthesized powder. The powder size was measured to be around 249 ± 32 nm. The EDS maps in Fig. 2(a) show a relatively uniform distribution of Pd while some evidence of unreacted Sn can be observed. This can be caused by inherent agglomeration of the nanosized powder which provides pockets for unreacted Sn deposition [8].
After the pretreatment, the resultant powders were treated in Co bath for obtaining FeCo coating on the powder surface. After 10 minutes of plating, the process was stopped and the powders were filtered, rinsed and dried. The Fig. 2 (b) shows the SEM and EDS mapping of the powder after electroless plating step. The size of the coated powder was measured to be 254±47 nm. However, the difference in size between pre-coated and coated powder was statistically insignificant. The EDS analysis confirmed a uniformly distributed presence of Co on the powder surface. This confirmed the successful plating of Co  The XRD analysis of the washed, pretreated and plated sample was carried out to confirm the phase composition of the powder. The XRD patterns are shown in Fig. 3. The XRD pattern of the as-prepared powder consisted of a main Nd2Fe14B phase along with secondary phases of Nd2Fe17 and α-Fe. The presence of secondary phase is inherent within the adapted synthesis procedure. After the calcination step, the fiber is made up of a Fe2O3, FeNdO3 and NdBO3 composite [22]. During the reduction, the composite oxide reduce together to form Nd2Fe14B. However, by reducing the boron content within the oxide fibers, the presence of Nd2Fe17 and α-Fe phases can be induced. The primary α-Fe phase peak can be clearly be seen at the 44.7 o . The presence of Nd2Fe17 peaks is however difficult to establish visually due to the presence of strong overlap with the Nd2Fe14B peaks. In the XRD pattern of the sensitized and activated powder, the α-Fe peaks were greatly reduced. The low pH of 1-2 used during pretreatment caused the dissolution of excess α-Fe while only Nd2Fe14B and Nd2Fe17 peaks remain. Finally, in the XRD patterns obtained after plating shows a reappearance of peak at 44.7 o . This peak is presumed to be of the deposited FeCo phase as no separate Co peaks were observed alongside. The quantitative analysis of the FeCo plated XRD pattern confirmed a greatly diminished Nd2Fe17 content in the final powder.

Process As spun Calcined Reduced Washed Pre-treated Deposited
As the FeCo phase and α-Fe have a similar XRD pattern, it is difficult to distinguish between the two to confirm the composition of the coated phase. The TEM analysis of the powder surface was conducted to further verify the elemental composition and observe the layer thickness. Figure 4 shows TEM image superimposed with an EDX line map of electroless plated Nd2Fe14B powder. A thin layer of under 10 nm thickness can be observed at the powder surface. There is a clear interface present between the core and the shell phase. The EDX line map confirmed the presence of Fe and Co within the shell. In core region, the Nd and Fe concentrations become dominant. This confirms the formation of a core-shell structure formation between hard and soft magnetic phases. The thickness of the soft phase is sufficiently thin for developing exchange spring magnetic effect [16].
The formation of FeCo layer as well as the preservation of the Nd2Fe14B phase confirms that the galvanic substitution progressed at the expense of Nd2Fe17 during the plating process. The reported results confirm the possibility of obtaining soft phase coating on Nd2Fe14B permanent magnetic powder by careful design of the reaction process. Based on these promising initial results, additional fine tuning will be carried out to attain a powder with ideal exchange spring magnetic effect. The content of the metastable phase needs to be adjusted to increase the Fe concentration within the shell phase. Additionally, the agglomeration of the nanosized powder needs to be controlled for obtaining a thorough coating of the shell phase around each individual Nd2Fe14B particle.

Conclusions
Electroless plating of a soft magnetic FeCo phase on a hard magnetic Nd2Fe14B nanoparticles has been successfully reported. The starting nanocomposite hard magnetic powder was synthesized using a modified electrospinning technique. The powder consisted of a primary Nd2Fe14B phase along with a secondary phase of metastable Nd2Fe17 phase. The starting powder surface was then subjected to sensitization and activation treatment followed by electroless plating in a Co bath. During the electroless plating, the Nd2Fe17 phase preferentially ionized leaving behind only Nd2Fe14B powder. Consequently, the ionized Fe from the Nd2Fe17 phase co-deposited along with Co on to the surface of Nd2Fe14B powder to form a soft magnetic shell. The XRD and TEM analysis confirmed that the deposited shell was of FeCo phase while the core was of Nd2Fe14B phase. The thickness of the shell was measured to be under 10 nm. The designed process opens up a vital avenue for developing Nd2Fe14B based spring magnetic materials which can possess unmatched magnetic energy product. Further research needs to be conducted for tuning the structural and compositional features of the shell to obtain optimal magnetic properties.