Limits of grain boundary engineering in nanocrystalline Nd-Fe-B melt-spun ribbons
Graphical abstract
Introduction
Magnetic performance to weight ratio makes Nd-Fe-B polymer-bonded magnet indispensable in automotive applications [1], [2], [3]. As the magnetic powder is blended with a non-magnetic binder, the remanent magnetization is diluting as the volume percent of the binder is increasing. Therefore, they are classified as medium-performance magnets [4]. However, the coercivity of the Nd-Fe-B magnet is not related to the magnetic powder/non-magnetic binder ratio but to the chemistry and microstructural features in Nd-Fe-B alloy [5]. In automotive applications, operating temperatures typically exceeds 150 °C; thus the coercivity must withstand such temperatures [6], [7]. Nd-Fe-B melt-spun ribbons are composed of randomly oriented Nd2Fe14B grains within the size of the single magnetic domain, i.e. 0.3 µm [8], [9]. Therefore, they have a higher potential for increase in coercivity than sintered Nd-Fe-B magnets in which typical grain size is 5–10 µm [10], [11]. Besides grain-size refinement, there are other options to influence the coercivity of Nd-Fe-B magnets [12].
First is to sufficiently decouple the Nd2Fe14B grains by uniformly few nanometers thin Nd-rich phase through the complete or through the pseudopartial GB wetting [13], [14]. Second is by infiltration of low eutectic temperature Nd-based alloys, which typically causes thicker Nd-rich grain boundary phase [15]. Nd-based alloy is infiltrated along the grain boundaries (GB) which prevent physical contact between the grains, leading to weaker intergrain exchange coupling. Third option is the grain–boundary diffusion process (GBDP), where heavy rare earth (HRE)-rich shells are formed in outer parts of grains [16]. HRE-rich regions have higher magnetocrystalline anisotropy, therefore they protect magnetically weak regions.
In our previous reports, we have shown that the coercivity of the sintered Nd-Fe-B magnet can be drastically improved by the electrophoretic deposition (EPD)-modified GBDP [11], [17]. EPD offers many advantages over a simple dip-coating in terms of cost, ease of application, control and uniformity of the coating [18], [19]. Therefore, we have applied our highly efficient EPD-modified GBDP to Nd-Fe-B commercial ribbons with different amounts of DyF3 powder. We investigated how the quantity of DyF3 reflects the coercivity behavior. Transmission electron microscope (TEM) analyses were employed to understand the diffusion mechanism in melt-spun ribbons and to locate the Dy-rich regions.
Section snippets
Experimental
Commercially available HRE-free Nd-Fe-B melt-spun ribbons (typically 40 µm) were coated with DyF3 powder via EPD. The EPD was carried out at 20 V in ethanol-based DyF3 suspension as indicated in Fig. 1 (a). Different deposition times were used and consequently, various amounts of DyF3 powder were deposited to the surface of the ribbons. Coated ribbons were thermally treated in a vacuum furnace at 650 °C or 700 °C for 15 min, 30 min, or 60 min to enable the diffusion along the grain boundaries
Results and discussion
Coercivities of Nd-Fe-B ribbons treated with various amounts of DyF3 are shown in Fig. 1 (b). Fig. 1 (c) shows the comparison of demagnetization curves of commercial Nd-Fe-B melt-spun ribbons and EPD-modified GBDP ribbons with the highest achieved coercivity, µ0Hci = 2.19 T. Based on magnetic results, the optimum experimental parameters were 0.5 wt% of DyF3 and heat-treatment at 650 °C for 60 min, followed by rapid cooling. No coercivity-enhancement was observed on samples treated at 700 °C,
Conclusions
We have applied EPD-modified GBDP with DyF3 on Nd-Fe-B melt-spun ribbons. The improvement of the µ0Hci was 8%, which is not as high as in the case of EPD-modified GBDP with DyF3 powder in Nd-Fe-B sintered magnets (30%). Comparative investigations of grain-boundaries in sintered Nd–Fe-B magnets and Nd-Fe-B ribbons revealed the limits of the grain boundary engineering. Thin and dry grain boundaries in Nd-Fe-B ribbons appeared to be the crucial limiting factor for the diffusion efficiency. Unlike
CRediT authorship contribution statement
Marko Soderžnik: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Visualization, Writing - original draft, Writing - review editing. Bojan Ambrožič: Data curation, Formal analysis, Investigation, Writing - review editing. Kristina Žagar Soderžnik: Data curation, Formal analysis, Investigation, Writing - review editing. Matic Korent: Data curation, Formal analysis, Investigation, Visualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
CENN Nanocenter Slovenia is acknowledged for using FIB FEI HeliosNanoLab 650. Authors acknowledge the Postdoctoral Basic Research project ID Z2-7215 and ID PR-08336 which were financially supported by the Slovenian Research Agency.
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