Using a grain-boundary diffusion process (GBDP) involving the electrophoretic deposition (EPD) of submicron TbF3 powder, we substantially increased the coercivity of sintered Nd-Fe-B permanent ...magnets. The experiments used magnets with low heavy-rare-earth (HRE) content (HRE = 1.2 wt%) and a coercivity of 790 kA/m (at 75 °C). After experiencing optimized conditions at 875 °C for 10 h and subsequent annealing at 500 °C for 1 h, the coercivity was increased to 1536 kA/m (at 75 °C). This value is 1.94 times higher than that for a sintered magnet, without post-sintering heat treatment. Furthermore, a vibration test revealed satisfactory adhesion of the TbF3 powder to the surface of the magnet with no detected reduction in coercivity. Using field emission gun scanning electron microscopy (FEG-SEM) with an energy dispersive spectroscope (EDS), we confirmed the formation of various secondary intergranular phases and the core-shell-type microstructure, which increases the coercivity. The Tb content in the magnet, exposed to the EPD-based GBDP, was controlled by inductively coupled plasma optical electron spectroscopy (ICP-OES). The additional Tb detected in the magnet due to the described technology was only 0.12 wt%.
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•EPD-modified diffusion process on Nd-Fe-B ribbons was applied for the first time.•8% coercivity-improvement was measured, while remanence remained unchanged.•Thin and dry grain ...boundaries were crucial limits for the diffusion efficiency.•The lack of intergranular phase in Nd-Fe-B melt-spun ribbons was discussed.
Grain boundaries in rapidly quenched Nd-Fe-B melt-spun ribbons are of major importance for achieving superior coercivity. Using the so-called grain boundary diffusion process (GBDP) initiated by the electrophoretic deposition (EPD) of DyF3, we have improved the coercivity for 8%, while the remanence remained almost unchanged. Coercivity-improvement is attributed to higher magnetocrystalline anisotropy of Dy-rich regions, which were detected in outer parts of the Nd–Fe–B grains. Electron microscopy study revealed significant limitation hindering the diffusion efficiency; thin and dry grain-boundaries.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In a novel process to enhance the coercivity we have electrophoretically deposited DyF3 powder onto the surface of an as-sintered Nd–Fe–B magnet as the initial step in the grain-boundary diffusion ...process. After a conventional heat treatment at 850 and 500 °C the coercivities were higher than in the case of simple dipping after a typical 10 h, with Hci values exceeding 1600 kA/m for a 200 μm-thick deposited layer. The electrophoretic deposition (EPD) process is quick, easily controllable in terms of thickness and can be used to deposit the rare earth fluoride powder on the surface of complex and irregularly shaped magnets. Since the amount of deposited powder can be tailored to maximise the coercivity while minimising the quantity of expensive heavy rare earth there is no wasted powder, making the diffusion process, which takes place after the sintering process, more environmentally friendly and potentially cheaper than conventional dipping.
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► A novel technique to enhance coercivity via the GBDP. ► Controllable layer thickness for heavy rare earths. ► Maximised coercivity with minimum losses of material. ► Environmentally friendly and low-cost.
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•Nd-Fe-B hot-deformed magnets were produced by low pressure spark plasma sintering (SPS).•Low pressure SPS gave comparable magnetic properties than the conventional process.•Cone-like ...shape of the magnet was observed in the initial stages for the first time.•The experiments were perfectly interpreted in the frame of Stoner-Wohlfarth model.
We have produced hot-deformed Nd-Fe-B magnets from commercial (MQU-F) Nd-Fe-B ribbons. The spark-plasma-sintering technique was used to deform the samples under low pressures of 40 MPa. The initial stages of the hot-deformation process were investigated in terms of microstructures and magnetic properties. Hot-deformed magnets with different deformation ratios were produced and the dependence of the remanence on the deformation ratio was determined. In the initial stages of the hot-deformation process, a cone-like shape of the magnet was observed for the first time. The experimental data can be qualitatively interpreted by applying the basic Stoner–Wohlfarth model.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Revealed dominant core-shell mechanism for grain boundary diffusion processed permanent magnet.•A structure-chemistry-magnetic-property analysis of Nd-Fe-B magnet used for electric vehicles and wind ...turbines.•Gaining high efficiency of Nd-Fe-B via grain boundary engineering.•Importance of high coercivity permanent magnets for production of electric components on a macro and nanoscale.
We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Anisotropic sintered magnets based on the Nd2Fe14B phase doped with Tb were prepared using a grain-boundary diffusion process (GBDP) in order to enhance their coercivity. A FEGSEM microstructural ...analysis revealed that these GBDP magnets had a core-shell structure, where thin, Tb-rich, (NdTb)2Fe14B shells are formed on the original matrix Nd2Fe14B grains after diffusion of the Tb. This shell thickness varies from a few tens of nanometres in the middle of the magnet up to a few micrometers near the edge. The exact chemical composition of these shells was determined using EDS and WDS electron-probe microanalyses, which were modified and optimized for submicrometer scale analyses. When analyzing the common Nd–Lα, Tb–Lα and Fe–Kα lines a mutual multiple overlap in the EDS spectra is present and, as a result, an accurate quantitative analysis was only feasible when using WDS. Using this technique we were able to achieve a lateral analytical resolution of 0.4μm. A further improvement in resolution, down to 0.15μm, was realized with a dedicated set-up using low-voltage EDS, analyzing the “atypical” low-energy Nd–Mα, Tb–Mα and Fe–Lα lines. Quantitative analyses confirmed that the reaction phase (NdxTb1−x)2Fe14B is formed after the diffusion of Tb with the equilibrium concentration of Tb being equal to x≈0.5, i.e., with the atomic ratio of Nd/Tb equal to 1/1. We also found that a relatively sharp Tb concentration gradient from the shell to the core occurs within a length of ≈0.5μm, while the Fe concentration remains unchanged. In terms of magnetic properties, the Tb-doping significantly increased coercivity by ≈30% while the remanence remained at the same value as in the undoped Nd–Fe–B.
► Nd–Fe–B sintered magnets were doped with Tb using grain-boundary diffusion process. ► A tiny core-shell reaction phase was formed around the Nd2Fe14B matrix grains. ► EDS and WDS analyses confirmed (Nd0.5Tb0.5)2Fe14B equilibrium shell composition. ► Coercivity of Tb-doped Nd–Fe–B increases by 30% without a drop in remanence.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In this work, we present a newly developed, economically efficient method for processing rare-earth Nd-Fe-B magnets based on spark plasma sintering. It makes us possible to retain the technologically ...essential properties of the produced magnet by consuming about 30% of the energy as compared to the conventional SPS process. A magnet with anisotropic microstructure was fabricated from MQU F commercial ribbons by low energy consumption (0.37 MJ) during the deformation process and compared to the conventionally prepared hot-deformed magnet, which consumed 3-times more energy (1.2 MJ). Both magnets were post-annealed at 650 °C for 120 min in a vacuum. After the postannealing process, the low-energy processing (LEP) hot-deformed magnet showed a coercivity of 1327 kAm-1, and remanent magnetization of 1.27 T. In comparison, the highenergy processing (HEP) hot-deformed magnet had a coercivity of 1337 kAm-1 and a remanent magnetization of 1.31 T. Complete microstructural characterization and detailed statistical analyses revealed a better texture orientation for the HEP hot-deformed magnet processed by high energy consumption, which is the main reason for the difference in remanent magnetization between the two hot-deformed magnets. The results show that, although the LEP hot-deformed magnet was processed by three times lower energy consumption than in a typical hot-deformation process, the maximum energy product is only 8 % lower than the maximum energy product of a HEP hot-deformed magnet.
Permanent magnets are vital components in the rapidly-developing renewable energy sector, where the motors for electric vehicles and the generators in wind turbines require strong magnets with the ...ability to operate at temperatures well over 100°C. To achieve high coercivity, remanence and consequently high energy product at elevated temperatures the addition of heavy rare earth (HRE) to the basic Nd-Fe-B composition is needed. To minimize the amount of critical elements such as dysprosium or terbium the grain boundary diffusion process is applied on sintered magnets. Instead of using dipping in slurry of HRE or expensive sputtering a new chemical process was invented - electrophoretic deposition (EPD).
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