Coercivity Increase of the Recycled HDDR Nd-Fe-B Powders Doped with DyF3 and Processed via Spark Plasma Sintering & the Effect of Thermal Treatments

Abstract

The magnetic properties of the recycled hydrogenation disproportionation desorption recombination (HDDR) Nd-Fe-B powder, doped with a low weight fraction of DyF3 nanoparticles, were investigated. Spark plasma sintering (SPS) was used to consolidate the recycled Nd-Fe-B powder blends containing 1, 2, and 5 wt.% of DyF3 grounded powder. Different post-SPS sintering thermal treatment conditions (600, 750, and 900 °C), for a varying amount of time, were studied in view of optimizing the magnetic properties and developing characteristic core-shell microstructure in the HDDR powder. As received, recycled HDDR powder has coercivity (HCi) of 830 kA/m, and as optimally as SPS magnets reach 1160 kA/m, after the thermal treatment. With only 1–2 wt.% blended DyF3, the HCi peaked to 1407 kA/m with the thermal treatment at 750 °C for 1 h. The obtained HCi values of the blend magnet is ~69.5% higher than the starting recycled HDDR powder and 17.5% higher than the SPS processed magnet annealed at 750 °C for 1 h. Prolonging the thermal treatment time to 6 h and temperature conditions above 900 °C was detrimental to the magnetic properties. About ~2 wt.% DyF3 dopant was suitable to develop a uniform core-shell microstructure in the HDDR Nd-Fe-B powder. The Nd-rich phase in the HDDR powder has a slightly different and fluorine rich composition i.e., Nd-O-F2 than in the one reported in sintered magnets (Nd-O-F). The composition of reaction zone-phases after the thermal treatment and Dy diffusion was DyF4, which is more abundant in 5 wt.% doped samples. Further doping above 2 wt.% DyF3 is ineffective in augmenting the coercivity of the recycled HDDR powder, due to the decomposition of the shell structure and formation of non-ferromagnetic rare earth-based complex intermetallic compounds. The DyF3 doping is a very effective single step route in a controlled coercivity improvement of the recycled HDDR Nd-Fe-B powder from the end of life magnetic products.

Optimized Microstructure and Improved Magnetic Properties of Pr-Dy-Al-Ga Diffused Sintered Nd-Fe-B Magnets

“…Meanwhile, Al penetrates into the selected grain facets from the grain boundaries with a high concentration at the grain edges. With a cooling effect coming in place, lighter Al atoms get transported inwards due to the low melting point while matrix restructuring happens, known as core-shell morphology [ 27 ]. Although the core-shells are not obvious, a higher concentration of Pr at the grain boundaries takes precedence of surface diffusion by the substitution of Nd atoms, resulting in the intergranular region becoming richer with Nd and hard phase grains taking composition (Pr, Nd) 2 Fe 14 B.…”

Spark Plasma Sintering as an Effective Texturing Tool for Reprocessing Recycled HDDR Nd-Fe-B Magnets with Lossless Coercivity

“…The SPS operation was performed under 2 × 10 −2 mbar dynamic vacuum. A constant pressure of 40 MPa and relatively slow heating rate of 50 • C/min was maintained up to the peak forging temperature in order to avoid high current density in the cermet dies, which otherwise may induce undesirable microstructural changes (grain growth, Nd-rich GB, and oxide phase transitions) [38]. Under these optimal settings, the heating rate was kept constant until the holding temperature in the range of 700-800 • C was reached, but the pressure in the final 50 • C step was subsequently increased to peak conditions of 100-150 MPa within one minute.…”

Limitations in the Grain Boundary Processing of the Recycled HDDR Nd-Fe-B System

“…The effectiveness of hydrogen gas towards decrepitating (HD) and disproportionation (HDDR) of the end-of-life rare-earth (RE) scrap has convincing been proven in previous studies [ 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. Contrary to the theoretical models, the HDDR Nd-Fe-B system lacks high coercivity as a translation of high magnetocrystalline anisotropy ( µ 0 H a ) ~ 7.2 T because of crystal structure inhomogeneities, grain morphology, surface defects, oxidation, nonferromagnetic grain boundaries and localized exchange interactions at the grain interfaces, which restrict their usability [ 13 , 14 , 15 , 16 , 17 ]. Therefore, rather confining the potential application of the recycled HDDR Nd-Fe-B powders to polymer bonded magnets only [ 15 , 18 , 19 , 20 ], we have recently demonstrated Spark Plasma Sintering (SPS) as a convenient method to practically fabricate bulk magnets with magnetic properties at par with the end-of-life sintered magnets [ 21 , 22 , 23 ] or commercial grade HDDR Nd-Fe-B powder [ 24 ].…”

“…Contrary to the theoretical models, the HDDR Nd-Fe-B system lacks high coercivity as a translation of high magnetocrystalline anisotropy ( µ 0 H a ) ~ 7.2 T because of crystal structure inhomogeneities, grain morphology, surface defects, oxidation, nonferromagnetic grain boundaries and localized exchange interactions at the grain interfaces, which restrict their usability [ 13 , 14 , 15 , 16 , 17 ]. Therefore, rather confining the potential application of the recycled HDDR Nd-Fe-B powders to polymer bonded magnets only [ 15 , 18 , 19 , 20 ], we have recently demonstrated Spark Plasma Sintering (SPS) as a convenient method to practically fabricate bulk magnets with magnetic properties at par with the end-of-life sintered magnets [ 21 , 22 , 23 ] or commercial grade HDDR Nd-Fe-B powder [ 24 ]. Several researchers have further explored the possibilities of improving the coercivity in the HDDR Nd-Fe-B system, with alloying additions [ 25 , 26 , 27 , 28 , 29 ], by mechanical milling [ 30 , 31 , 32 , 33 , 34 ] and via tailoring the disproportionation-desorption-recombination parameters [ 10 , 11 , 35 , 36 , 37 , 38 , 39 , 40 , 41 ].…”