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

  • ️Tue Jan 01 2019

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

Awais Ikram et al. Materials (Basel). 2019.

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.

Keywords: HDDR Nd2Fe14B; coercivity; doping DyF3; rare earth permanent magnets; recycling; spark plasma sintering.

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Conflict of interest statement

The authors hereby declare no conflict of interest.

Figures

Figure 1
Figure 1

SEM backscattered mode images of (a) Hydrogenation disproportionation desorption recombination (HDDR) powder particles, (b) DyF3 nanoparticles, (c) DyF3 grains and (d) recycled HDDR powder blended with DyF3 nanoparticles.

Figure 2
Figure 2

SEM backscattered electron (BSE) images of as-SPS samples with DyF3 in (a) 1 wt.% (b) 2 wt.% and (c) 5 wt.%.

Figure 3
Figure 3

Shows the magnetic properties in DyF3 doped magnets before and after annealing at 600 °C for 6 h, (a) coercivity and (b) remanence.

Figure 4
Figure 4

Variation in the magnetic properties of SPS reprocessed blend of DyF3 doped recycled HDDR powder with thermal treatment temperatures of 900 °C (a) coercivity, (b) remanence; and 750 °C (c) coercivity, (d) remanence.

Figure 5
Figure 5

Shows the microstructure of DyF3 doped samples after thermal treatment for 6 h at 750 °C, (a) 1 wt.% doped samples with two zone microstructure (inset A shows core-shell zone and inset B shows normal HDDR microstructure), (b) at higher magnification 1% doped samples; (c) 2% DyF3 blend samples and (d) uniform core-shell structure formation throughout the microstructure; (e) 5 wt.% DyF3 samples, with excessive growth zone of DyNdFe14B shells at the expense of matrix phase clearly shown in (f).

Figure 6
Figure 6

Signifies the reaction zones (RZ) in the microstructure of doped samples after the thermal treatment at 750 °C for 6 h, (a) relatively small RZ in 1 wt.% DyF3 samples, (b) 2 wt.% doped samples have the optimal magnetic properties but RZ contains RE-F4 (rare earth fluoride) and Nd-O-F2 (oxyfluoride) phases, (c–f) the relatively wider RZs of 5 wt.% doped samples containing additional interphase compounds along with rare earth fluorides and oxyfluoride based Nd-rich phase.

Figure 7
Figure 7

The mechanism of core-shell structure formation in the HDDR Nd-Fe-B system: (a) blend of HDDR Nd-Fe-B powder and DyF3 particels, (b) liquid phase sintering, lack of long range Dy diffusion, (c) Dy diffuses with the liquid phase and F reacts with Nd-rich phases; and (d) the (Dy,Nd)2Fe14B core-shell structures form up upon solidification.

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