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; M Farhan Mehmood; Zoran Samardžija; Richard Stuart Sheridan; Muhammad Awais; Allan Walton; Saso Sturm; Spomenka Kobe; Kristina Žužek Rožman

doi:10.3390/ma12091498

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

Materials (Basel). 2019 May 8;12(9):1498. doi: 10.3390/ma12091498.

Authors

Awais Ikram^{1

2}, M Farhan Mehmood^{3

4}, Zoran Samardžija⁵, Richard Stuart Sheridan⁶, Muhammad Awais⁷, Allan Walton⁸, Saso Sturm^{9

10}, Spomenka Kobe^{11

12}, Kristina Žužek Rožman^{13

14}

Affiliations

¹ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. rana.awaisikram@yahoo.com.
² Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia. rana.awaisikram@yahoo.com.
³ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. farhan@ijs.si.
⁴ Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia. farhan@ijs.si.
⁵ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. zoran.samardzija@ijs.si.
⁶ School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. R.S.Sheridan.1@bham.ac.uk.
⁷ School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. m.awais@bham.ac.uk.
⁸ School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. A.Walton@bham.ac.uk.
⁹ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. saso.sturm@ijs.si.
¹⁰ Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia. saso.sturm@ijs.si.
¹¹ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. spomenka.kobe@ijs.si.
¹² Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia. spomenka.kobe@ijs.si.
¹³ Department for Nanostructured Materials, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. tina.zuzek@ijs.si.
¹⁴ Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia. tina.zuzek@ijs.si.

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 (H_Ci) 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 H_Ci peaked to 1407 kA/m with the thermal treatment at 750 °C for 1 h. The obtained H_Ci 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.

Grants and funding

674973/Horizon 2020 Framework Programme