Particle size dependent sinterability and magnetic properties of recycled HDDR Nd–Fe–B powders consolidated with spark plasma sintering

Awais Ikram, Farhan Mehmood, Richard Sheridan, Muhammad Awais, Allan Walton, Anas Eldosouky, Saso Sturm, Spomenka Kobe, Kristina Zuzek Rozman

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The dependence of the magnetic properties on the particle size of recycled HDDR Nd-Fe-B powders were investigated, with the aim to assess the reprocessing of the end-of-life scrap magnets. The as received recycledHDDR powder had coercivity (HCi) = 830 kA/m and particles in the range from 30 – 700 µm (average 220 µm). After burr milling, the average particle size was reduced to 120 µm and subsequently the HCi of fine (milled) powder was595 kA/m. Spark plasma sintering (SPS) was exploited to consolidate the nanograined HDDR powders and limit abnormal grain coarsening. The optimal SPS-ing of coarse HDDR powder at 750 OC for 1 min produced fully densemagnets with HCi = 950 ± 100 kA/m which further increased to 1200 kA/m via thermal treatment at 750 OC for 15 mins. The burr milled fine HDDR powder under similar SPS conditions and after thermal treatment resulted in HCi = 940 kA/m. The fine powder was further sieved down from 630 to less than 50 µm mesh size, to evaluate the possible reduction in HCi in relation to the particle size. The gain in oxygen content doubled for < 50 µm sized particles as compared to coarser fractions (> 200 µm). The XRD analysis for fractionated powder indicated an increase in Nd2O3 phase peaks in the finer (< 100 µm) fractions. Similarly, the HCi reduced from 820 kA/m in the coarse particles (> 200 µm) to 460 kA/m in the fine sized particles (< 100 µm). SPS was done on each HDDR powder fraction under the optimal conditions to measure the variation in HCi and density. The HCi of SPS-ed coarse fraction (> 200 µm) washigher than 930 kA/m and it fell abruptly to just 70 kA/m for the fine sized particles (< 100 µm). The thermal treatment further improved the HCi to > 1000 kA/m only up to 100 µm sized fractions with > 90% sintered density. The fulldensification (> 99%) was observed only in the coarse fractions. The loss of coercivity and lack of sinterability in the fine sized particles (< 100 µm) is attributed to a very high oxygen content. This implies that during recycling, the HDDR powder particles can be sized down only up to ≥ 100 µm.
Original languageEnglish
Pages (from-to)90-99
Number of pages10
JournalJournal of Rare Earths
Issue number1
Early online date23 Jul 2019
Publication statusPublished - Jan 2020


  • reprocessing end-of life scrap
  • rare earth permanent magnets
  • HDDR
  • Nd2Fe14B
  • spark plasma sintering
  • coercivity
  • recycling
  • Spark plasma sintering
  • Rare earth permanent magnets
  • Nd Fe B
  • Reprocessing end-of life scrap
  • Recycling

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • General Chemistry


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