Particle size dependent sinterability and magnetic properties of the recycled HDDR Nd-Fe-B powders consolidated with Spark Plasma Sintering

Research output: Contribution to journalArticle

Authors

  • Awais Ikram
  • Farhan Mehmood
  • Anas Eldosouky
  • Saso Sturm
  • Spomenka Kobe
  • Kristina Zuzek Rozman

Colleges, School and Institutes

External organisations

  • Jozef Stefan Institute, Ljubljana, Slovenia
  • Jožef Stefan Institute

Abstract

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 recycled HDDR 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 was 595 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 dense magnets 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) was higher 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 full densification (> 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.

Details

Original languageEnglish
JournalJournal of Rare Earths
Publication statusAccepted/In press - 14 Feb 2019

Keywords

  • Reprocessing End-of Life Scrap, Rare Earth Permanent Magnets, HDDR, Nd2Fe14B, 38 Recycling, Spark Plasma Sintering, Coercivity