| Transmission Electron Microscopy for efficient recycling of Nd-Fe-B permanent magnets Saso Sturm1; Awais Ikram2; Xuan Xu2; Spomenka Kobe3; Kristina Zuzek Rozman2; Farhan Mehmood2; 1HEAD OF DEPARTMENT FOR NANOSTRUCTURED MATERIALS, Ljubljana, Slovenia; 2JOZEF STEFAN INSTITUTE, Ljubljana, Slovenia; 3JOSEF STEFAN INSTITUTE, Ljubljana, Slovenia; PAPER: 281/SISAM/Regular (Oral) SCHEDULED: 11:45/Fri. 25 Oct. 2019/Dr. Christian Bernard ABSTRACT: The transition towards future green and e-mobility technologies, based on permanent magnets, is unavoidably linked to a stable supply of (heavy) rare earth elements. In the recent past, the transition towards green and e-mobility technologies has been hindered due to various geo-political and economic reasons. One promising way to address this problem is to develop strategies for various efficient magnet recycling and reprocessing routes that turn magnet waste into new functional magnets with little or negligible loss of overall magnetic performance [1]. The current recycling techniques, however, such as hydrogenation disproportionation desorption recombination (HDDR) processing, remelting, spark plasma sintering (SPS) and electrochemical deposition, inevitably perturb the desired microstructure. Detecting and understanding the underlying chemical and physical mechanisms is thus the key to optimize the overall magnetic performance of the recycled/reprocessed magnets. These mechanisms are typically associated with the structural/chemical properties of different crystal phases and defects, including dislocations, grains, and interphase boundaries. In recent years, state-of-the-art Transmission Electron Microscopy, which typically includes Scanning Transmission Electron Microscopy (STEM) with different spectroscopy techniques, such as Energy Dispersive X-ray Spectroscopy (EDXS) and Electron Energy-Loss Spectroscopy (EELS), has become an indispensable tool for characterization of rare earth-based permanent magnets. Transmission Electron Microscopy has become indispensable since it provides correlative structural and chemical information along with atomic-scale spatial resolution. In this presentation the above-described analytical techniques will be demonstrated against the most challenging research problems that were encountered during the development of two conceptually different recycling routes. In one approach, end-of-life Nd-Fe-B permanent magnets were first transformed in powder form by a HDDR process. The HDDR process was further used to fabricate initial dense reprocessed bulk magnet by spark plasma sintering (SPS) at different temperatures (ranging from 650<sup>0</sup>C to 850<sup>0</sup>C).Finally, a conventional heat treatment route at 750<sup>0</sup>C for 15 minutes was used on the bulk magnet that was fabricated. By applying advanced TEM, i.e. atomic-scale Z-contrast STEM, combined with EDXS and EELS, the resulting magnetic properties were critically assessed against various types of structural and compositional discontinuities down to the atomic-scale. We believe these discontinuities control the microstructure evolution during the SPS processing route. An alternative reprocessing route for Nd-Fe-based magnets that we have developed is based on electro-co-deposition of Nd and Fe from an ionic-liquid (1-ethyl-3-methylimidizolium dicyanamide) electrolyte. In order to reveal the deposition mechanism, the chemistry and phase composition of the deposit were investigated by means of analytical TEM. This showed that Nd(III) is reduced to Nd(0) during the electrodeposition process. Furthermore, we were able to confirm that the deposition of the Nd–Fe starts with the sole deposition of Fe, followed by the co-deposition of Nd–Fe. These new insights into the electrodeposition process represent an important step closer to efficient recycling of rare earths in metallic form at a mild temperature. This is a sustainable and economically viable route based on the green-chemistry approach, thus providing a genuine alternative to high-temperature molten-salt electrolysis. References: [1] This work was supported by the European Union's EU Framework Programme for Research and Innovation Horizon 2020 under Grant Agreement No 674973 (DEMETER). |
