Abstract
The wide application of Nd-Fe-B permanent magnets, in addition to rare-earth metal resource constraints, creates the necessity of the development of efficient technologies for recycling sintered Nd-Fe-B permanent magnets. In the present study, a magnet-to-magnet recycling process is considered. As starting materials, magnets of different grades were used, which were processed by hydrogen decrepitation and blending the powder with NdHx. Composition inhomogeneity in the Nd2Fe14B-based magnetic phase grains in the recycled magnets and the existence of a core-shell structure consisting of a Nd-rich (Dy-depleted) core and Nd-depleted (Dy-enriched) shell are demonstrated. The formation of this structure results from the grain boundary diffusion process of Dy that occurs during the sintering of magnets prepared from a mixture of Dy-free (N42) and Dy-containing magnets. The increase in the coercive force of the N42 magnet was shown to be 52%. The simultaneous retention of the remanence, and even its increase, were observed and explained by the improved isolation of the main magnetic phase grains as well as their alignment.
Highlights
The demand for rare-earth permanent magnets continues its strong and steady growth, due to their application in existing and future energy systems
The aim of the present study is to investigate the possibility of recycling Nd-Fe-B magnets using hydrogen decrepitation, a blending powder procedure and mixtures of magnets of different grades
Magnets, which can be used in the recycling process, since it effectively ensures the utilization of rare earth elements from waste magnets and saves rare earth resources
Summary
The demand for rare-earth permanent magnets continues its strong and steady growth, due to their application in existing and future energy systems. The rare-earth metals (REMs) are among the most critical elements, in particular, from the viewpoint of their availability This fact determines the problem of the development of efficient technologies for recycling sintered Nd-Fe-B permanent magnets, which is closely related to the development of new approaches to the formation of high-coercivity and high-performance states of the Nd-Fe-B permanent magnet materials. One approach to solving the problem consists of using the grain boundary modification (GBM) of sintered magnet materials [1,2], which includes grain-boundary diffusion (GBD) and grain-boundary structuring (GBS) These processes effectively increase the coercivity of Nd-Fe-B magnets with a small amount of rare earth additives; Materials 2020, 13, 3049; doi:10.3390/ma13143049 www.mdpi.com/journal/materials. GBM can be realized via the careful addition of compounds or blended elements and the application of hydrogen [5]
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