Enhancement of thermal stability of Nd-Fe-B sintered magnets with tuned Tb-diffused microstructures via temperature control

초록

We investigate the magnetic properties and thermal stability of Tb-diffused Nd-Fe-B magnets prepared at various grain-boundary diffusion temperatures and additional heat-treatment temperatures. These heat-treatment processes improved the coercivity of Tb-diffused Nd-Fe-B magnets than that of the base magnets. The diffusion temperature was found to play a key role in controlling the magnetic thermal stability; temperature variations induced precise changes in the decoupled Tb-diffused microstructures in the magnets. The magnet fabricated at a high diffusion temperature showed the best coercivity at room temperature but poor thermal stability. This was due to the formation of Tb-rich (Tb, Nd)(2)Fe14B phases with high magnetocrystalline anisotropy produced inside the grains and less core-shell structures during diffusion at a higher temperature. The best thermal stability was observed for the magnet prepared at a lower diffusion temperature. This magnet had more well-formed core-shell structures than the remaining magnets. By analyzing its microstructure, using electron microscopy and a micromagnetic equation, it was found that a decoupled microstructure with diffused Tb atoms was mainly responsible for the better thermal magnetic stability. Results obtained herein suggest that an optimized diffusion temperature can provide a magnet with good thermal stability. (C) 2020 Elsevier B.V. All rights reserved.

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