| Summary: | 碩士 === 國立中正大學 === 物理系研究所 === 105 === The effect of grain boundary diffusion process on sintered NdFeB magnets (5 mm in thickness) by coating with various heavy-rare-earth alloy powder which containing two or three elements, and low melting points are studied. At first, we identify the compounds of alloys based on phase diagrams and DTA data. According to previous studies, the alloy powder with lower melting point exhibits better flow ability during diffusion and improves the effect of diffusion. Secondly, we compare the effect of coating different alloy powders made by rolling ball mill(RBM) or high-energy ball mill(HEBM) process. The powders made by HEBM process were finer than RBM process. However, HEBM contain less crystallinity and the morphology is granular. On the other hand, the shape of powders which made by RBM process are longer and flatter. In general, the better magnetic properties were measured by coating RBM process powders after GBD process. Thirdly, the different diffusing conditions are adopted to reduce the lattice diffusion (decrease the reduction of Br). The optimal heat treatment parameters from low temperature to high temperature slowly and holding high temperature for a period of time, which leads to the outstanding magnetic properties especially for the cobalt containing alloy powders (ΔiHc~11 kOe ). In the fourth part, hydrogen decrepitating process is adopted to get the finer powders. As the result, diffusion by 4.6 μm C powders presents higher ΔiHc than that diffused by 38.8 μm C powders. Finally, the H-grade magnets coated with alloy powders by GBD process exhibit better magnetic properties than UH-grade magnets (including value of β and the coercivities at room temperature and high temperatures, respectively).
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