| In order to meet the requirement of rare earth permanent magnetic material on electric vehicles, wind power generators, energy-efficient appliances and other newly-developing fields and promote the efficient use of the rare earth resources, it is an important goal in the research fields of sintered Nd-Fe-B magnets to enhance the comprehensive magnetic properties of the magnet and reduce the amount of heavy rare earth (HRE) elements required to fabricate high-coercivity magnets. In this paper, a new grain boundary diffusion (GBD) technique of electrophoretic deposition (EPD) was developed, the impact of the key process and the content of rare earth on the magnetic properties was studied, the optimum diffusion process of HRE compounds such as DyF3 and TbF3 was clarified, the optimum diffusion amount of the magnets with different rare earth content was optimized, the mechanism of coercivity enhancement of the GBD processed magnet was elucidated, the employed properties of the GBD processed magnets were studied, the high-coercivity sintered Nd-Fe-B magnet with low amount of the HRE elements was prepared, and efficiently utilization of the HRE elements was achieved. This work has important guidance for the large-scale manufacturing and application of the GBD processed magnets by EPD.The main results and achievements are summarized as follows:1. Using EPD technique, the coating of the HRE compounds with control over the thickness and the uniformity was prepared on the sintered Nd-Fe-B magnet successfully. The optimum diffusion temperature and time of the DyF3 diffusion by the grain boundary diffusion process using EPD method was clarified. Through controlling the process parameters such as temperature and time, the coercivity was increased from 16.1 to 22.8 kOe. A significant increment in coercivity of more than 6.5 kOe was achieved with less than 1.2 wt.% Dy in the HRE-free original magnets, and the HRE utilization was more than three times higher than conventional powder metallurgy process. Dy concentrations variation versus distance from the magnet surface in the GBD processed magnet was accurately characterized. Micro structure analysis suggested that the increase of the coercivity was closely related to the concentration and the distribution of Dy. During the diffusion, Dy diffused into the magnet mainly through the grain boundaries, and Dy favors to substitute for Nd to form the (Nd, Dy)2Fe14B core-shell phases with higher magnetocrystalline anisotropy field in the outer region of Nd2Fe14B grains, which increased the coercivity significantly. Furthermore, excess Nd substituted by Dy is ejected from the matrix grains and prefer to move to the grain boundaries. As a result, the ejected Nd makes the grain boundary phase become more continuous, uniform and thicker by the Dy diffusion process. This continuous and thick Nd-rich grain boundary phase weakens the exchange coupling between the matrix grains, which enhanced the coercivity further.2. Taking advantage of EPD to control over the thickness and uniformity of the coatings, the impact of the effective DyF3 content and the thickness of the magnets on the magnetic and micro structural properties of DyF3-coated sintered Nd-Fe-B magnets were systematically investigated and the limitation that grain boundary diffusion process was only suitable for magnet slice was solved preliminarily. Coercivity of the DyF3 EPD-coated magnet increased from 16.10 to 24.04 kOe with increasing the effective DyF3 content. Through optimizing the DyF3 content, a significant increment in coercivity of ?8.0 kOe was achieved with less than 1.3 wt.% Dy in the HRE-free original magnets, which had obvious advantage compared with other grain boundary diffusion methods. Since the diffusion depth of Dy is limited and the diffusion effect subjected to the thickness of the magnets, the coercivity gradually decreased with increasing the thickness of the magnets. The increment of the coercivity was ?3.26 kOe with the thickness of 8.5 mm, which was superior to the best results at present. It further confirmed that EPD is a highly cost-effective process as part of producing thicker high-coercivity Nd-Fe-B-based magnets with accurately control the DyF3 content coated on the surface of the magnet.3. Using EPD technique, the changes in the magnetic and microstructural properties of TbF3 GBD processed magnets with different content of rare earth (RE) in the original sintered Nd-Fe-B magnets were systematically investigated, and the HRE diffusion behavior and the mechanism of coercivity enhancement were elucidated. From the results of magnetic properties, the coercivities of the magnets with different RE content first increased gradually then became stable or decreased with increasing the effective TbF3 content coated on the surface of the magnets. In the original magnet with RE=30 wt.%, the increment in coercivity was maximum, and the increment of 10.07 kOe was achieved with less than 0.81 wt.% Tb, which realized the efficiently use of the HRE elements. Microstructure analysis suggested that in the sample with RE=30 wt.%, the RE content was lower, the grain boundary phase was fewer and not continuous. After diffusion, near the surface of the GBD processed magnet, the (Nd, Tb)2Fe14B core-shell phases was formed. Furthermore, the core-shell distribution of Tb was maintained to a depth of 500 ?m, and the grain boundary was much more continuous. As to the sample with RE=34 wt.%, the improvement of the grain boundary phase after diffusion was not as good as the sample with RE=30 wt.%, therefore the increment of the coercivity was lower than the sample with RE= 30 wt.%. However, the coercivity of the sample with RE=34 wt.% reached up to 28.12 kOe with less than 1.44 wt.% Tb, which was the highest value of the coercivity in the HRE-free original magnets by grain boundary diffusion process at present. This work has important guidance for optimizing the optimum diffusion amount of the magnets with different rare earth content and preparing high-coercivity sintered Nd-Fe-B magnet with low amount of the HRE elements.4. The impact of grain boundary diffusion process on the employed properties of TbF3-coated sintered Nd-Fe-B magnets with different content of rare earth was systematically investigated. From the magnetic properties at high temperature, the temperature coefficient of coercivity decreased obviously and the thermal stability was improved in the GBD processed magnets. At high temperature, the irreversible loss of magnetic flow decreased with increasing the RE content, and the irreversible loss of magnetic flow decreased obviously in the GBD processed magnets, which greatly increased the operating temperature of the GBD processed magnets. The electrical resistivity in the GBD processed magnets was improved because of there were a lot of fluoride existed at the grain boundary. |
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