Effects Of Grain Boundary Structure And Chemistry On The Coercivity Of NdFeB Materials

Nd-Fe-B based magnets exhibit excellent magnetic properties and are being widely used in heavy-duty applications such as generators and electric vehicles. In these applications, magnets with a high coercivity are required since the motors often operate at a high temperature (²00°C). In this thesis, emphasis is placed on the grain boundary structure and chemistry. The effects of secondary tempering on structure near the grain boundary, the effects of intergranular additions of fluorides additives on coercivity and microstructure, the microstructure and chemistry of diffusion processed NdFeB magnets were investigated. Moreover, the relationship between the microstructure and magnetic properties of NdFeB film, and the effect of Dy film on the microstructure were also investigated.The magnetic properties of secondary tempered magnets is better that the magnets without tempering, so the structure of grain boundary in the sintered magnet with and without secondary tempering were investigated by HRTEM and NBDs. It is found the grain boundary phase is continuous and there is not fixed orientation relationship between the matrix phase and the grain boundary phase in the tempered magnets. The thickness of the region of reduced anisotropy of the Nd2Fe14B grain in secondary tempered magnets was decreased. The MFM results confirm that the applied magnetic field which make the magnets to reach saturation is larger than that in the hysteresis loop. The Nd2Fe14B grains are in a single domain state when they are magnetized to saturation. The incomplete-thermal-demagnetization at high sample temperature accompanies with the growth of reversal domains and the decrease of domain width.The properties and microstructure of the magnets with fluorides additions were investigated. It was shown that when the amount of GdF₃?YF₃ and MgF₂ additions were 0.1 wt. %,, the coercivity was increased. The electrical resistivity was also slightly improved with fluorides additions. For the (Nd_0.8Pr_0.2)_14.5Dy_1.2Fe_balB₆ magnets, there is a 400 kA/m increase in coercivity and the coercivity increases to 2018 kA/m by 2.0 wt.% additions of DyF₃. Compared with the magnets by other processes, the increase of coercivity per unit weight percentage of Dy in the present process is almost doubled. F in the grain boundary phase decreased the thickness of the region of reduced anisotropy of the Nd₂Fe₁₄B grain adjacent to grain boundary. When the F content approached to about 30 at. % in the grain boundary phase, the ordered ROF phase was formed. There is slight enrichment of Dy on the surface of Nd₂Fe₁₄B grains(matrix phase). The continuous grain boundary phase as well as the (Nd, Dy)₂Fe₁₄B phase with well-de?ned crystal structure may contribute to the increase of coercivity.The coercivity of the magnets after DyF₃ diffusion process increased. The added value of coercivity in as-sintered magnet after diffusion process is higher than that in the two-stage tempered magnet, which is due to the more continuous grain boundary phase upon further annealing at 900 oC. It indicates that a continuous grain boundary phase is helpful to DyF₃ diffusion process. When sufficiently diffused, there is no enrichment of Dy in the grain boundary phase. The excess Nd in Nd₂Fe₁₄B grains substituted by Dy led to the Nd-O phase at the grain boundary and on the surface of the magnet.A [Si/Mo(50 nm)/Nd-Fe-B(400 nm)/Mo(50 nm)] film with relatively good remanence (B_r?=11.7 kG, H_ic?= 13.7 kOe and (BH)_max?=31.0 MGOe) was prepared on a Si substrate heated at 650?. The relationship between the microstructure and remanence was investigated. Moreover, according to the idea of diffusion process in NdFeB sintered magnets, the diffusion mechanism of Dy in the NdFeB films was investigated. It was found the (110) orientated Mo grains were proved to promote the growth of c-axis texture of the Nd₂Fe₁₄B grains. Dy diffused from the grain boundary and the Nd-O phase of the NdFeB film, the excess Nd in Nd₂Fe₁₄B grains substituted by Dy increases the mount of Nd-O phase. The formed Nd-O phase between the Nd₂Fe₁₄B grains is helpful to the decoupling effect which will contribute to the increase of coercivity.