| Due to excellent permanent magnetic properties at room temperature, sintered neodymium-iron-born（NdFeB） permanent magnetic materials have found a wide range of applications. In the 21 st century, the industries of the electronics, information, medical equipment, hybrid electric vehicles（HEVs） and wind generators have a rapid development. In order to meet the requirements of the magnets used in above fields, the NdFeB magnets should have enhanced properties at the elevated temperatures. As a result, more and more efforts have been put towards to improving both their coercivity and thermal stability.Magnetic properties of sintered NdFeB magnets are dependent not only on intrinsic properties of the main hard magnetic Nd2Fe14 B phase, but also on the characteristics, volume fraction, and distribution of grain boundary phases. It is possible to improve properties of sintered NdFeB magnets by modifying microstructures and compositions of both the main phase and intergranular phases. Enhanced coericivity has currently been achieved by the partial substitution of Dy for Nd in NdFeB based magnetic materials and the anisotropy field of the hard phase has been enhanced. However, this coercivity enhancement by Dy substitution can only be achieved at the expense of the remanence and maximum energy product, due to the reduction in magnetization Ms because of the antiferromagnetic coupling of Dy atoms with Fe atoms in the（Nd,Dy）2Fe14B lattice. Moreover, the scarcity of Dy resources has motivated extensive research into the enhancement of coercivity while minimizing the Dy usage.Aiming at improving properties and reducing the materials cost for NdFeB based permanent magnetic materials, the study demonstrated in this thesis include three parts. First, the microstructure, grain boundary structure and coercivity of sintered Nd Fe B magnets have been improved by the optimization of the additional heat treatment; Second, the grain boundary diffusion process based on heavy rare earth（HRE） metals and their compounds has been employed to modify the grain boundary structure and improve magnetic properties; Third, a new non-RE compound grain boundary solid diffusion technology has been developed for enhancing coercivity while further reducing the Dy usage and the cost of sintered NdFeB magnets.Firstly, in order to better understand the effect of grain boundary phases and their microstructure on magnetic properties, especially the coercivity-microstructure relationship of sintered NdFeB magnets, the microstructure and coercivity improvement of commercially sintered NdFeB magnets after optimized additional heat treatment have been investigated. By optimized additional heat treatment at 900℃/1h and 550℃/1h, the coercivity of the NdFeB sintered magnet increased from 1399 kA/m to 1560 kA/m. It is found that the formation of continuous, clear and smooth thin layers of the Nd-rich phase surrounding the main phase is important for the high coercivity of sintered NdFeB magnets. The effect of various grain boundary phases on the coercivity has been studied by the finite-difference micromagnetic simulation of the magnetization reversal process based on experimentally observed electronic microscope images. The results show that the nucleation of reversed magnetic domains occurs at the interface between the main phase and large intergranular phase, and the presence of these non-ferromagnetic phases is expected to cause high stray field. It is also clear that the thin grain boundary layer between neighboring Nd2Fe14 B grains can prevent the reversed magnetic domains from expanding into neighboring grains, since the domain wall cannot cross the nonmagnetic grain boundary phase. The study in this chapter has importance in understanding the coercivity-microstructure relationships of sintered NdFeB magnets.The coercivity of melt spun and sintered NdFeB based permanent magnetic materials was enhanced by the traditional Dy2O3 grain boundary diffusion process with a less expense of remanence. In details, the coercivity has been enhanced from 1678 kA/m to 2086 kA/m for melt spun NdFeB powders and from 965 k A/m to1154 kA/m for sintered NdFeB magnets. The effects of diffusion process parameters on magnetic properties are discussed. The mechanism of coercivity enhancement, element distribution and the microstructure of magnets have been also investigated. The results show that most of Dy atoms are enriched in the edge of the Nd2Fe14 B grains, which indicates the enhancement of coercivity is due to the thin Dy-rich shell surrounding the Nd2Fe14 B phase grains after the diffusion process. It can be concluded that the grain boundary diffusion process is an effective way to increase coercivity with little heavy rare earth. Based on the microstructure observations and microstructure parameter analysis, the underlying mechanism of grain boundary diffusion can be understood. Firstly, the volume diffusion promotes the substitution of Dy for Nd in the Nd2Fe14 B phase, resulting in the formation of Dy-enriched shells surrounding the Nd2Fe14 B, which can inhibit the nucleation of reversed magnetic domains. Secondly, the formed Nd-rich phase is helpful to the exchange decoupling effect. Thirdly, the pinning strength of the grain boundary in domain wall motion is enhanced. The study in this chapter provides a guideline to understand the mechanism of grain boundary diffusion process.In order to get rid of dependence on rare earths for the grain boundary diffusion process, a non-RE compound grain boundary solid diffusion approach for enhancing magnetic properties and corrosion resistance of Dy-free sintered Nd Fe B magnets has been proposed. It is found that the coercivity, temperature stability and corrosion resistance of sintered NdFeB magnets can be effectively improved by MgO solid diffusion process. The coercivity has been enhance from 1094 kA/m to 1170 kA/m. The microstructure investigations show that MgO entered mainly into the intergranular regions and modified the grain boundary phase composition and structure, which can inhibit the nucleation of reversed magnetic domains. The formation of Nd-O-Fe-Mg phase increases the chemistry stability of grain boundary phases and the pinning force for magnetic domain wall motion. It is believed that the presented non-RE compound grain boundary diffusion process has great significance in further reducing the cost of NdFeB permanent magnetic materials.Based on above mentioned non-RE compound grain boundary diffusion process, using ZnO as the diffusion medium, the ZnO diffusion process and its effects on properties of sintered NdFeB have been studied. By process optimization, the coercivity of Dy-free sintered NdFeB magnets has been enhanced from 1085 kA/m to 1290 kA/m. This new grain boundary diffusion process has a lot of advantages, such as grain boundary phase transformation, microstructural modification, intergrain exchange decoupling, which are responsible to the enhanced coercivity and corrosion resistance. It is found that the ZnO play a critical role as a reaction medium in the grain boundary diffusion of sintered NdFeB magnets. Furthermore, the relationship between coercivity and morphology of various Nd-rich phases was analyzed by SEM and TEM observations, and the underlying mechanism of non-RE compound grain boundary solid diffusion for sintered NdFeB magnets has been discussed. These studies in Chapters 5 and 6 develop a new system of grain boundary diffusion process for high performance-to-price ratio NdFeB based magnets and have great significance in the development of rare earth permanent magnetic materials. |
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