Changjiang Yan

Enhanced Magnetic Properties of Sintered Ce–Fe–B-Based Magnets by Optimizing the Microstructure of Strip-Casting Alloys

We present an effort to enhance the magnetic properties of sintered Ce-Fe-B-based magnets by optimizing the microstructure of strip-casting (SC) alloys. The results indicated the microstructure in the Ce30.5Febal.B1 SC alloy was characteristic for peritectic solidification and Ce2Fe14B coexisted with CeFe2 and primary Fe-rich phase. After alloying a little amount of Ho and Nd, the microstructure of Ce-Fe-B strip was optimized toward that of Nd-Fe-B strip. Experimental results indicated that all magnetic properties, especially the energy product, were enhanced after optimizing the microstructure of SC alloy. Fracture surface images of the magnets illustrated the grain boundary structure was also modified due to optimization of the microstructure of SC alloys.

Effect of hydriding degree on the microstructure and magnetic properties of sintered NdFeB magnets

The effects of hydriding degree on the microstructure and magnetic properties of sintered NdFeB magnets have been studied. The degree of crushing depends on the absorption content of hydrogen and affects the magnetic properties of final magnet. Insertion of hydrogen atoms leads to a significant increase of the unit cell volume. And the crush mechanism depends on the internal stress resulting from differences in the expansion rates of the two phases. The remanence of final magnets increases monotonously while the coercivity decreases sharply with the increasing of hydriding degree, attributed to the strip fracture features and the morphology of particles.

Design and fabrication of Dy-free sintered permanent magnets with high coercivity

The purpose of this study was to save heavy rare earth Dy in NdFeB production. By ways of optimizing preparation process and replacing Nd with Pr, Dy-free magnet with high coercivity was successfully prepared, the nominal composition is Pr14B6Co2.2Cu0.2Al1Fe76.7 (at %). The effect of Pr on magnetic performance and microstructure of the magnet were investigated systemically, alloy Nd13.65B6Co2.2Cu0.2Al1Fe76.9 (at %) was prepared by the same preparation process as comparison. The results indicated that magnet Pr14B6Co2.2Cu0.2Al1Fe76.7 has much higher coercivity than that of magnet Nd13.65B6Co2.2Cu0.2Al1Fe76.9. Microstructure investigation showed that the presence of small grains, thin and smooth RE-rich film, associated with higher coercivity.

Enhanced Temperature Stability of Coercivity in Sintered Permanent Magnet by Substitution of Ce for Didymium

Effects of partial substitution of Ce for didymium (Pr-Nd alloy, abbreviated to Di) on elevated temperature magnetic properties in sintered (Di_1-xCe_x)_27.5Dy₃Al_0.1Cu_0.1Fe_bal.B (wt.%, x = 0 ~ 0.56) magnets were investigated in this paper. Temperature stability of remanence was found to decrease in Ce substitution. However, the temperature stability of coercivity was largely improved when x ≥ 0.24. Temperature dependence of magnetocrystalline anisotropy field (H_A) was improved as Ce substitution. Microstructural observation and micromagnetic analysis indicated that a microstructure of magnets was modified when x ≥ 0.24. The enhancement of temperature stability of coercivity was due to the improvement of temperature dependence of (H_A) and the modification of microstructure.

Enhanced Thermal Stability of Nd–Fe–B Sintered Magnets by Intergranular Doping Y 72 Co 28 Alloys

The intergranular addition process using Y₇₂Co₂₈ eutectic alloy powders has been applied to (Pr, Nd)₂₇Dy₄Fe_67.65Cu_0.15Al_0.2B₁(wt.%) sintered magnets, resulting in an enhanced thermal stability. It was found that the coercivity of magnets at 100 °C increased while the addition of Y₇₂Co₂₈ is 1 wt.%, whereas the magnetic properties decreased slightly at room temperature due to lower magnetocrystalline anisotropy field (H_A ) of Y₂Fe₁₄B. The temperature coefficient of remanence (α ) and coercivity (β) for the magnets improved significantly. Microstructure observation indicated that Y atoms prefer to enter into the 2-14-1 phase and form a core-shell structure with an outer layer of (Nd, Dy, Y)₂(Fe, Co)₁₄B. Magnetic domain and intrinsic properties analysis suggested that the improved thermal stability resulted from the modified grain boundaries and the introduction of Y in matrix phases.

Enhanced Electric Resistivity of Die-Upset Magnets By Segmented NdF 3 Addition

We report an approach to improve the electric resistivity of Nd-Fe-B die-upset magnets without sacrificing their magnetic properties very much. NdF3 fine powders were segmentally added in the die filling with MQU-F powders and then were die-upset for magnets. Experimental results indicated that electric resistivity monotonically increases with the increase of NdF3 addition, which is due to the formation of continuous segmented fluoride layers. When NdF3 content reached 5.3 wt%, the electric resistivity almost increases by eight times. Magnetic properties exhibit no drastic decrease after the addition of NdF3 and coercivity increases when adding 2.12 wt% of NdF3.

Preparation and Component Characterization of Nd-Fe-B Reference Materials

In this study, a series of Nd-Fe-B reference materials for rapid testing were prepared. The content of various elements in NdaRbMcFe100-a-b-c-xBx (R includes La, Ce, Gd, Ho, Co, Dy, Tb, Pr; M includes Nb, Zr, Al, Cu, Ga) are: a = 8 ~ 27 wt%, b = 0 ~ 10 wt%, c = 0 ~ 2 wt%, x = 0 ~ 1.6 wt%. ICP-OES analysis results show that these Nd-Fe-B reference materials are homogeneous. The standard deviation (SD) of results tested by ICP-OES in three different laboratories for confirming the standard values is less than 0.05% while the element content is equal to or less than 1 wt%, the relative standard deviation (RSD) is less than 5% while the element content is more than 1 wt%. These reference materials were used to establish a rapidly quantitative testing method by X-ray fluorescence spectrometer (XRF). It was shown that the composition characterization results by XRF are close to the results by ICP-OES and the comparison results by other laboratories (RSD <; 4%). And the RSD of the precision on XRF is less than 1.5% (n = 11). All the data proved that the rapid method by XRF was accurate and stable. Meanwhile, the testing time for one Nd-Fe-B sample by XRF (about 15 minutes) is shorter obviously than that by ICP-OES (about one day). This rapid quantitative testing method can be applied in Nd-Fe-B industrial production to control product quality and realize online analysis for saving time.

Coercivity Enhancement of HDDR Hot-Pressed Magnets by NdCu Diffusion Treatment

In order to improve the coercivity of hot-pressed magnets with hydrogenation disproportionation desorption recombination (HDDR) powders being the precursor, diffusion processing of an NdCu eutectic alloy is employed for grain boundary modification, resulting in an increase of coercivity from 15.4 to 17.8 kOe, which leads to a better thermal stability with the temperature coefficient (β) changing from -0.549%/K to -0.519%/K. It is evidently observed that Nd-rich grain boundary layers are formed between the adjacent matrix phase grains as well as in the junction parts after diffusion treatment, which is responsible for the coercivity enhancement of the HDDR hot-pressing magnets. The influence of the microstructure on the coercivity has been further investigated to reveal that the Nd-rich phases occurred during the boundary modification process of the NdCu eutectic alloy decouple the exchange interactions between the grains.