Aru Yan

The preparation of sintered NdFeB magnet with high-coercivity and high temperature-stability

The NdFeB magnets with high intrinsic coercivity have been produced by using the conventional powder metallurgy method (including SC, HD and JM) of sintered NdFeB magnets. The effects of grain boundary phases on the microstructure and magnetic properties of as-sintered and annealed magnets have been tried to investigate. Also the Curie temperature of the magnets was studied. By adopting suitable component ratio of some heavy rare-earth atoms and some micro-quantity additives, we have prepared high-coercivity sintered NdFeB magnets with magnetic properties of jHc=36.3kOe, Br=11.7kGs and (BH)max=34.0MGOe. The temperature coefficient of residual magnetic flux of the magnets (between 20 and 200°C) is -0.113%/°C, while the temperature coefficient of intrinsic coercivity -0.355%/°C. The Curie temperature of the magnets is about 342°C.

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 Heat Treatment on Microstructure and Thermal Stability of Nd–Fe–B Sintered Magnets

The effect of heat treatment on the microstructure and thermal stability of sintered Nd-Fe-B magnets was investigated. The results show that the microstructure of the magnets was greatly affected by heat treatment. The intrinsic coercivity (H_cj) was increased and the temperature coefficient of coercivity (β) was decreased by heat treatment. As the annealing temperature changes from 500°C to 770°C, H_cj first increases and reaches a maximum, then starts to decrease, but β remains unchanged from 500°C to 680°C and increases rapidly when annealed over 680°C. A magnet with H_cj= 30.55 kOe, β (from 20°C to 200°C) = -0.4079%°C was obtained after annealed at 620°C for 2 h. The relationship between the annealing temperature and magnetic properties was studied.

Phase structure, magnetic properties, and magnetization behavior of Sm (Co,Zr) 7/α- (Fe-Co) nanocomposite made by mechanical alloying

The Sm(Co,Zr)7/α-(Fe–Co) nanocomposite permanent magnets are produced by mechanical alloying and subsequent annealing. The soft magnetic phases were introduced by two different processes. In the first process, the crushed as-cast SmCo6.8Zr0.2 powders were blended and milled with different weight percent of fine iron powders (SCZ+Fe) and in the second process, the crushed as-cast (SmCo6.8Zr0.2)1−xFex (x=0,0.1,0.25,0.5,0.75) powders (SCZF) were milled directly. XRD analysis showed that the phase structure of annealed SCZ+Fe are consisted of SmCo7 phase and α-(Fe,Co) phase, while the composites of annealed SCZF are composed of α-(Fe,Co) and SmCo hard phases that varied with different Fe content. The best energy product of the annealed SCZF and SCZ+Fe are 11.3 MGOe (SCZF-3) and 10 MGOe (SCZ+Fe-1), respectively, resulting from the very strong exchange coupling between the hard phase and the soft phase. According to the Henkel-plots of SCZF-3 and SCZ+Fe-1, a stronger intergrain exchange coupling effect was obse...

Coercivity enhancement of sintered Nd-Fe-B magnets by chemical bath deposition of TbCl3

The chemical bath deposition (CBD) and the grain boundary diffusion method were combined to diffuse the heavy rare earth for obtain the thick magnets with high coercivity and low heavy rare earth. The jet mill powders were soaked into the alcohol solution of 0.2 wt. % TbCl3. A thin layer of TbCl3 was wrapped to the surface of (PrNd)2Fe14B powder particles. The coercivity of magnet is increased from 11.89 kOe to 14.72 kOe without significant reduction of remanence after grain boundary diffusion in the sintering and the annealing processes. The temperature coefficients of the remanence and the coercivity are improved by the substitution of PrNd by Tb in the surface of grains. The highly accelerated temperature/humidity stress test (HAST) results indicate that the CBD magnet has poor corrosion resistance, attributing to the present of Cl atoms in the grain boundaries.

Effect of Rare-Earth Content on Coercivity and Temperature Stability of Sintered Nd-Fe-B Magnets Prepared by Dual-Alloy Method

The influence of rare earth content on coercivity and temperature stability with dual-alloy method has been investigated. It is found that the coercivity of magnets increases rapidly firstly and then slightly with the increasing of rare earth content from the range of 29.5 wt% to 33.5 wt%. The coercivity and the temperature coefficient of coercivity will get better with rare earth content of 31.5 wt%. The result of magnetic anisotropy field of magnet revealed that it almost has no change with different rare-earth content. Scanning electron microscopy (SEM) and Transmission electron microscope (TEM) revealed that continuous thin layers of Nd-rich phase about 4 nm were found along the grain boundaries in the annealed sample at 31.5 wt% rare earth content, while the grain boundary width of magnet exceeds 8 nm at 33.5 wt%.

Effect of Temperature Gradient on the Microstructure of the Rapidly Solidified Nd–Fe–B Strips

High-performance Nd-Fe-B magnets with nominal composition of Nd13.25Dy0.2Fe78.87Co1Cu0.1Al0.5B6.08 (at%) have been produced by using strip casting technique. It was found that the temperature gradient of alloy in the solidification stage determines the grain size and the grain alignment of the alloy strips. The growth direction of grains parallels the direction of heat flow during solidification stage. The average solidifying rate of strips is more than 103°C/s, which can effectively depress the formation of α-Fe dendrites. It is observed that the concentration of rare earth has a significant decrease from free side to wheel side of the strip. The results of magnetic properties of final magnets produced with different strips indicate that the nonuniform phase structure can deteriorate the squareness of demagnetization curve and the temperature coefficient of coercivity. Effects of the temperature gradient in the solidification process on the microstructure of alloy strips, magnetic properties, and thermal stability of the final magnets are investigated systematically.

Coercivity enhancement of (Nd,Ce)-Fe-B sintered magnets by doping Nd-Fe additives

The effect of Nd-Fe additives on magnetic properties and microstructure in (Nd,Ce)-Fe-B sintered magnet has been investigated. By doping 3wt% Nd-Fe additives, the coercivity of the magnet increases from 10.56kOe to 12.69kOe with slight remanence decrease. Microstructure observation reveals that the volume fraction of the grain boundary phase increases accompanying with the thickening of the thin grain boundary between the adjacent grains. The RE6Fe13Cu phase which has low melting temperature develops at the triple junction position and the out shell of the matrix grains get magnetically hardened with 3wt% Nd-Fe additives. The results of the dynamic magnetic domains between the original magnet and processed magnet elucidate that the formation of reversed magnetic domains are more difficult for the processed magnet which is the direct evidence to clarify the coercivity enhancement.

Dysprosium Diffusion Behavior and Microstructure Modification in Sintered Nd-Fe-B Magnets via Dual-Alloy Method

Dysprosium diffusion and microstructure alteration behaviors in sintered Nd-Fe-B magnets via dual-alloy method have been investigated. The sintered magnets were prepared by blending Dy-free and Dy-rich powders with varying proportions. Results indicate that Dy element has already diffused from Dy-rich powders into the 2:14:1 phase of Dy-free powders and tended to homogenize in the bulk magnet, attributed to the coalescence of grains and dysprosium element concentration gradient. In addition, Dy and Pr/Nd elements counterdiffusion process redistributes the grain boundaries homogeneously. The fracture surfaces of magnets prepared via dual-alloy method almost appear intergranular fracture. It demonstrates that both of (Nd/Pr, Dy)2Fe14B phase formation and microstructure alteration have favored to coercivity enhancement and thermal stability improvement of the magnets.

Effect of DyHx addition on the magnetic properties and microstructure of Nd14.1Co1.34Cu0.04FebalB5.84 magnets

The sintered Nd14.1Co1.34Cu0.04FebalB5.84 permanent magnets with DyHx addition have been prepared by powder-blending technique. The magnetic properties and microstructures of the magnets were investigated. It was found that Dy element is enriched in the intergranular phase, being evenly distributed throughout the grain boundary and improving the orientation of magnets as a lubricant, and 1.2% to 2% DyHx additions are proved to be very effective in improving the permanent magnetic properties of Nd14.1Co1.34Cu0.04FebalB5.84 magnets. Further analysis revealed that the increase of coercivity and slight decrease of remanence due to DyHx addition can be attributed to the reduction of the primary phase volume, formation of the (NdDy)-rich phase and grain refinement of the main phase.