Zhaohui Guo

Effect of Co on the thermal stability and magnetic properties of AlNiCo 8 alloys

The magnetic properties, microstructure, and reversible temperature coefficient of magnetic flux of Alnico 8 alloys with the different Co and Ti have been investigated in this paper. The Alnico alloys containing 34, 36, 38, and 40 mass percent cobalt were fabricated by the conventional casting method including thermomagnetic treatment. The transmission electron microscopy (TEM) testing shows that there are more α1 phases particles with the diameter of less than 10 nm appearing in the matrix with the increasing of the content of Co and Ti. The thermal stability of Alnico 8 alloys deteriorates with the increasing of the content of Co and Ti, which are caused mainly by the decrease of the degree of particle alignment and particle perfection.

Investigation on microstructure and magnetic properties of Sm2Co17 magnets aged at high temperature

The Sm2Co17 magnet is the most promising candidate for high temperature applications. The microstructure evolutions and losses in the magnetic properties of the magnet in high temperature aging status have been investigated. The Sm(CobalFe0.22Cu0.068Zr0.025)7.75 magnets were prepared using the conventional powder sintering method. The magnet samples were isothermally aged at 500°C, 600 °C, and 700 °C for 72 h, respectively. The magnetic properties and the demagnetization curve were kept invariable for the magnet samples aged at 500 °C. The coercivity Hcj of the magnet samples decreased with increasing aging temperature. The Hcj decreased from 29.2 kOe for the original status samples to 10.8 kOe for the samples aged at 700 °C. The cell structure in the magnet is not destroyed after aging at 700 °C for 72 h. The deterioration of the magnetic properties and the demagnetization-curve squareness was caused by an increasing lattice mismatch between the 2:17R cell phase and the cell-boundary 1:5H phase, and by a...

Rare earth magnets resisting eddy currents

Magnets with high electrical resistance were prepared by electrically isolating Sm/sub 2/Co/sub 17/ and Nd/sub 2/Fe/sub 14/B powder particles. This is achieved by a polymer mixture consisting of epoxy polyester-insulant (EPI), polyvinylbutyral (PVB), epoxy resin (ER) and silane coupling (SC). The properties of interest for anisotropic Sm/sub 2/Co/sub 17/ and isotropic Nd/sub 2/Fe/sub 14/B magnets respectively are /spl rho/=1/spl times/10/sup 5/ /spl Omega/-cm/sup 2//m, Br=6.8 kG, iHc=10.0 kOe, (BH)max=9.2 MGOe in 5%EPI, 4%PVB, 2%ER, 1%SC and /spl rho/=9.4/spl times/10/sup 4/ /spl Omega/-cm/sup 2//m, Br=4.5 kG, iHc=9.5 kOe, (BH)max=13 MGOe in 4%EPI, 3%PVB, 2%ER, 1%SC. The insulated magnets exhibit far better magnetic stability than those of sintered and bonded magnets after magnetizing in radio frequency field. The insulated Sm/sub 2/Co/sub 17/ magnet has the best performance in resisting eddy current. At 500 kHz frequency, 830 Oe magnetizing-field and in one minute, the irreversible loss of recoil remanence is 1.47% and 22.2% respectively for insulated Sm/sub 2/Co/sub 17/ and Nd/sub 2/Fe/sub 14/B magnets.

Preparation and magnetic properties of hard-magnetic (CoPt)/soft-magnetic (FeCo) composite nanocable array

An array of hard-magnetic (CoPt)/soft-magnetic (FeCo) composite nanocable has been fabricated in the pores of porous anodic aluminum oxide templates: CoPt nanotubes were first prepared by high temperature chemical reduction method; FeCo alloy was then electrodeposited into the CoPt nanotubes. The morphology and structure of the nanocable array were characterized by scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The magnetic measurements showed that two apparent kinks appear in the hysteresis loop after the FeCo alloy was deposited into the CoPt nanotubes, which may be attributed to the hard-magnetic phase being partly or even completely decoupled from the soft-magnetic phase.

Enhancement of coercivity for melt-spun SmCo7−xTax ribbons with Ta addition

The effect of Ta addition on the structure and magnetic properties of melt-spun SmCo7−xTax (x=0–0.6) ribbons has been investigated. The intrinsic coercivity increases dramatically from 1.9 kOe for Ta-free ribbon to 9.8 kOe for SmCo6.7Ta0.3 ribbon, while further increasing Ta results in little enhancement of the coercivity. XRD patterns reveal that SmCo7−xTax ribbons crystallize in the pure TbCu7-type structure for low Ta substitution of x≤0.3 and a minor phase of TaCo3 appears when x>0.3. According to Rietveld structure refinement, the ternary alloying element Ta prefers to occupy the 2e site of TbCu7-type structure. The mechanism of the coercivity enhancement has been discussed based on crystal structure and microstructure.The effect of Ta addition on the structure and magnetic properties of melt-spun SmCo7−xTax (x=0–0.6) ribbons has been investigated. The intrinsic coercivity increases dramatically from 1.9 kOe for Ta-free ribbon to 9.8 kOe for SmCo6.7Ta0.3 ribbon, while further increasing Ta results in little enhancement of the coercivity. XRD patterns reveal that SmCo7−xTax ribbons crystallize in the pure TbCu7-type structure for low Ta substitution of x≤0.3 and a minor phase of TaCo3 appears when x>0.3. According to Rietveld structure refinement, the ternary alloying element Ta prefers to occupy the 2e site of TbCu7-type structure. The mechanism of the coercivity enhancement has been discussed based on crystal structure and microstructure.

Fractal study for the fractured surface of Nd-Fe-B permanent magnets

Fractal study was used to the analysis of the fractured surface of commercial N50-type Nd-Fe-B sintered magnets. The strain-stress properties of the specimens indicate that the elastic and plastic deformations occur simultaneously during intergranual cracking. The microfractography of the specimens exhibits typical brittle fracture pattern with some toughness dimples, indicating that plastic deformation has happened in some local areas. A “line-measuring dimension” Dline was selected to discuss the fracture behavior. The calculated Dline is about 1.28 and 1.29 for the specimens with c-axis parallel and perpendicular to the applied force direction, respectively. The line-measuring dimension analysis indicates that the fracture feature may be isotropic for our studied specimens, which is a bit inconsistent with previous report on the fracture characterization of Nd-Fe-B sintered magnets. More comprehensive fractal study is further needed to confirm the fracture behavior in detail in the future.

The corrosion mechanism of the sintered (Ce, Nd)-Fe-B magnets prepared by double main phase and single main phase approaches

The sintered (Ce, Nd)-Fe-B magnets were produced widely by Double Main Phase (DMP) method in China as the magnetic properties of the DMP magnets are superior to those of single main phase (SMP) magnets with the same nominal composition. In this work, the microstructure and corrosion mechanism of the sintered (Ce0.2Nd0.8)30FebalB (wt.%) magnets prepared by DMP and SMP method were studied in detail. Compared to SMP magnets, the DMP magnets have more positive corrosion potential, lower corrosion current density, larger electron transfer resistance, and lower mass loss of the free corrosion experiment in 0.5mol/l Na2SO4 aqueous solution. All of the results show that the DMP magnets have better corrosion resistance than SMP magnets. The back scattered electron images show that the crystalline grains of the DMP magnets are sphericity with a smooth surface while the SMP ones have plenty of edges and corners. Besides, the distribution of Ce/Nd is much more uneven in both magnetic phase and rare earth (Re)-rich ph...

Effect of deformation ratios on grain alignment and magnetic properties of hot pressing/hot deformation Nd-Fe-B magnets

The magnetic properties, microstructure and orientation degrees of hot pressing magnet and hot deformation Nd-Fe-B magnets with different deformation ratios have been investigated in this paper. The remanence (Br) and maximum magnetic energy product ((BH)max) were enhanced gradually with the deformation ratio increasing from 0% to 70%, whereas the coercivity (HCj) decreased. The scanning electron microscopy (SEM) images of fractured surfaces parallel to the pressure direction during hot deformation show that the grains tend to extend perpendicularly to the c-axes of Nd2Fe14B grains under the pressure, and the aspect ratios of the grains increase with the increase of deformation ratio. Besides, the compression stress induces the long axis of grains to rotate and the angle (θ) between c-axis and pressure direction decreases. The X-ray diffraction (XRD) patterns reveal that orientation degree improves with the increase of deformation ratio, agreeing well with the SEM results. The hot deformation magnet with ...

Effect of

Nd-Fe-B composite magnets with different amounts of CaF2 have been prepared by sintering at different temperatures. The microstructure and densities of the Nd-Fe-B composite magnets were investigated by scanning electron microscope and Archimedes method. The electrical and magnetic properties were measured with a four probe electrical resistivity meter and a NIM-2000HF hysteresisgraph meter. The results show that CaF2 was distributed in the grain boundary. The electrical resistances of the composite magnets increased with the increase of the amounts of CaF2, while the magnetic properties of the magnets deteriorated markedly. The electrical resistances of the Nd-Fe-B composite magnets with 3, 6 and 9 wt.% as-bought CaF2 sintered at 1050°C were 270, 320 and 400 μΩ cm, which increased approximately 17%, 39% and 74% over that of the magnet without CaF2 ( 230 μΩ cm). The maximum energy products, (BH)max, of the composite magnets with 3, 6 and 9 wt.% as-bought CaF2 sintered at 1050°C were 41.03, 35.77 and 28.66 MGOe, respectively. The densities of the composite magnets decreased with the increase of the amounts of CaF2. High energy ball milling can refine CaF2 to nanoscale and thus improve its distribution in the composite magnets. The (BH)max of the Nd-Fe-B magnets with 3 wt% CaF2 first increased and then decreased with the increase of milling time. But the electrical resistivities of the sintered Nd-Fe-B magnets with the CaF2 without milling and after high energy ball milling have little change.

{\hbox{CaF}}_{2}

Magnetic domains are strong correlated to the macro-magnetic properties of magnetic materials. Among the methods to revealing magnetic domains, magnetic force microscopy (MFM) is a powerful tool with high lateral resolution and easy sample preparation. In this review, we summarize the recent magnetic domain investigations on three typical kinds of rare earth permanent magnets: high coercivity Nd-Fe-B sintered magnets, hot-deformed Nd-Fe-B magnets and 2:17-type Sm-Co sintered magnets. The influence of temperature, fabrication procedure parameters on these magnetic materials is mainly discussed. For Nd-Fe-B sintered magnets, the remanent state specimens show single domains and formation and grownup of reversal domains occur during thermal demagnetization process. The magnetic configurations of nano-scale anisotropic hot-deformed Nd-Fe-B magnets are typical interaction domains. For 2:17-type Sm-Co sintered magnets, the zig-zag domain walls distributed along the cell boundaries are revealed by MFM.