Increase Of Maximum Energy Product Research Articles

The effect of agarose agent on the structural, magnetic and optical properties of barium hexaferrite nano-particles synthesized by sol-gel auto-combustion method

The main goal of this study is to improve the structural, magnetic and optical properties of the M-type barium hexaferrite nano-particles by addition of agarose as a natural agent, which were synthesized by the sol-gel auto-combustion method. For this purpose, properties of the synthesized samples were characterized by DTA-TGA, XRD, FT-IR, FE-SEM, TEM, VSM, and UV–Vis-DRS analyses. The DTA-TGA thermal analyses showed that agarose as a fuel agent could reduce the starting temperature of crystallization in the precursors. Also, based on these results, the temperature of 800 °C was chosen as the calcination temperature of the precursors. XRD patterns confirmed the formation of pure M-type barium hexaferrite without any impurities after calcination at 800 °C. The presence of agarose caused a more homogeneous distribution of metal cations Fe3+ and Ba2+ and the precursor was more easily and completely transformed into hexagonal barium ferrite in the presence of this agent. Also, the Rietveld refinement reported a decrease in average crystallite size from 49.4 to 29.6 nm by adding agarose. In addition, FE-SEM and TEM images showed that the agarose agent significantly reduces the average particle size and also prevents agglomeration of particles. Moreover, magnetic studies indicated that the presence of agarose led to the simultaneous increase of coercivity and remanence magnetization, resulting in an increase of maximum energy product from 0.81 to 0.92 MGOe. Also, the band gap energy (Eg) of the synthesized samples was found in the range of 2.36–2.76 eV. Finally, the significant effect of the agarose agent on the properties of BaFe12O19 nano-particles was clearly explained.

Comparison and process study of hot-pressed and hot-deformed Nd-Fe-B magnets prepared by amorphous and nanocrystalline powders

Hot-pressed and hot-deformed Nd-Fe-B magnets are generally made from rapidly quenched powder after crystallization treatment. In this work, the amorphous melt-spun ribbons are also employed directly as the precursor. The amorphous and crystallized powders with the composition of Nd28.47Pr0.56FebalB0.97Co3.63Al0.13Nb0.56Zr0.44 were employed for fabricating hot-pressed and hot-deformed magnets. The influences of processing parameters on the microstructure and magnetic properties have been investigated. For the hot-pressed magnets produced from both crystallized and amorphous powders, the grain pre-orientation and magnetic properties including coercivity, remanence and maximum energy product increase with the increasing temperature, pressure and holding time in the ranges of 650–800 °C, 300–600 MPa and 30–150 min, respectively. The hard magnetic properties of the hot-pressed magnets obtained from amorphous powder are slightly higher than those from crystalline powder due to the smaller grain size of the former. The magnetic properties of hot-deformed magnets are mainly determined by the deformation temperature and not influenced by the state of the powder and hot-pressed precursors. The hot-pressed magnets and hot deformed magnets with different microstructures exhibit different magnetization mechanisms. The optimal hot-deformation temperature is found to be 750 °C for obtaining a high maximum energy product of 39 MGOe. The present results suggest that the rapidly quenched amorphous powder can be used to not only directly prepare hot-pressed magnets with higher magnetic properties, but also to reduce the process step for hot-deformed Nd-Fe-B magnets.

Effect of heating rate on densification and magnetic properties of MnZn ferrites sintered by multiphysical fields coupling methodology

This work has provided a novel idea of fabricating MnZn bulk ferrite materials. MnZn ferrite powders with a composition of Zn0·8Mn0·2Fe2O4 (wt-%) were sintered under coupled multiphysical fields applied by Gleeble-1500D thermal simulation instrument. Cylindrical bulk ferrites with a diameter of 2·5 mm and a height of 2·5 mm were prepared, and the effect of heating rate on densification and magnetic properties of MnZn ferrites was studied. The results show that the densification and magnetic properties of MnZn ferrite were directly affected by the heating rates. A relative density of up to 98·5% was achieved when the heating rate of 100°C s−1 was used. Such magnetic properties as maximum energy product and remanence of MnZn ferrite sintered under coupled multiphysical fields are improved when compared to samples produced by conventional sintering. In particular, a 204·39% increase in maximum energy product is obtained. Finally, the densification mechanisms of multiphysical fields sintering method are discussed.

Significant changes in crystallization kinetics of Nd2Fe14B/α-Fe nanocomposites induced by Nb addition

The effects of Nb addition on the crystallization kinetics, microstructure and magnetic properties of melt-spun Nd2Fe14B/α-Fe nanocomposite magnets have been studied. The analysis of crystallization kinetics for the alloys indicates that the growth of crystallites in the Nb-doped alloy needs more activation energy than that in the Nb-free alloy. This crystallization behaviour contributes to the formation of a finer and more homogeneous microstructure in the Nb-doped alloy than in the Nb-free alloy. And the Nb addition makes the grains more equiaxed shape. The intrinsic coercivity and the maximum energy product increase from 600 kA/m, 105 kJ/m3 for Nd10Fe84B6 alloy to 750 kA/m, 120 kJ/m3 for Nd10Fe83Nb1B6 alloy, respectively.

Study on crystallization behavior and microstructure of melt-spun Nd2(Fe,Nb)14B/α-Fe alloys

The effects of Nb addition on the crystallization behavior, microstructure, and magnetic properties of melt-spun Nd2Fe14B/α-Fe nanocomposite magnets have been studied. The addition of Nb can significantly improve the thermal stability of amorphous phase in as-spun alloys, narrow the range between the onset crystallization temperature and the optimal annealing temperature, restrain the initial formation and growth of α-Fe and Nd2Fe14B. A finer and more homogeneous microstructure can be obtained in the Nb-doped alloy than in the Nb-free alloy. And the Nb addition makes the grains more equiaxed shape. The Nd10Fe83Nb1B6 alloy annealed at 715 °C for 10 min exhibits the improved magnetic properties, Br = 0.90 T, iHc = 750 kA/m, (BH)max = 120 kJ/m3, the intrinsic coercivity and the maximum energy product increase by 25% and 14%, compared with the Nd10Fe84B6 alloy.

Effect of Cu doping and annealing treatment on the microstructure and magnetic properties of nanocrystalline single-phase Nd-Fe-B alloys

The effects of Cu addition and annealing treatment on the magnetic properties and microstructure of Nd 12.3Fe 81.7– x Cu x B 6 ( x=0–1.2) ribbons melt-spun and annealed were systematically investigated by the methods of vibrating sample magnetometer (VSM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Optimum magnetic properties were achieved by annealing melt-spun Nd 12.3Fe 81.5Cu 0.2B 6 ribbons at 550°C for 15 min, which only contained Nd 2Fe 14B phase. The remanence, coercive force, and maximum energy product increase by 18.4%, 36.2%, and 49% respectively compared with those of Cu-free samples. The significant improvement in magnetic properties originates from the finer grains of the samples by introducing Cu, which leads to the stronger exchange-coupling between neighboring grains.

Effects of Nb and Zr additions on crystallization behavior, microstructure and magnetic properties of melt-spun (Nd,Pr) 2Fe 14B/α-Fe alloys

Nanocomposite (Nd,Pr) 2Fe 14B/α-Fe magnets were prepared by crystallizing the amorphous melt-spun ribbons. The effects of Nb and Zr additions on the crystallization behavior, microstructure, and magnetic properties of (Nd 0.4Pr 0.6) 8.5Fe 85.5− x − y Nb x Zr y B 6 ( x = 0 , 0.5 , 1 ; y = 0 , 0.5 , 1 ) alloys were studied. The microstructure and phase analysis shows that for (Nb,Zr)-free ribbons, α-Fe first precipitates, followed by the crystallization from amorphous to metastable phase before final transformation to the 2 : 14 : 1 phase and α-Fe. However, the combined Nb and Zr additions can increase the precipitating temperature of α-Fe, restrain the formation and growth of α-Fe, avoid the formation of metastable phase, refine and uniform the grain size, and thus greatly enhance intergrain exchange coupling between the hard and soft magnetic phases (verified by δM– H plot). After optimal thermal treatment, the coercivity and maximum energy product increase from 411 kA/m, 107 kJ/m 3 for (Nd 0.4Pr 0.6) 8.5Fe 85.5B 6 sample to 534 kA/m, 144 kJ/m 3 for (Nd 0.4Pr 0.6) 8.5Fe 84.5Zr 0.5 Nb 0.5B 6 sample, respectively.

Micromagnetic analysis of the magnetization reversal processes in Sm(Co,Cu,Fe,Zr)z magnets*

The influence of microstructure on magnetic properties of Sm(Co,Cu,Fe,Zr)z magnets is investigated by micromagnetic finite_element method. The demagnetization curves at different temperatures are also simulated. The calculated results show that the saturation polarization Js depends on the relative proportions of 2∶17 phase and 1∶5 phase. The coercivity and the maximum energy product increase with increasing 2∶17type cell size, while they decrease wi th increasing width of 1∶5type cell boundary. The coercivity analysis shows t hat the magnetization reversa l in Sm(Co,Cu,Fe,Zr)z magnets is mainly controlled by nucleation at cell bounda ry. The abnormal temperature dependence of coercivity is due to the fact that the cell boundary becomes less magnetic even nonmagnetic and the nucleation is difficult with increasing temperature.

Magnetically soft phase in magnetization reversal processes of nanocompositeSm2Fe15Ga2Cx/α−Fepermanent magnetic materials

The magnetic properties and magnetization reversal processes of nanocomposite ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{15}{\mathrm{Ga}}_{2}{\mathrm{C}}_{x}/\ensuremath{\alpha}\ensuremath{-}\mathrm{Fe}$ permanent magnet materials have been investigated. Remanence increases and intrinsic coercivity decreases drastically with increasing magnetically soft phase \ensuremath{\alpha}-Fe content. About 5 vol. % excess of \ensuremath{\alpha}-Fe results in a significant increase in maximum energy product from 5.6 MG Oe for single-phase nanocrystalline alloys to 8.2 MG Oe for two-phase nanocomposites. A further increase in \ensuremath{\alpha}-Fe content leads to a decrease of the maximum energy product owing to the rapid decrease of coercivity and the deterioration of the squareness of the hysteresis loop. The nucleation field for reversing the magnetically hard phase decreases monotonically with increasing \ensuremath{\alpha}-Fe content. Remanence enhancement, a high degree of reversibility, and high ratios of remanence coercivity to intrinsic coercivity are observed in two-phase nanocomposites.

Microstructure refinement and significant improvements of magnetic properties in Pr 2Fe 14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb) 2(Fe,Nb,Zr) 14B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.

Microstructure refinement and improvements of magnetic properties ofPr2(Fe, Co)14B/α-(Fe, Co) nanocomposites by additionalGa

The effect of additive Ga on the microstructure and magnetic properties ofnanocomposite Pr2(Fe,Co)14B/α-(Fe,Co) ribbons has been investigated. One per cent Ga addition was found toimprove significantly its magnetic properties. The remanence and maximumenergy product increase from 1.14 T, 17 MG Oe for Ga-free samples to 1.22 T,22.2 MG Oe for Ga-doped samples. The significant improvements of magneticproperties originate from the refinement of the grains of the samples byintroducing Ga, which leads to a stronger exchange coupling between themagnetically hard and soft phases in comparison with that inGa-free samples.

NITROGENATION OF THE SAMARIUM–COBALT INTERMETALLIC, Sm2Co7

The effects of nitrogenation on the 2:7 samarium–cobalt intermetallic, Sm2Co7 , are studied. The uptake of nitrogen by Sm2Co7 increases monotonically with the processing temperatures. The saturation magnetizations of Sm2Co7Nx are seen to increase initially with the nitrogen absorption, followed by decrease upon further nitriding. The behaviors of M s of the nitrides are explained within the framework of the simple Stoner picture of itinerant electron magnetism. The coercive force and maximum energy product increase with the nitrogen absorption.

The influence of the powder particle size on the anisotropic properties of NdFeB magnets produced by single-stage hot deformation

Anisotropic NdFeB magnets were prepared by single-stage hot deformation using rapidly quenched isotropic powder as raw material. The influence of the particle size of the raw material on the magnetic properties of the anisotropic NdFeB magnets was studied. The best hard magnetic properties were achieved for the particle size range of 45–100 μm. The highest increase of maximum energy product was about 17% due to the increase of remanence by 11% compared with unscreened powders. The effects of particle size on the magnetic properties were discussed on the basis of hot workability and oxidation.

Microstructure refinement and improvements of magnetic properties of two-phase exchange-coupled Sm2Fe15Ga2Cx/α-Fe nanocomposites by additional Zr

The effect of additive Zr on the microstructure and magnetic properties of nanocomposite Sm2Fe15Ga2Cx/α-Fe permanent magnet has been investigated. A few percent of Zr addition was found to improve significantly its magnetic properties. After an appropriate heat treatment, the remanence, intrinsic coercivity, and maximum energy product increase from 6.4 kG, 5.5 kOe, and 5.6 MGOe for the Zr-free sample to 7.2 kG, 7.7 kOe, and 9.0 MGOe for the Zr-containing one, respectively. The significant improvements of magnetic properties originate from the microstructure refinement by introducing Zr. The “exchange-spring” behavior and the exchange-coupling between the magnetically hard phase Sm2Fe15Ga2Cx and the soft one α-Fe are discussed as well.

“Interaction Anisotropy” Model of the Structure of Alnico Magnet Alloys

The permanent magnet properties of Alnico alloys have been attributed to the shape anisotropy of an elongated single-domain magnetic precipitate oriented by cooling in a field. This paper reports measurements of the angular variation of the magnetic properties of highly directional crystal-oriented Alnico 5 DG,Columax, and Ticonal XX which are inconsistent with the simple shape anisotropy model. A domain “interaction anisotropy” model is proposed which treats the structure as a complex interconnected single-domain network. The interaction between single-domain elements within this structure minimizes magnetostatic and exchange energy by introducing local variations in particle domain configuration; these nucleate buckling magnetization reversal at particle irregularities and cross-links. In its simplest form this structure can be visualized as a series of H-shaped single-domain size units. The rather complex behavior of such a structure has been analyzed by measuring the magnetic properties of analogous physical models consisting of arrays of pivoted magnets. This analysis suggests that the progressive increase in maximum energy product from 6 million gauss-oersteds for Alnico 5 DG and 8 million gauss-oersteds for Columax to twelve million gauss-oersteds for Ticonal XX is associated with the suppression of buckling through the reduction of particle irregularities and cross-links. Electron micrographs of the structures of these materials and their observed angular variation of magnetic properties are consistent with this new model.