Effect of high magnetic field on the crystallization of Nd_2Fe_(14)B/α-Fe nanocomposite magnets

Nd8.1Dy0.9Fe76.95Co8.55B5.5 nanocomposite magnets annealed with and without a 10 T magnetic field were investigated in this article. The ribbons with coexisting amorphous and crystalline phases were selected to do this study. The resuits of Moessbauer spectroscopy revealed that the content of α--Fe increased when annealed in high strength magnetic field. The size of the grains also increased considerably after the application of magnetic annealing. All these led to the decrease of the magnetic properties, especially the coercivity of the ribbons.

Effect of Gallium Addition on Magnetic Properties of Nd_2Fe_(14)B-Based/α-Fe Nanocomposite Magnets

The influence of Ga addition on the crystallization behavior and the magnetic properties of nanocomposite Nd2Fe14B-based/α-Fe magnets was investigated. It was found that the addition of 0.2% did not change the crystallization temperature of amorphous alloy, but the magnetic properties were improved significantly because of the strong exchange coupling interaction between the hard and soft magnetic phases. The optimum magnetic properties with iHc = 600. 3 kA· m^-1, B r = 0.75 T, and （BH）max = 88.03 kJ· m^-3 were obtained in bonded Nd9.5（FeCoZr）83.8 Ga0.3 B6.5 magnet with 15 m·s^- 1 wheel speed and 670 ℃ annealing treatment. The apparent improvement of magnetic properties originates from the grain refinement calculated using the Scherrer formula from corresponding XRD patterns and the excellent rectangularity of the demagnetization curve.

Preparation and Magnetic Properties of Melt-Spinning Nd_2Fe_(14)B/α-Fe Nanocomposite Magnets

Nd_(11)Fe_(71)Co_8V_(1.5)Cr_1B_(7.5) magnet was prepared by melt-spinning and subsequently annealed. The effects of the wheel speed on the magnetic properties and microstructure were studied. The results reveal that fine nanocomposite microstructure consisting of Nd_2Fe_(14)B and α-Fe phases can be developed at an optimum wheel speed of about 21 m·s^(-1). After optimal annealing (640 ℃×4 min), magnetic properties of B_r=0.64 T, (()_jH_c)=903.5 kA·m^(-1) and (BH)_(max)=71 (kJ·m^(-3)) were obtained for the bonded magnets. The addition of Cr element significantly reduces grain size, increasing the intrinsic coercivity and maximum magnetic energy product.

Computer Simulation of Effect of Intergrain Exchange Interaction on Magnetic Properties of Nanocomposite Magnets

Effects of the intergrain exchange interaction on magnetic properties of nanocomposite magnets were investigated by using the computer simulation based on the micromagnetic theory. The simulation was carried out under the assumptions that the strength of the intergrain exchange interaction is weaker than that of the intragrain exchange interaction, that inhomogeneous nanostructures result in the distribution of the strength of the intergrain exchange interaction, and that there exists nonmagnetic intergranular phase (NMIP) between grain boundaries. The distribution of the strength of the intergrain exchange interaction was simulated by the lognormal distribution with the standard deviation of σ.The calculations for Nd 2Fe 14B/α-Fe nanocomposite magnets reveal that a suitably weak intergrain exchange interaction and small grain size enable us to improve magnetic properties. It is also found that a Nd 2Fe 14B/α-Fe nanocomposite magnet has a potential of a (BH) max value exceeding 300 kJ·m -3. On the other hand, the calculations for Nd 2Fe 14B/Fe 3B nanocomposite magnets reveal that the distribution of the strength of the intergrain exchange interaction deteriorates magnetic properties significantly. Particularly, this tendency is remarkable, when the grain size L is larger than its optimum value, 11 nm. The existence of nonmagnetic boundary layers accelerats this tendency. At σ=0.2, the calculated demagnetization curve for the model magnet composed of Nd 2Fe 14B(36%)/Fe 3B(54%)/NMIP(10%) (Valume fraction) grains (L=15 nm) agrees with that obtained experimentally for a Nd 2Fe 14B/Fe 3B nanocomposite magnet. These results suggest importance of refinement of grain size, suppression of a nonmagnetic intergranular phase, and preparation of homogeneous nanostructure for superior magnetic properties.

Structure and Magnetic Properties of Nd-Fe-B/α-Fe Nanocomposite Magnets by Co, Nb, Dy Substitutions

Structure and magnetic properties of the nanocomposite magnets prepared by mechanical al loying procedure with composition 55 wt pct Nd (Fe0.92B0.08)5.5+45 wt pct α-Fe,55 wt pct Nd(Fe0.8-.Co0.12Nbx B0.08)5.5+45 wt pct α-Fe (x=0.00, 0.01- 0.03) and 55 wt pct (Nd0.9Dy0.1) (Fe0.77Co0.12Nb0.03B0.08)5.5+45 wt pct α-Fe were studied. It was found that substitution of Co for Fe could significantly improve the permanent magnetic properties of the nanocomposite magnets and typically, the maximum magnetic energy product was increased from 104.8 kJ/m3 (13.1 MGOe) to 141.6 kJ/m3 (17.7 MGOe). In contrast to the case of conventional nominally single-phase magnets, the addition of Nb results in promoting the growth of α-Fe grain and is thus unfavorable for the improvement of permanent magnetic properties of the nanocomposites. Although the addition of Dy can increase the coercivity of the magnets, the increase of magnetic anisotropy of hard phase leads to decrease of the critical grain size of soft phase. Additionally it causes the difficulty of preparing the nanocomposites because it is more difficult to control the grain size of soft phase to meet the requirement of appropriate exchange coupling between hard and soft grains

Micromagnetism simulation on effects of soft phase size on Nd2Fe14B/α-Fe nanocomposite magnet with soft phase imbedded in hard phase

In this study, micromagnetism simulation by using timte dltterence method is cameo out on the Ncl21-el415/a-Fe nanocomposite magnet with soft phase imbedded in hard phase. The effects of soft magnetic phase size （S） on the magnetic properties and magnetic reversal modes are systematically analyzed. As S increases from 1 nm to 48 nm, the remanence （Jr） increases, while the coercivity （Hci） decreases, leading to the result that the magnetic energy prod- uct [（BH）max] first increases slowly, and then decreases rapidly, peaking at S = 24 nm with the （BH）max of 72.9 MGOe （1 MGOe = 7.95775 kJ.m-3）. Besides, with the increase of S, the coercivity mechanism of the nanocomposite magnet changes from nucleation to pinning. Furthermore, by observing the magnetic moment evolution in demagnetization pro- cess, the magnetic reversal of the soft phase in the nanocomposite magnet can be divided into three modes with the increase of S： coherent rotation （S 〈 3 nm）, quasi-coherent rotation （3 nm≤S 〈 36 nm）, and the vortex-like rotation （S ≥36 nm）.

Crystallization kinetics of amorphous Nd_(3.6)Pr_(5.4)Fe_(83)Co_3B_5 and preparation of α-Fe/Nd_2Fe_(14)B nanocomposite magnets by controlled melt-solidification technique

The crystallization kinetics of amorphous (Nd3.6Pr5.4Fe83Co3B5) and the preparation of α-Fe/Nd2Fe14B nanocomposite magnets by controlled melt-solidification of (Nd3.6Pr5.4Fe83Co3B5) was investigated by employing DTA, XRD, and TEM. The results show that a metastable intermediate phase (Nd8Fe27B24) prior to α-Fe and Nd2Fe14B phases is crystallized as the amorphous Nd3.6Pr5.4Fe83Co3B5 is heated to 1 223 K. The crystallization activation energy of α-Fe and Nd8Fe27B24 phases is larger at the beginning stage of crystallization, and then it decreases with crystallized fraction x for the former and has little change when x is below 70% for the latter, which essentially results in an α-Fe/Nd2Fe14B microstructure with a relatively coarse grain size about 20-60 nm and a non-uniform distribution of grain size in the annealed alloy. The α-Fe/Nd2Fe14B nanocomposite magnets with a small average grain size about 14 nm and a quite uniform grain size distribution were prepared by controlled melt-solidification of (Nd3.6Pr5.4Fe83Co3B5) at a wheel speed of 20 m·s-1 during melt-spinning. The magnets show a high maximum energy product of （BH）max = 194 kJ·m-3, which is nearly twice of that of the nanocomposite magnets made by annealing the amorphous Nd3.6Pr5.4Fe83Co3B5 precursor alloy.

Morphology and magnetic properties of Sm_(2)Co_(7)/α-Fe nanocomposite magnets produced by high energy ball milling and spark plasma sintering

In this article,the Sm_(2)Co_(7)/α-Fe nanocomposite magnets were prepared by high energy ball milling and spark plasma sintering method.The effect of soft phase content on the magnetic properties was studied.Up to 30 wt% α-Fe was added into Sm_(2)Co_(7) matrix without the decrease of remanence.Optimal energy product(BH)max of 9.2 MGOe was obtained with 20 wt% α-Fe.TEM observation shows that the grain size of α-Fe is 20-50 nm which ensures a good coupling effect between soft and hard phase.One more thing needs to be mentioned is that there exists inter-diffusion between Sm-Co phase and α-Fe phase.Moreover,our results can also illustrate that the Sm_(2)Co_(7)/α-Fe nanocomposite magnets are able to acquire better magnetic properties than the SmCo_(5)/α-Fe magnets prepared by the same process due to the large domain width of Sm_(2)Co_(7) phase.

Microstructure and magnetic properties of(SmCo+FeCo)/NdFeB multicomponent nanocomposite magnets fabricated by HPTC with change in heating temperature and composition

We report the fabrication of bulk anisotropic(SmCo+FeCo)/NdFeB multicomponent nanocomposite magnets using high-pressure thermal compression(HPTC).The correlations among microstructure,magnetic properties,heating temperature and composition of the HPTC nanocomposite magnets were studied.The HPTC magnet made under variable temperatures(VT),with 19 wt% of FeCo phase,exhibits a maximum energy product of 32 MGOe,which is much higher than that(14 MGOe) for the HPTC magnet made under a constant temperature(CT).When the FeCo content increases to^23 wt%,the HPTC magnet made under VT still remains a high energy product of about 25 MGOe.With increasing NdFeB content,the(SmCo+FeCo)/NdFeB multicomponent nanocomposite magnets exhibit an enhanced magnetic anisotropy and coercivity.This work is beneficial to fabricating high-performance and low-cost permanent magnets for practical applications.

Nanocrystalline and nanocomposite permanent magnets by melt spinning technique

The melt-spinning technique offers an opportunity for tailoring magnetic properties by controlling the structures and microstructures in both single-phase and composite magnets.This review first broadly discusses the principle of cooling control,amorphization,crystallization,annealing,and consolidation of the melt-spun ribbons.The phase,microstructure,and magnetic properties of popular single-phase nanocrystalline magnets are reviewed,followed by the nanocomposite magnets consisting of magnetically hard and soft phases.The precipitation-hardened magnetic materials prepared by melt spinning are also discussed.Finally,the role of intergrain exchange coupling,thermal fluctuation,and reversible/irreversible magnetization processes are discussed and correlated to the magnetic phenomena in both single-phase and nanocomposite magnets.

Preferred Orientation in Nanocomposite Permanent Magnet Materials

Melt-spun （Nd11.4Fe82.9B5.7）0.99M1 ribbons （M = Zr, Nb, Ga, Zr＋ Ga, Nb ＋ Ga）were prepared by melt-spinning technique. Ga addition is found to be effective for the orientation of c-axis of Nd2Fe14B grains perpendicular to the ribbon plane. Better magnetic properties can be achieved by adding both the two kinds of elements Zr ＋ Ga, Nb ＋ Ga, and it is found that the preferred orientation is further improved. The alignment degree changes with ribbon thickness and is highest when ribbon thickness is 120 μm. Heat treatment can improve the texture degree, but lead to coarser grains. Cryogenic treatment is first applied for the treatment of nanocomposite Nd2Fe14B/α-Fe melt-spun ribbons. The effects on magnetic properties and texture degree of nanocomposite magnets after cryogenic treatment were studied. The result shows that cryogenic treatment is beneficial to the enhancement of texture degree of melt-spun ribbon and the grain size has no obvious change.

Effects of magnetic annealing on crystallization and magnetic properties of Nd_2Fe_(14)B/α-Fe nanocomposite magnets

Crystallization and magnetic properties of Nd2Fe14B/α-Fe nanocomposite magnets have been investigated by annealing the as-spun ribbons with magnetic field.The crystallization process was accelerated by field annealing.The hysteresis loop became to be fat by magnetic annealing at 645 oC for 4 min,which was 690 oC for ribbons annealing without magnetic field.The relative content of α-Fe phase was increased from the results of XRD.The strength of the magnetic field had no obvious influence on the remanence and coercivity,but modified the squareness of hysteresis loop.

Effect of Zirconium Content on Microstructure and Magnetic Properties of Nanocomposite Bonded Magnets

Five kinds of bonded magnets with compositions of Nd(10.5)Fe(78.4-x)Co(5)Zr(x)B(6.1) (atom percentage x = 0, 1.0, 1.5, 2.0, 2.5) were prepared by rapid quenching, post heat treatment and mould-pressing. The microstructure and crystallization behavior were studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and atomic force microscopy (AFM). The results suggest that high content of Zr can increase the glass formation ability (GFA) of alloys. When the content of Zr is controlled at a certain level, Fe,Zr with high melting point is formed in the alloys, and grain size is reduced consequently. At the same time, because of Zr addition, the coercivity and squareness of demagnetization loop are obviously improved, and the energy product is accordingly increased. As a result, optimal magnetic properties of Nd(10.5)Fe(78.4-x)Co(5)Zr(x)B(6.1) (B(t) = 0. 659 T, H(cj) = 628 kA center dot m(-1), H(cb) = 419 kA center dot m(-1) (BH)(m) 73 kJ center dot m(-3)) are obtained when x = 2.

Coercivity enhancement of Nd_2Fe_(14)B/α-Fe nanocomposite magnets through neodymium diffusion under annealing

The coercivity enhancement of ball-milled Nd2Fe14B/α-Fe nanocomposite magnets was investigated. It was found that the coerciv- ity could be enhanced through mixing a small amount of Nd powder with as-milled Fe-rich Nd-Fe-B powders. The annealed samples were investigated by means of X-ray diffraction, scanning electron microscopy and magnetic measurement systems. Under annealing, some of Nd powders promoted the formation of hard magnetic phase Nd2Fe14B. On the other hand, a few of Nd would diffuse into the interface of Nd2FelaB/α-Fe nanocomposite to compensate for the loss of the interracial magnetic anisotropy. These two features are all beneficial to the coercivity.

Texture Evolution in Nanocomposite Nd_2Fe_(140B/α-Fe Magnets Prepared by Direct Melt Spinning

Texture evolution in nanocomposite Nd_2Fe_ 14B/α-Fe magnets prepared by direct melt spinning was investigated. The free surface and wheel-contacted surface exhibit different texture direction. Modification of composition not only enhances magnetic properties, but also changes texture direction of the ribbon. Low temperature heat treatment can increase the magnetic properties to some extent, and high temperature annealing decreases the magnetic properties. Both low and high temperature heat treatment have effects on grain orientation, but the difference still exists between the two surfaces of the ribbon. So it is infeasibility to prepare anisotropic Nd_2Fe_ 14B/α-Fe nanocomposite magnets by direct melt spinning.

B effects on crystallization and magnetic properties of Pr_(9.44)Fe_(90.56-x)B_x(x=7.16, 4.76) nanocomposite magnets

Magnetic properties, phase evolution and microstructures of Pr9.44Fe90.56-xBx （x=7.16 and 4.76） nanocomposite magnets have been investigated. Both as-spun ribbons exhibit two-step demagnetization curves, indicating the feature of partial crystallization. By using di？erential scanning calorimeter （DSC） measurements, remained amorphous phase of ribbon with x=7.16 shows a higher crystallization temperature. At optimum annealing temperature of 650℃ for 10 min, better magnetic properties, Mr=1.01 T, jHc=796 kA/m and （BH）max=136 kJ/m3 were obtained in the sample with x=4.76. Transmission electron microscopy （TEM） observation confirms that the sample with x=4.76 possesses much finer grain size, which causing a better exchange coupling between soft and hard magnetic grains.

High-performance α-Fe/Pr_2Fe_(14)B-type nanocomposite magnets fabricated by direct melt spinning

High-performance α-Fe/Pr2FelnB-type nanocomposite magnets based on the compositions of PrsFes6B6 microalloyed with Co, Nb and C were fabricated by direct melt spinning. The coercivity was greatly improved from 5.5 kOe for the Pr8Fe86B6 ribbons to 7.4 kOe for the Pr8Fe85NbB5C ribbons. The balanced high coercivity and remanence were obtained in Pr8Fe75Co10NbB5C ribbons due to the Co substitution for Fe, which led to the significant improvement of magnetic properties in these ribbons. A remanence ratio of 0.82, a coercive field of 6.6 kOe and a maximum energy product of 26.2 MGOe in melt-sptm Pr8Fe75Co10NbB5C ribbons were ob- tained at room temperature.

Microstructure and magnetic properties of directly quenched Nd_2Fe_(14)B/α-Fe nanocomposite materials at different temperatures

Directly quenched Nd9.5Fe81Zr3B6.5 nanocomposite permanent magnets were prepared under different melt treatment conditions, i.e., the melt temperature was varied prior to ejection onto the quenching wheel. The effect of quenching temperature on the microstructure and magnetic properties of the alloys was studied by X-ray diffractometry, transmission electron microscopy and magnetization measurements. It is found that a finer and more uniform microstructure can be obtained directly from the melt quenched at lower temperature. With increasing initial quenching temperature, the optimal quenching speed decreases and the microstructure of the ribbons becomes coarser and more irregular. As a result, the magnetic properties of the alloys are deteriorated. It is believed that the break of the pre-existing Nd2Fe14B clusters and decrease in number of the developing nuclei of Nd2Fe14B phase with increase in quenching temperature may be the causes for the change of the microstructure and the magnetic properties of the ribbons.

Absorption performance of DMSA modified Fe_3O_4@SiO_2 core/shell magnetic nanocomposite for Pb^(2+) removal

The purpose of this study is to explore the adsorption performance of meso-2,3-dimercaptosuccinic acid(DMSA)modified Fe3O4@SiO2 magnetic nanocomposite(Fe3O4@SiO2@DMSA)for Pb2+ions removal from aqueous solutions.The effects of solution pH,initial concentration of Pb2+ions,contact time,and temperature on the amount of Pb2+adsorbed were investigated.Adsorption isotherms,adsorption kinetics,and thermodynamic analysis were also studied.The results showed that the maximum adsorption capacity of the Fe3O4@SiO2@DMSA composite is 50.5 mg/g at 298 K,which is higher than that of Fe3O4 and Fe3O4@SiO2 magnetic nanoparticles.The adsorption process agreed well with Langmuir adsorption isotherm models and pseudo second-order kinetics.The thermodynamic analysis revealed that the adsorption was spontaneous,endothermic and energetically driven in nature.

Effect of Nd_2Fe_(14)B/α-Fe Nanocomposite Permanent Magnets with a Combined Co and Nb Additions

Influence of Co＋Nb on the Nd8Fe82Co3Nb1B6 nanocomposite magnets was investigated by adding Co element combined with Nb element. Results show that the high temperature stability of two phases is increased. Adding Co＋Nb could improve the glass forming ability of the alloy, reduce the size of grains, increase the exchange coupling ability of two phases, and obviously increase the magnetic properties of the alloy. The optimal magnetic properties are Br=1.14 T, Hcj=320 kA/m, （BH）max=109.3 kJ/m^3