Permanent Magnet Properties Research Articles

Effects of Cu nanopowders addition on magnetic properties and corrosion resistance of sintered Nd–Fe–B magnets

Abstract Sintered permanent magnets have been prepared by blending Nd–Fe–B jet-milled powders with Cu nanopowders (0, 0.05, 0.1 and 0.2 wt%) before pressing in a perpendicular magnetic field. Permanent magnetic properties, Curie temperature ( T c ), microstructures and corrosion resistance of the sintered magnets were investigated. Microstructural characterization revealed that the grain size of Nd 2 Fe 14 B phase was refined and more homogeneous, and the grain boundaries became clear and smooth by the addition of Cu nanopowders, resulting in the improvement of coercivity H c j . Moreover, the corrosion resistance of Nd–Fe–B magnets was also improved. However, T c remained nearly unchanged with Cu addition, which was consistent with the elemental analysis results. Cu was not detected in the matrix phase Nd 2 Fe 14 B, but concentrated in the intergranular region.

Hydrogen-induced diffusion phase transformations in Nd2Fe14B and Sm2Fe17 magnetically hard alloys

The Nd 2 Fe 14 B and Sm 2 Fe 17 magnetically hard alloys become thermodynamically unstable under the action of hydrogen and suffer hydrogen-induced direct and inverse phase transformations at elevated temperatures. The kinetics of these transformations is investigated. It is shown that they are diffusion-controlled and develop according to the mechanism of nucleation and growth. The hydrogen treatment of materials is a new branch of materials science [1] based on the specific features of hydrogen and hydrogen-induced phase transformations. In Nd 2 Fe 14 B- and Sm 2 Fe 17 -type magnetic materials, the hydrogen-induced diffusion phase transformations occur due to the different affinities of the atoms of different components of materials to hydrogen. Since the alloys consist of strong hydride- and nonhydride-forming elements, they lose their thermodynamic stability as a result of saturation with hydrogen. At low temperatures, these materials exist in the nonequilibrium (metastable) state. At elevated temperatures (T > (0.20 – 0.45) T m , where T m is the melting point), the diffusion of large atoms of the material becomes possible, and we observe the realization of hydrogen-induced diffusion phase transformations. The alloys decompose into rare-earth hydrides ( Nd H x or Sm H x ) and the α -phase of iron. In the Nd 2 Fe 14 B alloy, we also observe the formation of iron boride (Fe 2 B). The evacuation of hydrogen from the alloy after the direct transformation requires the thermodynamically inverse phase transformation resulting in the formation of the Nd 2 Fe 14 B or Sm 2 Fe 17 initial phase. As a result of the direct and inverse phase transformations, the microstructure is refined, which improves the magnetic properties of permanent magnets prepared from the alloys treated as indicated above [2]. In what follows, we generalize the results of investigations of the kinetics of hydrogen-induced direct and inverse diffusion phase transformations in the Nd 2 Fe 14 B or Sm 2 Fe 17 alloys and discuss the nature of these transformations within the framework of solid-state physics.

Magnetic properties, microstructure, and phase evolution of Pr xFe bal.Ti yB 20−x ( x=4–9; y=2.5–5) nanocomposites

Magnetic properties, microstructure, and phase evolution of Pr lean and boron-enriched Pr x Fe bal.Ti y B 20− x ( x=4–9; y=2.5–5) melt-spinning ribbons with nanostructures have been investigated. Based on thermal magnetic analysis (TMA), for y=2.5, two phases, namely Pr 2Fe 14B and α-Fe, were found for ribbons with x=9, while additional two metastable phases, Pr 2Fe 23B 3 and Fe 3B, existed for x=4, 7 and 8. With the decrease of Pr content, the remanence increases but coercivity decreases. The optimal properties of B r=9.5 kG, i H c=10.7 kOe, and ( BH) max=17.8 MG Oe are achieved in Pr 9Fe bal.Ti 2.5B 11 nanocomposites. On the other hand, higher Ti substitution for Fe in Pr 7Fe bal.Ti y B 13 ribbons could refine the grain size and suppress the metastable Pr 2Fe 23B 3 and Fe 3B phases effectively. The excellent permanent magnetic properties are mainly dominated by the nanoscaled microstructures and the coexistence of sufficient magnetically soft phases, Fe 3B, Pr 2Fe 23B 3 and α-Fe, with magnetically hard Pr 2Fe 14B phase.

Thermophysical properties of Nd-, Er-, YNi-alloys

Abstract Nd – Fe – B alloys are widely used in many technical applications due to their excellent permanent magnetic properties. YNi2B2C intermetallic compounds receive much attention because of the coexistence of superconductivity and magnetic ordering phenomena. Er2PdSi3 alloys with hexagonal AlB2-type crystallographic structure are of special interest because of their complex magnetic ordering, Kondo and heavy fermion behavior. The thermophysical properties (electrical conductivity, thermoelectric power, thermal conductivity and viscosity) of these liquid multicomponent intermetallic alloys have been measured in order to supply necessary information for crystal growth and casting processes. Good reproducibility of the results has been obtained.

Effect of pulsed magnetic field treatment on the magnetic properties for nanocomposite Nd 2Fe 14B/α-Fe alloys

Nanocomposite alloys with the compositions of Nd 8.5Fe 77.6− x Co 5Zr 2.7Ga x B 6.2 ( x = 0, 0.3, 0.6, 0.8, 1.0) were prepared by melt spinning. It was found that a small amount of 0.6 at.% Ga addition is beneficial to improve the hard magnetic properties of the alloy. The melt-spun Nd 8.5Fe 77Co 5Zr 2.7Ga 0.6B 6.2 ribbons were annealed in a pulsed magnetic field. The results revealed that the remanence J r and the maximum energy product ( BH) max were obviously improved when annealed below the Curie temperature of the alloy. The best magnetic properties of J r = 0.975 T, i H c = 582 kA/m, and ( BH) max = 131 kJ/m 3 were obtained for the ribbons annealed at 300 °C in a pulsed magnetic field. The ( BH) max was enhanced by 23.8% compared with that of the ribbons without magnetic annealing. It provides a new way to improve the magnetic properties of permanent magnets produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys.

Order-disorder transformations in Sm–Co and Sm–Co–ZrC systems with 2-17 stoichiometry

This work investigates the order-disorder transformations utilizing time-resolved x-ray diffraction at the Advanced Photon Source. The metastable phases that can form during the order-disorder transformations in the Sm–Co intermetallics have the potential to produce materials with enhanced permanent magnetic properties. The high-temperature experiments transformed the disordered alloys with the TbCu7-type structure obtained by rapid solidification into the mixture of the hexagonal and rhombohedral ordered structures at 1375K. The ordering process and the role of the ZrC alloying on the phase formation and ordering transformations were examined. The results showed the formation of shoulders of the fundamental peaks and superlattice peak broadening prior to complete ordering. The lattice parameters expanded linearly up to the onset of ordering, after which an abrupt change of slope was observed. The change in slope suggests a nucleation and growth mechanism for the ordering transformation. The effect of ZrC alloying promoted the formation of the disordered structure in the as-solidified state and lowered the onset of the ordering temperature.

Modelling of a planar magnetic micropusher for biological cell manipulations

The improvement of the efficiency and the automation of biological cell technologies are currently of great importance. One way is to build biological micro-factories which are able to perform complete biotechnological processes automatically. This technology requires the development of new automatic cell transport system to feed work stations in micro-factories. An original magnetic cell micropusher is described in this paper. The ferromagnetic pusher which is submerged in the biological medium follows the movement of a permanent magnet located in the air. This paper focuses on the modelling of the dynamic behaviour of the micropusher according to the magnet trajectory. The generic model proposed is able to determine pusher trajectory according to the micropusher magnetic properties and the permanent magnet shape and properties. This simulation tool will permit to optimize and to study cell trajectory control in further works.

Magnetic properties of rare-earth permanent magnets under a high radiation environment

An study on the demagnetization of rare-earth permanent magnets under high radiation environment is started from the microscopic point of view. The demagnetization of NEOMAX is successfully induced by the well defined neutron field produced by the 5 MW reactor in Kyoto University. Preliminary TDPAC measurement of 111Cd(←111In) in NEOMAX, including demagnetized one, is reported.

Magnetically nonlinear dynamic model of synchronous motor with permanent magnets

This paper deals with a magnetically nonlinear two-axis dynamic model of a permanent magnet synchronous motor (PMSM). The geometrical and material properties of iron core and permanent magnets, the effects of winding distribution, saturation, cross-saturation and slotting effects are, for the first time, simultaneously accounted for in a single two-axis dynamic model of a three-phase PMSM. They are accounted for by current- and position-dependent characteristics of flux linkages. These characteristics can be determined either experimentally or by the finite element (FE) computations. The results obtained by the proposed dynamic model show a very good agreement with the measured ones and those obtained by the FE computation.

A New Standstill Position Detection Technique for Nonsalient Permanent-Magnet Synchronous Motors Using the Magnetic Anisotropy Method

This paper describes a new method to estimate the standstill position of permanent-magnet synchronous motors (PMSMs). The method is based on the anisotropic properties of permanent magnets and is therefore referred to as a magnetic anisotropy method (MAM). It differs significantly from all standard standstill position detection methods previously presented in the literature. It is totally independent of saliency and saturation effects and can therefore be used in many types of PMSMs where the standard methods would fail. The paper fully explains the experimental results and briefly analyzes the physical phenomena behind the method

Influence of dynamic crystallization on exchange-coupled NdFeB nanocrystalline permanent magnets

Dynamic crystallization was introduced to improve the magnetic properties of NdFeB nanocrystalline permanent magnets by optimizing microstructure. The microstructure was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It has been determined that, compared with the conventional heat treatment, dynamic crystallization can shorten the crystallization time. Moreover, dynamic crystallization can refine grains, enhance the exchange-coupled interaction among grains, and promote the magnetic properties. As a result, the optimal magnetic properties of Nd 10.5(FeCoZr) 83.4B 6.1 ( B r = 0.685T, H ci = 732 kA·m −1, H cb = 429 kA·m −1, (BH) m = 75 kJ·m −3) are obtained after dynamic crystallization heat treatment at 700 °C for 10 min.

The effect of compound addition Dy 2O 3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy 2O 3 and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy 2O 3 or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy 2O 3 additions result in the increase in the H ci and temperature dependence due to the increase of T c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy 2O 3 and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N eff and the smaller Φ phase.

Microstructure and magnetic properties of electrodeposited CoPtP alloys

CoPtP alloys have been grown by electrochemical process in an external magnetic field in order to enhance the perpendicular magnetic properties. The coercivity and squareness of the alloys were improved as the external magnetic field was increased. The alloys have a mosaic microstructure consisting of grains with size in the range of 10–15nm. The grains are separated by highly well defined 1–2nm grain boundaries. The alloys exhibit a columnar microstructure with a hcp [002] fiber texture. The films exhibit significant perpendicular magnetic anisotropy and permanent magnetic properties in the perpendicular direction, with coercivity of up to 6.1kOe and squareness of up to 0.9.

Study on the magnetic properties, phase evolution and microstructure of Pr xFe 82.2−yTi yB 17.8−x ( x=5–11.8; y=2.5–5.5) nanocomposites

Pr lean and boron-enriched ingots, i.e., Pr x Fe 82.2− y Ti y B 17.8− x ( x=5 to 11.8; y=2.5 to 5.5), were adopted for preparing melt-spun ribbons with nanostructures. Based on thermal magnetic analysis (TMA), for y=2.5, two phases, namely Pr 2Fe 14B and α-Fe, were found for ribbons with x=8.9, 10.7 and 11.8, while additional two metastable phases, Pr 2Fe 23B 3 and Fe 3B, prevailed for x=6 and 7.1. With the decrease of Pr content, remanence increases but coercivity decreases. The optimal properties of B r=9.5 kG, i H c=9.6 kOe, and (BH) max=17.7 MGOe are achieved in Pr 8.9Fe 79.7Ti 2.5B 8.9 nanocomposites. Meanwhile, better magnetic properties for easy magnetization at lower field, i.e. i H c=6.8 kOe, B r=9.9 kG and (BH) max=16.1 MGOe, are obtained in Pr 7.1Fe 79.7Ti 2.5B 10.7. The excellent permanent magnetic properties are mainly dominated by the nanoscaled microstructures and the coexistence of sufficient magnetically soft phases, Fe 3B, Pr 2Fe 23B 3 and α-Fe, with magnetically hard Pr 2Fe 14B phase.

Corrosion protection of Nd–Fe–B magnets by silanization

Nd–Fe–B type permanent magnets present excellent magnetic properties. However, they are highly sensitive to the attack of corrosive environment. The aim of the present work is to describe the improvement of corrosion resistance of such magnets by dodecyl trimethoxysilane (DTMS) treatment. Electrochemical impedance spectroscopy (EIS) tests showed that, after treated by either DTMS, phosphate or chromate precursor, the protectiveness of Nd–Fe–B alloys was apparently increased. Corrosion performance of magnet treated with silane only was slightly inferior to those of phosphatized or chromatized ones. However, significant improvement in corrosion protection was achieved after two-step treatments, i.e. by top-coating silane films on phosphate or chromate under-layer. The depth profiles of element concentrations on surfaces measured by secondary ion mass spectroscopy (SIMS) indicate a two-layer surface structure for two-step treated samples. Co-existing of silane and phosphate or chromate was detected in the top-layer, suggesting the interaction between the two components. This results in the formation of more stable structure of hybrid organ–inorganic coatings against the attack of corrosive species.

Modeling of microstructure changes in Fe−Cr−Co magnetic alloy using the phase-field method

During the last decade, the phase-field method has emerged in many fields if materials science as a powerful tool to simulate and predict complex microstructure development. In this study, the microstructure changes during thermomagnetic treatment and step aging of the Fe−Cr−Co magnetic alloy are modeled using phase-field methods. Using phase-field simulation, the model reasonably represented microstructural changes in the Fe−Cr−Co system quantitatively. In particular, it is shown that gradual step aging is important to obtain the lamellar microstructure that provides excellent permanent magnetic properties. Modeling of microstructure development with the framework of the phase-field method is shown to be a very effective strategy to predict and analyze complex microstructure formation.

Magnetic Properties and Microstructure of (Fe 0.5+yPt 0.5-y) zB 100-z(y=0–0.2; z= 82 and 84) Melt Spun Ribbons

The Fe-B/FePt-type nanocomposite ribbons with strong exchange-coupling and high permanent magnetic properties have been developed by the melt-spinning method. For ternary (Fe 0.5+yPt 0.5-y) zB 100-z (y=0–0.2; z=82 and 84), multiple phases, namely γ 1, Fe 2B and Fe 3B, with nanoscaled grain sizes (25–35 nm) can readily form in ribbons after an isothermal annealing at 500 °C. Strong exchange-coupling between grains was found in these ribbons, giving rise to an enhancement of not only the remanence but also the maximum energy product. An iH c of more than 12 kOe has been obtained in (Fe 0.6Pt 0.4) 82B 18 (B r= 6.6 kG, (BH) max=8.9 MGOe), and a B r of 9.4 kG, ft of 7.5 kOe and (BH) max of 14.0 MGOe can be achieved in (Fe 0.675Pt 0.325) 84B 16. Magnetically soft Fe 2B and Fe 3B were found to present in the matrix, as evidenced by high resolution transmission electron microscopy. Because of the higher saturation magnetization of Fe 3B than Fe 2B, the coexistence of Fe 3B with Fe 2B and γ 1 phases was believed the main cause to result in the improved remanence and magnetic energy product of the (Fe 0.675Pt 0.325) 84B 16 ribbons.

Effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets

The effect of γ-ray irradiation on the magnetic properties of NdFeB and Fe–Cr–Co permanent magnets has been investigated. The magnetic flux loss of two kinds of magnets before and after irradiation was measured. Results show that the effect of γ-ray irradiation on the magnetic properties of sintered NdFeB is not so obvious as that on Fe–Cr–Co magnet. Irradiation-induced damage from γ-ray for the Fe–Cr–Co magnets was characterized for the first time. The decline of permanent magnetic properties of Fe–Cr–Co magnet induced by γ-ray irradiation is reversible except for the maximum energy product ( BH) max. The difference of coercivity mechanism between these two kinds of permanent magnets is responsible for the different dependence of magnetic properties loss induced by γ-ray irradiation.

Improved perpendicular anisotropy and permanent magnet properties in Co-doped Nd-Fe-B films multilayered with Ta

Multilayers of 200-nm thick Co-free or Co-doped Nd-Fe-B with 10-nm thick Ta were prepared by sputtering on heated glass substrates and investigated into the microstructure, texture and hard magnet properties. The perpendicularly textured Nd/sub 2/Fe/sub 14/B grains evolved in the multilayers with sizes neatly controlled by the thickness of the unit layer. Co was found to dissolve in the Nd/sub 2/Fe/sub 14/B grains, resulting in the enhanced c-axis texture possibly due to the modulated surface energy of the Nd/sub 2/Fe/sub 14/B phase. It was also found that Co boosts the crystallization and grain growth of the Nd/sub 2/Fe/sub 14/B phase, rapidly leading to the relatively stable multilayered structure where the dimension of the Nd/sub 2/Fe/sub 14/B grains is exactly defined by the thickness of the unit layer. Such effect of Co suppresses the local variation of the microstructure and minimizes the deterioration of the permanent magnet properties of the multilayer with an increased thickness of 10-/spl mu/m.

Passive effects of rare-earth permanent magnets on flexible conductive structures

The paper presents a theoretical and experimental study of vibrating structures where paramagnetic or diamagnetic systems interact with rare-earth passive magnets. The theoretical model of the system is focused on the damping properties of permanent magnets and on their interactions with the dynamic behaviour of an Euler–Bernoulli beam. In particular, the magnetic model is based on the analogy of the equivalent currents method in a quasi-static open-circuit-type configuration and it is used to determine the influence of eddy currents on the dynamic behaviour of conducting material structures. The magnetic effects are characterised by a viscous-type damping and by a stiffening dynamic effect of the structure, called “phantom effect”. The authors present the experimental outcomes for uniform cantilever clamped-free beams of different kinds of paramagnetic or diamagnetic conducting materials. It appears that the system frequency response can be modified by the presence of a pair of concordant or discordant permanent magnets of high residual induction settled at the free end. Through the comparison between theoretical and experimental results, the paper demonstrates the validity of the model, that is able to describe both the above mentioned effect of dynamic stiffening of the structure and the considerable localised damping properties in paramagnetic or diamagnetic materials having low electric resistivity.