Low Coercive Force Research Articles

Comparative Study on Variable Flux Memory Machines With Parallel or Series Hybrid Magnets

Variable flux memory (VFM) permanent magnet (PM) machines exhibit an additional degree of freedom for control, i.e., PM magnetization state, and thus excellent flux controllability. Moreover, the hybrid PM topologies having variable PM (VPM) with low coercive force and constant PM (CPM) with high coercive force at the same time are employed to improve the torque density and the flux controllability. The parallel and series connections between the two different kinds of PMs are both feasible. Based on equivalent magnetic circuits, two-dimensional finite element analyses and experiments, the VFM machines with these two connection types are investigated and compared in this paper. The results reveal that the VPM with series-connected CPM is beneficial for more stable working point and higher torque density. A pair of VFM prototypes with parallel and series hybrid PMs are manufactured and tested to validate the analyses.

Magnetization State Regulation Characteristic Study of Series Hybrid Permanent Magnet Axial Field Flux-Switching Memory Machine

Due to the short axis, high torque density, and wide permanent magnet (PM) flux linkage regulation range, the series hybrid PM axial field flux-switching memory machine (SHPM-AFFSMM) has been paid much attention. In order to extend its speed range, the way to generate an effective current pulse to regulate the magnetization state of the low coercive force PM is the key issue. The magnetization state regulation (MSR) method characterized with bidirectional and quantitative current pulse is presented to investigate the MSR characteristics of the SHPM-AFFSMM. Based on the proposed MSR method, the MSR characteristics of the researched machine, including the inductance characteristics, cogging torque, and permanent magnet flux density, are investigated by simulation and experiment.

Amorphous formation and magnetic properties of Co-containing Fe[sbnd]Si[sbnd]B[sbnd]P[sbnd]Cu nanocrystalline alloys

Fe84.3-xCoxSi4B8P3Cu0.7 (x = 0, 2, 4, 6, 8, and 10 at.%) alloy ribbons were fabricated by melt spinning. Results showed that the FeCoSiBPCu alloys added with 0–10 at.% Co showed improved amorphization. The modified alloys displayed not only high thermal stability but also high heat treatment temperature in the range of 150–160 K. Compared with the other Co-containing alloys, the alloys added with 2–4 at.% Co exhibited a higher saturation magnetization (Ms) of approximately 181–194 emu/g and a lower coercive force (Hc) of approximately 10.2–12.3 A/m, thus showing improved magnetic properties in the melt-spun state. When annealed before the secondary onset crystallization temperature, all alloys crystallized a single α-Fe phase. After annealing at 833 K for 10 min, the alloys added with 8–10 at.% Co showed excellent soft magnetic properties, including a high Ms of 198–200 emu/g and a low Hc of 2.6–2.9 A/m.

A Novel Dual-Sided PM Variable Flux Memory Machine

Memory machine (MM) was regarded as a promising candidate for wide-speed-range applications [1]–[6]. Since the low coercive force (LCF) PM is employed in MM, the air-gap flux can be flexibly adjusted by changing the magnetization state (MS) of PM with a temporary current pulses. The speed range can be subsequently extended with high efficiency maintained. According to the magnetizing current pattern, MMs can be generally categorized into AC- [1]–[3] and DC-magnetized [4]–[6] types. The former MM normally employs vector-control algorithm to apply d-axis current pulse in stator windings to vary the magnetization level of LCF PMs, while the latter one adopts auxiliary DC magnetizing coils to facilitate the magnetization control. Besides, the rotor-PM topologies of MMs have structures similar to conventional interior PM machines, which shows comparable torque density with those conventional surface-mounted PM machines . Nonetheless, LCF magnets on the rotor generally suffer from to armature reaction demagnetization effect, and the integration of armature and magnetizing functions in the stator winding results in complicated online FW control. On the other hand, the stator-PM MMs have the advantages of easy online magnetization control, and a simple and robust salient rotor as well as easy thermal management can be obtained. Nevertheless, the fact that two kinds of PMs, i.e., NdFeB and LCF PMs, as well as two sets of windings are located on stator leads to excessively crowded stator space and low torque density. Therefore, this paper attempts to propose a novel dual-sided PM memory machine (DSPM-MM) by combing the distinct advantages of “high torque density” of rotor-PM MM and “simple online PM flux control” of stator-PM MM. In the proposed design, the consequent-pole NdFeB PMs are placed in the rotor, while LCF PMs are mounted between the adjacent stator teeth to enable flexible air-gap flux adjustment. The machine topology and operating principle are introduced and addressed, respectively. Then, the electromagnetic performance is analyzed, which confirms the feasibility of the proposed design.

Comparative Study of Hybrid PM Memory Machines Having Single- and Dual-Stator Configurations

In this paper, the memory flux principle is extended to switched flux structures, forming two newly emerged switched flux memory machines (SFMMs) with single-stator (SS) and dual-stator (DS) configurations. Two types of permanent magnets (PMs), i.e., NdFeB and low coercive force PMs, are located in the stationary part. Thus, the developed machines can achieve easy online PM magnetization control, excellent air-gap flux control, and acceptable torque capability. In order to address the issue about the limited stator space encompassing dual PMs and magnetizing coils in the SS-SFMM, a DS design is further developed, where all excitations are placed on a separate inner stator to improve the torque density. A comparative study between the SFMMs with SS and DS structures is established. The investigated machine topologies and operating principle are described first based on a “U”-shaped hybrid PM arrangement, and the PM sizing of the DS machine is optimized with a simplified magnetic circuit model. In addition, the electromagnetic characteristics of the SFMMs with SS and DS structures are investigated and compared by a finite-element (FE) method. The FE results are validated by the experiments on two fabricated prototypes.

Structure and magnetic analyses of hexaferrite Sr1−xLaxFe22+Fe163+O27 prepared via the solid-state reaction

W-type ferrite Sr1−xLaxFe22+Fe163+O27 (x = 0.00, 0.05, 0.08, 0.10, 0.13 and 0.15) powders and magnets were prepared via the solid-state reaction in nitrogen. Phase components of the powder samples were examined by X-ray diffraction (XRD). XRD results demonstrated the appearance of pure phase hexagonal crystal structure and calculate the lattice constants c and a, c/a ratio, cell volume (Vcell), X-ray density (dX-ray) and crystallite size (D) parameters according to XRD data. Scanning electron microscopy (SEM) was used to observe the morphology of the magnets. SEM analyses revealed the hexagonal morphology of the ferrites. The magnetic characteristics of magnetic powders were measured by a vibrating sample magnetometer (VSM), and the magnetic characteristics of magnets were also measured by a permanent magnetic measuring equipment. The results indicate that, the synthesized samples with a low coercive force value and a considerable saturation magnetization are best able to make optical read-out of the magnetic storage information in erasable video and audio discs.

4D Printing of Complex Structures with a Fast Response Time to Magnetic Stimulus.

A poly(dimethylsiloxane)/Fe (PDMS/Fe) composite ink was developed for reversible four-dimensional (4D) printing to create magnetically responsive three-dimensional (3D) structures with a fast response time for their structure evolvements under external magnetic field. These 3D structures could obtain or lose high magnetization immediately by the on or off of an external magnetic field due to the low magnetic coercive force and the high magnetic permittivity of soft magnetic Fe particles in this composite ink, whereas PDMS in this composite ink served as the flexible matrix component to ensure their shape recovery. As exampled by a 3D butterfly with the fast flapping of its wings under an external magnetic field, complex 3D structures created by 4D printing with this PDMS/Fe ink could have the reversible magnetically stimulated structure evolvement property and develop designed magnetically induced motions with a fast response time for various magnetomechanical applications. Furthermore, structure evolvements of these 3D structures could also induce structure-related property changes, as demonstrated by a 3D terahertz photonic crystal (3D-TPC) device with remotely tunable terahertz properties, which could create novel functionalities for various functional devices created by 4D printing from this kind of ink through external magnetic field stimulation.

Electromagnetic and mechanical properties of carbonyl iron powders-PLA composites fabricated by fused deposition modeling

3D printing is an emerging additive manufacturing method for effectively fabricating materials with complex structures. Fused deposition modeling (FDM) is one of the most popular 3D printing techniques, usually used for making thermoplastic parts. However, this method has been rarely considered for designing and fabricating radar wave absorbing structures, which can be very helpful to achieving intricate structures with good mechanical and wave absorbing properties. To verify the validity of this method, carbonyl iron particles filled thermoplastics composites are fabricated by FDM in this study. The morphological, magnetic, electromagnetic and mechanical properties of the composites are investigated by scanning electron microscopy, vibrating sample magnetometer, broad band dielectric spectrometer and universal testing machine. The composites exhibit the hysteretic loops of typical soft ferromagnetism with relatively low remanence, coercive force, and hysteresis losses. The saturation magnetization per unit mass of iron powders of the composites does not have a linear relation with filler contents. The obtained composites materials possess slightly improved tensile strength and significantly improved total elongations than materials obtained by traditional manufacture methods. This 3D printing process will enlighten a rational design of composites with complex shapes, and can also be extended to other thermoplastics based composites, including ABS, PC, and PA.

Effect of Co-substitution on the Vibrational, Magnetic, and Dielectric Properties of Copper Ferrites Obtained by Microwave-Assisted Hydrothermal Method

Co-substituted copper ferrites have been synthesized by a microwave-assisted hydrothermal method. X-ray diffraction confirms that Cu1.0Fe2.0O4 presents both tetragonal and CuO monoclinic phases, whereas the Co-substituted copper ferrites are cubic single-phase structures. All materials were crystallized as ultrafine particles with very low sublattice distortions, and the lattice constants vary with the degree of substitution. Fourier-transform infrared and Raman spectroscopies suggest a short range of the crystal lattices. Magnetic analyses demonstrated that each of the ferrites exhibits soft magnetic behavior with low hysteresis loss and coercive force. Electrochemical impedance data indicate a strong relative permittivity and weak dielectric loss tangent to the Co-substituted samples.

Magnet-Frozen-Permeability FEA and DC-Biased Measurement for Machine Inductance: Application on a Variable-Flux PM Machine

This paper proposes a magnet-frozen-permea-bility finite-element method and an experimental approach for inductance analysis of a variable-flux permanent magnet machine. The machine studied uses Alnico magnets, whose nonlinear magnetic properties deteriorate the inductance calculation accuracy. Besides the machine core permeability is frozen, the magnet-frozen-permeability approach freezes nonlinear B–H curves and nonlinear permeabilities of the magnets in the finite-element analysis. Because the soft magnets have low coercive force, conventional symmetric inductance measurement currents may demagnetize them. This paper presents a zero-start dc-biased current trajectory tracking approach. This approach uses positive current, which avoids demagnetization during the inductance measurement. Meanwhile, based on flux linkage-current curves, influences of resistance can be eliminated. Cross-magnetization effects are evaluated in the two proposed methods. Inductances of these two methods are compared and show good agreements.

Asynchronous responses of mechanical and magnetic properties to structure relaxation for FeNbB bulk metallic glass

Asynchronous responses of mechanical and magnetic properties to structure relaxation for the Fe71Nb6B23 bulk metallic glass were systematically investigated. It is interesting that this ternary alloy can combinedly exhibit outstanding magnetic and mechanical properties, especially good ductility, after optimally annealing in structure relaxation stage for eliminating the internal stress and homogenizing the microstructure. The alloy exhibits low coercive force of 1.6 A/m, high effective permeability of 15 × 103, high fracture strength of 4.2 GPa and good plastic strain of 1.8%. It is also found that responses of mechanical and magnetic properties to structure relaxation are asynchronous. The glass transition and crystallization will greatly deteriorate the magnetic and mechanical properties. Here we propose a physical picture and demonstrate that the primary structure factors determining magnetic and mechanical properties are different. This work will bring a promising material for application and a new perspective to study the effect of annealing-induced structure relaxation on mechanical and magnetic properties.

MAGNETIC PROPERTIES AND MAGNETOCALORIC EFFECT OF Fe90-xPrxZr10 RAPIDLY QUENCHED ALLOYS

In this paper, we present the results of studying magnetic properties and magnetocaloric effect of Fe90-xPrxZr10 (x = 1, 2 and 3) rapidly quenched alloys. The alloy ribbons with thickness of about 15 µm were prepared by melt-spinning method on a single roller system. X-ray diffraction patterns of the ribbons manifest their almost amorphous structure. Thermomagnetization measurements show that the Curie temperature of the alloys can be controlled to be near room temperature by changing concentration of Pr (x). When the concentration of Pr is increased, saturation magnetization of the alloys increased from 48 emu/g (with x = 1) to 66.8 emu/g (with x = 2). All the ribbons reveal soft magnetic behavior with low coercive force (Hc < 42 Oe). The magnetic entropy change of the alloys, |∆Sm|max > 0.9 J.kg-1K-1 in magnetic field change DH = 12 kOe, shows large magnetocaloric effect at phase transition temperature. On the other hand, the working temperature range is quite large (dFWHM ~ 70 K) revealing an application potential in magnetic refrigeration technology of these alloys.

Super-high hardness of (Fe,Co)-B-Si-Zr/Hf bulk glassy alloys

In this work, the alloying effect of similar atom substitution of Co for Fe on the glass-forming ability as well as mechanical and magnetic properties of Fe71.7-xCoxB16.7Si8.3Zr3.3 and Fe72.5-yCoyB16.7Si8.3Hf2.5 (x, y = 0, 5, 10, 15, 20 and 25) glassy alloys has been studied. Our experiments show that the substitution of Co for Fe causes a decrease in the glass-forming ability. By increasing the content of Co, the critical glass formation size gradually decreases from 2.5 mm to 1 mm. Nano-indentation results indicate that both the Young's modulus and hardness increase with increasing Co content in the bulk glassy alloys. The Fe51.7Co20B16.7Si8.3Zr3.3 bulk glassy alloy exhibits a microhardness, Hv = 13.5 GPa, and a Young's modulus, E = 230 GPa, the microhardness being the highest among all the Fe-based metallic bulk materials reported to date. All of the glassy alloys exhibit good soft magnetic properties with high saturation magnetization (1.12–1.33 T) and low coercive force (0.6–5.2 A/m).

Synthesis of PPy/RGO-based hierarchical material with super-paramagnetic behavior and understanding its robust photo current driven by visible light

In this work, a PPy/RGO-based hierarchical material, Fe3O4@Pt@PPy@RGO, was constructed by enveloping hollow Fe3O4 microspheres and Pt nano-particles (NPs) into polypyrrole (PPy) shell, and then binding the core-shell nano-composites with graphene oxide (GO) membranes by UV irradiation for 12 h, which simultaneously reduced GO membranes to reduced graphene oxide (RGO). Photo-current detection show that visible light illumination triggered robust photo current from the PPy/RGO-based hierarchical material. Applying additional voltages did no harm to the hierarchical material, but enhanced its photo currents up to 5 times. Based on Raman, XPS and water contact angle test, it is clarified that the robust visible-light-driven performance of Fe3O4@Pt@PPy@RGO is attributed to the synergistic interactions between PPy and RGO. On one hand, its visible-light-driven performance stems from energy level matching and strong π-π stacking interaction between PPy and RGO. Driven by visible light irradiation, photo electrons were excited from PPy and injected into RGO spontaneously. Those injected photo-electrons migrated through RGO membranes rapidly to form photo current. On the other hand, its robust electron transporting and high structural stability originated from strong π-π conjugating between PPy and RGO, which alleviated surface stress in RGO and PPy to make the hierarchical material exhibit low surface energy and strong interfacial binding. Protected by PPy shell, hollow Fe3O4 microspheres maintained their super-paramagnetic nature in Fe3O4@Pt@PPy@RGO, making the hierarchical material exhibit low magnetic remanence and low coercive force at room temperature. The Fe3O4@Pt@PPy@RGO could be dispersed into water to form magnetic fluid, and separated from the fluid conveniently for recycling use by weak magnetic attraction.

Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics

NiCuZn ferrite with superior magnetic performance is vital ceramic material in multilayer chip inductors (MLCI) applications. In this study, low-temperature-sintered Ni0.22Cu0.2Zn0.58Fe2O4 ferrite ceramic doped with 1.0 wt% Bi2O3-x wt% Nb2O5 (where x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized via solid-state reaction method. Effects of Bi2O3-Nb2O5 additives on microstructures and magnetic properties of NiCuZn ferrite ceramics sintered at 900 °C were systematically investigated. Results indicate that an appropriate amount of Bi2O3-Nb2O5 composite additives can significantly promote grain growth and densification of NiCuZn ferrite ceramics when sintered at low temperatures. Specifically, samples doped with 1.0 wt% Bi2O3 and 0.4 wt% Nb2O5 additives exhibited excellent initial permeability (~ 410 @ 1 MHz), high cutoff frequency (~ 10 MHz), high saturation magnetization (~ 54.92 emu/g), and low coercive force (~ 20.32 Oe). These observations indicate that NiCuZn ferrite ceramics doped with appropriate amounts of Bi2O3-Nb2O5 additives are great candidate materials for MLCI applications.

Recent advances in variable flux memory machines for traction applications: A review

This paper overviews the recent advances in variable flux memory machines (VFMMs) for traction applications with particular reference to newly emerged machine topologies and related control strategies. Due to the use of flux memorable low coercive force (LCF) magnets, the air-gap flux of VFMM can be flexibly varied via a magnetizing current pulse. Thus, the copper loss associated with the flux weakening current and high-speed iron loss can be significantly reduced, and hence high efficiency can be achieved over a wide speed and torque/power operation. These merits make VFMM potentially attractive for electric vehicle (EV) applications. Various novel VFMMs are reviewed with particular reference to their topologies, working principle, characteristics and related control techniques. In order to tackle the drawbacks in the existing VFMMs, some new designs are introduced for performance improvement. Then, the electromagnetic characteristics of an exemplified EV-scaled switched flux memory machine and various benchmark traction machine choices, such as induction machine, synchronous reluctance machines, as well as commercially available Prius 2010 interior permanent magnet (IPM) machine are compared. Finally, the key challenges and development trends of VFMM are highlighted, respectively.

Ductile FeNi-based bulk metallic glasses with high strength and excellent soft magnetic properties

Fe-based bulk metallic glasses (BMGs) have attracted great attention due to their excellent soft magnetic properties and high fracture strength, but few applications have been materialized as structural materials because of their brittleness at room temperature. Here, we successfully synthesized an Fe39Ni39B14.2Si2.75P2.75Nb2.3 BMG which exhibits large plastic strain of 7.8%, high fracture strength of 3.35 GPa and excellent soft magnetic properties, i.e., rather high saturation flux density of 0.88 T, low coercive force of 0.7 A/m and high permeability of 20800. The results indicated that the mutual repulsion between ductility and strength could be renovated in the Fe-based BMGs by the rearrangement of atomic configurations through Ni addition. With proper combination of non-directional metal-metal bonds and directional metal-metalloid bonds, the mechanical properties of FeNi-based BMGs can be improved. Our studies provide a guideline in designing ductile FeNi-based BMGs with high strength, large GFA and excellent soft magnetic properties.

Investigation of magnetic phase transition and magnetocaloric effect of (Ni,Co)-Mn-Al melt-spun ribbons

Magnetic phase transition, magnetocaloric effect and critical parameters of Ni50-xCoxMn50-yAly (x = 5 and 10; y = 17, 18 and 19) rapidly quenched ribbons have been studied. X-ray diffraction patterns exhibit a coexistence of the L21 and 10M crystalline phases of the ribbons. Magnetization measurements show that all the samples behave as soft magnetic materials with a low coercive force less than 60 Oe. The shape of thermomagnetization curves considerably depends on Co and Al concentrations. The Curie temperature (TC) of the alloy ribbons strongly increases with increasing the Co concentration and slightly decreases with increasing the Al concentration. The Ni45Co5Mn31Al19 and Ni40Co10Mn33Al17 ribbons reveal both the positive and negative magnetocaloric effects. Under magnetic field change (ΔGH) of 13.5 kOe, the maximum magnetic entropy change (|ΔSm|max) of the Ni45Co5Mn31Al19 ribbon is about 2 and -1 J·kg−1·K−1 for negative and positive magnetocaloric effects, respectively. Basing on Arrott - Noakes and Kouvel - Fisher methods, critical parameters of the Ni45Co5Mn31Al19 ribbon were determined to be TC ≈ 290 K, β ≈ 0.58, γ ≈ 0.92 and δ ≈ 2.59. The obtained values of the critical exponents indicate that the magnetic order of the alloy ribbon is close to the mean-field model.

Synthesis of Hybrid Magnet Memory Machines Having Separate Stators for Traction Applications

This paper proposes two novel partitioned stator (PS) memory machines based on flux-reversal and switched-flux topologies, respectively. The proposed machines geometrically feature two separate stators with armature winding and hybrid PM excitations, i.e., NdFeB and low coercive force PMs. The proposed PS machines can offer increased available inner stator space for hybrid PM, which is beneficial to the torque improvement. The machine topologies are introduced first in terms of possible PM types with parallel magnetic circuits. The design considerations for hybrid PM are analytically analyzed. Then, the electromagnetic characteristics of the two proposed machines are comprehensively compared by using finite element (FE) method. The commercially available Toyota Prius 2010 machine is referenced as a benchmark for comparison with the optimal PS machine having identical overall dimension in potential context of traction applications. Finally, The FE analyses are validated by experiments on a small-scaled demonstration machine.

Low temperature magnetic properties of basalts containing near ~TM30 titanomagnetite

The paper is devoted to studying the mineral composition and magnetic properties, mainly at the cryogenic temperatures, of the Middle–Late Devonian basalts from North Timan. The magnetic minerals in these basalts are dominated by intermediate-composition titanomagnetites (TM25–TM30) which demonstrate unusual magnetic properties in a wide temperature range. At room temperature, a low coercive force coexists with relatively high Mrs/Ms ratios. At cryogenic temperatures, the dependences of magnetic susceptibility on the temperature and frequency of the applied field are characteristic of this titanomagnetite composition, whereas the remanent saturation magnetization acquired at 2 K is destroyed at significantly lower temperatures compared to the synthetic analogs. The obtained results again highlight the necessity of studying the low-temperature properties of titanomagnetite samples with a controlled composition and grain size.