Strontium Ferrite Magnets Research Articles

Recent Developments in 3D Printing of Rare-Earth-Free Permanent Magnets

This article reviews the advances in additive manufacturing of magnetic ceramics and alloys without rare-earth elements. Near-net-shaped permanent magnets with varying shapes and dimensions overcome traditional limitations of the cast, sintered, and bonded magnets. The published articles are categorized based on material types and 3D printing techniques. Selective laser melting and electron beam melting were predominantly used to produce alnico magnets. In addition to the electron beam melting, manganese aluminium-based alloys were successfully printed by fuse filament fabrication. By incorporating magnetic powders in polymers and then printing via extrusion, the fuse filament fabrication was also used to produce strontium ferrite magnets. Moreover, hard ferrites were printed by stereolithography and extrusion free-forming, without drawing composites into filaments. Magnetic properties in some cases are comparable to those of conventional magnets with the same compositions. Currently, available software packages can simulate magnetic fields for designing magnets and optimize the integration in electrical machines. These developments open up opportunities for next-generation permanent magnet applications.

Reactive flash sintering of SrFe12O19 ceramic permanent magnets

Reactive flash-sintering technique has been used in order to obtain strontium ferrite magnets from a mixture of SrCO3 and Fe2O3 commercial powders. This technique allows preparing sintered SrFe12O19 at a furnace temperature of just 973 K during just 2 min by applying a modest field of 40 V cm-1, instead of the conventional sintering process employed in ferrite magnet manufacturing that demands high temperature and long dwell times. Analysis of structural and magnetic properties were performed as a function of time in which the flash event was held. Mössbauer spectra show the existence of five different kinds of local environments, confirming the formation of strontium hexaferrite. The resulting samples exhibit comparable magnetic properties to the state-of-the-art ferrite magnets. In particular, produced samples reach a coercivity of 0.4 T and a specific saturation magnetization of 70 Am2 kg-1.

FeCo Nanowire–Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products

Due to the issues associated with rare-earth elements, there arises a strong need for magnets with properties between those of ferrites and rare-earth magnets that could substitute the latter in selected applications. Here, we produce a high remanent magnetization composite bonded magnet by mixing FeCo nanowire powders with hexaferrite particles. In the first step, metallic nanowires with diameters between 30 and 100 nm and length of at least 2 μm are fabricated by electrodeposition. The oriented as-synthesized nanowires show remanence ratios above 0.76 and coercivities above 199 kA/m and resist core oxidation up to 300 °C due to the existence of a >8 nm thin oxide passivating shell. In the second step, a composite powder is fabricated by mixing the nanowires with hexaferrite particles. After the optimal nanowire diameter and composite composition are selected, a bonded magnet is produced. The resulting magnet presents a 20% increase in remanence and an enhancement of the energy product of 48% with respect to a pure hexaferrite (strontium ferrite) magnet. These results put nanowire–ferrite composites at the forefront as candidate materials for alternative magnets for substitution of rare earths in applications that operate with moderate magnet performance.

An overview on nucleation theories and models

Several different models for coercivity are discussed. There are two main situations: i) nanocrystalline magnets, with grain size bellow the single domain particle size, and ii) magnets with grain size above single domain particle size. The described theories and models are general, and can be applied in either NdFeB magnets, SmFeCoCuZr or strontium ferrite magnets. The spring effect observed in isotropic nanocrystalline magnets can be explained with the Stoner-Wohlfarth model. Modifications of the Stoner-Wohlfarth model are necessary to take into account the effect of interaction between grains. When the grain size is above the single domain size, energy considerations show that nucleation should occur at the surface of grains. Nucleation is interpreted as a two-step process, where domain wall displacement occurs for grain size above single domain size, after a nucleus is first formed. The effect of grain size on the coercive field is discussed.

Life cycle assessment of permanent magnet electric traction motors

Ongoing development of electrified road vehicles entails a risk of conflict between resource issues and the reduction of greenhouse gas emissions. In this study, the environmental impact of the core design and magnet material for three electric vehicle traction motors was explored with life cycle assessment (LCA): two permanent magnet synchronous machines with neodymium-dysprosium-iron-boron or samarium-cobalt magnets, and a permanent magnet-assisted synchronous reluctance machine (PM-assisted SynRM) with strontium-ferrite magnets. These combinations of motor types and magnets, although highly relevant for vehicles, are new subjects for LCA. The study included substantial data compilation, machine design and drive-cycle calculations. All motors handle equal take-off, top speed, and driving conditions. The production (except of magnets) and use phases are modeled for two countries – Sweden and the USA – to exemplify the effects of different electricity supply. Impacts on climate change and human toxicity were found to be most important. Complete manufacturing range within 1.7–2.0 g CO2-eq./km for all options. The PM-assisted SynRM has the highest efficiency and lowest emissions of CO2. Copper production is significant for toxicity impacts and effects on human health, with problematic emissions from mining. Resource depletion results are divergent depending on evaluation method, but a sensitivity analysis proved other results to be robust. Key motor design targets are identified: high energy efficiency, slender housings, compact end-windings, segmented laminates to reduce production scrap, and easy disassembly.

Event Related Brain Activation in Experiments, Analysis and Neurofeedback

Several different models for coercivity are discussed. There are two main situations: i) nanocrystalline magnets, with grain size bellow the single domain particle size, and ii) magnets with grain size above single domain particle size. The described theories and models are general, and can be applied in either NdFeB magnets, SmFeCoCuZr or strontium ferrite magnets. The spring effect observed in isotropic nanocrystalline magnets can be explained with the Stoner-Wohlfarth model. Modifications of the Stoner-Wohlfarth model are necessary to take into account the effect of interaction between grains. When the grain size is above the single domain size, energy considerations show that nucleation should occur at the surface of grains. Nucleation is interpreted as a two-step process, where domain wall displacement occurs for grain size above single domain size, after a nucleus is first formed. The effect of grain size on the coercive field is discussed.

Hysteresis Modeling of Bonded Anisotropic Ferrite Magnets

The modeling of hysteresis curves of bonded ferrite magnets is discussed. Hysteresis of anisotropic magnets were calculated according to the Stoner-Wohlfarth Model, for the cosn (theta) distribution. The crystallographic texture has significant effect on the hysteresis curve. Two different samples were examined, one isotropic and another anisotropic. The anisotropy field of strontium ferrite magnets was determined to be 19.5 kOe. The Mr/Ms ratio of the anisotropic bonded magnet was estimated as 0.8.

Recycling of Strontium Ferrite Waste in a Permanent Magnet Manufacturing Plant

Residues resulting from the manufacture of strontium ferrite magnets have been recycled for further use in magnet fabrication instead of disposal as waste. The quality of the recycled ferrite powder has been tested and compared to that of the new starting ferrite material. The magnetic properties of the recycled powder not only match those of the starting material acquired by the company for the production of magnets but exceed them. A coercivity value 3.5 times larger than that of the new starting ferrite powder, accompanied by a 25% increase in remanence, makes this material a new and improved ferrite product to re-enter the production chain in the factory with an extended applications range. This improvement is proven to be due to tuning of the morphology and microstructure through processing and subsequent heat treatment. The use of processing conditions in the same range as those typically used in the preparation of ferrite powders and magnets, in combination with the superior magnetic quality of the...

High Technology Applications of Barium and Strontium Ferrite Magnets

Ceramic magnets as barium ferrite or strontium ferrite have many applications in high technology. One of the reasons is the low cost when compared to competitor materials, as Alnico, MnBi, MnAl or NdFeB. In this study, the advantages and disadvantages of Ba and Sr ferrite magnets are discussed. One clear advantage is that ferrites are already oxides, and do not present the corrosion problems typical of NdFeB and other metallic alloys. As ferrites are oxides, the processing is much easier and cheaper. For example sintering can be done at air, and milling under wet condition. One of the main conclusions is the excellent ratio cost/benefit of ferrites, giving advantage in many applications. Special attention is given for application of ferrites in high efficiency motors.

Effect of magnetic field on dust charging and corresponding probe measurement

The effect of external magnetic field on the Langmuir probe measurement and dust charging are studied in low-pressure hydrogen plasma. The experiment is performed in a dusty plasma device where plasma is created by the hot cathode filament discharge technique. A strong Strontium ferrite magnet is used inside the plasma, near the dust zone. The plasma parameters are measured at different distances from the magnet with the help of Langmuir probe system. It is observed that even at “low magnetic field case,” where rLe≥rP, rLi〉〉rP; the electron collection by the probe deviates strongly from the actual value, until rLe≥10rP. The observations of electron energy probability function show that at higher magnetic field, the Langmuir probe collects only the higher energy electrons compared to the low energy electrons. Both Quasi-neutrality condition and capacitance model are used separately to calculate the charge accumulated on the dust grain. Introducing the reduction factor on quasi-neutrality condition, it is observed that the influence of magnetic field on dust charge is almost negligible for “low magnetic field” case. The dust charge calculated from quasi-neutrality condition matches well with the experimentally observed dust current results, within the experimental error range. However, capacitance model deviates from the experimental results at higher magnetic field.

Improvement of charged particles transport across a transverse magnetic filter field by electrostatic trapping of magnetized electrons

A study on the transport of charged particles across a magnetic filter field has been carried out in a double plasma device (DPD) and presented in this manuscript. The DPD is virtually divided into two parts viz. source and target regions by a transverse magnetic field (TMF) which is constructed by inserting strontium ferrite magnets into two stainless steel rectangular tubes. Plasma electrons are magnetized but ions are unmagnetized inside the TMF region. Negative voltages are applied to the TMF tubes in order to reduce the loss of electrons towards them. Plasma is produced in the source region by filament discharge method and allowed to flow towards the target region through this negatively biased TMF. It is observed that in the target region, plasma density can be increased and electron temperature decreased with the help of negatively biased TMF. This observation is beneficial for negative ion source development. Plasma diffusion across the negatively biased TMF follows Bohm or anomalous diffusion process when negative bias voltage is very less. At higher negative bias, diffusion coefficient starts deviating from the Bohm diffusion value, associated with enhanced plasma flow in the target region.

Maximum energy product at elevated temperatures for hexagonal strontium ferrite (SrFe12O19) magnet

The electronic structure of hexagonal strontium ferrite (SrFe12O19) was calculated based on the density functional theory (DFT) and generalized gradient approximation (GGA). The GGA+U method was used to improve the description of localized Fe 3d electrons. Three different effective U (Ueff) values of 3.7, 7.0, and 10.3eV were used to calculate three sets of exchange integrals for 21 excited states. We then calculated the temperature dependence of magnetic moments m(T) for the five sublattices (2a, 2b, 12k, 4f1, and 4f2) using the exchange integrals. The m(T) of the five sublattices are inter-related to the nearest neighbors, where the spins are mostly anti-ferromagnetically coupled. The five sublattice m(T) were used to obtain the saturation magnetization Ms(T) of SrFe12O19, which is in good agreement with the experimental values. The temperature dependence of maximum energy product ((BH)max(T)) was calculated using the calculated Ms(T).

A study of hybrid bonded magnets of Sm-Co and Sr-ferrite using a mixture design

The present paper deals with the study of hybrid polymer bonded magnets of strontium ferrite and samarium cobalt. A three-component constrained mixture design, employing ten runs for the experiment, was used. The magnetic properties such as remanence (Br), coercivity (Hc), and energy product (B.H)max were used as the response. The results are modeled in the form of mathematical equations to predict the response as a function of the samarium cobalt and strontium ferrite permanent magnet powders percentage and polymer content. The predicted results from the obtained mathematical equation are compared with the experimentally measured values.

Direct-Drive Synchronous Generators with Excitation from Strontium-Ferrite Magnets: Efficiency Improvement

Direct-Drive Synchronous Generators with Excitation from Strontium-Ferrite Magnets: Efficiency Improvement The authors consider the possibility to raise the specific power of synchronous generators with excitation from inexpensive permanent magnets. For this purpose, it is proposed to use tooth-wise windings and permanent magnets based on inexpensive magneto-hard material, e.g. strontium-ferrite. The magnets are to be placed between the rotor teeth, the alternate polarity of which is facing the air-gap. This provides a simpler and cheaper technology of making such a generator and improves its reliability. The proposed rational bevelling of the stator teeth not only raises the specific power of the generator but also reduces the level of noise and vibrations, extends the longevity of the magnets and bearings as well as facilitates the starting torque of the electric machine, e.g. if it is employed as wind generator.

Anisotropic Ferrite Produced with Coarse Particles Milled from Sintered Magnets

Anisotropic ferrite sintered magnets are usually produced by the sintering of wet pressed bodies previously oriented by a magnetic field. It is well known that the orientation is efficient for very fine single crystalline particles. The use of dry pressing reduces the orientation capability, since the mobility of fine particles can be reduced by their agglomeration. However, the use of coarse powders obtained by milling sintered anisotropic magnets should facilitate the orientation during dry pressing. These coarse particles are composed of grains with the same crystallographic directions, so they could be oriented by a magnetic field. The goal of this paper is to investigate this possibility. In this work the sintered strontium ferrite magnets were milled and the obtained powder was classified, oriented and dry isostatically pressed. Sintering was conducted in a dilatometer furnace and shrinkage was monitored. The magnetic properties of the samples were evaluated by VSM. Microstructure was also evaluated. Magnetic properties were dependent of the particle size, and orientation and shrinkage were quite low for coarse particles.

Performance comparison of U-Core and round-stator single-phase permanent-magnet motors for pump applications

Strontium-ferrite-based permanent magnets (PMs) are attractive for applications in electric machines due to several reasons. Their price is some two orders of magnitude less than the price of rare-earth magnets. They are chemically inert, which makes them suitable for applications in aggressive environments. Strontium-ferrite magnets have high specific electric resistance, so they do not experience thermal problems due to eddy-current losses. On the other hand, their low residual flux density imposes the need for special machine construction when high air-gap flux density is needed. It is shown in this paper how a synchronous motor with strontium-ferrite PMs can efficiently replace an induction motor in low-power applications, and why a round stator is a clear winner against a U-core PM motor.

Ｓｒフェライトプラスチックボンド磁石における結晶配向度の向上

There have been developed various hard ferrite powders with a high magnetocrystalline anisotropy which are very useful for the bonded ferrite magnets. However, the conventional bonded hard ferrite magnets have not given the most of their intrinsic anisotropy. In this study, strontium ferrite (SrO.6Fe2O3) plastic bonded magnet was fabricated by the powder injection molding method using a metal mold in which the closed magnetic circuit may induce. The pellet was manufactured by heating strontium ferrite powder having small specific area. From the observation of crystal-orientation of the obtained specimens, it was proved that the anisotropic magnetic field induced from the closed magnetic circuit in the metal mold directly attributed to the improvement of coercive force of the plastic bonded strontium ferrite magnet. Further, the mechanism of enhancement of crystal-orientation degree was discussed.

Use of Supercritical Extraction Debinding to Obtain Sintered Strontium Ferrite Magnets by Powder Injection Moulding

Use of Supercritical Extraction Debinding to Obtain Sintered Strontium Ferrite Magnets by Powder Injection Moulding

Modeling and control of a new horizontal-shaft hybrid-type magnetic bearing

This paper reports on the development of a new horizontal-shaft hybrid-type magnetic bearing system. The bearing system will be used for a horizontal-shaft machine. The rotor is levitated due to the repulsive force between a stator and a rotor permanent magnet (PM). A lower cost and higher radial stiffness have been achieved by using a strontium-ferrite magnet on the rotor and an Nd-Fe-B PM above and below the rotor magnet. A finite-element analysis was performed to calculate the levitation force and radial stiffness. An upper stator magnet subtending an angle of 45/spl deg/ provides the best compromise between a large levitation force and radial stiffness. A model for the horizontal-shaft hybrid magnetic bearing system has been developed and includes the effect of the rotor dynamics and the electromagnetic forces. An integral servocontroller was designed to stabilize the axial position. The controller has been implemented in a digital signal processor. Experimental results performed on a prototype system are in agreement with the theoretical results.

Magnetic properties of anisotropic Sr-La system ferrite magnets

An experiment was carried out to investigate the effect of La/sub 2/O/sub 3/ on the magnetic and physical properties of strontium ferrite magnets. It was found that the La/sub 2/O/sub 3/ addition to SrO-6Fe/sub 2/O/sub 3/ (stoichiometric composition) was very useful in stabilizing the magnetoplumbite structure and that these ferrites of the Sr-La system had excellent permanent-magnet properties. Various compositions were tested, and the optimum conditions for making magnets and some properties of typical specimens were determined. The semisintering condition was 1250 degrees C for 1 h in oxygen, and the sintering condition 1300 degrees C for 0.5 h in oxygen. Magnetic and physical properties are presented and discussed. The ((SrO)0.143(Fe/sub 2/O/sub 3/)/sub 0.857/)/sub 98/(La/sub 2/O/sub 3/)/sub 2/ compound magnet exhibited the highest value of (BH)/sub max/, and this value was equivalent to or higher than that of ordinary Ba and Sr ferrites. >