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TRANSCRIPT
Journal of Materials Processing Technology 189 (2007) 1–12
Review
Soft magnetic composite materials (SMCs)
H. Shokrollahi ∗, K. JanghorbanDepartment of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran
Received 4 July 2006; received in revised form 1 January 2007; accepted 20 February 2007
Abstract
Soft magnetic composites (SMCs), which are used in electromagnetic applications, can be described as ferromagnetic powder particles surroundedby an electrical insulating film. SMC components are normally manufactured by conventional PM compaction combined with new techniques,such as two step compaction, warm compaction, multi-step and magnetic annealing followed by a heat treatment at relatively low temperature.These composite materials offer several advantages over traditional laminated steel cores in most applications. The unique properties includethree-dimensional (3D) isotropic ferromagnetic behavior, very low eddy current loss, relatively low total core loss at medium and high frequencies,possibilities for improved thermal characteristics, flexible machine design and assembly and a prospect for greatly reduced weight and productioncosts. With expanded applications of soft magnetic composite materials expected in the future, a review of the magnetic properties, characteristics,
processing and applications of SMCs is presented in this article.© 2007 Published by Elsevier B.V.K
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eywords: Soft magnetic composites; Core loss; Magnetic properties; Organic coating; Inorganic coating
ontents
1. Literature review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Magnetic characteristics and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24. Core loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Hysteresis loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.2. Eddy current loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46. Classification of soft magnetic composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Sintered soft magnetic materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56.2. Powder cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57.1. Compaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57.2. Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Materials selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78.1. Pure iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78.2. Fe–Ni alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78.3. Fe–Si alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88.4. Fe–Co alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Particle size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810. Coated iron-based powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Organic coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 910.2. Inorganic coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
∗ Corresponding author.E-mail address: [email protected] (H. Shokrollahi).
924-0136/$ – see front matter © 2007 Published by Elsevier B.V.oi:10.1016/j.jmatprotec.2007.02.034
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. Literature review
Magnetic materials have revolutionized our lives. Theseaterials are used in electronic, computer and telecommu-
ication industries. During the last decades different typesf magnetic materials have been used including pure ironnd its alloys, such as Fe–Ni, Fe–Ni–P, Fe–Nd–B and Fe–Sind soft and hard ferrites, such as Ni–Zn, Mn–Zn and Baerrites. Different aspects of processing, properties, effect ofdditives on magnetic properties and applications of theseerrites were discussed by many researchers [1–50] includ-ng these authors who studied the effects of additives, suchs V2O5 and MoO3 in Mn–Zn–ferrites for low consumptionamps and high frequency applications [39,40]. New materialsncluding amorphous materials, amorphous wires, nanocrys-alline materials and today’s soft magnetic composite materialsre the latest development in magnetic history [46–83]. Its worth reviewing the highlight of the magnetic materialsevelopment very briefly, before going to more details aboutMCs.
The idea of using iron–resin composites for soft magneticpplications is not new. It appeared more than 100 years ago butron–resin composites have been rarely used because their prop-rties, the processing technology for making parts and real needsor these materials were not sufficiently developed. However,hese limitations were being overcome with the development ofmproved raw materials and new shaping technologies. Theseomposites find increasing use in electrical motors, replacingxisting laminate materials [53,56,59,65,68–73]. These materi-ls are being developed to provide materials with competitiveagnetic properties (good relative permeability and magnetic
aturation), but with high electrical resistivity [49]. Insulatedron powder (Fig. 1) offers several advantages over traditionalteel in some applications, for example, the isotropic nature ofhe SMC combined with the unique shaping possibilities opensp for 3D-design solutions [55,56]. In recent years, effects ofarticle size, particle composition (Fe–Ni, Fe–Ni–Co, Fe–Si)1–8], compaction parameters (warm compaction, pressure,ubricant), resin and wet chemical methods for creation of insu-ation layer around particles have been verified [2,6,7,53,60]. A
iterature survey in the field of soft magnetic composite materialss given in Table 1, where a summary of some recent develop-ents is presented.
ig. 1. A schematic diagram of the component elements of a powder core [65].
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ls Processing Technology 189 (2007) 1–12
. Introduction
During the last several years, interest in the study of soft mag-etic composites (SMCs) has been increasing at an acceleratingate, stimulated by recent advances in materials synthesis andharacterization techniques and the realization that these mate-ials exhibit many unique and interesting physical and chemicalroperties with a number of potential technological applications.hey play a key role in power distribution, make possible theonversion between electrical and mechanical energy, underlieicrowave communication, and provide both the transducers
nd the active storage material for data storage in informationystems.
New developments in powder composites make SMC mate-ials interesting for applications in electrical machines, whenombined with new machine design rules and new produc-ion techniques. These composites have several advantages,uch as reduction in weight and size. Weight can be reducedhrough several types of technology improvements; in materi-ls, design techniques and fabrication processes. To establishhe design rules, one must pay attention to electromagnetic lossharacteristics of SMC materials. Several different series of iron-ased SMCs are: (1) pure iron powder with resin, (2) sinteredron-based powders, (3) pure iron powder with additions of Zn-tearate and carbon, (4) iron-based powder alloys (Fe, Ni, Co,i), (5) commercially available iron powder “Somaloy” [70–75].mong these, the composite materials minimize fringing fluxue to their distributed air gap.
An interesting example of recent commercial progress forMC applications is the BDC-motor, shown in Fig. 2 for ABS
ype brake systems produced by Asian Seiki Co. Ltd., Japan65]. The SMC machine must be designed using short magneticath length and minimum weight. In some applications, suchs magnetic cores and magnetic machines, these composites areble to replace electrical steel sheets or ferrites [63].
. Magnetic characteristics and properties
Two key characteristics of an iron core component are itsagnetic permeability and core loss characteristics. The mag-
etical, electrical and mechanical characteristics depend on thereparation and processing of the components. In addition theaterials purity, shape and size of particles influence the overallagnetic response.Two basic types of soft magnetic materials are extensively
sed, depending on the application and its requirements. Theseaterials are:
(a) Ferrimagnetic materials, which are based on ceramic oxidesof some metals, such as ferrites and are applicable to fre-quencies from a few kilohertzs to well over 80 MHz.
b) Ferromagnetic materials based on iron and nickel, whichare for lower frequency applications, <2 kHz, consist of
iron-based alloys which have low to medium frequencyapplications in electric machines. SMCs are ferromagneticmaterials with significantly improved medium to high fre-quency properties which made them a viable alternative to
H. Shokrollahi, K. Janghorban / Journal of Materials Processing Technology 189 (2007) 1–12 3
Table 1Literature survey in the field of soft magnetic composite materials
Researchers Subject Year Reference
Lefebvre, L. Philippe Sylvain Soft magnetic composites (complex shape production and isotropic magnetic behavior, high resistivity) 1993 [49]P. Laurent, G. Viau Effect of the magnetic fraction on the complex susceptibility of soft magnetic composite materials 1996 [50]M.E. McHenry, M.A. Willard Amorphous and nanocrystalline materials (review paper) 1999 [42]P. Gilbert, H.G. Phan Development of soft magnetic composites (low-loss applications) 2002 [5]L. Pennander, A. Jack Soft magnetic iron powder material AC properties and their application in electrical machines 2003 [65]I. Chicinas, O. Geoffroy Soft magnetic composite based on mechanically alloyed nanocrystalline Ni3Fe 2005 [8]E. Bayramli, O. Golgelio Powder metal development (electrical motors) 2005 [56]Y.G. Guo, J.G. Zhu 3D and 2D vector magnetic properties of soft magnetic composite materials 2005 [55,57,58]A. Hamler, V. Gorican The use of SMCs in synchronous electric motor 2006 [59]K. Janghorban, H. Shokrollahi Effect of different compaction methods on the magnetic properties of SMCs 2006 [61,62]
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4.1. Hysteresis loss
At low frequencies the hysteresis loss is the main core losspart and can be reduced by large particle size, higher purity
Fig. 2. Commercial ABS motor: (a) original lam
steel laminations in a range of new applications, such asrotating machinery, sensors and fast switching solenoids.
The unique properties of the soft magnetic composite mate-ials include magnetic and thermal isotropy, very low eddyurrent loss and relatively low total core loss at low to highrequencies, high magnetic permeability, high remanent mag-etization, high resistivity, reduction in size and weigh, largenisotropy constant, low coercivity and high Curie temperature77]. Soft magnetic composites are isotropic, have improvedigh frequency performance and can be compacted to 3D-shapessing the established PM compaction route. The soft magneticomposites have also certain limitations: on a part they haveaximum permeability and magnetic induction than laminates,
n the other part the powder metallurgical procedures used inrincipal for the obtaining of soft powdered cores are not suitableor all sizes and shapes of the core components.
Fig. 3 shows the applicable regions for soft magnetic mate-ials used in AC magnetic fields. Soft ferrite has low core lossn the high frequency region, but due to its low magnetic fluxensity, it has the drawback of requiring a large core. Elec-rical steel sheets have high flux density, but electrical sheetsannot be used in the high frequency region due to excessiveore loss. Powder cores are magnetic materials which cover theegion where the former two magnetic materials cannot be used69].
. Core loss
Traditionally, the contributions to the dissipation in magneticaterials are classified into three categories:
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motor and (b) improved new SMC design [65].
. Hysteresis loss (Ph);
. Eddy current loss (Pe);
. Residual loss (Pr).
The residual losses are not too well understood and perhapsepresent an expression of our ignorance of the system. Residualosses are a combination of relaxation and resonant losses. Theseosses are only important at very low induction levels and veryigh frequencies and can be ignored in power applications. Theotal core loss of a magnetic device is the sum of the eddy currentosses and hysteresis losses [13].
ig. 3. The applicable regions for soft magnetic materials used in AC magneticelds [69].
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f the iron in the particles and stress relieving heat treatment.ysteresis loss can be expressed by [13]:
h = f
∮H dB
here Ph is the hysteresis loss, f the frequency, H the magneticeld strength and B is the magnetic induction. In an iron powderaterial, impurities in the iron particles and stressed regions give
ise to pinning sites that hinder domain wall motion. The coer-ive force raised by these causes can be reduced by using a highurity iron for the particles and provide a heat treatment proce-ure following the compaction to improve the stressed regions.nother source of hindering the domain wall motion is possiblerain boundaries inside the particles. The heat treatment proce-ure following the compaction is the main step to be taken toeduce hysteresis loss.
.2. Eddy current loss
Eddy current loss is due to electrical resistance losses withinhe core caused by the alternating electric field. When eddy cur-ents are induced in materials, two main effects are observed:ncomplete magnetization of the material (skin effects) andncrease in core losses. Eddy current loss can be expressed as13]:
e = CB2f 2d2
ρ
here Pe is the eddy current loss, C the proportionality constant,the flux density, f the frequency, ρ the resistivity and d is the
hickness of the material.Eddy current loss can be minimized in a number of ways.
irst, resistivity is increased by addition of Si to the iron pow-ers. Another common technique to reduce the eddy current losss to use thinner laminates. The latest technique is the insulatedron powder (Fig. 1) with smallest eddy current loss. The insu-ating coating of every particle gives very small eddy currentaths inside a particle and a relatively high resistivity of theulk material. The small non-magnetic distances between every
article act as air gaps and decrease the permeability of the bulkaterial.Fig. 4 shows the total losses for ring shaped components withn-coated and coated iron powder particles [65]. The effect of
ig. 4. The total losses for ring shaped components with un-coated and coatedron powder particles [65].
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oating in reducing the normalized total loss is significant atigher temperatures. Fig. 5 lists concept for iron loss reduction63].
. Applications
The uses for soft magnetic materials are typically classifieds either DC or AC applications [53–75]. DC or direct cur-ent applications are characterized by a constant applied fieldfrom a battery type device). The most common DC applica-ions are found in automobiles. Key magnetic characteristicsor DC applications are permeability, coercive force, and satu-ation induction. For AC or alternating current applications, aariable field is applied. The materials for AC electromagneticircuits require high induction and low dynamic (eddy current)osses. These are strongly influenced by the work frequency andnduction, and also by the magnitude of density and electricalesistivity of materials. Key magnetic parameters in AC applica-ions are permeability, saturation, and total core losses resultingrom the alternating magnetic field. However, the recent intro-uction of polymer coated iron powers has opened the door for/M to be utilized in AC applications. These polymer coatedowders are usually used in the as pressed condition; i.e. no sin-ering is required. In applications where saturation induction ishe key magnetic parameter, these applications are ideal for thenalloyed iron materials.
Soft magnetic composites containing iron powder find usen a variety of applications including; design of the com-osite core with three dimensional isotropic ferromagneticehavior, powder cores for switch reluctance power supplies,C out put chokes, resonant inductors, sintered materials for
ntilock braking sensors, electromagnetic actuation devices,ubstitute for laminate steels in brushless DC motors, rotating
achineries, low frequency filters, induction field coils, mag-etic seal systems, transformers coil, magnetic field shielding,igh temperature applications, such as in aircraft engine electricomponents.
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Table 2Parameter dependence on composition and the fabrication process [85]
I II III IV V
Permeability ↑ ↓ ↓ ↑ ↑Maximum flux density ↑ ↓ ↓ ↑ ↑Coercivity ↑ – – ↑ ↓Resistivity ↓ ↑ ↑ ↓ ↓Thermal conductivity ↑ ↓ ↓ ↑ ↑Strength ↓ ↓ ↑ ↑↓a ↑(I) Increasing particle size; (II) addition of lubricant; (III) addition of binder;(IV) increasing compaction pressure; (V) heat treatment.
H. Shokrollahi, K. Janghorban / Journal of
These materials can be used in alternators, generators andlectric motors, which are used in a wide variety of applicationsnvolving power tools, such as drills, saws, sanding and grindingevices and yard tools, such as edgers and trimmers, just to namefew such tools.
. Classification of soft magnetic composites
Soft magnetic parts produced by PM processes can be broadlyivided into sintered magnetic cores, which are manufactured byhe conventional PM process and powder compressed magneticores which are manufactured without sintering.
.1. Sintered soft magnetic materials
The DC magnetic properties of sintered compacts are deter-ined by the chemical composition of the material and the
ensity and crystal grain size of the sintered part. Pure ironintered compacts display comparatively high magnetic fluxensity. In general, the flux density of iron-based materialshows a strong relationship with the purity of the material andensity of the sintered part.
Accordingly, high flux density can be obtained by using highurity iron powders and manufacturing a high density sinteredompact by applying warm compaction, die wall lubricant andigh compaction techniques.
In sintered parts, addition of a small amount of phosphor (P)o pure Fe powder encourages grain growth, making it possibleo produce sintered compacts with a coarser grain size [77].
In addition to the material composition and density of theintered compact, the AC magnetic properties of sintered com-onents are also strongly related to the shape of the part.xamples are electromagnetic actuators, used in various kindsf motors in AC magnetic fields. Core loss occurs when softagnetic materials are used in an AC magnetic field.
.2. Powder cores
Powder cores are made from magnetic powder particlespproximately 100 �m in size, which are insulated individually.n manufacturing these types of cores, the iron-based powdersith a size of around 100 �m are insulated with an inorganic
nsulating layer, and the powder is mixed with a small amount ofn organic resin as a binder. The mixture is then compacted andeat treated. In this case, the heat treatment must be performedt a temperature which will not destroy the inorganic insulatingayer or the organic resin binder. This means that densification asresult of the sintering process, as in sintered magnetic materi-ls, cannot be expected with powder cores [77]. Therefore, highensity must be realized in the compaction process.
. Processing
Soft magnetic powders are the main component of SMCs thatre covered by an insulation layer, Fig. 1. Depending on how theombination of materials and processing parameters are chosen,wide range of properties can be obtained.
(tmi
rials Processing Technology 189 (2007) 1–12 5
Soft magnetic composites are produced by traditional powderompaction techniques followed by a heat treatment at low tem-eratures which does not destroy the insulating layer betweenhe iron particles. Different magnetic and mechanical proper-ies are obtained depending on binder, lubricant additives andrganic coatings on the iron particles as well as warm or coldompaction.
Preparation of SMCs consists of the following steps: (a) pro-iding a low carbon powder of a soft magnetic material selectedrom the group consisting of an atomized or sponge powder ofssentially pure iron or an iron-based prealloyed powder con-aining Si, Ni, Al or Co, (b) providing the particles of the powderith an electrically insulating layer, (c) mixing the powder of the
lectrically insulated particles with a lubricant, (d) compactinghe powder to a composite body and (e) heating the compositeody at a temperature between 400 and 700 ◦C. Fig. 6 showshe sample preparation flow chart [77]. The key to lowering theoercive force is either by developing a coating that can with-tand annealing temperatures or applying a compaction methodhat does not introduce the deleterious cold work. One tech-ique to eliminate the deleterious effects of cold working theron powder was pioneered by Dr. Kugimiya [49]. In this tech-ique, the surface of the powder was first oxidized; the powderas then hot pressed to full density (this material is referred
o as “NANOCON” material). This processing produced highulk resistivity with excellent permeability and low total coreosses.
Table 2 shows the typical changes of some material proper-ies due to variations in composition and the fabrication process85]. For a given application a compromise between the parame-ers in the table defines the composition and fabrication processhat gives the optimum behavior. In a material with a binderhe strength increases with increasing compaction while thepposite behavior occurs in a material without binder.
.1. Compaction
a In a material including a binder the strength increases up to typically 200 ◦CGelinas et al., 1998) and (Jansson, 2000), due to an improved distribution ofhe binder, further increase deteriorates the binder and the strength decreases. A
aterial without a binder (including a lubricant) shows a monotonously increasen strength with increasing temperature (Jansson, 1998).
6 H. Shokrollahi, K. Janghorban / Journal of Materials Processing Technology 189 (2007) 1–12
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ow temperature anneal to reduce the internal stress of the ironowder.
New compaction technologies are being explored that couldliminate the cold working of the iron powders. New tooling andubricant techniques are needed to significantly reduce the needor premixed powder lubrication. Generally two types of sam-les are produced for magnetic measurements; these are rodsnd toroids. According to the magnetic tester, size and dimen-ions can be varied. The combination of heated powder andigh compaction pressure result in flow of the polymer formingcontinuous matrix around the iron powder particles.
Density has a significant effect on the part performance.igher density P/M parts exhibit increased permeability and
aturation induction without any degradation of the coerciveorce. Techniques to increase the part density include doubleress/double sinter, warm compaction, or restriking a fully sin-ered part. Table 3 lists encapsulated green strength and densityata [75]. High velocity compaction (HVC) increases the com-etitiveness of SMC as well as expands the range of applicationss high density components with enhanced electromagneticroperties can be mass produced [54]. In the manufacture ofowder components, new compaction methods, such as warm
ompaction and two step compaction are being explored thatould eliminate the cold working of the iron powder and couldncrease the material density. Density and residual stresses havesignificant effect on the magnetic losses and part perfor-
tewa
able 3ncapsulated green strength and density data [75]
rocess parametersPowder temperature RT 75 ◦FDie temperature RT 150 ◦F
tandard P/M: Admixed lubeDensity (g/cm3)
Compacted at 50 (psi) 6.92 6.94TRS (psi) 2250 2310
icroencapsulated powderDensity (g/cm3)
Compacted at 50 (psi) 7.04 7.08TRS (psi) 3448 3224
on flow chart [77].
ance. Warm compaction and two step compaction methodsncrease magnetic induction, magnetic permeability and densitynd decrease core losses [61].
.2. Annealing
Annealing is required to minimize the deleterious effects ofold work on the magnetic performance of the core material.esults of an annealing process can greatly vary by the processistory of the material, the underlying material composition andther factors.
The hysteresis loss is partly due to stresses introduced in theaterial at compaction. In order to reduce hysteresis, a stress
elieving heat treatment most often follows the compaction, buthe heat also degrades the insulation between powder particlesnd thus increases the presence of eddy currents in the material.
Fig. 7 shows effect of annealing on the magnetic prop-rties. Annealing is effective for controlled development ofnduced anisotropy, adjustment of a well defined domain struc-ure, controlled microstructural changes and nanocrystallization.enerally, annealing can be classified into three categories:ulti-step thermal annealing, magnetic field annealing and
hermal–magnetic field annealing. Recently, the effect of differ-nt annealing conditions on the magnetic properties of SMCsas investigated [86]. It was found that magnetic loss of
nnealed powder was smaller than that of unannealed powder
125 ◦F 175 ◦F 225 ◦F 225 ◦F250 ◦F 350 ◦F 450 ◦F 550 ◦F
6.97 6.97 7.1 7.052750 3100 3,250 4,150
7.07 7.15 7.18 7.254343 4732 10,670 16,660
H. Shokrollahi, K. Janghorban / Journal of Mate
aidlcfhtatca
8
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Fig. 7. Effect of annealing on the magnetic properties [84].
fter 100 h ball-milling process. Annealing treatments can elim-nate residual stresses and some internal defects and help toomain growth and domain wall movement. Also, the magneticoss of samples which had two steps milling and annealing pro-ess was smaller than the samples with one-step process. It wasound that at low frequencies (<10 kHz) the magnetic loss in theigh-temperature magnetic annealed state is smaller than that inhe low-temperature magnetic, magnetic annealed and withoutnnealing states. On the contrary, the magnetic loss in the highemperature-magnetic annealed state is larger at high frequen-ies (>10 kHz) than the others. Fig. 8 shows the magnetic losss a function of frequency (at low frequencies) [86].
. Materials selection
The proper choice of metallic powder is different for AC andC magnets and must be dealt with separately. It is a commonnowledge that the magnetic properties of powders are a func-ion of their chemical composition, melting practice, hardening
rocess and heat treatment. The magnetically soft alloys mustombine as many as possible of the following characteristics atoderate cost:ig. 8. Magnetic loss as a function of frequency (logarithmic level) at lowrequencies (<10 kHz).
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rials Processing Technology 189 (2007) 1–12 7
1) low hysteresis losses;2) low eddy current losses;3) high permeability at low field strength;4) high saturation value;5) no aging effects;6) uniform magnetic characteristics.
oft magnetic powders or iron-based alloys are one of the com-onents of the SMCs that are currently replacing electrical steelheets or ferrites in some applications. The most common func-ion of magnetic iron-based alloys is as cores in power andistribution transformers.
Pure iron is the most prototypical soft magnetic material. Itas a very high saturation flux density, Bs = 2.2 T, and its cubicnisotropy leaves it with a relatively small magnetocrystallinenisotropy, K1 ≈ 4.8 × 104 J/m3, and small magnetostrictiononstants, λ1 0 0 = 21 × 10−6, λ1 1 1 = −20 × 10−6 [87]. The termron applies not only to substantially pure iron but to the wellnown alloys used for such purposes. Fe alloys contain up to0 wt% of one or more of the elements, such as Al, Si, Cr, Nb,o, Ni and Co. Alloyed irons provide higher magnetic perme-
bility and lower total core losses and result in devices havingigher efficiencies than devices using pure iron cores [88]. Addi-ion of elements to iron increases resistivity but eddy losses aretill too high, even at 50 Hz.
Carbon impurities and stress can be large contributors to theysteresis loss. Most steels used for laminations have low carbonontent, and often 1–3% silicon by weight. To provide optimalagnetic performance, these alloys possess very low levels of
arbon, nitrogen, and oxygen. They rely on various additions ofhosphorus, nickel, silicon, or cobalt to optimize permeability,oercive force, or induction. Table 4 illustrates DC magneticroperties for PM materials [75].
.1. Pure iron
The irons or electrical irons are low carbon alloys that offerlittle more magnetic permeability than the iron–cobalt alloys.hey have been used in relays, solenoids and magnets in vac-um equipments, particularly in direct current magnetic fieldpplications.
For applications, taking into account the purity, a powder withower content of C (<0.01 wt%), S (0.01 wt%) and O2 as wells H2 content <0.06 wt% was produced with the trade name ofP200HD by Iron Powder Plant from Buzau, and PERMITE 75
nd SOMALOY by HOganas [89]. Fig. 9 compares the initialux density curves for two iron specimens with two purities.he superior behavior of 99.99% pure Fe is evident [84].
.2. Fe–Ni alloys
The nickel–iron alloys possess the highest permeability by farf all the soft magnetic alloys. The nickel–iron alloys exhibit the
east amount of flux density. These alloys, therefore, are consid-red foremost for applications requiring high permeability andather low flux density. Properties vary over composition range.ptimum composition must be selected for a particular appli-
8 H. Shokrollahi, K. Janghorban / Journal of Materials Processing Technology 189 (2007) 1–12
Table 4DC magnetic properties for PM materials [75]
Alloy system Density (g/cm3) Maximum permeability Coercive force (Oe) Maximum induction (G) Resistivity (�� cm)
Fe 6.8–7.2 1800–3500 1.5–2.5 10–13 10F 1.2–F 0.8–5 0.2–
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e/P 6.8–7.4 2500–6000e/Si 6.8–7.3 2000–60000 Ni/50 Fe 7.2–7.6 5000–1500
ation. High Ni content alloys have high permeability; around0 wt% Ni has high saturation magnetization and low Ni contentave high electrical resistivity.
Magnetic Fe–Ni alloys are generally called permalloys. Orig-nally permalloy was the registered trademark for certain Fe–Nilloys, but it has now become a generic term. There are threeajor Fe–Ni compositions of technical interests: (1) 78% Ni
ermalloys (e.g., Supermalloy, Mumetal, Hi–mu 80) where theighest initial permeability is required. (2) 65% Ni permalloyse.g., A Alloy, 1040 Alloy) which show a strong response toeld annealing while maintaining K1 ≈ 0.3) 50% Ni permalloyse.g. Deltamax) which have high flux density (Bs = 1.6 T) [86].ll the FCC Fe–Ni alloys with Curie temperatures in excess of00 ◦C respond very well to magnetic field heat treatments sohat B–H loops with a variety of shapes can be achieved.
.3. Fe–Si alloys
These grades all have more hardness and electrical resistivityhan the irons. They have been found suitable for alternating
agnetic field applications, such as relays and solenoids. Theselloys are for applications requiring very low hysteresis loss,igh permeability, low residual magnetism, and freedom from
agnetic aging.6.5 wt% Si–Fe alloy is a well known alloy, because of thexcellent soft magnetic properties, such as high saturation mag-etization, near zero magnetostriction and high resistivity, which
ig. 9. Initial flux density curves for two iron specimens: (a) 99.9% pure Fe andb) 99.99% pure Fe [84].
hrtetitti
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2.0 10–14 301.2 9–13 600.5 9–14 45
uggests that the core loss should be reduced compared to thosef comparable alloys with 3–4 wt% silicon [84].
.4. Fe–Co alloys
Iron–cobalt alloys have the highest magnetization satura-ion of all known magnetic alloys as shown in Fig. 10. Theron–cobalt alloys, with slightly improved permeability, gener-lly have been preferred for their high magnetic saturation ofux density. This property maximizes the amount of magnetismvailable for magnetic circuits. Alloys in this family have beensed most frequently for aerospace motor and generator lam-nations, electromagnets, high performance transformers and
agnetic bearings.Influence of ternary additions on the saturation of Fe–Co
lloys, such as Ti, V, Cr, Ni, Cu was found to be detrimental3], with the exception of Mn. Table 5 also shows the effect of
and Nb addition on the saturation induction of Fe–Co-basedlloys after furnace cooling from 760 ◦C. No gain was resultedrom these alloys.
. Particle size
Low frequency permeabilities of small particle compositesave larger values than those of large particle ones and naturalesonance frequency of small particle composites is lower thanhat of large particle ones. Magnetic nanoparticles show a vari-ty of unusual magnetic behavior compared to bulk materials orhin film systems [42], mostly due to surface/interface effects,
ncluding symmetry breaking, electronic environment/chargeransfer, and magnetic interactions. When the size of the par-icles is reduced below the single domain limit (∼15–20 nm forron oxide), they exhibit superparamagnetism at room tempera-ig. 10. The Slater–Pauling curve showing the mean atomic moment for aariety of binary alloys as a function of their composition [3].
H. Shokrollahi, K. Janghorban / Journal of Mate
Table 5Saturation magnetization for different Fe–Co-based alloys after furnace cooling[3]
Alloy Saturationinduction (T)
Estimated volume fractionof second phase (%)
FeCo–2 V 2.32 –FeCo–3.6 V 2.29 Not measuredFeCo–1 Nb 2.34 7FF
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ure followed by a spin-glass like transition at low temperature42]. Soft magnetic nanocrystalline alloys have high coerciv-ty and low remanence magnetization. Two important factorso improve the remanent magnetization are the nanocrystallinerain size and the degree of coherence across interphase bound-ries (it should be sufficient to enable adjacent phases to bexchange coupled). Fig. 11 shows variation of coercivity (HC)ith particle size (D) for magnetic material [84].Once the magnetic particle is in a nanometer size, the eddy
urrent produced within the particle is also negligibly small.herefore, conductivity of the magnetic constituent is no longerfactor in the material selection consideration and metallic mate-
ial selection consideration and metallic materials can be useds magnetic phase. The advantages of magnetic nanocompositenclude: (1) reduction in total core power losses, (2) the highux capabilities at elevated temperatures that the nanocompos-
te cores are expected to support, thereby enabling manufacturef smaller power devices, and (3) broadband devices.
0. Coated iron-based powders
SMCs are basically pure iron powder particles coated withvery thin, electrically insulated layer (Fig. 1). Good insula-
ion and fine particles are generally required to minimize eddyurrents in high frequency applications. At low frequency, insu-ation is less critical but nevertheless needed in order to minimizehe negative effect of the eddy currents on the magnetization ofhe material. There are two principal technologies that are the
ack bone of the coating industries [90]:(a) liquid coating technology (wet), which has been applied formore than two centuries;
ig. 11. Schematic diagram showing variation of coercivity (HC) with particleize (D) for magnetic material [84].
csi
Fa
rials Processing Technology 189 (2007) 1–12 9
b) powder coating technology, which has been applied on anindustrial scale for some 30 years.
In general, insulating coatings are classified into two mainategories, Inorganic and organic coatings. Inorganic coatingsan be subdivided into several categories; metallic oxide coat-ngs (such as Fe2O3), phosphate coatings (zinc phosphate, ironhosphate and manganese phosphate), and sulfate coatings.rganic coatings can be divided into two categories, thermo-lastic coatings and thermosetting coatings.
0.1. Organic coatings
To provide the maximum magnetic permeability the amountf interparticle insulation should be minimized and iron contentaximized. Effect of epoxy content on the core loss is shown
n Fig. 12. It is evident that high epoxy content >6.5 vol% is notonstructive at higher frequencies. Cores made from polymer-onded iron particles should have as low a polymer content ass possible which unfortunately tends to reduce the physicaltrength of the core. A summary of typical magnetic proper-ies for a variety of isolated particle materials is given in Table 678].
It has been proposed to coat magnetic core iron particlesith polymers in a number of ways including: (1) dispersing
he particles in a solution of the polymer dissolved in a solventnd driving off the solvent, (2) polymerizing the polymer in situn the surface of the particles, and (3) coating the particles influidized bed with the polymer dissolved in an appropriate
olvent [90].Unfortunately, the more common polymers that one might
xpect to survive hostile environments, do not have the processbility characteristics needed to completely coat the particlesnd/or to readily mold high density, high strength cores withhe desired physical and magnetic properties. Indeed most poly-
ers otherwise suitable for hostile environments are thermosetshich after having been once cured about the iron particle can-ot be dissolved, reprocessed or compression/injection molded.n the other hand, most thermoplastics which might be both
annot practically be coated uniformly and continuously ontomall iron particles primarily because they are either essentiallynsoluble in industrially acceptable solvents (for example, crys-
ig. 12. Core loss/cycle vs. frequency at 1 T; green samples compacted withoutdmixed lubricant [56].
10 H. Shokrollahi, K. Janghorban / Journal of Materials Processing Technology 189 (2007) 1–12
Table 6Typical mechanical and magnetic properties for several isolated particle materials [78]
Material Polymercoating (w/o)
Oxidecoating
Compaction temperature(powder/die) (◦C)
Curing temperature(◦C)
Density at 690MPa (g/cm3)
Strength(MPa)
μi μm Hc
(Oe)B (T)
SC100 0.75 No 150/260 315 7.20 210 100 400 388 1.09SC120 0.60 No 150/260 315 7.30 210 120 425 380 1.12SC600 0.25 No 150/260 315 7.40 100 140 600 380 1.27TC80 0.75 Yes 150/260 315 7.15 210 80 210 380 0.77AP500 LS – Yes Cold 480 7.22 35 85 425 243 1.27AP500 HS Cured – Yes Cold 150 7.15 95 80 230 380 0.86AP500 HS Warm Press – Yes Cold/150 None 7.45 90 90 300 388 0.99A 480L 650
tras
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P500 High Perm – Yes Cold/150CM – Yes Cold/260
alline thermoplastics), do not coat the particles well, cannot beeadily handled in a heated condition preparatory to moldingnd/or have too high a melt viscosity for proper filling out of thehaping die during molding.
The selection of a thermoset instead of a thermoplastic isone to minimize the effect of the temperature variations on theagnetic and mechanical properties of the composites. There areany advantages that make the choice of applying thermosetting
owder coatings so attractive to the coating companies. Thesere:
a) powder is immediately ready for use;b) less powder waste during the application process;c) reduced health hazard in case of exposure of operators;d) superior cured-film properties;e) lower capital investment costs.
ome of the thermosets which are used for coating are listed inable 7 [91].
0.2. Inorganic coatings
Inorganic coatings including several inorganic compounds,uch as, phosphates (zinc/iron/manganese phosphates), oxides,
able 7ome of the thermosets which are used for coating [91]
ame Descriptions
poxy powders For high gloss and smooth coatings with excellentadhesion, flexibility and hardness, solvent andchemical resistance
crylic powders Widely used in surface coatings, with good glossand color retention on exterior exposure, heat andalkali resistance
olyester powders General performance between epoxy and acrylicpowders, excellent durability, high resistance toyellowing under ultra-violet light
poxy-polyesterhybrid powders
Epoxy powders containing a high percentage ofspecial polyester resin (sometimes exceeding50%) with resistance to overbake yellowing andweatherability, main backbone of the powdercoating industries
olyurethanepowders
Good all-round physical and chemical properties,good exterior durability
rio
aiwultsdpcmcicsguwp
7.20 35 80 520 307 1.297.25 25 125 245 356 0.80
ulfates can be used for creating of electrically insulated parti-les. There are two general methods for applying an inorganicoating on the iron-based particles, wet chemical and dry chemi-al methods. In wet chemical method, a suitable inorganic layer,uch as Fe–Zn phosphate precipitates on the metallic surface.n dry process, the metallic powders, for example, are oxi-ized in a furnace at a suitable temperature and atmosphere.t should be considered that, wet chemical processing of pow-ers compared to a bulk material is more difficult and needs theontrol of time, temperature and bath composition. For exam-le, phosphating time for powders is very shorter than bulkaterials. Three principal types of phosphate coatings that are
n general use are based on zinc, iron and manganese. Theseoatings can be applied by spray or immersion, and mechanicallloying.
1. Conclusions
The ideal soft magnetic material is an isotropic media withery high magnetic permeability, low coercivity and high satu-ation induction. In addition, the material could be easily shapednto three-dimensional structures in order to fully take advantagef the material’s isotropic nature.
SMCs do in many respects resemble the ideal material,s they are isotropic materials consisting of small insulatingron particles. Intricate 3D-shapes could be obtained by theell-established, cost-effective P/M-compaction processes. Sat-ration induction is close to the laminates and eddy currentosses are significantly lower due to the smaller size of the par-icles (typically 5–200 �m) compared to the thickness of theteel sheets (normally 200–1000 �m). However, SMCs have aistributed air-gap, leading to lower permeabilities and further,lastic deformation of the particles that takes place during theompaction step, results in higher hysterisis losses. A heat treat-ent after compaction partly relieves the stresses. Raising the
ompaction pressure (to increase density) increases the mechan-cal strain induced in the iron particles, resulting in higheroercive force and hysteresis loss unless the material is given atress-relieving heat-treatment. The latest iron–resin composite
rades attempt to combat these effects by reducing the vol-me of resin, combining the insulating material characteristicsith lubricity to improve compressibility, and oxidizing the ironarticle surfaces to increase resistivity.
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H. Shokrollahi, K. Janghorban / Journal of
The SMC concept is very flexible as the final propertiesf component depend on the composition of the iron powder,oating, lubricant/binder, compaction and heat treatment. Usingew techniques in compaction (such as warm and two stepsompaction), annealing conditions (such as two steps anneal-ng/magnetic annealing), new powder compositions (Fe–Ni,e–Co, Fe–Si alloys), nano particle size, and suitable insulation
ayers, improve the magnetic properties of SMCs.
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