Hybrid Phosphate-Alumina Iron-Based Core-Shell Soft Magnetic Composites Fabricated by Sol-Gel Method and Ball Milling Method - MDPI

metals
Article
Hybrid Phosphate-Alumina Iron-Based Core-Shell
Soft Magnetic Composites Fabricated by Sol-Gel
Method and Ball Milling Method
Yifan Pan 1 , Liwei Qian 1 , Xiang Wang 2 , Jingguang Peng 2, * and Wei Lu 1, *
 1 Shanghai Key Lab of D&A for Metal-Functional Materials, School of Material Science and Engineering,
 Tongji University, 201800 Shanghai, China; panyifan1017@163.com (Y.P.); qianlw_tj@163.com (L.Q.)
 2 Shanghai Automotive Powder Metallurgy Co., 201900 Shanghai, China; xiangw@shautopm.com.cn
 * Correspondence: jingguangp@shautopm.com.cn (J.P.); weilu@tongji.edu.cn (W.L.);
 Tel.: +86-21-6958-1508 (W.L.)
 



 Received: 9 January 2020; Accepted: 10 February 2020; Published: 17 February 2020 


 Abstract: Novel Fe-based soft magnetic composites (SMCs) with hybrid phosphate-alumina layers
 were prepared by both sol–gel and ball-milling methods. The effects of the fabrication methods and
 the addition of Al2 O3 particles on the microstructure and the soft magnetic performance of SMCs
 were studied. The formation of the hybrid phosphate-Al2 O3 shell not only leads to the decrease of
 the total core loss, but also results in the reduction of the permeability and saturation magnetization.
 However, the degree of decrease caused by the different methods were not identical. The sample
 with 8% Al2 O3 prepared by the sol–gel method showed the best magnetic performance, exhibiting a
 high-amplitude permeability (µa ) of 85.14 and a low total core loss (Ps) of 202.3 W/kg at 50 mT and
 100 kHz. The hysteresis loss factor and the eddy current loss factor were obtained by loss separation.
 The results showed that the samples with the same Al2 O3 content prepared by different methods
 exhibited almost the same total core loss. However, the contribution of the hysteresis loss and the
 eddy current loss showed an obvious difference in behavior because of the change of the particle
 shapes and the refinement of the particle size during the ball-milling process.

 Keywords: soft magnetic composites; sol–gel method; ball-milling method; loss separation

1. Introduction
 The eddy currents which appear in ferromagnetic materials are caused by the magnetization
in an alternating current (AC) magnetic field as a consequence of Faraday’s law. The ferromagnetic
materials that were magnetized in an AC magnetic field would internally-generate the eddy current, a
long-existing barrier that limited the applications of the devices at medium to high frequency because
of the increase in eddy current loss and the reduction of the permeability [1,2].
 Owing to their superior properties, including high permeability, high-frequency stability, 3D
isotropy magnetic field, high productivity and, especially, the high electrical resistivity, iron-based
soft magnetic composites (SMCs) with a special core–shell structure [3] have attracted broad academic
interest and investigations in order to satisfy the requirements of applications in power electronics,
transformers and electric motors at medium to high frequency [4,5]. The iron-based SMCs are
conventionally prepared by coating insulating materials onto the pure iron matrix to block the path
of closed-current loops and the compaction and annealing process [6]. Therefore, the selection of
appropriate insulating materials with high electric resistivity and good adhesive strength is vital to
minimize the eddy current loss of the SMC cores [7].
 The coating layers can be divided into two principal types: organic materials and inorganic
materials. Organic materials such as silicon resin [8] and epoxy [9] are the most popular insulating

Metals 2020, 10, 257; doi:10.3390/met10020257 www.mdpi.com/journal/metals

Metals 2020, 10, 257 2 of 10

compounds due to their high electric resistivity and good flexibility. However, the application of
organic materials in insulating layers is restricted due to their low thermal stability, which may not
permit high-temperature heat treatment to eliminate the internal stress that is introduced during
the preparation process. Thus, inorganic materials, including phosphates [10], oxides such as SiO2 ,
Al2 O3 , ZrO2 , Fe3 O4 [11–13] and ferrites such as Ni-Zn ferrites and Mn-Zn ferrites [14,15], have
gained greater attention and are increasingly considered as promising insulating materials because
of their superior electrical resistance and temperature resistance. Peng et al. [15] fabricated Mn-Zn
ferrite-coated iron-based composite particles by a co-precipitation method and studied the soft
magnetic properties of the composites. Wu et al. [16] prepared Fe/SiO2 composite particles via a
reverse micro-emulsion method and investigated the effects of the addition amounts of SiO2 and the
heat-treatment temperatures on the permeability and the total core loss of the composites. Though the
above composite particles [15,16] have been fabricated, it was difficult to ensure the homogeneity of the
coatings to be applied in industrialization. In addition, the insulating layer is easily peeled off because
of the poor adhesiveness to the matrix, which would result in the direct contact between core particles.
In comparison to the oxides and nano-ferrites, phosphates show the advantage of better adhesiveness
to the iron matrix, good homogeneity of coatings and the potential of industrialization because of the
simple preparation process and more limited equipment required. However, a previous paper [17] has
reported that the phosphate coatings would crystalize or decompose at a relatively high temperature
(above 500 ◦ C), dramatically decreasing the electric resistivity. Huang et al. [18] investigated the effect
of annealing temperatures on the composition and the magnetic properties of the phosphate-coated
SMCs; their results showed that phosphate coatings degrade into iron phosphide after annealing at
600 ◦ C, and the magnetic properties worsened as a result. Owing to the high bond energy of the Al-O
covalent bond, Al2 O3 possess high electrical resistivity and high temperature resistance. However,
neither the single phosphate coating nor the alumina could satisfy the requirement of the application
at high frequency. Therefore, hybrid phosphate–alumina iron-based soft magnetic composites are
promising to effectively reduce the total core loss to broaden the frequency of industrial applications.
 Owing to the advantages such as low equipment requirements, easy operation and economic
efficiency, the mechanical milling method and the sol–gel method are two of the most widely used
methods to prepare SMCs. In this work, novel hybrid phosphate–alumina iron-based SMCs were
fabricated first by phosphating and then by coating with Al2 O3 via two methods, namely sol–gel
and mechanical milling. The hybrid-coated SMCs retained relatively high permeability and showed
very low core loss. The effects of the Al2 O3 content and the different preparation methods on the soft
magnetic properties of the composites were studied in detail.

2. Preparation and Characterization
 Gas-atomized iron powders with a particle size ranging from 75 µm to 250 µm were conducted
to achieve the desired size ratio, annealed at 700 ◦ C for some time, twice cleaned in absolute ethyl
alcohol, and dried at 60 ◦ C for five hours before the passivation. Then, 50 g iron powders were mixed
with a certain amount of ethanol-diluted orthophosphate acid with a concentration of 0.01 g/mL, and
the mixture were stirred in a solution of 65 ◦ C until the solvent completely evaporated to obtain the
phosphate-coated particles.
 The sol–gel method and the ball-milling method were used to prepare Fe @ phosphate particles to
obtain powders with the hybrid coating. A certain amount of aluminum oxides were dissolved in
100 mL of ethanol and was magnetically stirred at room temperature for 1 h to obtain the aluminum
sol. Then, some as-prepared Fe/phosphate powders were dispersed in the solution using a spiral
mixer for 15 min. A mixture of distilled water and ethanol (1:3 (V/V)) was added dropwise, acetic acid
was added to adjust the pH value to around four, and the mixture was continuously stirred for 1.5 h.
Subsequently, the suspension was separated and washed with ethanol three times and dried. Finally,
the powders were calcinated at 600 ◦ C for 2 h under a nitrogen atmosphere.

Metals 2020, 10, 257 3 of 10

 As for the ball-milling method, the weight ratio of ball to powders was set to 10:1 and a mixture
of the as-prepared Fe/phosphate powders and the α-Al2 O3 nanoparticles with the weight percent of
2 wt%–8 wt% were carried out at a speed of 200 rpm for 30 min. All of the above powders were mixed
with the lubricant of Zinc stearate (0.5 wt%) and were pressed at an uniaxial pressure of 800 MPa at
room temperature to obtain the ring-shaped compacts with an external diameter of 20 mm, an inside
diameter of 12 mm and height of approximately 3.4 mm.
 An X-ray diffractometer using Cu-Kα radiation was used to analyze the phases of the samples.
The morphology and composition of the powders were detected by a scanning electron microscopy
(SEM, Nova NanoSEM 450, FEI, USA) equipped with an energy-dispersive X-ray Spectroscope (EDS,
 Metals 2019, 9, x FOR PEER REVIEW 3 of 11
Ultra, EDAX, USA), whose resolutions were 1.0 nm at 15 kV and 0.8 nm at 30 kV. All of the SEM images
were magnified 1000 times and a metallographic analyzer produced by a Shanghai optical instruments
 2 wt%–8 wt% were carried out at a speed of 200 rpm for 30 min. All of the above powders were mixed
factory was used to characterize the microstructure. The soft magnetic properties of the ring-shaped
 with the lubricant of Zinc stearate (0.5 wt%) and were pressed at an uniaxial pressure of 800 MPa at
compacts were determined by a hysteresis looper (MATS 2010SA) at a frequency ranging from 5 kHz
 room temperature to obtain the ring-shaped compacts with an external diameter of 20 mm, an inside
to 100 kHz and a magnetic excitation level of 0.05 T; the uncertainty of measured µa and total loss
 diameter of 12 mm and height of approximately 3.4 mm.
were less than 3% and 5%, respectively. The vibrating sample magnetometer (VSM) was conducted to
 An X-ray diffractometer using Cu-Kα radiation was used to analyze the phases of the samples.
measure the saturation magnetization of the compacts; the accuracy and repeatability were less than
 The morphology and composition of the powders were detected by a scanning electron microscopy
1% and 5%, respectively.
 (SEM, Nova NanoSEM 450, FEI, USA) equipped with an energy-dispersive X-ray Spectroscope (EDS,
 Ultra,
3. EDAX,
 Results and USA), whose resolutions were 1.0 nm at 15 kV and 0.8 nm at 30 kV. All of the SEM
 Discussion
 images were magnified 1000 times and a metallographic analyzer produced by a Shanghai optical
 instruments
3.1. SMCs with factory
 Singlewas used to
 Phosphate characterize the microstructure. The soft magnetic properties of the
 Coating
 ring-shaped compacts were determined by a hysteresis looper (MATS 2010SA) at a frequency ranging
 Figure 1 shows the SEM images of the raw iron powders, the phosphated powders, and the EDS
 from 5 kHz to 100 kHz and a magnetic excitation level of 0.05 T; the uncertainty of measured μa and
(energy disperse spectrum) analysis and maps of the phosphated powders. Results suggest that the
 total loss were less than 3% and 5%, respectively. The vibrating sample magnetometer (VSM) was
coated powders keep the same irregular shape and only show a rougher surface than the raw iron
 conducted to measure the saturation magnetization of the compacts; the accuracy and repeatability
powders. Figure 1c, which displays the EDS analysis of the selected area of the phosphated iron
 were less than 1% and 5%, respectively.
particle, indicates the existence of Fe, O and P on the surface of the phosphated particles. In addition,
the EDS maps show that the iron, oxygen and phosphate elements distribute on the surface of the
 3. Results and Discussion
coated powders. Therefore, it can be concluded that the phosphate insulating layers were formed
during
 3.1. SMCsthe with
 phosphating process.Coating
 Single Phosphate

 Figure1.1.SEM
 Figure SEMimages
 images of of
 thethe powders
 powders (a) pure
 (a) pure iron powder,
 iron powder, (b) phosphated
 (b) phosphated iron powder,
 iron powder, (c) EDS
 (c) EDS analysis
 analysis
 of of thearea
 the selected selected area maps
 and EDS and EDS maps
 of (d) iron,of(e)
 (d)oxygen
 iron, (e)
 andoxygen and (f) phosphorus
 (f) phosphorus of phosphated
 of phosphated powders.
 powders.

 Figure 1 shows the SEM images of the raw iron powders, the phosphated powders, and the
EDS (energy disperse spectrum) analysis and maps of the phosphated powders. Results suggest
that the coated powders keep the same irregular shape and only show a rougher surface than the raw
iron powders. Figure 1c, which displays the EDS analysis of the selected area of the phosphated iron
particle, indicates the existence of Fe, O and P on the surface of the phosphated particles. In addition,
the EDS maps show that the iron, oxygen and phosphate elements distribute on the surface of the
coated powders. Therefore, it can be concluded that the phosphate insulating layers were formed

Metals 2020, 10, 257 4 of 10

 Figure 2 reveals the influence of the phosphating on the permeability and the total core loss of the
compacts at 50 mT external magnetic field. The permeability of the as-coated sample is 127.7 at 5 kHz
and is not as high as that of the pure iron sample due to the magnetic dilution effect of the non-magnetic
materials [19]. The presence of non-magnetic phosphate between the magnetic cores plays the same role
as air gaps, which would create a demagnetizing field in the entire magnetic circuit, resulting in lower
permeability at low frequency. However, owing to the greater frequency stability, the permeability
of the coated sample is superior to that of the pure iron sample at 100 kHz. At high frequency,
the phosphate with strong resistivity leads to a lower eddy current. The increased eddy current caused
 Metals 2019, 9, x FOR PEER REVIEW 4 of 11
by the high-frequency AC field in the pure iron sample circulates in the opposite direction to the
applied field, which results in the decrease of the excited magnetic field and permeability [17,20].

 Figure 2. The effects of the phosphating process on the soft magnetic properties of the samples (a) the
 amplitude permeability and (b) the core loss.
 Figure 2. The effects of the phosphating process on the soft magnetic properties of the samples (a) the
 amplitude permeability and (b) the core loss.
 The total core loss of the as-coated sample (shown in Figure 2b) is approximately two thirds of the
pure iron sample at 100 kHz. This can be explained by the blocking effect of the phosphate insulation
 Figure 2 reveals the influence of the phosphating on the permeability and the total core loss of
layer on the inter-particle eddy current loss, which is the main contributor to the total core loss at
 the compacts at 50 mT external magnetic field. The permeability of the as-coated sample is 127.7 at 5
high frequency.
 kHz and is not as high as that of the pure iron sample due to the magnetic dilution effect of the non-
 magnetic
3.2. SMCs materials
 with Hybrid [19]. The presence of Coating
 Phosphate–Alumina non-magnetic phosphate between the magnetic cores plays
 the same role as air gaps, which would create a demagnetizing field in the entire magnetic circuit,
 The XRD
 resulting patterns
 in lower of the pure
 permeability iron
 at low particles However,
 frequency. and the hybrid-coated particles
 owing to the greater are displayed
 frequency in
 stability,
Figure 3. The diffraction peaks at 2θ = 44.667 ◦ , 65.000◦ and 82.366◦ correspond to the (110), (200) and
 the permeability of the coated sample is superior to that of the pure iron sample at 100 kHz. At high
(211) planesthe
 frequency, of the α-Fe phase
 phosphate with (JCPDS 06-0696). leads
 strong resistivity The XRD patterns
 to a lower of the
 eddy coated
 current. Thepowders
 increasedareeddy
 not
obviously different from those of the original. Only the diffraction peak intensity
 current caused by the high-frequency AC field in the pure iron sample circulates in the opposite of the α-Fe phases
of the hybrid-coated
 direction powders
 to the applied viawhich
 field, the ball-milling
 results inmethod decreases
 the decrease of slightly
 the excitedand no other diffraction
 magnetic field and
peaks could be observed.
 permeability [17,20]. This could be explained primarily by the possibility that the content of the
phosphate layer is too small to be detected, or that the phosphate layer is amorphous
 The total core loss of the as-coated sample (shown in Figure 2b) is approximately two thirds of [18]. Previous
research
 the purehas ironreported
 sample that
 at 100amorphous
 kHz. ThisAlcan 2 O3be would transfer
 explained by to
 theδ-Al 2 O3 , θ-Al
 blocking 2 O3of
 effect and
 theα-Al 2 O3 at
 phosphate
750 ◦ C, 1000 ◦ C, 1100 ◦ C, respectively [21]. Therefore, preparing the Al O particles by the sol–gel
 insulation layer on the inter-particle eddy current loss, which is the main contributor to the total core
 2 3
method calcining at 600 ◦ C could only obtain amorphous Al2 O3 . As for the ball-milling method,
 and frequency.
 loss at high
the broadening of the α-Fe peak may be due to the small crystallites and their lattice distortions [22].
The
 3.2. grain
 SMCsrefinement
 with Hybridand the plastic deformation
 Phosphate–Alumina Coatingduring the ball-milling process would also decrease
the intensity of the diffraction peaks.

permeability [17,20].
 The total core loss of the as-coated sample (shown in Figure 2b) is approximately two thirds of
the pure iron sample at 100 kHz. This can be explained by the blocking effect of the phosphate
insulation layer on the inter-particle eddy current loss, which is the main contributor to the total core
loss at high frequency.
Metals 2020, 10, 257 5 of 10

3.2. SMCs with Hybrid Phosphate–Alumina Coating

Metals 2019, 9, x FOR PEER REVIEW 5 of 11

 Figure 3. The XRD patterns of the powders (a) pure iron; (b) hybrid coating via the ball-milling
 method with 8% Al2O3; (c) hybrid coating via the sol–gel method with 8% Al2O3.

 The XRD patterns of the pure iron particles and the hybrid-coated particles are displayed in
Figure 3. The diffraction peaks at 2θ = 44.667°, 65.000° and 82.366° correspond to the (110), (200) and
(211) planes of the α-Fe phase (JCPDS 06-0696). The XRD patterns of the coated powders are not
obviously different from those of the original. Only the diffraction peak intensity of the α-Fe phases
of the hybrid-coated powders via the ball-milling method decreases slightly and no other diffraction
peaks could be observed. This could be explained primarily by the possibility that the content of the
 Figure 3. The XRD patterns of the powders (a) pure iron; (b) hybrid coating via the ball-milling method
phosphate layer is too small to be detected, or that the phosphate layer is amorphous [18]. Previous
 with 8% Al2 O3 ; (c) hybrid coating via the sol–gel method with 8% Al2 O3 .
research has reported that amorphous Al2O3 would transfer to δ-Al2O3, θ-Al2O3 and α-Al2O3 at 750
°C, 1000
 SEM°C, 1100 and
 images °C, respectively
 EDS maps of[21]. Therefore, preparing
 the hybrid-coated powders theviaAlball
 2O3 milling
 particles
 arebyexhibited
 the sol–gel method
 in Figure 4.
and calcining at 600 °C could only obtain amorphous Al O . As for the ball-milling
A change of the particle shape from the irregular shape to a flake shape after the ball-milling process
 2 3 method, the
broadening
could of the α-Fe
 be observed peak may
 in Figures be due
 4a and to the small
 2a. Traces crystallites
 of aluminum, and their
 oxygen lattice distortions
 and phosphorus [22]. The
 are uniformly
grain refinement and the plastic deformation during the ball-milling process would
detected on the particle surface in the EDS maps (Figure 4b–d), suggesting the formation of the also decrease the
intensity of the diffraction peaks.
core–shell structured Fe @ phosphate @ Al2 O3 SMCs.

 Figure 4. SEM images of
 Figure 4. of (a)
 (a) hybrid-coated
 hybrid-coated powders
 powders with
 with8%
 8%Al
 Al22O
 O33 via ball milling and EDS maps of
 of
 (b)
 (b) aluminum,
 aluminum, (c)
 (c) oxygen
 oxygen and
 and (d)
 (d) phosphorus.
 phosphorus.

 Figure 5a–d and
 SEM images indicates that increasing
 EDS maps the Al2 O3powders
 of the hybrid-coated content via
 results in thicker
 ball milling areand more in
 exhibited uniform
 Figure
insulating layers. No obvious insulating layers could be seen with the addition
4. A change of the particle shape from the irregular shape to a flake shape after the of 2% Al O . Insulating
 2 3ball-milling
layers
processwere
 couldevident with additional
 be observed in Figure 4aamounts of 6%
 and Figure 2a.Al2 O3 , but
 Traces some partsoxygen
 of aluminum, of the and
 matrix particles
 phosphorus
still contacted directly. Further increasing the additional amount of Al O to 8%
are uniformly detected on the particle surface in the EDS maps (Figure 4b–d), suggesting the
 2 3 leads to complete
insulating
formation layers. However,structured
 of the core–shell more poresFecould be observed
 @ phosphate @ Alwith the increasing Al2 O3 content.
 2O3 SMCs.

Metals 2020, 10, 257 6 of 10
Metals 2019, 9, x FOR PEER REVIEW 6 of 11
 Metals 2019, 9, x FOR PEER REVIEW 6 of 11

 Figure 5.
 Figure 5. Metallographs
 Metallographs of the hybrid-coated compacts by the ball-milling
 ball-milling method
 method with
 with various
 various
 Figure 5. Metallographs of the hybrid-coated compacts by the ball-milling method with various
 additional amounts
 additional amounts of Al22O33 (a) 2%; (b) 4%; (c) 6%; (d) 8%.
 of Al
 additional amounts of Al2O3 (a) 2%; (b) 4%; (c) 6%; (d) 8%.
 To further
 Figure 5a–danalyze the that
 indicates microstructure: the Al
 increasing the SEM
 2O3 (scanning electron
 content results in microscopy) images
 thicker and more of the
 uniform
 Figure 5a–d indicates that increasing the Al2O3 content results in thicker and more uniform
polished surface of the Fe @ phosphate
 insulating layers. No obvious insulating layers @ Al O SMC compact were characterized.
 2 3 could be seen with the addition of 2% Al2O3. In addition,
 insulating layers. No obvious insulating layers could be seen with the addition of 2% Al2O3.
the EDS maps
 Insulating werewere
 layers shown to verify
 evident withtheadditional
 microstructure (Figure
 amounts 6). Al
 of 6% The2Oiron matrix
 3, but someparticles
 parts ofwere firmly
 the matrix
 Insulating layers were evident with additional amounts of 6% Al2O3, but some parts of the matrix
encapsulated
 particles still by a thin coating
 contacted with
 directly. a core–shell
 Further structure
 increasing the via the ball-milling
 additional amount method.
 of Al2O3theto fact that the
 8% leads to
 particles still contacted directly. Further increasing the additional amount of Al2O3 to 8% leads to
Pcomplete
 and Al elements
 insulatingonlylayers.
 distributed around
 However, the matrix
 more pores powder
 could be and the Fe element
 observed distributed
 with the in almost
 increasing Al2O3
 complete insulating layers. However, more pores could be observed with the increasing Al2O3
the whole visual area could strongly prove the nature of the composite interface microstructure.
 content.
 content.

 Figure 6. SEM images of (a) the polished surface of Fe @ phosphate @ Al2O3 SMC compact and EDS
 Figure 6. SEM
 Figure images
 6. SEM of (a)of
 images the
 (a)polished surface
 the polished of Fe of
 surface @ phosphate @ Al2@
 Fe @ phosphate O3AlSMC compact
 2O3 SMC and EDS
 compact and EDS
 maps of (b) iron, (c) phosphorus and (d) aluminum.
 mapsmaps
 of (b)ofiron, (c) phosphorus
 (b) iron, and (d)
 (c) phosphorus andaluminum.
 (d) aluminum.
 To further
 Figure analyze
 the the microstructure: thematrix
 SEM (scanning electron microscopy) images of the
 To 7further
 reveals hysteresis
 analyze loops of the
 the microstructure: the SEMiron(scanning
 particles and the
 electron hybrid-coated
 microscopy) imagesparticles
 of the
polished
prepared surface
 by the of the Fe
 ball-milling@ phosphate
 method @ Al
 with the2O3 SMC compact were characterized. In addition, the
 polished surface of the Fe @ phosphate @ various additional
 Al2O3 SMC compact amounts of Al2 O3 . The
 were characterized. Insaturated
 addition, the
EDS maps were
magnetization shown to verifyisthe microstructure
 205(Figure 6).
 AllThe iron matrix particles weredisplay
 firmly
 EDS maps of rawshown
 were pure iron approximately
 to verify emu/g.
 the microstructure (Figure theThe
 6). hybrid-coated
 iron matrix powders
 particles were firmly
encapsulated
excellent by a thin
 properties coating with a core–shell structure via the160ball-milling method. the
 It isfact that
 encapsulated byofasaturated magnetization
 thin coating of approximately
 with a core–shell structure via emu/g
 the to 190
 ball-millingemu/g.
 method. widely
 the fact that
the P and Al elements only distributed around the matrix powder and the Fe element distributed in
 the P and Al elements only distributed around the matrix powder and the Fe element distributed in

Metals 2020, 10, 257 7 of 10

accepted
 Metals 2019,that the saturated
 9, x FOR magnetization mainly depends on the composition and the purity of7 of
 PEER REVIEW the
 11
materials [1], and that introducing non-magnetic phosphate and α-Al2 O3 would dilute the percentage
 almost
of magneticthe phases
 wholeandvisual area
 reduce could
 the Ms as a strongly
 result. Theprove the nature
 decreasing trend ofof
 thethe composite
 saturated interface
 magnetization
 microstructure.
in Figure 7 is due to the increasing content of Al2 O3 in insulating layers.

 Metals 2019, 9, x FOR PEER REVIEW 7 of 11

 almost the whole visual area could strongly prove the nature of the composite interface
 microstructure.

 Figure 7.
 Figure 7. Magnetic
 Magnetic hysteresis
 hysteresis loops
 loops of
 of the
 the pure
 pure iron
 iron and
 and the
 the hybrid-coated
 hybrid-coated powders
 powders prepared
 prepared by
 by the
 the
 ball-milling
 ball-milling method
 methodwith
 withthe
 thevarious
 variousadditional
 additionalamount
 amountof ofAl
 Al22O
 O33.

 Figure
 Figure 87 reveals
 reveals the
 the amplitude permeability
 hysteresis loops as a function
 of the matrix of frequency
 iron particles and theofhybrid-coated
 the samples prepared
 particles
by the ball-milling method (Figure 8a) and the sol–gel method (Figure 8b) with
 prepared by the ball-milling method with the various additional amounts of Al2O3. The saturated the same additional
amount Figure
 of Al7.2 O
 Magnetic hysteresis loops of the pure
 of iron and the hybrid-coated powders
 preparedprepared bothby the
 magnetization 3 . raw
 of The pure
 amplitude
 iron ispermeability
 approximately the
 205hybrid-coated
 emu/g. All theSMCs
 hybrid-coated by
 powders methods
 display
 ball-milling method with the various additional amount of Al2O3.
exhibit excellent
 excellent propertiesfrequency stability
 of saturated at the measured
 magnetization frequency, which
 of approximately could be
 160 emu/g to attributed
 190 emu/g.toItthe better
 is widely
integrity of the insulation layers shown in Figure 5. Increasing the
 accepted that the saturated magnetization mainly depends on the composition2 and content of Al O causes a tendency
 3 the purity of the
 Figure 7 reveals the hysteresis loops of the matrix iron particles and the hybrid-coated particles
of permeability
 materials [1], anddecrement, and the
 that introducing permeability
 non-magnetic of the samples
 phosphate and α-Alwith
 2O3 8%
 wouldAl2dilute
 O3 reach 77.24 (ball
 the percentage
 prepared by the ball-milling method with the various additional amounts of Al2O3. The saturated
milling method)
 of magnetic phases andand
 85.14 (sol-gel
 reduce method),
 the Ms respectively,
 as a result. at 100 kHz,
 The decreasing trendwhich
 of the are ~82.4%magnetization
 saturated and ~90% of
 magnetization of raw pure iron is approximately 205 emu/g. All the hybrid-coated powders display
that of phosphate-coated
 in Figure powders content
 7 is due to the increasing (93.7). The
 of Alresults could be explained
 2O3 in insulating layers. by the following reasons.
 excellent properties of saturated magnetization of approximately 160 emu/g to 190 emu/g. It is widely
On the one hand, the permeability has a positive correlation with the Ms and the magnetism would be
 accepted that the saturated magnetization mainly depends on the composition and the purity of the
diluted by the non-magnetic Al2 O3, as shown in Figure 6. On the other hand, a higher porosity with
 materials [1], and that introducing non-magnetic phosphate and α-Al2O3 would dilute the percentage
an increasing Al2 O3 content could be concluded from Figure 5. The decreasing magnetic permeability
 of magnetic phases and reduce the Ms as a result. The decreasing trend of the saturated magnetization
is usually attributed to the inter-particle gap effect, as a source of the demagnetizing field [1]. As a
 in Figure 7 is due to the increasing content of Al2O3 in insulating layers.
result, the density of the SMCs reduces and results in the decreasing of the permeability.

 Figure 8. The amplitude permeability of the hybrid-coated samples by (a) ball milling and (b) sol–gel
 with the same additional amount of Al2O3.

 Figure 8 reveals the amplitude permeability as a function of frequency of the samples prepared
 Figure 8. The amplitude permeability of the hybrid-coated samples by (a) ball milling and (b) sol–gel
by the ball-milling
 with the 8.
 same
 method (Figureof8a) and the sol–gel method (Figure 8b) with the same additional
 Figure Theadditional
 amplitudeamount Al2of
 permeability O3the
 . hybrid-coated samples by (a) ball milling and (b) sol–gel
amount of Al2O3. The amplitude permeability of the hybrid-coated SMCs prepared by both methods
 with the same additional amount of Al2O3.
exhibit excellent frequency stability at the measured frequency, which could be attributed to the
better integrity of the insulation layers shown in Figure 5. Increasing the content of Al2O3 causes a
 Figure 8 reveals the amplitude permeability as a function of frequency of the samples prepared
tendency of permeability decrement, and the permeability of the samples with 8% Al2O3 reach 77.24
 by the ball-milling method (Figure 8a) and the sol–gel method (Figure 8b) with the same additional
(ball milling method) and 85.14 (sol-gel method), respectively, at 100 kHz, which are ~82.4% and
 amount of Al2O3. The amplitude permeability of the hybrid-coated SMCs prepared by both methods
 exhibit excellent frequency stability at the measured frequency, which could be attributed to the

Metals 2020, 10, 257 8 of 10

 The difference of the permeability caused by the fabrication technique could mainly be attributed to
the grain size. As it is accepted that the magnetization is mainly down to the movement and the rotation
of the domain walls, the required energy of the domain wall movement is less than that of the domain
wall rotation. As the grain size decreases, the number of domain walls decreases and the contribution
ofMetals
 the domain
 2019, 9, xwall rotation
 FOR PEER to the magnetization is greater than that of the domain wall movement [22].
 REVIEW 8 of 11
The effect of the ball-milling process is to decrease the grain size; therefore, the permeability of the
SMCs
 ~90%fabricated
 of that of by this method is not
 phosphate-coated as high(93.7).
 powders as thatThe
 prepared
 resultsbycould
 the sol–gel method.by the following
 be explained
 reasons.
 Figure On9 the one hand,
 depicts the permeability
 the influence of the Al2has O3 acontent
 positiveoncorrelation
 the total corewithloss
 the of
 MstheandSMCs
 the magnetism
 samples
 would be
prepared by diluted by the
 the different non-magnetic
 methods. It can beAl 2O3, that
 seen as shown
 the effectin of
 Figure 6. On the methods
 the preparation other hand, a higher
 on the total
 porosity
core loss iswith an increasing
 slight. Compared Al 2O3 the
 with content could be concluded
 phosphate-coated samples,from theFigure 5. The
 total core decreasing
 loss magnetic
 of hybrid-coated
 permeability
samples decreasesis usually
 by ~49% attributed
 (202.3 atto100 thekHzinter-particle
 by the sol–gelgapmethod)
 effect, asand a source
 ~47% of the at
 (210.8 demagnetizing
 100 kHz by
 field
the [1]. As a result,
 ball-milling the density
 method) with 8%ofAl theO
 2 3SMCs
 . To reduces
 further and
 explore results
 the in the
 effects decreasing
 of the Al Oof
 2 3 the permeability.
 content and the
 The difference of the permeability caused by the fabrication technique could mainly be
fabrication methods on the total core loss of SMCs samples, the total core loss curves were investigated
byattributed to the grainmethod.
 the loss separation size. AsTheit is total
 acceptedcore that
 loss the
 (Pt )magnetization
 at a weak fieldisand mainly down tolow
 a relatively thefrequency
 movement
 and the
could rotation ofand
 be separated thewritten
 domainaswalls,
 [4,23]:the required energy of the domain wall movement is less than
 that of the domain wall rotation. As the grain size decreases, the number of domain walls decreases
 2
 and the contribution of the domain = Phrotation
 Ptolwall + Pe ≈ to f +K
 Kh the ef
 magnetization is greater than that of(1) the
 domain wall movement [22]. The effect of the ball-milling process is to decrease the grain size;
where f is the
 therefore, thefrequency, the constant
 permeability of the SMCsKh and Ke are the
 fabricated byhysteresis
 this method lossisfactor
 not asand theaseddy
 high that current
 preparedlossby
factor, respectively.
 the sol–gel method. K h and K e are obtained based on Equation (1) from the curves of the total core loss
and the loss-separation fitting.

 Figure 9. The total core loss of the hybrid-coated samples prepared by (a) ball milling and (b) sol–gel
 with the same
 Figure 9. Theaddition
 total coreamount
 loss of of
 theAlhybrid-coated
 2 O3 . samples prepared by (a) ball milling and (b) sol–gel
 with the same addition amount of Al2O3.
 Table 1 displays the Kh , Ke and the standard error of the samples with different additional amounts
of the Al2 O3 fabricated by different methods. The Kh shows a slight increasing trend with the elevated
 Figure 9 depicts the influence of the Al2O3 content on the total core loss of the SMCs samples
Al2 O3 amount, which is attributed to the higher porosity, as shown in Fig. 5. However, the Ke exhibits
 prepared by the different methods. It can be seen that the effect of the preparation methods on the
an opposite, decreasing trend. The Ke of the sample with 8% Al2 O3 is only one-quarter of that of the
 total core loss is slight. Compared with the phosphate-coated samples, the total core loss of hybrid-
phosphate-coated sample. The Kh and Ke of samples prepared by the ball-milling method is higher
 coated samples decreases by ~49% (202.3 at 100 kHz by the sol–gel method) and ~47% (210.8 at 100
and lower, respectively, than that of samples by the sol–gel method at the same Al2 O3 content.
 kHz by the ball-milling method) with 8% Al2O3. To further explore the effects of the Al2O3 content
 and the fabrication methods on the total core loss of SMCs samples, the total core loss curves were
 Table 1. Kh , Ke and their standard error of samples with various content of Al2 O3 by different methods.
 investigated by the loss separation method. The total core loss (Pt) at a weak field and a relatively low
 frequency
 Content of could Khbe separated
 StandardandKwritten
 h (Ball as [4,23]:
 Standard Ke Standard Ke (Ball Standard
 Al2 O3 (%) (Sol-Gel) Error Milling) Error (Sol-Gel) Error (10−4 ) Milling) Error (10−4 )
 0 1.4924 0.05382 
 1.4924 0.05382
 
 0.02547
 4.979 0.02547 4.979 (1)
 2 1.2677 0.03233 1.3307 0.04673 0.02764 2.990 0.01841 9.737
 where4 f is the 1.3128
 frequency, the constant
 0.02956 1.3841
 and are 0.01706
 0.03153
 the hysteresis loss
 2.735
 factor and the eddy
 0.01453 2.917
 current
 loss factor,
 6 respectively.
 1.1807 and 1.4550
 0.02567 are obtained
 0.02317 based on equation
 0.01248 2.375(1) from0.00902
 the curves 2/143
 of the total
 8
 core loss 1.3268
 and the 0.02202
 loss-separation 1.5068
 fitting. 0.02202 0.00731 2.037 0.00631 2.037

 Table 1. Kh, Ke and their standard error of samples with various content of Al2O3 by different
 methods.

 Cont Standard Standard Standard Standard
 ent 
 error error error (10-4) error (10-4)
 of
 (sol- (ball

Metals 2020, 10, 257 9 of 10

 Density, particle size and residual stress play important roles in hysteresis loss [24]. The influence
of the ball-milling process on particles could be divided into two parts: particle size and particle shape.
Figures 2a and 4a suggest that the morphology of the particles transforms from irregular shape to flake
shape after the ball-milling process, which deteriorates the compressibility of the powders and results
in the lower density and higher porosity. In addition, the smaller particles in the compacts which were
caused by the impact effect of balls on particles shows more unfavorable surfaces and defects. Owing
to the pinning effect of these defects, the hysteresis loss of the samples prepared by the ball-milling
method is higher than that of the samples prepared by the sol–gel method.
 As for the eddy current loss factor (Ke ), it is dominated by the electric resistivity, which could by
elevated by increasing the intrinsic resistivity, defects, residual stress and porosity or by decreasing
the particle size [24]. Coating an insulating Al2 O3 layer on the phosphate iron powders directly
increases the intrinsic resistivity, and the electric resistivity is positively correlated to the Al2 O3 content.
However, the increased number of the pores and grain boundaries and the reduced particle size which
resulted from the ball-milling process account for the different Ke of various SMC samples. Therefore,
compared to the sample prepared by the sol–gel method, the contribution of the eddy current loss to
the total core loss for the sample prepared by the ball milling is smaller.

4. Conclusions
 Novel hybrid phosphate–alumina iron-based soft magnetic composites (SMCs) were successfully
fabricated by both the sol–gel method and the ball-milling method. The hybrid insulating layers
effectively enhanced the frequency stability of the permeability and reduced the total core loss in the
SMCs. The SMCs prepared by both methods retained above 85% of the permeability and showed only
approximately 50% of the total core loss of the phosphated samples prepared under the same process
conditions. The loss separation results show that the contribution of the Ph and the Pe to the total
core loss is inverse. The hysteresis loss in the samples prepared by the ball-milling method is greater
than that prepared by the sol–gel method. These results suggest that the hybrid phosphate–alumina
coating is promising, and can be used to obtain high-performance SMCs for electromagnetic devices at
high frequency.

Author Contributions: Data curation, Y.P., L.Q. and X.W.; Investigation, Y.P.; Methodology, Y.P.; Resources, J.P.
and W.L.; Supervision, W.L.; Writing – original draft, Y.P.; Writing – review & editing, W.L. All authors have read
and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China (Grant No 51971162,
U1933112, 51671146), the Program of Shanghai Technology Research Leader (18XD1423800, 19XD1432100),
Shanghai Automobile Industry Science and Technology Development Fund (No.1831) and the Fundamental
Research Funds for the Central Universities.
Conflicts of Interest: The authors declare no conflict of interest.

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