Enhanced Magnetic Properties of Fesial Soft Magnetic Composites Prepared by Utilizing PSA As Resin Insulating Layer

polymers

Article Enhanced Magnetic Properties of FeSiAl Soft Magnetic Composites Prepared by Utilizing PSA as Resin Insulating Layer

Hao Lu 1,2, Yaqiang Dong 2,3,* , Xincai Liu 1,*, Zhonghao Liu 1,2, Yue Wu 2, Haijie Zhang 2, Aina He 2,3, Jiawei Li 2,3 and Xinmin Wang 2

1 Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China; [email protected] (H.L.); [email protected] (Z.L.) 2 Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, CAS Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China; [email protected] (Y.W.); [email protected] (H.Z.); [email protected] (A.H.); [email protected] (J.L.); [email protected] (X.W.) 3 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (Y.D.); [email protected] (X.L.)

Abstract: Thermosetting organic resins are widely applied as insulating coatings for soft magnetic powder cores (SMPCs) because of their high electrical resistivity. However, their poor thermal stability and thermal decomposition lead to a decrease in electrical resistivity, thus limiting the annealing temperature of SMPCs. The large amount of internal stress generated by soft magnetic composites during pressing must be mitigated at high temperatures; therefore, it is especially important to ﬁnd organic resins with excellent thermal stabilities. In this study, we prepared SMPCs using poly-silicon- Citation: Lu, H.; Dong, Y.; Liu, X.; Liu, Z.; Wu, Y.; Zhang, H.; He, A.; Li, containing arylacetylene resin, an organic resin resistant to high temperatures, as an insulating layer. ◦ J.; Wang, X. Enhanced Magnetic With 2 wt % PSA as an insulating layer and annealed at 700 C for 1 h, the FeSiAl SMPCs achieved 3 Properties of FeSiAl Soft Magnetic the best magnetic properties, including the lowest core loss of 184 mW/cm (measured at 0.1 T and Composites Prepared by Utilizing 50 kHz) and highest permeability of 96. PSA as Resin Insulating Layer. Polymers 2021, 13, 1350. https:// Keywords: soft magnetic composites; poly-silicon-containing arylacetylene resin; stability; core loss doi.org/10.3390/polym13091350

Academic Editor: Tetsu Mitsumata 1. Introduction Received: 28 March 2021 Metal soft magnetic powder cores (SMPCs) are prepared by traditional powder met- Accepted: 15 April 2021 allurgy by mixing a metal magnetic powder and an insulating medium. Because of the Published: 21 April 2021 addition of an insulating medium, SMPCs exhibit high resistivity and low loss [1–5]. The power density and switching frequency of SMPCs are between those of silicon steel and Publisher’s Note: MDPI stays neutral ferrite, which just fills the gap during this period. Therefore, they are widely used in with regard to jurisdictional claims in published maps and institutional affil- main transformers, high-frequency chokes, automobiles, large-capacity switching power iations. supplies, and electromagnetic chargers. The increasing transmission efficiency in the 5G era requires magnetic materials that can operate at higher frequencies. However, the eddy- current loss (Pcv) of soft magnetic materials at high frequencies cannot be avoided. Pcv generates a large amount of heat and causes the magnetic performance to decrease, which greatly restricts applications in high-end products. The key to reducing Pcv is to prepare Copyright: © 2021 by the authors. an insulating layer with a high resistivity and strong binding force to the magnetic pow- Licensee MDPI, Basel, Switzerland. der [6,7]. Many attempts have been made to select insulating coating materials, which are This article is an open access article distributed under the terms and mainly categorized as organic or inorganic. Polyimide, phenol, epoxy, parylene, silicone, conditions of the Creative Commons and acrylic resins have been used as organic coating materials for SMPCs and amorphous Attribution (CC BY) license (https:// ribbons [8–19]. The advantage of organic coatings is their good coating integrity, but their ◦ creativecommons.org/licenses/by/ heat-resistant temperatures are limited, generally less than 500 C. Amorphous ribbons 4.0/). require polymers with multiple functional properties such as high insulation, corrosion

Polymers 2021, 13, 1350. https://doi.org/10.3390/polym13091350 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1350 2 of 10

resistance, increased sensitivity, and high-temperature resistance. Thus, existing polymers require further research and development. Inorganic coating materials include phosphate, oxides, sodium silicate, and ferrite [20–29]. To fully release the stress generated during the pressing process, SMPCs generally require heat treatment at temperatures above 600 ◦C. High-temperature heat treatment destroys the structure of the organic coating layer and increases Pcv between the magnetic powder particles, which is particularly severe at high frequencies. To improve the soft magnetic properties, we must develop new insulating coating materials with high resistivity, thermal stability, and bonding strength. Poly-silicon-containing arylacetylene resin (PSA), a kind of organic–inorganic hybrid polymer, has the advantages of high electrical insulation, super high thermal stability, and excellent bonding strength [30,31], making it an ideal magnetic powder core insulating material. However, there are no related reports on the preparation of SMPCs using PSA as an insulating layer. In this study, to further improve the high thermal stability and magnetic properties of FeSiAl SMPCs, PSA was used as an insulating layer, and the effects of the PSA content and heat treatment on the magnetic properties of the SMPCs were systematically examined.

2. Materials and Experimental Method Gas-atomized FeSiAl (85 wt % Fe–9.6 wt % Si–5.4 wt % Al) powder with an average particle size of 45 µm was purchased from Changsha Hualiu Powder Metallurgy Com- pany. PSA was provided by the Special Functional Polymer Laboratory of the East China University of Science and Technology. Epoxy (EP) and silicone resin (SI) were supplied by Shandong Haoshun Chemical. A silane coupling agent (KH550), which was used as a surface pretreatment for the magnetic particles to enhance their binding strength with the organic resin, was supplied by Nanjing Shuguang Chemical Group. The FeSiAl powders were surface-treated in KH550 at 1% of the magnetic particle mass. Subsequently, PSA was dissolved in acetone at mass ratios of 1, 2, 3, and 4 wt % relative to the FeSiAl powder and mechanically stirred for 0.5 h. The mixture was then stirred under ultrasonic conditions for 2 h, and the solution temperature was maintained below 60 ◦C. The hybrid magnetic particles were then dried in a vacuum oven at 120 ◦C for 24 h. For comparison, we prepared magnetic powders coated with SI and EP resin with a mass ratio of 2 wt % relative to the FeSiAl powder. The magnetic particles were pressed and shaped at 1800 MPa to obtain annular samples (20.3 mm outer diameter, 12.7 mm inner diameter, and 5.0 mm height). To study the effect of the annealing temperature, the ring-shaped samples were annealed at 400, 500, 600, and 700 ◦C for 1 h. In addition, the performances of the three resin-coated powder cores were compared at 600 ◦C. The surface morphology of the powdered core samples after annealing was observed using scanning electron microscopy (SEM, EVO18). The relative molecular mass and distribution of the PSA were measured using a gel permeation chromatograph (HLC 8320, Tosoh, Japan) with high-performance liquid-chromatography-grade tetrahydrofuran as the solvent. The thermal resistance of the PSA as an insulating layer was studied using thermogravimetry (TG). The characteristic functional groups of the PSA and its existence on the surface of the magnetic powders were analyzed using an intelligent Fourier infrared spectrometer (FTIR) in the range of 500 to 3500 cm−1. The corrosion resistances of the samples were compared using the salt-spray test, and the corrosion resistance of the heat- treated samples was tested using a salt-spray test box. The NaCl concentration in the salt-spray box was set to 0.35 wt %, and the temperature was room temperature. The corrosion potentials of the samples were tested using an electrochemical workstation. The effective permeability of the SMPCs was tested using an impedance analyzer (Agilent 4294A, America), and the power loss of the SMPCs was measured using an AC B-H loop tracer (MATS–2010SA, America) with a measurement range of 10–100 kHz. The hysteresis loops were detected using a vibrating sample magnetometer (Lakeshore 7410, America). PolymersPolymers 20212021,, 1313,, x 1350 FOR PEER REVIEW 3 3of of 11 10

3.3. Results Results 3.1.3.1. Analysis Analysis of of the the PSA PSA TheThe sample preparation processprocess is is illustrated illustrated in in Figure Figure1, where1, where the the coating coating mechanism mecha- nismis marked is marked as steps as 1steps and 1 2. and The 2. silanol The silanol produced produced by the hydrolysisby the hydrolysis of KH550 of connectedKH550 con- to nectedthe surface to the of surface the magnetic of the magnetic powder powder through through the hydroxyl the hydroxyl group [group32,33], [32,33], and the and end the of endKH550 of KH550 connected connected to the to PSA the surface PSA surface via intermolecular via intermolecular forces, forces, thereby thereby strengthening strengthen- the ingbinding the binding force between force between the coating the coating layer and layer magnetic and magnetic powder powder [34]. [34].

FigureFigure 1. 1. SchematicSchematic diagram diagram of of sample sample preparation preparation and and coating coating mechanism. mechanism.

FigureFigure 22 showsshows thethe TG TG analyses analyses of of the the PSA PSA and and the the basic basic structure structure of theof the PSA PSA measured meas- uredat a heatingat a heating rate ofrate 10 of◦C/min 10 °C/min in a in nitrogen a nitrogen atmosphere. atmosphere. Two Two weight-loss weight-loss stages stages ofthe of thePSA PSA were were detected detected during during the the entire entire process. process. The The volatilization volatilization of of solvent solvent impurities impurities at ◦ at100–300 100–300C °Cresulted resulted in in a a mass mass loss loss of of 0.4%, 0.4%, and and a a massmass lossloss ofof 4.4%4.4% in the second stage ◦ atat 500–900 500–900 °CC was was mainly mainly caused caused by by the the decomposition decomposition of the of thePSA. PSA. Considering Considering that thatthe heat-treatmentthe heat-treatment temperature temperature of magnetic of magnetic powder powder cores cores is generally is generally between between 600 and 600 and700 ◦ °C,700 thisC, thismass mass loss losswas was considered considered completely completely acceptable. acceptable. The The results results of of the the TG TG analysis analysis showedshowed that that the the PSA PSA had had excellent excellent thermal thermal stability. stability. The The alkynyl alkynyl group group of of the the PSA PSA main main chainchain underwent underwent a a crosslinking crosslinking reaction reaction under under heating heating to to form form an an aromatic aromatic ring ring structure. structure. RuptureRupture of the molecular chainchain of of the the crosslinked crosslinked polymer polymer required required the the breaking breaking of multipleof mul- tiplechemical chemical bonds, bonds, which which effectively effectively inhibited inhibi theted pyrolysis the pyrolysis of PSA, of PSA, and the and introduction the introduc- of tionsilicon of silicon increased increased the thermal-decomposition the thermal-decomposition temperature temperature of the resin.of the resin.

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Figure 2. PSA structure and thermogravimetric curve.

FigureFigure 2. PSA 3 shows structure the and infrared ther thermogravimetricmogravimetric spectrum of curve. the PSA in the range of 500 to 3500 cm-1. Among the peaks, 3295 cm-1 is the stretching vibration of alkyne hydrogen (C≡CH), 2154 −1 cm-1 is Figurethe stretching3 3shows shows thevibration the infrared infrared of spectrum -C spectrum≡C-, 2969 of the cmof PSA the-1 is in PSAthe the asymmetric rangein the of range 500 stretching to of 3500 500 cm to vibration 3500. Among cm -1. − − 1 -1 ≡ 1 of Amongthe-CH peaks,3 in theSi-CH 3295 peaks,3, cm and 3295 1591,is thecm 1570, stretching is the 1470, stretching vibration and 1402 vibration of cm alkyne-1 are of the hydrogen alkyne C=C hydrogenskeletal (C CH), stretching (C2154≡CH), cm vi-2154is −1 brationscmthe-1 stretching is onthe the stretching benzene vibration vibration ring. of -C The≡C-, of peak 2969-C≡ atC-, cm 1245 2969is cm cm the-1 -1corresponds asymmetric is the asymmetric stretching to flexural stretching vibration vibration vibration of out- -CH3 −1 in Si-CH , and 1591, 1570, 1470, and 1402 cm are the C=C-1 skeletal stretching vibrations on sideof -CHthe -CH3 in3 Si-CH3 plane3, inand Si-CH 1591,3 [30].1570, The 1470, relative and 1402 molecular cm are mass the C=Cand distributionskeletal stretching of PSA vi- −1 the benzene ring. The peak at 1245 cm corresponds-1 to flexural vibration outside the -CH werebrations measured on the by benzene gel permeation ring. The chromatography. peak at 1245 cm Thecorresponds measured to M flexuraln of PSA vibration was 1804, out- 3 plane in Si-CH [30]. The relative molecular mass and distribution of PSA were measured andside M wthe was -CH 2509.3 plane3 Combined in Si-CH with3 [30]. infrared The relative analysis, molecular these resultsmass and proved distribution that the ofsub- PSA by gel permeation chromatography. The measured M of PSA was 1804, and M was 2509. stancewere was measured PSA. by gel permeation chromatography.n The measured Mn of PSAw was 1804, andCombined Mw was with 2509. infrared Combined analysis, with these infrared results provedanalysis, that these the substanceresults proved was PSA. that the sub- stance was PSA.

Figure 3. FTIR spectra for PSA. Figure 3. FTIR spectra for PSA. Figure3.2. Magnetic 3. FTIR Propertiesspectra for PSA. Figure4 shows the magnetization curves of the FeSiAl powders coated with different amounts of PSA. The saturation magnetization (Ms) of the coated magnetic powder was

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Polymers 2021, 13, 1350 5 of 10 3.2.3.2. MagneticMagnetic PropertiesProperties FigureFigure 44 showsshows thethe magnetizationmagnetization curvescurves ofof thethe FeSiAlFeSiAl powderspowders coatedcoated withwith differentdifferent amountsamounts ofof PSA.PSA. TheThe satusaturationration magnetizationmagnetization ((MMss)) ofof thethe coatedcoated magneticmagnetic powderpowder waswas slightlyslightly reduced, reduced, but but the the effect effect was was within within a a controllable controllable range range and and had had little little influence inﬂuenceinfluence on on thethe properties. properties. This This shows shows that that PSA PSA as asas an anan in insulatinginsulatingsulating coating coating material material did did not not deteriorate deteriorate MMss ofof thethe magneticmagnetic powderpowder cores.cores.

FigureFigure 4. 4. MagnetizationMagnetization curves curves of of magnetic magnetic powder powder coat coat with with different different contentcontent ofof PSA.PSA.

FigureFigure 5 55 shows showsshows thethethe FITRFITRFITR spectraspectraspectra ofofof thethethe magneticmamagneticgnetic powderspowderspowders coatedcoatedcoated withwithwith differentdifferentdifferent PSAPSAPSA −1 contentscontents and and the the originaloriginal powder.powder. EachEachEach curvecurvecurve hadhadhad strong strongstrong absorption absorptionabsorption peaks peakspeaks at atat 3456 34563456 cm cmcm-1-1 −1 andand 1640 1640 cm cm-1-1 correspondingcorrespondingcorresponding toto to -OH-OH -OH groups,groups, groups, mainlymainly mainly duedue due toto to waterwater water absorbedabsorbed on on the the surface surface −1 ofof the the magnetic magnetic powderspowders andand thethe hydrolysishydrolysishydrolysis ofofof KH550. KH550.KH550. The TheThe peaks peakspeaks at atat2853 28532853 and andand 2922 29222922 cm cmcm-1-1 −-11 werewere derived derived fromfrom thethethe stretchingstrestretchingtching vibration vibrationvibration of ofof -CH -CH-CH222 bonds, bonds,bonds, and andandthe thethepeak peakpeak at atat 1384 13841384 cm cmcm-1 waswas attributedattributed to to the the deformation deformation vibration vibration of of C-H. C-H. Absorption Absorption peaks peaks belonging belonging to to PSA PSA were were −1 detecteddetected atat 3310,3310, 1570,1570, andand and 12651265 1265 cmcm cm-1-1,, whilewhile, while thesethese these peakspeaks peaks werewere were absentabsent absent fromfrom from thethe the spectrumspectrum spectrum ofof theofthe the originaloriginal original powder.powder. powder. ItIt cancan It canbebe concludedconcluded be concluded thatthat thatPSAPSA PSA waswas coated wascoated coated ontoonto ontothethe surfacesurface the surface ofof thethe of mag-mag- the neticmagneticnetic powders.powders. powders.

FigureFigure 5. 5. FTIRFTIR spectra spectra for for powder powder coated coated with with varied varied content content of of PSA. PSA.

Figure6a,b show the effective permeability ( µe) of the samples coated with 1–4 wt % PSA contents and 2 wt % EP and SI after heat treatment at 600 ◦C. At a resin content of 2 wt %, the PSA-coated sample had a medium µe of 87 at 1 MHz. In contrast, the Polymers 2021, 13, x FOR PEER REVIEW 6 of 11

Figure 6a,b show the effective permeability (μe) of the samples coated with 1–4 wt % PSA contents and 2 wt % EP and SI after heat treatment at 600 °C. At a resin content of 2 wt %, the PSA-coated sample had a medium μe of 87 at 1 MHz. In contrast, the SI-coated Polymers 2021, 13, 1350 6 of 10 sample had the lowest μe of 53 at 1 MHz. The EP-coated samples showed the highest magnetic permeability, largely because EP has poor thermal stability, and significant decomposition causes the volume fraction of the nonmagnetic phase to decrease, resulting in a higher magnetic permeability. This indicates that the PSA had a weak magnetic dilution SI-coated sampleeffect had and the was lowest suitableµ fore of use 53 as at an 1 insulating MHz. The coating EP-coated material. With samples increasing showed PSA con- the highest magnetictent, permeability, μe of the samples largely at 1 MHz because decreased EP has from poor 89 to thermal 74. However, stability, the frequency and signiﬁcant stability decomposition causesimproved the at volume the same fractiontime. There of are the two nonmagnetic main reasons for phase this. The to decrease, magnetic permeabil- resulting ity of magnetic powder cores can be calculated by the following equation [35]: in a higher magnetic permeability. This indicates that the PSA had a weak magnetic 4−1 dilution effect and was suitable for use as an=1+ insulating coating1− material., With increasing(1) 4 + −1 PSA content, µe of the samples at 1 MHz decreased from 89 to 74. However, the frequency stability improvedwhere at μ the is the same intrinsic time. permeability There are of twothe material, main reasonsN is the demagnetization for this. The factor, magnetic and p is the percentage of nonmagnetic materials in the composite. Based on Equation (1), the permeability of magneticmagnetic permeability powder cores of the can magnetic be calculated powder cores by thecontinuously following decreased equation with [35 in-]: creasing nonmagnetic materials. On the other hand, with increasing PSA content, the particle surface thickness became4π uneven.(µ − 1 )As a result, the combination of powders was µe = 1 + (1 − p), (1) looser, the gaps between particles4π + Nincreased,(µ − 1) and the density decreased, thereby reducing the effective magnetic permeability of the powder cores. Figure 7a shows the appearance where µ is the intrinsicof the samples permeability after heat of treatment the material, at 600 °C.N Theis the EP-coated demagnetization sample had serious factor, cracks, and p is the percentagethe of SI-coated nonmagnetic sample had materials slight cracks, in the and composite. the PSA-coated Based sample on hadEquation a smooth surface. (1), the This is because EP and SI have poor thermal stabilities; they decompose rapidly at high magnetic permeabilitytemperatures, of the and magnetic the generated powder gas escapes cores an continuouslyd destroys the decreasedoriginal topographic with increas- struc- ing nonmagneticture. materials. On the other hand, with increasing PSA content, the particle surface thickness becameTo intuitively uneven. compare As a the result, corrosion the combinationresistances of the of samples, powders a 24 was h salt-spray looser, test the gaps between particleswas performed, increased, and andthe results the density are shown decreased, in Figure 7b. thereby The EP-coated reducing samples the effectivewere severely corroded, with a thick antirust layer that easily fell off; the SI-coated samples were magnetic permeability of the powder cores. Figure7a shows the appearance of the samples moderately corroded;◦ and the PSA-coated samples were only slightly corroded. This is after heat treatmentdirectly at related 600 C.to the The thermal EP-coated stability sampleof the resins. had The serious insulating cracks, layer of the the SI-coatedEP-coated sample had slightsample cracks, was and incomplete the PSA-coated and thin after sample heat treatment, had a smooth resulting surface. in poor oxygen This isconcentra- because EP and SI have poortions thermalin different stabilities; areas on the they surface decompose of the magnetic rapidly powder. at high Therefore, temperatures, Cl− easily ad- and the generated gassorbed escapes onto andthe surface destroys defects the and original ultimately topographic caused crevice structure. corrosion. The PSA decomposed only slightly during heat treatment, and thus it had excellent corrosion resistance.

Polymers 2021, 13, x FOR PEER REVIEW 7 of 11 Figure 6. TheFigure effective 6. The effective permeability permeability of different of different content content ((a)a) and different different resins resins (b). (b).

Figure 7. Surface morphology of the sample after annealing (a), and morphology after salt-spray corrosion (b). Figure 7. Surface morphology of the sample after annealing (a), and morphology after salt-spray

corrosion (b). Figure 8a,d show the core loss (Pcv) of the samples coated with 1–4 wt % PSA and 2 wt % EP and SI after heat treatment at 600 °C. The 2 wt % PSA sample had the lowest Pcv of 319 and 608 mW/cm3 at 60 and 100 kHz, respectively. To explore how the loss was reduced, we performed a loss-separation analysis, and the hysteresis loss (Ph) and eddy- current loss (Pe) after separation are shown in Figure 8b,c and Figure 8e,f, respectively. Under a magnetic field of 0.1 T, Ph of the samples was not significantly different, although that of the PSA-coated samples was slightly lower. Ph was dominant in the low-frequency range and mainly originated from the rotation of the magnetic domain during the magnetization process. The magnitude of Ph was proportional to the area of the DC hysteresis loop, which can be expressed by the following empirical formula:

=, (2)

where Kh is the hysteresis coefficient, Bm is the maximum induction field or test field strength, ƒ is the test frequency, and α is the simulation coefficient. Both Kh and α are related to the coercivity of the material itself, and the coercivity of the powder cores can be reduced by annealing heat treatment and a uniform insulating coating. Under the same heat-treatment temperature, Ph will not make much difference. Pe dominates at higher frequencies and is proportional to the square of ƒ. It is generally composed of interparticle eddy-current loss (Pinter) and intraparticle eddy-current loss (Pintra), and can be expressed as [25,36,37]: ℎ = + = + , (3) 20 where h is the thickness of the sample, d is the diameter of the magnetic powder particles, β is the geometric coefficient, and ρSMCs and ρpowder are the resistivities of the powder core and the magnetic powder, respectively. Among them, the particle-size ratio used in this study is beneficial for reducing Pinter. Compared with the samples coated with PSA, the samples coated with EP and SI resins had higher Pe values of 88 and 74 mW/cm3, respectively, because of their poor thermal stability. The resins decomposed at high temperatures and destroyed the coating layer. The above results indicate that a coating with an appropriate PSA content is beneficial in obtaining SMPCs with low-loss characteristics.

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To intuitively compare the corrosion resistances of the samples, a 24 h salt-spray test was performed, and the results are shown in Figure7b. The EP-coated samples were severely corroded, with a thick antirust layer that easily fell off; the SI-coated samples were moderately corroded; and the PSA-coated samples were only slightly corroded. This is directly related to the thermal stability of the resins. The insulating layer of the EP-coated sample was incomplete and thin after heat treatment, resulting in poor oxygen concen- trations in different areas on the surface of the magnetic powder. Therefore, Cl− easily adsorbed onto the surface defects and ultimately caused crevice corrosion. The PSA decomposed only slightly during heat treatment, and thus it had excellent corrosion resistance. Figure8a,d show the core loss ( Pcv) of the samples coated with 1–4 wt % PSA and 2 wt % EP and SI after heat treatment at 600 ◦C. The 2 wt % PSA sample had the lowest 3 Pcv of 319 and 608 mW/cm at 60 and 100 kHz, respectively. To explore how the loss was reduced, we performed a loss-separation analysis, and the hysteresis loss (Ph) and eddy-current loss (Pe) after separation are shown in Figure8b,c and Figure8e,f, respectively. Under a magnetic ﬁeld of 0.1 T, Ph of the samples was not signiﬁcantly different, although that of the PSA-coated samples was slightly lower. Ph was dominant in the low-frequency range and mainly originated from the rotation of the magnetic domain during the magnetization process. The magnitude of Ph was proportional to the area of the DC hysteresis loop, which can be expressed by the following empirical formula:

α Ph = KhBm f , (2)

where Kh is the hysteresis coefficient, Bm is the maximum induction field or test field strength, ƒ is the test frequency, and α is the simulation coefficient. Both Kh and α are related to the coercivity of the material itself, and the coercivity of the powder cores can be reduced by annealing heat treatment and a uniform insulating coating. Under the same heat-treatment temperature, Ph will not make much difference. Pe dominates at higher frequencies and is proportional to the square of ƒ. It is generally composed of interparticle eddy-current loss (Pinter) and intraparticle eddy-current loss (Pintra), and can be expressed as [25,36,37]: 2 ! h d 2 2 2 Pe = Pinter + Pintra = + π Bm f , (3) βρSMCs 20ρpowder where h is the thickness of the sample, d is the diameter of the magnetic powder particles, β is the geometric coefficient, and ρSMCs and ρpowder are the resistivities of the powder core and the magnetic powder, respectively. Among them, the particle-size ratio used in this study is beneficial for reducing Pinter. Compared with the samples coated with PSA, the samples 3 coated with EP and SI resins had higher Pe values of 88 and 74 mW/cm , respectively, because of their poor thermal stability. The resins decomposed at high temperatures and destroyed the coating layer. The above results indicate that a coating with an appropriate PSA content is beneficial in obtaining SMPCs with low-loss characteristics. In summary, at a heat-treatment temperature of 600 ◦C, the 2 wt % PSA-coated sample had the lowest Pcv. Therefore, we used 2 wt % PSA-coated samples to explore the effect of the heat-treatment temperature on the magnetic properties. The effective permeability, core loss, and loss separation of the samples annealed at 400–700 ◦C are shown in Figure9. The magnetic permeability continued to rise for two reasons. First, high-temperature heat treatment eliminated the internal stress generated during the pressing process, removed the pinning effect of internal stress on the domain walls, reduced the rotation resistance of the magnetic domain, and reduced Ph. The higher the temperature was, the more internal stress was released, and the better the magnetic properties of the powder core. However, there was also a limit to how much the magnetic properties could be improved by high temperatures. If the temperature was too high, overburning occurred, destroying the insulating layer and increasing Pe. PSA has a higher temperature limit because of its excellent heat resistance. The other reason for the increasing magnetic permeability was that the slight PSA decomposition reduced the volume fraction of the nonmagnetic phase. Polymers 2021, 13, 1350 8 of 10

The core loss of the sample at 10–100 kHz continued to decrease, mainly due to the decrease in Ph. Pe did not decrease with increasing temperature, which may have been caused by Polymers 2021, 13, x FOR PEER REVIEWthe increase in Pe between particles caused by the partial decomposition of the PSA.8 of At11 700 ◦C, the powder core had the best magnetic performance, with an effective permeability of 96 at 1 MHz and core loss of 184 mW/cm3 (measured at 50 kHz and 0.1 T).

PolymersFigure 2021 8., 13 The, x FOR total PEER core REVIEW loss (a ,d), hysteresis loss (b,e), and eddy-current loss (c,f) as function of the frequency for the9 of 11 Figure 8. The total core loss (a,d), hysteresis loss (b,e), and eddy-current loss (c,f) as function of the frequency for samples.the samples.

In summary, at a heat-treatment temperature of 600 °C, the 2 wt % PSA-coated sample had the lowest Pcv. Therefore, we used 2 wt % PSA-coated samples to explore the effect of the heat-treatment temperature on the magnetic properties. The effective permeability, core loss, and loss separation of the samples annealed at 400–700 °C are shown in Figure 9. The magnetic permeability continued to rise for two reasons. First, high-temperature heat treatment eliminated the internal stress generated during the pressing process, removed the pinning effect of internal stress on the domain walls, reduced the rotation resistance of the magnetic domain, and reduced Ph. The higher the temperature was, the more internal stress was released, and the better the magnetic properties of the powder core. However, there was also a limit to how much the magnetic properties could be improved by high temperatures. If the temperature was too high, overburning occurred, destroying the insulating layer and increasing Pe. PSA has a higher temperature limit because of its excellent heat resistance. The other reason for the increasing magnetic permeability was that the slight PSA decomposition reduced the volume fraction of the nonmagnetic phase. The core loss of the sample at 10–100 kHz continued to decrease, mainly due to the decrease in Ph. Pe did not decrease with increasing temperature, which may have been caused by the increase in Pe between particles caused by the partial decomposition of the PSA. At 700 °C, the powder core had the best magnetic performance, with an effective permeability of 96 at 1 MHz and core loss of 184 mW/cm3 (measured at 50 kHz and 0.1 T).

Figure 9. The effective permeability ( a), the total core loss ( (b),), hysteresis hysteresis loss loss ( (cc),), eddy-current eddy-current loss loss (d) as a function of the frequency for the samples. (d) as a function of the frequency for the samples.

4. Conclusions

The PSA showed excellent thermal stability and had little effect on Ms of the magnetic powders. Thus, it can be used as an insulating coating material for SMPCs. Owing to the high thermal stability of the PSA, it remained on the surface of the magnetic powder after heat treatment, which not only increased the allowable heat-treatment temperature and permeability of the powder core, but also reduced the core loss. The samples coated with 2 wt % PSA and treated at 700 °C achieved the best magnetic properties, with a maximum permeability of 96 and Pcv of 184 mW/cm3 (measured at 50 kHz and 0.1 T). In addition, the corrosion resistance of the powder core was significantly improved.

Author Contributions: Conceptualization, H.L. and X.L.; methodology, Z.L., Y.W., H.Z.; resources, Y.D.; data curation, H.L.; writing—original draft preparation, H.L.; writing—review and editing, H.L. and X.L.; supervision, J.L., A.H., and X.W.; All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Youth Innovation Promotion Association CAS (Grant No. 2021294), the Science and Technology Service Network Initiative of the Chinese Academy of Sciences (Grant No. KFJ-STS-QYZD-2021-07-002), the S&T Innovation 2025 Major Special Program (Grant Nos. 2018B10085 and 2019B10090), the National Natural Science Foundation of China (Grant No. U1809216), and the K. C. Wong Magna Fund at Ningbo University. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Sunday, K.J.; Taheri, M.L. Soft magnetic composites: Recent advancements in the technology. Met. Powder Rep. 2017, 72, 425– 429, doi:10.1016/j.mprp.2016.08.003. 2. Silveyra, J.M.; Ferrara, E.; Huber, D.L.; Monson, T.C. Soft magnetic materials for a sustainable and electrified world. Science 2018, 362, eaao0195, doi:10.1126/science.aao0195.

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4. Conclusions

The PSA showed excellent thermal stability and had little effect on Ms of the magnetic powders. Thus, it can be used as an insulating coating material for SMPCs. Owing to the high thermal stability of the PSA, it remained on the surface of the magnetic powder after heat treatment, which not only increased the allowable heat-treatment temperature and permeability of the powder core, but also reduced the core loss. The samples coated with 2 wt % PSA and treated at 700 ◦C achieved the best magnetic properties, with a maximum 3 permeability of 96 and Pcv of 184 mW/cm (measured at 50 kHz and 0.1 T). In addition, the corrosion resistance of the powder core was signiﬁcantly improved.

Author Contributions: Conceptualization, H.L. and X.L.; methodology, Z.L., Y.W., H.Z.; resources, Y.D.; data curation, H.L.; writing—original draft preparation, H.L.; writing—review and editing, H.L. and X.L.; supervision, J.L., A.H., and X.W.; All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Youth Innovation Promotion Association CAS (Grant No. 2021294), the Science and Technology Service Network Initiative of the Chinese Academy of Sciences (Grant No. KFJ-STS-QYZD-2021-07-002), the S&T Innovation 2025 Major Special Program (Grant Nos. 2018B10085 and 2019B10090), the National Natural Science Foundation of China (Grant No. U1809216), and the K. C. Wong Magna Fund at Ningbo University. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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