Research articles  研究文章
Industry-oriented Fe-based amorphous soft magnetic composites with SiO2-coated layer by one-pot high-efficient synthesis method
以單盤高效合成方法為基於行業的Fe基無定形軟磁複合材料，帶有SIO 2塗層層

https://doi.org/10.1016/j.jmmm.2020.166924Get rights and content  取得權利和內容
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Highlights  亮點

  • •
    One-pot highly efficient synthesis method for SiO2-coating with only 1.9 h was developed.
    僅開發了僅開發了僅1.9 h的SIO 2塗層的一柱高效合成方法。
  • •
    An industry-oriented automated coating device was designed.
    設計了面向行業的自動塗料設備。
  • •
    Eddy current loss contributes only 13.5 ~ 18.1% of total loss when TEOS concentration over 0.09 ml/g.
    當TEOS濃度超過0.09 mL/g時，渦流損失僅貢獻總損失的13.5〜18.1％。
  • •
    Fe-based amorphous SMCs insulated using TEOS concentration of 0.09 ~ 0.14 ml/g exhibit excellent magnetic properties.
    使用0.09〜0.14 mL/g的TEOS濃度絕緣的基於Fe的非晶SMC具有出色的磁性特性。

Abstract  抽象的

A new one-pot highly efficient synthesis method implemented using an industry-oriented roboticized coating device was designed and developed for obtaining high quality SiO2-coating on the surface of commrercial Fe78Si13B9 (FeSiB) amorphous flaky powder at the room temperature. Hereinto, the conventional instilling method was modified as directly pouring the ultra-diluted precursor by anhydrous ethanol into the reaction mixture, favorable for both simple operation and shortening the total manufacture time to 1.9 h (including the drying time of about 0.7 h). Comparison and analysis on the soft magnetic properties of the FeSiB amorphous powder cores insulated using different precursor concentration within the range of 0 ~ 0.25 ml/g were investigated in detail. SEM, EDS, and FTIR were employed to characterize the core-shell structure of magnetic powders. The increment of pinning sites induced by the raising precursor concentration can cause a reduction in effective permeability, while result in the increases of hysteresis loss and excess loss. The eddy current loss contributes only about 13.5% ~ 18.1% of total core loss for the coated powder cores under the TEOS concentration over 0.09 ml/g, indicating that the corresponding magnetic composites have a good electro-insulating layer. This work offered a referable idea for efficiently coating other inorganic insulations on the magnetic powders within different shapes.
設計和開發了一種新的使用面向行業的機器人化塗層裝置實施的新型單鍋高效合成方法，用於在室溫下在通用FE 78 SI 13 B 9 （FESIB）無定形片狀粉末的表面上獲得高質量的SIO 2塗層。在這裡，常規灌輸方法被修改為將無水乙醇直接倒入反應混合物中，有利於簡單操作和將總製造時間縮短至1.9 h（包括約0.7 h的干燥時間）。詳細研究了對使用不同的前體濃度絕緣的FESIB無定形粉末芯的軟磁性能的比較和分析。使用SEM，ED和FTIR來表徵磁粉的核心殼結構。由升高前體濃度引起的固定位點的增加會導致有效的滲透率降低，同時導致磁滯損失和多餘損失的增加。在TEOS濃度下，渦流損失僅貢獻約13.5％〜18.1％的TEOS濃度在0.09 mL/g的濃度下，表明相應的磁複合材料具有良好的電構造層。這項工作提供了一個引用的想法，可以有效地塗上不同形狀內磁粉的其他無機絕緣。

Keywords  關鍵字

One-pot synthesis method
Soft magnetic composites
Magnetic properties
Industry-oriented coating device
Fe-based amorphous flaky powders

一鍋合成方法
軟磁複合材料
磁性特性
面向行業的塗料設備
基於Fe的無定形片狀粉末

1. Introduction  1。簡介

To satisfy the intrinsic requirements of high-frequency application and miniaturization of power electronic devices, the intergranular insulated soft magnetic composites (SMCs) with low core loss and relatively high magnetic flux density have attracted growing attention from scientists and producers over the last few decades [1], [2], [3]. Accompanying with the unique advantages of good DC-bias property, three-dimensional physical property isotropy, favorable frequency stability of effective permeability, near-net shaping with high-dimensional accuracy beneficial for the reduction in mass production cost [3], [4], [5], SMCs have been extensively used in the electromagnetic components in our industry and daily life, such as transformer, inductor, active filter, switching regulator and so on [6], [7].
為了滿足高頻應用和電力電子設備的微型化的內在要求，晶間絕緣軟磁複合材料（SMC）具有低核心損失和相對較高的磁通量密度，在過去的幾十年中引起了科學家和生產商的日益增長的關注[1] ，[1]， [2] ，[3]， [3] 。伴隨著良好的DC偏差特性的獨特優勢，三維的物理性各向同性，有效滲透性的頻率穩定性，近網狀形狀具有高維準確性有益於質量生產成本的降低[3] ， [4] ， [5] [5] 6] ， [7] 。
It is reported that about 9% of generated electrical energy is lost as heat dissipation during electromagnetic transmission and distribution [3]. As we known, the total energy loss (Wt) of SMCs is composed of hysteresis loss (Wh), eddy current loss (We), and excess loss (Wexc). The Wh arises from the internal residual stress caused by the increased density of defects and dislocations generated in the cold compaction process for SMCs [8]. The subsequent annealing heat treatment at an enough high temperature is the effective mean to release the residual stress by reducing the imperfections in SMCs, thus leading to the decrease in Wh. This fact requires that the thin insulation layer coated on the surface of ferromagnetic powder must withstand the high annealing temperature (usually above 450 °C) to ensure the non-significant increase in We. The We can be generally decomposed into two types: the inter-particle and intra-particle eddy current losses, and the former’s magnitude is dominated by the uniformity and quality of surface-insulated coating on the magnetic particles [9]. As a consequence, adopting a suitable insulated coating material has become in the focus of interest in the preparation of SMCs.
據報導，隨著電磁傳輸和分佈期間的熱量耗散，約有9％的產生的電能丟失了[3] 。如我們所知，SMC的總能量損失（ W T ）由磁滯損失（ W H ），渦流損失（ W e ）和多餘損失（ W EXC ）組成。 W h是由SMC冷壓縮過程中產生的缺陷和位錯密度增加引起的內部殘留應力[8] 。隨後在足夠高溫下退火的熱處理是通過減少SMC的缺陷來釋放殘餘應力的有效均值，從而導致W H的降低。這一事實要求在鐵磁粉末上覆蓋的薄絕緣層必須承受高退火溫度（通常高於450°C），以確保W e的不顯著增加。通常可以分解為兩種類型：顆粒間和粒子內渦流損失，前者的幅度由磁性顆粒上表面絕緣塗層的均勻性和質量主導[9] 。結果，採用合適的隔熱塗層材料已成為SMC準備的重點。
Considering that the organic coatings, for instance silicon resin and phenolic resin, exhibit a low thermal resistance (almost less than 200 °C), the inorganic coatings, such as Al2O3 [8], [10], SiO2 [7], [11], TiO2 [12], which can offer an advantage of tolerating high annealing temperature for a long time, have been proposed as the electro-insulation for SMCs. Hereinto, chemical sol-gel method was normally employed to obtain a uniform inorganic coating on the surface of ferromagnetic particle. However, the reported inorganic coating procedures up to now faced two salient problems: low preparation efficiency (total preparation time exceeds 5 h [10,11,13,14]) and relatively high synthesis temperature (more than 50 °C [10,13,14]), which are not unfavorable to popularize in industrial application. Recently, to avoid the reduction in effective permeability caused by the two kinds of non-magnetic coatings above, the soft magnetic coatings of NiZn- [15] and MnZn- [16], [17] ferrites prepared via chemical co-precipitation method were putted forward for SMCs owing to their high permeability and resistivity. Nevertheless, the ferrite, as a typical oxide ceramic, exhibits the properties of high brittleness and low adhesion, easily leading to the magnetic aggregation and the decrement in green compact density of SMCs [18]. Moreover, the preparation of ferrite coating also faces the problems of low efficiency and high synthesis temperature [17], [18] like the inorganic coatings above. Furthermore, it is not suitable to coat an in-situ fine ferrite layer on the surface of Fe-based amorphous magnetic powders [19] owing to that the non-magnetic FeB phase will precipitate at a high calcination temperature (usually greater than 500 °C [20]) for obtaining favorable magnetic properties of the ferrite coatings.
考慮到有機塗層（例如矽樹脂和酚醛樹脂）表現出較低的熱阻力（幾乎小於200°C），因此無機塗料（例如Al 2 O 3 [8] ，[10]， [10] ，Sio 2 [7]，[11] ， [11] ，Tio 2 [12] ，可以為長時間的高溫提供高度的溫度，以至於為高高的溫度提供了高度的啟動，並提出了一個頻繁的溫度。在這裡，通常採用化學溶膠方法在鐵磁顆粒表面獲得均勻的無機塗層。但是，據報導的無機塗料程序現在面臨兩個顯著問題：較低的製備效率（總製備時間超過5 h [10,11,13,14]）和相對較高的合成溫度（超過50°C [10,13,14]），這些溫度在工業應用中不受歡迎。最近，為了避免上面兩種非磁性塗層引起的有效滲透性的降低，將Nizn- [15]和Mnzn- [16]的軟磁性塗層提前提出，因為它們是通過化學共沉澱方法製備的，由於SMC的高滲透率和電阻率。 然而，作為典型的氧化物陶瓷的鐵素體表現出高耐磨性和低粘附性的特性，很容易導致磁聚集和SMC的綠色緊湊密度的降低[18] 。此外，鐵氧體塗層的製備也面臨著低效率和高合成溫度的問題[17] ， [18]，例如上面的無機塗層。此外，在基於Fe的非晶磁粉表面上塗上原位的細鐵礦層[19] ，因為非磁性Feb相會在高鈣化溫度（通常大於500°C [20] ）下沉澱，以獲得獲得甲氧化鐵礦塗層的有利磁性特性。
In the previous works, we successfully fabricated a high-quality SiO2 insulating layer on the surface of Fe-based magnetic flaky powder via sol-gel method with a total preparation time of about 2.6 h at the room temperature [7], [21]. In this investigation, a new one-pot highly efficient synthesis method for obtaining uniform SiO2-coating on the surface of commercial Fe78Si13B9 (at.%, FeSiB) amorphous flaky powder at the room temperature and its corresponding industry-oriented experimental coating equipment were designed and developed. The influence of addition amount of precursor on the soft magnetic properties of SMCs was studied in detail, and the loss separation analysis on the associated SMCs above was also carried out.
在以前的工作中，我們通過溶膠 - 凝膠方法成功地在室溫下的總製備時間約為2.6小時[7] ， [21] ，成功地在基於Fe的磁性片狀粉末的表面上製造了高質量的SIO 2絕緣層。在這項研究中，一種新的單鍋高效合成方法，用於在商業Fe 78 Si 13 B 9 （at。％，FESIB）在室溫及其相應的行業導向的實驗塗層設備上獲得均勻的SIO 2塗層。詳細研究了前體添加量對SMC的軟磁性能的影響，並且還進行了上面相關SMC的損耗分離分析。

2. Experimental section  2.實驗部分

2.1. Materials and reagents
2.1。材料和試劑

The commercial FeSiB amorphous flaky powders with particle size distribution ranging of 45 ~ 165 μm (−325 ~ −100 mesh) fabricated from ribbon pulverization was provided by China Amorphous Technology CO., LTD as the original powder substrate, wherein the thickness of the flaky powders is about 20 μm. Tetraethyl orthosilicate (TEOS) with a concentration of 99 wt%, as the single-source precursor, and polyvinylpyrrolidone (PVP) with an average molecular weight of 130 kg·mol−1, as the surface active agent, were purchased from Aldrich. Aqueous ammonia solution (NH3·H2O, 25 wt%), anhydrous ethanol (EtOH), acetone and silicon resin were from the Guangzhou Chemical Reagent Co., Ltd. Deionized water was prepared by LDF-II water purification system (Ludao Instruments Co, Ltd, Shanghai, China) in laboratory.
中國無需粉狀粉狀的商業FESIB無定形片狀粉末，粒度分佈的範圍為45〜165μm（-325〜 -100元），由中國無定形技術Co。，作為原始粉末基材，由中國粉狀技術公司提供，其中片狀粉末粉的厚度約為20毫米。濃度為99 wt％的四乙基矽酸鹽（TEOS），作為單源前體和平均分子量為130 kg·mol -1的聚乙烯基吡咯烷酮（PVP），作為表面活性劑，是從Aldrich購買的。水溶液溶液（NH 3 ·H 2 O，25 wt％），無水乙醇（ETOH），丙酮和矽樹脂來自廣州化學試劑有限公司，有限公司，由LDF-II II II水淨化系統（Ludao Instruments Co，Ludao Instruments Co，Ltd，Ltd，Shanghai，Shanghai，Charane，Shanghai，Chere）製備。

2.2. Fabrication of core-shell structural FeSiB/SiO2 magnetic flaky powders
2.2。核殼結構FESIB/SIO 2磁性片狀粉末的製造

Fig. 1 presents the one-pot synthesis procedure for the core-shelled magnetic powders and its corresponding industry-oriented experimental coating device. It is shown that the coating device consists of programmable logic controller (PLC) system, synchronous lifting system, and mechanical stirring system. In a typical one-pot synthesis procedure as shown in Fig. 1, Step I: 1500 g of the amorphous magnetic flaky powders were added into the mixture of PVP (38 g) and EtOH (1400 ml) in the cylinder with a speed of 300 revolutions per minute (rpm) for 20 min at room temperature. In this step, the elevator button on the PLC control panel was used to control the position and rotate speed of mechanical stirrer. Step II: a certain aqueous ammonia dissolved in 350 ml deionized water was added into the cylinder through the hole on its flat plate to adjust the pH value within the range of 8–10 under the continuously mechanical stirring for 10 min. Step III: predetermined concentrations of TEOS with 0.04, 0.09, 0.14, 0.20 and 0.25 ml/g (relative to the mass of the magnetic powders) diluted in the EtOH (ranging from 420 ml to 3000 ml, with the volume ratio (TEOS/EtOH) of 1:7 ~ 1:8) were poured into the reaction mixture solution under the continuously mechanical stirring with a speed of 400 rpm for 40 min regulated on the PLC control panel. Based on the steps above, the supernatant in the cylinder was first discharged via the hydrant at the bottom of the cylinder as shown in Fig. 1. And then, 1500 ml deionized water or EtOH was added into the cylinder to rinse the coated powders with a mechanical stirring speed of 200 rpm at least twice times. At last, the obtained coated powders were dried in a drying oven at 95 °C for 40 min. As compared to the mentioned preparation information in the previous work [11], [13], [14], [22], the new one-pot synthesis method implemented using the designed experiential coating equipment exhibits fewer total preparation time of approximately 1.9 h (including the drying time of about 0.7 h), and higher yield of core-shelled powders of 1500 g. The main factors leading to the phenomena above can be attributed to: i) the conventional instilling method was instead of directly pouring the diluted TEOS into the mixture solution in the reactor, which not only simplifies the operations but also shortens the manufacture time, ii) the unique blade structure with double blades perpendicular to each other, as seen in Fig. 1, is beneficial to the improvement of reaction kinetics.
圖1給出了核心殼磁粉及其相應面向行業的實驗塗料裝置的一鍋合成程序。結果表明，塗​​料設備由可編程邏輯控制器（PLC）系統，同步提升系統和機械攪拌系統組成。如圖1所示，在典型的單鍋合成過程中，步驟I ：1500 g無定形磁性片狀粉末在圓柱體中的PVP（38 g）和EtOH（1400 mL）的混合物中加入，在室溫下每分鐘300轉（RPM）的速度為20分鐘。在此步驟中，使用PLC控制面板上的電梯按鈕來控制機械攪拌器的位置和旋轉速度。第II步：通過其平板上的孔中將溶解在350 mL去離子水中的某些水性氨中添加到圓柱體中，以在連續機械攪拌下在8-10範圍內調節pH值10分鐘。步驟III：具有0.04、0.09、0.14、0.20和0.25 mL/g的預測濃度（相對於磁粉的質量）在ETOH中稀釋（範圍為420 ml至3000 ml，範圍從40毫升到3000 ml，體積比（TEOS/ETOH）在1：7〜1：8）均在40分鐘下均連續攪拌PLC控制面板。基於上面的步驟，首先通過圓柱體底部的消防栓排出圓柱體中的上清液，如圖1所示。 然後，將1500 mL去離子水或EtOH添加到圓柱體中，以200 rpm的機械攪拌速度至少兩次沖洗塗層粉末。最後，將所得塗層的粉末在95°C的干燥烤箱中乾燥40分鐘。與先前工作中提到的準備信息相比[11] ， [13] ， [14] ， [22] ，使用設計的經驗式塗料設備實施的新的單鍋合成方法顯示出更少的總準備時間約為1.9 h（包括約0.7 h的干燥時間），以及較高的核心殼粉末粉的收益率為1500 g。導致上述現象的主要因素可以歸因於：i）傳統的灌輸方法不是將稀釋的Teos直接倒入反應器中的混合溶液中，這不僅簡化了操作，還縮短了製造時間，而且還縮短了製造時間；
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Fig. 1. Schematic of the one-pot synthesis procedure for the core-shelled magnetic powders and its corresponding experimental coating device.

2.3. Preparation of FeSiB/SiO2 magnetic flaky powders cores

1.4 wt% of thermosetting silicon resin dissolved in acetone, as an organic binder, was added to the coated magnetic powders by continuously stirring with glass rod until the acetone totally evaporated, followed by adding zinc stearate (0.6 wt%) as a lubricant agent. The 1500 g of collected composite powders under different TEOS concentrations were then compacted into toroidal powder cores with dimension of Φ26.9 mm × Φ14.8 mm × 11.2 mm under a pressure of 2050 MPa with a holding time of 5 s at room temperature. Finally, the compacted cores were orderly cured at 150 °C for 1 h, impregnated in the silicon solution under a vacuum condition for 0.2 h and annealed at the temperature of 450 °C for 0.5 h under an air atmosphere to release the internal stress induced by the cold pressing. As a comparison sample, the magnetic particles coated alone with silicon resin, marked as R-SMCs, was also prepared.

2.4. Characterizations

Phase identification and analysis of magnetic powders was analyzed by a X-ray diffraction (XRD, Rigaku MiniFlex600X, Japan) with Cu Kα radiation at a range of 2θ = 20 ~ 90° with a step size of 0.05°. The core-shell structure of powders was characterized by a scanning election microscopy (SEM, Phenom ProX) coupled with an energy-dispersive spectrometer (EDS), and a Fourier transform infrared (FTIR, Bruker Uecior-22) spectrum within a range of 400 ~ 4000 cm−1. Surface morphologies of the raw and coated powders were performed using SEM. Cores’ resistivity and green compact density were separately measured by an AC impedance spectrum and the principle of Archimede. The core loss was measured by a B-H curve analyzer (Iwastu SY-8218). The complex permeability spectra from 1 kHz to 120000 kHz at 0.1 V, and DC-bias property were collected by an LCR meter (Wayne Kerr 6500B) for all the powder core specimens. Real-part (μ′, equal to the effective permeability, μe) and imaginary part (μ″) of complex permeability can be calculated by the following equations.(1)μ′=Lsleμ0N2Ae(2)μ′′=Racleωμ0N2Aewhere Ls, Ae and le are the inductance, the cross-section area, and the mean flux density path length of the ring sample cores, respectively. Rac is the AC resistance of the core, ω is the angular frequency (2πf), and N is the total number of coil turns. μ0 is the permeability of vacuum and it is 4π × 10−7 H/m.

3. Results and discussions

3.1. Characterization of the SiO2-coated amorphous powders

Fig. 2 displays the SEM images of the magnetic flaky powders before and after chemical coating process with the TEOS concentrations ranging from 0 to 0.25 ml/g. It is observed that the flaky powders were in the polygonal shape with smooth edges, favorable for the uniform insulation coating. The surface of raw FeSiB magnetic flaky powders is clean, shining and smooth. The inset in Fig. 2(a) gives the XRD pattern of the raw flake powders. Only a diffuse halo pattern at around 2θ = 45° typical for the formation of amorphous structure, without any other obvious diffraction peaks, can be observed. After chemical coating process, the surface of coated powder became somber and rough compared with the raw amorphous flaky powder as shown in Fig. 2(b)~(f). At the TEOS concentration of 0.04 ml/g, it is seen that the deposition coated on the flaky powder surface is non-uniform. With the elevating TEOS concentration to 0.14 ml/g, a uniform and dense coating layer was presented on the surface of the amorphous flaky powder. Nevertheless, the surface of flaky powder became coarse and was gathered lots of granular aggregates as the TEOS concentration is greater than 0.20 ml/g.
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Fig. 2. SEM images for the surface morphologies of magnetic flaky powders before (a) and after chemical coating process with TEOS concentrations of (b) 0.04 ml/g, (b) 0.09 ml/g, (c) 0.14 ml/g, (d) 0.20 ml/g, (b) 0.25 ml/g.

To identify the core-shell structure of the coated FeSiB amorphous flaky powders, Fig. 3 presents the EDS elemental distribution maps and EDS spectrums of the as-coated flaky powders under the TEOS concentrations of 0.04 ml/g and 0.14 ml/g. The EDS maps of Fig. 3(a) reveal that only elements Fe and Si exist in the uncoated regions, while the elements of Fe, Si, and O simultaneously appear in the coated region on the surface of a typical unevenly coated flaky powder under the TEOS concentration of 0.04 ml/g. For the case of 0.14 ml/g of the TEOS concentration, the Fe, Si and O elements are uniformly distributed on the surface of the coated magnetic powders. The signals of C element in the EDS patterns primarily originated from the conducting resin to fix the magnetic powders on the specimen stage. It is also observed from the EDS patterns that the O and Si signals on the surface of as-coated powders were obviously enlarged as compared to the raw powders. Hereinto, the mass fractions of O and Si elements separately increase from 0.94 wt% to 9.63 wt%, and from 2.72 wt% to 11.42 wt% as the TEOS concentration is up to 0.14 ml/g, which implies that the coating layer should be SiO2.
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Fig. 3. SEM images, EDS elemental distribution maps and (c) EDS spectrums of the as-coated flaky powders with the TEOS concentrations of (a) 0.04 ml/g and (b) 0.14 ml/g.

To further confirm the composition of the coating layer on the surface of magnetic powder, Fig. 4 displays the recorded FTIR spectra for the magnetic flaky powders before and after chemical coating process with the TEOS concentrations ranging from 0 to 0.25 ml/g. No obvious absorption peak is observed for the raw magnetic powders, while some characteristic absorption peaks appear after chemical coating process. The strongest absorption peaks centered at around 1091 cm−1 is regarded as the asymmetric stretching vibrations of Sisingle bondOsingle bondSi, while the symmetric stretching vibrations of Sisingle bondOsingle bondSi causes the absorption peaks at 803 cm−1 and 464 cm−1 [13]. The peak located at about 952 cm−1 is contributed to the stretching vibration of Si-OH [23]. The absorption peak at 1629 cm−1 and the broad absorption band with the center at 3441 cm−1 can be attributed to the overlapping Osingle bondH stretching vibrations from the excessive amounts of the hydroxyl groups during the hydrolysis and condensation processes [9]. The peak at 2349 cm−1 is associated with the stretching vibration of Cdouble bondO caused by the atmospheric CO2 [24]. These presented absorption peaks further testify that the SiO2-insulated layer was settled on the surface of the FeSiB amorphous magnetic flaky powder.
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Fig. 4. FTIR spectra of the magnetic flaky powders before and after chemical coating process with different TEOS concentrations.

3.2. Magnetic properties of FeSiB/SiO2 magnetic flaky powders cores

Fig. 5 gives the frequency dependences of real-part (μ′) and imaginary-part (μ″) of complex permeability for the SMCs insulated with SiO2 under different concentrations of TEOS and their corresponding green compact densities and electrical resistivities. No distinct reduction in the μ′ for the powder cores chemically coated using TEOS concentration greater than 0.09 ml/g was observed even increasing the frequency up to 3000 kHz (3 MHz). Oppositely, as seen in Fig. 5(a), the progressive attenuations in the μ′ began to emerge when the frequency over 60 kHz and 800 kHz for the R-SMCs (prepared using the magnetic particles coated alone with silicon resin) and the coated powder cores under the TEOS concentration of 0.04 ml/g (marked as SMCs-4), respectively. To explain this phenomenon, the relaxation frequency (fr) and the insulation situation of powder cores should be synthetically considered. According to the Visser’s coherent model, the μ′ is inversely proportional to fr (Eq. (3) [25]). The μ″ reveals the energy losses relating to the hysteresis response and eddy currents [26], and its peak value corresponds to the fr. It is seen that the R-SMCs and the SMCs-4 separately exhibit the fr at 14.3 MHz and 106.6 MHz. With elevating the TEOS concentration over 0.09 ml/g, the fabricated SMCs did not attain their fr in the maximum frequency test range of the employed LCR meter. This can be attributed to the magnetic powders favorably insulated with each other beneficial for enhancement of electrical resistivity of SMCs (as seen in Fig. 5(b)), leading to an increase in the fr as shown in the Eq. (4) [26].(3)μ′=ui11+f2/fr2(4)fr=ρb2πμ0μid2where ρb is the bulk electrical resistivity, μi is the initial permeability and d is the effective particle dimension.
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Fig. 5. Frequency dependences (a) of real-part and imaginary-part of complex permeability for the powder cores insulated with SiO2 under different concentrations of TEOS and their corresponding green compact densities and electrical resistivities (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Moreover, with elevating the TEOS concentration from 0 to 0.25 ml/g, the μ′ gradually decreases from 87.3 to 56.2. Generally, the intensity of effective demagnetizing field, which exhibits a directly proportional correlation with the volume fraction of non-magnetic phase (Vn, Eq. 5) in core specimen, has a significant influence upon the μ′ of SMCs [1]. As depicted in Fig. 5(b), the density shows the same reducing tendency as the μ′ of SMCs, in which the density decreases from 5.732 to 5.694 g/cm3 as the TEOS concentration increases from 0 to 0.25 ml/g.(5)Vn=rt-rbrt×100%where rt and rb represent the true and apparent bulk densities of cores, respectively.
Core loss plays a significant role on the conversion efficiency of powder cores in the power application. According to the Bertotti’s loss separation theory [27], the total core loss (Pcv, in kW/m3) is composed of the hysteresis loss (Ph), eddy current loss (Pe), and excess loss (Pexc), and can be expressed as follows.(6)Pcv=Ph+Pe+Pexc
The value of Ph is equal to the product of the quasi-static hysteresis loop area and frequency, thus can be described as below.(7)Ph=∮Bfμ0μedB=KhBmxfwhere Kh and x are the simulated coefficients.
The Pe normally consists of inter- and intra-particle eddy current losses, namely Peinter and Peintra, and can be obtained by the following equations [16], [28]. The Pexc originates from the eddy currents mainly caused by the domain wall branching and bowing [1], [4], [29], and can be calculated by subtracting the Ph and Pe from Pcv.(8)Pe=Peinter+Peintra=πdeBmf2β1ρb+πdpBmf2β2ρp=KeBm2f2(9)β1=61-0.633whtanh1.58hw(10)Ke=πde2β1ρb+πdp2β2ρpwhere ρb, ρp are the electrical resistivities of SMCs and magnetic particles, and w, h are the side lengths of rectangular cross section of powder cores, respectively. dp represents the thickness of magnetic flaky powders, and the β2 is a granular geometrical factor, for the flaky powders, β2 = 6 [11]. de is the effective eddy current dimension, which is the shorter edge of the rectangular cross section of cores. Ke is the eddy current loss coefficient.
Fig. 6(a) shows the mesh surfaces of total core loss versus the frequency (f) and the maximum magnetic flux density (Bm) for the R-SMCs and the powder cores insulated with SiO2 using different concentrations of TEOS. As compared to the R-SMCs, the SiO2-insulated powder cores under the TEOS concentration below 0.20 ml/g exhibit a relatively low Pcv in the entire testing ranges of Bm and f due to their higher electrical resistivity (as seen in Fig. 5(b)), while the coated powder core specimen under the TEOS concentration of 0.25 ml/g shows a higher Pcv when the f below 70 kHz due to its higher Vn, leading to a larger hysteresis loss. Moreover, with the increasing TEOS concentration, the core’s Pcv decreases first and then increases, and exhibits the minimum Pcv of 765 kW/m3 at 200 kHz for Bm = 0.08 T when the TEOS concentration is in the range of 0.09 ~ 0.14 ml/g as shown in the Fig. 6(b). To further understand the change rule of Pcv, frequency dependencies of Ph, Pe, and Pexc for the corresponding cores above were separately depicted in Fig. 6(c), (e) and (f).
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Fig. 6. Mesh surfaces (a) of total core loss versus frequency and the maximum magnetic flux density, frequency dependency of total core loss (b), hysteresis loss (c), eddy current loss (e) and excess loss (f), fitting results for the Ph/f vs. Bm (d) and KexcBmy vs. Bm (g), and DC-bias field dependence (h) of percent permeability for the powder cores insulated with SiO2 using different concentrations of TEOS.

In regard to Ph as presented in Fig. 6(c), the linear slope (Ph/f) of the function of pH vs. f is the quasi-static hysteresis energy loss (unit in J/m3), representing the cores’ energy loss per volume during a single complete magnetization cycle, which can be obtained by measuring the area of the quasi-static hysteresis loop. And then, the nonlinear power fitting curves (Eq. 7) of Ph/f vs. Bm, as well as the coefficients Kh and x can be obtained for the powder cores insulated with SiO2 using different concentrations of TEOS as depicted in Fig. 6(d). With the raising TEOS concentration from 0 to 0.25 ml/g, the Ph gradually increases from 201 kW/m3 to 460 kW/m3 at 200 kHz for Bm = 0.08 T, which is mainly owing to the increase in the volume fraction of non-magnetic phase in the powder cores. It is also noted that the SMCs’ Pcv is dominated by Ph when the TEOS concentration is greater than 0.09 ml/g.
Clearly, as seen in Fig. 6(e), the powder cores chemically insulated using TEOS concentration over 0.09 ml/g exhibit a much smaller value of Pe compared with those of the R-SMCs and the SMCs-4, which is mainly due to the higher electrical resistivity shown in Fig. 5(b). Moreover, it is also observed that the Pe of the coated powder cores under the TEOS concentration ranging of 0.09 ~ 0.25 ml/g contributes only about 13.5% ~ 18.1% of Pcv due to their high electrical resistivities. This fact indicates that a high quality electro-insulating layer was presented on the magnetic particle surface for the coated core specimens under the TEOS concentration over 0.09 ml/g.
Referring to the Pexc, it is known that the Pexc largely depends on the frequency, operating magnetic flux density, number of active magnetic objects, and cross section area of the material perpendicular to the magnetic flux [1], [16]. Thus, the Pexc can be fitted by following formula [11].(11)Pexc=KexcBmyfzwhere Kexc, y and z are the simulated coefficients.
Fig. 6(f) presents the simulated results of Pexc vs. f for the powder cores insulated with SiO2 using different concentrations of TEOS. And then, the simulated coefficient of KexcBmy under the different maximum magnetic flux intensities can be obtained. Thus, the corresponding nonlinear power fitting curves of KexcBmy vs. Bm, as well as the coefficients Kexc and y can be gained as shown in Fig. 6(g). Moreover, it is observed that the Pexc shows the same increasing variation tendency as the Ph the with the elevating TEOS concentration. This is because of that both Ph and Pexc are sensitive of cores’ density, which determines the volume fraction of non-magnetic phase in the powder cores (Eq. 3), thus affecting the number of pinning sites that hinder the domain wall motion. Based on the discussion above, Table 1 summarizes the fitted coefficients of each formula of different partial losses for the FeSiB amorphous magnetic powder cores insulated under different concentrations of TEOS.

Table 1. Fitted coefficients of each formula of different partial losses for the FeSiB amorphous magnetic powder cores insulated under concentrations of TEOS.

TEOS concentration, ml/gPhPeWexc
KhxKeKexcyz
0178.972.054.300.922.122.12
0.04272.512.081.531.102.092.10
0.09345.602.090.5441.192.072.08
0.14325.602.060.5321.282.092.04
0.20318.612.010.5251.351.982.05
0.25345.401.980.5231.471.972.03
Except for the effective permeability and total core loss, DC-bias property is another important performance index determining the size and rate power of powder cores used in the power applications. Fig. 6(h) shows the DC-bias field dependence of percent permeability, defined as the ratio between the μ′ under a DC-bias field and the μ′ at zero-DC-bias field, for the powder cores insulated with SiO2 under different concentrations of TEOS. With the increasing TEOS concentration from 0 to 0.25 ml/g, the percent permeability at an applied field of 100 Oe increases from 48.3% to 76.4%, showing the same tends of the Ph (Fig. 6(c)) and the Pexc (Fig. 6(f)), while exhibiting a complete reversal of variation tendency of the μ′ (Fig. 5(a)). These phenomena can be contributed to the interface-pinning effect that impedes the movement of domain wall [30]. Combined with the result of Pe in Fig. 6(e), it is important to balance the five magnetic performance indexes above by regulating different process parameters for achieving the optimal design of powder cores toward high-frequency application and miniaturization of power electronic devices.

4. Conclusions

Under the newly designed one-pot highly efficient synthesis method and its corresponding automated experimental coating equipment, 1500 g of commercial Fe-based amorphous flaky powders with a high-quality SiO2-coating layer can be obtained only using the total preparation time of 1.9 h (including the drying time of about 0.7 h) at the room temperature, suitable for the popularization in industrial application. The effect of addition amount of precursor on the powder cores’ soft magnetic properties was systematically investigated. A uniform and dense insulating layer can be obtained within the TEOS concentration range of 0.09 ~ 0.14 ml/g. With further elevating TEOS concentration over 0.2 ml, the SiO2 coating layer on the surface of amorphous flaky powder became coarse and generated lots of granular aggregates. Under this situation, the eddy current loss has a little impact on the total core loss as compared to the hysteresis loss and residual loss in the entire testing frequency range when the TEOS concentration over 0.09 ml/g. As compared to the other powder core specimens, the Fe-based amorphous powder cores insulated under the TEOS concentration range of 0.09 ~ 0.14 ml/g exhibit excellent comprehensive soft magnetic properties, including the core loss of 765 kW/m3at 200 kHz for Bm = 0.08 T, a relatively high and stable permeability of 67.5 even increasing the frequency up to 3 MHz, and an good DC-bias property of 71.4%.

CRediT authorship contribution statement

Haibo Sun and Xin Liu are responsible for conceptualization and investigation; Ce Wang and Zhili Guo are responsible for data curation, formal analysis; Dongchu Chen and Weihong Chen are responsible for funding acquisition and resources; Haibo Sun writing - original draft; Jian Wang is responsible for Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by the Guangdong Provincial Science and Technology Program (Grant No. 2019A050510050), Guangdong province key project of foundation and application foundation research joint fund (Grant No. 2019B1515120020) and Foshan Municipal Key Technology Tackling Program (Grant No. 1920001001392).

References

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C. Wang and Z. Guo contributed equally to this work.
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