Properties Optimization of Soft Magnetic Composites Based on the Amorphous Powders with Double Layer Inorganic Coating by Phosphating and Sodium Silicate Treatment

Abstract

Core-shell structured amorphous FeSiBCr@phosphate/silica powders were prepared by phosphating and sodium silicate treatment. The soft magnetic composites (SMCs) were fabricated based on these powders. The effects of phosphoric acid (H₃PO₄) concentration and annealing temperature on their properties were investigated. During the phosphating process, the powder coated with a low concentration of H₃PO₄-ethanol solution leads to uneven phosphate coating, while the peeling of phosphate coating occurs for the high H₃PO₄ concentration. Using 0.5 wt.% phosphoric solution, a uniform and dense insulation layer can be formed on the surface of the powder, resulting in increased resistivity and the reduced eddy current loss of the amorphous soft magnetic composites (ASMCs). This insulation layer can increase the roughness of the powder surface, which is beneficial to the subsequent coating of sodium silicate. By optimizing sodium silicate treatment, a complete and uniform SiO₂ layer can be formed on the phosphated powders well, leading to double layer core-shell structure and excellent soft magnetic properties. The magnetic properties of amorphous SMCs can be further improved by post annealing due to the effectively released residual stress. The enhanced permeability and greatly reduced core loss can be achieved by annealing at 773 K, but the deterioration of magnetic properties occurs as the annealing temperature over 798 K, mainly due to the increase of α-Fe(Si) and Fe₃B phases, which hinder the domain wall displacement and magnetic moment rotation. The excellent soft magnetic properties with permeability μ_e = 35 and core loss P_s = 368 kW/m³ at 50 mT/200 kHz have been obtained when the SMCs prepared with the powders coated by 0.5 wt.% H₃PO₄ and 2 wt.% sodium silicate were annealed at 773 K.

1. Introduction

Soft magnetic composites (SMCs), consisting of soft magnetic powders and insulation binder, have found increasing applications in various electronic and electrical components due to their stable permeability and low core loss at high frequency [1,2,3,4]. With the development of magnetic devices towards miniaturization, high frequency, and high efficiency, SMCs with high magnetic saturation, stable permeability, and low core loss are urgently required [5,6,7]. Currently, there are various types of magnetic powders for SMCs, including pure iron (Fe), Fe-Si, Sendust (Fe-Si-Al), Flux (Fe-Ni), MPP (Fe-Ni-Mo), Fe-based amorphous, and nanocrystalline alloys. The Fe-based amorphous alloys such as FeSiBCr prepared by gas atomization exhibit high potential to work as the ideal core materials due to their good sphericity, low coercivity, and high saturation magnetization.

The preparation of SMCs involves coating an insulation layer on the metallic powders followed by powder forming and curing. Both inorganic coatings and organic coatings are employed. Inorganic coatings including oxides (SiO₂ [8,9,10], MgO [11], TiO₂ [12], Al₂O₃ [13], ZrO₂ [14]), ferrites (MnZn ferrite [15], NiZn ferrite [16]) and inorganic salts exhibit high temperature resistance and high electric resistivity. They are generally prepared by in-situ chemical synthesis or coating a mixture of inorganic nanoparticles and organic resin on the powder. However, it was found that, for the oxide coating, a uniform and dense layer tightly bound to the magnetic powder is difficult to prepare. Soft magnetic ferrites can be used for coating, which exhibits high resistivity and high magnetization, but the brittle ferrite layer is easy to break during the pressing process. The inorganic salt layer can be prepared by direct reaction of the acid with the magnetic powder [15]. These coatings have the advantages of high density, high uniformity, high resistivity, and strong bonding with the magnetic powder. The common acids include HNO₃ [17,18] and H₃PO₄ [19,20], among which H₃PO₄ is frequently used for industrial production. However, for the amorphous Fe-based alloy powders, it is difficult to prepare an insulating layer on their surfaces by H₃PO₄ treatment [21] since amorphous alloys generally show excellent corrosion resistance [22].

It has been reported [8] that silica (SiO₂) coating can work as effective high-resistive thin layer for the magnetic powders. Sol-gel method is generally employed for preparing SiO₂ coating on Fe-based powder, but this process is complicated since four kinds of chemicals including tetraethoxysilane (TEOS) are needed [23]. Recently, FeSiCr powder was coated with a SiO₂ layer by reacting carbon dioxide with sodium silicate, which indicates that sodium silicate (Na₂SiO₃) can be used as a silicon source [24]. However, the insulating layer prepared by sodium silicate is not dense (in particular for the atomized amorphous powder with the smooth surface).

In this work, in order to obtain a uniform and dense insulation layer on the amorphous Fe-based alloy powder, an inorganic double layer coating is proposed, which was prepared by phosphating followed by SiO₂ coating. The phosphating layer by H₃PO₄ treatment is employed to increase the surface roughness of the powder, which is beneficial for depositing a uniform and dense insulation SiO₂ layer. By this approach, we prepared core-shell structured amorphous FeSiBCr@phosphate/SiO₂ powders, which then were made into magnetic cores. The effects of H₃PO₄ concentration and annealing temperature on the soft magnetic properties of SMCs were investigated. This method is suitable for the industrial production of amorphous SMCs with stable permeability and low core loss.

2. Experimental Procedures

Commercial gas atomized Fe₈₇Si_5.5B₄Cr_3.5 amorphous powders were supplied by Tiz-Advanced Alloy Technology Co., Ltd (Quanzhou, China). The average particle size D50 of the raw powders is approximately 20 µm and the apparent density of powders is 3.5–4.0 g/cm³. The saturation magnetization (M_s) of the raw powders is 148 Am²/kg. Sodium silicate (Na₂SiO₃) with modulus of 1 is purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). To prepare SMCs, the FeSiBCr powders (30 g) were first dispersed in H₃PO₄-ethanol solution (30 g) with various H₃PO₄ concentrations (0 wt.%, 0.25 wt.%, 0. 5 wt.%, 0.75 wt.%, 1.0 wt.%) under constant stirring at 55 °C until the ethanol solvent was completely evaporated for surface passivation. After washing and drying, the phosphated powders were obtained. The phosphatized powders were then dispersed in 2 wt.% sodium silicate aqueous solution (15 g) and stirred at 328 K for 20 min. The obtained magnetic powders were filtered and washed with deionized water for three times. The treated powders were mixed with 2.5 wt.% silicone resin and 0.5 wt.% epoxy resin-acetone solution and stirred at room temperature until the acetone solvent evaporated completely for binding. The bonded powders were compacted into toroidal-shaped amorphous SMCs with the dimensions of Φ20 × Φ12 × 4 mm under 1200 MPa for 5.5 s. The SMCs were cured at 453 K for 1 h. For annealing treatment, the selected amorphous SMCs were heated at different temperatures (748, 773, 798 K) for 1 h under Ar atmosphere.

The surface morphologies of the powders were observed by scanning electron microscopy (SEM, FEI Quanta 200, Hillsboro, OR, USA). Fourier transform infrared spectrometer (FTIR, VERTEX 70, Bruker, Billerica, MA, USA) was used to investigate the phase constitution of the insulation layer. The soft magnetic properties were obtained by LCR meter (IM3536, Hioki, Nagano, Japan) and soft magnetic AC measurement device (MATS-3010SA, Linkjoin, Loudi, China). The electrical resistivity was measured by a high resistance weak current tester (ST2643, Jingge, SuZhou, China). The phase structures of powders were confirmed by X-ray diffraction (XRD, Philips X’ Pert, PANalytical, Almelo, The Netherlands) using Cu Κα radiation at room temperature in the range of 20–80°. The magnetic hysteresis loops were measured at room temperature using a vibrating sample magnetometer (VSM, East Changing, Beijing, China). The radial crush strength was measured by core rupture test machine (FL-8621, Feiling, Dongguan, China).

3. Results and Discussion

3.1. The Formation of Double Layer Structure on the Powder

Figure 1 shows the SEM images of the amorphous FeSiBCr powders before and after different treatments. In Figure 1a, the original powder shows smooth surface, which is unfavorable for the coating of sodium silicate through physical contact. The surface of the powder after phosphatized in a 0.5 wt.% H₃PO₄-ethanol solution becomes rough, as shown in Figure 1b. The rough phosphate structure can improve the adhesion of sodium silicate. After treated only by sodium silicate, the powder surface exhibits loose and porous structure (Figure 1c). A dense insulating layer can be formed on the surface of the powder after two-step treatment by both H₃PO₄ and sodium silicate, as shown in Figure 1d. As we know, the uniform and dense insulation layer on the powders is beneficial to the soft magnetic properties of SMCs.

FTIR analysis results for the amorphous FeSiBCr powder and the powders after different treatments are shown in Figure 2. FTIR Curve (a) for the original powders shows no distinct band. Bands of 2915 cm⁻¹ (Curve (b) and (c)) are attributed to the-CH₃ group from residual C₂H₅OH. The characteristic peaks of phosphate are observed at the band of 1065 cm⁻¹and 540 cm⁻¹ for the powders treated by H₃PO₄-ethanol solution (Curve (b)). The bands at 1640 cm⁻¹and 1065 cm⁻¹ are assigned to the bending mode of P-OH bonds and the symmetric stretching vibrations of P-O bonds [25], respectively. The results indicate that the phosphate has been formed on the surface of the magnetic powder after H₃PO₄ treatment. On Curve (c) for the powders coated by sodium silicate-aqueous solution, the broad absorption band at 1038 cm⁻¹ originates from the symmetric vibration absorption of Si-O-Si [26], which confirms that the obtained coating layer on the powders is SiO₂. On Curve (d) for the powders treated with H₃PO₄-ethanol followed by sodium silicate solution, the characteristic bonds of both phosphate and SiO₂ can be detected, indicating that both phosphate and SiO₂ layers are formed on the powders. Compared with phosphate and SiO₂ coating alone, the hybrid phosphate-SiO₂ layer exhibits significantly decreased intensity of the absorption peak corresponding to -OH at 3450 cm⁻¹, indicating that the -OH of phosphate binds to the -OH of sodium silicate. The results thus suggest that the P-O-Si bond may be formed on the powder, which can enhance the binding of SiO₂ and magnetic powder.

The evolution of the core-shell structure on the magnetic powder can be illustrated in Figure 3. In the first step 1, H₃PO₄ reacts with the FeSiBCr powder according to Equation (1), and the first layer of phosphate can be formed on the powder [27,28]. In the second step, Si(OH)₄ dehydrates and condenses with the -OH on the phosphated powder surface to form a networked SiO₂, and the second shell on the powder is formed. Hydrolysis reaction and condensation reaction can be expressed by Equations (2) and (3) [29]. As a result, the core-shell structured FeSiBCr@phosphate/SiO₂ powders can be prepared.

Fe²⁺ + HPO₄ ²⁻ → FeHPO₄
3Fe²⁺ + 2PO₄³⁻ → Fe₃(PO₄)₂

(1)

4Fe²⁺ + O₂ + 4PO₄³⁻ + 4H⁺ → 4FePO₄ +2H₂O
Na₂O·nSiO₂ + (2n+1) H₂O → 2NaOH + nSi(OH)₄

(2)

$$\mathrm{n}\mathrm{S}\mathrm{i}{(\mathrm{O}\mathrm{H})}_4\to [\mathrm{S}\mathrm{i}{(\mathrm{O}\mathrm{H})}_4]\mathrm{n}\stackrel{-2{\mathrm{H}}_2\mathrm{O}}{\to }\begin{array}{ccccccc}& |& & & & |& \\ -& \mathrm{S}\mathrm{i}& -& \mathrm{O}& -& \mathrm{S}\mathrm{i}& -\\ & |& & & & |& \\ & \mathrm{O}& & & & \mathrm{O}& \\ & |& & & & |& \\ -& \mathrm{S}\mathrm{i}& -& \mathrm{O}& -& \mathrm{S}\mathrm{i}& -\\ & |& & & & |& \end{array}$$

(3)

3.2. Effects of Phosphated by Various H₃PO₄ Concentrations

Figure 4a–d show the SEM images of the powders phosphated by the H₃PO₄ with different concentrations (0.25 wt.%, 0.5 wt.%, 0.75 wt.%, 1.0 wt.%). For the powder phosphated with 0.25 wt.% H₃PO₄, P and O elements are evenly distributed on the powder surface, as shown in Figure 4a with EDS result. It is believed that a phosphating layer is formed. Comparing the morphologies in Figure 4a–d, the higher concentration of H₃PO₄ leads to the rougher surface of the powder. When the H₃PO₄ concentration increases to 1 wt.%, a large amount of sheet structures appears. EDS testing shows that these sheet structures contain abundant P, O, and Fe elements, presumably phosphate, as shown in Figure 4d. The appearance of the sheet structured phosphate may destroy the integrity of the layer and reduce the density of the SMCs. Thus, it is necessary to control the nucleation and precipitation of the flaky phosphate to make the phosphate coating strong and dense.

Table 1. Effective permeability µ_e, Q and core loss P_s of the SMCs with 0.5 wt.% H₃PO₄-ethanol solution and different concentration of sodium silicate-aqueous solution.

Sample (Powder Treatment)	Effective Permeability (μe)	Q	Ps (kW/m3)
			50 mT/100 kHz	50 mT/200 kHz
Phosphatized 0.5 wt.%	37.8	42	301.0	703.6
Phosphatized 0.5 wt.% + Sodium silicate 1 wt.%	36.4	35	243.6	576.3
Phosphatized 0.5 wt.% + Sodium silicate 2 wt.%	33.4	50	136.4	368.0
Phosphatized 0.5 wt.% + Sodium silicate 3 wt.%	32.0	45	187.1	490.3
Phosphatized 0.5 wt.% + Sodium silicate 4 wt.%	29.0	35	194.3	564.6

shows the effective permeability µ_e, quality factor Q and core loss P_s of the magnetic cores prepared from the powders treated with 0.5 wt.% H₃PO₄-ethanol solution followed by different concentrations of sodium silicate solution. All cores were annealed at 773 K for 1 h. It was found that an appropriate amount of SiO₂ can help to form a uniform and complete insulation layer on the powders, which will not completely break during the pressing process. However, more SiO₂ with high hardness may result in decreased density and deteriorated magnetic properties [30]. The core based on powders experienced 2 wt.% sodium silicate solution offer the optimal performance with high permeability, high Q, and low magnetic loss. Therefore, in the following experiments, for studying the effects of H₃PO₄ concentration on the properties of SMCs, 2 wt.% sodium silicate-aqueous solution were selected for coating SiO₂.

Figure 5 shows the magnetic properties of SMCs fabricated by the amorphous FeSiBCr powders treated with various H₃PO₄-ethanol solutions followed by 2 wt.% sodium silicate-aqueous solution and annealed at 773 K. For all samples, the permeability decreases as the frequency increases, mainly due to the weakening effect of eddy currents generated by induced electromotive force on the applied magnetic field at high frequencies (Figure 5a). It decreases from ~37.8 to ~29.0 with increasing H₃PO₄ concentration from 0 to 1 wt.%. Increasing H₃PO₄ concentration leads to more non-magnetic material, resulting in lower permeability. When the H₃PO₄ concentration is 0.5 wt.%, the SMCs has the beat frequency stability, as shown in Figure 2. The insulating layer prepared by 0.5 wt.% H₃PO₄ concentration followed by 2 wt.%Na₂SiO₃ has the beat coating effect and can effectively isolate eddy currents between powders.

Q value is the quality factor, which is an important indicator to measure the high-frequency performance of soft magnetic composites. The Q value can be expressed by the ratio of the real permeability μ of the soft magnetic composite to the imaginary permeability μ’, as well as the ratio of the inductance and the equivalent loss resistance, expressed as Equation (4):

$$Q=\frac{\mu \prime }{\mu ″}=(\frac{2\pi {fL}_s}{R_s})$$

(4)

where L_s is the inductance and R_s is the equivalent loss resistance. In general, the higher the Q value on behalf of powder cores, the lower the rate of energy loss. The Q value of the sample first increases then decreases with increasing frequency (Figure 5b). The SMCs treated with 0.5 wt.% H₃PO₄ concentration exhibits a maximum peak value of 50, which shows a significant performance improvement compared to those without H₃PO₄ treatment.

The frequency dependence of core loss (P_s) for the SMCs prepared with different H₃PO₄ concentrations in the range of 20 kHz to 200 kHz at B_m = 50 mT are shown in Figure 5c. The P_s of all samples increase rapidly with increasing frequency. The lowest P_s of 368 kW/m³ at 200 kHz is obtained for the SMC with 0.5 wt.% H₃PO₄ concentration, decreased by 41% compared to that without phosphating. P_s decreases from 628 to 368 kW/m³ until the H₃PO₄ concentration increases from 0 to 0.5 wt.%. As the concentration of H₃PO₄ exceeds 0.5 wt.%, the P_s shows a rapid increase. As we know, P_s is mainly determined by the hysteresis loss (P_h), the eddy loss (P_e) and residual loss (P_r), which can be expressed by Equation (5) [31]:

$$P_{\mathrm{s}\left(\mathrm{total}\right)}=P_h+P_e+P_r\approx f{{\displaystyle \oint }}^{\text{}}HdB+\frac{C{B}^2{d}^2{f}^2}{\rho }+{\mathrm{K}}_{\mathrm{r}}{\mathrm{Bxf}}^{1.5}$$

(5)

$$P_{\mathrm{e}}=P_e{}^{\mathrm{Intra}}+P_e{}^{\mathrm{Inter}}=\frac{{(\pi d_{\mathrm{powder}}{B}_m)}^2}{20{\rho }_{\mathrm{powder}}{R}_{\mathrm{powder}}}{f}^2+\frac{{(\pi d_{\mathrm{eff}}{B}_m)}^2}{\beta {\rho }_{\mathrm{s}}{R}_{\mathrm{s}}}{f}^2$$

(6)

where H is the magnetic field, B is the magnetic flux density, f is the frequency, C is the constant, d is the effective diameter of powders and ρ is the resistivity. K_r is residual loss coefficient related to the material, and x is the coefficient related to the magnitude and frequency of the applied magnetic field. P_r is a combination of relaxation and resonant losses. These losses are only important at very low induction levels and very high frequencies and can be ignored in power applications [1,32,33]. Eddy current losses include intra-particle and inter-particle eddy current losses, as shown in Equation (6), d_powder is the effective size of the powder, ρ_powder is the density of the powder, and R_powder is the resistivity of the powder. d_eff is the effective size of the eddy current, ρ_s is the density of the SMCs, and R_s is the resistivity of the SMCs. P_e^Intra is mainly related to the resistivity and particle size of the powder. P_e^Inter is mainly related to the degree of insulation between powder particles, that is, the resistivity of SMCs. In this paper, by using different insulation coating processes, the Rs of the SMCs can be effectively improved, thereby reducing the P_e^Inter.

P_s can be separated into P_h and P_e based on above equation and the results are presented in Figure 5d and Figure 5e, respectively. When the H₃PO₄ concentration exceeds 0.5 wt.%, P_h and P_e increase with the increase of H₃PO₄ concentration. This can be understood since the excess iron phosphate precipitates between the soft magnetic composite particles [34]. Sodium silicate can react with dehydration and condensation on the detached phosphate layer, which worsens the coating effect of the magnetic powder, leading to the increase of P_h and P_e. The reduced P_e is mainly due to the electrical resistivity reduction resulting from the destruction of the layer, as shown in Figure 6.

The magnetic composite consists of core and surface layer. For a spherical magnetic particle coated with a layer, the saturation magnetization (M_S) can be expressed as Equation (7),

$$M_{\mathrm{S}}=\frac{M_{\mathrm{SP}}·V_{\mathrm{P}}+M_{\mathrm{SL}}·V_{\mathrm{L}}}{V_{\mathrm{P}}+V_{\mathrm{L}}}=M_{\mathrm{SP}}-\frac{6t}{d}(M_{\mathrm{SP}}-M_{\mathrm{SL}})$$

(7)

where d and t represent the diameter of magnetic particle and the thickness of surface layer, respectively. M_SP and M_SL are the saturation magnetizations of the particle and layer, respectively. V corresponds to the volume [35]. The hysteresis loops of FeSiBCr powders treated by different processes are recorded in Figure 7. The powders show a typical characteristic of soft magnetic material, including a high saturated magnetization and a low coercivity. The maximum M_s of raw FeSiBCr powders is 148 Am²/kg. After treated by H₃PO₄ (0.5 wt.%) and sodium silicate (2 wt.%), the M_s decreases to 145 Am²/kg and to 137 Am²/kg, respectively. However, when coated with phosphate followed by SiO₂, M_s is 143 Am²/kg, and this value is somewhere between the phosphate and SiO₂ coating alone. Compared with the layer of SiO₂, the layer of hybrid phosphate and SiO₂ has a lower M_SL due to the increase of non-magnetic substances. The increased M_S value for the powders coated with phosphate and SiO₂ indicated that the thickness t of the phosphate-SiO₂ layer is less than that of SiO₂ layer according to Equation (5). The results show that the double layer coating has a higher density, which gives a smaller thickness. As a result, the SMC prepared with core-shell structured FeSiBCr@phosphate/SiO₂ powders exhibits the best soft magnetic properties.

3.3. Effects of Annealing Treatment

Figure 8a shows the variation of the effective permeability (μ_e) with frequency for SMCs treated by H₃PO₄ (0.5 wt.%) and sodium silicate (2 wt.%) followed by annealing at different temperatures. With the increasing annealing temperature from 748 to 773 K, the μ_e increases from 28.1 to 33.4. In general, the increased annealing temperature can decrease the internal stress of magnetic powder core, thereby reducing the difficulty of magnetic domain reversal and increasing the permeability [36]. The internal stress for SMCs after annealing at 748 K is released incompletely. With further elevating the annealing temperature to 773 K, the internal stress within the particles was almost removed, leading to the increase of μ_e. When the annealing temperature increases to 798 K, the μ_e of the SMCs decreases sharply to 28.0. For the SMCs annealed at 773 K, the μ_e reaches the highest value of 33.4, increased by 43%, compared with the μ_e of 18.9 for that without annealing treatment. Similarly, due to the stress release, the quality factor (Q) shows a peak value of 50 at a frequency of 3 MHz after annealing at 773 K (Figure 8b). The minimum core loss is also obtained for the SMC annealed at 773 K (Figure 8c).

To illustrate the reasons for the deterioration of magnetic properties after annealing above 798 K, the magnetic powders were tested for XRD, as shown in Figure 7. For the original powders without annealing, only one broad diffraction peak can be observed, indicating a typical amorphous structure [37]. For the powders annealed at the temperatures below 748 K, a slightly intense α-Fe (Si) phase begins to appear. The crystallization occurred after the sample annealed at 773 K, with the precipitation of α-Fe(Si) and Fe₃B phases. When the annealed temperature rises to 798 K, the intensity of Fe₃B phase increases. Compared with the soft magnetic phase of α-Fe(Si). the Fe₃B phase has a higher anisotropic field, resulting in decreased soft magnetic properties of the SMCs [38]. The average grain size D of α-Fe(Si) annealed at 773 K is approximately 18 nm calculated by the Scherrer formula. These nano-sized grains are randomly dispersed in the residual amorphous matrix. When D < L_ex (L_ex is value of ferromagnetic exchange length, about 30~40 nm for α-Fe), with the reduction of grain size, the effective anisotropy shows a sharp decrease and the impact is negligible. The magnetic domain structure exhibits a vortex structure determined by the exchange coupling energy and demagnetization energy jointly, the coercive force and residual magnetism are close to zero. Therefore, the magnetic powders after annealing at 773 K have excellent soft magnetic properties [39,40].

3.4. DC-Bias Properties

The presence of DC current or voltage components in an AC power system is called DC bias. The DC-bias property is important for SMC since almost all of the powder cores are used in a DC-bias field. Figure 9 shows the percentage of permeability as a function of the DC-bias field for SMC cores, which were made from the powders coated with different H₃PO₄ concentrations followed by 2 wt.% sodium silicate solution and annealed at 773 K. The permeability decreases with the increase of the bias field for all cores, since the SMCs approach to magnetic saturation at high DC field. In addition, with the increase of H₃PO₄ concentrations from 0 to 0.75 wt.%, the DC-bias performance of the core, defined by the percentage of reduced permeability, increased from 75.3 to 88.2% at 4000 A/m. The results indicate that the H₃PO₄ and sodium silicate double-layer coating can significantly improve the DC bias performance of SMCs. In particular, the SMCs prepared with H₃PO₄ concentrations of 0.5 and 0.75 wt.% exhibit relatively superior DC-bias performances (over 80%). The reason is that the voids and gaps in the powder cores can pin the domain wall in the magnetizing process and prevent the propagation of domain movement between particles, then suppressing the decrease of permeability, which is beneficial in achieving a stable percent permeability [41].

3.5. Radial Crushing Properties

The SMCs based on powders coated by different H₃PO₄ concentrations and 2 wt.% sodium silicate-aqueous solution followed by annealed at 773 K is employed for radial crush testing. The test method is shown in Figure 10a, where the radial crush strength of a ring shape specimen is determined by applying radial pressure. Figure 10b shows the radial crush strength of various SMCs. The strength first increases then decreases with the increasing concentration. The Si(OH)₄ produced by hydrolysis of sodium silicate_, through dehydration and condensation between -OH can form networked SiO₂ between the magnetic powders, which can increase the binding force between the magnetic powders. When the powders coated by 2 wt.% sodium silicate-aqueous solution alone, the binding force between the powder is poor due to the weak binding force between SiO₂ and powders, which is manifested by the low radial crush strength of the SMCs. As the H₃PO₄ concentration increases, the radial crush strength increases since the rough surface of the phosphate can increase the binding force with SiO₂, while the binding force between the powders also increases. However, when the H₃PO₄ concentration exceeds 0.5 wt.%, the radial crush strength of the sample decreases, since part of the SiO₂ forms on sheets detached from phosphate layer.

4. Conclusions

The core-shell structured amorphous FeSiBCr@phosphate/SiO₂ powders were prepared by H₃PO₄ and sodium silicate treatments for fabricating high performance SMCs. The processes of phosphating, sodium silicate coating and annealing treatment have been investigated in detail. A complete and uniform phosphate/SiO₂ doubled layer has been prepared on the amorphous powder by 0.5 wt.% H₃PO₄ followed by 2 wt.% sodium silicate treatments. When the H₃PO₄ concentration exceeds to 0.5 wt.%, the phosphate sheets were detached from phosphate layer, leading to poor powder coating efficiency and poor performance of SMC. After annealing at 773 K, the SMCs show improved permeability and greatly reduced core loss due to the effectively released residual stress. However, when the annealing temperature is 798 K, the soft magnetic properties including permeability and core loss were deteriorated due to the increasing precipitation of Fe₃B phase. The excellent magnetic properties with permeability μ_e = 33.5 and core loss P_s = 368 kW/m³ at 50 mT and 200 kHz have been achieved in the SMC prepared by optimized process. i.e., 0.5 wt.% H₃PO₄ -ethanol solution and 2 wt.% sodium silicate-aqueous solution treatment followed by 773 K annealing, which promises great potential applications of electronic components at medium and high frequencies.