Effects of the Gas-Atomization Pressure and Annealing Temperature on the Microstructure and Performance of FeSiBCuNb Nanocrystalline Soft Magnetic Composites

Abstract

FeSiBCuNb powders prepared by the gas atomization method generally exhibit a wide particle size distribution and a high degree of sphericity. In addition, the correspondingly prepared nanocrystalline soft magnetic composites (NSMCs) perform good service stability. In this paper, effects of the gas-atomization pressure and annealing temperature on the microstructure and soft magnetic properties of FeSiBCuNb powders and NSMCs are investigated. The results show that the powders obtained by a higher gas-atomization pressure possess a larger amorphous ratio and a smaller average crystallite size, which contribute to the better soft magnetic performance of the NSMCs. After being annealed at 550 °C for 60 min, the NSMCs show a much better performance than those treated by the stress-relief annealing process under 300 °C, which indicates that the optimization of the soft magnetic properties resulting from the precipitation of the α-Fe(Si) nanocrystalline largely overwhelms the deterioration caused by the grain growth of the pre-existing crystals. In addition, the annealed NSMCs prepared by the powders with the gas-atomization pressure of 4 MPa show the best performance in this work, μ_e = 33.32 (f = 100 kHz), H_c = 73.08 A/m and P_cv = 33.242 mW/cm³ (f = 100 kHz, B_m = 20 mT, sine wave).

1. Introduction

FeSiBCuNb nanocrystalline soft magnetic composites (NSMCs) show excellent soft magnetic properties, such as a high saturation magnetic induction (B_s) and permeability (μ_e), and low coercivity (H_c) and loss (P_cv). They are also widely used in electronic devices such as sensors, inductors, and transformers [1,2,3]. Compared with flaky powders prepared after melt-spinning and strip breakage, the gas-atomized powders show a slightly lower amorphous ratio, but slightly higher sphericity, wider particle size distribution and lower preparation energy consumption [4]. As the spherical powders are insulation coated easily, the correspondingly prepared NSMCs generally have good stability, and a low eddy current loss (P_e) and magnetostrictive coefficient (λ_s) [5]. Furthermore, the wide particle size distribution of the gas-atomized powders makes them pile up easily, which is beneficial in improving the μ_e [6].

The parameters of gas atomization play a key role in the preparation of soft magnetic powders with high quality. Shi et al. [7] found that, with the increasing gas-atomization pressure, the gas kinetic energy per unit mass of melt increased, and the average particle size of the obtained FeCrMoNiPBCSi powders were smaller. Gao et al. [8] studied the effects of the melt flow rate, gas-atomization pressure and melt superheat on the AlSi10Mggas-atomization powders. They showed, that with a larger gas-atomization pressure and a higher melt superheat, the melt flow rate tended to be smaller, which led to the smaller average particle size of the powders. Alvarez et al. [9] studied the relationship between the cooling rate and the gas atomization process parameters, such as the droplet temperature, gas temperature and thermal conductivity and particle size of powders. Ciftci et al. [10] found that the gas consumption and the crystalline fraction of the FeCoPSiNb powders could be reduced by using a higher gas temperature. However, there are few studies that focus on the influence of the gas-atomization pressure on the microstructure of FeSiBCuNb powders as well as on the soft magnetic properties of the corresponding NSMCs systematically.

Nanocrystalline soft magnetic materials are generally obtained by annealing amorphous soft magnetic materials at an appropriate temperature. The annealing process also has an effective effect on the soft magnetic properties of nanocrystalline materials [11,12]. Zhao et al. [13] systematically studied effects of the annealing temperature on the B_s and H_c of FeSiBCuNbP nanocrystalline powders and the μ_e and P_cv of the corresponding NSMCs. The results showed that, when the annealing temperature was between 400 °C and 500 °C, only the α-Fe soft magnetic nanocrystalline phase precipitated, and the NSMCs showed good comprehensive soft magnetic properties. However, when the annealing temperature was 550 °C, the comprehensive soft magnetic properties deteriorated sharply due to the precipitation of the hard magnetic phase. Meng et al. [14] systematically studied the effect of annealing time on the properties of the nanocrystalline Fe₈₃Si₄B₁₀P₂Cu₁ ribbons. Li et al. [15] studied the effect of annealing time on the properties of the Fe-B-P-C-Cu nanocrystalline alloy. They found that, when the annealing time was too long, the grain size increased excessively although the residual stress could be released, which resulted in the increase of H_c and the decrease of μ_e. Luo et al. [16] systematically studied the effect of magnetic-field annealing on the properties of FeCoSiBCu amorphous powders. They found that magnetic-field annealing could refine grains by increasing the nucleation rate of nanocrystalline grains, which could improve the soft-magnetic properties. It is worth noting that the cooling rate during the gas atomization process is not high enough. In addition, in most cases, the obtained powders are composed of nanocrystalline and amorphous phases. Thus, both crystallization of the amorphous matrix and grain growth of the nanocrystalline may occur during the annealing process, where the latter is harmful to the soft magnetic properties of the NSMCs. However, there is little research in this regard.

In this work, FeSiBCuNb spherical powders are prepared by the gas atomization method. Effects of the gas-atomization pressure on the microstructure and soft magnetic properties of FeSiBCuNb powders are systematically and quantitatively studied. Then, the corresponding NSMCs are prepared. The effects of the gas-atomized powders with different average particle sizes and amorphous ratios on the soft magnetic properties of FeSiBCuNb NSMCs, as well as effects of the annealing temperature on the soft magnetic properties of the NSMCs, are then investigated.

2. Experimental Procedure

2.1. Preparation of the Powders

The commercial 1K107 Fe_73.5Si_13.5B₉Cu₁Nb₃ bulk alloy is used as raw material. The raw alloy ingot is supplied by Advanced Technology (Bazhou, China) Special Powder Co., ltd. The smelting temperature is 1200 °C. Both the smelting and gas-atomization processes are carried out in an argon atmosphere. The gas-atomization pressures are selected as 2 MPa, 3 MPa and 4 MPa, respectively. Fine powders passed through a 325-mesh sieve are used for the study. For convenience, the powders without annealing treatment are named as P2-raw, P3-raw, and P4-raw. A portion of the powders are annealed at 300 °C or 550 °C. Accordingly, the powders after annealing treatment are named as PX-Y, where X and Y refer to the corresponding gas-atomization pressure and the annealing temperature, respectively.

2.2. Preparation and Heat-Treatment of the NSMCs

The fine powders are made into the NSMCs. REN60 silicone resin is selected as the insulating coating agent and adhesive, and acetone is selected as the solvent. Firstly, 3 wt.% REN60 silicone resin is uniformly dispersed in acetone by ultrasonic and mechanical stirring. Secondly, the powders are added to the mixed solution and stirred continuously until the acetone is completely volatilized. Then, the insulated powders are put into the mold after drying in a 60 °C oven for 60 min. The annular NSMCs are obtained under 1400 MPa. The pressurization rate is 360 MPa/min and the holding time is 1 min. Lastly, the NSMCs are cured at 180 °C for 60 min. Similar to the naming method of powders, the NSMCs are named as CX-Y.

2.3. Characterization Techniques

The particle size distribution of the powders is tested using a laser particle size analyzer (Bettersize2600, Bettersize Instruments Ltd., Dandong, China). The phase microstructure is analyzed using X-ray diffraction (Rigaku D/max-rB, Cu Kα). Differential scanning calorimetry (TA Instruments, New Castle, DE, USA) is used to characterize the crystallization behavior of powders with a heating rate of 40 °C/min. A scanning electron microscope (Hitachi Regulus 8100, Chiyoda, Japan) is used to test the morphology of the powders. A soft magnetic direct current measuring instrument (DSMC-8200SD, Loudi, China) is used to test the H_c of the NSMCs. The μ_e and DC-bias performance of the NSMCs are tested by a Lenz capacitor resistance meter (LCR-8210, Taiwan, China, 10 kHz~1 MHz). The P_cv of the NSMCs is measured by a soft magnetic alternating current analyzer (MAST-3000SA, Loudi, China, 10 kHz~1 MHz) at 20 mT.

3. Results and Discussion

3.1. Structure and Soft-Magnetic Properties of the Gas-Atomized Powders

The particle size distribution and the symbolic size of the powders with different gas-atomization pressures are shown in Figure 1. One sees from Figure 1 that the FeSiBCuNb powders prepared at a higher gas-atomization pressure not only are finer, but also show a narrower particle size distribution. With the increasing gas-atomization pressure, the gas kinetic energy per unit mass of melt becomes higher. As a result, the crushing of melt is more sufficient, and the average particle size of the powders is finer. P4-raw is shown to be the finest powder, with a D₅₀ = 15.19 μm.

All the SEM images of the powders are shown in Figure 2, where the SEM images for all of the raw powders made by different gas-atomization pressures with both low and high magnifications are supplemented. The SEM images of P2-raw, P3-raw, and P4-raw at low magnification are shown in Figure 2a–c, respectively, while the corresponding images with high magnification are shown in Figure 2d–f, respectively. One sees from Figure 2 that the powders possess excellent sphericity and a smooth surface, which indicates that the powders can easily be coated, and the correspondingly prepared NSMCs should have good stability and relatively low loss. Moreover, there are almost no satellite powders around.

To identify the contained phases in the FeSiBCuNb powders, the XRD patterns are shown in Figure 3. One sees from Figure 3 that there is a broad diffuse peak corresponding to the amorphous phase, while sharp peaks corresponding to the (110), (200), and (211) crystal planes of α-Fe (Si) appear distinctly at 2θ = 44.8°, 65.3°, and 82.7°, respectively [13]. With the increasing gas-atomization pressure, the intensity of these three peaks decreases, indicating the decrease of the crystalline ratio. In addition, when the pressure is 4 MPa, there is only a very small diffraction peak at 2θ = 44.8°. To quantify the results of the XRD pattern, both the volume fraction of the amorphous phase (V_am) and the average crystallite size (d) are estimated according to Equation (1) [17] and Equation (2) [18], respectively.

$$V_{\mathrm{a}\mathrm{m}}=\frac{I_{\mathrm{a}\mathrm{m}}}{I_{\mathrm{c}\mathrm{r}}+I_{\mathrm{a}\mathrm{m}}}$$

(1)

$$d=\frac{K\lambda }{\beta \cos \theta }$$

(2)

where I_am and I_cr are integral intensities of the diffraction peaks of the amorphous phase and crystalline phase, respectively, K is the shape factor equal to 0.94, λ is the wave-length of the X-ray used, β is the full width at half maxima, and θ is the angle between the incident and the scattered X-ray. The calculated V_am values of P2-raw, P3-raw, and P4-raw are about 56%, 62%, and 92%, respectively, while the estimated d are 36 nm, 28 nm and 22 nm, correspondingly.

In order to find the proper annealing temperature, the crystallization behavior of the powders is shown in the DSC curves in Figure 4. One sees that there are two peaks in each DSC curve. The first exothermic peak corresponds to the precipitation of the α-Fe (Si) phase, while the second peak corresponds to the hard-magnetic phase, including the Fe-boron phases [19,20,21,22,23]. It can be seen that the gas-atomization pressure has little effect on the onset temperatures (T_x1, T_x2) and the peak temperatures (T_p1, T_p2) for both the primary process and the secondary crystallization, as the position of the peaks barely changes with the increasing gas-atomization pressure. Similarly, the gas-atomization pressure does not effectively affect the thermal stability of the powders. Furthermore, the temperature intervals between two peaks for all the powders are relatively large (>135 °C), indicating the good nanocrystallization-forming ability of the FeSiBCuNb system to some extent [24,25].

The hysteresis loops of the powders are shown in Figure 5. It is well known that the microstructure, noticeably the crystallite size, essentially determines the hysteresis loop of a ferromagnetic material. While d is lower than 100 nm, the H_c of the powders is positively correlated with d⁶ [26]. One sees that the H_c of P2-raw, P3-raw, and P4-raw are 1407.1 A/m, 721.9 A/m, and 288.2 A/m, respectively. The H_c of materials can be simply represented as follows [27]:

$$H_{\mathrm{c}}=\frac{P_{\mathrm{c}}{K}_1{d}^6}{J_{\mathrm{s}}{A}^3}$$

(3)

where P_c is a constant, J_s is the saturation magnetization, K₁ is the magneto-crystalline anisotropy constant, and A is the ferromagnetic exchange constant between adjacent grains. As mentioned above, the powders prepared with a higher gas-atomization pressure show a smaller d. Thus, according to Equation (3), the H_c of the powders should decrease sharply with the increase of the gas-atomization pressure, which is in accordance with the results shown in Figure 5. The B_s of P2-raw, P3-raw, and P4-raw are 1.25 T, 1.18 T, and 1.22 T, respectively. There is no significant association between the gas-atomization pressure and B_s, since B_s is mainly related to the content of the ferromagnetic elements.

3.2. Soft-Magnetic Properties of the NSMCs

After preparing the gas-atomized powders into the NSMCs, the effects of the annealing temperature on the soft magnetic properties of the NSMCs are investigated. The dependence of frequency (f) on μ_e for the NSMCs is shown Figure 6a. One sees that all the samples exhibit excellent high-frequency stability of the μ_e, which keeps at a constant value below 100 kHz. One sees from Figure 6 that, after the heat treatment, the μ_e of FeSiBCuNb NSMCs prepared by the gas-atomized powder with a higher gas-atomization pressure is shown to be larger. The μ_e of FeSiBCuNb NSMCs also increases with the increasing annealing temperature. Figure 6b shows the μ_e of the NSMCs at f = 100 kHz. Among all the samples, the C4-550 has the highest μ_e, which is 33.32 (f = 100 kHz).

As is known, μ_e is closely related to residual stresses, defects, and the content of non-magnetic materials, [28] which can be expressed by Equation (4).

$${\mu }_{\mathrm{e}}=\frac{B_{\mathrm{s}}{}^2}{a{K}_1+b{\lambda }_{\mathrm{s}}\sigma }=\frac{{\mu }^{\prime }d}{{\mu }^{\prime }c+d}$$

(4)

where a and b are constants, $\sigma$ is the residual stress, ${\mu }^{\prime }$ is the intrinsic permeability of the soft magnetic powders, and c is the non-magnetic material content. The non-magnetic material content mainly includes the air gaps and defects between powders during forming and the organic insulating coating agent, since these materials contain no magnetic elements. Generally, the c is mainly related to the electrical resistivity of the NSMCs.

According to Equation (4), the μ_e of the FeSiBCuNb NSMCs with a smaller d should be higher, which is in accordance with the results shown in Figure 3 and Figure 6. Meanwhile, as the powders under a higher gas-atomization pressure show a smaller average particle size (see Figure 1), they pile up more easily. Thus, there is less air gap in the NSMCs made by the finer powders, which reduces the c and then causes the increase of μ_e to some extent. In other words, under the same annealing treatment, the higher μ_e of FeSiBCuNb NSMCs made by the powders with a higher gas-atomization pressure should be attributed to the lower d and c.

In order to discover the effect of annealing treatment on the d of the NSMCs as well as to exclude the effect introduced by the insulation coating agent, the XRD patterns of the gas-atomized powders with different annealing treatments are shown in Figure 6. Comparing Figure 7a with Figure 3, one sees that the amorphous ratio and the d of the powders barely change after being annealed at 300 °C, where the slightly decreased amorphous ratio and slightly increased d result from the grain growth of the pre-exist nanocrystalline in the amorphous matrix. However, comparing Figure 7a with Figure 7b, one sees that the amorphous ratio decreases to 0% and the d decreases sharply for all the powders after being annealed at 550 °C, which indicates that large amounts of α-Fe (Si) nanocrystalline grains with a smaller d have precipitated from the amorphous matrix, though further grain growth of the pre-existing nanocrystalline is also promoted.

It is known that annealing at an appropriate temperature can not only release the residual stress caused by crystallization and cold pressing, but also promote the precipitation of nanocrystalline. Relief of the residual stress in the NSMCs reduces the pinning effect and the magnetic anisotropy, which can reduce λ_s and then improve μ_e [10], while the precipitation of nanocrystalline effectively increases the μ_e, as λ_s of the α-Fe (Si) nanocrystalline is lower than that of the amorphous matrix. Thus, it can be speculated that the increased μ_e of the NSMCs after being annealed at 300 °C should mainly result from the relief of the residual stress, since both the amorphous ratio and the d barely change compared with those without annealing treatment. Comparing those after annealing at 300 °C, the largely increased μ_e of the NSMCs after annealing at 550 °C should be mainly attributed to the obviously decreased d of the gas-atomized FeSiBCuNb powders, according to Equation (4). Furthermore, with the increasing annealing temperature, the evaporation of the insulation coating agent causes the reduction of c to some extent, which also leads to the increase of μ_e.

The DC-bias performance of the NSMCs is shown in Figure 8. One sees that the NSMCs made by the powders with a higher gas-atomization pressure show a lower DC-bias performance. However, the DC-bias performance worsens with the increasing annealing temperature, which is opposite to the trend for μ_e, as the higher the μ_e is, the greater the magnetic induction intensity of the NSMCs will be under the same applied magnetic field. In turn, it is easier for the magnetic induction to reach saturation. When the applied magnetic field is 100 Oe without annealing, the best DC-bias performance in this study can reach 85%, corresponding to C2-raw (see Figure 8), while the worst is about 57%, as the precipitation of the nanocrystals in C4-550 tends to largely increase the μ_e. The NSMCs with excellent DC-bias performance can be widely used in a high current field.

The dependence of the frequency on the H_c and the P_cv of the NSMCs is shown in Figure 9a,b, respectively. One sees from Figure 9 that a similar trend can be found between H_c and P_cv. In addition, both the H_c and P_cv of the NSMCs prepared by the powders with a higher gas-atomization pressure are shown to be smaller under the same annealing treatment. Compared with those without heat treatment, the P_cv of FeSiBCuNb NSMCs decreases after being annealed at 300 °C, and further decreases after being annealed at 550 °C. As is known, P_cv includes hysteresis loss (P_h), P_e and residual loss (P_r) [29]. In addition, P_r generally results from magnetization relaxation and resonance of the domain walls, which can be ignored in most cases [30]. Thus, the P_cv of the NSMCs can be represented as follows:

$$P_{\mathrm{cv}}=P_{\mathrm{h}}+P_{\mathrm{e}}=K_{\mathrm{h}}{B}_{\mathrm{m}}^{1.6}f+K_{\mathrm{e}}{B}_{\mathrm{m}}^2{f}^2$$

(5)

where K_h and K_e are the hysteresis loss coefficient and the eddy current loss coefficient, respectively, and B_m is the maximum magnetic induction strength. Due to the presence of the insulation coating, the P_e of the NSMCs is relatively small. As a result, the P_h is dominant for the P_cv of the NSMCs, and the K_h or P_h is proven to be positively correlated with the H_c of the NSMCs. According to Equation (3), The H_c decreases with the decreasing d. Thus, under the same annealing treatment, the lower P_cv of the FeSiBCuNb NSMCs made by the powders with a higher gas-atomization pressure should be attributed to the lower H_c of the raw powders, as these powders show a smaller d and higher V_am. Compared with the NSMCs without annealing, both H_c and P_cv of the NSMCs decrease after being annealed at 300 °C, and decrease more after being annealed at 550 °C. As mentioned previously, the stress-relief annealing treatment at 300 °C only has a slight effect on grain growth but largely reduces the residual stress in the NSMCs, which leads to the reduction of the K_h and H_c of the NSMCs. Meanwhile, the nanocrystallization annealing treatment under 550 °C can not only further reduce the residual stress but also promote the precipitation of α-Fe (Si) nanocrystals from the amorphous matrix, which in turn further decreases the H_c and P_cv.

Figure 8. The DC-bias characteristics of the NSMCs.

Figure 8. The DC-bias characteristics of the NSMCs.

4. Conclusions

In this work, effects of the gas-atomization pressure on the size distribution, microstructure and magnetic properties of FeSiBCuNb powders, as well as the effects of annealing temperature on the soft magnetic properties of the corresponding NSMCs, are systematically studied. The following conclusions are as follows:

(1)

The obtained powders contain the amorphous phase and the α-Fe (Si) phase. With the increasing gas-atomization pressure, the soft-magnetic properties of the corresponding powders and NSMCs tend to be better, which can be attributed to the smaller d and the larger amorphous ratio of the powders. The powders prepared by the 4 MPa gas-atomization pressure without annealing treatment show the highest amorphous ratio of 92% in this study.

(2)

After annealing treatment, the μ_e of NSMCs increases compared with the raw NSMCs, while H_c and P_cv decrease. In addition, the nanocrystallization annealing treatment at 550 °C can largely optimize the soft magnetic properties of the NSMCs, which is better than the stress-relief annealing treatment at 300 °C. It can thus be suspected that the improvement of soft magnetic properties resulting from the precipitation of the α-Fe(Si) nanocrystals largely overwhelms the deterioration caused by the grain growth.

(3)

After being annealed at 550 °C, the NSMCs made by the powders using the 4 MPa gas-atomization pressure show the best performance among this work with μ_e = 33.32 (f = 100 kHz), H_c = 73.08 A/m, and P_cv = 33.242 mW/cm³ (f = 100 kHz, B_m = 20 mT, sine wave), resulting from the stress relief and the smallest d of the precipitated α-Fe (Si) nanocrystalline.