Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12 − xPx nanocrystalline soft-magnetic alloys

Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12 − xPx nanocrystalline soft-magnetic alloys

Thin Solid Films 519 (2011) 8283–8286 Contents lists available at ScienceDirect Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e...

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Thin Solid Films 519 (2011) 8283–8286

Contents lists available at ScienceDirect

Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12 − xPx nanocrystalline soft-magnetic alloys A.D. Wang a, H. Men a, B.L. Shen a,⁎, G.Q. Xie b, A. Makino b, A. Inoue b a Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 519 Zhuangshi Road, Ningbo 315201, China b Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan

a r t i c l e

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Available online 6 April 2011 Keywords: Soft-magnetic material Fe-based Nanocrystalline alloy High Fe content Grain size

a b s t r a c t The P content dependences of the crystallization behavior, thermal stability and soft-magnetic properties of high Fe content Fe83.3Si4Cu0.7B12 − xPx (x= 0 to 8) nanocrystalline soft-magnetic alloys were investigated. P addition is very effective in widening the optimum annealing temperature range and refining of bcc-Fe grain size in addition to the increasing of nanocrystalline grain density. Uniform nanocrystalline bcc-Fe grains with average size of about 20 nm and number density of 1023–1024 /m3 were prepared at around x = 6–8 for the annealed Fe83.3Si4Cu0.7B12 − xPx alloys. The coercivity Hc markedly decreases with increasing x and exhibits a minimum at around x = 6–8, while the saturation magnetic flux density Bs shows a slight decrease. Fe83.3Si4Cu0.7B6P6 nanocrystalline alloy exhibits excellent soft-magnetic properties with a high saturation magnetic flux density Bs of 1.77 T, low coercivity Hc of 4.2 A/m and high effective permeability μe of 11,600 at 1 kHz. © 2011 Elsevier B.V. All rights reserved.

1. Instruction Fe-based nanocrystalline soft-magnetic materials have aroused wide interests as soon as the first development of Fe73.5Cu1Nb3Si13.5B9 alloy by crystallizing amorphous ribbons prepared using single roller melt-spinning method [1–3]. Compare with the conventional widely used Si-steel, Fe-based nanocrystalline soft-magnetic materials exhibit excellent soft-magnetic properties such as quite low coercivity (Hc), high permeability (μ e) and low core loss [4–8]. Therefore, the representative nanocrystalline soft-magnetic alloys Fe–Cu–Nb–Si–B (Finemet) and Fe–Zr–B (Nanoperm) have been widely used for industrial applications [4,9–11]. Nevertheless, with the aim of realization of miniaturization and higher efficiency of electrical machinery and apparatus, soft-magnetic materials should be further improved. New nanocrystalline soft-magnetic materials with high saturation magnetic flux density Bs and low core loss are strongly desired. Since the amorphous precursor is needed in preparation traditional Fe-based nanocrystalline alloys [1,12,13]. The Fe content of the developed nanocrystalline soft-magnetic alloys is always low because a large amount of metal and metalloid elements such as Nb, Zr, Mo, B, C, P and Si are added to prepare amorphous precursor and realize uniform nanocrystalline structure [4,11,14]. The Bs of the conventional Fe-based nanocrystalline alloys is at most 1.7 T. It is well known that high Bs competitive with Si-steel can be obtained in FeCo-based nanocrystalline

⁎ Corresponding author at: Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail address: [email protected] (B.L. Shen). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.03.110

alloys by the substitution of Co for Fe in Fe-based nanocrystalline alloys [15]. However, the Co addition increases material cost and Hc markedly. Therefore, high Fe content Fe-based nanocrystalline soft-magnetic alloys without metal elements addition other than Cu are desirable for practical use. According to these viewpoints, the recently reported FeSiBCu and FeSiBPCu soft-magnetic alloys with high magnetic flux density of about 1.8 T and 1.9 T respectively [16,17]. Owing to the strong magnetic coupling interaction between the fine crystalline grains through the remaining ferromagnetic amorphous phase, the magnetocrystalline anisotropy is low and the alloys exhibit good soft-magnetic properties [7,18,19]. In this study, as Bs basically depends on Fe content, high Fe content Fe83.3Si4B12 − xPxCu0.7 nanocrystalline soft-magnetic with different P content (x= 0–8) were synthesized. The effect of P addition on thermal stability, crystallization behavior, and microstructure were explored as well as the dependence of P content on soft-magnetic properties. Whereafter the alloy compositions were optimized with the aim of further improving the soft-magnetic properties. 2. Experiment Muticomponent alloy ingots with the nominal compositions of Fe83.3Si4B12 − xPxCu0.7 (x = 0–8) were prepared by induction melting mixtures of Fe (99.99 mass %), Si (99.99 mass %), B (99.5 mass %), Cu (99.99 mass %), and premelted Fe3P (99.9 mass %) in a high purity argon atmosphere. Amorphous ribbons with about 20 μm in thickness and approximately 1 mm in width were produced by single-roller melt-spinning method. Thermal properties of amorphous ribbons were evaluated with a differential scanning calorimeter (DSC) at a

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heating rate of 0.67 K/s under high purity argon flow. The amorphous specimens were subjected to annealing at various temperatures for series time in a pipe furnace under argon atmosphere. The microstructures of the melt-spun and annealed ribbons were identified by X-ray diffractometry (XRD) with Cu Kα radiation. Some annealed ribbons with representative compositions and magnetic properties were identified by transmission electron microscopy (TEM). Mean grain size of a nanocrystalline grain was estimated by using Scherrer's equation from the full width at half maximum of the bcc (110) reflection peak from the specimens. Saturation magnetic flux density (Bs) was tested with a vibrating sample magnetometer (VSM) under a maximum applied field of 800 kA/m. Coercivity (Hc) were measured by using a B–H loop tracer under a field of 1000 A/m. Effective permeability (μ e) at 1 kHz was measured with a vector impedance analyzer under a field of 0.4 A/m. Density was measured by the Archimedean method with n-tridecane. All the property measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) as-spun ribbons with different P content. As can be seen from the patterns, for all as-spun ribbons, only one broad peak around 2θ = 45° could be noticed, indicating that these samples were amorphous (in XRD scale). The DSC curves measured at a heating rate of 0.67 K/s in a high purity argon flow for Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) melt-spun ribbons are shown in Fig. 2. It is clear that the replacement of B by P brings about a change in the crystallization behavior. The crystallization temperature onset (Tx1) of the first peak corresponds to the precipitation of bcc-Fe(Si) phase decreases gradually with the increasing of P content. According to a previous study, it is known that the precipitation of bcc-Fe(Si) is triggered by the Cu-enriched clusters precipitated during annealing process and/or the primary crystals in the as-quenched state [3,12,20]. Hence, in this alloy system, the replacement of B by P is benefit for the formation of a high density of nucleation sites which affects the nanocrystallization and nanostructure [21]. Tx2 can be considered as a parameter of the thermal stability of remaining amorphous state above Tx1 [22]. It has been already reported in the representative nanocrystalline soft magnetic alloys [5] that the amorphous precursor with a large Tx2–Tx1 can change to a uniform nanocrystalline structure without any compounds by annealing within the temperature region between the two peaks. The appropriate annealing temperature range for the crystallization of bcc-Fe(Si) nanocrystalline grains is also widen. Moreover, since B and P hardly solidifies in the bcc-Fe(Si) phase, the replacement of B by P in the amorphous matrix increases with increasing number

Fig. 1. XRD patterns of the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) melt-spun ribbons.

Fig. 2. DSC curves of Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) melt-spun ribbons measured at a heat rate of 0.67 K/s.

of nanocrystalline grains. This suggests that the grain growth is suppressed by the increase of P content in the amorphous matrix phase. Because melt-spun Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) alloy ribbons annealed for 2 min exhibit the lowest coercive (Hc), we focus on the 2 min annealed alloys in the present study. The annealing temperature (TA) dependence of Hc for melt-spun Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) alloy ribbons is shown in Fig. 3. The Hc dependence on TA for all the alloy ribbons annealed under 460 °C shows similar tendency, whereas that for the alloy ribbons annealed at higher temperature exhibits quite different tend lines. For all alloys, Hc decreases gradually with increasing TA up to 460 °C, and reached their minimum of about 3 A/m at TA = 460 °C. In the alloys with x = 0, 2 and 4, marked enhancement of Hc occurs at 460 °C. On the other hand, Hc shows little change and keeps a low value for the alloy with x = 6 and 8 annealed between 460 and 520 °C. Accordingly, P addition is very effective in widening the optimum annealing temperature range of the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) alloy. Since soft-magnetic property greatly relates with microstructure, it was expected that there are remarkable differences among microstructures of the alloys [8,18,23]. In order to ascertain the microstructure change during the annealing process in the alloys with different P content, all samples annealed were identified with XRD. According to the XRD patterns, no obvious crystallization happened in all samples annealed at TA b 460 °C. The decrease of Hc may attribute to the elimination of stress fields which act as pinning centers for domain walls [24]. On the

Fig. 3. Annealing temperature (TA) dependence of Hc for the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) ribbons annealed for 2 min.

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other hand, XRD patterns annealed at 480, 500 and 520 °C show clear crystallization peaks. These specimens are identified as compatible phases of bcc-Fe and amorphous phases, no other phases were confirmed. The P content dependence of the average grain size (D) estimated from Scherrer's equation is shown in Fig. 4. The D of the sample shows a marked decrease trend with the increasing of P content under x b 6. While for the alloy with x = 6 and 8, the D of nanocrystalline bcc-Fe grains is smaller than 25 nm. It has been pointed out that effective magnetocrystalline anisotropy decreases with D owing to the magnetic coupling between nanocrytalline grains through the ferromagnetic amorphous phase [8,18,23]. The increment of Hc is proportional to D6 in the present alloy system, that behavior is similar to other nanocrystalline soft-magnetic alloy systems [8,25]. Fig. 5. shows the changes of soft-magnetic properties as a function of P content for the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) nanocrystalline ribbons annealed at 520 °C for 2 min. In Fig. 5(a), as mentioned above, Hc decreases gradually with increasing x up to 4, and reached their minimum of about 4 A/m at x = 6 and 8. The P content dependence of Bs is shown in Fig. 5(b). Note that in the present alloy system, Bs decreases gradually with increasing x up to 6 and decreases obvious to lower than 1.75 T. As mentioned above, the present alloys consist of crystalline bcc-Fe(Si) and amorphous phases, B and P is main in the amorphous phase. Bs is expressed as Bs = Bsc R + Bsa(1 − R), where R, Bsc and Bsa are crystallinity (R), the saturation magnetic flux densities of the crystalline and amorphous phases, respectively. As Bsc of the bcc-Fe phase is almost constant, the decrease of Bs is expected owing to the slight reduction of crystallinity and marked decrease of saturation magnetic flux densities of amorphous phases caused by P substation of B which has been confirmed in the previous study [26]. In Fig. 5(c), effective permeability (μ e) increases obviously to more than12000 at x = 6–8. In conclusion, P content with x = 6 exhibits good soft-magnetic properties. Fig. 6 shows the TEM bright field images, selected area electron diffraction (SAED) patterns and grain size distributions of melt-spun Fe83.3Si4B12 − xPxCu0.7 alloy ribbons with x = 0 and 6 annealed at 520 °C for 2 min. From the SAED patterns, the crystalline phases are identified as bcc-Fe(Si) phase, without any compound phases. The TEM bright field image shows that the Fe83.3Si4Cu0.7 B12 alloy without P addition with relatively high Hc of 48 A/m includes coarse bcc-Fe(Si) grains with average diameter of about 35 nm. However, the TEM image for the Fe83.3Si4Cu0.7B6P6 alloy shows a uniform nanocompound structure with fine grain size of about 21 nm. The grain size distribution of the alloy with x = 6 is more uniform and the standard deviation (σ) deceases from 0.272 to 0.237. In addition, it is clear that the number density of Fe83.3Si4Cu0.7B6P6 is much higher than that of

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Fig. 5. The changes of (a) coercive force (Hc), (b) saturate magnetization (Bs) and permeability (μ e) as a function of P content for the Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) nanocrystalline ribbons annealed at 520 °C for 2 min.

the alloy without P addition, which is about 1023–1024/m3 and almost the same as the grain density of Fe–Cu–Nb–Si–B nanocrystalline alloy. Here, we discuss the reason why remarkable effect of P addition on the crystallization behavior, magnetic property and microstructure. In Fe–Cu–Nb–Si–B nanocrystalline alloy, Yoshizawa suggested that Cu clusters form prior to nanocrystallization leading to an increase in the local concentration of Fe in the vicinity of these clusters, thus leading to the formation of bcc-Fe crystals [1,2]. The Driving force for Cu-rich nucleation site is the interaction force among atoms. According to the recent studies, it was considered that the existence of primary crystals in the as-quenched state was necessary for obtaining uniform nanocompound structure alloys such as Fe–Cu–B, Fe–Cu–Si–B and Fe–Cu–Si–B–P [15,18,19]. The mixing enthalpy between the constituent elements is considered [26]. It is noted that the mixing enthalpy between Fe and Cu is positive (+13 kJ/mol), suggesting that there are repulsive interactions existing between Fe and Cu. On the other hand, attractive interactions existing between P and Cu for their mixing enthalpy is negative (−9 kJ/mol). Therefore, the addition of P is effective in facilitating the formation of Curich nucleation site and or primary crystals in melt-spinning process. During the crystallization, large amounts of the clusters could act as the nucleation site for the bcc-Fe grains which presumably results in the decrease in the bcc-Fe grain size and the improvement of the softmagnetic properties for the nanocrystalline alloys. 5. Conclusion

Fig. 4. Dependence of bcc-Fe grain size on the P content of Fe83.3Si4Cu0.7B12 − xPx (x = 0–8) alloy ribbons annealed at 480, 500 and 520 °C for 2 min.

In conclusion, the effect of P addition on the soft-magnetic properties, crystallization behavior and microstructure of high Fe content soft-magnetic nanocrystalline Fe83.3Si4Cu0.7B12 − xPx (x= 0 to 8) were investigated. As a result, Hc decreases and μe increases markedly with increasing P content, accompanied with significant reduction crystalline grain size. Excellent magnetic properties such as a low Hc of about 4.2 A/m and a high Bs of 1.77 T were obtained in the alloy with x = 6. In the present alloy system, more than 6% P addition favor nucleation of primary crystals in high number density.

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Fig. 6. TEM bright field images, selected area electron diffraction (SAED) patterns and grain size distribution of melt-spun Fe83.3Si4B12 − xPxCu0.7 alloy ribbons annealed at 520 °C for 2 min (a)x = 0 and (b)x = 6.

Acknowledgment This work was supported by the National 863 project (Grant No. 2009AA03Z214), the National Science Fund for Distinguished Young Scholars (Grant No. 50825103) and the “Hundred of Talents Program” (Grant No. KGCX-2-YW-803) by Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

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