Application of Fe₇₈Si₉B₁₃ amorphous particles in magnetorheological fluids

Abstract

Fe₇₈Si₉B₁₃ amorphous particles are applied in the preparation of silicon oil-based magnetorheological (MR) fluids. The structure and morphology of the Fe₇₈Si₉B₁₃ amorphous particles were characterized by X-ray diffraction and scanning electron microscopy, respectively. The magnetic properties of the Fe₇₈Si₉B₁₃ amorphous particles and the MR fluids prepared with Fe₇₈Si₉B₁₃ amorphous particles were characterized by vibrating sample magnetometry. The MR effect of the MR fluids was investigated with a rotational rheometer. The results of VSM show that the saturation magnetization of Fe₇₈Si₉B₁₃ amorphous particles is 169.48 emu g⁻¹, accompanying low remanence and low coercivity. The results of rotational rheometry manifest that the MR fluids with Fe₇₈Si₉B₁₃ amorphous particles have an improved magnetorheological response compared with magnetic fluids with Fe₃O₄ nanoparticles under an applied magnetic field.

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Introduction

Magnetic fluids have received widespread attention, especially for their magnetorheological properties. Magnetic fluids are widely used in many fields such as sealing,^1,2 target drug delivery,^3,4 lubrication,⁵ removal of water pollutants,⁶ dampers,^7,8 brakes.⁹ It has been known since the mid-20th century that there are two main types of magnetic fluids: ferrofluids and MR fluids.¹⁰ Ideal ferrofluids are stable colloidal suspensions with ultrafine (5–10 nm) magnetic particles, and magnetic fluids are usually named MR fluids when magnetic particles are on the micrometer scale.^10,11

The magnetic performance of a magnetic fluid is mainly determined by the magnetic particles and the application field of the magnetic fluid is closely related with the property of the liquid carrier. Silicone oil is frequently chosen as the liquid carrier due to its excellent thermostability and low volatility as well as its low viscosity-temperature coefficient.^12–14 Fe₃O₄ particles are popularly selected to prepare magnetic fluids due to Fe₃O₄ particles own numerous superiorities such as easy preparation, low cost and good biological compatibility. Nevertheless, Fe₃O₄ particles have low magnetic energy-density per unit volume, resulting in low saturation magnetization of magnetic fluids with Fe₃O₄ particles, which in turn limits the magnetorheological response of MR fluids under an applied magnetic field. The saturation magnetization of Fe₃O₄ particles is commonly no more than 80 emu g⁻¹.^15–20 However, magnetic fluids with higher saturation magnetization are necessary for many applications.^8,21–23 For instance, MR brake is a device to transmit torque by the shear stress of MR fluids and the shear stress is determined by the saturation magnetization of magnetic particles. MR fluids with carbonyl iron particles were used to make MR brake.²³ However, the remanence and coercivity are high in their research. Of course, there are high grade carbonyl iron particles to be used to prepare MR fluids.^24,25

Magnetic particles are expected to be synthesized with high saturation magnetization, low remanence and low coercivity simultaneously. The Fe-based amorphous materials have broad prospects due to the low cost of raw materials together with their unique magnetic properties.^15,26,27 Application of Fe-based amorphous alloys for efficient transformer cores is becoming more widespread due to the excellent soft magnetic properties.²⁸ The perfect soft magnetic properties of Fe-based amorphous particles make it have development potential in the application of magnetic fluids.¹⁵ Nevertheless, there are few researches that apply amorphous particles in the preparation of MR fluids.

We prepared silicon oil-based MR fluids with Fe₇₈Si₉B₁₃ amorphous particles. The experiment results prove that Fe₇₈Si₉B₁₃ amorphous particles and MR fluids with Fe₇₈Si₉B₁₃ amorphous particles have high saturation magnetization together with low remanence and low coercivity. Moreover, the viscosity of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles shows faster average growth rate and MR fluids with Fe₇₈Si₉B₁₃ amorphous particles show stronger magnetic response than that of magnetic fluids with Fe₃O₄ particles under an applied magnetic field.

Experimental

Preparation of magnetic particles and magnetic fluids

Fe₇₈Si₉B₁₃ amorphous particles were made by two different methods. Fe-based alloy ingots with the nominal composition of Fe₇₈Si₉B₁₃ were prepared by high frequency induction electric furnace protected by purity argon atmosphere. The ingots were remelted three times and stirred by graphite rod. The Fe₇₈Si₉B₁₃ ribbons were produced by single-roller melt spinning method onto a copper wheel with circumferential speed of 37 m s⁻¹ at an argon atmosphere. Then the Fe₇₈Si₉B₁₃ ribbons were milled for 24 h and 84 h with a rotation speed of 360 rpm by high-energy planetary ball mill. This kind of amorphous particle is called as ribbon amorphous particle in this work. The other amorphous particles were made by the following method: the Fe₇₈Si₉B₁₃ ingots were remelted three times by high frequency induction electric furnace protected by purity argon atmosphere. The Fe₇₈Si₉B₁₃ particles were produced by high pressure argon gas atomisation at a pressure of 7 MPa. The particles were sieved and the sizes below 10 μm were used. Then the sprayed particles were milled for 24 h, 60 h and 84 h with a rotation speed of 360 rpm by high-energy planetary ball mill. This kind of amorphous particle is called as spraying amorphous particle. The Fe₃O₄ particles were obtained by chemical coprecipitation method. And the black Fe₃O₄ particles were obtained according to the following reaction:

Fe²⁺ + 2Fe³⁺ + 8NH₃·H₂O = Fe₃O₄ + 8NH₄⁺ + 4H₂O

Certain amounts of the spraying amorphous particles and Fe₃O₄ particles were firstly dispersed in deionized water (100 mL). Then a certain amount of oleic acid (0.8 mL) was added to coat on the surface of the particles with mechanical stirring and supersonic dispersion for 40 min. Next, the coated particles were dispersed in silicon oil with mechanical stirring and supersonic dispersion for about 1.5 h, respectively. The processes were operated at around 50 °C. Ultimately, the two kinds of magnetic fluids with a volume concentration (5%) were prepared.

Characterization

The structure of magnetic particles was identified by X-ray diffraction (XRD; Rigaku D/MAX-2500/PC) carried out with D/max-rB, using Cu Kα radiation, and differential scanning calorimeters (DSC; NETZSCH, DSC 404). The morphology and size of the Fe₇₈Si₉B₁₃ amorphous particles were characterized by scanning electron microscope (SEM; JAPAN, SU-70) and the morphology of the Fe₇₈Si₉B₁₃ amorphous particles before and after coating process was analyzed by transmission electron microscopy (TEM; JAPAN, JEOL, JEM-2100F). The magnetic hysteretic curves of the particles and magnetic fluids were measured by a vibrating sample magnetometer (VSM; United States, LDJ, LS7307–9309) at room temperature. The MR effect of the MR fluids was investigated with rotational viscometer (Theta industries, Inc. USA DV-III) and rotational rheometer (Anton Paar Germany GmbH, Physica MCR301).

Results and discussion

The XRD patterns of the three kinds of particles are shown in Fig. 1. From Fig. 1(a), it can be seen that only one broad diffraction peak in the 2θ = 45° is detected, showing an amorphous structure, without any crystalline peaks. From Fig. 1(b), it can be seen that the particles are only partial transformation to the amorphous state. With increasing milling time, the XRD diffraction intensity decreases and the XRD characteristic peak broadens. The XRD patterns show that the initially sharp diffraction lines of Fe are considerably broadened and an amorphous structure is seen to emerge from the initial diffraction peaks as the milling process goes on. This is due to the high speed collision process among steel balls, tank wall and particles in the progress of milling, resulting in a decrease in the size of the particles and a formation of the abundant defects in the particles. This provides the channel for alloying elements transfer, ultimately leading to the disorder of the particles and enhancement of the amorphous characteristics. Fig. 1(c) is the XRD pattern of Fe₃O₄ particles as it indicates a series of diffraction peaks for (220), (311), (400), (422), (511) and (440) planes. There are no obvious impurity peaks and the samples can be indexed to the cubic spinel structures.

Fig. 1 The XRD patterns of (a) Fe₇₈Si₉B₁₃ ribbons and Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 24 h and 84 h (b) Fe₇₈Si₉B₁₃ spraying amorphous particles and spraying amorphous particles after being milled for 24 h, 60 h and 84 h (c) Fe₃O₄ particles.

The DSC curves of the Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 24 h and 84 h are shown in Fig. 2. The two exothermic peaks during heating progress demonstrate two stages crystallization of the amorphous particles, which reveal that the particles remain amorphous even after being milled for 24 h and 48 h. The results correspond well with the XRD patterns of Fe₇₈Si₉B₁₃ ribbon amorphous particles as mentioned in Fig. 1(a).

Fig. 2 DSC curves of Fe₇₈Si₉B₁₃ ribbon amorphous particles with a heating rate of 20 K min⁻¹.

Fig. 3(a), (c) and (e) show the SEM images of Fe₇₈Si₉B₁₃ spraying amorphous particles and Fe₇₈Si₉B₁₃ ribbon amorphous particles, with a typical size in the range of 4–10 μm. Fig. 3(a) is the image of Fe₇₈Si₉B₁₃ spraying amorphous particles and the particles are regular spheres. The SEM images demonstrate that the change of the size of Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h is little. Fig. 3(b), (d) and (f) are the EDS spectrograms corresponding to each picture in Fig. 3(a), (c) and (e). The spectrograms show that the element content almost remains unchanged. TEM images of Fe₇₈Si₉B₁₃ spraying amorphous particles before and after the coating process are shown in Fig. 3(g) and (h).

Fig. 3 The SEM images and the SEM-EDX spectrograms of (a) and (b) Fe₇₈Si₉B₁₃ spraying amorphous particles (c) and (d) Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h and (e) and (f) Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 84 h; (g) and (h) TEM images of Fe₇₈Si₉B₁₃ spraying amorphous particles before and after the coating process.

Fig. 4 shows the magnetic properties of (a) Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 24 h and 84 h and (b) Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 0 h, 24 h, 60 h and 84 h. It turns out that the saturation magnetization of Fe₇₈Si₉B₁₃ ribbon amorphous particles and Fe₇₈Si₉B₁₃ spraying amorphous particles decreases slightly with increasing milling time.

Fig. 4 The magnetic hysteresis curves of (a) Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 24 h and 84 h and (b) Fe₇₈Si₉B₁₃ spraying amorphous particles and spraying amorphous particles after being milled for 24 h, 60 h and 84 h.

The magnetic hysteresis curves of Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 84 h, Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h and Fe₃O₄ nanoparticles are shown in Fig. 5. The results indicate that the saturation magnetization of Fe₇₈Si₉B₁₃ spraying amorphous particles and Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 84 h are 167.01 emu g⁻¹ and 169.48 emu g⁻¹, respectively, while the saturation magnetization of traditional Fe₃O₄ particles is just 46.89 emu g⁻¹ in our research. What's more, the remanence and coercivity of Fe₇₈Si₉B₁₃ spraying amorphous particles are just 2.5 emu g⁻¹ and 36 Oe, respectively. At the same time, the remanence and coercivity of Fe₇₈Si₉B₁₃ ribbon amorphous particles are just 1.4 emu g⁻¹ and 19 Oe. The possible reason that Fe₇₈Si₉B₁₃ ribbon amorphous particles own low remanence and low coercivity is that the atoms are spatial disordered without macroscopical magnetic anisotropy. As the Fe₇₈Si₉B₁₃ spraying amorphous particles are only partial transformation to the amorphous state originally, the remanence and coercivity of Fe₇₈Si₉B₁₃ spraying amorphous particles is slightly higher than that of Fe₇₈Si₉B₁₃ ribbon amorphous particles. Considering the excellent soft magnetic properties of Fe₇₈Si₉B₁₃ amorphous particles, the particles could be used to prepare magnetic fluids with high saturation magnetization, low remanence and low coercivity.

Fig. 5 The magnetic hysteresis curves of Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 84 h, Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h and Fe₃O₄ particles; the inset are magnified views of the hysteresis loops at low applied fields.

Magnetic properties for Fe₇₈Si₉B₁₃ ribbon amorphous particles, Fe₇₈Si₉B₁₃ spraying amorphous particles and Fe₃O₄ particles have been given in Table 1. Numerical values of saturation magnetization (Ms), remanence (Mr) and coercivity (Hci) of the three kinds of particles are shown in Table 1.

Table 1 Specific magnetic properties of Fe₇₈Si₉B₁₃ ribbon amorphous particles, Fe₇₈Si₉B₁₃ spraying amorphous particles and Fe₃O₄ particles

Magnetic properties	Fe78Si9B13 ribbon amorphous particles		Fe78Si9B13 spraying amorphous particles				Fe3O4 particles
	24 h	84 h	0 h	24 h	60 h	84 h
Ms (emu g^−1)	176.46	169.48	178.00	174.91	170.30	167.01	42.41
Hci (Oe)	25	19	38	50	30	36	4.8
Mr (emu g^−1)	4	1.4	2.5	3.8	2.13	2.5	3.65

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The magnetic properties of MR fluids are shown in Fig. 6. The results show that the saturation magnetization of MR fluids with Fe₇₈Si₉B₁₃ ribbon amorphous particles and Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h is 3.7 emu g⁻¹ and 3.5 emu g⁻¹, respectively. The saturation magnetization of magnetic fluids with the same Fe₃O₄ particles volume fraction is about 0.8 emu g⁻¹. This demonstrates that excellent magnetic properties of magnetic particles will contribute to excellent magnetic properties of MR fluids.

Fig. 6 The magnetic hysteresis curves of MR fluids with Fe₇₈Si₉B₁₃ ribbon amorphous particles, Fe₇₈Si₉B₁₃ spraying amorphous particles after being milled for 84 h and magnetic fluids with Fe₃O₄ particles.

Therefore, because of the excellent magnetic properties of Fe₇₈Si₉B₁₃ amorphous particles, the MR fluids can provide larger shear stress compared with magnetic fluids prepared with Fe₃O₄ particles, which can produce greater torque and friction. In the meantime, as the MR fluids we made have low remanence and low coercivity, the agility to react is insured. These MR fluids should have potential applications, especially in the devices to transmit torque and friction by the shear stress of magnetic fluids,²² such as clutch, brake, damper and so on.

The viscosity versus temperature of MR fluids under applied magnetic strength of 0 Gs, 500 Gs and 1100 Gs are measured by a rotational viscometer. A simplified schematic of the rotational viscometer is shown in Fig. 7(a). The chain-like structures in the absence of magnetic field and under an external magnetic field is shown in Fig. 7(b) and (c). Fig. 8 manifests that the viscosity of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles increases when the temperature decreases and magnetic strength increases. The effect of temperature on viscosity mainly depends on the viscosity-temperature property of silicon oil and the influence of magnetic strength on viscosity mainly depends on the magnetic property of the particles. In addition, the viscosity of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles is about two to three times higher than that of magnetic fluids with Fe₃O₄ nanoparticles under an applied magnetic field. With the increase of magnetic strength from 0 Gs to 1100 Gs, the average growth rate of the viscosity of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles is 9.64 cp per Gs, which is much faster than that of magnetic fluids with Fe₃O₄ particles. The reason might be interpreted as follows. The suspended particles will be polarized under an applied magnetic field and the dipole–dipole interaction between the particles causes the formation of linear chains, parallel to the applied field. And the chain-like structures inhibit the deformation of magnetic fluids. Therefore, the field-induced particles in the form of chain-like structures result in the increasing of the viscosity of MR fluids. The linking of chain-like structures becomes stronger with the increase of magnetic strength. However, the effect of the magnetic strength can approach a limit which is determined by the saturation magnetization of the particles. Therefore, the higher saturation magnetization of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles contributes to the formation of much stronger chainlike structures.

Fig. 7 (a) Schematic diagram of the rotational viscometer; (b) and (c) the chain-like structures in the absence of magnetic field and under an external magnetic field, respectively.

Fig. 8 (a) The viscosity versus temperature of magnetic fluids with Fe₇₈Si₉B₁₃ amorphous particles and Fe₃O₄ particles (b) the curve of magnetic strength dependence of viscosity at about 30 °C, 50 °C, 70 °C.

The rheological properties of magnetic fluids can be measured by a rotational rheometer. And a simplified schematic of the rotational rheometer is shown in Fig. 9(a). The chain-like structures in the absence of magnetic field and under an external magnetic field is shown in Fig. 9(b) and (c). Shear stress and viscosity as a function of shear rate of magnetic fluids with Fe₇₈Si₉B₁₃ particles (filled symbols) and Fe₃O₄ particles (open symbols) under different magnetic strengths on a logarithmic scale are shown in Fig. 10(a) and (b). Both the magnetic fluids with Fe₇₈Si₉B₁₃ particles and Fe₃O₄ particles show an increase in shear stress and shear viscosity with increasing magnetic strength. However, applied magnetic field affects magnetic fluids of Fe₇₈Si₉B₁₃ particles more significantly than that with Fe₃O₄ particles. In Fig. 10(a), in absence of magnetic field, the shear stress of both the magnetic fluids with Fe₇₈Si₉B₁₃ particles and Fe₃O₄ particles exhibited a linear increase with increasing shear rate. Under a magnetic field, magnetic fluids with Fe₇₈Si₉B₁₃ particles show a greater increase than that with Fe₃O₄ particles in shear stress. Typical Bingham behavior can be observed for MR fluids with Fe₇₈Si₉B₁₃ particles and the shear stress show a wide plateau range over wide shear rate ranges. The possible reason is that the chain-like structures of particles are formed via dipole–dipole interactions under an applied magnetic field, resulting in the solid-like properties of the MR fluids.^29,30 Fig. 10(b) shows that the shear viscosity of both magnetic fluids decreases with increasing shear rate, which is called the shear thinning behavior. The yield stress of the magnetic fluids can be obtained by linear extrapolation method and the intercept of each fitting curve is considered as the yield stress of the magnetic fluids under the corresponding magnetic field. Therefore, the yield stress of the two magnetic fluids under different magnetic field is obtained in Fig. 10(c). For example, at 250 kA m⁻¹, the yield stresses of magnetic fluids with Fe₇₈Si₉B₁₃ particles and Fe₃O₄ particles are 4.85 kPa and 36 Pa, respectively. It indicates that the yield stress of both magnetic fluids increases with increasing magnetic strength, especially the MR fluids with Fe₇₈Si₉B₁₃ particles. The reason is that chain-like structure is well formed under the applied magnetic field and the force between particles becomes stronger with increasing magnetic strength. What's more, the yield stress of magnetic fluids is attributed to the magnetization intensity of the magnetic particles.³¹ Fe₇₈Si₉B₁₃ particles are more easily magnetized than Fe₃O₄ particles under the same magnetic strength, increasing the magnetic interactions between the particles. Similarly, higher magnetic strength also increases the magnetic interactions between the particles. This means that the chain-like structures of magnetic fluids with Fe₇₈Si₉B₁₃ particles under higher applied magnetic field aren't easily broken.

Fig. 9 (a) Schematic diagram of the rotational rheometer; (b) and (c) the chain-like structures in the absence of magnetic field and under an external magnetic field, respectively.

Fig. 10 (a) and (b) Shear stress and viscosity as a function of shear rate of magnetic fluids with Fe₇₈Si₉B₁₃ particles (filled symbols) and Fe₃O₄ particles (open symbols) under different magnetic strengths; (c) yield stress as a function of magnetic strength for magnetic fluids with Fe₇₈Si₉B₁₃ particles and the inset is yield stress as a function of magnetic strength for magnetic fluids with Fe₃O₄ particles.

Conclusions

In the present work, silicon oil-based magnetic fluids with Fe₇₈Si₉B₁₃ ribbon amorphous particles, Fe₇₈Si₉B₁₃ spraying amorphous particles and Fe₃O₄ particles were prepared, respectively. The results indicate that with increasing milling time, the amorphous character of Fe₇₈Si₉B₁₃ spraying amorphous particles is enhanced. The saturation magnetization is 167.01 emu g⁻¹ for Fe₇₈Si₉B₁₃ spraying amorphous particles and 169.48 emu g⁻¹ for Fe₇₈Si₉B₁₃ ribbon amorphous particles after being milled for 84 h. The saturation magnetization of Fe₇₈Si₉B₁₃ ribbon amorphous particles is approximately 4 times than that of Fe₃O₄ particles. At the same time, the remanence and coercivity of Fe₇₈Si₉B₁₃ spraying amorphous particles is 1.18 emu g⁻¹ and 19.3 Oe. The remanence and coercivity of Fe₇₈Si₉B₁₃ ribbon amorphous particles is only approximately 1/3 of that of Fe₇₈Si₉B₁₃ spraying amorphous particles. The perfect magnetic properties of Fe₇₈Si₉B₁₃ amorphous particles contribute to the perfect magnetic properties of the magnetic fluids. The saturation magnetization of MR fluids with Fe₇₈Si₉B₁₃ amorphous particles is also approximately 4 times than that of the magnetic fluids with Fe₃O₄ particles. The saturation magnetization has a direct impact on the viscosity when the MR fluids are under an applied magnetic field. And the MR fluids with Fe₇₈Si₉B₁₃ amorphous particles show a stronger magnetorheological response than magnetic fluids with Fe₃O₄ particles.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundational of China (Grant no. 51571130).

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