Magnetoresistance Characteristics of Magnetorheological Gel under a

Mar 3, 2014 - Tsubasa Oguro , Hiroyuki Endo , Takehito Kikuchi , Mika Kawai , Tetsu Mitsumata. Reactive and Functional Polymers 2017 117, 25-33 ...
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Magnetoresistance Characteristics of Magnetorheological Gel under a Magnetic Field Miao Yu,*,† Benxiang Ju,† Jie Fu,† Shuzhi Liu,† and Seung-Bok Choi‡ †

The Key Lab for Optoelectronic Technology and Systems, Ministry of Education, Chongqing University, Chongqing 400044, China Department of Mechanical Engineering, Smart Structures and Systems Laboratory, Inha University, Incheon 402-751, Republic of Korea

‡

ABSTRACT: A kind of magnetoresistance material was prepared, named as magnetorheological gel (MRG). The MRG samples were fabricated by dispersing carbonyl iron particles (CIP) into the polyurethane gel, and several experimental devices were prepared to investigate the magnetoresistance characteristics of MRG. The magnetoresistance characteristic was systematically tested and the influence of the magnetic field was analyzed. It is found that the resistance value of MRG with the CIP content of 70 wt % can be decreased from 7.56 to 2.44 MΩ with increasing of magnetic field from 0.1 T to 1 T. The experimental results have also proved that CIP content and the composition of matrix have a greater impact on the magnetoresistance characteristics. Because of the interaction between the magnetic force of CIP and motion resistance under a magnetic field, the obvious hysteresis phenomenon was observed and recorded. Lastly, it was observed that the magnetoresistance can be changed reversibly by controlling the magnetic field.

1. INTRODUCTION Magnetorheological fluid (MRF), magnetorheological elastomer (MRE), and magnetorheological foam, comprise a class of smart material, whose rheological properties can be controlled by the external magnetic field.1−13 The most well-known MR material is MRF, whose yield stress can be changed by an external magnetic field. On the basis of this property, MRF can be applied on dampers,14−16 brakes,17 actuators,18 polishers,19 isolators,20 and so on. In MRF, iron particles are suspended in a liquid carrier fluid, and the sedimentation of iron particles leads to a significant decrease of the MR effect and application stability. This problem can be overcome by replacing the matrix with a high viscosity material. A new generation of MR material, known as magnetorheological gel (MRG), was proposed by Wilson et al. and Fuchs et al.21,22 This kind of material has the advantage of being able to control initial viscosity and reduce the sedimentation of iron particles in matrix, thus, improving the stability of the system. The properties of the polymer carrier play a key role in determining the behavior of the MRG. Wei et al. reported the rheological properties of MRG under both steady and oscillation shear using a MR rheometer: the MRG exhibited high static shear yield stress (60.8 kPa, at 573mT), dynamic shear yield stress (83.9 kPa, at 573mT) and a wide variation range (static shear yield stress, 6−62 kPa; dynamic shear yield stress, 15−85 kPa).23 A very strong relative increase of storage modulus, up to 6000% was obtained by An et al.24 These results indicate that MRG is a sensitive MR material under a magnetic field. On the basis of the above excellent characteristics, MRG can be successfully used in vibration control and damping devices.21,25 Apart from the application of mechanical properties, conductive polymer composites based on conductive filler in an insulating polymer matrix have attracted a great deal of scientific and industrial interest for several decades,26−28 and © 2014 American Chemical Society

the electric conductivity of MR materials has also attracted considerable attention. The pressure-dependent conductivity of MRE was discovered, its resistance diminishing with the increase of the compression force.29,30 Structurally, MRE can be thought of as solid analogue of MRF.31 Ferromagnetic particles are mixed with a polymer matrix and then exposed to a magnetic field during the vulcanization of the mixture. This is called anisotropic MRE, in which iron particles are firmly locked in the rubber matrix and form chain-like structures along the direction of the external magnetic field.32,33 Previous studies have shown that the change in resistance with pressure was found to be an order of magnitude larger for a MRE than for the same volume fraction of fillers dispersed randomly in the polymer. The filler particles have a high surface roughness, and when particles are brought into contact under pressure, the electric current takes place via microcontacts between asperities. Our interest in MRG comes from its matrix with off-state viscosity, different from MRE, and several groups also reported the possible rearrangement inside MRG under the effect of a magnetic field. This means iron particles in MRG can be moved freely by a magnetic field force.24,34 Owing to the movement of particles under a magnetic field, changes in the conductivity of MRG occur. Compared with the MRF and MRE, MRG is considered as the intermediate system between MRF and MRE.24 In addition, the most noteworthy is that the particle chains of MRE within the elastomer composite are intended to always operate in the preyield regime, while those of MRG typically operate within a postyield continuous shear or flow regime. Most of the studies on MRG focus on their mechanical Received: Revised: Accepted: Published: 4704

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The status of MRG under different tilt angles was recorded by using photographs at room temperature as shown in Figure 1. If the beaker was tilted, the surface of MRG was also parallel to the bottom of the beaker at the beginning. However, the surface adapted to gravity and became level to the horizontal plane about several minutes later; it is very clear that MRG is a kind of material with high off-state viscosity, and it also can be seen as the analogue of MRF. In this paper, two groups of MRG were prepared. The first group of MRG was based on different CIP content, and samples with CIP content of 50 wt %, 60 wt %, and 70 wt % were fabricated to investigate the influence of CIP content on magnetoresistance characteristic. In addition, to investigate the influence of polyurethane matrix on magnetoresistance characteristics, MRG samples were fabricated with different matrix compositions with the same CIP content of 70 wt %. The composition of two groups of MRG samples were listed in Table 1.

properties, but studies on the electro-conductive characteristics of MRG have rarely been reported. In this study, we have synthesized a series of MRG samples, and prepared the experimental devices based on these MRG samples. The influence factors for the magnetoresistance of MRG were systematically discussed and analyzed.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. Diphenylmethane diisocyanate (MDI: 4,4- ≈ 50%, 2,4- ≈ 50%, Yantai Wanhua Polyurethanes Co. Ltd., China) and castor oil (CO: Sinopharm Chemical Reagent Co. Ltd., China) were used as the main raw materials for preparation of the MRG matrix. CO was distilled at 110 °C under a vacuum drying oven for about 1 h before use; excess water was evaporated in this step. Stannous octoate (Sinopharm Chemical Reagent Co. Ltd., China) was used as a catalyst. The filler particles were carbonyl iron particles (type: JCF2-2, Jilin Jien Nickel Industry Co. Ltd., China) with the size distribution: D50 = 5−8 μm, and these particles were used as received without further purification. 2.2. Preparation of MRG. The polyurethane matrix of MRG was mainly synthesized from CO and MDI, and there were three steps in the synthesis of the MRG samples. First, CO was poured into a beaker and the temperature was increased to 80 °C. Five minutes later, MDI was injected into the beaker, and the mixture was stirred for about 10 minutes being well mixed. The mole ratio of MDI and CO was calculated by the following formula: m /933 g·mol−1 nOH = CO nNCO mMDI /250 g·mol−1

Table 1. Composition of the MRG Samples MRG sample

mCO/mMDI

CIP (wt %)

first group

100:10 100:10 100:10 100:5 100:10 100:15

50 60 70 70 70 70

second group

2.3. Experimental Device Fabrication. The schematic diagram and photograph of the experimental device used for study of the magnetoresistance effect of MRG are shown in Figure 2. It mainly included MRG, electrodes, plastic sheet, and

(1)

Where nOH is the mole of −OH group,nNCO is the mole of −NCO group, mCO is the weight of CO and mMDI is the weight of MDI. Second, the mixture was mixed with CIP. At the same time, stannous octoate was dripped into a mixture with stirring as catalyst. At last, as soon as the CIP and the matrix were well mixed, the mixture was placed in a drybox to vulcanize for about 1 h at the temperature of 80 °C. After that, the sample was placed several days at room temperature, and then the MRG sample was obtained. Photographs of the MRG sample are shown in Figure 1.

Figure 2. The schematic diagram and photograph of the experimental device.

insulation layer. MRG was encapsulated in between the two electrodes as a kind of conductive medium, which was fixed in a space with the dimensions of 10 mm side length and 1 mm thickness. The experimental device was packaged by insulation layer to avoid the effect of interference factors. 2.4. Characterization. A homemade test system was designed and commissioned to characterize the magnetoresistance characteristics of MRG, as shown in Figure 3. The experimental device of MRG was fixed between the two magnetic poles, and the spacing of the magnetic poles was set to 10 mm. A magnetic field with variable magnetic flux density was generated by a kind of commercial electromagnet (Type: SB-130, Changchun Yingpu Magnetoelectricity Technology Co. Ltd., China), and the range of magnetic flux density can be varied from 0 to 2 T by adjusting the multifunction DC power supply (type: JDAW-2012F, Changchun Yingpu Magneto-

Figure 1. Photographs of the MRG sample. 4705

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The magnetoresistance is infinity in the absence of a magnetic field, and the experimental devices can be seen as insulator. That is because the CIP are homogeneously distributed in the matrix with off-state viscosity, these CIP exhibit a typical spherical structure (Figure 4) and they are very stable in the MRG sample. The space between CIP was filled with polymer matrix and led to CIP out of contact. However, in the presence of a magnetic field, the magnetoresistance is dependent on an external magnetic field, and it was found that the magnetoresistance of all experimental devices kept a decreasing trend when the magnetic flux density varied from 0.1 T to 1 T. Taking the experimental device with CIP content of 70 wt % as an example, the magnetoresistance can be changed from 7.56 to 2.44 MΩ with an increase of the magnetic field from 0.1 T to 1 T. Furthermore, the increment of CIP also significanlyt affects magnetoresistance, the magnetoresistance variation of CIP-50 wt % is lower than that of the CIP-60 wt %, while the CIP content is increased from 50 wt % to 60 wt %, the absolute variation of magnetoresistance is 3.53 and 5.45 MΩ, respectively. Interestingly, in the comparison experiment, above a CIP content of 60 wt %, the CIP effect for magnetoresistance is not obvious. When the magnetic field exceeds 0.5 T, the magnetoresistance of CIP-70 wt % is higher than that of CIP-60 wt %. In combination with the aforementioned results, this can be explained by the interactions among the CIP under the effect of a magnetic field. A typical experiment was done to illustrate the interaction force of CIP along the direction of the magnetic field. Figure 5 is the photograph of the MRG sample without and with the influence of a magnetic field.

Figure 3. The schematic diagram of the characterization system for testing magnetoresistance characteristics of MRG.

electricity Technology Co. Ltd., China). An inductance, capacitance, resistance (LCR) meter (type: U1732B, Agilent Technologies, Inc. USA) can be used to test the magnetoresistance value of the MRG sample under the different magnetic fields. In addition, a constant resistance, experimental device and voltage source form a series circuit to monitor the variation of magnetoresistance under the constant magnetic field. An Agilent oscilloscope (type: DSOX2024A, Agilent Technologies, Inc. USA) was used to record voltage signal of both ends of constant resistance, and this signal characterized the change of magnetoresistance of MRG. In this study, the experiments were all carried out at room temperature.

3. RESULTS AND DISCUSSION MRG is mainly composed of CIP and polymer gel. Prior studies have shown that the CIP and polymer gel have a greater impact on mechanical properties of MRG under the magnetic field.23,25 The study on the electrical properties of MRG was reported in this paper to further evaluate and understand the characteristics of MRG. The influence of CIP content and polymer gel on the magnetoresistance characteristics was experimentally studied and analyzed as shown in the following sections. 3.1. Effect of Magnetic Field and CIP Content on Magnetoresistance. In the first group, three kinds of experimental devices based on MRG samples were fabricated with CIP content of 50 wt %, 60 wt %, and 70 wt %. The magnetoresistance of MRG was measured by the test system (Figure 3), and the result is shown in Figure 4.

Figure 5. Influence of a magnetic field on a MRG sample: MR device of MCR 301 rheometer (a), without magnetic field (b), with magnetic field (c).

As can be seen in Figure 5a, a constant magnetic field was generated by an electromagnet and can be imposed on the MRG sample. The surface morphology of MRG was subjected to significant changes under a constant magnetic field of 0.5 T. Many peak structures were formed, and the direction was perpendicular to the surface of electromagnet, as shown in Figure 5c. If no magnetic field was applied, the CIP were randomly suspended in the polymer matrix, and the MRG was in the free-state, as shown in Figure 5b. In addition, an aliquot of the MRG sample was placed between two glass slides to investigate the microstructure by using a digital microscope (model VHX-600, Keyence Co.). Image analysis techniques

Figure 4. The magnetoresistance of MRG with different CIP content. 4706

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Figure 6. Observation sample of MRG and image processing results of the microstructures (a); the sketch of CIP rearrangement with increasing magnetic field (b); the profile of the cross section of structures depending on the increasing of magnetic field (c).

enhances the aggregation capabilities of the chain structures, bringing the CIP into closer contact. Because of the magnetic field force, the existing chain structures, which produce a force tend to attract discrete CIP around the chain structures. With increasing magnetic field, the CIP structures will be enhanced and further strengthen the contact between the CIP. Hence, CIP structures can be seen as a kind of conductor, the crosssectional area of a conductor can be substantially increased with increasing magnetic field, and can be confirmed by Figure 6c. As seen with eq 2, this will result in a decreasing resistance value due to increasing magnetic field. A comparison of the three experimental devices shows that CIP content has a greater impact on magnetoresistance. In contrast to Figure 4, the resistance value of the CIP-50 wt % is higher than that of the experimental device with CIP-60 wt % remarkably. It is known that the higher is the CIP content, the smaller is the interval between particles. CIP can be considered as the dipoles (m) and along with the direction of the magnetic field, the magnitude of the force is given by36

were also used to analyze the microstructures of MRG under the effect of a magnetic field. The biggest issue is one referred to as “segmentation”; image segmentation was utilized to distinguish the object from the background, and converts the microstructure photograph to a “binary” image, that is, white or black, as shown in Figure 6. The black represents the matrix regions, and the white is the CIP. Figure 6a shows the results of a threshold segmentation microstructure image; the formation of chain structures could be detected at the different magnetic flux density. It is remarkable that the CIP of MRG self-assemble to chainlike or columnar structure with their longitudinal axis parallel to the external magnetic field. When a critical density of the magnetic field was achieved, a strong interaction between the CIP was formed to rearrange the particles. Owing to the structures along with the direction of magnetic field, the normal force was generated among the CIP, and FN ∝ H2.35 Further increase of the H results in a higher FN, and a great number of discrete CIP were concentrated in surrounding chain structures. With an increasing magnetic field, many chainlike structures associated to form thicker columnar structures. A sketch of the influence of magnetic field on changing particle structures is shown in Figure 6b. CIP change their positions, and each of them falls into a new equilibrium state. The particles do not form narrow chain structures because the particles agglomerate under a higher magnetic field, which thickened the columnar structures formation. For the agglomerated particles, Figure 6c shows the profile of the cross section of chain or columnar structures depending on the increase of the magnetic field. The strong magnetic field and the high magnetization of the CIP can result in an increasing cross-sectional area of the structures in MRG, and this result also can be confirmed in Figure 6a. As soon as the magnetic field is applied, the CIP in the polymer matrix are constrained by a magnetic field force and then rearranged to form chain structures parallel to the direction of magnetic field, which further reduce the spacing between CIP. Because the chain structures of CIP cover the whole spacing between the electrodes of the experimental devices, the spacing of the electrodes is fixed and denoted by L, L = 1 mm. Therefore, the resistance value can be written as R = 0.001ρ /S

F=

3μ0 m2 2πr 4

(3)

in which37−39

m=

4 3 πa μ0 μ1χH 3

(4)

where a is the CIP radius, χ is the susceptibility of particles, H is the magnetic field strength, μ0 = 4π × 10−7Hm−1, μ1 is the permeability of MRG, and r is the interval between the CIP. Equation 3, which reflects the relationship between the interval and interaction force of CIP under the effect of magnetic field. In this case, with higher CIP content, it increases the capability to move the location of CIP to form chain or columnar structures, lead to decrease the resistance value. However, when CIP content continues to increase, the magnetic fielddependent resistance is relatively stable. There must be a competition between the interaction force of CIP and the motion resistance of MRG under a magnetic field, which leads to the magnetoresistance dependence on both the CIP content and viscosity with increasing magnetic field. The viscosity of MRG can be characterized by using a commercial rheometer (model: MCR301, Anton Paar) under the effect of a magnetic field. The principle of the main test part and the result are all shown in Figure.7.

(2)

where ρ is the resistivity, S represents the effective conductive cross-sectional area of the MRG, and the S is mainly determined by a cross-sectional area of chain or columnar structures in this work. The increment of magnetic field 4707

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increase of the ratio of CO and MDI under the magnetic field sweep. From the structure analysis, this result is due to the change of matrix viscosity, and it can be noted that this matrix mainly was composed of soft segments and hard segments. The viscosity is determined by the ratio of soft segment and hard segment content. The hard segments of matrix are provided by MDI, while the soft segments are composed of CO. Hard segments are homogeneously distributed in the soft segments, which easily assemble together due to strong polarity between hard segments, and play a decisive role in viscosity of the matrix. The viscosity of MRG under magnetic field sweep is shown in Figure 8, which also proves that a higher hard segment content leads to a higher viscosity of MRG; therefore, the CIP will be bound in a matrix with a higher hard segment content and hardly be able to move when a magnetic force is applied. This strongly suggests that rearrangement of the CIPs occurs with difficulty in MRG with higher viscosity, and the number of chain structures decreases with an increase of the number of hard segments. As a result, the resistance value tends to a higher value under the same magnetic field. 3.3. Hysteresis of Magnetoresistance. Experiments have confirmed that the motion resistance of MRG plays a role of hindering the CIPs rearrangement. For MRG, the hysteresis of magnetoresistance can be generated by the interaction between magnetic force and motion resistance. Thus, the study of hysteresis can give us a comprehensive understanding of magnetoresistance characteristics of MRG. A method to investigate the hysteresis characteristics of magnetoresistance under the magnetic field was used. The experimental schematic diagram is shown in Figure 3, and the test results are shown in Figure 9. Figure 3 shows the series circuit built for the experiment. The constant voltage source was set to V = 10 (V). Therefore, the magnetoresistance of the experimental device can be characterized as

Figure 7. The viscosity of MRG with different CIP content under a magnetic field.

In Figure 7, the magnetic field sweep was carried out under the rotational shear mode with shear rate of 10 (1/s), and the higher CIP content of the sample leads to a higher viscosity. Under the lower CIP content sample, the movement of CIP is mainly determined by the magnetic force, while the viscosity of MRG with higher CIP content increases rapidly, the stronger motion resistance is obtained. Therefore, when CIP content is increased to a critical value, the magnetic force and motion resistance approaches balance to hinder the movement of the CIP, the resistance value cannot be decreased by increasing the CIP content. 3.2. Effect of Polymer Matrix on Magnetoresistance. The polymer matrix was mainly composed of CO and MDI, in order to investigate the matrix affects the magnetoresistance characteristics under the magnetic field. Three kinds of polyurethane matrix were fabricated by adjusting the mass ratio of CO and MDI. In this group, MRG samples were all prepared by using the CIPs content of 70 wt % sample to avoid the effect of the CIPs content on magnetoresistance. The experimental devices were also tested with different matrix to investigate the magnetoresistance characteristics. Figure 8 shows the relationship between magnetic field and resistance value. Figure 8 clearly indicates that a different matrix has also a crucial impact on magnetoresistance characteristics. The magnetoresistance of experimental devices increased with an

R1 = R 2(V − V2)/V2

(5)

where R1 is the magnetoresistance of experimental device, V2 represent the voltage of both ends of constant resistance, and R2 = 4.14 MΩ. The value of V2 reflects the variation of R1. In the hysteresis experiment, the magnetic flux density was set to 1 T. As soon as the magnetic field was turned on, the voltage signal of R2 was recorded by oscilloscope at the same time. When the voltage signal achieved a saturated state, the magnetic field was closed. After the signal acquisition, the voltage signal was transferred to a computer for processing and analysis. The hysteresis characteristic of R1 is the magnetoresistance based on the first group of MRG, This is shown in Figure 9a. It can be seen from Figure 9a that the voltage increases with time first, and then reaches a steady-state value. A rough estimation of the hysteresis time can be obtained in Figure 9a, about 9 s under the effect of a constant magnetic field. This strongly suggests that rearrangement of the CIPs occurs in the polymer matrix. After the magnetic field is turned on, the induced CIP−CIP interactions will decrease the distance between neighboring CIPs slowly. That will bring a remarkable change in R1, and lead to the increasing of the voltage of R2. At the same time, the viscosity of the MRG also increases to a steady value quickly, CIPs moved with difficulty in the matrix under high viscosity. A new balance is formed between motion resistance and the interaction force of CIPs, while the voltage of R2 remains stable. When the magnetic field was turned off, the interaction force of the CIPs disappeared

Figure 8. The magnetoresistance of MRG based on the second group of samples. 4708

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Figure 9. Hysteresis characteristics of magnetoresistance: The voltage variation of constant resistance was tested under the magnetic field of 1 T, with the magnetic field subsequently being shut down. The experimental devices based on the first group of MRG (a); the voltage variation of the experimental devices based on the second group of MRG, and the same test conditions was applied (b).

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and the viscosity of MRG decreased quickly due to decreasing magnetic force. The CIPs can change their locations very easily, which destroy the chain structures in the matrix. In that case, the voltage of R2 shows a decreasing trend with increasing time, as shown in Figure 9a. Furthermore, Figure 9b shows that hysteresis characteristics of magnetoresistance based on a second group of MRG have a phenomenon similar with that of Figure 9a. From the experimental results above, it can be seen that the magnetoresistance of experimental devices has an obvious time-dependent and reversible characteristic under the magnetic field.

4. CONCLUSIONS A systematic study on the experimental devices based on MRG was carried out to investigate the magnetoresistance characteristics of MRG under a magnetic field. The resistance of MRG can be changed by varying the magnetic flux density, and keeps a decreasing trend when the magnetic flux density varies from 0.1 T to 1 T. An increment of CIP content also significantly affects magnetoresistance, while the CIP content is increased to a higher value, and the magnetic field-dependent resistance is relatively stable. The magnetoresistance is also influenced by different ratios of CO and MDI in matrix, which can be increased by increasing of the ratio of CO and MDI. These results strongly indicate that rearrangement of the CIPs occurs in the MRG and leads to the variation of magnetoresistance. The hysteresis phenomenon of magnetoresistance is generated by the interaction between the magnetic force and motion resistance, and a hysteresis time of about 9 s is obtained under the effect of constant magnetic field. Because of its magnetoresistance characteristics under a magnetic field, MRG can be used for magnetoresistors, sensors, and other electrical devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 23 65111016. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was obtained from NSFY (No. 61203098); National Technology R&D Program of the Ministry of Science and Technology (No. 2012BAF06B04); The Fundamental Research Funds for the Central Universities (No. CDJZR11128801). 4709

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dx.doi.org/10.1021/ie4040237 | Ind. Eng. Chem. Res. 2014, 53, 4704−4710