Experimental Study on Piezoresistivity and Magnetoresistivity of Magnetorheological Elastomers

Weiqiang Yea, Yi-Min Dengb and Wei Wangc

Ningbo University, Ningbo, Zhejiang Province 315211, P.R. China

aywqql@163.com, bdengyimin@nbu.edu.cn, cwei_wang_008@163.com

Keywords: Magnetorheological elastomer, test rig, piezoresistivity, sensitivity, magnetoresistivity

Abstract. As a new type of smart materials, magnetorheological elastomer (MRE) has become a

hot current research focus. However, the piezoresistivity and magnetoresistivity of MRE have not

been well studied. In this paper, this was done by using a test rig developed by the authors. The

experimental results showed that the conductivity of the MRE responded sensitively to the applied

pressure, and a linear relationship between the resistivity of the MRE and the applied pressure can

be observed within a certain range. Besides, the sensitivity of piezoresistivity is different among

different ratios of metal content, and it becomes more obvious when using the nickel content. And,

the magnetoresistivity of MRE is not obvious in a range of low magnetic field intensity, and there is

also no hysteresis phenomenon about magnetoresistivity.

Introduction

Magnetorheological elastomers (MREs) are smart materials made by aligning magnetic

microparticles (such as iron and nickel) inside a liquid polymer before the curing process has started.

Compared with the magnetorheological fluid, MREs not only have many characteristics such as

controllable behavior and fast response, but also have some unique advantages, such as good

stability, low production cost and so on. As such, MREs and their application are becoming a hot

research topic.

So far, there have been very few researches reported about electromagnetic properties of MREs.

Kchit and Bossis [1-2] studied piezoresistivity of MREs by experiment and found that the

piezoresistivity of magnetorheological elastomers not only have some repeatability, but also change

significantly (could change several orders of magnitude ) by the variation of stress, and it has good

linear relationship in a certain range. Wang et al. [3] from Nevada University analyzed the sensing

behavior of MREs and established their electrical equivalent model. In addition, Bossis et al. [4]

conducted a preliminary experimental study about the magnetoresistivity of MREs, and found that

the resistance of MREs varied significantly with the variation of magnetic field, and there showed

hysteresis phenomena with the applied field.

MREs may be used in manufacturing sectors as a sensing element for stress-sensitive

applications, and as a magnetic field sensor after in-depth studying about their piezoresistivity and

magnetoresistivity. However, current researches are primarily focused on studying the sensitive

components of pressure-sensitive devices and magnetic devices with nano-composite materials

[5-7]. Therefore, a further research of the piezoresistivity and magnetoresistivity of the

magnetorheological elastomers are necessary, so that new sensors can be developed for the industry.

Experimental Study about Piezoresistivity

Materials. 1# powder of iron and 2# powder of nickel (3-5 size, from Jiangyou Hebao

Nanomaterial Co., Ltd.). The elastomer we used is RTV-2 from Senli Silicone Rubber Co., Ltd. We

also used silicone oil and curing agent (also from Senli Silicone Rubber Co., Ltd.) as additives.

Preparation. Use pre-structured way to prepare magnetorheological elastomers at room

temperature. First, prepare mixture composed of the nickel powder, silicone rubber and silicone oil

at room temperature, and add a moderate amount of curing agent, then put the mixture into the

prior-made mold and place them in a vacuum box. After pumping air out of the box, thus out of the

Applied Mechanics and Materials Vols. 37-38 (2010) pp 444-447Online available since 2010/Nov/11 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.37-38.444

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.6.218.72, Rutgers University Libraries, New Brunswick, United States of America-29/06/14,15:43:49)

magnetorheological mixture, for a number of times, the mixture is finally put it in a static magnetic

field for curing. We may obtain good-quality MRE in 24 hours. Fig. 1 shows the route of the MRE

preparation process.

Fig. 1 Preparation flow chart of magnetorheological elastomers

Test Method of Piezoresistivity. Fig. 2 is the schematic diagram of test rig for piezoresistivity of

MRE. We used two coppers with their thickness being about 1mm as the electrodes, in order to

measure the resistance of MRE. The SD2002/5 digital ohm meter (from Qianfeng Electronic

Instrument Co., Ltd.) has been used, with the direction of a built-in current being parallel to the

direction of the applied pressure. The ohmmeter has a RS232C interface, which is connected with

the computer through its serial port. The communication protocol of RS232 interface has one start

bit (0), 8-bit data (LSB first), 1 stop bit (1), and has no parity, 4800 baud rate. A real-time resistance

collection program was developed by using visual C++ programming language, which can be used

to receive the data from the SD2002/5 constantly, and the data were finally stored in computer in

text format. The pressure signal was measured by using a load cell in real time and was collected by

DSP system and saved in the computer for data processing.

1—load cell 2,4—electrodes 3—MRE sample

Fig. 2 Schematic diagram of the test rig for piezoresistivity

Experimental Results and Discussion

Piezoresistivity of MRE with Different Content Proportions. Fig. 3 represents the data of

piezoresistivity of MRE with different nickel content proportions (the weight fraction of nickel are

60%, 50%, 40%). From the figure we can see that the resistance of magnetorheological elastomer

responded with the variation of pressure significantly (with several orders of magnitude) in a certain

range of pressure, and has a good linear relationship. The sensitivity of piezoresistivity is different

among different magnetorheological elastomers. Assume the absolute values of slope corresponding

to these nickel proportions, within the pressure range (0-0.5MPa), are K1, K2, K3 respectively, we

can observe that K3> K2> K1. This shows that for the magnetorheological elastomer, the more the

content of nickel, the greater sensitivity of piezoresistivity it will be, and vice versa. However, when

the content of nickel powder is less than 40%, the piezoresistivity will not be obvious, even

disappear. And, as the pressure increases, the resistance of magnetorheological elastomer will

always remain infinite.

Make vacuum Move the mixture into mold Mix the magnetorheological fluid

Put into a static magnetic field Get the MRE Open the mold Seal mold

Applied Mechanics and Materials Vols. 37-38 445

Fig. 3 Piezoresistivity of MRE with different content proportions

Piezoresistivity of MRE with Same Content Proportion but Different Soft Magnetic

Materials. Fig. 4 shows the data of piezoresistivity of MRE with the same proportion (the weight

proportion of nickel and iron are both 60%) but different soft magnetic materials. As can be seen

from the diagram, the sensitivity of the piezoresistivity is significantly different between the MRE

samples with different soft magnetic materials. Assume the absolute value of slope with both MRE

samples are KFe, KNi, within a certain pressure range (0-0.05MPa), then obviously we can see that

KNi > KFe. This may be due to the fact that, there must be more oxidized particles in MRE samples

base on iron particles than nickel particles during the preparation of magnetorheological elastomers.

The tunnel effect of MRE sample with the iron particles is not so significant compared to the

sample with nickel powder during compression, because of the thicker insulation oxide. Therefore,

to study the application of sensitive components for some sensitive sensoring applications, we can

opt for nickel powder as the soft magnetic materials in the preparation of magnetorheological

elastomers.

Fig. 4 Piezoresistivity of MRE with different soft magnetic materials

Experimental Study about Magnetoresistivity

Test Method of magnetoresistivity. The test rig of magnetoresistivity is almost the same as the one

shown in Fig. 2. The magnetic field was provided by a coil, and its direction is parallel to the

built-in current of SD2002/5 digital ohmmeter. The test was done at room temperature, and the

initial voltage value on the coil was 1V. The coil voltage was changed gradually, with each of the

voltage being maintained for about 1 minute. Record the value of the corresponding magnetic field

and that of the resistance of the sample respectively by the HT20 Tesla meter (from Shanghai

Prosper Magnet Technology Co., Ltd.) and the SD2002/5 Digital ohmmeter. Make the voltage

change from 1V to 6V then back to 1V, thus measuring a total of 12 times.

Experimental Results and Discussion. Fig. 5 shows the data of magnetoresistivity of MRE. As

can be seen from the figure, the resistance of the sample decreases as the magnetic field increases,

but the magnetoresistivity is not very significant, and there showed no hysteresis phenomenon. This

result was different from the result reported by Bossis et al. [4] (Bossis et al. found that the

magnetoresistivity of magnetorheological elastomers is significant, and there is hysteresis

phenomenon about the magnetoresistivity). The main reasons may be as following. 1) The elastic

properties of the soft magnetic materials and the content of the metal in MRE are both quite

different from those used by Bossis et al [4]. 2) The maximum value of the magnetic field applied in

446 Advances in Engineering Design and Optimization

this experiment is about 160mT. However, that used in the study of Bossis et al. [4] is about 600mT.

Therefore, in order to achieve better test results and good performance of MRE with significant

magnetoresistivity and hysteresis phenomenon, much more experimental research should be done.

Fig. 5 Magnetoresistivity of MRE

Conclusions

The studies reported in the previous sections show that (1) The piezoresistivity of an MRE sample

changes significantly (in several orders of magnitude) when it is compressed, which shows that the

piezoresistivity is very sensitive in the magnetorheological elastomers, and it has a good linear

relationship within a certain range of pressure variance.

(2) The sensitivity of piezoresistivity varies with different proportions of nickel particles in

magnetorheological elastomers within a certain range of pressure, and it increases as the nickel

content increases.

(3) The sensitivity of piezoresistivity differs with different soft magnetic materials, and the

piezoresistive sensitivity of MRE with nickel powder is much greater than that with iron powder.

(4) The magnetoresistivity of magnetorheological elastomers is not obvious in a range of low

magnetic field intensity, and there is also no hysteresis phenomenon about magnetoresistivity.

Further research should be done to prepare better quality magnetorheological elastomers, and

do more experimental research and analysis about piezoresistivity and magnetoresistivity of

magnetorheological elastomers. A theoretical model should be established through analysis and

comparison of the experimental results. In addition, a set of appropriate magnetorheological

elastomer samples should be selected for engineering application, such as for the development of

sensitive pressure sensors, magnetic sensors, etc.

Acknowledgements: This work was supported by the project “MRE Application Research in

Vibration Reduction” from Zhejiang Provincial Education Department, China.

References

[1] N. Kchit and G. Bossis: Journal of Physics: Condensed Matter, (2008) No. 20, pp. 1-5.

[2] N. Kchit and G. Bossis: Journal of Physics D: Applied Physics, (2009) No. 42, pp. 1-8.

[3] X. Wang and F. Gordaninejad: Journal of Mechanical Design, ASME Transaction (2009) No.

131, pp. 1-6.

[4] G. Bossis and C. Abbo: International Journal of Modern Physics B, Vol. 6-7 (2001) No. 15, pp.

564-573.

[5] Y.H. Song, Q. Zheng and X.S. Yi: Acta Materiea Compositae Sinica, Vol. 2 (1999) No. 16, pp.

46-51 (In Chinese).

[6] P. Wang, T.H. Ding, F. Xu and Y.Z. Tan: Chinese Journal of Sensors and Actuators, (2004) No.

1, pp. 15-18 (In Chinese).

[7] W. Pan, P. Zhai and L.Z. Liu: Chinese Journal of Materials Research, Vol. 4 (1997) No. 11, pp.

397-401 (In Chinese).

Applied Mechanics and Materials Vols. 37-38 447

Advances in Engineering Design and Optimization 10.4028/www.scientific.net/AMM.37-38 Experimental Study on Piezoresistivity and Magnetoresistivity of Magnetorheological Elastomers 10.4028/www.scientific.net/AMM.37-38.444

DOI References

[2] N. Kchit and G. Bossis: Journal of Physics D: Applied Physics, (2009) No. 42, pp. 1-8.

doi:10.1088/0022-3727/42/10/105506