Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral Oil in Alternative Magnetic Field

Transcription

1 J. Ind. Eng. Chem., Vol. 12, No. 5, (2006) SHORT COMMUNICATION Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral Oil in Alternative Magnetic Field Ioan Bica Department of Physics, West University of Timisoara, Bd. V. Parvan, no. 4, , Timisoara, Romania Received March 30, 2006; Accepted July 3, 2006 Abstract: A magnetorheological suspension (MRS) obtained by thermal decomposition of Fe 2 (CO) 9 in mineral oil with stearic acid was prepared. The mean diameter of the microparticles was 2.10 µm at a standard deviation of 0.40 µm. For volume fractions of 0.06 and 0.30, the MRS is conductive only for alternative magnetic fields with a minimal effective intensity of 60 ka/m. The experimental results are discussed. Keywords: magnetoresistance, magnetorheological suspension, magnetic field, iron microparticles, electrical conductivity, alternative current Introduction 1) Magnetorheological suspensions (MRSs) are known under the generic name of intelligent fluids [1,2]. They are obtained from mixtures consisting of a liquid phase media (e.g., mineral oil, silicon oil), tensioactive substances, and ferro- or ferrimagnetic microparticles [3-15]. When mixed with graphite microparticles [16,17], MRSs become electroconductive under the action of an external magnetic field, similar to the behavior of electroconductive polymers subjected to an electric field [18]. Recent research [19] has shown that the MRSs obtained through thermal decomposition of Fe 2 (CO) 9 in mineral oil with stearic acid become electroconductive in a timeconstant magnetic field. The electrical conductivity of an MRS depends on the intensity and direction of the external magnetic field [17]. In this paper, we show that the MRSs obtained by the procedure described in Ref. [5] are conductive in an alternative magnetic field. The electrical conductivity characterizing MRS magnetoresistances occurs in narrow stripes [20]. In addition, we are in the position to prepare sensors for the detection of fringe fields in magnetic heads [21] as well as warfare agents [22]. To whom all correspondence should be addressed. ( ibica2@yahoo.com) Sample Preparation A mixture consisting of kg ±5 % Fe 2 (CO) 9 powder with granulation ranging between 4.5 and 5.2 µm, kg ± 5 % mineral oil (Aneron/Merck type), and kg ± 2 % stearic acid was treated thermally [5] for s at the temperature of 510 K ±10 %. An MRS was obtained, having iron microparticles dispersed in mineral oil (Figure 1). The possible losses of mineral oil with stearic acid can be compensated using the supplier, as described in Ref. [5]. Experimental Device The experimental device used for the study of the electrical conductivity of MRSs in an alternative magnetic field is described in Ref. [17]. The overall configuration of the device is shown in Figure 2. The device consists of a coil 1, placed on a core 2. Between the polar parts 3 and 4 there is a magnetoresistance (electric resistance with MRS). The magnetoresistance is built by means of the glass tube 5 and the electrodes 6. Between the electrodes there is the MRS (position 7 in Figure 2). The body, consisting of the MRS, has its length and diameter equal to m± 10 %. The electrodes of the magnetoresistance are provided

2 Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral oil in Alternative Magnetic Field 807 Figure 1. Iron microparticles in mineral oil. Cummulative frequency function of the diameter, d of the iron particles: d m, mean diameter; σ, standard deviation. Figure 3. Magnetization M function of the intensity H ef for the MRS with different volume fractions ø. Figure 2. Experimental device: 1-coil; 2-magnetic core; 3 and 4-polar parts; 5-glass tube; 6-non-magnetic electrodes; 7-MRS; 8-digital ohmmeter (type DT9208 A); OX-coordinate axis; H - direction of the magnetic field intensity vector. with tightness fittings (not specified in Figure 2). They are connected to the digital ohmmeter 8 in Figure 2. The magnetoresistance is fixed and positioned between the polar parts 3 and 4 by means of a positioning fixing subassembly. It is not presented in Figure 2. Results and Discussion The MRS used for building the magnetoresistance was prepared as shown under 2. By adding mineral oil, MRSs were obtained with volume fractions of ø 1 = 0.30 and ø 2 = The MRS magnetization curves obtained by means of the magnetometer VSM 880 are shown in Figure 3. The value H ef. of the intensity of the alternative magnetic field was measured by means of a Hall sonde [23]. Magnetoresistances with MRSs were built having two values of ø(ø 1 = 0.30 and ø 2 = 0.06). The resistance R of the magnetoresistances was registered at the

3 808 Ioan Bica Figure 4. Variation of R by time t and H ef. : a)ø 1=0.3; b) ø 2 = moment of application of H and at intervals of 1 s, respectively, up to 45 s to determine effective intensities (H ef. ) of the magnetic field as a parameter. For longitudinal magnetic fields with H eff. 60 ka/m, R ø1 and R ø2. For transversal magnetic fields ( H Ox - Figure 2), namely from H ef = 60 ka/m, however, the phenomenon of electrical conductivity in the MRS occurs. The variation of R with respect to t for various values of the transversal magnetic field (H ef. ) is shown in Figure 4. The electrical conductivity of the MRS body is calculated using the relation [17] σ = 250 R( MΩ) With the values of R(t) in Figure 4 inserted into Eq. (1), σ(t) is obtained in the form of the graphs in Figure 5. It results from Figure 5 that the electrical conductivity of the MRS depends on ø and H ef. Thus, for ø 1 = 0.30, the electrical conductivity in the MRS arises at intervals of (1) Figure 5. Variation of the conductivity σ of MRS by time t and different values of Hef., for: a) ø 1 = 0.3; b) ø 2 = time after the application of H, of 9 s, for H ef. = 60 ka/m; 4 s, for H ef. = 70 ka/m; instantaneously, for 80 H ef. (ka/m) 90. For ø 2 = 0.06, however, conductivity in the MRS occurs upon application of transversal H, from H ef. = 70 ka/m. For H ef. = 60 ka/m, conductivity in the MRS occurs with a 4-s delay from the moment of application of H. The variation velocity of σ is higher during of the first 5 s after the application of H and then it exhibits a slow variation with time. The electrical conductivity σ (Figure 5) is considerably influenced by ø for fixed H ef.. If we denote as σ ø1 and σ ø2 the electrical conductivities for the volume fractions ø 1 and ø 2, respectively, of the MRS, then the relation α = σ ø2 σ ø1 (2) describes the time variation shown in Figure 6.

4 Electrical Conductivity of Magnetorheological Suspensions Based on Iron Microparticles and Mineral oil in Alternative Magnetic Field 809 The electrical conductivity in MRS occurs only in transversal magnetic field; The moment of onset of electrical conductivity in the MRS is determined by H ef. for a fixed value of ø (Figure 4); The electrical conductivity in the MRS is considerably influenced by the value ofø for fixed values of H ef. and t (Figures 5 and 6). Acknowledgment Figure 6. Plots of α as function of the time t from the application of the alternative magnetic field, for various values of H ef. It is observed from Figure 6 that α was greater than 1 for 0 t(s) 45 and 60 H ef. (ka/m) 80. At H ef. = 90 ka/m, however, we observe α = 1.44 for t = 0 s; α 2.21 for t = 2.5 s; α 1 for 2.5 < t(s) 33; α = 1.44 for t = 45 s. In an alternative magnetic field, the magnetic particles perform oscillatory movements along the directions identical and parallel to the direction of H. Permanently, the iron particles are placed at high distances, for which R(t) [19]. Thus, along directions parallel to H (longitudinal magnetic field), no electrical conductivity occurs in the MRS. The vibrations of the chains of magnetic dipoles propagate along directions perpendicular on the direction of H. In MRSs, fields of mechanical stresses occur [24]. Under the actions of mechanical stresses, the iron microparticles become positioned closer together. The initial thickness of the oil layer is inversely proportional to H ef. and it diminishes with time [19]. As a result [19], the value of R is dependent on H ef. and on the time t for a fixed value of ø. The passage from ø 1 to ø 2 (ø 2 < ø 1 ) has as its effect the diminution of the interactions between the iron microparticles during the migration in alternative magnetic field ( H Ox, Figure 2). The result is that, for fixed H ef. and t, at ø 2 <ø 1, the electrical conductivity of the MRS increases (Figure 6). Conclusions The MRSs obtained by thermal decomposition of Fe 2 (CO) 9 in mineral oil with stearic acid exhibit electrical conductivity in an alternative magnetic field; I am thankful to E. Papp, C. T. Cheveresan, and D. Baltateanu of the West University of Timisoara for interesting discussions. I also thank F. Stoian and O. Marinica of the Politehnica University of Timisoara (Complex Fluids Department) for assistance in performing the magnetorheological measurements. References 1. K. Shimada, Y. Akagami, S. Kamiyama, T, Fujita, T. Miyazaki, and A. Shibayama. In Proc. of Eighth Int. Conf. on ER and MR Fluids, G. Bossis, Ed., World Scientific Publishing (2002), pp E. Sudo. M. Funaoka, and H. Nishiyama. In Proc. of Eighth Int. Conf. on ER and MR Fluids, G. Bossis, Ed., World Scientific Publishing Co. (2002), pp Cl. Korman, H. M. Laun, and H. J. Richter, Int. J. Mod. Phys. B, 10, 3167 (1996). 4. A. J. Margida, K. D. Weiss, and J. D. Carlson, Int. J. Mod. Phys. B, 10, 3335 (1996). 5. I. Bica, Mater. Sci. Eng. B, 98, 89 (2003). 6. H. J. Choi, M. S. Cho, and K. To, Physica A, 254, 272 (1998). 7. J. W. Kim, S. G. Kim, H. J. Choi, and M. S. Jhon, Macromol. Rapid Commun., 20, 450 (1999). 8. I. S. Sim, J. W. Kim, H. J. Choi, C. A. Kim, and M. S. Jhon, Chem. Mater., 13, 1243 (2001). 9. M. S. Cho, Y. H. Cho, H. J. Choi, and M. S. Jhon, Langmuir, 19, 5875 (2003). 10. M. S. Cho, H. J. Choi, and W. S. Ahn, Langmuir, 20, 202 (2004). 11. a) M. S. Cho, H. J. Choi, and M. S. Jhon, Polymer, 46, (2005); b) J. H. Sung, M. S. Cho, H. J. Choi, and M. S. Jhon, J. Ind. Eng. Chem., 10, 1217 (2004). 12. M. S. Cho and H. J. Choi, Mater. Sci. Forum, 449, 1201 (2004). 13. S. T. Lim, M. S. Cho, I. B. Jang, H. J. Choi, and M. S. Jhon, IEEE Trans. Magn., 40, 3033 (2004). 14. S. T. Lim, M. S. Cho, In B. Jang, and H. J. Choi, J. Magn. Magn. Mater., 282, 170 (2004).

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