Dielectric and electrical properties of electrorheological carbon suspensions

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Abstract

Measurements of electrorheological (ER) effects, dielectric properties, and electrical conductivity are made on an ER suspension composed of carbonaceous particles and silicone oil to understand the mechanism which governs the ER effect. In the frequency dependence of the dielectric permittivities, a dielectric dispersion is observed in the first-order dielectric permittivity, while in the third-order dielectric permittivity a resonance peak, which is due to shear-induced particle rotation, is recognized, showing that such dielectric properties are closely related to ER properties of the suspension. An electrical conduction mechanism, which is responsible for the ER effect, is also investigated from the dependence of dc electrical conductivity on electric-field strength and temperature.

Graphical abstract

In the spectra of third-order dielectric permittivity, a negative peak due to shear-induced particle rotation is observed.

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Introduction

Suspensions composed of small dielectric particles (∼ μm) and electrically nonconducting liquids show dramatic rheological changes on application of the strong electric field of a few kilovolts per millimeter [1], [2], [3], [4]. This kind of rheological change, electrorheological (ER) effect, was first found by Winslow [1] and since then extensive engineering and scientific work has been done. Owing to the large rheological change with a short response time of order of milliseconds, the ER fluids are expected as potential functional fluids applicable to some mechanical devices such as clutch, damper, and valve [5], [6]. The application of the electric field induces an interfacial polarization at the interface between the particle and the liquid. The polarization thus formed interacts with those at nearby particles, leading to the chain-like and/or columnar structures along the electric field, which are responsible for the rheological change. In such a way, the dielectric properties of the ER fluids are so closely related to the ER effect that many dielectric studies have been made to understand the mechanism of the ER effect [7], [8], [9], [10]. In these dielectric studies, the measurements are normally made at low electric fields, giving us information about the magnitude and the relaxation time of the polarization. However, it is expected that the dielectric property at high electric fields may differ from that at low electric fields owing to a nonlinear dielectric behavior when increasing the electric field strength. If we consider that the ER effect occurs at high fields, to understand the dielectric behavior when the ER effect occurs, it is desirable to measure the dielectric properties at high fields. In our previous studies on TiO2 [11], cellulose [12], and BaTiO3 [13] suspensions, it is suggested that shear-induced particle rotation occurs even at high fields where the chain-like structure is formed. This phenomenon is evidenced by a resonance peak in the spectra of the ER effect [11] or of the ER effect and the first- and the third-order dielectric permittivities [12], [13]. The dielectric studies at high fields, thus, give us some insights not obtainable from the ER measurements, but are not enough. To shed further light on the ER mechanism, a microscopic understanding of the electrical conduction process is also necessary, since it is closely related to the ER effect.

In the present study, measurements of the ER effects, first- and third-order dielectric permittivities, and dc conductivity are made on the carbon suspension to understand the ER mechanisms. In the frequency dependence of the first- and third-order dielectric permittivities, different behaviors from those in cellulose and BaTiO3 suspensions [12], [13] are observed. In the first-order dielectric permittivity, a dielectric dispersion, which consistently explains the frequency dependence of the ER effect, is observed. However, the resonance peak due to the shear-induced particle rotation is not recognized in the first-order permittivity but only in the third-order permittivity. These results are discussed on the basis of two characteristic times, relaxation time of the interfacial polarization and the characteristic time of the particle rotation. From the measurements of the electrical conductivity, it is suggested that the conduction process is a thermally activated one and the activation energy, which may be associated with the accumulation of the charge near the interface and hence the interfacial polarization, is dependent on the electric field strength.

Section snippets

Experimental

The commercial ER suspension BA-1 (Bridgestone Co, Japan), in which 30 vol% of carbonaceous particles are dispersed in 10 cS silicone oil, was used for the measurements. The particles are prepared by pulverizing spherocrystals obtained from coal-tar pitch. The average size of the particles is 0.9 μm with their size ranging from 0.4 to 1.4 μm. The ER and the dielectric properties were measured simultaneously with a double cylinder type of viscometer [14]. A small ac voltage from a function

ER effect

Shear stress vs shear rate relationships when varying the amplitude of the electric field (20 Hz) are depicted in Fig. 1. In the absence of the electric field, the shear stress is almost proportional to the shear rate, indicating that the flow is Newtonian. On the other hand, when the electric field is applied the flow becomes non-Newtonian with the induced shear stress Δτ (field-induced change in the shear stress) being almost independent of the shear rate. As this figure shows, Δτ increases

Summary

ER effects, dielectric properties, and dc electrical conduction are investigated for the ER suspension composed of carbonaceous particles and silicone oil. The ER effect and first-order dielectric permittivity decrease at high frequencies, which are explained in terms of the relaxation of the interfacial polarization characterized by a relaxation time of ∼160 μs. In addition, effects of the shear-induced particle rotation are recognized in ε1 and ε3 with some decrease in ε1 at low frequencies

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