Dielectric properties of dielectrophoretically assembled particulate-polymer composites
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Dielectric properties of dielectrophoretically assembled particulate-polymer composites C. P. Bowen, R. E. Newnham, and C. A. Randall Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 30 January 1997; accepted 28 July 1997)
The dielectrophoretic effect is a phenomenon in which dipole-dipole interactions are induced between particles in a suspension by an electric field.1–5 This dipole interaction leads to the formation of chains or fibrils parallel to the applied electric field. Recently, the dielectrophoretic effect has been shown to be a possible composite assembly technique permitting property changes to be induced with the appropriate electric fields.6,7 The results presented in this paper show that the dielectrophoretic assembly process can be used to engineer anisotropy into composite materials. Various filler materials are aligned in a thermoset polyurethane matrix and the dielectric properties are measured. Comparisons are drawn between the dielectrophoretically assembled composites and those processed conventionally in the absence of an electric field. Dielectric properties are modeled with modified mixing laws and discussed in relation to the composite microstructure and the alpha relaxations of the polymer phase.
I. INTRODUCTION
Chemical processing techniques have produced a wide variety of submicron to nanosized particulates of metallic, semiconducting, and insulating materials. The availability of these materials is increasing with the commercialization of these powders. Recent interest in functional composite materials has led to the development of processing methods to assemble these inorganic submicron particulates in polymeric matrices. One interesting approach to assembly is through the control of electrostatic forces applied to colloidal suspensions. When an electric field is applied to a suspension, two basic phenomena can occur: (i) if there is a net charge on the particle, the particle will undergo electrophoretic migration, and (ii) if dipoles are induced, particles will interact through a dipolar field and undergo a uniaxial coagulation process sometimes referred to as mutual dielectrophoresis.1–5 Both electrophoretic and dielectrophoretic assembly techniques are of interest for electroceramic composites.8,9 Dielectrophoretic assembly can be induced by applying a high alternating field across a particle suspension in uncured thermoset polymers. The assembly conditions depend on the electrode space charge polarization and the conductance of each polymer chemistry. For example, aligned particulates of any dielectric character (insulator, semiconductor, or conductor) can be obtained from a randomly dispersed distribution connectivity whenever a sufficiently large dipole-dipole force is induced.10 Dielectrophoretically aligned oxide particles in a silicone elastomer matrix are shown in Fig. 1. In a polyurethane matrix optimum alignment can be established with frequencies ,10
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