A study of the frequency dependence of the dielectrophoretic effect in thermoset polymers
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A study of the frequency dependence of the dielectrophoretic effect in thermoset polymers C. P. Bowen, T. R. Shrout, R. E. Newnham, and C. A. Randall Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 25 September 1996; accepted 12 February 1997)
Ceramic-polymer composites with a 1-3 connectivity can be created via a novel process called dielectrophoretic assembly. The process involves an electric field which is applied to a suspension of ceramic particles in an uncured thermoset polymer matrix. Under appropriate conditions, the applied electric field acts to induce a spatial redistribution of the particles into a chained or fibril structure. It was shown previously that the electrorheological response and fibril microstructure are dependent on both the frequency and magnitude of the applied alternating electric field.3 This paper will show that the frequency dependence of the uncured thermoset polymer suspensions results from the complex electrical phenomena specific to each thermoset system. Specifically, it will be shown through low field dielectric measurements and high field current-voltage analysis that the dielectrophoretic effect can be limited by electrode polarization, ionic conductivity, and space charge relaxation. It is the frequency dependence of these limiting phenomena that gives rise to the observed frequency dependence in the dielectrophoretic force of attraction being utilized to drive particulate assembly.
I. INTRODUCTION
Recently, dielectrophoresis has been shown to be a possible means to create novel structures that enhance the properties of particulate-polymer composites.1–3 This structure is achieved by applying an electric field to a suspension of dielectric particles in an uncured thermoset polymer which perturbs the system in such a way as to redistribute the particles into a chain-like structure. Depending on how the electric field is applied to the suspension, two phenomena can occur, namely, electrophoresis and dielectrophoresis. Electrophoresis refers to the migration of charged particles toward the electrode of opposite charge whereas dielectrophoresis refers to a dipolar interaction causing migration of particles (charged or uncharged) to regions of higher field strength.4 Owing to the dielectric mismatch between the particle and fluid phases, the application of an electric field to a suspension results in field gradients being produced locally around each particle. The particles are also polarized by this local field which, through dipole-dipole interactions, destabilizes the colloidal equilibrium and induces a uniaxial flocculation of particles into fibrils in the direction of the applied electric field.4–6 It was demonstrated previously that in a variety of thermoset polymer systems, the uniaxial flocculation could be “tuned” under alternating electric fields to produce structures that were dependent on the applied frequency.3 In those experiments, it was foun
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