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DIELECTRIC PERMITTIVITIES AT X-BAND FREQUENCIES OF CONDUCTIVE FIBERS ALIGNED INPOLYMER MATRICES YUNG-SHOU HO*, J. LODGE**, AND P.E. SCHOEN*** * University of Florida, MD Chemistry Dept., Gainesville, FL **SFA Inc., Landover, ***Naval Research Laboratory Code 6090, Washington D.C. ABSTRACT: Composites have been formed of Cycom, nickel-coated graphite fibers, aligned at concentrations of up to 6 weight percent in polymer matrices. Dielectric constants up to approximately 70 have been measured for these samples at x band frequencies. The results have been compared with a single non-interacting particle theory. Introduction: The trend to minimize the size of electronic components has created a demand for materials with high dielectric constants. One approach to create such a material has been to mix conducting particles (i.e. nickel, silver, copper, etc.) into an insulating matrix. The metal particles are polarizable and increase the permittivity of the composite material.[1] The disadvantage of this approach is that large amounts of metal particles are needed to achieve significant values of dielectric response. In contrast, high aspect ratio conducting fibers that can be aligned magnetically in an insulating matrix will not only provide a higher dielectric constant at substantially smaller particle concentrations but also offer strongly anisotropic behavior. We have been studying the dependence of the dielectric constant of these fiber composites on the length and concentration of the fibers, upon their alignment and dispersion, and on matrix dielectric properties. The results will be compared with the predictions of a single, non-interacting particle theory that has been developed to predict the dielectric behavior of composites at low fiber concentrations. Experiment: Cycom, a nickel coated graphite fiber from American Cyanamid Co., with two different lengths (381 and 762 lm) and 8 gIm diameter was used as the conducting filler material. The nickel coating is about 400nm thick and has a conductivity of 1.4 x 107/ohm-m. Two different polymer matrices were investigated. One was Epoxy 815 from Shell Co., and the other was a mixture of 30% Epoxy 815 and 70% Beetle 216-10 resin (ureaformaldehyde in butanol) from American Cyanamid Co. Various concentrations of the fibers were mixed with the monomer. After a mixture was homogeneously blended and degassed, it was cast in a mold and aligned in a magnetic field. The samples were cured in the magnetic field for 24 hours at room temperature and cut into shape for dielectric measurement. Two techniques (waveguide reflection/transmission, and cavity resonance) have been used for microwave dielectric measurement.[2] Samples cut into rectangular shapes were pressed into x-band waveguide, with fiber alignment in the plane of the waveguide cross-section, either parallel or perpendicular to the microwave electric vector. Scattering S parameters were measured for each sample on a Hewlett-Packard 8510 network analyzer. Other samples were cast and aligned in blocks, from which were cut