The permittivity at X-band frequencies of nickel-coated graphite fibers in an epoxy matrix
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P. Schoen Center for Bio/Molecular Science & Engineering, Code 6900, Naval Research Laboratory, Washington, DC 20375-5348 (Received 3 May 1993; accepted 27 August 1993)
In this study, we have investigated the microwave dielectric behavior of a composite formed by embedding nickel-coated graphite fibers in an epoxy matrix. Permittivities of composites in the X-band frequency range as a function of fiber concentration, fiber length, and the degree of fiber aggregation were studied. Fiber aggregation was reduced significantly by the addition of silica particles to the composite mixture before epoxy curing. Predictions from the mean field theory fit the experimental data well at dilute fiber concentrations.
I. INTRODUCTION Predicting the permittivity of a heterogeneous system consisting of two or more constituents with distinct properties is, in general, a complicated problem. The parameters affecting the permittivity of the composite include the permittivity of each constituent, the particle shape and size distributions, volume loading, and aggregation of the constituents. Varying one or two of these parameters can change the permittivity of the composite significantly. For the most commonly used formulas for calculating composite permittivity, i.e., the mixture formula, the integral formula, and the upper and lower bounds approximation, the parameters include only the permittivity of the constituents, the loading fractions, and the depolarization factors (which relate to the geometry of the constituents). The effect of aggregation of the constituents is not in general considered. This may be justifiable for ordered systems with constituents periodically arranged, for which contacts between particles occur only when one of the constituents is present in high concentration (leading to percolation behavior). However, for a disordered system with constituents randomly distributed (especially for high aspect ratio particles), aggregation can occur at dilute concentration. Doyle and Jacobs1 have shown that this aggregation is a key factor for abnormal permittivity enhancement of particulate (small particle) composites. In this study, we have investigated the dependence of the permittivity of composites with aligned fibers on the fiber concentration, fiber length, as well as fiber ag-
a) Currently
at Naval Research Laboratory, Code 6900, Washington, DC 20375-5348.
246 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 1, Jan 1994
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gregation. The advantage of using metal fibers instead of metal particulates is their strong shape anisotropy. With their high aspect ratio, these fibers give the composite a much higher permittivity in the alignment direction for much less filler concentration than would a particulate system. Thus, they present the possibility for lightweight, high permittivity composites with substantial dielectric anisotropy. Adding a third constituent (silica powder) increases the viscosity of the precured composite mixture, which improves the fiber dispersion in the matri
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