Artificial dielectrics of conductive fibers in polymers: Effects of viscosity and matrix composition on permittivity and
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Artificial dielectrics of conductive fibers in polymers: Effects of viscosity and matrix composition on permittivity and loss Yung-Shou Hoa) Biochemistry Department, Georgetown University, Washington DC 20375
P. Schoen Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington DC 20375 (Received 9 December 1994; accepted 2 October 1995)
We report the microwave dielectric properties of composites made of nickel-coated graphite fibers in two different types of silicone polymers. The influence of matrix viscosity on the fiber alignment, fiber filamentation in the composites, and how those affect the permittivity and loss tangent of the composite are discussed. The dependence of the permittivity and loss of the composites on the permittivity and loss of the matrix will also be compared.
I. INTRODUCTION
The interest to miniaturize electronic components has created a search for new materials with specific dielectric properties, such as high permittivity and low loss tangent.1 Since the dimensions of microwave devices (such as microstrips, filters, resonators, etc.) are roughly proportional to the reciprocal of the square root of the dielectric constant, use of high permittivity material as substrates in these devices allows them to be miniaturized. One way to achieve a high dielectric constant material is to mix conductive particles (i.e., nickel, silver, copper, etc.) in a matrix to form a composite.2–4 The metal powder particles are polarizable and increase the permittivity of the composite material. 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 can be aligned in an insulating matrix, which will not only provide a higher dielectric constant at substantially smaller filler concentrations, but also offer strongly anisotropic behavior. When the sizes of conductive fillers in a composite are much smaller than the wavelength of radiation, the dielectric behavior of the composite can be characterized as an effective dielectric function. This dielectric function depends upon several factors, such as the filler concentration, the conductivity and morphology of the fillers, the dielectric properties of the matrix, and the dispersion of the fillers in the matrix. Various mixing formulas, which are commonly used in predicting the permittivity of a composite consisting of two or more constituents with distinct properties, were summarized in previous articles.5,6 Most of these predictions were a)
Currently at Naval Research Laboratory, Code 6900, Washington, DC 20375-5348. J. Mater. Res., Vol. 11, No. 2, Feb 1996
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limited to systems of low filler concentration, or with very specific microstructure (filler dispersion). The effect of dispersion and aggregation of fillers in the matrix was never fully discussed. In this report, we will show the importance of the
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