FABRICATION AND RADIO-FREQUENCY CHARACTERIZATION OF HIGH DIELECTRIC LOSS TUBULE-BASED COMPOSITES NEAR PERCOLATION

Microscopic lipid tubules with an average aspect ratio (length/diamete r) of approximately 12 were metallized electrolessly with copper or ni ckel-over-copper, and mixed with vinyl to make foot-square composite d ielectric panels. As loadings increased the metal tubule composites di splayed an onset of electrical percolation with accompanying sharp inc reases in real and imaginary permittivities. Gravity-induced settling of the tubules while the vinyl was drying increased true loading densi ty at percolation threshold for nickel/copper tubules to similar to 12 vol %. This threshold was at a significantly lower loading density th an that previously measured for percolation by composites containing s pherical conducting particles. Qualitatively, the shape of the composi te permittivity versus loading density curves followed predictions by the effective-mean field theory for conducting stick composites. Chang es in permittivity of the vinyl panels were observed for several days after fabrication, and were apparently associated with solvent evapora tion from the matrix. [S0021-8979(98)05723-5].