Microwave Properties of Non-Percolating Metal-Insulator Composites
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MICROWAVE PROPERTIES OF NON-PERCOLATING METAL-INSULATOR COMPOSITES I.S. JACOBS*, H.J. PATCHEN*, S.A. MILLER*, F.J. RACHFORD**, AND J.O. HANSON*** * ** ***
GE Corporate Research and Development, Schenectady, NY 12301 Naval Research Laboratory, Washington, DC 20375 GE Re-entry Systems Department, Philadelphia, PA 19101
ABSTRACT Permittivity and permeability have been studied at microwave frequencies (1 to 20 GHz) on artificial dielectrics of metal particles randomly loaded in a polymer up to 40 vol%. Ni-Cr alloy powder (-1 to 37 prm) in various size fractions was prepared by gas-water atomization, providing an oxide coating for interparticle isolation. The alloy is ferromagnetic below 165 K, enabling a magnetic measure of volume loading. The complex permeability results are in very good agreement with calculations of induced magnetic dipole effects as functions of frequency and particle diameter. Permittivity results are independent of particle size and essentially constant with frequency. When compared with a highly successful effective spherical cluster model and a well-founded empirical curve, the present alloy powder exhibits a residual upward deviation attributed to nonspherical morphology. In sum, the microwave electromagnetic behavior of such non-percolating composites is rather well understood. INTRODUCTION A random-packed metal-insulator composite is a limiting case of the class of artificial dielectrics which has attracted attention for well over a century. It relates to cermet-topology materials, to solar collectors, to models for the various mathematically equivalent phenomena in heterogeneous media, as well as to recently discussed far-infrared absorption anomalies [1]. We have sought a greater understanding of the electromagnetic properties of such composites by studying a well-dispersed system (see also [2]), a correlated (i.e., coated), non-percolating random array of quasi-spherical conducting particles loaded in an insulator up to 40 vol%. While the giant absorption anomalies were found largely in the infrared on submicron metal particles, we have chosen larger particles (1 to 37 gm) for study at microwave and selected millimeter wavelengths. We find that the permittivity and permeability behaviors of such composites can be well understood for the selected range of parameters. A brief description is presented here. A complete report will be submitted elsewhere. SAMPLE SELECTION AND PREPARATION There were several criteria for the conducting particles in this investigation. We required that the metal powder be nonmagnetic at room temperature but ferromagnetic at low temperature to facilitate determination of the volume loading, p, in the composites. The latter is key to quantifying the dielectric behavior. We also required that the powder be susceptible to developing a thin insulating oxide layer on each particle to prevent percolation. Powder preparation of the selected alloy was by gas-water atomization, in which a molten metal is forced through a specially designed nozzle and the liquid metal str
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