Microwave Dielectric Properties of Elastomers Containing Particulate Metal Fillers
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MICROWAVE DIELECTRIC PROPERTIES OF ELASTOMERS CONTAINING PARTICULATE METAL FILLERS BARRY G.M. HELME Department of Materials Engineering and Materials Design, University of Royal Nottingham, University Park, Nottingham, NG7 2RD, England, UK. Society/SERC Research Fellow on secondment from Quasar Microwave Technology Ltd., Moorswater, Liskeard, Cornwall, PL14 4LQ, England, UK. ABSTRACT Elastomers have been manufactured containing varying concentrations of particles of Ag/Al, Ag/Cu, Ag/Ni, Ag/glass (G3), Ni/graphite, Ni, and Ag. Their dielectric and microwave properties have been measured between 0.4 GHz and 40 GHz and at temperatures between ambient and 100°C. The filler particles are small relative to the microwave wavelength but larger than the skin depth. Sheets of these elastomers of up to 2 mm thickness gave been subjected to cyclical stretching of up to 40% elongation and up to 10 cycles. The microwave properties have been measured as a function of numbers of stretching cycles and composition of elastomer. Some correlations were found between the composition, the microwave properties and the number of stretch cycles completed. It was found in general that repeated stretching degraded the initially high conductivity of the loaded elastomers. This effect was also revealed by the increase in microwave transmissivity through the sheets, and a decrease in reflectivity. Formulations of elastomer-metal particle mix have recently been produced which have a microwave transmissivity more Phan 80 dB below the incident signal, for a 2 mm thick sample, even after 10 stretching cycles of 20% elongation. This property is of great value for the use of the elastomer as a reflective microwave shield. INTRODUCTION 1Elastomers are well known for their electrically insulating properties (p >100a cm) and their ability to be repeatedly stretched by significant percentages of their liner dimensions while being able to relax to their original shape. They are used extensively for flexible elastic environmental shielding and gaskets. Elastomers and polymers in general may be modified so that their resistivities are greatly reduced and they become relatively good electrical conductors (WO n cm). Such materials have important applications including use as elastic and rigid electromagnetic interference shielding [1], electrostatic discharge protection [2], conductive adhesives [3], thermistors [4], piezoelectrics [5], selective polymer heaters [6], capacitors [7], thick film resistors (8], and threshold switches [9]. The present work is concerned with electromagnetic interference shielding. The resistivity is achieved either by doping the insulator with suitable chemical impurities [10], or by mixing a fine powder of good conductor with the material before curing. The ],atter method was used to produce conductive elastomers which are the subject of this work. Many experimental investigations have been published concerning the electrical properties of conductive loaded polymers and elastomers [11,12]. The conductive particles may be of a typ
