Dielectric Properties Measurements and Data
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DIELECTRIC PROPERTIES MEASUREMENTS AND DATA
JOSEPH A. CARPENTER,
JR.
Ceramics Division, National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT This paper reviews measurement techniques and sources of data for the dielectric constant and loss factor of solid materials. Accurate values of such properties are basic to intelligent design of materials processing schemes utilizing electromagnetic energy. Emphasis is on techniques and data for these properties in the "microwave" range of frequencies of roughly 108 to 1011 Hz and at elevated temperatures. INTRODUCTION Accurate values of the dielectric constant and the loss factor are needed for the intelligent design of schemes utilizing electromagnetic energy for processing or interrogation of solid materials. The inadequacies of the currently available data have been recognized [1,2]. This paper is intended to provide an overview of the techniques for making such measurements and the sources of data currently available. It is hoped that the currently available data will serve as bases on which measurements of better data will be made in the future. TECHNIQUES A variety of techniques are used for measurements in the "microwave" frequency band over the ranges of temperatures, dielectric constants and loss factors of interest. Three reviews (3-5] covering most of the techniques are recommended for background information. Two international round-robins to determine the expected accuracies of these methods have been conducted (3,6] which indicate that most can provide 1% accuracy in dielectric constants on the order of 2 to 10 and 2-5% in loss factors as low as 10-3 to 10-4. Similar to reference 5, the present paper groups the techniques under three categories: lumped element, resonant cavity, and transmission line. Lumped Element In these techniques the sample is inserted between and in contact with two electrodes (flat plate or coaxial) connected to an analyzing circuit which measures capacitance and conductance (see reference 7 for definitions). The dielectric constant is determined from the capacitance measured with the specimen inserted, the capacitance measured without the specimen inserted, and the specimen's thickness and electroded area. In many systems one or more of the electrodes is adjustable so as to maintain the same capacitance with the specimen inserted as without. The loss factor is determined from the conductance and the frequency of the signal. Modern analysis systems are available which perform such calculations automatically. These techniques have been standardized [8] as ASTM D 150, which along with [3,9] should be consulted for details of the theory and experimental procedures involved. Schemes suitable for high temperature measurements have been described [10]. Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
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These techniques can make measurements on a single sample over a wide range of frequencies up to about 108 or 109 Hz, the upper limit to the frequency range being set mainly by the requireme
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