Author
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NELSON, STUART |
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Submitted to: Institute of Electrical and Electronics Engineers
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/27/2001 Publication Date: N/A Citation: N/A Interpretive Summary: The electrical characteristics of materials, known as dielectric properties, are important in determining how materials interact with electric fields. In the application of radio-frequency and microwave heating of materials, such as foods and other relatively poor conductors of electricity, these properties determine how rapidly the materials will absorb energy from the high-frequency electric fields. These properties are also important because they can be used with appropriate low-power instruments to rapidly determine quality factors such as moisture content in cereal grains. In research on the use of radio-frequency and microwave measurements for sensing moisture content in grains, the packing density of the grain kernels can have an influence on the dielectric properties similar to that of changes in moisture content. Therefore, the effects of density must be determined for reliable measurements of moisture content. Certain dielectric mixture equations have been studied in connection with this research and used for the modeling of dielectric properties of grains. This paper describes tests with powdered coal and limestone samples that were measured for the Bureau of Mines in their mine safety research on prevention of explosions in coal mines. It uses these measurements and the calculation of the dielectric properties, also known as permittivities, to determine the accuracy of certain theoretical dielectric mixture equations in calculating the permittivities of powdered and granular materials. One mixture equation is proven to be superior in predicting the dielectric properties when compared to measured values, strengthening the justification for use of this mixture equation in further research on granular and particulate materials. Technical Abstract: The permittivities of pulverized samples of coal and limestone were measured over a range of bulk densities at 11.7 GHz and 20 degrees C, and were used, along with particle densities, determined by pycnometer measurements, and the Landau & Lifshitz, Looyenga dielectric mixture equation to determine the solid material permittivities. Subsequently, similar measurements were taken on a 35%-65% coal-limestone mixture, and the Landau & Lifshitz, Looyenga equation and the Complex Refractive Index dielectric mixture equation were used to calculate the permittivities of powdered coal, limestone, and the 35%-65% mixture at various bulk densities. Results compared to the measured permittivities showed that the Landau & Lifshitz, Looyenga equation provided much better estimates than the other equation, with errors of 0.15% in predicting the dielectric constants of powdered coal, 1.4% in predicting the dielectric constants of powdered limestone, and 0.5% in predicting the dielectric constants of the coal-limestone mixture. |
