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A Technique for Measuring the Dielectric Properties of Minerals at Microwave Heating Frequencies Using an Open-Ended Coaxial Line

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A Technique for Measuring the Dielectric Properties of Minerals at Microwave Heating Frequencies Using an Open-Ended Coaxial Line REPORT OF INVESTIGATIONS/1994 I PLEASE DO NOT REMOVE FRCM LIBRARY UBRARY SPOKANE RESEARCH CENTER RECttVED 4 U6 BUREAU OF MNES E. 315 AVE. SPOKANE, WA 86247 A Technique for ~easuringthe Dielectric Properties of Minerals at Microwave Heating Frequencies Using an Open-Ended Coaxial Line By J. B. Salsman and S. P. Holderfield U.S. Department of the Interior Mission Statement As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally-owned public lands and natural resources. This includes fostering sound use of our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the environmental and cultural valucs of our national parks and historical places; and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to ensure that their development is in the best interests of all our people by encouraging stewardship and citizen participa- tion in their care. The Department also has a major responsibility for American Indian reservation communities and for people who live in island territories under U.S. administration. Report of Investigatians 9519 A Technique for Measuring the Dielectric Properties of Minerals at Microwave Heating Frequencies Using an Open-Ended Coaxial Line By J. B. Salsman and S. P. Holderfield UNITED STATES DEPARTMENT OF THE INTERIOR Bruce Babbitt, Secretary BUREAU OF MINES This report is based on work done under an agreement between the University of Alabama and the U.S. Bureau of Mines. Internation$ Standard Sei-ial Number ISSN 1066-5552 CONTENTS Page Abstract ........................................................................... Introduction ....................................................................... Measurement of the dielectric properties of minerals .......................................... Thotyofmeaswement ............................................................. Determination~fC~andC,.......................................................... Selection of reference materials for determining capacitances .................................. Determination of density effects ....................................................... Mathematical relationships ........................................................... Equipment ......................................................................... Coaxialline ...................................................................... Networkanalyzer .................................................................. Heatingdevice .................................................................... Densitycharnber .................................................................. Experimentalprocedure ............................................................... Samplepreparation ............................................................... Calibration of network analyzer ....................................................... Dielectric property measurements ...................................................... Results ............................................................................ Conclusions ........................................................................ Referenws. ........................................................................ Appen&.-Abbreviations and symbols used in this report ...................................... ILLUSTRATIONS Equivalent circuit of a sample at the end of a coaxial lime ................................... Cross section of a typical coaxial line .................................................. End of the coaxial line where the spacer was placed ....................................... Cohal line constructed for this measurement method ..................................... Impedance standards constructed for calibration of test equipment ............................ Nettvork analyzer in Smith chart diplay mode ........................................... Heaters used in the measurement system to heat mineral samples to temperatures of 325 "C ......... Insulating box used to house the heating apparatus and mineral samples to alleviate sample oxidation dwhgtheheathgcycle ........................................................... Holding apparatus constructed for measuring dielectric properties of minerals as a function of temperature and density .......................................................... Density variation apparatus ......................................................... Complete measurement system ...................................................... Dielectric constant of chalcopyrite at diferent frequencies and temperatures ..................... Loss factor of chalcopyrite at different frequencies and temperatures ........................... Dielectric constant of chalcocite at different frequencies and temperatures ....................... Loss factor of chdcocite at different frequencies and temperatures ............................ Dielectric constant of chalcocite at different temperatures for two discrete frequencies .............. Loss factor of chalcocite tit different temperatures for two discrete frequencies ................... Dielectric constant of cobaltite at different temperatures and frequencies ........................ Loss factor of cobaltite at different temperatures and frequencies ............................. Dielectric constant of chalcopyrite .................................................... Loss factor of chalcopyrite .......................................................... 1. Comparison of dielectric constant and loss factor of measured and calculated data for a 0.2-N saline solution ....................................................................... mm millimeter GHz gigahertz pet percent pm micrometer g/cins gram per cubic cecenheter kHz kilohertz "6 degree Celsius MHz megahertz Reference to specific products does not imply endorsement by the US. Bureau of Mines, A TECHNIQUE FOR MmSURtNG THE DIELECTRIC PR8PEfa"TIES OF MINERALS AT MICROWAVE HMTING FREQUENCIES USING AN OPENWENDEDCOAXIAL LIME By J. 8. ~alsman'and S. P. ~oldertield~ ABSTRACT As part of the research effort on investigating the effects of microwave energy on the chernieal and physical properties of minerals and ores, the U,S, Bureau of Mines (USBM) Tuscaloosa Research Center has conducted research to measure the dielectric properties of these materials, The objective was to establish a reliable database for use in predicting the effects of microwave heahg on a wide rmp of minerals. The dielectric constants and loss factors of minerals, commonly referred to as the dielectric prop- erties, were determined utilizing the theory of microwave propagation through an open-endedl, ;air-filled line that was terminated at its open end with the particular mineral under investigation. Zo this phase of microwave research, the USBM measured the dielectric properties of powdered min- erals with medium to high electrical conductivities in the frequency range of 300 MHz to 3 GHz and as a function of temperature from 25 to 325 "C. Since the minerals were prepared as powders, tech- niques were used to relate the measured dielectric properties of the powdered minerals to the dielectric propertim of the mineral at its theoretical or natural density, "&&at enajneer, Tusrxrloosa Researeh Center, U.S. Bureau of Mines, Tuscaiochsa, AL (naw with Salt Lake Gity Research Center, U.S. Bureau af Mines, Salt Lake City, UJJUT), 'mernlca~enajneer, Tuscal- Research Center. INTRODUCTION Major research efforts are underway worldwide on the data is that until the recent development of advanced elec- use of microwave energy in various processing techniques. tronic equipment such as the network analyzer, it was Deterrents to these research efforts are the lack of suf- very difficult to develop successful measuring systems in ficient data on the electrical properties of materials (in- this frequency range. This range is too high for radio- cluding minerals), and the lack of personnel trained in frequency techniques and is at the low-frequency end of microwave heating technology. It is very likely that micro- the microwave spectrum. However, it is this frequency wave energy may offer many advantages to minerals bene- range that is of interest, for it includes the frequencies ficiation and processing techniques; however, it is difficult allocated by the Federal Communications Commission for to exploit these advantages without an understanding of use in industrial, scientific, medical, and domestic appli- the electrical properties of minerals. As part of its mission cations (915 MHz, 2.45 GHz, and 22 GHz); the widely to advance research in processing techniques, the U.S. used microwave heating frequencies are 915 MHz and Bureau of Mines (USBM) is studying the use of micro- 2.45 GHz. wave energy. Microwave technology is beginning to find many suc- The electrical properties of a mineral are those prop- cessful uses in industrial applications other than food prep- erties that characterize how that mineral is affected by aration or communications. Recent investigations of the an electric field, and these properties may, and often do, use of microwave energy as a heat source in materials- change with frequency. The electrical properties of min- processing techniques have shown several advantages over erals are also temperature dependent. At the microwave conventional methods of heating. For example, studies heating frequencies of 300 MHz to
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