Dielectric Dispersion Measurements of Salt-Water Saturated Porous Glass Compared with Local Porosity Theory
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DIELECTRIIC DISPERSION MEASUREMENTS OF SALT-WATER SATURATED POROUS GLASS COMPARED WITH LOCAL POROSITY THEORY. B. D. HANSEN*, E. HASLUND*, R. HILFER*,**,*** and B. NOST*, *Department of Physics, University of Oslo, 0316 Oslo, Norway, **Institut fUr Physik, Universit~it Mainz, 6500 Mainz, Germany. ***Center for Advanced Study, Norwegian Academy of Science, P.O.Box 7606, 0205 Oslo, Norway
INTRODUCTION A recent study [1] of the dielectric frequency response of a two component composite performed on a single specimen shows that local porosity theory LPT [2] represents a substantial improvement compared with other theories predicting the complex dielectric dispersion [3,4,5]. The purpose of the present work is to extend this investigation to a systematic study on several specimens with different compositions. Sintered porous glass beads, saturated with salt water, are a very suitable model system for dielectric measurements. The composition is varied by changing the porosity through sintering the glass beads to different degrees and the other component can be varied by changing the fluid, leaving the pore structure unchanged. The LPT introduces a new method for characterizing the random pore structure through the so called "local porosity distribution function" and "the local percolation porbability function". Knowing these quantities, the complex dielectric frequency dispersion can be calculated without fitting parameters. By using image processing techniques on digitized pictures of the pore structure, we have measured for the first time the variation of the local porosity distribution with bulk porosity. The local percolation probability function of the specimens is determined by fitting the LPT to the observed dielectric dispersion. A more comprehensive test of the LPT has to await measurements of the local percolation probability function.
EXPERIMENTAL PROCEDURE Specimen Preparation Glass powder of spherical grains of diameter approximately 0.09 mm were fused in a steel cylindrical container at 700 C, making slugs of diameter 35 mm and length 70 mm. From homogeneous regions of the slugs, determined by x-ray tomography, the specimens were cut and ground to a cylinder shape of diameter approximately 25 mm and length 10 mrm. Different porosities were obtained by varying the sintering time. The study includes 5 specimens of porosity 4.8%, 8.5%, 12.2%, 14.7%, and 26.5%. The porosity is detennined by weighing the specimens dry and saturated with water and also the buoyancy of the watersaturated specimens was measured. Electrical Measurements The effective conductivity and permittivity of the salt-water saturated specimens were obtained by measuring the two-electrode admittance of a parallel plate condenser with the specimen between the plates. The stray capacitances were corrected for by measuring the admittance of the condenser with and without the specimen at the same plate separation. The stray capacitance is taken to be the difference between the capacitance of the empty condenser and an ideal parallel plate condens
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