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Dielectric properties of porous materials

Dielectric properties of porous materials
Typ:
Ort:

IFG, Build. 330, Room 356

Zeit:

12.06.2015, 5pm

Beginn: 12. June 2012
Dozent:

Gunnar A. Niklasson 

A review will be given of dielectric and conductivity phenomena in porous materials. In most cases of practical interest these materials can be viewed as a mixture of solid grains, water (containing ions) and sometimes air. The electrical conductivity consists of two parts, namely the bulk conductivity due to motion of ions in water-filled pores and a surface conductivity due to ion motion in the electrochemical double layer at the pore interfaces. The surface contribution dominates in the case of low conductivity of the pore fluid. Dielectric properties at high frequencies can to some extent be understood from effective medium theories, or mixing laws, obtained from solutions of the electrostatic problem for simple model geometries. Effective medium theories have frequently been applied in the high MHz to low GHz region to quantify water content in porous materials. However, such a procedure has large uncertainties, since there exists no universal effective medium theory and results are sensitive to the actual microstructure of the sample. Effective medium concepts are also able to describe the so called Maxwell-Wagner relaxation, which usually occurs at quite high frequencies. It is a result of the different conductivities of the constituents since ions from the pore liquid can more easily approach the pore interfaces and thus be blocked there. At frequencies in the kHz region and lower it appears that diffusive effects within the electrochemical double layer at the interfaces dominate the dielectric response. As the frequencies get lower, the ions have time to explore more and more of the complex pore network and finally give rise to electrode polarisation at the electrodes.

Several of the concepts described above will be illustrated with measured results from our laboratory. We have over the years studied some model materials containing single or multiple pores in a polycarbonate foil, and “artificial sandstones” consisting of silica spheres glued together with epoxy. Studies more related to various applications include dielectric measurements on cement mortar and natural sandstone (Lemunda) from the Visingsö formation in Sweden.