Porosity

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Porosity is important characteristics of porous materials. Our model DT-900 is capable of characterizing this parameter by measuring high frequency electric conductivity of porous material saturated with conducting liquid.

Application of high frequency (MHz scale) electric field is a novel feature of this instrument. It allows to:

Monitor the full porous space wetted with a liquid, including dead ends probes, which is impossible for traditional DC and low frequency methods.

Eliminate electrodes polarization, which allows very simple probe design – flat face touching probe.

Experimental raw data output of the measurement is so-called “formation factor”, which is ratio of the porous material conductivity to the conductivity of the liquid that wets this material. There is a well-known and well-verified theory that predicts formation factor. This theory was developed more than a hundred years ago by Maxwell and Wagner, and yields the surprisingly simple expression for the formation factor:

 

where P is the porosity.

Figure below shows the measured “formation factor” (conductivity ratios Ks/Km) as a function of the total porosity for a series of materials.

Formation factor for various heterogeneous and porous materials.Green triangles correspond to porous sandstones, blue diamonds correspond to CPG porous silica, red diamonds correspond to porous alumina, brown squares correspond to aluminium hydroxide dispersions at different voleme fractions prepared with equilibrium dilution. The solid line represents the prediction of Maxwell-Wagner theory.

We would like to stress that this method would yield total porosity of the sample, that consists of inter-particle porosity and porosity inside of the particles that includes contribution of all wetted pores even with dead ends. In order to extract material porosity one would have to assume inter-particle porosity. We have found that 40% value for this parameter agrees well with independent data.