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Laboratory investigation of the thermoelectric effect in clean silica sands, A

Meyer, Clinton
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Abstract
The effect of thermoelectric coupling on self-potential was investigated by applying a temperature gradient to unconsolidated sands fully saturated by saline (NaCl) solutions. Thus far, very few published investigations have taken into account the effect of temperature on the electrodes and the ionic strength of the pore water, generating spurious diffusion potentials typically on the same order of magnitude as the thermoelectric effect. For this investigation, 25 experiments are conducted to test the influence of salinity upon the thermoelectric coupling coefficient over four orders of magnitude in salinity variation. The intrinsic thermoelectric coefficient ranged from -0.4 mV °C-1 (10-3 M NaCl) to +0.9 mV °C-1 at very low salinities (10-4 M). Silica sands with median grain sizes of 0.72 mm and 0.2 mm were used, with corresponding surface conductivities of 1.52x10-5 (S m-1) and 5.43x10-5 (S m-1) , respectively. In order to isolate the thermoelectric response associated with the temperature gradient, raw self-potential measurements were corrected for the diffusion potentials arising from different ionic strengths within our sand tank using a commonly accepted model. Our experimental data can be reproduced by a simple model accounting for the effect of surface conductivity due to the electrical double layer coating the surface of the grains. Our results indicate that Hittorf transport numbers changing with salinity need to be considered when developing a holistic model. Furthermore, when considering the polarity and magnitude of the thermoelectric response, salinity and grain size are important factors.
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