Thermoelectricity and Thermodiffusion in Magnetic Nanofluids: Entropic Analysis

Abstract

An analytical model describing the thermoelectric potential production in magnetic nanofluids (dispersions of magnetic and charged colloidal particles in liquid media) is presented. The two major entropy sources, the thermogalvanic and thermodiffusion processes are considered. The thermodiffusion term is described in terms of three physical parameters; the diffusion coefficient, the Eastman entropy of transfer and the electrophoretic charge number of colloidal particles, which all depend on the particle concentration and the applied magnetic field strength and direction. The results are combined with well-known formulation of thermoelectric potential in thermogalvanic cells and compared to the recent observation of Seebeck coefficient enhancement/diminution in magnetic nanofluids in polar media.

Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions – anisotropy of the Soret coefficient under a magnetic field

“…15 for D m (H) in Ref. 45 , including derivatives of the NP's chemical potential with respect to temperature. Note that here the Eastman entropy of transfer of the NP's is supposed independent onH.…”

“…Finally we present the in-field anisotropy of the diffusion coefficient D m and of the Soret coefficient S T . The field-dependence of S T is here adjusted with a mean field model 45 , similar to the one previously developed to describe the in-field D m -anisotropy 46,47 .…”

Structural, Thermodiffusive and Thermoelectric Properties of Maghemite Nanoparticles Dispersed in Ethylammonium Nitrate

“…It can be shown that at the other extreme condition, the Soret equilibrium, when the thermodiffusive motions of all particles are completed, the Seebeck coefficient Se st only depends on the redox couple's reaction entropy [27] (here, it is assumed that the Eastman entropy of transfer of redox species and that of counterions are small, and that the ionic strength at the hot and cold electrodes are not greatly modified)…”

“…In ionic, aqueous ferrofluids, the magnitude and the direction of S T are known to depend strongly on the ionic environment; i.e., the pH level, the counterion types and concentrations, the effective charge number of individual particles, etc. Despite such complex natures, the existing theoretical models [23][24][25][26][27] have been successfully applied to describe experimental observations [28][29][30][31] in the case of colloidal suspensions prepared in weak electrolytes. The link between the thermodiffusive motion and the resulting Seebeck potential has also been verified in ferrofluids based on weak electrolytes [18,19].…”

Inversion of thermodiffusive properties of ionic colloidal dispersions in water-DMSO mixtures probed by forced Rayleigh scattering

“…When submitted to a gradient of temperature rT , ionic nanoparticles (NPs) dispersed in a polar solvent are subjected both to the Ludwig-Soret e↵ect [1], which induces a gradient of volume fraction r and a Seebeck e↵ect [2,3], which induces an electric field E. This can be used for potential thermoelectric applications in terms of heat recovery [4][5][6][7][8][9][10]. In stationary conditions, both r and E are proportional to rT and the NPs migrate either towards the cold region or towards the hot region depending on the sign of their Soret coe cient S T , given (at high dilution) by r = S T rT .…”