Thermoelectric energy recovery at ionic-liquid/electrode interface

A Thermally Chargeable Capacitor containing a binary solution of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide (EMIMTFSI) in acetonitrile is electrically charged by applying a tempera- ture gradient to two ideally polarisable electrodes. The corresponding thermoelectric coefficient is -1.7 mV/K for platinum foil electrodes and -0.3 mV/K for nanoporous carbon electrodes. Stored electrical energy is extracted by discharging the capacitor through a resistor. The measured capacitance of the electrode/ionic- liquid interface is 5 micro $\mu$F for each platinum electrode while it becomes four orders of magnitude larger $\approx 36$ mF for a single nanoporous carbon electrode. Reproducibility of the effect through repeated charging-discharging cycles under a steady-state temperature gradient demonstrates the robustness of the electrical charging pro- cess at the liquid/electrode interface. The acceleration of the charging by convective flows is also observed. This offers the possibility to convert waste-heat into electric energy without exchanging electrons between ions and electrodes, in contrast to what occurs in most thermogalvanic cells.Comment: 8 pages, 11 figure

Experimental Evidence of Fluctuation-Dissipation Theorem Violation in a Superspin Glass

We present the experimental observation of the fluctuation-dissipation theorem (FDT) violation in an assembly of interacting magnetic nanoparticles in the low temperature superspin glass phase. The magnetic noise is measured with a two-dimension electron gas Hall probe and compared to the out of phase ac susceptibility of the same ferrofluid. For "intermediate" aging times of the order of 1 h, the ratio of the effective temperature $T_{\rm eff}$ to the bath temperature T grows from 1 to 6.5 when T is lowered from $T_g$ to 0.3 $T_g$, regardless of the noise frequency. These values are comparable to those measured in an atomic spin glass as well as those calculated for a Heisenberg spin glass

Out-of-equilibrium dynamics in superspin glass state of strongly interacting magnetic nanoparticle assemblies

International audienceInteracting magnetic nanoparticles display a wide variety of magnetic behaviors ranging from modified superparamagnetism, superspin glass to possibly, superferromagnetism. The superspin glass state is described by its slow and out-of-equilibrium magnetic behaviors akin to those found in atomic spin glasses. In this article, recent experimental findings on superspin correlation length growth and the violation of the fluctuation-dissipation theorem obtained in concentrated frozen ferrofluids are presented to illustrate certain out-of-equilibrium dynamics behavior in superspin glasses

Crystalline Order Effects on the Magnetic Properties of Superlattices made of Cobalt Nanocrystals

International audienceWe present the magnetic property evolution in long-range ordered fcc supracrystals of Co nanocrystals (NCs) induced mainly by nanocrystallinity. By increasing the nanocrystalline order of individual NCs, ac susceptibility measurements indicate a significant increase in the 'interaction-to-anisotropy' energy ratio, a trend favourable for the formation of SFM state in these 3D supracrystals

On the <i>ac</i> Measurements of the Electrical Conductivity of Dilute Colloidal Electrolytes

The details of ac-conductivity measurements in a colloidal electrolyte cell with flat control gate electrodes are discussed. The use of impedance diagnostics in studies of transport phenomena in such systems requires caution in the definition of the thickness of the screening accumulation layers that appear along the metal-electrolyte interfaces. The value of this characteristic length λ0 critically depends on the volume fraction ϕ⊙ of colloidal particles in the bulk of the electrolyte. Accounting for the dependence λ0(ϕ⊙) makes it possible to consistently explain the large discrepancy in the available experimental data regarding the influence of the colloidal fraction on the effective conductivity of the colloidal suspension

Magnetothermopower of hole doped manganites La2−2x_{2-2x}Sr1+2x_{1+2x}Mn2_2O7_7 (0.3⟨x⟨0.5)(0.3\langle x\langle 0.5)

URL: http://www-spht.cea.fr/articles/s00/10

Enhanced Thermoelectric Power in Ionic Liquids

International audienceThe thiolate/disulfide organic redox couple (McMT−/BMT), derived from 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT), recently proposed as an alternative to usual inorganic couples in dye-sensitized solar cells, exhibits a remarkable reversibility and stability in a thermogalvanic cell containing a 1-ethyl-3-methyimidazolium tetrafluoroborate/acetonitrile (EMIMBF4/AN) mixture. The thermoelectric power depends strongly on concentrations of both the ionic liquid and the redox couple. It is enhanced by a factor of six at high ionic-liquid concentrations. The control of these parameters can be used to enhance the thermopower and the thermoelectric efficiency of future ionic-liquid thermocells

Two stage Seebeck effect in charged colloidal suspensions

International audienceWe discuss the peculiarities of the Seebeck effect in stabilized electrolytes containing the colloidal particles. Its unusual feature is the two stage character, with the linear increase of differential ther-mopower as the function of colloidal particles concentration $n_\odot$ during the first stage and dramatic drop of it at small $n_\odot$ during the second one (steady state). We show that the properties of the initial state are governed by the thermo-diffusion flows of the mobile ions of the stabilizing electrolyte medium itself and how the colloidal particles participate in formation of the electric field in the bulk of suspension. In its turn the specifics of the steady state in thermoelectric effect we attribute to considerable displacements of the massive colloidal particles in process of their slow thermal diffusion and break down of their electroneutrality in the vicinity of electrodes

Charged colloids at the metal–electrolyte interface

International audienceWe discuss the peculiarities of the structure of the interface between a metal and a stable colloidal dispersion of charged nanoparticles in an electrolyte. It is demonstrated that a quasi-2D ionic structure of elevated density arises in its vicinity due to the effect of electrostatic image forces. The stabilized colloidal particles, being electroneutral and spatially distributed objects in the bulk of the electrolyte and approaching the interface, are attracted to it. In their turn, the counterions forming their coat partially retract into the 2D-layer, which results in an acquisition by the colloidal particle of the effective charge eZ*≫e and which, together with its mirror image, creates the electric dipole. The formed dipoles, possessing the moments directed perpendicularly to the interface, form the gas with repulsion between particles. The intensity of this repulsion, evidently, depends on the value of the effective charge eZ* acquired by the nanoparticle having lost a number of counterions. It can be related to the value of the excess osmotic pressure P$_{osm}$ measured in the experiment. On the other hand, this effective charge can be connected by means of the simple geometric consideration with the structural charge eZ of the nanoparticle core being in the bulk of the electrolyte