Magnetically enhancing the Seebeck coefficient in ferrofluids

Ion specific tuning of nanoparticle dispersion in an ionic liquid: a structural, thermoelectric and thermo-diffusive investigation

Dispersions of charged maghemite nanoparticles (NPs) in EAN (ethylammonium nitrate) a reference Ionic Liquid (IL) are studied here using a number of static and dynamical experimental techniques; small angle scattering (SAS) of X-rays and of neutrons, dynamical light scattering and forced Rayleigh scattering. Particular insight is provided regarding the importance of tuning the ionic species present at the NP/IL interface. In this work we compare the effect of Li+, Na+ or Rb+ ions. Here, the nature of these species has a clear influence on the short-range spatial organisation of the ions at the interface and thus on the colloidal stability of the dispersions, governing both the NP/NP and NP/IL interactions, which are both evaluated here. The overall NP/NP interaction is either attractive or repulsive. It is characterised by determining, thanks to the SAS techniques, the second virial coefficient A2, which is found to be independent of temperature. The NP/IL interaction is featured by the dynamical effective charge ξeff0 of the NPs and by their entropy of transfer ŜNP (or equivalently their heat of transport ) determined here thanks to thermoelectric and thermodiffusive measurements. For repulsive systems, an activated process rules the temperature dependence of these two latter quantities.

A numerical study on the interplay between the intra-particle and interparticle characteristics in bimagnetic soft/soft and hard/soft ultrasmall nanoparticle assemblies

A mesoscopic scale approach and the Monte Carlo (MC) method have been employed to study the exchange bias behaviour of MnFe₂O₄ (soft)/maghemite (soft) and CoFe₂O₄ (hard)/maghemite (soft) nanoparticles (NPs) of size ∼ 3 nm in dense and diluted assemblies at low temperatures. The analysis of our MC results clearly shows that in the powder samples the contribution to the exchange bias field (H _ex) and the coercivity (H _c) comes mainly from the intraparticle core/shell structure in the hard/soft sample and that the interplay between the internal characteristics and the interparticle interactions is more important in the soft/soft samples where the dipolar strength is enhanced. In the diluted frozen ferrofluid samples where interparticle exchange interactions are absent and the role of the dipolar interactions is not significant the exchange bias effects are reduced, and they come from the intra particle structure. The variation of H _ex and H _c with the applied cooling field well reproduces the experimental findings and sheds light on the key mechanisms of the observed magnetic behaviour. Our results demonstrate the possibility to control the magnetic behaviour of nanostructures by using properly chosen core/shell bimagnetic nanoparticles.

Extensive Analysis on the Thermoelectric Properties of Aqueous Zn-Doped Nickel Ferrite Nanofluids for Magnetically Tuned Thermoelectric Applications

Because of the ever-increasing consumption of energy, higher-efficiency thermoelectric materials are in more demand. In this regard, initially, ZnxNi(1-x)Fe2O4 nanoparticles were synthesized using a co-precipitation technique and investigated using Rietveld refinement and Brunauer-Emmett-Teller (BET) analyses. Then, they were dispersed in water to obtain stable 1 vol % of aqueous ZnxNi(1-x)Fe2O4 nanofluids and their viscous, electrical, thermal, and thermoelectric properties were analyzed in the absence and presence of a magnetic field. The Rietveld refinement revealed the formation of single phase spinel ferrite structures and cationic distribution of ZnxNi(1-x)Fe2O4 nanoparticles, whereas BET analysis revealed the surface area of the nanoparticles. The viscosity studies proved the pseudo-plastic shear thinning behavior and dipole-dipole interactions of ZnxNi(1-x)Fe2O4 nanofluids. The electrical conductivity, thermal conductivity, and Seebeck coefficient studies revealed that the maximum enhancement was observed for the Zn0.2Ni0.8Fe2O4 nanofluid, which was attributed to the enhanced electrical double layer formation and Brownian motion of nanoparticles in the nanofluid. The enhancement in the properties of synthesized nanofluids in the presence of a magnetic field was attributed to the formation of chain-like structures, which was substantiated through the magneto-viscosity studies. The thermoelectric energy conversion efficiency of ZnxNi(1-x)Fe2O4 nanofluids was calculated which showed that the maximum enhancement of 27% was observed in the Zn0.2Ni0.8Fe2O4 nanofluid at 770 G. The observed results proved that the synthesized nanofluids are magnetically tunable thermoelectric materials which are suitable for waste heat energy harvesting applications.

Thermodiffusion anisotropy under a magnetic field in ionic liquid-based ferrofluids

Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide - EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at Φ = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m-1) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

Multifunctional Magnetic Nanocolloids for Hybrid Solar-Thermoelectric Energy Harvesting

Present environmental issues force the research to explore radically new concepts in sustainable and renewable energy production. In the present work, a functional fluid consisting of a stable colloidal suspension of maghemite magnetic nanoparticles in water was characterized from the points of view of thermoelectrical and optical properties, to evaluate its potential for direct electricity generation from thermoelectric effect enabled by the absorption of sunlight. These nanoparticles were found to be an excellent solar radiation absorber and simultaneously a thermoelectric power-output enhancer with only a very small volume fraction when the fluid was heated from the top. These findings demonstrate the investigated nanofluid's high promise as a heat transfer fluid for co-generating heat and power in brand new hybrid flat-plate solar thermal collectors where top-heating geometry is imposed.

Magnetically tuned thermoelectric behavior of Zn-doped magnetite nanofluids

In the present work, magneto thermoelectric behavior of the Zn-doped magnetite nanofluids is reported. Thermal and electrical conductivity studies have been done, compared and determined to be in line with the theoretical models. Thermoelectric voltage measurements have been carried out in the fluid samples for quite a number of temperature differences at various magnetic fields, and the Seebeck coefficient is calculated from the obtained measurements. It is observed that the fluid samples, which includes magnetite nanoparticle with zinc dopant concentration x = 0.2 shows better enhancement in electrical conductivity, mild enhancement in thermal conductivity and higher Seebeck coefficient value among all the samples. Also, a higher enhancement of 26% is observed in the Seebeck coefficient value of the same sample with an application of 770 G magnetic field. Hence, this is identified as a potential candidate for energy harvesting purposes such as thermoelectric generators in automobile systems, industries and etc.

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

Ethylammonium nitrate (ionic liquid) based ferrofluids with citrate-coated nanoparticles and Na + counterions were synthesized for a wide range of nanoparticle (NP) volume fractions ( Φ ) of up to 16%. Detailed structural analyses on these fluids were performed using magneto-optical birefringence and small angle X-ray scattering (SAXS) methods. Furthermore, the thermophoretic and thermodiffusive properties (Soret coefficient S T and diffusion coefficient D m ) were explored by forced Rayleigh scattering experiments as a function of T and Φ . They were compared to the thermoelectric potential (Seebeck coefficient, Se) properties induced in these fluids. The results were analyzed using a modified theoretical model on S T and Se adapted from an existing model developed for dispersions in more standard polar media which allows the determination of the Eastman entropy of transfer ( S ^ NP ) and the effective charge ( Z 0 e f f ) of the nanoparticles.