Towards high-performance electrochemical thermal energy harvester based on ferrofluids

The ionic liquid-based thermo-electrochemical cells receive increasing attention as an inexpensive alternative to solid-state thermo-electrics for waste heat harvesting applications. Recently, it has been demonstrated that magnetic nanoparticles (MNPs) in liquid-based thermoelectric materials result in enhancement of the Seebeck effect opening new perspectives to the design of a thermoelectric device with relatively high efficiency and cost effectiveness. Here, the role of an interacting assembly of MNPs in the thermoelectric signal is studied for the first time. Based on a thermodynamic approach, an analytic expression has been derived for the Seebeck coefficient that includes the inter-particle magnetic interactions in the assembly and the nanoparticle's magnetic characteristics (saturation magnetization, magnetic anisotropy). Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm is performed to calculate their contribution to the Seebeck coefficient, for diluted assemblies of \u3b3-Fe2O3 and CoFe2O4 nanoparticles, materials commonly used in ferrofluids. The results demonstrate the increase of the size and temperature range of the Seebeck coefficient with the increase of nanoparticles\u2019 magnetic anisotropy paving the way for the detailed study of the magneto-thermal effects in high-performance thermoelectric materials based on ferrofluids

Micromagnetic simulation of a ferromagnetic particle

In this work, the magnetic behaviour of a ferromagnetic particle has been investigated by means of micromagnetic modelling, using the Finite Element Method. The simulations were performed on an ellipsoidal particle with uniaxial magnetocrystalline anisotropy by varying the anisotropy constant, the shape and dimensions of the particle. The results indicate the critical particle size for different reversal modes. Above a critical size the formation and motion of domain walls is clearly observed. The associated nucleation and coercive fields are estimated from the demagnetization curves

Effect of organic coating on the charge distribution of CoFe2O4 nanoparticles

We have studied the charge distribution of organic surfactant coated CoFe2O4 spherical nanoparticles together with the uncoated ones, using density functional theory (DFT)calculations. The coatings used are the diethylene glycol (DEG)and oleic acid (OA). Our results showed that the coating influences more the charge of the Co atoms and less the Fe atoms charge while the influence is very small in the O atoms and that the effect is more pronounced in the charge of the DEG coated particles. The average electrostatic potential and electric field have higher strength of interaction in the coated particles, especially the ones with the DEG coating

A New Approach to Include Surface Contributions in Micromagnetic Simulations of Nanoparticles

AbstractIn this work a micromagnetic model is presented for ferromagnetic nanoparticles where the surface is treated as a single effective layer and not as a separate shell. The model consists of two coupled Partial Differential Equations (PDE), one for the magnetization vector of the bulk volume and the second for the outer nodes. The strength of the coupling depends on the effective width of the layer. Simulations were made by means of the Finite Element Method (FEM). For a comparison FEM for core/shell type and atomistic Monte Carlo simulations were also performed. Our results show that Hs, the field where reversal takes place, varies as ∼1/D, where D is the particle's radius, with the anisotropy strength for any anisotropy direction. Moreover the computational cost of the effective layer model is lower than the core shell one, thus can be easily extended to larger particles where dipolar interactions should also be taken into account

Effect of albumin coating on the magnetic behavior of Mn ferrite nanoclusters

The effect of clustering induced by albumin coating on the magnetic behaviour of ultra-small MnFe2O4 nanoparticles has been systematically investigated and compared with that in pure Mn ferrite nanoparticle dense assembly, using a mesoscopic scale approach and numerical simulations reproducing the experimental findings well. Our results provide evidence that in the coated system, the interplay between intra-particle and intra-cluster exchange interactions strongly affects the exchange bias and coercive field values, with the dipolar interactions playing a minor role. Instead, the albumin coating does not affect the thermal stability of the observed superspin glass phase, the freezing temperature being similar in the coated and uncoated systems

Optimising the magnetic performance of Co ferrite nanoparticles via organic ligand capping

Ferrofluids of CoFe2O4 nanoparticles are gaining increasing interest due to their enhanced heating performance in biomedical applications (e.g. in magnetic hyperthermia as mediators for cancer treatment) or in energy applications (e.g. magneto-thermo-electric applications). Until now, the effect of an organic surfactant on the magnetic particle behaviour has been unintentionally overlooked. Here, we present the counterintuitive magnetic effect of two representative organic ligands: diethylene glycol (DEG) and oleic acid (OA) bonded at the surface of small (approximate to 5 nm in size) CoFe2O4 particles. The combined results of the bulk dc susceptibility, local-probe Mossbauer spectroscopy and physical modelling, which is based on electronic structure calculations and Monte Carlo simulations, reveal the effect of different ionic distributions of the particles due to the different surfactant layers on their magnetic behaviour. They result in an unexpected increase of the saturation magnetisation and the blocking temperature, and a decrease of the coercive field of DEG coated CoFe2O4 nanoparticles. Our work provides a pathway for the production of colloidal assemblies of nanocrystals for the engineering of functional nano-materials

Charge distribution on the water/γ-Fe 2 O 3 interface

International audienceWe have studied the charge distribution in the γ-Fe 2 O 3 interface with H 2 O, for two different structures (films and spherical nanoparticles) with Density functional (DFT) molecular dynamics calculations. Our results show that the adsorption energy depends on the shape of the surface and in the case of the films also on the orientation of the crystal and that the ionic state of iron atoms increases with the addition of water in both structures while the magnetic moments of the structures do not show any significant change. The mean displacement of the charge with temperature is significant only in the spherical nanoparticles. The average electrostatic potential decreases with the addition of water and shows an oscillatory behavior near the surface.

Effect of albumin mediated clustering on the magnetic behavior of MnFe2O4 nanoparticles: Experimental and theoretical modeling study

Over the last two decades, iron oxide based nanoparticles ferrofluids have attracted significant attention for a wide range of applications. For the successful use of these materials in biotechnology and energy, surface coating and specific functionalization is critical to achieve high dispersibility and colloidal stability of the nanoparticles in the ferrofluids. In view of this, the magnetic behavior of clusters of ultra-small MnFe2O4 nanoparticles covered by bovine serum albumin, which is known as a highly biocompatible and environmentally friendly surfactant, is investigated by magnetization measurements, and numerical simulations at an atomic and mesoscopic scale. The coating process with albumin produces a change in the structure, actual size and shape distribution of clusters of exchange coupled particles, giving rise to a distribution of blocking temperatures. The coated system exhibits a superspin glass (SSG) behavior with the SSG freezing temperatures similar to the uncoated ones, providing evidence that the strength of the dipolar interactions is not affected by the presence of the albumin. The DFT calculations show that the albumin coating reduces the surface anisotropy and the saturation magnetization in the nanoparticles leading to lower values of the coercive field in agreement with the experimental findings. Our results clearly demonstrate that the albumin coated clusters of MnFe2O4 particles are ideal systems for energy and biomedical applications since colloidal and thermal stability as well as biosafety is obtained through the albumin coating