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Cobalt ferrite (CoFe2O4) nanoparticles from 2 nm to 15 nm were synthesized using a solvothermal procedure. In this non‐aqueous method, acetophenone was used as solvent and the particle size was controlled by changing the reaction temperature from 120 °C to 180 ºC. Nanoparticles show good crystallinity and uniformity above 140 °C, as determined by X‐ray diffraction and transmission electron microscopy. Magnetic measurements indicate superparamagnetic behaviour. Zero‐field‐cooling (ZFC) dc‐magnetisation experiments at 100 Oe show blocking temperatures ranging from 215 K to more than 305 K. Saturation magnetisation increases with particle size and a small decrease is observed when increasing the temperature. The magnetocaloric effect shown by these cobalt ferrite nanoparticles is reasonably high for nanomaterials. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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The influence of the applied magnetic field on the magnetocaloric properties of a fine magnetic particle system has been studied using a Monte Carlo technique. By simulating zero field cooling (ZFC) curves under different strengths of the applied magnetic field, we have analyzed the variation of the entropy for temperatures above the maximum of the ZFC curves, where the process is reversible. The entropy curves have been observed to behave in a different fashion at low values of the magnetic field, where the peak only slightly shifts to higher temperatures with increasing fields. For larger fields, the peak rapidly shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. It is also observed that the blocking temperature as a function of the magnetic field shows the feature of a change from a bell‐like shape to a monotonically decreasing function, resembling what is found experimentally for intermediate values of the sample concentration. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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The structure and dynamics of a multi-subunit deformable particle are investigated by Brownian dynamics simulations. Pairwise hydrodynamic interactions between spherical subunits are described by the Rotne-Prager diffusion tensor and non-hydrodynamic interactions by means of an attractive energy parameter, E. In the absence of shear flow, the average simulated particle structure ranges from 'stringy' (€ <, kT) to compact, with a transition region in between. Time-dependent numerical results are presented for the average subunit coordination number and the average particle perimeter area as a function of increasing shear rate. Particle elongation in the flow field leads to a reduction in the effective fractal dimension of the particle. The greatest sensitivity to shear rate occurs in the stringy4ompact transition regime. Substantially different dynamic behaviour is obtained when the simulation is repeated without t h e inclusion of hydrodynamic interactions.Recently, we introduced' a new deformable particle model suitable for dynamic simulation consisting of a set of discrete interchangeable spherical subunits. Subject to the connectedness of the whole multi-subunit structure, the subunits are free to move relative to one another like particles in a loose floc of interacting colloidal spheres which cannot fall apart. This is different from conventional chain-like models of linear or branched macromolecules which have fixed bonds holding the subunits together. Brownian dynamics simulation of the deformable particle with 30 subunits has shown' that, by changing the value of a single energy parameter E, the average configuration of the subunits can be made to change from a stringy structure (E + kT) to a roughly spherical compact structure (E % k T ) ; in the transition region (E = 2.5

We report a computer simulation study of diffusion influenced reactions in a disorder medium constituting by immobile spherical obstacles when the concentration of reagents is smaller than the concentration of obstacles. We found that the compartmentalization of the embedding medium leads to a strong decrease of the rate of the first collision between reagents and a strong increase of the rate of recollision after a no-reactive encounter. The behavior of the full rate of reaction depends on the probability that a collision leads to reaction (value of the activation energy) and on the relationship between the decrease of the rate of collision and the increase of the rate of recollision. Thus, totally diffusion controlled reactions are always unfavored in these mediums, while partially diffusion controlled reactions with very high activation energy are more favored in mediums with a bigger degree of compartmentalization.