Sašo Gyergyek

Colloidal stability of oleic- and ricinoleic-acid-coated magnetic nanoparticles in organic solvents.

The colloidal stability of oleic- and ricinoleic-acid-coated nanoparticles in organic solvents with dielectric constants ε(r) ranging from 2.0 to 9.8 was studied. Although the acids are structurally similar, there is an OH group in the ricinoleic acids tail, a marked improvement in the colloidal stability of the ricinoleic-acid-coated magnetic nanoparticles in moderately polar organic solvents and monomer methyl methacrylate was observed as a result. The bonding of both acids provokes a significant change in the surface properties of the iron-oxide nanoparticles. A clear shift from a strong electron-donor to a weak electron-donor was confirmed with the bonding of the oleic acid. The effect of ricinoleic acid bonding is even more dramatic: a clear shift toward a weak electron-acceptor is evident. A detailed analysis of the total energy of interaction, including the vOCG theory, between two particles was used to describe the different behaviors of the coated nanoparticles. In the case of the oleic acid nanoparticles in an apolar medium, such as decane, a small net attraction of ∼0.84k(B)T, which is insufficient to cause nanoparticles agglomeration, exists. In polar media the net attraction is larger than 1.5k(B)T, resulting in precipitation of the oleic-acid-coated nanoparticles. The same findings apply to the ricinoleic-acid-coated nanoparticles, but only when dispersed in the apolar medium. In the polar medium an additional repulsion due to polar solvation forces exists, resulting in a decrease of the net attraction to as low as ∼0.14k(B)T.

Orientational Order-Magnetization Coupling in Mixtures of Magnetic Nanoparticles and the Ferroelectric Liquid Crystal

We have studied the ferroelectric SmC* phase of SCE9 ferroelectric liquid crystal (LC) mixtures with magnetic nanoparticles (NPs). The impact of the NPs on the Goldstone and soft mode dielectric response has been determined by the dielectric spectroscopy measurements. The possible indirect coupling between the magnetic moments and the electrical polarization has been verified by measuring the impact of the electrical field on the magnetic susceptibility via SQUID susceptometer measurements. The disordering effects on the ferroelectric phase transition have been studied by the high resolution calorimetry. Similar disordering effects have been found as in the case of the aerosil particles [1, 2].

Preparation of Nanosized Copper and Cadmium Chalcogenides by Mechanochemical Synthesis

Nanosized copper and cadmium chalcogenides have been synthesized by mechanochemical route from elementary precursors, using a SPEX 8000M ball mill. Most reactions were carried out in air, while some of the reactions had to be performed under nitrogen to avoid oxidation. The obtained nanoparticles were characterized by X-ray powder diffraction (powder XRD) and transmission electron microscopy (TEM). Following products were obtained and characterized: CuS, CuSe2, Cu7Te5, CdS, CdSe, and CdTe, with particle sizes within 8 and 19 nm.

The effect of magnetic nanoparticles upon the smectic-A to smectic-C* phase transition

ABSTRACT We report on wide-angle X-ray scattering measurements along the smectic-A to chiral ferroelectric smectic-C* phase transition of the liquid crystal SCE9 and its mixture with maghemite magnetic nanoparticles of typical dimension 20 nm. The temperature profiles of the tilt angle are fitted by an extended mean-field model. Neither pre-transitional order effects nor variations in the SmA layer thickness are observed, indicating a rather negligible influence of these nanoparticles upon the molecular orientation at the smectic-A to smectic-C* phase transition of SCE9. These results are very different from what was observed for smaller CdSe nanoparticles (3.5 nm) where both a dilation of the smectic layers in the SmA phase and a crossover behaviour for the smectic-A to smectic-C* transition away from tricriticality have been observed for analogous concentrations. GRAPHICAL ABSTRACT

Anisotropic microrheological properties of chain-forming magnetic fluids

In an external magnetic field, magnetic colloids transform from an isotropic fluid to an anisotropic viscoelastic material. Using passive microrheology, we measured the microrheological properties of a magnetic fluid as a function of direction and magnitude of external magnetic field. The effective microviscosity strongly depends on the magnitude of the external field, while it is almost independent of its direction. The measured effective storage modulus varies significantly within the sample and depends both on the direction and the magnitude of the external magnetic field. It vanishes in a zero field, while in a non-zero field it is larger by a factor of 2 to 4 in the direction along the field than perpendicular to it. The non-zero value of the storage modulus, which indicates the formation of a viscoelastic fluid, appears at magnetic fields at which the dynamic light scattering experiments reveal the formation of elongated structures.

Multiferroic Behaviour in Mixtures of the Ferroelectric Liquid Crystal and Magnetic Nanoparticles

A dielectric spectroscopy, magnetic susceptibility and high resolution calorimetry have been carried out in the vicinity of the ferroelectric smectic C* phase of SCE9 ferroelectric liquid crystal (LC) mixtures with magnetic nanoparticles (NPs) to determine the impact of the magnetic nanoparticles on the Goldstone and soft mode and to study the disordering effects on the ferroelectric phase transition. It was verified via SQUID susceptometer that the indirect coupling between the NPs magnetic moments and the LC electrical polarization exists in these mixtures.

Mixtures of Magnetic Nanoparticles and the Ferroelectric Liquid Crystal: New Soft Magnetoelectrics

A mixture of maghemite ferromagnetic nanoparticles and the ferroelectric liquid crystal Felix 015/100 was experimentally analysed in the vicinity of the ferroelectric smectic C* (SmC*) phase. In order to verify the existence of magnetoelectricity in prepared soft material, a SQUID-based magnetometer was used. It was shown that the orientation of magnetic nanoparticles is directly coupled to the liquid crystal molecular director field. Such coupling allows possibility of indirect coupling between the nanoparticle magnetization and liquid crystal polarization. This makes such mixtures potential for new soft magnetoelectrics.

Phase Ordering in Mixtures of Liquid Crystals and Nanoparticles

We have studied the coupling interaction between liquid crystal (LC) molecules and nanoparticles (NPs) in LC=NPs mixtures. Using a simple phenomenological approach, possible structures of the coupling term are derived for strongly anisotropic NPs. The coupling terms include (i) an interaction term promoting the mutual ordering of the LC molecules and the NPs, and (ii) the Flory-Huggins-type term enforcing the phase separation. Both contributions exhibit the same scaling dependence on the diameter of the NPs. However, these terms only exist for a finite degree of nematic LC ordering. The magnetic response due to the LC-NPs coupling is probed experimentally for a mixture of weakly anisotropic magnetic NPs and a ferroelectric LC. A finite coupling effect was observed in the ferroelectric LC phase, suggesting such systems can be used as soft magnetoelectrics.

Preparation of a Superparamagnetic Nanocomposite with a High Content of Magnetic Iron Oxide in a PMMA Matrix by Precipitation Polymerization

We have prepared nanocomposites containing large amounts of superparamagnetic nanoparticles dispersed in a polymethyl methacrylate matrix. The preparation was divided into three steps. In the first step, maghemite nanoparticles were synthesized using coprecipitation from aqueous solutions, followed by coating with oleic acid (OA). In the second step, the OA-coated nanoparticles were dispersed in n-decane to prepare a stable, concentrated suspension. Finally, methyl methacrylate was added to the suspension and in situ polymerization was carried out at elevated temperatures. The content of nanoparticles was controlled by varying the nanoparticles/monomer ratio. The main focus was on controlling the nanocomposites homogeneity. The nanocomposites were characterized using X-ray powder diffractometry, TEM, SEM, thermogravimetry, FT-IR, NMR and magnetic measurements. The TEM analysis showed that the nanoparticles were well dispersed in the polymer matrix. They retained their superparamagnetic nature even when encapsulated by polymer with concentrations up to 48 wt%. The high loading of magnetic nanoparticles resulted in relatively high saturation magnetizations of the nanocomposites, up to 31 emu/g.

Indirect Magnetoelectric Coupling in Mixtures of Magnetite and Ferroelectric Liquid Crystal

The existence of an indirect magnetoelectricity in a soft composite material prepared as a mixture of magnetite ferromagnetic nanoparticles (NPs) and the ferroelectric liquid crystal (FLC) Felix 015/100 was verified by using a SQUID-based magnetometer. It was observed that the orientation of nanoparticles is directly coupled to the liquid crystal director field. Such a coupling allows the possibility of indirect coupling between the magnetic and ferroelectric order. This gifts the mixtures of FLC and magnetic nanoparticles (MNPs) with a great potential for soft indirect magnetoelectrics.