Dorothée Vinga Szabó

Polymer-nanoparticle composites: From synthesis to modern applications

The addition of inorganic spherical nanoparticles to polymers allows the modification of the polymers physical properties as well as the implementation of new features in the polymer matrix. This review article covers considerations on special features of inorganic nanoparticles, the most important synthesis methods for ceramic nanoparticles and nanocomposites, nanoparticle surface modification, and composite formation, including drawbacks. Classical nanocomposite properties, as thermomechanical, dielectric, conductive, magnetic, as well as optical properties, will be summarized. Finally, typical existing and potential applications will be shown with the focus on new and innovative applications, like in energy storage systems.

Synthesis and Magnetic Properties of Nanostructured Maghemite

Nanocrystalline maghemite, {gamma}{endash}Fe{sub 2}O{sub 3}, can be synthesized in a microwave plasma using FeCl{sub 3} or Fe{sub 3}(CO){sub 12} as the precursor. Electron microscopy revealed particle sizes in the range of 5 to 10 nm. In general, this material is superparamagnetic. The magnetic properties are strongly dependent on the precursor. In both cases the production process leads to a highly disordered material with the consequence of a low magnetization. The assumption of a disordered structured is also supported by electron energy loss (EEL) and Moessbauer spectroscopy. The structure of this material shows a nearly identical number of cations on tetrahedral and octahedral lattice sites. {copyright} {ital 1997 Materials Research Society.}

Magnetic properties of nanocrystalline Cr2O3 synthesized in a microwave plasma

Abstract Nanocrystalline Cr 2 O 3 particles were synthesized in a microwave plasma using chromium hexacarbonyl as precursor. Electron microscopy revealed that the particle diameter is, depending on the process parameters, in the range from 7 to 9 nm or from 5 to 6 nm. In the case of the larger particles, additionally elongated ones with a diameter from 6 to 7 nm and a length from 10 to 15 nm, were found. The larger particles were found to be single crystals. The smaller particle fraction showed a granular substructure, resembling to a polycrystalline particle. The size of these granules is in the range from 2 to 3 nm. The structure of the material was determined by electron diffraction to be the rhombohedral corundum structure. In the temperature range from 10 to 300 K the magnetic behavior can be described by a modified Langevin function but with a magnetic moment having a thermally activated component. In any event, in this temperature range the assumption Kv ⪡ kT is valid, where K is the anisotropy energy and v the particle volume. The thermally activated magnetization can be attributed to the material with the granular substructure. The size of the magnetic domains was found to be identical with the electronmicroscopically determined size of the granules.

Synthesis of nanocrystalline MoS2 and WS2 in a microwave plasma

Abstract This paper describes the synthesis of nanoparticulate MoS2 and WS2. These compounds are of special interest, as they form layered structures. The synthesis is performed by the reaction of the hexacarbonyls with H2S in a microwave plasma. The resulting particles are in the range of 5 to 15 nm in diameter. It is remarkable that, in contrast to other nanoparticles, these particles contain step dislocations in the {0002} planes and point defects, causing bending of the lattice planes. Additionally, particles exhibiting nested fullerene-like structures and polyhedron-shaped crystals were found.

Nanocoated particles: a special type of ceramic powder

Abstract Ceramic particles with sizes below 10 nm consisting of two different oxides in the core and the coating have been prepared by using a two-step microwave plasma process. Precursors for the synthesis are the chlorides. The structures occurring in such particles are demonstrated in the system alumina (Al2O3)-zirconia (ZrO2). When the alumina kernels covered with zirconia are small, or alternatively when the alumina coating on the zirconia kernels are thin, no special structural features are observed. In this case, the alumina is glassy and the zirconia is crystallized. In contrast, particles consisting of a crystallized zirconia core and a crystallized γ-alumina coating exhibit structural defects similar to dislocations that adjust to the different structures. These could be sessile dislocations in the {211} plane of the γ-alumina. Additionally, maghemite (γ-Fe2O3) particles coated with cubic zirconia were studied. The coating of this material is free of dislocations.

Coated Nanoparticles: A New Way to Improved Nanocomposites

This paper introduces the new concept of coated nanoparticles as starting material for improved nanocomposites. The very special properties of nanomaterials often are properties of isolated particles. After combining nanoparticles to a macroscopic workpiece, usually these special properties are lost. Therefore, to obtain macroscopic parts exhibiting the properties of the isolated particles it is necessary to avoid or at least reduce the interaction of the particles. This can be achieved by coating each individual particle with a second ceramic or polymer layer. This type of materials can be synthesised only by using the microwave plasma process, because in this process the particles leave the plasma zone with electrical charges thwarting agglomeration. Additionally, by proper selection of the coating material it is possible to avoid grain growth during densification of the powder by sintering or hot pressing. As an example of application and as a proof of concept, the properties of macroscopic superparamagnetic parts are explained. Possibly, coated nanoparticles are the only starting material to produce macroscopic parts showing the very special properties of nanomaterials.

Synthesis and properties of nanocrystalline superparamagnetic γ-Fe2O3

Abstract Nanocrystalline maghemite (γ-Fe 2 O 3 ) was synthesized in a microwave plasma energized by a 915 MHz generator. Precursor for this synthesis is water free FeCl 3 . The resulting particles were in the size range from 4 to S nm. Magnetic measurements revealed that the material synthesized by the microwave plasma route is superparamagnetic. At a temperature of 300 K and a magnetic field of 1 T the magnetization of this material was 2.7 Am 2 /kg. At 10 K this value increased to 4.3 Am 2 /kg. Additionally, measurements of the Mossbauer effect were performed. As expected, these measurements showed a pure quadrupole splitting at room temperature and two predominantly magnetic sites at 4 K. The blocking temperature is around 80 K.

Spin-glass freezing of maghemite nanoparticles prepared by microwave plasma synthesis

Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Neel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant Keff. Keff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv ≈ 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivity and exchange bias shows a sharp increase toward low temperatures which is due to enhanced surface anisotropy. The source of this enhanced magnetic anisotropy comes from the disordered surface spins which get frozen at low temperatures. Memory effects and thermoremanent magnetization experiments also support the existence of spin-glass behaviour. All these magnetic measurements signify either magnetic blocking or surface spin-glass freezing at high and low temperatures, respectively.Magnetic properties of 6 nm maghemite nanoparticles (prepared by microwave plasma synthesis) have been studied by ac and dc magnetic measurements. Structural characterization includes x-ray diffraction and transmission electron microscopy. The temperature scans of zero field cooled/field cooled (ZFC/FC) magnetization measurements show a maximum at 75 K. The ZFC/FC data are fitted to the Brown-Neel relaxation model using uniaxial anisotropy and a log-normal size-distribution function to figure out the effective anisotropy constant Keff. Keff turns out to be larger than the anisotropy constant of bulk maghemite. Fitting of the ac susceptibility to an activated relaxation process according to the Arrhenius law provides unphysical values of the spin-flip time and activation energy. A power-law scaling shows a satisfactory fit to the ac susceptibility data and the dynamic critical exponent (zv ≈ 10) takes value between 4 and 12 which is typical for the spin-glass systems. The temperature dependence of coercivi...

Nanoparticles from compounds with layered structures

Abstract Layered structures of the type B8, C6, and C7 excel in the fact that the binding within the layers between the metal and the non-metal is of the covalent type and between the layers of the van der Waals type. This leads, in the case of nanoparticles, to a large number of dangling bonds on the periphery of the layers. In the well-studied case of graphite, the formation of onion-like structures and nanotubes reduces the free energy of the system. It is shown that the formation of these morphologies is very general for small particles. This is shown for BN, MoS2, WS2, WSe2, and MoSe2. SnS2 and ZrSe2 are special exceptions, as nanoparticles of these compounds preferably form rope-like structures. These experiments lead to the general rule that nanoparticles made of layered compounds try to form closed structures.

Ceramic nanoparticles coated with polymers based on acrylic derivatives

Ceramic nanoparticles are synthesized by a microwave plasma process and coated with a polymer layer generated in situ by photopolymerization. Acrylic and methacrylic monomers are the most suitable precursors for polymer coatings. In the case of coatings derived from MMA, only a small quantity of the polymeric material is soluble in organic solvents. GPC analysis of the dissolved part reveals the predominance of oligomers. According to FT-IR spectra, a substantial number of MMA-ester groups is transformed into carboxylates, providing strong adhesion of the polymer to the ceramic core insoluble in THF. Composites with PTFE-analog coating exhibiting enhanced thermal and chemical stability are obtained from the condensation of perfluorinated alkanes on metal oxide nanoparticles.