Repositório RCAAP

We utilize a kinetic approach to the problem of wave propagation in dusty plasmas, taking into account the variation of the charge of the dust particles due to inelastic collisions with electrons and ions. The components of the dielectric tensor are written in terms of a finite and an infinite series, containing all effects of harmonics and Larmor radius. The formulation is quite general and valid for the whole range of frequencies above the plasma frequency of the dust particles, which are assumed motionless. The formulation is employed to the study of electrostatic waves propagating along the direction of the ambient magnetic field, in the case for which ions and electrons are described by bi-Maxwellian distributions. The results obtained in a numerical analysis corroborate previous analysis, about the important role played by the dust charge variation, particularly on the imaginary part of the dispersion relation, and about the very minor role played in the case of electrostatic waves by some additional terms appearing in the components of the dielectric tensor, which are entirely due to the occurrence of the dust charge variation.

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One step (direct) and multisteps synthesis methods of metal and oxide nanoparticles are presented including both bottom-up and top-down procedures. Chemical methods involving the polyol process allows to get either isotropic or anisotropic nanoparticles. Nanowires, nanorods and dumbells can be generated either by heterogeneous nucleation or by using a template (mesoporous silica). Top-down procedures are illustrated by laser irradiation of immersed bulk targets (Ag, Au) which generates self-organized nanostructures or colloidal solutions. Laser irradiation of these colloidal solutions modifies the size distribution and the shape of nanoparticles. Bimetallic nanoparticles are generated from mixtures of monometallic colloidal solutions. Surface-mediated methods involving a host support and an invited phase are also presented. They lead to the formation of metal or oxide clusters and nanoparticles. The speciation of these entities is correlated with the observed catalytic performances.

The nature of gold species in Au/CeX Zr1-X O2 catalysts was successfully investigated using the visible and near infrared photoacoustic spectroscopy and Fourier transform infrared spectroscopy of adsorbed carbon monoxide. It was found that the visible and near infrared photoacoustic spectroscopy is a powerful technique for investigating the changes of gold particle size due to addition of zirconium and the oxidation state of gold particles. It was also noted that the increase of zirconium in the solids makes gold incorporation more difficult due to the electronic shielding caused by zirconium, decreasing the probability of replacing cerium by gold. For the samples studied, the particle size ranged from 20-30 nm and the Au+, Au3+ and Auº species were found in the catalysts.

This short review aimed to give to reader's brief applications of polymer colloids in biomedical area such as therapy and medical diagnosis as also developed in our laboratory. Indeed, the polymer particles and composite particles are commonly used in immunoassays as solid phase supports, for the encapsulation of active agents and for the immobilization of biomolecules such as oligonucleotides, proteins or antibodies. In the area of composite particles, magnetic particles bearing immobilized biomolecules are used in biomedical diagnosis such as immunoassay, specific nucleic acids concentration, cell labelling and separation and in numerous biotechnological applications.

Electronic and magnetic properties of nanoscale materials are closely related to the atomic arrangement at the interface shared by different chemical elements. A very precise knowledge of the surface/interface structure is then essential to properly interpret the new properties coming out. Of a particular interest is the relationship between structure, morphology and magnetic properties of exchanged-coupled interfaces in ferromagnetic (FM) and antiferromagnetic (AF) materials. The interaction at the AF/FM interface modifies the magnetic switching properties of the FM film, which turn out to be a usefull property on new magnetic devices technology. We present here an investigation of the buried exchange-coupled interface [NiO/CoO]/[PtCo] grown on a Pt(111) single crystal. The magneto-optical Kerr effect reveals a strong coupling at the interface, by an increasing coercivity, and a spin reorientation of the FM film when ordering occurs in the AF layer. The combination of grazing incidence X-ray diffraction, X-ray reflectivity and soft X-ray resonant magnetic scattering yields a comprehensive description of the system.

We discuss in this study the advantages and limitations of the osmotic stress method that enables to set the osmotic pressure to a given system. By investigating aqueous suspension of monodisperse silica nanoparticles of radius 78 at an ionic strength of 10-2 mol/L, we show that the method is very accurate to probe the phase behavior of colloidal suspensions because it allows to prepare samples all along the equation of state of the system at constant ionic strength without any aggregation and with a well defined structure, as shown by Small-Angle Neutron Scattering (SANS) experiments. However the method fails to yield crystalline structures, since solid samples obtained are always glassy, even when the fluid-solid transition is crossed with small successive jumps of 1000 Pa. This phenomenon comes from the kinetics of the process which exhibits in our experimental conditions an exponential decay time with a characteristic time of ≈ 3 hours that induces a very strong change of the volume fraction of the suspension in the early stages of the stress. When the jump of pressure is very important, the system is frozen in the vicinity of the dialysis bag and forms a dense shell that eventually prevents some spatial regions of the sample to reach equilibrium. In this case, the osmotic stress forces the sample to get a structure very spatially heterogeneous at macroscopic scale.

We report on experimental observations of self-assemblies in colloidal dispersions of clay nanoplatelets and magnetic nanoparticles. Visual observations have been combined with small angle X-ray scattering (SAXS) in the study of several composites at a fixed clay concentration in the dilute regime, and varying ferrofluid concentrations. Our visual observations which encompass macroscopic separation in gravitational- and magnetic field, indicate that all samples present a concentrated phase and a diluted one. SAXS data obtained from each phase are consistent with the interpretation that the scattering contribution from the clay nano-platelets in the samples can be neglected in comparison with the magnetic particle contribution. The analysis of the scattered intensity is performed combining two models, one based on the global scattering function and the other allowing the extraction of the structure factor of the mixtures. The parameters of the size distribution of magnetic nanoparticles determined by both methods are in good agreement. The structure factor of the mixtures shows that on a local scale, the mixtures behave like a gas of isolated magnetic nanoparticles. It also indicates the presence of interactions between magnetic nanoparticles mediated by the presence of Laponite platelets. Such interactions could be attributed with a progressive partial phase separation between spheres and discs rather than to the formation of dense aggregates.

Fischer-Tropsch synthesis is a part of Gas-to Liquids (GTL), Biomass-to-Liquids (BTL) and Coal to Liquids (CTL) technologies, which produce alternatives clean fuels from natural gas, biomass and coal. The catalytic performance of cobalt catalysts in Fischer-Tropsch synthesis strongly depends on the size of cobalt particles in nanoscale range (6-30 nm). Cobalt catalysts are usually prepared via incipient wetness impregnation using cobalt salts (cobalt precursors). Catalyst preparation involves several important steps. The paper shows that decomposition of cobalt precursors is often a crucial step in the catalyst design; slower rate of decomposition of cobalt precursors favors smaller size of cobalt particles, higher cobalt dispersion and enhances catalytic performance in Fischer-Tropsch synthesis.

The Spray Pyrolysis (SP) process has been employed at moderate temperature (470-970 K) for the synthesis of micro- and nano-particles of hydrated metal oxides and of related nanocomposites. It has been applied to iron nitrate solutions, without or with the addition of sodium chloride as a flux. After removal of the flux, nanocomposites of ferrihydrite (5Fe2O3,9H2O) / hematite (α- Fe2O3) / and an amorphous Fe2O3 are obtained, with surface area 140 m²/g. It has been applied also to a sol of Aluminum-tri-sec-butoxide, without or with activation by europium or terbium ions. Boehmite (γ-AlOOH) powders are synthesized at 470 K, with surface area equal to 180 m²/g. Transition alumina (γ-Al2O3) are synthesized at 970 K. Nanocomposites made by reaction of the boehmite or γ-Al2O3 particles with the amino acid asparagin (ASN) have been prepared and characterized. The luminescence of the Eu3+ or Tb3+ doped nanocomposites has been investigated: they can be considered as bio-compatible luminescent nanoparticles.

In Fe-Cu alloys prepared by pulsed electrodeposition, Fe clusters form directly within a non-magnetic Cu matrix. The clusters contain around 140 Fe atoms and are superparamagnetic at room temperature. Below 160 K, it is proposed that the clusters order into a so-called superferromagnetic arrangement. This behaviour is ascribed to the persistence of weak exchange interactions mediated by isolated Fe atoms dispersed within the non-magnetic Cu matrix.

Lubrication strategies developed long time ago employed amphiphilic molecular lubricant additives to confer to lubricating oils or greases specific properties such as friction reduction, antiwear, anticorrosion. The understanding of their action and the development of various type of nanoparticles are at the origin of new lubrication strategies using composite or functionalized nanoparticles in dispersion in lubricating oils or greases to confer to the lubricants the needed functions. The present work is concerned with the presentation of these new approaches. Some examples will be developed to show the main aspects of the mechanisms of action of these new nano-additives.

Model self-assembled networks of telechelic polymer C18 - PEO(35k) - C18 in water have been studied. The rheology of such transient networks has been investigated as a function of polymer concentration, and a typical percolation law has been observed. The network structure has been characterised by Small Angle Neutron Scattering in D2O, where the interactions between micelles formed by the hydrophobic C18-stickers of the polymer give rise to a peak in the scattered intensity. These model networks have then been used as a matrix for the incorporation of silica nanoparticles (R = 10 nm), and we have checked individual dispersion by scattering using contrast variation. The rheological response of the networks is considerably modified by the presence of the silica nanoparticles, and in particular an interesting dependence of the relaxation time on silica concentration has been found. The analogy in reinforcement behaviour of such a self-assembled, viscoelastic, and aqueous system with model experiments of elastomers filled with nanoparticles is discussed by comparison to a silica-latex system.

Polymeric nanoparticle systems (nanocapsules and nanospheres) present potential applications as drug delivery systems. Nevertheless, their full applications have not been exploited due to their limited shelf life when stored in aqueous medium. Drying polymeric nanoparticles using spray-drying represents a promising platform to improve the physicochemical stability of formulations and/or to control the release of hydrophilic and lipophilic drugs. This article presents a brief overview of the most recent and ongoing research in the use of spray-drying process to prepare and/or to dry polymeric nanoparticles formulations intended for drug administration.

A dynamical freezing, which is the analogous of a glass transition, is observed at large concentrations in aqueous dispersions of maghemite nanoparticles. We study experimentally the structure and the dynamical properties of two dense ferroglass-formers in two very distinct states of interparticle interaction, either strongly repulsive or attractive. The static structure of the magnetic colloidal dispersions is probed by means of Small Angle X-ray Scattering both in and without the presence of an external magnetic field. Translational dynamics of the repulsive glassformer are investigated by X ray Photon Correlation Spectroscopy. Slow dynamics and aging properties, which both become anisotropic under an applied field, are here investigated.

A new kind of thermo-responsive particles were prepared by the self-assembly technique, comprising poly(hydroxyvalerate-co-hydroxybutirate)-b-poly(N-isopropylacrylamide)/ (PHBHV-b-PNIPAAm) block copolymers. The hydrophilic part PNIPAAm was synthesized by Reversible Addition- Fragmentation chain Transfer (RAFT) polymerization. Particles with core-shell morphology were obtained with hydrophilic outer shells and hydrophobic inner cores. Dexametasone acetate (DexAc) was used as a model drug with an encapsulation efficiency of 77%. The release of DexAc in aqueous solution was strongly dependent on temperature, suggesting that PHBHV-b-PNIPAAm particles can be used as a thermo-responsive carrier material with external control in a drug release system.

β-cyclodextrin (β-CD) is widely used as a component of pharmaceutical formulations, classically to improve the solubility and oral bioavailability of poorly water-soluble drugs through formation of drug/β-CD inclusion complexes. Unexpectedly, the association of the highly water-soluble drug meglumine antimoniate (MA) with β-CD turned this antimonial compound orally-active in a murine model of leishmaniasis. To get insight into the mechanisms responsible for the enhanced oral efficacy of MA, the MA/β-CD composition was characterized physicochemically, using thermogravimetry, circular dichroism, mass spectrometry (ESI-MS), osmometry and photon correlation spectroscopy. The freeze-dried MA/β-CD was found to form nanoassemblies in water, as a result of multiple non-inclusion interactions between MA and β-CD, which behave as a sustained release system of the MA drug.

The use of the scanning tunneling microscope (STM) for the investigation of Kondo adatoms on normal metallic surfaces reveals a Fano-Kondo behavior of the conductance as a function of the tip bias. In this work, the Doniach-Sunjic expression is used to describe the Kondo peak and we analyze the effect of a complex Fano phase, arising from an external magnetic field, on the conductance pattern. It is demonstrated that such phase generates local oscillations of the Fano-Kondo line shape and can lead to the suppression of anti-resonances.

The phase behavior of acidic samples of EDL-MF based on cobalt ferrite nanoparticles with controlled mean sizes was investigated at constant temperature and in absence of magnetic field. By monitoring the nanoparticle charge by pH adjustments, we constructed an experimental pH-dependent phase diagram for all samples that revealed sol, gel thixotropic or coagulated phases in different pH regions. Then, by using an extended DLVO potential we analyzed quantitatively the observed phase diagram in function of pH and nanoparticle mean size.

Synthesis of polymorphous of nanoparticulate alumina has been the subject of many works. The stabilization of these nanoparticles in aqueous suspensions is necessary in many technologic application and industrial products. Recently, a number of routes have been developed using various dispersants to prepare these nanoparticles dispersed in aqueous solution. In this work, nanoparticulate alumina prepared by precipitation and freezing drying was investigated as potential material for aqueous dispersion. These nanoparticles were dispersed in methylethylglycol (MEG), polyvinylalcohol (PVA) and polymethacrylate (PMA). The alumina nanoparticles and the dispersions were characterized physical-chemically and tested in the preparation of thin films.