V. Ciupina

Nanoscale powders of different iron oxide phases prepared by continuous laser irradiation of iron pentacarbonyl-containing gas precursors

Abstract Different iron oxide-based nanosized powders were obtained by the IR laser synthesis from gas phase reactants by using iron pentacarbonyl vapor as iron donor. In the flowing gas mixture, oxidizers (like nitrous oxide and air) and sensitizers (like sulfur hexafluoride and ethylene) have been used. A major amount of γ-Fe2O3 was obtained after a series of experiments in which α-Fe2O3 and iron fluorides were the dominant phases. The mean size of the obtained γ-iron oxide particles was in the range 3–7 nm. The structural characterization of the new elaborated materials points out to the crucial influence that the experimental parameters (particularly the combined choice of the oxidizer/sensitizer agents) have on the specific oxide phase formation as well as on its purity.

Biocompatibility and bioactivity enhancement of Ce stabilized ZrO2 doped HA coatings by controlled porosity change of Al2O3 substrates

Al(2) O(3) substrates with controlled porosity were manufactured from nanosized powders obtained by plasma processing. It was observed that when increasing the sintering temperature the overall porosity was decreasing, but the pores got larger. In a second step, Ce stabilized ZrO(2) doped hydroxyapatite coatings were pulsed laser deposited onto the Al(2) O(3) substrates. It was shown that the surface morphology, consisting of aggregates and particulates in micrometric range, was altered by the substrate porosity and interface properties, respectively. TEM studies evidenced that Ce stabilized ZrO(2) doped HA particulates ranged from 10 to 50 nm, strongly depending on the Al(2) O(3) porosity. The coatings consisted of HA nanocrystals embedded in an amorphous matrix quite similar to the bone structure. These findings were congruent with the increased biocompatibility and bioactivity of these layers confirmed by enhanced growing and proliferation of human mesenchymal stem cells.

Growth and characteristics of tantalum oxide thin films deposited using thermionic vacuum arc technology

Tantalum pentoxide (Ta2O5) thin films were synthesized using thermionic vacuum arc (TVA) technology. TVA is an original deposition method using a combination of anodic arc and electron gun system for the growth of thin films from solid precursors under vacuum of 10−6 Torr. The properties of the deposited Ta2O5 thin films were investigated in terms of wettability, refractive index, morphology, and structure. The surface free energy was determined by means of surface energy evaluation system indicating a hydrophilic character and the refractive index was measured by Filmetrics F20 device. The morphology was determined from bright field transmission electron microscopy (TEM) image performed by Philips CM 120 ST TEM system. It exhibits nanoparticles of 3–6 nm diameter smoothly distributed. Selected area electron diffraction pattern revealed the contrast fringes given by complex polycrystalline particles included in the amorphous film. The measured fringes could be indexed using monoclinic structure of Ta2O5.

Photochemistry Aspects of the Laser Pyrolysis Addressing the Preparation of Oxide Semiconductor Photocatalysts

The laser pyrolysis is a powerful and a versatile tool for the gas-phase synthesis of nanoparticles. In this paper, some fundamental and applicative characteristics of this technique are outlined and recent results obtained in the preparation of gamma iron oxide () and titania () semiconductor nanostructures are illustrated. Nanosized iron oxide particles (4 to 9 nm diameter values) have been directly synthesized by the laser-induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). Temperature-dependent Mossbauer spectroscopy shows that mainly maghemite is present in the sample obtained at higher laser power. The use of selected samples for the preparation of water-dispersed magnetic nanofluids is also discussed. nanoparticles comprising a mixture of anatase and rutile phases were synthesized via the laser pyrolysis of - (vapors) based gas-phase mixtures. High precursor concentration of the oxidizer was found to favor the prevalent anatase phase (about 90%) in the titania nanopowders.

Silicon Dicarbide Synthesis

A method for the synthesis of the hypothetical silicon dicarbide structure is proposed. This structure with tetragonal symmetry has been synthesized in matrixes of hardened novolac with silicon powder. Thermal treatment up to 1200°C induces in these matrixes spiroconjugation features with subsequent organizing in tetragonal symmetry. XRD, HRTEM patterns and SAED confirmed that the hypothetical structures proposed are synthetically realizable.

ZnO nanoparticles obtained by hydrothermal method at low temperature

The structural properties and morphology of ZnO nanoparticles obtained by hydrothermal method were studied. ZnO samples were obtained by hydrothermal method, in soft synthesis conditions, temperature of solution about 70°C, in presence of a bidentate ligand or a tensioactive agent. The resulted oxides morphologies were compared with the morphologies of ZnO samples obtained in absence of ligands or tensioactive agents. Samples present a hexagonal phase of ZnO with lattice parameters about a=0.32nm and c=0.5nm, values confirmed by XRD measurements. Morphological properties are studied using bright field images, measuring the nanoparticles diameter and nanopellets size.

Facet oxidation and degradation of AlGaAs/GaAs pulsed laser diodes

Oxide films grow in atmosphere as a result of oxidation reaction and mass transport of the element through the oxide film. In most III-V compounds (as GaAs) semiconductor oxides of III-element and V-element are formed individually. The AlGaAs/GaAs laser diodes were exposed to facet oxidation at mirror surfaces in normal environmental conditions for a storage process. It was observed a decrease of optical output power in the early stage of experiment together with a constancy of the ratio P/P(O) for the long time term (month). We suggest the development at the mirror laser facet of a natural oxide film as Ga2O3 and GaAsO. We present a SEM (Scanning Electron Microscopy) image of a natural oxide film grown in atmosphere at the surface of AlGaAs as laser facet, as well as a picture of a facet oxidation of a laser device. The EDS (Electron Dispersion Spectrum) of a natural oxide put into evidence the O (Kα) signal. Dielectric coating using Al2O3 and a-Si layers protected a part of laser devices, and at the mirror surface a reflectivity of 70% in the emission range of the laser (890 nm) was measured. The emitted power for dielectric coated devices was constant, so the rapid degradation process was slowed down.

Iron-carbon nanocomposite obtained by laser-induced gas-phase reactions

Iron-carbon composite nanopowders have been synthesized by the CO2 laser pyrolysis of gas-phase reactants. The experimental device allows for a very low reaction time and a rapid freezing that creates nanoscale-condensed particles. Iron pentacarbonyl and ethylene-acetylene mixtures were used as iron and carbon precursors. In a two-steps experiment, the reaction products may present themselves as iron-based nanoparticles dispersed in a carbon matrix. By a careful control of experimental parameters and radiation geometries we demonstrate the feasibility of an efficient and well-controlled, single-step technique for the production of iron-based nano-cores embedded in carbon layers. Highly dispersed nanoparticles, narrow size distributions and particles with about 4.5 - 6 nm mean diameters were obtained. Electron microscopy and Raman spectroscopy were used in order to analyze the structure and composition of the obtained nanopowders as well as their Soxhlet residue.

TEM study of CaTiO3 synthesized by sol-gel method

Nanometer-sized calcium titanate (CaTiO3) particles were synthesized by the modified sol-gel method, from Ca(CH3COO)2 and Ti(C4H90) an ideal cation stoichiometry for CaTiO4 perovskite. EDTA was used like chelating agent. XRD analysis for their crystal structure and TEM for their texture characterized the materials obtained after heating in the range 600 - 10000C. The TEM photographs reveal nanoparticles in the size range 50 - 45 nm with a mean diameter found at 120 nm.

Carbon-encapsulated iron nanoparticles prepared by laser pyrolysis: characterization and catalyzers for carbon nanotubes and nanofibers

The embedding of Fe-based nanoparticles in carbon layers allows novel physical and catalyzing properties due to inertness and resistance to external detrimental conditions. We have prepared almost spherical carbon encapsulated iron nanoparticles with narrow size distribution, via laser co-pyrolysis method in which the CW CO2 laser beam irradiates a gas mixture containing iron pentacarbonyl (vapors) and ethylene/acetylene hydrocarbons. Specific flow geometries were used in order to synthesize iron particle first followed by stimulate hydrocarbon decomposition at iron surfaces. High-resolution transmission electron microscopy images reveal the core-shell feature of synthesized nanostructures with around 2 nm thick carbon layers and 3-7 nm diameters iron-based core dimensions. The mean diameter could be experimentally controlled. It was found a decreasing trend of particle size with the decreasing of pressure and total reactant gas flow. EELS, EDAX and Raman spectroscopy analysis confirm the simultaneous presence of carbon and iron. The nanoparticles were seeded onto Si wafer and further used as substrates for laser induced CVD carbon nanotubes growth. Depending on laser power density, nanotubes or nanofibres are formed, in strong dependence with the location of iron based nanoparticles on Si substrates as revealed by SEM analysis.