Emanation Thermal Analysis of SiC Based Materials

Results of emanation thermal analysis (ETA) characterizing microstructure changes of SiC based materials during heat treatment in argon are demonstrated. This method made it possible to reveal fine changes of the texture of SiC nano-sized powders, SiC micro-sized powders and SiC whiskers under in situconditions of the heating. ETA curves can serve as fingerprints of the respective samples.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal behaviour of advanced composite materials based on SiC fibres

Emanation thermal analysis (ETA) was used for characterization of thermal behaviour of SiC_f/SiC composites on heating in argon and air, respectively. Effect of gas environment (argon, air) and helium ions implantation on the microstructure development of the SiC_f/SiC composite prepared by chemical vapour infiltration (CVI) from Nicalon CG fibres was investigated under in situ conditions of heating. The annealing of near surface structure irregularities was observed in the range 280-700°C and evaluated by means of the mathematical model, assuming that the structure irregularities served as diffusion paths for radon. The ETA reflected the formation of amorphous silica and its subsequent crystallization to crystoballite. Morphology of the SiC_f/SiC samples before and after the heat treatments was characterized by means of SEM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterization of titania based nanowires

A new method for preparation of titania nanowires with diameter around 10 nm and length up to 2–3 μm is described. The precursor was prepared from sodium titanate by adding ethylene glycole (EG) and heating at temperature of 198°C for 6 h under reflux. The sodium titanate glycolate formed by this way aggregated into 1D nanostructures and was subsequently transformed into titania glycolate during a chemical treatment with 98% sulfuric acid. Titania nanowires with variable amount of anatase and rutile were prepared by heating to temperatures in the range 350–1000°C. The precursor as well as titania based samples were characterized by X-ray diffraction, Infrared spectroscopy, Scanning electron microscopy, High resolution transmission microscopy, Thermogravimetry, Differential thermal analysis, Evolved gas analysis and Emanation thermal analysis. The nitrogen adsorption/desorption was used for surface area and porosity determination. The photoactivity of the prepared titania samples was assessed by the photocatalytic decomposition of 4-chlorophenol in an aqueous slurry under UV irradiation of 365 nm wavelength.     

IR laser-induced metal ablation and dielectric breakdown in benzene

IR laser-induced irradiation of Co and Ni sheets leads to metal ablation and when carried out in gaseous benzene (1–10 Torr) to dielectric breakdown which is accompanied by metal plasma and deposition of nanostructured carbon. The metal plasma (metal atoms and ions) as well as transients of benzene decomposition (neutral and ionic carbon and C₂ species) were detected by optical emission spectra. Different features of carbon deposited at benzene pressure 5–10 Torr on a distant glass and on the irradiated metal sheets were revealed by FTIR, Auger and Raman spectroscopy and electron microscopy and explained by surface assisted carbonization. The reported process suggests a novel approach to gas-phase carbonization of organic molecules.     

Zinc oxide prepared by homogeneous hydrolysis with thioacetamide, its destruction of warfare agents, and photocatalytic activity

Zinc sulfide (ZnS) nanoparticles were prepared by homogeneous hydrolysis of zinc sulfate and thioacetamide (TAA) at 80 degrees C. After annealing at a temperature above 400 degrees C in oxygen atmosphere, zinc oxide (ZnO) nanoparticles were obtained. The ZnS and ZnO nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Brunauer-Emmett-Teller (BET)/Barrett-Joyner-Halenda (BJH) methods were used for surface area and porosity determination. The photocatalytic activity of as-prepared zinc oxide samples was determined by decomposition of Orange II dye in aqueous solution under UV irradiation of 365 nm wavelength. Synthesized ZnO were evaluated for their non-photochemical degradation ability of chemical warfare agents to nontoxic products.     

Aerogel nanoscale magnesium oxides as a destructive sorbent for toxic chemical agents

An autoclave hypercritical drying procedure has been used to prepare precursors of MgO from Mg(OCH₃)₂. This material was prepared with a specific surface area of 1200 m² g ¹. The dehydrated materials consisted of much smaller crystallites than conventionally prepared MgO and were free of OCH₃ groups. The precursors and samples of magnesium oxide were taken for experimental evaluation of their reactivity with mustard. The largest percentage of the conversion mustard into non-toxic products after the elapse of the reaction was 77%.     

Photocatalytic properties of Ru-doped titania prepared by homogeneous hydrolysis

Nanocrystalline titania particles doped with ruthenium oxide have been prepared by the homogenous hydrolysis of TiOSO₄ in aqueous solutions in the presence of urea. The synthesized particles were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED) and Nitrogen adsorption-desorption was used for surface area (BET) and porosity determination (BJH). The photocatalytic activity of the Ru-doped titania samples were determined by photocatalytic decomposition of Orange II dye in an aqueous slurry during irradiation at 365 nm and 400 nm wavelengths.      

Reductive dissolution of microparticulate manganese oxides

Electrochemical dissolution of immobilised microparticulate Mn(III,IV) oxides in slightly acidic solution (pH 4.4) was found to be a very general reaction, which is responsible for well-defined voltammetric peaks. Dissolution of six Mn(III,IV) oxides is initiated by the reduction of Mn(IV) to Mn(III) in the solid phase, which is followed by a massive dissolution via further reduction of Mn(III) to Mn(II), which finally yields soluble Mn²⁺. The reactivity of manganese oxides depends on their structure: the most reactive are amorphous (δ-MnO₂) and layered structures (birnessite); more resistant toward reductive dissolution are α- and λ-MnO₂ and electrochemical manganese dioxide; and least reactive is β-MnO₂. Reductive dissolution of LiMn₂O₄ resembles that of λ-MnO₂, whereas CaMnO₃ dissolves via a different reaction mechanism.     

IR laser photodeposition of a-Fe/Si films developing nanograins of ferrisilicate, iron disilicide and rare hexagonal iron upon annealing

IR laser-induced gas-phase photolysis of Fe(CO)(5)-SiH(4) mixtures occurs as SiH(4)-photosensitized decomposition of Fe(CO)(5) is accelerated by products of this decomposition and it results in deposition of amorphous Si/Fe nanocomposite films. Analyses of the deposited and subsequently annealed solid films were made by FTIR, Raman and X-ray photoelectron spectroscopy, X-ray diffraction and electron microscopy. The deposited films are amorphous, contain crystalline nanostructures of iron silicide FeSi(2) and undergo atmospheric oxidation in topmost layers to iron oxide and hydrogenated silicon oxide. Upon annealing they develop nanocrystalline structures of ferrisilicate, Fe(1.6)SiO(4), carbon-encaged iron disilicide, FeSi(2), and very rare hexagonal (high-pressure) Fe surviving at ambient conditions. The mechanism of formation of these nanostructures is discussed in terms of gas-phase and solid-phase reactions.