János Szépvölgyi

Particle size distribution and dislocation density determined by high resolution X-ray diffraction in nanocrystalline silicon nitride powders

Two silicon nitride powders were investigated by high resolution X-ray diffraction. The first sample was crystallized from the powder prepared by the vapour phase reaction of silicon tetrachloride and ammonia while the second was a commercial powder produced by the direct nitridation of silicon. Their particle size and dislocation density were obtained by the recently developed modified Williamson‐Hall and Warren‐Averbach procedures from X-ray diffraction profiles. Assuming that the particle size distribution is log-normal the size distribution function was calculated from the size parameters derived from X-ray diffraction profiles. The size distributions determined from TEM micrographs were in good correlation with the X-ray results. The area-weighted average particle size calculated from nitrogen adsorption isotherms was in good agreement with that obtained from X-rays. The powder produced by silicon nitridation has a wider size distribution with a smaller average size than the powder prepared by vapour phase reaction. The dislocation densities were found to be between about 10 14 and 10 15 m 2 . Published by Elsevier Science S.A. All rights reserved.

Formation of boron nitride thin films on β-Si3N4 whiskers and α-SiC platelets by dip-coating

Abstract Thin films of boron nitride have been deposited on the surface of β-Si 3 N 4 whiskers and α-SiC platelets by dip-coating in aqueous and methanolic boric acid solutions, respectively followed by drying at room temperature, heating in nitrogen flow, and finally nitriding in an ammonia flow at 1000 °C. X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy and scanning electron microscopy (SEM) studies proved the formation of boron nitride surface coating. The XPS studies indicated that the methanolic boric acid solution was a more effective precursor as compared to the aqueous solution. Transformation of H 3 BO 3 to BN was much more efficient on the surface of SiC platelets than on that of Si 3 N 4 whiskers.

Hollow alumina microspheres prepared by RF thermal plasma

Thermal plasma treatment of ceramic particles of different shape makes them spherical. The spherical particles are hollow or porous under certain experimental conditions. The objective of the present work was to investigate how the water content and porosity of raw materials and the composition of the gas phase influence the size of the hollows. We concluded that the porous structure is the major factor contributing to the formation of hollow spheres, while high initial water content is not a prerequisite for the formation of internal cavities.

Thermal plasma synthesis of zinc ferrite nanopowders

Ž. Nanosized zinc ferrite spinel powders of various compositions were produced in a radiofrequency RF thermal plasma from oxide mixtures and from co-precipitated hydroxides. Bulk and surface chemical compositions of the products were measured by ICP-AES and XPS. Phase conditions were determined by XRD. Morphology was investigated by SEM and TEM. In spite of the short residence time of reagents in the plasma reactor, a high degree of spinel formation was achieved. The Zn enrichment on the surface determined by quantitative XPS is attributed to a thin ZnO layer on the surface of the zinc ferrite particles. In zinc ferrous ferrites, an inhomogeneous distribution of Zn inside the grains may also exist. q 2001 Elsevier Science B.V. All rights reserved.

Simultaneous calcination and spheroidization of gibbsite powders in an RF thermal plasma

Abstract Thermal treatment of gibbsite (Al(OH)3) powders of different particle size was studied in inductively coupled thermal plasma operating at normal pressure. Calcination and spheroidization took place simultaneously although spheroidization did not occur with coarse feed particles. Microsized or nanosized Al2O3 particles of different phase composition were produced depending on the size of injected grains. The microsized powders were mainly formed by melt solidification. In the formation of nanopowders, however, evaporation of small grains followed by homogeneous nucleation from the vapor phase and grain growth seem to be the dominant processes.

Efficient synthesis of fullerenes in RF thermal plasma reactor

Abstract Formation of fullerene soot was studied in an inductively coupled, radiofrequency (RF) thermal plasma reactor. A previously developed kinetic model of fullerene formation was applied to determine synthesis conditions leading to high fullerene yield. The experimental results verified the kinetic model. Maximum yield of 4.1% was obtained in particular conditions. It corresponded to a fullerene production rate of 6.4 g h −1 .

Palladium Nanoparticle–Graphene Catalysts for Asymmetric Hydrogenation

We report for the first time the application of palladium nanoparticle-graphene (Pd/Gn) catalysts in the asymmetric hydrogenation of aliphatic α,β-unsaturated carboxylic acids using cinchonidine as chiral modifier. Pd/Gns were prepared by deposition–precipitation from the aqueous phase over graphite oxide and subsequent simultaneous reduction of both the support and the metal precursor with NaBH4. The materials obtained were characterized by ICP optical emission spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. We demonstrate that the Pd/Gns modified by cinchonidine can act as efficient catalysts in the asymmetric hydrogenation of α,β-unsaturated carboxylic acids for producing optically enriched saturated carboxylic acids.Graphical Abstract

The effect of heat-treatment on the grain-size of nanodisperse plasmathermal silicon nitride powder

Nanodisperse silicon nitride has been synthesized by vapor phase reaction of silicon tetrachloride and ammonia in a thermal plasma reactor and crystallized at temperatures 1250, 1350, 1450 and 1500°C. The average grain-size and the dislocation density of the samples were determined by the recently developed modified Williamson-Hall and Warren-Averbach procedures from X-ray diffraction profiles. A new numerical method provided log-normal grain-size distributions from the size parameters derived from X-ray diffraction profiles. It has been shown that the average grain-size in the amorphous phase is lower than that observed in the crystalline fraction. On the other hand, the average grain-size in the crystalline fraction decreases up to 1450°C while it increases during heat-treatment at 1500°C. The size distribution and the area-weighted average grain-size obtained by X-rays were in good agreement with those determined by TEM and from the specific surface area, respectively. The dislocation density was found to be of the order of 1014 and 1015 m−2.

Activation of amorphous Cu-M (M = Ti, Zr or Hf) alloy powders made by mechanical alloying

Abstract Amorphous Cu-M alloy powders (Cu 40 Ti 60 , Cu 50 Zr 50 and Cu 65 Hf 35 ) produced by mechanical alloying are activated by being applied as catalyst in the transformation of various alcohols at elevated temperature (523–573 K). During the dehydrogenation of 2-propanol to acetone and the transformations of allyl alcohol to form propanal and 1-propanol active, stable and selective catalysts are generated from Cu-Zr and Cu-Hf. DSC, XRD and XPS data indicate bulk crystallization and copper segregation to the surface. Due to surface titanium enrichment Cu-Ti proved to be inferior to the other two alloys.

Comparative Study of the Decomposition of CCl4 in Cold and Thermal Plasma

Decomposition of carbon tetrachloride was studied in an inductively coupled thermal plasma reactor and in a low temperature, non-equilibrium plasma reactor, in neutral and oxidative conditions, respectively. In neutral conditions formation of solid soot, aliphatic- and cyclodienes was observed in equilibrium, and products, such as Cl2 and C2Cl6 were detected in non-equilibrium plasma. Feeding of oxygen into the thermal plasma reactor depressed both soot and dienes formation and induced the formation of oxygen containing intermediates and products. GC-MS analyses of the gaseous products and the extract of the soot referred to as complex decomposition and recombination mechanism at given conditions. Presence of oxygen in the low temperature plasma reactor results in the formation of carbonyl compounds as intermediers. CO2 and Cl2 revealed as final products of CCl4 decomposition in cold plasma.