Zoltán Károly

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.

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 .

Guest escape and uptake in nonporous crystals of a gold(I) macrocycle

The nonporous gold(I) diphosphine complex [Au(2)(cis-dppe)(2)](NO(3))(2) [1, cis-dppe = cis-1,2-bis(diphenylphosphino)ethylene] is robust enough to trap guests, but at the same time, it is flexible enough to allow guest release without destruction of its crystal lattice. This nonporous gold(I) compound 1 is also efficient at capturing and releasing carbon dioxide in a controlled manner.

Spark plasma sintering of Si3N4/multilayer graphene composites

Abstract Mulitlayer graphene reinforced silicon nitride composites were prepared by spark plasma sintering to investigate the effect of the graphene addition on mechanical properties. The composites contained multilayer graphene (MLG) in various (0, 1, 3 and 5 wt%) content. Significantly higher fracture toughness of 8.0 MPa m1/2 was obtained at 1% MLG content, however, on further increasing the graphene content the toughness did not increase, but dropped to the value of the monolithic silicon nitride. The maximum hardness of 18.8 MPa was also obtained at 1% MLG, while at higher MLG contents it gradually decreased. Graphical Abstract

Decomposition of Chlorobenzene by Thermal Plasma Processing

Decomposition of chlorobenzene as a model molecule of aromatic chlorinated compounds was studied in radiofrequency thermal plasma both in neutral and oxidative conditions. Optical emission spectroscopy was applied for the evaluation of the plasma excitation and molecular rotational-vibrational temperature. Atomic (C, H, O) and molecular (CH, OH, C2) radicals were identified, while the morphology of the formed soot was characterized by electron microscopy. Organic compounds adsorbed on the surface of the soot after plasma processing were comprised of various polycyclic aromatic hydrocarbons (PAH) and chlorinated PAH molecules. Their amount was greatly affected by experimental conditions, especially the oxygen content and plate power. The higher input power reduced the ring number of the PAH molecules. Addition of oxygen significantly reduced the amount of both PAHs chlorinated PAH molecules but enhanced the formation of polychlorinated benzene compounds.

Synthesis of Mullite from Laboratory Waste Silica through Transferred Arc Plasma Processing Method

Transferred arc plasma (TAP) processing could be an economic and time saving processing method for waste treatment and recycling of chemical laboratory solid wastes. In this work, three different waste silica powders derived from chemical laboratories which are adsorbed with catalytic amount of ruthenium, palladium, and ferrocene derivatives are recycled in addition with alumina to form mullite (3Al2O3·2SiO2) by TAP processing technique and as well as in conventional method for comparison. The phase and microstructure formation of the processed samples were analyzed by X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) images, respectively. The results show that palladium adsorbed silica has significantly enhanced the formation and densification of mullite rather than ruthenium- and ferrocene-adsorbed silica in TAP processing. The SEM images show that the different kinds of microstructures developed in plasma arc processing mullite due to the direction of the plasma arc formation and solidification.

Reduction of Metallurgical Wastes in an RF Thermal Plasma Reactor

Recovery of metals from iron and zinc oxides, as well as from zinc-containing metallurgical wastes, such as flue dust from the Siemens–Martin process and sludge from hot galvanizing, has been studied in an rf thermal plasma reactor under reducing conditions. The product composition was estimated by thermodynamic calculations based on the minimization of the Gibbs free enthalpy. Effects of the plate power of rf generator and the feed rate of powder on the chemical and phase composition of products have been investigated in detail. It has been proved that the rf thermal plasma treatment makes possible to produce unstable species in thermodynamic terms: metallic zinc was gained in the reaction of ZnO and hydrogen. The gradient cooling along the plasma reactor led to the segregation of the iron and zinc compounds. Valuable products were made from the particular wastes by a single step thermal plasma processing.

Optical emission spectra analysis of thermal plasma treatment of poly(vinyl chloride)

Abstract Decomposition of poly(vinyl chloride) (PVC) was investigated in radiofrequency thermal plasma in neutral, oxidative and reductive conditions. Optical emission spectroscopy (OES) was applied for the characterization of the plasma column. OES was used to identify active plasma components such as excited atoms, ions, radicals and molecules. The spectra were dominated by molecular C2, CN, OH, and CH bands, and atomic H, Ar, C, Cl and O lines. Emission intensities of main species were monitored versus various experimental parameters. The rotational-vibrational temperatures determined from different bimolecular species were considered in the range of 2000–6400 K. Solid soot samples were collected and purified to investigate the possibility of graphene formation as a by-product of the decomposition process. Graphical Abstract

Synthesis of Mullite by Means of Transferred and Nontransferred Arc Plasma Melting

Arc plasma melting technique is a simple method for the synthesis of high melting point materials. In this article, mullite was synthesized by transferred arc plasma (TAP) and nontransferred arc plasma (non-TAP) melting processes, and the results were compared. The mixes of alumina and silica powders (3:2 mole ratios) were ball milled for four hours and then melted in an arc plasma torch, used in transferred and nontransferred mode, at 5 kW input power and two minutes of processing time. Argon gas was used as a plasma-forming gas. The crystalline phases and the microstructural features of the melted samples were determined by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. A complete crystallization of mullite, with dense, thick whiskers-shaped crystals, was achieved in TAP processing of the alumina/silica system. On the other hand, the non-TAP process produced porous mullite along with a small amount of residual alumina phase. Differential thermal analysis (DTA) curves of the synthesized mullite samples allowed a deeper understanding of the mechanisms of mullite formation during the two different processes.