O. V. Belousova

Preparation of nickel nanoparticles for catalytic applications

Spherical oxidized nickel particles 15 to 200 nm in average size have been produced by a crucibleless aerosol method involving metal vapor condensation in an inert gas flow and oxidation processes. The particles have been characterized by scanning electron microscopy, X-ray microanalysis, X-ray diffraction, BET surface area measurements, and vibrating-sample magnetometry. The process parameters have been optimized for the preparation of particles with tailored size, specific surface area, and saturation magnetization. A dc electric field applied to the condensation zone during the oxidation process reduces the size and increases the extent of oxidation of the particles. We have studied low-temperature oxidation of carbon monoxide and propane on nickel nanopowders differing in particle size and extent of oxidation. The nanoparticles with optimized characteristics have been shown to have a marked catalytic effect on these processes.

Electric field-assisted levitation-jet aerosol synthesis of Ni/NiO nanoparticles

Ni/NiO powdered nanoparticles with average sizes 10–30 nm were prepared by a levitation-jet method involving the condensation of Ni metal vapour in a mixture of helium with various amounts of air or oxygen. The process was undertaken with the application of a DC electric field up to 6.5 kV cm−1. The particles were characterized by X-Ray diffraction, transmission electron microscopy, BET adsorption and vibrating sample magnetometry. It was found that the intensity of the applied electric field and partial oxygen pressure correlated with the main structural and magnetic parameters of the nanoparticles, such as average particle size, residual ratio of nickel, coercivity and maximum magnetisation. The specific surface area of the particles correlated with the magnitude of the external electric field. Room-temperature hysteresis loops of weakly oxidized nanoparticles show ferromagnetic-like behaviour, whereas the strongly oxidized ones exhibit a low-field ferromagnetic feature superimposed to a paramagnetic signal, regardless of the particle size. Magnetic measurements allowed for the estimation of the residual metal Ni content in the powdered nanoparticles, which can be as low as 0.04 at.% depending on oxygen partial pressure and external electric field strength.

Ferromagnetic Zn/ZnO nanoparticles

Spherical zinc particles ranging in average size from 275 to 760 nm and covered with platelike zinc oxide particles on the order of 10 nm in size have been prepared by levitation-jet aerosol synthesis through condensation of zinc vapor in an inert-gas flow. The nanoparticles have been characterized by transmission electron microscopy, X-ray diffraction, BET measurements, and vibrating-sample magnetometry. The results indicate that the observed ferromagnetic ordering is due to changes in unit-cell volume on the surface of the nanoparticles. High-temperature magnetization data demonstrate that the ferromagnetic ordering of the nanoparticles persists up to 750 K.

Ferromagnetic nanoparticles in Sn-O system

Tin oxide nanoparticles ranging in average size from 12 to 315 nm have been prepared by levitation-jet aerosol synthesis through condensation of tin vapor in a flow of inert gases and oxygen (air). The nanoparticles have been characterized by transmission electron microscopy, X-ray diffraction, BET measurements, vibrating-sample magnetometry, and Raman scattering spectroscopy. The results indicate that the nanoparticles may exhibit room-temperature ferromagnetism, with their magnetization having a maximum at O: Sn = 1. The ferromagnetic order is tentatively attributed to the presence of localized states on the Sn/SnO and SnO/SnO2 interfaces.

Levitation jet synthesis of nickel ferrite nanoparticles

Pseudospherical nickel ferrite particles 25 to 70 nm in average size were prepared by a crucibleless aerosol method through cocondensation of Fe and Ni vapors in an inert-gas flow containing a small amount of air. The particles were characterized by transmission electron microscopy, X-ray microanalysis, X-ray diffraction, BET measurements, and vibrating-sample and SQUID magnetometry. The results were used to optimize process parameters for the preparation of particles with a tailored size, specific surface area, and saturation magnetization. A dc electric field applied to the condensation zone can serve to improve the phase purity of nickel ferrite nanoparticles, reduce their size, and change their Curie temperature.

Catalytically active magnetic nanoparticles in the Cu-O system

Cu-O nanoparticles 11 to 215 nm in average size have been prepared by a levitation-jet process in a He flow with air additions and characterized by scanning electron microscopy, X-ray diffraction, BET measurements, vibrating-sample magnetometry, and optical spectroscopy in the UV, visible, and IR spectral regions. The nanoparticles were pseudospherical in shape and consisted predominantly of Cu2O and CuO. Magnetization measurements showed that the nanoparticles exhibited soft-magnetic ferromagnetic behavior (up to 0.06 A m2/kg in terms of maximum magnetization). The nanoparticles were also investigated in a high-temperature air flow reactor as a promising catalytic material for propane oxidation to CO2. In the case of two-phase nanoparticles, the total conversion temperature was found to decrease by 70 K as the nanoparticle size decreased from 110 to 51 nm, with a minimum temperature of 450 K for single-phase Cu2O particles 29 nm in size. The increase in catalytic activity seems to be related to the behavior of localized electron states on the CuO/Cu2O interface, which are responsible for the room-temperature ferromagnetism of the nanoparticles.

Synthesis of copper ferrite nanoparticles

Pseudospherical copper ferrite particles 20 to 90 nm in average size were prepared by an aerosol method through condensation of iron and copper vapors in an inert-gas flow, followed by the oxidation of the resulting two-phase powder under heterogeneous combustion conditions to an almost single-phase product. The nanoparticles were characterized by scanning electron microscopy, X-ray diffraction, BET measurements, and vibrating-sample magnetometry. Analysis of the X-ray diffraction data and the behavior of the magnetization of reaction intermediates and final synthesis products in the range 400–1100 K made it possible to propose models for the nanostructure of the particles and establish the likely sequence of the observed phase transformations.

Generation of electric potentials during heterogeneous combustion in systems containing VI group elements

The behavior of frontally burning heterogeneous systems containing chromium, molybdenum, and tungsten, which are used in the self-propagating high-temperature synthesis of complex oxide materials, was studies by the potentiometric method. It was revealed that the peak emf between the combustion wave front and the synthesis products can be as high as 2 V, depending on the chemical composition of the initial stock. A mobile registering probe moving with the velocity of combustion wave propagation makes it possible to substantially increase the time of emf recording

Some approaches to collecting electric voltage generated by SHS reactions

We suggest two approaches to collecting the electromotive force (Ec) generated by SHS reactions such as the use of mechanical systems with a movable voltage-collecting electrode and the so-called battery assembly. The use of the battery system comprising of two reactive systems generating voltage of opposite polarity afforded to increase Ec up to 2.3 V.

SHS of a superoxide in the Na-Fe-Y-O system

For the first time, complex oxide NaFeYO7 + x was prepared by SHS (Tc = 750–1200 K, U = 1–4 mm/s) in the Na-Fe-Y-O system. Explored was the influence of reagents ratio on combustion limits for the mixtures under study. Combustion products were characterized by XRD, SEM, EDX, chemical analysis, and magnetic measurements. Single-phase NaFeYO7 + x was obtained at an excess of sodium in green mixtures. To stabilize the product, the water-washing procedure was used just after synthesis. The synthesized material has a cubic crystal structure with a cell parameter close to that of sodium yttrate NaYO2, it exhibited very weak magnetic properties, and overly-stoichiometric oxygen content. The product is heat resistant up to 450 K. The synthesized product is a new inorganic superoxide with good technological perspectives for use as a solid oxygen-containing material.