A. I. Medvedev

Properties of oxide nanopowders prepared by target evaporation with a pulse-periodic co2 laser

The design and characteristics of a setup for producing metal oxide nanopowders with an output of up to 20 g/h are discussed. The grain mean size in the powders is 15 nm, and the radiation power consumption is 30—40 (W h)/g. Y2O3-stabilized ZrO2 (YSZ) and Al2O3 + YSZ nanopowders are prepared by target evaporation with a pulse-periodic CO2 laser, followed by vapor condensation in an air stream. The mean power, peak power, and efficiency of the pulse-periodic CO2 laser, excited by a combined discharge, are, respectively, 1 kW, 10 kW, and ≈10%. Data for the powder specific surface, grain shape, and grain size distribution, as well as results of X-ray phase and structure analysis, are reported.

Properties of powders produced by evaporating CeO2/Gd2O3 targets exposed to pulsed-periodic radiation of a CO2 laser

The characteristics (phase composition, grain shape, grain size distribution, and specific surface area) of Ce0.78Gd0.22O2-δ nanopowders produced by exposing the target to pulsed CO2 laser radiation are reported. Reasons for a threefold increase in the output of the experimental powder-preparation unit (up to 60 g/h) with the characteristic grain size (≈10 nm) remaining unchanged are discussed.

Synthesizing aluminum nanoparticles in an oxide shell

It is shown that a protective oxide shell can be formed on the surface of aluminum nanoparticles by the controlled addition of oxygen to the working inert gas of the installation while powders are being synthesized by the method of electrical explosion of wire. The specific surface area of the powder, the concentration of the aluminum metal, and the phase composition of the powders are given as a function of the added oxygen at different values of the energy injected to the wire. The powders are analyzed using BET, XRD, SEM, and TEM methods; the aluminum metal concentration is determined by the volumetric technique. The proposed method considerably reduces the degree of agglomeration, refines the powders, and eliminates their pyrophoricity by as early as at the synthesis stage. Today the obtained results are used for making a dense constructional ceramic material based on aluminum oxide.

Properties of nickel oxide nanopowders prepared by electrical explosion of a wire

The properties of NiO nanopowders prepared by electrical explosion of a wire in an oxygen-containing atmosphere are presented. Most of the NiO nanopowders are found to be oxygen-enriched, the excess of oxygen depending mainly on the nickel vapor concentration. The dependences of the powder particle size on the oxygen concentration and overheating of the exploding metal are discussed. The powder nanoparticles are both single-crystal and polycrystalline with a rhombohedral lattice and have different shapes (from cubic to spherical). There typical sizes range from 15 to 50 nm, depending on the explosion conditions.

Iron oxide nanopowders prepared by the electroexplosion of wire

We have studied the effect of process parameters (supplied energy, oxygen concentration in the gas mixture, and others) on the particle size and yield of iron oxide nanopowder obtained by the electroexplosion of wire. Under optimal conditions, up to 25% of the particles in the powder were 15–20 nm in size (specific surface of up to 100 m2/g). Using several independent characterization techniques, we determined the phase composition, elemental composition, and structure of the material in the bulk and on the surface of the particles. The majority phase in the powder (90–98 wt %) is the metastable oxide γ-Fe2O3, which is sufficiently stable under ordinary storage conditions. The powder also contains the stable oxide α-Fe2O3. The effect of annealing in air and helium on the phase composition and carbon content of the powder is examined. The results are interpreted in terms of redox and diffusion processes.

Electrochemical properties of cathodes made of (La,Sr)(Fe,Co)O3 containing admixtures of nanoparticles of cupric oxide and intended for fuel cells with a solid electrolyte based on ceric oxide

The electric and electrochemical characteristics of cathodes made of La0.6Sr0.4Fe0.8Co0.2O3−δ (LSFC) and intended for fuel cells with electrolytes based on ceric oxide are studied. Adding cupric oxide into the LSFC cathode is shown to exert a favorable effect of the properties of the LSFC-CuO/SDC electrode system, where SDC stands for the CeO2-Sm2O3 electrolyte. The effect produced by cupric oxide when added in the form of nanopowder is perceptibly greater than in the case of micropowdered CuO. Adding a mere 0.5 wt % of nanopowdered CuO reduces the LSFC cathode resistance nearly tenfold. The cathode’s adhesion to the electrolyte substantially improves as well, which makes it possible to lower the cathode’s firing temperature by 100°C. The maximum of electrochemical activity is intrinsic to cathodes containing 2 wt % CuO, with the caking temperature of 1000°C. According to a 2011-h life test of the LSFC-SDC composite cathodes containing nanopowdered CuO, temporal stability of their electrochemical characteristics improves with the SDC content. The time dependences of the polarization resistance of cathodes containing 40–50 wt % SDC look like decaying exponential curves. The steady-state polarization resistance, calculated on the basis of this, is equal to 0.1–0.2 ohm cm2. At an overvoltage of less than 100 mV, the cathodes may provide for a current density of 0.5–1.0 A cm−2.

pH dependence of the composition of electrodeposited Co films in aqueous sulfate solutions

The electrodeposition of cobalt onto gold is studied as a function of the pH of the electrolyte. Electrochemical quartz crystal microgravimetry is used to identify the material deposited. Using a Co sulfate solution without additives, it is found that for pH <-3.2, Co metal deposits. For pH > 3.2 and at sufficiently negative applied potential, first Co hydroxide deposits and then Co metal. The addition of boric acid, H3BO3 to cobalt sulfate solutions leads to different results: measurements done up to pH 5.5 indicate that only Co metal deposits, showing that boric acid prevents the formation of cobalt hydroxide.

Properties of the translucent ceramics Nd: Y2O3 prepared by pulsed compaction and sintering of weakly aggregated nanopowders

A translucent cubic yttria ceramic material doped by neodymium, namely, 1Nd: Y2O3, with particles of micrometer size (5–17 μm) and clearly defined boundaries is synthesized from nanopowders prepared by laser-induced evaporation with the use of magnetic pulsed compaction and vacuum sintering. Owing to the high activity of nanoparticles, the sintering is performed at temperatures below 1750°C without densifying additives. An increase in the sintering temperature to 900°C leads to an increase in the visual transparency of the ceramic materials and a decrease in the radiation attenuation coefficient. The samples of the translucent ceramics are characterized by rather large values of the microhardness (11.8 GPa) and the fracture toughness as compared to those of single crystals of the same composition. The fracture toughness of the ceramic material increases by a factor of approximately 2.5 with a decrease in the average crystallite size from 5.0 to 0.6 μm.