N. N. Dremova

Synthesis and electrical properties of Gd2MO5 (M = Zr, Hf)

Polycrystalline Gd2ZrO and Gd2HfO5 have been prepared by heat-treating coprecipitated oxide mixtures, and their order-disorder phase transitions have been studied in the range 20–1600°C. The materials have been shown to consist of nanostructured grains with a nanodomain size of ∼40 nm. Their electrical conductivity has been determined by impedance spectroscopy in air between 300 and 1000°C. The 1000°C conductivities of Gd2ZrO5 and Gd2HfO5 are 3.7 × 10−3 and 1.8 × 10−3 S/cm, and the respective effective activation energies are 1.37 and 1.56 eV.

Activation of metallic aluminum by tin and gallium chlorides in oxidation with water

We have studied the effect of gallium chloride and tin chloride solutions on the water oxidation of aluminum at SnCl2 concentrations of 0.68 and 6.32 wt %, GaCl3 concentrations of 0.56 and 2.67 wt %, and MCln: Al(M = Sn, Ga; n = 2, 3) molar ratios from 0.017 to 0.3. The results indicate that, when aluminum is oxidized in the presence of these salts, the reaction rate and hydrogen yield increase with reaction temperature and salt concentration and reach the highest levels when a mixture of gallium and tin chlorides is used. The reaction products are identified and the likely mechanism of the processes involved in the oxidation of aluminum is discussed.

Structural and electrical properties of nanostructured fluorite-like R2 + xTiO5 + 1.5x (R = Y, Er; 0 ≤ x ≤ 1)

We have studied the detailed structure and electrical properties of single-crystal and polycrystalline fluorite-like R2 + xTiO5 + 1.5x (R = Y, Er; 0 ≤ x ≤ 1) materials and found the first evidence for the formation of nanodomains ≏ 40–1000 nm in size, which are coherent with the matrix and have various degrees of structural order. The formation of the nanodomains is driven by internal stress. Using impedance spectroscopy, we have determined the electrical conductivity of the materials in air at temperatures from 300 to 1000°C. The 1000°C electrical conductivity of Y2TiO5 (Er2TiO5) and Y2.44TiO5.67 (Er2.44TiO5.67) is 1.86 × 10−3 (1.35 × 10−3) and 9.98 × 10−4 (8.13 × 10−4) S/cm, respectively. The effective activation energy for electrical conduction in Y2TiO5 (Er2TiO5) and Y2.44TiO5.67 (Er2.44TiO5.67) is 1.16 (1.21) and 1.25 (1.21) eV, respectively.

Preparation of titanium diboride nanopowders of different particle sizes

Reactions between titanium and microcrystalline boron powders in a Na2B4O7 ionic melt at temperatures from 700 to 850°C and those between TiCl4 and NaBH4 at temperatures from 300 to 750°C and hydrogen pressures of up to 10 MPa, with no solvent, have been studied by X-ray diffraction, scanning electron microscopy, thermogravimetry, and elemental analysis. The results demonstrate that TiB2 formation occurs at t 〉 730°C and 550°C, respectively. According to scanning electron microscopy data, the TiB2 powder consists of particles 70–75 and 35–50 nm size, and the crystallite size evaluated from X-ray diffraction data is 55 and 30 nm, respectively, in agreement with the equivalent particle diameters obtained from the specific surface area of the TiB2 powders: 60 and 45 nm, respectively.

Oxidation behavior of TiB2 micro- and nanoparticles

The oxidation of TiB2 particles (75 to 1500 nm in size) has been studied at temperatures of up to 1000°C by thermogravimetry, X-ray diffraction, X-ray photoelectron spectroscopy, IR frustrated total internal reflection spectroscopy, and energy dispersive X-ray analysis. The oxidation onset was observed between 210 and 475°C, depending on the particle size. This distinction can presumably be accounted for in terms of the deformation produced by the Laplace pressure. Oxidation at temperatures under 1000°C leads to the formation of the rutile phase of TiO2 and boron oxide (B2O3). Moreover, at a temperature of ≃ 1000°C titanium borate, TiBO3, was observed to form. Under all of the conditions examined, the oxidation reaction does not reach completion and the oxidation products contain unreacted TiB2.

Preparation of hafnium diboride nanopowders in an anhydrous Na2B4O7 ionic melt

Reactions between hafnium powder and microcrystalline boron in a Na2B4O7 ionic melt have been studied at temperatures from 600 to 850°C. The results demonstrate that nanoparticulate hafnium diboride forms starting at 750°C. According to electron microscopy data, the HfB2 powder obtained at 850°C consists of nearly spherical particles 50–55 nm in diameter, which agrees with the equivalent particle diameter (≃ 60 nm) evaluated from the specific surface area of the HfB2 and with the crystallite size (≃55 nm) determined from X-ray diffraction data.

Corrosion resistance of nanostructured films of titanium diboride in mineral acid solutions

The reactions of nanostructured TiB2 films (mean grain size is 3.3 ± 1.1 nm) with mineral acids (HCl, H2SO4, H3PO4, and HNO3) of different concentrations have been studied. It is found that the dissolution of the films is a congruent process showing linear kinetics for the time periods in the range from 300 to 1500 min. The corrosion depth parameters of nanostructured TiB2 films in mineral acids are determined, and their corrosion resistance is graded on a ten-point scale. The studied films show the lowest corrosion resistance in their reaction with nitric acid solutions, and the highest resistance is observed in phosphoric acid solutions.

A Study of a Novel Nanocomposite Material Based on Reduced Graphene Oxide and Poly( o -Phenylenediamine)

The formation of a nanocomposite coating based on reduced graphene oxide (RGO) and poly(o-phenylenediamine) (PPD) in the chemical reaction of graphene oxide (GO) and o-phenylenediamine (OPD) has been studied by cyclic voltammetry (CVA), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). It has been shown that the action of cyclic polarization provides a more uniform structure of the nanocomposite films (decrease in the corrugation effect). In addition, it has been found that the polarization of GO films containing the OPD monomer distributed inside to the PPD synthesis potentials leads to the formation of uniformly distributed globular structures, which most probably correspond to a polymer electrochemically synthesized in the space between the GO nanosheets.

Formation of zirconium diboride nanoparticles as a result of reaction between zirconium tetrachloride and sodium borohydride

We have studied reaction between ZrCl4 and NaBH4 at temperatures between 300 and 725°C. The results demonstrate that single-phase zirconium diboride nanoparticles are formed starting at 575°C. According to electron microscopy data, the ZrB2 powder obtained at 575 and 725°C consists of variously shaped particles, some of which are almost spherical, ranging in diameter from ~10 to 20 and from 25 to 35 nm, respectively. These values agree with the equivalent particle diameters evaluated from the measured specific surface area of ZrB2, ~14 and ~32 nm, respectively, and with the crystallite size extracted from X-ray diffraction data: Dhkl ~ 13 and 28 nm.

High-temperature carbonization of humic acids and a composite of humic acids with graphene oxide

Humic acids (HAs) isolated from high-moor peat have been studied by magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG), and Raman spectroscopy. A composite of HAs with graphene oxide (GO) has been prepared for the first time, and the thermal carbonization (900°C) of both HAs and the HA–GO composite has been carried out. With the use of mass spectrometry, it has been found that CO2 and H2O molecules are mainly released from HAs into the gas phase at a low temperature (to 150°C). At higher temperatures, carbon monoxide and different low-molecular-weight hydrocarbons also begin to be released. From microscopic examinations, it follows that HA forms small agglomerates with sharply outlined edges as a result of carbonization, whereas the composite forms only large aggregates.