Papers by Author: S.A. Petrova

Abstract: Electrochemical behaviour of mechanoactivated β-MnO₂ powders has been studied by the method of cyclic voltammetry with a carbon-paste electroactive electrode. Mechanical activation was carried out by dry grinding in an AGO-2 planetary ball mill. It was found that the grinding process results in a mechanochemical effect in the surface layer of the oxide particles: Mn (IV) cations are reduced to Mn (III). Voltammetry test detects that mechanical activation of β-MnO₂ leads to a new state, which is characteristic for the γ-modification of manganese dioxide (β-MnO₂ γ-MnO₂).

Abstract: An investigation has been undertaken of the structural characteristics of the manganese oxides to understand these characteristics affected by mechanochemical treatment conditions. Chemically pure manganese (II, III) oxides and their mixtures were used as the initial components.

Abstract: It is shown that investigations of the isotope exchange kinetics in nanocrystalline oxides allows one not only to solve problems connected with determination of process parameters and characteristic scenarios, but also to substantially extend experimental opportunities in analysis of diffusion properties of oxides. A specific behavior of the oxygen isotope exchange was studied in nanocrystalline LaMnO3 and ZrO2:Y2O3 oxides. The former oxide is characterized by very small values of oxygen volume diffusion coefficients, while the latter one is, on the contrary, an ionic conductor. The study was carried out using powdered nanomaterials prepared from original ceramic materials by grinding in planetary mills or by laser evaporation. The study revealed future trends in the use of nanomaterials for analysis of the diffusion kinetics in oxides. It is demonstrated that in this case the sensitivity of traditional isotope methods increases considerably and fundamentally new opportunities are provided for analysis of processes on the "gas phase  solid state" interface as well as for the study of volume diffusion in polycrystals. Considering a strong dependence of the isotope exchange rate on the particle size, it is also topical to conduct studies dealing, along with determination of kinetic parameters of the process, with the use of the obtained data for certification of dimensional characteristics of oxide nanopowders.

Abstract: Specific features of the structural phase transitions of the first order were investigated in nanosized crystals with Jahn-Teller ions. As an example the phase transitions of martensite type with changes in symmetry from a cubic to a tetragonal one have been considered. The Kanamori model was used to take into account the size of nanocrystallites and the distribution of cations over non-equivalent crystallographic sublattices in such systems. It was shown the temperature and the latent heat of the transition decrease significantly for the nanoscaled grains. A possibility of multi-phase state for nanocrystalline materials was considered.

Abstract: The electrochemical behavior of perovskite type LaMnO3 (LMO) oxides with different mean particle size was studied by voltammetry with the use of a carbon paste electroactive electrode. Three stages of electrochemical reduction were recognized. The first two of them are related to the release of oxygen from the crystal lattice in the range of two side nonstoichiometry of LaMnO3±δ whereas the final stage is conditioned by the decomposition of LaMnO3-δ into new phases. The nature of these phases and their formation mechanisms are different for nano- and microparticles. The utmost size effect appears on cathodic curves recorded from the stationary potential. The effect is not only due to the size factor but also due to the difference in electrochemical properties of nano- and microparticles. While the decomposition of LaMnO3 microparticles proceeds into La2O3 and MnO oxides, the nanoparticles decompose through the intermediate stage of Mn3O4 formation in accordance with the transformation sequence principle.

Abstract: The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.