Robert Grigorievich Zakharov

Oxygen Isotope Exchange between Gaseous Phase Enriched with 18O Isotope and Nanocrystal Oxides LaMnO3+δ Obtained by Severe Plastic Deformation

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.

Phase composition and thermal properties of Ce1 − xLnxO2 − d (Ln = Sm, Pr) solid solutions

Ce1 − xLnxO2 − d (Ln = Sm, Pr; x = 0–0.30) oxides have been prepared through pyrolysis of polymer-salt systems, and their particle size and specific surface area have been determined. Their sintering behavior and thermal expansion have been studied using dilatometry. The thermal expansion of the Pr-containing materials is shown to vary considerably with temperature. X-ray diffraction data (obtained at various temperatures and oxygen pressures) suggest that the Ce1 − xPrxO2 − d oxides undergo a low-temperature phase transition, presumably a reversible decomposition of one solid solution to two others. The structural parameters of the phases involved and the oxygen nonstoichiometry of the samples are determined.

Specific Features of Jahn-Teller Structure Phase Transitions in Nanocrystalline Materials

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.

Influence of the Transition to Nanoscaled State on Electrochemical Properties of LaMnO3+δ Oxide

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.

Kinetics of redox processes in manganese oxides

The redox processes in ceramic and powder samples of α-Mn2O3 (kurnakite) and β-Mn3O4 (hausmannite) have been studied in the temperature range 20–1000°C. The results demonstrate that the conversion of α-Mn2O3 to β-Mn3O4 in the binary system Mn-O is essentially irreversible, even if there are nucleation centers in the form of the higher oxide α-Mn2O3 during the oxidation of hausmannite. It is shown that the high reactivity of hausmannite ground in a WC mill is not due to a mechanochemical effect. Grinding-induced contamination has a significant effect on the oxidation behavior of the system: the impurities incorporated into the sample during grinding in a mill favor complete (WC contamination) or partial (Fe contamination) conversion of hausmannite to kurnakite. Hausmannite contaminated with tungsten carbide oxidizes by a catalytic mechanism. In the case of Fe impurities, oxidation follows a solid-state mechanism, through the formation of restricted FexMn2−xO3 solid solutions.

Mechanical Activation of Mn-O Oxides: Structural Phase Transitions, Magnetism and Oxygen Isotope Exchange

The mechanically activated oxides MnO2, Mn2O3, Mn3O4 and MnO were studied under ambient and non-ambient temperature conditions. Regimes of mechanical treatment were found that allowed one to preserve the single-phase state without an essential change of oxide chemical composition. New data about the temperatures and sequence of phase transitions in mechanically activated manganese oxides as well as about the kinetics of isotope exchange were obtained. Magnetic properties of the treated oxides were measured in the temperature range of 4-300К. It is shown that mechano-activation essentially influenced the temperature and field dependences of magnetization, temperatures of magnetic phase transitions and leads to the appearance of additional magnetic phases. New data on the rates of surface reactions during isotope exchange and oxygen self-diffusion coefficients are obtained.

Effect of Mechanical Activation on the Electrochemical Behavior of MnO2

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).

Structural Phase Transitions in Mechanoactivated Manganese Oxides

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.