Papers by Keyword: Nanocrystalline Oxides

Abstract: The structure and properties of a composite consisting of Mg–Y–Nd–Zr alloy (WE43) and various oxides are studied. The particles of the WE43 powder were coated by the nanocrystalline oxide layer by means of a wet chemical deposition process. After that the powder is compressed into solid samples and deformed using high pressure torsion at room temperature. A second phase is present both in pure WE43 alloy and in the one with deposited oxides. We observed that the modification of the alloy by the oxide layer deposition and deformation by high pressure torsion changes the phase composition and properties of the samples.

Abstract: The following nanocrystalline binary oxide systems were studied: Mg-Ti, Ni-Ti, Zr-Al, as well as some pure and doped unary oxides. The xerogels were heated at a constant T (200 to 1600°C) for 3 to 6 hours. There was a threshold tempearture for oxide formation and in many cases the products were metastable nanocrystalline phases, depending on the grain size and composition, including doping. The oxide phases of Ni-Ti, Mg-Ti, and Zr-Al, formed at 900 °C are different from those formed at higher temperature. New ranges of solid solutions and the formations of higher temperature structures were found. A transition phase can be defined as a structure formed at relative low tempearture, irreversibly transforming at higher temperature into an equilibrium phase of the same elemental composition. Some low temperature transition phases have a structure similar to that of a high temperature equilibrium phase, e.g., (the equilibrium phase is given in parentheses) tetragonal ZrO2 (monoclinic) and low-T qandilite-like solid solutions (qandilite + geikielite). Others are unique with no representation in the equilibrium phase diagram, e.g., gamma-like alumina (corundum) and anatase (rutile), which are formed as nanocrystalline oxides due to a low growth rate caused either by a low temperature of calcination or due to additives. To asses the importance of crystal size in the stabilization of transition phases, the following studies were undertaken: (a) XRPD analysis of all unary, doped and binary compositions; (b) the evolution of transition phases in HT XRPD of the Mg titanates; (c) the phase evolution was studied with time at temperatures were mixtures of transition and equilibrium phases were found; (d) the retention of pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high tempetature; (e) additional evidence from HRTEM, SEM and DTA experiments was also collected. A model, correlating the size effect with the unusual phases and structures is proposed.

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.