Papers by Keyword: Nonstoichiometry

Abstract: Effects of cation nonstoichiomtry on crystal structure parameters, microstructure, and dielectric properties of ceramics (Na_0.5+xBi_0.5)TiO₃ and (Na_0.5-xBi_0.5)TiO₃ with Bi/Na<1 and of solid solutions [(Na_0.5Bi_0.5)_1-xK_x]TiO₃ and (Na_0.5Bi_0.5)(Ti_1-xMg_x)O₃ with x = 0 – 0.1 have been studied. Changes in the unit cell parameters and microstructure of the samples observed in ceramics prepared correlate well with both preparation conditions and radii of substituting cations. Ferroelectric phase transitions near ~ 400 K and ~ 600 K were confirmed ib the systems studied. Phase transitions near ~ 400 K demonstrate a pronounced relaxor behavior determined by the presence of polar regions in a nonpolar matrix. Besides, additional anomalies related to presence of relaxing dipoles formed by oxygen vacancies were observed on temperature dependences of dielectric permittivity at temperatures higher than 700 K. The results obtained confirmed that increase in nonstoichiometry lead to increase in ionic conductivity of the samples.

Abstract: The marker method in studying the formation mechanism and defect structure of higher oxide during oxidation of lower oxide has been discussed. The approach to this problem needs specific treatment, both in experimental procedure and in the interpretation of results. It has been shown that the correct results of marker experiments in the case of highly defected substrates can be obtained, if these substrates before the marker deposition process are submitted to homogenization under highest oxidant activity, at which they remain stable at a given temperature. In addition, the nonstoichiometry must be considered in formulating appropriate chemical reactions, being the basis for foreseeing the location of markers in the interior of reaction product. The other very important problem consists in the possibility of the formation of reaction product not only on the surface of oxidized substrate but also inside of this substrate. In such a situation, the formulation of final conclusions concerning the crystalline lattice disorder from marker position should be combined with considerations of chemical reactions and transport processes occurring in a given substrate.

Abstract: Nonstoichiometry and chemical diffusion in Co₃O₄ oxide have been studied as a function of temperature (973-1173 K) and oxygen pressure (30-10⁵ Pa), using thermogravimetric techniques. It has been found that at very low oxygen pressures, close to the dissociation pressure of the oxide, interstitial cations and quasi-free electrons are the predominant point defects, while at high pressures cation vacancies and electron holes predominate. This behaviour is reflected in complex dependence of the deviation from stoichiometry, y, in the Co_3±yO₄ oxide on oxygen pressure. At low pressures, namely, deviation from stoichiometry decreases with increasing oxygen pressure, reaching virtually constant value in intermediate pressures and increases at highest pressure range. Finally, these data as well as the results of kinetic rate measurements of Co_3±yO₄ formation have been utilized in calculating the chemical diffusion coefficient as a function of temperature.

Abstract: The influence of residual stress, thermal stress and chemically induced expansion stress etc... on the fracture damage of solid oxide fuel cells (SOFCs) were investigated by using nondestructive testing method and numerical stress-strain analyses under operating conditions. In order to estimate stress-deformation behavior of cell/stack of SOFCs, mechanical properties of SOFC elements were evaluated under controlled high temperature and oxygen partial pressure conditions. In addition to deformation and mechanical damage behavior were observed by using acoustic emission method.

Abstract: Structure phase transitions in nonstoichiometry oxides with perovskite structure were considered. The microscopic description of cooperative Jahn-Teller transition was suggested and the effects caused by mechanical activation of oxide systems (their conversion into nanostructural state, acceleration of oxidation-renewal reactions and immiscibility) were analized. The correspondence of the theoretical results to the data of experiment on phase states of mechanically activated oxide NdMnO₃ is shown.

Abstract: Oxidation mechanism of nonstoichiometric cuprous oxide (Cu2 yO) has been studied as a function of temperature (973-1273 K) and oxygen pressure (1-105 Pa), using microthermogravimetric, marker and two-stage oxidation techniques. The last method consisted in using heavy oxygen isotope 18O in order to confirm the results, obtained with marker technique. It has been found that in early stages of reaction not exceeding 30 hours, CuO formation on the surface Cu2 yO follows approximately cubic rate law. In later stages, during long-term oxidation, lasting hundreds of hours, oxidation process follows strictly parabolic kinetics with the rate, being independent of Cu2 yO pre-treatment. Marker and two-stage oxidation experiments have shown that the growth process of CuO layer proceeds by the outward diffusion of cations, clearly indicating that cation sublattice of CuO is predominantly defected.

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: Calcium ruthenates were prepared in different ratios of Ru to Ca (RRu/Ca = 0.5～1.4) by spark plasma sintering. CaRuO3 in a single phase was obtained at RRu/Ca = 1.0. At RRu/Ca < 1.0, a mixture of CaRuO3 and CaO was obtained, whereas CaRuO3 with second phase of RuO2 was obtained at RRu/Ca > 1.0. The density at RRu/Ca < 1.0 were 80-85% and slightly increased with increasing RRu/Ca. The density significantly increased up to 95% with increasing RRu/Ca from 1.1 to 1.4, suggesting that the second phase of RuO2 was effective to densify CaRuO3. The density of CaRuO3 in a single phase was 82% at most. The lattice parameters of CaRuO3 decreased with increasing RRu/Ca from 0.7 to 1.0, showing a nonstoichiometric range of Ca1+δRuO3+δ.

Abstract: Titinium carbide (TiCx) was produced by self-propagating high temperature synthesis (SHS) method. The morphology and non-stoichiometric number of the SHS product were observed by scanning electron microscopy and neutron diffractometry, respectively. Tubular titanium carbide with hole inside was formed with different non-stoichiometric number (x), which value increased with combustion temperature.

Abstract: The preparation of single-phase CuLaO2 with delafossite-type structure by means of the solid-state reaction method was investigated using X-ray diffraction. The results showed that notwhistanding the fact that there was a trace of metallic copper, nearly single-phase CuLaO2 was obtained by using La(OH)3 as a lanthanum source and by firing the mixed powder with nonstoichiometric composition ratio of La(OH)3:Cu2O =1:1.425 in a vacuum at 1273 K for 10 h. The measurement of electrical conductivity and Seebeck coefficient showed that CuLaO2 thus obtained was a p-type semiconductor and had a Seebeck coefficient of approximately 70 /V/K.