Showing papers by "Vladislav V. Kharton published in 2007"

Chemically Induced Expansion of La2NiO4+δ-Based Materials

Abstract: The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+δ (M = Fe, Co, Cu; x = 0−0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10-4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (eC) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10-3 to 6 × 10-3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10-6 K-1, does not exceed 5%. The high...

Spin-state ordering and magnetic structures in the cobaltites YBaCo{sub 2}O{sub 5+{delta}} ({delta}=0.50 and 0.44)

Abstract: The antiferromagnetic-ferromagnetic phase transition in $\mathrm{Y}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.50}$ and $\mathrm{Y}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.44}$ cobaltites with different types of oxygen-ion ordering in the $[\mathrm{Y}{\mathrm{O}}_{0.5∕0.44}]$ layers has been studied by neutron powder diffraction in combination with group-theoretical analysis. As a result, the crystal and magnetic structures above and below the phase transformation temperature ${T}_{i}$ were determined and successfully refined. In both cases, the proposed models involve a spin-state ordering between diamagnetic (${t}_{2g}^{6}{e}_{g}^{0}$, $S=0$) and paramagnetic (${t}_{2g}^{4}{e}_{g}^{2}$, $S=2$) ${\mathrm{Co}}^{3+}$ ions with octahedral coordination. Electronic ordering results in a nonzero spontaneous magnetic moment in the high-temperature magnetic phases with isotropic negative exchange interactions. In the case of $\mathrm{Y}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.5}$, the phase transformation does not change the $Pmma$ $(2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p})$ symmetry of the crystal structure. The wave vectors of magnetic structures above and below ${T}_{i}$ are $\mathbit{k}=0$ and $\mathbit{k}={\mathbit{c}}^{*}∕2$, respectively. In the case of $\mathrm{Y}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.44}$ a crossover $P4∕nmm$ $(3\sqrt{2}{a}_{p}\ifmmode\times\else\texttimes\fi{}3\sqrt{2}{a}_{p}\ifmmode\times\else\texttimes\fi{}2{a}_{p})\ensuremath{\rightarrow}I4∕mmm$ $(3\sqrt{2}{a}_{p}\ifmmode\times\else\texttimes\fi{}3\sqrt{2}{a}_{p}\ifmmode\times\else\texttimes\fi{}4{a}_{p})$ was involved to solve the low-temperature magnetic structure. The wave vectors in both high-temperature and low-temperature magnetic phases are $\mathbit{k}=0$. Mechanisms of the phase transformation in both compositions are discussed in the light of obtained magnetic structures. The proposed spin configurations were compared with other models reported in literature.

Oxygen permeability, stability and electrochemical behavior of $$ \Pr _{2} {\text{NiO}}_{{4 + \delta }} $$ -based materials

Abstract: The high-temperature electronic and ionic transport properties, thermal expansion and stability of dense $$ \Pr _{2} {\text{NiO}}_{{4 + \delta }} ,\Pr _{2} {\text{Ni}}_{{0.9}} {\text{Fe}}_{{0.1}} {\text{O}}_{{4 + \delta }} $$ and $$ \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $$ ceramics have been appraised in comparison with K2NiF4-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073–1223 K leads to reversible decomposition of Pr2NiO4-based solid solutions into Ruddlesden–Popper type Pr4Ni3O10 and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of $$ \Pr _{2} {\text{NiO}}_{{4 + \delta }} $$ ceramics at 1173–1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of $$ {\text{La}}_{2} {\text{NiO}}_{{4 + \delta }} $$ . The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973–1073 K, porous $$ \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $$ electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to $$ {\text{La}}_{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $$ , whilst cathodic reduction decreases their performance.

Stability and oxygen transport properties of Pr2NiO4+δ ceramics

Abstract: The oxygen permeability and stability of dense Pr 2 NiO 4+ δ ceramics have been appraised in comparison with K 2 NiF 4 -type lanthanum nickelate In oxidizing atmospheres, Pr 2 NiO 4+ δ exhibit an extensive oxygen uptake and decomposition into the Ruddlesden–Popper-type Pr 4 Ni 3 O 10 and praseodymium oxide at temperatures below 900 °C At 900–950 °C when the K 2 NiF 4 -type praseodymium nickelate is stable in the ceramic membranes placed under an oxygen partial pressure gradient, the steady-state oxygen permeability of Pr 2 NiO 4+ δ is similar to La 2 NiO 4+ δ The phase changes on cooling lead to considerably higher oxygen permeability, which becomes comparable to that of perovskite-type cobaltites, and to noticeable isothermal expansion The stability of Pr 2 NiO 4+ δ in reducing atmospheres, estimated from the p (O 2 ) dependencies of total conductivity and Seebeck coefficient, is lower than that of La 2 NiO 4+ δ , probably due to a smaller size of the A-site cation

Raman spectra of apatites: La10−x Si6−y (Al,Fe)y O26±δ

Abstract: Raman spectra of eight polycrystalline apatites of the general formula La10−xSi6−yM′yO26 ± δ with M′ = Al or Fe were obtained at 300 K. Raman spectra of La10Si4Fe2O26 and La9.83Si4.5Al1.5O26 were investigated in the range 80–1000 K and 80–623 K, respectively. Tentative assignments of bands to stretching and bending modes of SiO4 tetrahedra and to M'O vibrations are proposed. Except for the two new bands, which appear around 700 cm−1 when Al is replaced by Fe, only some band broadenings and relative intensity changes are observed as a function of the rate of O5 or La vacancies. Most of the bands soften and broaden continuously when raising the temperature. This is an indication that the Al- and Fe-substituted apatites do not undergo any structural change up to 1000 K. Above 1000 K, the broad and weak shoulder observed at 850 cm−1 for La10Si4Fe2O26 is replaced by a strong band at 868 cm−1, suggesting that SiO4 tetrahedra undergo a structural modification. All compounds show the same residual band broadening at 80 K. This suggests that there is a small rate of static disorder preferentially related to the solubility of Al and Fe in the Si sublattice rather than to other defects. Moreover, the observation of FeO modes indicates that the dynamics of the solid solution obeys the so-called two-mode behavior. The occurrence of FeO stretching vibrations 150 cm−1 lower than for those of SiO suggests that the coordination number of iron could be larger than 4, particularly for the Fe4+ species. Copyright © 2006 John Wiley & Sons, Ltd.

Oxygen Nonstoichiometry and Ionic Conductivity of Sr3Fe2-xScxO7-δ

Abstract: The substitution of scandium for iron in the Ruddlesden−Popper Sr3Fe2-xScxO7-δ (x = 0−0.3) system increases tetragonal unit-cell volume and oxygen nonstoichiometry and decreases partial p- and n-type electronic conductivities studied in the oxygen partial pressure range from 1 × 10-20 to 0.7 atm at 973−1223 K. The solubility of Sc3+ corresponds to approximately x ≈ 0.35. The relatively low, temperature-activated hole mobility indicates a small-polaron mechanism of the electronic transport, as for undoped Sr3Fe2O7-δ. The atomistic computer simulations showed that major contribution to the ionic conductivity is provided by the oxygen sites surrounded by iron cations in the perovskite-type layers of Sr3(Fe,Sc)2O7-δ structure, whereas stable ScO6 octahedra are essentially excluded from the oxygen diffusion processes. Minimum migration energy, 0.9−1.4 eV, was found for nonlinear pathways formed by the apical O1 sites linking iron−oxygen polyhedra along the c-axis and equatorial O3 positions in the perovskite-t...

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

Abstract: The solubility of Ti 4+ in the lattice of apatite-type La 9.83 Si 6− x Ti x O 26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La 9.83 Si 6− x Ti x O 26.75 ( x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p (O 2 ) range from 10 −20 to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H 2 –90%N 2 gradients at 700–950 °C, suggesting the stabilization of Ti 4+ in the apatite structure. Titanium addition in La 9.83 Si 6− x Ti x O 26.75 ( x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p (O 2 ) = 10 −20 atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr 4+ in the La 9.83 Si 6− x Zr x O 26.75 system was found to be negligible, while the maximum concentration of Ce 4+ in La 9.4− x Ce x Si 6 O 27− δ is approximately 5% with respect to the number of lanthanum sites.

Effect of BaO Addition on Crystallization, Microstructure, and Properties of Diopside-Ca-Tschermak Clinopyroxene-Based Glass-Ceramics

Abstract: This work presents the processing and the characterization of diopside–Ca-Tschermak clinopyroxene-based glass–ceramics (GCs), where partial (i.e., 5%, 10%, and 20%) substitution of Ba for Ca was attempted in the composition of CaMg0.8Al0.4Si1.8O6. Sintering, crystallization, microstructure, and properties of the new materials were investigated under different heat-treatment conditions concerning heating rates (5–40 K/min), temperatures (850°–1000°C), and times (1–300 h). The good matching of thermal expansion coefficients and the strong, but not reactive, adhesion to yttria-stabilized cubic zirconia, in conjunction with the low level of electrical conductivity and oxygen permeability, indicate that the investigated GCs are suitable for further experimentation as candidates for solid oxide fuel cells sealants.

Methane oxidation over mixed-conducting SrFe(Al)O3−δ–SrAl2O4 composite

Abstract: The steady-state CH4 conversion by oxygen permeating through mixed-conducting (SrFe)07(SrAl2)03Oz composite membranes, comprising strontium-deficient SrFe(Al)O3−δ perovskite and monoclinic SrAl2O4-based phases, occurs via different mechanisms in comparison to the dry methane interaction with the lattice oxygen The catalytic behavior of powdered (SrFe)07(SrAl2)03Oz, studied by temperature-programmed reduction in dry CH4 at 523–1073 K, is governed by the level of oxygen nonstoichiometry in the crystal lattice of the perovskite component and is qualitatively similar to that of other perovskite-related ferrites, such as Sr07La03Fe08Al02O3−δ While extensive oxygen release from the ferrite lattice at 700–900 K leads to predominant total oxidation of methane, significant selectivity to synthesis gas formation, with H2/CO ratios close to 2, is observed above 1000 K, when a critical value of oxygen deficiency is achieved The steady-state oxidation over dense membranes at 1123–1223 K results, however, in prevailing total combustion, particularly due to excessive oxygen chemical potential at the membrane surface In combination with surface-limited oxygen permeability, mass transport limitations in a porous layer at the membrane permeate side prevent reduction and enable stable operation of (SrFe)07(SrAl2)03Oz membranes under air/methane gradient Taking into account the catalytic activity of SrFeO3−δ-based phases for the partial oxidation of methane to synthesis gas and the important role of mass transport-related effects, one promising approach for membrane development is the fabrication of thick layer of porous ferrite-based catalyst at the surface of dense (SrFe)07(SrAl2)03Oz composite

Oxygen ion transport numbers: assessment of combined measurement methods

Abstract: Several modifications of the faradaic efficiency and electromagnetic field (EMF) methods, taking electrode polarisation resistance into account, were considered based on the analysis of ion transport numbers and p-type electronic conductivity of $$ {\left( {{\text{La}}_{{0.9}} {\text{Sr}}_{{0.1}} } \right)}_{{0.98}} {\text{Ga}}_{{0.8}} {\text{Mg}}_{{0.2}} {\text{O}}_{{3 - \delta }} $$ ceramics at 973–1,223 K. In air, the activation energies for p-type electronic and oxygen ionic transport are 115 ± 9 and 71 ± 5 kJ/mol, respectively. The oxygen ion transference numbers vary in the range 0.992–0.999, increasing when oxygen pressure or temperature decreases. The apparent electronic contribution to the total conductivity, estimated from the classical faradaic efficiency and EMF techniques was considerably higher than true transference numbers due to a non-negligible role of interfacial exchange processes. The modified measurement routes give reliable and similar results when p(O2) values at the electrodes are high enough, whilst decreasing the oxygen pressure leads to a systematic error for all techniques associated with measurements of concentration cell EMF. This effect, presumably due to diffusion polarisation, increases with decreasing temperature. The most reliable results in the studied p(O2) range were provided by the modified faradaic efficiency method.

Mixed conductivity, thermal expansion and defect chemistry of A-site deficient LaNi0.5Ti0.5O3- δ

Abstract: This work is focused on the analysis of defect chemistry and partial electronic and oxygen ionic conductivities of A-site deficient La 1− x Ni 0.5 Ti 0.5 O 3− δ ( x = 0.05 and 0.10). The orthorhombic-to-rhombohedral phase transition was monitored by means of dilatometry and high-temperature X-ray diffractometry. The average thermal expansion coefficients vary in the range (8.5–13.0) × 10 −6 K −1 , increasing with temperature and A-site deficiency. The ion transference numbers determined by the Faradaic efficiency measurements are lower than 0.1% at 900–975 °C in air. Activation energies of the oxygen ionic conductivity at 897–977 °C are 430 and 220 kJ/mol for x = 0.05 and 0.10, respectively. Atomistic simulation demonstrated high stability of ternary defect clusters formed by the vacant sites in the A-sublattice, oxygen vacancies and Ni 3+ cations, which leads to a very low level of mixed conductivity.

Behavior of (La,Sr)CoO3- and La2NiO4-based ceramic anodes in alkaline media: compositional and microstructural factors

Abstract: The behavior of dense ceramic anodes made of perovskite-type $$ {\text{La}}_{{1 - x - y}} {\text{Sr}}_{x} {\text{Co}}_{{1 - z}} {\text{Al}}_{z} {\text{O}}_{{3 - \delta }} $$ (x = 0.30–0.70; y = 0–0.05; z = 0–0.20) and K2NiF4-type $$ {\text{La}}_{2} {\text{Ni}}_{{1 - x}} {\text{Me}}_{x} {\text{O}}_{{4 + \delta }} $$ (Me = Co, Cu; x = 0–0.20) indicates significant influence of metal hydroxide formation at the electrode surface on the oxygen evolution reaction (OER) kinetics in alkaline solutions. The overpotential of cobaltite electrodes was found to decrease with time, while cyclic voltammetry shows the appearance of redox peaks characteristic of Co(OH)2/CoOOH. This is accompanied with increasing effective capacitance estimated from the impedance spectroscopy data, because of roughening of the ceramic surface. The steady-state polarization curves of $$ {\left( {{\text{La}},{\text{Sr}}} \right)}{\text{CoO}}_{{{\text{3}} - \delta }} $$ in the OER range, including the Tafel slope, are very similar to those of model Co(OH)2–La(OH)3 composite films where the introduction of lanthanum hydroxide leads to decreasing electrochemical activity. La2NiO4-based anodes exhibit a low electrochemical performance and poor stability. The effects of oxygen nonstoichiometry of the perovskite-related phases are rather negligible at high overpotentials but become significant when the polarization decreases, a result of increasing role of oxygen intercalation processes. The maximum electrocatalytic activity to OER was observed for A-site-deficient $$ {\left( {{\text{La}}_{{0.3}} {\text{Sr}}_{{0.7}} } \right)}_{{0.97}} {\text{CoO}}_{{3 - \delta }} $$ , where the lanthanum content is relatively low and the Co4+ concentration determined by thermogravimetric analysis is highest compared to other cobaltites. Applying microporous layers made of template-synthesized nanocrystalline $$ {\left( {{\text{La}}_{{0.3}} {\text{Sr}}_{{0.7}} } \right)}_{{0.97}} {\text{CoO}}_{{3 - \delta }} $$ leads to an improved anode performance, although the effects of microstructure and thickness are modest, suggesting a narrow electrochemical reaction zone. Further enhancement of the OER kinetics can be achieved by electrodeposition of cobalt hydroxide- and nickel hydroxide-based films.

Defect Formation and Transport in La0.95Ni0.5Ti0.5O3‐δ.

Abstract: Deficiency in the A sublattice of orthorhombic perovskite-type La1−xNi0.5Ti0.5O3−δ, with maximum at x = 0.07 – 0.08 , is compensated by the formation of trivalent nickel and oxygen vacancies. The atomistic computer simulations showed that these defects are trapped near the A-site cation vacancies, resulting in the stabilization of Ni3+ cations and low electronic and oxygen-ionic transport. The average thermal expansion coefficient of La0.95Ni0.5Ti0.5O3−δ ceramics, calculated from dilatometric data in air, increases from 8.6 × 10 −6 K −1 at 300–800 K to 12.0 × 10 −6 K −1 at 1300–1500 K. The data on Seebeck coefficient and total conductivity, predominantly p-type electronic, suggest a broadband mechanism of hole transport. The activation energies for the hole and ionic conductivities are 89 and 430 kJ/mol, respectively. The oxygen ion transference numbers determined by the faradaic efficiency measurements in air, vary in the range 9.5 × 10 −5 – 8.1 × 10 −4 at 1173–1248 K, increasing with temperature. Reducing oxygen partial pressure leads to a moderate decrease of the conductivity, followed by phase decomposition in the p ( O 2 ) range 9 × 10 −11 to 8 × 10 −9 atm at 1073–1223 K. The low- p ( O 2 ) stability limit of La0.95Ni0.5Ti0.5O3−δ perovskite was found between that of La3Ni2O7 and Ni/NiO boundary.