Oxygen-deficient perovskite-related (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ as oxygen electrode materials for SOFC/SOEC

Perovskite-related Ln2NiO4+δ (Ln = La, Pr, Nd) nickelates with layered Ruddlesden-Popper combine redox stability with noticeable oxygen stoichiometry changes, yielding enhanced mixed transport and electrocatalytic properties. These unique features are promising for applications as oxygen electrodes with good electrochemical performance in reversible SOFC/SOEC (solid oxide fuel/electrolysis cell) systems. To date, most efforts were focused on oxygen-hyperstoichiometric Ln2NiO4+δ-based phases, whereas nickelates with oxygen-deficient lattice remain poorly explored. Recent studies demonstrated that the highest electrical conductivity in (Ln2-xSrx)2NiO4±δ series at elevated temperatures is observed for the compositions containing ~ 60 at.% of strontium in A sublattice [1,2]. The present work was focused on the characterization of (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates for the possible use as materials for reversible oxygen electrodes. The ceramic materials were prepared by Pechini method with repeated annealings at 650-1200°C and sintered at 1250-1300°C for 5 h under oxygen atmosphere. Variable-temperature XRD studies confirmed that all studied compositions retain tetragonal K2NiF4-type structure in the temperature range 25-900°C. The results of thermogravimetric analysis showed that the prepared nickelates has oxygen-deficient lattice under oxidizing conditions at temperatures above 700°C. Partial substitution of nickel by cobalt or iron results in a decrease of p-type electronic conductivity and the concentration of oxygen vacancies in the lattice (Fig.1), but also suppresses dimensional changes associated with microcracking effects (due to anisotropic thermal expansion of tetragonal lattice). Electrochemical performance of porous (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ electrodes in contact with Ce0.9Gd0.1O2-δ solid electrolyte was evaluated at 600- 800°C employing electrochemical impedance spectroscopy and steady-state polarization (anodic and cathodic) measurements.publishe

Oxygen-deficient Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates as oxygen electrode materials for SOFC/SOEC

Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4±δ (M = Ni, Co, Fe) nickelates have been characterized as prospective oxygen electrode materials for solid electrolyte cells. XRD studies showed that these oxides retain tetragonal K2NiF4-type structure in air until at least 900°C. Average thermal expansion coefficients of Nd0.8Sr1.2Ni0.8M0.2O4±δ calculated from the structural data are in the range 14.5-15.8 ppm/K. TGA studies revealed that these nickelates are oxygen-deficient in air at temperature above 700°C but tends to oxygen stoichiometry or minor excess on cooling. Incorporation of cobalt or iron into nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency and electrical conductivity. Electrochemical impedance spectroscopy studies of symmetrical cells showed that porous Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ electrolyte exhibit quite similar performance, with lowest values of polarization resistance (0.8 Ohm×cm2 at 800°C) observed for M = Ni. The polarization resistance can be further decreased (down to 0.04 Ohm×cm2 at 800°C for M = Ni) by surface modification with PrOx.publishe

ВЛИЯНИЕ МЕТОД А СПЕКАНИЯ НА МИКРОСТРУКТУРУ КЕРА МИКИ ND0,4SR1,6NIO

Two different methods have been employed for fabrication of Nd0.4Sr1.6NiO4–d ceramics with focus on the microstructure and density of ceramic samples. Conventional sintering at 1100–1300 °C has been found to yield porous materials. Rapid grain growth at ≥1 300 °C induces the development of microcracks associated with a strongly anisotropic expansion ofNd0.4Sr1.6NiO4–d tetragonal lattice. On the contrary, spark plasma sintering (SPS) at 1100 °C enables fabrication of dense gastight ceramics, but is accompanied by the structural transformation from tetragonal (I4/mmm) to orthorhombic (Immm) symmetry due to oxygen losses from the lattice under low-p(O2) conditions of SPS process. The post-treatment conditions were optimized to oxidize sintered samples and to restore tetragonal structure while preserving gas-tightness of ceramics.Исследовано влияние методов свободного спекания и плазменного искрового спекания (ПИС) на микроструктуру керамики Nd0,4Sr1,6NiO4–d. Установлено, что керамика, изготовленная свободным спеканием, является пористой независимо от температуры отжига. Керамика, полученная методом плазменного искрового спекания, – газоплот-ная. однако в результате применения восстановительной атмосферы вакуума в процессе ПИС происходит структурное превращение тетрагональной ячейки I4/mmm в орторомбическую Immm. В результате проведенной серии термообработок керамики были найдены оптимальные условия последующего отжига керамики, приводящие к восстановлению тетрагональной структуры оксида (Immm→I/4mmm) и сохранению газоплотности керамики Nd0,4Sr1,6NiO4–d

ТЕРМИЧЕСКОЕ РАСШИРЕНИЕ, ЭЛЕКТРИЧЕСКАЯ ПРОВОДИМОСТЬ И КИСЛОРОДНАЯ НЕСТЕХИОМЕТРИЯ НИКЕЛАТОВ La2-xSrxNiO4-δ КАК ПОТЕНЦИАЛЬНЫХ КАТОДНЫХ МАТЕРИАЛОВ TОТЭ

La2-xSrxNiO4-δ (x = 1.0–1.6) nickelates were evaluated as potential cathode materials for solid oxide fuel cells, with focus on the structural stability, oxygen nonstoichiometry and electrical conductivity under oxidizing conditions. All studied ceramic materials were found to preserve K2NiF4-type tetragonal structure under oxidizing conditions at 25–900  °С.La2-xSrxNiO4-δ (x = 1.0–1.6) nickelates demonstrate oxygen deficiency at temperatures above 500 °С, with oxygen nonstoichiometry increasing with temperature and strontium content. The electrical conductivity is p-type and show metallic-like behavior under oxidizing conditions at 500–1000 °С. The highest conductivity values, 220 S/cm at 900 °С and 440  S/cm at 600 °С in air, are measured for La0,8Sr1,2NiO4-δ ceramics. While the high-temperature XRD studies revealed strongly anisotropic thermal expansion of La2-xSrxNiO4-δ crystal lattice, the lattice volume show nearly linear dependence on temperature, with average linear thermal expansion coefficients varying in the range (14.2–15.6) · 10-6 K-1.Оксиды системы La2-xSrxNiO4-δ (x = 1,0–1,6) были исследованы в качестве  потенциальных катодных материалов для твердооксидных топливных элементов. Были изучены структурная стабильность, кислородная нестехиометрия и электрическая  проводимость. Установлено, что все оксиды сохраняют кристаллическую структуру типа K2NiF4 в окислительных условиях в температурной области 25–900 °С. Оксиды данной системы являются дефицитными по кислороду при температурах выше 500 °С, и  кислородная нестехиометрия повышается с ростом температуры и увеличением содержания стронция. Исследованные никелаты обладают псевдометаллической электропроводностью p-типа в окислительных условиях при температурах 500–1000 °С. Наибольшая электрическая проводимость характерна для La0,8Sr1,2NiO4-δ (220 См/см при 900 °С и 440 См/см при 600 °С). Методом высокотемпературной рентгеновской дифракции установлено, что никелаты La2-xSrxNiO4-δ проявляют анизотропное термическое расширение кристаллической решетки, однако объемное термическое расширение носит практически линейную зависимость от температуры; значения линейных коэффициентов термического расширения составляют (14,2–15,6) · 10-6 K-1

Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine

[This corrects the article DOI: 10.1186/s13054-016-1208-6.]

Experimental reproduction of double-peaked P-wave

Eur Heart J 2000; 21 [Absrt. Suppl.]: 55

A thermogravimetric study of thermal dehydration of copper hexacyanoferrate by means of model-free kinetic analysis

The kinetics of thermal dehydration of K2x/3Cu[Fe(CN)6]2/3·nH2O was studied using thermogravimetry for x = 0.0 and 1.0. Data from both non-isothermal and isothermal measurements was used for model-free kinetic analysis by the Friedman and KAS methods. The water content was determined to be n = 2.9 – 3.9, with an additional ~10% of water, likely surface adsorbed, that leaves very fast when samples are exposed to a dry atmosphere. The determined activation energies are 19 kJ (mol H2O)-1 for x = 0.0 and 16 kJ (mol H2O)-1 for x = 1.0. The dehydration is adequately described as a diffusion controlled single step reaction following the D3 Jander model. The determined dehydration enthalpy is, 11 kJ (mol H2O)-1 for x = 0.0 and 27 kJ (mol H2O)-1 for x = 1.0, relative to that of water. The increase with increasing x is evidence for that the H2O molecules form bonds to the incorporated K+ ions

The 3R3\mathit{R} polymorph of CaSi2\mathrm{CaSi_{2}}

The Zintl phase CaSi2 commonly occurs in the 6R structure where puckered hexagon layers of Si atoms are stacked in an AA′BB′CC′ fashion. In this study we show that sintering of CaSi2 in a hydrogen atmosphere (30 bar) at temperatures between 200 and 700 °C transforms 6R-CaSi2 quantitatively into 3R-CaSi2. In the 3R polymorph (space group R-3m (no. 166), a=3.8284(1), c=15.8966(4), Z=3) puckered hexagon layers are stacked in an ABC fashion. The volume per formula unit is about 3% larger compared to 6R-CaSi2. First principles density functional calculations reveal that 6R and 3R-CaSi2 are energetically degenerate at zero Kelvin. With increasing temperature 6R-CaSi2 stabilizes over 3R because of its higher entropy. This suggests that 3R-CaSi2 should revert to 6R at elevated temperatures, which however is not observed up to 800 °C. 3R-CaSi2 may be stabilized by small amounts of incorporated hydrogen and/or defects