Showing papers by "Vladislav V. Kharton published in 2013"

A (3 + 3)-dimensional "hypercubic" oxide-ionic conductor: type II Bi2O3-Nb2O5.

Abstract: The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated “hypercubic” structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an “inflated” pyrochlore, in which corner-connected NbO6 octahed...

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

Abstract: The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

SrO‐Containing Diopside Glass–Ceramic Sealants for Solid Oxide Fuel Cells: Mechanical Reliability and Thermal Shock Resistance

Abstract: A series of glasses and glass–ceramics (GCs) aiming at applications as sealants for solid oxide fuel cells (SOFCs) were synthesized by partial substitution of Ca for Sr in the diopside-Ba disilicate composition. X-ray diffraction in conjunction with the Rietveld-RIR technique were employed to quantify the crystalline (diopside and Sr-diopside) and amorphous phases in the glasses sintered/heat treated at 850 °C in humidified 10%H2–90%N2 gas mixture for 250 h. Weibull modulus varied in the range 11.6–34.4 implying toward good mechanical reliability of synthesized GCs. Thermal shock resistance of model electrochemical cells made of yttria-stabilized zirconia, gadolinia-doped ceria, and lanthanum gallate based solid electrolytes, hermetically sealed by one diopside-based composition, was evaluated employing quenching from 800 °C in air and water. Suitable thermal expansion coefficient, mechanical reliability, and strong adhesion to stabilized zirconia and metallic interconnects, are all suggesting a good suitability of the sealants for SOFC applications.

Reference Electrodes for Solid-Electrolyte Devices

Abstract: Solid electrolytes are a class of solids where electrical conduction occurs predominantly via migration of ions and the overall level of ionic transport is high enough for practical applications, whilst electronic contribution to the total electrical conductivity is negligible. This term is commonly used for the materials with ion transport numbers (t ion) equal to or higher than 0.99 under given external conditions. The range of conditions (temperature, pressure, and thermodynamic activities of components) where t ion ≥ 0.99 is often called the electrolytic domain. A large number of solid electrolytes (SEs) with mono-, bi-, and trivalent ion charge carriers are known. The groups important from the practical point of view include [1–5]: