I. E. Kuritsyna

Analysis of electric properties of ZrO2-Y2O3 single crystals using teraherz IR and impedance spectroscopy techniques

The data obtained by impedance spectroscopy (1 Hz to 32 MHz) and broad-band dielectric spectroscopy (30 GHz-150 THz) are presented for crystals based on zirconia doped by 1.5–30 mol % Y2O3 or 10 mol % Sc2O3 and 1 mol % Y2O3. The maximum of ionic conductivity is confirmed for the latter composition in the working temperature range of solid oxide fuel cells where the doping by scandium and yttrium oxides makes it possible to obtain isotropic single crystals. Dependences of dielectric permeability and high-frequency conductivity of materials on the composition of crystals and temperature are presented.

Microstructural and Electrochemical Study of Charge Transport and Reaction Mechanisms in Ni/YSZ Anode

The microstructure and chemical changes in the Ni-YSZ heterojunctions were studied by scanning, transmission, and highresolution electron microscopy and X-ray diffraction. It was shown that new nano-size Ni grains are formed at Ni-YSZ interface at the cell operation. The following two stage reaction mechanism was proposed for hydrogen oxidation on the nano-size Ni grains: 1. NiO + Н2 ⇒ Н2О + Ni; 2. O(YSZ) + Ni ⇒ NiO + 2e. This mechanism implies that the SOFC will operate during some time even after switching off the hydrogen flow. We have observed and investigated current-voltage characteristics of SOFC after cutting off the hydrogen pass through anode chamber. Electrochemical properties of SOFC were investigated as a function of Ni volume content for two different geometry of anode current collector. Good correlations of experimentally measured SOFC characteristics with results of numeric calculations were found.

Protective coatings based on Mn-Co spinel for current collectors of solid oxide fuel cells

The method of electrostatic spray pyrolysis was designed to apply protective coatings based on Mn-Co spinel to ferrite stainless steels (Crofer22APU and 08Kh18T1). The comparative thermogravimetric (TG) studies of ferrite stainless steels with and without protective coatings were carried out. The electrochemical characteristics of protective coatings exposed to long current loading were studied. The formation processes of Cr2O3 oxide films were studied at the contact of ferrite stainless steel with La0.8Sr0.2MnO3 ionic and electronic conductor. The coatings of Mn-Co spinel were shown not preventing formation of continuous oxide film on the stainless steel surface.

Stability and functional properties of Sr0.7Ce0.3MnO3 − δ as cathode material for solid oxide fuel cells

Studies of oxygen diffusion, interphase exchange, specific electric conductivity, and thermal expansion showed that perovskite-like Sr0.7Ce0.3MnO3 − δ (SCMO) as a potential cathode material for solid oxide fuel cells (SOFCs) has considerable advantages over the conventional materials based on lanthanum-strontium manganites. To prevent the interactions of SCMO with solid electrolyte membranes of stabilized zirconia and lanthanum gallate, it is necessary to deposit protective layers of solid solutions based on cerium oxide, which do not form new phases in contact with SCMO and electrolytes. The trials of model SOFCs with porous SCMO-based cathodes demonstrated satisfactory electrochemical and endurance characteristics of these electrodes.

Structure and properties of the crystals of solid electrolytes (ZrO2)1 – x – y (Sc2O3) x (Y2O3) y (x = 0.035–0.11, y = 0–0.02) prepared by selective melt crystallization

Single crystals of solid electrolytes of the (ZrO2)1–x–y(Sc2O3)x(Y2O3)y (x = 0.035–0.11, y = 0‒0.02) system were grown by selective melt crystallization. Stabilization of ZrO2 only with Sc2O3 in the concentration range 9–11 mol % Sc2O3 did not afford crystals with a cubic structure, and only the introduction of additional Y2O3 stabilizers afforded uniform transparent single-phase cubic crystals. All the crystals under study had high microhardness, but low crack resistance. The ion conductivity of crystals with 6 and 9 mol % Sc2O3 (6ScZr and 9ScZr, respectively) is comparable to that of 8 mol % Y2O3-stabilized ZrO2 (8YSZ), which is the most suitable electrolyte in the ZrO2–Y2O3 binary system. The specific conductivity of crystals containing 8–10 mol % Sc2O3 and 1–2 mol % Y2O3 exceeds that of other materials including 8YSZ. The maximum conductivity in the given range of compositions is inherent in the cubic phase with 10 mol % Sc2O3 and 1 mol % Y2O3 (10Sc1YZr).

Ceramic membranes based on scandium-stabilized ZrO2 obtained by tape casting technique

Results of studies of solid-electrolyte membranes with the composition of 89 mol % ZrO2-10 mol % Sc2O3-1 mol % CeO2 obtained using the technique of slip casting on a moving tape are presented. Optimization of technological parameters of membrane casting and sintering allowed manufacturing parallel plane gastight plates with the thickness of 200–250 μm that were tested in model solid oxide fuel cells (SOFC) of planar design with standard electrodes based on nickel-containing cermets and lanthanum-strontium manganite. It is shown that though conductivity of such membranes is lower as compared to that of compacted and sintered compacted samples due to diffusion of the aluminum oxide admixture in the course of the manufacturing process, power density of SOFC is sufficiently high and reaches 430 mW/cm2 at 850°C.

Structure and Transport Properties of Zirconia-Based Solid Solution Crystals Co-Doped with Scandium and Cerium Oxides

The crystals of (ZrO2)1–x(Sc2O3)x(СeO2)0.01 solid solutions (x = 0.08–0.10) were obtained by directional crystallization. The crystals of the grown composites were semitransparent, opalescent, and without cracks and had varying microstructure in the bulk. In the range of compositions under study, it was impossible to obtain optically homogeneous, fully transparent crystals. The crystals grown at a growth rate of 10 mm/h had a nonuniform distribution of ceria along the length of the ingot. The introduction of ceria in an amount of 1 mol % increased the conductivity of the crystals, but the increase in the specific electric conductivity depended on the Sc2O3 content and the phase composition of the crystals. The highest conductivity was inherent in the (ZrO2)0.89(Sc2O3)0.10(CeO2)0.01 crystals.

Electrotransport Characteristics of Ceramic and Single Crystal Materials with the (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01 Composition

The comparative analysis of electrotransport characteristics and structure of ceramic and single crystal solid electrolytes with the (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01 composition is carried out before and after their life tests. It is shown that before the life tests, the specific conductivities of single-crystal and ceramic materials virtually coincide. During the 3000 h life tests, the specific ionic conductivity decreases for both single crystal and ceramic samples down to about 0.1 S cm–1 but the degradation of conductivity in single crystal proceeds more slowly as compared with the ceramic material. The reason for degradation of electrotransport characteristics in the single crystal is associated with the transition of its bulk structure from the t′′ phase to a phase with the higher degree of tetragonality, whereas in the ceramic material, in addition to the latter process, a rhombohedral phase appears presumably along grain boundaries.

Change in the mechanism of conductivity in ZrO2-based crystals depending on the content of stabilizing Y2O3 additive

The interrelationship between the structure, phase composition, and transport characteristics of solid electrolytes based on ZrO2 has been studied as dependent on the content of stabilizing Y2O3 additive. It is established that twin boundaries do not lead to the appearance of additional mechanism of ionic conductivity acceleration in ZrO2–Y2O3 crystals. The maximum conductivity has been observed in ZrO2–(8–10) mol % Y2O3 crystals containing a t” phase, in which oxygen atoms are displaced from high-symmetry positions characteristic of the cubic phase.

Influence of the yttria dopant on the structure and properties of (ZrO2)0.91–x (Sc2O3)0.09(Y2O3) х (x = 0–0.02) crystals

We have studied the influence of the yttrium oxide (Y2O3) dopant (1 and 2 mol %) on the phase composition, structure, and electrical properties of ZrO2–9 mol % Sc2O3 solid solution. Stabilization of ZrO2 jointly with 9 mol % Sc2O3 and 2 mol % Y2O3 is shown to allow the acquisition of high phase stability transparent homogeneous crystals with a cubic structure. Their mechanical grinding is established to cause no change in the phase composition of these crystals, whereas the powders retain the initial fluorine structure. The powders preserved the original structure of the fluorite crystals. All the probed crystals reveal high microhardness and low fracture toughness. Increasing the Y2O3 concentration in the crystals led to a reduction of the maximum loads on the indenter, which the sample withstood without cracking. As is shown, the specific conductivity exhibits nonmonotonic behavior depending on the Y2O3 concentration in the crystals. Increasing the Y2O3 content to 2 mol % in the solid electrolyte reduces the conductivity of the crystals in the entire temperature range that is attributed to a decrease in the carrier mobility due to the increasing ion radius of the stabilizing ion.