S. N. Shkerin

Synthesis of dense ceramics of single-phase mayenite (Ca12Al14O32)O

The nanoporous material mayenite was synthesized by two methods: solid-phase and self-propagating high-temperature (combusting) (SHS) syntheses. The structure of the materials was determined by the X-ray diffraction analysis and IR and Raman spectroscopy.

Structure and phase transitions of (La, Sr)(Ga, Mg)O3−α solid electrolyte

Abstract Electrical conductivity, Raman scattering (RS) spectra, and thermal expansion of the solid solution based on lanthanum gallate having the formula La0.88Sr0.12Ga0.82Mg0.18O2.85 (LSGM12–18) were studied as a function of temperature. X-ray diffraction analysis (XDA) of the samples at room temperature was performed. A dilatometric examination demonstrated that phase transitions of the second order took place at 775±10 and 880±20 K. The transition at 880 K was confirmed both by Raman scattering spectra and by a change in the activation energy of the electrical conductivity. This phase transition is probably due to a change of the oxygen sublattice symmetry.

Phase transition in mayenite Ca12Al14O33

A phase transition in Ca12Al14O33 has been discovered and investigated by thermogravimetric analysis, differential scanning calorimetry, dilatometry, and high-temperature X-ray diffraction. The phase transition occurs at 922 ± 45 K (ΔH = −406 ± 13 kJ/mol, ΔS = −440 ± 14 J/(mol K)) and is presumably a first-order one. It does not change the symmetry of the cationic subsystem. The phase transition is difficult to reveal because the material changes its mass, probably by releasing water bound in several different ways.

Model gauze electrode O2, Pt/O2-: Effect of the cell's pretreatment and the single-crystal electrolyte orientation

The polarization resistance of a model platinum gauze electrode in contact with a single-crystal electrolyte of doped zirconium dioxide is studied by impedance spectroscopy. Geometric characteristics of metal/electrolyte contact (contact area, triple-phase boundary length) are easily determined. The electrode pretreatment affects results of measurements in identical conditions. Cathodic and anodic polarizations increases its activity. The partial polarization resistance of a low-frequency relaxation process (R2) alters jumpwise, which suggests that there exist an initial state and a state after a current treatment. During a prolonged storage, R2 alters with time and tends to regain the initial state. The polarization resistance of the initial electrode depends on the electrolyte orientation. No such dependence is observed for the current-treated electrode.

Synthesis of an anode material based on mixed calcium vanadatomolybdate and its stability in contact with solid electrolytes

Mixed calcium vanadatomolybdate CaV0.5Mo0.5O3 was synthesized, and its phase composition was studied in relation to the annealing temperature in the interval 600–1150°C. The heat treatment at 1150°C leads to the formation of a single-phase product of perovskite structure. Transformations in the systems consisting of CaV0.5Mo0.5O3 and solid electrolytes 0.9ZrO2-0.1Y2O3, BaZr0.95Y0.05O3, and La0.88Sr0.12Ga0.82Mg0.18O2.85 (weight ratio 1: 1) in the temperature interval 900–1250°C were studied using X-ray diffraction analysis. The temperatures at which calcium vanadatomolybdate starts to react with the electrolyte materials are considerably higher than the operation temperatures of the solid oxide fuel cells. Recommendations are formulated on the prospects for using CaV0.5Mo0.5O3 as an anode material for solid oxide fuel cells, primarily for those with the electrolyte based on lanthanum gallate.

Ceramics with mayenite structure: Molecular sieve for helium gas

It was found that mayenite {ie536-1} has selective transmission of helium gas, which is associated with its unique structural features. The permeability of helium through mayenite is (16.5–17.0) × 10−8 cm2 s−1 Pa−1.

Studies of a Model Mesh Electrode O2,Pt/O2- under Galvanostatic Polarization

The impedance of model platinum mesh electrode in contact with a doped zirconium-dioxide-based single-crystal electrolyte is studied by impedance spectroscopy under constant-current polarization. The electrode is distinguished for its readily determinable geometrical characteristics of the metal/electrolyte interface, in particular, the length of the triple-phase boundary and the electrode contact area. Three different orientations of the electrolyte face are studied. The impedance dispersion is discussed in terms of the electrode prehistory, orientation, and potential.

Studying the O2, metal/O2- (solid electrolyte) electrode system with use of model electrodes: The exchange current density determination

Cells with model gold and platinum electrodes are studied by an impedance spectroscopy method. Distinguishing features of these electrodes are a large characteristic size of the metal/electrolyte contact (up to 100 μm) and such well definable geometrical parameters as the length of triple-phase boundary (TPB) and the contact area. E. Shouler discovered that the electrolyte resistance, measured in an electrochemical cell, varies with the oxygen activity. Modern view is that this is connected with the TPB expansion. The expansion parameters determined here depend on the electrolyte prehistory and reach a few tens of micrometers. Specific (referred to a unit TPB length) values of polarization conductivities of both electrodes are obtained. Depending on the electrode prehistory, they amount to (2–6.5) × 10–4 and (3–22)× 10–4 S cm–1 for Pt and Au, respectively, at 977°C in an oxygen atmosphere. The polarization conductivity, calculated per unit active area of electrolyte, is independent of the Pt electrode prehistory. Calculated exchange current densities are compared with radioisotope assay results.

Polarisation resistance of the O2, Au/O2− and H2-H2O, Au/O2− electrode systems

Gold electrodes with known contact geometries were studied using impedance spectroscopy. From these data it was possible to determine the specific polarisation conductivity per unit length of three-phase boundary (TPB). The values were found to be (3÷22)×10−4 S·cm−1 dependent on the electrode history in pure oxygen at 977 °C and 2×10−6 S·cm−1 at 977 °C in “pure” hydrogen (PO2=10−20 atm at 1001 °C). The results are compared with previous data obtained for platinum electrodes.

Photoinduced Oxidation of a Surface of CaV0.5Mo0.5O3 − δ

CaV0.5Mo0.5O3 − δ with the structure of perovskite is synthesized in an atmosphere of humid hydrogen. It is found that when the material under study is moved into air, a nonequilibrium state forms due to the enhanced concentration of oxygen over the surface of the highly reduced oxide. It is shown that photoinitiated oxidation is observed on the surface of the material under the action of a low-power laser beam (50 mW), where it selectively occurs under a green laser beam (λ = 532 nm). This phenomenon is observed for such materials for the first time.