Abstract: The p(O2)-T- diagrams of La2Ni1-xMxO4+ (M=Co and Cu, x= 0-0.20), determined by the coulometric titration technique at 923-1223 K in the oxygen partial pressure range 10-4 to 0.6 atm, can be adequately described by equilibrium processes of oxygen intercalation into the rock-salt type layers and hole localization on B-site cations forming 3+ oxidation states. For the hole activity, a non-ideal solution model taking into account the repulsion of p-type electronic charge carriers can be used. The electrostatic repulsion excludes occupation of nearest neighboring sites and leads to splitting of the energy levels for more distant sites. The affinity of Ni and Cu cations with respect to the hole localization is similar and cannot be statistically separated analyzing the oxygen nonstoichiometry data only. On the contrary, cobalt cations tend to remain in the trivalent state and Co3+ should be treated as a separate type of charged point defect. Oxygen vacancies formed in the perovskite-like layers due to intrinsic Frenkel disorder have no essential effect on the oxygen thermodynamics. As expected, the thermodynamic functions governing the intercalation-related processes are independent of defect concentrations.
Abstract: Due to relatively high oxygen permeability, lanthanum-strontium ferrite phases are of
interest as ceramic membrane materials for the partial oxidation of natural gas. This work was focused on the study of perovskite-type ferrites co-doped with Sr2+ and Ce4+ or Nb5+, with particular emphasis on the ionic transport and thermodynamic stability limits at low oxygen chemical potentials. Dense membranes of La0.5-2xCexSr0.5+xFeO3-δ (x = 0 - 0.2) and La0.5-2ySr0.5+2yFe1-yNbyO3-δ
(y = 0 - 0.1) were characterized employing X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, oxygen permeation and faradaic efficiency studies, and the measurements of total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10-20 to 0.5 atm. The incorporation of Ce4+ or Nb5+ was found to decrease thermal expansion and electronic transport parameters, whereas the ionic conductivity behavior is complex, indicating the relevance of redox interactions of the variable-valence cations and the concentration of mobile oxygen vacancies.
Abstract: The influence of B-site substitution on electroconducting properties of perovskite-type solid solutions (La0.5Li0.5)[Ti1-x(M0.5Nb0.5)x]O3 with M = Al, Ga, have been studied. A decrease in the conductivity due to the unit cell contraction with increasing x in case of M = Al has been revealed. In case of Ga and Nb substitutions for Ti the unit cell volume has been found to increase with increasing x while the conductivity decreases due to the impurity phases presence. The high
and low temperature anomalies on the temperature dependences of dielectric characteristic related to both the relaxation effect and phase transitions have been revealed.
Abstract: The structure, microstructure, electric transport and oxygen permeation properties of
(La,Sr)(Ga,Mg,Fe)O3-δ ceramics with perovskite or brownmillerite structures have been studied. Oxygen permeability values of ~0.1 cm3/cm2·min has been revealed at 1000 K for the ceramics studied. The correlation of electroconducting properties of the LaGaO3-based ceramics with perovskite and brownmillerite structures to their oxygen permeability characteristics has been proved.
Abstract: The dependence of electrical conductivity of pyrochlore type Yb2+xTi2-xO7-x/2 materials shows that these materials are predominantly ionic conductors and are potential solid electrolytes for high temperature electrochemical applications. However, a detailed impedance study as a function of temperature revealed that a proper assessment of these materials must take into account significant effects of microstructure, composition and thermal history. At intermediate temperatures,
the impedance spectra can be described by a series association of well separated contributions, and these could be ascribed to the properties of grain interiors, internal interfaces, and external electrode/material interfaces. The bulk and grain boundary properties were thus assessed and related to changes in microstructure and composition. The firing conditions also exert significant effects on the transport properties, especially for compositions which might deviate from the expected single phase range.
Abstract: New oxide-ion conductors Ln2+xTi2-xO7-x/2 (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln2O3· 64.5 mol.% TiO2)-LANTIOX with a disordered pyrochlore structure are obtained by coprecipitation with next thermal annealing at 1600°C. Their ionic conductivity in air at 740°C attains 5·10-3 S/cm for Ln2.096Ti1.904O6.952 (Ln= Lu, Yb), 10-3 S/cm for Ln2,096Ti1,904O6,952 (Ln=Tm, Er, Ho) and 10-
4S/cm for Dy2,096Ti1,904O6,952 due to the presence of defects in both the cation and anion sublattices. The materials contain ~4.8-10.6% LnTi anti-site defects. Ln2.096Ti1.904O6.952 ceramics are shown to be stable in the temperature range of 1400 to 1700°C. 1400°C-samples Ln2+xTi2-xO7-x/2 have lower ionic conductivity then 1600°C and 1690°C samples.
Abstract: This paper highlights the importance of micropore size of carbonous materials in the
storage capacity of hydrogen. A study is made of a series of carbon fibers with different burn off, activated by a new procedure in which supercritical CO2 is used, as well as of several activated carbons. The best storage results at 77 K and 1 bar corresponded to the activated carbon fibers. The maximum value obtained for these materials was 2.86wt%, significantly higher than that obtained previously by other authors. The study shows that storage increases rapidly with the degree of
activation of the fiber and is closely linked to the micropores. Micropores of around 0.6 nm are those responsible for the greater increase in storage. This study confirms that activation with supercritical CO2 may lead to microporous solids with enormous capacity for adsorbing H2.
Abstract: In this work we investigate the effect of hydrogen absorption/desorption at room
temperature on the structural, magnetic and microstructural characteristics of Zr(Fe0.8Cu0.2)2 and Zr(Fe0.8Co0.1Cu0.1)2. Hydrating kinetics of the as cast bulk samples have been examined while in both samples an anomalous behavior at the absorption part of the P-C curve has been observed. Co doped sample absorbs higher hydrogen content at lower pressure. Crystal structure analysis has
been performed by using the Rietveld method. Co free sample has saturation magnetization of 50.85 Am2/kg while Co doped sample has 54.04 Am2/kg at external field of 1.8 T. After hydrogenation the magnetization decreases. Thermomagnetic analysis in the range of 4.2 to 1100 K reveals that the Curie temperature of AB2 phases is in the range of 510 to 550 K. Scanning electron microscopy
with energy dispersive x-ray spectroscopy was used in order to examine the composition and surface morphology of the bulk samples. The grain size reduces due to hydrogenation and this is attributed to the more hysteretic magnetization curve.
Abstract: The work concerns new polymeric materials for electrolytes synthesized at the
Electrotechnical Institute. Poly(styrene-4-sulfonate)-grafted poly(vinylidene fluoride) (PVDF-g- PSSA) membranes proton-exchange membranes were synthesised by free radical graft polymerization of polystyrene into PVDF films (50 *m thick) followed by sulfonation. Physical and electrochemical properties of that membranes were investigated. The membranes were tested as electrolytes in a single cell configuration system. The influence of working condition, as gas
humidity and cell temperature on fuel cell performance were measured. Obtained performances were lower in comparison with commonly used electrolyte materials. At this stage of development the synthesized materials are not suitable for fuel cell application. The results gave authors information for selection membrane with the best parameters and chemical process configuration.
Abstract: The methods of dimensional control for solid surfaces are usually contact methods.
However, in fragile and reduced hardness surfaces, these methods present critical disadvantages. The method presented in this work aims to offer resources for the optimization of graphite components machined with laser through the measurement of the channels’ depth and shape. In this scope, the laser triangulation technique demonstrated to be a very useful tool in the quality control of graphite diffusers plates for fuel cell prototyping and in their future production.