Slavko Bernik

Characterisation of LaNi1 − xCoxO3 as a possible SOFC cathode material

The perovskite with nominal composition LaNi0.6Co0.4O3 was evaluated for the use as a possible solid oxide fuel cell (SOFC) cathode. Thick film layers, printed and fired on yttria stabilised zirconia (YSZ) electrolyte, exhibit good characteristics, i.e. a porous microstructure, a temperature expansion coefficient of 11.9 × 10−6/K, and low, temperature independent, sheet resistivity. However, LaNi0.6Co0.4O3 reacts with YSZ at lower temperatures than other perovskites which are usually used for SOFC cathodes.

Electrical and microstructural characterisation of (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 as possible SOFC cathode materials

The perovskites with nominal compositions (La0.8Sr0.2)(Fe1 − xAlx)O3 and (La0.8Sr0.2)(Mn1 − xAlx)O3 (x from 0 to 0.94) were evaluated as possible solid oxide fuel cell (SOFC) cathodes. Cell parameters of solid solutions were calculated. The electrical and microstructural characteristics and high temperature interactions with YSZ were studied. As compared with ‘pure’ perovskites, doping with strontium and aluminium decreases and increases their specific resistivity, respectively. The incorporation of alumina and strontium oxide substantially reduces the sinterability resulting in a rather porous, fine grained microstructure. The reaction rate between perovskite materials and YSZ at high temperatures is higher for lanthanum manganites than for lanthanum ferrites, and the partial exchange of cations on ‘B’ sites with aluminium decreases the reaction rate.

Microstructural and electrical characteristics of Y2O3-doped ZnO–Bi2O3-based varistor ceramics

Abstract The microstructural and electrical characteristics of ZnO–Bi 2 O 3 -based varistor ceramics doped with Y 2 O 3 in the range from 0 to 0.9 mol% have been investigated. The addition of Y 2 O 3 resulted in the formation of a fine-grained Bi–Zn–Sb–Y–O phase along the grain boundaries of the ZnO grains which inhibits the grain growth. The mean ZnO grain size decreased from 11.3 to 5.4 μm with increasing amounts of Y 2 O 3 . The threshold voltage ( V T ) of the ceramics increased from 150 to 274 V/mm, the non-linear coefficient α was not influenced and remained at approximately 40, and the leakage current also increased with the amount of Y 2 O 3 added. On the basis of the Mukae et al. (Mukae, K., Tsuda, K. and Nagasawa, I., Capacitance-vs-voltage characteristics of ZnO varistors. J. Appl. Phys ., 1979, 50 , 4475–4476) Schottky barrier model of ZnO varistors, the addition of Y 2 O 3 resulted in a slight increase in the density of interface states ( N S ) and a more pronounced increase in the donor density ( N D ), causing a decrease of the barrier height ( Φ B ) and the depletion layer width (t). The increase of the leakage current ( I L ) with higher amounts of Y 2 O 3 added can be ascribed to the increase in donor density (N D ) as well as to the increased amount of Y 2 O 3 -containing phase at the grain boundaries of ZnO.

Interactions between a thick film LaMnO3 cathode and YSZ SOFC electrolyte during high temperature ageing

Abstract Reactions between thick film LaMnO 3 cathodes and YSZ substrates were investigated by ageing at 1450 °C. Also, subsolidus phase equilibria in the La 2 O 3 Mn 2 O 3 ZrO 2 system were confirmed by X-ray powder diffraction analysis. A La 2 Zr 2 O 7 phase formed on the YSZ/LaMnO 3 interface. The Mn 2 O 3 released in the reaction partly diffused in to YSZ and partly evaporated. Diffusion of Y and Zr into LaMnO 3 was not detected. After a prolonged period of ageing (100 h) the cathode layer is separated along most of the YSZ/LaMnO 3 interface with only a few sintered contacts. On the surface of large pores where LaMnO 3 separated from the YSZ substrate “islands” of La silicate phase was found. Silica originated from grain boundaries in YSZ. The presence of La silicate phase could be a reason for separation of the LaMnO 3 layer from YSZ substrate after prolonged high-temperature ageing.

The characteristics of ZnO–Bi2O3-based varistor ceramics doped with Y2O3 and varying amounts of Sb2O3

ZnD-Bi 2 O 3 -based varistor samples doped with 0.45 mol% of Y 2 O 3 and varying amounts of Sb 2 O 3 in the range from 1.8 to 0.0 mol% were fired at 1230 °C. Only in the samples co-doped with Sb 2 O 3 did doping with Y 2 O 3 resulted in the formation of a fine-grained Bi-Zn-Sb-Y-O phase (the Y 2 O 3 -containing phase) at the grain boundaries, which very effectively hinders the grain growth. Despite of a decrease in the amount of added Sb 2 O 3 from 1.8 to 0.45 mol% and a significant decrease in the amount of spinel phase the samples had a similar ZnO grain size and a threshold voltage of 200 V/mm. The results confirmed that doping with Y 2 O 3 is a very promising route for the production of fine-grained high-voltage ZnO-Bi 2 O 3 -based varistor ceramics, and determining the proper amounts of added Sb 2 O 3 and Y 2 O 3 is of great importance.

Subsolidus phase equilibria in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems

Subsolidus equilibria in air in the NiO–CeO2 and La2O3–CeO2–Fe2O3 systems were studied. In the NiO–CeO2 and Fe2O3–CeO2 systems, no binary compound and no solid solubility were detected. La2O3 was soluble in CeO2, forming a cubic fluorite solid solution up to La0.5Ce0.5O1.75, whereas no solid solubility of CeO2 in La2O3 was detected. In the La2O3–CeO2–Fe2O3 system, no ternary compound was found. The tie line is between LaFeO3 and CeO2.

Electrical and microstructural characteristics of ZnO–Bi 2 O 3 -based varistors doped with rare-earth oxides

ZnO-based varistor samples with a relatively high Sb 2 O 3 to Bi 2 O 3 ratio of 5 were fired at 1200 °C and found to have a high threshold voltage ( V T ) of 280 V/mm and a low energy-absorption capacity of 50 J/cm3. The introduction of rare-earth oxides (REO) increased the energy-absorption capacity of Pr 6 O 11 - and Nd 2 O 3 -doped samples to 110 J/cm3 while their threshold voltage ( V T ) remained slightly above 300 V/mm. Doping with Pr 6 O 11 and Nd 2 O 3 altered the formation of the spinel phase and significantly changed its particle size and distribution which, as a result, had a positive effect on the energy-absorption capacity of the REO-doped samples. Doping with small amounts of Pr 6 O 11 and Nd 2 O 3 appears to be promising for the preparation of ZnO-based varistors with a high breakdown voltage and a high energy absorption capacity.

Some characteristics of Al2O3- and CaO-modified LaFeO3-based cathode materials for solid oxide fuel cells

Abstract Al 2 O 3 - and CaO-modified LaFeO 3 were tested as possible solid oxide fuel cell (SOFC) cathode materials. Their electrical and structural characteristics were studied. In the La(Fe 1- x Al x )O 3 system, specific resistivities increase with increasing concentration of Al 2 O 3 . The sintered materials are porous and the average grain diameters decrease with increasing Al 2 O 3 content. In the La 1- x Ca x FeO 3 system, a new single phase, iso-structural with LaFeO 3 was observed for x = 0.5. The room temperature resistivities of La 1- x Ca x FeO y decrease with increasing values of x , up to x = 0.5. The resistivity of La 0.5 Ca 0.5 FeO y is more than four orders of magnitude lower than the resistivity of LaFeO 3 .

Microstructural engineering of ZnO-based varistor ceramics

In ceramic materials, special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of polycrystalline materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic is related to a preparatory stage of phase transformations, manifested through the evolution of chemically induced structural faults. In non-centrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that is triggered by the addition of the specific IB-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional TEM techniques, new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of an IB was determined by micro-diffraction, providing the most reliable identification of crystal polarity in non-centrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other technique. Knowing the local crystal chemistry of IBs, we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time, the grains with IBs dominate the entire microstructure via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to the formation of special boundaries.

SUBSOLIDUS PHASE EQUILIBRIA IN THE LA2O3-GA2O3-CEO2 SYSTEM

Subsolidus equilibria in air in the La 2 O 3 –Ga 2 O 3 –CeO 2 system were studied with the aim of obtaining information on possible interactions between a LaGaO 3 -based solid electrolyte and CeO 2 during preparation of the anode in solid oxide fuel cells. No ternary compound was found. The tie lines are between La 4 Ga 2 O 9 and the end of the CeO 2 solid solution range with composition La 0.5 Ce 0.5 O 1.75 and between the LaGaO 3 and CeO 2 range of solid solutions.