Anthony Petric

Superior Oxygen Ion Conductivity of Lanthanum Gallate Doped with Strontium and Magnesium

The solid solution range of the La(Sr)Ga(Mg)O{sub 3} perovskite was investigated and the oxygen ion conductivities of corresponding compositions were measured. The solubility of strontium could be extended by the addition of magnesium ions to establish a balance in the oxygen vacancies associated with A and B sites. In this way, the Sr limit in the solid solution was increased to more than 20% of the La content. Doping with either Sr or Mg enhanced oxygen ion conductivity, but whereas Sr doping yielded decreased activation energy with increasing concentration, Mg produced a higher activation energy with increased Mg content. La{sub 0.8}Sr{sub 0.2}Ga{sub 0.9}Mg{sub 0.1}O{sub 2.85} was found to have the highest conductivity apart from CeO{sub 2}-based electrolytes at temperatures below 700 C, while La{sub 0.8}Sr{sub 0.2}Ga{sub 0.85}Mg{sub 0.15}O{sub 2.825} has the highest conductivity except for Bi{sub 2}O{sub 3} at temperatures above 700 C.

Evaluation of La–Sr–Co–Fe–O perovskites for solid oxide fuel cells and gas separation membranes

Perovskites based on alkaline-earth containing lanthanum cobaltites exhibit high electronic and ionic conductivities but their application as cathodes for solid oxide fuel cells (SOFC) or gas separation membranes may be limited by thermal expansion and low stability of certain compositions. To identify the optimum composition regime, the following three series in the system La1−xSrxCo1−yFeyO3−δ were studied: (a) La1−xSrxCoO3−δ, (b) La0.3Sr0.7Co1−yFeyO3−δ and (c) La1−xSrxCo0.2Fe0.8O3−δ. All samples were investigated by four probe dc conductivity measurements and dilatometry. For samples in group (a), La1−xSrxCoO3−δ, the conductivity at 800°C increases with Sr content to a peak value at x=0.3 and then decreases continuously. The thermal expansion coefficient (TEC) decreases first, then increases again. A minimum of TEC with α30–1000°C=18.5×10−6 K−1 was obtained at x=0.2. For samples of series (b), both the electrical conductivity and TEC decrease with iron content. Even though these materials have high electronic and ionic conductivities, it is difficult to utilize them as fuel cell cathode materials because their thermal expansion is not compatible with other cell components. These materials are, however, promising for gas separation membranes.

Electrical Properties of Yttrium-Doped Strontium Titanate under Reducing Conditions

The electrical conductivity of SrTiO 3 containing rare earth dopants (Y,La,Pr,Sm,Nd,Gd,orYb)was measured ar 600-900 C in reducing atmospheres. An unusually high conductivity was observed for the yttrium-doped samples compared to those with rare earth dopants. particularly at the composition of the solubility limit Sr 0.88 Y 0.08 TiO 3-δ where the conductivity at 800°C was 64 S/cm. The conductivity was confirmed to be n-type by thermopower measurements. The oxygen deficiency was determined to be 1% by thermogravimetric analysis. Increasing the strontium vacancy concentration to Sr 0.86 Y 0.08 TiO 3-δ effected a further improvement in conductivity to 82S/cm. This material has high structural stability over a broad range of temperature (up to 1400°C) an oxygen partial pressure (1-10 20 atm). A simple defect model was developed to explain the change of electrical conductivity as a function of yttrium content and oxygen deficiency.

Electrophoretic deposition of ceramic materials for fuel cell applications

La0.8Sr0.2Ga0.875Mg0.125O3-x (LSGM), La0.8Sr0.2Co0.2Fe0.8O3-d (LSCF), yttria stabilized zirconia (YSZ) and (Ce0.8Gd0.2)O1.9 (CGO) were electrophoretically deposited on Ni foils and Ni-yttria stabilized zirconia substrates prepared by tape casting. It was demonstrated that the ethyl alcohol‐phosphate ester‐polyvinyl butyral system is an eAective solvent‐dispersant‐binder system for electrophoretic deposition of these materials. The influence of dispersant, binder and current density on deposition eAciency and deposit morphology was studied. The microstructure of the deposits was examined by electron microscopy. The proposed solvent‐ dispersant‐binder medium for electrophoretic deposition of LSGM, LSCF, YSZ and CGO has important advantages and implications in fuel cell design. # 2000 Elsevier Science Ltd. All rights reserved.

Electrochemical deposition of ceria and doped ceria films

Abstract Cathodic electrodeposition of CeO 2 and Ce 1− x Gd x O 2− y was performed from aqueous and mixed ethyl alcohol-water solutions of CeCl 3 and GdCl 3 on Ni foil and Ni–yttria stabilized zirconia substrates. The influence of hydrogen peroxide on the electrodeposition process was studied. Electrochemical intercalation of poly(diallyldimethylammonium chloride) into the deposits was demonstrated and the mechanism of intercalation is discussed. The experimental results indicate that the polymer acts as a binder, providing better adhesion of the organoceramic deposits and reducing cracking while allowing for film formation on porous substrates. The deposits were studied by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy.

Electrolytic and electrophoretic deposition of CeO2 films

Abstract Cerium oxide films of thickness 0.1–300 μm were deposited on Ni substrates via cathodic electrolytic and electrophoretic deposition. The films were studied by X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Experimental results obtained by both deposition methods are compared. The influence of deposition parameters and additives on deposition yields and film morphologies is studied and discussed.

Electrophoretic deposition of electrolyte materials for solid oxide fuel cells

Electrophoretic deposition of electrolyte materials for solid oxide fuel cells, including La0.8Sr0.2Ga0.875Mg0.125O3−x, yttria stabilized zirconia and (Ce0.8Gd0.2)O1.9, was studied under various experimental conditions. The use of phosphate ester as a dispersant and poly (vinyl butyral) as a binder enabled high deposition rate and formation of crack-free, adherent deposits. Electrodeposition rates were quantified in experiments performed at constant current and constant voltage modes from suspensions in ethanol, isopropanol and mixed ethanol—isopropanol solvents. The microstructure of as prepared and sintered deposits was studied by electron microscopy. The bath composition was optimized to enable formation of dense deposits.

Effect of heating rate on the combustion synthesis of Ti-Al intermetallic compounds

Titanium aluminide compounds were synthesized by the thermal explosion mode of self-propagating high-temperature synthesis (SHS). The effects of heating rate on the combustion characteristics and the microstructures of the products were studied. It was found that the low density of the reacted sample was due to the outgassing of water vapour and other gases, which were released by dissociation of hydrated aluminium oxides. Higher heating rates resulted in a product of higher density and single-phase microstructure. At lower heating rates, the reaction product was a mixture of phases for TiAl and Ti3Al reactions. A liquid (Al)-solid (Ti) reaction mechanism is predicted for slow heating while a solid-solid mechanism is expected for high heating rates. The origin of porosity in the product is also discussed.

Conductivity and stability of SrVO3 and mixed perovskites at low oxygen partial pressures

Abstract The electrical conductivity and chemical stability of ABO 3 (where the A-site contains La or Sr, and the B-site contains Ni, V, Ti, Nb, Mo or W and combinations thereof) have been studied in air and reducing atmospheres. Neither the rare-earth vanadates, such as LaVO 3 , nor the alkaline-earth vanadates, with the exception of SrVO 3 , are good electronic conductors. SrVO 3 shows excellent electronic conductivity of 1000 S/cm at 800°C and an oxygen partial pressure of 10 −20 atm, but is unstable under more oxidizing conditions. However, strontium-doped lanthanum vanadates are stable over a range of oxygen partial pressures of 10 −14 –10 −20 atm. La 0.7 Sr 0.3 VO 3− δ has a conductivity of 50 S/cm at 800°C and P O 2 =10 −20 atm. However, none of the compounds studied have both adequate high-temperature electrical conductivity in reducing atmospheres and stability to cycling between oxidizing/reducing conditions, to serve as anodes in solid oxide fuel cells.

Electrolytic deposition of zirconia and zirconia organoceramic composites

Abstract Thin films of zirconia and an organoceramic composite consisting of zirconium hydroxide and poly(diallyldimethylammonium chloride) (PDDA) were obtained via cathodic electrodeposition. Films up to 10-μm-thick were obtained on Ni and porous Ni–yttria-stabilized zirconia (YSZ) cermet substrates. By varying the current density, concentrations of PDDA and zirconium salt, the amount of the deposited material and its composition could be controlled. The deposits were studied by thermogravimetric (TG) analysis and scanning electron microscopy (SEM). A mechanism of electrochemical intercalation of the cationic polymer into zirconium hydroxide deposit is discussed.