Timothy R. Armstrong

Electrochemical properties of mixed conducting perovskites La1-xMxCo1-yFeyO3-δ (M = Sr, Ba, Ca)

Perovskite compositions in the system La 1-x M x Co 1-y Fe y O 3-δ (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia.Perovskite compositions in the system La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility and concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia. This material is an attractive candidate for several important applications, including solid oxide fuel cell cathodes.

Electrochemical properties of mixed conducting perovskites La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca)

Perovskite compositions in the system La 1-x M x Co 1-y Fe y O 3-δ (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia.Perovskite compositions in the system La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. This loss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility and concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttria-stabilized zirconia. This material is an attractive candidate for several important applications, including solid oxide fuel cell cathodes.

Processing and Electrical Properties of Alkaline Earth‐Doped Lanthanum Gallate

Oxides exhibiting substantial oxygen ion conductivity are utilized in a number of high-temperature applications, including solid oxide fuel cells, oxygen separation membranes, membrane reactors, and oxygen sensors. Alkaline earth-doped lanthanum gallate powders were prepared by glycine/nitrate combustion synthesis. Compacts of powders synthesized under fuel-rich conditions were sintered to densities greater than 97% of theoretical. Appropriate doping with Sr or Ba on the A-site of the perovskite structure, and Mg on the B-site, resulted in oxygen ion conductivity higher than that of yttria-stabilized zirconia (YSZ), and high ionic transference numbers. Doping with Ca and Mg resulted in lower conductivity than YSZ. Thermal expansion coefficients of the doped gallates were higher than that of YSZ.

Effect of A-site cation nonstoichiometry on the properties of doped lanthanum gallate

Lanthanum gallate doped with Sr and Mg (LSGM) was synthesized using a combustion synthesis technique. The synthesized powders were sintered to high density in air, although excessively high sintering temperatures led to bloating of samples, possibly due to the volatilization of Ga2O from the perovskite structure. The electrical conductivity of sintered LSGM tended to decrease with increasing A/B cation nonstoichiometry. Under oxidizing conditions, the conductivity was almost completely ionic, but in reducing atmospheres a substantial electronic component was observed. It is likely that this electronic conduction resulted from the introduction of electronic charge carriers via the partial reduction of Ga from the trivalent to the divalent state. The flexural strength of LSGM with an A/B cation ratio of 1.00 was measured to be ∼150 MPa at room temperature; the strength decreased to ∼100 MPa at higher temperatures (600–1000°C). The fracture toughness, as measured by notched beam analysis, was ∼2.0–2.2 MPa√m at room temperature, decreasing to ∼1.0 MPa√m at 1000°C.

Dimensional Instability of Doped Lanthanum Chromite

Lattice expansion phase stability, and dimensional stability of doped lanthanum chromites have been examined over a wide range of temperatures and oxygen partial pressures. Reduction of doped lanthanum chromite resulted in a linear expansion of the sample that was dependent on the acceptor (Sr, Ca) concentration, temperature, oxygen partial pressure, and oxygen content within the sample. Additional doping with aliovalent B-site additives significantly reduced lattice expansion in reducing environments. The lattice expansion in reducing environments was directly related to the loss of lattice oxygen and the simultaneous reduction of Cr{sup 4+} to Cr{sup 3+} to maintain electroneutrality.

Influence of Cobalt and Iron Additions on the Electrical and Thermal Properties of ( La , Sr ) ( Ga , Mg ) O 3 − δ

(La,Sr)(Ga,Mg)O 3-δ (LSGM) perovskite compositions doped with Co or Fe were prepared by a combustion synthesis technique. Small doping levels of Co or Fe (≤10 mol % on the B-site) resulted in increased electrical conductivity at low temperatures (due to the introduction of electronic charge carriers into the lattice), but at high temperatures the electrical conductivity was dominated by ionic conduction and was similar in magnitude to that of the base LSGM composition. Higher additions of Co or Fe resulted in much higher electrical conductivity in which the electronic component was dominant. A reversible weight loss at elevated temperatures was observed in all of the compositions as the lattice oxygen stoichiometry decreased with increasing temperature. This loss of lattice oxygen correlated with a reduction in the rate of increase (or even a net decrease) in electrical conductivity with increasing temperature. This behavior was attributed to the elimination of electronic charge carriers (and a reduction in valence of some of the Co or Fe cations in the material) as lattice oxygen vacancies were formed. The thermal expansion of the materials increased substantially with increasing Co content, while only slight increases in thermal expansion were observed with increasing Fe content.

Synthesis and crystallization of yttrium-aluminium garnet and related compounds

Amorphous oxide combustion products with compositions corresponding to Y4Al2O9, YAlO3, and Y3Al5O12 were synthesized by the glycine-nitrate process and heat-treated to induce crystallization. The crystalline structure of the resulting powders was determined by powder X-ray diffraction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttrium-to-aluminium ratio, and the heat-treatment conditions. Heat treatment for short durations resulted in incompletely crystalline powders that consisted of a mixture of Y4Al2O9, YAlO3, and Y3Al5O12 phases, regardless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature generated both pure-phase, monoclinic Y4Al2O9 and Y3Al5O12 with the garnet structure. Prolonged heat treatment at high temperature failed to generate pure-phase orthorhombic YAlO3. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.

Mechanical properties of calcium- and strontium-substituted lanthanum chromite

Mechanical properties of acceptor (calcium and strontium)-substituted lanthanum chromites are reported as a function of composition, temperature and environment. The strength dependence on temperature for these perovskite conductors was found to depend on the acceptor type, with the calcium-substituted chromites showing a significant reduction in strength with increasing temperature, while the strength of strontium-substituted chromites was essentially invariant with temperature. The decrease in strength observed upon annealing in highly reducing environments was correlated to changes in lattice structure, stoichiometry and fracture morphology. A significant observation was the decrease in the cohesive strength of the grains relative to grain boundaries, beyond a critical oxygen vacancy concentration in the chromites. The structural changes in the chromite lattice upon reduction also resulted in decreased fracture toughness.

Sintering of lanthanum chromite using strontium vanadate

Abstract Small proportions (5 and 10 wt.%) of strontium vanadate (Sr 3 (VO 4 ) 2 ) were added to strontium-doped lanthanum chromite (La 0.85 Sr 0.15 CrO 3 ) to produce high density fuel cell interconnect materials in air at 1550°C without adversely affecting the desirable properties of the material. Compositions investigated were shown to have good electrical conductivity at SOFC operating temperatures in air and reducing environments, phase stability from room temperature to 1000°C, negligible thermal expansion mismatch with yttria-stabilized zirconia electrolytes and relatively low dilation at p O 2 10 −16 atm.

Mechanical properties of alkaline earth-doped lanthanum gallate

Lanthanum gallate doped with alkaline earths was prepared from combustion-synthesized powders. Mechanical properties of the doped gallates were evaluated as a function of composition and temperature. The indentation fracture toughness of Sr-substituted gallates was significantly better than the Ca- and Ba-substituted materials, but the toughness of all the doped gallates was significantly lower than yttria-stabilized zirconia, a typical electrolyte material. Small improvements in room temperature toughness and strength were measured in (La0.9Sr0.1)xGa0.8Mg0.2O3−δ, (“LSGM-1020”) samples with significant A-site cation non-stoichiometry (x = 0.9). The flexural strength of stoichiometric LSGM-1020 decreased from ≈150 MPa at room temperature, to ≈100 MPa at higher temperatures (600–1000°C). The notched-beam fracture toughness of LSGM-1020 decreased from ≈2.0–2.2 MPa√m at room temperature, to ≈1.0 MPa√m at 600°C. The decrease in mechanical properties over this temperature range was correlated to changes in crystal structure that have been identified by neutron diffraction. These crystallographic changes were also accompanied by significant changes in the thermal expansion behavior and elastic modulus. For off-stoichiometric LSGM-1020 with A/B cation stoichiometry of 0.90, strength and toughness also decreased with temperature, but the retained toughness (≈1.5 MPa√m) at elevated temperatures was higher than the toughness of the stoichiometric LSGM material.