Defect Structure and Electrical Properties of La1−ySryFe1−xAlxO3−δ

La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were studied as potential materials for solid-oxide fuel cell (SOFC) cathodes. The phase relations in the LaFeO₃–SrFeO_3−δ–LaAlO₃ system were investigated by X-ray powder diffraction analysis. The defect structure of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites was investigated by Mössbauer spectroscopy and weight-loss analysis. Relations between the nonstoichiometry and the conductivity of the La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ perovskites were investigated. The incorporation of aluminum ( x ) into LaFe_{1− x} Al_xO₃ was found to have no influence on the defect structure but to decrease the conductivity. The incorporation of strontium ( y ) into La_{1− y} Sr _y Fe_{1− x} Al _x O_3−δ promotes the formation of anion vacancies and Fe⁴⁺ that lead to higher conductivity.     

Alkoxide Route to La0.5Sr0.5CoO3 Films and Powders

An all-alkoxide route to films and nano-phase powders of the La_0.5Sr_0.5CoO₃ perovskite is described. To our knowledge, this is the first purely alkoxide-based route to (La_{1− x} Sr _x )CoO₃, and it yields phase-pure and elementally homogeneous perovskite at 700°C by heating at 2°C/min. At 700°C, a cubic unit cell was obtained with a _c=3.853Å, and after further heating to 1000°C, a rhombohedral cell could be indexed: a _r=5.417 Å, α_r=59.94°. Ninety to 130 nm thick films of La_0.5Sr_0.5CoO₃ were obtained by spin coating. The gel-to-oxide conversion was studied in some detail, using thermo-gravimetric analysis, differential scanning calorimetry, powder X-ray diffraction, IR spectroscopy, and transmission electron microscope equipped with an energy-dispersive X-ray spectrometer.     

Electrical Conduction Behavior of La1+xSr1−xGa3O7–δ Melilite-Type Ceramics

Ceramics of the melilite-type compound La_{1+ x} Sr_{1− x} Ga₃O_7−δ were prepared by conventional ceramic processing. Samples prepared represented the entire homogeneity region of the phase (i.e., x =−0.15 to 0.60). Electrochemical characterization under variable temperature and atmospheric conditions in the vicinity of air entailed four-point direct-current conductivity measurements and electromotive force measurements. La_{1+ x} Sr_{1− x} Ga₃O_7−δ samples exhibited a p -type behavior with generally increased conductivity with increased substitution of lanthanum for strontium, which reached a saturation value of ∼10⁻¹ S·cm⁻¹ at 950°C.     

30 kbar Phase Equilibria of the La2O3–SrO–CuO System at 950°C

Phase equilibria of the La₂O₃–SrO–CuO system have been determined at 950°C at 30 kbar (3 GPa). Stable phases at the apexes of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂, CuO₄ and La₂Cu₂O₅ in the LaO_1.5–CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO–SrO binary. The La_{2– x} Sr _x -CuO_4–δ solid solution is stable for 0.00 is ≤ x ≤ 1.29, the La_{2– x} Sr_{1+ x} Cu₂O_6+δ solid solution is stable for 0.03 ≤ x ≤0.20, the La_{2– x} Sr _x Cu₂O_5–δ solid solution is stable for 0.00 ≤ x ≤1.08, and the La _x Sr_{14– x} Cu₂₄O₄₁ solid solution is stable for 0.00 ≤ x ≤ 6.15. The 30 kbar phase diagram differs from the 1 atm (0.1 MPa) and 10 kbar (1 GPa) results principally in the absence of La_{1– x} Sr_{2+ x} Cu₂O_5.5+δ as a stable phase and the extended range of the La_{2– x} Sr _x Cu₂O_5–δ solid solution at 30 kbar.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Chemical Solution Deposition of Transparent and Metallic La0.5Sr0.5TiO3+x/2 Films Using Topotactic Reduction

Metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films were prepared by the chemical solution deposition (CSD) method using topotactic reduction processing. The use of Si powder as the reducing agent was facile and allowed easy manipulation. It was observed that metallic (resistivity at 300 K ∼2.43 mΩ cm) and transparent (∼80% transmittance at visible light) La_0.5Sr_0.5TiO_{3+ x /2} films could be obtained with an annealing temperature of 900°C, which was significantly lower than the hydrogen reduction temperature (∼1400°C). The successful preparation of metallic and transparent La_0.5Sr_0.5TiO_{3+ x /2} films using CSD has provided a feasible route for depositing other perovskite-structured functional layers on La_0.5Sr_0.5TiO_{3+ x /2} films using this low-cost all CSD method.     

Stability of Bi2Sr2Ca1Cu2O8 ±δ Thick Films at Elevated Oxygen Pressures and Temperatures

Bi₂Sr₂Ca₂Cu₂O_8±δ-type compound thick films were exposed to oxygen-argon-gas mixtures (1% to 20% oxygen gas) at elevated pressures (up to 207 MPa) and temperatures (500° to 940°C) for times ranging from 5 to 96 h. At a sufficiently high oxygen fugacity and temperature, Bi₂Sr₂Ca₁Cu₂O_8±δ decomposed via a solid-state reaction. Room-temperature X-ray diffractometry and electron probe microanalysis of decomposed films revealed the presence of Bi₂(Sr,Ca)₂-Cu₁O_6±θ ro-type compound, Bi₂Sr₂,Ca₁O_8±δ-type compound, and CuO. Bi₂Sr₂Ca₁Cu₂O_8±δ decomposition was accompanied by a modest weight gain, which was consistent with an oxidation reaction. The solid-state decomposition reaction could be reversed by heat treatment of decomposed films at 860°C in pure, flowing oxygen at ambient pressure.     

La(Sr)FeO3 SOFC Cathodes with Marginal Copper Doping

(La_0.8Sr_0.2)_0.98Fe_0.98Cu_0.02O_3−δ can be sintered directly onto YSZ (without the need for a protective ceria interlayer). Though subject to an extended "burn-in" period (∼200 h), anode-supported YSZ cells using the Cu-doped LSF achieve power densities ranging from 1.3 to 1.7 W/cm² at 750°C and 0.7 V. These cells have also demonstrated 500 h of stable performance. The results are somewhat surprising given that XRD indicates an interaction between (La_0.8Sr_0.2)_0.98Fe_0.98-Cu_0.02O_3−δ and YSZ resulting in the formation of strontium zirconate and/or monoclinic zirconia. The amount and type of reaction product was found to be dependent on cathode and electrolyte powder precalcination temperatures.     

Low-Temperature Synthesis and Properties of LaMnO3±d and La0.67R0.33MnO3±d (R = Ca, Sr, Ba) from Citrate Precursors

LaMn_{1− y} ³⁺Mn _y ⁴⁺O_3±d and La_0.67R_0.33Mn_{1− y} ³⁺Mn _y ⁴⁺O_3±d (R = Ca, Sr, Ba) phases were synthesized at 350°C by using very reactive, amorphous precursors obtained from the stoichiometric citrate solutions. The chemical process was optimized with respect to the solution concentration, pH, and additives. The precursor reactions were investigated as a function of the cation stoichiometry and the additive by simultaneous thermal and thermogravimetric analysis and X-ray diffraction. The reaction pathway was found to be independent of the cation stoichiometry, but related to the acid or base additive. The annealing temperature was systematically increased in the 350–1200°C interval and the La_0.67Sr_0.33MnO_3±d properties (i.e., crystal sizes, Mn average valence, Curie temperature, magnetization, magnetic susceptibility) were measured and found to vary consistently as a function of it.     

Compressive Creep Performance of SrFeO3

Compressive creep performance of strontium-deficient and strontium-excess SrFeO_3-δ materials has been investigated in the temperature range 800°–1000°C and in the stress range 2.5–25 MPa. The absolute densities of the strontium-deficient and strontium-excess materials are 4.99 and 5.25 g/cm³, respectively, which corresponds to porosities of ∼2% and 5%, respectively. Both materials contain secondary phases because of the cation nonstoichiometry. The creep rate is faster for the strontium-deficient material than the strontium-excess material. The stress exponent is approximately unity, and the activation energy is 260± 30 kJ/mol for both materials. The results can be explained by a cation diffusion mechanism. The present findings are discussed in relation to previous sintering data and the possible application of these materials as oxygen-permeable membranes.     

Ba0.5Sr0.5Zn0.2Fe0.8O3−δ Perovskite Oxide as a Novel Cathode for Intermediate-Temperature Solid-Oxide Fuel Cells

A cobalt-free perovskite oxide, Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF), was investigated as a novel cathode material for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). Room-temperature nonstoichiometry of BSZF was as high as 0.412, which was favorable for oxygen–reduction reaction. At 700°C, the polarization resistances were as low as 0.22 and 0.13 Ω·cm², respectively, for pure and impregnated BSZF cathodes under open-circuit conditions, suggesting that oxygen-deficient BSZF was a promising cathode candidate for IT-SOFCs.     

Phase Equilibria of the La2O3-SrO-CuO System at 950°C and 10 kbar

Phase equilibria of the La₂O₃-SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂CuO₄ in the LaO_1.5-CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO-SrO binary. The La_2-xSr _x CuO_4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La_2-xSr_1+xCu₂O_6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La_8-xSr _x Cu₈O_20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La _x Sr_14-x-Cu₂₄O₄₁ solid solution is stable where 0 ≤ x ≤ 6, and the La_1+xSr_2-xCu₂O_5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La₂O₃-SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges.     

La1+xSr1–xGa3O7–δ Melilite-Type Ceramics - Preparation, Composition, and Structure

Ceramic samples of the melilite-type La_{1+ x} Sr_{1– x} Ga₃O_7–δ ( x =−0.15 to 0.60) compound were prepared by conventional ceramic processing. Sintering characteristics and microstructural evolution were studied. A phase diagram study was performed to establish the solid solubility limits as a function of the La:Sr ratio. Structural investigations of the Dalton composition as well as strontium- and lanthanum-rich samples entailed X-ray, neutron, and electron diffraction techniques at ambient and elevated temperatures. The homogeneity region was remarkably broad ( x =−0.15 to 0.60) with no changes in space group observed. A small shrinkage of the unit cell was found with increased lanthanum content. Phase transitions at ambient and intermediate temperatures did not occur.     

Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates

Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO₂–La₂O₃ (–Gd₂O₃) systems. All melts crystallized upon quenching in oxygen. Hf₂La₂O₇ pyrochlore and Gd_0.5Hf_0.5O_1.75 fluorite phases were identified. Gd_0.5Hf_0.5O_1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO₂- La₂O₃ melts contained 31.6–34.2 mol% La₂O₃. The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na₂O·4MoO₃ melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf₂La₂O₇ and −48.8±4.7 kJ/mol for Hf₂Gd₂O₇. The enthalpy of the pyrochlore–fluorite phase transition in Hf₂Gd₂O₇ is 23.6±3.1 kJ/mol.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Superplastic Deformation in Fine-Grained YBa2Cu3O7−x

The superplastic behavior of YBa₂Cu₃O_{7− x} ceramic superconductors was studied. Large compressive deformation over 100% strain was measured in the temperature range of 775°–875°C, with a strain rate of 1 × 10⁻⁵ to 1 × 10⁻³/s, and a grain size of 0.5–1.4 μm. The nature of the deformation was investigated in terms of three deformation parameters: the stress exponent ( n ), the grain size exponent ( p ), and the activation energy ( Q ). The measured values of these parameters were n = 2 ± 0.3, p = 2.7 ± 0.7, and Q = 745 ± 100 kJ/mol. With the aid of the deformation map, the deformation mechanism was identified as grain boundary sliding accommodated by grain boundary diffusion. The conclusion is consistent with the microstructural observations made by SEM and TEM: the invariance of equiaxed grain shape, the absence of significant dislocation activity, no grain boundary second phases, and no significant texture development.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

Homogeneity Region of Strontium- and Magnesium-Containing LaGaO3 at Temperatures between 1100° and 1500°C in Air

Phase stability studies were performed within the quasi-ternary system LaGaO₃-SrGaO_2.5-"LaMgO_2.5". Emphasis was cast on the temperature dependence of the homogeneity region of La_{1− x} Sr _x Ga_{1− y} Mg _y O_3−δ perovskite solid solutions. Isothermal sections were determined at 1100°, 1250°, 1400°, and 1500°C in a static air atmosphere. The single-phase homogeneity region was found to considerably diminish with decreasing temperature, indicating a reduction of the solid solubility of Sr and Mg, and below 1100°C the doped perovskite becomes unstable. Consequently, the cubic perovskite phase was found to exist only at elevated temperatures and for high Sr and Mg amounts. Sample preparation was performed by the mixed-oxide process as well as by a modified combustion synthesis.     

Electrophoretic Deposition of Dense La0.8Sr0.2Ga0.8Mg0.115Co0.085O3−δ Electrolyte Films from Single-Phase Powders for Intermediate Temperature Solid Oxide Fuel Cells

La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O_3−δ (LSGMC) powders were prepared by polymeric precursor synthesis, using either polyvinyl alcohol (PVA) or citric acid (CA) as complexing agents. The powders were synthesized using different ratios between the complexing agent and the cations dissolved in solution. The obtained polymer gel precursors were dried and calcined at temperatures between 1000° and 1450°C. Single-phase LSGMC powders were obtained at a firing temperature of 1450°C, using PVA and a molar ratio between the hydroxylic groups and the total cations of 3:1. Phase-pure LSGMC powders were used to sinter (1490°C, 2 h) thick pellets. The functional properties of LSGMC pellets were assessed by electrochemical impedance spectroscopy. The electrical conductivity values and the apparent activation energies in different transport regimes were in agreement with literature data. The same LSGMC powders were deposited by electrophoretic deposition (EPD) on a green membrane containing lanthanum-doped ceria (La_0.4Ce_0.6O_{2− x} , LDC), a binder, and carbon powders. The LSGMC/LDC bi-layer obtained by EPD was cofired at 1490°C for 2 h. A dense and crack-free 8-μm-thick LSGMC film supported on a porous skeleton of LDC was obtained. The combined use of proper powder synthesis and film processing routes has thus proven to be a viable way for manufacturing anode-supported LSGMC films.     

Effect of High-Temperature, High-Oxygen-Fugacity Annealing on the Stability of the (Bi,Pb)2Sr2Ca2Cu3O10 ±δ-type Compound

The stability of the (Bi,Pb)₂Sr₂Ca₂Cu₃O_10±δ-type compound has been evaluated under conditions of elevated temperature (500°-860°C) and elevated oxygen fugacity (i.e., in O₂/Ar gas mixtures containing ≤120% O₂, at total pressures of 5207 MPa). At sufficiently high oxygen fugacities and temperatures, the (Bi,Pb)₂Sr₂Ca₂Cu₃O_10±δ-type compound transformed into a mixture of a strontium-rich (Bi,Pb)₁-(Sr,Ca,Cu)₂O_y-type compound, a calcium-rich (Bi,Pb)₂-(Sr,Ca,Cu)₂O_y-type compound, CuO, and a small amount of (Sr,Ca)O. The decomposition of the (Bi,Pb)₂Sr₂Ca₂-Cu₃O_10±δ-type compound was accompanied by a 2%-3% weight gain, which was consistent with an oxidation reaction. The conditions of oxygen fugacity and temperature leading to decomposition, and the resulting decomposition products, are compared for the (Bi,Pb)₂Sr₂Ca₂Cu₂O_10±δ-type and Bi₂Sr₂Ca₁Cu₂O_8±Ψ-type compounds.