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.     

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.     

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.     

High-Temperature Creep Behavior of Mixed Conducting La0.5Sr0.5Fe1−xCoxO3-δ (0.5≤x≤1) Materials

Steady-state compressive creep rate of La_0.5Sr_0.5Fe_0.5Co_0.5O_3−δ (LSFC) and La_0.5Sr_0.5CoO_3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N₂ compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

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.     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

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.     

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.     

Synthesis and Dielectric Properties of Subsituted Lanthanum Aluminate

LaAlO₃ was chemically modified in order to ascertain the effects of substitution of larger cations on the compound's slight rhombohedral distortion from cubic symmetry—a property that often degrades the performance of LaAlO₃ substrates for epitaxial high-temperature superconducting films. La_{1– x} Sr _x Al_{1– x} Ti_xO₃ (x = 0–05, 0.10, 0.15, 0.25),La_{1– x} Sr_xAl_1–xZr_xO₃, La_{1– x} Sr _x Al_1–xMg_xO_3–2xF _2x and La_{1– x} ,Sr_xAl_{1– x} Sc _x O_{3– x} (x = 0.05, 0.10, 0.15) were prepared in polycrystalline form and 1–2 mm single crystals of the Sr,Ti- and Sr,Zr-substituted systems were grown using a PbO–PbF₂–B₂O₃–PbO₂ flux. Shifts in the peak positions of the X-ray powder diffraction patterns confirmed subsitution of the larger cations. The diffraction patterns were also typified by the line-broadening and the decrease in the rhombohedral splitting at all doping levels. The dielectric constant of LaAlO₃ was unchanged for all of the fluoride-containing systems and for the 5% Sr, Ti- and Sr,Zr-systems.     

Pyrochlore Phases in the System ZnO–Bi2O3–Sb2O5: I. Stoichiometries and Phase Equilibria

Two cubic pyrochlore phases exist in the system ZnO–Bi₂O₃–Sb₂O₅. Neither has the supposed "ideal" stoichiometry, Zn₂Bi₃Sb₃O₁₄. One, P ₁, is a solid solution phase, Zn_{2+ x} Bi_{2.96−( x − y )}Sb_{3.04− y} O_14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P ₂, is a line phase, Zn₂Bi_3.08Sb_2.92O_13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P ₁ and P ₂ in the ZnO–Bi₂O₃–Sb₂O₅ phase diagram have been determined.     

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La_1−xSr_x)_zFe_1−yMn_yO_3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO₂) doped with 8 mol% yttria (Y₂O₃) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO₂ at 1000°C, as well as of La₂Zr₂O₇ and SrZrO₃ at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y₂O₃-stabilized ZrO₂ (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.     

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.     

Preparation and Electrical Properties of New Oxide Ion Conductors Ce6−xGdxMoO15−δ (0.0≤x≤1.8)

A series of oxide ion conductors Ce_{6− x} Gd _x MoO_15−δ (0.0≤ x ≤1.8) have been prepared by the sol–gel method. Their properties were characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Raman, IR, X-ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The XRD patterns showed that the materials were single phase with a cubic fluorite structure. The conductivity of Ce_{6− x} Gd _x MoO_15−δ increases as x increases and reaches the maximum at x =0.15. The conductivity of Ce_4.5Gd_1.5MoO_15−δ is σ_t=3.6 × 10⁻³ S/cm at 700°C, which is higher than that of Ce_4.5/6Gd_1.5/6O_2−δ (σ_t=2.6 × 10⁻³ S/cm), and the corresponding activation energy of Ce_4.5Gd_1.5MoO_15−δ (0.92 eV) is lower than that of Ce_4.5/6Gd_1.5/6O_2−δ (1.18 eV).     

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.     

Nonisothermal Crystallization Kinetics of BixY3−xFe5O12 (0.25≤x≤1.00) Prepared from Coprecipitation Process

The nonisothermal crystallization kinetic of Bi _x Y_{3− x} Fe₅O₁₂ (0.25≤ x ≤1.00) powders prepared by coprecipitation process has been investigated. The activation energy of crystallization was calculated by differential scanning calorimetry at different heating rates. The activation energies of crystallization of Bi _x Y_{3− x} Fe₅O₁₂ system are 492, 447, 377, and 353 kJ/mol and the Avrami constant n are 3.49, 2.25, 2.48, and 2.33 for x =0.25, 0.50, 0.75, and 1.00, respectively. The Avrami exponent values (1< n <3) were consistent with surface and internal crystallizations occurring simultaneously for 0.50≤ x ≤1.00, the value ( n >3) for the Avrami exponent was consistent with bulk crystallization domination in Bi _x Y_{3− x} Fe₅O₁₂ system. The results reveal that increasing the substitution amount of bismuth for yttrium would significantly decrease activation energy in Bi _x Y_{3− x} Fe₅O₁₂ system.     

Electrical Characterization of Sr0.97Ti1−xFexO3−δ by Complex Impedance Spectroscopy: I, Materials with Low Iron Contents

Impedance spectroscopy has been used to characterize Sr_0.97Ti_{1− x} Fe _x O_3−δ materials. This technique was suitable to evaluate the grain and grain-boundary conductivities of samples with ≤5% iron. The role of grain boundaries was changed by sintering in different conditions, and this was interpreted by accounting for microstructural differences. The corresponding relaxation times were also used to confirm the interpretation of complex impedance spectra. The values of grain-boundary conductivity and the corresponding values of activation energy obtained by this method were relatively close to predictions obtained on assuming a p → n inversion at the grain-boundary core, as proposed by Waser and coauthors.1–3 The temperature dependence of relaxation times confirmed this finding.     

Factors Affecting the Preparation of Sr2Fe2−xMoxO6

Samples of Sr₂Fe_{2− x} Mo _x O₆were prepared by solid-state reaction in air and 5%H₂–95%N₂. X-ray diffractometry was used to identify the phases and evaluate the lattice parameters. It is found that molybdenum ions can dissolve in the SrFeO₃ even if the sample is heated in air but the solubility is limited. The solubility can be enhanced by heating the sample in low oxygen partial pressure, which is attributed to the larger ionic radii of Fe³⁺ and Mo⁵⁺ than that of Fe⁴⁺. The degradation of Sr₂Fe_{2− x} Mo _x O₆in water and air is also reported.     

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.     

Effect of La Doping on the Phase Conversion, Microstructure Change, and Electrical Properties of Bi2Fe4O9 Ceramics

Undoped and La-doped Bi₂Fe₄O₉ ceramics were synthesized using a soft chemical method. It is observed that in calcining La-doped Bi₂Fe₄O₉, Bi(La)FeO₃ phase rather than Bi_{2− x} La _x Fe₄O₉ gradually increases with increasing La doping content. The phase conversion from mullite-type structure of Bi₂Fe₄O₉ to rhombohedrally distorted perovskite one of Bi(La)FeO₃ with increasing La doping content indicates that La doping can stabilize the structure of BiFeO₃. This is further evidenced that Bi₂Fe₄O₉ can be directly converted to Bi(La)FeO₃ by heating the mixtures of nominal composition of Bi₂Fe₄O₉/ x La₂O₃. Furthermore, the microstructure changes and the room temperature hysteresis loops and leakage current for Bi_{2− x} La _x Fe₄O₉ with x =0 and 0.02 were characterized.