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.     

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.     

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.     

Thermodynamic Evaluation for the Y2O3–BaO–CuOx System

The thermodynamic data for the Y₂O₃–BaO–Cu₂O–CuO quaternary system were optimized from measured thermodynamic data. A two-sublattice model for ionic solution was used to express the Gibbs free energy of the liquid phase, and a two-sublattice regular solution model was used for the nonstoichiometric YBa₂Cu₃O_6+δ superconducting compound. The optimized thermodynamic data were used to calculate the phase diagrams of the Cu₂O–CuO binary system and the CuO _x –Y₂Cu₂O₅ and CuO _x –BaCuO₂ quasi-binary systems. The results were in good agreement with reported measured data. The liquidus projection and isothermal and vertical sections of the Y₂O₃–BaO-CuO _x quasi-ternary system were calculated. The effect of oxygen pressure on some reaction temperatures was predicted by calculating them at various oxygen pressures, and the oxygen contents (6 +δ) in YBa₂Cu₃O_6+δ were calculated at various temperatures and oxygen pressures. The results were compared with experimental data.     

Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium

Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd₂O₃ corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T _c superconductor phase, and has the formula Ba_2–xNd_1+xCu₃O_6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba₂NdCu₃O_6+z, the transformation from the high- T _c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba_2+2x-Nd_4–2xCu_2–xO_10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd₂O₃ and Nd₂O₃–CuO_x systems are also presented. Standard X-ray diffraction patterns of BaNd₂–CuO₅ and (Nd_1.9Ca_0.1)CuO_4–z are provided.     

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.     

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.     

Solid Phase Relations at 950°C in La-Ba-Cu-O

Subsolidus phase relations in the La₂O₃–BaO–CuO system were studied at 950°C. Three previously reported binary compounds exist (La₂CuO₄, BaLa₂O₄, and BaCuO₂) and five previously reported ternary phases occur (La_2-xBa_xCuO_4-(x/2)+δ, La_4-2xBa_2+2xCu_2-xO_10-2x, La_2-xBa_1+xCu₂O_6-(x-2), La_3-xBa_3+xCu₆O_14±δ, and La₄BaCu₅O_13+δ). Of the seven phases in the diagram, all but BaLa₂O₄, BaCuO₂, and La₄BaCu₅O_13+δ were shown to exhibit significant ranges of solubility. The diagram is important in that both >30 K (La_2-xBa_xCuO_4-(x/2)+δ) and >90 K (La_3-xBa_3+xCu₆O_14+δ, x=1) superconductors occur.     

Phase Equilibria of the SrO–CaO–CuO System at 950°C in 1 atm of Oxygen and at 10 kbar

Phase equilibria in the CuO-rich (≥33% CuO) portion of the SrO–CaO–CuO system have been determined at 950°C in 1 atm of pure oxygen and at 10 kbar (1 bar = 10⁵ Pa). Three solid-solution series occur under these conditions. There is a complete solid solution between the Ca₂CuO₃ and Sr₂CuO₃ endmembers at both 1 atm and 10 kbar, as has been previously noted in experiments conducted in air. Another solid-solution series extends continuously between SrCuO₂ and (Sr_0.38Ca_0.62)CuO₂ in 1 atm of oxygen, but is limited to between SrCuO₂ and (Sr_0.64Ca_0.36)CuO₂ at 10 kbar. At 10 kbar, more Ca-rich phases in this solid-solution series are not stable, but (Sr_0.3Ca_0.7)CuO₂ and (Sr_0.1–0.16Ca_0.9–0.84)CuO₂ are stable at 1 atm. The third solid-solution series ranges between Sr_l4Cu₂₄O₃₈ and (Sr_0.41Ca_O.59)₁₄-Cu₂₄O₃₈ in 1 atm of oxygen; the Ca-rich limit of this solid solution changes only slightly to (Sr_0.39Ca_0.61)₁₄Cu₂₄O₃₈ at 10 kbar.     

Quantitative Electron Microscopy of (Bi,Pb)2Sr2Ca2Cu3O10+δ/Ag Multifilament Tapes During Initial Stages of Annealing

The microstructural and compositional evolution during initial annealing of a superconducting (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ/Ag tape is studied using quantitative transmission electron microscopy. Special attention is devoted to the occurrence of Pb-rich liquids, which are crucial for the Bi₂Sr₂CaCu₂O_8+δ to (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ transformation. Ca and/or Pb-rich (Bi,Pb)₂Sr₂CaCu₂O_8+δ grains dissolve into a liquid, which reacts with Ca-rich phases to increase the liquid's Ca-content. This leads to (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ formation. Apparently, a Ca/Sr ratio of around 1 is sufficient to keep (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+δ nucleation going. It is confirmed that Ag particles are transported from the Ag-sheath into the oxide core by the liquid and not by mechanical treatment of the tape.     

Synthesis and Characterization of Sr2Ce2Ti5O16 in the System SrO-CeO2-TiO2

The ternary system SrO-CeO₂-TiO₂ was investigated using X-ray diffractometry. The formation of a new compound, Sr₂Ce₂Ti₅O₁₆, was established, and its compatibilities with SrO, SrCeO₃, and SrTiO₃ were studied. The results revealed the existence of a series of compounds Sr_6–12xCe_6xTi₅O₁₆ and solid solutions Sr_2+nCe₂Ti_5+nO_16+3n ( n ≤ 6).     

Subsolidus Relations in the La2O3─CuO─CaO Phase Diagram and the La2O3─CuO Binary Join

The phase diagram for the CuO-rich part of the La₂O₃─CuO join was redetermined. La₂Cu₂O₅ was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu₇O₁₉ had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La₂O₃─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La_1.9Ca_1.1Cu₂O_5.9 and LaCa₂Cu₃O_8.6, were stable at these temperatures, with three binary phases, Ca₂CuO₃, CaCu₂O₃, and La₂CuO₄. La₂Cu₂O₅ and La₈Cu₇O₁₉ were stable only at 1020°C, and did not support solid-solution formation.     

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.     

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.     

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.     

Phase Equilibria and Superconducting Properties of BiSr2YCu2O7 (1212 Phase)

Phase equilibria of the quasi-quaternary system BiO_1.5–SrO-YO_1.5–CuO have been studied at a temperature of 950°C in air, with special regard to the 1212 phase. The 1212 phase reveals only very small changes in the cation ratio. Single-phase samples exist for (Bi_0.24–0.36Cu_0.42-0.55)–Sr₂Y_1.27Cu₂O _y compositions. The bismuth-rich composition of the 1212 phase is in thermodynamic equilibrium with a liquid and the 2212 phase, whereas the copper-rich composition is in equilibrium with five other phases. The influence of combined calcium and lead doping also has been studied. Exceeding the calcium saturation of the 1212 phase increases the amount of 2212 as a secondary phase. Single-phase 1212 samples do not show any superconductivity in either the as-prepared or the post-annealed state. The only compositions with bulk superconductivity are those with calcium and lead doping after annealing at a temperature of 980°C. The superconductivity is attributed to the 2212 phase crystallizing from the melt during slow cooling.     

Superconducting Properties of Barium-Doped (Bi,Pb)2Sr2Ca2Cu3Ox Glass-Ceramics

The Ba-doped superconducting (Bi,Pb)₂Sr_{2- x} Ba _x Ca₂Cu₃O _y and (Bi,Pb)₂Sr₂Ca_{2- x} Ba _x Cu₃O _y (0 ≦ x ≦ 1.0) were prepared by using a melt-quenching method, and the effect of Ba additions on the glass-forming ability and the crystalline phase was examined. The glass-forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO₃ as well as CaO was observed in the melt-quenched sample of (Bi,Pb)₂SrBaCa₂Cu₃O _y . BaPbO₃ crystals were precipitated in all glass-ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high- T _c phase, and (Bi,Pb)₂Sr_1.4Ba_0.6Ca₂Cu₃O _y glass-ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with a T _c of 103 K, although BaPbO₃ and the low- T _c phase were still largely present.     

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.     

Formation Mechanism of MoO3-Doped YSr2Cu3Oy Using the Citrate Process

The formation mechanism of the synthesis of MoO₃-doped YSr₂Cu₃O _y powders using the citrate process was investigated. It was shown that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ played a crucial role in forming the superconducting phase. It was found that the precursor phase (Sr_{1- x} Y _x )₁₄Cu₂₄O₄₁ interacted with water and decomposed when it was heavily milled and heated.     

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.