Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

The thermal expansion of La_0.9Sr_0.1Cr_{1- x} M _x O₃(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤ x ≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La_0.9Sr_0.1CrO₃was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La_0.9Sr_0.1Cr_{1- x} M _x O₃ perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La_0.9Sr_0.1Cr_0.96Co_0.02Al_0.02O₃ were 10.5 × 10⁻⁶/°C in air, 10.7 × 10⁻⁶/°C under He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 11.8 × 10⁻⁶/°C in H₂ atmosphere.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

Metastable Crystal Structure of Strontium- and Magnesium-Substituted LaGaO3

The metastable crystal structure of strontium- and magnesium-substituted LaGaO₃ (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La_0.825Sr_0.175Ga_0.825Mg_0.175O_2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8. At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α _a ,ortho = 10.81 × 10⁻⁶ K⁻¹, α _b ,ortho = 9.77 × 10⁻⁶ K⁻¹, and α _c ,ortho = 9.83 × 10⁻⁶ K⁻¹ (25°–400°C), and for the cubic modification to be α_cubic= 13.67 × 10⁻⁶ K⁻¹ (500°–1000°C).     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

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.     

Chemical Preparation of Pure and Strontium- and/or Magnesium-Doped Lanthanum Gallate Powders

Powder compositions of LaGaO₃, La_0.9Sr_0.1GaO_2.95, and La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 were prepared via a Pechini-type process that uses citric acid and ethylene glycol. The calcination behavior of the precursor powders of the above-mentioned phases was studied in the temperature range of 200°–1400°C in an air atmosphere. Characterization of the powder samples were performed using several processes, including X-ray diffractometry, thermogravimetry/differential thermal analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, inductively coupled plasma–atomic emission spectroscopy, and carbon and nitrogen analyses.     

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.     

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.     

Fabrication and Characterization of Ce0.8Sm0.2O1.9 Microtubular Dual-Structured Electrolyte Membranes for Application in Solid Oxide Fuel Cell Technology

Samaria-doped ceria (SDC, Ce_0.8Sm_0.2O_1.9) ceramic powders of submicrometer size were synthesized by a sol–gel auto-combustion method. From these powders microtubes with a dual structure comprising of a dense layer and a porous substrate layer were fabricated in a single step through a phase inversion/sintering technique. A sintering temperature in excess of 1450°C is required for SDC to achieve gastight microtubes. The mechanical strength of the SDC microtubes increases with increasing sintering temperature and may attain up to 208 MPa when sintered at 1500°C. Electrical impedance spectroscopy studies indicate that the SDC microtubes have electrical conductivities of 4.46 × 10⁻⁴–0.072 S/cm and corresponding activation energy of 81.9 kJ/mol at temperatures between 400° and 800°C. Full fuel cells were fabricated by coating Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) on to the inner surface and a Ni-SDC cermet on to the outer surface of the gastight microtubes to act as the cathode and the anode, respectively. The resultant BSCF|SDC|Ni-SDC microcells have a stable output maximum of 106 mW/cm² at 750°C when hydrogen and air were used as fuel and oxidant gas, respectively.     

Co2+-Substitution Effect in Ce-TZP/La M-type Hexaferrite Composites

A Ce-TZP/platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO powders. Platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)₁₂O₁₉ composites was expanded widely by substituting Co²⁺ ions for Fe²⁺ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)₁₂O₁₉ composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Thin Yttrium-Stabilized Zirconia Electrolyte Solid Oxide Fuel Cells by Centrifugal Casting

A centrifugal casting technique was developed for depositing thin 8-mol%-yttrium-stabilized zirconia (YSZ) electrolyte layers on porous NiO-YSZ anode substrates. After the bilayers were cosintered at 1400°C, dense pinhole-free YSZ coatings with thicknesses of ∼25 μm were obtained, while the Ni-YSZ retained porosity. After La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-Ce_0.9Gd_0.1O_1.95 (GDC) or La_0.8Sr_0.2MnO₃ (LSM)-YSZ cathodes were deposited, single SOFCs produced near-theoretical open-circuit voltages and power densities of ∼1 W/cm² at 800°C. Impedance spectra measured during cell tests showed that polarization resistances accounted for ∼70%–80% of the total cell resistance.     

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.     

Doped Lanthanum Gallate Film Solid Oxide Fuel Cells Fabricated On a Ni/YSZ Anode Support

A colloidal deposition without any binder was developed to prepare a dense La_0.8Sr_0.2Ga_0.85Mg_0.15O_3−δ (LSGM) film on porous NiO/YSZ substrates, using an incompletely crystallized LSGM powder as starting material. Both the dense LSGM film with a thickness of 15 μm and the required phase composition of the LSGM were achieved simultaneously by sintering at 1400°C for 6 h. The conductivity of the supported LSGM film attained 0.102 S/cm at 800°C, which was comparable with those of the self-supported LSGM films. The maximum power density of the LSGM film cell was 480 at 800°C and 614 mW/cm² at 850°C, respectively.     

Fabrication of an Anode-Supported Gadolinium-Doped Ceria Solid Oxide Fuel Cell and Its Operation at 550°C

Ce_0.9Gd_0.1O_{2− x} (CGO) layers were deposited onto nonconductive porous NiO–CGO supports by electrophoretic infiltration, and then compacted by isostatic pressing to achieve a high packing density of the deposited layer. The bilayers were sintered to give dense CGO layers at 1290°C in air. A fuel cell comprising an La_0.6Sr_0.4Co_0.2Fe_0.8O_{3− x} cathode, a 10-μm CGO electrolyte, and a Ni–CGO anode was tested at 550°C with humidified 10% H₂ and air. The cell showed an open circuit voltage of 0.86 V and delivered a steady current of about 470 mA/cm² at a terminal voltage of 0.24 V.     

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.     

Electrophoretic Codeposition of La0.6Sr0.4Co0.8Fe0.2O3−δ and Carbon Nanotubes for Developing Composite Cathodes for Intermediate Temperature Solid Oxide Fuel Cells

Carbon nanotubes (CNTs)/La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) composite films have been fabricated by electrophoretic codeposition on Ce_0.9Gd_0.1O_1.95 (CGO) substrates. CNTs are used as a sacrificial phase to produce ordered porous LSCF cathodes for intermediate temperature solid oxide fuel cells. The synthesis of LSCF powder by a modified sol–gel route is presented. The possible mechanism of formation of CNT/LSCF composite nanoparticles in suspension is discussed. Moreover the optimal suspension composition and the conditions for achieving successful electrophoretic deposition (EPD) of CNTs/LSCF composite nanoparticles were evaluated. Experimental results showed that the CNTs were homogeneously distributed and mixed with LSCF nanoparticles forming a mesh-like structure, which resulted in a highly porous LSCF film when the CNTs were burned out during heat treatment in air at 800°C for 2 h. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques were employed to characterize the microstructure of the precursors and of the composite films.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

Synthesis and Characterization of Sr- and Mg-Doped LaGaO3 Tapes

The study focuses the processing of La_{1− x} Sr _x Ga_{1− y} Mg _y O_{3− d} (LSGM) tapes and the characterization of their microstructure, chemical composition, and sinter behavior during annealing. Dilatometric runs show that sintering of the tapes conclude at about 1500°C. This finding correlates with the observed sharp increase of the crystallinity of LSGM at higher temperatures. The porosity of the tapes has been found to be minimum at 1525°C. Increasing sintering temperatures cause a loss of Sr, Mg, and Ga, which is presumably due to evaporation of the elements. The Vickers hardness of the tapes sintered at 1500°C was measured to be about 900.     

Effect of Oxygen Partial Pressure During Liquid-Phase Sintering on the Dielectric Properties of 0.9MgTiO3–0.1CaTiO3

Microstructural evolution and microwave dielectric properties of liquid-phase-sintered 0.9MgTiO₃–0.1CaTiO₃ dielectric ceramic material have been investigated as a function of oxygen partial pressure (     ) during sintering. Sintering in a weakly reducing atmosphere (     =10⁻¹⁴ atm) generally increased the density, permittivity, quality factor ( Q × f ), and the temperature coefficient of resonance frequency (τ_f), but further reducing atmosphere down to     of 10⁻¹⁴ atm generally decreased Q × f and τ_f. When the 5 wt% lithium borosilicate glass-added specimen was sintered at 950°C and     =10⁻¹⁴ atm, it demonstrated a permittivity of 18.8, Q × f of 19 000 GHz, and τ_f of 10 × 10⁻⁶ K⁻¹.