Strength degradation and failure limits of dense and porous ceramic membrane materials

Thin dense membrane layers, mechanically supported by porous substrates, are considered as the most efficient designs for oxygen supply units used in Oxy-fuel processes and membrane reactors. Based on the favorable permeation properties and chemical stability, several materials were suggested as promising membrane and substrate materials: Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ, La_0.6?xSr_0.4Co_0.2Fe_0.8O_3?δ (x = 0, 0.02) and Ce_0.9Gd_0.1O_1.95?δ. Although membranes operate at elevated temperatures, the ends of tubes in certain three-end concepts remain almost at room temperature. The current work concentrates on the failure potential of these membrane parts, where in a complex device also the highest residual stresses should arise due to differences in thermal expansion. In particular, sensitivity of the materials to subcritical crack growth was assessed since the long-term reliability of the component does not only depend on its initial strength, but also on strength degradation effects. The results were subsequently used as a basis for a strength–probability–time lifetime prediction.     

X-ray single phase LSGM at 1350 °C

Synthesis of X-ray-phase-pure (La_1−xSr_xGa_1−yMg_yO_3−δ, LSGM, where x = 0.1, y = 0.15 and 0.17) powders were achieved at temperatures as low as 1350 °C via organic precursor method using tartaric acid as the carrier material. LSGM materials were characterized for their phase purity, crystallization and electrical properties. Pellets sintered at 1350 °C for 6 h were single phase and dense (>99%). Electron microscopy analysis of X-ray single-phase pellets revealed MgO precipitates with sizes ranging from 50–300 nm. Phase formation and distribution in this complicated multi-cation-oxide system as a function of temperature were reported and discussed. Amorphous LSGM first crystallizes at 625 °C. However, elimination of undesired phases require higher temperatures. Impedance measurements as a function of temperature up to 545 °C revealed that the X-ray phase pure pellets may have extrapolated ionic conductivity values as high as 0.14–0.16 S/cm at 800 °C. Possible implications of limited MgO solubility on the ionic conductivity are presented.     

Electronic conductivity,oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ is approximately 15%. The incorporation of Al³⁺ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr_0.7Ce_0.3Mn_0.9Al_0.1O_3−δ, measured in the oxygen partial pressure range from 10⁻¹⁸ to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O₂) below 10⁻⁴ to 10⁻² atm. The oxygen permeability of dense Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO₃-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ ceramics in air are (10.8–11.8) × 10⁻⁶ K⁻¹.     

Ceramic materials for H2 transport membranes applicable for gas separation under coal-gasification-related conditions

This work focuses on the synthesis, characterization and testing of mixed protonic–electronic conducting membrane materials for H₂ separation from gas mixtures capable of operating in a membrane reactor at temperatures higher than 600 °C. La_5.5WO_12−δ and selected substituted barium zirconates with stoichiometries BaCe_0.5Zr_0.4Y_0.1O_3−δ and BaCe_0.2Zr_0.7Yb_0.08Ni_0.02O_3−δ were therefore characterized and tested under coal-gasification-related conditions at 600–900 °C. Sintered samples of the synthesized substituted barium zirconates were characterized by measuring the total conductivity and the thermal expansion coefficients. Also particle size distributions, BET surface-areas and elemental analysis of the starting powders, including commercial La_5.5WO_12−δ were specified. The compounds were exposed to syngas with steam, as well as to an atmosphere mainly consisting of CO₂. The microstructure and phase composition of the membrane materials were studied by SEM, EDX and XRD before and after exposure. BaCe_0.2Zr_0.7Yb_0.08Ni_0.02O_3−δ shows a very promising chemical stability from 600 °C to 900 °C and La_5.5WO_12−δ at 900 °C.     

Influence of anode pore forming additives on the densification of supported BaCe0.7Ta0.1Y0.2O3−δ electrolyte membranes based on a solid state reaction

We describe a solid state reaction for the preparation of both NiO–BaCe_0.7Ta_0.1Y_0.2O_3?δ anode substrates and BaCe_0.7Ta_0.1Y_0.2O_3?δ (BCTY10) electrolyte membranes on porous NiO–BCTY10 anode substrates. The amounts of the pore forming additive in the substrates showed a significant influence on the densification of the BCTY10 membranes. After sintering at 1450 °C for 5 h, the BCTY10 membrane on a NiO–BCTY10 anode containing 30 wt.% starch achieved a high density and showed adequate chemical stability against H₂O and CO₂. The chemical stability of BCTY10 was even better than that of BaCe_0.7Zr_0.1Y_0.2O_3?δ. With a mixture of BaCe_0.7Zr_0.1Y_0.2O_3?δ (BZCY7) and La_0.7Sr_0.3FeO_3?δ (LSF) as a cathode, a single fuel cell with 12 μm thick BCTY10 electrolyte generated maximum power densities of 142, 93, 29 mW/cm² at 700, 600 and 500 °C, respectively. The electrolyte resistance and interfacial polarization resistance of the cell under open circuit conditions were also investigated.     

Crystal structure,thermal expansion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)

Ba_xSr_1−xCo_0.8Fe_0.2O_3−δ (0.3 ≤ x ≤ 0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x ≤ 0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10⁻⁶ °C⁻¹. The conductivity were larger than 30 S cm⁻¹ above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba_0.4Sr_0.6Co_0.8Fe_0‘2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ are potential cathode materials.     

Characterization of Ba0.5Sr0.5M1−xFexO3−δ (M = Co and Cu) perovskite oxide cathode materials for intermediate temperature solid oxide fuel cells

Ba_0.5Sr_0.5Co_1?xFe_xO_3?δ (x = 0.2, 0.6, and 0.8) and Ba_0.5Sr_0.5Cu_1?xFe_xO_3?δ (x = 0.6 and 0.8) perovskite oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells. All the samples synthesized by a citrate–EDTA complexing method were single-phase cubic perovskite solid solutions. Then, the thermal expansion coefficient, electrical conductivities, the oxygen vacancy concentrations, the polarization resistances (R_p), and the power densities were measured. An increase in the Co content resulted in a decrease in the polarization resistance, the electrical conductivities at low temperatures, and the inflection point of the thermal expansion coefficient, but it led to an increase in the electrical conductivities at high temperatures, the oxygen vacancy concentrations, and the maximum power densities. The Cu-based system has similar behavior to the Co-based system; yet, in terms of the electrical conductivities, high Cu content gave a better result than low content for the entire range of temperatures.     

Sol–gel synthesis of La0.6Sr0.4CoO3−x and Sm0.5Sr0.5CoO3−x cathode nanopowders for solid oxide fuel cells

Nano-powders of La_0.6Sr_0.4CoO_3?x (LSC) and Sm_0.5Sr_0.5CoO_3?x (SSC) compositions, which are being investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) with La(Sr)Ga(Mg)O_3?x (LSGM) as the electrolyte, were synthesized by low-temperature sol–gel method using metal nitrates and citric acid. Thermal decomposition of the citrate gels was followed by simultaneous DSC/TGA methods. Development of phases in the gels, on heat treatments at various temperatures, was monitored by X-ray diffraction. Sol–gel powders calcined at 550–1000 °C consisted of a number of phases. Single perovskite phase La_0.6Sr_0.4CoO_3?x or Sm_0.5Sr_0.5CoO_3?x powders were obtained at 1200 °C and 1300 °C, respectively. Morphological analysis of the powders calcined at various temperatures was done by scanning electron microscopy. The average crystallite size of the powders was ～15 nm after 700 °C calcinations and slowly increased to 70–100 nm after heat treatments at 1300–1400 °C.     

Evaluation of the La0.75Sr0.25Mn0.8Co0.2O3−δ system as cathode material for ITSOFCs with La9Sr1Si6O26.5 apatite as electrolyte

In the past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C.Apatite materials (La_10?xSr_xSi₆O_27?x/2) are attractive candidates for solid electrolytes, with a high ionic conductivity at 700 °C, a chemical and a dimensional stability for a pO₂ ranging from 10^?25 to 0.2 atm. A perovskite oxide (La_0.75Sr_0.25Mn_0.8Co_0.2O_3?δ) has been used as a cathode material.Symmetrical cathode/electrolyte/cathode cells were fabricated by stacking layers obtained by tape casting of apatite and perovskite powders and co-sintering at 1400 °C for 2 h in air.Impedance spectroscopy measurements were performed on these cells in order to determine the electrode resistance. It has been shown that the latter decreases with the porosity content of the cathode and with the use of a composite material (apatite/perovskite) instead of a simple perovskite.     

Effects of La doping on electrical conductivity,thermal expansion and electrochemical performance in LaxSr1–xCo0.9Sb0.1O3–δ cathodes for IT–SOFCs

Perovskite oxides La_xSr_1–xCo_0.9Sb_0.1O_3–δ (LSCSbx, x=0.0–0.8) are investigated as IT–SOFC cathodes supported with La_0.9Sr_0.1Ga_0.8Mg_0.2O_3–δ (LSGM) electrolyte. All LSCSbx oxides have a tetragonal distorted perovskite structure with s.g. P4/mmm, while a La₂Co₂O₅ impurity phase was observed within La doping levels at x=0.6–0.8. The LSCSb0.4 has a good chemical compatibility with LSGM electrolyte for temperatures up to 1050 °C. XPS examinations indicate the existence of Co³⁺/Co⁴⁺ mixed valence states in LSCSbx. The conductivity increases with La doping and the LSCSbx with x=0.4 exhibits the highest electrical conductivity (e.g., 673–1637 S cm⁻¹ at 300–850 °C). The thermal expansion coefficient (TEC) decreases from 25.89×10^–6 K^–1 for x=0.0 to 18.5×10^–6 K^–1 for x=0.6 at 30–900 °C. Among the LSCSbx compositions, the LSCSb0.2 exhibits the lowest polarization resistance (R_p), which is merely 0.069 Ω cm² at 700 °C. The maximum power density of the cell with LSCSb0.2 cathode on 300 µm thick LSGM electrolyte attains 564 mW cm^–2 at 850 °C, which is higher than that of SrCo_0.9Sb_0.1O_3–δ (SCSb) cathode. All of the results indicate that LSCSb0.2 is a promising material for application in IT–SOFCs cathodes.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

Chemical stability and electrical properties of Nb doped BaCe0.9Y0.1O3−δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe_0.9?xNb_xY_0.1O_3?δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550–750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe_0.87Nb_0.03Y_0.1O_3?δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10^?2 S cm^?1 in wet hydrogen at 650 °C compared to 1.01×10^?2 S cm^?1 for undoped electrolyte.     

Structure and electrical conductivity of BaCe0.7In0.1A0.2O3−δ (A = Gd,Y) ceramics

BaCe_0.7In_0.1A_0.2O_3?δ (A = Gd, Y) ceramics were synthesized by solid state reaction method. The microstructure and electrical properties of BaCe_0.7In_0.1A_0.2O_3?δ ceramics were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance analysis at intermediate temperatures of 773–1073 K in different atmospheres. All BaCe_0.7In_0.1A_0.2O_3?δ ceramics exhibit a cubic perovskite structure. Relative densities of BaCe_0.7In_0.1A_0.2O_3?δ ceramics are above 92%. BaCe_0.7In_0.1Gd_0.2O_3?δ and BaCe_0.7In_0.1Y_0.2O_3?δ ceramics exhibit an excellent chemical stability against boiling water. The conductivity values of BaCe_0.7In_0.1Gd_0.2O_3?δ are higher than those of BaCe_0.7In_0.1Y_0.2O_3?δ in both air and dry hydrogen atmospheres. The highest conductivity is 4.6 × 10^?2 S cm^?1 for BaCe_0.7In_0.1Gd_0.2O_3?δ ceramic in air at 1073 K. BaCe_0.7In_0.1Gd_0.2O_3?δ ceramic with a conductivity value of 1.0 × 10^?2 S cm^?1 at 823 K in both air and dry hydrogen atmospheres is considered as a promising alternative for electrolytes of SOFC in view of decreasing the operating temperature and keeping both high conductivity and good chemical stability.     

Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells

Nanocrystalline SOFC cathode materials of perovskite family, La_1?xSr_xM_1?yCo_yO₃, where 0 < x ≤ 0.5, 0 < y ≤ 0.8 (M is transitional metal = Mn or Fe), have been synthesized at a relatively low temperature by combustion synthesis using alanine as a novel fuel. Detailed X-ray powder diffraction analyses show 47–96% phase purity in the as-synthesized powder and upon calcination at ～825 °C single-phase material is obtained wherein the nanocrystallinity (crystallite size ～19–24 nm) is retained. Densification studies of the materials are carried out within 900–1100 °C. The coefficient of thermal expansion (CTE) of these cathodes is measured. Electrical conductivity of the cathodes sintered at different temperatures are measured in the temperature range 700–900 °C and correlated with the density of the sintered materials. The electrochemical performances of Ni-YSZ anode-supported SOFC having YSZ electrolyte (～10 μm) with CGO interlayer (～15 μm) are studied with the developed cathodes in the temperature range 700–800 °C using H₂ as fuel and oxygen as oxidant. Highest current density of ～1.7 A/cm² is achieved during testing at 800 °C measured at 0.7 V with a cathode composition of La_0.5Sr_0.5Co_0.8Fe_0.2O₃. Precipitation of nanocrystalline grains over the core grains in porous microstructure of this cathode might be one of the reasons for such high cell performance.     

Single step reactive sintering and chemical compatibility between La9Sr1Si6O26.5 and selected cathode materials

Apatite-type silicates are considered as promising electrolytes for solid oxide fuel cells (SOFC). However more studies on the chemical compatibility of these materials with common SOFC electrodes are required. Here, we report the synthesis of single phase La₉Sr₁Si₆O_26.5 composition by reactive sintering at 1650 °C for 10 h. Fully dense pellets showed very high oxide-anion conductivity, 25 mS cm^?1 at 700 °C. Furthermore, the chemical compatibility of La₉Sr₁Si₆O_26.5 with some selected cathode materials has also been investigated. The lowest reaction temperatures were determined to be 1100 °C, 1000 °C and 900 °C for La_0.8Sr_0.2MnO_3?δ, La₂Ni_0.8Cu_0.2O₄ and La_0.6Sr_0.4Co_0.8Fe_0.2O₃, respectively. The segregation of minor amounts of SiO₂ seems to be a key limiting factor that must be overcome. Finally, these cathode materials were deposited over dense oxy-apatite pellets and the area specific resistances in symmetrical cells were determined. These values, at 700 °C, were 14.4 and 2.6 Ω cm² for La_0.8Sr_0.2MnO_3?δ and La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ, respectively. Furthermore, the area specific resistances are notably improved 0.6 Ω cm² when a 50 wt.% composite of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ and Ce_0.8Gd_0.2O_1.9 is used.     

Preparation of stabilized Gd-doped BaPrO3 materials by Zr substitution

The synthesis of a Gd-doped BaPrO₃–BaZrO₃ solid solution has been investigated in an effort to stabilize the BaPrO₃ perovskite, a system with poor chemical stability and endurance for humid, CO₂ and reducing conditions. The phases BaZr_xPr_1?xGd_0.3O_3?δ (x = 0–0.7, Δx = 0.1) have been prepared by acrylamide combustion synthesis. The solid solution is characterized by X-ray powder diffraction, scanning electron microscope (SEM) and durability against CO₂, moisture and reducing conditions investigated. Crystal structure parameters were obtained from X-ray powder diffraction data and results show its symmetry increases by increasing Zr content. BaZr_0.1Pr_0.6Gd_0.3O_3?δ has a maximum weight increase due to reaction with CO₂ of nearly 13%, while this is reduced below 2% for BaZr_0.5Pr_0.2Gd_0.3O_3?δ. The material appears to be unaltered by 5%H₂/Ar for x ≥ 0.5, and under wet Ar for x ≥ 0.4, and thus render a higher chemical stability and confirms the stabilizing role of BaZrO₃. However, microstructure investigations reveal gadolinium segregation due to the high doping level of the perovskite.     

Conductivity and electrochemical performance of (Ba0.5Sr0.5)0.8La0.2CoO3−δ cathode for intermediate-temperature solid oxide fuel cell

This study reports the successful preparation of a single-phase cubic (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ perovskite by the citrate–EDTA complexing method. Its crystal structure, thermogravimetry, coefficient of thermal expansion, electric conductivity, and electrochemical performance were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Its coefficient of thermal expansion shows abnormal expansion at 300 °C, which is associated with the loss of lattice oxygen. The maximum conductivity of a (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ electrode is 689 S/cm at 300 °C. Above 300 °C, the electronic conductivity of (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ decreases due to the formation of oxygen vacancies. The charge-transfer resistance and gas phase diffusion resistance of a (Ba_0.5Sr_0.5)_0.8La_0.2CoO_3?δ–Ce_0.8Sm_0.2O_1.9 composite cathode are 0.045 Ω cm² and 0.28 Ω cm², respectively, at 750 °C.     

SrCo1−xSbxO3−δ cathode materials prepared by Pechini method for solid oxide fuel cell applications

In this study, SrCo_1?ySb_yO_3?δ powders were prepared by a modified Pechini method. According to the study results, the cubic Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics was obtained as 10% of cobalt ions were substituted by antimony ions. Doping of Sb³⁺ ions appeared both to stabilize the Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics and to enhance densification and retard grain growth. The coefficient of thermal expansion of the SrCo_1?xSb_xO_3?δ ceramics increased with the content of the antimony ions, ranging from 10.17 to 15.37 ppm/°C at temperatures lower than the inflection point (ranging from 450 °C to 550 °C) and from 22.16 to 29.29 ppm/°C at higher temperatures. For the SrCo_0.98Sb_0.02O_3?δ ceramic, electrical conductivity reached a maximum of 507 S/cm at 450 °C. The ohmic and polarization resistances of the single cell with the pure SrCo_0.98Sb_0.02O_3?δ cathode at 700 °C read respectively 0.298 Ω cm² and 0.560 Ω cm². The single cell with the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode appeared to reduce the impedances with the R₀ and R_P at 700 °C reading respectively 0.109 Ω cm² and 0.127 Ω cm². Without microstructure optimization and measured at 700 °C, the single cells with the pure SrCo_0.98Sb_0.02O_3?δ cathode and the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode, demonstrated maximum power densities of 0.100 W/cm² and 0.487 W/cm². Apparently, SrCo_1?ySb_yO_3?δ is a potential cathode for use in IT-SOFCs.     

Study on stability and electrical performance of yttrium and bismuth co-doped BaCeO3

Yttrium and bismuth co-doped BaCeO₃ (BaCe_0.8?xY_0.2Bi_xO_3?δ, x=0, 0.1, 0.3, 0.5, abbreviated as B0, B10, B30, B50, respectively) powders were successfully synthesized by the citrate–EDTA auto-ignition method and dense ceramics were also obtained. The investigations were mainly focused on the chemical stability and electrical performance of the bismuth doped BaCe_0.8Y_0.2O_3?δ. The phase and thermal analysis of the powders demonstrated that the appropriate amount of Bi dopant can well stabilize BaCeO₃ (e.g. Bi=0.3 and 0.5) and prevented them from boiling water damage, whilst the stability in CO₂-containing atmosphere degraded with the increase of the bismuth content. It was demonstrated that in wet air, the conductivities of B30 and B50 reached as high as 0.20 S/cm and 0.71 S/cm at 700 °C, respectively. And both were one order of magnitude higher than that of B0. An electron conduction mechanism for the bismuth doped BaCeO₃ specimen was proposed. Yttrium and bismuth co-doping exerted influences on both the chemical stability and electrical performance.