Novel composite interconnecting ceramics La0.7Ca0.3CrO3−δ/Ce0.8Sm0.2O1.9 for solid oxide fuel cells

In this paper, an interconnecting ceramic for solid oxide fuel cells was developed, based on the modification from La_0.7Ca_0.3CrO_3−δ by addition of Ce_0.8Sm_0.2O_1.9. It is found that addition of small amount Ce_0.8Sm_0.2O_1.9 into La_0.7Ca_0.3CrO_3−δ dramatically increased the electrical conductivity. For the best system, La_0.7Ca_0.3CrO_3−δ + 5 wt.% Ce_0.8Sm_0.2O_1.9, the electrical conductivity reached 687.8 S cm⁻¹ at 800 °C in air. In H₂ at 800 °C, the specimen with 3 wt.% Ce_0.8Sm_0.2O_1.9 had the maximal electrical conductivity of 7.1 S cm⁻¹. With the increase of Ce_0.8Sm_0.2O_1.9 content the relative density increased, reaching 98.7% when the Ce_0.8Sm_0.2O_1.9 content was 10 wt.%. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce_0.8Sm_0.2O_1.9 content, ranging from 11.12 × 10⁻⁶ to 12.46 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Therefore, this material system will be a very promising interconnect for solid oxide fuel cells.     

Effects of gallia additions on sintering behavior of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> ceramics prepared by commercial powders

The densification behavior and grain growth of Ce_0.8Gd_0.2O_1.9 ceramics were investigated with the gallia concentration ranging from 0 to 10 mol%. Both the sintered density and grain size were found to increase rapidly up to 0.5 mol% Ga₂O₃, and then to decrease with further additions. Under the same sintering conditions, the samples with 3 mol% Ga₂O₃ and less exhibited a higher sintered density, as compared to the one without Ga₂O₃ addition. However, a pinning effect on grain growth was found at 2 mol% Ga₂O₃. In the dopant content range of 0 to 10 mol%, 0.5 mol% Ga₂O₃ was the optimum doping level in promoting densification and grain growth of commercially available powders of Ce_0.8Gd_0.2O_1.9.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Properties and performance of La1.6Sr0.4NiO4+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite cathodes were prepared successfully using combustion synthesis method for intermediate temperature solid oxide fuel cells. The chemical compatibility, thermal expansion behavior, electrical conductivity and electrode performance were studied. The X-ray diffraction of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite result proved a slight reaction between La_1.6Sr_0.4NiO_4+δ and Ce_0.8Sm_0.2O_1.9. Both the thermal expansion coefficient and the electrical conductivity of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 decreased with increasing Ce_0.8Sm_0.2O_1.9 content. AC impedance spectroscopy measurements indicated that the addition of 30 wt% Ce_0.8Sm_0.2O_1.9 to La_1.6Sr_0.4NiO_4+δ exhibited the lowest polarization resistance (0.238 Ωcm²) at 800 °C in air, which was only one fourth of the La_1.6Sr_0.4NiO_4+δ electrode measured at the same temperature.     

Influence of the additives and processing conditions on the characteristics of dense SnO2-based ceramics

Three different SnO₂-based powder mixtures, containing 2 wt% CuO as sintering aid and Sb₂O₃ in amounts from 0 to 4 wt% as activator of the electrical conductivity, were sintered to high density at temperatures in the range 1000–1400°C and soaking times from 1 to 6 h. Densification behaviour and microstructure development are strongly dependent on the presence of CuO, that gives rise to a liquid phase, and on Sb₂O₃ that retards the liquid phase formation and hinders grain growth. Cu and Sb cations can enter s.s. in the SnO₂ network with different oxidation states and in different positions, depending on the sintering conditions. The characteristics of the grain boundary phase, of the SnO₂ solid solutions and their modification depending on thermal treatments were analyzed. The electrical resistivity values varied in a wide range from 10^–1 to 10⁴ cm, depending on starting composition and processing conditions: in terms of the final density and of the electrical conductivity, the optimal sintering conditions were found to be 1200°C, for 1–3 h. The electrical resistivity was related to the microstructural features, particularly to the characteristics of the resulting SnO₂-based solid solutions.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

Pressureless sintering of SiC-TiC composites with improved fracture toughness

Composites of SiC-TiC containing up to 45 wt% of dispersed TiC particles were pressureless sintered to 97% of theoretical density at temperatures between 1850°C and 1950°C with Al₂O₃ and Y₂O₃ additions. An in situ-toughened microstructure, consisted of uniformly distributed elongated -SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase, was developed via pressureless sintering route in the composites sintered at 1900°C. The fracture toughness of SiC-30 wt% TiC composites sintered at 1900°C for 2 h was as high as 7.8 MPa·m^1/2, owing to the bridging and crack deflection by the elongated -SiC grains.     

Functional nano-composite oxides synthesized by environmental-friendly auto-combustion within a micro-bioreactor

Nano-crystalline Sm_0.2Ce_0.8O_1.9 (SDC), Gd_0.2Ce_0.8O_1.9 (GDC), La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) and NiO (65 wt.%)–GDC powders for applications in solid-oxide fuel cells were synthesized by employing cotton fibers as the micro-bioreactor. As compared with the conventional glycine-nitrate (GN) combustion method, this novel process allows the preparation process to be more environmental-friendly. Furthermore, the particle size of obtained powders could be smaller, attributing to the blocking effect of cellulose on suppressing the particle contact during synthesis. By this innovative method, SDC powder with a particle size as small as ～10 nm, was obtained, which can be easily sintered to dense electrolytes at 1350 °C. The process is also capable for large-scale synthesis.     

Sintering and electrical properties of Ce<Subscript>0.8</Subscript>Sm<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> film prepared by spray pyrolysis and tape casting

Ce_0.8Sm_0.2O_1.9 (SDC) powder was synthesized by spray pyrolysis at 650 °C. XRD results showed that phase-pure SDC powder with an average crystallite size of 11 nm was synthesized. SDC electrolyte film was prepared by tape casting and sintered at different temperatures of 1,300, 1,400 and 1,500 °C for 2 h, respectively. The SDC electrolyte film was relatively denser and showed finer microstructure at relatively lower temperature of 1,400 °C, which might be due to the high sintering activity of the spray pyrolysis SDC powder. The ionic conductivity of the SDC electrolyte film sintered at 1,400 °C reached a maximum value of 9.5 × 10⁻³ S cm⁻¹ (tested at 600 °C) with an activation energy for conduction of 0.90 eV.     

Sintering behaviour and ac conductivity of dense Ce0.8Gd0.2O1.9 solid electrolyte prepared employing single-step and two-step sintering process

High density sub-microcrystalline Ce_0.8Gd_0.2O_1.9 (CGO) ceramics using an unconventional two-step sintering (TSS) process have been successfully obtained. Impedance spectroscopy has been used to measure conductivity of CGO samples between 283 and 422 °C. The CGO ceramic samples sintered using the TSS process have been found to deliver superior physical and electrical properties compared to those processed employing the single-step sintering process.     

Evolution of the microstructure of undoped and Nb-doped SrTiO3

Undoped and Nb-doped SrTiO₃ specimens with excess titania compositions were prepared by sintering in air at 1420 or 1480 °C. Large grains due to liquid-phase sintering were obtained for undoped specimens containing 0.6 mol % excess titania and fired at 1480 °C. On the other hand uniform fine grains were observed for samples fired at 1420 °C, resulting from grain-growth inhibition due to exsolved TiO₂ second phase. The solubility of excess titania seemed less than 0.2 mol% under our experimental conditions. The microstructural behaviour of Nb-doped SrTiO₃ could be explained well by the Sr-vacancy compensation model. According to this model, the solubility of excess titania in SrTiO₃ increased with Nb₂O₅ dopant concentration. Thus, for specimens which had high excess titania compositions and were sintered at 1480 °C, large grains were observed when the Nb content was low enough to retain sufficient excess titania-forming liquid phase. For specimens having the same compositions and fired at 1420 °C, uniform fine grains were obtained due to grain growth inhibition by the exsolved TiO₂ second phase, when the Nb content was low. If the excess titania was less than the solubility determined by the amount of Nb dopant, Ruddlesden-Popper-type phases were believed to be formed and resulted in poor densification. Although excess titania was the major factor in determining the grain size of the specimens, the niobium dopant enhanced grain growth.     

Study of Ln0.6Sr0.4Co0.8Mn0.2O3-δ (Ln=La, Gd, Sm or Nd) as the cathode materials for intermediate temperature SOFC

Perovskite type oxides Ln_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ (Ln=La, Gd, Sm, or Nd) have been prepared by the solid state reaction of corresponding oxides. The crystal parameters of the compositions were determined by XRD powder diffraction, which revealed that all the compositions have orthorhombic structure. The reaction test of all samples with Ce_0.8Gd_0.2O_1.9 was carried out at 1200 °C for 48 h, and no reaction product was detected by XRD. The change in mass of La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ as a function of temperature was determined by thermogravimetric analysis (TGA). The electrical conductivity of the sintered samples were measured as a function of temperature from 200 to 1000 °C. The highest conductivity of about 1400 S cm⁻¹ was found in La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ. The cathodic polarization of these oxides electrodes deposited on Ce_0.8Gd_0.2O_1.9 tablet was studied at 500-800 °C in air.     

Microstructure and properties of the sintered composite prepared by hot pressing of TiN-coated alumina powder

Alumina powders (average grain size: 50 m) coated with TiN film of thickness 0.5 and 1.2 m were prepared by rotary powder-bed chemical vapour deposition for 15 and 90 min, respectively. These Al₂O₃-TiN composite powders were hot-pressed at 1800 °C and 40 MPa for 30 min. The microstructure of the Al₂O₃-TiN sintered composite was composed of a TiN network homogeneously distributed on the grain boundaries of alumina. The mechanical properties (hardness, bending strength and fractured toughness) and thermal conductivity of the sintered composite were found to depend on the composition and microstructure of the sintered composite, even with a small content (3–7 wt%) of TiN. The resistivity of the sintered composite was 10^–1-10^–3 cm. The relatively high electrical conductivity of the Al₂O₃-TiN composite was caused by the grain boundary conduction of TiN.     

Synthesis and properties of Ba0.5Sr0.5(Co0.6Zr0.2)Fe0.2O3−δ perovskite cathode material for intermediate temperature solid-oxide fuel cells

A highly stable perovskite cathode material, Ba_0.5Sr_0.5(Co_0.6Zr_0.2)Fe_0.2O_3−δ (BSCZF) for intermediate temperature solid-oxide fuel cells (IT-SOFCs) was synthesized via the improved EDTA-citric acid complexing technique combined with high-temperature sintering. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical impedance spectra (EIS) measurements. An electrolyte-supported BSCZF/SDC/Ni-SDC fuel cell was fabricated to evaluate the performance of the material. The XRD study indicates that the sintering temperature higher than 950 °C is sufficient to the formation of clean single BSCZF perovskite phase. Due to the incorporation of Zr ions, BSCZF perovskite exhibit lower electrical conductivity with higher activation energy but higher structural stability than the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) parent oxide. The maximum electrical conductivity of BSCZF attains 16.9 S cm⁻¹ at around 540 °C. Impedance spectra showed that the ASRs of BSCZF cathode on samaria doped ceria (Ce_0.8Sm_0.2O_1.9, SDC) electrolyte are low but are still slightly larger than those of BSCF at similar conditions. The BSCZF/SDC/Ni-SDC cell exhibited a stable output with the maximum power densities of 30, 75, 139 and 241 mW cm⁻² at 550, 600, 650 and 700 °C, respectively. Due to the high electrochemical performances as well as the excellent stability, BSCZF perovskite may be an attractive cathode material for IT-SOFCs.     

Electrochemical performance of (La,Sr)(Co,Fe)O3−δ–(Ce,Sm)O2−θ–CuO composite cathodes for intermediate temperature solid oxide fuel cells

La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ–CuO composite cathodes were studied for the potential application in intermediate temperature solid oxide fuel cells. Ce_0.8Sm_0.2O_2?θ electrolyte with porous Ce_0.8Sm_0.2O_2?θ interlayer was successfully prepared by one-step sintering process. The effect of interlayer between cathode and electrolyte and CuO on the electrochemical performance of the composite cathodes was investigated by AC impedance spectroscopy. The application of interlayer decreased the area specific resistance of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode. The addition of CuO to La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ reduced the phase formation temperature of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ by 150 °C and the addition of CuO to La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode reduced the optimal calcination temperature of the cathode to 800 °C. The composite cathode with 2 mol% CuO calcined at 800 °C exhibited the lowest area specific resistance of 0.05 Ω cm² at 700 °C in air, which was reduced by 67% compared with that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode. The studies of the corresponding single cell performance, thermal expansion and thermal cycling behaviors further indicated that the composite cathode with 2 mol% CuO could be a promising cathode material.     

Stable suspensions of doped ceria nanopowders for electrophoretic deposition of coatings for solid oxide fuel cells

We have studied nonaqueous suspensions of Ce_0.8Sm_0.2O_1.9 (CSO), Ce_0.8Gd_0.2O_1.9 (CGO), and Се_0.8(Sm_0.75Sr_0.2Ba_0.05)_0.2O_2–δ (CSSBO) nanopowders produced by laser evaporation of a target. The nanoparticles were nearly spherical in shape and their average diameter was 9, 7, and 15 nm, respectively. Using ultrasonic processing, we obtained stable nanopowder suspensions in an isopropanol + acetylacetone mixed medium, investigated their particle size composition, evaluated their zeta potential as a function of pH, and obtained potentiometric titration curves. The starting nanopowder suspensions have been shown to be weakly acidic and have a rather high initial zeta potential. During titration of the nanopowder suspensions with 0.17 N KOH in isopropanol, no isoelectric point was observed. The maximum positive values of the zeta potential, favorable for electrophoretic deposition (EPD), were reached in weakly acidic media in the pH range 4–6. Using EPD, we obtained a coating from a stable self-stabilized CSSBO suspension (ζ = +31 mV, pH 4.0), which was then sintered in air at a temperature of 1400°C. Our results demonstrate that the starting nonaqueous suspensions of the CSO, CGO, and CSSBO nanopowders suit well for producing gas-tight, homogeneous solid oxide fuel cell coatings by EPD.     

Electrical behavior of Gd-doped CeO2 electrolyte ceramics sintered from nanopowders prepared by mechanical alloying process

The densification behavior of nanocrystalline Gd-doped ceria electrolyte, synthesized via mechanical alloying process, was investigated by means of the conventional pressure less sintering and the two-step sintering methods. The effect of the heating rate and the amounts of dopant on the sinterability of Ce_1−x Gd _x O_(2−δ) x = 0.2 (2GDC) and x = 0.3 (3GDC) oxides was studied, which indicated that the gadolinium retards densification and grain growth in the final state of the conventional sintering and 2GDC samples reach 94% density at 1,550 °C. Subsequent investigation on the grain growth in the fully densified ceramics showed that lowering of the heating rate and increasing of the soaking time reduce the effect of dopant and cause samples to be densified to the higher theoretical density (97%) at lower temperatures (1,400 °C). Fully dense Gd-doped ceria ceramics with finest grain size (900–1,100 nm) can be obtained by two-step sintering method. Electrical conductivity measurement in the GDC samples was studied by impedance spectroscopy. The grain boundary conductivity in these specimens obtained by two-step sintering method was compared with normal sintered specimens. It is concluded that the reduced conductivity observed in the two-step sintering specimen is attributable to the microstructure changes obtained by increased of grain boundary resistivity.     

Preparation and electrical behavior study of the ceramic interconnect La0.7Ca0.3CrO3−δ with CeO2-based electrolyte Ce0.8Sm0.2O1.9

Based on the conventional interconnect La_0.7Ca_0.3CrO_3−δ, a novel ceramic interconnect for intermediate temperature solid oxide fuel cells was developed. In the air, the electrical conductivities of La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 at 600, 700 and 800 °C were 96.7, 146.3 and 687.8 S cm⁻¹, respectively, which increased significantly as compared with La_0.7Ca_0.3CrO_3−δ under the same conditions. Similarly, in pure hydrogen, La_0.7Ca_0.3CrO_3−δ + 3%Ce_0.8Sm_0.2O_1.9 possessed the maximal electrical conductivities which were 4.2, 5.3 and 7.1 S cm⁻¹, respectively at 600, 700 and 800 °C. The crystal structures of La_0.7Ca_0.3CrO_3−δ, La_0.7Ca_0.3CrO_3−δ + 5%Ce_0.8Sm_0.2O_1.9 and La_0.7Ca_0.3CrO_3−δ + 10%Ce_0.8Sm_0.2O_1.9 were single phase with hexagonal symmetry, cubic phase plus some doped ceria impurity and orthorhombic phase plus some doped ceria impurity, respectively. The difference between the crystal structures may account for the difference between the electrical conductivities. The electrical conductivities and sinterability of La_0.7Ca_0.3CrO_3−δ were increased by introducing Ce_0.8Sm_0.2O_1.9, whereas the other properties were not influenced.     

Preparation and performance of intermediate-temperature fuel cells based on Gd-doped ceria electrolytes with different compositions

In this work, the effect of two frequently used Gd_xCe_1−xO_2−x/2 electrolytes (x = 0.1 and x = 0.2) on the performance of fuel cells operated at intermediate temperature was studied. The microstructures of ceria electrolytes responsible for the performance were discussed. Electrochemical measurements of as-prepared cells showed that the cell with Gd_0.2Ce_0.8O_1.9 electrolyte had a better performance than that of Gd_0.1Ce_0.9O_1.95. It can be concluded that the increase of grain boundary conductivity of Gd_0.2Ce_0.8O_1.9 electrolyte contributes to its better cell performance.     

Effects of aluminium on the electrical and mechanical properties of PTCR BaTiO3 ceramics as a function of the sintering temperature

The effect of AI additions on the electrical behaviour of positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramic sintered in air at temperatures ranging between 1220 and 1400° C have been investigated. Two batches of material, both showing a PTCR effect, were prepared identically except that additions of AI₂O₃ (0.55 mol %) were made to one of them. It has been confirmed that the presence of aluminium results in an increase in the temperature at which the maximum resistivity, _max, occurs as well as reducing the sintering temperature, in the presence of silicon, to 1240° C. Additionally, direct comparisons between the two materials have demonstrated that such additions result in an increase of 100% in the minimum resistivity, _min, at sintering temperatures beyond 1280° C. A similar increase in _max for sintering temperatures below 1360° C and a five-fold reduction in the ratio of _max/_min in samples sintered above 1320° C have also been attributed to the presence of aluminium. It was further found that aluminium increases the average grain size by 30% and promotes the formation of a liquid phase.