A Novel Method for Fabrication of Y2O3-Stabilized ZrO2 Electrolyte Films

Gas-tight Y₂O₃-stabilized ZrO₂ (YSZ) films were prepared on NiO–YSZ and NiO–SDC (Sm_0.2Ce_0.8O_1.9) anode substrates by a novel method. A cell, Ni–YSZ/YSZ(10 μm)/LSM–YSZ, was tested with humidified hydrogen as fuel and ambient air as oxidant. The maximum power densities of 1.64, 1.40, 1.06, and 0.60 W/cm² were obtained at 850°, 800°, 750°, and 700°C, respectively. With methane as fuel, a cell of Ni–SDC/YSZ (12 μm)/LSM–YSZ exhibited the maximum power densities of 1.14, 0.82, 0.49, and 0.28 W/cm² at 850°, 800°, 750°, and 700°C, respectively. The impedance results showed that the performance of the cell was controlled by the electrode polarization rather than the resistance of YSZ electrolyte film.     

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.     

Densification and Cell Performance of Gadolinium-Doped Ceria (GDC) Electrolyte/NiO–GDC Anode Laminates

A thin film (60 μm thick) of a gadolinium-doped ceria (GDC) electrolyte was prepared by the doctor blade method. This film was laminated with freeze-dried 42 vol% NiO–58 vol% GDC mixed powder and pressed uniaxially or isostatically under a pressure of 294 MPa. This laminate was cosintered at 1100 °–1500 °C in air for 4–12 h. The laminate warped because of the difference in the shrinkage of the electrolyte and electrode during the sintering. A higher shrinkage was measured for the electrode at 1100 °–1200 °C and for the electrolyte at 1300 °–1500 °C. The increase of the thickness of anode was effective in decreasing the warp and in increasing the density of the laminated composite. The maximum electric power density with a SrRuO₃ cathode using 3 vol% H₂O-containing H₂ fuel was 100 mW/cm² at 600 °C and 380 mW/cm² at 800 °C, respectively, for the anode-supported GDC electrolyte with 30 μm thickness.     

Plasma-Sprayed YSZ/Ni–LSGM–LSCo Intermediate-Temperature Solid Oxide Fuel Cells

An intermediate-temperature solid oxide fuel cell based on YSZ/Ni anode, LSGM electrolyte, and lanthanum strontium cobaltite (LSCo) cathode coatings were sequentially deposited onto a porous Ni substrate by atmospheric plasma spraying (APS). The spray parameters for each coating are well selected. The sprayed YSZ/Ni anode having a novel nanostructure with advantageous triple phase boundaries after hydrogen reduction shows a good electrocatalytic activity for hydrogen oxidation reactions. Dense LSGM with a thickness of about 60 μm and a conductivity of about 0.053 S/cm at 800°C shows a good gas tightness and gives an open circuit voltage value >1 V. The sprayed LSCo cathode with a thickness of 10–20 μm and a porosity of about 25% keeps the right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions after plasma spraying and heat treatment at about 1000°C for 1 h. A maximum output power density of the sprayed cell achieved 365 mW/cm² at 800°C, 250 mW/cm² at 750°C, and 180 mW/cm² at 700°C. The results show that the use of APS cell allowed the reduction of the operating temperature to below 750°C.     

Processing of Composite Thin Film Solid Oxide Fuel Cell Structures

A solid oxide fuel cell (SOFC) structure is proposed in which a composite thin film cathode substrate supports a dense thin film electrolyte with a thickness of less than 1 μm. The cathode substrate has a graded porosity achieved through the partial sintering of a spin-coated CeO₂ colloidal suspension. The resulting surface has a pore size and surface roughness which allowed a fully dense ZrO₂:16%Y (YSZ) electrolyte to be spin-coated directly from a polymeric precursor without capillary forces removing the precursor from the surface of the porous substrate. Using this process, fuel cell structures were constructed with temperatures not exceeding 800°C. The porous CeO₂ interlayer should allow for decreased ohmic losses, as well as decreased reactions between the YSZ and the cathode substrate. In addition, the nanocrystalline grain sizes should allow for increased catalytic activity on the cathode. Calculated ohmic losses indicated the resistance of the CeO₂ interlayer limited the power of the structure, which was minimized by impregnating the porous layer with a mixed-conducting perovskite. The final structure shows significantly reduced ohmic losses as calculated at 400°C.     

Thermal Treatment of Titanate Derivatives Synthesized by Ion-Exchange Reaction

TiO₂ fibers were formed by thermal treatment of layered H₂Ti₄O₉ (hydrous titanium dioxide) and KHTi₄O₉ synthesized by ion-exchange reactions. The calcination of the former at 900° and 1050°C for 3 h yielded TiO₂ fibers with anatase and rutile phases, whose length and diameter were 15–20 and 2–5 μm and 10–15 and 3–5 μm, respectively. The thermal treatment of the latter at temperatures of 250° to 500°C yielded pure K₂Ti₈O₁₇, which tended to decompose to K₂Ti₆O₁₃ and TiO₂ at temperatures >600°C. At 1050°C, K₂Ti₆O₁₃ phase was formed with rutile TiO₂ fibers, whose length and diameter were 10–20 and 1–3 μm, respectively.     

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.     

Fabrication and Characterization of Anode-Supported Tubular Solid-Oxide Fuel Cells by Slip Casting and Dip Coating Techniques

High-performance anode-supported tubular solid-oxide fuel cells (SOFCs) have been successfully developed and fabricated using slip casting, dip coating, and impregnation techniques. The effect of a dispersant and solid loading on the viscosity of the NiO/Y₂O₃–ZrO₂ (NiO/YSZ) slurry is investigated in detail. The viscosity of the slurry was found to be minimum when the dispersant content was 0.6 wt% of NiO/YSZ. The effect of sintering temperature on the shrinkage and porosity of the anode tubes, densification of the electrolyte, and performance of the cell at different solid loadings is also investigated. A Ni/YSZ anode-supported tubular cell fabricated from the NiO/YSZ slurry with 65 wt% solid loading and sintered at 1380°C produced a peak power output of ∼491 and ∼376 mW/cm² at 800°C in wet H₂ and CH₄, respectively. With the impregnation of Ce_0.8Gd_0.2O₂ (GDC) nanoparticles, the peak power density increased to ∼1104 and ∼770 mW/cm² at 800°C in wet H₂ and CH₄, respectively. GDC impregnation considerably enhances the electrochemical performance of the cell and significantly reduces the ohmic and polarization resistances of thin solid electrolyte cells.     

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.     

Preparation of Yttria-Stabilized Zirconia Thin Films on Strontium-Doped LaMnO3 Cathode Substrates via Electrophoretic Deposition for Solid Oxide Fuel Cells

A yttria-stabilized zirconia (YSZ) thin film on an La_0.8Sr_0.2MnO₃ porous cathode substrate was prepared, using electrophoretic deposition (EPD) to fabricate a solid oxide fuel cell (SOFC). The electrical conductivity of an La_0.8Sr_0.2MnO₃ substrate is satisfactorily high at room temperature; therefore, YSZ powder could be deposited electrophoretically onto an La_0.8Sr_0.2MnO₃ substrate without any extra surface treatment, such as a metal coating. Successive repetition of EPD and sintering was required to obtain a film without gas leakage, because of the thermal expansion coefficient mismatch between the YSZ and the La_0.8Sr_0.2MnO₃ substrate. On the other hand, the electromotive force of the oxygen concentration in the cell that used YSZ film prepared via EPD increased and attained the theoretical value when the number of deposition and calcination cycles was increased. Six or more successive repetitions were required to obtain a YSZ film without gas leakage. A planar-type SOFC was fabricated, using nickel as the anode and YSZ film (∼10 μm thick) that had been deposited onto the La_0.8Sr_0.2MnO₃ substrate as the electrolyte and cathode. The cell exhibited an open circuit voltage of 1.0 V and a maximum power density of 1.5 W/cm². Thus, the EPD method could be used as a colloidal process to prepare YSZ thin-film electrolytes for SOFCs.     

Ceria–Yttria-Stabilized Zirconia Composite Ceramic Systems for Applications as Low-Temperature Electrolytes

Ceramic samples of Ce_{1− x} Gd _x O_{2− y} and yttria-stabilized zirconia (YSZ) were prepared using solid-state methods. Polished faces of disks of these materials were held in intimate contact in a reaction cell at temperatures ranging from 1000° to 1300°C for durations up to 72 h. XRD, SEM, and microprobe Raman techniques were used to analyze the resulting reactions and ion diffusion. No reaction was observed at 1000°C after 72 h between the 10-mol%- and 20-mol%-Gd-doped CeO₂ and the YSZ. However, at 1300°C, a mixing region 25 μ m wide occurred, resulting in a cubic phase, where Zr⁴⁺ ions diffused into the CeO₂.     

Space Charge-Mediated Ionic Transport in Yttria-Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria-stabilized zirconia (YSZ)–Al₂O₃ composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al₂O₃ in YSZ was minimal and nanosize Al₂O₃ of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al₂O₃ grains were strained.     

Dense Alumina–Zirconia Coatings Using the Solution Precursor Plasma Spray Process

For the first time, dense coatings have been made by the solution precursor plasma spray (SPPS) process. The conditions are described for the deposition of dense Al₂O₃–40 wt% 7YSZ (yttria-stabilized zirconia) coatings; the coatings are characterized and their thermal stability is evaluated. X-ray diffraction analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 72 and 56 nm, respectively. The as-sprayed coating has a 95.6% density and consists of ultrafine splats (1–5 μm) and unmelted spherical particles (<0.5 μm). The lamellar structure, typical of conventional plasma-sprayed coatings, is absent at the same scale in the SPPS coating. The formation of a dense Al₂O₃–40 wt% 7YSZ coating is favored by the lower melting point of the eutectic composition, and resultant superheating of the molten particles. Phase and microstructural thermal stabilities were investigated by heat treatment of the as-sprayed coating at temperatures of 1000°–1500°C. No phase transformation occurs, and the grain size is still in the nanometer range after the 1500°C exposure for 2 h. The coating hardness increases from 11.8 GPa in the as-coated condition to 15.8 GPa following 1500°C exposure due to a decrease in coating porosity.     

Synthesis of Magnesium Aluminate Spinel Platelets from α-Alumina Platelet and Magnesium Sulfate Precursors

Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al₂O₃ and 1–8 μm (average 3 μm) MgSO₄ heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al₂O₃ platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al₂O₃ platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina.     

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.     

High Electrical Conductivity and High Fracture Strength of Sc2O3-Doped Zirconia Ceramics with Submicrometer Grains

The microstructure, crystal phase, electrical conductivity, and mechanical strength of less than 7-mol%-Sc₂O₃-doped zirconia ceramics fabricated by comparatively low-temperature sintering at 1200–1300°C for 1 h were investigated. Zirconia ceramics having a uniform microstructure (grain size < 0.5 μm) stabilized with 6 mol% Sc₂O₃ showed high electrical conductivity (0.15 S/cm at 1000°C) and high fracture strength (660 MPa). With the increase of Sc₂O₃ content from 3.5 to 7 mol%, the grain size, fracture strength, and electrical conductivity at 1000°C changed from 0.2 to 0.5 μm, 970 to 440 MPa, and 0.07 to >0.2 S/cm, respectively. Sc₂O₃-doped zirconia polycrystals with high fracture strength and high electrical conductivity are promising candidates for the electrolyte material of solid oxide fuel cells.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Creep Deformation in an Alumina–Silicon Carbide Composite Produced via a Directed Metal Oxidation Process

Flexural creep studies were conducted in a commercially available alumina matrix composite reinforced with SiC particulates (SiC_p) and aluminum metal at temperatures from 1200° to 1300°C under selected stress levels in air. The alumina composite (5 to 10 μm alumina grain size) containing 48 vol% SiC particulates and 13 vol% aluminum alloy was fabricated via a directed metal oxidation process (DIMOX(tm))† and had an external 15 μm oxide coating. Creep results indicated that the DIMOX Al₂O₃–SiC_p composite exhibited creep rates that were comparable to alumina composites reinforced with 10 vol% (8 (μm grain size) and 50 vol% (1.5 μm grain size) SiC whiskers under the employed test conditions. The DIMOX Al₂O₃–SiC_p composite exhibited a stress exponent of 2 at 1200°C and a higher exponent value (2.6) at ≥ 1260°C, which is associated with the enhanced creep cavitation. The creep mechanism in the DIMOX alumina composite was attributed to grain boundary sliding accommodated by diffusional processes. Creep damage observed in the DIMOX Al₂O₃-SiC_p composite resulted from the cavitation at alumina two-grain facets and multiple-grain junctions where aluminum alloy was present.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.