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.     

Diffusion of Sputtered Vanadium, Nickel, and Silver in Lead Magnesium Niobate Ceramic

The sputtering of nickel-vanadium (Ni-V) and silver (Ag) to form thin films on lead magnesium niobate (PMN) ceramic substrates has been studied. An empirical equation was derived which correlated the average Ni-V film thickness with three sputtering variables: input power, argon pressure, and sputtering time. The films were dense, smooth, and feature less. Upon annealing at 800°C in argon, both Ni and V diffused into PMN; but reverse diffusion from the ceramic into the metal film was not observed. Vanadium diffused at a faster rate than Ni, leaving pores in the film. The diffusion coefficient of Ni in PMN was determined to be 4.1 × 10⁻¹² cm²/s, whereas the diffusion coefficient of V was determined to be between 3.5 × 10⁻⁹ and 2.9 × 10⁻¹¹ cm²/s at 800°C. Sputtered Ag was also dense, smooth, and featureless. However, the diffusion of Ag at 800°C in PMN was much slower than that of either Ni or V.     

Preparation of CuAlO2 Films by Wet Chemical Synthesis

CuAlO₂ is a delafossite-type compound and is a known p -type semiconductor. Transparent CuAlO₂ thin films were prepared using a sol–gel technique. The films with an Al/Cu atomic ratio of 1.0 consisted of CuAlO₂, Cu₂O, and CuO after heat treatment at 800°–900°C in nitrogen gas. The electrical resistivity of the film heated at 800°C was 250 Ω·cm.     

Oxidation Resistance Coating of LSM and LSCF on SOFC Metallic Interconnects by the Aerosol Deposition Process

Conductive La_0.8Sr_0.2MnO₃ (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by the aerosol deposition method, for use as an oxidation resistance-coating layer in the metallic interconnector of a solid oxide fuel cell. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 100 h. The surface of the bare SS after annealing at 800°C for 100 h was covered with Cr₂O₃ and Fe₃O₄ oxide scales, and the electrical conductivity was sharply decreased. However, the LSM- and LSCF-coated SSs showed a surface microstructure with almost no oxidation and maintained good electrical conductivity after annealing at 800°C for 100 h. The area-specific resistance (ASR) of LSM- and LSCF-coated alloys after 100 h of oxidation at 800°C was 20.6 and 11.7 mΩ·cm², respectively.     

Superior Perovskite Oxide-Ion Conductor; Strontium- and Magnesium-Doped LaGaO3: III, Performance Tests of Single Ceramic Fuel Cells

The performance of La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) as an optimized electrolyte of a solid oxide fuel cell was tested on single cells having a 500-µm-thick electrolyte membrane. The reactivity of NiO and LSGM suggested use of an interlayer to prevent formation of LaNiO₃. The interlayer Sm-CeO₂ was selected and sandwiched between the electrolyte and anode. Comparison of Sm-CeO₂/Sm-CeO₂+ Ni and Sm-CeO₂+ Ni as anodes showed that Sm-CeO₂/Sm-CeO₂+ Ni gave an exchange current density 4 times higher than that of Sm-CeO₂+ Ni. The peak power density of the interlayered cell is 100 mW higher than that of the standard cell without the interlayer. This improvement is due to a significant reduction of the anode overpotential; the overpotential of the cathode La_0.6Sr_0.4CoO_3-delta (LSCo) remained unchanged. Comparison of the peak power density in this study and with that of a previous study, also with a 500-µm-thick electrolyte, indicates a factor of 2 improvement, i.e., from 270 mW/cm² to 550 mW/cm² at 800°C. The excellent cell performance showed that an LSGM-based thick membrane SOFC operating at temperatures 600° < T _op < 800°C is a realistic goal.     

Pressure-Assisted Combustion Synthesis of Dense Layered Ti3AlC2 and its Mechanical Properties

A near-single-phase Ti₃AlC₂ ternary carbide was synthesized from 3Ti–1.1Al–1.8C powder blend, both by the wave propagation and thermal explosion (TE) modes of self-propagating high-temperature synthesis. The application of a moderate (28 MPa) pressure immediately after TE at 800°C (reactive forging) yielded a 95% dense material containing, in addition to Ti₃AlC₂, an appreciable amount of TiC_{1− x} . By adjusting the starting composition, a 99% dense material containing up to 90 vol.% Ti₃AlC₂ was obtained. The material had a fine-layered microstructure with Ti₃AlC₂ grain size not exceeding 10 μm. The samples were readily machinable and had a high compressive strength of ∼800 MPa up to 700°C.     

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.     

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.     

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.     

Spin Coating of a Ca(Ti,Fe)O3 Dense Film on a Porous Substrate for Electrochemical Permeation of Oxygen

A dense Ca(Ti_0.75Fe_0.25)O_3−α (Ca(Ti,Fe)O₃) film, which is a mixed conductor of oxide ions and electrons/holes, was prepared on a porous CaTiO₃ substrate by a spin-coating method. The calcined Ca(Ti,Fe)O₃ powder with 2–3 μm grain sizes was mixed with a dispersant in ethanol to form the slurry for spin coating. A uniform Ca(Ti,Fe)O₃ green film was obtained at 1000–1500 rpm on the rotating porous CaTiO₃ substrate, which had an average pore diameter of a few micrometers. The optimum sintering conditions for the spin-coated films were a soaking temperature of 1235°C and a holding time of 2 h, in air. A dense, sintered Ca(Ti,Fe)O₃ film }20–50 μm thick was prepared by repeating the coatingsintering process. The gas-tight film prepared on the porous substrate exhibited higher electrochemical permeation of oxygen at an operating temperature of 1000°C compared with that of thicker, sintered Ca(Ti,Fe)O₃ disks.     

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.     

Oxidation Behavior of Ferritic Steel Alloy Coated with Highly Dense Conducting Ceramics by Aerosol Deposition

Conducting La_0.8Sr_0.2MnO₃ (LSM) ceramic layers with a thickness of ∼10 μm were deposited on ferritic stainless steel (SS) by aerosol deposition for use as an oxidation resistance coating layer in the metallic interconnects of solid oxide fuel cell. The microstructural evolution and electrical properties of the LSM-coated SS were observed. The coated layers were fairly dense without pores or cracks, and maintained good adhesion even after oxidation at 800°C for 1000 h in air atmosphere. The surface of the bare SS after heat treatment at 800°C for 1000 h was covered with Cr-containing oxide scales, and the electrical conductivity was sharply decreased. However, the LSM-coated SS alloy showed a surface microstructure with almost no chromic oxide formation and maintained good electrical conductivity after the heat treatment. Close observation of the interface between LSM and SS indicated the presence of ∼500-nm-thick Cr₂O₃ and MnCr₂O₄ spinel phase, which may have caused the long-time deterioration of the interconnector performance. The area-specific resistance of the LSM-coated alloy after heat treatment at 800°C for 1000 h was 10.4 mΩ·cm².     

Microstructure Characterization in Superplastically Deformed Silicon Nitride

Fully dense silicon nitride (Si₃N₄) has been produced by hot isostatically pressing α-Si₃N₄ powder at 1740°C under 160 MPa, with 0.5 wt% Y₂O₃ and 0.5 wt% Al₂O₃ as sintering additives. The sintered material was composed of very fine (0.5 μm) and equiaxed grains, as required for superplasticity. Before deformation, a very small amount of intergranular glassy silicate-based film was detected by transmission electron microscopy at the two-grain and triple-point junctions. Compression tests with a strain value of up to }0.5 were conducted in nitrogen in the temperature range of 1600°–1700°C. The observation of a shear-thickening phenomenon and the presence of a transition from a mild to a strong strain hardening at 20 MPa were attributed to the occurrence of rigid contacts between the grains. The angular distribution of the observed strain whorls was used to evidence the increase of rigid contacts between the grains, under the local expulsion of the wetting liquid film, with increases in compressive stress.     

Low-Temperature Densification of Zirconium Diboride Ceramics by Reactive Hot Pressing

Zirconium diboride–silicon carbide ceramics with relative densities in excess of 95% were produced by reactive hot pressing (RHP) at temperatures as low as 1650°C. The ZrB₂ matrix was formed by reacting elemental zirconium and boron. Attrition milling of the starting powders produced nanosized (<100 nm) Zr particulates that reacted with B below 600°C. The reaction resulted in the formation of nanoscale ZrB₂ crystallites that could be densified more than 250°C below the temperatures required for conventional ZrB₂ powder. Because of the low-temperature densification, the resulting ZrB₂ grain sizes were as small as 0.5±0.30 μm for specimens densified at 1650°C and 1.5±1.2 μm for specimens densified at 1800°C. Vickers hardness, elastic modulus, and flexure strength of fully dense materials produced by RHP were 27, 510, and 800 MPa, respectively.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Preparation of Submicrometer A12O3 Powder by Gas-Phase Oxidation of Tris (acetylacetonato) aluminum (111)

Fine A1₂O₃ powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al₂O₃ at 1000° and 1200°C, and δ-Al₂O₃ at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas.     

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).     

Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.