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.     

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.     

Reactive Ceria Nanopowders via Carbonate Precipitation

Nanocrystalline CeO₂ powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce³⁺ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO₃·2.5H₂O, decomposes to CeO₂ at temperatures ≥300°C. The CeO₂ powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.     

Annealing of Irradiation-Induced Thermal Conductivity Changes in ThO2-1.3 wt% UO2

The thermal conductivities of sintered pellets of ThO₂-1.3 wt% U0₂ were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 10¹⁴ to 4.7 × lo^L7 fissions/cm³. Each fission fragment damaged a region of 2.2 × 10^-16 cm³ with the reduction in conductivity saturating by about 10¹⁷ fissions/cm³. Samples having exposures from 10¹⁵ to 10¹⁶ fissions/cm³ were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.     

Modeling Electrophoretic Deposition on Porous Non-Conducting Substrates Using Statistical Design of Experiments

Statistical design of experiments was used to model electrophoretic deposition of yittria-stabilized zirconia (YSZ) particles on porous, non-conducting NiO–YSZ substrates. A 2³–full-factorial matrix with three repetitions of the centerpoint was augmented with six axial runs and two additional centerpoints to form an inscribed central composite design. Fixed ranges of substrate firing temperature (1100°–1300°C), deposition voltage (50–300 V), and deposition time (1–5 min) were used as the independent design variables to model responses of YSZ deposition thickness, area-specific interfacial resistance (ASR), and power density. Regression equations were determined, which were used to optimize deposition parameters based on the desired responses of low interfacial polarization resistance and high-power density. Low substrate firing temperature (1100°C) combined with a low voltage (50 V) and minimal deposition time (1 min) resulted in a 6 μm-thick YSZ film, a power density of 628 mW/cm², and an ASR of 0.21 Ω·cm². Increasing the substrate firing temperature, voltage, and time to 1174°C, 215 V, and 3 minutes, respectively, reduced the ASR to 0.19 Ω·cm², increased YSZ film thickness to 25 μm, but had only a negligible effect on power density (600 mW/cm²).     

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.     

Wetting of TaC by Liquid Cu and Liquid Ag

Wetting of TaC^0.97±0.01 by liquid Cu and liquid Ag was studied by the sessile drop method. The cosine of the contact angle increased linearly with increasing temperature in both systems. The critical surface energy for spreading was 1098 ergs/cm² at 1605°C for Cu and 688 ergs/cm² at 2160°C for Ag. The work of adhesion, which was 1759 ergs/cm² at the melting point for Cu and 321 ergs/cm² at the melting point for Ag, increased parabolically with increasing temperature in both systems. The surface energy of TaC_0.97±0.01 was estimated to be 1804±706 ergs/cm².     

Wetting of Ceramic Oxides by Molten Metals Under Ultrahigh Vacuum

Contact angles, surface free energies, and work of adhesion were determined by a sessile drop technique for the wetting of MgO, Al₂O₃, and SiO₂ by In, Ga, and Sn at 10^-10 torr. The surface free energies of In, Ga, and Sn were 540, 632, and 573 ergs/cm² (±5%), respectively, at their melting points. Works of adhesion and equilibrium contact angles for wetting of MgO by In are 172 ergs/cm² and 133° by Ga, 356 ergs/cm² and 116° by Sn, 278 ergs/cm² and 121°. For wetting of Al₂O₃ by In, they are 237 ergs/cm² and 124° by Ga, 226 ergs/cm² and 130° by Sn, 257 ergs/cm² and 123°. For wetting of SiO₂ by In, they are 208 ergs/cm² and 128° by Ga, 260 ergs/cm² and 126° by Sn, 252 ergs/cm² and 124°.     

Sodium Diffusion in SiO2 Glass

Diffusion of the radioactive tracer ²²Na in a commercial SiO₂ glass was investigated from 170° to 1000°C. The temperature dependence curve had discontinuities at about 573° and 250°C. The resulting Arrhenius equations are D = 3.44 × 10² exp(-21.1 kcal/RT) cm²/sec between 1000° and 573°C, D = 0.398 exp(-25.8 kcal/RT) cm²/sec between 573° and 250°C, and D = 2.13 exp(-28.3 kcal/RT) cm²/sec between 250° and 170°C. The two anomalies are discussed in terms of "quartz-like" and "cristobalite-like" precrystalline elements in the structure of the glass. Comparison of the Na diffusion in SiO, glass with that in soda-silica and soda-lime-silica glasses shows that SiO₂ glass occupies a boundary position with respect to these systems. A possible diffusion mechanism is discussed.     

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.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Thermal Diffusivity of Be4B, Be2B, and BeB6

The thermal diffusivities of polycrystalline Be₄B, Be₂B, and BeB₆ were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be₄B, Be₂B, and BeB₆ varied from 0.13 to 0.072 to 0.031 cm²/s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm²/s at 1000°C. Heat transport in BeB₆ is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be₄B and Be₂B. Analytical expressions for the thermal diffusivities (α) of Be₄B and Be₂B at 200° to 1000°C and of BeB₆ at 25° to 1500°C are: α(Be₄B)=1/(5.83+9.05×10 3 T ), α(Be₂B)=1/(10.92+1.40×10 2 T ), and α(BeB₆)=5.60×10 4+5.72/ T +77.3/T²-4.09×10⁴/T³ cm²/s.     

High-Temperature Hall Measurements on Cerium Dioxide Thin Films

Simultaneous Hall and conductivity measurements have been performed on sputtered polycrystalline thin films and on bulk ceramic specimens of nearly stoichiometric CeO₂ in the temperature range between 900° and 1040°C. The measurements have been performed in air using low-frequency alternating current. In the case of the bulk ceramic specimens, an upper limit for the carrier mobility of ≤0.2 cm²/(V·s) has been obtained, which is in accordance with data from the literature for bulk samples. The conductivity of the thin films (l/1Ω·m) at 1000°C) is in accordance with data from the literature for bulk ceramics. The carrier density derived from the Hall measurements (3 × 10¹⁶/cm³ at 1000°C) increases with increasing temperature, whereas the Hall mobility (4 cm²(V·s) at 1000°C) decreases with increasing temperature. These values differ from literature data for bulk ceramic specimens. The difference may be duelo the small grain diameters (∼200 nm) in the 1-μm-thick thin films.     

Isothermal Oxidation Behavior of Mo-Si-B Intermetallics at 1450°C

Five Mo-Si-B multiphase intermetallic compositions were synthesized and oxidized isothermally at 1450°C in flowing air. Average mass change rates were strongly dependent on sample composition, particularly boron content. An Mo₅Si₃ matrix material containing 1.6 wt% boron exhibited parabolic mass gain with a rate of 5.3 × 10^-4 mg²(cm⁴.h), while a similar material with 0.14 wt% boron oxidized rapidly in a linear manner at a rate of -3.3 mg/(cm².h). Oxidation rates of the Mo-Si-B intermetallics were compared to that of MoSi₂ oxidized at 1450°C under identical conditions.     

Slip-Cast Ce0.8Sm0.2O1.9 Cone-Shaped SOFC

Cone-shaped Sm-doped CeO₂ (Ce_0.8Sm_0.2O_1.9, SDC) electrolyte cylinders have been fabricated using the slip-casting technique. A single solid oxide fuel cell has been prepared by applying a Sm_0.5Sr_0.5CoO₃ cathode on the outside of the cylinders and a NiO–SDC (7:3 wt%) anode on the inside. The open circuit voltage of the cell was 0.93 V at 400°C, and a maximum power density of about 300 mW/cm² at 700°C was obtained with humidified hydrogen (3% H₂O) as the fuel and ambient air as the oxidant. Impedance results showed that the performance of the cell was mainly influenced by the ohmic resistance of the electrolyte.     

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.     

Solid-State Storage of Radioactive Krypton in a Silica-Based Matrix

A solid silica-based matrix containing 30 cm³ of Kr (STP)/cm³ of glass was prepared by sintering 96% SiO₂ with 28% porosity under 140 MPa krypton pressure. The glass was heated to 850° or 900°C and held at temperature until the glass density was ∼2 g/cm³. At 420°C, only 0.7% of the krypton would be released after one half-life of ⁸⁵Kr (10.7 years). At T>600°C, release of krypton is accompanied by crack development, comminution, and glass softening. Advantages and disadvantages of this technique for radioactive gas storage and diffusion data are presented.     

Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth

Textured Sr_0.53Ba_0.47Nb₂O₆ ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr₂Nb₅O₁₅ template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid-state-synthesized SrNb₂O₆ and BaNb₂O₆ powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm², saturation polarizations of 21 μC/cm² (60%–85% of the single-crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single-crystal value), and room-temperature pyroelectric coefficients of 2.9 × 10⁻²μC·(cm²·°C)⁻¹ (52% of the single-crystal value). These results show that TGG is a viable option for accessing single-crystal properties in polycrystalline ceramics.     

Microstructure and Grain-Boundary Effect on Electrical Properties of Gadolinium-Doped Ceria

The microstructural evolution and grain-boundary influence on electrical properties of Ce_0.90Gd_0.10O_1.95 were studied. The nanoscale powders synthesized from a semibatch reactor exhibited 50% green density and 92% sintering density at 1200°C (∼200°C lower than previous studies). Impedance spectra as a function of temperature and grain size were analyzed. The Ce_0.90Gd_0.10O_1.95 with finest grain size possessed highest overall grain-boundary resistance; this contribution was eliminated at temperatures >600°C, regardless of grain size. The grain conductivity was independent of grain size and was dependent on temperature with two distinct regimes, indicative of the presence of Gd'_Ce− V _o^∘∘ complexes that dissociated at a critical temperature of ∼580°C. The activation energy for complex dissociation was ∼0.1 eV; the value for the grain-boundary was ∼1.2eV, which was size independent.     

Durability and Properties in Alkali Aluminofluorophosphate Glasses

Glass-forming and properties of glasses in the systems AI-Li-Na-K-P-O-F are presented. Useful durability, which is defined as a weight less in deionized water at 95°C of <1 mg/cm².h, limits the range of properties available. These boundaries are represented by T_gs of 305°±25°C, density of 2.63±0.02 g/cm³, and refractive indices of 1.46±0.01.