Solubility and Transport of NiO Cathodes in Molten Carbonate Fuel Cells

Equilibrium NiO solubility measurements were made in Li₂CO₃/K₂CO₃ mixtures as a function of temperature, ambient gas environment, and salt composition for gases containing 3.1% H₂O. The equilibrium solubility was found to increase with increasing temperature, CO₂ partial pressure, and cation fraction Li⁺ and to decrease slightly with increasing O₂ partial pressure. These results were coupled with theoretical predictions of (1) O₂, H₂, and CO₂ activities and (2) compositional gradients which develop across the electrolyte structure of an operating carbonate fuel to explain the two distinct regions of nickel precipitation across the fuel cell. Precipitation occurs near the cathode because NiO solubility decreases commensurate with electrolyte compositional gradients, whereas the second region deeper within the electrolyte matrix results from a decreased O₂ activity.     

Oxygen Exchange and Diffusion in Calcia-Stabilized Zirconia

Oxygen diffusion was measured by the sectioning technique on single-crystal and polycrystalline specimens of Zr_0.858Ca_0.142CO_1.858. The diffusion coefficient, which may be represented by the equation
D = 0.01818^+0.068_-0.015 exp - (31, 200 × 4300)/ RT cm²/sec
agrees with that calculated from the electrical conductivity and an oxygen transport number of 1. The oxygen surface exchange coefficient may be represented by
α= 0.078^+0.443_-0.066 exp - (22,800 × 4400/ RT cm/sec
The quantitative influence of ignoring the exchange rate on the apparent diffusion coefficient is discussed.     

Oxidation of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite

The recession rates for 10^-6-m-thick C interfaces in chemical vapor infiltrated SiC reinforced with Nicalon fibers were calculated from thermogravimetric data, assuming all of the mass losses were due to C oxidation, and found to be consistent with the measured recession distances of the C interface, which were surprisingly uniform across the composite. Agreement between the two approaches for a microstructurally complex material indicates thermogravimetric analysis could be an important tool for understanding environmental effects in ceramic composites with reactive interfaces. Mass losses were linear within the first 1.08× 10⁴ s to 2.16× 10⁴ s between 1073 and 1373 K and between 3.1× 10² and 2.5× 10³ Pa O₂. Calculated reaction orders with respect to O₂ were between 0.5 and 1.0 at 1373 K, and activation energies were about 50 kJmol^-1. Analysis of the kinetic data and estimates of gas boundary layer thickness suggest the mechanism for the C-interface oxidation involved reaction control, but the possibility of diffusion control for some conditions cannot be ruled out.     

Defect Chemistry and Electrical Properties of Thallium Oxide Single Crystals

Ehdectrical resistivity and Hall voltage were measured between 4.2 and 300 K on T1₂O₃ crystals annealehd at 550°C for 24 h under oxygen pressures of 2×10⁴ to 10⁷ Pa. The carrier concentration varied from 7.97×10²⁰ to 5.08×10²⁰ cm⁻³, the low-temperature Hall mobility from 131 to 189 cm²/V.s, and the Fermi level from 7.1×10⁴ to 5.05×10⁴ J/mol above the bottom of the conduction band as P ₀₂ was increased from 2×10⁴ to 10⁷ Pa. The dependence of Fermi level on carrier concentration and P _0l was consistent with a parabolic density-of-states function describing the conduction band. Over the entire region of oxygen pressure investigated, Fermi-Dirac statistics were required to describe the dependence of carrier concentration on P ₀₂.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Depth Profile Measurement by Secondary Ion Mass Spectrometry for Determining the Tracer Diffusivity of Oxygen in Rutile

Secondary ion mass spectrumètry has been applied for measuring the tracer diffusivity of oxygen in the c direction of single-crystal rutile for a temperature range of 1150 to 1450 K at 6000 Pa pressure of oxygen gas. Specimens diffusion-annealed in oxygen gas containing ¹⁸O were subsequently continuously sputtered and analyzed for ¹⁶O and ¹⁸O. The tracer diffusivity was determined from the depth profile of ¹⁸O, taking into account a surface exchange reaction of oxygen. The tracer diffusivity in Cr₂O³-doped rutile was 3 to 8 times larger than that in pure rutile. For pure rutile, the diffusivity is expressed by D (m²/s)=3.4×10⁻⁷, exp [-251(kJ/mol)/ RT ], and for 0.08 mol% Cr₂O³-doped rutile, by D (m²/s)= 2.0×10⁻⁸ exp[-204(KJ/mol)/ RT ]. The Cr₂O³ doping had a catalytic effect on the rate constant of the surface exchange reaction on the c surface. The rate constant is represented, for pure rutile, by K (m/s)= 2.4×10⁻¹ exp[-246(KJ/mol)/ RT ], and for 0.08 mol% Cr₂O³-doped rutile, k (m/s)= 3.5×10⁻⁵ exp[-131(KJ/mol)/ RT ].     

Oxygen Tracer Diffusion in Lanthanum-Doped Single-Crystal Strontium Titanate

Strontium titanate (SrTiO₃) is known as a good high-temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, ¹⁸O diffusion has been investigated in lanthanum-doped SrTiO₃, single crystals in the temperature range 700° to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 × 10⁻¹⁵ and 1 × 10⁻¹³ cm²/s have been calculated from the SIMS results. Low surface enrichment of ¹⁸O compared to the ¹⁸O concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.     

High-Thermal-Expansion Polycrystalline Leucite Ceramic

A high-thermal-expansion ceramic consisting of leucite crystals was prepared by sintering leucite powder. The densification was promoted by adding Li₂CO₃ or Na₂CO₃. The leucite ceramic obtained had a thermal expansion coefficient of about 2.4 × 10⁻⁵/°C from room temperature to 600°C. Some of the Li⁺ and Na⁺ ions were incorporated into the leucite crystal lattice to form solid solutions, and the characteristic tetragonal-cubic inversion point shifted from ca. 600° to 650°C.     

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.     

Thermodynamic Study of the Cu-Sr-O System

The Gibbs energies of formation of the inter-oxide compounds in the Cu-Sr-O system have been obtained from solid-state electrochemical measurements in the temperature range 900 to 1300 K. Cells employing yttria-stabilized zirconia or single-crystal calcium fluoride as electrolyte were used in studies of SrCu₂O₂, Sr₂CuO₃, SrCuO₂, and "Sr₃Cu₅O_{8+ z} ." The oxygen potential vs stoichiometry dependence was studied, particularly for this last compound, by thermogravimetry at a few selected temperatures in the oxygen partial pressure interval 1 × 10¹ to 1 × 10⁵ Pa. Based on the results of emf measurement, thermogravimetric study, and X-ray powder diffraction, phase relations in this system were calculated.     

Synthesis of Nanoparticulate Silica Composite Membranes by the Pressurized Sol–Gel Technique

A crack-free silica composite membrane has been synthesized from a nanoparticulate silica sol (particle diameter <10 nm) by a pressurized sol–gel coating technique developed in this study. The microporous silica layers with an estimated pore radius of 0.78 nm were deposited inside the pores (average pore size of 0.1 μm) of slip cast a-alumina support tubes. The microstructure of the coated layer was controlled by adjusting sol properties and pressurizing conditions. The room-temperature intrinsic permeability of N₂ through the silica membrane layer after heat treatment at 200°C is about 4.9 × 10⁻¹² mol·m/m²·s· Pa, and the mechanism of gas transport is Knudsen flow. The thermal stability of the silica composite membrane is excellent up to 500°C.     

Thermal Expansion of the Hexagonal (6H) Polytype of Silicon Carbide

The thermal expansion of the hexagonal (6H) polytype of α-SiC was measured from 20° to 1000°C by the X-ray diffraction technique. The principal axial coefficients of thermal expansion were determined and can be expressed for that temperature range by second-order polynomials: α¹¹= 3.27 × 10^–6+ 3.25 × 10^–9T – 1.36 × 10^–12 T ² (1/°C), and ş₃₃= 3.18 × 10^–6+ 2.48 × 10^–9 T – 8.51 × 10^–13 T ² (1/°C). The σ₁₁ is larger than α₃₃ over the entire temperature range while the thermal expansion anisotropy, the δş value, increases continuously with increasing temperature from about 0.1 × 10^–6/°C at room temperature to 0.4 × 10^–6/°C at 1000°C. The thermal expansion and thermal expansion anisotropy are compared with previously published results for the (6H) polytype and are discussed relative to the structure.     

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.     

Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Open-circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO₂. The open-circuit emf data were used to delineate the low- p o₂ ionic domain boundary for "pure" ThO₂, which is presented as a log P_θ line on a log Po₂-1/ T diagram. In addition the ionic conductivity, σ_ion, and the high-Po₂ log P_θ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po₂^I/4 dependence of σ_p, the p-type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 10³/4.575T, log σ_ion= 1.9−44.3×10³/4.575T, and log P_θ=−1.0−31.4 × 10³/4.575T; for the second batch, log Pθ=11.2−219.7 × 10³/4.575T, log σ_ion= 1.7−41.6 × 10³/4.575T, and log Pθ=0.6−40.4 × 10³/4.575T. The oxygen permeability of ThO₂ tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.     

Volatilization of 137Cs and 106Ru from Borosilicate Glass Containing Actual High-Level Waste

Volatility of ¹³⁷ Cs and ¹⁰⁶Ru from borosilicate glass containing actual high-level waste was measured in an almost closed stainless-steel canister. The temperature dependence of the volatility of ¹³⁷Cs was close to that obtained in our previous study using ¹³⁴Cs. The volatility of ¹⁰⁶Ru was about one-fifth that of ¹³⁷Cs at 600° and 800°C. The air contamination by ¹³⁷Cs and ¹⁰⁶Ru in the canister at 400°C was estimated at 1.8 × 10² and 2 × 10 Bq/cm³, respectively, when it was assumed that the glass contained a realistic amount of ¹³⁷Cs and ¹⁰⁶Ru expected in commercial waste glass. These results are useful for predicting safety in a storage facility under operation.     

Linear and Nonlinear Optical Properties of TeO2 Glass

A thin plate of TeO₂ glass of 5.0 × 4.0 × 0.25 mm³ size, which was large enough for various optical measurements, was obtained by a rapid quenching method. The linear refractive index was measured as a function of wavelength from 486.1 to 1000 nm. The refractive index at 486.1 nm was as high as 2.239. The optical energy band gap was estimated as 3.37 eV from the optical absorption spectrum. The third-order nonlinear optical susceptibility, χ⁽³⁾, was determined by the third-harmonic generation (THG) method. The χ⁽³⁾ value was as high as 1.4 × 10⁻¹² esu, about 50 times as large as that of SiO₂ glass. The results are discussed based on Lines' model in which an influence of cationic empty d -orbitals on the nonlinear properties was taken into account.     

Concurrent Measurement of Volume, Grain-Boundary, and Surface Diffusion Coefficients in the System NiO-Al2O3

Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al₂O₃ system were measured concurrently by using a diffusion couple consisting of an A1₂O₃ bicrystal and an NiO single crystal. The A1₂O₃ bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H₂ atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D _s = 7.41×10^-2 exp (-35,200/ RT ), D _gb = 2.14×10^-1 exp (-63,100/ RT ) (tilt angle =30°), and D _v = 1.26×10⁴ exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.     

Sintering of Spherical Glass Powder under a Uniaxial Stress

The sintering of spherical borosilicate glass powder (particle size 5 to 10 μm) under a uniaxial stress was studied at 800°C. The experiments allowed the measurement of the kinetics of densification and creep, the viscosities for creep and bulk deformation, and the sintering stress which was found to increase with density. The data show excellent qualitative agreement with Scherer's theory of viscous sintering. In addition, the quantitative comparison between theory and experiment shows good agreement; the measured viscosity of the bulk glass was ∽1×10⁹ P (∽1×10⁸ Pa·s) compared to ∽3×10⁹ P (∽3 Pa·s) obtained by fitting the data with Scherer's theory.     

Deviation from Stoichiometry in Cr2O3 at High Oxygen Partial Pressures

The deviation from stoichiometry, δ, in Cr₂−δO₃ was measured by a tensivolumetric method in the high pO₂ range of ≊10⁴ to 10⁴ Pa at 1100°C. The value of δ, or chromium vacancy concentration, was≊9×10⁻⁵ mol/mol Cr₂O₃ in air for Cr₂O₃ with 99.999% purity. The chemical diffusion coefficient, DT, determined from equilibration data was ≊4.6× cm²·s⁻¹ at 1100°C for pO₂= 2.2 ×10¹ Pa. The self-diffusion coefficient of Cr ions was calculated from and δ and found to be≊1.6×10-17 cm²-s⁻¹, in good agreement with recently measured values.     

Effect of Microstructure on Pulse Electrical Strength of MgO

Pulse electrical strengths of hot-pressed MgO containing 0.3 wt% LiF were measured. Strengths as high as 5.9×10⁸ V/in. were obtained on samples 99.8% of theoretical density with 1-to 3-μm grain size. An increase of either porosity or grain size resulted in a monotonic decrease in strength. Single-crystal strengths of 4.0×10⁶ V/in. were obtained by polishing samples chemically. A simple recessed-electrode configuration eliminated corona effects.