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.     

Grain Boundary Diffusion of Magnesium in Zirconia

This paper reports the transport kinetics of Mg in cubic yttria-stabilized zirconia (containing 10% mol of Y₂O₃ (10YSZ)) involving the bulk and the grain boundary diffusion coefficients. The diffusion-controlled concentration profiles of Mg were determined using secondary ion mass spectrometry (SIMS) in the range 1073–1273 K. The determined bulk diffusion coefficient and the grain boundary diffusion product may be expressed as the following functions of temperature, respectively: D = 5.7 exp[(−390 kJ/mol)/ RT ] cm²·s⁻¹ and D 'αδ= 3.2 × 10⁻¹⁵ exp[(−121 kJ/mol)/ RT ] cm³·s⁻¹, where α is the segregation enrichment factor and δ is the boundary layer thickness. The grain boundary enhancement factor decreases with temperature from 10⁵ at 1073 K to 10³ at 1273 K.     

Jonker "Pecir" Analysis of Oxide Superconductors

Data for the normal state of the superconducting oxide systems YBa₂Cu₃O_6+y, and the nonsuperconducting system La₃Ba₃Cu₆O_12.5+y, when plotted as thermoelectric coefficient versus logarithm of conductivity, exhibit Jonker "pear" behavior, confirming the semiconducting character of these materials. The symmetry of the plots indicates similar conduction parameters (density-of-states (DOS), transport constants, and mobilities) for p - and n -type mechanisms. Band gaps on the order of 0.5 eV are obtained. DOS—mobility products for the above-mentioned systems and La_2-xBa_xCuO₄ and Bi₂Sr₂CaCu₂O₈ scale roughly according to the density of copper ions in these materials and fall in the range 10²⁰ to 10²¹ (cm·V·s)⁻¹. Assuming DOS equal to copper content results in mobilities on the order of 0.15 cm²·V⁻¹· s⁻¹. These results are discussed in terms of possible semiconduction mechanisms.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Vaporization Thermodynamics and Enthalpy of Formation of Aluminum Silicon Carbide

The vaporization thermodynamics of aluminum silicon carbide was investigated using Knudsen effusion mass spectrometry. Vaporization occurred incongruently to give Al( g ), SiC( s ), and graphite as reaction products. The vapor pressure of aluminum above (Al₄SiC₄+ SiC + C) was measured using graphite effusion cells with orifice areas between 1.1 × 10⁻²and 3.9X10⁻⁴ cm². The vapor pressure of aluminum obtained between 1427 and 1784 K using an effusion cell with the smallest orifice area, 3.9X10⁻⁴ cm², is expressed as
log p (Pa) =−(18567 ± 86) ( K/T ) + (12.143 ± 0.054)
The third-law calculation of the enthalpy change for the reaction Al₄SiC₄( s ) = 4Al( g ) + SiC( hex ) + 3C( s ) using the present aluminum pressures gives Δ H °(298.15 K) = (1455 ± 79) kJ·mol⁻¹. The corresponding second-law result is Δ H °(298.15 K) = (1456 ± 47) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from the elements calculated from the present vaporization enthalpy (third-law calculation) and the enthalpies of formation of Al( g ) and hexagonal SiC is Δ H °_f= -(221 ± 85) kJ·mol⁻¹. The standard enthalpy of formation of Al₄SiC₄( s ) from its constituent carbides Al₄C₃( s ) and SiC( c, hex ) is calculated to be Δ H °(298.15 K) = (38 ± 92) KJ·mol⁻¹.     

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°.     

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.     

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.     

Interdiffusion and Phase-Boundary Reaction Between Silicate Glass Surface and Aqueous CsCl Solution

Technical silicate glass samples with fresh fracture surfaces or HF-etched surfaces were treated in aqueous CsCl at 90°C for 10 to 85 min. Reaction profiles were determined quantitatively using a SIMS technique. The Cs₂O concentration at the glass surface increased with increasing time, followed by penetration of Cs⁺ ions into the glass. The profiles can be described assuming a model which contains both a surface phase-boundary process and an interdiffusion process in the glass for which D=5.10⁻¹⁷ cm² s⁻¹.     

"Classical" Superplasticity of SiAlON Ceramics

"Classical" superplasticity of SiAlON ceramics with the nominal composition (Y _x Li _y )_{0.6/(3 x + y )}Si_8.9Al_3.1O_2.5N_13.5 is reported in this study. During deformation, these materials exhibit little microstructural evolution, with negligible growth of elongated β'-phase grains and minimal texture formation. Excellent formabilities are obtained in the temperature range of 1500°-1600°C in compression, where a strain rate of 10⁻² s⁻¹ has been achieved, and in punch stretching, where a strain rate of 1.2 × 10⁻³ s⁻¹ has been used successfully. Flow stresses are found to be dramatically affected by the viscosity of the grain-boundary phase and decrease as the amount of lithium addition increases. Fracture stress also is compositionally dependent and decreases as the lithium content increases. As the overall formability is determined by the competition between fracture and deformation, maximum formability coincides with a maximum ratio of fracture stress to forming stress at an intermediate lithium composition. Finally, all materials exhibit higher room-temperature bend strength after postforming annealing. Thus, transient superplastic deformation does not impair the ultimate mechanical properties of the materials.     

Compressive Properties of Yttrium Oxide

Compressive properties of polycrystalline yttrium oxide (Y₂O₃) were studied by slow-strain-rate experiments (ε= 5.7 × 10^–6 s⁻¹) between room temperature and 1600°C. It was shown that Y₂O₃ fails in a brittle manner up to 1000°C, and at 1200°C and above plastic deformation becomes dominant. Plastic deformation of Y₂O₃ takes place exclusively by dislocation motion. Maximum stress, yield stress, and elastic modulus decrease with increasing temperature, although the decrease at temperatures above 1000°C is much more pronounced.     

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).     

Superconducting Characteristics of Polycrystalline Magnesium Diboride Ceramics Fabricated by a Spark Plasma Sintering Technique

Highly densified MgB₂ superconductors were successfully fabricated using a spark plasma sintering (SPS) technique, and their superconductivity with respect to microstructural evolution was evaluated. Full densification with final density close to the theoretical density was achieved at a temperature of 1000°C within a total SPS processing time of 40 min. Both an MgB₂ specimen sintered at 1000°C for 30 min and one sintered at 1050°C for 10 min exhibited a high critical transition temperature ( T _c) similar to that of an MgB₂ single crystal (39 K), and a very sharp superconducting transition width (Δ T ) less than 0.5 K. In addition, high critical current densities ( J _c) of 7.7 × 10⁵ A/cm² in a field of 0.6 T at 5 K and of 8.3 × 10⁴ A/cm² in a field of 0.09 T at 35 K were obtained. These excellent superconducting characteristics of the SPS-processed MgB₂ are attributed to uniformly distributed secondary MgO phase nanoparticles and well-developed dislocations in the microstructure that may act effectively as extrinsic flux pinning sites, resulting in the strong pinning force showing a high J _c of 8.7 × 10⁴ A/cm² even in the condition of a field of 4 T at 5 K.     

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.     

Effects of Temperature and Strain Rate on the Plastic Deformation of Fully Dense Polycrystalline Y1Ba2Cu3O7−x Superconductor

The knowledge of the steady-state stress for plastic deformation as a function of temperature and strain rate is essential for hot-forming superconducting material into commercially useful shapes. In this paper, results are presented on the experimental determination of the rheology of fully dense polycrystalline Y₁Ba₂Cu₃O_7−x superconducting material at temperatures ranging from 750° to 950°C and strain rates of 10⁻⁴, 10⁻⁵, and 10⁻⁶ s⁻¹. The data are best fitted by a power law: ε(s⁻¹)=8.9 × 10⁻¹⁷. (s⁻¹) σ^2.5 (Pa) exp [−2.01 × 10⁵(J·mol⁻¹)|RT]. X-ray analysis shows that the superconducting material retains its phase composition after nearly 70% total strain of the sample. A strong anisotropy in the resistivity of the deformed samples is observed because of the development of a preferred orientation of the a or b axis of Y₁Ba₂Cu₃O_7−x orthorhombic perovskite single crystals perpendicular to the principal maximum compressive stress.     

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.     

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.     

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B₂O₃–Bi₂O₃—CdO. It exhibited the following dielectric and piezoelectric properties: K_p= 0.52 to 0.58, Q_m= 1000, ε^T₃₃/ε₀= 800 to 1000, tan δ= 50 × 10⁻⁴, ρ= 7.56 to 7.64 g/cm³. Ceramics with the aid of suitable dopants (CdO, SiO₂, and excess PbO) in the Pb-(Ni_1/3Nb_2/3)O₃—PZT family could be sintered at 860° to 900°C. For these materials, K_p= 0.56 to 0.61, Q_m= 1000, ε^T₃₃/ε₀= 1500 to 2000, tan δ≤ 50 × 10⁻⁴, ρ= 7.80 to 8.03 g/cm³. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.     

Extension of Glass-Forming Region in a ZrF4-Based System by Rapid Melt Quenching

It was shown that rapid melt quenching can extend the glass-forming region of a ZrF₄–BaF₂–YF₃–AlF₃ composition considerably. Cooling at a rate of ∼10² K · s⁻¹ prevented glass formation at compositions less than about 25 mol% BaF₂, whereas quenching at a rate of ∼2×10⁶ K · s⁻¹ resulted in glasses with a BaF₂ content as low as about 13 mol%. This high quenching rate was achieved by a specially designed twin-roller apparatus.     

Properties and Performance of Cation-Doped Ceria Electrolyte Materials in Solid Oxide Fuel Cell Applications

Cation-doped CeO₂ electrolyte has been evaluated in single-cell and short-stack tests in solid oxide fuel cell environments and applications. These results, along with conductivity measurements, indicate that an ionic transference number of ∼0.75 can be expected at 800°C. Single cells have shown a power density >350 mW/cm². Multicell stacks have demonstrated a peak performance of >100 mW/cm² at 700°C using metallic separators.