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.     

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.     

Kinetics of Precipitation and Measurements of Strain in the System MgO-Al2O3-Cr2O3

A quantitative X-ray technique for measuring precipitation strains has not been previously applied in metallic or oxide systems. The Warren-Averbach analysis of strain was used to determine the buildup of elastic strain energy in the spinel crystalline solution matrix (gross composition = 60 mol% MgAl₂O₂+ 40 mol% Cr₂O₃) during the isothermal (1135°C) precipitation of a metastable (coherent) monoclinic phase. The elastic strain energy of the spinel crystalline solution matrix increased to a maximum of about 3.1 × 10⁷ ergs/cm³ for a reaction time of 8 h. There was a marked decrease in the elastic strain energy during the initial precipitation of the equilibrium corundum crystalline solution with the composition (Al³⁺_0.72 Cr³⁺_0.25)O₃. An overall diffusion activation energy for precipitation of the mono-clinic phase was approximately 86 kcal/mol.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

Cation Tracer Diffusion in Cr2O3 and Cr2O3-0.09 wt% Y2O3

The cation diffusivities in the lattice and along dislocations and grain boundaries have been measured on sintered polycrysals of Cr₂O₃; and Cr₂Cr₂O₃-0.09 wt% Y₂O₃ at1100°C and at the pO₂ corresponding to that of Cr/Cr₂O₃ equilibrium at that temperature. Results for lattice and dislocation diffusivities in pure Cr₂O₃ are in good agreement with previous work. The present results indicate that yttrium additions have negligible effect on lattice and dislocation diffusion. However, grain-boundary diffusion in pure Cr₂O₃ is significantly slower than grain-boundary diffusion in Cr₂O₃-0.09 wt% Y₂O₃. The results are discussed in terms of their implications for the reactive-element effect in high-temperature oxidation of chromium-containing alloys.     

Electrical Conduction in Sintered α-Sb2O4

Electrical conductivity and thermoelectric power were measured on sintered α-Sb₂O₄ at 250° to 780°C. Oxygen partial pressure dependence of the conductivity and sign of the Seebeck coefficient showed α-Sb₂O₄ to be a p -type semiconductor above 600°C in the oxygen pressure range of lo⁵ to 10² Pa. A hopping conduction was proposed from very small hole mobility with an activation energy of 18 kJ/mol.     

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.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

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.     

Synthesis of Phase-Pure Pb(ZnxMg1–x)1/3Nb2/3O3 up to x= 0.7 from a Single Mixture via a Soft-Mechanochemical Route

Phase-pure perovskite Pb(Zn _x Mg_{1– x} )_1/3Nb_2/3O₃ solid solution (PZ _x M_{1– x} N) is obtained for x ≦ 0.7 by heating a milled stoichiometric mixture of PbO, Mg(OH)₂, Nb₂O₅, and 2ZnCO₃·3Zn(OH)₂·H₂O at 1100°C for 1 h. Percent perovskite ( f _P) with respect to total crystalline phase decreases with increasing temperature of subsequent heating then increases to 900°C for the mixtures where x ≦ 0.8 and milled for 3 h. For mixtures with x = 0.9 and x = 1, f _P decreases monotonically. Curie temperature increases almost linearly with increasing x up to x = 0.7. The maximum dielectric constant at 1 kHz is 2×10⁴ and 1.7×10⁴ for the mixture with x = 0.4 and x = 0.7, respectively. The stabilization mechanism of strained perovskite is discussed.     

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.     

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.     

Superconductive Properties of the YBa2Cu4O8 Superconductor

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa₂Cu₄O₈ (124) at 850°C in flowing oxygen at 1 atm (≅10⁵ Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were characterized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be ∼5 × 10⁴ A/cm², and the superconducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics

Purified air is passed over a specimen of YBa₂Cu₃O_{7– x} at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10⁻⁵ Pa for BaO( g ), 1.2 × 10⁻⁴ Pa for Cu( g ), and 2.2 × 10⁻⁵ Pa for CuO( g ) are obtained under 2.1 × 10⁴ Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.     

Microstructure and Nonlinear Properties of Microwave-Sintered ZnO-V2O5 Varistors: II, Effect of Mn3O4 Doping

The microstructure and nonlinear current-voltage characteristics of Mn₃O₄-doped ZnO-V₂O₅ ceramics, microwave-sintered at 800°-1200°C for 10 min, have been investigated. A high density (96% of the theoretical density) has been achieved. The incorporation of Mn₃O₄ additives does not significantly alter the densification behavior of the ZnO-V₂O₅ materials, but rather pronouncedly increases the nonlinear coefficient (α= 23.5) and markedly suppresses their leakage current density ( J _L= 2.4 10^-6 A/cm²). On the other hand, the intrinsic properties of the materials, including the Schottky barrier height (Phi_b) and the donor density ( N _d), are only moderately modified; that is, Phi_b= 1.16 eV and N _d= 5.4 10¹⁷/cm³. X-ray diffractometry analyses and energy-dispersive X-ray microanalyses (via scanning electron microscopy) indicate that the V₂O₅ species facilitate the densification and the development of microstructure via the formation of a liquid phase (Zn₃(VO₄)₂) along the grain boundaries, whereas the Mn₃O₄ species markedly enhance the nonohmic behavior of the ZnO-V₂O₅ materials by forming the surface states along the grain boundaries.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

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.     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity