Effects of Thermal Expansion Mismatch Stresses on th Room-Temperature Fracture of Boron Carbid

Calculation of apparent fracture energies (γ_a) using the applied failure stress and fractographically determined flaw sizes (C) in B₄ C shows γ_a decreasing with C , once C < 100 μm. This is attributed to increasing contributions of microstructural stresses (σ_i) due to thermal expansion anisotropy ( 1 × 10⁻⁶°C⁻¹). Extrapolation of this σ_i contribution to C ∼ G (the grain size) and calculation from maximum thermal expansion mismatch are in reasonable agreement. e.g., giving σ∼ 1000 MPa. Strength-mirror size data also show deviations at higher strengths-smaller mirror sizes consistent with both the occurrence and estimated level of such microstructural stress contributions to failure.     

Enhancement of Fracture Properties of Wustite by Precipitation

Both the fracture-initiation energy, γ_i, and the fracture strength, σ _f , of 99.5% dense wustite (grain size 23 μm) were substantially increased by (1) precipitating coherent-coplanar Fe₃O₄, and (2) decomposing the wustite eutectoidally to α-Fe+Fe₃O₄, forming a continuous α-Fe network at the former wustite grain boundaries. The increase in γ_i, and σ_f resulting from the presence of 13 vol% Fe₃O₄ in wustite was attributed to the difference in cleavage habits between the precipitates and matrix and to crack-front pinning. The continuous α-Fe network increased the fracture stability in that fracture was not catastrophic but progressed in steps. The improvement in γ _i and σ _f was attributed mainly to the plastic work in fracturing the α-Fe.     

Transient Thermal Stress Behavior in ZrO2-Toughened Al2O3

The initial strength of (σ_i) and thermal shock resistances (Δ T_c and σ_r/σ_i), as determined by quench tests, of Al₂O₃-ZrO₂ composites are increased by increasing amounts of tetragonal ZrO₂ second phase for contents of up to ∼15 vol%. For composites with ≤9 vol% ZrO₂ the increases in σ_r and Δ T_c reflect the increase in γ_IC with addition of ZrO₂ However, for ZrO₂contents >9 vol%, the thermal shock resistances (Δ T_c and σ_r/σ_i) and σ_i are also affected by machining-induced microcracking in the surface of the samples. For ZrO₂ contents >14 vol%, bulk microcracking can become extensive and result in a degradation of σ_i and Δ T_c .     

ASYMPTOTIC EXPANSIONS FOR THE DISTRIBUTIONS OF SERIAL CORRELATIONS

Abstract. Let X ₁, …, X _n be a random sample from a population with a distribution function F and let E ( X ₁) = 0, E ( X ₁²) < ∞. Let r ₁=Σ _{t =1} ^{n -1} X _t X _{t +1}/Σ _{t =1} ^{n -1}( X _t ²+ X _{t +1}²). We derive a proper Edgeworth type expansion for the sampling distribution of r ₁ under the assumption that F is a mixture of Gaussian distributions of one of two given types. The result can easily be extended to the sampling distributions of serial correlations of arbitrary lag s .     

Determination of Transport Properties of Alumina Oxide Scale

By measuring the potential difference between the two interfaces of an alumina scale developed on a FeCrAl alloy, or by applying an electric field during its growth, values of t_i, σ_i, and σ_e along with their temperature dependence were determined. A residual potential, V₀, which was measured between the two oxide interfaces, is related to the oxygen chemical potential gradient on the scale. Alumina growth is controlled by oxygenion diffusion (by either V" or O"_i), and the transport properties are modified by the amount and distribution of impurities.     

Preparation and Characterization of Samaria-Doped Ceria Electrolyte Materials for Solid Oxide Fuel Cells

The microstructure, thermal expansion, mechanical property, and ionic conductivity of samaria-doped ceria (SDC) prepared by coprecipitation were investigated in this paper. The results revealed that the average particle size ranged from 10.9±0.4 to 13.5±0.5 nm, crystallite dimension varied from 8.6±0.3 to 10.7±0.4 nm, and the specific surface area distribution ranged from 62.6±1.8 to 76.7±2.2 m²/g for SDC powders prepared by coprecipitation. The dependence of lattice parameter, a, versus dopant concentration, x , of Sm³⁺ ion shows that these solid solutions obey Vegard's rule as a ( x )=5.4089+0.10743 x for Ce_{1− x} Sm _x O_{2−1/2 x} . For SDC ceramics sintered at 1500°C for 5 h, the bulk density was over 95% of the theoretical density; the maximum ionic conductivity, σ_800°C=(22.3±1.14) × 10⁻³ S/cm with minimum activation energy, E _a=0.89±0.02 eV, was found in the Ce_0.80Sm_0.20O_1.90 ceramic. A dense Ce_0.8Sm_0.2O_1.9 ceramic with a grain size distribution of 0.5–4 μm can be obtained by controlling the soaking time at 1500°C. When the soaking time was increased, the microhardness of Ce_0.8Sm_0.2O_1.9 ceramic increased, the toughness slightly decreased, which was related to grain growth with the soaking time.     

Experimental-Statistical Method for Investigation of Multicomponent Yttrium Garnet Systems

In the present work, some problems are discussed arising in the construction and study of a mathematical model describing the synthesis of multicomponent yttrium garnets with predetermined microwave parameters. To construct the model, the following formula was used for Ca-V YIG substituted with In and Cr: Y₃–_2xCa_2xFe5–( x+y+z ) In_y Cr_zV_xO₁₂. For this purpose, ferrogarnets were synthesized differing in x, y, and z with their values chosen so as to obtain real roots of a set of equations of the type Y_k=Σ B _iX_i+Σ B_ij X_iX_j+Σ B_ijkX_iX_jX _k , where Y_k are the ferrogarnet microwave properties. The graphical presentation of these solutions as triple diagrams is a very convenient way of obtaining ferrogarnets with predetermined microwave characteristics. On the other hand, the possibility is created of mathematically predicting and determining the influence of the different components on the yttrium ferrogarnet parameters as a function of their concentration.     

Creep Behavior and Grain-Boundary Sliding in Polycrystalline A12O3

The creep properties of polycrystalline A1₂O₃ (grain size 14 to 65 μm) were examined under compressive stresses of between 4,000 and 18,000 psi (27.6 and 124 MPa) in the range 1600° to 1700°C. Two distinct types of behavior were observed. The creep rate of medium-grained specimens (14 to 30 μm) could be described by ασ^1.2 / d² where σ is the applied stress and d is the grain size. These results are consistent with the Nabarro-Herring creep mechanism. For the coarse-grained (65 μm) specimens, the creep rate was related to the stress by ασ^2.6. This behavior was not related to cracking; instead, a dislocation mechanism was thought to be rate-controlling. Considerable evidence for grain-boundary sliding was seen, and measurements showed that grain-boundary sliding contributed between 46 and 77% of the total strain in the 3 medium-grained specimens examined and between 38 and 50% in the 3 coarsegrained specimens examined.     

Fracture Properties of Polycrystalline Lithia-Stabilized β"- Alumina

The critical stress-intensity factor, K_1C , and the fracture strength, σ _f , have been investigated on both hot-pressed and sintered lithia-stabilized β "-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K_1C and σ _f were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (<120 μm), the strength was slightly dependent on grain size.
For larger grain sizes, the strength decreased rapidly with increasing grain size and the fracture mode was almost entirely transgranular. The K_1C values for sintered β "-alumina were in the same range as those obtained on the hot-pressed material.     

Influence of Impurity Particles on the Fracture of UO2

Three types of second-phase precipitates were identified by scanning electron microscopy of helical specimens of nominally pure UO₂, i.e. (1) bonded or weakly bonded complex Si particles precipitated during sintering from impurities in the starting material, (2) more strongly bonded Al₂O₃-UO₂ surface particles that formed on the external surfaces during sintering, and (3) metallic Fe or U phases. The influence of these particles on the room-temperature fracture stress, σ _f , was investigated. Particles of types (1) and (2) were as large as 300 μm and caused up to 100% reduction in σ _f , whereas the metallic phases had no significant effect. The critical-strain-energy-release rate, G_c , determined from the measured flaw sizes and associated fracture stresses, was ∼3600 ergs/cm². Purification procedures that reduced the impurity particle sizes and number density improved the strength significantly.     

Design, Fabrication, and Characterization of Silicon Nitride Particle-Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration

Silicon nitride particle-reinforced silicon nitride matrix composites were fabricated by chemical vapor infiltration (CVI). The particle preforms with a bimodal pore size distribution were favorable for the subsequent CVI process, which included intraagglomerate pores (0.1–4 μm) and interagglomerate pores (20–300 μm). X-ray fluorescence results showed that the main elements of the composites are Si, N, and O. The composite is composed of α-Si₃N₄, amorphous Si₃N₄, amorphous SiO₂, and a small amount of β-Si₃N₄ and free silicon. The α-Si₃N₄ transformed into β-Si₃N₄ after heat treatment at 1600°C for 2 h. The flexural strength, dielectric constant, and dielectric loss of the Si₃N_4(p)/Si₃N₄ composites increased with increasing infiltration time; however, the pore ratios decreased with increasing infiltration time. The maximum value of the flexural strength was 114.07 MPa. The dielectric constant and dielectric loss of the composites were 4.47 and 4.25 × 10⁻³, respectively. The present Si₃N_4(p)/Si₃N₄ composite is a good candidate for high-temperature radomes.     

Structure Evolution in Hydrothermally Processed (<100°C) BaTiO3 Films

Thin films of cubic BaTiO₃ were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)₂ or a mixture of NaOH and BaCl₂. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions. BaTiO₃ formation was facilitated by increasing [OH^-], [Ba²⁺], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO₃ particles. Films processed at relatively low [OH^-], [Ba²⁺], and temperatures were coarse grain and opaque, but increasing [OH^-], [Ba²⁺], and temperature caused the grain size to decrease, resulting in transparent films.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Reformulation of an Aqueous Alumina Slip Based on Modification of Particle-Size Distribution and Particle Packing

Six alumina casting slips with particle-size distributions varying from 44 to 0.1 μm were examined. Particle packing was calculated using the approach of Andreasen. Viscosity, green density, and pore-size distribution were measured. It was found that contouring the intermediate size distribution for particles finer than 15 μm provided the most desirable viscosity for slips composed of wide size distributions. For slips containing 50 vol% solids, the lowest viscosity obtained was 196 × 10⁻³ N · s/m² (with a two-component size distribution), and a green density of 2.52 g/cm³ (65% of theoretical) was achieved with a ternary system. These casts had bimodal pore-size distributions centered around approximately 1 and 0.1μm.     

Synthesis and Electrical Properties of Dense Ce0.9Gd0.1O1.95 Ceramics

Uniform spherical powders of Ce_0.9Gd_0.1O_1.95 with an average diameter of 250 nm were obtained at 700°C from a sol-gel process of mixing nitrates and ethylene glycol. Broadening of the X-ray peaks of the fluorite structure reveals a small crystallite size within the powders. Sintering in air of pressed pellets of the powders at 1585°C gives ceramics of 99% theoretical density with grain sizes 1-10 µm and a cubic unit cell a = 5.422 ± 0.034 Å. The electrical conductivity σ=σ_o+σ_e has two components. In air or argon, the electronic component σ_e is negligible and the oxide-ion conductivity σ_o is not described by a classical Arrhenius equation; a pronounced curvature at T similar/congruent T * has been observed in the Arrhenius plot of the bulk conductivity. The system could be modeled by a condensation of mobile oxygen vacancies into ordered clusters below a temperature T * similar/congruent 583 ± 45°C and a motional enthalpy Δ H _m= 0.63 ± 0.01 eV for the vacancies. A measured trapping energy Δ H _t(1 - T/T *) has Δ H _t= 0.19 ± 0.01 eV = 2.57 kT *. In a reducing atmosphere, σ_e exhibits a small-polaron motional enthalpy Δ H _p= 0.40 ± 0.08 eV for transfer of a 4 f electron from a Ce³⁺ to a Ce⁴⁺ ion and a Δ H _p+Δ H _pt= 0.51 ± 0.04 eV at T < T *.     

Experimental Procedure for Determining Transport Properties of Oxide Scales

To determine the transport properties of oxide scales growing on an alloy, a laboratory apparatus was developed which allows plotting of characteristic V = f (i) curves and oxidation with an applied electric field. The apparatus and formalism used are described; it appears that such experimental procedures allow determination of the mean ionic transport number, ti; the conductivity values σ, σ_i, and σ,_e; and the effective charge, Z*, of the moving species in the oxide scale (A1₂O₃ in the present case).     

Porous Glass-Ceramics with a Skeleton of the Fast-Lithium-Conducting Crystal Li1+xTi2−xAlx(PO4)3

Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li₂O─CaO─TiO₂─Al₂O₃–P₂O₅ system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ and β-Ca₃(PO₄)₂ phases. The median pore diameter and surface area of the resulting porous Li_{1+ x} Ti_{2− x} Al _x (PO₄)₃ glass-ceramics were approximately 0.2 μm and 50 m²/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO₃ aqueous solution was 8 × 10⁻⁵ S/cm at 300 K or 2 × 10⁻² S/cm at 600 K.     

Driving Force for Sintering of Particles with Anisotropic Surface Energies

Crystalline aggregates at constant temperature and pressure are unstable toward shape changes for which     , where δG is a dzgerential change in the Gibbs free energy, σ_iA_i is the free energy of surface or interface i, and γ_ih_i is the free energy of edge, ledge, or dislocation defect i. The expression reduces to the familiar expression for the driving force for early-stage sintering. Line defects play central roles in driving the sintering of crystalline solids.     

Diffusional Creep and Kinetic Demixing in Yttria-Stabilized Zirconia

The creep behavior of fine-grained yttria-stabilized zirconia with 25 mol% Y₂O₃ has been characterized as part of an investigation of kinetic demixing in solid-solution oxides which are subjected to a nonhydrostatic state of stress. At temperatures between 1400° and 1600°C, the steady-state strain rate of (Zr_0.6Y_0.4)O_1.8 samples with average grain sizes between 2.5 and 14.5 μm can be summarized by the flow law ɛ= 6.5 × 10⁻⁷σ^1.2 exp[−550 (kJ/mol)/ RT ] d ^−2.2 (s⁻¹) for stresses in the range 8 to 60 MPa, where σ is in pascals and d is in meters. This flow law indicates that deformation occurs by a Nabarro-Herring creep mechanism in which the creep rate is limited by cation lattice diffusion. Kinetic demixing was not observed in deformed polycrystalline samples even though diffusional creep was rate limited by cation lattice diffusion. This result can be explained if the cation diffusivities are approximately equal or if extensive grain rotation occurs during diffusional creep.     

Dynamics of Formation of Self-Organized Mesoporous AlO(OH)·αH2O Structure in Al-Metal Surface Hydrolysis in Humid Air

In humid air, a nascent Al-metal surface (S) with a surface Hg²⁺ catalyst hydrolyzes in divided reaction centers (micelles) with a vigorous exothermic reaction, {Al + 2OH⁻}+αH₂O → AlO(OH)·αH₂O + 3e⁻+ H⁺. It yields amorphous AlO(OH)·αH₂O with a huge ∼90% porosity with α= 0.25. The primary driving forces of the reaction are the chemical potential μ_e between the reaction species, the mechanical stress ς induced in expansion of S, and the flow of the reaction species. They drive it in a common direction perpendicular to S. The heat released in it flows primarily along S. It disrupts and stops the directional hydrolysis if the local temperature in the micelle reaches a critical value T _c (hot spot). The hot spot cools to the operating value T ₀, and the reaction restarts and runs over to T _c in a periodic manner, at a time scale of Δ t _i∼ 5 s, per the dynamics of hot spots, forming a self-organized mesoporous structure of 15–50-nm diameter ellipsoidal shaped particles (halo) separated through 3–5-nm pores. A pore, in continuous formation of the sample, forms in disrupted reaction during the hot spot as it cools from T _c to T ₀. The result is modeled in terms of the microstructure and dynamics of the hot spots.