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.     

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.     

Control of Dielectric Constant and Loss in Alumina Ceramics

The dissipation factors and dielectric constants of alumina ceramics containing less than 100 ppm of impurities and of specimens doped with Si, Ti, Ca, Mg, Fe, and Cr ions were measured in the region 10² to 8.5 × 10⁹ cps and 25° to 875° C. Multiple regression analysis of the data at 500° C and 10⁶ cps showed a linear relation between impurity concentration and tan δ, with a correlation coefficient of 0.93. Si ions caused the greatest rise of tan δ, Mg and Ti were second, Ca third, and Cr and Fe had no significant influence. These effects diminished with rising frequency and became negligible in the microwave region. Activation energies of conduction for pure and doped alumina were estimated from measurements of δ at 10⁵ cps and 500°C. Values between 1.2 and 1.6 ev were calculated for all compositions except the one containing Mg²⁺, for which 2.0 ev was obtained. At low frequencies, the dielectric constant (k') rose exponentially with temperature, reflecting a similar rise in the number of free charge carriers contributing to interfacial polarization. At higher frequencies the temperature variation of k' fell to a shallow positive slope of about 120 ppm per °C. This coefficient was not influenced by low concentrations of impurities but could be effectively compensated without excessive loss by additions of 10 to 20% SrTi0₃.     

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.     

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.     

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.     

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     

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.     

Paramagnetic Resonance and Optical Absorption in Gamma-Ray Irradiated Alpha Quartz: The "Al" Center

The optical absorption spectra between 1.0 and 8.5 eV and the electron paramagnetic resonance spectra at 10 GHz of a synthetic crystal of α-quartz grown on a z plate were measured as a function of ⁶⁰Co γ-ray irradiation and thermal treatment. After initial irradiation, an estimate of a lower limit for the concentration of centers contributing to the resolved optical bands gave N ≳10¹⁶ cm^-3 for each band. An estimate of the concentration of spins of each of the distinct paramagnetic species gave N <3×10¹⁵ cm^-3 with the exception of the "Al" center for which N ∼3×10¹⁵ cm^-3. Thermal bleaching and re-irradiation of the crystal showed that the defects with bands in the 1.0 to 6.5 eV region were independent of the defects with bands in the 5.5 to 8.5 eV region and that undetected precursor defect configurations were altered by heating to form those defects whose optical bands (5.5 to 8.5 eV) increased after the second irradiation. The only correlation observed between paramagnetic defects and defects producing optical absorption bands was between the "Al" center and a band at 8.45 eV; the correlation was quite weak.     

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.     

Space-Charge Development in Glass

Data are given that graphically demonstrate space-charge build-up in a glass containing alkali and in a glass relatively free of alkali. The observed potential distributions show a decided asymmetry and have large potential drops near the electrodes with a linear potential change near the center of the sample. The observations qualitatively fit the expected theoretical distributions, according to Proctor and Sutton, for material having cations mobile and anions essentially immobile. Crude estimates arising from comparison with the theory yield carrier concentration near 2 × 10¹⁶ per cm³. mobility of the cation near 3 × 10^-6 cm.² per volt second, and diffusion constant near 2 × 10^-6 cm.² per second. These mobility and diffusion values are two orders of magnitude larger than values computed from conductivity data or computed from measurements of the diffusion coefficient for sodium cations, both in simple alkali silicates. According to the theory, these higher values are perhaps reasonable, since they are associated with only the more mobile portion of the cation population.     

Influence of Atmosphere on the Final-Stage Sintering Kinetics of Ultra-High-Purity Alumina

The influence of sintering atmosphere on the final-stage sintering of ultra-high-purity alumina has been investigated. Model final-stage microstructures were tailored via a latex sphere impregnation and burnout technique. Critical experiments have been conducted to quantitatively examine the influence of the oxygen partial pressure on the final-stage sintering kinetics. Samples were sintered at 1850°C in either dry hydrogen ( P o₂∼ 3 × 10⁻¹⁷ atm) or wet hydrogen P o₂∼ 5 × 10⁻¹⁰ to 2 × 10⁻¹¹ atm), and their microstructures were characterized as a function of sintering time, Sintering in dry hydrogen decreased the susceptibility of the final-stage microstructure to pore/boundary breakaway. In the kinetic analysis, the variation in the number of pores per grain, N _g, was taken into account. It was found that in both atmospheres, the densification rate was controlled by grain boundary diffusion, and that sintering in dry hydrogen increased the densification rate by a factor of 2.25. In addition, it was determined that the grain growth rate in both atmospheres was controlled by the rate of surface diffusion of matter around the pores and that sintering in dry hydrogen enhanced the grain growth rate by a factor of 5.6. The overall effect of the dry hydrogen atmosphere was that it enhanced the coarsening rate relative to the densification rate by a factor of 2.5, and consequently shifted the grain size-density trajectory to much lower densities for a given grain size.     

Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10⁻⁸ m²·K·W⁻¹. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T = 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10⁻⁸ m²·K·W⁻¹ in dense alumina and 2.2 × 10⁻⁸ m²·K·W⁻¹ in alumina containing a pore volume fraction of 0.3.     

Effect of Al Doping on the Electric and Dielectric Properties of CaCu3Ti4O12

Al-doped CaCu₃Ti_{4− x} Al _x O_{12− x /2} (CCTO, x =0–0.1) ceramics were prepared by the solid-state reaction, and their electric and dielectric properties were investigated. Al doping has been shown to reduce the dielectric loss remarkably while maintaining a high dielectric constant. At x =0.06, the loss tangent (tan δ) was below 0.06 over the frequency range of 10²–10⁴ Hz, and the dielectric constant was 41 000 at 10 kHz. Impedance spectra indicated that Al doping increased the resistivity of the grain boundary by an order of magnitude. The improvement of the dielectric loss in Al-doped CCTO was attributed to the enhanced grain boundary resistivity.     

Improvement in Wear Resistance of High-Strength and High-Toughness Silicon Nitride Modified by Ion Implantation

Surface modification by ion implantation has been conducted to improve the tribological properties of a high-strength and high-fracture-toughness unidirectionally aligned silicon nitride (UA-SN). B⁺, N⁺, Si⁺, and Ti⁺ ions were implanted into the planes parallel and normal to the grain alignment of the UA-SN with a fluence of 2 × 10¹⁷ ions/cm² at an energy of 200 keV. The ion implanted UA-SN showed a dramatic improvement in wear resistance. For example, the specific wear rate of the Si⁺-implanted specimen in the direction parallel to the grain alignment was reduced to a value of 3 × 10⁻¹⁰ mm²/N, equal to 1/20 of the unimplanted one. Cross-sectional transmission electron microscopy indicates the high wear resistance was attributed to the amorphous surface caused by the ion implantation.     

The Influence of ZnF2 Doping on the Electrical Properties and Microstructure in Bi2O3–ZnO-Based Varistors

ZnO varistors with different amounts of ZnF₂ from 0.00 to 0.80 mol% were prepared using a solid-state reaction technique, to explore the potential application of ZnO. The F-doping effects on the microstructure and electrical properties of ZnO-based varistors were investigated. The average grain size of ZnO increased from 4.93 to 6.48 μm as the ZnF₂ content increased. Experimental results showed that as the ZnF₂ content increased, the breakdown voltage decreased from 617 to 367 V/mm, and the nonlinear coefficient did not change much. However, a slight increase was observed in the leakage current. Besides, when the ZnF₂ content increased, the donor concentration increased from 0.669 × 10¹⁸ to 8.720 × 10¹⁸ cm⁻³. The study indicated that ZnF₂ played a similar role as sintering aids to promote grain growth and the substitutional F atoms in the bulk served as a donor to increase the donor concentration.     

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.     

Investigation of Some Glasses for High-Level Gamma-Radiation Dosimeters

Several glass systems were evaluated as dosimeters to measure integrated doses in the range from 10⁶ to 10⁹ rads by measuring optical density as a function of radiation dose. The two more promising glasses of those studied were Corning's Code 8392 and a special high-antimony glass. The maximum dosage that can be measured with these two glasses has not yet been determined. In exposures up to 3 × 10⁸ rads, there was continuous increase in optical density with dose, and no evidence of saturation at the highest doses used. Glasses also were prepared that darkened only slightly at dosages up to about 8.7 × 10⁷ rads. Such glasses may have utility for measuring dosages above 10⁹ rads.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).