Lattice Modification in Ion-Implanted Ceramics

The effect of ion implantation on alumina (Al₂O₃) and silicon carbide (SiC) was investigated by Rutherford backscattering (RBS), indentation hardness, fracture toughness, transmission electron microscopy (TEM), and linear scratching with a diamond stylus. The implanted (10^l6 to 10¹⁷ Cr.cm⁻² at 280 to300 keV, 1 to 4 |MX 10^l6Ti.cm ⁻²at 150 keV,2 |MX 10¹⁶Zr.cm⁻² at 150 keV) AI₂O₃ lattice is significantly damaged but remains crystalline; the lattice hardness increases, and the scratched surface is less sensitive to fracture. The implanted (10¹³ to 10^l6 N.cm⁻² at 62 keV, 10¹⁴ to 10^l6 Cr.cm ⁻² at 280 keV) Sic lattice becomes amorphous to a depth of 250 nm, becomes less hard, and deforms without fracture when scratched. The variation in Rutherford backscattered spectra and in surface hardness with annealing are reported and interpreted in terms of lattice defects for the Al₂O₃ specimens.     

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

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.     

Electrical Breakdown Strength of Alumina at High Temperatures

The dc electrical strength of sapphire and poly crystalline alumina was studied up to 1400°C. The electrical strength was essentially identical for both materials. It was > 10⁶ V/cm at room temperature and decreased gradually with temperature up to 900°C (2.6×10⁵ V/ cm), then dropped rapidly to 2×10⁴ V/cm at 1400°C for a sample thickness of ∼ 100 μm. The electrical strength decreased with the sample thickness. It was inversely proportional to the thickness for samples thicker than ∼ 600 μm at 1200°C. The breakdown behavior was explained on the basis of a thermal breakdown model.     

Partially Reduced Cuprous Oxide Nanoparticles Formed in Porous Glass Reaction Fields

This paper describes the formation of cuprous oxide (Cu₂O) particles ∼10 nm in diameter and their reduction by hydrogen gas in porous glasses. Nanoparticles of Cu₂O coated with metallic copper are expected to show high third-order optical nonlinear susceptibilities (χ⁽³⁾). The porous glass is a medium in which nanoparticles can be partially reduced without forming agglomerations. Images obtained using transmission electron microscopy showed that particles with the desired core-shell structure were actually formed, even though some particles were not reduced uniformly from the surface. The χ⁽³⁾/α (α: absorbance) values in relevant conditions (10⁻¹³ esu·cm) were similar to that of copper because of the formation of a byproduct of copper nanoparticles on reduction.     

Silicon Nitride/Molybdenum Disilicide Composite with Superior Long-Term Oxidation Resistance at 1500°C

The long-term high-temperature cyclic oxidation (100 cycles, 10⁴ h, 1500°C) of a Si₃N₄ material and a Si₃N₄/MoSi₂ composite, both fabricated with Y₂O₃ as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO₂ formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm² after 10⁴ h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si₃N₄ material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si₃N₄/MoSi₂ composites for long-term structural applications at elevated temperatures up to 1500°C.     

Enhanced Oxygen Diffusion at 1400°C in Deformed Single-Crystal Magnesium Oxide

The rate of oxygen difusion at 1400°C into pure, deformed, and scandium-doped MgO was measured using a secondary-ion mass spectrometer with ¹⁶O^- primary ions as sputtering agents to monitor the penetration of ¹⁸O into the sample from an ¹⁸O-enriched surface layer. In samples doped with scandium, oxygen diffusivity did not differ significantly from the value, 1.3 × 10-¹⁵ cm²/s, found for the pure sample. In deformed samples, the diffusion coefficient was 5.8 × 10^-15 cm²/s .     

Early-Stage Thermal Oxidation of Carbothermal β-Sialon Powder

Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state ²⁹Si and ²⁷Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but ²⁹Si and ²⁷Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol⁻¹, suggesting the rate-limiting step to be due to dissolution of O₂ in an amorphous silica surface layer on the β-sialon particles.     

Inadequacies of Viscosity Theories for a Vitreous KNO3-Ca(NO3)2 Melt

The viscosity of a 0.60KNO₃-0.40Ca(NO₃)₂ melt was measured from 10^9.6 to 10¹⁴ P by a beam-bending method. In this range, the viscosity exhibited Arrhenius behavior, with an activation enthalpy of 138 kcal/mol. These data joined smoothly with capillary and rotational viscometer data from 10⁻² to 10⁸ P previously reported for this system. The temperature dependence of shear sound wave velocities at 75.2 MHz was measured, and the temperature dependence of the shear modulus from 60° to 120°C was calculated for this melt. The Fulcher equation, In _n=A+B/(T-T₀ ), described the temperature dependence of the viscosity of this melt poorly, indicating severe deficiencies in the viscosity theories which predict an equation of this form.     

Transformation Range Viscosity of Fluorozirconate Glasses

Transformation range viscosity was measured for multicomponent fluorozirconate glasses in the system 0.56ZrF₄·(0.34-x)BaF₂·0.06LaF₃·0.04AlF₃·xRF, where R =Li, Na, K, or Cs . The results indicate that the viscosity of these glasses can be described by an Arrhenius equation over the viscosity range from 10⁷ to 10¹² Pa·s with an activation energy of the order of 650 to 850 kJ. In general, the effects of alkali fluoride additions on the viscosity of fluorozirconate glasses are comparable to those in silicate systems.     

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.     

Oxidation Behavior of Si-C-O Fibers (Nicalon) under Oxygen Partial Pressures from 102 to 105 Pa at 1773 k

Polycarbosilane-derived SiC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10² to 10⁵ Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 ×10² Pa and actively oxidized under an oxygen partial pressure of 10² Pa. Under oxygen partial pressures from 2.5 × 10² to 10³ Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer. Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was lost under the active-oxidation condition.     

Effects of Samarium Oxide Addition on the Phase Composition, Microstructure, and Electrical Resistivity of Aluminum Nitride Ceramics

The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm₂O₃ additive were investigated. The phase composition was approximately consistent with that estimated from the Sm₂O₃–Al₂O₃ phase diagram using the amount of added Sm₂O₃ and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm₂O₃ additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10¹⁰–10¹²Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.     

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.     

Electrical Properties of Gallium-Doped Zinc Oxide Films Prepared by RF Sputtering

Ga-doped polycrystalline ZnO films on glass substrates were prepared by sputtering the targets, which had been prepared by sintering disks consisting of ZnO powder and various amounts of Ga₂O₃, to investigate the effects of gallium doping and sputtering conditions on electrical properties. Optimizing the RF power density, argon gas pressure, and gallium content, transparent Ga-doped ZnO films with resistivity less than 10⁻³Ω·cm were obtained. Electron concentrations for undoped and Ga-doped ZnO films were on the order of 10¹⁸ and 10²¹/cm³, respectively. The Ga-doped ZnO films became degenerate when the electron concentration exceeded ∼ 10¹⁹/cm³, and the optical band gap increased with increasing carrier concentration because of the increase of Fermi energy in the conduction band.     

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.     

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 ].     

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.     

Determination of the Oxygen Self-Diffusion Coefficients in Y2O3-Containing Tetragonal Zirconia Polycrystals by Raman Spectrometric Monitoring of the 16O-18O Exchange Reaction

A new method of determining the oxygen self-diffusion coefficients (D) in oxides has been developed. The method is based on Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. By using the time dependent ¹⁸O2 concentration measured in a quasi in situ manner by Raman spectroscopy, the oxygen self-diffusion coefficients are calculated. The calculation takes into account the influence of the surface exchange reaction and the limited gas volume. The result obtained in 2.8 mo1% Y₂O₃-containing tetragonal zirconia polycrystals is D = 1.55 (^+0.07_-0.07) x 10^-2 exp[(-120.0 0.4) (kJ/mol) / RT ] cm²/sec. It was demonstrated that Raman spectroscopy is useful tool for determining the oxygen self-diffusion coefficients in oxides.     

The Influence of Piezoelectric Grain Resonance on the Dielectric Spectra of LiNbO3 Ceramics

It has often been suggested that the higher frequency dispersion ( f ∼10⁷∼10⁸ Hz) evident in the dielectric response of high-permittivity ferroelectric ceramics might be associated with piezoelectrically driven resonant mechanical motion of individual grains. The hypothesis certainly appears reasonable, but so far no direct verification appears to have been attempted. Our discovery that the individual crystallite resonances are well preserved in poled LiNbO₃ bicrystals suggested that we explore the dispersion of LiNbO₃ ceramics. In both conventionally sintered and uniaxially hot-pressed ceramics, the dielectric dispersion around 10⁸ Hz has obvious resonant character. Using a modified ECAP computer program demonstrated that both the real and imaginary components of the impedance can be quantitatively described using the equivalent circuit models for the coupled individual grain resonances.