Transparent Cr4+-Doped YAG Ceramics for Tunable Lasers

Transparent Cr⁴⁺:YAG (Y₃Al_SO₁₂) ceramics doped with Ca and Mg as counterions and SiO₂ as a sintering aid were fabricated by a solid-state reaction method using high-purity powders of Al₂O₃, Y₂O₃, and Cr₂O₃. The mixed powder compacts were sintered at 1750°C for 10 h in oxygen, or 1750°C for 10 h under vacuum, and then annealed at 1400°C for 10 h in oxygen. Cr-doped YAG ceramics sintered in oxygen had a brown color and characteristic absorption by Cr⁴⁺ ions, whereas these YAG ceramics sintered under different conditions (vacuum + oxygen) had a green color and absorption at ∼590 and 430 nm by Cr³⁺ ions. The absorption behavior of YAG ceramics sintered in oxygen was almost equivalent to that of Cr⁴⁺:YAG single crystals fabricated by the Czochralski method.     

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.     

High-Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine-grained (Co_0.5Mg_0.5)O and (Co_0.25Mg_0.75)O has been characterized as part of an investigation of kinetic demixing in solid-solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain-boundary diffusivity, D _o ^b is independent of oxygen partial pressure. The values of ω D _o ^b , where ω is the grain-boundary width, that have been determined from the steady-state diffusional creep rates are given by ω D _o ^b =4.7×10⁻⁸ exp[-230 (kJ/mol)/ RT ] (cm³/s) for (Co_0.5Mg_0.5)O in the range 950° to 1200°C and ω D _o ^b =7.4 × 10⁻⁸ exp[-263 (kJ/mol)/ RT ] (cm³/s) for (Co_0.25Mg_0.75)O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation-climb-controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.     

Effect of Oxygen Partial Pressure on the Dielectric Properties and Microstructures of Cofired Base-Metal-Electrode Multilayer Ceramic Capacitors

The dielectric properties, dopant distributions, and microstructures of BaTiO₃-based multilayer ceramic capacitors (MLCCs) sintered in H₂–N₂–H₂O atmospheres with     =10^−7.5 Pa (BMX-7.5) and     =10^−9.5 Pa (BMX-9.5) were studied, and the effects of oxygen partial pressures were analyzed. Dielectric measurements showed that BMX-7.5 had a lower dielectric constant at temperatures above 20°C, but a higher dielectric constant at temperatures below 10°C when compared with BMX-9.5. The coexistence of core–shell and core grains was observed in bright field (BF) transmission electron microscopy images in both types of capacitors. Triple-point and grain boundary phases were observed more frequently in BMX-9.5 than in BMX-7.5, and energy-dispersive X-ray spectrometer point-by-point analysis revealed that these second phases contained high concentrations of dopants such as Si, Y, and Ca. The dopant concentration in the shell regions in BMX-7.5 was higher than that in similar regions in BMX-9.5. Smeared and twisted grain boundaries with fringes observed in both types of MLCCs indicated that the shell regions in both samples were formed either by diffusion of foreign ions into BaTiO₃ or by crystallization of grain boundary and triple-point liquid phases. It was deduced that the partial pressure of oxygen in the sintering atmosphere influenced the microstructures, dopant distributions, and core–shell ratios of the grains in these materials.     

Effect of Composition on the Oxygen Tracer Diffusion in Transparent Yttrium Aluminium Garnet (YAG) Ceramics

The grain boundary structure and oxygen tracer diffusion in transparent yttrium aluminum garnet (YAG) ceramics varying from 2% excess of Y₂O₃ to 0.5% excess of Al₂O₃ were studied. The characterization of the specimens is as follows: (i) For the Y₂O₃-excess specimen, a second phase (yttrium aluminum perovskite: YAP) containing silicon in the grain boundary was found, (ii) For the Al₂O₃-excess specimen, both aluminum-rich particles (alumina) and a silicon-rich segregant layer were observed in the grain boundary. The volume diffusion of the oxygen tracer is little influenced by the excess composition. In contrast, the grain boundary diffusion of the oxygen tracer is suppressed in the Y₂O₃-excess specimens, compared to Al₂O₃-excess specimens. These differences are thought to result from the chemical reaction between the second phase and the intergranular liquid phase during the sintering.     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

Oxygen Ion Diffusion in Single-Crystal and PoIycrystaIIine Yttrium Iron Garnet

The oxygen ion self-diffusion coefficient was determined for single-crystal and polycrystalline yttrium iron garnet (Y₃Fe₅O₁₂). The rate of exchange between oxygen gas enriched with the stable isotope ¹⁸O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single-crystal and 8μ polycrystalline Y₃Fe₅O₁₂ between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI₂O₃ where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D = 0.40 exp (-65.4/RT) cm² per second.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Chemical Diffusion in Nickel Oxide

The chemical diffusion coefficient was measured for undoped, single-crystalline NiO at 900° to 1200°C and within an oxygen partial pressure of 10⁻⁵ to 0.21 atm. Electrical conductivity was used to monitor the reequilibration kinetics after the oxygen pressure was suddenly changed over the initially equilibrated NiO crystal. The chemical diffusion coefficient was calculated vs the reequilibration degree indicating the most stable range of investigations. The chemical diffusion coefficient value is virtually the same for the oxidation and reduction experiments, giving, respectively: D_chem.=(1.64 × 10⁻²) exp[-(22,480±800)/RT] D_chem.=(9.68 × exp[-(21,430±2600)/RT] It has been stated that chem is independent of the oxygen pressure and thus of oxide composition. The electrical conductivity depends on the oxygen partial pressure in the power (l/n) = (1/5.45), indicating that doubly ionized cation vacancies are the predominant defects. Deviation from the linear dependence of log α vs logp^o2 was observed at <10⁻⁵ atm, indicating formation of anion vacancies of interstitials.     

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.     

Atomic Structures and Energies of Σ7 Symmetrical Tilt Grain Boundaries in Alumina Bicrystals

Symmetrical Σ7 tilt grain boundaries of alumina (Al₂O₃) were studied using bicrystals. Three types of Σ7 boundaries were successfully fabricated, that is, rhombohedral twin (Σ7{1[Onemacr]02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {4[Fivemacr]10} and {2[Threemacr]10} (Σ7{4[Fivemacr]10} and Σ7{2[Threemacr]10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Σ7{1[Onemacr]02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Σ7{2[Threemacr]10} and Σ7{4[Fivemacr]10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grain-boundary planes.     

Effects of Yttrium Doping α-Alumina: I, Microstructure and Microchemistry

The influence of yttrium doping on the microstructure and microchemistry of hot-pressed α-alumina was investigated using a combination of electron microscopy techniques. The implications of microstructure and microchemistry on the improved creep behavior of the doped material are discussed. The samples doped only with yttrium had a bimodal grain-size distribution that was strongly correlated to the frequency and distribution of Y₃Al₅O₁₂ (YAG) precipitates in the microstructure. Yttrium segregated to most of the grain boundaries, with a normal excess concentration of Gamma= 3.3 ± 0.9 atoms/nm² at random boundaries. Two types of twin boundaries (Sigma3 and Sigma7) accommodated no yttrium. None of the boundaries or triple-point junctions contained a glassy grain-boundary phase. Strong interaction of the grain boundaries and dislocations with YAG precipitates indicated a pinning mechanism by the precipitates. Yttrium doping did not appear to favor formation of special boundaries in α-alumina.     

Oxygen Diffusion in Vapor-Deposited Indium Oxide Films

Diffusion measurements were made between 1000 and 1300 K on films of In₂O₃ by electrochemical techniques using an yttria-stabilized zirconia solid electrolyte. At atmospheric oxygen pressures, diffusion invariably occurred via an oxygen interstitial mechanism. The diffusion coefficient decreased with decreasing oxygen pressure with a P _O2^1/2 dependence. Near 1.0 Pa P _O2the mechanism began to change to a vacancy mechanism which dominated at P _O2 <10⁻³ Pa, where diffusivity increased with further decrease in oxygen pressure according to a P _O2^−1/2 dependence. Activation energies for interstitial and vacancy diffusion were 166 and 190 kJ/mol, respectively.     

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.     

Migration of Intergranular Boundaries in Cubic Zirconia-Yttria Induced by Magnesia Addition

When partially sintered cubic ZrO₂–10 mol% Y₂O₃ specimens are heat-treated at 1500°C with 5MgO·95(ZrO₂–10 mol% Y₂O₃) powder mixture in the pores, the intergranular boundaries, which may be either grain boundaries or thin glass films, migrate, leaving behind them a new solid solution slightly enriched with MgO but depleted of Y₂O₃. The boundary curvatures generally increase during the migration and the average migration distance initially increases linearly with the heat treatment time. The observed boundary migration behavior is similar to that observed previously in a number of metallic systems, and its driving force is believed to arise from the coherency strain in the thin solute diffusion zone ahead of the moving boundaries.     

Grain-Boundary Diffusion of 51Cr in MgO and Cr-Doped MgO

The grain-boundary diffusion product, D'δ , of ⁵¹Cr in MgO and Cr-doped MgO as a function of grain-boundary orientation and point-defect concentration was determined at T =1200° to 1450°C. A large degree of anisotropy was found in the grain-boundary diffusion behavior in MgO. The ratio of D'δ_|| parallel to D'δ_‖ perpendicular to the growth direction, D'_||/D'_‖ , is 10² for a 5° (100) tilt boundary, decreased to ∼2 in boundaries with tilt angles > 10°. The decrease in D'_||/D'_‖ is due to a large increase in D'_‖ with increasing tilt angle. The results indicate that grain-boundary diffusion in MgO is connected to the orientation of dislocations and the mechanism is one of dislocation pipe diffusion. The grain-boundary diffusion product D'δ increases with increasing Cr concentration in MgO and is ∼4 times larger for MgO containing 0.56 at. % Cr than for the undoped MgO. For all bicrystals studied, the activation energies are within 180 ± 20 kJ/mol which is 60% of the activation energy for ⁵¹Cr diffusion in undoped MgO.     

Oxygen Self-Diffusion in Single-Crystal Mn-Zn Ferrite

Self-diffusion coefficients for the oxygen ion in single-crystal Mn-Zn ferrite were determined by the gas-solid isotope exchange technique. The oxygen volume diffusion coefficients can be expressed as D =6.70 × 10⁻⁴ exp (-330 (kJ /mol) /RT)m₂/s (>1350°C), D=3.94 × 10⁻¹⁰ exp (−137 (kJ/mol)/RT)m²/s (1100° to 1350°C), and D=7.82 × 10⁴ exp (−507 (kJ/mol)/RT)m²/s (<1100°C).     

Electrical Conductivity of Y2O3 as a Function of Oxygen Partial Pressure in Wet and Dry Atmospheres

The electrical conductivity of polycrystalline Y₂O₃ has been studied as a function of the partial pressure of oxygen (10^–14 to 10⁵ Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P₂O₅. Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P _O2 and decreases with increasing P_H2O. At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P _O2 and increases with increasing P _H2O. In the Y₂O₃ samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P _H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P _O2 and low P _H2O, the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm². s^–1. V^–1     

Preparation and Optical Properties of Transparent Eu3+:Y3Al5(1−x)Sc5xO12 Ceramics

Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu³⁺ for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ was observed when 10 at.% Al³⁺ was substituted by Sc³⁺. The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc³⁺ doping, is the main reason for the reduced peak splitting.     

Effect of Sc Substitution for Al on the Optical Properties of Transparent Ce:YSAG Ceramics

Transparent Ce:YAG (Ce:Y₃Al₅O₁₂) and Ce:YSAG (Ce:Y₃Sc _x Al_{5− x} O₁₂) ceramics have been successfully fabricated using Sc³⁺ to substitute for Al³⁺ in Ce:YAG. The effect of Sc substitution on the luminescent properties of Ce:YAG has been investigated. At the substitution level of 20 at.% of Sc³⁺ for Al³⁺, the emission intensity of Ce:YSAG is the highest. Meanwhile, the doping of Sc into Ce:YAG lattice broadened both the absorption and emission bands, which is believed to be due to the inhomogeneous broadening of the 5 d energy level of Ce³⁺ caused by the Sc³⁺ substitution.