Synthesis and Characterization of Aurivillius Phases in the Bi–Ag–Ti–O System

New Aurivillius phases in the Bi–Ag–Ti–O system were investigated by means of a solid-state reaction and X-ray diffraction. We found that the oxygen partial pressure has a significant influence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag_0.5Bi_8.5Ti₇O₂₇ was observed after firing in an O₂ flow, but a single-phase material is difficult to obtain. A single-phase compound of the four-layer Aurivillius phase Ag_0.5Bi_4.5Ti₄O₁₅ was obtained on firing in an oxygen partial pressure of 10 bar (1 × 10⁶ Pa). The dielectric properties (at 1 MHz) of the Ag_0.5Bi_4.5Ti₄O₁₅ compound were as follows: T _max=687°C, ɛ _r =166 (∼20°C), and tan δ=0.004 (∼20°C).     

Nonlinear Optical Properties of B2O3-Based Glasses: M2O-B2O3(M = Li, Na, K, Rb, Cs, and Ag) Binary Borate Glasses

The third-order nonlinear optical susceptibilities χ⁽³⁾ of M₂O-B₂O₃ (M = Li, Na, K, Rb, Cs, and Ag) binary borate glasses have been measured by the third harmonic generation (THG) method. It is found that the enhancement of χ⁽³⁾by the structural change of BO₃ units to BO₄ units is small, while the enhancement of χ⁽³⁾ due to the formation of non- bridging oxygen is rather significant. The effects of alkali cations on the χ⁽³⁾ of alkali borate glasses are discussed in terms of the M-O bond character, focusing on the covalency of Li₂O-B₂O₃ glasses. Comparison of the χ⁽³⁾ values for Cs₂O-B₂O₃ and Ag₂O-B₂O₃ glasses which contain cations of comparable polarizability reveals that the χ⁽³⁾ value is much greater for Ag₂O-B₂O₃ glasses than for Cs₂O-B₂O₃glasses, which is possibly due to the great contribution of Ag(4 d_z2 + 5 s + 5 p_z ) hybrid orbitals to the nonlinear optical response.     

Self-Radiation Damage in Curium-244 Oxide and Aluminate

Self-radiation from α-decay of ²⁴⁴Cm causes lattice swelling, phase transformations, and loss of structure in several Cm compounds. In Cm oxides, the percentage swelling at saturation decreases with decreasing symmetry: Swelling in fluorite (fee) CmO₂ (∼0.3%) is approximately twice that in bee C -type Cm₂O₃; monoclinic Cm₂O₃ does not swell. CmO₂ swells with time according to Δa/α=3.0×10⁻³(1–e^−1.7×104λ_t), essentially the same relation as for PuO₂ and AmO₂. The rate of structure loss of Cm oxides decreases with increasing temperature of prior heatings. In CmAlO₃ the metamict state is produced before swelling reaches saturation. Radiation-induced transformation at room temperature produces metastable phases that normally exist at higher temperatures, e.g. bcc C -type Cm₂O₃ transforms to hexagonal A-type Cm₂O₃, and rhombohedral CmAlO₃ becomes cubic.     

Populations and Emission Properties of the 5I6 and 5I7 Levels in Ho3+ Doped into PbO–Bi2O3–Ga2O3 Glasses

Emission properties of PbO–Bi₂O₃–Ga₂O₃ glasses doped with Ho³⁺ were investigated for fiber-optic amplification at the 1.18 μm wavelength region. When the glasses were doped with Ho³⁺ ions only, there was a weak emission at 1.18 μm with a lifetime of ∼200 μs. However, when Yb³⁺ ions were codoped, the lifetime of the 1.18 μm emission increased to 630 μs together with a significant increase in intensity. A similar enhancement in the intensity and lifetimes was realized for the 2.05 μm emission. These effects are due to energy transfer from the Yb³⁺:²F_5/2 to the Ho³⁺:⁵I₆ level. Devitrification of the ternary PbO–Bi₂O₃–Ga₂O₃ glasses was efficiently suppressed by adding 10 mol% GeO₂. Optimum Ho³⁺ concentration was ∼0.4 mol%, whereas Yb³⁺ ions can be added up to the solubility limit.     

Enhancement of Giant Dielectric Response in CaCu3Ti4O12 Ceramics by Zn Substitution

Zn-substituted CaCu₃Ti₄O₁₂ ceramics were synthesized by solid-state sintering. Their microstructures and dielectric properties were investigated. Ca(Cu_{1− x} Zn _x )₃Ti₄O₁₂ single-phase structures were obtained up to x =0.1, and the Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ mixed-valent structure was enhanced with increasing Zn substitution. The giant dielectric response was significantly enhanced by Zn substitution. The dielectric constant increased with increasing x , and a giant dielectric constant plateau as high as ∼9 × 10⁴ was achieved for x =0.1 at 10 kHz, while that for x =0 was ∼3 × 10⁴. The enhanced giant dielectric response was profoundly concerned with the modified mixed-valent structure.     

The ZrB2 Volatility Diagram

A volatility diagram was calculated for temperatures of 1000, 1800, and 2500 K to understand the oxidation of ZrB₂. Applying the diagram, it can be seen that exposure of ZrB₂ to air produces ZrO₂ (cr) and B₂O₃ (l) over the temperature range considered. The pressure of the predominant vapor species was predicted to increase from ∼10⁻⁶ Pa at 1000 K, to 344 Pa at 1800 K, and to ∼10⁵ Pa at 2500 K. Predictions were consistent with experimental observations that ZrB₂ exhibits passive oxidation below 1200 K, but undergoes active oxidation at higher temperatures due to B₂O₃ (l) evaporation.     

Analysis of Infrared Reflection Spectra of (Mg1−xZnx)Al2O4 Microwave Dielectric Ceramics

Infrared reflection spectra of (Mg_{1− x} Zn _x )Al₂O₄ ceramics were analyzed by Kramers–Kroning analysis and classical oscillator model simulation. The dielectric properties were extrapolated down to the microwave range using the classical oscillator model for fitting the dielectric function. According to structure analysis, the losses originating from bend vibration and stretch vibration of the bond between A-site cation and oxygen anion dominated the whole dielectric losses of the spinel ceramics. The coexistence of Mg and Zn deteriorated the intrinsic dielectric properties due to the bond asymmetry thus introduced. The calculated Qf (∼10⁵ GHz) was much higher than the measured ones (∼10⁴ GHz), suggesting that the extrinsic loss was significant. Therefore, the microwave dielectric properties of MgAl₂O₄ and ZnAl₂O₄ could be improved much by microstructure modification, and the little superiority in their solution compared with the end-members was due to microstructure improvement.     

Energy Transfer Process for the Blue Up-Conversion in Calcium Aluminate Glasses Doped with Tm3+ and Nd3+

Excitation of Tm³⁺ to ³ H ₄ using the 791 nm pump source showed the frequency up-converted blue emission (∼480 nm) due to the Tm³⁺:¹ G ₄→³ H ₆ transition in Tm³⁺/Nd³⁺ codoped CaO·Al₂O₃ glasses. Intensity and lifetime changes with rare-earth concentrations suggested the efficient energy transfer of Tm³⁺:³ H ₄→ Nd³⁺:⁴ F _5/2 and Nd³⁺:⁴ F _3/2→ Tm³⁺:¹ G ₄. The latter transfer enabled Tm³⁺ to reach its ¹ G ₄ level, and the blue emission became possible through the ¹ G ₄→³ H ₆ transition. Quantitative analysis with rate equations proved that these two transitions were the most efficient among all the possible energy transfer routes between Tm³⁺ and Nd³⁺. Calculated up-conversion efficiency of the Tm³⁺/Nd³⁺ combination in CaO·Al₂O₃ glass was 6.6 × 10⁻³, and it was ∼4 orders of magnitude larger than those reported for other oxide glasses.     

Thermodynamic Assessment of the Silver–Oxygen System

An assessment of the silver–oxygen system has been made, and a consistent set of thermodynamic parameters has been optimized. The calculated thermodynamic properties and phase relations are in good agreement with the experimental data. Ag₂O is the only phase that is commonly found within the system. In air, it decomposes to silver and oxygen gas at 420 K. There is a eutectic between silver and Ag₂O at a temperature of 804 K, an oxygen partial pressure ( P _O2) of 526 bar (5.26 × 10⁷ Pa), and an oxygen mole fraction in the liquid phase of 0.25. Uncertainties remain on the Ag₂O liquidus for P _O2 > 10⁸ Pa. An ionic two-sublattice model has been used to describe the liquid phase. This work is part of a study of interactions between compounds from the bismuth-strontium-calcium-copper-oxygen system and silver.     

Grain Growth in Fe3O4

Sintering and microstructural evolution were studied in Fe₃O₄ as a model system for spinel ferrites. Fe₃O₄ powder, purified by the salt-crystallization method, was sintered to ∼99.5% density in a CO-CO₂ atmosphere. The p _O2 Of the sintering atmosphere drastically affects the microstructure (grain size) of sintered Fe₃O₄ without significantly affecting density. The measured grain-boundary mobilities, M , of Fe₃O₄ fit the equation M=M ₀( T ) p _O2^−1/2 with M ₀( T ) = 2.5×10⁵ exp[-(609kJ·mol-¹/ RT ](m/s)(N/m²)^−l. The grain-boundary migration process appeared to be pore-drag controlled, with lattice diffusion of oxygen as the most likely rate-limiting step.     

Experimental Phase Equilibria in the PbOx-CaO System

Phase equilibria in the PbO _x -CaO system at oxygen partial pressures of 1.01 10⁵ and 1.01 10³ Pa were experimentally investigated. The temperatures of the eutectic reaction between PbO and PbCa₂O₄ and of the peritectic melting of PbCa₂O₄ were measured via differential thermal analysis at five oxygen partial pressures. The phase transition in air between PbO and Pb₃O₄ was not affected by the coexistence of PbCa₂O₄.     

Seeding of the Reaction-Bonded Aluminum Oxide Process

The effect of the initial α-Al₂O₃ particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al₂O₃ to α-Al₂O₃, and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al₂O₃ particles are fine (∼0.2 μm, i.e., 2.9 × 10¹⁴γ-Al₂O₃ particles/cm³), then they seed the phase transformation. The fine α-Al₂O₃ decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics.     

Quantitative Identification of Phonon Modes Controlling the Multiphonon Relaxation in Heavy-Metal Oxide Glasses

The phonon mode(s) controlling the multiphonon relaxation (MPR) in PbO–Bi₂O₃–Ga₂O₃ glass was analyzed, and the effect of GeO₂ addition on the MPR process was investigated. MPR rates were obtained from the lifetimes of the Tm³⁺:³ H ₄ level in glasses over the temperature range 20–280 K. In PbO–Bi₂O₃–Ga₂O₃ glass, phonons from the bending vibration between GaO₄ tetrahedra (∼550 cm⁻¹) controlled the MPR process. On the addition of GeO₂, the phonon mode at ∼770 cm⁻¹ due to the stretching vibration of GeO₄ tetrahedra started to affect the MPR process. Phonon modes controlling the MPR process in PbO–Bi₂O₃–Ga₂O₃–GeO₂ glass were both 550 cm⁻¹ and 770 cm⁻¹.     

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).     

Structure and Nonlinear Optical Properties of Lanthanide Borate Glasses

The third–order nonlinear optical susceptibility, χ⁽³⁾, of lanthanide (lanthanum, praseodymium, neodymium, and samarium) borate glasses has been measured by the third harmonic generation method. The structure of the present glass system has been studied by infrared and Raman spectroscopic methods. The network structures of the present Ln₂O₃–B₂O₃ glasses have been confirmed to be basically similar to each other. Praseodymium, neodymium, and samarium borate glasses exhibit χ⁽³⁾ values that are larger than lanthanum borate glasses, because of the optical resonance effect, in accordance with the f – f transition. Especially, the χ⁽³⁾ value for 30Pr₂O₃·70B₂O₃ glass is 1.8 × 10⁻¹² esu, which is a factor of ∼60 larger than that of SiO₂ glass. This striking enhancement of χ⁽³⁾ is mainly attributed to the large transition moment to the first excitation state.     

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.     

Oxygen Evolution During MnO-Mn3O4 Dissolution in a Borosilicate Melt

Foam evolution during dissolution of MnO-Mn₃O₄ pellets and powders in borosilicate glass was recorded photographically. The pellets were placed horizontally in transparent crucibles, covered with molten glass, and held at 1150°C. If the Mn₃O₄ content in pellets was more than 31 wt%, they developed foam after an initial foamless period. The length of the foamless period decreased and the duration of foaming increased as the Mn₃O₄ content increased. Batches prepared from MnO-Mn₃O₄ powders and frit, and soaked at 1150°C, foamed without an initial foamless period. The foam developed and collapsed before the set temperature was established within the melt and rose to a higher level than foam produced by pellets. Thermogravimetry of batches heated in 1 atm (∼10⁵ Pa) of O₂ shows oxidation at 400° to 600°C followed by mass loss due to volatilization and oxygen evolution.     

Low-Silica Glasses Based on Calcium Aluminates

An exploratory study of the effects of compositional variation on glass formation in low-silica aluminate compositions revealed that CaO is essential for glass formation at ordinary quenching rates. Other oxides, such as Li₂O, MgO, BaO, ZnO, Na₂O, K₂O, BeO, B₂O₃, and PbO, can be present in the glass in limited amounts (in some cases up to 30 mol%). A necessary, but insufficient, condition for easy glass formation is that the ratio of oxygen ions to network-forming cations (assumed to be Si⁴⁺ and Al³⁺) be ∼2.5. The glass-forming compositions were also characterized by liquidus temperatures below ∼1500°C (2732°F). Young's modulus ranged from 13.0 to 17.8×10⁶ psi. Glasses containing network-modifying cations with high field strengths generally had the higher moduli. Strength (420,000 to 650,000 psi in 0.3-mil fibers), static fatigue, viscosity, annealing, and surface tension were studied to a limited extent for the composition (in mol%) 30Al₂O₃-4SiO₂-60CaO-6MgO.     

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.