Outgassing of ZrF4-Based Glasses

The nature and quantities of the chemical species evaporating from the surface of fluorozirconate glasses heated to above T_g were investigated. ZrF4 vaporized from a ZBLAN glass whereas ZrF₃. ZrF₂ and ZrF vaporized from a ZBL glass. Other gases found include HF, HCl, O₂, CO₂, and atomic fluorine.     

Point Defect Modeling of La2CuO4-Based Superconductors

A point defect modeling study of the oxygen nonstoichiometry and high-temperature electrical properties of La_{2− x} AE _x CuO_{4− y} (AE = Ba, Sr, and Ca) systems indicates that, whereas oxygen excess ( y < 0) due to oxygen interstitials predominates as x → 0, oxygen deficit ( y > 0) results primarily from dopant-vacancy associates at large x ; isolated oxygen vacancies are apparently minority defect species. The onset of oxygen deficiency due to dopant-vacancy associates occurs at x ∼ 0.15 in the AE = Ba and Sr systems but at x ∼ 0.05 in the AE = Ca system. Ideal ionic point defect behavior persists to x ∼ 0.4 in the AE = Ba and Ca systems and to x ∼ 0.5 in the AE = Sr system. A premature flattening of the electrical properties above x ∼ 0.2 occurs in the AE = Ba and Sr systems. Reasons for the extensive range of ideal ionic behavior and for the onset of degenerate electronic behavior are discussed, as are ramifications for high- T_c superconductivity in these materials.     

BiNbO4-Based Glass–Ceramic Composites for Microwave Applications

The effect of a bespoke glass sintering aid, 0.3Bi₂O₃–0.3Nb₂O₅–0.3B₂O₃–0.1SiO₂ (BN1), developed from the base ceramic composition, BiNbO₄ (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛ_r=38, a MW quality factor Q × f ₀=17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τ_f=−10 ppm/°C. The high Q × f ₀, ɛ_r, and low τ_f, coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics.     

Joining Si3N4-Based Ceramics with Oxidation-Formed Surface Layers

A simple processing technique has been developed for joining Si₃N₄-based ceramics. Thin (<5 μm thick), amorphous, or partially crystalline SiO₂-based surface layers were formed, via low-temperature oxidation (at 1200°C), on the faces to be joined. Joining of the surface-coated pieces could then be performed in an inert environment at typical sintering/joining temperatures (i.e., 1700°C), with or without applied gas pressure, via a transient viscous/liquid phase. This method was most effective for Si₃N₄ ceramics with single oxide sintering additives when a thin (∼1 μm thick), highly smooth (RMS roughness <60 nm) SiO₂ layer was formed, and essentially 'pore-free' joints could be formed. However, the method was less suitable for a multi-additive SiAlON material under current experimental conditions, as relatively high roughness (RMS roughness >400 nm) oxide scales formed, leaving residual porosity at the joint interface.     

Surface and Bulk -OH Infrared Absorption in ZrF4- and HfF4-Based Glasses

Infrared absorption at the 3400 cm⁻¹ -OH peak has been measured as a function of thickness for several ZrF₄-BaF₂-LaF₃ and HfF₄-BaF₂LaF₃ glasses to separate contributions from bulk and surface -OH. For glasses melted under CCl₄ reactive atmosphere, the peak is due almost entirely to surface -OH. In ambient atmosphere the -OH peak exhibited no time dependence over a 30 d period, indicating a very small rate of surface attack by atmospheric H₂O.     

Formation Reaction of ZrO2 Scatterers in ZrF4-Based Fluoride Fibers

This paper clarifies the formation reaction of ZrO₂ crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF₂ purified by sublimation. BaO reacts with ZrF₄ to form ZrO₂ at 600°C during a glass-melting process. The ZrO₂ formation reaction is influenced by H₂O. Ba(OH)₂, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF₄ to form ZrO₂ at 450° to 520°C. When low-oxide-content BaF₂ is used for fiber preparation, scatterers significantly decrease.     

可见光分解水制氢催化剂研究进展

光解水制氢能否实用化取决于太阳光的有效利用率,研究开发可见光化的光催化剂成为当前光催化剂研究中的重要课题。介绍了光解水制氢的反应机理,叙述了近年来半导体光催化剂在利用可见光方面的研究进展。认为应该寻找本身具有较高的氢生成活性中心的光催化剂,实现不负载Pt等贵金属光解水制氢;循环使用牺牲剂,或无须牺牲剂实现可见光的光解水。如能开发利用海水制氢,则这对于淡水资源和土地资源日益稀缺的我国来说尤为重要。     

Interaction of BiNbO4-Based Low-Firing Ceramics with Silver Electrodes

Several commercial silver electrode pastes have been evaluated for use with BiNbO₄-based low-firing ceramics. After electrode application and subsequent cofiring, the electrode pattern on the ceramic was no longer visible. A bismuth-rich second phase was present at the electrode/BiNbO₄ interface, and silver was detected in the second phase. Systematic X-ray diffractometry analysis of the mixtures between different silver pastes and doped/undoped BiNbO₄ revealed an interaction between the silver and the BiNbO₄. This reaction was responsible for the disappearance of the silver electrode patterns.     

Melting Apparatus for the Synthesis of Bulk ZrF4-Based Fluoride Glass

Heavy-metal fluoride glasses are currently prepared by melting of the solid fluoride precursors. We have constructed a melting facility for the synthesis of ZrF₄-based fluoride glass cullet. This stand-alone system provides a controlled (dry and reactive) atmosphere and in situ quenching of the melts. The glass is melted in a vitreous carbon crucible, which is mounted in a silica tube flow reactor. The reactor is resistively heated with a tube furnace and purged with He and SF₆ or Cl₂. It was found that of the latter two halogenating species only Cl₂ prevents effectively the formation of black inclusions in the melt (ZrF₃ and ZrF₂ particles). With this melting apparatus we have routinely melted 250-g batches of optically clear ZrF₄–BaF₂–LaF₃–AlF₃–NaF glass samples.     

Decomposition Kinetics of CaYb2O4

The kinetics of decomposition of CaYb₂O₄ were studied using quantitative analysis by X-ray diffraction methods and a modified method of "standard binary mixtures." This type of method was required as total composition remained constant with only the phase assemblage changing. The experimental factors considered in obtaining meaningful results from such a procedure are covered. The linearity of the fraction decomposed or concentration unreacted versus time curves at various temperatures revealed that the decomposition proceeded according to "zero-order" kinetics. The activation energy for this process was 100 kcal/mole.     

Near-Net-Shape Fabrication of Ti3AlC2-Based Composites

A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO₂, and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d ₅₀≈0.7 μm) and inter-agglomerate pores ( d ₅₀≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti₃AlC₂, TiAl₃, Al, and Al₂O₃, which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa.     

High-Temperature Electrical Transport and Thermoelectric Power of Partially Substituted Ca3Co4O9-Based Ceramics

We present high-temperature electrical transport and thermoelectric power of K and/or La-doped Ca₃Co₄O₉-based ceramics. Temperature and doping dependence of electrical transport properties indicate that both K and La substitutions can increase electrical conductivity at a high temperature, which are mainly related to an increase in carrier concentration and carrier mobility. A metallic-to-semiconducting transition temperature ( T ^*) can be observed and shifts to a lower temperature as the doping content increases. However, activation energy is almost independent of the doping. All of the samples show positive thermoelectric power, and the temperature dependence of the thermoelectric power exhibits a simple linear behavior at T > T ^*. Our experimental data indicate that all of the Ca₃Co₄O₉-based ceramic samples exhibit large thermoelectric power (∼120 to 180 μV/K).     

Spark Plasma Sintering of LiTi2(PO4)3-Based Solid Electrolytes

Solid electrolytes, LiTi₂(PO₄)₃ (LTP), Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), and Li_1.3Al_0.3Ti_1.7(PO₄)_2.9(VO₄)_0.1 (LATPV), were prepared by conventional sintering (CS) and spark plasma sintering (SPS) methods, and the Li⁺ ion conductivity of the sintered pellets was examined using an impedance analyzer. SPS remarkably improved the densification compared to CS and resulted in dense ceramics (95–97% of theoretical density) irrespective of the substituted ions. The highest conductivity of 2.6 × 10⁻⁴ S/cm was found for the LATPV specimen sintered by spark plasma at 1100°C. LATP and LATPV exhibited an order of magnitude higher ionic conductivity than LTP in the specimens of similar densities. The results demonstrated that the enhanced conductivity in substituted LTP is not due to the enhanced densification alone. The other possible explanations are discussed in terms of bottleneck size, lithium content, and grain boundary characteristics.     

Tunable Trap Levels Observed in La and Eu Codoped CaAl2O4-Based Phosphor

CaAl₂O₄-based phosphors with Eu²⁺ and La³⁺ codoped have been prepared by the traditional solid-sintering method, and the photoluminescence intensity can be efficiently enhanced by codoped La³⁺ ions. Positron annihilation spectroscopy proves that a large amount of vacancies exist in the CaAl₂O₄ host and the complex defects form on increasing the concentration of doped La³⁺ ions. The thermoluminescence spectra demonstrate that some new trap levels with various trap depths are produced on increasing the concentration of the doped La³⁺ ions, which are correlated to the La³⁺-associated complex defects in the CaAl₂O₄-based phosphor.     

Point Defects of La2CuO4-Based Ceramics, Part I: Oxygen Interstitials

Theoretical and experimental studies are reported on oxygen excess defect structure of lanthanum cuprate, La₂CuO_4+δ. The local density functional formalism is applied to theoretical analysis. The self-consistent discrete variational method has been used to find energy levels, densities of states, charge transfer, wavefunctions, and potentials for a fragment consisting of N (30–44) atoms embedded in the infinite crystal. Calculations have been performed on three interstitial oxygen models, with and without local relaxations. These models include sites proposed on the basis of neutron diffraction and interstitial dimers [O₂]^q. The dimer charge q is found to be consistent with that of normal oxygens, and not -1 as proposed in "superoxide" models. Electrostatic energies are used to estimate the relative stabilities of the proposed interstitial structures. The experimental Seebeck coefficient also shows that the charge of interstitial oxygen is -2. Effective atomic configurations for the ideal lattice are compared with those in the vicinity of the defects, and defect-related spectro-scopic consequences are examined.     

Decomposition Reactions in CaCu3Ti4O12 Ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics sintered in air at 1115°C for 3 and 24 h have been heat treated in N₂ at 1000°C. Surface layers develop on the outer regions of the ceramics, and a combination of X-ray diffraction and analytical electron microscopy has been used to establish the phase content of the layers. A model to explain the formation of the surface layers is proposed based on decomposition of CCTO into a mixture of CaTiO₃, TiO₂, and Cu₂O. The role of limited decomposition in the development of electrically inhomogeneous CCTO ceramics prepared at elevated temperatures in air is discussed.     

GeO2-Based Sol–Gel Films

This paper describes the preparation of new oxide host materials for Er³⁺ based on GeO₂ by the sol-gel method. The characterization of these new materials is reported, with particular regard to the crystallization behavior: the tendency of GeO₂ gels to crystallize during thermal treatment, in fact, is a critical problem. The investigated systems, powders and films, are GeO₂-SiO₂, with different compositions, and 80GeO₂20M _x O _y , with M = Nb, Sn, Ti, Te.     

Ferroelastic Behavior of LaCoO3-Based Ceramics

LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics show a nonelastic stress–strain behavior during four-point bending experiments where hysteresis loops are observed during loading–unloading cycles. Permanent strain is stored in the material after unloading, and a mechanism related to ferroelastic domain switching in the rhombohedral perovskite is proposed. Domain switching in the materials has been confirmed using X-ray diffractometry. Fracture toughnesses of La_0.8Ca_0.2CoO₃ measured using single-edge notched beam and single-edge V-notched beam methods coincide and are equal to 2.2 MPa·m^1/2 at room temperature and decrease to ∼1 MPa·m^1/2 at temperatures >300°C. A decrease in fracture toughness is consistent with ferroelastic behavior, because the rhombohedral distortion decreases with increasing temperature.     

PbZr0.4Ti0.6O3-Based Reflectors with Tunable Peak Wavelengths

Based on the phase separation, a series of quasiperiodic PbZr_0.4Ti_0.6O₃ (PZT) multilayers consisting of the ordered alignment of the dense PZT and porous PZT layers have been grown using one single precursor. The multilayers exhibit good performance as a dielectric mirror, and the position of the reflection maximum can be easily tuned over the whole visible range through varying the spinning rate. More than 86% optical reflectivity and less than 19 nm photonic stop-band width have been achieved at room temperature. The quasiperiodic PZT multilayers can be used as dielectric reflectors, interference filters, and optical cavities.