Radiation-induced color centers in γ-irradiated glasses in the Na2O-Nb2O5-P2O5 system

The specific features of the induced optical absorption spectra of glasses in the 45Na₂O · xNb₂O₅ · (55 ? x)P₂O₅ system with Nb₂O₅ contents x = 5, 10, 20, 25, 30, and 35 mol % are investigated as functions of the irradiation dose and the heat treatment time. The spectra are decomposed into Gaussian components with the use of computer processing. It is revealed that the glass composition and the irradiation dose affect the number, type, and parameters of the bands associated with the PO ₄ ^2? hole-type centers, electron-type color centers of the phosphate matrix, [Nb^(5+)?] one-electron centers, and [Nb^(5+)?-O-Nb^(5+)?] two-electron centers. The inference is made that heat treatment at temperatures close to the glass transition point T _g leads to the formation of groupings with a structure similar to structural motifs of NaNbO₃ crystals. It is demonstrated that these groupings are responsible for the induced optical absorption in the near-IR spectral range.     

Modeling nonisothermal crystallization in a BaO∙2SiO2 glass

The accuracy of a differential thermal analysis (DTA) technique for predicting the temperature range of significant nucleation is examined in a BaO∙2SiO₂ glass by iterative numerical calculations. The numerical model takes account of time-dependent nucleation, finite particle size, size-dependent crystal growth rates, and surface crystallization. The calculations were made using the classical and, for the first time, the diffuse interface theories of nucleation. The results of the calculations are in agreement with experimental measurements, demonstrating the validity of the DTA technique. They show that this is independent of the DTA scan rate used and that surface crystallization has a negligible effect for the glass particle sizes studied. A breakdown of the Stokes-Einstein relation between viscosity and the diffusion coefficient is demonstrated for low temperatures, near the maximum nucleation rate. However, it is shown that accurate values for the diffusion coefficient can be obtained from the induction time for nucleation and the growth velocity in this temperature range.     

The CO2–CeO2 interaction and its role in the CeO2 reactivity

The interaction between CO₂ and CeO₂ and its role in the surface reactivity of alumina-supported cerium oxide has been studied by programmed thermodesorption (TPD) of CO₂ and FTIR spectroscopy. The performance of Ce/Al₂O₃ systems was then analyzed for the propane oxidation in presence of CO₂. The results have shown that the catalytic activity decreased when carbonate species are formed at the surface of CeO₂. This behavior was attributed to the presence of CO₂ from three different sources: contamination before use, during the handling of the samples, contamination proceeding from the reactants and from CO₂ produced by the reaction itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase relations in ZrO2–La2 Zr2O7–Y2 Zr2O7system

Abstract

The phase relations in the system ZrO₂–La₂Zr₂O₇– Y₂Zr₂O₇ have been investigated using X-ray diffraction. Mixed oxide phase assemblages were prepared by hydrolysing zirconium butoxide with solutions of Y and La nitrates, followed by drying, calcining, and sintering. The cubic zirconia phase can accept into solid solution the larger, non-cubic stabilising lanthanum ion in the presence of a suitable proportion of the cubic stabilising oxide of yttrium. As the amount of the larger rare earth element ion is increased formation of pyrochlore and tetragonal type compounds is favoured.     

Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system

Liquidus line in the high-Bi₂O₃-containing region of the TiO₂–Bi₂O₃ system was determined experimentally. The equilibrating and quenching technique with subsequent electron probe microanalyser (SEM-EDS) microanalysis were employed. Based on the data, liquidus line was constructed between 60 and 92 mol% Bi₂O₃. The current results showed a higher solubility of Bi₂O₃ in the liquid phase in equilibrium with the Bi₄Ti₃O₁₂ compound compared with the existing phase diagram. In addition, differential scanning calorimetry (DSC) was used to estimate the transformations covering the composition range from 60 to 95 mol% Bi₂O₃. Further, the phase diagram of the TiO₂–Bi₂O₃ system was calculated using a quasichemical model for the liquid phase. The thermodynamic properties of the intermediate compounds were estimated from the data of TiO₂ and Bi₂O₃ pure solids.     

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CeO₂–Y₂O₃ as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO₂–0.5 mol% Y₂O₃–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, is obtained as 832.5 K and the M_s temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.     

Enhancement of the Low Temperature Activity in NO Reduction in Lean Conditions by SMSI Effect in Pt/CeO2–ZrO2 on Alumina Catalyst

A treatment with H₂ at 800°C followed by mild reoxidation (<300°C) increases the low temperature (<200°C) steady-state activity in the reduction of NO by propene in the presence of O₂ of a Pt(1.5%)/(ZrO₂–CeO₂ on Al₂O₃) catalyst. The effect is not present for a lower temperature treatment with H₂ or for reoxidation above 300°C. Transient reactivity tests together with characterization data (H₂-TPR, O₂ uptake, H₂ chemisorption) indicate that the high temperature reduction modifies the characteristics of ceria–zirconia and induced metal–support interaction effects, both responsible of the enhanced catalytic activity at low temperature.     

How different dopants leads to difference in photocatalytic activity in doped TiO2?

This study explores the significance of dopant location in a doped TiO₂ nanostructure in ascertaining its photocatalytic properties. The un-doped TiO₂, boron-doped TiO₂ (B–TiO₂) and nitrogen-doped TiO₂ (N–TiO₂) photocatalysts were synthesized (with variable dopant concentrations) via sol-gel method. The photocatalysts were further characterized for structural, surface, and physico-chemical properties in reference to their influence on photocatalytic properties. The results of X-ray diffraction (XRD), micro Raman, Energy dispersive X-ray technique (EDX), X-ray photoelectron spectroscopy (XPS), and Fourier Transform infrared spectroscopy (FTIR) confirmed the existence of B and N atoms in the TiO₂ crystal lattice. The results also indicated that the B and N doping promoted the formation of rutile phase in doped TiO₂. Further, B doping leads to decrease in the surface area whereas N doping leads to increase in surface area of TiO₂. The UV–Vis DRS analysis revealed that a red shift in absorption band edge occurs upon B and N doping. The band gap values also decreased to 2.96 and 2.27 eV in B–TiO₂ and N–TiO₂, respectively in comparison to 2.98 for un-doped TiO₂. The photocatalytic degradation studies of diclofenac sodium (DCLF) were conducted to examine the effect of dopant role on the efficiency of doped photocatalyst. B–TiO₂ exhibited maximum photocatalytic activity by degrading 98% of DCLF in comparison to N–TiO₂, which showed 95% degradation.     

Insights into the ternary eutectic microstructure formed in intercolony regions in Al2O3–ZrO2(Y2O3) system

Al₂O₃/ZrO₂(Y₂O₃) pseudo-binary eutectic melt may solidify in the dendritic form at the low G/v ratio (thermal gradient G divided by growth rate v) associated with the Y₂O₃ microsegregation along the monovariant line. Especially, in the system with high Y₂O₃ content (≥4.5 mol%), an Al₂O₃/ZrO₂/YAG ternary eutectic in situ microstructure growing with a “plane front” will appear at the boundaries of dendritic structures. The volume fraction of the ternary eutectic increases with the increase in Y₂O₃ content. It was found that the amount of ternary eutectic can be well predicted by the Scheil equation (or nonequilibrium lever rule).     

Enhanced photocatalytic activity by tailoring the interface in TiO2–ZrTiO4 heterostructure in TiO2–ZrTiO4–SiO2 ternary system

To clarify the interfacial structural of TiO₂–ZrTiO₄ heterostructure and their function in improving the catalytic behavior, TiO₂–ZrTiO₄–SiO₂ photocatalyst with mesoporous structure, a large surface area (188.54 m²g^-1) and an appropriate band gap energy (3.06 eV) was synthesized by in-situ reaction between TiO₂ and ZrO₂ at 800 °C in TiO₂–ZrO₂–SiO₂ ternary system. The concomitant effects of loading amount of TiO₂ and calcination temperatures on the interfacial interaction, and the corresponding interfacial effects on the photodegradation performance of the catalyst were investigated. Results show that an optimal amount of TiO₂ is crucial in affecting the degree of interfacial interactions. Photocatalytic activity is significantly enhanced with the strong interfacial interaction between TiO₂ and ZrTiO₄ by forming heterojunction. A higher amount of TiO₂ and calcination temperature will lead to the decrease in BET value, increase in pore size and band gap value, as well as the coarsening and aggregation of particles which exhibit negative influence on the interfacial interaction between TiO₂ and ZrTiO₄. In the photodegradation process, the prepared catalyst with a appropriate TiO₂ molar ratio (Ti:Zr:Si, 5:1:6) exhibited a RhB-adsorption of 60.7% in dark reaction, and a degradation rate of 95% after visible light irradiation for 60 min. In addition, the probable degradation mechanism for the improvement in the degradation efficiency was discussed in detail.     

Phase formation and crystal structure determination in the Y2O3–TeO2 system prepared in an oxygen atmosphere

The solid state synthesis of phases in the Y₂O₃–TeO₂ system under oxygen atmosphere is presented here as a method of obtaining pure material starting from simple oxides such as Y₂O₃ and α-TeO₂. In the composition range of 0–100% of TeO₂, the compounds Y₆TeO₁₂, Y₂TeO₆, Y₂Te₄O₁₁, Y₂Te₅O₁₃ and Y₂Te₆O₁₅ were synthesized after annealing at 770–1100 °C. The reactions of decomposition of the phases at higher temperatures are also reported.The crystal structure of Y₆TeO₁₂ has been refined by the Rietveld method from X-ray powder diffraction data. It crystallizes with the rhombohedral space group R-3 (S.G. no. 148) as the homologous compound In₆TeO₁₂. It is based on a three dimensional arrangement of isolated TeO₆ octahedra (TeO = 1.94 Å) and edge-shared YO₇ polyhedra (YO bond lengths ranging from 2.18 to 2.65 Å).     

Corrosion protection of steel in molten Li2CO3–K2CO3 and Na2CO3–K2CO3 mixtures in a hydrogen-containing atmosphere

The electrochemical behaviour of TiN-, TiN–AlN-, Cr- and CrN-coated 316L stainless steel in molten Li₂CO₃–K₂CO₃ and Na₂CO₃–K₂CO₃ melts in a reducing gaseous atmosphere (10% H₂–90% N₂) was studied using voltammetry and scanning electron microscopy combined with energy-dispersed X-ray analysis in the temperature range of 600–730 C. To facilitate the identification of the electrochemical reactions the voltammetric behaviour of stainless steel, titanium, nickel and gold was also investigated. Voltammetric characteristics obtained at AlN–TiN coated electrodes showed no anodic reactions at potentials more negative than that of CO^2– ₃ oxidation. Cr- and CrN-coated electrodes demonstrated a suppressed anodic dissolution after the first steady state voltammetric cycle. The voltammograms obtained for the other electrodes studied displayed the corresponding anodic metal-dissolution waves. TiN, AlN, Cr and CrN coatings seem to be the most promising as corrosion-resistant materials for the anodic compartments of molten carbonate fuel cells.     

Near Constant Loss Dielectric Response in 2Bi2O3–B2O3 Glasses

Electrical conduction and relaxation phenomena in bismuth borate glasses in the composition 2Bi₂O₃–B₂O₃ (Bi₄B₂O₉) were investigated. Dielectric studies carried out on these glasses revealed near constant loss response in the 1 kHz–1 MHz frequency range at moderately high temperatures (300–450 K) associated with relatively low loss (tan δ = 0.006) and high dielectric constant (ε_r′ = 37) at 1 kHz, 300 K. The variation in AC conductivity with temperature at different frequencies showed a cross over from near constant loss response characterized by local ion vibration within the potential well to universal Jonscher's power law dependence triggered by ion hopping between potential wells or cages. Thermal activation energy for single potential well was found to be 0.48 ± 0.05 eV from cross over points. Ionic conduction and relaxation processes were rationalized by modulus formalism. The promising dielectric properties (relatively high ε_r′ and low tan δ) of the present glasses were attributed to high density (93% of its crystalline counterpart), high polarizability, and low mobility associated with heavy metal cations, Bi³⁺.     

Crystallization of Glasses in the K2O–Nb2O5–SiO2System

The changes in the structure and phase composition of glasses in the K₂O–Nb₂O₅–SiO₂system upon their heat treatment in the temperature range 700–800°C are studied by the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction. It is demonstrated that the crystallization is the primary process giving rise to microinhomogeneities in glasses due to heat treatment. Nanocrystals of an unidentified niobium-containing phase precipitate in glasses with the formation of regions with a decreased content of potassium and niobium oxides. An increase in the duration of heat treatment at the studied temperatures results in an increase in the size of nanocrystals without change in their phase composition. This is accompanied by the disappearance of diffusion zones, which leads to a decrease in the SAXS intensity in the range of small scattering angles and, correspondingly, to a decrease in the light scattering intensity.     

Hot Photoluminescence in γ-In2Se3 Nanorods

The energy relaxation of electrons in γ-In₂Se₃ nanorods was investigated by the excitation-dependent photoluminescence (PL). From the high-energy tail of PL, we determine the electron temperature (T _e) of the hot electrons. The T _e variation can be explained by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The high-quality γ-In₂Se₃ nanorods may be a promising material for the photovoltaic devices.     

Mandelshtam–Brillouin Scattering in As2S3 and GeS2 Chalcogenide Glasses

Mandelshtam–Brillouin and Rayleigh scattering in As₂S₃ and GeS₂ chalcogenide glasses is investigated. Longitudinal hypersonic velocities, adiabatic elastooptical constants (p ₁₂) _ad at frequencies 15 GHz, extinction coefficients, and scattering losses are determined.     

Electrocatalytic Activity of Ti/TiO_2 Electrodes in H_2SO_4 Solution

Ti/TiO_2 electrodes were prepared with the polymeric precursor method (PPM). The structureand morphology of Ti/TiO_2 electrodes were examined with XRD and ESEM. The voltammetric charge(q~*) of Ti/TiO_2 electrodes as cathode in 0.5 mol/L H_2SO_4 solution was investigated with cyclicvoltammetry. It was found that the electrocatalytic activity of the Ti/TiO_2 electrodes was affected by thestructure and morphology of the Ti/TiO_2 electrodes, in other words, was affected by the calcinationconditions of preparing the electrodes. The value of q_in~* was considerably larger than that of q_out~*, whichmeans that the 'inner' active surface area was much larger than the 'outer' active surface area, and'inner' active surface played a main role in the electrocatalytic activity of the Ti/TiO_2 electrodes.     

Subsolidus phase relations in the SnO2–CuSb2O6 binary system

The SnO₂–CuSb₂O₆ binary system plays a significant role in the SnO₂–Sb₂O₃–CuO ternary system, delimiting the compositional ranges and their sensor and catalytic properties. The initial compositions expressed as (1−x) SnO₂–xCuSb₂O₆ were treated both non-isothermally using DTA up to 1500 °C and iso-thermally at temperatures between 1000 and 1200 °C. Phase analysis of the samples was determined by XRD and IR spectroscopy. It was established that SnO₂–CuSb₂O₆ is a pseudobinary system with a solid solubility limit of the end members. The determined lattice parameters of the unit cell of the obtained rutile and trirutile solid solutions obeyed Vegard's rule. From IR spectral investigations it was inferred that Sn⁴⁺ might be preferentially incorporated on Sb⁵⁺ sites into the CuSb₂O₆ lattice.     

Electrocatalytic Activity of Ti/TiO2 Electrodes in H2SO4 Solution

Ti/TiO2 electrodes were prepared with the polymeric precursor method (PPM). The structure and morphology of Ti/TiO2 electrodes were examined with XRD and ESEM. The voltammetric charge (q*) of Ti/TiO2 electrodes as cathode in 0.5 mol/L H2SO4 solution was investigated with cyclic voltammetry. It was found that the electrocatalytic activity of the Ti/TiO2 electrodes was affected by the structure and morphology of the Ti/TiO2 electrodes, in other words, was affected by the calcination conditions of preparing the electrodes. The value of q*in was considerably larger than that of q*out, which means that the ‘inner’ active surface area was much larger than the ‘outer’ active surface area, and ‘inner’ active surface played a main role in the electrocatalytic activity of the Ti/TiO2 electrodes.