Lamellar-interpenetrated Al−Si−Mg/Al2O3−ZrO2 composites prepared by freeze casting and pressureless infiltration

Using freeze casting and pressureless infiltration methods, we prepared lamellar Al−Si−Mg/Al₂O₃−ZrO₂ composites with initial ceramic loading of 30 vol% and different Al₂O₃:ZrO₂ weight ratios (Al₂O₃:ZrO₂=1:9, 3:7, 5:5, 7:3 and 9:1). The resultant composites inherited the lamellar structure of the Al₂O₃−ZrO₂ scaffolds, and the thickness of both metal and ceramic layers showed a trend of first increase and then decrease with increasing Al₂O₃ content. During pressureless infiltration, multiple chemical reactions took place between ZrO₂ and the Al−12Si−10Mg alloy and the main reaction products were (Al_1−m, Si_m)₃Zr, Al₂O₃ and ZrSi₂ phases. The degree of the reaction depended on the ZrO₂ content in the ceramic composition. In general, the compressive strength of the composites decreased with increasing Al₂O₃ content, but three-point bending strength showed a first decrease and then increase. When Al₂O₃:ZrO₂=1:9, the compressive and bending strength of the composites reached about 997±60 MPa and 426±10 MPa, respectively. A simple model was proposed to illustrate the fracture mode and toughening mechanism of the composites.     

Hydrothermal synthesis and characterization of nanorods “LixV2−δO4−δ·H2O”

Nanorods “Li_xV_2−δO_4−δ·H₂O” were hydrothermally synthesized with starting agents LiOH·H₂O and V₂O₅, and reducing agent hydrazine monohydrate (NH₂NH₂·H₂O) under alkaline condition at 160 °C. The samples were characterizated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The nanorods obtained have diameters from 80 to 100 nm with length up to several micrometers. Molecular coordination and assembly mechanism can be assumed to explain the formation of one-dimensional nanorods.     

CrO4 2− Ions Adsorption by Fe-Modified Pozzolane

Abstract

A Fe-modified pozzolane was prepared and employed for the removal of CrO₄ ^2? ions from aqueous solution under batch type experiments as a function of contact time, initial concentration of metal ion and temperature. The adsorption isotherms are described by means of Freundlich, Langmuir, and Dubinin-Radushkevich (D-R) models. The pozzolane was characterized by XRD diffraction analysis. The results showed that the maximum adsorption capacity of Fe-modified pozzolane for CrO₄ ^2? ions was (3.23 ± 0.01)×10^?3 mmol g^?1. The adsorption was found to be initial concentration and temperature dependent. The thermodynamic parameters values such as ΔH⁰, ΔG⁰, and ΔS⁰ were obtained to predict the nature of adsorption. These values show that the adsorption reaction is endothermic and spontaneous. The results show that the Fe-modified pozzolane, that is an easily available material, can be successfully used as adsorbent of anionic species, such as CrO₄ ^2? ions in aqueous solutions, and can be an alternative for more costly adsorbents used for chromates removal in wastewater treatment processes.     

Synthesis,characterization and catalytic properties of La2−x Sr x NiO4−δ

Various compositionsx in the catalyst system La_2–x Sr _x NiO_4– have been prepared by conventional techniques and characterized by X-ray powder diffraction, electron microscopy and BET surface measurements. The catalytic properties of these catalysts have been tested in the propylene oxidation reaction. The catalytic activity can be correlated with the oxygen content and with the strontium substitution.     

Combustion Synthesis of PrBa2Cu3O7−δ and YBa2Cu3O7−δ cuprate materials

PrBaCuO and YBaCuO cuprate materials were prepared from cooper, barium peroxide, and yttrium/praseodymium oxide by SHS and standard solid-state synthesis. SHS reactions were carried out using relatively large cooper particles (< 63 im) to obtain small product samples (13 mm in diameter). High ambient temperature was used to stabilize a combustion front in the ignited pellets. Explored was the effect of cooper particles size, starting density, and ambient temperature on phase evolutions in synthesized materials.     

A corrosion mechanism of titanium diboride in KF−AlF3−Al2O3 melt

TiB₂ samples were exposed to molten KF?AlF₃?Al₂O₃: 54.8-42.1-3.1 mol% salt, at 680 °C for 50, 100 and 200 h. The corroded samples of TiB₂ were investigated by SEM-EDX, EBSD, XRD, FT-IR and MAS NMR analysis. Corrosion was noted to occur predominantly as pitting attacks on the surface of the investigated materials. An inter-crystal and trans-crystal corrosion were identified on the cross-sections of the samples. A perturbation of TiB bonds was detected (SEM-EDX and NMR analysis), at which a formation of orthorhombic TiO₂ was also identified (EBSD analysis). The subsequent NMR, XRD and FT-IR analysis of the behaviour of TiB₂ powder in molten KF?AlF₃?Al₂O₃ supports the statement about the formation of orthorhombic TiO₂ and mullite type of aluminium borates.     

Preparation and electrical properties of (1−x)SrBi2Nb2O9−xBiFeO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, (1−x)SrBi₂Nb₂O₉−xBiFeO₃ [(1−x)SBN−xBFO] (x=0.0, 0.03, 0.05, 0.07, 0.10) were prepared by a conventional solid-state reaction method. The crystal structure, microstructure and electrical properties were systematically investigated. All compositions formed layered perovskite structure without any detectable secondary phases. Plate-like morphology of the grains which is characteristic for layer-structure Aurivillius compounds was clearly observed. The excellent electrical properties (e.g., d₃₃~19 pC/N, 2P_r~18.8 μC/cm²) and a high Curie temperature (e.g., T_c~449 °C) were simultaneously obtained in the ceramics with x=0.05. Additionally, thermal annealing studies indicated that the BFO modified SBN ceramics system possessed stable piezoelectric properties, demonstrating that the modified SBN-based ceramics are the promising candidates for high-temperature applications.     

Thermodynamic Properties of Aqueous Aerosols to High Supersaturation: I—Measurements of Water Activity of the System Na+−Cl−−NO− 3−SO2− 4−H2O at ~ 298.15 K

ABSTRACT

Water activity/concentration relationships of aqueous NaCl, NaNO₃, Na₂SO₄ and their mixtures from dilute concentration to high supersaturation have been determined at room temperature in an electrodynamic balance. Using a dynamic measurement technique based on the evaporation of the droplets, a set of water activity measurements of a solution droplet can be obtained in less than an hour. The water activities of the mixtures are compared with theoretical predictions of the Pitzer-Simonson-Clegg (PSC) model, the Zdanovskii-Stokes-Robinson (ZSR) equation, and the Kusik and Meissner (KM) model. Comparisons of the data with model predictions indicate that the PSC, ZSR, and KM approaches agree well among themselves and with the experimental data. All three models give similar maximum (±0.02) and standard (0.01) deviations in mass fraction of the solute (mfs). These deviations are close to the experimental error of ±(0.01–0.02). The agreement of the models and the need for single-component water activities at different temperatures, as input for all models, suggest that a high priority should be given to such experiments in the future.     

Perovskite forming ceramics of the system SrxLa1−x TiIVx+yCoIIyCoIII1−x−2yO3 for NTC thermistor applications

The paper reports the preparation, structure and electrical properties of oxide ceramic semiconductors based on the series Sr_xLa_1−x Ti^IV_x+yCo^II_yCo^III_1−x−2yO₃ with perovskite type stucture: 0<x<1, 0<y<(1−x)/2. The study starts from LaCoO₃ which is highly conductive yielding metallic condutivity above 330°C. The upset trigonal distortion of LaCoO₃ is reduced when Sr^II/Ti^IV is substituted for La^III/Co^III corresponding to increasing values of x and also when 2 Co^III are introduced for Ti^IV/Co^II into the lattice corresponding to increasing values of y. At high values of x and y othorhombic distortion occurs. At the same time, the interaction between the Co^III atoms of LaCoO₃ is increasingly interrupted providing increasing values of the the resistivity value ρ_25°C and of the B_25/100°C value deduced from measurements at 25 and 100°C according to ρ(T )=ρ_25°C e^B/T. The range of variation of x an y makes possible to prepare ceramics with desired electrical properties within the limits of ρ_25°C=1.1 Ωcm, B=1910 K and ρ_25°C=1 bis 8×10⁶ Ωcm at B-values up to 6500 K. Dependent on composition, NTC ceramics for thermistor or insurance applications are accessible. Thermistors do not show aging even at higher temperature, e.g. at 500°C, provided the single phase state is achieved as a result of mixed oxide preparation and sintering. Hence, high temperature thermistor applications are also made possible. The semiconductor behavior can be understood using the conventional polaron state hopping model.     

A novel TiO2/ZrxTi1−xO2 composite photocatalytic films

TiO₂/Zr_xTi_1−xO₂ composite films have been prepared by sol–gel method and their photocatalytic activity and stability have been investigated for the first time. Their surface morphology and average surface roughness are characterized by AFM. TiO₂ P25 and pure sol–gel TiO₂ films have also been prepared and investigated for comparison. Films with smaller crystallite size and larger surface roughness have been obtained by introduction of Zr_xTi_1−xO₂ intermediate layers between TiO₂ layers and substrate in the composite films. The results show that the photocatalytic activity and stability of the composite films are higher than those of pure sol–gel TiO₂ and TiO₂ P25 films.     

On the Role of Calcination Temperature in Pt-SO 4 2− /ZrO2−Al2O3 Preparation and Catalytic Behaviors During the n-Hexane Hydroisomerization

Effects of calcination temperature for Pt-SO ₄ ^2? /ZrO₂?Al₂O₃ (PSZA) catalysts in n-hexane hydroisomerization were investigated by N₂-adsorption, XRD, TG-DTA, FTIR, XPS and H₂-TPR. An optimum calcination temperature is helpful to complete the crystallization process, resulting in better distribution of alumina into zirconia crystal and producing new acid centers responsible for higher catalytic activity.     

VSe2−ySy electrodes in lithium and lithium-ion cells

Step potential electrochemical spectroscopy data of lithium anode cells using VSe2–ySy(0y0.5) compounds as cathode material and LiClO4 solutions in PC as electrolyte were studied. On increasing y, a lower extension of the intercalation process was observed, due to the loss of a second insertion step corresponding to lithium ion occupancy of tetrahedral sites. In contrast, the reaction rate increased with sulfur content. VSe2–ySy/LiCoO2 lithium-ion cells were cycled to test the electrochemical behaviour of the vanadium chalcogenides as anodic material. A better capacity retention was observed for y = 0.3. The effect of current density on the performance of the cells was also evaluated.     

Deactivation of H-mordenite and ZrO2/SO2−4 during n-butane isomerization

The catalyst deactivation in the isomerization of n-butane was studied on H-mordenite and ZrO₂ promoted by SO²⁻₄ ion. The deactivation of H-mordenite was the result of coke deposition on the catalyst pores and the deactivation of ZrO₂/SO²⁻₄ resulted from the decrease in the oxidation state of sulphur in the surface complex. The reoxidation of sulphur, producing again the SO²⁻₄ ion, led to the recovery of the catalytic activity of ZrO₂/SO²⁻₄ and coke burning resulted in the recovery of the H-mordenite activity.     

Ketonization of fatty methyl esters over Sn−Ce−Rh−O catalyst

A mixture of methyl esters of fatty acids obtained by transterification of nonerucic rape oil was ketonized. The starting material, diluted with methanol, was converted at atmospheric pressure over a catalyst that contained Sn, Ce, and Rh oxides in a molar ratio of 90:9:1. At a temperature of 385°C ketones were obtained with a total yield of 63% at the 96% conversion of starting material. The reported experiments prove that catalysts other than iron that are active in ketonization of primary alcohols can be successfully used in ketonization of esters of fatty acids. The kind of diluent used plays a crucial role in the conversion.     

Production of hydrogen by steam reforming of methanol over Cu/CeO2 catalysts derived from Ce1−xCuxO2−x precursors

The Ce_1−xCu_xO_2−x mixed oxides were synthesized using the coprecipitation method, and were reduced to form the Cu/CeO₂ (cop) catalysts for the steam reforming of methanol. All the Cu-containing catalysts tested in this study showed high selectivities to CO₂ (over 97%) and H₂. A 3.9 wt% Cu/CeO₂ (cop) catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513 K, which was higher than those over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/Al₂O₃ (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the 3.9 wt% Cu/CeO₂ (cop) catalyst was due to the highly dispersed Cu metal particles and the Cu⁺ species stabilized by the CeO₂ support.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

Synthesis and characterization of Bi1.5Zn0.92Nb1.5−xSnxO6.92−x/2 pyrochlore ceramics

The morphological, compositional, structural, dielectric and electrical properties of Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 ceramics have been investigated by means of scanning electron microscopy (SEM), X-ray energy dispersion spectroscopy (EDS), X-ray diffraction (XRD), temperature and frequency dependent dielectric constant and temperature dependent conductivity measurements for Sn-contents in the range of 0.00 ≤ x ≤ 0.60. It was shown that single phase of the pyrochlore ceramics can only be obtained for x ≤ 0.25. Above this value a ZnO phase appeared in the XRD patterns and SEM micrographs as well. An increase in the lattice constant and in the temperature coefficient of dielectric constant and a decrease in the dielectric constant values with increasing Sn content was observed for the ceramics which exhibited a single phase formation. A temperature dependent but frequency invariant dielectric constant was observed for this type of ceramics. The lowest electrical conductivity and highest dielectric constant was observed for the sample which contains 0.06 Sn. The Bi_1.5Zn_0.92Nb_1.5?xSn_xO_6.92?x/2 pyrochlore ceramic conductivities are thermally active above 395 K. For temperatures greater than 395 K, the conductivity activation energy which was found to be 0.415 eV for the pure sample increased to 1.371 eV when sample was doped with 0.06 Sn.     

Antiferromagnetism and heat capacity of NaCo2−xCuxO4 ceramics

Polycrystalline samples of NaCo_2−xCu_xO₄ (x=0, 0.01, 0.03 and 0.05) were synthesized in two different ways: 1) by a mechanochemically assisted solid-state reaction method (MASSR) and 2) by a citric acid complex method (CAC). In this work we examined the influence of these synthesis routes and small Cu concentrations on magnetic properties and the heat capacity of sintered samples. The magnetic susceptibility (χ) of all samples followed the Curie-Weiss law in the temperature range between 50 K and 300 K, while a negative Weiss constant (θ) implied an antiferromagnetic interaction. According to the magnetic susceptibility data, a peak around 30 K indicating the presence of Co₃O₄ as a secondary phase appeared for all MASSR samples and CAC samples with Cu content above 1%. The effective magnetic moment (μ_eff) of CAC samples was lower than the theoretical, spin only value obtained for the Co⁴⁺ ion in the low spin state indicating the presence of low spin Co³⁺(S =0). These values were also lower compared to the values obtained for MASSR samples. The highest μ_eff of 1.75 μ_B/atom Co was obtained for the undoped MASSR sample. The heat capacity of CAC samples at 2 K decreased with Cu concentration due to lowering of the electronic specific heat coefficient (γ). The highest γ of 63.9 mJ/molK² was obtained for the undoped CAC sample. This reduction in γ values was the result of the decrease of the density of state and/or mass enhancement factor.