Sol-gel synthesis of titania-alumina catalyst supports

Titanium oxide is a good candidate as new support for hydrotreating (HDT) catalysts, but has the inconvenience of presenting small surface area and poor thermal stability. To overcome these handicaps TiO2-Al2O3 mixed oxides were proposed as catalyst support. Here, the results concerning the preparation, characterization and testing of molybdenum catalyst supported on titania-alumina are presented. The support was prepared by sol-gel route using titanium and aluminum isopropoxides, chelated with acetylacetone (acac) to promote similar hydrolysis ratio for both the alcoxides. The effect of nominal complexing ratios [acac]/[Ti] and of sol aging temperature on the structural features of nanometric particles was analyzed by quasi-elastic light scattering (QELS) and N-2 adsorption isotherm measurements. These characterizations have shown that the addition of acac and the increase of aging temperature favor the full dispersion of primary nanoparticles in mother acid solution. The dried powder presents a monomodal distribution of slit-shaped micropores, formed by irregular packing of platelet primary particles, surface area superior to 200 m(2) g(-1) and mean pore size of about 1 nm. These characteristics of porous texture are preserved after firing at 673 K. The diffraction patterns of sample fired above 973 K show only the presence of anatase crystalline phase. The crystalline structure of the support remained unaltered after molybdenum adsorption, but the surface area and the micropore volume were drastically reduced. (C) 2002 Published by Elsevier B.V. B.V.

Photo-induced transformations in chalcogenide nanocomposite layers

Investigations of photo-induced structural transformations (PST) and related changes of optical parameters in amorphous chalcogenide layers were further developed towards the establishment of their dependence on the compositional modulation of the material at nanoscale-dimensions (similar to3-10 nm) and possible improvement of optical recording parameters as well. Besides the known amorphous-amorphous PST, photo-stimulated interdiffusion and crystallization in multilayer structures were found as a useful method for amplitude-phase optical relief formation. The last two types of PST were influenced by size restrictions and efficiently operated by the composition and by the modulation period of the layered nanocomposite. Experimental evidences were obtained in Se-, AsSe-, Se0.4Te0.6-containing layered or quasi zero-dimensional structures based on As2S3 or SiOx and MgF2 matrix. Comparison was made with As2S3- and GeS2-based multicomponent layers, containing Se, Te and Ga. (C) 2004 Elsevier B.V. All rights reserved.

SYNTHESIS OF ULTRA-FINE CRYSTALLINE ZRXTI1-XO4 POWDER BY POLYMERIC PRECURSOR METHOD

Reactive ZrxTi1-xO4 (x=0.65, 0.50 and 0.35) powder was prepared by the polymeric precursor method. Studies by X-ray diffraction (XRD), nitrogen adsorption/desorption, and thermogravimetric analysis (TG) showed that powders with high crystallinity (>90%) and high surface areas (>40 m(2)/g) are obtained after calcination at 700 degrees C for 3 h. Infrared spectroscopy and XRD results showed that these titanates nucleate from the amorphous phase with no intermediate phases, at low temperature (450 degrees C).

Thermal decomposition of crystalline Ni-II-Cr-III layered double hydroxide: A structural study of the segregation process

A structural study of the thermal evolution of Ni0.69Cr0.31(OH)(2)(CO3)(0.155)(.)nH(2)O into NiO and tetragonal NiCr2O4 is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns. The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides. It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of the parent Cr-III-doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations play in the segregation of crystalline spinels is also discussed.

Nonperturbative approach to potentials in impenetrable boxes based on quasi-exact solvability

In this work we develop an approach to obtain analytical expressions for potentials in an impenetrable box. In this kind of system the expression has the advantage of being valid for arbitrary values of the box length, and respect the correct quantum limits. The similarity of this kind of problem with the quasi exactly solvable potentials is explored in order to accomplish our goals. Problems related to the break of symmetries and simultaneous eigenfunctions of commuting operators are discussed.

Morphological analysis of pits formed on Al 2024-T3 in chloride aqueous solution

It has been used a new image analysis method, based on segmentation by shape parameters, for pits morphology examination from Al 2024 aluminum-copper alloy in chloride aqueous solution. Corrosion behavior of this alloys in naturally aerated 3.5% NaCl solution has been investigated through open circuit potential measurements. Afterwards, pits have been characterized by image analysis taking density and size measurements right from corroded surfaces. Morphological investigation has been conducted for profiles, cut orthogonally from mean surface planes, and observed through light microscopy. Image analysis data could demonstrate that pits are wider than deep, evoluting for conical, quasi-conical or irregular shapes. Most pits have presented a quasi-conical morphology, but the wider ones have evoluted to an irregular shape influenced by sub-surface microstructure. Image analysis based on shape segmentation could enhance the differences on morphological behavior. (C) 2004 Elsevier B.V. All rights reserved.

Coating residual stress effects on fatigue performance of 7050-T7451 aluminum alloy

The tendency of the aircraft industry is to enhance customer value by improving performance and reducing environmental impact. In view of availability, aluminum alloys have a historically tendency to faster insertion due to their lower manufacturing and operated production infrastructure. In landing gear components, wear and corrosion control of many components is accomplished by surface treatments of chrome electroplating on steel or anodizing of aluminum. One of the most interesting environmentally safer and cleaner alternatives for the replacement of hard chrome plating or anodizing is tungsten carbide thermal spray coating, applied by the high velocity oxy fuel (HVOF) process. However, it was observed that residual stresses originating from these coatings reduce the fatigue strength of a component.An effective process as shot peening treatment, considered to improve the fatigue strength, pushes the crack sources beneath the surface in most of medium and high cycle cases, due to the compressive residual stress field induced. The objective of this research is to evaluate a tungsten carbide cobalt (WC-Co) coating applied by the high velocity oxy fuel (HVOF) process, used to replace anodizing. Anodic films were grown on 7050-T7451 aluminum alloy by sulfuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on axial fatigue strength of anodic films grown on the aluminum alloy surface is to degrade the stress-life performance of the base material. Three groups of specimens were prepared and tested in axial fatigue to obtain S-N curves: base material, base material coated by HVOF and base material shot peened and coated.Experimental results revealed increase in the fatigue strength of Al 7050-T7451 alloy associated with the WC 17% Co coating. on the other hand, a reduction in fatigue life occurred in the shot peened and coated condition. Scanning electron microscopy technique and optical microscopy were used to observe crack origin sites, thickness and coating/substrate adhesion. (c) 2007 Elsevier B.V. All rights reserved.

Luminescence in semi-crystalline zirconium titanate doped with lanthanum

This paper aims to describe the synthesis of the semi-crystalline and crystalline powder of lanthanum doped with zirconium titanate (65/35), LZT through Pechini method. The analysis done by Raman demonstrated that semi-crystalline phase at 550 degrees C and crystalline phase after 600 degrees C were formed. The XRD pattern shows the ZrTiO4 phase formation demonstrating that La substitutions into the lattice take place. The calcined powder at different temperatures shows a semi-crystalline phase presenting photoluminescence effect when processed at low temperatures. From 300 to 400 degrees C a broadband is observed at 563 nm and 568 nm, respectively. Defects creation such as: Zr3+ center dot Vo(center dot center dot) and Ti3+ - V-O(center dot center dot), Zr and Ti replaced by La with vacancy formation, impurities and imperfections contributed to the photoluminescence effect. However, the main emission is due to a reverse Ti4+ -> O2- or/and Zr4+ -> O2- transition that occur within a regular titanate or zirconate eight-fold coordination [BO8-delta], B = Zr4+, Ti4+. (c) 2006 Elsevier B.V. All rights reserved.

Influence of silver additions on the Cu-13 wt pct Al alloy hardness.

The influence of silver additions on the Cu-13 wt. pot. Al alloy hardness was studied for additions in the range 0 to 16 wt. pot Ag. The results indicated a pronounced hardness increase with the silver content and an influence of the quenching temperature. Data obtained from scanning electron microscopy indicated that the formation of silver-rich precipitates, wich change with the quenching temperature, seems to produce the changes on alloys hardness.

Influence of silver additions on the thermal behavior of the Cu-8 mass% Al alloy

The influence of additions of 2, 4, 6, 8, 10 and 12 mass% Ag on the thermal behavior of the Cu-8 mass% Al alloy was studied using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicate that the presence of silver introduces new thermal events, due to the formation of a silver-rich phase and, for additions of 10 and 12 mass% Ag, it is possible to verify the formation of the gamma (1) phase (Cu9Al4) and the metastable transitions which are only observed in alloys with a minimum of 9 mass% Al.

Effect of commercial mouthwashes on the corrosion resistance of Ti-10Mo experimental alloy

The purpose of this work was to evaluate the effect of three commercial mouthwashes on the corrosion resistance of Ti-10Mo experimental alloy. Experiments were made at 37.0 +/- 0.5 degrees C in a conventional three-compartment double wall glass cell containing commercial mouthwashes. Three mouthwashes with different active ingredients were tested: ( I) 0.05% sodium fluoride + 0.03% triclosan; (II) 0.5 g/l cetylpyridinium chloride + 0.05% sodium fluoride; (III) 0.12% chlorohexidine digluconate. The assessment of the individual effect of active ingredients was studied by using 0.05% sodium fluoride. Commercially pure titanium (CP Ti) was used as control. Microstructures from Ti-10Mo experimental alloy and CP Ti were also evaluated using optical microscopy. Ti-10Mo as-cast alloy shows the typical rapidly cooled dendrites microstructure (beta phase) while CP Ti has exhibited a metastable martensitic microstructure. Electrochemical behavior of dental materials here studied was more affected by mouthwash type than by Ti alloy composition or microstructure. In both alloys passivation phenomenon was observed. This process may be mainly related to Ti oxides or other Ti species present in spontaneously formed film. Small differences in passive current densities values may be connected with changes in film porosity and thickness. Protective characteristics of this passive film are lower in 0.05% sodium fluoride + 0.03% triclosan mouthwash than in the other two mouthwashes tested.