Mass fractal characteristics of silica sonogels as determined by small-angle x-ray scattering and nitrogen adsorption

A sample series of silica sonogels was prepared using different water-tetraethoxysilane molar ratio (r(w)) in the gelation step of the process in order to obtain aerogels with different bulk densities after the supercritical drying. The samples were analyzed by means of small-angle x-ray-scattering (SAXS) and nitrogen-adsorption techniques. Wet sonogels exhibit mass fractal structure with fractal dimension D increasing from similar to2.1 to similar to2.4 and mass-fractal correlation length xi diminishing from similar to13 nm to similar to2 nm, as r(w) is changed in the nominal range from 66 to 6. The process of obtaining aerogels from sonogels and heat treatment at 500degreesC, in general, increases the mass-fractal dimension D, diminishes the characteristic length xi of the fractal structure, and shortens the fractal range at the micropore side for the formation of a secondary structured particle, apparently evolved from the original wet structure at a high resolution level. The overall mass-fractal dimension D of aerogels was evaluated as similar to2.4 and similar to2.5, as determined from SAXS and from pore-size distribution by nitrogen adsorption, respectively. The fine structure of the secondary particle developed in the obtaining of aerogels could be described as a surface-mass fractal, with the correlated surface and mass-fractal dimensions decreasing from similar to2.4 to similar to2.0 and from similar to2.7 to similar to2.5, respectively, as the aerogel bulk density increases from 0.25 (r(w)=66) up to 0.91 g/cm(3) (r(w)=6).

Continuous pasteurization of egg yolk: Thermophysical properties and process simulation

This article presents empirical correlations to predict the density, specific heat, thermal conductivity and rheological power-law parameters of liquid egg yolk over a temperature range compatible with its industrial thermal processing (0-61 C). Moreover, a mathematical model for a pasteurizer that takes into account the spatial variation of the overall heat transfer coefficient throughout the plate heat exchanger is presented, as are two of its simplified forms. The obtained correlations of thermophysical properties are applied for the simulation of the egg yolk pasteurization, and the obtained temperature profiles are used for evaluating the extent of thermal inactivation. A detailed simulation example shows that there is a considerable deviation between the designed level of heat treatment and that this is predicted through process simulation. It is shown that a reliable mathematical model, combined with specialized thermophysical property correlations, provide a more accurate design of the pasteurization equipment that ensures effective inactivation, while preserving nutritional and sensorial characteristics.

Er3+ and Eu3+ containing transparent glass ceramics in the system PbGeO3-PbF2-CdF2

Glasses with composition 60PbGeO(3)-10PbF(2)-30CdF(2) (mol%) have been obtained in the bulk form with a high stability against crystallization. After doping them with 0.5 mol% of Er3+ or Eu3+ and appropriate heat treatment transparent glass ceramics could be obtained. Electronic spectroscopy, X-ray diffraction and transmission electron microscopy measurements have been made. beta-PbF2: Er3+/Eu3+ Single crystals, 5-10 nm in size, are detected in the otherwise transparent composite medium, the size of the particles and absence of clustering allowing for the increased transparency of the final materials. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Reversible holographic glasses using visible lasers

Antimony based glasses have been investigated for the first time regarding the possibility of holographic data storage using visible lasers sources. Changes in both refractive index and the absorption coefficient were measured using a holographic setup. The modulation of the optical constants is reversible by heat treatment. Bragg gratings were written under visible light of an Ar laser and erased thermally.

Short-range order kinetics in alpha-AgZn for various states of post-deformation defect annealing after cold-rolling

Kinetics of short-range ordering (SRO) in Ag with 21, 23 and 28 at% Zn is investigated by residual resistometry during isochronal and isothermal heat treatment for different states of post-deformation defect annealing after cold-rolling to about 30 and 60% thickness reduction. Resistivity changes due to pure ordering can be separated from the as-measured total resistivity change which includes defect annealing. Although the initial state of SRO of the as-rolled material can be estimated to be comparably low, for as-rolled and partially annealed states by appropriate thermal treatment evolution of SRO is achieved which corresponds quite well to that of recrystallized samples. It is observed, however, that quenched-in surplus vacancies contribute considerably to the ordering process for the recrystallized state and that this contribution is still increased by the grain growth during the final stage of annealing. It therefore turns out that SRO-kinetics under equilibrium vacancy conditions can be better observed in a state of post-deformation annealing, for which deformation induced point defects are annealed-out, but a relatively high dislocation density is still present to act as a vacancy sink. Copyright (C) 1996 Acta Metallurgica Inc.

Effects of synthesis and processing on supersaturated rare earth-doped nanometric SnO2 powders

This paper presents and discusses some of the results of the effects of processing on rare earth-doped nanosize SnO2. Several relevant factors that may influence the characteristics of the final product are studied. The influence of two preparation routes and two heat-treatment conditions on the incorporation of dopants is investigated. The route whereby a soluble salt is used as the dopant source is found to provide the highest degree of dopant incorporation, even under the least favorable heat-treatment conditions.

Epitaxial growth of LiNbO3 thin films in a microwave oven

Oriented LiNbO3 thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 degreesC for 10 min. A microstructural study indicated that the films treated at temperatures below 600 degreesC were homogeneous and dense, and the optical properties confirmed the good quality of these films. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Analyses of fatigue data in 4340 high strength steel after cadmium electroplating

Bending fatigue tests were carried out to clarify the effects of heat treatment parameters: temperature and time after cadmium electroplating on a high strength steel, to avoid hydrogen embrittlement. Temperatures heat of 190 degrees C, 230 degrees C, 250 degrees C and 300 degrees C at 3, 8 and 24 hours together with the base material electroplated, with and without heat treatment, resulted in 14 conditions studied with respect to fatigue behaviour. Statistical data analysis was performed to identify the best combination temperature/time regarding fatigue strength of the ABNT 4340 steel and the results obtained revealed that the fatigue strength depend on temperature/time conditions.

Microstructure and dielectric properties of (Ba,Sr)TiO3 thin film produced by the polymeric precursor method

BaxSr1-xTiO3 (x = 0.6) (BST) thin films were successfully prepared on a Pt(111)/TiO2/SiO2/Si(100) substrate by spin coating, using the polymeric precursor method. BST films with a perovskite single phase were obtained after heat treatment at 700 degrees C. The multilayer BST thin films had a granular structure will a grain size of approximately 60 nm. A 480-nm-thick film was obtained by carrying out five cycles of the spin-coating/heating process. Scanning electron microscopy and atomic force microscopy analyses showed that the thin films had a smooth, dense, crack-free surface with low surface roughness (3.6 nm). At room temperature and at a frequency of 100 kHz, the dielectric constant and the dissipation factor were, respectively, 748 and 0.042. The high dielectric constant value was due to the high microstructural quality and chemical homogeneity of the thin films obtained by the polymeric precursor method.

Characterization of ZnO-degraded varistors used in high-tension devices

The effects of the degradation process on the structural, microstructural and electrical properties of ZnO-based varistors were analyzed. Rietveld refinement showed that the BiO2-x phase is affected by the degradation process. Besides the changes in the spinel phase, the degradation process also affects the lattice microstrain in the ZnO phase. Scanning electron microscopy analysis showed electrode-melting failure, while wavelength dispersive X-ray spectroscopy qualitative analysis showed deficiency of oxygen species at the grain boundaries in the degraded samples. Atomic force microscopy using electrostatic mode force illustrated a decrease in the charge density at the grain boundaries of the degraded sample. Transmission electron microscopy showed submicrometric spinel grains embedded in a ZnO matrix, but their average grain size is smaller in the degraded sample than in the standard one. Long pulses appeared to be more harmful for the varistors' properties than short ones, causing higher leakage current values. The electrical characteristics of the degraded sample are partially restored after heat treatment in an oxygen-rich atmosphere. (C) 2006 Elsevier Ltd. All rights reserved.

Cracks developed during SrTiO3 thin-film preparation from polymeric precursors

Strontium titanate (SrTiO3) thin films were prepared by dip-coating Si(111) single-crystal substrates in citrate solutions of ethylene glycol, considering several citric acid/ethylene glycol (CA/EG) ratios. Measurements of intrinsic viscosity indicate that increasing the amount of EG increases the precursors' polymeric chains and increases the weight loss. After deposition the substrates were dried on a hotplate (approximate to 150 degrees C); this was followed by heat treatment at temperatures ranging from 500 to 700 degrees C using heating and cooling rates of 1 degrees C min(-1). SEM and optical microscopy investigations of the sintered films obtained from different CA/EG ratios indicate that there is a critical thickness above which the films present cracks. This critical thickness for SrTiO3 films deposited on the Si(111) substrate is about 150 nm, Measurements of crack spacing as a function of film thickness indicate that the origin of cracks cannot be explained by the elastic behavior of the film but rather by the viscoelastic relaxation of the film during pyrolysis and sintering. Copyright (C) 1999 John Wiley & Sons, Ltd.

Bulk photochromism in a tungstate-phosphate glass: A new optical memory material?

In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.

Characterization of the reversible photoinduced optical changes in Sb-based glasses

Changes occurring in absorption coefficients when glasses in the SbPO4-WO3 binary system were irradiated by light, at the edge of the absorption band, were measured in real time. These glasses present good thermal and optical properties and photoinduced changes in the absorption coefficients are reversible by heat treatment around 150 degrees C. Subsequent recording/erasing cycles could be made without sample degradation. The sensitivity of the induced optical changes was studied for different wavelengths, light powers and energy of light dose exposures, and for different compositions of the glasses. The changes in the absorption coefficients of the glass samples were accompanied by a color change from yellow to blue, and were also characterized by visible spectroscopy. The color changes occurred through the entire volume of the glass (similar to 2 mm thickness) for the Ar-ion laser lines at the edge of the absorption band. (c) 2006 Elsevier B.V. All rights reserved.

Multi-layered LiNbO3 films prepared by a polymeric precursor method

The polymeric precursor method was used to prepare multi-layered LiNbO3 films. The overall process consists of preparing a coating solution from the Pechini process and the deposited film is subsequently heat-treated. Two-layered films were prepared by this process, onto (0001) sapphire substrates. Two different routes were investigated for the heat-treatment. The amorphous route consisted of performing, after each deposition, a pre-treatment at low temperature to eliminate the organic material. In this case, the crystallization heat-treatment was performed only after the two layers had been deposited. on the other hand, a process layer-after-layer crystallization was used. Both routes led to (0001) LiNbO3 oriented films. However, only the film prepared by the layer-after-layer crystallization presented an epitaxial growth and a crack-free morphology. Moreover, the layer-after-layer crystallization process led to a film exhibiting the best optical properties. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Study of electrical, chemical and structural characteristics of superconductor thin film obtained by polymeric precursors method

Superconductor films of the BSCCO system have been grown by dip coating technique with good success. The chemical method allows us to grow high temperature superconductor thin films to get better control of stoichiometry, large areas and is cheaper than other methods. There is a great technological interest in growth oriented superconductor films due anisotropic characteristics of superconductor materials of high critical temperature, specifically the cuprates, as we know that the orientation may increase the electrical transport properties. Based on this, the polymeric precursor method has been used to obtain thin films of the BSCCO system. In this work we have applied that method together with the deposition technique known as dip coating to obtain Bi-based superconductor thin films, specifically, Bi1.6Pb0.4Sr2.0C2.0Cu3.0Ox+8, also known as 2223 phase with a critical temperature around 110 K. The films with multilayers have been grown on crystalline substrates of LaAlO3 and orientated (100) after being heat treated around 790 degrees C - 820 degrees C in lapse time of 1 hour in a controlled atmosphere. XRD measurements have shown the presence of a crystalline phase 2212 with a critical temperature around 85 K with (001) orientation, as well as a small fraction of 2223 phase. SEM has shown a low uniformity and some cracks that maybe related to the applied heat treatment. WDS has also been used to study the films composition. Different heat treatments have been used with the aim to increase the percentage of 2223 phase. Measurements of resistivity confirmed the presence of at least two crystalline phases, 2212 and 2223, with T-c around 85 K and 110 K, respectively.