Structural and electrical characterization of semiconducting barium-lead-titanate ceramics

BaTiO3 is usually doped to achieve the temperature stability required by device applications, as well as to obtain a large positive temperature coefficient anomaly of resistivity (PTCR). Uniform distribution of dopants among the submicron dielectric particles is the key for optimal control of grain size and microstructure to maintain a high reliability. The system Ba0.84Pb0.16TiO3 was synthesized from high purity BaCO3, TiO2, PbO oxide powders as raw materials. Sb2O3, MnSO4 and ZnO were used as dopants and Al2O3, TiO2 and SiO2 as grain growth controllers. Phase composition was analyzed by using XRD and the microstructure was investigated by SEM. EDS attached to SEM was used to analyze phase composition specially related to abnormal grain growth. Electrical resistivities were measured as a function of temperature and the PTCR effect characterized by an abrupt increase on resistivity.

Structural and surface properties of thermally prepared Ti/RhxTi(1-x)Oy electrodes: I - influence of preparation parameters

Electrodes of RhxTi(1-x) O-y nominal composition were prepared by thermal decomposition of the chloride or nitrate precursor salts dissolved in strongly acidic medium and applied by brush to both sides of a Tidegrees support. A systematic study of the influence of calcination temperature and time as well as oxygen flux was conducted. The coatings were characterised by SEM, EDAX, XRD, open circuit potential measurements and cyclic voltammetry (CV). Visible-ultraviolet spectrophotometry was employed to identify the chemical form of the precursor in solution while thermogravimetric analysis (TGA) was used to assess the decomposition temperature ranges. Optimisation of the coating preparation parameters showed coatings obtained from [Rh(H2O)(6)](NO3)(3) precursor dissolved in HNO3 1:2 (v/v) and fired at 430 degreesC for 2 h in a 5 1 min (-1) oxygen stream-furnished stable electrodes having the highest electrochemically active surface area. (C) 2002 Elsevier B.V. Ltd. All rights reserved.

Structural and microstructural behaviour of SnO2 dense ceramics doped with ZnO and WO3

SnO2-based varistors doped with ZnO and WO3 were prepared by mixed oxide method. Experimental evidence shows that the increase in ZnO amount increases the volume and microstrain of unit cell while the WO3 promotes a decrease. The effect of ZnO and WO3 additives could be explained by the substitution of Sn4+ by Zn2+ and W6+. The addition of WO3 inhibits the grain growth due to the segregation in the grain boundary without influence in the densification of the samples. Besides that, an increase in the electrical resistance of the SnO2-ZnO-WO3 system was observed independent of the WO3 concentration. (c) 2005 Elsevier B.V. All rights reserved.

Structural and physicochemical characteristics of lintnerized native and sour cassava starches

The comprehension of the structure of starch granules is important for the understanding of its physicochemical properties. Native and sour cassava starches after being analyzed with respect to their pasting properties and baking expansion capacity, were treated with 2.2 N HCl at 38 degreesC for a maximum of nine days. The starch granules remaining after lintnerization were analyzed for amylose content and intrinsic viscosity, by X-ray diffraction, scanning electron microscopy and chromatographic analysis. The results indicated that the acid hydrolysis on all starches occurred in two steps. The first one, with high hydrolysis rate, was characterized by a quick degradation of the amorphous part of the granules whereas the second step, with lower hydrolysis rate, was characterized by a higher resistance of the organized areas of the granules to acid treatment. Most of the amylose chains were found in the amorphous areas of starch granules only a small percentage was involved in the crystalline regions. The microscopic and chromatographic analysis demonstrated that the acid hydrolysis was not able to disrupt the entire granular crystalline structure. Fermented starch showed amylose and/or amylopectin chain fractions resistant to pullulanase, probably due to structural alterations during fermentation.

Preparation, structural and optical characterization of BaWO4 and PbWO4 thin films prepared by a chemical route

Polycrystalline BaWO4 and PbWO4 thin films having a tetragonal scheelite structure were prepared at different temperatures. Soluble precursors such as barium carbonate, lead acetate trihydrate and tungstic acid, as starting materials, were mixed in aqueous solution. The thin films were deposited on silicon, platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure of the thin films were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and specular reflectance infrared Fourier transform spectroscopy, respectively. Nucleation stages and surface morphology evolution of thin films on silicon substrates have been studied by atomic force microscopy. XRD characterization of these films showed that BaWO4 and PbWO4 phase crystallize at 500 degreesC from an inorganic amorphous phase. FTIR spectra revealed the complete decomposition of the organic ligands at 500 degreesC and the appearance of two sharp and intense bands between 1000 and 600 cm(-1) assigned to vibrations of the antisymmetric stretches resulting from the high crystallinity of both thin films. The optical properties were also studied. It was found that BaWO4 and PbWO4 thin films have Eg = 5.78 eV and 4.20 eV, respectively, of a direct transition nature. The excellent microstructural quality and chemical homogeneity results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of BaWO4 and PbWO4 thin films. (C) 2003 Elsevier Ltd. All rights reserved.

Structural and magnetic transformation of monodispersed iron oxide particles in a reducing atmosphere

Uniform metal iron ellipsoidal particles of around 200 nm in length were obtained by reduction and passivation of alumina-coated alpha-Fe2O3 (hematite) particles under different conditions of temperature and hydrogen flow rate. The monodispersed hematite particles were prepared by the controlled hydrolysis of ferric sulfate and further coated with a homogeneous thin layer of Al2O3 by careful selection of the experimental conditions, mainly pH and aluminum salt concentration. The reduction mechanism of alpha-Fe2O3 into alpha-Fe was followed by x-ray and electron diffraction, and also by the measurements of the irreversible magnetic susceptibility. The transformation was found to be topotactic with the [001] direction of hematite particles, which lies along the long axis of the particles, becoming the [111] direction of magnetite and finally the [111] direction of metal iron. Temperature and hydrogen flow rate during the reduction have been found to be important parameters, which determine not only the degree of reduction but also the crystallite size of the final particles. Magnetic characterization of the samples shows that the only parameters affected by the crystallite size are the saturation magnetization and magnetic time-dependence effect, i.e., activation volume. (C) 2002 American Institute of Physics.

Structural and phenomenological characterization of the thermoreversible sol-gel transition of a zirconyl aqueous precursor modified by sulfuric acid

The thermoreversible sol-gel transition is well-known in biological and organic polymeric systems but has not been reported for inorganic systems. In this paper we put in evidence a thermoreversible sol-gel transition for zirconyl chloride aqueous solutions modified by sulfuric acid in the ratio 3:1 Zr:SO4. The synthesis conditions are detailed and a variety of experimental techniques (turbidimetry, dynamic rheology, and EXAFS) have been employed for investigating the thermal reversibility and the chemical structure of this new material. Turbidimetric measurements performed for solutions containing different concentrations of precursor have evidenced that the sol-gel transformation temperature increases from 50 to 80 degrees C as the concentration of zirconyl chloride decreases from 0.22 to 0.018 mol L-1. A more detailed study has been done for the sample with [Zr] = 0.156 mol L-1, in which the sol-gel-sol transformation has been repeated several times by a cyclic variation of the temperature. The mechanical properties of this sample, evaluated by measuring the storage and the loss moduli, show a change from liquid like to viscoelastic to elastic behavior during the sol-gel transition and vice versa during the gel-sol one. In situ EXAFS measurements performed at the Zr K-edge show that no change of the local order around Zr occurs during the sol-gel-sol transition, in agreement with the concept of physical gel formation. We have proposed for the structure of the precursor an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded in surface by complexing sulfate ligands, the sulfate groups act as a protective layer, playing a key role in the linking propagation among primary particles during sol-gel-sol transition.

Structural and functional characterization of myotoxin I, a Lys49 phospholipase A(2) homologue from Bothrops moojeni (Caissaca) snake venom

Myotoxin-I (MjTX-I) was purified to homogeneity from the venom of Bothrops moojeni by ion-exchange chromatography on CM-Sepharose. Its molecular weight, estimated by SDS-PAGE, was 13,400 (reduced) or 26,000 (unreduced). The extinction coefficient (E-1.0 cm(1.0 mg/ml)) of MjTX-I was 1.145 at lambda = 278 nm, pH 7.0, and its isoelectric point was 8.2 at ionic strength mu = 0.1. When lyophilized and stored at 4 degrees C, dimeric, trimeric, and pentameric forms of the protein were identified by SDS-PAGE. This heterogeneous sample could be separated into three fractions by gel filtration on Sephadex 6-50. The fractions were analyzed by isoelectric focusing, immunoelectrophoresis, and amino acid composition, which indicated that heterogeneity was the result of different levels of self-association. Protein sequencing indicated that MjTX-I is a Lys49 myotoxin and consists of 121 amino acids (M-r = 13,669), containing a high proportion of basic and hydrophobic residues. It shares a high degree of sequence identity with other Lys49 PLA(2)-like myotoxins, but shows a significantly lower identity with catalytically active Asp49 PLA(2)s. The three-dimensional structure of MjTX-I was modeled based on the crystal structures of three highly homologous Lys49 PLA(2)-like myotoxins. This model showed that the amino acid substitutions are conservative, and mainly the beta-wing region, and the C-terminal extended random coil. MjTX-I displays local myotoxic and edema-inducing activities in mice, and is lethal by intraperitoneal injection, with an LD50 value of 8.5 +/- 0.8 mg/kg, In addition, it is cytotoxic to myoblasts/ myotubes in culture, and disrupts negatively charged liposomes. In comparison with the freshly prepared dimeric sample, the more aggregated forms showed significantly reduced myotoxic activity. However, the edema-inducing activity of MjTX-I was independent of molecular association. Phospholipase A(2) activity on egg yolk, as well as anticoagulant activity, were undetectable both in the native and in the more associated forms. His, Tyr, and Trp residues of the toxin were chemically modified by specific reagents. Although the myotoxic and lethal activities of the modified toxins were reduced by these treatments, neither its edema-inducing or Liposome-disrupting activities were significantly altered. Rabbit antibodies to native MjTX-I cross-reacted with the chemically modified forms, and both the native and modified MjTX-I preparations were recognized by antibodies against the C-terminal region 115-129 of myotoxin II from B. asper, a highly Lys49 PLA(2)-homologue with high sequencial similarity. (C) 2000 Academic Press.

Structural and electronic properties of PbTiO3 slabs: a DFT periodic study

Structural and electronic properties of the bulk and relaxed surfaces (TiO2 and PbO terminated) of cubic PbTiO3 are investigated by means of periodic quantum-mechanical calculations based on density functional theory. It is observed that the difference in surface energies is small and relaxations effects are most prominent for Ti and Ph surface atoms. The electronic structure shows a splitting of the lowest conduction bands for the TiO2 terminated surface and of the highest valence bands for the PbO terminated slab. The calculated indirect band gap is: 3.18, 2.99 and 3.03 eV for bulk, TiO2 and PbO terminations, respectively. The electron density maps show that the Ti-O bond has a partial covalent character, whereas the Pb-O bonds present a very low covalency. (C) 2004 Elsevier B.V. All rights reserved.

Some structural and physicochemical characteristics of tuber and root starches

Starches from six different species (cassava, arrowroot, sweet potato, yam, canna and ginger) were isolated and some structural and physicochemical characteristics analysed and correlated. Phosphorous and amylose contents were determined using a colorimetric method and measuring iodine affinity, respectively. Molecular weight distributions of starches were analysed by Sepharose CL 2B. Granular shape and size distribution were performed using an image analyser system attached to a light microscope. Swelling power was determined at 60, 70, 80 and 90 degrees C. Pasting and thermal properties were measured using a rapid viscoanalyser, and a differential scanning calorimeter, respectively. Phosphorous content varied from 0.007 to 0.031% for cassava and canna starches, respectively. Yam, canna and ginger starches displayed higher amylose contents (32.6, 31.7 and 26.5%, respectively) than cassava, arrowroot and sweet potato starches (19.8, 20.8 and 22.6%, respectively). These last three starches displayed amylose molecules of higher molecular weight than those shown for yam, canna and ginger starches. Canna starch showed higher proportions of longer branch chains of amylopectin than others starches. The size and shape of granules were quite variable among all starches and the average size of granules varied from 13.9 to 42.3 mu m for sweet potato and canna, respectively. Swelling power, pasting, and thermal properties were affected by structural characteristics of the starches.

Structural and ferroelectric properties of Pb1-xSrxTiO3 thin films

Strontium-modified lead titanate (PST) thin films with composition Pb1-xSrxTiO3 (0.10 < x &LE; 0.60) were grown on Pt/Ti/SiO2/Si substrates using a soft chemical process. The crystallization of the PST thin films was achieved by heat treatment at 600&DEG;C. The structural and microstructural modifications in the films were studied using X-ray diffraction (XRD) and atomic force microscopy, respectively. The XRD study shows that the lattice parameters of polycrystalline PST thin films calculated from X-ray data indicate a decrease in lattice tetragonality with the increase in strontium content in these films. This indicates a gradual change from tetragonal to cubic structure. By atomic force microscopy analysis, the average grain size of the thin films was systematically reduced with the increase in Sr content. The dielectric property of the thin films was found to be strongly dependent on the Sr concentration. With 60 at.% Sr content, a ferroelectric to paraelectric phase transition was observed at room temperature.

The effects of ZnGa2O4 formation on structural and optical properties of ZnO : Ga powders

Gallium-doped zinc oxide (ZnO:Ga 1, 2 3, 4 and 5 at%) samples were prepared in powder form by modifying the Pechini method. The formation of zinc gallate (ZnGa2O4) With the spinel crystal structure was observed even in ZnO:Ga 1 at% by X-ray diffraction. The presence of ZnGa2O4 in ZnO:Ga samples was also evidenced by luminescence spectroscopy through its blue emission at 430 nm, assigned to charge transfer between Ga3+ at regular octahedral symmetry and its surrounding O2- ions. The amount of ZnGa2O4 increases as the dopant concentration increases, as observed by the quantitative phase analysis by the Rietveld method. (C) 2006 Elsevier B.V. All rights reserved.

Structural and morphological characterization of Pb1-xBaxTiO3 thin films prepared by chemical route: An investigation of phase transition

Detailed room temperature micro-Raman scattering, X-ray diffraction, atomic force microscopy and specular reflectance infrared Fourier transform spectroscopy studies were carried out on soft chemical by processed Pb1-xBaxTiO3 thin films. The micro-Raman spectra pointed the existence of a stable tetragonal ferroelectric phase in the entire composition range (0 < x <= 1). The infrared reflectance spectra showed that the frequency of several peaks decreases as the Ba2+ concentration increases. These features are correlated to a decrease in the tetragonal distortion of the TiO6 octahedra as the Ba2+ concentration increases. Furthermore, as x increases from 0.70 to 1.0, the Raman spectrum shows an evolution towards the well-known Raman spectrum of the tetragonal BaTiO3. Therefore, we demonstrated that the combination of solid solution PbTiO3-BaTiO3 with a grain size in the order of 30-40 nm supports the tetragonal ferroelectric phase at room temperature. (C) 2007 Elsevier B.V. All rights reserved.

Free activity in the cage associated with body weight and restoration of bone structural and mechanical properties in growing rats after hindlimb unloading

We investigated the importance of daily free activity in the cage and body weight gain during the recovering of bone structural and mechanical properties in growing rats after hindlimb unloading. Eight-week-old male Wistar rats were randomly divided into control (CG, n=24) and suspended (SG, n=24) groups. Animals from SG underwent a four-week hindlimb unloading period by tail-suspension. Animals from CG and those from SG after release were kept in collective cages and sacrificed at the age of 12, 16 and 20 weeks. Both femurs were removed and its area, bone mineral density (BMD), resistance to failure and stiffness were determined. Four-week hindlimb unloading decreased (p < 0.05) body weight (CG, 373.00 +/- 9.47 vs. SG, 295.86 +/- 9.19 g), BMD (CG, 0.19 +/- 0.01 vs. SG, 0.15 +/- 0.01 g/cm(2)), bone resistance to failure (CG, 147.75 +/- 5.05 vs. SG, 96.40 +/- 5.95 N) and stiffness (CG, 0.38 +/- 0.01 vs. SG, 0.23 +/- 0.02 N/m). Eight weeks of free activity in cage recovered (p > 0.05) the body weight (CG, 472.75 +/- 14.11 vs. SG, 444.75 +/- 18.91 g), BMD (CG, 0.24 +/- 0.01 vs. SG, 0.22 +/- 0.01 g/cm(2)), bone resistance to failure (CG, 195.73 +/- 10.06 vs. SG, 178.45 +/- 8.48 N) and stiffness (CG, 0.56 +/- 0.02 vs. SG, 0.47 +/- 0.03 N/m) of SG animals. Body weight correlated strongly with bone structural and mechanical properties (p < 0.0001). In conclusion, free activity in the cage associated with body weight gain restored bone structural and mechanical properties in growing rats after hindlimb unloading.

Study of the annealing temperature effect on the structural and luminescent properties of SrWO4:Eu phosphors prepared by a non-hydrolytic sol-gel process

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)