Evaluation of rare earth oxides doping SnO2.(Co1/4,Mn3/4)O-based varistor system

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

Influence of strains and defects on ferroelectric and dielectric properties of thin-film barium-strontium titanates

Pristine, W and Mn 1% doped Ba(0.6)Sr(0.4)TiO(3) epitaxial thin films grown on the LaAlO(3) substrate were deposited by pulsed laser deposition (PLD). Dielectric and ferroelectric properties were determined by the capacitance measurements and X-ray diffraction was used to determine both residual elastic strains and defect-related inhomogeneous strains-by analyzing diffraction line shifts and line broadening, respectively. We found that both elastic and inhomogeneous strains are affected by doping. This strain correlates with the change in Curie-Weiss temperature and can qualitatively explain changes in dielectric loss. To explain the experimental findings, we model the dielectric and ferroelectric properties of interest in the framework of the Landau-Ginzburg-Devonshire thermodynamic theory. As expected, an, elastic-strain contribution due to the epilayer-substrate misfit has an important influence on the free-energy. However, additional terms that correspond to the defect-related inhomogeneous strain had to be introduced to fully explain the measurements.

Magnetic oxides of spinel structure: Study of CoxCuyMnzO4 and MgxNiyMnzO4 manganites (x+y+z=3)

Systematic studies in manganites of spinel structure have been undertaken. We report on the magnetic properties of two particular cases, in which one of the transition metals, Mg2+ is non-magnetic (NiMgxMn2-xO4) or presents a stable oxidation state, Cu2+ (CoxCuyMnzO4, x + y + z = 3). The magnetic behaviour is described with respect to varying contents of cobalt, copper or manganese. A ferrimagnetic transition is observed at 110-120 K, which depends on the cobalt content. Presence of copper increases the coercive field by a factor of ten with respect to the parent compound NiMn2O4. (c) 2006 Elsevier B.V. All rights reserved.

OXIDES AS HETEROGENEOUS PROMOTERS FOR LIQUID-PHASE HYDROCARBONYLATION REACTIONS WITH IODOCARBONYLRUTHENIUM CATALYSTS

Basic or acid oxides, used as heterogeneous promoters of carbonylruthenium catalysts in liquid-phase hydrocarbonylation reactions on oxygenated substrates, strongly affect the activity and selectivity of the catalytic system. Concurrent or successive reactions of simple carbonylation, homologation, hydrogenation to hydrocarbons, and etherification take place to varying extents. Carbonylation and etherification are favored by acid oxides and homologation and hydrogenation by basic oxides. This behavior is related to the formation and stabilization by the oxides of H+ and H- hydridocarbonylruthenium catalytic species, whose relative concentrations in solution depend on the nature of the oxide. Heterogeneous oxides are easily separated and recycled from the reaction mixture. Their use simplifies the catalytic system and allows one to direct the catalytic process toward the target product.

Influence of the oxygen adsorbed on tin varistors doped with Co, Mn and Cr oxides

The non-ohmic properties of the 98.90% SnO2+(1-x)%CoO+0.05% Cr2O3+0.05% Nb2O5+x% MnO2 varistor system (all of them in mol %), as well as the influence of the oxidizing and reducing atmosphere on this system were studied in this work. Experimental evidence indicates that the electrical properties of the varistor depend on the defects that occur at the grain boundary and on the adsorbed oxygen species such as O''(2), O'(2), O in this region. Thermal treatments at 900 degreesC in oxygen and nitrogen atmospheres indicated such a dependence with the values of the non-linearity coefficient (alpha) increasing under oxygen atmosphere, being reduced in nitrogen atmosphere and restored after a new treatment in oxygen atmosphere, presenting a reversibility in the process. EDS analysis accomplished by SEM showed the distribution of the oxides in the varistor matrix. (C) 2002 Kluwer Academic Publishers.

Experimental and theoretical study of the ferroelectric and piezoelectric behavior of strontium-doped PZT

Theoretical data using ab initio perturbed ion calculation were compared with ferroelectric and piezoelectric experimental data of strontium doped PZT. Various concentrations of SrO in PZT at constant temperature and sintering time were carried out. Experimental results, such as the remanent polarization, P-R of 6.9-8.9 muC/Cm-2, the coercive field, E-C of 6.6-7.8 kVcm, and the planar coupling factor, Kp of 0.45-0.53, were compared with the energy of Zr4+ and Ti4+ ion dislocation and the lattice interaction energy which show that strontium increment in PZT alter the energies and increase the values of piezoelectric and ferroelectric variables. Calculations of lattice energy of the rhombohedral phase show that a phase non-stability is coincident with increasing experimental values of the P-R, E-C and Kp. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Influence of the rare-earths oxides doped on the SnO2CoOMnO2Ta2O5 varistor system

The tin dioxide is an n-type semiconductor, which exhibits varistor behavior with high capacity of absorption of energy, whose function is to restrict transitory over-voltages without being destroyed, when it is doped with some oxides. Varistors are used in alternated current fields as well as in continuous current, and it can be applied in great interval of voltages or in great interval of currents. The electric properties of the varistor depend on the defects that happen at the grain boundaries and the adsorption of oxygen. The (98.90-x)%SnO2.0.25%CoO+0.75%MnO2+0.05%Ta2O5+0.05%Tr2O3 systems, in which Tr=La or Nd. Current-voltage measurements were accomplished for determination of the non-linear coefficient were studied. SEM microstructure analysis was made to evaluate the microstructural characteristics of the systems. The results showed that the rare-earth oxides have influenced the electrical behavior presented by the system. (C) 2002 Kluwer Academic Publishers.

Use of strontium in the activation of bovine oocytes reconstructed by somatic cell nuclear transfer

As an important step in the nuclear transfer (NT) procedure, we evaluated the effect of three different treatments for oocyte activation on the in vitro and in vivo developmental capacity of bovine reconstructed embryos: (1) strontium, which has been successfully used in mice but not yet tested in cattle; (2) ionomycin and 6-dimethylammopurine (6-DMAP), a standard treatment used in cattle; (3) ionomycin and strontium, in place of 6-DMAP. As regards NT blastocyst development, no difference was observed when strontium (20.1%) or ionomycin/6-DMAP (14.4%) were used. However, when 6-DMAP was substituted by strontium (3), the blastocyst rate (34.8%) was superior to that in the other activation groups (p < 0.05). Results of in vivo development showed the possibility of pregnancies when NT embryos activated in strontium were transferred to recipient cows (16.6%). A live female calf was obtained when ionomycin/strontium were used, but it died 30 days after birth. Our findings show that strontium can be used as an activation agent in bovine cloning procedures and that activation with a combination of strontium and ionomycin increased the in vitro developmental capacity of reconstructed embryos. This is the first report of a calf produced by adult somatic cell NT in Latin America.

Use of strontium for bovine oocyte activation

Strontium efficiently activates mouse oocytes, however, there is limited information on its use in cattle. Thus, the objective of this study was to establish a suitable protocol for activating bovine oocyte with strontium. For pronuclear development, the absence of calcium and magnesium in the activation medium (TALP) with 10 and 50mM strontium (34.4 and 53.1%, respectively) was superior to the complete TALP (6.5 and 19.4%, respectively). In all activation media, better results were observed with 25 and 50 mM strontium (21.9-53.1 and 19.4-53.1%, respectively). Incubation for 4 h promoted similar results in all strontium concentrations. However, strontium at 15, 20, and 25 mM for 6 and 8 h (40.7, 46.7, and 48.3%, and 29.3, 48.3, and 40.7%, respectively) were superior to control (15.5 and 10%, respectively). After in vitro maturation for 26 h, strontium (S; 20 mM in Ca2+ and Mg2+-free TALP for 6 h), ionomycin + strontium (IS), and strontium + ionomycin (SI) (60, 63.3, and 65%, respectively) were similar in pronuclear development and superior to ionomycin (I; 5 mu M for 5 min; 36.7%). In treatments S and I, only 1 PN zygotes were observed. In treatment S, most of them had 1 and 2 PB (35.7 and 60.7%, respectively), and in treatment I, 0, 1, and 2 PB (14.3, 57.1, and 28.6%, respectively). Most of the zygotes in treatment IS and SI were 1 PN 2 PB (77.4 and 61.6%, respectively). The number of oocytes with clusters of cortical granules was similar in all treated groups (11-29%). Cortical granule exocytosis in treatment IS (68%) was similar to S (54%) and superior to 1, SI, and control (27, 45, and 5.0%, respectively). Cleavage and blastocyst rates were similar for S, I, IS, and SI treatments (61.7-76.7, and 8.3-13.3%, respectively) and the same was observed for ICM, TE, and total cell number, and ICM/total cell ratio (22-25, 64-69, and 86-95, and 0.26-0.27). In conclusion, strontium may be efficiently applied for bovine oocyte activation at 20 mM in Ca2+-and Mg2+-free TALP medium for 6 h.

Thermal decomposition and rehydration of strontium oxalate: morphological evolution

Undoped and Eu3+ doped monohydrate strontium oxalate samples were precipitated under ultrasound and conventional stirring and were heated at different temperatures. All samples were characterized by X-ray powder diffraction (XDR), infrared spectra (IR) and scanning electron microscopy (SEM). Monohydrate, dehydrate oxalates and carbonate particles are ellipsoids indicating a topotatic process. Particle size decrease is observed when ultrasound stirring and europium. doping are used and rehydration of strontium oxalate results in uniform hexagonal particle shape. An oxide and carbonate mixture is obtained from oxalates treated at 1050 degreesC and its suspension in water undergoes incomplete hydrolysis. The products from this incomplete hydrolysis present dendrite shape particles only when the former is precipitated under ultrasound stirring. In this process, surface energy is important for particle dispersion and ultrasound supplies activation energy to oxalate precursor. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Barium strontium titanate powder obtained by polymeric precursor method

Pure barium strontium titanate powder, with Ba/Sr ratio of 80/20 was prepared by the polymeric precursor method (also called Pechini process). The powder was obtained after a calcination at 800 degreesC for 8 h and characterized by XRD, IR, BET and SEM. The requirements to avoid barium carbonate as a secondary phase are presented and discussed in detail. (C) 2003 Elsevier B.V. All rights reserved.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140nm)/Ti (10 nM)/SiO2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV-vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV-vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Barium strontium titanate nanocrystalline thin films prepared by soft chemical method

Barium strontium titanate (Ba0.65Sr0.35TiO3) nanocrystalline thin films, which were produced by the soft chemical method, were crystallized at low temperature using a domestic microwave oven. A SiC susceptor were used to absorb the microwave energy and rapidly transfer the heat to the film. Low microwave power and short time have been used. The films obtained are crack-free, well-adhered, and fully crystallized. The microstructure displayed a polycrystalline nature with nanograin size. The metal-BST-metal structure of the thin films treated at 700 degrees C show food electric properties. The ferroelectric nature of the BST35 thin film was indicated by buttertly- shaped C-V curves. The capacitance-frequency curves reveal that the dielectric constant may reach a value up to 800 at 100kHz. The dissipation factor was 0.01 at 100kHz. The charge storage density as function of applied voltage graph showed that the charge storage densities are suitable for use in trench type 64 Mb ( 1-5 mu C/cm(2) and 265 Mb (2-11 mu C/cm(2)) DRAMs. (C) 2007 Elsevier Ltd. All rights reserved.