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)

The role of chain scission and chain branching in high density polyethylene during thermo-mechanical degradation

The mechanical and thermo-oxidative degradation of high density polyethylene (HDPE) was measured in a twin-screw extruder using various processing conditions. Two types of HDPE, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturation were analysed. Mild screw profiles, having mainly conveying elements, have short mean residence times then profiles with kneading discs and left hand elements. Carbonyl and traps-vinylene group concentrations increased, whereas vinyl group concentration decreased with number of extrusions. Higher temperature profiles intensified these effects. The thermo-mechanical degradation mechanism begins with chain scission in the longer chains due to their higher probability of entanglements. These macroradicals then react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the levels of extrusion temperature, shear and vinyl end groups content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than does the Ziegler-Natta type. (C) 2004 Elsevier Ltd. All rights reserved.

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.

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.

Evaluation of Philips and Ziegler-Natta high-density polyethylene degradation during processing in an internal mixer using the chain scission and branching distribution function analysis

The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 degrees C. Standard rheograrns using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of chain scission and branching (n(s)) was calculated from MWD curves and its plotting versus functional groups' concentration showed that chain scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain scission and branching distribution function (CSBDF) values showed that longer chains undergo chain scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than the Ziegler-Natta's type at the same processing condition. (c) 2006 Elsevier Ltd. All rights reserved.

Kinetic evaluation of the pyrolysis of high density polyethylene over H-AlMCM-41 material

The AlMCM-41 material with Si/Al=50 was synthesized by hydrothermal method, using cethyltrimethylammonium as template. The protonic H-AlMCM-41 acid form was obtained by ion exchange with ammonium chloride solution and subsequent calcination. The characterization of the material by several techniques showed that a good-quality MCM-41 material was obtained. High-density polyethylene (HDPE) has been submitted to thermal degradation alone, and in presence of the exchanged H-AlMCM-41 catalyst at a concentration of 1: 1 in mass (H-AlMCM-41/HDPE). The reactor was connected on line to a gas chromatograph connected to a mass spectrometer. This process was evaluated by thermogravimetry (TG), from 350 to 600degreesC, under helium dynamic atmosphere, with heating rates of 5.0; 10.0 and 20.0 degreesC/min. From TG curves, the activation energy, calculated using a multiple heating rate integral kinetic method, decreased from 225.5 KJ.mol(-1), for the pure polymer (HDPE), to 184.7 KJ.mol(-1), in the presence of the catalyst (H-AlMCM-41/HDPE).

Synthesis and properties of branched polyethylene/high-density polyethylene blends using a homogeneous binary catalyst system composed of early and late transition metal complexes

Branched polyethylene/high-density polyethylene blends (BPE/HDPE) with a wide range of molecular weights, melt flow indexes (MFI), and intrinsic viscosity were prepared using the homogeneous binary catalyst system composed by Ni(alpha-diimine)Cl-2 (1) (alpha-diimine = 1,4-bis(2,6-diisopropylphenyl)-acenaphthenediimine) and {Tp(Ms*)} TiCl3 (2) (Tp(Ms*)=hydridobis(3-mesitylpyrazol-1-yl)(5-mesityl-pyrazol-1-yl)) activated with MAO and/or TIBA in hexane at two different polymerization temperatures (30 and 55 degreesC) and by varying the nickel loading molar fraction (x(Ni)). At all Temperatures, a non-linear correlation between the x(Ni) and the productivity was observed, suggesting the occurrence of a synergistic effect between the nickel and the titanium catalyst precursors, which is more pronounced at 55 degreesC. The molecular weight of the BPE/HDPE blends considerably decreases with increasing Al/M molar ratio. The melt flow indexes (MFI) and intrinsic viscosities (eta) are strongly affected by x(Ni), but the melting temperatures are nearly constant, 132 +/- 3 degreesC. Dynamic mechanical thermal analysis (DMTA) shows the formation of different polymeric materials where the stiffness vanes according, to the x(Ni) and temperature used in the polymerization reaction. The surface morphology of the BPE/HDPE blends studied by scanning electron microscopy (SEM) revealed a low miscibility between the PE phases resulting in the formation of a sandwich structure after etching with o-xylene.

An investigation of metal oxides which are photoluminiscent at room temperature

Amorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trends. (C) 2004 Elsevier B.V. All rights reserved.

Combination of nickel and titanium complexes containing nitrogen ligands as catalyst for polyethylene reactor blending

Ethylene was polymerized using a combination of Ni(diimine)Cl-2 (1) (diimine = 1,4-bis(2,6-di-isopropylphenyl)-acenaphthenediimine) and {Tp(Ms)*} TiCl3 (2) (Tp(Ms)* = hydridobis(3-mesitylpyrazol-1-yl)(5-mesityl-pyrazol-1-yl)) compounds in the presence of methyl-aluminoxane (MAO) at 30 degrees C. The productivity reaches a maximum at X-Ni = 0.75 (1400 kg of PE/mol[M] . h), and the produced polyethylene (PE) showed maximal melt flow index (0.13 g/10 min) and minimal intrinsic viscosity (2.24 dL/g) compared to polyethylenes obtained with different values of nickel loading fractions (X-Ni). Productivity intrinsic viscosity data, as well as melt flow index measurements markedly depend upon the content of the late transition metal, thus suggesting a synergic effect between nickel and titanium catalysts.