Nature of potential barrier in (Ca-1/4,Cu-3/4)TiO3 polycrystalline perovskite

The nonohmic electrical features of (Ca-1/4,Cu-3/4)TiO3 perovskite ceramics, which have very strong gigantic dielectric is believed originate from potential barriers at the grain boundaries. In the present study, we used the admittance and impedance spectroscopy technique to investigate (Ca-1/4,Cu-3/4)TiO3 perovskite ceramics with low nonohmic electrical properties. The study was conducted under two different conditions: on as-sintered ceramics and on ceramics thermally treated in an oxygen-rich atmosphere. The results confirm that thermal treatment in oxygen-rich atmospheres influence the nonohmic properties. Annealing at oxygen-rich atmospheres improve the nonohmic behavior and annealing at oxygen-poor atmospheres decrease the nonohmic properties, a behavior already reported for common metal oxide nonohmic devices and here firstly evidenced for the (Ca-1/4,Cu-3/4)TiO3 perovskite related materials. The results show that oxygen also influences the capacitance values at low frequencies, a behavior that is indicative of the Schottky-type nature of the potential barrier. (c) 2006 Elsevier Ltd. All rights reserved.

Effect of combined bending strain and thermal cycling on the voltage-current characteristic curves of Bi-2223 tapes

High critical temperature superconductors are evolving from a scientific research subject into large-scale application devices. In order to meet this development demand they must withstand high current capacity under mechanical loads arising from thermal contraction during cooling from room temperature down to operating temperature (usually 77 K) and due to the electromagnetic forces generated by the current and the induced magnetic field. Among the HTS materials, the Bi2Sr2Ca2Cu3Ox, compound imbedded in an Ag/AgMg sheath has shown the best results in terms of critical current at 77 K and tolerance against mechanical strain. Aiming to evaluate the influence of thermal stress induced by a number of thermal shock cycles we have evaluated the V-I characteristic curves of samples mounted onto semicircular holders with different curvature radius (9.75 to 44.5 mm). The most deformed sample (epsilon = 1.08%) showed the largest reduction of critical current (40%) compared to the undeformed sample and the highest sensitivity to thermal stress (I-c/I-c0 = 0.5). The V-I characteristic curves were also fitted by a potential curve displaying n-exponents varying from 20 down to 10 between the initial and last thermal shock cycle.

Structural organization and thermal properties of the Sb2O3-SbPO4 glass system

The structural organization of Sb2O3-SbPO4 glasses has been studied by FTIR, Raman, P-31 MAS and spin echo NMR, Mossbauer and X-ray absorption spectroscopy (EXAFS and XANES at K and L-3,L-1-Sb edges). The combined results can be explained in terms of two potential mechanisms describing the change of the Sb(m) local environment upon incorporation of Q((4))-type phosphate. The formation of the latter species requires anionic compensation that may be adjusted by (a) formation of non bridging oxygen or (b) formation of SbO4E- groups (E = non-bonding electron pair). The second model is favored.

Synthesis and thermal study of the prodrug of oxamniquine

Oxamniquine polymeric prodrug with potential antischistosomal activity was prepared using dextran T-70 as a carrier, which was analysed by (HNMR)-H-1, C-13 NMR and IR spectroscopy. The formation of the oxamniquine salt was confirmed by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) which showed a different thermal behaviour when compared to the physical mixture.

Dielectric thermal analysis as a tool for quantitative-evaluation of the viscosity and the kinetics of epoxy resin cure

Dielectric thermal analysis has been proved as a valuable tool for monitoring the epoxy curing process and the related rheological properties in the fabrication of polymer-matrix composite materials. This technique also has the potential to be applied in the monitoring of magnet impregnation processes as well as in quality control. In this work we present the quantitative evaluation of the viscosity changing and the curing kinetics for a commercial Stycast epoxy resin system at different temperatures through the impedance analysis. The results showed correlation between the real component of the complex impedance and the isothermal reaction extent. Comparing the dielectric analysis result with the viscosity measured by rotational rheometer we observed a similar behavior reported for dynamic mechanic analysis. The results comparison have shown that the kinetics parameters obtained from DSC and DETA analysis showed different sensitivities related to the characteristics of curing stages. We concluded that the dielectric thermal analysis should be applied in quantitative evaluation of cure kinetics.

Quercetin-PVP K25 solid dispersions: Preparation, thermal characterization and antioxidant activity

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Effects of thermal annealing on the semi-insulating properties of radio frequency magnetron sputtering-produced germanate thin films

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Sewage sludge coke estimation using thermal analysis

The imposition of more stringent legislation by CETESB in the State of So Paulo (Brazil) governing the disposal and utilization of sewage sludge, coupled with the growth in its generation has prompted a drive for alternative uses of sewage sludge. One option that is especially promising, due to its potential to valorize sludge, is its conversion into carbonaceous adsorbents or coke for industrial effluents treatment. Thus, a methodology is presented to estimate the coke produced from the sludge of a sewage treatment station using thermal analysis. The used sewage sludge, which comes from aerobic treatment, was collected in the wastewater treatment station of Barueri, one of the largest of the So Paulo metropolitan area. The sludge samples were collected, dried, ground, and milled until they passed an ABNT 200 sieve. The inert ambient used during its thermal treatment produces inorganic matter and coke as residual materials. Coke formation occurs in the 200-500 A degrees C range and, between 500 and 900 A degrees C, its thermal decomposition occurs. The highest formation of coke occurs at 500 A degrees C.

Thermal effects on the catalysis by a magnetic field

We show that the formation of condensates in the presence of a constant magnetic field in 2+1 dimensions is extremely unstable. It disappears as soon as a heat bath is introduced with or without a chemical potential. The value of the condensate as well as other observables are shown to become nonanalytic at finite temperature.

Influence of thermal treatments in the corrosion behaviour of HVOF Cr 3C2-NiCr coatings

Thermal spray coatings as Cr3C2-NiCr obtained by high velocity oxy-fuel spraying (HVOF) are mainly applied due to their behaviour against aggressive erosive-abrasive and corrosive atmospheres and their thermal stability at high temperatures [1]. In order to increase the corrosion protection that it offers to the substrate trying to close the interconnected pores, it is possible to apply a thermal treatment with the gun during the spraying of the coating. This treatment could be applied in different ways. One of these ways consists of spraying only a few layers of coating followed by thermal treatment and finally the spray of the rest of layers. This thermal treatment on spraying is studied related to the corrosion properties of the system. The study comprises the electrochemical characterisation of the system by open circuit potential (OC), polarisation resistance (Rp), cyclic voltammetry (CV) and impedance spectroscopy measurements (EIS). Optical and scanning electron microscopy characterisation (OM and SEM) of the top and cross-section of the system has been used in order to justify the electrochemical results.

Purification and biochemical characterization of two xylanases produced by Aspergillus caespitosus and their potential for kraft pulp bleaching

Two extracellular xylanases produced by the thermotolerant fungus Aspergillus caespitosus grown in sugar cane bagasse were purified and characterized. Estimated molecular masses were 26.3 and 27 kDa (xyl I); 7.7 and 17.7 kDa (xyl II) for gel filtration and SDS-PAGE, respectively. Optimal temperature for both xylanases was 50-55°C. Optimal pH was 6.5-7.0 for xyl I, and 5.5-6.5 for xyl II. The thermostability (T half) at 55°C was 27.3 min (xyl I) and >90 min (xyl II). Xylanase activity was inhibited by several ions. β-mercaptoethanol activated 59 and 102% xyl I and xyl II activities, respectively. These enzymes preferentially hydrolyzed birchwood xylan, and the K m and V max values were 2.5 mg/ml and 1679 U/mg protein (xyl I), and 3.9 mg/ml and 113 U/mg protein (xyl II). The action of both xylanases mainly that of xyl II, on kraft pulp reduced kappa number and increased pulp viscosity. © 2004 Elsevier Ltd. All rights reserved.

Thermal studies of chitin-chitosan derivatives

New poly(azo) amino-chitosan compounds were obtained from the azo coupling reaction of N-benzyl chitosan and diazonium salts. The thermal behavior of these compounds was studied by thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG), TG coupled with a Fourier-transform infrared, and differential scanning calorimetry (DSC). TG/DTG curves of chitin-chitosan polymer showed two thermal events attributed to water loss and decomposition of the polysaccharide after cross-linking reactions. Thermal analysis of the poly(azo) amino-chitosan compounds showed that the decomposition temperatures decreased when compared to the starting chitin-chitosan and N-benzyl chitosan. DSC results showed an agreement with the TG/DTG analyses. Thermal behavior of poly(azo) amino-chitosans suggest that these compounds could be considered as potential thermal sensors. © 2012 Akadémiai Kiadó, Budapest, Hungary.

The role of Bi2O3 on the thermal, structural, and optical properties of tungsten-phosphate glasses

Glasses in the ternary system (70 - x)NaPO3-30WO 3-xBi2O3, with x = 0-30 mol %, were prepared by the conventional melt-quenching technique. X-ray diffraction (XRD) measurements were performed to confirm the noncrystalline nature of the samples. The influence of the Bi2O3 on the thermal, structural, and optical properties was investigated. Differential scanning calorimetry analysis showed that the glass transition temperature, Tg, increases from 405 to 440 C for 0 ≤ x ≤ 15 mol % and decreases to 417 C for x = 30 mol %. The thermal stability against devitrification decreases from 156 to 67 C with the increase of the Bi2O3 content. The structural modifications were studied by Raman scattering, showing a bismuth insertion into the phosphate chains by Bi-O-P linkage. Furthermore, up to 15 mol % of Bi 2O3 formation of BiO6 clusters is observed, associated with Bi-O-Bi linkage, resulting in a progressive break of the linear phosphate chains that leads to orthophosphate Q0 units. The linear refractive index, n0, was measured using the prism-coupler technique at 532, 633, and 1550 nm, whereas the nonlinear (NL) refractive index, n 2 was measured at 1064 nm using the Z-scan technique. Values of 1.58 ≤ n0 ≤ 1.88, n2 ≥ 10-15 cm 2/W and NL absorption coefficient, α2 ≤ 0.01 cm/GW, were determined. The linear and NL refractive indices increase with the increase of the Bi2O3 concentration. The large values of n0 and n2, as well as the very small α2, indicate that these materials have large potential for all-optical switching applications in the near-infrared. © 2012 American Chemical Society.

A simple procedure for the preparation of laponite and thermoplastic starch nanocomposites: Structural, mechanical, and thermal characterizations

The aim of this article is to propose advances for the preparation of hybrid nanocomposites prepared by the combination of intercalation from solution and melt-processing methods. This research investigates the effect of the laponite RDS content on the thermal, structural, and mechanical properties of thermoplastic starch (TPS). X-ray diffraction was performed to investigate the dispersion of the laponite RDS layers into the TPS matrix. The results show good nanodispersion, intercalation, and exfoliation of the clay platelets, indicating that these composites are true nanocomposites. The presence of laponite RDS also improves the thermal stability and mechanical properties of the TPSmatrix due to its reinforcement effect which was optimized by the high degree of exfoliation of the clay. Thus, these results indicate that the exfoliated TPS-laponite nanocomposites have great potential for industrial applications and, more specifically, in the packaging field. © The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.

Sintering of porous alumina obtained by biotemplate fibers for low thermal conductivity applications

In this research report, a sintering process of porous ceramic materials based on Al2O3 was employed using a method where a cation precursor solution is embedded in an organic fibrous cotton matrix. For porous green bodies, the precursor solution and cotton were annealed at temperatures in the range of 100-1600°C using scanning electron microscopy (SEM) and thermogravimetric (TG) analysis to obtain a porous body formation and disposal process containing organic fibers and precursor solution. In a structure consisting of open pores and interconnected nanometric grains, despite the low porosity of around 40% (calculated geometrically), nitrogen physisorption determined a specific surface area of 14m2/g, which shows much sintering of porous bodies. Energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analytical methods revealed a predominant amount of α-Al2O3 in the sintered samples. Thermal properties of the sintered Al2O3 fibers were obtained by using the Laser Flash which resulted in the lower thermal conductivity obtained by α-Al2O3 and therefore improved its potential use as an insulating material. © 2012 Elsevier Ltd.