Comparative degradation of ZnO- and SnO(2)-based polycrystalline non-ohmic devices by current pulse stress

The degradation behaviour of SnO(2)-based varistors (SCNCr) due to current pulses (8/20 mu s) is reported here for the first time in comparison with the ZnO-based commercial varistors (ZnO). Puncturing and/or cracking failures were observed in ZnO-based varistors possessing inferior thermo-mechanical properties in comparison with that found in a SCNCr system free of failures. Both systems presented electric degradation related to the increase in the leakage current and decrease in the electric breakdown field, non-linear coefficient and average value of the potential barrier height. However, it was found that a more severe degradation occurred in the ZnO-based varistors concerning their non-ohmic behaviour, while in the SCNCr system, a strong non-ohmic behaviour remained after the degradation. These results indicate that the degradation in the metal oxide varistors is controlled by a defect diffusion process whose rate depends on the mobility, the concentration of meta-stable defects and the amount of electrically active interfaces. The improved behaviour of the SCNCr system is then inferred to be associated with the higher amount of electrically active interfaces (85%) and to a higher energy necessary to activate the diffusion of the specific defects.

Atividade peroxidásica em basófilos de Phrynops geoffroanus (Testudines Chelidae)

As peroxidases, presentes nos peroxissomos e lisossomos, pertencem às oxidases e atuam como catalítico para o peróxido de hidrogênio (H2O2), posteriormente decomposto pela oxidação de cossubstratos, evitando danos celulares.(¹) Foi aplicada a técnica da peroxidase(2) em esfregaços sanguíneos de Phrynops geoffroanus, comparando com sangue humano, para avaliação da atividade e controle da reação. O esfregaço sanguíneo humano apresentou marcações em neutrófilos, fagócitos com muitos lisossomos e peroxissomos (Figura 1). Nos esfregaços sanguíneos de Phrynops geoffroanus, as marcações apresentaram-se nos basófilos (Figura 2), que representam de 10% a 25% dos leucócitos de quelônios e possuem grande número de granulações citoplasmáticas,(3) sugerindo a presença de grande quantidade de enzimas e organelas como lisossomos e peroxissomos, possivelmente associadas a sua participação em reações imunes. A atividade peroxidásica representa resposta do organismo a ações ambientais danosas, servindo como marcador biológico.

Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: Multi-physics FEA modeling and monotonic testing

Objective. To determine the influence of cement thickness and ceramic/cement bonding on stresses and failure of CAD/CAM crowns, using both multi-physics finite element analysis and monotonic testing.Methods. Axially symmetric FEA models were created for stress analysis of a stylized monolithic crown having resin cement thicknesses from 50 to 500 mu m under occlusal loading. Ceramic-cement interface was modeled as bonded or not-bonded (cement-dentin as bonded). Cement polymerization shrinkage was simulated as a thermal contraction. Loads necessary to reach stresses for radial cracking from the intaglio surface were calculated by FEA. Experimentally, feldspathic CAD/CAM crowns based on the FEA model were machined having different occlusal cementation spaces, etched and cemented to dentin analogs. Non-bonding of etched ceramic was achieved using a thin layer of poly(dimethylsiloxane). Crowns were loaded to failure at 5 N/s, with radial cracks detected acoustically.Results. Failure loads depended on the bonding condition and the cement thickness for both FEA and physical testing. Average fracture loads for bonded crowns were: 673.5 N at 50 mu m cement and 300.6 N at 500 mu m. FEA stresses due to polymerization shrinkage increased with the cement thickness overwhelming the protective effect of bonding, as was also seen experimentally. At 50 mu m cement thickness, bonded crowns withstood at least twice the load before failure than non-bonded crowns.Significance. Occlusal "fit" can have structural implications for CAD/CAM crowns; pre-cementation spaces around 50-100 mu m being recommended from this study. Bonding benefits were lost at thickness approaching 450-500 mu m due to polymerization shrinkage stresses. (C) 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Mechanical properties and dimensional effects of ZnO- and SnO(2)-based varistors

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

Eletrodo modificado em filme de paládio para a determinação voltamétrica de fosfito

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

Caracterização e propriedades catalíticas da zeolita HZSM5 modificada com nióbio

HZSM5 zeolite was modified by exchanging proton by niobium (V). Several samples were obtained with various degrees of exchange. Pore volumes and acidity were measured to characterize these exchanged zeolites. Catalytic properties were evaluated with two reaction tests: m-xylene transformation and n-heptane cracking. The introduction of niobium on HZSM5 zeolite decreases the diffusion coefficient of 2-methyl-pentane and increases the zeolite acidity. The sample containing niobium are initially more active in cracking of n-heptane and m-xylene isomerization than HZSM5 alone.

EFFECT OF THE ADDITION OF CR AND NB ON THE MICROSTRUCTURE AND ELECTROCHEMICAL CORROSION OF HEAT-TREATABLE AL-ZN-MG ALLOYS

The effect of the addition of Cr and Nb on the microstructure and the electrochemical corrosion of the weldable, high-strength and stress corrosion cracking (SCC) resistant Al-5%Zn-1.67%Mg-0.23%Cu alloy (H) has been studied. Combined additions of the alloying elements, J (with Nb), L (with Cr) and O (with Cr and Nb) and different heat treatments, ST (cold-rolled), A (annealed), F (quenched), B (quenched and aged) and C (quenched in two steps and aged), to obtain different microstructures and hardness have been performed. To correlate the electrochemical corrosion with the microstructure of the specimens, corrosion potential (E(cor)) measurements in different chloride solutions were performed and optical microscopy, SEM, TEM and EDX were applied. In chloride solutions containing dissolved O-2 or H2O2, the present alloys were polarized up to the pitting attack. It was shown that the E(cor) measurements were very sensitive to the alloy composition and heat treatment, increasing in the order H < J < L < O < Al (for a given heat treatment) and F < A approximate to ST < B < C (for a given alloy). The MgZn2 precipitates of the annealed (A) and cold-rolled (ST) specimens were dissolved in chloride solutions containing oxidizing agents and pitting attack was shown to develop in the cavities where the precipitates were present. In the specimens B and C, the compositions of the precipitate free zones was found to be equal to that of the matrix solid solution and preferential intergranular attack was not evident, this being in agreement with their SCC resistance. The addition of Cr and Nb increased the pitting corrosion resistance. The effects of Cr and Nb were additive, that of Cr being predominant, either, in the E(cor) shift or in the increase in the pitting corrosion resistance.

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.

Simulação do comportamento não linear de materiais quase-frágeis via elementos finitos com fissura embebida

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Structural behaviour of three-pile caps subjected to axial compressive loading

This work presents a comparative analysis about the behaviour of pile caps supported by 3 piles subjected to axial loading. Piles with 20 cm and 30 cm diameters were analysed. The main reinforcement was maintained in all the specimens, however, the arrangement of the secondary reinforcement varied. The main reinforcement consisted of steel bars connecting the piles. The secondary reinforcement was made up of: (a) bars going through the piles and through the projection of the column, (b) bars forming a network, and (c) vertical and horizontal stirrups. The main objective was the observation of the pile cap behaviour regarding the cracks and the modes of rupture. The real scale specimens were subjected to experimental tests until failure by rupture. Instruments were placed with the aim to obtain the displacement of the bases, the strains in the main and secondary reinforcement bars, in the compression struts, in the lower and upper nodal zones and in the sides of the caps. None of the caps reached failure by rupture with a load less than 1.12 times the theoretical load. The specimens ruptured due to the cracking of the compression strut and/or the yielding of the reinforcement bars in one direction.

Monolithic diphasic gels of mullite by sol-gel process under ultrasound stimulation

Diphasic gel in the mullite composition was prepared from a colloidal sol of boehmite mixed with a hydrolyzed tetraethoxisilane (TEOS) solution. The boehmite sol was obtained by peptization of a poorly crystallized or very small mean crystallite size (∼34 Å) precipitate, resulting from the reaction between solutions of aluminum sulfate and sodium hydroxide. Ultrasound was utilized in the processes of the TEOS hydrolysis and the boehmite peptization, and also for complete homogenization of the mixture to gel. The wet gel is almost clear and monolithic. The gel transparency is lost on drying, when syneresis has ended, so that the interlinked pore structure starts to empty and is recovered upon water re-absorption. Cracking closely accompanies this critical drying process. Differential thermal analysis (DTA) and X-ray diffraction (XRD) show that the solid structure of the gel is composed of an amorphous silica phase, as a matrix, and a colloidal sized crystalline phase of boehmite. Upon heat treatment, the boehmite phase within the gel closely follows the same transition sequence as in pure alumina shifted towards higher temperatures. Orthorhombic mullite formation was detected at 1300°C. © 1998 Elsevier Science B.V. All rights reserved.

On the development of the electrochemical potentiokinetic method

The development of electrochemical potentiokinetic methods as applied to the testing of metals and alloys is followed from its early phases up to its latest advances relating to intergranular corrosion, SCC and pitting corrosion tests of stainless steels and special alloys and to the examination of their structure and properties. In assessing the susceptibility to intergranular and pitting corrosion by potentiokinetic polarization tests, the polarization curves which apply to the bulk of the alloy grains (the matrix) must be distinguished from those pertaining to grain boundaries. Cyclic polarization measurements such as the electrochemical potentiokinetic reactivation (EPR) test make it possible to derive the alloy's susceptibility to intergranular, pitting and crevice corrosion from characteristic potentials and other quantities determined in the 'double loop' test. EPR is rapid and responds to the combined effects of a number of factors that influence the properties of materials. The electrochemical p otentiokinetic tests are sensitive enough to detect structural changes in heat treated materials ranging far beyond the stainless steels alone, and can be used for non-destructive testing aimed at elucidating the properties and behavior of materials. © 2001 Elsevier Science Ltd. All rights reserved.

Maize germ recovery and quality using the intermittent milling and dynamic steeping process

The intermittent milling and dynamic steeping (IMDS) process is an alternative method developed for wet milling of maize. In this process, the steeping stage can be reduced to 5 h by soaking maize in water at 60°C for 2 h and cracking the kernels to remove solution components diffusional barriers with minimum germ damage. Maize was dynamically steeped in solutions with 0.0, 0.1, and 0.2% sulphur dioxide (SO2) and 0.00, 0.55% lactic acid. Germ recovery, germ damage, fibre in germ, oil content and uncracked kernels were determined. A conventional steeping procedure was also performed. Germ recovery was higher for all tests using both SO2 and lactic acid than for the others with best germ yield for concentrations of 0.2% SO2 and 0.55% lactic acid. Germ damage ranged from 7.4 to 18.2% for all tests. The presence of lactic acid in the steeping solution decreased the amount of fibre in germ fraction. Germ oil content ranged from 39.3% (0-0% SO2, 0.55% lactic acid) to 44.0% (0.2% SO2, 0.55% lactic acid) for all treatments using IMDS. The smallest difference was 5.5% between IMDS (0.2% SO2, 0.55% lactic acid) and the conventional 36 h steeping process. An average of 1.3% of kernels remained uncracked after IMDS process. © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.

Effects of copper vapor laser radiation on the root canal wall of human teeth: A scanning electron microscope study

Objective: The aim of this study is to analyze the effects of copper vapor laser radiation on the radicular wall of human teeth. Materials and Methods: Immediately after the crowns of 10 human uniradicular teeth were cut along the cement-enamel junction, a chemical-surgical preparation of the radicular canals was completed. Then the roots were longitudinally sectioned to allow for irradiation of the surfaces of the dentin walls of the root canals. The hemi-roots were separated into two groups: one (control) with five hemi-roots that were not irradiated, and another (experimental) with 15 hemi-roots divided into three subgroups that were submitted to the following exposure times: 0.02,0.05, and 0.1 s. A copper vapor laser (510.6 nm) with a total average power of 6.5 W in green emission, frequency of 16.000 Hz, and pulse duration of 30 ns was used. Results: The results obtained by scanning electron microscope analysis showed the appearance of a cavity in the region of laser beam impact, with melting, recrystallization, and cracking on the edges of the dentin of the cavity due to heat diffusion. Conclusions: We determined that the copper vapor laser produces significant morphologic changes in the radicular wall of human teeth when using the parameters in this study. However, further research should be done to establish parameters that are compatible with dental structure in order to eliminate thermal damages. © Mary Ann Liebert, Inc.

A preliminary physicochemical, exergetic and economic study of hydrogen production utilizing glycerol

This work has as objective to demonstrate technical and economic viability of hydrogen production utilizing glycerol. The volume of this substance, which was initially produced by synthetic ways (from oil-derived products), has increased dramatically due mainly to biodiesel production through transesterification process which has glycerol as main residue. The surplus amount of glycerol has been generally utilized to feed poultry or as fuel in boilers, beyond other applications such as production of soaps, chemical products for food industry, explosives, and others. The difficulty to allocate this additional amount of glycerol has become it in an enormous environment problem, in contrary to the objective of biodiesel chain, which is to diminish environmental impact substituting oil and its derivatives, which release more emissions than biofuels, do not contribute to CO2-cycle and are not renewable sources. Beyond to utilize glycerol in combustion processes, this material could be utilized for hydrogen production. However, a small quantity of works (theoretical and experimental) and reports concerning this theme could be encountered. Firstly, the produced glycerol must be purified since non-reacted amounts of materials, inclusively catalysts, contribute to deactivate catalysts utilized in hydrogen production processes. The volume of non-reacted reactants and non-utilized catalysts during transesterification process could be reutilized. Various technologies of thermochemical generation of hydrogen that utilizes glycerol (and other fuels) were evaluated and the greatest performances and their conditions are encountered as soon as the most efficient technology of hydrogen production. Firstly, a physicochemical analysis must be performed. This step has as objective to evaluate the necessary amount of reactants to produce a determined volume of hydrogen and determine thermodynamic conditions (such as temperature and pressure) where the major performances of hydrogen production could be encountered. The calculations are based on the process where advance degrees are found and hence, fractions of products (especially hydrogen, however, CO2, CO, CH4 and solid carbon could be also encountered) are calculated. To produce 1 Nm3/h of gaseous hydrogen (necessary for a PEMFC - Proton Exchange Membrane Fuel Cell - containing an electric efficiency of about 40%, to generate 1 kWh), 0,558 kg/h of glycerol is necessary in global steam reforming, 0,978 kg/h of glycerol in partial oxidation and cracking processes, and 0,782 kg/h of glycerol in autothermal reforming process. The dry reforming process could not be performed to produce hydrogen utilizing glycerol, in contrary to the utilization of methane, ethanol, and other hydrocarbons. In this study, steam reforming process was preferred due mainly to higher efficiencies of production and the need of minor amount of glycerol as cited above. In the global steam reforming of glycerine, for one mole of glycerol, three moles of water are necessary to produce three moles of CO2 and seven moles of H2. The response reactions process was utilized to predict steam reforming process more accurately. In this mean, the production of solid carbon, CO, and CH4, beyond CO2 and hydrogen was predicted. However, traces of acetaldehyde (C2H2), ethylene (C2H4), ethylene glycol, acetone, and others were encountered in some experimental studies. The rates of determined products obviously depend on the adopted catalysts (and its physical and chemical properties) and thermodynamic conditions of hydrogen production. Eight reactions of steam reforming and cracking were predicted considering only the determined products. In the case of steam reforming at 600°C, the advance degree of this reactor could attain its maximum value, i.e., overall volume of reactants could be obtained whether this reaction is maintained at 1 atm. As soon as temperature of this reaction increases the advance degree also increase, in contrary to the pressure, where advance degree decrease as soon as pressure increase. The fact of temperature of reforming is relatively small, lower costs of installation could be attained, especially cheaper thermocouples and smaller amount of thermo insulators and materials for its assembling. Utilizing the response reactions process in steam reforming, the predicted volumes of products, for the production of 1 Nm3/h of H2 and thermodynamic conditions as cited previously, were 0,264 kg/h of CO (13% of molar fraction of reaction products), 0,038 kg/h of CH4 (3% of molar fraction), 0,028 kg/h of C (3% of molar fraction), and 0,623 kg/h of CO2 (20% of molar fraction). Through process of water-gas shift reactions (WGSR) an additional amount of hydrogen could be produced utilizing mainly the volumes of produced CO and CH4. The overall results (steam reforming plus WGSR) could be similar to global steam reforming. An attention must to be taking into account due to the possibility to produce an additional amount of CH4 (through methanation process) and solid carbon (through Boudouard process). The production of solid carbon must to be avoided because this reactant diminishes (filling the pores) and even deactivate active area of catalysts. To avoid solid carbon production, an additional amount of water is suggested. This method could be also utilized to diminish the volume of CO (through WGSR process) since this product is prejudicial for the activity of low temperature fuel cells (such as PEMFC). In some works, more three or even six moles of water are suggested. A net energy balance of studied hydrogen production processes (at 1 atm only) was developed. In this balance, low heat value of reactant and products and utilized energy for the process (heat supply) were cited. In the case of steam reforming utilizing response reactions, global steam reforming, and cracking processes, the maximum net energy was detected at 700°C. Partial oxidation and autothermal reforming obtained negative net energy in all cited temperatures despite to be exothermic reactions. For global steam reforming, the major value was 114 kJ/h. In the case of steam reforming, the highest value of net energy was detected in this temperature (-170 kJ/h). The major values were detected in the cracking process (up to 2586 kJ/h). The exergetic analysis has as objective, associated with physicochemical analysis, to determine conditions where reactions could be performed at higher efficiencies with lower losses. This study was performed through calculations of exergetic and rational efficiencies, and irreversibilities. In this analysis, as in the previously performed physicochemical analysis, conditions such as temperature of 600°C and pressure of 1 atm for global steam reforming process were suggested due to lower irreversibility and higher efficiencies. Subsequently, higher irreversibilities and lower efficiencies were detected in autothermal reforming, partial oxidation and cracking process. Comparing global reaction of steam reforming with more-accurate steam reforming, it was verified that efficiencies were diminished and irreversibilities were increased. These results could be altered with introduction of WGSR process. An economic analysis could be performed to evaluate the cost of generated hydrogen and determine means to diminish the costs. This analysis suggests an annual period of operation between 5000-7000 hours, interest rates of up to 20% per annum (considering Brazilian conditions), and pay-back of up to 20 years. Another considerations must to be take into account such as tariffs of utilized glycerol and electricity (to be utilized as heat source and (or) for own process as pumps, lamps, valves, and other devices), installation (estimated as US$ 15.000 for a plant of 1 Nm3/h) and maintenance cost. The adoption of emission trading schemes such as carbon credits could be performed since this is a process with potential of mitigates environment impact. Not considering credit carbons, the minor cost of calculated H2 was 0,16288 US$/kWh if glycerol is also utilized as heat sources and 0,17677 US$/kWh if electricity is utilized as heat sources. The range of considered tariff of glycerol was 0-0,1 US$/kWh (taking as basis LHV of H2) and the tariff of electricity is US$ 0,0867 US$/kWh, with demand cost of 12,49 US$/kW. The costs of electricity were obtained by Companhia Bandeirante, localized in São Paulo State. The differences among costs of hydrogen production utilizing glycerol and electricity as heat source was in a range between 0,3-5,8%. This technology in this moment is not mature. However, it allows the employment generation with the additional utilization of glycerol, especially with plants associated with biodiesel plants. The produced hydrogen and electricity could be utilized in own process, increasing its final performance.