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Various Temperatures (various + temperature)
Selected AbstractsCobalt(II) Complexes with Substituted Salen-Type Ligands and Their Dioxygen Affinity in N,N -Dimethylformamide at Various TemperaturesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2005Andreas Huber Abstract Several unsymmetrically substituted salen-type cobalt(II) Schiff-base complexes CoL [H2L = 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-hexadiene (1); 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (2); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-(2-hydroxyphen-yl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (3); 1-(2-hydroxyphenyl)-6-methyl-2,5-diaza-1,5-nonadien-8-one (4); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-methyl-2,5-di-aza-1,5-nonadien-8-one (5); 1-(2-hydroxyphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (6); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (7)] were prepared and characterized by their UV/Vis absorption spectra, magnetic moments, and oxidation potentials. Except for complex 4 (irreversible oxidation with t½ , 3 h), complexes 1,3 and 5,7 are remarkably resistant against irreversible auto-oxidation in air-saturated N,N -dimethylformamide (DMF) at ambient temperature. To characterize the Lewis acidity of the cobalt center in 1,7, the equilibrium constant Kpy was determined for monoadduct formation with pyridine (CoL + pyCoL·py). An O2 -sensitive optode was used to determine the Henry constant, KH, for the system O2/DMF in the temperature range 298,228 K. The formation of 1:1 adducts of complexes 1,7 with O2 in DMF, as characterized by the equilibrium constant K, was followed spectrophotometrically in the temperature range 298,228 K. The parameters ,Ho, ,So, and K are reported. At 298 K, K ranges from 21.9 M,1 (5) to 155 M,1 (7). The overall spectroscopic information, including EPR spectra obtained with frozen solutions of 3 and 7 in O2 -saturated DMF, confirm that the 1:1 adducts CoL·O2 are cobalt(III) superoxo compounds. The symmetrically substituted salen complex8 [H2L = 1,6-bis(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,4,4-tetramethyl-2,5-diaza-1,5-hexadiene in 8] is shown to catalyze the oxidation of triphenylphosphane and 2,6-di- tert -butylphenol by O2 in DMF at ambient temperature. The correlation of the data obtained for K, Kpy, and the oxidation potential E½ is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Morphology and Roughness of Silver Deposit Formed by Cementation at Various Temperatures in Pure Sulfuric AcidHELVETICA CHIMICA ACTA, Issue 11 2006Grzegorz Abstract The morphology and surface roughness of silver (Ag) deposit formed on metallic copper (Cu) by cementation conducted in a 0.5M H2SO4 solution was investigated at various temperatures above 25°. The influence of the presence or absence of oxygen (O2) on Ag morphology was studied at an initial Ag+ concentration of 20,mg/dm3. An analysis of distribution diagrams of the surface height calculated from scanning-electron-microscope (SEM) top-view images was performed. The cementation reaction results in a non-homogeneous Ag deposit formed on the surface independently of the presence or absence of O2 in solution. The Ag deposit covers Cu mainly with a uniform and compact layer with separated germs of predendrites, but also a huge ,fern-leaf-shaped' and ,lycopodium-twigs-shaped' dendrites appear occasionally on the surface. The presence of O2 in the system and temperature do not affect significantly the morphology of Ag dendrite as well as a deposit formed on the smooth part of the surface. The roughness of surface with Ag cement varies with temperature only under aerobic conditions where the enhanced Cu corrosion increases the size of anodic sites. The results obtained from the surface-height-distribution diagrams constructed for anaerobic conditions showed that the reaction between Cu+ and Ag+ does not start in the bulk of the solution even at the highest studied temperature. [source] Tricalcium Silicate T1 and T2 Polymorphic Investigations: Rietveld Refinement at Various Temperatures Using Synchrotron Powder DiffractionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2004Vanessa K. Peterson The lattice parameters, cell volume, and structure of a sample of phase pure triclinic tricalcium silicate were determined using in situ, high-temperature synchrotron powder diffraction and full-profile Rietveld refinement. The temperature range covered was from ambient to 740°C. Evidence of superstructure was found. The T2 type structure with disordered SiO4 tetrahedra was observed, and an average structure for the subcell (P, a= 11.7416(2) Å, b= 14.2785(2) Å, c= 13.7732(2) Å, ,= 105.129(1)°, ,= 94.415(1)°, and ,= 89.889(1)°) is presented. Differential thermal analysis and X-ray fluorescence was also performed. [source] Growth and characterization of Nd, Yb , yttrium oxide nanopowders obtained by sol-gel methodCRYSTAL RESEARCH AND TECHNOLOGY, Issue 12 2007A. Rzepka Abstract Nanopowders of Y2O3 pure, doped and codoped by Nd3+, Yb3+ were obtained by sol-gel method. Solution with ethylene glycol was choosed as the proper solution where crystallites of powder with Nd and Yb dopants had the same size. Finally the one-phased compounds of Y2O3 doped 0.5 at% Nd and 1, 2 or 4 at% Yb were obtained. Grain growth and their morphology were investigated in various temperature and time of heating. The changes of crystallite sizes and lattice constants in relation to the heating time and temperature for the composition Y2O3 doped 0.5 at% Nd and 2 at% Yb are presented. Y2O3 containing 0,5 at% of Nd exhibits intense luminescence bands centered at 920 nm, 1100 nm and 1360 nm whereas a single band at about 1020 nm appears in samples co-doped with neodymium and ytterbium. Luminescence spectra recorded did not depend on the sample preparation procedure and size of grains. OH impurity affects critically the relaxation dynamics of luminescent ion in nanopowders. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Thermal and EPR investigations of thallium gallium disulphide single crystalCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2009M. Acikgoz Abstract In this research, the results of the differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) investigations of TlGaS2 single crystal are presented. Specific heat capacity (Cp) anomalies of layered TlGaS2 have been obtained by using a new DSC technique for such crystals. Remarkable heat capacity anomalies have been revealed at the temperatures of 137.7 K, 174.5 K and 238.5 K. It is found that the anomalies appear at maximum with a small deviation (by 3-4%) from the regular values, and Cp discontinuity amounted to approximately 5%. Additionally, EPR spectra of Fe doped TlGaS2 single crystals have been recorded at various temperatures down to 6 K for different orientations of the applied magnetic field. Transformations of present EPR spectra are not sufficient for the confirmation of structural phase transitions, in contrast to the cases in iso structural TlInS2 and TlGaSe2 compounds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Studies on multiphased mixed crystals of NaCl, KCl and KICRYSTAL RESEARCH AND TECHNOLOGY, Issue 1 2009M. Priya Abstract Multiphased mixed crystals of NaCl, KCl and KI were grown by the melt method, for the first time. Densities and refractive indices of all the grown crystals were determined and used for the estimation of the composition in the crystal. Atomic absorption spectroscopic measurements were done to estimate the metal atom contents in the crystal. Lattice parameters and thermal parameters (Debye-Waller factor, mean square amplitude of vibration, Debye temperature and Debye frequency) were determined from the X-ray powder diffraction data. DC and AC electrical measurements were done at various temperatures ranging from 40 to 150°C. Activation energies were also estimated. The observed lattice parameters showed that the system exhibits three phases each nearly corresponds to NaCl, KCl and KI. The thermal and electrical parameters show a highly nonlinear bulk composition dependence. Results are reported. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nucleation kinetics and growth aspects of semi organic non-linear optical bis thiourea cadmium acetate single crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2006R. Sankar Abstract Nucleation parameters such as metastable zone width, induction period and interfacial energy have been determined for the aqueous solution growth of bis thiourea cadmium acetate (BTCA) single crystals. Solubility of BTCA has been determined for various temperatures. Metastable zone width and induction period values have been estimated in order to optimize the growth parameters. The interfacial tension values derived from experimentally determined induction period are found to be comparable with theoretical values. Bulk crystals of BTCA have been grown using the optimized growth parameters. The grown crystals have been subjected to structural, optical and mechanical property studies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preparation and characterization of ZnO nanofibers by electrospinningCRYSTAL RESEARCH AND TECHNOLOGY, Issue 5 2006R. Siddheswaran Abstract Electrospun ZnO nanofibers were obtained by calcinating PVA/Zinc Acetate composite fibers at various temperatures. Atomic Force Microscopy (AFM) revealed that the ZnO fibers have diameters in the range of 100-200 nm. The fibers were characterized by FT- IR, TGA-DTA, and XRD studies. The XRD results showed that the crystal structure and the morphology of the fibers were largely dependent on the calcination temperature (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2005C. Viswanathan Abstract Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (Tsb = 30°, 400°C). X-ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 , 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (Egopt) 1.56 eV; where as the films formed at 400°C were found to have a Egopt of 1.92 eV. The increase in the value of Egopt with Tsb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current -Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The significance of overlapping plant range to a putative adaptive trade-off in the black bean aphid Aphis fabae ScopECOLOGICAL ENTOMOLOGY, Issue 4 2004C. R. Tosh Abstract., 1. This study continues to explore the analysis of a putative adaptive trade-off in the utilisation of host plants Vicia faba and Tropaeolum majus by the aphid, Aphis fabae. These plants are utilised exclusively by the subspecies Aphis fabae fabae and A. f. mordwilkoi respectively, and this plant-use system has been studied previously as a potential source of disruptive selection. 2. Here the potential of these two host plants to generate disruptive selection is considered given common utilisation of the abundant host plant, Rumex obtusifolius, by both subspecies. 3. The life history of subspecific clones is quantified in the laboratory on V. faba, T. majus, and R. obtusifolius at various temperatures and used to parameterise a temperature-driven simulation model of aphid population development. 4. Accuracy of the model is tested using a field experiment, and fitness of clones on specific and common host is simulated using temperature data from a number of English sites. 5. The model gives a close quantitative fit to field data and makes the following predictions: performance of A. f. fabae is higher on the specific host than the common host under all tested thermal regimes; and performance of A. f. mordwilkoi is superior on the specific host in warm years but inferior in cold years. 6. Given the great abundance of R. obtusifolius relative to T. majus, the model predicts that the plant utilisation system has little potential to consistently promote hybrid dysfunction. This adds further weight to the assertion that the plant utilisation system studied can offer little insight into the evolutionary processes involved in subspecific differentiation and probably contains a host plant/host plants acquired after the evolution of reproductive barriers. [source] Toxicokinetics of waterborne bisphenol a in landlocked salmon (Salmo salar m. Sebago) eggs at various temperaturesENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 10 2001Jani O. Honkanen Abstract Eye-pigmented eggs of landlocked salmon were exposed to waterbone [14C]-labeled bisphenol A at four temperatures (2, 6, 8, and 12°C). Both in accumulation and depuration experiments, the eggs were exposed to a bisphenol A concentration of 1 ,g/L for 196 h. In the depuration experiment, the exposed eggs were placed into clean water for 96 h. At each sampling time, the eggs were dissected into three parts (eggshell, embryo, and yolk sac), and all of these parts were weighed and analyzed separately in a liquid scintillation counter. The results show that waterborne bisphenol A accumulates in developing salmon eggs. Accumulation of bisphenol A showed a clear trend both in whole eggs and in dissected parts: the higher the temperature, the higher the uptake rate constant and the concentration of bisphenol A. Steady state was reached only in the embryos at the lowest temperature; in other cases, accumulation was linear. Depuration was significant only in the embryos at 2 and 6°C. As the accumulation and the depuration results show, the elimination of bisphenol A was evidently more significant at lower temperatures. [source] Thermodynamic Study of the Binding of Methyltrioxorhenium with Pyridine and Its Derivatives in Benzene SolutionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005S. Masoud Nabavizadeh Abstract A spectrophotometric study of the interaction of methyltrioxorhenium (MTO) with pyridine and its derivatives in benzene solution has been carried out at various temperatures. The stability constants of the resulting 1:1 complexes were determined by analysis of spectrophotometric data and found to vary in the order 3,4-Me2Py > 4- tBuPy > 4-MePy > 3-MePy > 4-BenzylPy > Py > 3-PhPy > 3-C(O)OMePy > 3-ClPy. The enthalpy and entropy of adduct formation were determined from the temperature dependence of the stability constants. All complexes formed were enthalpy stabilized but entropy destabilized. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Physicochemical evaluation of silica-glass fiber reinforced polymers for prosthodontic applicationsEUROPEAN JOURNAL OF ORAL SCIENCES, Issue 3 2005Gökçe Meriç This investigation was designed to formulate silica-glass fiber reinforced polymeric materials. Fused silica-glass fibers were chosen for the study. They were heat-treated at various temperatures (500°C, 800°C and 1100°C), silanized, sized and incorporated in two modified resin mixtures (A and B). The flexural properties in dry and wet conditions were tested and statistically analyzed, and the content of residual methyl methacrylate (MMA) monomer, dimensional changes with temperature, water sorption and solubility were determined. Woven fibers [36.9% (wt/wt)], heat-treated at 500°C, gave the highest strength values for the polymeric composites (an ultimate transverse strength of 200 Mpa and a flexural modulus of 10 GPa) compared with the fibers heat-treated at other temperatures. There was no statistically significant difference in the measured flexural properties between resins A and B regarding fiber treatment and water storage time. These fiber composites had a small quantity of residual MMA content [0.37 ± 0.007% (wt/wt)] and very low water solubility, indicating good biocompatibility. It was suggested that silica-glass fibers could be used for reinforcement as a result of their anticipated good qualities in aqueous environments, such as the oral environment. [source] Elimination Mechanisms in the Aminolysis of Sulfamate Esters of the Type NH2SO2OC6H4X , Models of Enzyme InhibitorsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2008William J. Spillane Abstract The kinetics of the reaction of 4-nitrophenyl sulfamate NH2SO2OC6H4NO2 -4 (1a) in acetonitrile (ACN) with a series of pyridines (pKa range ca. 8 units) and alicyclic amines (pKa range ca. 3.6 units) has been studied in the presence of excess amine at various temperatures. The compounds 1a,1f are important as model substrates for the medicinally important sulfamate esters 667-coumate and emate and analogues. Pseudo-first-order rate constants (kobsd.) have been obtained mainly by the release of 4-nitrophenol/4-nitrophenoxide. Slopes of plots of kobsd. vs. [amine] gave second-order rate constants (k2), and Brönsted plots were biphasic for the aminolysis (with alicyclic amines) with an initial slope ,1 = 0.53 and a subsequent slope ,2 = 0.19. The change in slope occurs near the first pKa of 1a (17.9) in ACN. Leaving-group effects were probed by using the same series of phenyl sulfamates, i.e. 1a,f and the alicyclic amines N -formylpiperazine and pyrrolidine. The reactions were considered to be dissociative in nature involving E2- and E1cB- type mechanisms with the phenyl sulfamate anion 2 being involved in pyridine and in the weaker alicyclic amines (,1 segment) and a phenyl sulfamate dianion 3 being involved with the stronger alicyclic bases (,2 segment). The calculation of Leffler indices (,) for bond-forming (base···H+) and bond-breaking (S,OAr) steps allows fuller interpretation of the mechanisms occurring, which are seen as having the N -sulfonylamines, HN=SO2 and ,N=SO2 on the reaction pathways leading to products. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] The influence of arbuscular mycorrhizal colonization and environment on root development in soilEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2003D. Atkinson Summary The production of fine roots is one of the principal means by which carbon, fixed during photosynthesis, enters the soil, and quantifying the production for particular combinations of environmental and biotic factors is important for predicting the sequestration of carbon in the soils of grassland ecosystems. Arbuscular mycorrhizal fungi (AMF) can have a major effect on the production of roots, and we studied how colonization by AMF affects the lifespan of roots. Twenty per cent of control roots of Trifolium repens survived for longer than 42 days whereas 37% survived that long in AMF-colonized plants. The overall survival of the roots of Lolium perenne was less than in T. repens: around 10% of roots survived beyond 42 days and this was not affected by AMF colonization. Previous studies have shown that lifespans of roots can be affected by temperature. We tested the hypothesis that these observations are linked to a change in the morphology of the root system caused by temperature and also by AMF. We found that inoculation with AMF in a microcosm study using Plantago lanceolata grown at various temperatures, with and without AMF, showed no clear effect of AMF on branching patterns. Temperature had a significant effect on total lengths, numbers and branching rates of some higher orders of roots. Total lengths of both secondary and tertiary roots grown at 27°C were about double those of plants grown at 15°C. Colonization by AMF tended to reduce this effect. Evidently the effect of colonization by AMF on root lifespan depends on the species. Increased branching, and thus a greater proportion of ephemeral roots, was responsible for shortening the lives of the roots at increased temperature, which suggests a strong link between lifespan and morphology. [source] Crack-healing behaviour and resultant high-temperature fatigue strength of machined Si3N4/SiC composite ceramicFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 1 2008Y.-S. JUNG ABSTRACT The crack-healing behaviour of machining cracks in Si3N4/20 wt% SiC composite was investigated. The machining cracks were introduced by a heavy machining process, during the creation of a semicircular groove. The machined specimens were healed at various temperatures and times in air. The optimized crack-healing condition of the machined specimens was found to be a temperature of 1673 K and a time of 10 h. The specimens healed by this condition exhibited almost the same strength as the smooth specimens healed. Moreover, the bending strengths and the fatigue limits of the machined specimens healed were systematically investigated at temperatures from room temperature to 1673 K. The machined specimens healed at the optimized condition exhibited an almost constant bending strength (,700 MPa) up to 1673 K. Also, the specimens exhibited considerably high cyclic and static fatigue limits at temperatures from 1073 to 1573 K. These results demonstrated that the crack-healing could be an effective method for improving the structural integrity and reducing machining costs of the Si3N4/SiC composite ceramic. [source] Temperature dependence of stress,lifetime fatigue curvesFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 12 2000J. Kohout By analysing several finite-life fatigue curves obtained at various temperatures, a relation describing the temperature dependence of fatigue strength is proposed, based on the Basquin equation. Accordingly, an approximate inverse law between fatigue strength and the square root of absolute temperature is demonstrated. Moreover, a boundary temperature above which creep begins to play a significant role is estimated. A simple extension of the relationship to the low-cycle fatigue region, allows one to determine predictive curves describing fatigue behaviour at various temperatures, even in cases where only a single fatigue curve at a given temperature plus the temperature dependence of tensile strength are known. [source] Possible involvement of an FKBP family member protein from a psychrotrophic bacterium Shewanella sp.FEBS JOURNAL, Issue 7 2004SIB1 in cold-adaptation A psychrotrophic bacterium Shewanella sp. strain SIB1 was grown at 4 and 20 °C, and total soluble proteins extracted from the cells were analyzed by two-dimensional polyacrylamide gel electrophoresis. Comparison of these patterns showed that the cellular content of a protein with a molecular mass of 28 kDa and an isoelectric point of four greatly increased at 4 °C compared to that at 20 °C. Determination of the N-terminal amino acid sequence, followed by the cloning and sequencing of the gene encoding this protein, revealed that this protein is a member of the FKBP family of proteins with an amino acid sequence identity of 56% to Escherichia coli FKBP22. This protein was overproduced in E. coli in a His-tagged form, purified, and analyzed for peptidyl-prolyl cis-trans isomerase activity. When this activity was determined by the protease coupling assay using N -succinyl-Ala-Leu-Pro-Phe- p -nitroanilide as a substrate at various temperatures, the protein exhibited the highest activity at 10 °C with a kcat/Km value of 0.87 µm,1·s,1. When the peptidyl-prolyl cis-trans isomerase activity was determined by the RNase T1 refolding assay at 10 and 20 °C, the protein exhibited higher activity at 10 °C with a kcat/Km value of 0.50 µm,1·s,1. These kcat/Km values are lower but comparable to those of E. coli FKBP22. We propose that a FKBP family protein is involved in cold-adaptation of psychrotrophic bacteria. [source] Fabrication and Electrochemical Properties of Epitaxial Samarium-Doped Ceria Films on SrTiO3 -Buffered MgO SubstratesADVANCED FUNCTIONAL MATERIALS, Issue 11 2009Simone Sanna Abstract Thin films of samarium-oxide-doped (20,mol%) ceria (SDC) are grown by pulsed-laser deposition (PLD) on (001) MgO single-crystal substrates. SrTiO3 (STO) prepared by PLD is used as a buffer layer on the MgO substrates to enable epitaxial growth of the fluorite-structured SDC film; the STO layer provides a proper crystalline match between SDC and MgO, resulting in highly crystalline, epitaxial SDC films grown in the (001) orientation. Film conductivity is evaluated by electrochemical impedance spectroscopy measurements, which are performed at various temperatures (400,775,°C) in a wide range of oxygen partial pressure (pO2) values (10,25,1,atm) in order to separate ionic and electronic conductivity contributions. At 700,°C, SDC/STO films on (100) MgO exhibit a dominant ionic conductivity of about 7,×,10,2,S cm,1, down to pO2 values of about 10,15,atm. The absence of grain boundaries make the SDC/STO/MgO heterostructures stable to oxidation-reduction cycles at high temperatures, in contrast to that observed for the more disordered SDC/STO films, which degraded after hydrogen exposure. [source] Size Effect on Properties of Varistors Made From Zinc Oxide Nanoparticles Through Low Temperature Spark Plasma SinteringADVANCED FUNCTIONAL MATERIALS, Issue 11 2009Léna Saint Macary Abstract Conditions for the elaboration of nanostructured varistors by spark plasma sintering (SPS) are investigated, using 8-nm zinc oxide nanoparticles synthesized following an organometallic approach. A binary system constituted of zinc oxide and bismuth oxide nanoparticles is used for this purpose. It is synthesized at room temperature in an organic solution through the hydrolysis of dicyclohexylzinc and bismuth acetate precursors. Sintering of this material is performed by SPS at various temperatures and dwell times. The determination of the microstructure and the chemical composition of the as-prepared ceramics are based on scanning electron microscopy and X-ray diffraction analysis. The nonlinear electrical characteristics are evidenced by current,voltage measurements. The breakdown voltage of these nanostructured varistors strongly depends on grain sizes. The results show that nanostructured varistors are obtained by SPS at sintering temperatures ranging from 550 to 600,°C. [source] Modelling carbon balances of coastal arctic tundra under changing climateGLOBAL CHANGE BIOLOGY, Issue 1 2003Robert F. Grant Abstract Rising air temperatures are believed to be hastening heterotrophic respiration (Rh) in arctic tundra ecosystems, which could lead to substantial losses of soil carbon (C). In order to improve confidence in predicting the likelihood of such loss, the comprehensive ecosystem model ecosys was first tested with carbon dioxide (CO2) fluxes measured over a tundra soil in a growth chamber under various temperatures and soil-water contents (,). The model was then tested with CO2 and energy fluxes measured over a coastal arctic tundra near Barrow, Alaska, under a range of weather conditions during 1998,1999. A rise in growth chamber temperature from 7 to 15 °C caused large, but commensurate, rises in respiration and CO2 fixation, and so no significant effect on net CO2 exchange was modelled or measured. An increase in growth chamber , from field capacity to saturation caused substantial reductions in respiration but not in CO2 fixation, and so an increase in net CO2 exchange was modelled and measured. Long daylengths over the coastal tundra at Barrow caused an almost continuous C sink to be modelled and measured during most of July (2,4 g C m,2 d,1), but shortening daylengths and declining air temperatures caused a C source to be modelled and measured by early September (,1 g C m,2 d,1). At an annual time scale, the coastal tundra was modelled to be a small C sink (4 g C m,2 y,1) during 1998 when average air temperatures were 4 °C above normal, and a larger C sink (16 g C m,2 y,1) during 1999 when air temperatures were close to long-term normals. During 100 years under rising atmospheric CO2 concentration (Ca), air temperature and precipitation driven by the IS92a emissions scenario, modelled Rh rose commensurately with net primary productivity (NPP) under both current and elevated rates of atmospheric nitrogen (N) deposition, so that changes in soil C remained small. However, methane (CH4) emissions were predicted to rise substantially in coastal tundra with IS92a-driven climate change (from ,20 to ,40 g C m,2 y,1), causing a substantial increase in the emission of CO2 equivalents. If the rate of temperature increase hypothesized in the IS92a emissions scenario had been raised by 50%, substantial losses of soil C (,1 kg C m,2) would have been modelled after 100 years, including additional emissions of CH4. [source] Morphology and Roughness of Silver Deposit Formed by Cementation at Various Temperatures in Pure Sulfuric AcidHELVETICA CHIMICA ACTA, Issue 11 2006Grzegorz Abstract The morphology and surface roughness of silver (Ag) deposit formed on metallic copper (Cu) by cementation conducted in a 0.5M H2SO4 solution was investigated at various temperatures above 25°. The influence of the presence or absence of oxygen (O2) on Ag morphology was studied at an initial Ag+ concentration of 20,mg/dm3. An analysis of distribution diagrams of the surface height calculated from scanning-electron-microscope (SEM) top-view images was performed. The cementation reaction results in a non-homogeneous Ag deposit formed on the surface independently of the presence or absence of O2 in solution. The Ag deposit covers Cu mainly with a uniform and compact layer with separated germs of predendrites, but also a huge ,fern-leaf-shaped' and ,lycopodium-twigs-shaped' dendrites appear occasionally on the surface. The presence of O2 in the system and temperature do not affect significantly the morphology of Ag dendrite as well as a deposit formed on the smooth part of the surface. The roughness of surface with Ag cement varies with temperature only under aerobic conditions where the enhanced Cu corrosion increases the size of anodic sites. The results obtained from the surface-height-distribution diagrams constructed for anaerobic conditions showed that the reaction between Cu+ and Ag+ does not start in the bulk of the solution even at the highest studied temperature. [source] Kinetics of the synthesis of propyl and butyl acrylates in the presence of some heteropolyacids as catalystsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2009Jerzy Skrzypek Esterification reactions of acrylic acid with n -propanol and n -butanol were carried out in the liquid phase in the presence of H3PW12O40 or H3PMo12O40 as a catalyst, at various temperatures, molar ratios of the reactants, and concentrations of the catalyst. The kinetic equations had a nonelementary form. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 12,17, 2009 [source] Study of association thermodynamics between crystal violet and sodium bis(2-ethylhexyl)sulfosuccinate and kinetics of basic fading of crystal violet in microemulsionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2008Zhi Yun Chen The thermodynamics of the association between 4,4,,4,-tris(dimethyl-amino)triphenylmethyl chloride (crystal violet or CV) and sodium bis(2-ethylhexyl)-sulfosuccinate (aerosol OT or AOT) in water/AOT/n -decane microemulsion and the kinetics of the basic hydrolysis of CV in a water-in-oil microemulsion were investigated by UV,vis spectroscopic measurements. An association model of CV and AOT was used to analyze the experimental data to obtain the association constants at various temperatures. By taking the association into account, the "actual" rate constants and the activation energies of the basic hydrolysis of CV in the media of water/AOT/oil were obtained. The difference in thermodynamics and kinetics between the two media of water/AOT/n -decane and water/AOT/isooctane is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 294,300, 2008 [source] The kinetics of complex formation between Ti(IV) and hydrogen peroxideINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2007Daniel W. O'Sullivan The kinetics of the formation of the titanium-peroxide [TiO2+2] complex from the reaction of Ti(IV)OSO4 with hydrogen peroxide and the hydrolysis of hydroxymethyl hydroperoxide (HMHP) were examined to determine whether Ti(IV)OSO4 could be used to distinguish between hydrogen peroxide and HMHP in mixed solutions. Stopped-flow analysis coupled to UV-vis spectroscopy was used to examine the reaction kinetics at various temperatures. The molar absorptivity (,) of the [TiO2+2] complex was found to be 679.5 ± 20.8 L mol,1 cm,1 at 405 nm. The reaction between hydrogen peroxide and Ti(IV)OSO4 was first order with respect to both Ti(IV)OSO4 and H2O2 with a rate constant of 5.70 ± 0.18 × 104 M,1 s,1 at 25°C, and an activation energy, Ea = 40.5 ± 1.9 kJ mol,1. The rate constant for the hydrolysis of HMHP was 4.3 × 10,3 s,1 at pH 8.5. Since the rate of complex formation between Ti(IV)OSO4 and hydrogen peroxide is much faster than the rate of hydrolysis of HMHP, the Ti(IV)OSO4 reaction coupled to time-dependent UV-vis spectroscopic measurements can be used to distinguish between hydrogen peroxide and HMHP in solution. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 457,461, 2007 [source] Kinetics and mechanism of decomposition of intermediate complex during oxidation of pectate polysaccharide by potassium permanganate in alkaline solutionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2003Khalid S. Khairou The kinetics of decomposition of an [Pect·MnVIO42,] intermediate complex have been investigated spectrophotometrically at various temperatures of 15,30°C and a constant ionic strength of 0.1 mol dm,3. The decomposition reaction was found to be first-order in the intermediate concentration. The results showed that the rate of reaction was base-catalyzed. The kinetic parameters have been evaluated and found to be ,S, = , 190.06 ± 9.84 J mol,1 K,1, ,H, = 19.75 ± 0.57 kJ mol,1, and ,G, = 76.39 ± 3.50 kJ mol,1, respectively. A reaction mechanism consistent with the results is discussed. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 67,72, 2003 [source] Cyanide-Bridged CrIII,NiII Superparamagnetic Nanoparticles,ADVANCED MATERIALS, Issue 10 2003L. Catala Using reverse micelles as nanoreactors facilitates the preparation of bimetallic CrIII,NiII cyanide-bridged nanoparticles that behave as superparamagnets with a blocking temperature of 5.5 K. The Figure presents a plot of magnetization vs. field for various temperatures. Redispersion in an organic solution allows the particles to be diluted in a polymer matrix (inset of Figure) leading to a shift of the blocking temperature down to 2 K. [source] Two-dimensional small-angle X-ray scattering of self-assembled nanocomposite films with oriented arrays of spheres: determination of lattice type, preferred orientation, deformation and imperfectionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2007Bernd M. Smarsly Mesostructured oxide films were prepared by dip-coating from colloidal solutions on ultrathin Si wafers and solidified by heating at various temperatures. Two-dimensional small-angle X-ray scattering measurements were carried out in transmission under selected tilt angles and evaluated by comparison with analytical expressions. The films are composed of oriented mesophases, the structures of which are defined in terms of lattice type, preferred orientation, deformation and imperfection, notably stacking faults. [source] In situ high-temperature powder diffraction study of reversible phase transitions in decagonal Al71.2Co12.8Ni16JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2003Christian Soltmann This experimental work is aimed at confirming and broadening findings of former investigations into the type I,S1 phase transition in Al,Co,Ni quasicrystals. In situ high-temperature synchrotron radiation measurements were carried out on Al71.2Co12.8Ni16 powder samples. The cell parameters were precisely determined at various temperatures in the range 290,1110,K. The data provided a new extended insight into the thermal expansion of Al71.2Co12.8Ni16 and into fundamental properties of the phase transition. [source] Small-angle X-ray scattering studies on oxide layer thickness at the porous silicon interfaceJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3-1 2003M. Björkqvist We have determined the thickness of an oxide layer at the p+-type porous silicon interface as a function of oxidation time, by using a small angle X-ray scattering (SAXS). The scattering experiments were carried out using a Kratky camera with a step-scanning device. Oxidation was achieved by storing the porous silicon samples in various temperatures under high relative humidity. The negative deviations from Porod's law were observed from the scattering curves of oxidized samples. The oxide layer thickness was determined from the scattering curve using a sigmoidal-gradient approximation for the diffuse boundary. The oxide layer thickness values as a function of oxidation time, obtained using SAXS are compared to measured weight increase values, caused by the oxidation. [source] | ||||||||||||||||||||||||||||||||||||||||