Argon–water system at low temperatures

The molecular dynamics method is used to simulate argon solutions in water and a thin water film–argon system at low temperatures. The correlation in motions of two closely spaced argon atoms is of another nature than the correlation of two neon atoms in a neon solid solution in ice II. The structure of hydrate shells of argon atoms contains five-membered rings composed of water molecules. The solubility of argon in a water film at low temperatures is noticeably higher than at room temperature. If a water film is first cooled to the glassy state and then argon atoms are added to it, then approximately as many argon atoms are absorbed on the film surface as they are present in a cooled film in equilibrium with the argon atmosphere. Argon atoms migrate from one pit to another on the rough surface of a solid water film.     

Development of solid–oxide fuel cell for reduced operating temperatures

A technique for formation of electrolyte thin films with the thickness of 6–10 μm of zirconia stabilized by yttria (YSZ) is developed on the basis of the method of chemical deposition from the vapor phase of organometallic compounds (MOCVD). Planar electrochemical cells based on film electrolyte with a supporting anode with the working surface area of 12 cm² were manufactured. A solid-oxide fuel cell (SOFC) based on two fuel cells was developed and its life cycle tests at reduced operating temperatures (<800°C) were carried out for 400 h. The maximum power density reached in the SOFC tests was 316 mW/cm².     

Non-oxidative dehydro-oligomerization of methane to higher molecular weight hydrocarbons at low temperatures

The non-oxidative dehydro-oligomerization of methane to higher molecular weight hydrocarbons such as aroma tics and C2 hydrocarbons in a low temperature range of 773-973 K with Mo/HZSM-5,Mo-Zr/HZSM-5 and Mo-W/HZSM-5 catalysts is studied.The means for enhancing the activity and stability of the Mo-containing catalysts under the reaction conditions is reported.Quite a stable methane conversion rate of over 10% with a high selectivity to the higher hydrocarbons has been obtained at a temperature of 973 K.Pure methane conversions of about 5.2% and 2.0% have been obtained at 923 and 873 K,respectively.In addition,accompanied by the C2-C3 mixture,tht- methane reaction can be initiated even at a lower temperature and the conversion rate of methane is enhanced by the presence of tne initiator of C2-C3 hydrocarbons.Compared with methane oxidative coupling to ethylene,the novel way for methane transformation is significant and reasonable for its lower reaction temperatures and high selectivity to the desired prod     

Liquid Co–Sn alloys at high temperatures: structure and physical properties

The Co–Sn system is an important subsystem for Sn-based anode materials of lithium-ion batteries. Experimental results on the physical–chemical properties of this system in the liquid state, however, are rather sparse. In this work, the atomic structure and structure-sensitive thermophysical properties (viscosity, electrical resistivity, and thermoelectric power) of liquid Co–Sn alloys were investigated in a wide temperature range with special attention to the melting-solidification region. The obtained experimental results were combined with differential thermal analysis (DTA) data in order to verify the liquidus curve in the Sn-rich part of the Co–Sn phase diagram.     

Sol–gel processing at increased temperatures: the formation of polyester nanocomposites

Polyester nanocomposites were prepared using sol–gel precursors, prehydrolyzed sols, or nanoparticles in polyester formulations. The different inorganic components were introduced in the early stages of the esterification reaction and a typical polymerization temperature program was applied leading to temperatures up to 240 °C at low pressures. The structural and physical properties of the final materials depend on the applied method for the introduction of the sol–gel materials. Silicon atoms were incorporated into the polyester chain if silicon tetraalkoxide was used as precursor. The silicon atoms represent branching points in the polymer structure. Prehydrolyzed sols that were prepared under acidic conditions were another source of silicon and formed larger inorganic aggregates in the polymer matrix. Nanoparticles prepared via the Stöber process were the third inorganic species in polyester formation. All three processing pathways produced different kinds of materials depending on the type of silica incorporated in the polyester networks but also with regard to the nanoscale structure of the materials. Both, composition and structure have a major influence on the final polyester nanocomposite properties. Model reactions between silicon tetraalkoxides and diols or diacids using the temperature program for the polyester formation showed that exchange reactions of the alkoxides and the alcohols or acids can occur and the obtained products can carry out side reactions in the polyester formation. The final materials show a homogeneous distribution of the silicon containing moieties in the polyester matrix. The viscosities and the branching degrees of the polymers changed dramatically compared to the pristine polymers by incorporation of the sol–gel precursors.     

Determination of debye temperatures of supported ion particles by m?ssbauer spectroscopy

The Debye temperatures of alumina- and titania-supported iron particles were determined using Mössbauer spectroscopy between 5 and 280 K. The Debye temperatures were obtained from a nonlinear optimization algorithm using the temperature dependence of the second order Doppler shift (SOD) and the resonant absorption area (RAA). To test its reliability, the same method was used to determine of an -iron-foil and pyrophoric iron.     

Simulating infrared spectra and hydrogen bonding in cellulose Iβ at elevated temperatures

We have modeled the transformation of cellulose Iβ to a high temperature (550 K) structure, which is considered to be the first step in cellulose pyrolysis. We have performed molecular dynamics simulations at constant pressure using the GROMOS 45a4 united atom forcefield. To test the forcefield, we computed the density, thermal expansion coefficient, total dipole moment, and dielectric constant of cellulose Iβ, finding broad agreement with experimental results. We computed infrared (IR) spectra of cellulose Iβ over the range 300-550 K as a probe of hydrogen bonding. Computed IR spectra were found to agree semi-quantitatively with experiment, especially in the O-H stretching region. We assigned O-H stretches using a novel synthesis of normal mode analysis and power spectrum methods. Simulated IR spectra at elevated temperatures suggest a structural transformation above 450 K, a result in agreement with experimental IR results. The low-temperature (300-400 K) structure of cellulose Iβ is dominated by intrachain hydrogen bonds, whereas in the high-temperature structure (450-550 K), many of these transform to longer, weaker interchain hydrogen bonds. A three-dimensional hydrogen bonding network emerges at high temperatures due to formation of new interchain hydrogen bonds, which may explain the stability of the cellulose structure at such high temperatures.     

Solubility of minoxidil in binary mixture of ethanol + water at various temperatures

ABSTRACT

The solubility of minoxidil in the aqueous binary mixtures of ethanol at different temperature are investigated and the obtained solubility data are fitted by using some cosolvency models including van’t Hoff equation, Yalkowsky model, Jouyban–Acree model and Jouyban–Acree–van’t Hoff model. The mean relative deviations (MRD%) are used to illustrate the models performance. Moreover, the apparent entropy, enthalpy, and Gibbs free energy of minoxidil dissolution process in the investigated solvent mixtures are computed using van’t Hoff and Gibbs equations. Finally, by means of the inverse Kirkwood–Buff integrals preferential solvation of minoxidil by water is observed in water-rich and ethanol-rich mixtures.     

Electrical conductivity properties of some O-substituted arylazo — Barbiturate complexes at different temperatures

Electrical conductivity properties of someo-substituted arylazo-barbiturate complexes at different temperatures are studied. The _o, E andE _g values are determined. The copper complexes derived from –H, –CH₃ and –OCH₃ substituted organic compounds are with semiconducting properties at low temperatures and insulator at high temperatures. All the carboxy and the cobalt and the the nitro complexes, and nickel-methyl complexes are of semiconducting behaviour. The data are explained in the light of extrensic and lattice vibrations and the width of the forbidden energy gap properties.

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Zusammenfassung Bei verschiedenen Temperaturen wurden die elektrischen Leitfähigkeitseigenschaften einiger o-substituierter Arylazobarbiturat-Komplexe untersucht und Werte für _o,E undE _g ermittelt. Die durch H-, CH₃- und OCH₃-Substitution der organischen Verbindungen erhaltenen Kupferkomplexe zeigen bei niedrigen Temperaturen Halbleiter- und bei hohen Temperaturen Isolatoreigenschaften. Alle Carboxy- und Nitrokomplexe sowie die Kobalt- und die Nickel-Methyl-Komplexe zeigen Halbleiterverhalten. Die Ergebnisse werden angesichts von Störstellen- und Gitterschwingungen und der Breite der verbotenen Energiezone erklärt.

     

Electron transport in CaMnO3 − δ at elevated temperatures: a mobility analysis

The drift mobility of electron charge carriers in oxygen non-stoichiometric manganite CaMnO_3???δ was calculated by combining the total electrical conductivity and oxygen non-stoichiometry data at 700–950 °С and oxygen partial pressure varying between 10^?6 and 1 atm. The carrier concentration changes with pressure and temperature were obtained with the help of the earlier-developed defect model involving reactions of oxygen exchange and thermal excitation of manganese sites. The activation energy for mobility is found to increase with oxygen non-stoichiometry. High-temperature electron transport properties of the manganite CaMnO_3???δ can be explained in terms of activated jumps of n-type small polarons in adiabatic regime. The relatively small mobility of charge carriers is explained by strong localization of polarons on manganese sites.     

Volumetric properties of the octyl methoxycinnamate + ethyl acetate solvent system at several temperatures

In this article, the excess molar volumes and partial molar volumes of components of the binary system octyl methoxycinnamate (OMC)?+?ethyl acetate (AcOEt) were investigated from density measurements on the entire range of mass fractions for this system at 298.15, 303.15, 308.15, 313.15, and 318.15?K. The results of excess molar volumes were fitted by the Redlich–Kister equation using third degree polynomials. The system exhibited negative excess volumes (up to ?0.926?cm³?mol^?1 at 318.15?K), probably due to interactions like dispersion forces between unlike molecules or some differences in the molar volumes of pure components. The effect of temperature on the different volumetric properties studied was also analysed. Besides, the volumetric thermal expansion coefficients were also calculated finding values varying from 7.53?×?10^?4?K^?1 for pure OMC up to 1.38?×?10^?3?K^?1 for pure AcOEt at 308.15?K.     

Dissolution and gelation of α-chitin nanofibers using a simple NaOH treatment at low temperatures

In this study, both α-chitin powders and nanofibers were successfully dissolved in 20 wt% NaOH solutions at low temperatures. Elemental analysis confirmed that this NaOH-freezing treatment did not cause deacetylation of chitin. After heating and neutralizing with water, chitin hydrogels could be prepared. X-ray diffraction and field emission scanning electron microscope data demonstrated that these two hydrogels formed typical regenerated microporous structures with low crystallinity, which was caused by the dissolution process. Based on this result, cold ethanol was used for α-chitin nanofibers during the initial stages of neutralization, which effectively prevented the dissolution and decrease in crystallinity and was even able to preserve the continuous network structure of original nanofiber. This gelation behavior seems to be attributed to interdigitation and aggregation between neighboring nanofibers in cold alkali solution leading to the shrinkage of the hydrogel. In general, by avoiding the dissolution process, highly crystalline hydrogel based on α-chitin nanofibers was prepared by a simple NaOH treatment without use of any other chemical solvents or cross-linking agents. We expect this new type of hydrogel could be promoted to wide applications and research studies as novel green nanomaterials.     

Effect of hydrogen-sulfide on the hydrogen permeance of palladium–copper alloys at elevated temperatures

The hydrogen permeance of several 0.1 mm thick Pd–Cu alloy foils (80 wt.% Pd–20 wt.% Cu, 60 wt.% Pd–40 wt.% Cu and 53 wt.% Pd–47 wt.% Cu) was evaluated using transient flux measurements at temperatures ranging from 603 to 1123 K and pressures up to 620 kPa both in the presence and absence of 1000 ppm H₂S. Sulfur resistance, as evidenced by no significant change in permeance, was correlated with the temperatures associated with the face-centered-cubic crystalline structure for the alloys in this study. The permeance of the body-centered cubic phase, however, was up to two orders of magnitude lower when exposed to H₂S. A smooth transition from sulfur poisoning to sulfur resistance with increasing temperature was correlated with the alloy transition from a body-centered-cubic structure to a face-centered-cubic structure.     

Kinetics of direct and water-mediated tautomerization reactions of nucleobases at low temperatures ⩽200 K

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well-known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H-O=C-N- $\rightleftharpoons$ O=C-N-H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5-aza-uracil (1,3,5-triazine-2,4(1H,3H)-dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions.     

Migration of perfluorinated compounds from paperbag to Tenax® and lyophilised milk at different temperatures

ABSTRACT

Migration tests of perfluoroalkyl substances (PFASs) from a grease-proof paperbag used for packaging of pet food have been carried out. No migration of perfluorocaboxylic acids (PFCAs), from PFBA to PFTeDA, to the simulant Tenax® was found after 10 days at 40°C. However, the increase of temperature in the range 80–160°C gave rise to the migration of the PFCAs. Finally, the migration to real foods such as lyophilised whole and low-fat milk samples at 80 and 120°C was studied. The results indicate that the migration percentages of the PFCASs into food samples are much higher than those obtained into Tenax®.