987 resultados para Thermodynamic


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The crystallization and phase transformation of amorphous Si3N4 ceramics under high pressure (1.0-5.0 GPa) between 800 and 1700 degreesC were investigated. A greatly enhanced crystallization and alpha-beta transformation of the amorphous Si3N4 ceramics were evident under the high pressure, as characterized by that, at 5.0 GPa, the amorphous Si3N4, began to crystallize at a temperature as low as 1000 degreesC (to transform to alpha modification). The subsequent alpha-beta transformation occurred completed between 1350 and 1420 degreesC after only 20 min of pressing at 5.0 GPa. In contrast, under 0.1 MPa N-2, the identical amorphous materials were stable up to 1400 degreesC without detectable crystallization, and only a small amount of a phase was detected at 1500 degreesC. The crystallization temperature and the alpha-beta transformation temperatures are reduced by 200-350 degreesC compared to that at normal pressure. The enhanced phase transformations of the amorphous Si3N4, were discussed on the basis of thermodynamic and kinetic consideration of the effects of pressure on nucleation and growth.

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Ultrasonic absorption coefficients were measured for butylamine in heavy water (D2O) in the frequency range from 0.8 to 220 MHz and at concentrations from 0.0278 to 2.5170 mol dm(-3) at 25 degrees C; two kinds of relaxation processes were observed. One was found in relatively dilute solutions (up to 0.5 mol dm(-3)), which was attributed to the hydrolysis of butylamine. In order to compare the results, absorption measurements were also carried out in light water (H2O). The rate and thermodynamic parameters were determined from the concentration dependence of the relaxation frequency and the maximum absorption per wavelength. The isotope effects on the diffusion-controlled reaction were estimated and the stability of the intermediate of the hydrolysis was considered while comparing it with the results for propylamine in H2O and D2O. Another relaxation process was observed at concentrations greater than 1 mol dm(-3) in D2O. In order to examine the solution characteristics, proton NMR measurements for butylamine were also carried out in D2O. The chemical shifts for the gamma- and delta-proton in butylamine molecule indicate the existence of an aggregate. From profiles of the concentration dependence of the relaxation frequency and the maximum absorption per wavelength of sound absorption, the source of the relaxation was attributed to an association-dissociation reaction, perhaps, associated with a hydrophobic interaction. The aggregation number, the forward and reverse rate constants and the standard volume change of the reaction were determined. It was concluded from a comparison with the results in H2O that the hydrophobic interaction of butylamine in D2O is stronger than that in H2O. Also, the isotope effect on this reaction was interpreted in terms of the solvent structure.

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The adsorption of an electroinactive product greatly influences an irreversible electrochemical reaction in three ways, including self-block, self-inhibition, and self-acceleration, and changes not only the heterogeneous electron-transfer rate constant but also the modified formal potential and electron-transfer coefficient of the electrochemical reaction. In order to study these adsorption effects, a double logarithmic method was suggested to be used in processing the potential-controlled thin layer spectroelectrochemical data. The result shows three types of double logarithmic plots for three kinds of adsorption effects. These double logarithmic plots can be a diagnostic criterion of the adsorption effects and enable us to determine some thermodynamic and kinetic parameters. The combination of nonlinear regression with double logarithmic method is a convenient way to examine the suggested mechanism and to extract more information from the limited experimental data. Some examples are given to test the theoretical results. (C) 1999 The Electrochemical Society. S0013-4651(98)05-012-5. All rights reserved.

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A simple double logarithmic method in potential-controlled thin-layer spectroelectrochemistry for an irreversible electrochemical process has been studied by numerical analysis and examined by experimental examples. This simple algorithm has a novel function offering some important information about the mechanism of a complex electrochemical process directly from a limited amount of potential-spectrum data, and can be used to distinguish different reaction mechanisms such as E, EC, EE, as well as to determine the electron-transfer coefficient, a, and the kinetically modified E-0'. Combination of the double logarithmic method with nonlinear regression provides a powerful tool to examine the proposed mechanism and also to estimate other thermodynamic and kinetic parameters. (C) 1999 The Electrochemical Society. S0013-4651(98)06-090-X. All rights reserved.

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After isothermal crystallization of the amorphous poly(ether ether ketone), double endothermic behaviour can be found through differential scanning calorimetry experiments. During the heating scan of semicrystalline PEEK, a metastable melt, which comes from the melt of the thinner lamellar crystal populations, can be obtained between these two endotherms. The metastable melt can recrystallize immediately just above the lower melting temperature and form slightly thicker lamellae than the original ones. The thickness and the perfection depend upon the crystallization time and the crystallization temperature. By comparing the TEM morphological observations of the samples before and after partial melting, it can be shown that lamellar crystals, having different thermodynamic stability, form during isothermal crystallization. After partial melting, only the type of lamellar crystal exhibiting the higher thermodynamic stability remains. Wide angle X-ray diffraction measurements shows a slightly change in the crystallinity of the samples before and after the partial melting. Small angle X-ray scattering results exhibit a change in the long period of the lamellar crystals before and after the partial melting process. The crystallization kinetics of the metastable melt can be determined by means of differential scanning calorimetry. The kinetic analysis showed that the isothermal crystallization of the metastable PEEK melt proceeds with an Avrami exponent of n = 1.0 similar to 1.4, reflecting that probably one-dimensional or an irregular line growth of the crystal occurred between the existing main lamellae with heterogeneous nucleation. (C) 1998 Elsevier Science Ltd. All rights reserved.

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Solvent extraction of Ce(IV), Th(IV) with Cyanex 923 in n-hexane from sulphuric acid medium was studied with the dependence of the extraction on acidity and temperature being investigated. The Ce(IV) and Th(IV) extraction mechanism was proposed by slope analysis and the IR spectra of purified Cyanex 923 saturated with Ce(IV) were employed to determine the composition of the Ce(IV) complex. The equilibrium constant and thermodynamic functions of Th(IV) extraction were calculated and the characteristics of the stripping of Ce(IV), Th(IV) from the loaded organic phase were studied. (C) 1998 Elsevier Science B.V. All rights reserved.

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The synergistic extraction of rare earths(III) with binary systems containing HBTMPTP and primary amine N1923 from sulfuric acid medium was observed, The syner gistic extraction factor(R) decreased with increasing atomic number of lanthanides. Through the methods of slope analysis, constant mole and saturation titration, the synergistic extraction stoichiometry was obtained, The thermodynamic function was calculated, The IR spectra of the saturated synergistic extraction completely confirmed the mechanism.

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A new kind of amphiphilic polymer(PAMC(16)B) has been synthesized where the amphiphilic moiety is attached as a side chain via the hydrophilic end to the polymer backbone. DSC, POM, and WAXD study revealed that the polysurfactant formed thermotropic liquid crystalline phase. The liquid crystalline domains appeared after annealed at the melt for a period of time, and evolved with annealing. It was indicated that the thermodynamic effect played a significant role on the self-aggregation of hydrophobic ends, so as to form liquid crystalline structure. X-ray investigation suggested that the mesophase structure could be described as smectic with lamellar type of packing, in which alkyl tails aggregated to form layer, A model of supermolecular structure was given.

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According to the thermodynamic equilibria between the low spin state Co(III) (t2g6e(g)0) ion and the high spin state Co3+ (t2g4e(g)2) ion and between the cobalt and manganese ions with different valence state and spin state, an approximate semiempirical f

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On the basis of the spin and valence state equilibria and superexchange interaction of the various cobalt ions in LaCoO3, an approximate semiempirical formula has been proposed and used to calculate magnetic susceptibilities of LaCoO3 over a wide temperature range (100-1200 K). The results indicate that there are thermodynamic equilibria between the low spin state Co(III) (t2g6e(g)0) ion, the high spin state Co3+ (t2g4e(g)2) ion, the Co(II) (t2g6e(g)1) ion and the Co(IV) (t2g5e(g)0) ion in LaCoO3. The energy difference between the low spin state Co(III) and the high spin state Co3+ is about 0.006 eV. The content of the low spin state Co(III) ion is predominant in LaCoO3 and the content of the high spin state Co3+ ion varies with temperature, reaching a maximum at about 350 K, then decreasing gradually with increasing temperature. At low temperature the contents of the Co(II) ion and the Co(IV) ion in LaCoO3 are negligible, while above 200 K the contents of both the Co(II) ion and the Co(IV) ion increase with increasing temperature; however, the content of the Co(II) ion always is larger than that of the Co(IV) ion at any temperature. These calculated results are in good agreement with experimental results of the Mossbauer effect, magnetic susceptibility and electrical conductivity of LaCoO3.

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A statistical thermodynamics theory of polydisperse polymer blends based on a lattice model description of a fluid is formulated. Characterization of a binary polydisperse polymer mixture requires a knowledge of the pure polymer system and the interaction energy. It is assumed that the intrinsic and interactive properties of polymer (for example, T*, P*, rho*, and epsilon(ij)*) are independent of molecular size. Thermodynamic properties of ternary and higher order mixtures are completely defined in terms of the pure fluid polymer parameters and the binary interaction energies. Thermodynamic stability criteria for the phase transitions of a binary mixture are shown. The binodal and spinodal of general binary systems and of special binary systems are discussed.

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The paramagnetic susceptibility of lanthanum manganite has been measured over a wide temperature range (100-1073 K). On the basis of the thermodynamic equilibria between the various manganese ions with different valence and spin states and the magnetic interactions between the various manganese ions, a semiempirical formula has been proposed to calculate the paramagnetic susceptibilities of lanthanum manganite at different temperatures. The results indicate that most of the discrepancies between the calculated and experimental reciprocal susceptibilities of lanthanum manganite are less than 10% and that the relative contents of the various manganese ions in lanthanum manganite vary with temperature. The relative content of the trivalent manganese ion with a high spin state is dominant over the whole temperature range, while be relative content of the tetravalent manganese ion with a high spin state decreases monotonously with increasing temperature. At 300 K the calculated relative content of the tetravalent manganese ion in lanthanum manganite is about 34%, which is in good agreement with the experimental result (30%). There are some divalent manganese ions present in lanthanum manganite from low temperature to high temperature. The ratio of the relative contents of the tetravalent and divalent manganese ions in the compound varies with temperature. Above 750 K the relative content of the tetravalent manganese ion is less than that of the divalent manganese ion. The variation in the electrical resistivity of lanthanum manganite with temperature has also been interpreted reasonably.