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71.
Manuel María Mahamud 《Applied Surface Science》2007,253(14):6019-6031
In this paper two series of active carbons obtained at different flow rates of the activating agent, CO2, are characterized in order to establish the different mechanisms of pore development during the activation step. This study complements previous works on textural development during the different steps in the process of obtaining active carbons: coal oxidation, coal pyrolysis and char gasification. As the characteristics of the original and intermediate materials are of capital importance in the pore development of active carbons, the properties of the active carbons, precursor chars and coals were considered and analyzed together. Mercury porosimetry and helium picnometry were used to determine classical textural parameters as well as to perform a more detailed study of the pore volume generation during the different conditions of the activation step. Data obtained from the mercury porosimetry determinations was also employed for fractal determinations according to the methodologies proposed by Friesen and Mikula, Zhang and Li and the procedure of Neimark. Average surface fractal dimensions as well as fractal profiles and local surface fractal dimensions were calculated. The use of different flow rates during the activation step produces changes not only in the ordinary textural parameters but also in the fractal characteristics of the active carbons. Activation at higher flow rates leads to smoother fractal profiles and also to lower values of the average surface fractal dimensions of the active carbons. 相似文献
72.
Yuri Bozhkov Antonio Carlos Gilli Martins 《Journal of Mathematical Analysis and Applications》2004,294(1):334-344
It is shown that a Lie point symmetry of the Lane-Emden system is a Noether symmetry if and only if its parameters belong to the critical hyperbola. 相似文献
73.
74.
Isabel Brandariz José L. Barriada Teresa Vilari?o Manuel E. Sastre de Vicente 《Monatshefte für Chemie / Chemical Monthly》2004,343(6):1475-1488
Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances. 相似文献
75.
We study the spectrum and eigenmodes of the QCD Dirac operator in a gauge background given by an instanton liquid model (ILM) at temperatures around the chiral phase transition. Generically we find the Dirac eigenvectors become more localized as the temperature is increased. At the chiral phase transition, both the low lying eigenmodes and the spectrum of the QCD Dirac operator undergo a transition to localization similar to the one observed in a disordered conductor. This suggests that Anderson localization is the fundamental mechanism driving the chiral phase transition. We also find an additional temperature dependent mobility edge (separating delocalized from localized eigenstates) in the bulk of the spectrum which moves toward lower eigenvalues as the temperature is increased. In both regions, the origin and the bulk, the transition to localization exhibits features of a 3D Anderson transition including multifractal eigenstates and spectral properties that are well described by critical statistics. Similar results are obtained in both the quenched and the unquenched case though the critical temperature in the unquenched case is lower. Finally we argue that our findings are not in principle restricted to the ILM approximation and may also be found in lattice simulations. 相似文献
76.
The authors derive a formula for the volume of a compact domain in a symmetric space from normal sections through a special submanifold in the symmetric space.This formula generalizes the volume of classical domains as tubes or domains given as motions along the submanifold.Finally,some stereological considerations regarding this formula are provided. 相似文献
77.
Carmen Izquierdo María Luisa Moyá Jose Luis Usero Julio Casado 《Monatshefte für Chemie / Chemical Monthly》1992,123(5):383-389
Summary A simple working method for the estimation of the aggregation numbers and core radii from weight measurements of sodium bis-2-ethylhexylsulphosuccinate (aerosol-OT orAOT)/decane/water microemulsions of several compositions is proposed.Starting out from the hypotheses that (i) the density of the water inside the droplets is the same as that of free water, (ii) all the surfactant is localized as interface, and (iii) the penetration of the organic phase into the interface is negligible, values for the aggregation number and core radii comparable to those determined using more sophisticated methods were obtained.The results/methodological simplicity ratio may be of great advantage in kinetic work on reaction mechanisms in microemulsions.
Abschätzung von Aggregationszahl und Radius von Mikroemulsionen
Zusammenfassung Es wird eine einfache Methode zur Abschätzung von Aggregationszahl und Core-Radius von Mikroemulsionen aus Gewichtsmessungen von Natriumbis-2-ethylhexylsulphosuccinat (aerosol-OT oderAOT)/Decan/Wasser-Mikroemulsionen verschiedener Zusammensetzung vorgeschlagen. Unter den Annahmen, daß (i) die Dichte von Wasser innerhalb der Aggregationen die gleiche als bei freiem Wasser ist, (ii) das ganze Emulsionsmittel in der Grenzfläche vorliegt und (iii) das Eindringen der organischen Phase in die Grenzfläche vernachlässigbar ist, können Werte für Aggregationszahlen und Core-Radien erhalten werden, die durchaus mit entsprechenden Werten aus aufwendigeren Methoden vergleichbar sind. Das Leistungsverhältnis (Ergebnisse/geringer Aufwand) ist sehr vorteilhaft und macht die Methode für kinetische Arbeiten zur Bestimmung von Reaktionsmechanismen in Mikroemulsionen interessant.相似文献
78.
The effect of gamma irradiation in air is investigated on four thermoplastic polyesters (PET, PBT, PEN, PCT-co-ET) in films containing aromatic rings, in order to evaluate the influence of aromatic density and the role of oxygen on radiation resistance. Physical-chemical-nuclear analyses were used to this purpose. EPR measurements were carried out to detect radical stability against oxygen permeation. Viscometric investigations reported a very similar trend for all the investigated polyesters: a chain break effect that decreases at the highest doses. FT-IR analyses focused on the formation of oxidized species. Positron annihilation spectra pointed out a decrease of the intensity of ortho-positronium formation, while its lifetime remains unchanged with radiation dose. 相似文献
79.
The phase diagram of the nematic mesophase present in the tetradecyltrimethylammonium bromide/sodium bromide/water ternary system was determined. A calamitic nematic mesophase (NC) was observed which extends to very high concentrations of electrolyte. The order parameters of the surfactant head group in the mesophases were studied by the NMR quadrupolar splitting of the deuterated surfactant. On increasing the temperature of nematic mesophases with low electrolyte concentrations, a phase separation occurs with the formation of a more highly ordered hexagonal phase and an isotropic phase. Diffusion measurements of the isotropic micellar solution by the NMR PFG method were used to estimate hydrodynamic radii at low surfactant concentrations and to study micelle diffusion as the concentration of the surfactant was increased to the liquid crystalline region. At higher surfactant concentrations, the diffusion coefficient reached a limiting value. The calamitic nematic mesophase in this surfactant/electrolyte/water system appears to be formed by long wormlike micelles. 相似文献
80.
Manuel Santos-Trigo 《ZDM》2007,39(5-6):523-536
Research programs in mathematical problem solving have evolved with the development and availability of computational tools. I review and discuss research programs that have influenced and shaped the development of mathematical education in Mexico and elsewhere. An overarching principle that distinguishes the problem solving approach to develop and learn mathematics is to conceptualize the discipline as a set of dilemmas or problems that need to be explored and solved in terms of mathematical resources and strategies. In this context, relevant questions that help structure and organize this paper include: What does it mean to learn mathematics in terms of problem solving? To what extent do research programs in problem solving orient curricular proposals? What types of instructional scenarios promote the students’ development of mathematical thinking based on problem solving? What type of reasoning do students develop as a result of using distinct computational tools in mathematical problem solving? 相似文献