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121.
Gina Pecchi Patricio Reyes M. Graciela Jiliberto T. López J. L. G. Fierro 《Journal of Sol-Gel Science and Technology》2006,37(3):169-174
Platinum-supported catalysts prepared by impregnation of mixed CeO2/Al2O3 and CeO2/ZrO2 oxides using the sol-gel method were characterized and used in the combustion of ethyl acetate. In each series, the effect
of CeO2 loading (3 and 5 wt%) was studied. Characterization data from the studied catalysts (specific area measurements, hydrogen
chemisorption, programmed temperature reduction (TPR), X-ray diffraction (XRD), photoelectron spectra (XPS) and transmission
electron microscopy (TEM)) revealed significant changes in porosity and metal dispersion in each series. The catalytic activity
of the solids, evaluated in the total combustion of ethyl acetate, exhibited a positive effect with the addition of Ce in
the zirconia series whereas no significant changes was observed in the alumina series. 相似文献
122.
A modified first-order kinetic law which takes into account defect decay during an ordering process was employed to predict the short-range-order kinetics of a quenched and a quenched-deformed Cu—5 at.% Zn alloy, in conjunction with experiments performed by isothermal calorimetry. The effective activation energy of point defect migration and its temperature dependence strongly suggest the contribution of bound vacancies to the ordering process. An estimate of 91.2 kJ mol–1 was made for the activation energy of solute—vacancy migration by applying an effective rate constant, a value in very good agreement with that obtained from previous non-isothermal experiments. The isothermal curves were utilized to determine the ordering energy: w=–2.90 kJ mol–1. In conjunction, a parametric study of the defect sink density was performed in order to assess its influence on the calculated isothermal curve profiles.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
123.
Andrs Vega Oscar Donoso‐Tauda Andres Ibaez Carlos A. Escobar 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(4):o199-o204
A series of five compounds containing the bicyclo[3.3.0]octa‐2,6‐diene skeleton are described, namely tetramethyl cis,cis‐3,7‐dihydroxybicyclo[3.3.0]octa‐2,6‐diene‐2,4‐exo,6,8‐exo‐tetracarboxylate, C16H18O10, (I), tetramethyl cis,cis‐3,7‐dihydroxy‐1,5‐dimethylbicyclo[3.3.0]octa‐2,6‐diene‐2,4‐exo,6,8‐exo‐tetracarboxylate, C18H22O10, (II), tetramethyl cis,cis‐3,7‐dimethoxybicyclo[3.3.0]octa‐2,6‐diene‐2,4‐exo,6,8‐exo‐tetracarboxylate, C18H22O10, (III), tetramethyl cis,cis‐3,7‐dimethoxy‐1,5‐dimethylbicyclo[3.3.0]octa‐2,6‐diene‐2,4‐exo,6,8‐exo‐tetracarboxylate, C20H26O10, (IV), and tetramethyl cis,cis‐3,7‐diacetoxybicyclo[3.3.0]octa‐2,6‐diene‐2,4‐exo,6,8‐exo‐tetracarboxylate, C20H22O12, (V). The bicyclic core is substituted in all cases at positions 2, 4, 6 and 8 with methoxycarbonyl groups and additionally at positions 3 and 7 with hydroxy [in (I) and (II)], methoxy [in (III) and (IV)] or acetoxy [in (V)] groups. The conformations of the methoxycarbonyl groups at positions 2 and 4 are exo for all five compounds. Each C5 ring of the bicyclic skeleton is almost planar, but the rings are not coplanar, with dihedral angles of 54.93 (7), 69.85 (5), 64.07 (4), 80.74 (5) and 66.91 (7)° for (I)–(V), respectively, and the bicyclooctadiene system adopts a butterfly‐like conformation. Strong intramolecular hydrogen bonds exist between the –OH and C=O groups in (I) and (II), with O...O distances of 2.660 (2) and 2.672 (2) Å in (I), and 2.653 (2) and 2.635 (2) Å in (II). The molecular packing is stabilized by weaker C—H...O(=C) interactions, leading to dimers in (I)–(III) and to a chain structure in (V). The structure series presented in this article shows how the geometry of the cycloocta‐2,6‐diene skeleton changes upon substitution in different positions and, consequently, how the packing is modified, although the intermolecular interactions are basically the same across the series. 相似文献
124.
Jose Patricio Díaz Varela Antoni Torrens Torrell 《Archive for Mathematical Logic》2006,45(8):1011-1020
?ukasiewicz implication algebras are {→,1}-subreducts of Wajsberg algebras (MV-algebras). They are the algebraic counterpart of Super-?ukasiewicz Implicational logics investigated in Komori, Nogoya Math J 72:127–133, 1978. The aim of this paper is to study the direct decomposability of free ?ukasiewicz implication algebras. We show that freely generated algebras are directly indecomposable. We also study the direct decomposability in free algebras of all its proper subvarieties and show that infinitely freely generated algebras are indecomposable, while finitely free generated algebras can be only decomposed into a direct product of two factors, one of which is the two-element implication algebra. 相似文献
125.
While every theorem has a proof in mathematics, in US geometry classrooms not every theorem is proved. How can one explain the practitioner’s perspective on which theorems deserve proof? Toward providing an account of the practical rationality with which practitioners handle the norm that every theorem has a proof we have designed a methodology that relies on representing classroom instruction using animations. We use those animations to trigger commentary from experienced practitioners. In this article we illustrate how we model instructional situations as systems of norms and how we create animated stories that represent a situation. We show how the study of those stories as prototypes of a basic model can help anticipate the response from practitioners as well as suggest issues to be considered in improving a model. 相似文献
126.
Stability of closed timelike curves in the Gödel universe 总被引:1,自引:0,他引:1
We study, in some detail, the linear stability of closed timelike curves in the Gödel universe. We show that these curves are stable. We present a simple extension (deformation) of the Gödel metric that contains a class of closed timelike curves similar to the ones associated to the original metric. This extension correspond to the addition of matter whose energy-momentum tensor is analyzed. We find the conditions to have matter that satisfies the usual energy conditions. We study the stability of closed timelike curves in the presence of usual matter as well as in the presence of exotic matter (matter that does satisfy the above mentioned conditions). We find that the closed timelike curves in the Gödel universe with or without the inclusion of regular or exotic matter are stable under linear perturbations. We also find a sort of structural stability. 相似文献
127.
Rodrigo?CastroEmail author Patricio L.?Felmer 《Communications in Mathematical Physics》2005,256(2):411-435
We consider the nonlinear Schrödinger equation with superlinear and subcritical nonlinearity. Assuming that the potential is radially symmetric we find radial sign-changing solutions of the equation that concentrate in a ball, as the parameter goes to zero. We study the asymptotic profile of these highly oscillatory solutions, completely characterizing their behavior by means of an envelope function.Partially supported by FONDECYT Lineas Complementarias Grant 8000010.Partially Supported by FONDAP Matemáticas Aplicadas and FONDECYT Lineas Complementarias Grant 8000010. 相似文献
128.
The stability of some Schiff-bases formed between PLP and different amino acids has been investigated in a wide range of pH. The kinetic constants of formation of these compounds and their hydrolysis rate constants have been determined. Results show that the α-position of the carboxyl group of amino acid plays an important role on the mechanism of water attack upon the C?N? bond. The absence of ionic groups in the surroundings of that bond must be an important factor of stability. Bulky hydrophobic substituents in the amino acid, near the amine part, protect the imine bond against hydrolysis. 相似文献
129.
The title problem is solved assuming that the thickness varies symmetrically with respect to the x-axis. The edges defined by are elastically restrained against rotation while the remaining edges are clamped or simply supported.Approximate expressions for four of the lower natural frequencies of vibration (including the fundamental) are given.A forced vibrations situation is also dealt with. 相似文献
130.
G.E. Barberis D. Davidov J.P. Donoso F.G. Gandra C. Rettori J.F. Suassuna 《Solid State Communications》1978,28(6):427-433
The resonance properties of the Γ6 ground state of Nd+3 in the type II Superconductors CeRu2, ThRu2 and LaRu2 are reported. A significant change of the g value and lineshape were observed for CeRu2:Nd upon going from the normal state to the superconducting state. The data in the superconducting state is discussed in terms of conduction electron's pairing mechanism and inhomogeneous field distribution mechanism in the superconducting state. A preliminary lineshape analysis is not completely consistent with previous theoretical predictions and experiments. 相似文献