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排序方式: 共有247条查询结果,搜索用时 15 毫秒
1.
Sergiu Muncaci Carlos Mattea Siegfried Stapf Ioan Ardelean 《Magnetic resonance in chemistry : MRC》2013,51(2):123-128
Frequency‐dependent NMR relaxation studies have been carried out on water (polar) and cyclohexane (nonpolar) molecules confined inside porous ceramics containing variable amounts of iron oxide (III). The porous ceramics were prepared by compression of powders mixed with iron oxide followed by thermal treatment. The pore size distribution was estimated using a technique based on diffusion in internal fields that exposed a narrow distribution of macropore sizes with an average pore dimension independent of iron oxide content. The relaxation dispersion curves were obtained at room temperature using a fast field cycling NMR instrument. They display an increase of the relaxation rate proportional to the iron oxide concentration. This behavior is more prominent at low Larmor frequencies and is independent of the polar character of the confined molecules. The results reported here can be fitted well with a relaxation model considering exchange between molecules in the close vicinity of the paramagnetic centers located in the surface and bulk‐like molecules inside the pores. This model allows the extraction of the transverse diffusional correlation time that can be related to the polar character of the confined molecules. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
2.
We present a study of fractional configurations in gravity theories and Lagrange mechanics. The approach is based on a Caputo
fractional derivative which gives zero for actions on constants. We elaborate fractional geometric models of physical interactions
and we formulate a method of nonholonomic deformations to other types of fractional derivatives. The main result of this paper
consists of a proof that, for corresponding classes of nonholonomic distributions, a large class of physical theories are
modelled as nonholonomic manifolds with constant matrix curvature. This allows us to encode the fractional dynamics of interactions
and constraints into the geometry of curve flows and solitonic hierarchies. 相似文献
3.
4.
In our work, we will explore the possibility of implementing the well-known Carr–Purcell–Meiboom–Gill pulse sequence to determine the pore size of porous ceramics with magnetic impurities. The proposed approach exploits the diffusion dependence of the spin-echo signal in the presence of internal gradients occurring as a result of susceptibility contrast between the porous matrix and the confined liquid. For calibrating the technique, a comparison of the pore size data with those extracted from the so-called DDIF technique (DDIF, decay due to diffusion in the internal fields) is performed. This approach can be applied for nondestructive in situ characterization of soils, concrete, biological tissues or other structures with micrometer pore size. 相似文献
5.
Methods in Riemann–Finsler geometry are applied to investigate bi-Hamiltonian structures and related mKdV hierarchies of soliton equations derived geometrically from regular Lagrangians and flows of non-stretching curves in tangent bundles. The total space geometry and nonholonomic flows of curves are defined by Lagrangian semisprays inducing canonical nonlinear connections (N-connections), Sasaki type metrics and linear connections. The simplest examples of such geometries are given by tangent bundles on Riemannian symmetric spaces G/SO(n) provided with an N-connection structure and an adapted metric, for which we elaborate a complete classification, and by generalized Lagrange spaces with constant Hessian. In this approach, bi-Hamiltonian structures are derived for geometric mechanical models and (pseudo) Riemannian metrics in gravity. The results yield horizontal/vertical pairs of vector sine-Gordon equations and vector mKdV equations, with the corresponding geometric curve flows in the hierarchies described in an explicit form by nonholonomic wave maps and mKdV analogs of nonholonomic Schrödinger maps on a tangent bundle. 相似文献
6.
Sergiu I. Vacaru 《International Journal of Theoretical Physics》2010,49(11):2753-2776
In this work, we apply the anholonomic deformation method for constructing new classes of anisotropic cosmological solutions in Einstein gravity and/or generalizations with nonholonomic variables. There are analyzed four types of, in general, inhomogeneous metrics, defined with respect to anholonomic frames and their main geometric properties. Such spacetimes contain as particular cases certain conformal and/or frame transforms of the well known Friedman-Robertson-Walker, Bianchi, Kasner and Gödel universes and define a great variety of cosmological models with generic off-diagonal metrics, local anisotropy and inhomogeneity. It is shown that certain nonholonomic gravitational configurations may mimic de Sitter like inflation scenarios and different anisotropic modifications without satisfying any classical false-vacuum equation of state. Finally, we speculate on perspectives when such off-diagonal solutions can be related to dark energy and dark matter problems in modern cosmology. 相似文献
7.
We find sufficient conditions for the absence of harmonic L 2 spinors on spin manifolds constructed as cone bundles over a compact Kähler base. These conditions are fulfilled for certain perturbations of the Euclidean metric, and also for the generalized Taub-NUT metrics of Iwai-Katayama, thus proving a conjecture of Vi?inescu and the second author. 相似文献
8.
Sergiu Rudeanu 《Journal of Optimization Theory and Applications》1969,4(4):253-259
In Ref. 1, methods for solving bivalent, linear or nonlinear programs were given, and various applications to operations research, graph theory, and related areas were described. The basic algorithm of Ref. 1 gives the maximum (minimum) of any real-valued functionf(x
1,...,x
n) with (0, 1) variables. It provides a parametric representation of all theN points wheref reaches its optimum. In all concrete applications, it turned out that the numberm of parameters actually appearing in the representation was the smallest integer such thatN2
m
.In this paper, we show that the latter property need not hold in all cases, but we can choose the parameters so as to obtain each optimizing point once, and only once. 相似文献
9.
Denisa Dumitrescu Sergiu Shova Constantin Draghici Marcel Mirel Popa Florea Dumitrascu 《Molecules (Basel, Switzerland)》2021,26(12)
3-Arylsydnones bearing fluorine and bromine atoms on the benzene ring were synthesized from N-nitroso-2-fluorophenylglycines and characterized by NMR spectroscopy. These were employed further in synthesis of the corresponding 1-(2-fluorophenyl)pyrazoles by 1,3-dipolar cycloaddition reaction with dimethyl acetylenedicarboxylate (DMAD) as activated dipolarophile. The sydnones as reaction intermediates were characterized by single crystal X-ray diffraction analysis showing interesting features such as halogen bonding as an important interaction in modeling the crystal structure. 相似文献
10.