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排序方式: 共有135条查询结果,搜索用时 15 毫秒
11.
An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations. 相似文献
12.
Joan A. Vaccaro 《Foundations of Physics》2011,41(10):1569-1596
An increasing number of experiments at the Belle, BNL, CERN, DAΦNE and SLAC accelerators are confirming the violation of time
reversal invariance (T). The violation signifies a fundamental asymmetry between the past and future and calls for a major
shift in the way we think about time. Here we show that processes which violate T symmetry induce destructive interference
between different paths that the universe can take through time. The interference eliminates all paths except for two that
represent continuously forwards and continuously backwards time evolution. Evidence from the accelerator experiments indicates
which path the universe is effectively following. This work may provide fresh insight into the long-standing problem of modeling
the dynamics of T violation processes. It suggests that T violation has previously unknown, large-scale physical effects and
that these effects underlie the origin of the unidirectionality of time. It may have implications for the Wheeler-DeWitt equation
of canonical quantum gravity. Finally it provides a view of the quantum nature of time itself. 相似文献
13.
We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO2. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO2 nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO2 matrix in the surface shell. 相似文献
14.
Vaccaro SR 《The Journal of chemical physics》2011,135(9):095102
The voltage dependence of the ionic and gating currents of a K channel is dependent on the activation barriers of a voltage sensor with a potential function which may be derived from the principal electrostatic forces on an S4 segment in an inhomogeneous dielectric medium. By variation of the parameters of a voltage-sensing domain model, consistent with x-ray structures and biophysical data, the lowest frequency of the survival probability of each stationary state derived from a solution of the Smoluchowski equation provides a good fit to the voltage dependence of the slowest time constant of the ionic current in a depolarized membrane, and the gating current exhibits a rising phase that precedes an exponential relaxation. For each depolarizing potential, the calculated time dependence of the survival probabilities of the closed states of an alpha helical S4 sensor are in accord with an empirical model of the ionic and gating currents recorded during the activation process. 相似文献
15.
CM Thaker S Rayaprol Krushna Mavani DS Rana MS Sahasrabudhe SI Patil DG Kuberkar 《Pramana》2002,58(5-6):1035-1039
The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO3 perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good
example of these properties is (La1−2x
Pr
x
Ca
x
)MnO3 (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T
c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using
X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for
understanding the structural, transport and magnetic properties are discussed in detail. 相似文献
16.
Wilson SM Wiberg KB Cheeseman JR Frisch MJ Vaccaro PH 《The journal of physical chemistry. A》2005,109(51):11752-11764
Cavity ring-down polarimetry (CRDP) has been exploited to interrogate the nonresonant optical activity (or circular birefringence) of prototypical organic compounds in the vapor phase, thereby revealing the intrinsic chiro-optical response evoked from isolated (solvent-free) molecules. Specific polarization rotation parameters have been measured at two distinct excitation wavelengths (355 nm and 633 nm) for a variety of gas-phase species drawn from the terpene, epoxide, and alkane/alkene families, with complementary solution-phase polarimetric studies serving to highlight the pronounced influence of solute-solvent interactions. Time-dependent linear response calculations performed at high levels of density functional theory have been enlisted to unravel the structural and electronic origins for observed behavior. Aside from elucidating the complex solvation processes that mediate chiro-optical phenomena taking place in condensed media, this study affords a critical assessment for emerging ab initio predictions of nonresonant optical activity and for their promising ability to assist in the determination of absolute molecular stereochemistry. 相似文献
17.
G. Vaccaro G. Buscarino S. Agnello G. Messina M. Carpanese F. M. Gelardi 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,76(2):197-201
In the present work we report an experimental investigation by electron paramagnetic resonance spectroscopy on the hyperfine
structure of the E'g_\gamma point defect, probing the local arrangement of the network (range-II order), and by Raman spectroscopy on the D1 and D2 lines, probing mean features of the network (range-III order). Our studies, performed on a-SiO2 samples thermally treated at 1000 °C in air for different time durations, show that changes of the hyperfine structure and
of the D1 and D2 lines occur in a correlated way. These results give strong evidence that the range-II and range-III order properties are
intimately related to each other and that these properties are
determined by the history of the material. 相似文献
18.
[AlCl(3) + 2THF] is a new catalytic system for the Diels-Alder cycloaddition under SFC and air atmosphere. By using equimolar amounts of reactants, this catalyst prevents the polymerization of the diene and allows the corresponding adducts to be isolated with high regio- and stereocontrol and in excellent yields. [reaction: see text] 相似文献
19.
Federica Valentini Giulia Brufani Benedetta Di Erasmo Luigi Vaccaro 《Current Opinion in Green and Sustainable Chemistry》2022
The great challenge for modern research is to define the most efficient tools to make more sustainable the industrial production and manufacturing. Among the different aspects that require attention the replacement of toxic and/or non-renewable solvents it is certainly playing a crucial role. Dealing with widely used dipolar aprotic solvents, among the different alternatives proposed in the literature γ-valerolactone (GVL) plays a pivotal role covering different application area. In this contribution, the benefits derived from the use of GVL as a circular, safe, biomass-derived reaction medium are highlighted covering most recent publications (2021). The presentation has been divided into three major sections: (i) biomass valorization, (ii) materials synthesis, manufacturing and recycle and (iii) new synthetic methodologies. 相似文献
20.
Mauro Vaccaro Gaetano Mangiapia Antonella Accardo Diego Tesauro Eliana Gianolio Henrich Frielinghaus Giancarlo Morelli Luigi Paduano 《Colloid and polymer science》2008,286(14-15):1643-1652
Two novel amphiphilic unimers containing an aliphatic hydrophobic chain (PDA) with two C≡C triple bonds and hydrophilic heads presenting the chelating agent DTPAGlu and the CCK8 bioactive peptide, respectively, have been prepared by solid phase synthesis. Aggregates obtained by mixing together PDA-DTPAGlu, or its Gd(III) complex, and PDA-L2-CCK8 in 70/30 molar ratio before and after a polymerization process carried out by UV irradiation have been structurally characterized by means of small angle neutron scattering. The relaxivity properties of aggregates containing Gadolinium complexes have also been investigated. Elongated mixed micelles have been observed, in which the relaxivity value r 1p for each Gadolinium complex, measured at 20 MHz and 298 K, is around 12 mM–1 s–1. 相似文献