全文获取类型
收费全文 | 213篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
化学 | 152篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 16篇 |
物理学 | 44篇 |
出版年
2023年 | 2篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 5篇 |
2014年 | 3篇 |
2013年 | 15篇 |
2012年 | 21篇 |
2011年 | 18篇 |
2010年 | 7篇 |
2009年 | 12篇 |
2008年 | 13篇 |
2007年 | 8篇 |
2006年 | 13篇 |
2005年 | 10篇 |
2004年 | 3篇 |
2003年 | 7篇 |
2002年 | 4篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有224条查询结果,搜索用时 359 毫秒
71.
Bipyridinium dications are versatile building blocks for the assembly of functional materials. In particular, their reliable electrochemical response has encouraged the design of electroactive films. Diverse and elegant experimental strategies to coat metallic and semiconducting electrodes with bipyridinium compounds have, in fact, emerged over the past two decades. The resulting interfacial assemblies span from a few nanometers to several micrometers in thickness. They incorporate from a single molecular layer to large collections of entangled polymer chains. They transport electrons efficiently from the electrode surface to the film/solution interface and vice versa. Electron self-exchange between and the physical diffusion of the bipyridinium building blocks conspire in defining the charge transport properties of these fascinating electroactive assemblies. Often, the matrix of electron-deficient bipyridinium dications can be exploited to entrap electron-rich analytes. Electrostatic interactions promote the supramolecular association of the guests with the surface-confined host matrix. Furthermore, chromophoric sites can be coupled to the bipyridinium dications to produce photosensitive arrays capable of harvesting light and generating current. Thus, thorough investigations on the fundamental properties of these functional molecule-based materials can lead to promising applications in electroanalysis and solar energy conversion, while contributing to advances in the basic understanding of electron transport in interfacial assemblies. 相似文献
72.
Alías FM Daff PJ Paneque M Poveda ML Carmona E Pérez PJ Salazar V Alvarado Y Atencio R Sánchez-Delgado R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(22):5132-5146
Nucleophilic attack of the beta-carbon of an Ir(III)-alkenyl functionality onto the alpha-carbon of a coordinated nitrile- or aldehyde occurs intramoleculary to yield initially iridacyclic structures. Nitriles give rise to isolable complexes that contain delocalized five-membered rings (iridapyrroles, e.g. 3'-8') in a reaction catalyzed by H2O (for some of these syntheses, Ir(III)-eta 3-allyl derivatives may be used as the source of the Ir(III)-alkenyl moiety). In contrast, the alkenyl-to-aldehyde C-C coupling gives transient iridacycles that evolve by a fast alkyl-to-alkylidene migration and beta-H elimination. The end products (13* and 14*) contain an elaborated chelating alkoxide-olefin ligand. Addition of [H(OEt2)2][BAr'4] to the iridapyrroles effects stereospecific protonation of the beta-ring carbon. Those iridapyrroles which contain an additional metal-alkyl functionality (e.g. 3a*, alkyl = C2H5) afford highly reactive cationic alkyl-alkylidene intermediates that evolve instantaneously by migratory insertion/beta-H elimination. The end products also contain an elaborated, chelating ligand, although this time with an olefin and imine terminus compared with the previous ligand. Contrary to this result, protonation of the hydride-iridapyrrole complex 8a* in weakly coordinating solvents permits isolation of two unusual cationic cis-hydride-alkylidene compounds 11*, which undergo reversible 1,2-H shifts. 相似文献
73.
Tight Distance-Regular Graphs and the Q-Polynomial Property 总被引:1,自引:0,他引:1
Arlene A. Pascasio 《Graphs and Combinatorics》2001,17(1):149-169
Let Γ denote a distance-regular graph with diameter d≥3, and assume Γ is tight (in the sense of Jurišić, Koolen and Terwilliger). Let θ denote the second largest or smallest eigenvalue
of Γ, and let σ0,σ1,…,σ
d
denote the associated cosine sequence. We obtain an inequality involving σ0,σ1,…,σ
d
for each integer i (1≤i≤d−1), and we show equality for all i is closely related to Γ being Q-polynomial with respect to θ. We use this idea to investigate the Q-polynomial structures in tight distance-regular graphs.
Received: January 30, 1998 Final version received: August 14, 1998 相似文献
74.
J. Castillo-Tejas A. Rojas-Morales F. Lpez-Medina Juan F.J. Alvarado G. Luna-Brcenas F. Bautista O. Manero 《ournal of non Newtonian Fluid Mechanics》2009,161(1-3):48-59
In this work, non-equilibrium molecular dynamics simulations are used to generate the flow of linear polymer chains (monomer-springs with FENE potential) and a Lennard–Jones fluid (Newtonian fluid) through a contraction–expansion (4:1:4) geometry. An external force field simulating a constant pressure gradient upstream the contraction region induces the flow, where the confining action of the walls is represented by a Lennard–Jones potential. The equations of motion are solved through a multiple-step integration algorithm coupled to a Nosé-Hoover dynamics [S. Nose, A unified formulation of the constant temperature molecular dynamics methods, J. Chem. Phys. 81 (1984) 511–519], i.e., to simulate a thermostat, which maintains a constant temperature. In this investigation, we assume that the energy removed by the thermostat is related to the viscous dissipation along the contraction–expansion geometry. A non-linear increasing function between the pressure drop and the mean velocity along the contraction for the linear molecules is found, being an order of magnitude larger than that predicted for the Lennard–Jones fluid. The pressure drop of both systems (the linear molecules and Lennard–Jones fluid) is related to the dissipated energy at the contraction entry. The large deformation that the linear molecules experience and the evolution of the normal stress at the contraction entry follow a different trajectory in the relaxation process past the contraction, generating large hysteresis loops. The area enclosed by these cycles is related to the dissipated energy. Large shear stresses developed near the re-entrant corners as well as the vortex formation, dependent on the Deborah number, are also predicted at the exit of the contraction. To our knowledge, for the first time, the excessive pressure losses found in experimental contraction flows can be explained theoretically. 相似文献
75.
In optical communications, four-dimensional (4D) modulation formats encode information onto the quadrature components of two arbitrary orthogonal states of polarisation of the optical field. Many analytical models available in the optical communication literature allow, within a first-order perturbation framework, the computation of the average power of the nonlinear interference (NLI) accumulated in coherent fibre-optic transmission systems. However, all such models only operate under the assumption of transmitted polarisation-multiplexed two-dimensional (PM-2D) modulation formats, which only represent a limited subset of the possible dual-polarisation 4D (DP-4D) formats. Namely, only those where data transmitted on each polarisation channel are mutually independent and identically distributed. This paper presents a step-by-step mathematical derivation of the extension of existing NLI models to the class of arbitrary DP-4D modulation formats. In particular, the methodology adopted follows the one of the popular enhanced Gaussian noise model, albeit dropping most assumptions on the geometry and statistic of the transmitted 4D modulation format. The resulting expressions show that, whilst in the PM-2D case the NLI power depends only on different statistical high-order moments of each polarisation component, for a general DP-4D constellation, several other cross-polarisation correlations also need to be taken into account. 相似文献
76.
77.
78.
79.
80.
For spin-polarized low-energy electrons diffracted from ferromagnetic Ni(001), the exchange-and the spin-orbit-induced scattering asymmetries Aex and Aso of two diffracted beams have been simultaneously measured at T = 300 K and at T = 520 K for several constant energies as functions of the polar angle of incidence. Corresponding “rocking curves” calculated by dynamical theory are in generally good agreement with the data. Analysis of Aso shows firstly that an energy-dependent exchange approximation is adequate, secondly that the topmost interlayer spacing is essentially bulk-like with a possible slight outward relaxation (1%), and thirdly that 3p → 3d excitation is significant. From Aex the following magnetic information is obtained: The surface magnetization for T = 300 K and 520 K is such as to imply, via extrapolation by mean field theory, a T = 0 value slightly enhanced (by + 5% ± 5%) with respect to the bulk magnetization. The spin asymmetries of localized and uniform inelastic processes are very small, leading, for incident electron energy 69 and 89 eV, to a combined inverse mean free path asymmetry of less than 0.3%. 相似文献