首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1246篇
  免费   41篇
  国内免费   4篇
化学   878篇
晶体学   3篇
力学   31篇
数学   208篇
物理学   171篇
  2023年   7篇
  2022年   26篇
  2021年   28篇
  2020年   25篇
  2019年   23篇
  2018年   16篇
  2017年   12篇
  2016年   50篇
  2015年   27篇
  2014年   49篇
  2013年   71篇
  2012年   95篇
  2011年   102篇
  2010年   62篇
  2009年   45篇
  2008年   77篇
  2007年   61篇
  2006年   59篇
  2005年   65篇
  2004年   54篇
  2003年   38篇
  2002年   46篇
  2001年   21篇
  2000年   17篇
  1999年   9篇
  1998年   18篇
  1997年   16篇
  1996年   13篇
  1995年   7篇
  1994年   7篇
  1993年   8篇
  1992年   8篇
  1991年   6篇
  1990年   7篇
  1989年   6篇
  1988年   5篇
  1987年   7篇
  1986年   12篇
  1985年   8篇
  1984年   6篇
  1983年   3篇
  1982年   4篇
  1981年   5篇
  1980年   8篇
  1979年   5篇
  1977年   4篇
  1974年   3篇
  1971年   3篇
  1967年   5篇
  1870年   2篇
排序方式: 共有1291条查询结果,搜索用时 15 毫秒
991.
Double‐stranded copper(II) string complexes of varying nuclearity, from di‐ to tetranuclear species, have been prepared by the CuII‐mediated self‐assembly of a novel family of linear homo‐ and heteropolytopic ligands that contain two outer oxamato and either zero ( 1 b ), one ( 2 b ), or two ( 3 b ) inner oxamidato donor groups separated by rigid 2‐methyl‐1,3‐phenylene spacers. The X‐ray crystal structures of these CuIIn complexes (n=2 ( 1 d ), 3 ( 2 d ), and 4 ( 3 d )) show a linear array of metal atoms with an overall twisted coordination geometry for both the outer CuN2O2 and inner CuN4 chromophores. Two such nonplanar allsyn bridging ligands 1 b – 3 b in an anti arrangement clamp around the metal centers with alternating M and P helical chiralities to afford an overall double meso‐helicate‐type architecture for 1 d – 3 d . Variable‐temperature (2.0–300 K) magnetic susceptibility and variable‐field (0–5.0 T) magnetization measurements for 1 d – 3 d show the occurrence of S=nSCu (n=2–4) high‐spin ground states that arise from the moderate ferromagnetic coupling between the unpaired electrons of the linearly disposed CuII ions (SCu=1/2) through the two anti m‐phenylenediamidate‐type bridges (J values in the range of +15.0 to 16.8 cm?1). Density functional theory (DFT) calculations for 1 d – 3 d evidence a sign alternation of the spin density in the meta‐substituted phenylene spacers in agreement with a spin polarization exchange mechanism along the linear metal array with overall intermetallic distances between terminal metal centers in the range of 0.7–2.2 nm. Cyclic voltammetry (CV) and rotating‐disk electrode (RDE) electrochemical measurements for 1 d – 3 d show several reversible or quasireversible one‐ or two‐electron steps that involve the consecutive metal‐centered oxidation of the inner and outer CuII ions (SCu=1/2) to diamagnetic CuIII ones (SCu=0) at relatively low formal potentials (E values in the range of +0.14 to 0.25 V and of +0.43 to 0.67 V vs. SCE, respectively). Further developments may be envisaged for this family of oligo‐m‐phenyleneoxalamide copper(II) double mesocates as electroswitchable ferromagnetic ‘metal–organic wires’ (MOWs) on the basis of their unique ferromagnetic and multicenter redox behaviors.  相似文献   
992.
The biological activation of N2 occurs at the FeMo‐cofactor, a 7Fe–9S–Mo–C–homocitrate cluster. FeMo‐cofactor formation involves assembly of a Fe6–8–SX–C core precursor, NifB‐co, which occurs on the NifB protein. Characterization of NifB‐co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent [4Fe–4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB‐co and deliver it to the NifEN protein, upon which FeMo‐cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB‐co‐loaded form provided an opportunity for Mössbauer analysis of NifB‐co. The results indicate that NifB‐co is a diamagnetic (S=0) 8‐Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the 57Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB‐co is either a 4Fe2+–4Fe3+ or 6Fe2+–2Fe3+ cluster having valence‐delocalized states.  相似文献   
993.
Chitosan is a N-containing biopolymer that can form high-quality films on glass, quartz, metals and other hydrophilic surfaces. Pyrolysis of chitosan films under argon at 800 °C and under inert atmosphere gives rise to high-quality single layer N-doped graphene films (over 99% transmittance) as evidenced by XPS, Raman spectroscopy, and TEM imaging.  相似文献   
994.
It is known that ab initio molecular dynamics (AIMD) simulations of liquid water at ambient conditions, based on the generalized gradient approximation (GGA) to density functional theory (DFT), with commonly used functionals fail to produce structural and diffusive properties in reasonable agreement with experiment. This is true for canonical, constant temperature simulations where the density of the liquid is fixed to the experimental density. The equilibrium density, at ambient conditions, of DFT water has recently been shown by Schmidt et al. [J. Phys. Chem. B, 113, 11959 (2009)] to be underestimated by different GGA functionals for exchange and correlation, and corrected by the addition of interatomic pair potentials to describe van der Waals (vdW) interactions. In this contribution we present a DFT-AIMD study of liquid water using several GGA functionals as well as the van der Waals density functional (vdW-DF) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)]. As expected, we find that the density of water is grossly underestimated by GGA functionals. When a vdW-DF is used, the density improves drastically and the experimental diffusivity is reproduced without the need of thermal corrections. We analyze the origin of the density differences between all the functionals. We show that the vdW-DF increases the population of non-H-bonded interstitial sites, at distances between the first and second coordination shells. However, it excessively weakens the H-bond network, collapsing the second coordination shell. This structural problem is partially associated to the choice of GGA exchange in the vdW-DF. We show that a different choice for the exchange functional is enough to achieve an overall improvement both in structure and diffusivity.  相似文献   
995.
a Enrico Bompiani in occasione del suo Giubileo scientifico. Lavoro eseguito nell' ambito del gruppo di rieerea N. 18 del C.N.R.  相似文献   
996.
Variable Metric Methods are Newton—Raphson-like algorithms for unconstrained minimization in which the inverse Hessian is replaced by an approximation, inferred from previous gradients and updated at each iteration. During the past decade various approaches have been used to derive general classes of such algorithms having the common properties of being Conjugate Directions methods and having quadratic termination. Observed differences in actual performance of such methods motivated recent attempts to identify variable metric algorithms having additional properties that may be significant in practical situations (e.g. nonquadratic functions, inaccurate linesearch, etc.). The SSVM algorithms, introduced by this first author, are such methods that among their other properties, they automatically compensate for poor scaling of the objective function. This paper presents some new theoretical results identifying a subclass of SSVM algorithms that have the additional property of minimizing a sharp bound on the condition number of the inverse Hessian approximation at each iteration. Reducing this condition number is important for decreasing the roundoff error. The theoretical properties of this subclass are explored and two of its special cases are tested numerically in comparison with other SSVM algorithms.This work has been done while this author was a visiting fellow at the Engineering Economic System Department, Stanford University.  相似文献   
997.
The sonolysis of water and some organic liquids such as ethylene glycol, methanol and chloroform in the presence of oxygen, at 20 and 475 kHz ultrasound frequencies has been investigated by the ESR-spin trapping technique. 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), 3,3,5,5-tetramethylpyrroline-N-oxide (TMPO) and N-tert-butyl-alpha-phenyl nitrone (PBN) were able to trap superoxide radical anion, generated as the result of the sonication of the organic media. The addition of superoxide dismutase (SOD) resulted in a dramatic decrease of the ESR signal intensity of the superoxide radical adduct. In addition, the thermolysis of the liquids under ultrasound was shown by ESR detection of the spin adducts of the radicals formed by homolytic fragmentation. Occasionally, the nature of the detected spin adduct was dependent on the sonication time or on the frequency of the ultrasonic radiation. Experiments carried out in the presence of 2-methyl-2-nitrosopropane (MNP) resulted in the detection of radicals originating from thermal decomposition of the spin trap, showing its lability under ultrasonic radiation.  相似文献   
998.
A Laguerre geometric local characterization is given of L-minimal surfaces and Laguerre deformations (T-transforms) of L-minimal isothermic surfaces in terms of the holomorphicity of a quartic and a quadratic differential. This is used to prove that, via their L-Gauss maps, the T-transforms of L-minimal isothermic surfaces have constant mean curvature \(H=r\) in some translate of hyperbolic 3-space \({\mathbb {H}}^3(-r^2)\subset \mathbb {R}^4_1\), de Sitter 3-space \({\mathbb {S}}^3_1(r^2)\subset \mathbb {R}^4_1\), or have mean curvature \(H=0\) in some translate of a time-oriented lightcone in \(\mathbb {R}^4_1\). As an application, we show that various instances of the Lawson isometric correspondence can be viewed as special cases of the T-transformation of L-isothermic surfaces with holomorphic quartic differential.  相似文献   
999.
In this paper,we characterize the sharp boundedness of the one-sided fractional maximal function for one-weight and two-weight inequalities.Also a new two-weight testing condition for the one-sided fractional maximal function is introduced extending the work of Martín-Reyes and de la Torre.Improving some extrapolation result for the one-sided case,we get weak sharp bounded estimates for one-sided fractional maximal function and weak and strong sharp bounded estimates for one-sided fractional integral.  相似文献   
1000.
One recently proposed criterion to separate two data sets in Classification is to use a hyperplane that minimizes the sum of distances to it from all the misclassified data points, where misclassification means lying on the wrong side of the hyperplane, or rather in the wrong halfspace. In this paper we study an extension of this problem: we seek the hyperplane minimizing the sum of concave nondecreasing functions of the distances of misclassified points to it. It is shown that an optimal hyperplane exists containing at least $d$ affinely independent points. This extends the result known for the minimization of the sum of distances, and enables to use combinatorial local-search heuristics for this problem. As a corollary, the same result is obtained for the approximation problem in which a hyperplane minimizing the sum of concave nondecreasing functions of the distances from a set of data points is sought.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号