全文获取类型
收费全文 | 26250篇 |
免费 | 3852篇 |
国内免费 | 1613篇 |
专业分类
化学 | 10945篇 |
晶体学 | 347篇 |
力学 | 6806篇 |
综合类 | 152篇 |
数学 | 2787篇 |
物理学 | 10678篇 |
出版年
2024年 | 24篇 |
2023年 | 300篇 |
2022年 | 501篇 |
2021年 | 590篇 |
2020年 | 786篇 |
2019年 | 626篇 |
2018年 | 634篇 |
2017年 | 674篇 |
2016年 | 899篇 |
2015年 | 952篇 |
2014年 | 1216篇 |
2013年 | 2104篇 |
2012年 | 1442篇 |
2011年 | 1725篇 |
2010年 | 1338篇 |
2009年 | 1590篇 |
2008年 | 1546篇 |
2007年 | 1729篇 |
2006年 | 1578篇 |
2005年 | 1281篇 |
2004年 | 1242篇 |
2003年 | 1113篇 |
2002年 | 939篇 |
2001年 | 798篇 |
2000年 | 751篇 |
1999年 | 644篇 |
1998年 | 622篇 |
1997年 | 587篇 |
1996年 | 512篇 |
1995年 | 438篇 |
1994年 | 394篇 |
1993年 | 346篇 |
1992年 | 305篇 |
1991年 | 229篇 |
1990年 | 211篇 |
1989年 | 170篇 |
1988年 | 159篇 |
1987年 | 107篇 |
1986年 | 114篇 |
1985年 | 106篇 |
1984年 | 103篇 |
1983年 | 45篇 |
1982年 | 94篇 |
1981年 | 44篇 |
1980年 | 28篇 |
1979年 | 13篇 |
1978年 | 17篇 |
1975年 | 7篇 |
1973年 | 9篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 772 毫秒
41.
Chemical bonding in isolated molecules and crystals of zwitterionic pentacoordinate silicon chelates
The electronic structures of a number of zwitterionic pentacoordinate silicon chelates were investigated using the results of X-ray diffraction studies and quantum-chemical calculatoins by the MPW1PW91/6-311G(d) method. The topological analysis of the electron density distribution function and the study in the framework of the natural bond orbital partitioning scheme showed that the character of chemical bonding in the axial fragments of the molecules under consideration changes from dative to three-center, four-electron as the silicon atom assumes a trigonal-bipyramidal coordination. 相似文献
42.
43.
强流脉冲电子束在材料中的能量沉积剖面、能量沉积系数和束流传输系数受其入射角的影响很大,理论计算了0.5~2.0MeV的电子束以不同的入射角在Al材料中的能量沉积剖面和能量沉积系数,并且还计算了0.4~1.4MeV电子束以不同入射角穿透不同厚度C靶的束流传输系数。计算结果表明,随着入射角的增大,靶材表面层单位质量中沉积的能量增大,电子在靶材料中穿透深度减小,能量沉积系数减小,相应的束流传输系数也减小;能量为0.5~2.0MeV的电子束当入射角在60°~70°时在材料表面层单位质量中沉积的能量较大。 相似文献
44.
We introduce a new scheme for the future application of Real-coded Lattice Gas (RLG) to the numerical simulation of suspended solid objects in a fluctuating fluid environment. The reproduction of Brownian motion for a single solid object is verified through the Gaussian distribution of its displacements. The effectiveness of the solid–solid interaction model is also confirmed in an N-body simulation. 相似文献
45.
A convective flow CO laser composed of polytetrafluoroethylene discharge tubes, nickel–chromium steel components, a cupric heat exchanger and other components is described. Operating in liquid nitrogen condition, output power of 60 W has been obtained from a non-selective cavity. 相似文献
46.
47.
We establish the existence and stability of multidimensional transonic shocks for the Euler equations for steady potential compressible fluids. The Euler equations, consisting of the conservation law of mass and the Bernoulli law for the velocity, can be written as a second-order, nonlinear equation of mixed elliptic-hyperbolic type for the velocity potential. The transonic shock problem can be formulated into the following free boundary problem: The free boundary is the location of the transonic shock which divides the two regions of smooth flow, and the equation is hyperbolic in the upstream region where the smooth perturbed flow is supersonic. We develop a nonlinear approach to deal with such a free boundary problem in order to solve the transonic shock problem. Our results indicate that there exists a unique solution of the free boundary problem such that the equation is always elliptic in the downstream region and the free boundary is smooth, provided that the hyperbolic phase is close to a uniform flow. We prove that the free boundary is stable under the steady perturbation of the hyperbolic phase. We also establish the existence and stability of multidimensional transonic shocks near spherical or circular transonic shocks.
48.
Noboru Nakatani 《Optical Review》2003,10(4):206-210
This paper describes a compulsorily phase locked differential interferometer using an orthogonally polarized light source of a modulated LD with high extinction ratio to reduce non-linearity of the interferometer caused by polarization cross-talk. The current modulated LD is used as a light source to make the interferometer compact and for the scanning phase of the interferometer. The interferometer is operated compulsorily at the maximum inclination point of the fringe intensity curve by fringe scanning and an electric system. A Wollaston prism of high extinction ratio (50 dB) is used to combine the polarizing beams and to make the polarization cross-talk very small. In one light source the polarized output beams are on the same propagation axis; in the other they have a small crossing angle (2.5 mrad ∼ 10 mrad) to completely exclude non-linearity of the interferometer causded by polarization cross-talk. Using jets of a gas mixture of nitrogen and ethylene, this interferometer was demonstrated to be useful in detecting the photothermal effect of a photothermal velocimeter under phase fluctuation in a turbulent flow. 相似文献
49.
介绍了车用压差式气体流量传感器的结构设计和电测电路,指出了传感器的使用范围,由实验测出了特性曲线.该仪器可检测车用发动机曲轴箱窜气量,适用于发动机不解体故障诊断. 相似文献
50.
Molecular electroactive monolayers have been produced from vinylferrocene (VFC) via light-assisted surface anchoring to H-terminated n- and p-Si(1 0 0) wafers prepared via wet chemistry, in a controlled atmosphere. The resulting Si-C bound hybrids have been characterized by means of XPS and AFM. Their performance as semiconductor functionalized electrodes and their surface composition have been followed by combining electrochemical and XPS measurements on the same samples, before and after use in an electrochemical cell. White-light photoactivated anchoring at short (1 h) exposure times has resulted in a mild route, with a very limited impact on the initial quality of the silicon substrate. In fact, the functionalized Si surface results negligibly oxidized, and the C/Fe atomic ratio is close to the value expected for the pure molecular species. The VFC/Si hybrids can be described as (η5-C5H5)Fe2+(η5-C5H4)-CH2-CH2-Si species, on the basis of XPS results. Electrochemical methods have been applied in order to investigate the role played by a robust, covalent Si-C anchoring mode towards substrate-molecule electronic communication, a crucial issue for a perspective development of molecular electronics devices. The response found from cyclic voltammograms for p-Si(1 0 0) functionalized electrodes, run in the dark and under illumination, has shown that the electron transfer is not limited by the number of charge carriers, confirming the occurrence of electron transfer via the Si valence band. The hybrids have shown a noticeable electrochemical stability and reversibility under cyclic voltammetry (cv), and the trend in peak current intensity vs. the scan rate was linear. The molecule-Si bond is preserved even after thousands of voltammetric cycles, although the surface coverage, evaluated from cv and XPS, decreases in the same sequence. An increasingly larger surface concentration of Fe3+ at the expenses of Fe2+ redox centers has been found at increasing number of cv’s, experimentally associated with the growth of silicon oxide. Surface SiO− groups from deprotonated silanol termination, induced by the electrochemical treatments, are proposed as the associated counterions for the Fe3+ species. They could be responsible for the observed decrease in the electron transfer rate constant with electrode ageing. 相似文献