全文获取类型
收费全文 | 494199篇 |
免费 | 4097篇 |
国内免费 | 1324篇 |
专业分类
化学 | 250593篇 |
晶体学 | 7754篇 |
力学 | 22697篇 |
综合类 | 13篇 |
数学 | 66088篇 |
物理学 | 152475篇 |
出版年
2020年 | 3214篇 |
2019年 | 3639篇 |
2018年 | 11909篇 |
2017年 | 12610篇 |
2016年 | 10257篇 |
2015年 | 5191篇 |
2014年 | 6909篇 |
2013年 | 18781篇 |
2012年 | 17111篇 |
2011年 | 27380篇 |
2010年 | 17416篇 |
2009年 | 17431篇 |
2008年 | 23952篇 |
2007年 | 27058篇 |
2006年 | 13964篇 |
2005年 | 18744篇 |
2004年 | 14500篇 |
2003年 | 13359篇 |
2002年 | 11301篇 |
2001年 | 12161篇 |
2000年 | 9323篇 |
1999年 | 7116篇 |
1998年 | 5843篇 |
1997年 | 5777篇 |
1996年 | 5511篇 |
1995年 | 5209篇 |
1994年 | 4893篇 |
1993年 | 4827篇 |
1992年 | 5647篇 |
1991年 | 5604篇 |
1990年 | 5376篇 |
1989年 | 5356篇 |
1988年 | 5408篇 |
1987年 | 5398篇 |
1986年 | 5136篇 |
1985年 | 6789篇 |
1984年 | 6925篇 |
1983年 | 5579篇 |
1982年 | 6032篇 |
1981年 | 5958篇 |
1980年 | 5635篇 |
1979年 | 5995篇 |
1978年 | 6171篇 |
1977年 | 6135篇 |
1976年 | 6152篇 |
1975年 | 5524篇 |
1974年 | 5599篇 |
1973年 | 5902篇 |
1972年 | 3811篇 |
1968年 | 3167篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
New tricyclic spacers, readily available through fourfold Mannich reaction of substituted dibenzyl ketones, were introduced into a series of ten H2O-soluble cyclophanes with spacious preorganized cavity binding sites. These spacers provide H2O-solubility with amine or crown-ether functionality remote from the cyclophane cavity while directing functional groups such as keto or OH groups in a precise geometrical array inside the cavity. The cyclophanes were designed to include organic substrates via a combination of apolar and specific polar functional group interactions. The X-ray crystal-structure analysis of the tritopic receptor 18 with one potential neutral-molecule and two cation-binding sites showed a large rectangular open cavity with dimensions of roughly 9 × 14 Å and a spacing of 9.7 Å between the O-atoms of two convergent C?O groups. Despite the binding-site preorganization, cyclophanes incorporating two of the new spacers did not show any substrate binding in aqueous solutions. The failure of these systems to function as receptors is mainly due to steric hindrance to important cyclophane aromatic ring-guest interactions. Also, the favorable solvation of the intracavity functionality may prevent the formation of complexes. Hybrid receptors constructed from the novel spacers and diphenylmethane units were found to bind flat aromatic substrates as well as bulky [4.2]paracyclophanes. The observed large differences in stability (ΔΔG°> 2 kcal mol?1) of the complexes formed by three structurally closely related hybrid receptors with convergent C?O, OH or CH2 groups and 6-hydroxynaphthalene-2-carbonitrile as guest can be explained by a strong solvation effect of the convergent functional groups on apolar inclusion complexation. 相似文献
32.
A. E. Dorokhov 《Czechoslovak Journal of Physics》2002,52(3):C79-C90
Nonperturbative nolocal structure of QCD vacuum is well described by instanton model. Specific helicity and flavor structure of zero modes of quarks, in instanton field allows simultaneously to explain some important features of low-and high-energy hadron phenomemology. The basic characteristics of hadron spectrum, partonic sum rules, heavyquark potential etc within the instanton liquid model are briefly discussed. 相似文献
33.
The dependence of the beam propagation factor (M
2 parameter) with the absorbed pump power in the case of monolithic microchip laser under face-cooled configuration is extensively
studied. Our investigations show that the M
2 parameter is related to the absorbed pump power through two parameters (α and β) whose values depend on the laser material properties and laser configuration. We have shown that one parameter arises due
to the oscillation of higher order modes in the microchip cavity and the other parameter accounts for the spherical aberration
associated with the thermal lens induced by the pump beam. Such dependency of M
2 parameter with the absorbed pump power is experimentally verified for a face-cooled monolithic microchip laser based on Nd3+ -doped GdVO4 crystal and the values of α and β parameters were estimated from the experimentally measured data points. 相似文献
34.
D. V. Vyalykh A. M. Shikin G. V. Prudnikova A. Yu. Grigor’ev A. G. Starodubov V. K. Adamchuk 《Physics of the Solid State》2002,44(1):164-170
This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy. 相似文献
35.
The problem of the vector and axial-vector dominance of weak interactions within the framework of the quark model of superconductivity type is discussed. 相似文献
36.
37.
Using an integral theory of grating diffraction we calculate efficiencies greater than 100% if a coating with gain is taken into account. A connection with guided modes is conjectured. The application in optical computing seems to be possible. 相似文献
38.
R. F. Bikbaev 《Theoretical and Mathematical Physics》1989,80(3):1004-1006
Physics and Mathematics Division with Computational Center of the Bashkir Branch of the USSR Academy of Sciences. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 80, No. 3, pp. 470–473, September, 1989. 相似文献
39.
40.
In this paper, the authors studied certain properties of the estimate of Liang and Krishnaiah (1985, J. Multivariate Anal. 16, 162–172) for multivariate binary density. An alternative shrinkage estimate is also obtained. The above results are generalized to general orthonormal systems. 相似文献