全文获取类型
收费全文 | 4454篇 |
免费 | 135篇 |
国内免费 | 16篇 |
专业分类
化学 | 3315篇 |
晶体学 | 103篇 |
力学 | 175篇 |
数学 | 111篇 |
物理学 | 535篇 |
无线电 | 366篇 |
出版年
2023年 | 32篇 |
2022年 | 60篇 |
2021年 | 76篇 |
2020年 | 92篇 |
2019年 | 107篇 |
2018年 | 118篇 |
2017年 | 88篇 |
2016年 | 144篇 |
2015年 | 104篇 |
2014年 | 194篇 |
2013年 | 455篇 |
2012年 | 284篇 |
2011年 | 281篇 |
2010年 | 165篇 |
2009年 | 149篇 |
2008年 | 248篇 |
2007年 | 249篇 |
2006年 | 175篇 |
2005年 | 181篇 |
2004年 | 160篇 |
2003年 | 150篇 |
2002年 | 117篇 |
2001年 | 50篇 |
2000年 | 50篇 |
1999年 | 44篇 |
1998年 | 29篇 |
1997年 | 33篇 |
1996年 | 46篇 |
1995年 | 34篇 |
1994年 | 38篇 |
1993年 | 42篇 |
1992年 | 39篇 |
1991年 | 32篇 |
1990年 | 43篇 |
1989年 | 35篇 |
1988年 | 27篇 |
1987年 | 35篇 |
1986年 | 34篇 |
1985年 | 43篇 |
1984年 | 41篇 |
1983年 | 33篇 |
1982年 | 28篇 |
1981年 | 25篇 |
1980年 | 26篇 |
1979年 | 21篇 |
1978年 | 18篇 |
1977年 | 23篇 |
1976年 | 17篇 |
1975年 | 22篇 |
1974年 | 12篇 |
排序方式: 共有4605条查询结果,搜索用时 31 毫秒
21.
Thirty eight compounds composed of achiral banana-shaped molecules belonging to three new homologous series have been synthesized and their mesomorphic properties investigated. The mesophases have been characterized using a combination of polarized light microscopy, differential scanning calorimetry, miscibility methods and X-ray diffraction measurements. All the compounds reported exhibit mesomorphic properties. Most of the homologues in the three series show B1 and B2 phases and the transition temperatures of these are lower in the fluoro substituted compounds. Some of the lower homologues of the fluoro substituted derivatives also show the nematic phase. 相似文献
22.
Multiple‐quantum 1H NMR spectroscopy has been finding a renewed interest for its possible applications in the analysis of mixtures of small molecules, due to its simplification properties. A crucial aspect of this application of multiple‐quantum NMR is the sensitivity of the spectrum intensity to the molecular structure and to the parameterization of the experiment, which could result in the missing of some components. We demonstrate that a general scheme to overcome this drawback consists in varying the experiment parameterizations over a small number of values, selected according the values of the couplings and the relaxation rates. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
23.
An efficient one-pot sequential intermolecular aza-Michael addition and Pd-catalyzed intramolecular Buchwald–Hartwig α-arylation of secondary amines have been investigated, for the synthesis of tetrahydroisoquinolines. This method is simple and furnished products in very good yield and also successfully applied for the synthesis of novel aza-spirotricylcic ethers. 相似文献
24.
B.V. Subba Reddy B. Bramha Reddy K.V. Raghavendra Rao J.S. Yadav 《Tetrahedron letters》2012,53(20):2500-2503
A tandem cycloisomerization/Friedel–Crafts alkylation of indoles has been achieved in a one-pot process to produce 2,5-disubstituted furans using gallium-catalyzed sequential nucleophilic addition onto metal-activated 1-alkynyl-2,3-epoxy acetates. The reaction proceeds efficiently under mild conditions with complete regioselectivity to afford the substituted furan derivatives in good yields with high diversity. 相似文献
25.
26.
27.
Bahni Ray P. Dinesh Sankar Reddy Dipankar Bandyopadhyay Sang Woo Joo Ashutosh Sharma Shizhi Qian Gautam Biswas 《Theoretical and Computational Fluid Dynamics》2012,26(1-4):311-318
Instability of a thin electrolyte film undergoing a direct current electroosmotic flow has been investigated. The film with a compliant electrolyte–air interface is flowing over a rigid charged substrate. Unlike previous studies, inclusion of the Maxwell stresses in the formulation shows the presence of a new finite wavenumber shear-flow mode of instability, alongside the more frequently observed long-wave interfacial mode. The shear mode is found to be the dominant mode of instability when the electrolyte–solid and electrolyte–air interfaces are of opposite charge or of same charge but have very large zeta-potential at the electrolyte–air interface. The conditions for mode-switch (interfacial to shear) and the direction of the travelling waves are discussed through stability diagrams. Interestingly, the analysis shows that when the interfaces are of nearly same zeta potential, the ‘free’ electrolyte–air interface behaves more like a ‘stationary’ wall because of the ion transport in the reverse direction of the flow. 相似文献
28.
29.
30.
Simultaneous grafting of 2-hydroxyethyl methacrylate and methyl methacrylate (3:1 and 3:2 mole ratios) onto insoluble collagen with ceric ammonium nitrate as initiator was attempted with a view to optimizing conditions for the preparation of hydrogels. The influence of monomer and initiator concentrations on the grafting reactions was investigated. The grafting results are discussed in the light of grafting efficiency and percentage of grafting. They were found to be lower when the 3:1 mole ratio of monomers was used. 相似文献