全文获取类型
收费全文 | 2986篇 |
免费 | 8篇 |
国内免费 | 247篇 |
专业分类
化学 | 2781篇 |
力学 | 11篇 |
数学 | 29篇 |
物理学 | 420篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 2篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 2篇 |
2014年 | 7篇 |
2013年 | 3篇 |
2012年 | 259篇 |
2011年 | 325篇 |
2010年 | 44篇 |
2009年 | 129篇 |
2008年 | 430篇 |
2007年 | 303篇 |
2006年 | 357篇 |
2005年 | 356篇 |
2004年 | 246篇 |
2003年 | 188篇 |
2002年 | 142篇 |
2001年 | 87篇 |
2000年 | 107篇 |
1999年 | 35篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 38篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 46篇 |
1992年 | 26篇 |
1991年 | 5篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1983年 | 2篇 |
1979年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有3241条查询结果,搜索用时 0 毫秒
991.
We present an ab initio study of the relaxations introduced in TiO (2) when a Cd impurity substitutes a Ti atom and an experimental test of this calculation by a perturbed-angular-correlation (PAC) measurement of the orientation of the electric-field gradient (EFG) tensor at the Cd site. The ab initio calculation predicts strong anisotropic relaxations of the nearest oxygen neighbors of the impurity and a change of the orientation of the largest EFG tensor component, V(33 ), from the [001] to the [110] direction upon substitution of a Ti atom by a Cd impurity. The last prediction is confirmed by the PAC experiment that shows that V(33 ) at the Cd site is parallel to either the [110] or the [1 1;0] crystal axis. 相似文献
992.
Baragaña B Blackburn AG Breccia P Davis AP de Mendoza J Padrón-Carrillo JM Prados P Riedner J de Vries JG 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(13):2931-2936
The cationic steroidal receptors 9 and 11 have been synthesized from cholic acid 3. Receptor 9 extracts N-acetyl-alpha-amino acids from aqueous media into chloroform with enantioselectivities (L:D) of 7-10:1. The lipophilic variant 11 has been employed for the enantioselective transport of N-acetylphenylalanine, a) through dichloromethane (DCM) and dichloroethane (DCE) bulk liquid membranes (U-tube apparatus), and b) through 2.5% (v/v) octanol/hexane via hollow fibre membrane contactors. Significant enantioselectivities and multiple turnovers were observed for both types of apparatus. 相似文献
993.
Corma A Galletero MS García H Palomares E Rey F 《Chemical communications (Cambridge, England)》2002,(10):1100-1101
Pyrene as a fluorophore has been covalently anchored on a delaminated ITQ-2 zeolite having a large external surface area (730 m2 g-1) and the resulting solid found to have a selective response as a heterogeneous sensor for I- in the presence of other halides. 相似文献
994.
Corma A García H Moussaif A Sabater MJ Zniber R Redouane A 《Chemical communications (Cambridge, England)》2002,(10):1058-1059
Two solid catalysts in which a chiral copper(II) bisoxazoline has been covalently anchored on silica and MCM-41 have been prepared; the solids are enantioselective catalysts (up to 92% ee) for the Friedel-Crafts hydroxyalkylation of 1,3-dimethoxybenzene with 3,3,3-trifluoropyruvate. 相似文献
995.
Korybut-Daszkiewicz B Wieckowska A Bilewicz R Domagała S Woźniak K 《Journal of the American Chemical Society》2001,123(38):9356-9366
[2]Catenane systems containing copper(II) and nickel(II) as metal centers have been self-assembled using tetraazamacrocyclic complexes and benzo-24-crown-8 as building blocks. A variety of methods, including X-ray crystallography, ESI mass spectrometry, (13)C and (1)H NMR, and electrochemistry, were applied to characterize these new face-to-face bismacrocyclic systems. Weak pi...pi interactions introduced by interlocking transition metal complexes with benzocrown moieties were shown to increase the communication (cooperativity) of metal centers. Introduction of the benzocrown increases the stability of the mixed valence state of the macrocyclic complex, which is reflected in high values of conproportionation constants. Moreover, this effect was found to be stronger than that obtained by shortening the length of the spacer between the two tetraazamacrocyclic subunits in the parent bismacrocycles. The extent of communication is larger for the nickel catenane than for the copper one. 相似文献
996.
García-Ripoll JJ Molina-Terriza G Pérez-García VM Torner L 《Physical review letters》2001,87(14):140403
In this paper we study a gaseous Bose-Einstein condensate and show the following: (i) A minimum value of the interaction is needed for the existence of stable persistent currents. (ii) Vorticity is not a fundamental invariant of the system, as there exists a conservative mechanism which can destroy a vortex and change its sign. (iii) This mechanism is suppressed by strong interactions. 相似文献
997.
The origin of life on Earth remains a fascinating mystery in spite of many theories existing on this subject. However, it seems that simple prebiotic molecules could play an essential role in the formation of more complex organisms. In our experiment, we synthesized a class of these molecules (quinones) bombarding frozen anthracene containing water with low energy hydrogen ions. This experiment roughly simulated the astrophysical conditions which one can find in the solar system. Thus, we can hypothesize that prebiotic molecules could be created by interaction of the solar wind with interplanetary dust grains. The delivery of these molecules to early Earth may have contributed to the generation of life on our planet. 相似文献
998.
Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy 总被引:13,自引:0,他引:13
Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator. 相似文献
999.
Molina-Terriza G Torner L Wright EM García-Ripoll JJ Pérez-García VM 《Optics letters》2001,26(20):1601-1603
We predict that vortex dipoles nested in light beams trapped in graded-index media can undergo closed Berry trajectories, yielding periodic vortex annihilations and revivals along the light-propagation direction. The vortex revivals from vortex-free wave fronts are mediated by Freund stationary point bundles that carry the necessary Poincaré-Hopf indices. Vortex spiraling and spontaneous generation of circular-edge dislocations are also found to occur. 相似文献
1000.
1 INTRODUCTION In recent years, chemists have paid their attention to the design and construction of extended metal- organic framework solids with potential applications, such as catalysts, nonlinear optical devices, ion ex- change, adsorption, sensors etc.[1, 2]. As one kind of polydentate ligands, isonicotinic acid has been exte- nsively employed to synthesize transition metal coo- rdination polymers[3, 4] with one-[5~8], two-[9~12] and three- dimensional[13~21] extended frameworks b… 相似文献