全文获取类型
收费全文 | 4632篇 |
免费 | 148篇 |
国内免费 | 30篇 |
专业分类
化学 | 3040篇 |
晶体学 | 15篇 |
力学 | 216篇 |
数学 | 793篇 |
物理学 | 746篇 |
出版年
2023年 | 20篇 |
2022年 | 57篇 |
2021年 | 74篇 |
2020年 | 61篇 |
2019年 | 71篇 |
2018年 | 56篇 |
2017年 | 59篇 |
2016年 | 128篇 |
2015年 | 115篇 |
2014年 | 146篇 |
2013年 | 255篇 |
2012年 | 328篇 |
2011年 | 388篇 |
2010年 | 203篇 |
2009年 | 212篇 |
2008年 | 270篇 |
2007年 | 278篇 |
2006年 | 297篇 |
2005年 | 283篇 |
2004年 | 225篇 |
2003年 | 219篇 |
2002年 | 201篇 |
2001年 | 66篇 |
2000年 | 71篇 |
1999年 | 46篇 |
1998年 | 61篇 |
1997年 | 65篇 |
1996年 | 62篇 |
1995年 | 44篇 |
1994年 | 52篇 |
1993年 | 34篇 |
1992年 | 28篇 |
1991年 | 24篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 11篇 |
1985年 | 25篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 18篇 |
1979年 | 22篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 16篇 |
1975年 | 11篇 |
1974年 | 9篇 |
排序方式: 共有4810条查询结果,搜索用时 31 毫秒
911.
Julio Garcia Michael Markl Susanne Schnell Bradley Allen Pegah Entezari Riti Mahadevia S. Chris Malaisrie Philippe Pibarot James Carr Alex J. Barker 《Magnetic resonance imaging》2014
Aims
The objective of this study was to evaluate the potential of 4D flow MRI to assess valve effective orifice area (EOA) in patients with aortic stenosis as determined by the jet shear layer detection (JSLD) method.Methods and Results
An in-vitro stenosis phantom was used for validation and in-vivo imaging was performed in 10 healthy controls and 40 patients with aortic stenosis. EOA was calculated by the JSLD method using standard 2D phase contrast MRI (PC-MRI) and 4D flow MRI measurements (EOAJSLD-2D and EOAJSLD-4D, respectively). As a reference standard, the continuity equation was used to calculate EOA (EOACE) with the 2D PC-MRI velocity field and compared to the EOAJSLD measurements. The in-vitro results exhibited excellent agreement between flow theory (EOA = 0.78 cm2) and experimental measurement (EOAJSLD-4D = 0.78 ± 0.01 cm2) for peak velocities ranging from 0.9 to 3.7 m/s. In-vivo results showed good correlation and agreement between EOAJSLD-2D and EOACE (r = 0.91, p < 0.001; bias: − 0.01 ± 0.38 cm2; agreement limits: 0.75 to − 0.77 cm2), and between EOAJSLD-4D and EOACE (r = 0.95, p < 0.001; bias: − 0.09 ± 0.26 cm2; limits: 0.43 to − 0.62 cm2).Conclusion
This study demonstrates the feasibility of measuring EOAJSLD using 4D flow MRI. The technique allows for optimization of the EOA measurement position by visualizing the 3D vena contracta, and avoids potential sources of EOACE measurement variability. 相似文献912.
C Dussouy L Bultel J Saguez A Cherqui M Khelifa E Grand P Giordanengo J Kovensky 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(32):10021-10028
The synthesis of four GlcNAc(β1→4)Glc disaccharides containing 2-O-acetyl and/or 6-sulfate groups was performed in high yields with total 1,2-trans stereoselectivity. These disaccharides were evaluated as candidates for insect chitinase inhibition and aphicidal activity. All the compounds prepared displayed physiological effects on M. persicae aphids; however, the inhibition of chitinases of different sources (bacteria, fungus, and aphid) followed different patterns according to subtle structural characteristics. 相似文献
913.
The element nitrogen is essential to life. Considerable attention is thus paid to the development of synthetic methods for its introduction into molecules. Nitrenes, long regarded as highly reactive but poorly selective species, have recently emerged as useful tools for the formation of C-N bonds. Practical metal-catalyzed protocols are now available for the preparation of amines through either the aziridination of alkenes or the C-H amination of alkanes. Recent results highlighted in this Minireview suggest that synthetic nitrene chemistry is maturing with a wider scope not limited to these two reactions. 相似文献
914.
915.
Dr. Hongpeng Jia Belinda Schmid Dr. Shi‐Xia Liu Michael Jaggi Philippe Monbaron Dr. Sheshanath V. Bhosale Shadi Rivadehi Prof. Steven J. Langford Dr. Lionel Sanguinet Dr. Eric Levillain Dr. Mohamed E. El‐Khouly Prof. Ysushi Morita Prof. Shunichi Fukuzumi Prof. Silvio Decurtins 《Chemphyschem》2012,13(14):3370-3382
A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P–TTF–P triads 1 a – c and asymmetric P–TTF dyads 2 a , b in good yields. These planar and extended π‐conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge‐transfer excitations within the donor–acceptor assemblies. Quantum‐chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxidation potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption measurements give insight into the excited‐state events and deliver corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a , b but not in triads 1 a – c . Clear evidence for a photoinduced and relatively long lived charge‐separated state (385 ps lifetime) is obtained for a supramolecular coordination compound built from the ZnP–TTF dyad and a pyridine‐functionalized C60 acceptor unit. This specific excited state results in a (ZnP–TTF)?+ ??? (C60py)?? state. The binding constant of ZnII ??? py is evaluated by constructing a Benesi–Hildebrand plot based on fluorescence data. This plot yields a binding constant K of 7.20×104 M ?1, which is remarkably high for bonding of pyridine to ZnP. 相似文献
916.
A series of 6π‐electron 4‐center species, E2N2 and E42+ (E=S, Se, Te) is studied by means of ab initio valence bond methods with the aims of settling some controversies on 1) the diradical character of these molecules and 2) the radical sites, E or N, of the preferred diradical structure. It was found that for all molecules, the cumulated weights of the two possible diradical structures are always important and close to 50 %, making these molecules comparable to ozone in terms of diradical character. While the two diradical structures are degenerate in the E42+ dications, they have on the contrary strongly unequal weights in the E2N2 neutral molecules. In these three molecules, the electronic structure is dominated by one diradical structure, in which the radical sites are the two nitrogen atoms, while the other diradical structure is much less important. The ordering of the various VB structures in terms of their calculated weights is confirmed by the relative energies of individual VB structures. In all cases, the major diradical structure (or both diradical structures when they are degenerate) is (are) the lowest one(s), while the covalent VB structures lie higher in energy. The vertical resonance energies are considerable in S2N2 and S42+, about 80 % of the estimated value for benzene, and diminish as one goes down the periodic table (S→Se→Te). This confirms the aromatic character of these species, as already demonstrated for S2N2 on the basis of magnetic criteria. This and the high weights and stabilities of one or both diradical structures in all systems indicates that aromaticity and diradical character do not exclude each other, contrary to what is usually claimed. Furthermore, it is shown that the diradical structures find their place in a collective electron flow responsible for the ring currents in the π system of these species. 相似文献
917.
Chiavarino B Crestoni ME Dopfer O Maitre P Fornarini S 《Angewandte Chemie (International ed. in English)》2012,51(20):4947-4949
Definitely different: the path towards sorting out a long-standing dichotomy in carbocation chemistry is disclosed by infrared multiple photon dissociation spectroscopy of tropylium and benzylium isomers of C(8)H(9)(+) ions. 相似文献
918.
Trinh TT Rozanska X Delbecq F Sautet P 《Physical chemistry chemical physics : PCCP》2012,14(10):3369-3380
The initial step for silicate and aluminosilicate condensation is studied in water in the presence of a realistic tetrapropylammonium template under basic conditions. The model corresponds to the synthesis conditions of ZSM5. The free energy profile for the dimer formation ((OH)(3)Si-O-Si-(OH)(2)O(-) or [(OH)(3)Al-O-Si-(OH)(3)](-)) is calculated with ab initio molecular dynamics and thermodynamic integration. The Si-O-Si dimer formation occurs in a two-step manner with an overall free energy barrier of 75 kJ mol(-1). The first step is associated with the Si-O bond formation and results in an intermediate with a five-coordinated Si, and the second one concerns the removal of the water molecule. The template is displaced away from the Si centres upon dimer formation, and a shell of water molecules is inserted between the silicate and the template. The main effect of the template is to slow down the backward hydrolysis reaction with respect to the condensation one. The Al-O-Si dimer formation first requires the formation of a metastable precursor state by proton transfer from Si(OH)(4) to Al(OH)(4)(-) mediated by a solvent molecule. It then proceeds through a single step with an overall barrier of 70 kJ mol(-1). The model with water molecules explicitly included is then compared to a simple calculation using an implicit continuum model for the solvent. The results underline the importance of an explicit and dynamical treatment of the water solvent, which plays a key role in assisting the reaction. 相似文献
919.
920.