全文获取类型
收费全文 | 1353948篇 |
免费 | 25837篇 |
国内免费 | 7801篇 |
专业分类
化学 | 672429篇 |
晶体学 | 20690篇 |
力学 | 75323篇 |
综合类 | 120篇 |
数学 | 243642篇 |
物理学 | 375382篇 |
出版年
2021年 | 13518篇 |
2020年 | 15940篇 |
2019年 | 16075篇 |
2018年 | 14562篇 |
2017年 | 13060篇 |
2016年 | 28577篇 |
2015年 | 21028篇 |
2014年 | 30397篇 |
2013年 | 74289篇 |
2012年 | 38326篇 |
2011年 | 36350篇 |
2010年 | 38602篇 |
2009年 | 40813篇 |
2008年 | 35809篇 |
2007年 | 31964篇 |
2006年 | 35507篇 |
2005年 | 29982篇 |
2004年 | 30243篇 |
2003年 | 28392篇 |
2002年 | 29076篇 |
2001年 | 27636篇 |
2000年 | 24396篇 |
1999年 | 22304篇 |
1998年 | 20917篇 |
1997年 | 20889篇 |
1996年 | 21040篇 |
1995年 | 19114篇 |
1994年 | 18528篇 |
1993年 | 18096篇 |
1992年 | 17957篇 |
1991年 | 18299篇 |
1990年 | 17506篇 |
1989年 | 17530篇 |
1988年 | 17138篇 |
1987年 | 17147篇 |
1986年 | 16056篇 |
1985年 | 22488篇 |
1984年 | 23746篇 |
1983年 | 19940篇 |
1982年 | 21610篇 |
1981年 | 20854篇 |
1980年 | 20197篇 |
1979年 | 20353篇 |
1978年 | 21663篇 |
1977年 | 21277篇 |
1976年 | 20974篇 |
1975年 | 19711篇 |
1974年 | 19313篇 |
1973年 | 19849篇 |
1972年 | 14332篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
871.
Pascher W. Den Besten J. H. Caprioli D. Leijtens X. Smit M. van Dijk R. 《Optical and Quantum Electronics》2003,35(4-5):453-464
Based on a rigorous vectorial analysis, a fast travelling-wave Mach–Zehnder modulator is modelled and designed. The cross-section of the semiconductor layer stack and the lossy electrodes are carefully modelled using the method of lines in order to investigate propagation characteristics, velocity and losses. This yields an accurate microwave and optical field distribution to explain the behaviour of the component. In order to enhance the modulation efficiency, design curves are derived and the cross-sectional dimensions for minimum microwave loss are determined. The loss of the optimized modulator agrees very well with small-signal measurements up to 40 GHz and HFSS simulations. The layerstack of the fabricated device is suitable for integration with InP multi-wavelength lasers. 相似文献
872.
R. Pizzoferrato L. Lagonigro T. Ziller A. Di Carlo R. Paolesse F. Mandoj A. Ricci C. Lo Sterzo 《Chemical physics》2004,300(1-3):217-225
We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φIR=4×10−4 is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er3+. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R0, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials. 相似文献
873.
M. V. Zyubin I. A. Rudnev V. A. Kashurnikov 《Journal of Experimental and Theoretical Physics》2003,96(6):1065-1077
A system of Abrikosov vortices in a quasi-two-dimensional HTSC plate is considered for various periodic lattices of pinning centers. The magnetization and equilibrium configurations of the vortex density for various values of external magnetic field and temperature are calculated using the Monte Carlo method. It is found that the interaction of the vortex system with the periodic lattice of pinning centers leads to the formation of various ordered vortex states through which the vortex system passes upon an increase or a decrease in the magnetic field. It is shown that ordered vortex states, as well as magnetic field screening processes, are responsible for the emergence of clearly manifested peaks on the magnetization curves. Extended pinning centers and the effect of multiple trapping of vortices on the behavior of magnetization are considered. Melting and crystallization of the vortex system under the periodic pinning conditions are investigated. It is found that the vortex system can crystallize upon heating in the case of periodic pinning. 相似文献
874.
875.
In his recent series of lectures, Prof. B. I. Plotkin discussed geometrical properties of the variety of associativeK-algebras. In particular, he studied geometrically noetherian and logically noetherian algebras and, in this connection, he
asked whether there exist uncountably many simpleK-algebras with a fixed finite number of generators. We answer this question in the affirmative using both crossed product
constructions and HNN extensions of division rings. Specifically, we show that there exist uncountably many nonisomorphic
4-generator simple Ore domains, and also uncountably many nonisomorphic division algebras having 2 generators as a division
algebra.
The first author is grateful to Professor B. I. Plotkin for communicating this problem to him and for stimulating conversations. 相似文献
876.
The formation of thinning filaments is commonly observed previously to the break-up of a very viscous jet. This paper shows that a fluid under capillary forces cannot break-up through the uniform collapse of a filament. 相似文献
877.
2-Formyl glycals undergo rapid condensation with arylhydrazines under solvent-free conditions to give the corresponding optically pure 4-substituted pyrazoles in good yields with high selectivity. The stereochemistry of the products was assigned by various NMR experiments. 相似文献
878.
879.
Ariel M. Sarotti 《Tetrahedron letters》2004,45(44):8203-8206
A two-step general approach to a new chiral auxiliary starting from levoglucosenone is reported. The compound is obtained by a [4+2] cycloaddition reaction with anthracene followed by a diastereoselective reduction of the C-2 keto function in high overall yield. The auxiliary has been used as chiral template in an asymmetric Diels-Alder reaction of the corresponding acrylic ester derivative with cyclopentadiene and shown to be efficient for asymmetric induction. 相似文献
880.
O. V. Koryakova V. V. Sattarova D. V. Sevenard O. G. Khomutov K. I. Pashkevich 《Russian Journal of General Chemistry》2003,73(1):141-144
The tautomeric equilibrium in substituted 5,6,7,8-tetrahydroquinazolines was studied by IR spectroscopy. The effect of various substitutents in the pyrimidine ring and the solvent on the state of the tautomeric equilibrium was evaluated. 相似文献