全文获取类型
收费全文 | 35348篇 |
免费 | 5507篇 |
国内免费 | 4798篇 |
专业分类
化学 | 20387篇 |
晶体学 | 424篇 |
力学 | 1280篇 |
综合类 | 296篇 |
数学 | 2948篇 |
物理学 | 9236篇 |
无线电 | 11082篇 |
出版年
2024年 | 102篇 |
2023年 | 772篇 |
2022年 | 1005篇 |
2021年 | 1356篇 |
2020年 | 1300篇 |
2019年 | 1340篇 |
2018年 | 1143篇 |
2017年 | 1242篇 |
2016年 | 1532篇 |
2015年 | 1708篇 |
2014年 | 2170篇 |
2013年 | 2621篇 |
2012年 | 3156篇 |
2011年 | 3226篇 |
2010年 | 2573篇 |
2009年 | 2562篇 |
2008年 | 2616篇 |
2007年 | 2356篇 |
2006年 | 2276篇 |
2005年 | 1801篇 |
2004年 | 1345篇 |
2003年 | 1109篇 |
2002年 | 1028篇 |
2001年 | 874篇 |
2000年 | 736篇 |
1999年 | 651篇 |
1998年 | 481篇 |
1997年 | 413篇 |
1996年 | 345篇 |
1995年 | 326篇 |
1994年 | 303篇 |
1993年 | 215篇 |
1992年 | 188篇 |
1991年 | 173篇 |
1990年 | 137篇 |
1989年 | 103篇 |
1988年 | 83篇 |
1987年 | 60篇 |
1986年 | 63篇 |
1985年 | 51篇 |
1984年 | 21篇 |
1983年 | 20篇 |
1982年 | 20篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1971年 | 4篇 |
1957年 | 4篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 797 毫秒
81.
82.
Ch Yan Poon 《Tetrahedron letters》2004,45(14):2985-2988
An efficient synthesis of the unique indolizino[7,6-c]quinoline carboskeleton of isaindigotidione has been achieved. This strategy employed l-proline and isatin as the main building blocks in the construction of the framework. Four transformations occurred in a one-pot operation to furnish the tetracyclic nucleus. 相似文献
83.
Hypersonic longitudinal sound velocities in five silicate and alumino-silicate liquids have been measured between 293 and 2550 K by Brillouin spectroscopy. Together with previous observations for four other glasses and liquids of the system SiO2-Al2O3-CaO-MgO, these results are used to discuss changes in hypersonic velocities in three adjacent temperature domains, i.e., below, in, and above the glass transformation range. The temperature dependence of Brillouin velocities is consistent with the observed variations with temperature of viscosity, density, and mean heat capacity for the same three temperature domains. These variations of physical properties of alumino-silicate liquids are qualitatively in agreement with the Inherent Structure Theory for liquids. 相似文献
84.
This paper presents an investigation on a single-mode-multimode-single-mode fiber structure. A one-way guided-mode propagation analysis for the circular symmetry waveguide is employed to model the light propagation and the approximated formulations are derived and evaluated concerning the accuracy. Phase conjunction of the multimode interference within the fiber structure is revealed. A simple way to predict and analyze the spectral response of the structure is presented through the space to wavelength mapping with the derived approximated formulations. The prediction of spectral response is verified numerically and experimentally. 相似文献
85.
Density functional method (DFT) (B3p86) of Gaussian98 has been used to optimize the structure of the Tc_2 molecule. The result shows that the ground state for Tc_2 molecule is an 11-multiple state and its electronic configuration is {}^{11}Σ_g^-, which shows the spin polarization effect of Tc_2 molecule of a transition metal element for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher energy states. So, that the ground state for Tc_2 molecule is an 11-multiple state is indicative of the spin polarization effect of Tc_2 molecule of a transition metal element: that is, there exist 10 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of Tc_2 molecule is minimized. It can be concluded that the effect of parallel spin of the Tc_2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell--Sorbie potential functions with the parameters for the ground state {}^{11}Σ_g^- and other states of Tc_2 molecule are derived. Dissociation energy D_e for the ground state of T_{c2} molecule is 2.266eV, equilibrium bond length R_e is 0.2841nm, vibration frequency ω_e is 178.52cm^{-1}. Its force constants f_2, f_3, and f_4 are 0.9200aJ·nm^{-2}, --3.5700aJ·nm^{-3}, 11.2748aJ·nm^{-4} respectively. The other spectroscopic data for the ground state of Tc_2 molecule ω_eχ_e, B_e, α_e are 0.5523cm^{-1}, 0.0426cm^{-1}, 1.6331×10^{-4}cm^{-1} respectively. 相似文献
86.
87.
88.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
89.
90.