全文获取类型
收费全文 | 22130篇 |
免费 | 4142篇 |
国内免费 | 5868篇 |
专业分类
化学 | 16999篇 |
晶体学 | 728篇 |
力学 | 1224篇 |
综合类 | 562篇 |
数学 | 2842篇 |
物理学 | 9785篇 |
出版年
2024年 | 63篇 |
2023年 | 296篇 |
2022年 | 819篇 |
2021年 | 782篇 |
2020年 | 889篇 |
2019年 | 844篇 |
2018年 | 862篇 |
2017年 | 1016篇 |
2016年 | 952篇 |
2015年 | 1163篇 |
2014年 | 1437篇 |
2013年 | 1863篇 |
2012年 | 1882篇 |
2011年 | 2062篇 |
2010年 | 1833篇 |
2009年 | 1815篇 |
2008年 | 1984篇 |
2007年 | 1766篇 |
2006年 | 1599篇 |
2005年 | 1315篇 |
2004年 | 1046篇 |
2003年 | 866篇 |
2002年 | 898篇 |
2001年 | 827篇 |
2000年 | 799篇 |
1999年 | 480篇 |
1998年 | 285篇 |
1997年 | 213篇 |
1996年 | 248篇 |
1995年 | 157篇 |
1994年 | 187篇 |
1993年 | 128篇 |
1992年 | 118篇 |
1991年 | 100篇 |
1990年 | 81篇 |
1989年 | 70篇 |
1988年 | 63篇 |
1987年 | 52篇 |
1986年 | 41篇 |
1985年 | 39篇 |
1984年 | 33篇 |
1983年 | 30篇 |
1982年 | 27篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
HaiHuiXIE XiaoYiWEI JiDongWANG MeiFangLIU RenZhouYANG 《中国化学快报》2003,14(6):588-590
A New Annonaceous acetogenin,squamostolide(1),was isolated from the seeds of Annona squamosa.Its structure was elucidated based on spectroscopic methods and comparison with known compounds.It is the first example of Annonaceous acetogenin with each of the two ends of the aliphatic chain bearing a γ-lactone.Thenew compound exhibited cytotoxic activity in vitro against bel-7402 and CNE2 human tumor cell lines. 相似文献
32.
本文使用变分方法证明了三维标准切触球的标准勒让德扭结,至少存在一个弦连结适应于标准切触结构的任何选定切触形式,这部分证明了著名的阿诺德弦猜想。 相似文献
33.
The structure of the title adduct comprises a phenanthroline derivative 2-phenyl-imidazo[4,5-f]1,10-phenanthroline and a methanol.The composition of the crystalline adduct was characterized as C19H12N4.CH3OH.It belongs to orthorhombic system,space group Pna21 with a=1.3693(4)nm,b=2.2988(7)nm,c=0.51338(15)nm,V=1.6160(8)nm^3.Z=4,and final R1=0.0423.wR2=0.1012 .Crystal structure shows that all the 19 carbon atoms and 4 nitrogen atoms are coplanar.The bond length data indicated that a very extensive conjugation system was formed.This conjugation makes the compound being a potentially excellent energy transformer used for luminescent materials. 相似文献
34.
35.
36.
本文对星载光学系统遮光罩消杂光结构的优化设计做了初步探讨。提出了遮光罩和挡光环消杂光设计的基本原则。给出了一种二级遮光罩结构寸的优化设计方法和一种遮光罩内壁上挡光环位置优化设计方法,并对一级和二级遮光罩进行了初步的分析比较。 相似文献
37.
Xiao‐Hui Liu Yan‐Guo Li Ying Lin Yue‐Sheng Li 《Journal of polymer science. Part A, Polymer chemistry》2007,45(7):1272-1281
The reversible addition–fragmentation chain transfer (RAFT) polymerization of acrylonitrile (AN) mediated by 2‐cyanoprop‐2‐yl dithiobenzoate was first applied to synthesize polyacrylonitrile (PAN) with a high molecular weight up to 32,800 and a polydispersity index as low as 1.29. The key to success was ascribed to the optimization of the experimental conditions to increase the fragmentation reaction efficiency of the intermediate radical. In accordance with the atom transfer radical polymerization of AN, ethylene carbonate was also a better solvent candidate for providing higher controlled/living RAFT polymerization behaviors than dimethylformamide and dimethyl sulfoxide. The various experimental parameters, including the temperature, the molar ratio of dithiobenzoate to the initiator, the molar ratio of the monomer to dithiobenzoate, the monomer concentration, and the addition of the comonomer, were varied to improve the control of the molecular weight and polydispersity index. The molecular weights of PANs were validated by gel permeation chromatography along with a universal calibration procedure and intrinsic viscosity measurements. 1H NMR analysis confirmed the high chain‐end functionality of the resultant polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1272–1281, 2007 相似文献
38.
39.
Modulation of Spin Distribution and Spin Transport by a Magnetic Field in a Quasi-One-Dimensional System with Spin--Orbit Coupling 下载免费PDF全文
The distributions of spin and currents modulated by magnetic field in a transverse parabolic confined two-dimensional electronic system with a Rashba spin--orbit coupling have been studied numerically. It is shown that the spin accumulation and the spin related current are generated by magnetic field if the spin--orbit coupling is presented. The distributions of charge and spin currents are antisymmetrical along the cross-section of confined system. A transversely applied electric field does not influence the characteristic behaviour of charge- and spin-dependent properties. 相似文献
40.
The mode equation for the dielectric waveguide with parabolic cross-section is derived using both the effective index method and WKB theory, and the expression of the mode propagation constant or effective index is solved directly from this mode equation, so that the mode-propagation properties of this kind of waveguide can be analysed conveniently with comparative accuracy. 相似文献