全文获取类型
收费全文 | 43690篇 |
免费 | 873篇 |
国内免费 | 329篇 |
专业分类
化学 | 21223篇 |
晶体学 | 303篇 |
力学 | 1729篇 |
综合类 | 12篇 |
数学 | 11234篇 |
物理学 | 10391篇 |
出版年
2023年 | 121篇 |
2022年 | 142篇 |
2021年 | 248篇 |
2020年 | 352篇 |
2019年 | 347篇 |
2018年 | 1387篇 |
2017年 | 1630篇 |
2016年 | 1210篇 |
2015年 | 1097篇 |
2014年 | 1040篇 |
2013年 | 1454篇 |
2012年 | 4209篇 |
2011年 | 3444篇 |
2010年 | 2458篇 |
2009年 | 2102篇 |
2008年 | 1438篇 |
2007年 | 1481篇 |
2006年 | 1391篇 |
2005年 | 5163篇 |
2004年 | 4477篇 |
2003年 | 2779篇 |
2002年 | 967篇 |
2001年 | 518篇 |
2000年 | 263篇 |
1999年 | 332篇 |
1998年 | 277篇 |
1997年 | 233篇 |
1996年 | 182篇 |
1995年 | 151篇 |
1994年 | 148篇 |
1993年 | 137篇 |
1992年 | 252篇 |
1991年 | 234篇 |
1990年 | 206篇 |
1989年 | 190篇 |
1988年 | 168篇 |
1987年 | 104篇 |
1986年 | 96篇 |
1985年 | 113篇 |
1984年 | 91篇 |
1983年 | 94篇 |
1982年 | 85篇 |
1981年 | 97篇 |
1979年 | 88篇 |
1978年 | 77篇 |
1977年 | 78篇 |
1976年 | 142篇 |
1975年 | 90篇 |
1974年 | 75篇 |
1973年 | 96篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
151.
We investigate the self-energy of one electron coupled to a quantized radiation field by extending the ideas developed in Hainzl (Ann. H. Poincaré, in press). We fix an arbitrary cut-off parameter Λ and recover the α2-term of the self-energy, where α is the coupling parameter representing the fine structure constant. Thereby we develop a method which allows to expand the self-energy up to any power of α. This implies that perturbation theory in α is correct if Λ is fix. As an immediate consequence we obtain enhanced binding for electrons. 相似文献
152.
153.
Efficient synthesis of ferrocenylenones using a Friedel-Crafts acylation reaction is described. Acryloyl, methacryloyl, crotonoyl, cinnamoyl, and β-methylcrotonoyl chlorides react with ferrocene in the presence of a Lewis acid (EtAlCl2 or EtAlCl2-Me3Al) to give the corresponding ferrocenylenones (acryloyl, methacryloyl, crotonoyl, cinnamoyl, and methylcrotonoylferrocenes) in good isolated yields. Besides ferrocenylenones, chloroactylferrocene is also synthesised by this method. 相似文献
154.
An adaptive pruning algorithm for the discrete L-curve criterion 总被引:1,自引:0,他引:1
Per Christian Hansen Toke Koldborg Jensen Giuseppe Rodriguez 《Journal of Computational and Applied Mathematics》2007
We describe a robust and adaptive implementation of the L-curve criterion. The algorithm locates the corner of a discrete L-curve which is a log–log plot of corresponding residual norms and solution norms of regularized solutions from a method with a discrete regularization parameter (such as truncated SVD or regularizing CG iterations). Our algorithm needs no predefined parameters, and in order to capture the global features of the curve in an adaptive fashion, we use a sequence of pruned L-curves that correspond to considering the curves at different scales. We compare our new algorithm to existing algorithms and demonstrate its robustness by numerical examples. 相似文献
155.
Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords butanolides in moderate/good yields. With epihalohydrins the reaction is regioselective and occurs at the less substituted end of the epoxide; the γ-haloalkyl-γ-butanolides thus obtained can be further transformed into various products. 相似文献
156.
The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures. 相似文献
157.
158.
159.
160.
Inés CarrancoJosé Luis D?&#x;az Oscar JiménezRodolfo Lavilla 《Tetrahedron letters》2003,44(46):8449-8452
The Lewis acid-catalyzed interaction of dihydropyridines with ethyl glyoxalate and primary aliphatic amines yields, depending on the reaction conditions, bicyclic aminals or rearranged tetrahydropyridines in one-pot multicomponent reactions. 相似文献