全文获取类型
收费全文 | 492篇 |
免费 | 17篇 |
国内免费 | 2篇 |
专业分类
化学 | 319篇 |
晶体学 | 1篇 |
力学 | 38篇 |
数学 | 44篇 |
物理学 | 109篇 |
出版年
2022年 | 6篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 15篇 |
2015年 | 8篇 |
2014年 | 9篇 |
2013年 | 19篇 |
2012年 | 29篇 |
2011年 | 24篇 |
2010年 | 12篇 |
2009年 | 13篇 |
2008年 | 20篇 |
2007年 | 23篇 |
2006年 | 36篇 |
2005年 | 21篇 |
2004年 | 23篇 |
2003年 | 17篇 |
2002年 | 22篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 12篇 |
1994年 | 5篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1989年 | 9篇 |
1987年 | 5篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1980年 | 4篇 |
1977年 | 3篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1970年 | 5篇 |
1968年 | 2篇 |
1941年 | 2篇 |
1892年 | 2篇 |
1891年 | 3篇 |
1890年 | 2篇 |
排序方式: 共有511条查询结果,搜索用时 31 毫秒
1.
2.
Daniel E. Vogler Ana Lorencak Julien M. Rey Markus W. Sigrist 《Optics and Lasers in Engineering》2005,43(3-5):527
A novel sensitive technique for the determination of losses in fiber cavities is presented. The method is based on the cavity ringdown scheme implemented in silica-based single-mode fibers. Bending losses of fiber cavities of different lengths have been measured showing all an oscillating behavior with respect to the curvature radius of the fiber as predicted by a theoretical model. The best minimum detectable absorbance per cavity pass achieved by this new method is 1.72×10−3 dB within a 10 m-long cavity. This limit suffices well for an accurate determination of optical bending losses even in bend-insensitive fibers. Furthermore, the comparison of the measured bending losses with a theoretical model allows the extraction of different fiber parameters. Good agreement has been found between the experimentally derived parameters and literature data. 相似文献
3.
R. Navarrete Casas A. García Rodríguez F. Rey Bueno A. Espínola Lara C. Valenzuela Calahorro A. Navarrete Guijosa 《Applied Surface Science》2006,252(17):6026-6030
In the present work, we have studied the adsorption of xanthine derivatives by activated carbon sorbents in aqueous solutions. The study comprised both kinetic, equilibrium and thermodynamic aspects. The kinetic results were reported in a previous paper; the equilibrium-related results are discussed here. The two types of carbon used exhibit some differences but the equilibrium isotherms obtained are all of the H-3 type in the classification of Giles. This suggests a high affinity of the sorbents for the sorbates. We also found that the overall adsorption process comprises more than one individual adsorption-desorption process of which one leads to the formation of a “monolayer” and the other to the “precipitation” of the sorbate on the sorbent surface (multilayer adsorption); the amount of sorbate adsorbed in monolayer form was seemingly greater in C-A14. 相似文献
4.
5.
Spin density redistribution between the paramagnetic centers Cu2+, N-O, and N’-O’ of bischelate complexes CuL2 of the copper ion with enaminoketone 3-imidazoline nitroxides (L) are investigated using an ab initio RHF (restricted Hartree-Fock)
approach. The most important channels of unpaired electron delocalization over the systems of π- and Σ-bonds in such complexes
are revealed. The conformation dependence of spin density localized on paramagnetic centers and delocalized due to a distortion
of the structure of the environment of the Cu2+ ion from square planar to tetrahedral is analyzed and explained.
Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 5, pp. 840–849, September-October, 1997. 相似文献
6.
J. L. Gascón M. Rodríguez J. A. Suárez Del Rey 《Journal of Radioanalytical and Nuclear Chemistry》1996,207(1):63-69
The determination of Am and Cm in typical waste streams from nuclear power plants using anion exchange chromatography has some drawbacks like the contamination by Pu and Po. This improved procedure solves these problems, and it has been applied with success to the analysis of Am and Cm in nuclear waste samples: ion exchange resins, ion exchange resins solidified with cement and evaporator concentrates. 相似文献
7.
A study of the conformations of the Z and E isomers of dinitro-1,8-p-menthane prepared by permanganate oxidation from the diamines, is reported. The configuration of the corresponding Z-diamine was established by the formation of a cyclic azodioxy derivative when a mixture of the diamines was oxidized. The 250 MHz proton NMR spectra were interpreted to reveal the conformations of the E and Z nitro derivatives. 相似文献
8.
Selected [2+2]-cycloadditions of three alkylvinylketenes 2 to one mono- and seven dialkyl-olefins 3 yielded eleven 2-alkyl-2-vinylcyclobutanones 4 (Tables 1 and 2). Three methods were compared, all involving in situ generation of the ketenes 2 by HCl-elimination from α,β-unsaturated acid chlorides 1 ; the most effective employed a large excess of olefin 3 and a high reaction temperature. The [2+2]-cycloadditions were fully regio- and stereoselective with respect to the olefin 3 , but less so with respect to the ketene 2 , so that - where possible - two stereoisomers of 4 resulted, namely A and B , whose configurations were determined from their 1H-NMR, spectra, mechanistic considerations and, in one case, 4f , by chemical correlation with a previously known cycloadduct 8 . 相似文献
9.
The [2 + 2]-cycloaddition of dichloroketene (prepared in situ from CCl3COCl and Zn(Cu)) with three alkynes 1a–c to form 2,3-dimethyl-( 2a ), 2,3-diethyl-( 2b ) and 3-butyl-4,4-dichlorocyclobut-2-enone ( 2c ) proceeds rapidly in the absence of POCl3. The primary products 2a–c rearrange in situ to the 2,4-dichlorocyclobut-2-enones 3a–c under the influence of ZnCL2 produced during the reaction. ZnCl2 converts both 2a and 3a into a 4:6 equilibrium mixture of the two; this isomerization does not occur with LiCl. The Cl-atoms of both 2a , b and 3a , b and of 2c may reductively be removed with Zn(Cu) in AcOH/pyridine to afford the alkylcyclobutenones 4a-c . Without pyridine, this reduction gives ca. 1:1 mixtures of the double-bond isomers 4 and 5 in low yields. The cyclobutenones 2c and 4c may be deuterated by CD3COOD in the presence of pyridine. D -Atom is introduced into 2c at C(4) and at C(γ), and into 4c at C(2) and C(4). A mechanism for this deuteration is considered, which does not involve a cyclobutadienolate 7 , but rather a cyclobutenolate of type 8 . The reductions of 2 and 3 to 4 might also pass through the same type of intermediate 8 . 相似文献
10.
Brough P Chiarelli R Pécaut J Rassat A Rey P 《Chemical communications (Cambridge, England)》2003,(21):2722-2723
Pyrimidinyl nitronyl nitroxides where the bis-N-oxy fragment is included in a six-membered ring were prepared from diacetonamine by a sequence of reactions including a Grignard reaction, a Ritter reaction and oxidation of the intermediate pyrimidine; the properties of the 2-phenyl-substituted representative are fully described. 相似文献