全文获取类型
收费全文 | 2512篇 |
免费 | 79篇 |
国内免费 | 7篇 |
专业分类
化学 | 1960篇 |
晶体学 | 7篇 |
力学 | 22篇 |
数学 | 310篇 |
物理学 | 299篇 |
出版年
2023年 | 15篇 |
2022年 | 14篇 |
2021年 | 31篇 |
2020年 | 58篇 |
2019年 | 39篇 |
2018年 | 14篇 |
2017年 | 14篇 |
2016年 | 72篇 |
2015年 | 62篇 |
2014年 | 77篇 |
2013年 | 124篇 |
2012年 | 158篇 |
2011年 | 163篇 |
2010年 | 89篇 |
2009年 | 78篇 |
2008年 | 143篇 |
2007年 | 116篇 |
2006年 | 119篇 |
2005年 | 123篇 |
2004年 | 132篇 |
2003年 | 90篇 |
2002年 | 98篇 |
2001年 | 47篇 |
2000年 | 44篇 |
1999年 | 35篇 |
1998年 | 28篇 |
1997年 | 41篇 |
1996年 | 39篇 |
1995年 | 38篇 |
1994年 | 46篇 |
1993年 | 42篇 |
1992年 | 36篇 |
1991年 | 24篇 |
1990年 | 25篇 |
1989年 | 26篇 |
1988年 | 29篇 |
1987年 | 16篇 |
1985年 | 29篇 |
1984年 | 17篇 |
1983年 | 9篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 19篇 |
1978年 | 11篇 |
1977年 | 18篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1973年 | 9篇 |
1970年 | 10篇 |
排序方式: 共有2598条查询结果,搜索用时 15 毫秒
31.
We consider a matched queueing network PH/M/coPH/PH/1, where the output of the first subsystem PH/M/c is regarded as one of the inputs of the second subsystem and matched with another input of PH-renewal process, and they are served by a single server with PH-service-distribution. The stationary state probabilities, the distribution of the virtual waiting times in the two subsystems, and the distribution of the total virtual waiting time are derived.The work was supported by the Max-Planck-Gesellschaft, Germany, and the Chinese Academy ofSciences. 相似文献
32.
Nonribosomal peptide synthetases (NRPS) are multimodular biocatalysts that bacteria and fungi use to assemble many complex peptides with broad biological activities. The same modular enzymatic assembly line principles are found in fatty acid synthases (FAS), polyketide synthases (PKS), and most recently in hybrid NRPS/PKS multienzymes. FAS as well as PKS are known to function as homodimeric enzyme complexes, raising the question of whether NRPS may also act as homodimers. To test this hypothesis, biophysical methods (size exclusion chromatography, analytical equilibrium ultracentrifugation, and chemical crosslinking) and biochemical methods (two-affinity-tag-system and complementation studies with enzymes being inactivated in different catalytic domains) were applied to NRPS subunits from the gramicidin S (GrsA-ATE), tyrocidine (TycB(1)-CAT and TycB(2-3)-AT.CATE), and enterobactin (EntF-CATTe) biosynthetic systems. These methods had revealed the dimeric structure of FAS and PKS previously, but all three NRPS systems investigated are functionally active as monomers. 相似文献
33.
Bauer RE Enkelmann V Wiesler UM Berresheim AJ Müllen K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(17):3858-3864
A series of first-generation polyphenylene dendrimers based on three different cores were prepared by Diels-Alder cycloaddition and their single-crystal structures were determined. Consisting exclusively of interlocked, twisted phenyl rings, these polyphenylene nanostructures have exciting structural and dynamic properties. Single crystals of dendrimers, suitable for X-ray structure analysis, were grown from different solvent mixtures by slow evaporation at room temperature. It should be pointed out that one of the described polyphenylene dendrimers represents up to now the biggest oligophenylene nanostructure from which crystallographic data is available. 相似文献
34.
Nitrobenzoxadiazole (NBD) derivatives are determined with limits of detection ranging down to 20 nmol l(-1) using liquid chromatography-mass spectrometry (LC-MS) with electron capture (EC) ionisation. An atmospheric pressure chemical ionisation (APCI) interface operated in the negative ion mode is used as ionisation source. Amine derivatives of 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBDCl) as well as the isocyanate derivatives of 4-nitro-7-piperazino-2,1,3-nitrobenzoxadiazole (NBDPZ) have been analysed using this technique. The parameters favouring electron capture mechanisms have been investigated thoroughly under consideration of the competing mechanism of deprotonation to allow a better understanding of the electron capture process and to improve selectivity of the analysis. 相似文献
35.
Petra Spitzer Bruno Rossi Yves Gaignet Stéphane Mabic Uwe Sudmeier 《Accreditation and quality assurance》2005,10(3):78-81
There is currently a major issue with the calibration of conductivity meters used for high purity water: the lack of availability of a reference material or reference methods for low conductivity ranges (conductivity below 1 S cm–1 at 25.0 °C, resistivity >1 M cm at 25.0 °C). This paper describes the current status of conductivity measurements in high purity water. A new and improved approach, currently being investigated, should allow us to make the calibration of conductivity meters used for low conductivity ranges traceable to the SI.Milipore, Milli-Q and Elix are registered trademarks of Millipore Corporation. 相似文献
36.
37.
The generation of thiirenes in the thermal and photochemical nitrogen elimination of 1,2,3-thiadiazoles is discussed in relation to oxirene formation. Isotope labelling experiments were performed with 4-phenyl-[4-13C]-1,2,3-thiadiazole ( 7 ) as the substrate. 相似文献
38.
Burlakov VV Arndt P Baumann W Spannenberg A Rosenthal U Parameswaran P Jemmis ED 《Chemical communications (Cambridge, England)》2004,(18):2074-2075
The 2,5-dititanabicyclo[2.2.0]hex-1(4)-ene (bis-titanocene-mu-(Z)-1,2,3-butatriene complex)3 is formed starting from [Cp2Ti(eta2-Me3SiC2SiMe3)] by in situ generated titanocene and 1,4-dichlorobut-2-yne via the 1-titanacyclobut-3-yne (2). 相似文献
39.
New experimental time-of-flight distributions are reported for Li+-N2 and Li2-CO at two center-of-mass energies of about 8 and 16 eV and large scattering angles θlab ? 120°. The Li+-N2 spectra show two widely spaced maxima, whereas the Li+-CO spectra show two and sometimes three maxima. The results are consistent with the model of rotational rainbows, and have also been analyzed in terms of an impulsive model involving collisions with the individual atoms of the molecules with energy-dependent masses. Classical trajectories for a simple model potential reveal only small contributions from vibrational excitation. 相似文献
40.
Uwe Klingebiel Dieter Bentmann Anton Meller 《Monatshefte für Chemie / Chemical Monthly》1978,109(5):1067-1073
Aminofluorosilanes react with lithiated amines undergoing LiF-elimination and substitution (1). The acyclic silicon-nitrogen-compound2 is isolated in the reaction with a difluorosilane after renewed lithiation.2 is cyclisated in the reaction with butyllithium by butane- and LiF-elimination (3). Aminofluorosilanes with bulky (4) or mesomeric stabilized (5) ligands form stable lithioaminofluorosilanes, which react with fluorosilanes giving substitution products (6, 7).6 and7 react with the lithium salt oftert-butylamin in a molar ratio of 12 to give8 and9 by intramolecular cyclisation.—The mass,1H and19F nmr spectra of the compounds are reported. 相似文献