全文获取类型
收费全文 | 6554篇 |
免费 | 524篇 |
国内免费 | 31篇 |
专业分类
化学 | 5095篇 |
晶体学 | 38篇 |
力学 | 174篇 |
综合类 | 1篇 |
数学 | 1066篇 |
物理学 | 735篇 |
出版年
2022年 | 27篇 |
2021年 | 54篇 |
2020年 | 68篇 |
2019年 | 117篇 |
2018年 | 69篇 |
2017年 | 67篇 |
2016年 | 216篇 |
2015年 | 243篇 |
2014年 | 245篇 |
2013年 | 411篇 |
2012年 | 321篇 |
2011年 | 332篇 |
2010年 | 292篇 |
2009年 | 299篇 |
2008年 | 338篇 |
2007年 | 285篇 |
2006年 | 271篇 |
2005年 | 247篇 |
2004年 | 287篇 |
2003年 | 228篇 |
2002年 | 296篇 |
2001年 | 155篇 |
2000年 | 146篇 |
1999年 | 75篇 |
1998年 | 65篇 |
1997年 | 83篇 |
1996年 | 90篇 |
1995年 | 66篇 |
1994年 | 76篇 |
1993年 | 78篇 |
1992年 | 70篇 |
1991年 | 44篇 |
1990年 | 46篇 |
1989年 | 50篇 |
1988年 | 45篇 |
1987年 | 43篇 |
1986年 | 42篇 |
1985年 | 55篇 |
1984年 | 70篇 |
1983年 | 61篇 |
1982年 | 76篇 |
1981年 | 74篇 |
1980年 | 80篇 |
1979年 | 86篇 |
1978年 | 82篇 |
1977年 | 88篇 |
1976年 | 78篇 |
1975年 | 75篇 |
1974年 | 80篇 |
1973年 | 59篇 |
排序方式: 共有7109条查询结果,搜索用时 15 毫秒
151.
Electrochromatography (EC) in microfluidic chips is emerging as an attractive alternative to capillary electrophoresis (CE) for on-chip separations. This review summarizes recent developments in the rapidly growing area of chip electrochromatography with a focus on "column" technologies. Relevant achievements are summarized according to the types of stationary phase used for the separations including open channels, microfabricated structures, and channels packed with beads or containing a porous monolith. The advantages and disadvantages of each, as well as practical aspects of their application, are discussed. The analytical performance of these devices is demonstrated with separations involving various families of compounds mostly in the reversed-phase chromatographic mode. 相似文献
152.
Graziano Bonetti Patrick Pibarot Alain Chaintreau Jean Paul Marion 《Journal of separation science》1992,15(5):305-306
The use of diethylene glycol monoethyl ether as a flavor solvent is restricted. Since existing methods of quantitation are tedious or require sophisticated multi-dimensional GC instrumentation, a simple method has been developed using low cost GC-MS. Monitoring of ions 31 and 45 enables quantitation of DEGEE in essential oils at levels down to 1 μg/g with accuracy equal to or better than 15%. 相似文献
153.
Poly(ethylene glycol) (PEG 3400) was used as a soluble polymeric support for the synthesis of a series of novel benzazepines. The key step for the preparation of these heterocycles was a phosphine-free palladium-catalyzed Heck reaction. Palladium nanoparticles formed during the course of the reaction were characterized. The presence of PEG 3400 influenced the outcome of the reaction in terms of selectivity. 相似文献
154.
Weeber JC Finot E Legay G Cathelat A Lacroute Y Dereux A 《Langmuir : the ACS journal of surfaces and colloids》2004,20(23):10179-10185
We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes. 相似文献
155.
Frère P Allain M Elandaloussi el H Levillain E Sauvage FX Riou A Roncali J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(4):784-792
The electrochemical and chemical oxidation of extended TTF 4 and 5 are analysed by cyclic voltammetry, Visible/NIR and ESR spectroscopies, and the X-ray structures of the new salts 5 x BF(4)(CH(2)Cl(2)) and 4 x ClO(4)(THF)(1/2) are presented. The effects of structural factors on the pi-dimerization or the disproportionation reaction of the cation radical are shown. The oxidation of compound 4 presents the successive formation of stable cation radical and dication species both in dichloromethane (DCM) and in a CH(3)CN/THF mixture. In contrast, for compound 5, the stability of the oxidation states strongly depends on the nature of the solvent. In DCM, the oxidation of 5 proceeds by two close one-electron transfers while in CH(3)CN/THF the dication is directly formed via a two-electron process. The X-ray structures of the two salts reveal the formation of pi-dimers of cation radical. While the dimer (5(2))(2+) is due mainly to pi-pi interactions between the conjugating spacer, the multiplication of the sulfur atoms in compound 4 contributes to stabilize the dimer by the combined effects of S-S and pi-pi interactions. Visible/NIR and ESR experiments confirm the higher tendency of 4(+)(.) to dimerize with the occurrence of dimer and monomer in solution, while for 5(+)(.) only the monomer is detected in DCM. On the other hand, by dissolution of 5 x BF(4)(CH(2)Cl(2)) in CH(3)CN, only the neutral and the dicationic states of compounds 5 are observed owing to the disproportionation reaction. 相似文献
156.
Synthesis of Novel Types of C-Glycosyl-derivatives from Acetylenic Sugars or Partial Synthetic Equivalents thereof The concept of synthetic equivalent is not unequivocal and, for example, α-fluoroenamines may behave in some cases as synthetic equivalents of ynamines and, in other situations, quite differently. Some uses of these α-fluoroenamines, ‘partial synthetic equivalents’ of ynamines, for the synthesis of C-glycosyl derivatives of coumarins, chromones, quinolines, quinolones, thietenes and isoxazoles are described. We also report the preparation of a series of mono- and diglycosylthiophenes by reacting HS? with mono- and diglycosylbutadiynes. Acetylenic sugar derivatives have also been used for the synthesis of a diglycosylazaphosphorine and a spirooxolenone. 相似文献
157.
A new effective approach to the synthesis of a small library of 2-amino-5-arylidene-1,3-thiazol-4(5H)-ones was reported using solvent-free reaction conditions under microwave irradiation. In the first step, rhodanines were subjected to Knoevenagel condensation with aryl aldimines according to a facile one-pot protocol. Then the (5Z)-5-arylidene rhodanine derivatives were transformed directly into the corresponding 2-amino-1,3-thiazol-4(5H)-ones by sulfur/nitrogen displacement reaction under microwaves with retention of configuration and good overall yields. 相似文献
158.
Bujoli B Lane SM Nonglaton G Pipelier M Léger J Talham DR Tellier C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(7):1980-1988
A new process for preparing oligonucleotide arrays is described that uses surface grafting chemistry which is fundamentally different from the electrostatic adsorption and organic covalent binding methods normally employed. Solid supports are modified with a mixed organic/inorganic zirconium phosphonate monolayer film providing a stable, well-defined interface. Oligonucleotide probes terminated with phosphate are spotted directly on to the zirconated surface forming a covalent linkage. Specific binding of terminal phosphate groups with minimal binding of the internal phosphate diesters has been demonstrated. The mixed organic/inorganic thin films have also been extended for use arraying DNA duplex probes, and therefore represent a viable general approach to DNA-based bioarrays. Ideas for interfacing mixed organic/inorganic interfaces to other bioapplications are also discussed. 相似文献
159.
Jean Favre-Bonvin katia Gluchoff-Fiasson Jacques Bernillon 《Tetrahedron letters》1982,23(18):1907-1908
Stearyl-velutinal, a new natural sesquiterpenoi̋d isolated from lactarius velutinus Bert., has the structure ; this very labile compound is responsible for the dark-blue reation of the fungus to the ‘sulfo-vanilic’ mixtures. 相似文献
160.
Andr Darchen Pascal Berthelot Claude Vaccher M. Nazareth Viana Michel Debaert Jean Louis Burgot 《Journal of heterocyclic chemistry》1986,23(5):1603-1607
Two-electron reduction of some substituted 1,2-dithiole-3-thiones 1 followed by alkylation of the dianionic intermediates leads through electrosynthesis to mixture of Z and E isomers of the corresponding substituted alkyl-(3-thioalkyl)-2-propenedithioates 2, 3 in satisfactory yield. The structure of those products was established by 13C and 1H nmr and mass spectroscopy. The isomers ratios were determined by nmr spectroscopy. 相似文献