全文获取类型
收费全文 | 616篇 |
免费 | 38篇 |
国内免费 | 2篇 |
专业分类
化学 | 411篇 |
晶体学 | 5篇 |
力学 | 7篇 |
数学 | 118篇 |
物理学 | 115篇 |
出版年
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 24篇 |
2020年 | 22篇 |
2019年 | 22篇 |
2018年 | 13篇 |
2017年 | 4篇 |
2016年 | 27篇 |
2015年 | 20篇 |
2014年 | 27篇 |
2013年 | 39篇 |
2012年 | 44篇 |
2011年 | 40篇 |
2010年 | 22篇 |
2009年 | 27篇 |
2008年 | 31篇 |
2007年 | 41篇 |
2006年 | 38篇 |
2005年 | 36篇 |
2004年 | 26篇 |
2003年 | 19篇 |
2002年 | 20篇 |
2001年 | 9篇 |
2000年 | 12篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有656条查询结果,搜索用时 453 毫秒
101.
Gonzalo Fiz Pontiveros Simon Griffiths Robert Morris David Saxton Jozef Skokan 《Combinatorica》2016,36(1):71-89
The Ramsey number r(K 3,Q n ) is the smallest integer N such that every red-blue colouring of the edges of the complete graph K N contains either a red n-dimensional hypercube, or a blue triangle. Almost thirty years ago, Burr and Erd?s conjectured that r(K 3,Q n )=2 n+1?1 for every n∈?, but the first non-trivial upper bound was obtained only recently, by Conlon, Fox, Lee and Sudakov, who proved that r(K 3,Q n )?7000·2 n . Here we show that r(K 3,Q n )=(1+o(1))2 n+1 as n→∞. 相似文献
102.
Dr. Carlos Vila Fares Ibrahim Amr Prof. Dr. Gonzalo Blay Prof. Dr. M. Carmen Muñoz Prof. Dr. José R. Pedro 《化学:亚洲杂志》2016,11(10):1532-1536
An efficient one‐pot asymmetric synthesis of pyrazoles bearing a chiral quaternary stereocenter has been developed. Quinine‐derived thiourea catalyzed the enantioselective addition of pyrazolones to isatin‐derived ketimines, providing the corresponding acetylated pyrazoles after in situ treatment with Ac2O/Et3N. The corresponding pyrazoles were afforded in high yields and excellent enantioselectivities. 相似文献
103.
Bourdelande JL Gallardo I Guirado G 《Journal of the American Chemical Society》2007,129(10):2817-2821
The determination of the standard potential of alkyl primary amines is reported for the first time using the nanosecond equilibrium method. The versatility and accuracy of the method demonstrates that it is not only an alternative to the classical and modern electrochemical methods, but also a powerful tool for quantifying inductive and/or solvation effects in a related family of compounds. Two different trends were observed depending on alkyl chain length. For "short-chain" alkyl primary amines, where the solvation around the amino group is expected to be the same, the standard potential value appears to follow a linear relationship with the number of carbon atoms, which indicates that the methylene group (-CH2-) causes an inductive effect that is responsible for the stabilization of the amine cation radical. Meanwhile, the E(o) rises slightly to a constant potential value 1.500 V for "long-chain" unbranched alkyl primary amines. This interesting result can be explained by a steric inhibition of solvation around the amino group due to a fold of the long alkyl chain following a solvent exclusion mechanism. 相似文献
104.
Jiménez-Osés G Aydillo C Busto JH Zurbano MM Peregrina JM Avenoza A 《The Journal of organic chemistry》2007,72(14):5399-5402
The use of a chiral serine equivalent as an excellent chiral building block has been demonstrated in the synthesis of alpha-benzylserine through a diastereoselective lithium enolate alkylation reaction and subsequent acid hydrolysis. The role of a coordinating countercation (lithium) in the alkylation reaction has been investigated. Theoretical studies have been performed in order to elucidate the stereochemical outcome of the alkylation process, which occurs with total retention of configuration. 相似文献
105.
Douhal A Angulo G Gil M Organero JA Sanz M Tormo L 《The journal of physical chemistry. B》2007,111(19):5487-5493
In this contribution, we report on studies of rotational and diffusional dynamics of 7-hydroxyquinoline (7HQ) within a reverse micelle (RM) containing different amounts of water. Analyzed in terms of the wobbling-in-a-cone model, the data reveal structural and dynamical properties of the nanopool. We clearly observed three regions in the behavior of confined water molecules within the RM hosting a double proton-transfer reaction between the probe and water. This observation remarkably reproduces the change of calculated water density within this life-mimicking medium. The number of water molecules per AOT head in the transition regions changes from 2 to 5, the latter being very near to the full solvation number (6) of the RM heads. Moreover, the H-bonds breaking and making within the RM to give new structures of the probe strongly affect the environment fluidization in different extents, reflected in different relaxation times of these structures; however, they are of similar sizes. We discuss the role of RM confinement and the proton-transfer dynamics on the behavior of water and their relationships to the packing of water molecules in the studied range of concentrations. 相似文献
106.
Analysis of the influence of the substitution on indolylmagnesium salts in the reaction with benzoyl chlo ride, acrylonitrile and methyl iodide, giving the C- and N-derivatives, have been carried out. The yield in the C- and N-product depends upon the electronic character and position of the substituent (methyl or phenyl) on the indole ring and of the ethereal solvent as well as the concentration and molar ratio of the reagents. The 2- or 3-phenyl substituted indolylmagnesium salts with acrylonitrile always gave the 1-(2-cyanoethyl)indole derivative. 相似文献
107.
A. Perea J. Gonzalo C.N. Afonso C. Vivien Ch. Leborgne 《Applied Physics A: Materials Science & Processing》1999,69(7):S141-S144
Fast photography of the plume produced by laser ablation of LiNbO3 in vacuum has been performed using an image intensified CCD (ICCD) camera in a time interval up to 2 7s after the laser pulse. Two differently oriented single crystalline LiNbO3 targets were used. The results show that although the emission intensity of the laser-generated plume initially depends on the crystalline orientation of the target, it reaches a stationary state after several minutes which is the same for both targets orientations. Under these stationary conditions, the angular distribution of the Li atoms is found to be broader than that of Nb atoms. The observed less forward directed expansion of the Li species may explain the poor Li content normally observed in films grown by laser ablation of LiNbO3 in vacuum. 相似文献
108.
R. del Coso A. Perea R. Serna J.A. Chaos J. Gonzalo J. Solis 《Applied Physics A: Materials Science & Processing》1999,69(7):S553-S556
The parameters that control the thickness distribution of thin films produced by pulsed laser deposition are investigated. It is found that highly asymmetric material distribution profiles can be obtained in vacuum when high energy densities are used and the target surface is positioned at the lens focus. The asymmetries are due to asymmetries in the spatial distribution of the beam. Under Ar gas pressure, the profile becomes symmetric and the distribution narrows as a consequence of collisions between the ejected species and the gas. In vacuum, decreasing the energy density, by decreasing the laser energy output or by moving the lens to defocus the beam, leads to symmetric distribution profiles. Nevertheless, in the first case a quite broad distribution and a low deposition rate are obtained, whereas in the second case the distribution is narrow and the deposition rate increases. 相似文献
109.
C.N. Afonso J. Gonzalo R. Serna J.C.G. de Sande C. Ricolleau C. Grigis M. Gandais D.E. Hole P.D. Townsend 《Applied Physics A: Materials Science & Processing》1999,69(7):S201-S207
Nanocomposite films formed by Cu nanocrystals (NCs) with sizes <10 nm embedded in an amorphous Al2O3 host have been grown by alternate pulsed-laser deposition both in vacuum and in a buffer gas (Ar) up to pressures of 0.1 Torr. The dimensions, dimension distributions, and shape of the NC produced in vacuum and in Ar up to pressures of 5᎒-3 Torr follow a similar trend as a function of the Cu areal density. This allows us to conclude that the nucleation and growth of the NC are dominated by processes occurring at the substrate surface rather than in the gas phase. For Ar pressures ̓᎒-2 Torr, the anisotropy of the NC is enhanced, the deposition rate decreases abruptly and a significant amount of the buffer gas is incorporated into the host, thus leading to the formation of a porous material. 相似文献
110.
Ana R. Millanao Aracely Y. Mora Nicols A. Villagra Sergio A. Bucarey Alejandro A. Hidalgo 《Molecules (Basel, Switzerland)》2021,26(23)
Broad antibacterial spectrum, high oral bioavailability and excellent tissue penetration combined with safety and few, yet rare, unwanted effects, have made the quinolones class of antimicrobials one of the most used in inpatients and outpatients. Initially discovered during the search for improved chloroquine-derivative molecules with increased anti-malarial activity, today the quinolones, intended as antimicrobials, comprehend four generations that progressively have been extending antimicrobial spectrum and clinical use. The quinolone class of antimicrobials exerts its antimicrobial actions through inhibiting DNA gyrase and Topoisomerase IV that in turn inhibits synthesis of DNA and RNA. Good distribution through different tissues and organs to treat Gram-positive and Gram-negative bacteria have made quinolones a good choice to treat disease in both humans and animals. The extensive use of quinolones, in both human health and in the veterinary field, has induced a rise of resistance and menace with leaving the quinolones family ineffective to treat infections. This review revises the evolution of quinolones structures, biological activity, and the clinical importance of this evolving family. Next, updated information regarding the mechanism of antimicrobial activity is revised. The veterinary use of quinolones in animal productions is also considered for its environmental role in spreading resistance. Finally, considerations for the use of quinolones in human and veterinary medicine are discussed. 相似文献