全文获取类型
收费全文 | 1661篇 |
免费 | 70篇 |
国内免费 | 11篇 |
专业分类
化学 | 1165篇 |
晶体学 | 14篇 |
力学 | 55篇 |
数学 | 154篇 |
物理学 | 354篇 |
出版年
2023年 | 8篇 |
2022年 | 22篇 |
2021年 | 35篇 |
2020年 | 36篇 |
2019年 | 37篇 |
2018年 | 28篇 |
2017年 | 25篇 |
2016年 | 53篇 |
2015年 | 49篇 |
2014年 | 51篇 |
2013年 | 77篇 |
2012年 | 120篇 |
2011年 | 132篇 |
2010年 | 93篇 |
2009年 | 93篇 |
2008年 | 138篇 |
2007年 | 104篇 |
2006年 | 83篇 |
2005年 | 85篇 |
2004年 | 73篇 |
2003年 | 57篇 |
2002年 | 66篇 |
2001年 | 21篇 |
2000年 | 39篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 14篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 8篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1970年 | 4篇 |
1966年 | 2篇 |
1965年 | 3篇 |
排序方式: 共有1742条查询结果,搜索用时 430 毫秒
991.
We investigated the effect of time delays on phase configurations in a set of two-dimensional coupled phase oscillators. Each oscillator is allowed to interact with its neighbors located within a finite radius, which serves as a control parameter in this study. It is found that distance-dependent time delays induce various patterns including traveling rolls, squarelike and rhombuslike patterns, spirals, and targets. We analyzed the stability boundaries of the emerging patterns and briefly pointed out the possible empirical implications of such time-delayed patterns. 相似文献
992.
The bicyclo[3.1.0]hexane scaffold can lock the conformation of a carbocyclic nucleoside into one of the two antipodal (north or south) conformations typical of conventional nucleosides that normally exist in a rapid, two-state equilibrium in solution. In a recent brief communication, we reported a practical method to access the requisite bicyclo[3.1.0]hexane pseudosugar for the north antipode via an intramolecular olefin-ketocarbene cycloaddition. The most attractive features of this synthesis was that a relatively complex synthon was obtained from simple and inexpensive starting materials and that the resulting racemic mixtures of purine nucleosides could be successfully resolved by adenosine deaminase (ADA) hydrolysis. In this work, we describe the development of a more general, lipase-catalyzed double-acetylation reaction, which could successfully resolve an earlier precursor, 4-(tert-butyldiphenylsilamethoxy)-1-(hydroxymethyl)bicyclo[3.1.0]hexan-2-ol [(+/-)-7], into enantiomerically pure (+)-diacetate 8 and (-)-monoacetate 9. The former diacetate was converted to the conformationally locked (north)-carbocyclic guanosine (+)-17 identical to the one obtained previously by ADA resolution. The present method represents a more general and efficient process applicable to the synthesis of all classes of (north) bicyclo[3.1.0]hexane nucleosides, including pyrimidine analogues. During the lipase-catalyzed resolution, we were able to demonstrate the presence of an unusual acetal-forming reaction that consumed small amounts of the unreactive monoacetate (-)-9. This side reaction was also enzyme-catalyzed and was triggered by the byproduct acetaldehyde generated during the reaction. 相似文献
993.
[structure: see text] Host-guest interactions between cucurbit[7]uril (CB7) and a series of dialkyl-4,4'-bipyridinium (viologen) dicationic guests were investigated by NMR spectroscopy. CB7 includes the aromatic nucleus of short chain viologens, but the mode of interaction is different with longer chain viologens due to the favorable hydrophobic interactions between the terminal alkyl substituents and the inner cavity of the host. A new pseudorotaxane was designed and synthesized on the basis of viologen-CB7 binding interactions. 相似文献
994.
995.
The electrogenerated chemiluminescence (e.c.l.) and electrochemistry of the laser dyes, coumarin-2, coumarin-30, rhodamine-6G (perchlorate), rhodamine-B (perchlorate), oxazine-1 (perchlorate), and Nile Blue (perchlorate) were studied in acetonitrile using 0.1 M tetra-n-butylammonium perchlorate (TBAP) as a supporting electrolyte. Rather low intensity e.c.l. was obtained for all dyes except Nile Blue. A study of the electrochemical oxidation and reduction of coumarin-30, oxazine-1 and rhodamine-6G using cyclic voltammetry and controlled potential coulometry demonstrated that chemical side reactions of the electrogenerated reactants are responsible for the low e.c.l. efficiency. In several cases the one-electron transfer reaction at the electrode is followed by a dimerization reaction. The neutral free radical formed on reduction of oxazine-1 was investigated by electron spin resonance spectroscopy and coupling constants for it are reported. Some experiments in which the e.c.l. of mixtures of the dyes with rubrene or 9,10-diphenylanthracene were determined are also described. 相似文献
996.
We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counterrotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles. 相似文献
997.
We study threshold features of a Brillouin-shifted Stokes comb generated in a distributed fiber Raman amplifier. When the input power of Brillouin pump is linearly increased in high Raman gain, the first Stokes wave grows exponentially at much lower threshold power and then experiences an appreciable power decrease in the vicinity of the Brillouin comb's threshold. This power reduction of the first Brillouin Stokes, which we did not see mentioned in previous reports, was caused by a power transfer to higher-order lines and initiated Brillouin comb generation. Moreover, the effects of Raman pump power and pumping direction on the threshold of a Brillouin comb are investigated. 相似文献
998.
[reaction: see text] Sequential treatment of the ring-closing metathesis reaction products with silica gel, activated carbon (50 equiv wt relative to the crude products), and column chromatography on silica gel efficiently removed dark brown ruthenium byproducts from the reaction mixture. After this treatment, colorless compounds could be obtained with a ruthenium level of 0.06-0.53 microg per 5 mg of product. 相似文献
999.
Foo YL Bratland KA Cho B Lim CW Baker J Wen JG Moon DW Greene JE 《Physical review letters》2003,90(23):235502
Alloy superlattice structures consisting of alternating Si-rich and C-rich layers form spontaneously during gas-source molecular beam epitaxy of Si(1-y)C(y) on Si(001) from constant Si2H6 and CH3SiH3 precursor fluxes at T(s)=725-750 degrees C. The self-organized patterning is due to a complex interaction among competing surface reactions. During growth of the initial Si-rich layer, strain-driven C segregation to the subsurface results in charge transfer from surface Si atom dangling bonds to C backbonds. This decreases the Si2H6 sticking probability, and, hence, the instantaneous deposition rate, thereby enhancing C segregation. The Si-rich layer continues until a critical C coverage is reached allowing nucleation of a C-rich layer which grows until the excess subsurface C is depleted. The process then repeats with periods tunable through the choice of T(s) and y(avg). 相似文献
1000.
Hongjin Jiang Kyoung-sik Moon Yangyang Sun C. P. Wong Fay Hua Tarasankar Pal Anjali Pal 《Journal of nanoparticle research》2008,10(1):41-46
Shape and size controlled gram level synthesis of tin/indium (SnIn) alloy nanoparticles and nanobundles is reported. Poly(N-vinylpyrrolidone)
(PVP) was employed as a capping agent, which could control the growth and structure of the alloy particles under varying conditions.
Transmission electron microscopy showed that unique SnIn alloy nanobundles could be synthesized from the bulk materials above
a certain concentration of PVP and below this concentration, discrete spherical nanoparticles of variable size were evolved.
The morphology and the composition of the as-synthesized SnIn alloy nanobundles were investigated by high-resolution transmission
electron microscopy (TEM). The possible mechanisms on the formation of these structures were discussed. 相似文献