全文获取类型
收费全文 | 2232篇 |
免费 | 90篇 |
国内免费 | 11篇 |
专业分类
化学 | 1604篇 |
晶体学 | 20篇 |
力学 | 51篇 |
综合类 | 1篇 |
数学 | 303篇 |
物理学 | 354篇 |
出版年
2022年 | 10篇 |
2021年 | 27篇 |
2020年 | 36篇 |
2019年 | 27篇 |
2018年 | 28篇 |
2017年 | 15篇 |
2016年 | 38篇 |
2015年 | 28篇 |
2014年 | 29篇 |
2013年 | 119篇 |
2012年 | 96篇 |
2011年 | 124篇 |
2010年 | 82篇 |
2009年 | 53篇 |
2008年 | 104篇 |
2007年 | 121篇 |
2006年 | 131篇 |
2005年 | 99篇 |
2004年 | 101篇 |
2003年 | 65篇 |
2002年 | 84篇 |
2001年 | 45篇 |
2000年 | 56篇 |
1999年 | 37篇 |
1998年 | 34篇 |
1997年 | 33篇 |
1996年 | 31篇 |
1995年 | 40篇 |
1994年 | 30篇 |
1993年 | 30篇 |
1992年 | 17篇 |
1991年 | 33篇 |
1990年 | 18篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 22篇 |
1985年 | 35篇 |
1984年 | 32篇 |
1983年 | 22篇 |
1982年 | 31篇 |
1981年 | 20篇 |
1980年 | 32篇 |
1979年 | 31篇 |
1978年 | 24篇 |
1977年 | 30篇 |
1976年 | 14篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 17篇 |
排序方式: 共有2333条查询结果,搜索用时 15 毫秒
161.
We describe the fabrication and characterization of a free-standing silica glass membrane waveguide formed using fiber fabrication processes. The membrane has a thickness of 0.6 microm and a width of 60 microm and is many meters long. The optical attenuation is measured as 0.4 dB/m. Such attenuation outperforms that of conventional planar waveguides by several orders of magnitude. 相似文献
162.
163.
We present the best known separation between tree-like and general resolution, improving on the recent exp(n
) separation of [2]. This is done by constructing a natural family of contradictions, of size n, that have O(n)-size resolution refutations, but only exp((n/log n))- size tree-like refutations. This result implies that the most commonly used automated theorem procedures, which produce tree-like resolution refutations, will perform badly on some inputs, while other simple procedures, that produce general resolution refutations, will have polynomial run-time on these very same inputs. We show, furthermore that the gap we present is nearly optimal. Specifically, if S (S
T
) is the minimal size of a (tree-like) refutation, we prove that S
T
= exp(O(S log log S/log S)).* This research was supported by Clore Foundation Doctoral Scholarship. Research supported by NSF Award CCR-0098197 and USA–Israel BSF Grant 97-00188. This research was supported by grant number 69/96 of the Israel Science Foundation, founded by the Israel Academy for Sciences and Humanities. 相似文献
164.
Carter EB Culver SL Fox PA Goode RD Ntai I Tickell MD Traylor RK Hoffman NW Davis JH 《Chemical communications (Cambridge, England)》2004,(6):630-631
The anions of the sweeteners saccharin and acesulfame form ionic liquids when paired with a variety of organic cations. 相似文献
165.
Investigations are reported that identify the biosynthetic source and origins of the pseudopterosins, pharmacologically important diterpene glycosides, in the gorgonian coral Pseudopterogorgia elisabethae. We report here the isolation of physiologically significant levels of endogenous pseudopterosins A, B, C, and D from purified symbionts identified as the dinoflagellate Symbiodinium sp. Biosynthetic studies in photosynthesizing symbiont isolates utilizing 14C-labeled inorganic carbon and the tritiated intermediate geranylgeranyl diphosphate yielded radiochemically pure pseudopterosins A through D and the first committed intermediate, elisabethatriene. The 14CO(2) uptake into the pseudopterosin pathway occurred at low levels compared to the 14CO(2) uptake into total lipids, suggesting a large reserve pool of the compounds. These results reveal for the first time that pseudopterosin biosynthesis is occurring within the algal symbiont and suggests the physiological implications of this biosynthesis. 相似文献
166.
Gebbink RJ Watanabe M Pratt RC Stack TD 《Chemical communications (Cambridge, England)》2003,(5):630-631
Cupric complexes of a novel phenanthroline-phenolate ligand have strongly distorted coordination geometries and electrochemical properties conducive to modeling the spectroscopy and reactivity of the enzyme galactose oxidase. 相似文献
167.
Phenyldimethylsilyllithium reacts with several N,N-dimethylamides, and the intermediates, formulated here as successively a carbene and an alpha-silyllithium species, may be trapped with nucleophiles and electrophiles, respectively, although not always with the nucleophile or electrophile of your choice. 相似文献
168.
169.
Intramolecular charge transfer and biomimetic reaction kinetics in galactose oxidase model complexes 总被引:2,自引:0,他引:2
One-electron oxidation of two structurally similar CuII-diphenolate complexes, 1 and 2, creates EPR-silent CuII-phenoxyl complexes [1]+ and [2]+ that mimic the oxidized form of the enzyme galactose oxidase (GOase). Both model complexes display novel NIR absorptions assigned to phenolate-phenoxyl charge transfer that resemble a tyrosinate-tyrosyl charge-transfer band observed in the enzymatic system. [1]+ and [2]+ react with benzyl alcohol to form 0.5 equivs of benzaldehyde per complex; biomimetic reduction to CuI-phenol complexes is not observed, but such species may exist transiently. Initial kinetic studies show that [2]+ reacts faster with benzyl alcohol than does [1]+, despite being a significantly weaker oxidant (DeltaE degrees = 370 mV). This acceleration is ascribed to mechanistic differences: [2]+ appears to bind substrate prior to the rate-determining step. Large, nonclassical kinetic isotope effects confirm C-H bond cleavage as the rate-determining step in the reactions of both [1]+ and [2]+ with benzyl alcohol, as is the case for GOase. 相似文献
170.
Schweiger M Seidel SR Arif AM Stang PJ 《Angewandte Chemie (International ed. in English)》2001,40(18):3467-3469