全文获取类型
收费全文 | 5036篇 |
免费 | 18篇 |
国内免费 | 20篇 |
专业分类
系统科学 | 11篇 |
丛书文集 | 18篇 |
教育与普及 | 4篇 |
理论与方法论 | 2篇 |
现状及发展 | 2210篇 |
研究方法 | 261篇 |
综合类 | 2540篇 |
自然研究 | 28篇 |
出版年
2013年 | 52篇 |
2012年 | 100篇 |
2011年 | 117篇 |
2010年 | 26篇 |
2008年 | 86篇 |
2007年 | 111篇 |
2006年 | 98篇 |
2005年 | 98篇 |
2004年 | 119篇 |
2003年 | 80篇 |
2002年 | 89篇 |
2001年 | 181篇 |
2000年 | 149篇 |
1999年 | 119篇 |
1992年 | 112篇 |
1991年 | 72篇 |
1990年 | 75篇 |
1989年 | 65篇 |
1988年 | 69篇 |
1987年 | 78篇 |
1986年 | 56篇 |
1985年 | 96篇 |
1984年 | 91篇 |
1983年 | 78篇 |
1982年 | 62篇 |
1981年 | 54篇 |
1980年 | 68篇 |
1979年 | 174篇 |
1978年 | 141篇 |
1977年 | 111篇 |
1976年 | 116篇 |
1975年 | 123篇 |
1974年 | 147篇 |
1973年 | 119篇 |
1972年 | 123篇 |
1971年 | 149篇 |
1970年 | 198篇 |
1969年 | 142篇 |
1968年 | 150篇 |
1967年 | 127篇 |
1966年 | 137篇 |
1965年 | 104篇 |
1964年 | 39篇 |
1959年 | 39篇 |
1958年 | 68篇 |
1957年 | 47篇 |
1956年 | 38篇 |
1955年 | 36篇 |
1954年 | 46篇 |
1948年 | 28篇 |
排序方式: 共有5074条查询结果,搜索用时 15 毫秒
361.
Detection of preinvasive cancer cells 总被引:12,自引:0,他引:12
362.
So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres-chemical species whose oxidation number, and hence electronic structure, can be changed reversibly-support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches. 相似文献
363.
Calignano A Kátona I Désarnaud F Giuffrida A La Rana G Mackie K Freund TF Piomelli D 《Nature》2000,408(6808):96-101
Smoking marijuana or administration of its main active constituent, delta9-tetrahydrocannabinol (delta9-THC), may exert potent dilating effects on human airways. But the physiological significance of this observation and its potential therapeutic value are obscured by the fact that some asthmatic patients respond to these compounds with a paradoxical bronchospasm. The mechanisms underlying these contrasting responses remain unresolved. Here we show that the endogenous cannabinoid anandamide exerts dual effects on bronchial responsiveness in rodents: it strongly inhibits bronchospasm and cough evoked by the chemical irritant, capsaicin, but causes bronchospasm when the constricting tone exerted by the vagus nerve is removed. Both effects are mediated through peripheral CB1 cannabinoid receptors found on axon terminals of airway nerves. Biochemical analyses indicate that anandamide is synthesized in lung tissue on calcium-ion stimulation, suggesting that locally generated anandamide participates in the intrinsic control of airway responsiveness. In support of this conclusion, the CB1 antagonist SR141716A enhances capsaicin-evoked bronchospasm and cough. Our results may account for the contrasting bronchial actions of cannabis-like drugs in humans, and provide a framework for the development of more selective cannabinoid-based agents for the treatment of respiratory pathologies. 相似文献
364.
Identification of genes that modify ataxin-1-induced neurodegeneration 总被引:55,自引:0,他引:55
365.
366.
1 Introduction Metal, metal oxide and metal compound nanoparticles (NPs) received considerable attention due to their unique properties: catalytic, magnetic, optical, electronic, etc. We believe that for different applications, there are preferable morphologies of NP-stabilizing medium composites. For example, small (1-3 nm) nanoparticles formed in micro/mesoporous hypercrosslinked polystyrene demonstrate excellent catalytic properties in various hydrogenation and oxidation reactions due to high surface... 相似文献
367.
Pigment granules in choroidal melanophores of the albino goldfish contained fine particulate materials which were in various degrees aggregated in clumps. Tyrosinase was considered to be present in an inhibited state in these pigment granules. 相似文献
368.
369.
A crude extract of the sciatic nerve, eluate of dialyzed nerve extract or 2 folin-positive sephadex fractions induced acetylcholine sensitivity of extrajunctional regions in extensor digitorum longus muscle fibres of the rat, when released from silastic plates implanted for 3--4 days onto the muscle surface. 相似文献
370.
In a survey of the intracellular bile salt oxidoreductase activity in fecal bacteria, 16 strains of nuclear dehydrogenating clostridia and 2 strains of non-nuclear dehydrogenating C. paraputrificum were demonstrated unable to oxidize cholate at any of the 3 OH groups. Since nuclear dehydrogenation at the delta-1 and delta-4 position requires a 3-oxo precursor steroid, it appears that these organisms require the presence of a 3 alpha-hydroxysteroid dehydrogenating organism for nuclear dehydrogenation. 相似文献