全文获取类型
收费全文 | 4474篇 |
免费 | 50篇 |
国内免费 | 62篇 |
专业分类
系统科学 | 89篇 |
丛书文集 | 245篇 |
教育与普及 | 206篇 |
理论与方法论 | 16篇 |
现状及发展 | 339篇 |
研究方法 | 605篇 |
综合类 | 3076篇 |
自然研究 | 10篇 |
出版年
2021年 | 9篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 26篇 |
2013年 | 37篇 |
2012年 | 271篇 |
2011年 | 333篇 |
2010年 | 134篇 |
2009年 | 105篇 |
2008年 | 387篇 |
2007年 | 370篇 |
2006年 | 399篇 |
2005年 | 429篇 |
2004年 | 267篇 |
2003年 | 238篇 |
2002年 | 183篇 |
2001年 | 147篇 |
2000年 | 254篇 |
1999年 | 120篇 |
1998年 | 44篇 |
1997年 | 22篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 14篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 16篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 20篇 |
1985年 | 17篇 |
1984年 | 19篇 |
1983年 | 26篇 |
1982年 | 22篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1972年 | 6篇 |
1971年 | 9篇 |
1970年 | 7篇 |
1966年 | 7篇 |
1959年 | 62篇 |
1958年 | 97篇 |
1957年 | 66篇 |
1956年 | 59篇 |
1955年 | 70篇 |
1954年 | 63篇 |
1948年 | 16篇 |
排序方式: 共有4586条查询结果,搜索用时 15 毫秒
201.
玟阐述了聚合材料(酚-甲醛齐聚物)的研究成果。这一成果可用于对在浸蚀介质中工作的零件进行涂覆处理。 相似文献
202.
列举了当前国际上一些主要相图计算程序的数值计算方法,即牛顿-拉夫森迭代法和单纯形法,分析了这些方法的优缺点,并提出了一种新的计算方法,该法既具有解几个变量的非线性方程组的牛顿-拉夫森法的优点,也具有可优化体系自由能的单纯形法的优点,并且避开了上述两法在计算相图的不足之处,该法适宜于寻求复杂二、三体系的稳定解。 相似文献
203.
Wabnitz H Bittner L de Castro AR Döhrmann R Gürtler P Laarmann T Laasch W Schulz J Swiderski A von Haeften K Möller T Faatz B Fateev A Feldhaus J Gerth C Hahn U Saldin E Schneidmiller E Sytchev K Tiedtke K Treusch R Yurkov M 《Nature》2002,420(6915):482-485
Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion. 相似文献
204.
Bennett IM Farfano HM Bogani F Primak A Liddell PA Otero L Sereno L Silber JJ Moore AL Moore TA Gust D 《Nature》2002,420(6914):398-401
Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant. 相似文献
205.
Gardner MJ Shallom SJ Carlton JM Salzberg SL Nene V Shoaibi A Ciecko A Lynn J Rizzo M Weaver B Jarrahi B Brenner M Parvizi B Tallon L Moazzez A Granger D Fujii C Hansen C Pederson J Feldblyum T Peterson J Suh B Angiuoli S Pertea M Allen J Selengut J White O Cummings LM Smith HO Adams MD Venter JC Carucci DJ Hoffman SL Fraser CM 《Nature》2002,419(6906):531-534
The mosquito-borne malaria parasite Plasmodium falciparum kills an estimated 0.7-2.7 million people every year, primarily children in sub-Saharan Africa. Without effective interventions, a variety of factors-including the spread of parasites resistant to antimalarial drugs and the increasing insecticide resistance of mosquitoes-may cause the number of malaria cases to double over the next two decades. To stimulate basic research and facilitate the development of new drugs and vaccines, the genome of Plasmodium falciparum clone 3D7 has been sequenced using a chromosome-by-chromosome shotgun strategy. We report here the nucleotide sequences of chromosomes 10, 11 and 14, and a re-analysis of the chromosome 2 sequence. These chromosomes represent about 35% of the 23-megabase P. falciparum genome. 相似文献
206.
207.
208.
209.
210.
A cat cloned by nuclear transplantation 总被引:1,自引:0,他引:1
Shin T Kraemer D Pryor J Liu L Rugila J Howe L Buck S Murphy K Lyons L Westhusin M 《Nature》2002,415(6874):859
Sheep, mice, cattle, goats and pigs have all been cloned by transfer of a donor cell nucleus into an enucleated ovum, and now we add the successful cloning of a cat (Felis domesticus) to this list. However, this cloning technology may not be readily extendable to other mammalian species if our understanding of their reproductive processes is limited or if there are species-specific obstacles. 相似文献