全文获取类型
收费全文 | 1019篇 |
免费 | 0篇 |
专业分类
系统科学 | 13篇 |
教育与普及 | 1篇 |
理论与方法论 | 18篇 |
现状及发展 | 368篇 |
研究方法 | 169篇 |
综合类 | 444篇 |
自然研究 | 6篇 |
出版年
2018年 | 9篇 |
2017年 | 10篇 |
2016年 | 12篇 |
2014年 | 11篇 |
2013年 | 10篇 |
2012年 | 58篇 |
2011年 | 72篇 |
2010年 | 26篇 |
2009年 | 6篇 |
2008年 | 39篇 |
2007年 | 52篇 |
2006年 | 54篇 |
2005年 | 51篇 |
2004年 | 27篇 |
2003年 | 28篇 |
2002年 | 41篇 |
2001年 | 32篇 |
2000年 | 41篇 |
1999年 | 34篇 |
1992年 | 31篇 |
1991年 | 10篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 23篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 11篇 |
1975年 | 8篇 |
1974年 | 11篇 |
1973年 | 12篇 |
1972年 | 14篇 |
1971年 | 15篇 |
1970年 | 14篇 |
1969年 | 12篇 |
1968年 | 9篇 |
1967年 | 11篇 |
1966年 | 8篇 |
1964年 | 6篇 |
1960年 | 4篇 |
1956年 | 6篇 |
1947年 | 4篇 |
排序方式: 共有1019条查询结果,搜索用时 15 毫秒
51.
Superattenuation in the tryptophan operon of Serratia marcescens 总被引:6,自引:0,他引:6
52.
Molecular structure of r(GCG)d(TATACGC): a DNA--RNA hybrid helix joined to double helical DNA 总被引:20,自引:0,他引:20
A H Wang S Fujii J H van Boom G A van der Marel S A van Boeckel A Rich 《Nature》1982,299(5884):601-604
The molecule r(GCG)d(TATACGC) is self-complementary and forms two DNA--RNA hybrid segments surrounding a central region of double helical DNA; its molecular structure has been solved by X-ray analysis. All three parts of the molecule adopt a conformation which is close to that seen in the 11-fold RNA double helix. The conformation of the ribonucleotides is partly determined by water molecules bridging between the ribose O2' hydroxyl group and cytosine O2. The hybrid-DNA duplex junction contains no structural discontinuities. However, the central DNA TATA sequence has some structural irregularities. 相似文献
53.
54.
55.
Summary The red anthraquinone derivative isolated from several Drechslera and Curvularia species has been identified as cynodontin (1,4,5,8-tetrahydroxy-2-methylanthraquinone), and not as 1,4,5,8-tetrahydroxy-2,6-dimethylanthraquinone as reported earlier. Both pigments showed similar physico-chemical properties. A good distinction could be obtained by gas-liquid chromatography. 相似文献
56.
Zusammenfassung Die Stereospezifität der Enzyme des Allantoinabbaus inStreptococcus allantoicus und die optische Drehung der Reaktionsprodukte wurde bestimmt. Allantoinase war aspezifisch, während Allantoate-amidohydrolase ausschliesslich (-)-Ureidoglykolate bildete. Diese Substanz wurde durch (-)-Ureidoglykolase hydrolysiert. 相似文献
57.
58.
59.
Sand can normally support a weight by relying on internal force chains. Here we weaken this force-chain structure in very fine sand by allowing air to flow through it: we find that the sand can then no longer support weight, even when the air is turned off and the bed has settled--a ball sinks into the sand to a depth of about five diameters. The final depth of the ball scales linearly with its mass and, above a threshold mass, a jet is formed that shoots sand violently into the air. 相似文献
60.
van Boekel R Min M Leinert Ch Waters LB Richichi A Chesneau O Dominik C Jaffe W Dutrey A Graser U Henning T de Jong J Köhler R de Koter A Lopez B Malbet F Morel S Paresce F Perrin G Preibisch T Przygodda F Schöller M Wittkowski M 《Nature》2004,432(7016):479-482
Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System. 相似文献