全文获取类型
收费全文 | 309篇 |
免费 | 9篇 |
国内免费 | 1篇 |
专业分类
系统科学 | 10篇 |
理论与方法论 | 4篇 |
现状及发展 | 143篇 |
研究方法 | 30篇 |
综合类 | 100篇 |
自然研究 | 32篇 |
出版年
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 7篇 |
2018年 | 16篇 |
2017年 | 16篇 |
2016年 | 10篇 |
2015年 | 12篇 |
2014年 | 7篇 |
2013年 | 9篇 |
2012年 | 20篇 |
2011年 | 28篇 |
2010年 | 10篇 |
2009年 | 5篇 |
2008年 | 3篇 |
2007年 | 12篇 |
2006年 | 11篇 |
2005年 | 7篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1997年 | 2篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1984年 | 2篇 |
1980年 | 2篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 5篇 |
1971年 | 6篇 |
1970年 | 2篇 |
1969年 | 3篇 |
1968年 | 9篇 |
1967年 | 3篇 |
1966年 | 1篇 |
1965年 | 3篇 |
1964年 | 1篇 |
1963年 | 4篇 |
1962年 | 2篇 |
1961年 | 1篇 |
1945年 | 1篇 |
排序方式: 共有319条查询结果,搜索用时 31 毫秒
61.
62.
本文对合成的嵌段聚醚酯化物的分子结构做了红外、高压液相色谱和~(13)C核磁共振分析,得出其构象和各种原子连接的方式,这些特征图谱为研究和生产此类型表面活性剂提供了可靠的检测依据。 相似文献
63.
Martins-Dias Patrícia Romão Luísa 《Cellular and molecular life sciences : CMLS》2021,78(10):4677-4701
Cellular and Molecular Life Sciences - About 11% of all human disease-associated gene lesions are nonsense mutations, resulting in the introduction of an in-frame premature translation-termination... 相似文献
64.
65.
Samuel Cardozo Ribeiro Diego Alves Teles Daniel Oliveira Mesquita Waltécio de Oliveira Almeida Luciano Alves dos Anjos Miriam Camargo Guarnieri 《Journal of Natural History》2019,53(39-40):2365-2377
ABSTRACTWe evaluated the activity period, thermal ecology, habitats, and preference for microhabitats of the skink Mabuya arajara in a humid forest environment on the slope of the Araripe Plateau, Brazil. A total of 283 lizards were observed. The lizards were found to be diurnally active (unimodal type pattern), with peak activity during the hottest hours of the day (1100 to 1200). About habitat use, the majority of individuals (73.8% N = 209) were recorded in the narrow transitional area, in the edges of the forest; while 26.4% of individuals were observed in open areas and no observations were made in dense forest. Seven different types of microhabitats were used, with fallen palm leaves (Arecaceae) being the preferred (47.7% N = 135). Considering all lizards, microhabitat niche breadth was 3.27. Males and females presented a high overlap (0.95) in microhabitat use. The average Tc recorded for M. arajara was 32.06 ± 2.72°C. Body size (SVL) did not influence body temperature (F = 0.51, P = > 0.05). Most of the animals were observed with their bodies completely exposed (67.84%, N = 192), 18.37% (N = 52) were semi-exposed and 13.79% (N = 39) were in shelters. Among animals with some degree of sunlight exposure, 57.59% (N = 163) were in shadows, 25.10% (N = 71) were under sunlight filtered by vegetation, and 17.31% (N = 49) were under direct sunlight.A review of the ecology of Mabuya spp. shows that several features appear to be conserved among members of the genus. 相似文献
66.
67.
Superplastic carbon nanotubes 总被引:1,自引:0,他引:1
Huang JY Chen S Wang ZQ Kempa K Wang YM Jo SH Chen G Dresselhaus MS Ren ZF 《Nature》2006,439(7074):281
The theoretical maximum tensile strain--that is, elongation--of a single-walled carbon nanotube is almost 20%, but in practice only 6% is achieved. Here we show that, at high temperatures, individual single-walled carbon nanotubes can undergo superplastic deformation, becoming nearly 280% longer and 15 times narrower before breaking. This superplastic deformation is the result of the nucleation and motion of kinks in the structure, and could prove useful in helping to strengthen and toughen ceramics and other nanocomposites at high temperatures. 相似文献
68.
Frazão C McVey CE Amblar M Barbas A Vonrhein C Arraiano CM Carrondo MA 《Nature》2006,443(7107):110-114
RNA degradation is a determining factor in the control of gene expression. The maturation, turnover and quality control of RNA is performed by many different classes of ribonucleases. Ribonuclease II (RNase II) is a major exoribonuclease that intervenes in all of these fundamental processes; it can act independently or as a component of the exosome, an essential RNA-degrading multiprotein complex. RNase II-like enzymes are found in all three kingdoms of life, but there are no structural data for any of the proteins of this family. Here we report the X-ray crystallographic structures of both the ligand-free (at 2.44 A resolution) and RNA-bound (at 2.74 A resolution) forms of Escherichia coli RNase II. In contrast to sequence predictions, the structures show that RNase II is organized into four domains: two cold-shock domains, one RNB catalytic domain, which has an unprecedented alphabeta-fold, and one S1 domain. The enzyme establishes contacts with RNA in two distinct regions, the 'anchor' and the 'catalytic' regions, which act synergistically to provide catalysis. The active site is buried within the RNB catalytic domain, in a pocket formed by four conserved sequence motifs. The structure shows that the catalytic pocket is only accessible to single-stranded RNA, and explains the specificity for RNA versus DNA cleavage. It also explains the dynamic mechanism of RNA degradation by providing the structural basis for RNA translocation and enzyme processivity. We propose a reaction mechanism for exonucleolytic RNA degradation involving key conserved residues. Our three-dimensional model corroborates all existing biochemical data for RNase II, and elucidates the general basis for RNA degradation. Moreover, it reveals important structural features that can be extrapolated to other members of this family. 相似文献
69.
70.
Laurance WF Oliveira AA Laurance SG Condit R Nascimento HE Sanchez-Thorin AC Lovejoy TE Andrade A D'Angelo S Ribeiro JE Dick CW 《Nature》2004,428(6979):171-175
Amazonian rainforests are some of the most species-rich tree communities on earth. Here we show that, over the past two decades, forests in a central Amazonian landscape have experienced highly nonrandom changes in dynamics and composition. Our analyses are based on a network of 18 permanent plots unaffected by any detectable disturbance. Within these plots, rates of tree mortality, recruitment and growth have increased over time. Of 115 relatively abundant tree genera, 27 changed significantly in population density or basal area--a value nearly 14 times greater than that expected by chance. An independent, eight-year study in nearby forests corroborates these shifts in composition. Contrary to recent predictions, we observed no increase in pioneer trees. However, genera of faster-growing trees, including many canopy and emergent species, are increasing in dominance or density, whereas genera of slower-growing trees, including many subcanopy species, are declining. Rising atmospheric CO2 concentrations may explain these changes, although the effects of this and other large-scale environmental alterations remain uncertain. These compositional changes could have important impacts on the carbon storage, dynamics and biota of Amazonian forests. 相似文献