全文获取类型
收费全文 | 1599篇 |
免费 | 13篇 |
国内免费 | 1篇 |
专业分类
系统科学 | 23篇 |
理论与方法论 | 10篇 |
现状及发展 | 882篇 |
研究方法 | 151篇 |
综合类 | 539篇 |
自然研究 | 8篇 |
出版年
2018年 | 25篇 |
2017年 | 17篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 22篇 |
2013年 | 19篇 |
2012年 | 60篇 |
2011年 | 74篇 |
2010年 | 39篇 |
2009年 | 18篇 |
2008年 | 86篇 |
2007年 | 72篇 |
2006年 | 82篇 |
2005年 | 68篇 |
2004年 | 60篇 |
2003年 | 59篇 |
2002年 | 54篇 |
2001年 | 35篇 |
2000年 | 49篇 |
1999年 | 29篇 |
1996年 | 12篇 |
1994年 | 19篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1987年 | 10篇 |
1985年 | 13篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 28篇 |
1979年 | 17篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 20篇 |
1975年 | 21篇 |
1973年 | 28篇 |
1972年 | 28篇 |
1971年 | 34篇 |
1970年 | 31篇 |
1969年 | 26篇 |
1968年 | 30篇 |
1967年 | 24篇 |
1966年 | 31篇 |
1965年 | 22篇 |
1964年 | 16篇 |
1963年 | 15篇 |
1961年 | 13篇 |
1956年 | 12篇 |
1947年 | 11篇 |
排序方式: 共有1613条查询结果,搜索用时 15 毫秒
151.
152.
Superconductivity at 52 K in hole-doped C60 总被引:4,自引:0,他引:4
Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60--yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice. 相似文献
153.
154.
Phylogenetic position of sponges in early metazoan evolution and bionic applications of siliceous sponge spicules 总被引:1,自引:0,他引:1
Werner E G MULLER WANG XiaoHong ZENG LingSen Heinz C SCHRODER 《科学通报(英文版)》2007,52(22):3029-3040
Sponges are the oldest and the simplest but not primitive multicellular animals. They represent the earliest evolutionary metazoan phylum still extant. It was a long and painful scientific process to posi-tion the most enigmatic and mysterious metazoan,the Porifera,into their correct phylogenetic place among the eukaryotes in general and multicellular animals in particular. As living fossils,sponges provide the best evidence for the early evolution of Metazoa. More recently,interest has been focused on the bionic applications of sponges' siliceous spicules,after the discovery of their unique structure and high fiber performance. In this review,the emergence of sponges,evolutionary novelties found in sponges,and the phylogenetic position of sponges in early metazoan evolution are highlighted. In ad-dition,the present state of knowledge on silicatein-mediated "biosilica" formation in marine sponges,including the involvement of other molecules in silica metabolism and their potential application in nanobiotechnology and medicine,is given. 相似文献
155.
156.
Summary In spring, the landing rate of flying European pine shoot beetles,Tomicus piniperda L., on injured Scots pine diminishes as colonization continues. This is due to olfactory cues that indicate progressive host degradation. Verbenone was shown to play a role in the beetle's recognition of this unsuitability of a formerly suitable host, since the compound was increasingly released from colonized tree sections as they aged, but not from uninfested sections. Also, the release of verbenone at natural rates in the forest inhibited the attraction of beetles to host monoterpenes. 相似文献
157.
K. Hölzle 《Cellular and molecular life sciences : CMLS》1947,3(4):149-151
Summary It has been found that cyclisation of -halogenated -naphthalene-thioglycolic acids can occur in -position with the elimination of the halogen. The influence of the temperature and of a further substituent (halogen) in the second nucleus are discussed. 相似文献
158.
G. Görgen C. Frößl W. Boland K. Dettner 《Cellular and molecular life sciences : CMLS》1990,46(7):700-704
Summary The terminally unsaturated hydrocarbons of the defensive secretion ofTribolium confusum are biosynthesized from fatty acids by oxidative decarboxylation. The process involves an enantiospecific cleavage of the C–H bond of thepro-(S) C(3)–H atom and simultaneous decarboxylation of the acid into an 1-alkene and carbon dioxide via ananti-periplanar transition state geometry (anti-elimination). The stereochemistry of this biotranformation is identical in all respects with the same reaction in higher plants. The mechanism seems to be of general importance for the biosynthesis of many vinylic substructures of natural products from oxygen-containing precursors. 相似文献
159.
160.
YidC mediates membrane protein insertion in bacteria 总被引:13,自引:0,他引:13
Samuelson JC Chen M Jiang F Möller I Wiedmann M Kuhn A Phillips GJ Dalbey RE 《Nature》2000,406(6796):637-641
The basic machinery for the translocation of proteins into or across membranes is remarkably conserved from Escherichia coli to humans. In eukaryotes, proteins are inserted into the endoplasmic reticulum using the signal recognition particle (SRP) and the SRP receptor, as well as the integral membrane Sec61 trimeric complex (composed of alpha, beta and gamma subunits). In bacteria, most proteins are inserted by a related pathway that includes the SRP homologue Ffh, the SRP receptor FtsY, and the SecYEG trimeric complex, where Y and E are related to the Sec61 alpha and gamma subunits, respectively. Proteins in bacteria that exhibit no dependence on the Sec translocase were previously thought to insert into the membrane directly without the aid of a protein machinery. Here we show that membrane insertion of two Sec-independent proteins requires YidC. YidC is essential for E. coli viability and homologues are present in mitochondria and chloroplasts. Depletion of YidC also interferes with insertion of Sec-dependent membrane proteins, but it has only a minor effect on the export of secretory proteins. These results provide evidence for an additional component of the translocation machinery that is specialized for the integration of membrane proteins. 相似文献