全文获取类型
收费全文 | 927篇 |
免费 | 21篇 |
国内免费 | 34篇 |
专业分类
系统科学 | 27篇 |
丛书文集 | 31篇 |
教育与普及 | 43篇 |
理论与方法论 | 8篇 |
现状及发展 | 21篇 |
研究方法 | 54篇 |
综合类 | 798篇 |
出版年
2024年 | 3篇 |
2023年 | 14篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 11篇 |
2019年 | 16篇 |
2018年 | 13篇 |
2017年 | 16篇 |
2016年 | 12篇 |
2015年 | 26篇 |
2014年 | 24篇 |
2013年 | 25篇 |
2012年 | 55篇 |
2011年 | 60篇 |
2010年 | 43篇 |
2009年 | 57篇 |
2008年 | 75篇 |
2007年 | 59篇 |
2006年 | 61篇 |
2005年 | 64篇 |
2004年 | 45篇 |
2003年 | 45篇 |
2002年 | 33篇 |
2001年 | 39篇 |
2000年 | 28篇 |
1999年 | 12篇 |
1998年 | 21篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1979年 | 1篇 |
1971年 | 4篇 |
1970年 | 6篇 |
1959年 | 6篇 |
1958年 | 5篇 |
1957年 | 2篇 |
1955年 | 2篇 |
1954年 | 1篇 |
1948年 | 5篇 |
1947年 | 1篇 |
排序方式: 共有982条查询结果,搜索用时 607 毫秒
61.
62.
The molecular mechanisms of congenital hypofibrinogenaemia 总被引:7,自引:0,他引:7
Maghzal GJ Brennan SO Homer VM George PM 《Cellular and molecular life sciences : CMLS》2004,61(12):1427-1438
Congenital hypofibrinogenaemia is characterized by abnormally low levels of fibrinogen and is usually caused by heterozygous mutations in the fibrinogen chain genes (, and ). However, it does not usually result in a clinically significant condition unless inherited in a homozygous or compound heterozygous state, where it results in a severe bleeding disorder, afibrinogenaemia. Various protein and expression studies have improved our understanding of how mutations causing hypo- and afibrinogenaemia affect secretion of the mature fibrinogen molecule from the hepatocyte. Some mutations can perturb chain assembly as in the 153 Cys Arg case, while others such as the B Leu Arg and the B414 Gly Ser mutations allow intracellular hexamer assembly but inhibit protein secretion. An interesting group of mutations, such as 284 Gly Arg and 375 Arg Trp, not only cause hypofibrinogenaemia but are also associated with liver disease. The nonexpression of these variant chains in plasma fibrinogen is due to retention in the endoplasmic reticulum, which in turn leads to hypofibrinogenaemia.Received 17 December 2003; received after revision 19 January 2004; accepted 21 January 2004 相似文献
63.
Chemical investigation of hassium (element 108) 总被引:5,自引:0,他引:5
Düllmann ChE Brüchle W Dressler R Eberhardt K Eichler B Eichler R Gäggeler HW Ginter TN Glaus F Gregorich KE Hoffman DC Jäger E Jost DT Kirbach UW Lee DM Nitsche H Patin JB Pershina V Piguet D Qin Z Schädel M Schausten B Schimpf E Schött HJ Soverna S Sudowe R Thörle P Timokhin SN Trautmann N Türler A Vahle A Wirth G Yakushev AB Zielinski PM 《Nature》2002,418(6900):859-862
The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes (269)Hs (refs 8, 9) and (270)Hs (ref. 10) in the fusion reaction between (26)Mg and (248)Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO(4), for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO(4). These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table. 相似文献
64.
Kapfhamer D Valladares O Sun Y Nolan PM Rux JJ Arnold SE Veasey SC Bućan M 《Nature genetics》2002,32(2):290-295
Rab3a is the most abundant Rab (ras-associated binding) protein in the brain and has a regulatory role in synaptic vesicle trafficking. Mice with a targeted loss-of-function mutation in Rab3a have defects in Ca(2+)-dependent synaptic transmission: the number of vesicles released in response to an action potential is greater than in wildtype mice, resulting in greater synaptic depression and the abolishment of CA3 mossy-fiber long term potentiation. The effect of these changes on behavior is unknown. In a screen for mouse mutants with abnormal rest-activity and sleep patterns, we identified a semidominant mutation, called earlybird, that shortens the circadian period of locomotor activity. Sequence analysis of Rab3a identified a point mutation in the conserved amino acid (Asp77Gly) within the GTP-binding domain of this protein in earlybird mutants, resulting in significantly reduced levels of Rab3a protein. Phenotypic assessment of earlybird mice and a null allele of Rab3a revealed anomalies in circadian period and sleep homeostasis, providing evidence that Rab3a-mediated synaptic transmission is involved in these behaviors. 相似文献
65.
66.
67.
Nanoscale structures need to be arranged into well-defined configurations in order to build integrated systems. Here we use a chemical-vapour deposition method with gas-phase catalyst delivery to direct the assembly of carbon nanotubes in a variety of predetermined orientations onto silicon/silica substrates, building them into one-, two- and three-dimensional arrangements. The preference of nanotubes to grow selectively on and normal to silica surfaces forces them to inherit the lithographically machined template topography of their substrates, allowing the sites of nucleation and the direction of growth to be controlled. 相似文献
68.
69.
Normal faulting in central Tibet since at least 13.5 Myr ago 总被引:16,自引:0,他引:16
Blisniuk PM Hacker BR Glodny J Ratschbacher L Bi S Wu Z McWilliams MO Calvert A 《Nature》2001,412(6847):628-632
Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb-Sr and 40Ar-39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle. 相似文献
70.
Pathogenic enterococci: new developments in the 21st century 总被引:8,自引:0,他引:8
Tendolkar PM Baghdayan AS Shankar N 《Cellular and molecular life sciences : CMLS》2003,60(12):2622-2636
Enterococci, traditionally viewed as Gram-positive commensal bacteria inhabiting the alimentary canals of humans and animals, are now acknowledged to be organisms capable of causing life-threatening infections in humans, especially in the nosocomial environment. The existence of enterococci in such a dual role is facilitated, at least in part, by its intrinsic and acquired resistance to virtually all antibiotics currently in use. Beginning with the initial identification of a streptococci of fecal origin in the late 19th century, enterococci have been studied for over a century now. A number of comprehensive reviews during this time have addressed various aspects of enterococci, including classification, biology, virulence, antibiotic resistance and so on. This review specifically addresses the important advances in the field of enterococcal research that have occurred since the beginning of the 21st century. Most notable among these developments have been the insights into enterococcal genomes and pathogenicity.Received 10 April 2003; received after revision 31 May 2003; accepted 3 June 2003 相似文献