全文获取类型
收费全文 | 28862篇 |
免费 | 82篇 |
国内免费 | 158篇 |
专业分类
系统科学 | 139篇 |
丛书文集 | 499篇 |
教育与普及 | 42篇 |
理论与方法论 | 100篇 |
现状及发展 | 13451篇 |
研究方法 | 1278篇 |
综合类 | 13197篇 |
自然研究 | 396篇 |
出版年
2013年 | 269篇 |
2012年 | 411篇 |
2011年 | 809篇 |
2010年 | 168篇 |
2008年 | 523篇 |
2007年 | 576篇 |
2006年 | 576篇 |
2005年 | 535篇 |
2004年 | 538篇 |
2003年 | 498篇 |
2002年 | 499篇 |
2001年 | 936篇 |
2000年 | 865篇 |
1999年 | 616篇 |
1992年 | 595篇 |
1991年 | 414篇 |
1990年 | 486篇 |
1989年 | 494篇 |
1988年 | 460篇 |
1987年 | 546篇 |
1986年 | 474篇 |
1985年 | 596篇 |
1984年 | 484篇 |
1983年 | 364篇 |
1982年 | 340篇 |
1981年 | 367篇 |
1980年 | 462篇 |
1979年 | 889篇 |
1978年 | 757篇 |
1977年 | 740篇 |
1976年 | 611篇 |
1975年 | 632篇 |
1974年 | 845篇 |
1973年 | 756篇 |
1972年 | 779篇 |
1971年 | 840篇 |
1970年 | 1073篇 |
1969年 | 813篇 |
1968年 | 820篇 |
1967年 | 795篇 |
1966年 | 681篇 |
1965年 | 472篇 |
1964年 | 156篇 |
1959年 | 249篇 |
1958年 | 440篇 |
1957年 | 292篇 |
1956年 | 259篇 |
1955年 | 246篇 |
1954年 | 239篇 |
1948年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The activation of ubiquitin and related protein modifiers is catalysed by members of the E1 enzyme family that use ATP for the covalent self-attachment of the modifiers to a conserved cysteine. The Escherichia coli proteins MoeB and MoaD are involved in molybdenum cofactor (Moco) biosynthesis, an evolutionarily conserved pathway. The MoeB- and E1-catalysed reactions are mechanistically similar, and despite a lack of sequence similarity, MoaD and ubiquitin display the same fold including a conserved carboxy-terminal Gly-Gly motif. Similar to the E1 enzymes, MoeB activates the C terminus of MoaD to form an acyl-adenylate. Subsequently, a sulphurtransferase converts the MoaD acyl-adenylate to a thiocarboxylate that acts as the sulphur donor during Moco biosynthesis. These findings suggest that ubiquitin and E1 are derived from two ancestral genes closely related to moaD and moeB. Here we present the crystal structures of the MoeB-MoaD complex in its apo, ATP-bound, and MoaD-adenylate forms, and highlight the functional similarities between the MoeB- and E1-substrate complexes. These structures provide a molecular framework for understanding the activation of ubiquitin, Rub, SUMO and the sulphur incorporation step during Moco and thiamine biosynthesis. 相似文献
993.
994.
Horses damp the spring in their step. 总被引:6,自引:0,他引:6
The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle-tendon units.These make the legs act like a child's pogo stick that is tuned to stretch and recoil at 2.5 strides per second. This mechanism is optimized by unique musculoskeletal adaptations: the digital flexor muscles have extremely short fibres and significant passive properties, whereas the tendons are very long and span several joints. Length change occurs by a stretching of the spring-like digital flexor tendons rather than through energetically expensive length changes in the muscle. Despite being apparently redundant for such a mechanism, the muscle fibres in the digital flexors are well developed. Here we show that the mechanical arrangement of the elastic leg permits it to vibrate at a higher frequency of 30-40 Hz that could cause fatigue damage to tendon and bone. Furthermore, we show that the digital flexor muscles have minimal ability to contribute to or regulate significantly the 2.5-Hz cycle of movement, but are ideally arranged to damp these high-frequency oscillations in the limb. 相似文献
995.
Axial volcano, which is located near the intersection of the Juan de Fuca ridge and the Cobb-Eickelberg seamount chain beneath the northeast Pacific Ocean, is a locus of volcanic activity thought to be associated with the Cobb hotspot. The volcano rises 700 metres above the ridge, has substantial rift zones extending about 50 kilometres to the north and south, and has erupted as recently as 1998 (ref. 2). Here we present seismological data that constrain the three-dimensional velocity structure beneath the volcano. We image a large low-velocity zone in the crust, consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust, and estimate the total magma volume in the system to be between 5 and 21 km3. This volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption (0.1-0.2 km3). We therefore infer that such volcanic events remove only a small portion of the reservoir that they tap, which must accordingly be long-lived compared to the eruption cycle. On the basis of magma flux estimates, we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years. 相似文献
996.
This paper describes the design of a unified framework for a multilingual text-to-speech (TTS) synthesis engine – Crystal. The unified framework defines the common TTS modules for different languages and/or dialects. The interfaces between consecutive modules conform to the speech synthesis markup language (SSML) specification for standardization, interoperability, multilinguality, and extensibility. Detailed module divisions and implementation technologies for the unified framework are introduced, together with possible extensions for the algorithm research and evaluation of the TTS synthesis. Implementation of a mixed-language TTS system for Chinese Putonghua, Chinese Cantonese, and English demonstrates the feasibility of the proposed unified framework. 相似文献
997.
The effect of Si on the austenite stabilization, martensite morphology, and magnetic properties in Fe-26%Ni-x%Si (x=3.5, 5, and 6) alloys have been studied by means of transmission electron microscopy (TEM) and M?ssbauer spectroscopy techniques. TEM observations reveal that the martensite morphology is closely dependent on the Si content. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field, and isomer shift values have been determined by Mssbauer spectroscopy. The M?ssbauer study reveals that the hyperfine magnetic field, the isomer shift values and the volume fraction of martensite decrease with increasing Si content. 相似文献
998.
There is substantial evidence that the martian volatile inventory and climate have changed markedly throughout the planet's history. Clues come from areas as disparate as the history and properties of the deep interior, the composition of the crust and regolith, the morphology of the surface, composition of the present-day atmosphere, and the nature of the interactions between the upper atmosphere and the solar wind. We piece together the relevant observations into a coherent view of the evolution of the martian climate, focusing in particular on the observations that provide the strongest constraints. 相似文献
999.
A large fraction of the mass of the interstellar medium in our Galaxy is in the form of warm (103-104 K) and cool (50-100 K) atomic hydrogen (H i) gas. Cold (10-30 K) regions are thought to be dominated by dense clouds of molecular hydrogen. Cold H i is difficult to observe, and therefore our knowledge of its abundance and distribution in the interstellar medium is poor. The few known clouds of cold H i are much smaller in size and mass than typical molecular clouds. Here we report the discovery that the H i supershell GSH139-03-69 is very cold (10 K). It is about 2 kiloparsecs in size and as massive as the largest molecular complexes. The existence of such an immense structure composed of cold atomic hydrogen in the interstellar medium runs counter to the prevailing view that cold gas resides almost exclusively in clouds dominated by molecular hydrogen. 相似文献
1000.