全文获取类型
收费全文 | 59712篇 |
免费 | 266篇 |
国内免费 | 543篇 |
专业分类
系统科学 | 1238篇 |
丛书文集 | 566篇 |
教育与普及 | 285篇 |
理论与方法论 | 530篇 |
现状及发展 | 32867篇 |
研究方法 | 880篇 |
综合类 | 21966篇 |
自然研究 | 2189篇 |
出版年
2013年 | 803篇 |
2012年 | 621篇 |
2011年 | 2666篇 |
2009年 | 597篇 |
2008年 | 823篇 |
2007年 | 936篇 |
2006年 | 1048篇 |
2005年 | 1198篇 |
2004年 | 2294篇 |
2003年 | 1925篇 |
2002年 | 1574篇 |
2001年 | 1539篇 |
2000年 | 1098篇 |
1999年 | 1030篇 |
1998年 | 644篇 |
1997年 | 773篇 |
1996年 | 532篇 |
1994年 | 684篇 |
1993年 | 695篇 |
1992年 | 967篇 |
1991年 | 858篇 |
1990年 | 941篇 |
1989年 | 731篇 |
1988年 | 727篇 |
1987年 | 726篇 |
1986年 | 772篇 |
1985年 | 988篇 |
1984年 | 835篇 |
1983年 | 720篇 |
1982年 | 811篇 |
1981年 | 848篇 |
1980年 | 961篇 |
1979年 | 1511篇 |
1978年 | 1355篇 |
1977年 | 1323篇 |
1976年 | 1176篇 |
1975年 | 1187篇 |
1974年 | 1120篇 |
1973年 | 1323篇 |
1972年 | 1400篇 |
1971年 | 1445篇 |
1970年 | 1670篇 |
1969年 | 1441篇 |
1968年 | 1335篇 |
1967年 | 1253篇 |
1966年 | 1076篇 |
1965年 | 850篇 |
1959年 | 519篇 |
1958年 | 679篇 |
1957年 | 574篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Grenvillian orogeny in the Southern Cathaysia Block: Constraints from U-Pb ages and Lu-Hf isotopes in zircon from metamorphic basement 总被引:2,自引:1,他引:2
LiJuan Wang JinHai Yu S.Y. O’Reilly W.L. Griffin Tao Sun ZhenYang Wei ShaoYong Jiang LiangShu Shu 《科学通报(英文版)》2008,53(19):3037-3050
Metamorphic basement rocks in the Cathaysia Block are composed mainly of meta-sediments with different ages. New zircon U-Pb geochronological results from the meta-sedimentary rocks exposed in the Zengcheng and Hezi areas, southern Cathaysia Block, show that they consist dominantly of early Neoproterozoic (1.0-0.9 Ga) materials with minor Paleo- to Mesoproterozoic and late Neoproterozoic (0.8-0.6 Ga) components, suggesting that the detritus mostly come from a Grenvillian orogen. The youngest detrital zircon ages place a constraint on the deposition time of these sediments in Late Neoproterozoic. Zircon Hf isotopic compositions indicate that the Grenvillian zircons were derived from the reworking of Mesoproterozoic arc magmatic rocks and Paleoproterozoic continental crust, implying an arc-continent collisional setting. Single-peak age spectra and the presence of abundant euhedral Grenvillian zircons suggest that the sedimentary provenance is not far away from the sample location. Thus, the Grenvillian orogen probably preexisted along the southern margin of the Cathaysia Block, or very close to the south. Similarity in the ages of Grenvillian orogeny and the influence of the assembly of Gondwana in South China with India and East Antarctic are discussed, with suggestion that South China was more likely linked with the India-East Antarctica continents in Early Neoproterozoic rather than between western Laurentia and eastern Australia. 相似文献
992.
从“十四五”保护与发展的生态关系角度,探讨了生态保护与绿色发展的十大生态关系,阐述了生态保护和绿色发展中的一系列有关生态关系的生态智慧及生态技术,并对长江、黄河等流域的生态保护和修复提出了相关建议。 相似文献
993.
金属基复合材料界面层阻尼功能研究 总被引:4,自引:0,他引:4
采用CVD技术制备了具有不同界面层的C-f/Al金属基复合材料,获得了一种界面层阻尼功能设计的新方法. 研究发现具有特殊界面层的C-f/Al复合材料的抗拉强度、弹性模量和阻尼性能比无界面层时都有明显增加,并且不同界面层的效果不同. 碳层对复合材料阻尼性能的提高效果最大,硅层的提高效果不如碳层,碳硅混合层的效果居中. 涂层的厚度也影响了阻尼提高的效果,较厚的碳层效果更好,这是由于提高了复合材料的阻尼应变振幅效应而产生的. 研究认为发生在界面层的微滑移是其主要的阻尼机制. 相似文献
994.
OH自由基与CS2反应机理的理论研究 总被引:4,自引:0,他引:4
用量子化学密度泛函理论(DFT)和G3方法,对OH自由基与CS2的反应进行了研究,在B3LYP/6-31G*,B3LYP/6-311 G(3df,3pd)G3计算水平上,优化了反应势能面上各驻点的几何构型,并对它们进行了内禀反应坐标(IRC)计算和振动分析,以确定中间体和过渡态的真实性,研究结果表明:OH自由基与CS2反应的产物为COS和SH,计算结果与实验结果一致。 相似文献
995.
一种新的图象分割方法 总被引:1,自引:0,他引:1
李小文 《华南师范大学学报(自然科学版)》1997,(3):1-19
大多数图象处理系统都需将目标从背景中分离出来。本文介绍一种新的自适应门限图象分割方法。门限曲面通过梯度值较大的象素点即可能的边缘点灰度值内插得到。 相似文献
996.
Rakesh K. Sahoo Arya Das Saurabh Singh Damin Lee Saroj K. Singh Rajaram S. Mane Je Moon Yun Kwang Ho Kim 《自然科学进展(英文版)》2019,29(4):410-415
A 3D porous carbon-manganese oxide ([email protected]) nanocomposite is successfully synthesized via a thermal plasma deposition method. The chemical bonds and compositions, phase structures, surface morphologies, etc. of as-obtained [email protected] nanocomposite were characterized by the various equipment, such as X-ray diffractometer, X-ray photoelectron spectroscopy, and electron microscopes. The electrochemical performances of the [email protected] nanocomposite electrode showed a specific capacitance of 780 F g?1 at a current density of 2 A g?1 and a capacitance retention rate of 99% after 5000 charge-discharge cycles at a high current density of 10 A g?1. These excellent capacitive performances may be attributed to the encapsulation of MnO nanoparticles by porous carbon sheets in the [email protected] MnO nanocomposite structure. It is believed that the carbon-encapsulated MnO nanoparticles can be protected from a volume deformation during the charge adsorption/desorption cycle and can be electrically improved by the encapsulated carbon sheets, resulting in better overall capacitive performance. In addition, this study also demonstrates the practical applicability by assembling a supercapacitor using the as-obtained [email protected] nanocomposite to glow a light emitting diode. 相似文献
997.
998.
J. L. Heilbron 《Annals of science》2019,76(3-4):387-389
999.
Luiz A. Rocha Joseph D. DiBattista Tane H. Sinclair-Taylor Michael L. Berumen 《Journal of Natural History》2020,54(15-16):1019-1023
ABSTRACT Here we document three cases of mimicry in coral reef fishes not previously reported in the literature involving two groupers (Epinephelus leucogrammicus and Plectropomus marisrubri) and a soapfish (Diploprion drachi) as mimics, and two wrasses (Larabicus quadrilineatus and Cheilinus quinquecinctus) and a blenny (Meiacanthus nigrolineatus) as models. All three cases are of aggressive mimicry, with a predatory species mimicking a harmless one, and in one of the cases, the mimicry is also Müllerian, where both the predator and harmless species are unpalatable. 相似文献
1000.
Programmable and autonomous computing machine made of biomolecules. 总被引:42,自引:0,他引:42
Devices that convert information from one form into another according to a definite procedure are known as automata. One such hypothetical device is the universal Turing machine, which stimulated work leading to the development of modern computers. The Turing machine and its special cases, including finite automata, operate by scanning a data tape, whose striking analogy to information-encoding biopolymers inspired several designs for molecular DNA computers. Laboratory-scale computing using DNA and human-assisted protocols has been demonstrated, but the realization of computing devices operating autonomously on the molecular scale remains rare. Here we describe a programmable finite automaton comprising DNA and DNA-manipulating enzymes that solves computational problems autonomously. The automaton's hardware consists of a restriction nuclease and ligase, the software and input are encoded by double-stranded DNA, and programming amounts to choosing appropriate software molecules. Upon mixing solutions containing these components, the automaton processes the input molecule via a cascade of restriction, hybridization and ligation cycles, producing a detectable output molecule that encodes the automaton's final state, and thus the computational result. In our implementation 1012 automata sharing the same software run independently and in parallel on inputs (which could, in principle, be distinct) in 120 microl solution at room temperature at a combined rate of 109 transitions per second with a transition fidelity greater than 99.8%, consuming less than 10-10 W. 相似文献