首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   15322篇
  免费   53篇
  国内免费   92篇
系统科学   96篇
丛书文集   110篇
教育与普及   95篇
理论与方法论   80篇
现状及发展   6155篇
研究方法   664篇
综合类   7987篇
自然研究   280篇
  2012年   222篇
  2011年   554篇
  2010年   109篇
  2009年   155篇
  2008年   257篇
  2007年   304篇
  2006年   281篇
  2005年   328篇
  2004年   381篇
  2003年   293篇
  2002年   294篇
  2001年   480篇
  2000年   484篇
  1999年   343篇
  1992年   298篇
  1991年   235篇
  1990年   254篇
  1989年   240篇
  1988年   249篇
  1987年   260篇
  1986年   227篇
  1985年   338篇
  1984年   251篇
  1983年   198篇
  1982年   174篇
  1981年   169篇
  1980年   211篇
  1979年   464篇
  1978年   381篇
  1977年   338篇
  1976年   301篇
  1975年   306篇
  1974年   398篇
  1973年   374篇
  1972年   324篇
  1971年   442篇
  1970年   566篇
  1969年   421篇
  1968年   398篇
  1967年   398篇
  1966年   425篇
  1965年   276篇
  1964年   104篇
  1959年   132篇
  1958年   250篇
  1957年   146篇
  1956年   155篇
  1955年   118篇
  1954年   99篇
  1948年   135篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
191.
192.
193.
正Published online:14 March 2014óScience China Press and Springer-Verlag Berlin Heidelberg 2014Erratum to:Chin.Sci.Bull.(2014)59(5–6):528–532DOI 10.1007/s11434-013-0060-1In the original publication of this paper,the first name and the last name of the first author has been documented  相似文献   
194.
The positive impacts of managing projects as a portfolio are quantified by comparing the value of the integrated risk of a project portfolio and the aggregation of single project risks implemented separately. Firstly, the integrated risk is defined by proposing risky events based on set theory. Secondly, as projects interact with each other in a project portfolio, the integrated risk is evaluated by using a Bayesian network structure learning algorithm to construct an interdependent network of risks. Finally, the integrated risk of a practical case is assessed using this method, and the results show that the proposed method is an effective tool for calculating the extent of risk reduction of implementing a project portfolio and identifying the most risky project, so as to assist companies in making comprehensive decisions in the phase of portfolio selection and portfolio controlling.  相似文献   
195.
196.
On-chip natural assembly of silicon photonic bandgap crystals.   总被引:20,自引:0,他引:20  
Y A Vlasov  X Z Bo  J C Sturm  D J Norris 《Nature》2001,414(6861):289-293
Photonic bandgap crystals can reflect light for any direction of propagation in specific wavelength ranges. This property, which can be used to confine, manipulate and guide photons, should allow the creation of all-optical integrated circuits. To achieve this goal, conventional semiconductor nanofabrication techniques have been adapted to make photonic crystals. A potentially simpler and cheaper approach for creating three-dimensional periodic structures is the natural assembly of colloidal microspheres. However, this approach yields irregular, polycrystalline photonic crystals that are difficult to incorporate into a device. More importantly, it leads to many structural defects that can destroy the photonic bandgap. Here we show that by assembling a thin layer of colloidal spheres on a silicon substrate, we can obtain planar, single-crystalline silicon photonic crystals that have defect densities sufficiently low that the bandgap survives. As expected from theory, we observe unity reflectance in two crystalline directions of our photonic crystals around a wavelength of 1.3 micrometres. We also show that additional fabrication steps, intentional doping and patterning, can be performed, so demonstrating the potential for specific device applications.  相似文献   
197.
Programmable and autonomous computing machine made of biomolecules.   总被引:42,自引:0,他引:42  
Y Benenson  T Paz-Elizur  R Adar  E Keinan  Z Livneh  E Shapiro 《Nature》2001,414(6862):430-434
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   
198.
199.
South-polar features on Venus similar to those near the north pole   总被引:1,自引:0,他引:1  
Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.  相似文献   
200.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号