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   30086篇
  免费   79篇
  国内免费   110篇
系统科学   118篇
丛书文集   275篇
教育与普及   89篇
理论与方法论   102篇
现状及发展   12608篇
研究方法   1168篇
综合类   15457篇
自然研究   458篇
  2013年   257篇
  2012年   372篇
  2011年   828篇
  2008年   458篇
  2007年   549篇
  2006年   538篇
  2005年   525篇
  2004年   550篇
  2003年   508篇
  2002年   509篇
  2001年   1018篇
  2000年   1017篇
  1999年   597篇
  1994年   343篇
  1992年   532篇
  1991年   452篇
  1990年   507篇
  1989年   501篇
  1988年   482篇
  1987年   483篇
  1986年   519篇
  1985年   594篇
  1984年   450篇
  1983年   446篇
  1982年   375篇
  1981年   363篇
  1980年   414篇
  1979年   947篇
  1978年   753篇
  1977年   687篇
  1976年   576篇
  1975年   673篇
  1974年   901篇
  1973年   739篇
  1972年   747篇
  1971年   971篇
  1970年   1135篇
  1969年   881篇
  1968年   907篇
  1967年   764篇
  1966年   730篇
  1965年   515篇
  1964年   164篇
  1959年   284篇
  1958年   511篇
  1957年   391篇
  1956年   293篇
  1955年   297篇
  1954年   284篇
  1948年   246篇
排序方式: 共有10000条查询结果,搜索用时 609 毫秒
331.
332.
333.
Vascular development is a dynamic process that relies on the coordinated expression of numerous genes, but the factors that regulate gene expression during blood vessel development are not well defined. ATP-dependent chromatin-remodeling complexes are gaining attention for their specific temporal and spatial effects on gene expression during vascular development. Genetic mutations in chromatin-remodeling complex subunits are revealing roles for the complexes in vascular signaling pathways at discrete developmental time points. Phenotypic analysis of these models at various stages of vascular development will continue to expand our understanding of how chromatin remodeling impacts new blood vessel growth. Such research could also provide novel therapeutic targets for the treatment of vascular pathologies.  相似文献   
334.
Liquid marbles.   总被引:5,自引:0,他引:5  
P Aussillous  D Quéré 《Nature》2001,411(6840):924-927
The transport of a small amount of liquid on a solid is not a simple process, owing to the nature of the contact between the two phases. Setting a liquid droplet in motion requires non-negligible forces (because the contact-angle hysteresis generates a force opposing the motion), and often results in the deposition of liquid behind the drop. Different methods of levitation-electrostatic, electromagnetic, acoustic, or even simpler aerodynamic techniques-have been proposed to avoid this wetting problem, but all have proved to be rather cumbersome. Here we propose a simple alternative, which consists of encapsulating an aqueous liquid droplet with a hydrophobic powder. The resulting 'liquid marbles' are found to behave like a soft solid, and show dramatically reduced adhesion to a solid surface. As a result, motion can be generated using gravitational, electrical and magnetic fields. Moreover, because the viscous friction associated with motion is very small, we can achieve quick displacements of the droplets without any leaks. All of these features are of potential benefit in microfluidic applications, and also permit the study of a drop in a non-wetting situation-an issue of renewed interest following the recent achievement of super-hydrophobic substrates.  相似文献   
335.
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   
336.
337.
338.
Immune recognition. A new receptor for beta-glucans.   总被引:26,自引:0,他引:26  
G D Brown  S Gordon 《Nature》2001,413(6851):36-37
The carbohydrate polymers known as beta-1,3-d-glucans exert potent effects on the immune system - stimulating antitumour and antimicrobial activity, for example - by binding to receptors on macrophages and other white blood cells and activating them. Although beta-glucans are known to bind to receptors, such as complement receptor 3 (ref. 1), there is evidence that another beta-glucan receptor is present on macrophages. Here we identify this unknown receptor as dectin-1 (ref. 2), a finding that provides new insights into the innate immune recognition of beta-glucans.  相似文献   
339.
S Berlin  H Ellegren 《Nature》2001,413(6851):37-38
We have taken a new approach to test the commonly accepted, but recently questioned, principle of clonal inheritance of vertebrate mitochondrial DNA (mtDNA) by relating its inheritance to a female-specific marker of nuclear DNA. Whereas this is impossible in organisms with male heterogamy (such as mammals), we show here that genealogies of mtDNA and the female-specific W chromosome of a bird species are completely concordant. Our results indicate that inheritance of mtDNA is free of detectable recombination effects over an evolutionary timescale.  相似文献   
340.
Superconductivity in CaCuO2 as a result of field-effect doping.   总被引:2,自引:0,他引:2  
Understanding the doping mechanisms in the simplest superconducting copper oxide-the infinite-layer compound ACuO2 (where A is an alkaline earth metal)-is an excellent way of investigating the pairing mechanism in high-transition-temperature (high-Tc) superconductors more generally. Gate-induced modulation of the carrier concentration to obtain superconductivity is a powerful means of achieving such understanding: it minimizes the effects of potential scattering by impurities, and of structural modifications arising from chemical dopants. Here we report the transport properties of thin films of the infinite-layer compound CaCuO2 using field-effect doping. At high hole- and electron-doping levels, superconductivity is induced in the nominally insulating material. Maximum values of Tc of 89 K and 34 K are observed respectively for hole- and electron-type doping of around 0.15 charge carriers per CuO2. We can explore the whole doping diagram of the CuO2 plane while changing only a single electric parameter, the gate voltage.  相似文献   
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