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   6347篇
  免费   13篇
  国内免费   53篇
系统科学   652篇
丛书文集   593篇
教育与普及   200篇
理论与方法论   11篇
现状及发展   525篇
研究方法   766篇
综合类   3664篇
自然研究   2篇
  2017年   12篇
  2015年   13篇
  2014年   22篇
  2013年   16篇
  2012年   346篇
  2011年   406篇
  2010年   102篇
  2009年   36篇
  2008年   365篇
  2007年   394篇
  2006年   521篇
  2005年   590篇
  2004年   428篇
  2003年   386篇
  2002年   345篇
  2001年   282篇
  2000年   448篇
  1999年   129篇
  1998年   54篇
  1997年   21篇
  1996年   21篇
  1995年   31篇
  1994年   38篇
  1993年   55篇
  1992年   44篇
  1991年   31篇
  1990年   25篇
  1989年   28篇
  1988年   18篇
  1987年   16篇
  1986年   29篇
  1985年   22篇
  1984年   14篇
  1983年   27篇
  1982年   21篇
  1981年   16篇
  1980年   14篇
  1979年   13篇
  1971年   12篇
  1970年   24篇
  1959年   97篇
  1958年   147篇
  1957年   115篇
  1956年   116篇
  1955年   87篇
  1954年   90篇
  1953年   67篇
  1952年   81篇
  1951年   64篇
  1948年   25篇
排序方式: 共有6413条查询结果,搜索用时 0 毫秒
71.
3MC syndrome has been proposed as a unifying term encompassing the overlapping Carnevale, Mingarelli, Malpuech and Michels syndromes. These rare autosomal recessive disorders exhibit a spectrum of developmental features, including characteristic facial dysmorphism, cleft lip and/or palate, craniosynostosis, learning disability and genital, limb and vesicorenal anomalies. Here we studied 11 families with 3MC syndrome and identified two mutated genes, COLEC11 and MASP1, both of which encode proteins in the lectin complement pathway (collectin kidney 1 (CL-K1) and MASP-1 and MASP-3, respectively). CL-K1 is highly expressed in embryonic murine craniofacial cartilage, heart, bronchi, kidney and vertebral bodies. Zebrafish morphants for either gene develop pigmentary defects and severe craniofacial abnormalities. Finally, we show that CL-K1 serves as a guidance cue for neural crest cell migration. Together, these findings demonstrate a role for complement pathway factors in fundamental developmental processes and in the etiology of 3MC syndrome.  相似文献   
72.
Designing fast oxide-ion conductors based on La2Mo2O9   总被引:1,自引:0,他引:1  
Lacorre P  Goutenoire F  Bohnke O  Retoux R  Laligant Y 《Nature》2000,404(6780):856-858
The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite, perovskites, intergrowth perovskite/Bi2O2 layers and pyrochlores. Here we report a family of solid oxides based on the parent compound La2Mo2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors, this material undergoes a structural transition around 580 degrees C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 x 10(-2) S cm(-1) at 800 degrees C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with beta-SnWO4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair--and which has a higher oxidation state--could be used as an original way to design other oxide-ion conductors.  相似文献   
73.
Smaglik P 《Nature》2004,428(6982):583
  相似文献   
74.
In recent years, QoS multicast routing has continued to be a very important research topic in the areas of networks. This paper presents a heuristic algorithm for the QoS multicast routing (HAQMR). This heuristic algorithm deals with delay and bandwidth constraints and has low cost. The HAQMR attempts to significantly reduce the overhead for constructing a multicast tree, the proof for correctness of the HAQMR is given, and the performance of the HAQMR is evaluated by simulations. The study shows that HAQMR provides an available approach to QoS multicast routing.  相似文献   
75.
76.
1 .INTRODUCTIONNewcommunication network si mulations are nee-dedin space based networkto demonstrate ensuingadvances in global operations and distributed re-source management . Theseinclude full-scale si mu-lation of networking, protocols ,information dis-tribution,and display to lead tointegrated demon-strations of full-scale land-,air-,sea-,and space-based network having explicit modelling of net-work dynamics ,inter-satellite links (ISLs) , up-downlinks ( UDLs) ,schedulers ,protocols …  相似文献   
77.
INTRODUCTIONThe micro-glass bead[1-2] is a new kind of silicate material, with good properties of chemical stability, mechanical strength as well as electro-resistance. When its diameter is below 0.08mm and the reflective index is in the range of 1.9~2.1, retro-reflective characteristics can be achieved. When a beam of ray from any direction irradiates the surface of the micro-glass beads, it can be converged at the special reflective layer consisted of focuses of micro-glass beads, owing …  相似文献   
78.
    
Mutations in PRKCSH, encoding the beta-subunit of glucosidase II, an N-linked glycan-processing enzyme in the endoplasmic reticulum (ER), cause autosomal dominant polycystic liver disease. We found that mutations in SEC63, encoding a component of the protein translocation machinery in the ER, also cause this disease. These findings are suggestive of a role for cotranslational protein-processing pathways in maintaining epithelial luminal structure and implicate noncilial ER proteins in human polycystic disease.  相似文献   
79.
80.
Neuenschwander P 《Nature》2004,432(7019):801-802
  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