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   13334篇
  免费   129篇
  国内免费   12篇
教育   8751篇
科学研究   2231篇
各国文化   111篇
体育   972篇
综合类   5篇
文化理论   102篇
信息传播   1303篇
  2021年   114篇
  2020年   170篇
  2019年   284篇
  2018年   334篇
  2017年   333篇
  2016年   271篇
  2015年   195篇
  2014年   270篇
  2013年   2480篇
  2012年   271篇
  2011年   251篇
  2010年   224篇
  2009年   213篇
  2008年   224篇
  2007年   228篇
  2006年   181篇
  2005年   185篇
  2004年   167篇
  2003年   174篇
  2002年   167篇
  2001年   206篇
  2000年   228篇
  1999年   193篇
  1998年   108篇
  1997年   109篇
  1996年   133篇
  1995年   103篇
  1994年   113篇
  1993年   123篇
  1992年   165篇
  1991年   172篇
  1990年   171篇
  1989年   182篇
  1988年   139篇
  1987年   139篇
  1986年   175篇
  1985年   179篇
  1984年   144篇
  1983年   141篇
  1982年   131篇
  1981年   115篇
  1980年   118篇
  1979年   158篇
  1978年   139篇
  1977年   116篇
  1976年   117篇
  1974年   117篇
  1973年   97篇
  1972年   90篇
  1971年   113篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Since its publication in 1967, the estimates of the US Panel on Invention and Innovation (“Charpie” Panel) on the cost structure of technological innovation have been widely quoted and almost universally accepted. However, two later studies, by Edwin Mansfield and by Statistics Canada, have found that R&D accounts for a far greater proportion of total innovation costs than that suggested by the Charpie Panel. The Canadian study may be particularly interesting for medium-sized industrial nations since it is likely that the costs of commercializing the results of R&D are affected by the size of the potential market or field of utilization of the innovation. The results of Mansfield and Statistics Canada imply that governments, in their encouragement of innovation by supporting R&D, have been following a better strategy than perhaps they knew.  相似文献   
992.
993.
994.
Acetaminophen (APAP) a commonly used drug for decrease the fever and pain but is capable to induced hepatotoxicity at over dose. This study was carried out to investigate the effect of APAP on the expression of anti-apoptotic and antioxidative defense genes, and whether aldose reductase over-expressing plasmid capable to protect against APAP-induced oxidative stress and cell death. APAP treatment induced oxidative stress and hepatotoxicity, and significantly increased aldose reductase mRNA and protein expression in mouse hepatocyte (AML-12). Unexpectedly, AML-12 cells over-expressing aldose reductase augmented APAP-induced reduction in cell viability, reactive oxygen species (ROS) production, glutathione (GSH) depletion and glutathione S-transferase A2 expression. Moreover, over-expression of aldose reductase potentiated APAP induced reduction on proliferating cell nuclear antigen, B cell lymphoma-extra large (bcl-xL), catalase, glutathione peroxidase-1 (GPx-1) and abolished APAP-induced B-cell lymphoma 2 (bcl-2) inductions. Further, over-expression of aldose reductase significantly abolished AMP activated protein kinase (AMPK) activity in APAP-treated cells and induced p53 expression. This results demonstrate that APAP induced toxicity in AML-12, increased aldose reductase expression, and over-expression of aldose reductase render this cell more susceptible to APAP induced oxidative stress and cell death, this probably due to inhibition AMPK or bcl-2 activity, or may due to competition between aldose reductase and glutathione reductase for NADPH.  相似文献   
995.
Simple as the outflow clock principle may seem for the measurement of time, its translation into an accurate clock raises formidable technical and conceptual problems. An attempt is made here to reconstruct the Egyptians' consciousness of these problems, and the success of their solutions.  相似文献   
996.
997.
During the period 1952 to 1965, there were strenuous world-wide efforts to produce an automated cotton spinning mill. These attempts, which involved the use of concepts developed outside the textile machinery industry (i.e. automatic transfer systems), were motivated by the need to reduce the very great amount of manual handling of packages between successive operations in the yarn manufacturing process.The case is unusual in that, while many of the developments were ‘technically successful’ in that installations were made which functioned satisfactorily in the mill), they were, without exception, commercially unsuccessful from the point of view of the machinery manufacturer and mill operator. This paper describes the economic forces which led to the attempts at automation, and explains why, after an expenditure in excess of £ 25 mill/on worldwide, commercial success was not achieved.The prime reason for failure lay in minor and major developments of existing individual machines, which had continued independently of automation research, and which had, to a great extent, obviated the economic need for automation. The surprising thing (in retrospect at least), is that, as production rates were increased step by step, development engineers working to produce automated systems saw, in the increasing speeds, only a challenge to their ingenuity — not a disappearance of the need for automatic transfer devices.A secondary reason for failure in the particular case of the Shirley Institute's automated card room sequence lay in the fact that the research was directed by a committee whose terms of reference were limited to pursuance of a defined goal — the development of an automated card room. The committee had no mandate to question the soundness of its objectives, and the work continued until a technically successful system was finally produced.  相似文献   
998.
This paper deals with the experimental aspects of ultrasonic delay lines in liquid media and is a sequel to a previous paper treating the general theory of such devices.Delay devices employing ultrasonic propagation through liquids have proved satisfactory for delay times of the order of 3 milliseconds or less. Such devices can be made broadband and give excellent reproduction of pulse shape. The design considerations for such devices are discussed and illustrated by actual examples.  相似文献   
999.
For some 300 years the modelling of uncertainty in the sciences has been totally dominated by probabilistic methods. Now, following major criticism of the theoretical foundations of probability theory, other approaches to dealing with uncertainty are being explored.  相似文献   
1000.
This paper describes the implementation and evaluation for scaling up a comprehensive early childhood teacher professional development program into 11 communities across 2 years with funding through state legislative actions. The comprehensive program had four major components based on results from a previous multi-condition random assignment study across four states. The previous results demonstrated that the most optimum approach for supporting children??s school readiness included: (1) teacher on-line professional development with facilitation, (2) classroom mentoring, (3) implementation of a research-based curriculum, and (4) technology-driven progress monitoring that informed instruction. The comprehensive professional development program was evaluated in a new state program designed to bring childcare, Head Start, and public school pre-kindergarten together into integrated partnerships. In Year 1, 220 teachers serving 3834 children were randomly assigned to either receive the comprehensive program or not. Teachers who served as controls in Year 1 received the program in Year 2, and those who received the program in Year 1 participated for an additional year in Year 2, allowing for examination of the effects of one versus 2 years of participation. The program improved teachers instructional practices relative to controls, and a second year of participation resulted in greater gains in children??s language and literacy. Results support the need for well-integrated, comprehensive professional development for early childhood educators.  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