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   74763篇
  免费   316篇
  国内免费   568篇
系统科学   1287篇
丛书文集   624篇
教育与普及   293篇
理论与方法论   547篇
现状及发展   37136篇
研究方法   1782篇
综合类   31784篇
自然研究   2194篇
  2013年   828篇
  2012年   913篇
  2011年   3075篇
  2009年   650篇
  2008年   1137篇
  2007年   1379篇
  2006年   1375篇
  2005年   1578篇
  2004年   2723篇
  2003年   2282篇
  2002年   2048篇
  2001年   2142篇
  2000年   1639篇
  1999年   1391篇
  1998年   654篇
  1997年   788篇
  1994年   705篇
  1993年   702篇
  1992年   1323篇
  1991年   1089篇
  1990年   1193篇
  1989年   988篇
  1988年   948篇
  1987年   984篇
  1986年   1041篇
  1985年   1246篇
  1984年   1026篇
  1983年   892篇
  1982年   932篇
  1981年   946篇
  1980年   1051篇
  1979年   2009篇
  1978年   1695篇
  1977年   1620篇
  1976年   1426篇
  1975年   1435篇
  1974年   1569篇
  1973年   1615篇
  1972年   1685篇
  1971年   1845篇
  1970年   2203篇
  1969年   1815篇
  1968年   1779篇
  1967年   1709篇
  1966年   1510篇
  1965年   1156篇
  1959年   648篇
  1958年   1024篇
  1957年   762篇
  1956年   640篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
191.
论述了交织编码的工作原理,给出了一种交织扩展Golay码的改进型软件设计方案,用TMS320C25实现了其编译码器并给出了模拟实验结果。实验结果表明,它能准确地纠正不大于72比特位的差错;处理128×128×8比特位的一幅图像数据的运行时间为0.412s.  相似文献   
192.
Summary Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics.  相似文献   
193.
翟展胜 《太原科技》2003,(2):62-62,64
通过分析影响光缆接续质量的因素,从减少本征影响、克服技术因素、优化综合处置、减弱和避开环境影响4方面针对性地提出了实际施工中应采取的施工方法,以控制和减小不利影响,提高光缆工程的接续质量,为建设高质量传输线路提供有力保证。  相似文献   
194.
195.
The genetic engineering of production traits in domestic animals   总被引:1,自引:0,他引:1  
  相似文献   
196.
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。  相似文献   
197.
Abstract: After a recursive multi-step-ahead predictor for nonlinear systems based on local recurrent neural networks isintroduced, an intelligent PID controller is adopted to correct the errors including identified model errors and accumulatederrors produced in the recursive process. Characterized by predictive control, this method can achieve a good controlaccuracy and has good robustness. A simulation study shows that this control algorithm is very effective.  相似文献   
198.
Dynamic Characteristics of Inter-Satellite Links in LEO Networks   总被引:1,自引:0,他引:1  
To establish an efficient inter-satellite link (ISL) in an LEO network, the effect of geometric characteristics of ISL on the ISLs and the devices on the LEO satellite should be examined. Because of the continuous movement of the LEO satellite, the time-varying behaviours of the ISL's geometric charactersistics continuously change with the changes of the satellite's position on the orbit. These dynamic geometric characteristics of the ISLs are important for ISL's performance analyzing and the design of the devices on the LEO satellite. This paper describes dynamic geometric characteristics of ISL, analyzes the impact of these regulations on the tracking system of the satellite's antenna and the power adjusting system of the satellite's transmitter, with the Iridium system as an example.  相似文献   
199.
H Schorle  T Holtschke  T Hünig  A Schimpl  I Horak 《Nature》1991,352(6336):621-624
Interleukin-2 (IL-2) is a lymphocytotropic hormone which is thought to have a key role in the immune response of mammalian cells. It is produced by a subpopulation of activated T-lymphocytes and acts in vitro as the principal auto- and paracrine T-cell growth factor (for reviews see refs 1-3). IL-2 is, however, not the sole T-cell growth factor, nor does it act exclusively on T cells, also promoting growth of NK cells and differentiation of B cells. A role for IL-2 in T-cell development has been postulated but remains controversial. Here we test the requirement for IL-2 in vivo using IL-2-deficient mice generated by targeted recombination. We find that mice homozygous for the IL-2 gene mutation are normal with regard to thymocyte and peripheral T-cell subset composition, but that a dysregulation of the immune system is manifested by reduced polyclonal in vitro T-cell responses and by dramatic changes in the isotype levels of serum immunoglobulins.  相似文献   
200.
It has long been realized that the problem of radar imaging is a special case of image reconstruction in which the data are incomplete and noisy. In other fields, iterative reconstruction algorithms have been used successfully to improve the image quality. This paper studies the application of iterative algorithms in radar imaging. A discrete model is first derived, and the iterative algorithms are then adapted to radar imaging. Although such algorithms are usually time consuming, this paper shows that, if the algorithms are appropriately simplified, it is possible to realize them even in real time. The efficiency of iterative algorithms is shown through computer simulations.  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