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   516417篇
  免费   57830篇
  国内免费   775篇
  2021年   4591篇
  2018年   5360篇
  2017年   5092篇
  2016年   7507篇
  2015年   11273篇
  2014年   12991篇
  2013年   17048篇
  2012年   20795篇
  2011年   21183篇
  2010年   13961篇
  2009年   12700篇
  2008年   18403篇
  2007年   18846篇
  2006年   17860篇
  2005年   17043篇
  2004年   17019篇
  2003年   15991篇
  2002年   15384篇
  2001年   19679篇
  2000年   19399篇
  1999年   15917篇
  1998年   6697篇
  1997年   6342篇
  1996年   5718篇
  1995年   5705篇
  1994年   5384篇
  1993年   5438篇
  1992年   12965篇
  1991年   12898篇
  1990年   12600篇
  1989年   12020篇
  1988年   11205篇
  1987年   10504篇
  1986年   10006篇
  1985年   9850篇
  1984年   8330篇
  1983年   7190篇
  1982年   5589篇
  1981年   5048篇
  1980年   4685篇
  1979年   7608篇
  1978年   6239篇
  1977年   5545篇
  1976年   5194篇
  1975年   5951篇
  1974年   6528篇
  1973年   6252篇
  1972年   5563篇
  1971年   5164篇
  1970年   4345篇
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
31.
The content of neurospecific proteins S-100, GFA and D2 was measured in malignant cerebral tumors by electrophoresis with the use of monospecific antisera. Concomitant measurement of proteins S-100 and GFA is a more reliable diagnostic criterion as to the tumor histogenesis than study of each protein alone. D2 protein appeared to be the most stable specific marker.  相似文献   
32.
33.
Zooplankton data collected during September 1995 in the NorthWest Atlantic at 4139'N, 4958'W (the location of the siteof the ‘Titanic’ wreck) were analysed. The regioninvestigated was characterized by a very sharp frontal zonebetween the Gulf Stream and the main stream of the LabradorCurrent. The total plankton biomass in the water column wasvery high. The macroplankton biomass values below the 600 mlayer were significantly higher as compared with the similarvalues measured before in other productive boreal regions ofthe Atlantic and Pacific oceans. A lot of dead mesoplanktonanimals occurred in the deep layers. The reason was that thecold-water mesoplankton advected by the Labrador Current diedoff intensively within the deep layers of the frontal zone andwere used as a food resource by the macroplankton carnivoresand scavengers that were very abundant there.  相似文献   
34.
The goal of this study was to develop a new implantable transducer for measuring anterior cruciate ligament (ACL) graft tension postoperatively in patients who have undergone ACL reconstructive surgery. A unique approach was taken of integrating the transducer into a femoral fixation device. To devise a practical in vivo calibration protocol for the fixation device transducer (FDT), several hypotheses were investigated: (1) The use of a cable versus the actual graft as the means for applying load to the FDT during calibration has no significant effect on the accuracy of the FDT tension measurements; (2) the number of flexion angles at which the device is calibrated has no significant effect on the accuracy of the FDT measurements; (3) the friction between the graft and femoral tunnel has no significant effect on measurement accuracy. To provide data for testing these hypotheses, the FDT was first calibrated with both a cable and a graft over the full range of flexion. Then graft tension was measured simultaneously with both the FDT on the femoral side and load cells, which were connected to the graft on the tibial side, as five cadaver knees were loaded externally. Measurements were made with both standard and overdrilled tunnels. The error in the FDT tension measurements was the difference between the graft tension measured by the FDT and the load cells. Results of the statistical analyses showed that neither the means of applying the calibration load, the number of flexion angles used for calibration, nor the tunnel size had a significant effect on the accuracy of the FDT. Thus a cable may be used instead of the graft to transmit loads to the FDT during calibration, thus simplifying the procedure. Accurate calibration requires data from just three flexion angles of 0, 45, and 90 deg and a curve fit to obtain a calibration curve over a continuous range of flexion within the limits of this angle group. Since friction did not adversely affect the measurement accuracy of the FDT, the femoral tunnel can be drilled to match the diameter of the graft and does not need to be overdrilled. Following these procedures, the error in measuring graft tension with the FDT averages less than 10 percent relative to a full-scale load of 257 N.  相似文献   
35.
36.
37.
38.
39.
Besides vobtusine and vobtusine-lactone, deoxyvobtusine was isolated from the leaves of Voacanga grandifolia (Miq. Rolfe. Spectral and chemical evi  相似文献   
40.
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