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   21408篇
  免费   1672篇
  国内免费   1530篇
  2024年   38篇
  2023年   220篇
  2022年   598篇
  2021年   1154篇
  2020年   761篇
  2019年   932篇
  2018年   913篇
  2017年   670篇
  2016年   949篇
  2015年   1300篇
  2014年   1571篇
  2013年   1723篇
  2012年   1894篇
  2011年   1761篇
  2010年   1083篇
  2009年   991篇
  2008年   1145篇
  2007年   1020篇
  2006年   875篇
  2005年   748篇
  2004年   579篇
  2003年   550篇
  2002年   458篇
  2001年   326篇
  2000年   327篇
  1999年   330篇
  1998年   190篇
  1997年   174篇
  1996年   182篇
  1995年   176篇
  1994年   158篇
  1993年   119篇
  1992年   159篇
  1991年   113篇
  1990年   110篇
  1989年   77篇
  1988年   54篇
  1987年   46篇
  1986年   35篇
  1985年   35篇
  1984年   17篇
  1983年   15篇
  1982年   16篇
  1981年   7篇
  1980年   4篇
  1979年   2篇
  1978年   1篇
  1975年   2篇
  1966年   1篇
  1950年   1篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
Probiotics and Antimicrobial Proteins - The present study investigated the effects of the dietary probiotic Clostridium butyricum (CB) on the growth, intestine digestive enzyme activity,...  相似文献   
993.
Monitoring the blood supply in the lower extremities is critical for individuals who are vulnerable to vascular dysfunction. Current clinical approaches are ineffective in observing hemodynamic changes in peripheral vessels. In this paper, we investigate the potential of photoacoustic tomography (PAT) as an alternative way to in vivo monitor hemodynamic changes in foot vessels. High spatial and temporal resolution maps of hemoglobin in major arteries and veins are shown. Results from twelve human subjects are presented here to visualize vascular perfusion of healthy volunteers in two age groups (young vs aged). Significant differences between the two groups are observed and verify the declining in vascular function with aging, highlighting the potential of PAT as a new tool to evaluate vascular function in the lower extremities.   相似文献   
994.
995.
This paper investigates the spatial heterogeneity of landowners’ willingness to supply three bioenergy crops: switchgrass, Miscanthus, and willow, in the northeastern United States. Spatial heterogeneity might arise for several reasons. For example, landowners closer to bioenergy processing plants might be more likely to be willing to supply bioenergy crops, and landowners who are more willing to supply bioenergy crops may be spatially clustered because they share similar land attributes, demographics, experiences, and/or values. Using high‐resolution GIS data related to the location of pellet plants utilizing bioenergy crops and survey data related to landowners’ characteristics including spatial location, we estimate a spatial probit model to explain the variation in individual‐specific reservation prices (RPs)—the feedstock price at which landowners become willing to supply a bioenergy crop. We find that respondents’ RP is lower the closer they live to their nearest pellet plant and spatial dependency is only present for switchgrass supply. We also identify three economic hotspots (areas with high potential supply and low RPs) for each bioenergy crop. We believe that bioenergy supply chains could be developed around these hotspots.  相似文献   
996.
997.
998.
999.
1000.
The oriental armyworm, Mythimna separata, is a serious agricultural pest in China. Seasonal and roundtrip migration has recently led to sudden, localized outbreaks and crop losses. To evaluate genetic differentiation between populations in eastern and western China and elucidate gene flow, the genetic structure of 20 natural populations from nine provinces was examined using seven microsatellite markers. The results indicated high genetic diversity. However, little to moderate (0 < FST < 0.15) genetic differentiation was detected, and there was no correlation between genetic distance and geographical distance. Bayesian clustering analysis identified three groups whereas discriminant analysis of principal components identified ten clusters that were considered as two clear‐cut clusters and one admixed group. Gene flow occurred frequently in most population pairs, and an asymmetrical migration rate was detected in several pairwise population comparisons. The bottleneck test showed that few populations had experienced recent bottlenecks. Correspondingly, large‐scale and long‐distance migration of M. separata has caused low genetic differentiation and frequent gene exchange. Our findings are important for studying genetic evolution and help to improve predictions of M. separata outbreaks in China.  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