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   766篇
  免费   7篇
  国内免费   10篇
系统科学   14篇
教育与普及   7篇
理论与方法论   4篇
现状及发展   138篇
研究方法   104篇
综合类   479篇
自然研究   37篇
  2019年   5篇
  2018年   5篇
  2017年   3篇
  2016年   3篇
  2015年   8篇
  2014年   8篇
  2013年   8篇
  2012年   31篇
  2011年   72篇
  2010年   19篇
  2009年   7篇
  2008年   45篇
  2007年   46篇
  2006年   52篇
  2005年   46篇
  2004年   27篇
  2003年   49篇
  2002年   41篇
  2001年   10篇
  2000年   17篇
  1999年   14篇
  1997年   5篇
  1995年   3篇
  1994年   3篇
  1992年   14篇
  1991年   12篇
  1990年   6篇
  1989年   12篇
  1988年   10篇
  1987年   5篇
  1986年   7篇
  1985年   8篇
  1984年   9篇
  1983年   7篇
  1982年   4篇
  1979年   15篇
  1978年   9篇
  1977年   5篇
  1976年   10篇
  1975年   10篇
  1974年   14篇
  1973年   13篇
  1972年   9篇
  1971年   9篇
  1970年   12篇
  1969年   8篇
  1968年   7篇
  1967年   11篇
  1966年   13篇
  1965年   4篇
排序方式: 共有783条查询结果,搜索用时 31 毫秒
1.
2.
3.
Thyroid cancer after Chernobyl.   总被引:17,自引:0,他引:17  
  相似文献   
4.
The aim of the work was to investigate the fate of injectant coal in blast furnaces and the origin of extractable materials in blast furnace carryover dusts. Two sets of samples including injectant coal and the corresponding carryover dusts from a full sized blast furnace and a pilot scale rig have been examined. The samples were extracted using 1-methyl-2-pyrrolidinone (NMP) solvent and the extracts studied by size exclusion chromatography (SEC). The blast furnace carryover dust extracts contained high molecular weight carbonaceous material, of apparent mass corresponding to 107?108 u, by polystyrene calibration. In contrast, the feed coke and char prepared in a wire mesh reactor under high temperature conditions did not give any extractable material. Meanwhile, controlled combustion experiments in a high-pressure wire mesh reactor suggest that the extent of combustion of injectant coal in the blast furnace tuyeres and raceways is limited by time of exposure and very low oxygen concentration. It is thus likely that the extractable, soot-like material in the blast furnace dust originated in tars is released by the injectant coal. Our results suggest that the unburned tars were thermally altered during the upward path within the furnace, giving rise to the formation of heavy molecular weight (soot-like) materials.  相似文献   
5.
南澳岛及其附近岛屿夏季鸟类的初步调查   总被引:4,自引:0,他引:4  
作者一行于 2 0 0 1年夏季对南澳及其附近岛屿的鸟类资源状况进行了调查研究 .结果发现 ,南澳及其附近岛屿有鸟类 51种 ,隶属 1 3目 2 8科 ,其中雀形目 2 3种 ,占所发现种类的 45% ,非雀形目 2 8种 ,占 55% ;国家一、二级保护动物各一种 ,分别为 :白腹军舰鸟 (F regataandrewsi)和岩鹭 (Egretta sacra) .按鸟类的栖息和迁徙习惯划分 ,有海鸟 8种 ,湿地候鸟 2种 ,湿地留鸟 9种 ,陆地候鸟 4种 ,陆地留鸟 2 8种 .在所有鸟类中以褐翅燕鸥 (Sterna anae-thetus)为优势种 ,数量最大 ,分布最广 .  相似文献   
6.
7.
8.
Low beta diversity of herbivorous insects in tropical forests   总被引:1,自引:0,他引:1  
Recent advances in understanding insect communities in tropical forests have contributed little to our knowledge of large-scale patterns of insect diversity, because incomplete taxonomic knowledge of many tropical species hinders the mapping of their distribution records. This impedes an understanding of global biodiversity patterns and explains why tropical insects are under-represented in conservation biology. Our study of approximately 500 species from three herbivorous guilds feeding on foliage (caterpillars, Lepidoptera), wood (ambrosia beetles, Coleoptera) and fruit (fruitflies, Diptera) found a low rate of change in species composition (beta diversity) across 75,000 square kilometres of contiguous lowland rainforest in Papua New Guinea, as most species were widely distributed. For caterpillars feeding on large plant genera, most species fed on multiple host species, so that even locally restricted plant species did not support endemic herbivores. Large plant genera represented a continuously distributed resource easily colonized by moths and butterflies over hundreds of kilometres. Low beta diversity was also documented in groups with differing host specificity (fruitflies and ambrosia beetles), suggesting that dispersal limitation does not have a substantial role in shaping the distribution of insect species in New Guinea lowland rainforests. Similar patterns of low beta diversity can be expected in other tropical lowland rainforests, as they are typically situated in the extensive low basins of major tropical rivers similar to the Sepik-Ramu region of New Guinea studied here.  相似文献   
9.
Brandon A 《Nature》2007,450(7173):1169-1170
  相似文献   
10.
The ability to maintain adequate nutrient intake is critical for survival. Complex interrelated neuronal circuits have developed in the mammalian brain to regulate many aspects of feeding behaviour, from food-seeking to meal termination. The hypothalamus and brainstem are thought to be the principal homeostatic brain areas responsible for regulating body weight. However, in the current 'obesogenic' human environment food intake is largely determined by non-homeostatic factors including cognition, emotion and reward, which are primarily processed in corticolimbic and higher cortical brain regions. Although the pleasure of eating is modulated by satiety and food deprivation increases the reward value of food, there is currently no adequate neurobiological account of this interaction between homeostatic and higher centres in the regulation of food intake in humans. Here we show, using functional magnetic resonance imaging, that peptide YY3-36 (PYY), a physiological gut-derived satiety signal, modulates neural activity within both corticolimbic and higher-cortical areas as well as homeostatic brain regions. Under conditions of high plasma PYY concentrations, mimicking the fed state, changes in neural activity within the caudolateral orbital frontal cortex predict feeding behaviour independently of meal-related sensory experiences. In contrast, in conditions of low levels of PYY, hypothalamic activation predicts food intake. Thus, the presence of a postprandial satiety factor switches food intake regulation from a homeostatic to a hedonic, corticolimbic area. Our studies give insights into the neural networks in humans that respond to a specific satiety signal to regulate food intake. An increased understanding of how such homeostatic and higher brain functions are integrated may pave the way for the development of new treatment strategies for obesity.  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