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   1091篇
  免费   3篇
  国内免费   11篇
系统科学   25篇
教育与普及   5篇
理论与方法论   13篇
现状及发展   113篇
研究方法   172篇
综合类   605篇
自然研究   172篇
  2021年   7篇
  2020年   4篇
  2019年   7篇
  2018年   7篇
  2017年   11篇
  2016年   15篇
  2015年   5篇
  2014年   12篇
  2013年   11篇
  2012年   94篇
  2011年   260篇
  2010年   33篇
  2009年   3篇
  2008年   79篇
  2007年   91篇
  2006年   58篇
  2005年   75篇
  2004年   73篇
  2003年   67篇
  2002年   70篇
  2001年   4篇
  2000年   8篇
  1999年   10篇
  1998年   4篇
  1997年   2篇
  1996年   2篇
  1995年   4篇
  1993年   3篇
  1992年   7篇
  1991年   4篇
  1989年   5篇
  1988年   2篇
  1985年   2篇
  1984年   2篇
  1983年   2篇
  1982年   3篇
  1980年   2篇
  1979年   5篇
  1978年   5篇
  1976年   2篇
  1975年   4篇
  1972年   3篇
  1971年   3篇
  1970年   3篇
  1969年   4篇
  1968年   6篇
  1967年   6篇
  1966年   4篇
  1961年   1篇
  1957年   1篇
排序方式: 共有1105条查询结果,搜索用时 31 毫秒
1.
We have each spent more than 50 years doing research that has had little impact. Even more lamentable is that our field, judgment and decision making (JDM), has on the whole had little impact during that span. We attribute that failure to the use of methodologies that emphasize testing models rather than looking for differences in behavior. The “cognitive revolution” led the field astray, toward the goal of studying model fit rather than comparing observable results. With modeling as the goal, experimentation was stultified. Simple tasks became dominant. Although a poor metaphor for real decision making, the gambling paradigm has lasted forever because the inputs to the decision are known to the researcher and thus easily modeled.  相似文献   
2.
 我1942年出生于苏格兰,它是一个非常小的国家,人口只有550万左右。我在农场里长大,是家里唯一的孩子,我的父母牺牲了很多自己的时间,对我的求学生涯给予了很多的支持。我天性热爱自由,喜欢跑来跑去地玩耍,也会发明游戏等。我的高中学校在爱丁堡,那所学校有很好的教学传统,教师非常优秀。除了学习拉丁语、法语、英语等语言,我还要上历史、地理、化学,还有哲学等课程。那个时候非常重视体育,我们总是组织军训。我在18岁,高中快要结束的时候,去了爱丁堡大学上学,并在那里获得学士学位。  相似文献   
3.
4.
5.

This article analyzes the angular spacing of the degree marks on the zodiac scale of the Antikythera mechanism and demonstrates that over the entire preserved 88° of the zodiac, the marks are systematically placed too close together to be consistent with a uniform distribution over 360°. Thus, in some other part of the zodiac scale (not preserved), the degree marks have been spaced farther apart. By contrast, the day marks on the Egyptian calendar scale are spaced uniformly, apart from minor errors. A solar equation of center is apparent which rises by nearly 2.7° over the preserved portion of the zodiac. The placement of the degree marks indicates that, in the preserved portion of the zodiac, the Sun was considered to run at a uniform pace of about 30° per synodic month, which is consistent with the Sun’s speed in the fast zone of the Babylonian solar theory of System A.

  相似文献   
6.
Flow and heat transfer of aqueous based silica and alumina nanofluids in microchannels were experimentally investigated. The measured friction factors were higher than conventional model predictions at low Reynolds numbers particularly with high nanoparticle concentrations. A decrease in the friction factor was observed with increasing Reynolds number, possibly due to the augmentation of nanoparticle aggregate shape arising from fluid shear and alteration of local nanoparticle concentration and nanofluid viscosity. Augmentation of the silica nanoparticle morphology by fluid shear may also have affected the friction factor due to possible formation of a core/shell structure of the particles. Measured thermal conductivities of the silica nanofluids were in approximate agreement with the Maxwell-Crosser model, whereas the alumina nanofluids only showed slight enhancements. Enhanced convective heat transfer was observed for both nanofluids, relative to their base fluids (water), at low particle concentrations. Heat transfer enhancement increased with increasing Reynolds number and microchannel hydraulic diameter. However, the majority of experiments showed a larger increase in pumping power requirements relative to heat transfer enhancements, which may hinder the industrial uptake of the nanofluids, particularly in confined environments, such as Micro Electro-Mechanical Systems (MEMS).  相似文献   
7.
We describe activity patterns, relative abundances and pollen transport by insect floral visitors in the Skukuza Ranger District, Kruger National Park, Republic of South Africa, based on field surveys conducted during the early rainy seasons of years 2006 to 2012 (inclusive). Diagnostic notes, illustrations and natural history observations are provided for species in the families Buprestidae, Cerambycidae, Cleridae, Lycidae and Scarabaeidae (Coleoptera), Muscidae, Platystomatidae and Tabanidae (Diptera), Apidae, Halictidae, Megachilidae, Sphecidae and Vespidae (Hymenoptera), and Pieridae and Thyrididae (Lepidoptera) that transported pollen of tree or shrub species. Observations on floral phenology and pollination syndrome are presented for 27 flowering tree or shrub species. To provide a foundation for studies of the effects of drought and climate change on pollination services in the Kruger National Park, we applied methods of food web analysis to characterise this plant-pollinator assemblage. The food web analysis shows moderate levels of redundancy in plant-pollinator interactions (with connectance values averaging 0.19 for plant species and 0.20 for pollinator species), suggesting that pollination services in this system may exhibit some resilience to environmental perturbations. Possible conservation strategies for maintaining pollination services in this ecological system are discussed, including habitat management to conserve larval or nesting areas for social and solitary bees and scarab beetles, as well as further inventories and long-term monitoring of pollinator species.  相似文献   
8.
9.
10.
主要探讨中药制剂四生散对人肺癌A5 4 9细胞株的增殖抑制作用以及其急性毒性.应用改进的MTT法测定四生散对肺癌细胞增殖的影响;采用琼脂糖凝胶电泳测定四生散对诱导肿瘤细胞凋亡的影响;再用分剂量组灌胃小鼠的方法,鉴定四生散的急性毒性.实验结果表明,四生散对人肺癌A5 4 9细胞株的增殖有较强的抑制作用,并具有明显的量效和时效相关性;经四生散处理后的A5 4 9细胞呈现凋亡特征性电泳条带;在3 2倍于人的剂量时,小鼠饮食和体重均在正常水平,并无异常,证实口服四生散无急性毒性.  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