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   35篇
  免费   4篇
  国内免费   8篇
现状及发展   7篇
研究方法   6篇
综合类   31篇
自然研究   3篇
  2021年   1篇
  2019年   1篇
  2018年   1篇
  2017年   4篇
  2016年   1篇
  2015年   1篇
  2014年   1篇
  2013年   6篇
  2012年   3篇
  2011年   4篇
  2010年   2篇
  2008年   2篇
  2007年   3篇
  2006年   2篇
  2005年   3篇
  2004年   1篇
  2003年   2篇
  2002年   1篇
  2001年   2篇
  2000年   1篇
  1990年   1篇
  1988年   1篇
  1981年   1篇
  1973年   2篇
排序方式: 共有47条查询结果,搜索用时 0 毫秒
41.
The curing sensitivity of concrete with cement Types 1, 3, and 5 as well as multiple powders consisting of cement, fly ash, and limestone powder was studied. Bottom ash was also used in the study as an internal curing agent and a partial substitution of fine aggregate. The curing sensitivity index was calculated by considering the performances of compressive strength and carbonation depth. Specimens were subjected to two curing conditions: continuously water-cured and continuously air-cured. The results show that cement Type 3 has a lower curing sensitivity, while cement Type 5 increases the curing sensitivity. For the mixes without bottom ash, the use of fly ash increases the curing sensitivity, while limestone powder reduces the curing sensitivity of concrete. The use of bottom ash in concrete reduces the curing sensitivity, especially at a lower mass ratio of water to binder. Concrete with limestone powder, together with bottom ash, is least sensitive to curing. The curing sensitivity calculated from carbonation depth also has a similar tendency as that derived by considering compressive strength. From the test results of compressive strength and curing sensitivity, bottom ash has been proven to be an effective internal curing agent.  相似文献   
42.
The protein predicted to be defective in individuals with Fanconi anemia complementation group J (FA-J), FANCJ, is a missing component in the Fanconi anemia pathway of genome maintenance. Here we identify pathogenic mutations in eight individuals with FA-J in the gene encoding the DEAH-box DNA helicase BRIP1, also called FANCJ. This finding is compelling evidence that the Fanconi anemia pathway functions through a direct physical interaction with DNA.  相似文献   
43.
Spoor F  Bajpai S  Hussain ST  Kumar K  Thewissen JG 《Nature》2002,417(6885):163-166
Early cetaceans evolved from terrestrial quadrupeds to obligate swimmers, a change that is traditionally studied by functional analysis of the postcranial skeleton. Here we assess the evolution of cetacean locomotor behaviour from an independent perspective by looking at the semicircular canal system, one of the main sense organs involved in neural control of locomotion. Extant cetaceans are found to be unique in that their canal arc size, corrected for body mass, is approximately three times smaller than in other mammals. This reduces the sensitivity of the canal system, most plausibly to match the fast body rotations that characterize cetacean behaviour. Eocene fossils show that the new sensory regime, incompatible with terrestrial competence, developed quickly and early in cetacean evolution, as soon as the taxa are associated with marine environments. Dedicated agile swimming of cetaceans thus appeared to have originated as a rapid and fundamental shift in locomotion rather than as the gradual transition suggested by postcranial evidence. We hypothesize that the unparalleled modification of the semicircular canal system represented a key 'point of no return' event in early cetacean evolution, leading to full independence from life on land.  相似文献   
44.
Résumé Nous avons démontré l'autofluorescence dans des neurones et neuroglies détachées du noyau de Deiters du lapin. Les cellules furent isolées et examinées dans une solution de 0.9% NaCl, sans fixation, deshydration ou addition de fluorochromes. La longueur d'onde de la fluorescence était 5400–5700 Å, et la fluorescence apparaissait d'habitude dans le cytoplasme et le nucléode. On a conclu qu'elle provient de la lipofuscine.  相似文献   
45.
Nummela S  Thewissen JG  Bajpai S  Hussain ST  Kumar K 《Nature》2004,430(7001):776-778
The origin of whales (order Cetacea) is one of the best-documented examples of macroevolutionary change in vertebrates. As the earliest whales became obligately marine, all of their organ systems adapted to the new environment. The fossil record indicates that this evolutionary transition took less than 15 million years, and that different organ systems followed different evolutionary trajectories. Here we document the evolutionary changes that took place in the sound transmission mechanism of the outer and middle ear in early whales. Sound transmission mechanisms change early on in whale evolution and pass through a stage (in pakicetids) in which hearing in both air and water is unsophisticated. This intermediate stage is soon abandoned and is replaced (in remingtonocetids and protocetids) by a sound transmission mechanism similar to that in modern toothed whales. The mechanism of these fossil whales lacks sophistication, and still retains some of the key elements that land mammals use to hear airborne sound.  相似文献   
46.
Summary The proportion of nitrogen obtained by wheat Pak-70 from N15 labelled urea applied at the rate of 120 kg N/ha was less than 35%. Leaching of nitrogen beyond the root zone was shown to be negligible. Most of the nitrogen was therefore left in the soil as residue. The increase in soil inorganic nitrogen values after the wheat harvest confirmed the presence of residual nitrogen.Acknowledgment. This work was sponsored jointly by the International Atomic Energy Agency and the Pakistan Atomic Energy Commission under Research Contract No. 1673/GS. The authors are grateful to the IAEA for providing some essential equipment and analyses of wheat samples for N15.  相似文献   
47.
节能减排目标下的企业应对行为研究   总被引:1,自引:0,他引:1  
基于企业视角,通过对Q市120家工业企业的抽样调查,分析节能减排指标对企业的影响,了解被调查企业对节能减排的主观态度和客观执行情况,探索企业节能减排工作中面临的问题和需求。重点考察了影响企业节能减排绩效的影响因素,例如企业规模、所有制、市场、企业原有环境绩效、上市情况等对节能减排活动的影响,以期为我国节能减排政策提供阶段性评估,提出在企业层面落实和促进节能减排目标实现的制度安排,并为长期的经济结构优化和可持续发展目标提供微观决策信息。  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