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   39592篇
  免费   115篇
  国内免费   185篇
系统科学   203篇
丛书文集   587篇
教育与普及   68篇
理论与方法论   148篇
现状及发展   17467篇
研究方法   1592篇
综合类   19158篇
自然研究   669篇
  2013年   364篇
  2012年   535篇
  2011年   1210篇
  2010年   235篇
  2008年   657篇
  2007年   769篇
  2006年   785篇
  2005年   718篇
  2004年   734篇
  2003年   675篇
  2002年   695篇
  2001年   1344篇
  2000年   1254篇
  1999年   807篇
  1992年   773篇
  1991年   571篇
  1990年   670篇
  1989年   684篇
  1988年   634篇
  1987年   679篇
  1986年   660篇
  1985年   792篇
  1984年   621篇
  1983年   530篇
  1982年   479篇
  1981年   490篇
  1980年   590篇
  1979年   1239篇
  1978年   1029篇
  1977年   985篇
  1976年   819篇
  1975年   869篇
  1974年   1202篇
  1973年   1017篇
  1972年   1053篇
  1971年   1211篇
  1970年   1522篇
  1969年   1116篇
  1968年   1142篇
  1967年   1068篇
  1966年   965篇
  1965年   674篇
  1964年   184篇
  1959年   366篇
  1958年   625篇
  1957年   424篇
  1956年   349篇
  1955年   338篇
  1954年   328篇
  1948年   234篇
排序方式: 共有10000条查询结果,搜索用时 762 毫秒
931.
932.
This paper explores the evolution of a trading zone by organizing the case study of turtle excluder devices within the model proposed by Collins et al. (2007). The case study offers evidence that trading zones do evolve and that the concepts of enforced and fractionated trading zones hold practical utility for describing and defining the complexities of actual exchanges. In this case a trading zone evolved from enforced to fractionated and ultimately diverged into two trading zones. For each step of the evolution I describe the forces that drove these transitions. Finally, I present an adapted trading zone model that is conceptually a better fit for the turtle excluder device case study.  相似文献   
933.
In higher vertebrates, sulfatases belong to a conserved family of enzymes that are involved in the regulation of cell metabolism and in developmental cell signaling. They cleave the sulfate from sulfate esters contained in hormones, proteins, and complex macromolecules. A highly conserved cysteine in their active site is post-translationally converted into formylglycine by the formylglycine-generating enzyme encoded by SUMF1 (sulfatase modifying factor 1). This post-translational modification activates all sulfatases. Sulfatases are extensively glycosylated proteins and some of them follow trafficking pathways through cells, being secreted and taken up by distant cells. Many proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates, which are sulfatase substrates. Indeed, sulfatases operate as decoding factors for a large amount of biological information contained in the structures of the sulfated sugar chains that are covalently linked to proteins and lipids. Modifications to these sulfate groups have pivotal roles in modulating specific signaling pathways and cell metabolism in mammals.  相似文献   
934.
935.
936.
This paper describes the design of a unified framework for a multilingual text-to-speech (TTS) synthesis engine – Crystal. The unified framework defines the common TTS modules for different languages and/or dialects. The interfaces between consecutive modules conform to the speech synthesis markup language (SSML) specification for standardization, interoperability, multilinguality, and extensibility. Detailed module divisions and implementation technologies for the unified framework are introduced, together with possible extensions for the algorithm research and evaluation of the TTS synthesis. Implementation of a mixed-language TTS system for Chinese Putonghua, Chinese Cantonese, and English demonstrates the feasibility of the proposed unified framework.  相似文献   
937.
An investigation of the diet of the Common Barn-owl ( Tyto alba ) on Antelope Island, Great Salt Lake, Utah, yielded four mammal species not previously known to occur on any island in the Great Salt Lake ( Microtus pennsylvanicus, M. montanus, Ondatra zibethicus , and a Sorex sp.). Two other species, known from other islands, were added to the list of fauna of Antelope Island ( Perognathus parvus and Reithrodontomys megalotis ). The barn owl diet on Antelope Island was remarkably like that of barn owls feeding in farmlands adjacent to the Great Salt Lake despite major vegetational differences.  相似文献   
938.
939.
The effect of Si on the austenite stabilization, martensite morphology, and magnetic properties in Fe-26%Ni-x%Si (x=3.5, 5, and 6) alloys have been studied by means of transmission electron microscopy (TEM) and M?ssbauer spectroscopy techniques. TEM observations reveal that the martensite morphology is closely dependent on the Si content. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field, and isomer shift values have been determined by Mssbauer spectroscopy. The M?ssbauer study reveals that the hyperfine magnetic field, the isomer shift values and the volume fraction of martensite decrease with increasing Si content.  相似文献   
940.
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