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   18270篇
  免费   97篇
  国内免费   227篇
系统科学   384篇
丛书文集   382篇
教育与普及   72篇
理论与方法论   62篇
现状及发展   6291篇
研究方法   793篇
综合类   10058篇
自然研究   552篇
  2014年   140篇
  2013年   280篇
  2012年   485篇
  2011年   1056篇
  2010年   360篇
  2009年   344篇
  2008年   533篇
  2007年   619篇
  2006年   612篇
  2005年   552篇
  2004年   407篇
  2003年   320篇
  2002年   326篇
  2001年   517篇
  2000年   570篇
  1999年   414篇
  1992年   302篇
  1991年   260篇
  1990年   263篇
  1989年   232篇
  1988年   215篇
  1987年   232篇
  1986年   208篇
  1985年   253篇
  1984年   237篇
  1983年   161篇
  1982年   172篇
  1981年   160篇
  1980年   199篇
  1979年   451篇
  1978年   343篇
  1977年   370篇
  1976年   267篇
  1975年   324篇
  1974年   449篇
  1973年   381篇
  1972年   363篇
  1971年   430篇
  1970年   497篇
  1969年   431篇
  1968年   414篇
  1967年   449篇
  1966年   387篇
  1965年   309篇
  1959年   155篇
  1958年   231篇
  1957年   179篇
  1956年   138篇
  1955年   154篇
  1954年   132篇
排序方式: 共有10000条查询结果,搜索用时 391 毫秒
881.
882.
883.
Zusammenfassung Dank einer neuen Anwendung von Lymphknotenextrakten (statt Röntgenextrakten, Corticosteroiden, Antilymphozytenserum usw.) wird eine erhebliche Reduktion der immunologischen Reaktion erzielt, wenn die Tiere mit diesen Extrakten vorbehandelt und gleichzeitig mit Antigenen behandelt werden.

This investigation was supported in part by U.S.P.H.S. Research Grant No. CA-02624 from National Cancer Institute; and in part by an institutional grant to Detroit Institute of Cancer Research from United Foundation of greater Detroit allocated through Michigan Cancer Foundation and the Detroit General Hospital Research Corporation, and Newaygo County Cancer Society.  相似文献   
884.
Riassunto È stata eseguita la registrazione di singole unità della parte dorsale del corpo genicolato laterale, in gatti non anestetizzati liberi di muoversi. Si dimostra che nella maggiore parte dei casi, le risposte ad un lampo di luce espresse in «spikes»/sec sono minime nel sonno sincronizzato, aumentano nella veglia e sono massime nel sonno desincronizzato. Qualora le risposte vengano riferite all'attività «spontanea» precedente il flash, il rapporto risposta-attività spontanea è maggiore nella veglia rispetto agli altri stati.

The research reported in this document has been sponsored by the Air Force Office of Scientific Research through the European Office of Aerospace Research (OAR), United States Air Force Grant AF EOAR 67-7 and by the Consiglio Nazionale delle Ricerche.

Moscow University, Chair of Higher Nervous Activity Physiology. Exchange Fellowship of the Italian Ministero degli Affari Esteri.  相似文献   
885.
886.
Zusammenfassung Chemisch synthetisiertes Galaktose-1,6-Diphosphat wird als Hemmer dès Glukose-1,6-Di-phosphats in der Phosphoglukomutase-Reaktion bestimmt.  相似文献   
887.
888.
889.
Control of spontaneously emitted light lies at the heart of quantum optics. It is essential for diverse applications ranging from miniature lasers and light-emitting diodes, to single-photon sources for quantum information, and to solar energy harvesting. To explore such new quantum optics applications, a suitably tailored dielectric environment is required in which the vacuum fluctuations that control spontaneous emission can be manipulated. Photonic crystals provide such an environment: they strongly modify the vacuum fluctuations, causing the decay of emitted light to be accelerated or slowed down, to reveal unusual statistics, or to be completely inhibited in the ideal case of a photonic bandgap. Here we study spontaneous emission from semiconductor quantum dots embedded in inverse opal photonic crystals. We show that the spectral distribution and time-dependent decay of light emitted from excitons confined in the quantum dots are controlled by the host photonic crystal. Modified emission is observed over large frequency bandwidths of 10%, orders of magnitude larger than reported for resonant optical microcavities. Both inhibited and enhanced decay rates are observed depending on the optical emission frequency, and they are controlled by the crystals' lattice parameter. Our experimental results provide a basis for all-solid-state dynamic control of optical quantum systems.  相似文献   
890.
The deposition of atmospheric dust into the ocean has varied considerably over geological time. Because some of the trace metals contained in dust are essential plant nutrients which can limit phytoplankton growth in parts of the ocean, it has been suggested that variations in dust supply to the surface ocean might influence primary production. Whereas the role of trace metal availability in photosynthetic carbon fixation has received considerable attention, its effect on biogenic calcification is virtually unknown. The production of both particulate organic carbon and calcium carbonate (CaCO3) drives the ocean's biological carbon pump. The ratio of particulate organic carbon to CaCO3 export, the so-called rain ratio, is one of the factors determining CO2 sequestration in the deep ocean. Here we investigate the influence of the essential trace metals iron and zinc on the prominent CaCO3-producing microalga Emiliania huxleyi. We show that whereas at low iron concentrations growth and calcification are equally reduced, low zinc concentrations result in a de-coupling of the two processes. Despite the reduced growth rate of zinc-limited cells, CaCO3 production rates per cell remain unaffected, thus leading to highly calcified cells. These results suggest that changes in dust deposition can affect biogenic calcification in oceanic regions characterized by trace metal limitation, with possible consequences for CO2 partitioning between the atmosphere and the ocean.  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