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   56449篇
  免费   253篇
  国内免费   549篇
系统科学   1243篇
丛书文集   357篇
教育与普及   291篇
理论与方法论   516篇
现状及发展   32521篇
研究方法   910篇
综合类   19223篇
自然研究   2190篇
  2013年   803篇
  2012年   635篇
  2011年   2674篇
  2009年   617篇
  2008年   819篇
  2007年   921篇
  2006年   1032篇
  2005年   1195篇
  2004年   2290篇
  2003年   1894篇
  2002年   1594篇
  2001年   1447篇
  2000年   957篇
  1999年   981篇
  1998年   642篇
  1997年   774篇
  1996年   534篇
  1994年   687篇
  1993年   689篇
  1992年   917篇
  1991年   785篇
  1990年   850篇
  1989年   669篇
  1988年   627篇
  1987年   666篇
  1986年   744篇
  1985年   861篇
  1984年   785篇
  1983年   668篇
  1982年   790篇
  1981年   832篇
  1980年   940篇
  1979年   1431篇
  1978年   1285篇
  1977年   1287篇
  1976年   1089篇
  1975年   1088篇
  1974年   1020篇
  1973年   1247篇
  1972年   1323篇
  1971年   1366篇
  1970年   1480篇
  1969年   1273篇
  1968年   1172篇
  1967年   1123篇
  1966年   921篇
  1965年   731篇
  1964年   479篇
  1958年   564篇
  1957年   486篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
181.
Progress on nuclear liquid gas phase transition (LGPT) or critical behavior has been simply reviewed and some signals of LGPT in heavy ion collisions, especially in NIMROD data, are focused. These signals include the power-law charge distribution, the largest fluctuation of the fragment observables, the nuclear Zipf law, caloric curve and critical exponent analysis etc.  相似文献   
182.
183.
Summary By means of intratracheal injection, particles of metalloid silicium about five microns in size are introduced into the lungs of guinea pigs, and the secondary parenchymatous reactions are studied from 10 minutes to 105 days. In the dust cells, the silicium is progressively broken up into very small particles of less than 1 micron in size. The cells do not show degeneration or mummification as the ordinary silica cells (Mavrogordato) do.  相似文献   
184.
人类发现自身陷入一种境地,在那里似乎没有真正意义的途径使我们从围绕自身的技术型企业中解脱出来,所以我们需要创造一个走向生态纪元的社会。很多社团近期已经热心于进入一个生态社会并在一些领域采取行动,这些领域便是新故事、生物区域主义以及生态特征。  相似文献   
185.
1 Results Rare earth doped waveguide amplifiers and devices have been demonstrated in silica, crystal and other glass hosts. These rare earth doped optical waveguide devices are based on inorganic materials. Many processing steps are required and can lead to long fabrication time and low yield.Polymer materials offer many distinct properties compared to inorganic materials, such as ease of fabrication, low production costs, simple processing steps, and compatibility with micro-fabrication technologies. ...  相似文献   
186.
    
Summary If amphibian gastrulæ are heated at 36 to 37°C for one hour, their development is blocked irreversibly; if, however, the dorsal lip is removed from these blocked gastrulæ and implanted in a normal gastrula, its development is resumed and the host is the site of a neural induction.  相似文献   
187.
Summary All sensory endings of the trigeminal nerve in the dura mater of the rat are formed by a small swelling, from which continues themetaterminal apparatus, fine filaments at the limit of visibility. Periodically the fine granules at the extremity of the filaments swell, whereas the fibrils themselves disappear, leaving a small, round, argentophil mass. Later the filaments reappear, then redeposit their debris, which persists for a time.Thus the metaterminal apparatus manifests cyclic variations, which recall the transitory existence of collaterals observed duringin vivo orin vitro development of nerve fibres.  相似文献   
188.
189.
Summary The preceding empirical relation was found between the surface tension of liquids, the specific weight of liquids, and the velocity of sound:=6.3 · 10–4 3/2. In place of the vèocity of sound can be used, with suitable changes in the equation, the compressibility or, in the case of solid substances, the elasticity of the material in question. The range of validity of this relation is so large that it is possible by means of it to calculate the surface tension of solid substances and of gases.  相似文献   
190.
Summary An alcaloid C16H19NO3 has been isolated fromErythrina tholloniana; the iodohydrate of this erythroidine has a melting point of 225°C and 239°C for its chlorhydrate. It has a powerful curare-like action on the frog or on its isolated sciatic-sartorius preparation; at a concentration of less than 1/1,000,000, a complete neuromuscular block is produced: the electrical stimulation of the motor nerve does not produce any contraction, but the muscle reacts by an end-plate potential having the same characteristics (shape, duration, possibility of summation) as the electrical waves produced in the same preparation curarized by ordinary curare or by quaternary ammonium derivatives. Decurarization by veratrine 1/200,000 is accompanied by the same electrical reactions as those which have been described in preparations treated by curare.On mammals, the alcaloid has little curariform activity; on the isolated phrenic-diaphragm preparation of the rat, incomplete block was produced at a concentration of 1/5000.  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