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   398458篇
  免费   3493篇
  国内免费   1332篇
化学   201491篇
晶体学   6608篇
力学   18733篇
综合类   5篇
数学   45909篇
物理学   130537篇
  2020年   3258篇
  2019年   3692篇
  2018年   4902篇
  2017年   4752篇
  2016年   6967篇
  2015年   4330篇
  2014年   6978篇
  2013年   17635篇
  2012年   13435篇
  2011年   16479篇
  2010年   11523篇
  2009年   11362篇
  2008年   14996篇
  2007年   15153篇
  2006年   14359篇
  2005年   12979篇
  2004年   12028篇
  2003年   10665篇
  2002年   10477篇
  2001年   12008篇
  2000年   9268篇
  1999年   7290篇
  1998年   5963篇
  1997年   5885篇
  1996年   5551篇
  1995年   5089篇
  1994年   4941篇
  1993年   4923篇
  1992年   5320篇
  1991年   5432篇
  1990年   5173篇
  1989年   4956篇
  1988年   5098篇
  1987年   4949篇
  1986年   4638篇
  1985年   6234篇
  1984年   6608篇
  1983年   5383篇
  1982年   5732篇
  1981年   5633篇
  1980年   5381篇
  1979年   5571篇
  1978年   5739篇
  1977年   5707篇
  1976年   5806篇
  1975年   5364篇
  1974年   5331篇
  1973年   5540篇
  1972年   3717篇
  1971年   3031篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures.  相似文献   
82.
The effect of traffic noise on sleep of young adults in their homes   总被引:1,自引:0,他引:1  
The disturbance of sleep by traffic noise is a major problem area in noise pollution. Extensive laboratory tests using multichannel electroencephalograms (EEG) have been carried out by many workers to determine the general response of people when exposed to noise during sleep. An experimental technique for obtaining results in people's homes has been developed using a simplified one-channel EEG. The responses of six people in the age range 19-24 were measured. A significant correlation was found between sleep disturbance and traffic noise when the noise is expressed in EPNdB. In the home experiments the subjects appeared to be approximately 10 dB less sensitive to noises than laboratory subjects for similar noise exposure. There also appeared to be some adaptation to the noise exposure; however, only one subject was tested for a period of 20 days and was insufficient to give definite results on adaptation.  相似文献   
83.
Radio Engineering and Electronics Institute, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 32, No. 9, pp. 1113–1124, September, 1989.  相似文献   
84.
85.
Summary Using a saturated non-boiling hydrocarbon mixture, the influence of two parameters on the results of field ionization mass spectrometry (FI-MS) measurements was studied: (a) the potential difference between the FI emitter and the counterelectrode; (b) the emitter temperature.Variation of the potential difference had only a minor effect on the average molecular mass measured and had no evident effect on the relative ring number distribution in the sample. In contrast, when the emitter temperature was increased, higher average molecular masses were recorded. Moreover, the average molecular masses shifted to higher ring numbers. In order to control the relationship between the described influences during mixture analysis, measuring instructions have been developed that enable the quantitative analysis of unknown saturated samples. However, average molecular mass of the mixture must be known.
Entwicklung einer quantitativen FI-MS-Methode zur Charakterisierung von gesättigten hoch- und nichtsiedenden Kohlenwasserstoffgemischen
  相似文献   
86.
87.
88.
Crystalline materials that are transparent in the vacuum UV spectral region and currently used have been reviewed. Transmission of crystals of solid solutions with the fluorite structure Ca1?x R xF2+x (R = Sc, Y, La, Yb, Lu) in the UV and vacuum UV spectral regions has been investigated. It is shown that application of different methods of purification of fluorides from some impurities can significantly improve the optical quality of fluoride multicomponent crystals in the short-wavelength spectral region.  相似文献   
89.
90.
In this paper we will present an overview of the use of photonic crystal fibers as fiber amplifiers. We will describe the basic concepts of optical amplification, and how to do numerical modelling of such components. We will then identify advantages and disadvantages of amplifiers based on PCF technology compared to conventional fibers, and then go into greater detail on some of these specific applications, such as low pump power amplifiers, and high-power double-clad amplifiers and lasers.  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