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   237451篇
  免费   2168篇
  国内免费   833篇
化学   120032篇
晶体学   4018篇
力学   10879篇
综合类   1篇
数学   26069篇
物理学   79453篇
  2020年   1895篇
  2019年   2140篇
  2018年   2785篇
  2017年   2706篇
  2016年   4043篇
  2015年   2511篇
  2014年   4115篇
  2013年   10312篇
  2012年   7838篇
  2011年   9747篇
  2010年   6836篇
  2009年   6763篇
  2008年   8999篇
  2007年   9050篇
  2006年   8572篇
  2005年   7741篇
  2004年   7130篇
  2003年   6290篇
  2002年   6211篇
  2001年   7129篇
  2000年   5427篇
  1999年   4272篇
  1998年   3601篇
  1997年   3516篇
  1996年   3351篇
  1995年   3044篇
  1994年   2988篇
  1993年   3081篇
  1992年   3186篇
  1991年   3359篇
  1990年   3119篇
  1989年   3045篇
  1988年   3086篇
  1987年   2972篇
  1986年   2811篇
  1985年   3839篇
  1984年   4034篇
  1983年   3212篇
  1982年   3432篇
  1981年   3353篇
  1980年   3257篇
  1979年   3355篇
  1978年   3448篇
  1977年   3365篇
  1976年   3469篇
  1975年   3156篇
  1974年   3166篇
  1973年   3205篇
  1972年   2242篇
  1971年   1854篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
    
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e).  相似文献   
73.
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   
74.
The auction algorithm for the transportation problem   总被引:1,自引:0,他引:1  
The auction algorithm is a parallel relaxation method for solving the classical assignment problem. It resembles a competitive bidding process whereby unassigned persons bid simultaneously for objects, thereby raising their prices. Once all bids are in, objects are awarded to the highest bidder. This paper generalizes the auction algorithm to solve linear transportation problems. The idea is to convert the transportation problem into an assignment problem, and then to modify the auction algorithm to exploit the special structure of this problem. Computational results show that this modified version of the auction algorithm is very efficient for certain types of transportation problems.  相似文献   
75.
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   
76.
77.
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   
78.
79.
80.
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   
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