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   280687篇
  免费   18432篇
  国内免费   858篇
化学   174853篇
晶体学   4037篇
力学   12839篇
综合类   1篇
数学   28489篇
物理学   79758篇
  2023年   4930篇
  2021年   4237篇
  2020年   6640篇
  2019年   4446篇
  2018年   5123篇
  2017年   3270篇
  2016年   9701篇
  2015年   8158篇
  2014年   9154篇
  2013年   15457篇
  2012年   10805篇
  2011年   10454篇
  2010年   10197篇
  2009年   10130篇
  2008年   9777篇
  2007年   9481篇
  2006年   8370篇
  2005年   7557篇
  2004年   6965篇
  2003年   6157篇
  2002年   6089篇
  2001年   7134篇
  2000年   5395篇
  1999年   4261篇
  1998年   3589篇
  1997年   3557篇
  1996年   3381篇
  1995年   3175篇
  1994年   3043篇
  1993年   3275篇
  1992年   3258篇
  1991年   3413篇
  1990年   3141篇
  1989年   3098篇
  1988年   3185篇
  1987年   3057篇
  1986年   2880篇
  1985年   3906篇
  1984年   4109篇
  1983年   3302篇
  1982年   3544篇
  1981年   3490篇
  1980年   3415篇
  1979年   3514篇
  1978年   3623篇
  1977年   3658篇
  1976年   3815篇
  1975年   3581篇
  1974年   3602篇
  1973年   3498篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
    
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   
12.
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   
13.
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   
14.
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   
15.
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   
16.
Considering the importance of the X-ray production cross sections for the determination of the element concentrations in a given material, we have measured them experimentally for Al Si, Sc, Ti, V, Fe, Co, Ni and Cu bombarded by protons with energies ranging from 40 to 180 keV. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
17.
18.
19.
We have simultaneously used adsorption isotherm volumetry and Fourier transform infrared spectroscopy in order to take the investigations on amorphous ice structure a step further, especially concerning porosity and annealing-induced modifications. We have studied surface reorganization during annealing and found that the number of surface sites decreases before crystallization, their relative ratios being different for amorphous and crystalline ice. We also present results confirming that ice can have a large specific surface area and nevertheless be non-microporous.  相似文献   
20.
单光子态的产生与测量   总被引:1,自引:0,他引:1  
报道了通过自发参量下转换产生单光子态的实验研究,使用中心波长425 nm的飞秒脉冲泵浦Ⅰ类非共线相位匹配的BBO晶体,在实验上得到单光子计数率为2.978×10-4,并分析了实验中的相关问题.  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