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   625273篇
  免费   6937篇
  国内免费   2095篇
化学   339786篇
晶体学   9079篇
力学   27110篇
综合类   25篇
数学   75723篇
物理学   182582篇
  2020年   4560篇
  2019年   4665篇
  2018年   5538篇
  2017年   5382篇
  2016年   9262篇
  2015年   6580篇
  2014年   9467篇
  2013年   26700篇
  2012年   20099篇
  2011年   24658篇
  2010年   16328篇
  2009年   16019篇
  2008年   22804篇
  2007年   23160篇
  2006年   22234篇
  2005年   20310篇
  2004年   18373篇
  2003年   16428篇
  2002年   16093篇
  2001年   17461篇
  2000年   13627篇
  1999年   10776篇
  1998年   9227篇
  1997年   8965篇
  1996年   8908篇
  1995年   8178篇
  1994年   8058篇
  1993年   7627篇
  1992年   8612篇
  1991年   8335篇
  1990年   8012篇
  1989年   7763篇
  1988年   8004篇
  1987年   7756篇
  1986年   7385篇
  1985年   10276篇
  1984年   10604篇
  1983年   8789篇
  1982年   9484篇
  1981年   9313篇
  1980年   9115篇
  1979年   9156篇
  1978年   9543篇
  1977年   9429篇
  1976年   9461篇
  1975年   9005篇
  1974年   8787篇
  1973年   9222篇
  1972年   5963篇
  1971年   4744篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity.  相似文献   
53.
54.
The electrical conductivity of hydrosols of ultradispersed diamonds was studied. The electromembrane method for cleaning and concentrating of ultradispersed diamonds in hydrosols was considered. The influence exerted by the concentration of the dispersed phase on the electrical characteristics of the cleaning process was analyzed. A mathematical relation making it possible to evaluate the output characteristics of the process and to determine the geometrical characteristics of the apparatus was proposed.  相似文献   
55.
The radiation emitted by charged, scalar particles in a Schwarzschild field with maximal acceleration corrections is calculated classically and in the tree approximation of quantum field theory. In both instances the particles emit radiation that has characteristics similar to those of gamma-ray bursters.  相似文献   
56.
This paper reports on the first experimental observation of quantum-well states and sp-type resonances in thin single-crystal gold, silver, and copper layers formed on single-crystal W(110) surfaces, which result from spatial localization of Bloch-type electronic wave functions in a quantum well with potential barriers at the vacuum/metal and metal/W(110) interfaces. The quantization of the valence-band electronic structure in Au/W(110), Ag/W(110), and Cu/W(110) systems was studied experimentally using angle-resolved photoelectron spectroscopy.  相似文献   
57.
58.
In this paper, the authors studied certain properties of the estimate of Liang and Krishnaiah (1985, J. Multivariate Anal. 16, 162–172) for multivariate binary density. An alternative shrinkage estimate is also obtained. The above results are generalized to general orthonormal systems.  相似文献   
59.
This paper presents an alternative to the beta continuous probability distribution for risk analysis. Particular attention has been given to two major applications of distributions, namely project management risk and critical path analysis (PERT). In conjunction with the beta, the triangular and normal distributions are frequently employed in order to give sufficient robustness to risk analysis. The beta distribution, as used in PERT, has a major theoretical implementation flaw. The new distribution was developed to give a possible alternative method of assessing risk. It is shown that the requirement to estimate the most pessimistic variate may be replaced by the probability to exceed the mode. Proposals for other simplifications in risk analysis are discussed. Practical means to validate the most appropriate distributions for risk analysis are outlined, and a cost-data case study is included.  相似文献   
60.
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