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   147995篇
  免费   1309篇
  国内免费   400篇
化学   80705篇
晶体学   2420篇
力学   6091篇
综合类   3篇
数学   13710篇
物理学   46775篇
  2021年   1040篇
  2020年   1195篇
  2019年   1259篇
  2018年   1644篇
  2017年   1694篇
  2016年   2554篇
  2015年   1570篇
  2014年   2364篇
  2013年   6086篇
  2012年   4870篇
  2011年   5974篇
  2010年   4207篇
  2009年   4221篇
  2008年   5754篇
  2007年   5554篇
  2006年   5411篇
  2005年   4979篇
  2004年   4480篇
  2003年   4152篇
  2002年   4032篇
  2001年   5566篇
  2000年   3994篇
  1999年   3086篇
  1998年   2121篇
  1997年   2233篇
  1996年   2094篇
  1995年   1930篇
  1994年   1937篇
  1993年   1759篇
  1992年   2235篇
  1991年   2335篇
  1990年   2155篇
  1989年   2113篇
  1988年   2094篇
  1987年   2100篇
  1986年   1928篇
  1985年   2388篇
  1984年   2421篇
  1983年   1920篇
  1982年   1932篇
  1981年   1764篇
  1980年   1750篇
  1979年   2040篇
  1978年   2107篇
  1977年   2060篇
  1976年   1983篇
  1975年   1867篇
  1974年   1849篇
  1973年   1888篇
  1972年   1240篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
242.
243.
5-Bromo- and 5-methoxy-3-ethylanthranils, respectively, were obtained by cyclization of o-nitrosopropiophenone under the influence of hydrogen bromide in benzene and hydrogen chloride in methanol. In these reactions, the starting nitroso ketone undergoes redox transformations that also proceed readily in an inert solvent in the absence of any cyclizing reagents.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 886–890, July. 1976.  相似文献   
244.
245.
In the present work, we use the binary insertion compound LiC12 to polymerize styrene, methyl methacrylate, butadiene, isoprene, and to copolymerize isoprene and styrene in various hydrocarbon solvents (aromatics and aliphatic) and etheral solvents. We show that the styrene polymerization in aromatic solvents gives better yields than in the etheral solvents, the polymer being atactic. Methyl methacrylate does not polymerize in toluene but does so completely in DME. More generally, the yields of polymerization are better with KC37 than with LiC12 because of the different capacities of the monomer to get into the carbon layers. The polymerization of dienes with LiC12 shows that the microstructures of the polymer obtained in π-or n-donor solvents are similar to the ones obtained by homogenous polymerization with Li cation in such solvents. However, for isoprene in cyclohexane, the results are different. The isoprene styrene copolymers are statistical ones and the mean length of styrene blocks is less than 5. The monomer interaction with the insertion compound and the growing chain geometry between the carbon layers are the facts which control either the stereospecificity of the polymerization or the selectivity of the copolymerization.  相似文献   
246.
247.
Conclusions The complexes of Co, Ni, Fe, and Cu with dimethyl- and diphenylglyoxime inhibit the initiated oxidation of cumene by reacting with the peroxide radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1342–1344, June, 1971.  相似文献   
248.
249.
In this paper, the NO-to-NO 2 conversion in various gaseous mixtures is experimentally investigated. Streamer coronas are produced with a dc-superimposed high-frequency ac power supply (10–60 kHz). According to NO x removal experiments in N 2 +NO x and N 2 +O 2 +NO x gaseous mixtures, it is supposed that the reverse reaction NO 2 +ONO+O 2 may not only limit NO 2 production in N 2 +NO x mixtures, but also increase the energy cost for NO removal. Oxygen could significantly suppress reduction reactions and enhance oxidation processes. The reduction reactions, such as N+NON 2 +O, induce negligible NO removal provided the O 2 concentration is larger than 3.6%. With adding H 2 O into the reactor, the produced NO 2 per unit removed NO can be significantly reduced due to NO 2 oxidation. NH 3 injection could also significantly decrease the produced NO 2 via NH and NH 2 - related reduction reactions. Almost 100% of NO 2 can be removed in gaseous mixtures of N 2 +O 2 +H 2 O+NO 2 with negligible NO production.  相似文献   
250.
The present state of a long term program is reviewed. It was started to elaborate a remote controlled automated radiochemical processing system for the neutron activation analysis of biological materials. The system is based on wet ashing of the sample, followed by reactive desorption of some volatile components. The distillation residue is passed through a series of columns filled with selective ion screening materials to remove the matrix activity. The solution is thus “stripped” from the interfering radioions, and it is processed to single-elements through group separations using ion-exchange chromatographic techniques. Some special problems concerning this system are treated. (a) General aspects of the construction of a (semi)automated radiochemical processing system are discussed; (b) Comparison is made between various technical realizations of the same basic concept; (c) Some problems concerning the “reconstruction” of an already published processing system are outlined.  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