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   1412271篇
  免费   30596篇
  国内免费   8515篇
化学   718345篇
晶体学   17311篇
力学   76565篇
综合类   165篇
数学   247782篇
物理学   391214篇
  2021年   13574篇
  2020年   16083篇
  2019年   16183篇
  2018年   14971篇
  2017年   13315篇
  2016年   29114篇
  2015年   21225篇
  2014年   30773篇
  2013年   74960篇
  2012年   42787篇
  2011年   43192篇
  2010年   40104篇
  2009年   41794篇
  2008年   41267篇
  2007年   37892篇
  2006年   41174篇
  2005年   34886篇
  2004年   34604篇
  2003年   31778篇
  2002年   32217篇
  2001年   32618篇
  2000年   27349篇
  1999年   23716篇
  1998年   21517篇
  1997年   21395篇
  1996年   21380篇
  1995年   19390篇
  1994年   18864篇
  1993年   18450篇
  1992年   18892篇
  1991年   19057篇
  1990年   18229篇
  1989年   18250篇
  1988年   17839篇
  1987年   17752篇
  1986年   16176篇
  1985年   22625篇
  1984年   23751篇
  1983年   19743篇
  1982年   21399篇
  1981年   20594篇
  1980年   19921篇
  1979年   20392篇
  1978年   21589篇
  1977年   21277篇
  1976年   21066篇
  1975年   19753篇
  1974年   19435篇
  1973年   19889篇
  1972年   14415篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
981.
982.
983.
984.
Levoglucosenone reacts with α-aminoazoles to yield azolo[1,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction oocurs much more smoothly than in the case of other α,β-unsaturated ketones. The reactions of levoglucosenone with β-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base results in the pyran ring closure, which has never been observed earlier in reactions of β-dicarbonyl compounds with α,β-unsaturated ketones under the conditions of basic catalysis. The structures of products were established by IR and NMR spectroscopy. For Part 3, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 553–558, March, 1997.  相似文献   
985.
The oxime and Schiff's base of N-furoylphenylhydroxylamine form complexes with Co2+, Cu2+, Zn2+ and Fe3+. These complexes were studied by means of the UV — Vis. spectra of nujol mulls, electron spin resonance (ESR) spectra and magnetic susceptibility to determine their stereochemistry. The complexes were characterized via elemental analyses, molar conductivities and thermogravimetric analyses. Organic reagents were used for the gravimetric determination of Co2+, Cu2+ and Fe3+ through the precipitation of their complexes. The compounds were also used for separation of a binary mixture of Cu2+ and Cu2+. Interferences were studied.  相似文献   
986.
987.
Thetrans-[Pt(NC5H4C(O)NHC2H4ONO2)2Cl2] complex (2) was prepared by the reaction of nicorandyl (N-nitroethoxynicotinamide), which is widely used in cardiology, with K2PtCl4 in water. The structure of2 was established by X-ray structural analysis. It was found that complex2 exhibits high antitumor activity, in particular, antimetastatic activity, unlike the analogous CuII complex with bromine atoms. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1672–1675, September, 1997.  相似文献   
988.
2 laser of wide tunability to pump 13CD3OD we could observe 10 new far infrared laser lines ranging from 109 μm to 472 μm. The lines were generated by pumping infrared absorbing transitions of large offset appertaining to the C-O stretching Q-branch. All lines were characterised in wavelength, polarisation, optimum pressure of operation and precise offset measurements. Received: 17 September 1996/Revised version: 18 November 1996  相似文献   
989.
990.
The concentration and chemical state of copper in the subsurface region of Cu/SiO2 composite films obtained by simultaneous magnetron sputtering from two sources (Cu and SiO2) are determined by x-ray photoelectron spectroscopy (XPS). It is established that copper in the as-grown film is primarily in the form of unoxidized atoms dispersed in a SiO2 matrix. Annealing of the film results in practically no oxidation, but about 70% of the copper atoms condense into metallic clusters with sizes below 10 Å in the subsurface region and about 50 Å in the bulk of the film. The changes in the binding energy of core electrons, and especially in the energies of Auger electrons, are so large in this situation that photoelectron and Auger spectroscopy are efficient methods for monitoring the chemical state of this composite material.  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