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   1421910篇
  免费   29914篇
  国内免费   7824篇
化学   721633篇
晶体学   20744篇
力学   76587篇
综合类   112篇
数学   248540篇
物理学   392032篇
  2021年   13569篇
  2020年   16051篇
  2019年   16176篇
  2018年   15247篇
  2017年   13582篇
  2016年   29238篇
  2015年   21228篇
  2014年   30753篇
  2013年   74986篇
  2012年   42536篇
  2011年   43223篇
  2010年   40100篇
  2009年   41699篇
  2008年   41133篇
  2007年   37896篇
  2006年   40846篇
  2005年   34869篇
  2004年   34512篇
  2003年   31783篇
  2002年   32231篇
  2001年   32506篇
  2000年   27359篇
  1999年   23702篇
  1998年   21515篇
  1997年   21412篇
  1996年   21380篇
  1995年   19414篇
  1994年   18882篇
  1993年   18484篇
  1992年   18928篇
  1991年   19117篇
  1990年   18276篇
  1989年   18295篇
  1988年   17920篇
  1987年   17820篇
  1986年   16768篇
  1985年   23121篇
  1984年   24199篇
  1983年   20249篇
  1982年   21843篇
  1981年   21082篇
  1980年   20439篇
  1979年   20808篇
  1978年   21963篇
  1977年   21599篇
  1976年   21378篇
  1975年   20069篇
  1974年   19759篇
  1973年   20148篇
  1972年   14644篇
排序方式: 共有10000条查询结果,搜索用时 281 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