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   403011篇
  免费   3361篇
  国内免费   1308篇
化学   187027篇
晶体学   6082篇
力学   21951篇
综合类   7篇
数学   68992篇
物理学   123621篇
  2021年   3133篇
  2020年   3333篇
  2019年   3764篇
  2018年   15117篇
  2017年   15152篇
  2016年   13126篇
  2015年   4935篇
  2014年   7287篇
  2013年   16142篇
  2012年   15848篇
  2011年   24624篇
  2010年   16288篇
  2009年   16642篇
  2008年   19418篇
  2007年   21412篇
  2006年   12306篇
  2005年   11709篇
  2004年   11247篇
  2003年   10515篇
  2002年   9601篇
  2001年   10601篇
  2000年   7905篇
  1999年   6092篇
  1998年   5130篇
  1997年   4928篇
  1996年   4668篇
  1995年   3973篇
  1994年   3893篇
  1993年   3790篇
  1992年   4146篇
  1991年   4365篇
  1990年   4143篇
  1989年   4065篇
  1988年   3835篇
  1987年   3970篇
  1986年   3739篇
  1985年   4673篇
  1984年   4720篇
  1983年   3920篇
  1982年   4013篇
  1981年   3734篇
  1980年   3691篇
  1979年   3955篇
  1978年   3935篇
  1977年   3924篇
  1976年   3894篇
  1975年   3690篇
  1974年   3626篇
  1973年   3672篇
  1972年   2620篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Determination of mercury (II) by substoichiometric extraction with malonic anilide into a mixture of ether and MIBK is reported. The method has been applied in the estimation of the metal in industrial effluents.  相似文献   
932.
The synthesis has been effected of new representatives of the imidazo[1,2-b]-1,2,4-triazepin-4-ones (-thiones). It has been shown with the aid of IR and UV spectroscopic and mass spectrometric methods that the lactam (thione) form is the predominant one for the compounds synthesized.For Communication 11, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 694–699, May, 1985.  相似文献   
933.
The nickel dithiophosphates and their corresponding disulphides are shown to be more effective uv stabilisers for LDPE than the closely related nickel xanthates and disulphides (dixanthogens). In general, the nickel complexes in both series are more effective uv stabilisers than the disulphides.Evidence is presented to show that the better performance of the dithiophosphates than the xanthates is associated with their higher solubility in the polymer. For the same reason, nickel dithiolates containing long alkyl substituents are more effective uv stabilisers than those containing shorter alkyl substituents.Thermal processing adversely affects the photo-stabilising performance of nickel dithiolates whereas the performance of the less efficient disulphides is enhanced under the same conditions. In the former case, the uv stability of the polymer is shown to be directly proportional to the amount of the nickel complex left after processing. In the case of the disulphides, on the other hand, the evidence suggests that increasing the severity of their processing increases the extent of oxidation to products which are more effective uv stabilisers than the parent disulphides.  相似文献   
934.
Benzophenone is photoreduced by pthalimidines and dihydro isoquinolones. The hydrogen atom α to the nitrogen atom is abstracted and radical coupling leads to adducts. CIDNP studies of these adducts show inversion of polarisation for the adduct on the N-alkyl chain when the nitrogen is bound to a benzylic methylene. This inversion is explained by considering that the radical on the N-alkyl chain derives from the radical on the ring.  相似文献   
935.
936.
Previously unknown solid complexes of Ti(IV), Zr(IV), and Zn(II) with chrysin were isolated, identified, and studied by thermogravimetric analysis and by UV-Vis, IR, and NMR spectroscopy.  相似文献   
937.
Reaction of furfuryl and tetrahydrofurfuryl alcohols with nitriles over copper oxide catalysts under a hydrogen pressure of 15 atm at a temperature of 230C gives N-alkylfurfuryl- (yield 46–50%) and N-alkyl-tetrahydrofurfurylamines (49–53%), and N-alkylpiperidines (28–41%). A reaction mechanism is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 23–26, January, 1988.  相似文献   
938.
939.
940.
Photoreactions of benzo(a)- and benzo(c)acridines in protic solvents were studied by CIDNP. Two routes to adducts are possible: direct recombination in the first radical pair (S state) or recombination of radicals after diffusion. The relative importance of each route seems to be linked to the presence of the benzo group and to solvent reactivity.  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