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   241537篇
  免费   2141篇
  国内免费   847篇
化学   122240篇
晶体学   4076篇
力学   11039篇
综合类   1篇
数学   27125篇
物理学   80044篇
  2020年   1878篇
  2019年   2117篇
  2018年   2782篇
  2017年   2714篇
  2016年   4045篇
  2015年   2512篇
  2014年   4115篇
  2013年   10554篇
  2012年   7870篇
  2011年   9829篇
  2010年   6919篇
  2009年   6851篇
  2008年   9127篇
  2007年   9130篇
  2006年   8658篇
  2005年   7874篇
  2004年   7226篇
  2003年   6398篇
  2002年   6382篇
  2001年   7227篇
  2000年   5502篇
  1999年   4328篇
  1998年   3657篇
  1997年   3578篇
  1996年   3420篇
  1995年   3089篇
  1994年   3050篇
  1993年   3145篇
  1992年   3254篇
  1991年   3431篇
  1990年   3180篇
  1989年   3105篇
  1988年   3154篇
  1987年   3044篇
  1986年   2872篇
  1985年   3950篇
  1984年   4144篇
  1983年   3313篇
  1982年   3541篇
  1981年   3482篇
  1980年   3370篇
  1979年   3452篇
  1978年   3550篇
  1977年   3469篇
  1976年   3561篇
  1975年   3243篇
  1974年   3250篇
  1973年   3294篇
  1972年   2297篇
  1971年   1884篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Conclusions The reactions of amides of arylphosphorous acids with acyl chlorides take place with replacement of the amide group and formation of amides of carboxylic acids and the corresponding aryl chlorophosphates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2131–2133, September, 1968.  相似文献   
982.
Summary The polarographic behaviour of nickel and cobalt in the mixed base electrolytes, urea-pyridine and urea-hydrazine has been studied. The effect of pH, concentration of supporting electrolyte and the presence of various other ions was investigated. Well defined polarograms for both the elements were obtained at pH 6.0 and 5.0 respectively. Attempts were made to utilize these observations in the separation of the two cations. While the difference in E 1/2 of the ions in urea-pyridine was greater than –0.3 volts that in urea-hydrazine was about –0.2 volts or slightly less even under the optimum conditions. In a mixed solution the waves for Co and Ni were therefore well developed and separated in the former base electrolyte. The polarograms in urea-hydrazine were, however, of little analytical use because of the washing away of the Ni wave due to the presence of even small amounts of cobalt.
Zusammenfassung Das polarographische Verhalten von Nickel und Kobalt in Harnstoff-Pyridin- und Harnstoff-Hydrazinlösung wurde untersucht und der Einfluß des pH-Wertes, der Leitsalzkonzentration sowie der Gegenwart verschiedener anderer Ionen festgestellt. Bei pH 6,0 bzw. 5,0 wurden für beide Elemente gut ausgebildete Polarogramme erhalten. Es wurde versucht, die erhaltenen Ergebnisse zur Trennung der beiden Ionen zu benutzen. In Harnstoff-Pyridinlösung beträgt der Unterschied der beiden Halbstufenpotentiale mehr als –0,3 V und man erhält gut ausgebildete und voneinander getrennte Stufen. In Harnstoff-Hydrazinlösung dagegen beträgt der Unterschied nur –0,2 V oder etwas weniger, selbst unter optimaler Bedingungen, und die Ni-Stufe wird schon durch kleine Kobaltmengen beeinträchtigt.
  相似文献   
983.
The relaxation time of the Kerr effect of nitrobenzene and m-nitrotoluene in various mixtures with carbon tetrachloride and various alcohols was determined by measuring the kinetics of the Kerr effect using picosecond laser techniques. These measurements yield information on the rotational motion of molecules in liquids. The relaxation time data are interpreted in terms of an effective local viscosity effect, pair correlation, and coupling of rotational motion with shear modes.  相似文献   
984.
The use of nuclide95Zr as radiotracer is often unfavourably influenced by the continuous accumulation of the radioactive daughter nuclide95Nb. The present paper gives a method showing how the daughter radioactivity can influence the value of the distribution coefficient of zirconium calculated from radioactivity measurements. A simple method of continuous purification of95Zr, which yields a product of the required degree of purity, is also described.  相似文献   
985.
The mass spectra of five polycyclic 1,4 benzodioxin derivatives have been studied. The breakdown patterns of the more important ions have been suggested by comparison with three compounds in a deuterated specific position. Some rearrangements are produced by specific hydrogen migration and a few others after scrambling.  相似文献   
986.
The pigments Luteoskyrin (Ls) and Rugulosin (Rg) might adopt a priori two extreme conformations, a planar and an angular one. To determine the predominant conformation in solution, the nature of the lowest energy transition of the chromophores and the presence of intramolecular H-bonding have been investigated. The solvent effects on electronic absorption and CD spectra indicated the π-π* nature of the lowest energy transition. Treatments of the electronic absorption results according to McRae and Kosower relations, IR absorption and PMR spectra suggested the presence of strong intra-molecular H-bonds. From these results it was concluded that Ls and Rg adopted in a variety of solvents the most planar conformation.  相似文献   
987.
The syntheses of 7′-methoxy- and 5′,7′,6-trimethoxy-coumarino(3′,4′:3,2)coumarones (IIa and IIb) are described.  相似文献   
988.
An accurate method for the determination of inorganic sulphate in synthetic detergents has been developed. After addition of a cationic surface-active agent (cetyl pyridinium chloride) to an anionic detergent, sulphate can be determined by means of indirect titration with EDTA, using a mixture of Metalphthalein and Naphthol Green B as indicator.  相似文献   
989.
In the interaction of ethioporphyrin and of several of its metal derivatives with negative ions of pyridine and anthracene in tetrahydrofuran solutions, an electron transfer takes place which results in the conversion of the pyridine and anthracene ions into neutral molecules, whereas the molecules of the pigment are first transformed into monovalent anions and subsequently into the di-, tri-, tetra-, and sometimes even into the hexavalent anions. In the first stage of this interaction, ferriethioporphyrin is reduced to ferro-ethioporphyrin, whereas the unmetallized pigment and its copper complex are transformed into the disodium derivative of ethioporphyrin. When the temperature is lowered, the rate of the electron transfer from pyridine ions to zinc-ethioporphyrin is decreased, and illumination leads to a substantial rate increase. The photochemical sensitivity of this system is possibly linked to the formation of stable complexes between pigment molecules and pyridine ions.  相似文献   
990.
The number-average molecular weights of beryllium 4-biphenyl(phenyl)phosphinate, di-n-pentylphosphinate, di-n-heptylphosphinate, and trifluoromethyl(phenyl)phosphinate are degraded by the presence of water in toluene. It is proposed that easily hydrolyzable P? O? P bonds contribute, in part, to the bonding of these polymers.  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