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   152808篇
  免费   1354篇
  国内免费   384篇
化学   82071篇
晶体学   2029篇
力学   6658篇
综合类   7篇
数学   15281篇
物理学   48500篇
  2020年   1114篇
  2019年   1173篇
  2018年   1476篇
  2017年   1451篇
  2016年   2468篇
  2015年   1548篇
  2014年   2426篇
  2013年   6317篇
  2012年   4932篇
  2011年   6266篇
  2010年   4283篇
  2009年   4293篇
  2008年   5694篇
  2007年   5772篇
  2006年   5409篇
  2005年   4996篇
  2004年   4410篇
  2003年   3853篇
  2002年   3886篇
  2001年   4341篇
  2000年   3320篇
  1999年   2572篇
  1998年   2242篇
  1997年   2186篇
  1996年   1982篇
  1995年   1899篇
  1994年   1888篇
  1993年   1726篇
  1992年   1993篇
  1991年   2121篇
  1990年   1917篇
  1989年   1919篇
  1988年   1859篇
  1987年   1755篇
  1986年   1693篇
  1985年   2195篇
  1984年   2326篇
  1983年   1897篇
  1982年   2078篇
  1981年   1966篇
  1980年   1889篇
  1979年   2030篇
  1978年   2197篇
  1977年   2067篇
  1976年   2113篇
  1975年   2019篇
  1974年   2074篇
  1973年   2061篇
  1972年   1354篇
  1971年   1211篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dimethyl α-bromo-o-benzenediacetate ( 1 ) condensed with hydrazine and acetylhydrazine to give respectively 1-carbomethoxy-2-amino-1,4-dihydro-3-(2H)isoquinolinone (2) and its N-acetyl derivative ( 9 ). Replacement of the bromine atom of 1 with the N-1-methylhydrazino ( 3 ) and the N-1-phenylhydrazino ( 5 ) groups occurred by allowing 1 to react respectively with methylhydrazine and phenylhydrazine. In the latter case alkylation by 1 at the N-2 also occurred which led to the formation of the 2-phenylaminoisoquinolinone ( 8 ). Derivatives 3 and 5 smoothly cyclized to the 1-earbomethoxy-5(H)-1,2,3,4-tetrahydro-2,3-benzodiazepin-4-ones 4 and 6 . A series of derivatives of 2 were also pharmacologically tested as antiinflammatory and CNS depressant agents.  相似文献   
992.
Copolymers of 1,2,2,2-tetrachloroethyl esters of unsaturated acids and halogenated N-phenyl maleimides with styrene were pyrolyzed; volatile products were analyzed with a mass spectrometer combined with a gas chromatograph. Hydrogen halide and carbon dioxide in the volatile products were determined during the thermal decomposition of copolymers in glass ampoules; the acyl chloride groups were determined in the residues. The thermal decomposition of copolymers of tetrachloroethyl esters with styrene sets in at ca. 230° by the release of chloral from the copolymer and splitting of some of the CCl bonds in the copolymer. The decomposition of copolymers of styrene with halogenated N-phenyl maleimides starts above 300° by depolymerization of the polystyrene chain sections and by splitting of some of the carbon-halogen bonds. At 310 and 500° for copolymers of tetrachloroethyl esters and at 500° for halogenated N-phenyl maleimides, there is radical dehydrohalogenation of the copolymers, with depolymerization of polystyrene blocks and splitting of carbon-carbon bonds in the main chain.  相似文献   
993.
The effect of substituents on the activation energy for primary dissociation processes in the molecular ions of mono- and para and meta di-substituted benzenes has been examined. Where the daughter ion retains the substituent group, variation of the energy of activation derives from a combination of the effects of substituents on the ionisation potential of the molecular ion and the appearance potential of the daughter ion. An equation relating the energy of activation for the fragmentation of the molecular ion of a mono-substituted benzene to that of related para and meta di-substituted benzenes is presented.  相似文献   
994.
995.
The phase diagram of the acrylic acid–acrylamide system as determined by DTA and the supporting x-ray diffraction data are presented. This diagram shows that the system forms a 1 : 1 addition compound which decomposes above 0°C. There is also one eutectic point at a 67 mole-% acrylic acid composition.  相似文献   
996.
Conclusions Cyclic alkoxysilanes — 1,3-dioxa-2-silacyclanes — and their oligomers are decomposed by organomagnesium compounds with the formation of the corresponding silanols (disiloxanes) and tetraalkylsilanols.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1283–1285, July, 1966.  相似文献   
997.
Zusammenfassung Das in der vorangehenden Mitt. vonE. Späth undF. Gandini-Kesztler beschriebene Nebenalkaloid wird auf synthetischem Wege mit dem 6-Hydroxy-peganin identifiziert. Die phenolische OH-Gruppe steht also, bezogen auf Formel III, nicht an der Stelle, welche dem CH3O des Harmins entspricht.  相似文献   
998.
A fairly general degenerate perturbation theory has been considered which involves two versions, i.e., the Rayleigh-Schrodinger and Brillouin-Wigner treatments. The simple recursive formulas have been found for effective Hamiltonians which allow one to split the degenerate energy levels in any orders of this theory.  相似文献   
999.
Abstract— The fiber-optic properties of etiolated plant tissues can be used to detect and characterize pigment absorption in vivo. Transmission spectra of light guided through several monocot and dicot etiolated tissues show a decreasing red/far red ratio with increasing tissue length. Absorption bands attributable both to vacuolar pigments such as anthocyanins and to chloroplast pigments lead to the conclusion that the guided light passes through both vacuole and cytoplasm. As etiolated tissue becomes green under white light treatment, the red/far red ratio also changes, the nature of the change depending upon the tissue involved. The blue/red ratio also changes both with increasing length of etiolated tissue and during the greening process, with the changes again dependent on the tissue involved. The spectral dependence of the light-guiding phenomenon in dark grown and green plants may have implications for physiological responses mediated by phytochrome.  相似文献   
1000.
It is proved that under the conditions of dynamic thermoanalytical examinations the course of a decomposition reaction is determined primarily by the gas- and heattransfer partial processes, i.e. indirectly by the experimental conditions. This raises the question of whether it is justified to make kinetic calculations based on the shapes of similar curves. A new measuring technique is reported which uses a slightly modified Derivatograph. With this new method decomposition reactions can be examined under “quasi-isothermal” and “quasi-isobaric” conditions. The pernicious effects of the gas- and heat-transfer processes are thus eliminated, and the shapes of the curves obtained provide the possibility of studying the decomposition reactions in a new and more thorough way.  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