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   7521篇
  免费   286篇
  国内免费   2篇
化学   3995篇
晶体学   46篇
力学   139篇
数学   1648篇
物理学   1981篇
  2021年   100篇
  2020年   123篇
  2019年   95篇
  2018年   169篇
  2017年   178篇
  2016年   412篇
  2015年   265篇
  2014年   302篇
  2013年   683篇
  2012年   367篇
  2011年   471篇
  2010年   271篇
  2009年   228篇
  2008年   239篇
  2007年   181篇
  2006年   141篇
  2005年   92篇
  2004年   96篇
  2003年   104篇
  2002年   118篇
  2001年   104篇
  2000年   80篇
  1999年   90篇
  1998年   89篇
  1997年   89篇
  1996年   84篇
  1995年   81篇
  1994年   74篇
  1993年   70篇
  1992年   71篇
  1991年   64篇
  1990年   59篇
  1989年   62篇
  1988年   67篇
  1987年   72篇
  1986年   66篇
  1985年   89篇
  1984年   83篇
  1983年   76篇
  1982年   69篇
  1981年   72篇
  1980年   84篇
  1979年   76篇
  1978年   80篇
  1977年   84篇
  1976年   64篇
  1974年   69篇
  1973年   75篇
  1966年   64篇
  1965年   66篇
排序方式: 共有7809条查询结果,搜索用时 15 毫秒
181.
Thermoanalytical techniques (TG, DTG, DTA and EGA) can be helpful in studying gel compositions and relating the chemical history and the role of the precipitant to the chemical properties of gels. The objective of our work was to determine the influence of the initial conditions in the reaction mixture on the product characteristics. Zirconia gels were prepared by the gelprecipitation method by the hydrolysis of zirconium tetrachloride dissolved in methanol. The substitution of water by methanol was chosen in order to obtain a reaction medium in which better control of the hydrolysis and condensation reactions of the zirconium precursor were achieved and thus also better control of the final properties of the dried zirconia gel-precipitates could be obtained. The nature of the hydrated zirconia gels obtained during the gelation process is strongly influenced by the conditions of the reaction mixture, e.g. concentration of the zirconium precursor, the mixing rate and particularly by the amount of water added. To distinguish between various types of water in the gel-precipitates, the furnace atmosphere and the partial water pressure were altered appropriately. It was shown that initial zirconium to water molar ratio has a significant effect on the properties of the final products. It was assumed that, according to the preparation conditions, three different types of water are present in the formed zirconia gel-precipitates. Modification of the dehydration processes of zirconia gel-precipitates enables control of the final microstructural and surface properties of the dried gels which are suitable for further catalysts support as well as mixed oxide preparation.  相似文献   
182.
183.
We speculate on a generalized dynamics described by an integral over action functionals that is a generalization of the standard functional integral. In a simple Gaussian case we obtain a certain differential equation for the measure of Feynman integral. We prove that the equation is satisfied for the spin zero field in one space-time dimension.  相似文献   
184.
Summary The syntheses andin vitro antibacterial and antifungal evaluation of certain (5-nitro-2-furyl)azomethines with different heterocyclic nuclei are described.
Die Anwendung von Lävulinsäure und 5-Nitro-2-furylmethylendiacetat in der Totalsynthese einiger neuer biologisch aktiver (5-Nitro-2-furyl)azomethine
Zusammenfassung Es wird die Synthese und diein-vitro-antibakterielle und antifungale Wirksamkeit für bestimmte (5-Nitro-2-furyl)azomethine mit verschiedenen heterocyclischen Kernen beschrieben.
  相似文献   
185.
Summary Good yields of substituted 5-(5R-2-furyl)-4H-pyrano[2,3-d]pyrimidines4 were obtained in the reaction of 2-ethoxymethyleneamino-3-cyano-4H-pyranes2 with ammonia. Compounds2 were prepared by the condensation of the starting 2-amino-5-acetyl-3-cyano-6-methyl-4H-pyranes1 with ethyl orthoformate. Spectral properties of the bicyclic system4 in relation to compounds3 are discussed.
  相似文献   
186.
Summary The formation of V2O5 fibrous microcrystals in the V2O5-NH3-H2O system has been investigated by electron microscopy. The morphology of these microcrystals is dependent on the concentration of the particular components in the colloidal system and on the time of aging. In some colloid systems the orientation of V2O5 fibrous crystals with their long axes parallel to one another is also possible. The microcrystal growth in these systems can be attributed to very effective conditions for the recrystallization of vanadium pentoxide crystals in equilibrium with ammonium vanadate in solution.With 3 figures  相似文献   
187.
JPC – Journal of Planar Chromatography – Modern TLC - The flowering tops of Hypericum hirsutum, H. montanum, H. perforatum subsp. angustifolium, H. perforatum subsp. perforatum, and H....  相似文献   
188.
The method is based on the oxidation of ethanol in the presence of alcohol oxidase, followed by oxidation of 2,2'-azino-di(3-ethylbenzthiazoline-6-sulfonate) in the presence of peroxidase.  相似文献   
189.
190.
The present paper deals with the application of direct injection enthalpimetry (DIE) in the study of the kinetics of azo coupling reactions. On the example of the coupling of diazotized aniline-4-sulphonic acid with 1,8-dihydroxynaphthalene-3,6-disulphonic acid, it was verified that the rate constants determined using DIE are comparable with those determined using spectrophotometry. Advantages result from the application of DIE in the study of the kinetics of azo coupling reactions.  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