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   208006篇
  免费   2242篇
  国内免费   552篇
化学   114893篇
晶体学   3688篇
力学   8100篇
综合类   11篇
数学   19807篇
物理学   64301篇
  2020年   1810篇
  2019年   2023篇
  2018年   2326篇
  2017年   2442篇
  2016年   3703篇
  2015年   2367篇
  2014年   3759篇
  2013年   9439篇
  2012年   6918篇
  2011年   8491篇
  2010年   5992篇
  2009年   5885篇
  2008年   7540篇
  2007年   7395篇
  2006年   7029篇
  2005年   6331篇
  2004年   5789篇
  2003年   5334篇
  2002年   5069篇
  2001年   6236篇
  2000年   4708篇
  1999年   3587篇
  1998年   2795篇
  1997年   2747篇
  1996年   2648篇
  1995年   2486篇
  1994年   2378篇
  1993年   2194篇
  1992年   2854篇
  1991年   2754篇
  1990年   2708篇
  1989年   2721篇
  1988年   2735篇
  1987年   2742篇
  1986年   2609篇
  1985年   3341篇
  1984年   3337篇
  1983年   2617篇
  1982年   2730篇
  1981年   2786篇
  1980年   2546篇
  1979年   2899篇
  1978年   2875篇
  1977年   2999篇
  1976年   2836篇
  1975年   2570篇
  1974年   2520篇
  1973年   2486篇
  1972年   1710篇
  1968年   1707篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The linear dispersion relation of a backward-wave oscillator (BWO), derived earlier by the authors, is modified to include effects of RF surface current at the beam-vacuum interface. This modified dispersion relation results in an unstable interaction between the slow cyclotron mode (SCM) and the structure mode in addition to the conventional Cherenkov instability caused by the slow space charge mode. Numerical analysis is then carried out using parameters of a BWO experiment at University of Maryland. Fine structure of the SCM instability is elucidated. The analysis indicates that BWO radiation would not be suppressed near cyclotron absorption in an infinitely long system.  相似文献   
72.
73.
Diffuse x-ray scattering (DXS) is used to study the formation of microdefects (MDs) in heat-treated dislocation-free large-diameter silicon wafers with vacancies. The DXS method is shown to be efficient for investigating MDs in silicon single crystals. Specific defects, such as impurity clouds, are found to form in the silicon wafers during low-temperature annealing at 450°C. These defects are oxygen-rich regions in the solid solution with diffuse coherent interfaces. In the following stages of decomposition of the supersaturated solid solution, oxide precipitates form inside these regions and the impurity clouds disappear. As a result of the decomposition of the supersaturated solid solution of oxygen, interstitial MDs form in the silicon wafers during multistep heat treatment. These MDs lie in the {110} planes and have nonspherical displacement fields. The volume density and size of MDs forming in the silicon wafers at various stages of the decomposition are determined.  相似文献   
74.
75.
Summary A non-destructive method is described for the determination of major and minor constituents in archeological specimens by energy-dispersive X-ray fluorescence. Homogeneity tests are made by measuring at various sites of the specimen. In the same way, mean values are obtained for inhomogeneous specimen without taking samples. For calibration, powder standards are used. In case of the determination of elements with numbers up to 14 (Si) a vacuum chamber is used and the dimensions of the specimens are limited by the dimensions of that vacuum chamber, whereas for the determination of elements from K up to U specimens of any size, form or weight are suitable.
Zerstörungsfreie Analyse von archäologischen Proben mit Hilfe der Energie-dispersiven Röntgenfluorescenzanalyse
Zusammenfassung Eine zerstörungsfreie Methode für die Bestimmung von Haupt- und Nebenbestandteilen in archäologischen Proben mit Hilfe der Energie-dispersiven Röntgenfluorescenzanalyse wird beschrieben. Für Homogenitätstests wird an mehreren Stellen der Probe gemessen. In der gleichen Weise werden für inhomogene Proben Mittelwerte erhalten ohne Probenahme. Für die Eichung werden Pulverstandards verwendet. Im Falle der Bestimmung von Elementen mit Ordnungszahlen bis 14 (Si) wird eine Vakuumkammer eingesetzt, und die Dimensionen der Proben sind durch die Dimensionen dieser Vakuumkammer begrenzt, während für die Bestimmung der Elemente K bis U Proben jeder Größe, jeder Form oder jeden Gewichts verwendbar sind.
  相似文献   
76.
Summary Besides functional behaviour of coatings their resistivity against corroding attack is a crucial criterion for quality. The application of special corroding and abrading tests yields after short times predictions about stability of the product in practical use. A number of tests have been applied to thin magnetic storage media. Besides tests of function and visual observations with light- and scanning electron microscopes, AES- and XPS-investigations were applied to discuss corrosion resistivity.  相似文献   
77.
78.
Ohne Zusammenfassung
Trace analysis of lead, cadmium and manganese in honey and sugar

Wir danken dem Fonds der Chemischen Industrie für finanzielle Unterstützung.  相似文献   
79.
80.
Catalytic activities of three transition metals, as iridium (III) chloride, rhodium (III) chloride and palladium (II) chloride, were compared in the oxidation of six aromatic aldehydes (benzaldehyde, p‐chloro benzaldehyde, p‐nitro benzaldehyde, m‐nitro benzaldehyde, p‐methoxy benzaldehyde and cinnamaldehyde), two hydrocarbons (viz. (anthracene and phenanthrene)) and one aromatic and one cyclic alcohol (cyclohexanol and benzyl alcohol) by 50% H2O2. The presence of traces (substrate: catalyst ratio equal to 1:62500 to 1:1961) of the chlorides of iridium(III), rhodium(III) and palladium(II) catalyze these oxidations, resulting in good to excellent yields. It was observed that in most of the cases palladium(II) chloride is the most efficient catalyst. Conditions for the highest and most economical yields were obtained. Deviation from the optimum conditions decreases the yields. Oxidation in aromatic aldehydes is selective at the aldehydeic group only and other groups remain unaffected. This new, simple and economical method, which is environmentally safe, also requires less time. Reactive species of catalysts, existing in the reaction mixture are also discussed. Copyright © 2007 John Wiley & Sons, Ltd.  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