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   323118篇
  免费   6193篇
  国内免费   4657篇
化学   163271篇
晶体学   5224篇
力学   15929篇
综合类   300篇
数学   45752篇
物理学   103492篇
  2020年   2106篇
  2019年   2229篇
  2018年   11386篇
  2017年   12342篇
  2016年   7721篇
  2015年   3920篇
  2014年   4172篇
  2013年   10001篇
  2012年   12088篇
  2011年   21936篇
  2010年   13830篇
  2009年   13730篇
  2008年   18880篇
  2007年   22435篇
  2006年   7752篇
  2005年   14013篇
  2004年   9612篇
  2003年   8684篇
  2002年   6454篇
  2001年   6570篇
  2000年   5242篇
  1999年   4025篇
  1998年   3139篇
  1997年   3102篇
  1996年   3164篇
  1995年   2867篇
  1994年   2706篇
  1993年   2530篇
  1992年   2986篇
  1991年   2868篇
  1990年   2805篇
  1989年   2802篇
  1988年   2741篇
  1987年   2720篇
  1986年   2539篇
  1985年   3400篇
  1984年   3402篇
  1983年   2825篇
  1982年   3015篇
  1981年   2937篇
  1980年   2846篇
  1979年   3005篇
  1978年   3253篇
  1977年   2994篇
  1976年   2957篇
  1975年   2800篇
  1974年   2749篇
  1973年   2794篇
  1968年   2026篇
  1967年   2165篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Poly(γ-benzyl-L-glutamate) (PBLG), was utilized as a surface coating agent in order to obtain parallel molecular orientation in thin layers of PBLG liquid-crystalline solutions confined between coated surfaces. It was found that in a thickness range much larger than the cholesteric pitch value, the texture was predominantly planar cholesteric. At film thickness comparable or smaller than the pitch, isotropic spherulites developed from a homeotropic nematic phase. Because of the coupling between the solution concentration and nematic director, as well as large differences in the elastic moduli of PBLG liquid-crystalline solutions, the long-range intermolecular interaction was unfavorable. Competition between parallel orientation induced by the surface coating of PBLG and inherent perpendicular orientation induced by PBLG molecules in the bulk would favor a short-range helix-coil transition mechanism, which results into a reentrant isotropic phase.  相似文献   
992.
We prove both geometric ergodicity and regular variation of the stationary distribution for a class of nonlinear stochastic recursions that includes nonlinear AR-ARCH models of order 1. The Lyapounov exponent for the model, the index of regular variation and the spectral measure for the regular variation all are characterized by a simple two-state Markov chain.  相似文献   
993.
A flow visualization of the two-dimensional rigid fling-clap motions of the flat-plate wing is performed to get the knowledge of fling-clapping mechanism that might be employed by insects during flight. In this numerical visualization, the time-dependent Navier-Stokes equations are solved for two types of wing motion; ‘fling followed by clap and pause motion’ and ‘cyclic fling-clapping motion’. The result is observed regarding the main flow features such as the sequential development of the two families of separation vortex pairs and their movement. For the ‘fling followed by clap and pause motion’, a strong separation vortex pair of counterrotation develops in the opening between the wings in the fling phase and they then move out from the opening in the following clap phase. For ‘the cyclic fling-clapping motion’, the separation vortex pair developed in the outside space in the clap phase move into the opening in the following fling phase. The separation vortex pair in the opening developed in the fling phase of the cyclic motion is observed to be stronger than those of the ‘fling followed by clap and pause motion’. Regarding the strong fling separation vortex and the weak clap separation vortex above it in the opening, the flow pattern of the fling phase of the cyclic fling and clap motion is different to that of the fling phase of the first cycle. The flow pattern of the third cycle of the cyclic fling-clapping motion is observed to be almost same as that of the second cycle. Therefore, a periodicity of the flow pattern is established after the second cycle.  相似文献   
994.
995.
The paper briefly describes the life and activities of F.A. Zander and Yu.V. Kondratyuk (A.I. Shargei), pioneers of domestic rocket engineering who made a significant contribution to the development of cosmonautics and rocket technologies. The main ideas and proposals of these prominent scientists and inventors in the field of rocket and space engineering, which found their application in space conquering, are considered. To the 120th anniversary of F.A. Zander and 110th anniversary of Yu.V. Kondratyuk  相似文献   
996.
The dispersion relation of magnetostatic waves tangentially magnetized to saturation ferrite film, with a “magnetic wall” condition (tangential component of microwave magnetic field is equal to zero) on one of the film surface and with a metal condition on the opposite surface is analyzed. The dispersion characteristics show that unidirectional magnetostatic waves appear in this structure: they can transfer energy in one direction only and fundamentally cannot transfer energy in the opposite direction. The dispersion-free propagation of magnetostatic waves also is possible in the structure in a wide frequency interval.  相似文献   
997.
An investigation of cold fusion of deuterium was performed by electrolyzing heavy water in a cell containing a palladium cathode while monitoring levels of tritium and neutron-capture gamma rays. No activity was detected that would indicate a sustained fusion reaction had taken place. Activities that were observed can be attributed to electrolytic enrichment and a normal cosmic-ray induced background.  相似文献   
998.
The solid solubility in the systems Y2O2S---La2O2S, Y2O2S---Gd2O2S, and Gd2O2S---La2O2S has been investigated. Solid solutions of all compounds, throughout the whole composition range, were readily obtained, using coprecipitated oxalates and a polysulfide flux.  相似文献   
999.
Data on the influence of the Stark Effect on an optically pumped FIR laser with CH3Cl or CH3Br as active medium are given for various wavelengths. A very simple method for an effective FIR power stabilisation based on the Stark Effect is described. Its working is illustrated with the EPR signal of a 10% diluted DAG sphere.  相似文献   
1000.
Stadia are popular models of chaotic billiards introduced by Bunimovich in 1974. They are analogous to dispersing billiards due to Sinai, but their fundamental technical characteristics are quite different. Recently many new results were obtained for various chaotic billiards, including sharp bounds on correlations and probabilistic limit theorems, and these results require new, more powerful technical apparatus. We present that apparatus here, in the context of stadia, and prove “regularity” properties.   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