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   19061篇
  免费   3299篇
  国内免费   1688篇
化学   13185篇
晶体学   179篇
力学   1040篇
综合类   67篇
数学   2028篇
物理学   7549篇
  2024年   63篇
  2023年   403篇
  2022年   712篇
  2021年   762篇
  2020年   740篇
  2019年   756篇
  2018年   739篇
  2017年   589篇
  2016年   984篇
  2015年   876篇
  2014年   1090篇
  2013年   1421篇
  2012年   1733篇
  2011年   1762篇
  2010年   1160篇
  2009年   1086篇
  2008年   1198篇
  2007年   1040篇
  2006年   990篇
  2005年   788篇
  2004年   602篇
  2003年   425篇
  2002年   434篇
  2001年   334篇
  2000年   279篇
  1999年   389篇
  1998年   276篇
  1997年   292篇
  1996年   284篇
  1995年   245篇
  1994年   209篇
  1993年   216篇
  1992年   143篇
  1991年   159篇
  1990年   129篇
  1989年   99篇
  1988年   88篇
  1987年   71篇
  1986年   68篇
  1985年   54篇
  1984年   57篇
  1983年   30篇
  1982年   32篇
  1981年   23篇
  1980年   26篇
  1978年   13篇
  1976年   16篇
  1975年   15篇
  1973年   15篇
  1968年   14篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.

Abstract  

The non-enzyme-catalyzed reaction of reduced glutathione (GSH) with two tumor cell cytotoxic cyclic chalcone analogues was investigated by reversed-phase high-performance liquid chromatography (RP-HPLC). HPLC analysis of the reaction mixtures indicated the formation of two diastereomeric chalcone–GSH adducts in each case, whose structural assignments were supported by MALDI-TOF-MS and HPLC–MS with electrospray ionization (ESI) measurements. Such reactivity accounts for the previously observed effect of the two cyclic chalcone analogues on the in vivo cellular thiol level of Jurkat T cells.  相似文献   
83.
Early detection of the red palm weevils (RPW) is a major challenge in agriculture among all kinds of palm trees due to the nature of the insect and the difficulty to trace them through their life stages associated with the tree life. Many methods have been applied for the weevil detection such as X-ray diffraction techniques, fluoroscopy and ultrasound. On the other hand, the idea of tomography has been used for other purposes such as the determination of the age of the tree and for applied environmental studies. Such technology can also reveal the weevil in principle. In this study, we explore the use of X-ray CT for weevil detection with the Monte Carlo method. A model of the stem of a palm tree is developed for simulations. MCNPX is chosen to carry out the simulations for the radiography tally in the code. The tally records the 2D data of the X-ray beams irradiating the tree model. An iterative reconstruction method for cone beam CT is applied to obtain the 3D slices of the tree model. We are exploring the minimum number of projection angles and the detectability of the weevil. We shall also report the sensitivity of weevil detection using X-ray CT with a large set of simulations with different weevil sizes and tree diameters.  相似文献   
84.
The reaction of [RuHCl(CO)(PPh3)3] with 4-phenylpyrimidine gave a new ruthenium(II) complex, namely [RuHCl(CO)(PPh3)2(pyrim-4-Ph)]. The complex has been studied by IR and UV?Cvis spectroscopy and by X-ray crystallography. The molecular orbitals of the complex have been calculated by density functional theory. The spin-allowed singlet?Csinglet electronic transitions of the complex have been calculated by time-dependent DFT, and the UV?Cvis spectrum of the compound has been discussed on this basis. The emission properties of the complex were also studied.  相似文献   
85.
This paper describes a rigorous a posteriori error analysis for the stochastic solution of non-linear uncertain chemical models. The dual-based a posteriori stochastic error analysis extends the methodology developed in the deterministic finite elements context to stochastic discretization frameworks. It requires the resolution of two additional (dual) problems to yield the local error estimate. The stochastic error estimate can then be used to adapt the stochastic discretization. Different anisotropic refinement strategies are proposed, leading to a cost-efficient tool suitable for multi-dimensional problems of moderate stochastic dimension. The adaptive strategies allow both for refinement and coarsening of the stochastic discretization, as needed to satisfy a prescribed error tolerance. The adaptive strategies were successfully tested on a model for the hydrogen oxidation in supercritical conditions having 8 random parameters. The proposed methodologies are however general enough to be also applicable for a wide class of models such as uncertain fluid flows.  相似文献   
86.
Based on the actual competition in the Chinese property insurance market, the repeated price game model for four oligarchs with different decision rules is built. On the basis of analyzing the stabilities of eight fixed points about the four-dimensional dynamic system, the Nash equilibrium and its local stable region are discussed mainly. Then the complexity of the four-dimensional discrete dynamic system and its evolutionary process are studied. Finally, the delay feedback control method is used to control the chaos. Numerical simulation results have shown that the influence which the change of price adjustment speed has on the movement of dynamic system has a sensitive dependence on the initial conditions, and there is a strong connection between the profit of each oligarch and the price adjustment speed or the control parameters, and the corresponding economic explanation to those phenomena will have important reference values to the realistic problems.  相似文献   
87.
Summary In the present paper, a diffraction integral formula for a misaligned optical system is derived. The interference effects of opticalelement arrays are studied by means of such a formula. The possibility of eliminating interference fringes is discussed. The conclusion is that the interference fringes could be eliminated when the αβγδ condition for arrays is satisfied. The synthetical aberration is simultaneously eliminated. However, in this case, the synthetical image coincides with the individual images. For some special application, the synthetical image does not coincide with the individual images and the αβγδ condition is not fulfilled. But the interference fringes may disappear in some special cases.
Riassunto La formula di Collins, che esprime il cammino ottico fra un punto del piano oggetto e un punto del piano immagine nel caso di un sistema ottico centrato, è generalizzata al caso di un sistema ottico disallineato. La formula cosí ottenuta è applicata allo studio del fenomeno d’interferenza che ha luogo nel piano della cosiddetta ?immagine sintetica? fra i vari fasci emergenti da una cortina di elementi ottici. Si discutono infine le condizioni in cui non si hanno frange d’interferenza sull’immagine sintetica.
  相似文献   
88.
89.
周伟江  马汉东 《计算物理》1993,10(4):422-428
绕弹体的超声速气流与发动机喷流相互作用,在尾部形成复杂的干扰流场。本文用有限差分法求解全N-S方程,对这一复杂流场进行了数值模拟,得到了实验观察到的各种流场结构及其随喷口压力比的变化规律。外流M=1.94,Re=2.2×105,喷流Mj=3.0,喷口压力比pj/p分别为1.03,0.527,0.15三种。差分算法为一种改进的Beam-Warming格式。计算底部压力和激波在喷流中心的反射位置与实验数据进行了比较,吻合较好。  相似文献   
90.
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