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   85050篇
  免费   515篇
  国内免费   381篇
化学   28138篇
晶体学   909篇
力学   6863篇
数学   32979篇
物理学   17057篇
  2022年   65篇
  2021年   78篇
  2020年   99篇
  2019年   141篇
  2018年   10530篇
  2017年   10350篇
  2016年   6275篇
  2015年   1028篇
  2014年   559篇
  2013年   939篇
  2012年   4129篇
  2011年   10875篇
  2010年   5922篇
  2009年   6311篇
  2008年   6892篇
  2007年   9055篇
  2006年   516篇
  2005年   1526篇
  2004年   1729篇
  2003年   2184篇
  2002年   1181篇
  2001年   384篇
  2000年   417篇
  1999年   252篇
  1998年   296篇
  1997年   217篇
  1996年   274篇
  1995年   211篇
  1994年   127篇
  1993年   139篇
  1992年   128篇
  1991年   110篇
  1990年   114篇
  1989年   114篇
  1988年   124篇
  1987年   107篇
  1986年   127篇
  1985年   153篇
  1984年   149篇
  1983年   107篇
  1982年   106篇
  1981年   113篇
  1980年   135篇
  1979年   114篇
  1978年   107篇
  1977年   89篇
  1976年   85篇
  1975年   71篇
  1974年   61篇
  1973年   72篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films were investigated using a number of techniques. A high saturation magnetic induction up to B s = 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed, which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of the composition. The text was submitted by the authors in English.  相似文献   
23.
Summary Carbon deposits on the surface ofRu/Fe2O3 catalysts used in the water-gas shift reaction have been investigated by Auger Electron Spectrometry. A correlation has been found between the thickness of the carbon deposit and the catalytic activity in WGSR. The carbon deposit covers the metallic active centers and blocks their contact with reagents. The dotting of the iron oxide support with sodium has been found to reduce the amount of carbon deposit. .   相似文献   
24.
A forecasting model is developed for the number of daily applications for loans at a financial services telephone call centre. The purpose of the forecasts and the associated prediction intervals is to provide effective staffing policies within the call centre. The model building process is constrained by the availability of only 2 years and 7 months of data. The distinctive feature of the data is that demand is driven in the main by advertising. The analysis given focuses on applications stimulated by press advertising. Unlike previous analyses of broadly similar data, where ARIMA models were used, a model with a dynamic level, multiplicative calendar effects and a multiplicative advertising response is developed and shown to be effective.  相似文献   
25.
This paper describes three case studies which investigated issues relating to the implementation and management of advanced information technology. The benefits derived from information technology were found to vary from one organization to another. These studies provide further evidence to support earlier research findings that senior management has an important role to play in the effective exploitation of information technology, and that the investment approach taken by an organization has a significant effect on the development of computer-based information systems. The effectiveness of a system depends not only on applying the appropriate technology, but also on how successfully technical and behavioural issues are resolved.  相似文献   
26.
Space-time directional Lyapunov exponents are introduced. They describe the maximal velocity of propagation to the right or to the left of fronts of perturbations in a frame moving with a given velocity. The continuity of these exponents as function of the velocity and an inequality relating them to the directional entropy is proved.  相似文献   
27.
In this paper, an MILP formulation is proposed for the design of a reverse logistics network based on a warehouse location–allocation model, which optimizes, simultaneously, the forward and reverse networks. A single product model with unlimited capacity is first defined. Subsequently, the model is extended to a multi-product capacitated recovery network model, where capacity limitations and a multi-product system can be considered. The proposed model is compared to published work in the field, where different model assumptions have been proposed. Two cases are described so as to gain a better insight into the model and allow a comparative analysis.  相似文献   
28.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models.  相似文献   
29.
We consider RKKY interaction in a quasi 2D system with nonparabolic dispersion. In our paper we calculate the RKKY range function assuming the in-layer confinement via effective dimensionality approach. We show, that indirect magnetic exchange in our system can be modelled by the effective spectral dimension which equals one.  相似文献   
30.
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