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   648463篇
  免费   7540篇
  国内免费   1992篇
化学   333913篇
晶体学   9684篇
力学   31199篇
综合类   22篇
数学   95600篇
物理学   187577篇
  2021年   4713篇
  2020年   5372篇
  2019年   5848篇
  2018年   16851篇
  2017年   16651篇
  2016年   16780篇
  2015年   7782篇
  2014年   11260篇
  2013年   27095篇
  2012年   23431篇
  2011年   34086篇
  2010年   22472篇
  2009年   22689篇
  2008年   28413篇
  2007年   30071篇
  2006年   20964篇
  2005年   19784篇
  2004年   18565篇
  2003年   17108篇
  2002年   15941篇
  2001年   17403篇
  2000年   13300篇
  1999年   10389篇
  1998年   8584篇
  1997年   8336篇
  1996年   8097篇
  1995年   7284篇
  1994年   7117篇
  1993年   6782篇
  1992年   7821篇
  1991年   7864篇
  1990年   7517篇
  1989年   7369篇
  1988年   7474篇
  1987年   7327篇
  1986年   6923篇
  1985年   9023篇
  1984年   9149篇
  1983年   7316篇
  1982年   7471篇
  1981年   7482篇
  1980年   6912篇
  1979年   7543篇
  1978年   7712篇
  1977年   7676篇
  1976年   7468篇
  1975年   6863篇
  1974年   6807篇
  1973年   6788篇
  1972年   4791篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
This paper addresses the issue of energy propagation features in ribbed panels over a wide frequency range. First, a tool for estimating the wave propagation characteristics of one- and two-dimensional structures by k-space analysis is presented. This tool uses a concept of Inhomogeneous Wave Correlation with sparse measured or extracted data, and leads to the estimation of θ-dependent wavenumbers. Here, the method is employed with the sparse simulated and measured normal velocities of a set of ribbed panels and new insights into their k-space behavior are highlighted. Behavior is essentially characterized at low frequencies by structural orthotropy. Comparisons with homogenized data show very good agreement. At higher frequencies, a new behavior pattern is observed and explained both numerically and experimentally. This is a multi-modal waveguide type of propagation in a direction parallel to the ribs.  相似文献   
942.
Structural hierarchy of rock masses and the mechanisms of its formation   总被引:1,自引:0,他引:1  
The formation mechanisms for the structural hierarchy in geological media are discussed. It is shown that the formation of such a hierarchy is caused by certain external and internal circumstances. The first ones consist in the fact that, because of external actions, the Earth’s poles continuously execute the translational and rotational motions responsible for a regular structure of rock fracture and for the appearance of the scale factor √2. The second ones consist in the fact that, under the action of many random factors and because of external actions, during the formation of geological media there appear dissipative structures and, hence, some self-similar fractal structures are formed.  相似文献   
943.
The aim of this paper is to investigate laminar-turbulent transition in a mixed convection phenomenon occurring in a horizontal rectangular duct. Indeed, laminar-turbulent transition is well known in the case of forced convection but the presence of secondary flow induced by natural convection on this transition is not well highlighted. In this study, we will not be concerned by determining a critical threshold value of a Reynolds number of transition but only to estimate the degree of turbulence in the transition regime, i.e. weak turbulence in the case of a mixed convection phenomenon. This is possible thanks to the application of the wavelet transform. The calculation of the Hölder exponent, associated with the maximum value of the singularity spectrum for various experimental conditions allows the degree of turbulence to be measured. The variation of the Hölder exponent versus heat flux and Reynolds number enables us to show that there are two ways to go towards turbulence: thermal by increasing heat flux and hydrodynamic by increasing fluid velocity.  相似文献   
944.
The chlorination of benzene, toluene, and o-xylene with molecular chlorine in the presence of the phthalocyanine complexes of different structures was studied. The transformations of the catalysts during the reaction were investigated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1644–1647, August, 2008.  相似文献   
945.
Oximes of 4-(4-pyrazolyl)-3-buten-2-onees obtained by successive reaction of 3-aryl-4-formylpyrazoles with acetone and hydroxylamine at the treatment with iodine suffered an oxidative cyclization yielding 3-aryl-4-(5-isoxazolyl)pyrazoles.  相似文献   
946.
947.
Possibilities of utilization of by-products formed in manufacture of epichlorohydrin are considered. A promising way to utilize wastes from production of epichlorohydrin by synthesis of 2,3-dichloropropene from 1,2,3-trichloropropane is suggested.  相似文献   
948.
Polymer films with a dispersed liquid crystal were prepared by photopolymerization of (meth)acrylic monomers. The electrooptical properties of these films were studied. The influence exerted by the composition of the monomer mixture, cross-linking agents, and chain-transfer agent on the liquid crystal drop size and on the transmission of the polymer-liquid crystal films was examined.  相似文献   
949.
950.
We study the short-term staffing problem of systems that experience random, non-stationary demand. The typical method to accommodate changes in arrival rate is to use historical data to identify peak periods and associated forecasting for upcoming time windows. In this paper, we develop a method that instead detects change as it happens. Motivated by an automatic call distributor system in a call centre with time-varying arrivals, we propose a change detection algorithm based upon non-homogeneous Poisson processes. The proposed method is general and may be thought of as a feed-forward strategy, in which we detect a change in the arrival process, estimate the new magnitude of the arrival rate, and assign an appropriate number of servers to the tasks. The generalized likelihood ratio statistic is used and a recommendation for its decision limit is developed. Simulation results are used to evaluate the performance of the detector in the context of a telephone call centre.  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