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   85261篇
  免费   1581篇
  国内免费   1120篇
化学   29352篇
晶体学   919篇
力学   7166篇
综合类   72篇
数学   32639篇
物理学   17814篇
  2023年   174篇
  2022年   253篇
  2021年   276篇
  2020年   369篇
  2019年   344篇
  2018年   10652篇
  2017年   10439篇
  2016年   6425篇
  2015年   1217篇
  2014年   795篇
  2013年   883篇
  2012年   4391篇
  2011年   11116篇
  2010年   6050篇
  2009年   6384篇
  2008年   7035篇
  2007年   9127篇
  2006年   574篇
  2005年   1599篇
  2004年   1718篇
  2003年   2170篇
  2002年   1163篇
  2001年   363篇
  2000年   439篇
  1999年   302篇
  1998年   319篇
  1997年   274篇
  1996年   345篇
  1995年   224篇
  1994年   183篇
  1993年   181篇
  1992年   130篇
  1991年   131篇
  1990年   121篇
  1989年   105篇
  1988年   107篇
  1987年   94篇
  1986年   94篇
  1985年   72篇
  1984年   64篇
  1983年   51篇
  1982年   60篇
  1981年   53篇
  1980年   57篇
  1979年   51篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
21.
22.
23.
24.
A first example of an Et2Zn mediated silylation of 1-aklynes is reported. A series of functional groups are tolerated in this reaction. Mechanistic studies support Zn alkynilides as intermediates in the reaction. This reaction protocol provides a practical method for the preparation of alkynylsilanes and expands the application of organometallic zinc in organic synthesis.  相似文献   
25.
26.
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   
27.
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   
28.
杂多酸催化合成乙酸乙酯   总被引:14,自引:0,他引:14  
回流吸附法制备的负载型12-钨磷酸(HPW/C)是合成乙酸乙酯的高效催化剂。最佳反应条件下乙醇转化率达96%,酯化选择性100%。催化剂的高活性是由于HPW在活性炭上高度分散,使活性位增多,有效酸量增大。HPW/C还具有热稳定性高和不易流失等优点。  相似文献   
29.
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   
30.
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   
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