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   567848篇
  免费   6013篇
  国内免费   1648篇
化学   274027篇
晶体学   7340篇
力学   29279篇
综合类   14篇
数学   96114篇
物理学   168735篇
  2021年   3939篇
  2020年   4330篇
  2019年   4673篇
  2018年   20350篇
  2017年   20603篇
  2016年   16603篇
  2015年   6935篇
  2014年   9017篇
  2013年   20808篇
  2012年   21701篇
  2011年   36109篇
  2010年   23209篇
  2009年   23464篇
  2008年   30555篇
  2007年   34527篇
  2006年   16701篇
  2005年   20809篇
  2004年   17408篇
  2003年   16181篇
  2002年   13718篇
  2001年   13780篇
  2000年   10576篇
  1999年   8082篇
  1998年   6769篇
  1997年   6457篇
  1996年   6372篇
  1995年   5660篇
  1994年   5545篇
  1993年   5354篇
  1992年   5942篇
  1991年   6060篇
  1990年   5778篇
  1989年   5600篇
  1988年   5515篇
  1987年   5425篇
  1986年   5104篇
  1985年   6607篇
  1984年   6751篇
  1983年   5357篇
  1982年   5449篇
  1981年   5423篇
  1980年   5004篇
  1979年   5434篇
  1978年   5570篇
  1977年   5406篇
  1976年   5371篇
  1975年   4937篇
  1974年   4906篇
  1973年   4954篇
  1972年   3486篇
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
111.
112.
Multi-sensor data fusion is an evolving technology whereby data from multiple sensor inputs are processed and combined. The data derived from multiple sensors can, however, be uncertain, imperfect, and conflicting. The present study is undertaken to help contribute to the continuous search for viable approaches to overcome the problems associated with data conflict and imperfection. Sensor readings, represented by belief functions, have to be fused according to their corresponding weights. Previous studies have often estimated the weights of sensor readings based on a single criterion. Mono-criteria approaches for the assessment of sensor reading weights are, however, often unreliable and inadequate for the reflection of reality. Accordingly, this work opts for the use of a multi-criteria decision aid. A modified Analytical Hierarchy Process (AHP) that incorporates several criteria is proposed to determine the weights of a sensor reading set. The approach relies on the automation of pairwise comparisons to eliminate subjectivity and reduce inconsistency. It assesses the weight of each sensor reading, and fuses the weighed readings obtained using a modified average combination rule. The efficiency of this approach is evaluated in a target recognition context. Several tests, sensitivity analysis, and comparisons with other approaches available in the literature are described.  相似文献   
113.
Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters.  相似文献   
114.
115.
116.
117.
Embedding endohdedral metallofullerenes (EMFs) into electron donor–acceptor systems is still a challenging task owing to their limited quantities and their still largely unexplored chemical properties. In this study, we have performed a 1,3‐dipolar cycloaddition reaction of a corrole‐based precursor with Sc3N@C80 to regioselectively form a [5,6]‐adduct ( 1 ). The successful attachment of the corrole moiety was confirmed by mass spectrometry. In the electronic ground state, absorption spectra suggest sizeable electronic communications between the electron acceptor and the electron donor. Moreover, the addition pattern occurring at a [5,6]‐bond junction is firmly proven by NMR spectroscopy and electrochemical investigations performed with 1 . In the electronically excited state, which is probed in photophysical assays with 1 , a fast electron‐transfer yields the radical ion pair state consisting of the one‐electron‐reduced Sc3N@C80 and of the one‐electron‐oxidized corrole upon its exclusive photoexcitation. As such, our results shed new light on the practical work utilizing EMFs as building blocks in photovoltaics.  相似文献   
118.
119.
120.
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