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   177779篇
  免费   2150篇
  国内免费   704篇
电工技术   3347篇
综合类   103篇
化学工业   28069篇
金属工艺   7783篇
机械仪表   5131篇
建筑科学   4417篇
矿业工程   875篇
能源动力   4701篇
轻工业   16429篇
水利工程   1706篇
石油天然气   3127篇
武器工业   2篇
无线电   19906篇
一般工业技术   33529篇
冶金工业   33323篇
原子能技术   4313篇
自动化技术   13872篇
  2021年   1319篇
  2019年   1255篇
  2018年   2102篇
  2017年   2087篇
  2016年   2194篇
  2015年   1579篇
  2014年   2731篇
  2013年   7813篇
  2012年   4574篇
  2011年   6390篇
  2010年   5064篇
  2009年   5913篇
  2008年   5892篇
  2007年   5860篇
  2006年   5051篇
  2005年   4712篇
  2004年   4490篇
  2003年   4158篇
  2002年   4132篇
  2001年   4147篇
  2000年   3940篇
  1999年   4079篇
  1998年   10486篇
  1997年   7437篇
  1996年   5705篇
  1995年   4310篇
  1994年   3637篇
  1993年   3585篇
  1992年   2615篇
  1991年   2553篇
  1990年   2426篇
  1989年   2448篇
  1988年   2383篇
  1987年   2136篇
  1986年   2075篇
  1985年   2374篇
  1984年   2198篇
  1983年   2019篇
  1982年   1892篇
  1981年   1952篇
  1980年   1800篇
  1979年   1819篇
  1978年   1779篇
  1977年   2110篇
  1976年   2694篇
  1975年   1559篇
  1974年   1551篇
  1973年   1609篇
  1972年   1354篇
  1971年   1262篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
71.

Background  

Bioelectrical Impedance (BIA) derived phase angle is increasingly being used as an objective indicator of nutritional status in advanced cancer. Subjective Global Assessment (SGA) is a subjective method of nutritional status. The objective of this study was to investigate the association between BIA derived phase angle and SGA in advanced colorectal cancer.  相似文献   
72.
The use of damage-sensitive features to evaluate structural condition or health is a very critical aspect of structural health monitoring. The purpose of this paper is to investigate the potential of two different damage-sensitive features for detecting damage. Different damage scenarios are simulated on a large-scale laboratory structure and a three-span highway bridge for demonstration. The features presented in this paper are the modal flexibility-based deflection and curvature both of which are obtained directly from dynamic properties. In the literature, flexibility associated with mode shapes and mode shapes curvatures have been mostly explored. In this study, multi-input–multi-output dynamic data are used to obtain modal flexibility, which is a close approximation to the actual flexibility. A main novelty is that the curvature is calculated from the deflected shapes using the modal flexibility as opposed to using modal vectors. In this paper, the theory of the methodology is explained and then experimental studies and results are presented. For the experimental studies, the laboratory specimen and the three-span bridge were gradually damaged. It is shown that both deflection and curvature are conceptual and physically meaningful features for damage detection and localization. The issues and the requirements for these features to perform successfully are also presented.  相似文献   
73.
A new type of a single-axis azimuthal tracker is presented. The novel feature of this tracker is the ability to move the collector’s plane in two directions through a special support structure. This structure consists of a sliding mechanism on the central axis and a curved window on the cylindrical surface coaxial to the central axis. Consequently, the proposed novel heliotrope behaves similarly to a two-axis tracker. Two different windows designed on the cylindrical surface may be used to provide very high efficiencies throughout a year. Several performance measurements have been conducted on this novel tracker, a polar tracker and the reference two-axis tracker. Pyranometers, appropriately calibrated, were installed on all three systems to record the global incoming irradiance on the collector’s plane. It is shown that the new tracker system can be very efficient since its plane intercepts, at least, 98% of the insolation with respect to a two-axis tracker. The proposed system can be utilized in solar-related applications (photovoltaic or thermal).  相似文献   
74.
75.
The results of the effect of ultrasonic treatment of ZnSe crystals on the structure of the energy spectrum of electronic states of centers with deep levels forming photoelectric and luminescent properties of this compound are presented. It is for the first time proved experimentally that the climb of edge dislocations under the effect of ultrasound leads to regrouping and generation of defects forming deep levels, which manifest themselves in phenomena of photosensitivity and radiative recombination.  相似文献   
76.
This letter presents a novel approach for organizing computational resources into groups within H.264/AVC motion estimation architectures, leading to reductions of up to 75% in the equivalent gate count with respect to state‐of‐the‐art designs.  相似文献   
77.
Copper slag is a by‐product generated during smelting to extract copper metal from the ore. The copper slag obtained may exhibit pozzolanic activity and may therefore be used in the manufacture of addition‐containing cements. In this paper the effect of the incorporation of the copper slag in cement is measured. Blends of copper slag with Portland cement generally possess properties equivalent to Portland cement containing fly ash, but very different to the silica fume incorporation. Copper slag and fly ash reduce the heat of hydration more effectively than silica fume in mortars. The replacement of 30% cement by copper slag reduces the flexural and compressive strength in a similar way to fly ash; however, after 28 days, the reduction is less than the percentage of substitution. Hydrated calcium aluminate phases were analysed using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The pozzolanic activity of copper slag is similar to that of fly ash and higher than silica fume. In the presence of low water/cement ratios, certain pozzolanic materials produce a very compact cement paste that limits the space available for hydration products, a determining factor in the formation of hydrated calcium aluminates. SEM was found to be a useful analytical technique when aluminates are formed and can be clearly detected by XRD. Copyright © 2008 Society of Chemical Industry  相似文献   
78.
79.
80.
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