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   4482篇
  免费   65篇
  国内免费   8篇
电工技术   182篇
综合类   7篇
化学工业   778篇
金属工艺   80篇
机械仪表   66篇
建筑科学   70篇
能源动力   107篇
轻工业   256篇
水利工程   20篇
石油天然气   10篇
无线电   527篇
一般工业技术   705篇
冶金工业   1328篇
原子能技术   125篇
自动化技术   294篇
  2023年   26篇
  2022年   62篇
  2021年   80篇
  2020年   42篇
  2019年   40篇
  2018年   36篇
  2017年   33篇
  2016年   65篇
  2015年   27篇
  2014年   78篇
  2013年   153篇
  2012年   107篇
  2011年   153篇
  2010年   106篇
  2009年   129篇
  2008年   132篇
  2007年   114篇
  2006年   111篇
  2005年   98篇
  2004年   105篇
  2003年   102篇
  2002年   90篇
  2001年   122篇
  2000年   91篇
  1999年   130篇
  1998年   549篇
  1997年   335篇
  1996年   241篇
  1995年   128篇
  1994年   135篇
  1993年   112篇
  1992年   63篇
  1991年   55篇
  1990年   54篇
  1989年   52篇
  1988年   53篇
  1987年   49篇
  1986年   46篇
  1985年   45篇
  1984年   37篇
  1983年   37篇
  1982年   40篇
  1981年   44篇
  1980年   23篇
  1979年   30篇
  1978年   20篇
  1977年   30篇
  1976年   53篇
  1973年   17篇
  1972年   13篇
排序方式: 共有4555条查询结果,搜索用时 15 毫秒
101.
As anthropogenic impacts on riverine ecosystems expand, both aquatic and terrestrial ecosystems are influenced over large spatiotemporal scales. We predicted how riparian bird communities changed in response to long‐term changes in floodplain landscapes such as woodland expansion (i.e. rapid increases in vegetation cover on gravel bars and the progress of vegetation succession due to a decrease in the frequency and magnitude of flood disturbance). To test the hypothesis that woodland expansion after dam construction reduces the abundance of gravel bar‐nesting birds and increases the abundance of forest‐nesting birds, we estimated historical changes between past and present bird abundances using species distribution models across multiple rivers that were either unregulated or regulated by dams. We created past and present vegetation maps from remote sensing images and used habitat quantities as explanatory variables in the species distribution models. As we hypothesized, the estimated abundance of gravel bar‐nesting birds decreased and that of forest‐nesting birds increased because of woodland expansion in some regulated rivers. This suggests that anthropogenic alterations of riverine conditions (e.g. dam construction) can affect terrestrial ecosystems (e.g. riparian bird communities) through changes in floodplains (e.g. woodland expansion). In addition, our findings highlight the efficacy of combining spatial and temporal analyses when examining long‐term ecological dynamics. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
102.
         下载免费PDF全文
A switched reluctance (SR) motor has a doubly salient pole structure. The performance of the SR motor strongly depends on the magnetic properties of the core material, since it consists only of an iron core and copper windings. This study experimentally evaluates the performance of an SR motor made of permendur (Fe–49% Co alloy), which has very high saturation flux density and low core loss. Two types of SR motors that have the same dimensions but different core materials are used: one motor is made of the conventional nonoriented Si steel, and the other is made of permendur. Comparison of the characteristics of the two motors shows that the performance of the permendur SR motor surpasses that of the conventional one in terms of output power and efficiency. © 2014 Wiley Periodicals, Inc. Electr Eng Jpn, 187(1): 51–57, 2014; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.22431  相似文献   
103.
         下载免费PDF全文
This paper investigates the mutual grounding impedance between vertical grounding electrodes based on field measurements and FDTD simulations. In the case of vertical electrodes, the mutual impedance between the electrodes is almost completely independent of the electrode length, and thus the induced voltage is nearly constant as the electrode length becomes longer. This characteristic is different from that of an overhead conductor, where the electromagnetic‐induced voltage is proportional to the conductor length. The greater the separation distance between the electrodes, the smaller the induced voltage, as in the case of an overhead conductor. The propagation speed increases as the separation increases. It is found that the speed is not necessarily proportional to the inverse of the relative permittivity of the earth.  相似文献   
104.
A study was made on increasing the coercive force in CoNiCr/Cr sputtered hard-disk media. A higher coercive force could be obtained when the substrate temperature was higher than 150°C and a negative bias voltage was applied to the substrate during sputtering. With optimized conditions, a coercive force of more than 2300 Oe was obtained. The increase in the coercive force is thought to be due to the enhancement of isolation between magnetic particles by developing the segregation of Cr in the CoNiCr film  相似文献   
105.
Propolis is a resinous substance collected by honeybees from various plant sources. The composition of propolis depends on time, vegetation, and the area of collection. This study examined the antioxidant activity of propolis from various areas of China: Heilongjiang, Neimongol, Hebei, Shandong, Shanxi, Gansu, Henan, Hubei, Sichuan, Hunan, Yunnan and Hainan. Ethanol extracts of propolis (EEP) were prepared and evaluated for their antioxidant activities by β-carotene bleaching, 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical-scavenging, and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation decolorization assays. Furthermore, the major constituents in EEP were identified by high-performance liquid chromatography (HPLC) analysis with a photodiode array (PDA) and mass spectrometric (MS) detection, and each component was quantitatively analyzed. All propolis samples except that from Yunnan had relatively strong antioxidant activity accompanied by high total polyphenol contents. Propolis with strong antioxidant activity contained large amounts of antioxidative compounds, such as caffeic acid, ferulic acid and caffeic acid phenethyl ester. On the other hand, propolis from Yunnan and Hainan had compounds not present in propolis from other areas.  相似文献   
106.
107.
A novel control algorithm using a time process chart that is capable of accurate control of cycloconverters is developed. This algorithm is obtained by making a straight-line approximation in a newly contrived phase plane. In spite of its rather simple procedures, this algorithm is capable of highly accurate control that is comparable to that of the conventional analog scheme. A six-pulse noncirculating current-type cycloconverter is controlled with a small-scale interface and a high-speed control program. Experimental results confirm the validity and usefulness of the proposed method. As far as the processing time is concerned, this method could be used to control a system with a larger pulse number, such as a 12 or 24-pulse system  相似文献   
108.
109.
This paper presents a novel parallel processing system for image synthesis using ray tracing. An object space is divided into parts (subspaces), each of which is allocated to a processor. The processor detects, simultaneously the intersections of the surfaces of each object and a fixed number of rays over the whole space, and calculates the local intensity on an object in each subspace. The global intensities of pixels on a screen are calculated by the other kind of processors simultaneously. We also present the optimal data structure, based on an adaptive division algorithm, for parallel processing of the object space.  相似文献   
110.
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