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   235836篇
  免费   4404篇
  国内免费   1253篇
电工技术   4488篇
综合类   1076篇
化学工业   35358篇
金属工艺   9285篇
机械仪表   7282篇
建筑科学   6390篇
矿业工程   1775篇
能源动力   5804篇
轻工业   22009篇
水利工程   2740篇
石油天然气   4343篇
武器工业   177篇
无线电   25337篇
一般工业技术   45878篇
冶金工业   43763篇
原子能技术   5385篇
自动化技术   20403篇
  2021年   2565篇
  2020年   1918篇
  2019年   2206篇
  2018年   3590篇
  2017年   3617篇
  2016年   3809篇
  2015年   2876篇
  2014年   4365篇
  2013年   11096篇
  2012年   6874篇
  2011年   8937篇
  2010年   7297篇
  2009年   8011篇
  2008年   8234篇
  2007年   8268篇
  2006年   6927篇
  2005年   6377篇
  2004年   5847篇
  2003年   5563篇
  2002年   5175篇
  2001年   5168篇
  2000年   5067篇
  1999年   5199篇
  1998年   13186篇
  1997年   9232篇
  1996年   7219篇
  1995年   5223篇
  1994年   4717篇
  1993年   4500篇
  1992年   3453篇
  1991年   3268篇
  1990年   3062篇
  1989年   3149篇
  1988年   2990篇
  1987年   2596篇
  1986年   2471篇
  1985年   2971篇
  1984年   2729篇
  1983年   2546篇
  1982年   2260篇
  1981年   2415篇
  1980年   2245篇
  1979年   2296篇
  1978年   2261篇
  1977年   2604篇
  1976年   3643篇
  1975年   1956篇
  1974年   1905篇
  1973年   1920篇
  1972年   1511篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
3.

To increase the efficiency of designing systems intended for monitoring surface cracks in aluminum structures during their working life, we have analyzed a two-dimensional symmetric problem on uniaxial extension of an Al-polyimide-Cu layered structure with ideal adhesion between layers and a model crack in the aluminum base. The problem has been first solved for a sample with the crack modeled by a zero-thickness notch using the ANSYS engineering simulation program package. It is shown that this setting of the problem can lead to inadequate results as manifested, in particular, by significantly overstated mechanical stresses in aluminum in the region of crack emergence on the surface. In order to eliminate this difficulty, we propose to use the structure with a model defect in the form of a notch of nonzero thickness in the initial unstressed state of the structure. Recommendations for selecting the thickness of a notch used in the model structure are given.

  相似文献   
4.
5.
We explore the possibility of characterizing sperm cells without the need to stain them using spectral and fluorescence lifetime analyses after multi-photon excitation in an insect model. The autofluorescence emission spectrum of sperm of the common bedbug, Cimex lectularius, was consistent with the presence of flavins and NAD(P)H. The mean fluorescence lifetimes showed smaller variation in sperm extracted from the male (tau m, τm = 1.54–1.84 ns) than in that extracted from the female sperm storage organ (tau m, τm = 1.26–2.00 ns). The fluorescence lifetime histograms revealed four peaks. These peaks (0.18, 0.92, 2.50 and 3.80 ns) suggest the presence of NAD(P)H and flavins and show that sperm metabolism can be characterized using fluorescence lifetime imaging. The difference in fluorescence lifetime variation between the sexes is consistent with the notion that female animals alter the metabolism of sperm cells during storage. It is not consistent, however, with the idea that sperm metabolism represents a sexually selected character that provides females with information about the male genotype.  相似文献   
6.
7.
8.
9.
Sequencing of rice genome has facilitated the understanding of rice evolution and has been utilized extensively for mining of DNA markers to facilitate marker-assisted breeding. Simple sequence repeat (SSR) markers that are tandemly repeated nucleotide sequence motifs flanked by unique sequences are presently the maker of choice in rice improvement due to their abundance, co-dominant inheritance, high levels of allelic diversity, and simple reproducible assay. The current level of genome coverage by SSR markers in rice is sufficient to employ them for genotype identification and marker-assisted selection in breeding for mapping of genes and quantitative trait loci analysis. This review provides comprehensive information on the mapping and applications of SSR markers in investigation of rice cultivars to study their genetic divergence and marker-assisted selection of important agronomic traits.  相似文献   
10.
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