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   246300篇
  免费   3695篇
  国内免费   1245篇
电工技术   4858篇
综合类   264篇
化学工业   36132篇
金属工艺   10957篇
机械仪表   7236篇
建筑科学   6365篇
矿业工程   488篇
能源动力   6354篇
轻工业   24622篇
水利工程   1863篇
石油天然气   1343篇
武器工业   3篇
无线电   31395篇
一般工业技术   45559篇
冶金工业   50296篇
原子能技术   3884篇
自动化技术   19621篇
  2021年   1716篇
  2019年   1554篇
  2018年   2508篇
  2017年   2411篇
  2016年   2654篇
  2015年   2054篇
  2014年   3350篇
  2013年   11056篇
  2012年   5703篇
  2011年   8069篇
  2010年   6491篇
  2009年   7154篇
  2008年   7736篇
  2007年   7791篇
  2006年   7113篇
  2005年   6607篇
  2004年   6304篇
  2003年   6313篇
  2002年   6292篇
  2001年   6414篇
  2000年   5829篇
  1999年   6448篇
  1998年   16596篇
  1997年   11630篇
  1996年   8981篇
  1995年   6708篇
  1994年   5927篇
  1993年   5738篇
  1992年   4064篇
  1991年   4010篇
  1990年   3594篇
  1989年   3512篇
  1988年   3447篇
  1987年   2916篇
  1986年   2842篇
  1985年   3419篇
  1984年   3059篇
  1983年   2749篇
  1982年   2549篇
  1981年   2572篇
  1980年   2433篇
  1979年   2275篇
  1978年   2236篇
  1977年   2815篇
  1976年   4176篇
  1975年   1901篇
  1974年   1797篇
  1973年   1786篇
  1972年   1461篇
  1971年   1318篇
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
41.
Potential mGAT4 inhibitors derived from the lead substance (S)-SNAP-5114 have been synthesized and characterized for their inhibitory potency. Variations from the parent compound included the substitution of one of its aromatic 4-methoxy and 4-methoxyphenyl groups, respectively, with a more polar moiety, including a carboxylic acid, alcohol, nitrile, carboxamide, sulfonamide, aldehyde or ketone function, or amino acid partial structures. Furthermore, it was investigated how the substitution of more than one of the aromatic 4-methoxy groups affects the potency and selectivity of the resulting compounds. Among the synthesized test substances (S)-1-{2-[(4-formylphenyl)bis(4-methoxyphenyl)-methoxy]ethyl}piperidine-3-carboxylic acid, that features a carbaldehyde function in place of one of the aromatic 4-methoxy moieties of (S)-SNAP-5114, was found to have a pIC50 value of 5.89±0.07, hence constituting a slightly more potent mGAT4 inhibitor than the parent substance while showing comparable subtype selectivity.  相似文献   
42.
43.
44.
ABSTRACT

The concept of digital game-based learning (DGBL) evolves rapidly together with technological enhancements of virtual reality (VR) and smart phones. However, the mental workload (MWL) that VR-training applications demand and motivational qualities originating from user experience (UX) should be identified in order to create effective and enjoyable training/learning challenges that fit with individual users’ capabilities. This study examined the effects of reality-based interaction (RBI) and VR on measures of student motivation and MWL, in a mental arithmetic game for secondary school pupils. In a randomised controlled trial with sixty school children, a mental arithmetic game was tested with three different interaction and two different presentation methods – VR RBI, VR head-mounted-display tapping and tablet flick-gesture. Results found a significant effect of RBI on MWL but no differences in enjoyment of training were found between VR-experience and tablet training-experience. In fact, adding the gaming-context to the mental arithmetic task created an enjoyable, motivating experience regardless of presentation or interaction-style.  相似文献   
45.
46.
47.
48.
Tian  J. Y.  Xu  G.  Hu  H. J.  Zhou  M. X. 《Strength of Materials》2019,51(3):439-449
Strength of Materials - A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation,...  相似文献   
49.

In-air epitaxy of nanostructures (Aerotaxy) has recently emerged as a viable route for fast, large-scale production. In this study, we use small-angle X-ray scattering to perform direct in-flight characterizations of the first step of this process, i.e., the engineered formation of Au and Pt aerosol nanoparticles by spark generation in a flow of N2 gas. This represents a particular challenge for characterization because the particle density can be extremely low in controlled production. The particles produced are examined during production at operational pressures close to atmospheric conditions and exhibit a lognormal size distribution ranging from 5–100 nm. The Au and Pt particle production and detection are compared. We observe and characterize the nanoparticles at different stages of synthesis and extract the corresponding dominant physical properties, including the average particle diameter and sphericity, as influenced by particle sintering and the presence of aggregates. We observe highly sorted and sintered spherical Au nanoparticles at ultra-dilute concentrations (< 5 × 105 particles/cm3) corresponding to a volume fraction below 3 × 10–10, which is orders of magnitude below that of previously measured aerosols. We independently confirm an average particle radius of 25 nm via Guinier and Kratky plot analysis. Our study indicates that with high-intensity synchrotron beams and careful consideration of background removal, size and shape information can be obtained for extremely low particle concentrations with industrially relevant narrow size distributions.

  相似文献   
50.
Wireless Personal Communications - Current mobile communications technology relies heavily on efficient design of antennas, where the operational characteristics of the wireless communication...  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