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   50855篇
  免费   4523篇
  国内免费   1635篇
电工技术   3635篇
技术理论   13篇
综合类   5411篇
化学工业   5280篇
金属工艺   1077篇
机械仪表   2591篇
建筑科学   10627篇
矿业工程   704篇
能源动力   909篇
轻工业   2252篇
水利工程   443篇
石油天然气   871篇
武器工业   452篇
无线电   5574篇
一般工业技术   5694篇
冶金工业   767篇
原子能技术   737篇
自动化技术   9976篇
  2025年   358篇
  2024年   920篇
  2023年   1001篇
  2022年   1590篇
  2021年   1735篇
  2020年   1407篇
  2019年   1285篇
  2018年   1193篇
  2017年   1458篇
  2016年   1678篇
  2015年   1891篇
  2014年   3244篇
  2013年   3091篇
  2012年   3699篇
  2011年   3665篇
  2010年   3185篇
  2009年   3336篇
  2008年   3004篇
  2007年   2957篇
  2006年   2633篇
  2005年   2102篇
  2004年   1654篇
  2003年   1439篇
  2002年   1191篇
  2001年   1057篇
  2000年   868篇
  1999年   725篇
  1998年   508篇
  1997年   378篇
  1996年   1354篇
  1995年   722篇
  1994年   537篇
  1993年   162篇
  1992年   164篇
  1991年   105篇
  1990年   93篇
  1989年   83篇
  1988年   75篇
  1987年   55篇
  1986年   51篇
  1985年   73篇
  1984年   62篇
  1983年   32篇
  1982年   33篇
  1981年   32篇
  1980年   19篇
  1979年   12篇
  1978年   15篇
  1977年   15篇
  1976年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
通过对相变增韧陶瓷及一种可切削玻璃-陶瓷动态疲劳(恒应力速率)试验中高应力速率区断裂应力下降现象的理论分析,发现这种现象与材料的阻力特性(R-curve)密切相关。确立的σ_f-σ理论关系能够很好地描述整个应力速率区间内的动态疲劳试验结果。高应力速率区σ_f-σ在双对数坐标下为负斜率直线,直线斜率为(m为阻力曲线KR=k(△a)~m的指数),断裂主要由材料阻力行为控制;低应力速率区,σ_f-σ在双对数坐标下为正斜率直线,直线斜率为 (n为应力腐蚀指数),断裂主要由材料应力腐蚀行为控制。建立了测定材料阻力特性的一种新方法,分别用这种方法及压痕/弯曲方法对一种可切削玻璃-陶瓷的阻力特性进行了实验测定,两种方法所得结果有很好的一致性。  相似文献   
2.
Common sense sometimes predicts events to be likely or unlikely rather than merely possible. We extend methods of qualitative reasoning to predict the relative likelihoods of possible qualitative behaviors by viewing the dynamics of a system as a Markov chain over its transition graph. This involves adding qualitative or quantitative estimates of transition probabilities to each of the transitions and applying the standard theory of Markov chains to distinguish persistent states from transient states and to calculate recurrence times, settling times, and probabilities for ending up in each state. Much of the analysis depends solely on qualitative estimates of transition probabilities, which follow directly from theoretical considerations and which lead to qualitative predictions about entire classes of systems. Quantitative estimates for specific systems are derived empirically and lead to qualitative and quantitative conclusions, most of which are insensitive to small perturbations in the estimated transition probabilities. The algorithms are straightforward and efficient.  相似文献   
3.
The syntheses of polyenynes as model compounds for poly(diacetylene)s (PDAs) are described. Variation of properties (UV–VIS, Raman, NMR and bond geometries) as a function of the chain length was investigated. After extrapolation to infinite chain length these data were compared to those for PDAs. From UV–VIS spectra a value of λ = 551 nm (2.25 eV) was calculated corresponding to the electronic transition of a single polyenyne chain. This energy is located at the low energy end of a yellow PDA solution spectrum. From Raman scattering v(C?C) = 2108–2128 cm?1 and v(C?C) = 1505–1532 cm?1 were calculated after extrapolation. Similarly sp-C13C NMR data yielded a shift of δ = 100 ppm. These data are almost identical to data known for yellow PDA solutions. Bond geometries are almost identical to those of poly(diacetylene)s and theoretical data.  相似文献   
4.
5.
OPGW应用问题的探讨   总被引:2,自引:1,他引:1  
根据多年的光纤复合架空地线(0PGW)应用经验与实践,从0PGW结构及组成、光纤余长、线路配合、金具配套、0PGW安装等方面,对0PGW应用中的若干问题进行详细的探讨。  相似文献   
6.
Istanbul is one of the most famous historical cities in the world. However, the project alignment selected as the best of a range of alignments cannot avoid passing beneath the historical and cultural heritages of Istanbul as well as under ancient and densely inhabited areas of the city. This paper will explain some of the challenges related to the bored tunnels.

Historical buildings are vulnerable. Yet many existing residential and office buildings are old and constructed on minimal foundations. As a consequence, it is vital that any drawdown of groundwater and any ground settlements have to be minimized.

In addition, the connection between the immersed and bored tunnels will be made directly and totally underground, without the usual intermediate shafts and beneath the deep waters of the Bosphorus Strait. This operation needs the utmost control of the tunnel excavation face to ensure its stability and to minimize water ingress. Based on such considerations, tunnel excavation by tunnel boring machine (TBM) using a slurry shield and having the ability to operate under high pore pressures was recommended as the method of excavation for the main running tunnels.

The paper will explain how the design team from Avrasyaconsult – the Employer’s Representative – arrived at the final minimum, specific and functional requirements of the bored tunneling works which are to be carried out using the ‘FIDIC EPC/Turnkey Project’ conditions.  相似文献   

7.
分析了α-Si∶H薄膜的质量和厚度对α-Si∶HTFT关键性指标的影响,深入、详细地讨论了其PECVD淀积工艺,并在实验的基础上确定了最佳淀积工艺参数,从而获得了高性能的75mm372×276像素α-Si∶HTFT有源矩阵  相似文献   
8.
The oligosaccharide component compositions of a series of non-reducing oligosaccharides extracted from the roots of Arnica montana L. has been determined by gel permeation chromatography (g.p.c.). The range of oligosaccharides present was found to extend beyond the octasaccharide previously reported. with 16% to 19% of the oligosaccharides having a degree of polymerisation of between 11 and 19 and almost 3% having a degree of polymerisation greater than 20. The chromatographic behaviour of this series of oligosaccharides is compared with that observed for series of D-gluco-oligosaccharides.  相似文献   
9.
A novel methanol-tolerant oxygen-reduction catalyst, Iridium-selenium (Ir-Se) chalcogenide, was synthesized by chemical precipitation in an organic solvent. Auger electron spectroscopy (AES) analysis confirmed that the synthesized Ir-Se chalcogenide had a chemical formula of Ir4Se. This chalcogenide showed strong catalytic activity towards the oxygen reduction reaction (ORR) and a high methanol tolerance. It was found that most of the oxygen could be directly reduced to water through a four-electron pathway with less than 10% hydrogen peroxide (H2O2) being produced during the ORR. The improvement in catalytic activity of the Ir-Se chalcogenide in comparison with that of pure Ir might be attributed to the effect of a bimetallic interaction.  相似文献   
10.
国外关于Zipf定律的实证研究的区域空间多采用国家层次,而国内关于Zipf定律的实证研究则涉及国家、跨省的经济区及行政省的不同层次,于是这就有可能存在经济区域空间划定是否合理的问题。现以长江三角洲地区为例,通过修正的引力模型计算城市之间的经济联系强度及潜能,以此进行长江三角洲经济区域的空间划定,并在划定的经济区域内进行Zipf定律的实证研究,以验证经济区域空间划定是否合理。研究结果显示了经济区域空间划定的合理性。  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