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   322867篇
  免费   8509篇
  国内免费   4289篇
电工技术   8528篇
技术理论   4篇
综合类   3867篇
化学工业   45806篇
金属工艺   15479篇
机械仪表   11611篇
建筑科学   10387篇
矿业工程   2775篇
能源动力   8994篇
轻工业   25531篇
水利工程   3480篇
石油天然气   6539篇
武器工业   570篇
无线电   40222篇
一般工业技术   58573篇
冶金工业   59102篇
原子能技术   6296篇
自动化技术   27901篇
  2022年   2861篇
  2021年   4265篇
  2020年   3147篇
  2019年   3360篇
  2018年   4777篇
  2017年   4875篇
  2016年   4938篇
  2015年   4693篇
  2014年   6888篇
  2013年   15184篇
  2012年   10587篇
  2011年   13614篇
  2010年   11248篇
  2009年   12516篇
  2008年   12420篇
  2007年   12363篇
  2006年   11112篇
  2005年   9967篇
  2004年   9029篇
  2003年   8456篇
  2002年   8158篇
  2001年   8265篇
  2000年   7523篇
  1999年   7965篇
  1998年   19846篇
  1997年   13634篇
  1996年   10612篇
  1995年   7668篇
  1994年   6531篇
  1993年   6397篇
  1992年   4342篇
  1991年   4165篇
  1990年   3935篇
  1989年   3730篇
  1988年   3556篇
  1987年   2977篇
  1986年   2953篇
  1985年   3256篇
  1984年   2948篇
  1983年   2675篇
  1982年   2528篇
  1981年   2586篇
  1980年   2319篇
  1979年   2259篇
  1978年   2156篇
  1977年   2614篇
  1976年   3606篇
  1975年   1840篇
  1974年   1813篇
  1973年   1852篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
This work investigates the impact of nine new product development (NPD) acceleration approaches (supplier involvement, lead user involvement, speeding up activities and tasks, reduction of parts and components, training and rewarding of employees, implementation of support systems and techniques, stimulating interfunctional cooperation, emphasis on the customer, and simplification of organizational structure) on development speed and new product profitability. Our findings from 233 manufacturing firms show that lead user involvement and training/rewarding of employees increase both development speed and profitability. Supplier involvement, speeding up activities and tasks, and a simplification of the organizational structure also enhance development speed, while an emphasis on the customer has an additional positive impact on new product profitability. Both new product speed and profitability increase firm financial performance. Our results further show that pioneers and fast followers should not select the same NPD acceleration approaches as the speed and profitability impact of the majority of the acceleration approaches depends on the new product strategy of the firm. These results are important as they provide guidance for pioneers and fast followers regarding which NPD acceleration approaches to select in order to enhance speed and profitability and, hence, firm financial performance.  相似文献   
82.
The TELEMAC project brings new methodologies from the Information and Science Technologies field to the world of water treatment. TELEMAC offers an advanced remote management system which adapts to most of the anaerobic wastewater treatment plants that do not benefit from a local expert in wastewater treatment. The TELEMAC system takes advantage of new sensors to better monitor the process dynamics and to run automatic controllers that stabilise the treatment plant, meet the depollution requirements and provide a biogas quality suitable for cogeneration. If the automatic system detects a failure which cannot be solved automatically or locally by a technician, then an expert from the TELEMAC Control Centre is contacted via the internet and manages the problem.  相似文献   
83.
The application of the Trefftz method for calculating wave forces on offshore structures is presented. Indirect and direct formulations using complete and non-singular systems of Trefftz functions for the Helmholtz equation are posed in this paper. An effective technique using different interpolation functions for the velocity potential and wave force are suggested to improve the computational accuracy of the wave force. The numerical examples show that the present method is highly efficient and accurate.  相似文献   
84.
85.
The effect of ion exchange conditions, such as Si/Al ratio, precursor copper salt, pH and concentration of the solution, on the catalytic activity in SCR of NO by propane and on the electronic state of copper ions in Cu-ZSM-5 has been studied. The NO conversion in NO SCR by C3H8 has been found to reach a maximum value at Cu/Al ratio about 0.37–0.4 and remain constant at higher Cu/Al.

ESR and UV–vis DR spectroscopy have been used to elucidate stabilization conditions of copper ions in Cu-ZSM-5 zeolites as isolated Cu2+ ions, chain copper oxide structures and square-plain oxide clusters. The ability of copper ions for reduction and reoxidation in the chain structures may be responsible for the catalytic activity of Cu-ZSM-5. These transformations of copper ions are accompanied by the observation of intervalence transitions Cu2+–Cu+ and CTLM of the chain structures in the UV–vis spectra.  相似文献   

86.
Filled fibrous sorbents modified with transition metal ferro- and ferricyanides capable to recover americium(III) and rare-earth metals(III) from nitric acid solutions were prepared. The most complete and rapid recovery is attained on complex-forming and cation-exchange sorbents (as filled fibrous materials) saturated with nickel ions. The sorbents are characterized by good kinetic properties and can be used for recovery of the above metals.  相似文献   
87.
88.
Water treatment using TiO2 semiconductor as a durable heterogeneous photocatalyst has been the focus of environmentalists in recent years. Currently, we developed an inexpensive and highly efficient approach for synthesizing nitrogen-doped TiO2 with lower band-gap energy that can respond to visible light. Doping on the molecular scale led to an enhanced nitrogen concentration of up to 21.8%. Reflectance measurements showed the synthesized N-doped TiO2 nanoparticles are catalytically active with the absorbance that extends into the visible region up to 600 nm. The water purification potential of this new class of compound was evaluated by studying the photodegradation of Acid Orange 7 (AO7) and E. coli. Experiments were conducted to compare the photocatalytic activities of N-doped TiO2 nanocatalysts and commercially available Degussa P25 power under identical solar light exposure. N-doped TiO2 demonstrated superior photocatalytic activities in both chemical compound degradation and bactericidal reactions. The result of this study shows the potential of applying new generations of catalyst for wastewater purification and disinfection.  相似文献   
89.
90.
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