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   423106篇
  免费   5477篇
  国内免费   1345篇
电工技术   7798篇
综合类   326篇
化学工业   60803篇
金属工艺   16410篇
机械仪表   12598篇
建筑科学   10334篇
矿业工程   2066篇
能源动力   11567篇
轻工业   35313篇
水利工程   4198篇
石油天然气   6708篇
武器工业   23篇
无线电   53436篇
一般工业技术   83473篇
冶金工业   81728篇
原子能技术   9305篇
自动化技术   33842篇
  2021年   3491篇
  2020年   2560篇
  2019年   3245篇
  2018年   5424篇
  2017年   5553篇
  2016年   5763篇
  2015年   3795篇
  2014年   6507篇
  2013年   19111篇
  2012年   10453篇
  2011年   14430篇
  2010年   11641篇
  2009年   13267篇
  2008年   13822篇
  2007年   13744篇
  2006年   12029篇
  2005年   10882篇
  2004年   10510篇
  2003年   10642篇
  2002年   9809篇
  2001年   10366篇
  2000年   9795篇
  1999年   10384篇
  1998年   26384篇
  1997年   18500篇
  1996年   14351篇
  1995年   10583篇
  1994年   9549篇
  1993年   9366篇
  1992年   6848篇
  1991年   6600篇
  1990年   6312篇
  1989年   6185篇
  1988年   5985篇
  1987年   5061篇
  1986年   4935篇
  1985年   5553篇
  1984年   5088篇
  1983年   4881篇
  1982年   4450篇
  1981年   4476篇
  1980年   4259篇
  1979年   4169篇
  1978年   3979篇
  1977年   4721篇
  1976年   6452篇
  1975年   3414篇
  1974年   3303篇
  1973年   3258篇
  1972年   2750篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
A layer-by-layer analysis of the structures, the phase compositions, and the defect substructures of differentially hardened rails has been carried by optical and transmission electron microscopy. It is found that the material volume of the rail head fillet is cooled faster than the volume located along the central axis.  相似文献   
54.
55.
Current ammonia production technologies have a significant carbon footprint. In this study, we present a process synthesis and global optimization framework to discover the efficient utilization of renewable resources in ammonia production. Competing technologies are incorporated in a process superstructure where biomass, wind, and solar routes are compared with the natural gas-based reference case. A deterministic global optimization-based branch-and-bound algorithm is used to solve the resulting large-scale nonconvex mixed-integer nonlinear programming problem (MINLP). Case studies for Texas, California, and Iowa are conducted to examine the effects of different feedstock prices and availabilities. Results indicate that profitability of ammonia production is highly sensitive to feedstock and electricity prices, as well as greenhouse gas (GHG) restrictions. Under strict 75% GHG reductions, biomass to ammonia route is found to be competitive with natural gas route, whereas wind and solar to ammonia routes require further improvement to compete with those two routes. © 2018 American Institute of Chemical Engineers AIChE J, 65: e16498 2019  相似文献   
56.
Wu  Li  Li  Jing  Fu  Caixia  Kühn  Bernd  Wang  Xiaolin 《Magma (New York, N.Y.)》2019,32(4):501-509
Magnetic Resonance Materials in Physics, Biology and Medicine - To investigate the value of using diffusion-weighted imaging (DWI) and intravoxel incoherent motion DWI (IVIM-DWI) to assess the...  相似文献   
57.
Metallurgical and Materials Transactions A - The influence of the chemical composition on the crystallization process, amorphous phase formation, and the anticorrosion properties of Al-Zr-Ni-Fe-Y...  相似文献   
58.
Fefelov  S. A.  Kazakova  L. P.  Bogoslovskiy  N. A.  Bylev  A. B.  Yakubov  A. O. 《Semiconductors》2020,54(4):450-453
Semiconductors - The current–voltage characteristics measured on Ge2Sb2Te5 thin films in the current mode are studied. The effect of multilevel recording is established when applying...  相似文献   
59.
60.
Sustainable and efficient food supply chain has become an essential component of one’s life. The model proposed in this paper is deeply linked to people's quality of life as a result of which there is a large incentive to fulfil customer demands through it. This proposed model can enhance food quality by making the best possible food quality accessible to customers, construct a sustainable logistics system considering its environmental impact and ensure the customer demand to be fulfilled as fast as possible. In this paper, an extended model is examined that builds a unified planning problem for efficient food logistics operations where four important objectives are viewed: minimising the total expense of the system, maximising the average food quality along with the minimisation of the amount of CO2 emissions in transportation along with production and total weighted delivery lead time minimisation. A four objective mixed integer linear programming model for intelligent food logistics system is developed in the paper. The optimisation of the formulated mathematical model is proposed using a modified multi-objective particle swarm optimisation algorithm with multiple social structures: MO-GLNPSO (Multi-Objective Global Local Near-Neighbour Particle Swarm Optimisation). Computational results of a case study on a given dataset as well as on multiple small, medium and large-scale datasets followed by sensitivity analysis show the potency and effectiveness of the introduced method. Lastly, there has been a scope for future study displayed which would lead to the further progress of these types of models.  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