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   446426篇
  免费   4964篇
  国内免费   1809篇
电工技术   7723篇
综合类   640篇
化学工业   65893篇
金属工艺   19415篇
机械仪表   13999篇
建筑科学   9744篇
矿业工程   3338篇
能源动力   10841篇
轻工业   32999篇
水利工程   5504篇
石油天然气   11961篇
武器工业   64篇
无线电   50650篇
一般工业技术   92776篇
冶金工业   80255篇
原子能技术   12286篇
自动化技术   35111篇
  2021年   3862篇
  2019年   3711篇
  2018年   6535篇
  2017年   6692篇
  2016年   7081篇
  2015年   4344篇
  2014年   7341篇
  2013年   19093篇
  2012年   11614篇
  2011年   15563篇
  2010年   12276篇
  2009年   13987篇
  2008年   14350篇
  2007年   14296篇
  2006年   12313篇
  2005年   11368篇
  2004年   10954篇
  2003年   10625篇
  2002年   10286篇
  2001年   10452篇
  2000年   9843篇
  1999年   10187篇
  1998年   25084篇
  1997年   17699篇
  1996年   13573篇
  1995年   10343篇
  1994年   9076篇
  1993年   9309篇
  1992年   6861篇
  1991年   6653篇
  1990年   6396篇
  1989年   6380篇
  1988年   6094篇
  1987年   5434篇
  1986年   5491篇
  1985年   6063篇
  1984年   5670篇
  1983年   5320篇
  1982年   4839篇
  1981年   4890篇
  1980年   4915篇
  1979年   4736篇
  1978年   4764篇
  1977年   5135篇
  1976年   6479篇
  1975年   4168篇
  1974年   4055篇
  1973年   4123篇
  1972年   3554篇
  1971年   3192篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
82.
This paper describes a sequential tripping strategy used in a wide area back-up protection expert system (BPES) to combat situations in which protection relays have maloperated or information is missing. The BPES is an innovative back-up protection scheme designed to prevent the occurrence of widespread blackouts. The BPES evaluates the certainty that transmission lines are likely to be affected by the fault and uses a sequential tripping strategy to isolate the fault if a firm decision is not available due to maloperated relays and/or missing information. The mode of analysis and the sequential tripping strategy ensures that the BPES will clear a fault at minimum risk to the network. An example is included to demonstrate how the certainty factor based sequential tripping strategy is employed by the BPES to clear a fault which occurred on the South Western part of the UK National Grid System  相似文献   
83.
S N Maitra 《Sadhana》1985,8(4):373-385
The burn time and burnout velocity of a multistage rocket flown vertically in vacuum with constant thrust tangential to the flight path and a prescribed initial/final thrust-to-weight ratio in an arbitrary stage have been determined. The present paper also deals with optimal staging under given conditions of flight.  相似文献   
84.
85.
86.
87.
A microprocessor-based system with 32 A/D, 24 D/A, and 16 ac load controllers, has been designed and built to monitor and control an ion beam thin-film deposition system. The A/D and D/A channels have electrical isolation of 7.5 kV between channels and between input and output. The microprocessor system keeps the ion beam deposition parameters stable for extended periods of operation and it is proposed as a means to greatly simplify switching from one deposition species to another to grow thin multilayer or alloy films.  相似文献   
88.
Existing duality principles in structural optimisation are briefly reviewed and then they are extended to structures with segment-wise constant cross-sections. All theories are discussed in the particular context of optimal plastic beam design with symmetric convex specific cost functions and are confirmed by independent calculations on illustrative examples. It is shown that the optimal solution is always associated with a displacement field in which the mean absolute curvature value for each segment equals the subgradient of the specific cost function, with respect to the maximum absolute moment value for that segment. Moreover, the dual problem consists of the maximisation of the difference of two terms: the first one is the integral of the product of load and deflection (external work), and the second is the sum of products of segment lengths and the mean complementary cost values (taken with respect to the mean absolute curvature for that segment). Finally, some tentative proposals for a class of non-convex optimisation problems are presented. For special cases, the proposed general statements reduce to theorems by Heyman, Foulkes and Hemp.  相似文献   
89.
P.J. Campion 《Measurement》1985,3(3):121-124
Two complementary national laboratory accreditation schemes are run by the National Physical Laboratory (NPL) to provide official recognition of competent British laboratories and an assurance of quality to their customers. The first of these, the British Calibration Service (BCS), was set up in 1966 to accredit laboratories to calibrate instruments, gauges and reference materials. In 1981 the National Testing Laboratory Accreditation Scheme (NATLAS) was formed to extend the service to all kinds of testing. Both BCS and NATLAS form an integral part of the UK national measurement system and were combined to form the National Measurement Accreditation Service on 1 October 1985.  相似文献   
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