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   434435篇
  免费   6144篇
  国内免费   2316篇
电工技术   8428篇
技术理论   5篇
综合类   1293篇
化学工业   68180篇
金属工艺   19339篇
机械仪表   13610篇
建筑科学   10025篇
矿业工程   3529篇
能源动力   9786篇
轻工业   36193篇
水利工程   5287篇
石油天然气   11949篇
武器工业   262篇
无线电   46241篇
一般工业技术   87211篇
冶金工业   77058篇
原子能技术   10827篇
自动化技术   33672篇
  2021年   4215篇
  2019年   3821篇
  2018年   6463篇
  2017年   6453篇
  2016年   6985篇
  2015年   4709篇
  2014年   7733篇
  2013年   18909篇
  2012年   12405篇
  2011年   16219篇
  2010年   13015篇
  2009年   14156篇
  2008年   15020篇
  2007年   15040篇
  2006年   13136篇
  2005年   11711篇
  2004年   10593篇
  2003年   10069篇
  2002年   9926篇
  2001年   10109篇
  2000年   9434篇
  1999年   9462篇
  1998年   21928篇
  1997年   15835篇
  1996年   12141篇
  1995年   9177篇
  1994年   8226篇
  1993年   8314篇
  1992年   6446篇
  1991年   6217篇
  1990年   6219篇
  1989年   6123篇
  1988年   5826篇
  1987年   5073篇
  1986年   5146篇
  1985年   5759篇
  1984年   5523篇
  1983年   5162篇
  1982年   4623篇
  1981年   4739篇
  1980年   4547篇
  1979年   4727篇
  1978年   4662篇
  1977年   4881篇
  1976年   6110篇
  1975年   4207篇
  1974年   3929篇
  1973年   4094篇
  1972年   3559篇
  1971年   3172篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
This paper demonstrates the use of computer simulation for topological design and performance engineering of transparent wavelength-division multiplexing metropolitan-area networks. Engineering of these networks involves the study of various transport-layer impairments such as amplifier noise, component ripple, chirp/dispersion, optical crosstalk, waveform distortion due to filter concatenation, fiber nonlinearities, and polarization effects. A computer simulation methodology composed of three main simulation steps is derived and implemented. This methodology obtains performance estimations by applying efficient wavelength-domain simulations on the entire network topology, followed by time-/frequency-domain simulations on selected paths of the network and finally Q-budgeting on an identified worst case path. The above technique provides an efficient tool for topological design and network performance engineering. Accurate simulation models are presented for each of the performance impairments, and the computer simulation methodology is used for the design and engineering of a number of actual metro network architectures  相似文献   
72.
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs  相似文献   
73.
In this letter, a concise process technology is proposed for the first time to enable the fabrication of good quality three-dimensional (3-D) suspended radio frequency (RF) micro-inductors on bulk silicon, without utilizing the lithography process on sidewall and trench-bottom patterning. Samples were fabricated to demonstrate the applicability of the proposed process technology.  相似文献   
74.
75.
76.
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   
77.
78.
The dynamic linewidth of 1.5 ?m ridge waveguide DFB lasers is shown to be reduced by shaping the pulse of the laser modulating waveform. Pulse shaping is performed by a second-order network designed to cancel the small-signal laser resonance. Results demonstrate a dynamic linewidth reduction from 1.4 ? to 0.55 ? FWHM for a 500 ps pulse.  相似文献   
79.
80.
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   
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