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   715288篇
  免费   9380篇
  国内免费   2124篇
电工技术   13158篇
综合类   681篇
化学工业   107669篇
金属工艺   26890篇
机械仪表   20696篇
建筑科学   17910篇
矿业工程   3025篇
能源动力   20077篇
轻工业   62126篇
水利工程   6898篇
石油天然气   11572篇
武器工业   46篇
无线电   84093篇
一般工业技术   138744篇
冶金工业   139068篇
原子能技术   13991篇
自动化技术   60148篇
  2022年   4254篇
  2021年   6507篇
  2020年   4777篇
  2019年   6021篇
  2018年   10190篇
  2017年   10000篇
  2016年   10518篇
  2015年   7237篇
  2014年   12102篇
  2013年   33249篇
  2012年   19181篇
  2011年   26528篇
  2010年   20952篇
  2009年   23642篇
  2008年   24360篇
  2007年   23914篇
  2006年   21081篇
  2005年   18911篇
  2004年   18255篇
  2003年   18124篇
  2002年   17170篇
  2001年   17456篇
  2000年   16201篇
  1999年   17161篇
  1998年   43600篇
  1997年   30582篇
  1996年   23541篇
  1995年   17783篇
  1994年   15564篇
  1993年   15384篇
  1992年   11099篇
  1991年   10508篇
  1990年   10267篇
  1989年   9855篇
  1988年   9258篇
  1987年   8203篇
  1986年   8032篇
  1985年   8921篇
  1984年   8322篇
  1983年   7555篇
  1982年   7076篇
  1981年   7051篇
  1980年   6770篇
  1979年   6529篇
  1978年   6201篇
  1977年   7463篇
  1976年   9855篇
  1975年   5290篇
  1974年   5017篇
  1973年   5050篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
InGaAs/GaAs(100) multiple-quantum-well-based inverted cavity asymmetric Fabry-Perot modulators are vertically integrated with GaAs/AlGaAs heterojunction phototransistors to yield all-optical photonic switches. The photonic switches using `normally on' modulator pixels exhibit an output on-off ratio of 12:1 with internal optical gain of 4 dB. The photonic switches using `normally off' modulator pixels yield similar contrast and gain, but exhibit intrinsic bistable behavior. The inverted cavity modulators employed permit utilizing the transparency of the GaAs substrate at the operating wavelength and offer advantages for fabricating large arrays for optical signal processing  相似文献   
992.
993.
Experimental tests showed that poststrike elevation in tongue-flicking rate (PETF) and strike-induced chemosensory searching (SICS) in the gila monster last longer than reported for any other lizard. Based on analysis of numbers of tongue-flicks emitted in 5-min intervals, significant PETF was detected in all intervals up to and including minutes 41–45. Using 10-min intervals, PETF lasted though minutes 46–55. Two of eight individuals continued tongue-flicking throughout the 60 min after biting prey, whereas all individuals ceased tongue-flicking in a control condition after minute 35. The apparent presence of PETF lasting at least an hour in some individuals suggests that there may be important individual differences in duration of PETF. PETF and/or SICS are present in all families of autarchoglossan lizards studied except Cordylidae, the only family lacking linguallly mediated prey chemical discrimination. However, its duration is known to be greater than 2-min only in Helodermatidae and Varanidae, the living representatives of Varanoidea. That prolonged PETF and SICS are typical of snakes provides another character supporting a possible a varanoid ancestry for Serpentes. Analysis of 1-min intervals showed that PETF occurred in the first minute. A review of the literature suggests that a pause in tongue-flicking and delay of searching movements are absent in lizards and the few nonvenomous colubrid snakes tested. The delayed onset of SICS may be a specific adaptation of some viperid snakes to allow potentially dangerous prey to be rendered harmless by venom following voluntary release after envenomation and preceding further physical contact with the prey.  相似文献   
994.
For pt.I see ibid., vol.17, no.1, p.95-101 (2002). This paper proposes a method for lateral stabilization of the levitated linear induction motor and also gives the analysis for calculating the stabilizing force in detail. The calculated force has been experimentally verified using the laboratory prototype described previously  相似文献   
995.
Multiple access for UWB impulse radio with pseudochaotic time hopping   总被引:5,自引:0,他引:5  
Pseudochaotic time hopping (PCTH) is a previously proposed encoding/modulation scheme for ultra-wideband (UWB) impulse radio. PCTH exploits concepts from symbolic dynamics to generate aperiodic spreading sequences, resulting in a noise-like spectrum. We present a multiple-access technique suitable for the PCTH scheme. In particular, we provide an analytical expression of the bit-error rate performance as a function of the number of users and validate it by simulation.  相似文献   
996.
We previously isolated a mutant cell that is the only mammalian cell reported to have a persistently low level of UDP-glucose. In this work we obtained a spontaneous revertant whose UDP-glucose level lies between those found in the wild type and the mutant cell. The activity of UDP-glucose pyrophosphorylase (UDPG:PP), the enzyme that catalyzes the formation of UDP-glucose, was in the mutant 4% and in the revertant 56% of the activity found in the wild type cell. Sequence analysis of UDPG: PP cDNAs from the mutant cell showed one missense mutation, which changes amino acid residue 115 from glycine to aspartic acid. The substituted glycine is located within the largest stretch of strictly conserved residues among eukaryotic UDPG:PPs. The analysis of the cDNAs from the revertant cell indicated the presence of an equimolar mixture of the wild type and the mutated mRNAs, suggesting that the mutation has reverted in only one of the alleles. In summary, we demonstrate that the G115D substitution in the Chinese hamster UDPG:PP dramatically impairs its enzymatic activity, thereby causing cellular UDP-glucose deficiency.  相似文献   
997.
Theoretical studies point to significant improvements in the performance of semiconductor laser amplifiers by injecting carriers with pulsed electric currents of sub-nanosecond duration. A pulsed Fabry-Perot amplifier (FPA) is most sensitive to input lightwave at the instant the carrier density is crossing the critical region, and gives a sharply pulsed sampling effect on the input lightwave signal. Compared with a FPA operating at subcritical electron density, the pulsed amplifier gives much higher gain, peak power, and bandwidth. In fact, pulsed operation of a FPA is also expected to give significantly higher gain and about the same peak output power as a traveling wave amplifier. Pulsed operation also improves the performance of a traveling wave amplifier by attenuating its internally reflected waves  相似文献   
998.
999.
1000.
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