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   13538篇
  免费   348篇
  国内免费   517篇
电工技术   314篇
综合类   26篇
化学工业   1573篇
金属工艺   1322篇
机械仪表   514篇
建筑科学   455篇
矿业工程   20篇
能源动力   532篇
轻工业   643篇
水利工程   96篇
石油天然气   145篇
武器工业   1篇
无线电   1836篇
一般工业技术   3846篇
冶金工业   1837篇
原子能技术   180篇
自动化技术   1063篇
  2023年   46篇
  2022年   87篇
  2021年   150篇
  2020年   100篇
  2019年   151篇
  2018年   249篇
  2017年   244篇
  2016年   302篇
  2015年   230篇
  2014年   327篇
  2013年   909篇
  2012年   555篇
  2011年   809篇
  2010年   615篇
  2009年   841篇
  2008年   739篇
  2007年   748篇
  2006年   690篇
  2005年   518篇
  2004年   555篇
  2003年   422篇
  2002年   401篇
  2001年   427篇
  2000年   399篇
  1999年   385篇
  1998年   730篇
  1997年   509篇
  1996年   443篇
  1995年   302篇
  1994年   243篇
  1993年   190篇
  1992年   145篇
  1991年   148篇
  1990年   114篇
  1989年   100篇
  1988年   77篇
  1987年   54篇
  1986年   55篇
  1985年   43篇
  1984年   45篇
  1983年   39篇
  1982年   29篇
  1981年   21篇
  1980年   21篇
  1979年   20篇
  1978年   22篇
  1977年   22篇
  1975年   12篇
  1974年   15篇
  1970年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
Characterizes classroom instruction (CRI) from a behavior analytic perspective. It is argued that effective teaching strategies also serve managerial functions through the development of stimulus control and the management of behavioral choice. The stimulus control properties of CRI are discussed, and research concerning the effects of antecedent events on children's academic performance is reviewed. A theory for predicting choices in behavior, known as matching theory, is presented that evolved out of experimental operant research. The characteristics of CRI that make it particularly suited to matching theory analysis are identified, and research applying matching theory to children's classroom behavior is reviewed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
103.
We studied the performance of a prototype electromagnetic calorimeter for the BELLE detector at the KEK proton synchrotron for an energy range of 0.25–3.5 GeV. The prototype consisted of an array of 6 × 5 CsI(Tl) crystals with 30 cm length (16.2 radiation lengths) and about 6 cm × 6 cm cross section. The scintillation light of each CsI(Tl) crystal was read out by two large-area PIN photodiodes and charge-sensitive preamplifiers attached at the rear face of the crystal. We measured the energy and position resolution for electrons and the e/π separation for two sets of matrix configurations: one corresponded to the center and the other to the edge of the barrel calorimeter. The overall performance measured by the test proves that the prototype calorimeter is satisfactory for the use in the BELLE detector.  相似文献   
104.
105.
106.
Archer  K.Q. Edwards  J.A. 《Electronics letters》1995,31(16):1320-1322
The performance of three GHARQ-II schemes, based on punctured convolutional codes, used over a simulated Rayleigh fading channel subject to additive white Gaussian noise are compared. Various fade rates and signal to noise ratios are considered  相似文献   
107.
108.
The wall-plug efficiency of a vertical-cavity surface-emitting laser is investigated. The optimum current which produces a maximum wall-plug efficiency is derived when the mirror reflectivity is fixed and the series resistance and parasitic leakage current are taken as variable parameters. The results are compared with experimental data. The optimum reflectivity in terms of both output power and wall-plug efficiency is determined for both unstrained and strained quantum lasers with fixed supply current and series resistance as a parameter. Temperature is also considered as a variable  相似文献   
109.
A series of analogues and homologues of spermine were synthesized, and their impact on MK-801 binding to the N-methyl-D-aspartate (NMDA) receptor was evaluated. These tetraamines encompass both linear and cyclic compounds. The linear molecules include norspermine, N1, N11-diethylnorspermine, N1,N12-bis(2,2,2-trifluoroethyl)spermine, homospermine, and N1,N14-diethylhomospermine. The cyclic tetraamines consist of the piperidine analogues N1,N3-bis(4-piperidinyl)-1,3-diaminopropane, N1,N4-bis(4-piperidinyl)-1,4-diaminobutane, N1,N4-bis(4-piperidinylmethyl)-1,4-diaminobutane, and N1,N4-bis[2-(4-piperidinyl)ethyl]-1,4-diaminobutane and the pyridine analogues N1,N3-bis(4-pyridyl)-1,3-diaminopropane, N1,N4-bis(4-pyridyl)-1,4-diaminobutane, N1,N4-bis(4-pyridylmethyl)-1,4-diaminobutane, and N1,N4-bis[2-(4-pyridyl)-ethyl]-1,4-diaminobutane. This structure-activity set makes it possible to establish the importance of charge, intercharge distance, and terminal nitrogen substitution on polyamine-regulated MK-801 binding in the NMDA channel. Four families of tetraamines are included in this set: norspermines, spermines, homospermines, and tetraazaoctadecanes. Calculations employing a SYBYL modeling program revealed that the distance between terminal nitrogens ranges between 12.62 and 19.61 A. The tetraamines are constructed such that within families cyclics and acyclics have similar lengths but different nitrogen pKa's and thus different protonation, or charge, states at physiological pH. The pKa values for all nitrogens of each molecule and its protonation state at physiological pH are described. The modifications at the terminal nitrogens include introduction of ethyl and beta,beta,beta-trifluoroethyl groups and incorporation into piperidinyl or pyridyl systems. The studies clearly indicate that polyamine length, charge, and terminal nitrogen substitution have a significant effect on how the tetraamine regulates MK-801 binding to the NMDA receptor. Thus a structure-activity basis set on which future design of MK-801 agonists and antagonists can be based is now available.  相似文献   
110.
In order to have a medical imaging examination for idiopathic hemifacial spasm before surgery, we designed a new method by using vertebroarterial DSA and vertebroarterial CTA based on an aired cisternapontis, 36 patients were examined since 1989 and the compression of the facial nerve from brain stem to porus acousticus internus was showed clearly in all the cases. The arteries responsible for the compression were cerebellar inferior posterior (55.6%), cerebellar inferior anterior (44.4%), auditory internus (25.0%) and vertebroarterial (11.1%), 36% of all the cases had more than one responsible arteries, 22.2% cases had only one compression point, 38.9% had two points, 25.0% had 3 and 13.9% had more. 85.7% of the compression points located in the root zone and 14.3% near the porus acousticus internus. 24 cases were treated with decompression surgery, it indicated that the compression conditions just met the medical imaging examinationg results. After surgery the symptom disappeared in 23 cases and remarkably released in one case with no death case. We believed that this method might be helpful to idiopathic hemifacial spasm decompression surgery and other surgeries in the CP angle.  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