全文获取类型
收费全文 | 24452篇 |
免费 | 249篇 |
国内免费 | 568篇 |
专业分类
电工技术 | 322篇 |
综合类 | 133篇 |
化学工业 | 2296篇 |
金属工艺 | 1000篇 |
机械仪表 | 1202篇 |
建筑科学 | 1463篇 |
矿业工程 | 612篇 |
能源动力 | 161篇 |
轻工业 | 4879篇 |
水利工程 | 790篇 |
石油天然气 | 461篇 |
武器工业 | 63篇 |
无线电 | 1897篇 |
一般工业技术 | 7945篇 |
冶金工业 | 414篇 |
原子能技术 | 227篇 |
自动化技术 | 1404篇 |
出版年
2021年 | 48篇 |
2019年 | 29篇 |
2018年 | 58篇 |
2017年 | 28篇 |
2016年 | 38篇 |
2015年 | 52篇 |
2014年 | 112篇 |
2013年 | 100篇 |
2012年 | 2179篇 |
2011年 | 2797篇 |
2010年 | 550篇 |
2009年 | 278篇 |
2008年 | 2043篇 |
2007年 | 1963篇 |
2006年 | 1715篇 |
2005年 | 1578篇 |
2004年 | 1340篇 |
2003年 | 1146篇 |
2002年 | 1059篇 |
2001年 | 869篇 |
2000年 | 948篇 |
1999年 | 670篇 |
1998年 | 536篇 |
1997年 | 423篇 |
1996年 | 407篇 |
1995年 | 362篇 |
1994年 | 338篇 |
1993年 | 250篇 |
1992年 | 289篇 |
1991年 | 268篇 |
1990年 | 271篇 |
1989年 | 223篇 |
1988年 | 177篇 |
1987年 | 229篇 |
1986年 | 234篇 |
1985年 | 238篇 |
1984年 | 215篇 |
1983年 | 193篇 |
1982年 | 203篇 |
1981年 | 157篇 |
1980年 | 133篇 |
1979年 | 89篇 |
1978年 | 68篇 |
1977年 | 28篇 |
1976年 | 26篇 |
1975年 | 27篇 |
1974年 | 26篇 |
1973年 | 26篇 |
1965年 | 29篇 |
1964年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
1.
Site-specific seismic hazard analysis is crucial for designing earthquake resistance structures, particularly in seismically active regions. Shear wave velocity ( V S) is a key parameter in such analysis, although the economy and other factors restrict its direct field measurement in many cases. Various V S–SPT– N correlations are routinely incorporated in seismic hazard analysis to estimate the value of V S. However, many uncertainties question the reliability of these estimated V S values. This paper comes up with a statistical approach to take care of such uncertainties involved in V S calculations. The measured SPT– N values from all the critical boreholes were converted into statistical parameters and passed through various correlations to estimate V S at different depths. The effect of different soil layers in the boreholes on the Vs estimation was also taken into account. Further, the average shear wave velocity of the top 30 m soil cover ( V S30) is estimated after accounting for various epistemic and aleatoric uncertainties. The scattering nature of the V S values estimated using different V S– N correlations was reduced significantly with the application of the methodology. Study results further clearly demonstrated the potential of the approach to eliminate various uncertainties involved in the estimation of V S30 using general and soil-specific correlations. 相似文献
2.
Daniel T. Payne Mandeep K. Chahal V clav B&rcaron ezina Whitney A. Webre Katsuhiko Ariga Francis D Souza Jan Labuta Jonathan P. Hill 《Frontiers of Chemical Science and Engineering》2020,14(1):28
Chiral 1,1’-binaphthyl-linked diporphyrin ‘tweezers’ (R)-1/(S)-1 and the corresponding zinc(II) complexes (R)-2/(S)-2 were prepared as chiral host molecules, and their utility for chiral analyses (especially enantiomeric excess (ee) determinations) were evaluated. Tris(1-n-dodecyl)porphyrins were used for the first time as the interacting units. Host capabilities of the diporphyrin tweezers were investigated by titrations with (R,R)- and (S,S)-cyclohexane-1,2-diamine (CHDA). The host molecules could be used as multichannel probes of ee by using UV-vis, circular dichroism (CD), fluorescence emission and 1H nuclear magnetic resonance (1H-NMR) methods. Chiral configurations could also be differentiated using CD or 1H-NMR spectroscopy. All three optical techniques give good resolution of ee with reasonable sensitivity considering the low concentrations used (ca. 10−6 mol·L−1). The ee determination of CHDA enantiomers using NMR spectroscopy is also possible because of the reasonably well separated resonances in the case of (R,R)- and (S,S)-CHDA. Non-metallated (R)-1/(S)-1 hosts could not be used to detect chiral information in a strongly acidic chiral guest. This work demonstrates the utility of 1,1’-binapthyl-linked chiral hosts for chiral analysis of ditopically interacting enantiomers. 相似文献
3.
Pratik P. BHATT Venkatesh K. R. KODUR Anuj M. SHAKYA Tarek ALKHRDAJI 《Frontiers of Structural and Civil Engineering》2021,15(1):177
This paper presents the results of fire resistance tests on carbon fiber-reinforced polymer (CFRP) strengthened concrete flexural members, i.e., T-beams and slabs. The strengthened members were protected with fire insulation and tested under the combined effects of thermal and structural loading. The variables considered in the tests include the applied load level, extent of strengthening, and thickness of the fire insulation applied to the beams and slabs. Furthermore, a previously developed numerical model was validated against the data generated from the fire tests; subsequently, it was utilized to undertake a case study. Results from fire tests and numerical studies indicate that owing to the protection provided by the fire insulation, the insulated CFRP-strengthened beams and slabs can withstand four and three hours of standard fire exposure, respectively, under service load conditions. The insulation layer impedes the temperature rise in the member; therefore, the CFRP–concrete composite action remains active for a longer duration and the steel reinforcement temperature remains below 400°C, which in turn enhances the capacity of the beams and slabs. 相似文献
4.
CAI Xiang bao 《中国邮电高校学报(英文版)》2003,10(2)
1 IntroductionMaterialswithPhotonicBandGaps (PBG’s)havebeenwidelystudiedboththeoreticallyandex perimentallyinthepastfew years[1~ 4] .Theexis tenceofgaps,which prohibitthepropagationofelectromagnetic (EM )wavesinacertainrangeoffrequencies,canhavesignificantimpactsbothinsci enceandtechnology .Manypracticalapplicationsofthesestructureshavebeensuggestedanddemon strated ,suchasPhotonicCrystal (PC)microcavi ties[5] ,infraredPC[6] ,PClens[7] ,suppressingspontaneousemission ,manipulatinglight… 相似文献
5.
1 Introduction High power laser diode arrays (LDA) have many advan- tages such as small volume, long working life, high slope efficiency and high optical density, so they have many applications in medical treatment, material pro- cessing, and for the pumping source of solid laser and etc. But unfortunately, the LDA can not be easy to use directly in these fields because of their poor beam quality and extremely asymmetric divergent beams (!x≈ 5°~10°and !y≈20°~35°, for example), so it … 相似文献
6.
The Algorithm and Its Application of a Multi-Channel Adaptive Decision-Feedback Equalizer 总被引:4,自引:2,他引:2
1 IntroductionOneofthekeyproblemsinhigh speeddigitalradiocommunicationistheInter Symbol Interfer ence (ISI)causedbythemulti pathbetweenthetransmitterandthereceiver.Becauseoftherelativemovementbetweenthereceiverandthetransmitterandthecontinuouschangeoftransmissionmedia,ISIisusuallytime variant.Inthepasttwotothreedecades,abroadstudyofthetype ,structureandadaptivealgorithmofadaptiveequalizershasbeencarriedouttomitigateISI[1~ 7] .Withtheintroductionofadaptiveantennasanddiversityreceptiontec… 相似文献
7.
An Improved Immune Genetic Algorithm for Solving the Optimization Problems of Computer Communication Networks 总被引:4,自引:0,他引:4
1 IntroductionIndesigningacomputercommunicationnet work ,thenetworkaveragedelayisanimportantpa rameterinthenetworkperformance .Inthispaper,weonlyconsiderM /M/1networks,whichmeansthatthemessageprocessingtimeisaprobabilisticdensityfunctionwithnegativepower,thegroupar rivalandsendingisofPoissiondistributionwithasinglequeue .Supposethatthenetworktopologicalstructureandtheestimatesoftheexternaltrafficrequirementsaregiven ,howtoselecttheoptimalroutestobeusedbythecommunicatingnodesinthenetworksoast… 相似文献
8.
最初于二十世纪二十年代投入运行的澳大利亚艾尔登(Eildon)水电站两台7.5MW混流式水轮机已经重新投入运行了。对机组恢复使用并在原来两倍的水头下运行的可行性进行了调查,发现这两台机组能够重新投运,并且可在更高压力下运行。本文介绍了这个非常规项目的机械试验、有限元分析和其它试验,这个项目后来在澳大利亚和新西兰获得了三项优秀工程奖。 相似文献
9.
1 IntroductionWiththerapiddevelopmentofInternet,thenetworkisbecominglargerandmorecomplex ,whichcallsformoresophisticatednetworkmanage mentstrategiestobeadoptedinordertoguaranteeadequatelevelsofperformance .Confrontedwiththepresentstatusofnetworkmanagement,manyschol arsmakegreateffortstoinventnewapproachesforthepurposeofimprovingtheperformanceofnet workmanagement.MobileAgenttechnologiesandtheirapplicationtonetworkmanagementaread vancedunderthisbackground .Althoughalotofworkhasstilltobedone ,t… 相似文献
10.
WANG You-guo WU Le-nan ZHANG Dan.School of Mathematics Physics Nanjing University of Posts Telecommunications Nanjing P. R. China .Department of Radio Engineering Southeast University Nanjing P. R. China .School of Telecommuncation Information Engineering Najing University of Posts Telecommunications Nanjing P.R. China 《中国邮电高校学报(英文版)》2006,13(1):79-83
1IntroductionSometi mes,noise can i mprove signal transmission orsignal processing in a nonlinear system.This phe-nomenonis the so-called Stochastic Resonance(SR)[1].Most studies of SRinvolve asubthresholdsignal througha single nonlinear systemand the non… 相似文献