全文获取类型
收费全文 | 34034篇 |
免费 | 3751篇 |
国内免费 | 1950篇 |
专业分类
化学 | 6994篇 |
晶体学 | 89篇 |
力学 | 1906篇 |
综合类 | 643篇 |
数学 | 8599篇 |
物理学 | 7112篇 |
无线电 | 14392篇 |
出版年
2024年 | 93篇 |
2023年 | 406篇 |
2022年 | 884篇 |
2021年 | 1136篇 |
2020年 | 1122篇 |
2019年 | 826篇 |
2018年 | 720篇 |
2017年 | 1085篇 |
2016年 | 1403篇 |
2015年 | 1318篇 |
2014年 | 2246篇 |
2013年 | 2078篇 |
2012年 | 2135篇 |
2011年 | 2252篇 |
2010年 | 1876篇 |
2009年 | 2087篇 |
2008年 | 2130篇 |
2007年 | 2188篇 |
2006年 | 2049篇 |
2005年 | 1647篇 |
2004年 | 1381篇 |
2003年 | 1355篇 |
2002年 | 1078篇 |
2001年 | 945篇 |
2000年 | 776篇 |
1999年 | 649篇 |
1998年 | 591篇 |
1997年 | 473篇 |
1996年 | 445篇 |
1995年 | 391篇 |
1994年 | 296篇 |
1993年 | 259篇 |
1992年 | 248篇 |
1991年 | 159篇 |
1990年 | 131篇 |
1989年 | 114篇 |
1988年 | 82篇 |
1987年 | 71篇 |
1986年 | 45篇 |
1985年 | 92篇 |
1984年 | 72篇 |
1983年 | 42篇 |
1982年 | 64篇 |
1981年 | 39篇 |
1980年 | 41篇 |
1979年 | 69篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 22篇 |
1974年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
机载行波管发射机设计的基本原则 总被引:3,自引:2,他引:1
论述了机载行波管发射机设计中需要遵循的5个基本原则,强调了行波管使用以及环境适应的重要性及优先地位,提出了对发射机指标理解的认识,以及最简设计与交叉设计的建议;介绍了机载行波管发射机设计过程中的几个主要步骤,对发射机主要性能参数进行了简单的分类与分析,并给出了一个行波管发射机设计的示例。 相似文献
52.
SHI Yan-rui CAO Pei-xia Shijiazhuang Post Telecommunication Technical College Shijiazhuang P.R. China. 《中国邮电高校学报(英文版)》2004,11(Z1)
1 Introduction ExpressMailService (EMS )wasopenedupbyChinaPostintheearly 80stomeettheneedsofeco nomicdevelopment.It sthemostcompetitiveoneofthepostservices.There recivilaviation ,Chinarail wayandotherprivateexpressdeliverycompaniesdomesticallyandUPS ,TNT ,DHLandTEDEXcompetingwithEMSintheinternationalmarket.OneofthemostpowerfulcompeteweaponofthesecompaniesisthelowerpricethanEMS[1 ] .BecauseofthefiercecompetitionEMSdeclinedfromthemo nopolistindomesticandinternationalmarketoftheear… 相似文献
53.
Recent years have witnessed a rapid growth of interest in the study of the dynamic behavior of replenishment rules of bullwhip effect. We prove that bullwhip effect and butterfly effect share a same the self-oscillation amplifying mechanism that is the ordering decisions the supplier self-oscillation amplify the perturbations brought by the errors in the processing of retailers' demand information. This results as an explicit self-similar structure of the sensitivity of the system to the initial values duty to the nonlinear mechanism. In this paper, the causes process of the bullwhip effect is described as the internal nonlinear mechanism and study on the complexity of bullwhip effect for order-up-to policy under demand signal processing. The methodology is based on fractal and chaotic theory and allows important insights to be gained about the complexity behavior of bullwhip effect. 相似文献
54.
多传感器目标的模糊识别 总被引:5,自引:0,他引:5
根据模糊理论,采用信息融合技术和模糊识别技术,讨论了在多类传感器网内进行多目标模糊识别的一般原理和方法。 相似文献
55.
56.
We discuss an error estimation procedure for the global error of collocation schemes applied to solve singular boundary value problems with a singularity of the first kind. This a posteriori estimate of the global error was proposed by Stetter in 1978 and is based on the idea of Defect Correction, originally due to Zadunaisky. Here, we present a new, carefully designed modification of this error estimate which not only results in less computational work but also appears to perform satisfactorily for singular problems. We give a full analytical justification for the asymptotical correctness of the error estimate when it is applied to a general nonlinear regular problem. For the singular case, we are presently only able to provide computational evidence for the full convergence order, the related analysis is still work in progress. This global estimate is the basis for a grid selection routine in which the grid is modified with the aim to equidistribute the global error. This procedure yields meshes suitable for an efficient numerical solution. Most importantly, we observe that the grid is refined in a way reflecting only the behavior of the solution and remains unaffected by the unsmooth direction field close to the singular point. 相似文献
57.
In electrodialysis using thin membranes and spacers, the compactness of the membrane cell-pairs leads to a small potential drop, and hence to energy saving. The spacer design itself has a great effect on the cost of the plants, since spacers act as turbulence promoters. A careful design, to increase the mass transfer coefficients, can reduce the membrane surface area required for a given application. Limiting current measurements, cell-pair resistance and pressure losses are presented for several thin spacers, for different flow-velocity values and feed water concentrations. It was possible to find an advantageous geometry of the separating mesh leading to substantial savings on investment and operation costs. 相似文献
58.
固体力学有限元体系的结构拓扑变化理论 总被引:2,自引:1,他引:1
本文是文[1]的继续.文[1]提出了杆件系统的结构拓扑变化理论和拓扑变化法本文将这一理论和方法推进到连续体有限元体系;且在此基础上揭示出有限元体系的一个新性质,称为基本位移之梯度的正交性定理,从而给出一套设计敏度的显式表达式,可直接用于计算. 相似文献
59.
《Electroanalysis》2006,18(24):2435-2440
Several hydroximethylfurfural (HMF) microbiosensors, based on a microelectrode configuration built by photolithographic techniques, have been developed. Two different biosensors based on the aldehyde dehydrogenase (ALDH) enzyme coupling with NADH oxidase (NOD) or diaphorase (Diaph) are described. Experimental variables were optimized using experimental design methodology, by central composite designs, taking the intensity registered for a given HMF solution as response. Given the reproducibility (residual standard deviation (RSD), 7.20%), repeatability (RSD=3%) and limit of detection (LOD)=1.68×10?4 mol dm?3 (α=β=0.05 and a replicate) reached by the ALDH‐Diaph microbiosensor under the optimum conditions, the analysis of HMF in honey was successfully accomplished. 相似文献
60.
A profit-maximizing supply chain network design model with demand choice flexibility 总被引:1,自引:0,他引:1
Zuo-Jun Max Shen 《Operations Research Letters》2006,34(6):673-682
We present a profit-maximizing supply chain design model in which a company has flexibility in determining which customers to serve. The company may lose a customer to competition if the price it charges is too high. We show the problem formulation and solution algorithm, and discuss computational results. 相似文献