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   31514篇
  免费   1411篇
  国内免费   1473篇
系统科学   2159篇
丛书文集   833篇
教育与普及   549篇
理论与方法论   373篇
现状及发展   975篇
研究方法   79篇
综合类   29428篇
自然研究   2篇
  2024年   134篇
  2023年   290篇
  2022年   519篇
  2021年   487篇
  2020年   352篇
  2019年   149篇
  2018年   941篇
  2017年   999篇
  2016年   743篇
  2015年   657篇
  2014年   954篇
  2013年   969篇
  2012年   1315篇
  2011年   1959篇
  2010年   1885篇
  2009年   1613篇
  2008年   1708篇
  2007年   1957篇
  2006年   1049篇
  2005年   1020篇
  2004年   946篇
  2003年   887篇
  2002年   957篇
  2001年   1000篇
  2000年   897篇
  1999年   1322篇
  1998年   1049篇
  1997年   1125篇
  1996年   1044篇
  1995年   858篇
  1994年   818篇
  1993年   708篇
  1992年   609篇
  1991年   567篇
  1990年   536篇
  1989年   503篇
  1988年   436篇
  1987年   266篇
  1986年   109篇
  1985年   41篇
  1984年   11篇
  1983年   1篇
  1980年   1篇
  1976年   1篇
  1967年   1篇
  1958年   2篇
  1955年   3篇
排序方式: 共有10000条查询结果,搜索用时 86 毫秒
991.
A theoretical study was carried out into membrane transport phenomena.Formulae for calculating the membrane transport resistance and transmembrane mass flux were given,variations in membrane resistance and moisture flux with the membrane sorption constant(C)under various humidity conditions were analyzed,and the value of C corresponding to the minimum membrane resistance or the maximum moisture flux was obtained.The results show that the membrane resistance and moisture flux relate not only to C but also to the relative humidities on both sides of the membrane.As C increases,membrane resistance initially decreases but then increases,i.e.,a minimum occurs,while the moisture flux first decreases and then increases,i.e.,a maximum occurs.The membrane resistance and moisture flux reach their extrema at the same value of C,which is determined by the relative humidities on both sides of the membrane.To reduce the membrane resistance,the value of C should be chosen based on the humidity conditions.  相似文献   
992.
We compute the superfluid density of a two-dimensional boson system with weak two-body repulsive interactions at zero temperature using one-loop perturbation theory in the weak coupling region. The boson fields are taken to be in continuum form in real space, and the interactions are approximated by a δ function of the distance between the bosons. We find that the one-loop fluctuations slightly decrease the superfluid density of a classical level. The superfluid density is approximated by the condensate density multiplied by the mass of the boson particle.  相似文献   
993.
A fully discrete implicit Euler upwind finite volume element method is derived and studied for one-dimensional semiconductor device. Upwind scheme is introduced to deal with the convection-dominated diffusion equations in the semiconductor model. With different time steps for the electrostatic potential and the other unknown quantities, the computational procedure of the method is obtained. The local mass conservation laws are preserved under the framework of the upwind finite volume element schemes. A first-order accuracy in the L 2-norm is proved. Numerical experiments are given to validate the usefulness and efficiency of the method.  相似文献   
994.
Over the past ten years,the enthusiasm to harness small RNA molecules,mainly small interfering RNAs(siRNAs) and microRNA mimics,into therapeutic weapons has never been quenched.However,a major obstacle hindering the clinical application of these small molecules is how to efficiently and precisely deliver siRNAs in vivo into target cells to silence target genes.An international collaborative study led by Song Erwei from the Memorial Hospital of Sun-YatSen University and Wang Jun from Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences of University of Science and Tech-  相似文献   
995.
I offer an analysis of the Principle of Sufficient Reason and its relevancy for the scientific endeavour. I submit that the world is not, and cannot be, rational—only some brained beings are. The Principle of Sufficient Reason is not a necessary truth nor a physical law. It is just a guiding metanomological hypothesis justified a posteriori by its success in helping us to unveil the mechanisms that operate in Nature.  相似文献   
996.
The semi-global stabilization problem for a class of nonlinear systems with state time-delay is addressed in this paper. By using Lyapunov-Krasovskii functional method and homogeneous domination approach, a homogeneous observer and an output feedback controller with a scaling gain are designed. Then the scaling gain is adjusted such that the closed-loop system is semi-global asymptotically stable. A numerical example is presented to illustrate the effectiveness of the obtained results in this paper.  相似文献   
997.
Automatically allocation of more attention to negative stimuli is called emotional negativity bias. An event-related potentials (ERPs) experiment investigated whether or not this bias was altered by positive mood arousal. The results suggested that the attention bias towards negative stimuli was attenuated when positive information was accessible.  相似文献   
998.
旅游业作为第三产业的支柱产业之一,低污染、低能耗的特点使其备受各级政府的重视。曲靖市旅游业拥有宜人的气候、丰富的资源、发达的经济等优势,但是也存在着观光旅游的景点知名度相对较低、基础性公共服务部门的服务意识薄弱、旅游配套设施不足等问题。为此,应通过科学规划,大力发展休闲游、自驾游等方式,以促进整个旅游业的产业升级。  相似文献   
999.
Two kinds of argon shroud protection devices with two different basic structures were designed and investigated. Industrial experiments and numerical simulations were used to examine the protection effect, and the mechanism of air entrapment during the casting of steel ingots was analyzed. The influence of the structure of the argon shroud protection device on the protection effect was investigated. An argon shroud protection device mounted to the nozzle holder on the bottom of the ladle does not provide a good protection effect because air can easily flow into the teeming system and cause reoxidation of molten steel during teeming. By contrast, an argon shroud protection device seated on the top of the central trumpet provides an excellent protection effect, where air has little chance of flowing into the teeming system during casting. The feasibilities of the argon shroud protection devices are discussed.  相似文献   
1000.
We study on the implementation flow of the radio computerized tomography (RCT) prediction method. A case in real cellular mobile radio (CMR) system together with the prediction results are also presented. As shown by the results, the RCT prediction method is marked for its convenience and rapidity, as well as its relative high precision even when the prediction procedure is highly simplified. Since it is developed according to the characteristics of wireless communication environments of our country and has concurrently merits from both statistical and deterministic prediction models, the RCT prediction method is in good agreement with engineering practices in cellular mobile communication in cities at home. Optimized by combining with other techniques, further improvement could be achieved in the stability and precision of the RCT prediction method which now serves as the core part of a software tool for commercial use in CMR system analysis and optimization. Biography: Yu Sheng-bing (1974-), male, Ph. D candidate, research interest: theory analysis and application development of radio propagation in wireless communication systems.  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