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   6442篇
  免费   181篇
  国内免费   497篇
系统科学   689篇
丛书文集   228篇
教育与普及   36篇
理论与方法论   22篇
现状及发展   80篇
综合类   6046篇
自然研究   19篇
  2024年   19篇
  2023年   43篇
  2022年   59篇
  2021年   69篇
  2020年   74篇
  2019年   63篇
  2018年   74篇
  2017年   92篇
  2016年   114篇
  2015年   165篇
  2014年   228篇
  2013年   231篇
  2012年   359篇
  2011年   370篇
  2010年   257篇
  2009年   326篇
  2008年   300篇
  2007年   406篇
  2006年   395篇
  2005年   389篇
  2004年   319篇
  2003年   325篇
  2002年   265篇
  2001年   252篇
  2000年   233篇
  1999年   214篇
  1998年   182篇
  1997年   187篇
  1996年   161篇
  1995年   144篇
  1994年   158篇
  1993年   131篇
  1992年   112篇
  1991年   89篇
  1990年   91篇
  1989年   95篇
  1988年   61篇
  1987年   38篇
  1986年   21篇
  1985年   8篇
  1955年   1篇
排序方式: 共有7120条查询结果,搜索用时 31 毫秒
101.
1Keytechnicalissuesinthegenerallayoutofnavi gationstructures ThegenerallayoutofTGPnavigationstructuresischaracterizedbyabigdis chargeatthedam ,ahighcontentofsedimentinflowandcomplicatedgeog raphyatthedamsite ,whereisneces sarybothtosolvethesedimentationandflowconditionfornavigationandtodealwiththerelationshipbetweennavigationstructuresontheonehandandpowergenerationandfloodcontrolontheoth er.1 1Selectionofthedamsiteandthearrangementoftheaxisoftheproject Thegenerallayoutofthenavigationstruct…  相似文献   
102.
以二(2-苯并咪唑亚甲基)胺(N3)为配体,合成了一种新型Cu(II)双核配合物作为超氧化物歧化酶(SOD)的模型化合物.采用元素分析、UV和IR光谱进行了表征,应用X射线衍射方法测定了晶体结构,并利用G98W程序在HF/LANL2DZ基组水平上对该双核配合物进行了量子化学计算.晶体结构分析表明,Cu(II)双核配合物的化学式量分子(C42H40N12O17Cl4Cu2)是以4,4′-联吡啶为桥的Cu(II)双核配合物单元,Cu(II)与配位原子形成变形八面体构型,轴向配位氧原子来源于高氯酸根.量化计算所得原子轨道贡献和原子净电荷布居分析结果与晶体结构中的配位情况相符.  相似文献   
103.
The aggregation behavior of various acid extractants in the solvent extraction systems of metal ions is re-examined and explained according to knowledge obtained in recent work. The conclusions are as follows. (1) Complexes formed by the extractants and metal ions can form reversed micelles in organic diluents, depending on the microstructures of the complexes. The dimers of the acid extractant cannot percolate to the metal-extractant aggregates, and the acid-salt complexes are always formed in the aggregates. The reversed micelles or the W/O microemulsions formed by different species cannot be associated with each other to form a unified aggregate. (2) In solvent extraction systems, hydration of the extractants and metal ions can be considered as the driving force of forming reversed micelles. (3) Information of the first approach to the insight of the bicontinuous microemulsion of NaDEHP shows that various components in the aqueous phase behave confined and very similar to the typical AOT/n-heptane W/O microemulsions. (4) In the extraction of lanthanide ions by the W/O microemulsion of sodium naphthenate, the saponification is a process of forming reversed micelle or W/O microemulsion, while the extraction step is a process of destroying reversed micelles or W/O microemulsion droplets.  相似文献   
104.
Pb isotopic geochemical study on the crustal structure of Tongbaishan   总被引:4,自引:0,他引:4  
Pb isotopic composition of the Tongbai complex, distributed in the Tongbaishan to the west of the Dabieshan, ranges between the Dabie core complex and the Dabie ultral-high pressure (UHP) metamorphic complex, the latter having more radiogenic Pb isotopic composition than the former. Granites from the Jigongshan pluton, which intruded mainly into the Tongbai complex, are distinct from the Tongbai complex but similar to the Dabie core complex in Pb isotopic composition, showing that the magma of the Jigongshan granites was derived from the partial melting of the Dabie core complex. According to Pb isotopic compositional variation model in the vertical crustal section and magma source from the Jigongshan pluton, it is suggested that the Tongbai complex was an upper rock serial of the Dabie core complex, which is beneath the Dabie UHP metamorphic complex in the crustal structure of the Tongbai-Dabie orogenic belt. The Tongbai complex was not well preserved in the Dabie area due to the high exhumed crustal section. However, the crustal section in the Tongbai area was exhumed less than that in the Dabie area, and the deep crust in the Tongbai area still contains the basement composition similar to the Dabie core complex. Therefore, the crustal basements from the Dabie to Tongbai areas are united. The present distribution of the basement blocks in different locations of the Tongbai-Dabie orogenic belt reflects different exposure of the crustal section.  相似文献   
105.
For simulating multi-scale complex flow fields it should be noted that all the physical quantities we are interested in must be simulated well. With limitation of the computer resources it is preferred to use high order accurate difference schemes. Because of their high accuracy and small stencil of grid points computational fluid dynamics (CFD) workers pay more attention to compact schemes recently. For simulating the complex flow fields the treatment of boundary conditions at the far field boundary points and near far field boundary points is very important. According to authors' experience and published results some aspects of boundary condition treatment for far field boundary are presented, and the emphasis is on treatment of boundary conditions for the upwind compact schemes. The consistent treatment of boundary conditions at the near boundary points is also discussed. At the end of the paper are given some numerical examples. The computed results with presented method are satisfactory.  相似文献   
106.
采用配体取代法合成出C60Ni(PPh3)2,利用元素分析、红外光谱、光电子能谱对产物进行表征,研究了产物的热稳定性和氯化还原性能。  相似文献   
107.
Gao  Shengli  Chen  Sanping  Yang  Xuwu  Shi  Qizhen 《科学通报(英文版)》2003,48(4):319-322
Seven novel solid complexes of the first-row transitional metal with L-α-histidine were synthesized, and their compositions were determined. The constant-volume combustion energies of the complexes were measured by a precision rotation bomb calorimeter. The standard molar enthalpies of combustion and the standard molar enthalpies of formation were calculated. The results indicated that the plots of the standard enthalpies of formation against the atomic number of the metal show a regularity of zigzag.  相似文献   
108.
从精神分析观点来看,卡夫卡与俄底浦斯情结之间有着显然的历史联系和必然的因果关系。俄底浦斯情结在卡夫卡身上实现了多层次的演绎和裂变。卡夫卡以他的现代主义创作和个人生活本身,对这种情结予以表现、深化和思考。  相似文献   
109.
本文通过化学分析的实验方法测定了第一过渡元素几种二价中心离子与EDTA配合形成的过渡金属配合物的稳定性顺序:Mn2+Zn2+,从实验数据得到证实,中心离子对配合物的稳定性影响与理论上的判断相吻合。  相似文献   
110.
基于复杂性研究的Internet安全模型   总被引:3,自引:0,他引:3  
回顾了基于复杂性科学和复杂网络研究的相关理论建立的几种Internet安全大范围模式行为的模型.从Internet复杂系统的Power-Law分布规律出发,论述了该分布规律对网络安全建模的影响,然后围绕网络安全的两个方面,即病毒传播和网络鲁棒性,分别论述了基于复杂系统和复杂网络研究成果建立的病毒传播的定量模型,以及网络鲁棒性与复杂网络结构的定性关系模型,最后还展望了未来该领域的研究前景.  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