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   28737篇
  免费   99篇
  国内免费   119篇
系统科学   195篇
丛书文集   454篇
教育与普及   43篇
理论与方法论   114篇
现状及发展   12270篇
研究方法   1387篇
综合类   13930篇
自然研究   562篇
  2013年   247篇
  2012年   490篇
  2011年   1062篇
  2010年   200篇
  2008年   561篇
  2007年   698篇
  2006年   623篇
  2005年   647篇
  2004年   700篇
  2003年   594篇
  2002年   640篇
  2001年   869篇
  2000年   836篇
  1999年   592篇
  1992年   565篇
  1991年   392篇
  1990年   431篇
  1989年   412篇
  1988年   406篇
  1987年   414篇
  1986年   455篇
  1985年   583篇
  1984年   406篇
  1983年   338篇
  1982年   298篇
  1981年   319篇
  1980年   371篇
  1979年   872篇
  1978年   681篇
  1977年   654篇
  1976年   522篇
  1975年   516篇
  1974年   773篇
  1973年   643篇
  1972年   705篇
  1971年   767篇
  1970年   1073篇
  1969年   840篇
  1968年   724篇
  1967年   748篇
  1966年   710篇
  1965年   530篇
  1964年   180篇
  1959年   269篇
  1958年   528篇
  1957年   363篇
  1956年   280篇
  1955年   255篇
  1954年   300篇
  1948年   186篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
81.
1 Results Strongly disordered metals with the high chemical and catalytic activity are often called as skeletal metals.Usually for their preparation the metallides of d-metal (which afterwards will be left in the “skeleton“) and chemically active component(s) are firstly synthesized.Then the last one(s) is (are) removed by the leaching with aqueous solutions of alkalis or carbonates.However,this method sometime fails,first of all,for the reactions,which should be realized in non-aqueous conditions.In th...  相似文献   
82.
1 Results Simultaneous micro-and nanostructuring was prepared on polyolefin surfaces by injection molding.The molds were made of electropolished aluminum foil where the micropatterns were structured with a custom made robot.Nanopatterns were subsequently created on the molds by oxidizing the aluminum surface electrochemically in polyprotic acid.The preparation technique allowed simultaneous control of the dimensions of the micro-and nanostructures.Structuring has a remarkable effect on the contact angle...  相似文献   
83.
84.
85.
Q Gu  F A Popp 《Experientia》1992,48(11-12):1069-1082
By considering an exciplex system consisting of collective molecules in interaction with both the 'pumping' fields and the biophoton fields, the two-level exciplex model and the three-level exciplex model are presented. They are useful for the investigation of the quasi-stationary behaviour of biophoton emission, and biophoton emission as a dynamic process in the presence of external perturbations. Our theoretical results predict a series of nonlinear effects, such as chaos, fractal behaviour, and non-equilibrium phase transition. These effects characterize the coherence nature of living systems. In our approaches, there are two important quantities f and x, which can be used to mark the working points of the two-level and three-level exciplex systems. All the influences of external perturbations on the exciplex systems, e.g. change of temperature, the addition of agents, exposure to light, etc., can be interpreted as shifts of the working points of the systems, leading to a diversity of nonlinear response of biophoton emission. In addition, the agreements of the theoretical results and the corresponding experimental observations on biophoton emission from biological systems in the presence of external perturbations are demonstrated.  相似文献   
86.
After a brief recall of the Sidelobe Canceler (SLC) working principle, including the derivation of a general formula for the Cancellation Ratio (CR), the effects of channel mismatching are investigated. In particular, curves providing CRvalues as a function of amplitude and phase channel mismatching, radar bandwidth, and jammer direction of arrival (JDOA) are provided for the cases of one and two auxiliary antennas. Subsequently, a time -space processor for performance restoration is analyzed in detail. In addition to the above mentioned quantities, the attainable CR value is expressed as a function of the space-time processor parameters. The contribution of the paper is related to the derivation of a number of mathematical equations of CR for several cases of practical interest for the radar engineer. In addition, several curves are presented to assist design of SLC systems.  相似文献   
87.
88.
M McClelland  A S Bhagwat 《Nature》1992,355(6361):595-596
  相似文献   
89.
90.
R A Silver  S F Traynelis  S G Cull-Candy 《Nature》1992,355(6356):163-166
Neurotransmission from mossy fibre terminals onto cerebellar granule cells is almost certainly mediated by L-glutamate. By taking advantage of the small soma size, limited number of processes and short dendrite length of granule cells, we have obtained high-resolution recordings of spontaneous miniature excitatory postsynaptic currents (m.e.p.s.cs) and evoked currents in thin cerebellar slices. Miniature currents have a similar time-course and pharmacology to evoked currents and consist of an exceptionally fast non-NMDA (N-methyl-D-aspartate) component (measured rise-time, 200 microseconds; estimated pre-filtered rise-time less than 100 microseconds; decay time constant, tau = 1.0 ms), followed by 50 pS NMDA channel openings that are directly resolvable. We could find no evidence for the recent proposal that miniature currents in granule cells are mediated solely by NMDA channels with a novel time course. The non-NMDA receptor component of m.e.p.s.cs has a skewed amplitude distribution, which suggests potential complications for quantal analysis. The difference in time course between the m.e.p.s.cs reported here and other synaptic currents in the brain could reflect differences in synaptic function or electrotonic filtering; the relative contribution of these possibilities has yet to be established.  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