全文获取类型
收费全文 | 27650篇 |
免费 | 97篇 |
国内免费 | 111篇 |
专业分类
系统科学 | 121篇 |
丛书文集 | 451篇 |
教育与普及 | 36篇 |
理论与方法论 | 94篇 |
现状及发展 | 12178篇 |
研究方法 | 1261篇 |
综合类 | 13247篇 |
自然研究 | 470篇 |
出版年
2013年 | 215篇 |
2012年 | 423篇 |
2011年 | 889篇 |
2010年 | 173篇 |
2008年 | 490篇 |
2007年 | 597篇 |
2006年 | 532篇 |
2005年 | 566篇 |
2004年 | 614篇 |
2003年 | 495篇 |
2002年 | 529篇 |
2001年 | 860篇 |
2000年 | 823篇 |
1999年 | 587篇 |
1992年 | 556篇 |
1991年 | 388篇 |
1990年 | 422篇 |
1989年 | 408篇 |
1988年 | 407篇 |
1987年 | 414篇 |
1986年 | 457篇 |
1985年 | 585篇 |
1984年 | 406篇 |
1983年 | 333篇 |
1982年 | 297篇 |
1981年 | 322篇 |
1980年 | 373篇 |
1979年 | 870篇 |
1978年 | 685篇 |
1977年 | 652篇 |
1976年 | 527篇 |
1975年 | 513篇 |
1974年 | 772篇 |
1973年 | 644篇 |
1972年 | 708篇 |
1971年 | 765篇 |
1970年 | 1076篇 |
1969年 | 841篇 |
1968年 | 723篇 |
1967年 | 748篇 |
1966年 | 713篇 |
1965年 | 530篇 |
1964年 | 182篇 |
1959年 | 268篇 |
1958年 | 525篇 |
1957年 | 361篇 |
1956年 | 279篇 |
1955年 | 255篇 |
1954年 | 297篇 |
1948年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
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
73.
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
74.
75.
76.
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
77.
A. Farina A. ProtopapaRadar Dpt Selenia S. p. A. Via Tiburtina Km. - Rome Italy 《系统工程与电子技术(英文版)》1992,(3)
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
78.
79.
80.