全文获取类型
收费全文 | 5699篇 |
免费 | 51篇 |
国内免费 | 30篇 |
专业分类
航空 | 2510篇 |
航天技术 | 1987篇 |
综合类 | 45篇 |
航天 | 1238篇 |
出版年
2022年 | 24篇 |
2021年 | 71篇 |
2019年 | 49篇 |
2018年 | 220篇 |
2017年 | 143篇 |
2016年 | 136篇 |
2015年 | 56篇 |
2014年 | 160篇 |
2013年 | 198篇 |
2012年 | 178篇 |
2011年 | 247篇 |
2010年 | 191篇 |
2009年 | 286篇 |
2008年 | 338篇 |
2007年 | 207篇 |
2006年 | 133篇 |
2005年 | 163篇 |
2004年 | 156篇 |
2003年 | 186篇 |
2002年 | 132篇 |
2001年 | 193篇 |
2000年 | 84篇 |
1999年 | 106篇 |
1998年 | 129篇 |
1997年 | 87篇 |
1996年 | 97篇 |
1995年 | 151篇 |
1994年 | 139篇 |
1993年 | 79篇 |
1992年 | 111篇 |
1991年 | 44篇 |
1990年 | 48篇 |
1989年 | 101篇 |
1988年 | 35篇 |
1987年 | 42篇 |
1986年 | 40篇 |
1985年 | 148篇 |
1984年 | 114篇 |
1983年 | 98篇 |
1982年 | 112篇 |
1981年 | 152篇 |
1980年 | 49篇 |
1979年 | 49篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1974年 | 24篇 |
1972年 | 26篇 |
1971年 | 21篇 |
排序方式: 共有5780条查询结果,搜索用时 0 毫秒
931.
针对混合型步进电机低速运行状态下脉动转矩较大的特点,应用鲁棒自适应控制策略来设计其低速伺服控制器,以实现高精度的跟踪控制性能.首先,根据混合型步进电机转矩形成的机理,揭示了其固有的脉动转矩对低速运动控制产生不利影响的动力学特征,并将它归结为两种结构不确定性,便于控制器的设计;其次,采取标准的自适应控制及其鲁棒化设计思想,借助于μ-修整策略来实现相应的控制律,确保脉动转矩的最大化补偿及伺服系统低速跟踪控制满足高性能的要求;最后对所建立的低速跟踪控制系统的性能进行了试验评估,并与作者前期的工作进行了比较,试验结果验证了其有效性与可行性. 相似文献
932.
High temperature effect on microflora of radish root-inhabited zone and nutrient solutions for radish growth. 总被引:2,自引:0,他引:2
E V Borodina L S Tirranen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(1):235-240
The effect of high temperatures (35 and 45 degrees C) on microflora of the root zone of radish plants grown in phytotron was evaluated by the response of microorganisms from 9 indicator groups. Phytotron air temperature elevated to 35 degrees C for 20 hours caused no significant changes in qualitative and quantitative composition of the root microflora in experimental plants. By the end of the experiment, the species diversity of microflora had changed. The amount of phytopathogenic microorganisms decreased which can be interpreted as more stable co-existence of microflora with plants. The numbers of microbes from other indicator groups was in dynamic equilibrium. The plants' condition did not deteriorate either. Exposure to the temperature of 45 degrees C for 7 hours have been found to change the numbers and species diversity in the radish root zone microflora. The microorganisms were observed to increase their total numbers at the expense of certain indicator groups. Bacteria increased spore forms at the stage of spores. Colon bacillus bacteria of increased their numbers by the end of experiment by an order. By the end of experiment the roots of experiment plants had microscopic fungi from Mucor, Aspergillus, Trichoderma, Cladosporium genera. The observed changes in the microbial complex seem to be associated with the changes of root emissions and general deterioration of the plants' condition. It is suggested that the response of the microorganisms can be indicative of the condition of plants under investigation. 相似文献
933.
A problem of modeling the incompressible viscous flow between two disks taking into account the rotation of one of them is considered. We study the effect of the disk angular velocity (flow regimes) in the interdisk spacing, finiteness (infiniteness) of disk radiuses, calculation region configuration as well as distances between disks on the flow characteristics, particularly, on the resistance moment coefficient. Numerical simulation is performed with the aid of Maple and ANSYS Fluent software. 相似文献
934.
H. J. Völk 《Space Science Reviews》2007,130(1-4):431-438
The dynamical and chemical effects of the Galactic Wind are discussed. This wind is primarily driven by the pressure gradient of the Cosmic Rays. Assuming the latter to be accelerated in the Supernova Remnants of the disk which at the same time produce the Hot Interstellar Medium, it is argued that the gas removed by the wind is enriched in the nucleosynthesis products of Supernova explosions. Therefore the moderate mass loss through this wind should still be able to remove a substantial amount of metals, opening the way for stars to produce more metals than observed in the disk, by e.g. assuming a Salpeter-type stellar initial mass function beyond a few Solar masses. The wind also allows a global, physically appealing interpretation of Cosmic Ray propagation and escape from the Galaxy. In addition the spiral structure of the disk induces periodic pressure waves in the expanding wind that become a sawtooth shock wave train at large distances which can re-accelerate “knee” particles coming from the disk sources. This new Galactic Cosmic Ray component can reach energies of a few×1018 eV and may contribute to the juncture between the particles of Galactic and extragalactic origin in the observed overall Cosmic Ray spectrum. 相似文献
935.
936.
937.
938.
A.A. Arellano-Baeza R.V. García M. Trejo-Soto A. Martinez Bringas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2009
Mexico is one of the most volcanically active regions in North America. Volcanic activity in central Mexico is associated with the subduction of the Cocos and Rivera plates beneath the North American plate. Periods of enhanced microseismic activity, associated with the volcanic activity of the Popocatépetl volcano are compared with periods, during which the microseismic activity was low. We detected systematical changes in the number of lineaments, associated with the microseismic activity due to lineament analysis of a temporal sequence of high resolution satellite images of the Popocatépetl volcano, provided by the ASTER/VNIR instrument. The Lineament Extraction and Stripes Statistic Analysis (LESSA) software package was used for the lineament extraction. In the future it would allow develop a methodology for detection of possible elevation of pressure in volcano edifice. 相似文献
939.
G. V. Kasatkin 《Cosmic Research》2005,43(6):395-403
The problem of studying a ring in the gravitational field of a center arose after the discovery of Saturn's rings by Galileo and subsequent discovery of the rings of other planets of the Solar System. Modern theoretical investigations of the existence and stability of planetary rings are mostly related to studies of plane differentially rotating discs [1]. As opposed to this line of research, this paper follows the approach established in classical works [2–4]. 相似文献
940.