全文获取类型
收费全文 | 2125篇 |
免费 | 269篇 |
国内免费 | 176篇 |
专业分类
航空 | 1161篇 |
航天技术 | 738篇 |
综合类 | 176篇 |
航天 | 495篇 |
出版年
2023年 | 10篇 |
2022年 | 31篇 |
2021年 | 49篇 |
2020年 | 42篇 |
2019年 | 40篇 |
2018年 | 43篇 |
2017年 | 30篇 |
2016年 | 48篇 |
2015年 | 52篇 |
2014年 | 83篇 |
2013年 | 79篇 |
2012年 | 104篇 |
2011年 | 116篇 |
2010年 | 116篇 |
2009年 | 138篇 |
2008年 | 162篇 |
2007年 | 79篇 |
2006年 | 96篇 |
2005年 | 96篇 |
2004年 | 68篇 |
2003年 | 92篇 |
2002年 | 95篇 |
2001年 | 104篇 |
2000年 | 61篇 |
1999年 | 73篇 |
1998年 | 71篇 |
1997年 | 51篇 |
1996年 | 57篇 |
1995年 | 39篇 |
1994年 | 56篇 |
1993年 | 35篇 |
1992年 | 41篇 |
1991年 | 25篇 |
1990年 | 20篇 |
1989年 | 51篇 |
1988年 | 19篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 20篇 |
1984年 | 29篇 |
1983年 | 22篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 6篇 |
1969年 | 4篇 |
排序方式: 共有2570条查询结果,搜索用时 62 毫秒
31.
32.
33.
本文基于带有界干扰的线性动力学模型,研究了卫星编队飞行中的相对位置控制问题。首先,在线性二次型最优控制的基础上,设计了一种非线性控制律,并使用李雅普诺夫稳定性理论证明了系统的稳定性。接着,通过对线性系统状态观测器进行改进,得到了一种非线性速度观测器,观测误差被证明是渐近收敛的。观测器与控制律的结合实现了无速度测量的控制,闭环系统被证明是渐近稳定的。文末的数值仿真验证了理论分析。 相似文献
34.
Morphological, contractile properties and myosin heavy chain (MHC) composition of rat soleus muscles were studied after 2 weeks of unloading (HS) and after 2 weeks of HS associated with selective deafferentation (HS + DEAF) at the level L4 and L5. The same significant reductions in muscle mass and tetanic tension were found after HS and HS + DEAF. However, the transformation of the slow-twitch soleus muscle towards a faster type characterized by a decrease in twitch time parameters and an increase in fast-twitch type MHC isoforms in HS did not appear in HS + DEAF conditions. Our results also showed that a pattern similar to firing rate of motoneurones innervating slow-twitch muscles inhibited the slow to fast fiber changes observed during HS. Nevertheless, neither the loss of mass or force output in the HS muscles were prevented by electrostimulation. Immobilization in a stretched position during HS maintained the muscle wet weight, mechanical and electrophoretical characteristics close to control values. We concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command. These basic data suggested that some experimental conditions such as electrostimulation or stretching, could participate in countermeasure programmes. 相似文献
35.
It has been suggested that a daily intake of fluid and salt supplements may be used to prevent bone demineralization in human subjects after prolonged exposure to hypokinesia (diminished muscular activity). Thus, the objective of this investigation was to evaluate the effect of fluid and salt supplementation in the prevention of development of osteoporosis in 64 Wistar rats with an initial body weight of 339-345 g, after exposure to 90 days of hypokinesia. They divided into 4 equal groups: the first group of rats placed under ordinary vivarium conditions and served as vivarium control; the second group were also placed under ordinary vivarium conditions but received daily fluid and salt supplements; the third group were subjected to pure hypokinesia, i.e. without the use of any preventive measures; and the fourth group were submitted to hypokinesia and received daily fluid and salt supplements. For the simulation of the hypokinetic effect the experimental group of rats were kept in small, individual, wooden cages. Through the experimental period the second and fourth group of rats received 8 ml/100 g body wt water and 5 ml 100 g body wt NaCl daily. By the end of the experimental period the animals were decapitated and the spongy matter of tibia and vertebrae of the rats were examined for changes referable to osteoporosis. It was found that the daily intake of fluid and salt supplements caused an increase in the volume density of primary spongiosa of bones. It was concluded that a daily intake of fluid and salt supplements may be used to prevent the development of osteoporosis in rats subjected to prolonged motor activity restriction. 相似文献
36.
37.
38.
39.
40.
Y A Clearwater 《Acta Astronautica》1988,17(2):217-222
The purpose and scope of the Habitability Research Group within the Space Human Factors Office at the NASA/Ames Research Center is described. Both near-term and long-term research objectives in the space human factors program pertaining to the U.S. manned Space Station are introduced. The concept of habitability and its relevancy to the U.S. space program is defined within a historical context. The relationship of habitability research to the optimization of environmental and operational determinants of productivity is discussed. Ongoing habitability research efforts pertaining to living and working on the Space Station are described. 相似文献