全文获取类型
收费全文 | 6935篇 |
免费 | 302篇 |
国内免费 | 387篇 |
专业分类
航空 | 3780篇 |
航天技术 | 2266篇 |
综合类 | 450篇 |
航天 | 1128篇 |
出版年
2021年 | 53篇 |
2019年 | 43篇 |
2018年 | 77篇 |
2017年 | 45篇 |
2016年 | 66篇 |
2015年 | 66篇 |
2014年 | 156篇 |
2013年 | 206篇 |
2012年 | 181篇 |
2011年 | 278篇 |
2010年 | 259篇 |
2009年 | 259篇 |
2008年 | 385篇 |
2007年 | 256篇 |
2006年 | 186篇 |
2005年 | 211篇 |
2004年 | 194篇 |
2003年 | 244篇 |
2002年 | 152篇 |
2001年 | 233篇 |
2000年 | 177篇 |
1999年 | 222篇 |
1998年 | 276篇 |
1997年 | 207篇 |
1996年 | 222篇 |
1995年 | 245篇 |
1994年 | 243篇 |
1993年 | 184篇 |
1992年 | 207篇 |
1991年 | 121篇 |
1990年 | 107篇 |
1989年 | 171篇 |
1988年 | 69篇 |
1987年 | 84篇 |
1986年 | 81篇 |
1985年 | 170篇 |
1984年 | 138篇 |
1983年 | 116篇 |
1982年 | 134篇 |
1981年 | 150篇 |
1980年 | 66篇 |
1979年 | 64篇 |
1978年 | 50篇 |
1977年 | 38篇 |
1975年 | 54篇 |
1974年 | 44篇 |
1973年 | 40篇 |
1972年 | 48篇 |
1971年 | 43篇 |
1969年 | 39篇 |
排序方式: 共有7624条查询结果,搜索用时 46 毫秒
151.
Timothy G. Hawarden Robert Crane Harley A. Thronson Jr. Alan J. Penny Anna H. Orlowska Thomas W. Bradshaw 《Space Science Reviews》1995,74(1-2):45-56
The designs of cold space telescopes, cryogenic and radiatively cooled, are similar in most elements and both benefit from orbits distant from the Earth. In particular such orbits allow the anti-sunward side of radiatively-cooled spacecraft to be used to provide large cooling radiators for the individual radiation shields. Designs incorporating these features have predictedT
tel near 20 K. The attainability of such temperatures is supported by limited practical experience (IRAS, COBE). Supplementary cooling systems (cryogens, mechanical coolers) can be advantageously combined with radiative cooling in hybrid designs to provide robustness against deterioration and yet lower temperatures for detectors, instruments, and even the whole telescope. The possibility of such major additional gains is illustrated by the Very Cold Telescope option under study forEdison, which should offerT
tel5 K for a little extra mechanical cooling capacity. 相似文献
152.
K Dose C Stridde R Dillmann S Risi A Bieger-Dose 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(3):203-210
A wide variety of terrestrial organisms, the so-called "anhydrobiotes," has learned to survive in a state of extreme dehydration in dry environments. Strategies for survival include the accumulation of certain polyols and nonreducing saccharides, which help to prevent damage to membranes and proteins, but at low water partial pressure DNA is also progressively damaged by various lesions, including strand breaks and cross-linking to proteins. These lesions, if they are not too numerous, can be repaired before the first replication step after rehydration, but long-term exposure to dry conditions finally diminishes the chances of survival as these lesions accumulate. If an organism has no chance to repair the accumulated DNA damage during intermittent periods of active life, survival will not exceed a few decades. The restriction of survival by dryness-induced DNA lesions is corroborated by new data on conidia of Aspergillus and the free plasmid pBR 322. Our results will be discussed with respect to the chance of finding dormant life or biochemical fossils on the surface of Mars. 相似文献
153.
Seven healthy subjects were submitted to a 42-day head down bedrest, where leg venous compliance (venous distensibity index VDI) and leg volumes were assessed by mercury strain gauge plethysmography with venous occlusion and optoelectronic plethysmography, respectively. Plethysmographic and volometric measurements were made, before, during (at days 1, 4, 7, 14, 21, 26, 34 and 41), and after bedrest (days 1, 4, 7, 11 and 30 of the recovery period). Results showed a continuous decrease in leg volumes throughout bedrest, when VDI increased until day 26 of bedrest, and then decreased afterwards. The recovery period was characterized by a rapid return of VDI to prebedrest levels while leg volumes progressively normalised. These results showed that leg venous compliance changes are not always dependant upon skeletal muscle changes, and that factors other than size of muscle compartment are able to determine increases in leg venous compliance during long-term bedrest. 相似文献
154.
To study the possible effect of simulated weightlessness on brain function state, the brain event-related potentials (ERPs) in a simple visual selective response task were compared between HDT and HUT in 9 normal subjects. The results were: The Target(T) and non-Target(NT) flash signals both induced significant slow positive potentials which were supposed to related to the attention state; the amplitude of the positive potentials in frontal regions decreased significantly especially for NT-ERPs during HDT comprared with that during HUT. The data reported provide new evidence indicating that more attention should be paid on the brain function study during space flight. 相似文献
155.
The article presents data concerning the osmolality and concentration of electrolytes and hormones regulating their balance for blood serum of 223 cosmonauts and astronauts. The obtained results allow us to judge the constancy of physicochemical parameters for the blood serum of healthy individuals and how they react to extreme conditions of space flight. The parameters used for evaluation included not just absolute values for the examined indices, but also how they responded to space flight, the dependence on baseline values and the interrelationship between ions. These data are important to predict the effect of exposure to extreme conditions and point to what extent the effect depends on the characteristics of the individual. 相似文献
156.
Highly mobile space suit material optimization 总被引:1,自引:0,他引:1
This paper discusses the factors that control the flexibility of fabric space suit elements by examining a bending model of a pressurized fabric tube. Results from the model are used to evaluate the current direction in highly mobile EVA glove research and suggest that changes are necessary in the suit and glove fabric selection methodology. 相似文献
157.
158.
本文提供了一个精确表达式,通过它可将亚声速位势流动仿射变换成不可压缩流动。同时给出了新的气动力计算公式。在流过Joukovski翼型的气动力计算中,它与用计算机提供的精确数值计算结果相比较,二者很符合。 相似文献
159.
通过杯杯型轴对称复合挤压两种宏观缺陷并存的上限分析表达式,研究了该变形方式下两种宏观缺陷并存区域与正挤部分、反挤部分变形程度和坯料相对余厚的关系。本文的模型及结论被实验结果所证实。 相似文献
160.
Bruckner J.M.H. Scott R.W. Rea F.G. 《IEEE transactions on aerospace and electronic systems》1973,(6):883-888
The estimation of a multimodal linear system whose mode-to-mode transitions are described by a finite-state Markov chain is described. The problem has application in studying separation standards in an air traffic control environment. An optimal solution is formulated which is computationally infeasible. A suboptimal estimator is then derived which closely approximates the optimal estimator. An example is presented to illustrate the technique. 相似文献