全文获取类型
收费全文 | 6868篇 |
免费 | 648篇 |
国内免费 | 487篇 |
专业分类
航空 | 3645篇 |
航天技术 | 2113篇 |
综合类 | 436篇 |
航天 | 1809篇 |
出版年
2023年 | 48篇 |
2022年 | 111篇 |
2021年 | 173篇 |
2020年 | 129篇 |
2019年 | 123篇 |
2018年 | 164篇 |
2017年 | 177篇 |
2016年 | 165篇 |
2015年 | 174篇 |
2014年 | 242篇 |
2013年 | 274篇 |
2012年 | 286篇 |
2011年 | 362篇 |
2010年 | 365篇 |
2009年 | 411篇 |
2008年 | 385篇 |
2007年 | 290篇 |
2006年 | 229篇 |
2005年 | 252篇 |
2004年 | 212篇 |
2003年 | 229篇 |
2002年 | 231篇 |
2001年 | 217篇 |
2000年 | 162篇 |
1999年 | 185篇 |
1998年 | 174篇 |
1997年 | 154篇 |
1996年 | 132篇 |
1995年 | 177篇 |
1994年 | 176篇 |
1993年 | 100篇 |
1992年 | 116篇 |
1991年 | 66篇 |
1990年 | 68篇 |
1989年 | 84篇 |
1988年 | 48篇 |
1987年 | 34篇 |
1986年 | 48篇 |
1985年 | 137篇 |
1984年 | 116篇 |
1983年 | 93篇 |
1982年 | 85篇 |
1981年 | 164篇 |
1980年 | 33篇 |
1979年 | 36篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1975年 | 34篇 |
1973年 | 31篇 |
1972年 | 42篇 |
排序方式: 共有8003条查询结果,搜索用时 93 毫秒
101.
102.
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
103.
104.
105.
106.
航天器振动试验中的动力吸振现象 总被引:1,自引:0,他引:1
振动环境试验是考验航天器中各分机力学品质的重要试验,分机适应力学环境的能力在设计阶段必须充分考虑。目前分机的力学环境条件往往是在考虑了一定的安全系数后,以分机安装处界面的加速度包络曲线给出的。然而一般来说,分机振动试验过程中的边界条件与分机实际工作状态的边界条件是不同的。具体地说就是在振动试验过程中分机通过试验转接托架与振动台面相连,实际工作状态是分机安装在安装件(卫星各舱板)上。由于试验转接托架与安装件的动力学特性不一样,根据多自由度弹簧质量阻尼系统的动力吸振原理,可以推出试验状态分机在固有频率处存在过试验现象。为了减少过试验对分机带来的危害,新的试验方法研究(加速度响应限幅控制、阻抗特性模拟法、限力技术等)已成为国内外航天工作者研究的热点。文章详细阐述了振动试验中动力吸振的机理及过试验产生的缘由。 相似文献
107.
Smirnova N. V. Lyakhov A. N. Setzer Yu. I. Osepian A. P. Meng C.-I. Smith R. Stenbaek-Nielsen H. C. 《Cosmic Research》2004,42(3):210-218
Spatial distributions of the electron density in the latitude range 60°–90° N were calculated on the basis of a physical model of the E and lower Fregions of the high-latitude ionosphere using statistical models of auroral proton and electron precipitation. It is shown that precipitating protons can play the key role in the ionization of the Eregion in the dusk and midnight sectors of the auroral oval. However, quantitative estimates of the contribution of protons to the ionization depend on the used statistical models of electron precipitation. Comparison of the electron density profiles calculated for two incoherent scatter radars, EISCAT (Tromsö) and ESR (Svalbard), for simultaneous precipitation of electrons and protons and for electron precipitation only show that the influence of protons is the most significant in the dusk sector over the EISCAT radar and in the midnight sector over the ESR radar. The results presented indicate the need to take protons into account when radar data are used to derive precipitating electron spectra. 相似文献
108.
根据固体火箭发动机内弹道计算模型,分析了使用大扩张比喷管地面静止试验性能与真空性能之间的联系,建立了由地面性能换算成真空性能的方法,计算出的真空推力、真空比冲等数据与实际相符。 相似文献
109.
In August, 1981, the Westerbork Synthesis Radio Telescope was used for 4 h to search for narrowband pulsing radio beacons in the direction of the Galactic Center. By using both the spatial discrimination and temporal stability available to an interferometric measurement, weak intermittent signals can be detected even in the face of the strong, naturally caused radiation from this region. A radio beacon within our bandwidth, centered on the 21 cm neutral hydrogen line, would be recognizable if it had a repetition period between 40 sec and 1/2 h. The rms sensitivity to point sources was approximately 50 mJy/cycle, and the detection limit was 500 mJy/cycle. The limit degrades for pulse widths < 0.02s. No repetitive signals were found. For a swept, narrow-band radio beacon constrained to the Galactic Disk (beamwidth = 0.02 rad), our detection limit corresponds to a transmitter power of 10(11) MW at the Galactic Center. 相似文献
110.
G. N. Boiko L. M. Erukhimov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):71-74
The creation of artificial plasma density irregularities (AI) with prescribed spectra in the ionospheric heating experiments is discussed. We show that periodic successions of powerful pulses, pumped into the F-region of the ionosphere lead to obtaining AI with the controlled stationary spatial spectrum and allow us to change power low spectral index 2β at least from 2β = 2.2 to 2β = 3.4 by changing of the powerful pulse time schedule. 相似文献