全文获取类型
收费全文 | 10086篇 |
免费 | 56篇 |
国内免费 | 21篇 |
专业分类
航空 | 4791篇 |
航天技术 | 3517篇 |
综合类 | 35篇 |
航天 | 1820篇 |
出版年
2021年 | 95篇 |
2019年 | 66篇 |
2018年 | 194篇 |
2017年 | 134篇 |
2016年 | 134篇 |
2015年 | 70篇 |
2014年 | 232篇 |
2013年 | 279篇 |
2012年 | 275篇 |
2011年 | 409篇 |
2010年 | 290篇 |
2009年 | 455篇 |
2008年 | 509篇 |
2007年 | 282篇 |
2006年 | 221篇 |
2005年 | 257篇 |
2004年 | 262篇 |
2003年 | 327篇 |
2002年 | 212篇 |
2001年 | 346篇 |
2000年 | 191篇 |
1999年 | 254篇 |
1998年 | 291篇 |
1997年 | 191篇 |
1996年 | 264篇 |
1995年 | 306篇 |
1994年 | 314篇 |
1993年 | 168篇 |
1992年 | 234篇 |
1991年 | 90篇 |
1990年 | 97篇 |
1989年 | 226篇 |
1988年 | 96篇 |
1987年 | 104篇 |
1986年 | 99篇 |
1985年 | 285篇 |
1984年 | 218篇 |
1983年 | 185篇 |
1982年 | 211篇 |
1981年 | 301篇 |
1980年 | 89篇 |
1979年 | 82篇 |
1978年 | 81篇 |
1977年 | 68篇 |
1976年 | 54篇 |
1975年 | 93篇 |
1974年 | 69篇 |
1972年 | 75篇 |
1971年 | 54篇 |
1970年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
881.
M. Grewing A. Vidal-Madjar A. Labeque C. Laurent A. Boksenberg C. Burgio S. di Serego Alighieri G.P. Whitcomb W. Burton C.I. Coleman R. Hoekstra C. Jamar J. Lequeux P. Rafanelli 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(4):147-151
Magellan is one of the future space projects being studied by the European Space Agency. The aim is to provide high resolution (λ/Δλ ≥ 2.5 × 104) spectra in the far and extreme UV (between 500 and 1550 Å) of faint galactic and extragalactic objects (V ≤ 16m). The instrument consists of a mechanical collimator, a concave holographic grating and a bidimensional photon-counting detector. A low resolution mode (λ/Δλ ≥ 103) will provide spectra of objects as faint as 18m.5. Magellan is planned as an observatory, operated in real time, and allowing interaction with the observer. 相似文献
882.
The results of a survey of data on failures of aircraft electronic and electrical components that was conducted to identify problematic components are reported. The motivation for the work was to determine priorities for future work on the development of accident investigation techniques for aircraft electrical components. The primary source of data was the Airforce Mishap Database, which is maintained by the Directorate of Aerospace Safety at Norton Air Force Base. Published data from the Air Force Avionics Integrity Program (AVIP) and Hughes Aircraft were also reviewed. Statistical data from these three sources are presented. Two major conclusions are that problems with interconnections are major contributors to aircraft electrical equipment failures, and that environmental factors, especially corrosion, are significant contributors to connector problems 相似文献
883.
The use of generators by the military is examined. The factors determining generator size and weight for a given power rating and the generator scaling laws are discussed. Size and weight reduction is shown to be primarily a function of improvements in material properties and cooling techniques 相似文献
884.
The application of magnetostriction in the design of a hybrid, electromechanical/hydraulic high-performance linear (large force and stroke) and/or rotational (large moment and angle) actuator is considered. The design concept combines the high power density of actuation possible with magnetostriction (approaching 0.1 HP/cm3 of the magnetostrictive material assuming 3 kHz excitation frequency) and the design flexibility of hydraulics. The objective of the research described was to validate the concept theoretically and to study alternatives and improvements. The system, as currently envisioned, offers very small packaging volume (approximately an order of magnitude smaller than conventional electromechanical systems), flexible packaging (relative location of the major system components is not critical), and easy control (precise control of actuation speed, quick reverse time, and inherent position lockup). The major technical problems associated with the design are outlined, and results of a computer simulation of a prototype actuator are presented 相似文献
885.
Hamlen R.P. Hoge W.H. Hunter J.A. O'Callaghan W.B. 《Aerospace and Electronic Systems Magazine, IEEE》1991,6(10):11-14
Advances in aluminum-air batteries are described. These include alloys that show higher efficiencies and therefore lower hydrogen evolution, low-cost air cathodes that can be fabricated in production quantities, and methods for handling the aluminum hydroxide reaction product. Emphasis is placed on the advances in air cathodes. The application of this technology to new products and the implications for electric vehicles are discussed 相似文献
886.
A smart system that uses state-of-the-art image processing techniques to detect and track targets moving in video or infrared imagery is presented. It is a general-purpose system that was developed to be independent of the application and the user interface. TCATS has successfully demonstrated a high detection capability and the ability to ignore exterior environmental conditions, such as cloud shadows moving through the scene, snow, blowing dust, rain and moderate camera motion. TCATS can handle many targets and retain the identity of each object from one frame to the next. Special rugged hardware has been developed to reduce the size, cost, and power consumption and to increase the processing rate, reliability, and environmental operating capability of the system. TCATS can be incorporated into an existing video assessment system without a major cost outlay intruder. The TCATS algorithms, custom-developed hardware, and preliminary field test results are described 相似文献
887.
Chistopher H.A. Gilman S. Hamlen R.P. 《Aerospace and Electronic Systems Magazine, IEEE》1993,8(5):7-10
The current status and thrust of the US Army Research Laboratory's battery and fuel cell R&D programs that support emerging electronic battlefield equipment applications are reviewed. Major technical barriers are identified along with the approaches proposed to solve these anticipated problems 相似文献
888.
A review is presented of current optoelectronics research and development at the NASA Goddard Space Flight Center for high-power, high-bandwidth laser transmitters; high-bandwidth, high-sensitivity optical receivers; pointing, acquisition, and tracking components; and experimental and theoretical system modeling at the NASA Goddard Space Flight Center. Program hardware and space flight opportunities are presented 相似文献
889.
S. I. Avdyushin M. B. Belotserkovsky V. Kim A. S. Koroteev A. M. Kulikov A. I. Livshits Yu. K. Melnikov R. I. Moisya N. N. Petrov H. A. Popov Yu. A. Romanovsky Yu. A. Utkin V. V. Chilap 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(12):163-167
Results of rocket experiments on study of plasma flows (PF) artificially injected by sources separated from vehicles and their effect on medium parameters in ionosphere at altitudes 160:230 km are presented.PF were injected comprising lithium ions with velocities 1,2 x 104 m/sec. and cesium-potassium ions with velocities (1,4–1,5)x103 m/sec. Mass flow rate in case of lithium PS is 2 mg/sec, and in case of cesium-potassium PS is 0,2 g/sec. During experiments mass-spectrometer measurements of ion medium content in ranges of different ion masses were held, disturbancies of electric fields with frequencies up to 20 kHz and electron flows with energies 0,7keV, 4,6keV and over 40 keV were controlled at distancies from 150m to (500–600)m between plasma source and scientific equipment. 相似文献
890.
Some of the problems in the development of pulsewidth power converters when the switching frequency is increased from hundreds of kilohertz to tens of megahertz are highlighted. The major goal is the establishment of a ten megahertz power chain designed to operate in a distributed system from a DC power bus of 24 to 48 V. Available components and materials are discussed, and possible topologies-flyback, forward, push-pull forward, and buck converters-are examined. The performance criteria chosen for this analysis are, in the order of preference, output requirements, size and weight, efficiency, and cost 相似文献