全文获取类型
收费全文 | 5142篇 |
免费 | 2篇 |
国内免费 | 16篇 |
专业分类
航空 | 2704篇 |
航天技术 | 1651篇 |
综合类 | 182篇 |
航天 | 623篇 |
出版年
2021年 | 32篇 |
2019年 | 27篇 |
2018年 | 125篇 |
2017年 | 65篇 |
2016年 | 43篇 |
2014年 | 82篇 |
2013年 | 114篇 |
2012年 | 111篇 |
2011年 | 176篇 |
2010年 | 114篇 |
2009年 | 196篇 |
2008年 | 206篇 |
2007年 | 124篇 |
2006年 | 91篇 |
2005年 | 94篇 |
2004年 | 120篇 |
2003年 | 144篇 |
2002年 | 185篇 |
2001年 | 207篇 |
2000年 | 93篇 |
1999年 | 123篇 |
1998年 | 159篇 |
1997年 | 100篇 |
1996年 | 141篇 |
1995年 | 178篇 |
1994年 | 148篇 |
1993年 | 98篇 |
1992年 | 130篇 |
1991年 | 59篇 |
1990年 | 66篇 |
1989年 | 127篇 |
1988年 | 61篇 |
1987年 | 64篇 |
1986年 | 53篇 |
1985年 | 157篇 |
1984年 | 129篇 |
1983年 | 115篇 |
1982年 | 123篇 |
1981年 | 160篇 |
1980年 | 58篇 |
1979年 | 47篇 |
1978年 | 58篇 |
1977年 | 38篇 |
1976年 | 40篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1972年 | 50篇 |
1971年 | 50篇 |
1970年 | 31篇 |
1969年 | 35篇 |
排序方式: 共有5160条查询结果,搜索用时 343 毫秒
811.
Johnson S.D. Witulski A.F. Erickson R.W. 《IEEE transactions on aerospace and electronic systems》1988,24(3):263-274
Because of their tolerance of transformer nonidealities, resonant converters are considered to be well-suited to high-voltage applications. The series and parallel resonant topologies, as well as a newly discovered hybrid resonant topology are compared for high-voltage applications. Design criteria which incorporate transformer nonidealities are developed and used in the construction of high voltage prototypes for each topology. It is found that the parallel topology leads to the lowest peak switch current and the most ideal behavior 相似文献
812.
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
813.
An analysis is done to determine the maximum power transfer conditions for full-wave rectifier circuits. Potential applications noted are implanted medical instruments, inductive power transfer to weapons, power transfer using space reflectors, and power generation in space. Three types of series impedances are considered: resistive/inductive (RL), resistive/capacitive (RC), and resistive/inductive/capacitive (RLC). The optimum ratio of ac-to-dc voltage output is determined for each type. For the case that involves all three impedance types, the optimum turning condition is also determined. The differential equations describing the circuits are solved in nondimensional form. The solutions involve partial differential equations, closed-form relationships, and simultaneous equations that are solved by numerical methods. The optimum ratio of peak ac-to-dc voltage ranges from 2.0 to 2.8, depending upon the circuit. The optimum turning differs significantly from the usual resonant conditions, especially for low Q. 相似文献
814.
The analysis of the transient and steady-state processes in LC choppers is presented. The method, used previously for the analysis of processes in grid-connected and self-commutated converters, is adjusted for analysis of LC chopper's processes. The difference equations describing these processes, are obtained and solved. The analytical solution of the problem and the conditions of the transient, steady-state, and stability existence in the chopper are found. Theoretical and experimental results are compared and satisfactory agreement is obtained 相似文献
815.
J.C. Henoux G. Chambe M. Semel B. Woodgate R. Shine J. Beckers 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(11):155-158
Linear polarization was observed in the S I 1437Å line in bright flaring points during the soft X-ray emission. The degree of polarization is about 25% and is detected at a signal to noise ratio of 2.9. The polarized electric vector is directed towards disk center to within 3°.This polarization could be due to collisional excitationm of S I by energetic electrons beamed in the vertical direction. We suggest that the heat flux in the region interconnecting the transition zone to the high chromosphere during the gradual phase of a flare could lead to an anisotropic excitation. Then the observed polarization would be due to vertical motions of the transition zone sweeping the preexisting chromosphere. 相似文献
816.
817.
R. S. Polidan 《Space Science Reviews》1989,49(3-4):85-94
In this paper we present new results from the Voyager ultraviolet spectrometers and the IUE spacecraft on V356 Sgr and Lyr. The V356 Sgr observations cover, in detail, two eclipses and include one IUE high dispersion SWP image. During both eclipses the total strength of the UV emission lines were found to be invariant. Also, an uneclipsed UV continuum was detected at wavelengths shorter than 1600 Å. The IUE high dispersion SWP spectrum revealed that the emission lines are extremely broad, almost symmetrical lines with weak, slightly blue shifted absorption components. No evidence of carbon is seen in the emission or absorption spectrum of V356 Sgr in eclipse. A model for the origin of the circumstellar matter in this binary system is presented. The Voyager ultraviolet observations of Lyr show a strong far-UV continuum that is detectable down to 912 Å The far-UV continuum flux level was variable on time scales shorter than the orbital period and displayed no obvious orbital modulation or eclipses. The spectral shape of the far-UV continuum closely resembles that of a UX UMa type cataclysmic variable. On 16 August 1985 an rapid brightening of the far-UV continuum was observed which was also reminiscent of cataclysmic variables. Analysis of the Lyr data suggest that the central object must be small, with a radius on the order of 1 R or less.This work is supported by NASA Grants NAGW-587 and NAG5-441. 相似文献
818.
The requirements and diagnostic applications for the V-22 tiltrotor aircraft are used to demonstrate the unique requirements of mechanical system diagnostic design. The rationale for the approach selected is explored, and it is shown how the mechanical system diagnostic requirements affect the avionics architecture and performance requirements. It is concluded that, in light of the underdeveloped nature of mechanical diagnostics technology and the extended time required for a given mechanical diagnostic design to mature, future avionics system designs need architecture that is adaptable enough to accommodate the evolving mechanical diagnostics 相似文献
819.
Parts of a study conducted to examine state-of-the-art power systems applicable to future military spacecraft are summarized. The study focused on burst-mode megawatt-class CW power, such as might be applied to SDIO directed energy systems, but lower-power, continuous-duty subsystems were included in less detail. A set of simple mass and volume algorithms has been developed to approximate several prime systems, and these were incorporated into a Lotus 1-2-3 spreadsheet. Among the power subsystems included in that study were primary batteries, alkaline primary fuel cells, and combustion turbogenerators. These systems, which are the most likely candidates for mobile battlefield power, are described 相似文献
820.
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