全文获取类型
收费全文 | 9259篇 |
免费 | 19篇 |
国内免费 | 45篇 |
专业分类
航空 | 4518篇 |
航天技术 | 3307篇 |
综合类 | 39篇 |
航天 | 1459篇 |
出版年
2021年 | 65篇 |
2019年 | 62篇 |
2018年 | 148篇 |
2017年 | 93篇 |
2016年 | 81篇 |
2014年 | 181篇 |
2013年 | 246篇 |
2012年 | 226篇 |
2011年 | 318篇 |
2010年 | 216篇 |
2009年 | 357篇 |
2008年 | 436篇 |
2007年 | 249篇 |
2006年 | 233篇 |
2005年 | 260篇 |
2004年 | 232篇 |
2003年 | 303篇 |
2002年 | 192篇 |
2001年 | 300篇 |
2000年 | 191篇 |
1999年 | 226篇 |
1998年 | 272篇 |
1997年 | 201篇 |
1996年 | 260篇 |
1995年 | 323篇 |
1994年 | 303篇 |
1993年 | 166篇 |
1992年 | 221篇 |
1991年 | 115篇 |
1990年 | 97篇 |
1989年 | 228篇 |
1988年 | 89篇 |
1987年 | 95篇 |
1986年 | 107篇 |
1985年 | 292篇 |
1984年 | 237篇 |
1983年 | 206篇 |
1982年 | 222篇 |
1981年 | 268篇 |
1980年 | 95篇 |
1979年 | 63篇 |
1978年 | 78篇 |
1977年 | 71篇 |
1976年 | 58篇 |
1975年 | 79篇 |
1974年 | 64篇 |
1973年 | 61篇 |
1972年 | 68篇 |
1971年 | 59篇 |
1970年 | 64篇 |
排序方式: 共有9323条查询结果,搜索用时 25 毫秒
71.
Jovanovic M.M. Lee F.C. Chen D.Y. 《IEEE transactions on aerospace and electronic systems》1986,(2):138-145
Reverse-bias safe operating area (RBSOA) of high-power Darlington transistors is characterized using a 120 A/1000 V nondestructive reverse-bias second breakdown tester designed and fabricated at Virginia Polytechnic Institute and State University. Elaborate RBSOA characteristics are generated with different forward/reverse base drives and collector current levels. The effects of elevated case temperature and second-base drive on RBSOA of four-terminal Darlington devices are also discussed. 相似文献
72.
73.
Jovanovic M.M. Farrington R. Lee F.C. 《IEEE transactions on aerospace and electronic systems》1990,26(2):326-336
The performance of the half-bridge (HB) zero-current-switched (ZCS) quasi-resonant converter (QRC) and zero-voltage-switched (ZVS) multiresonant converter (MRC) is compared with respect to their efficiency, input voltage range, semiconductor stresses, power density, and reliability. The efficiency of the HB ZVS-MRC at a given nominal input is shown to be highly dependent on the range of the input voltage, and it suffers when the converter has to be designed to cover a wide range. However, this is not the case for the HB ZCS-QRC. Experimental versions of the HB ZCS-QRC and HB ZVS-MRC were designed for the input voltage range from 150 to 350 V and a maximum output power of 100 W, under the same constraints, to facilitate their comparison 相似文献
74.
G. J. Peters 《Space Science Reviews》1989,49(3-4):9-22
The circumstellar plasma that produces H emission in Algol binaries has been investigated using phase-resolved, high dispersion data acquired from CCD and image tube detectors. Results are summarized in this paper, including discussions of the disk geometry and size, asymmetry in the distribution of material, long-term or non-phase dependent variability, mass outflow, the mean electron density, and how the latter properties vary with the system's period or location in the r-q diagram. Five systems which display permanent emission with periods ranging from 4.5 to 261 days (SW Cyg, UX Mon, TT Hya, AD Her, and RZ Oph) are intercompared. If P < 4.5 days, no permanent disks are observed, while if P > 6 days, stable disks with only slight long-term variations in their H brightness are seen. The most variable systems appear to be those in the 5 – 6 day range, but the star's position in the r-q diagram has the largest influence on its behavior. The trailing side of the accretion disk, where the gas stream impacts the inner disk, is usually brighter, and the leading side is often times more extended. The disk extends out to at least 95% of the Roche surface of the primary and is highly flattened (RP). Mass outflow near phase 0.5 is commonplace. 相似文献
75.
Meteorological control of the D region 总被引:1,自引:0,他引:1
J. Taubenheim 《Space Science Reviews》1983,34(4):397-411
After a short review of the characteristics of ionized state and meteorology of the mesopause region, the winter anomaly of the D region electron density and its variability are described as manifestations of meteorological control. A major mechanism is the redistribution of nitric oxide, another important mechanism is the strong temperature dependence of cluster ion formation rates. The meteorological control can be described either in a ‘concerted’ scenario of more or less independently acting mechanisms, or in a ‘unitary’ scenario where all mechanisms are regarded as effects of a common cause, viz., the strong winter vortex circulation of the middle atmosphere. 相似文献
76.
Susan M. P. McKenna-Lawlor 《Space Science Reviews》1982,32(1-2):235-250
McMath Plage 15266, which transited the solar disk during Carrington Rotation 1667, gave rise during its passage to a spectacular sequence of five proton producing flares. Solar circumstances leading up to the formation of the active plage are described. An account is given of the magnetic affiliations and optical characteristics of the flares themselves, and it is suggested that four of these events might be interpreted as two twin phase flares displaying secondary maxima and minima such that the second phase in each case could in some sense be deemed a consequence of phenomena initiated during the first phase. Those particle phenomena associated with the observed activity are reviewed, and it is suggested that the azimuthal propagation of solar cosmic rays in the corona may occur more efficiently for flares at eastern longitudes in which the magnetic axis is aligned in a roughly north to south rather than an east-to-west direction.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia. 相似文献
77.
A new family of constant false alarm rate (CFAR) processors is introduced. An Ll-CFAR forms its noise power estimate by linearly filtering ranked samples from the reference set; the weights of this combination, however, depend not only on the rank, but also on the relative proximity of the sample to the cell under test. From the class of Ll-CFARs may be chosen members which effectively censor spurious targets; members which exhibit impressive control of false alarm in the presence of a clutter edge; and members which are robust against both such inhomogeneities. While the design of such schemes is involved, their implementation is not significantly more burdensome than that of plain ordered statistic CFAR (OS-CFAR). After a discussion of the stochastic training of Ll-CFAR, the performance is thoroughly assessed under the most commonly encountered instances of environmental conditions, and compared with those of classical CFAR techniques 相似文献
78.
Investigation of corona initiation voltage at reduced pressures 总被引:1,自引:0,他引:1
Karady G.G. Sirkis M.D. Li Liu 《IEEE transactions on aerospace and electronic systems》1994,30(1):144-150
The results of an experimental study of the 60 Hz ac corona inception voltage at reduced pressures for a cylindrical conductor and a parallel ground plane are presented. Several spacings and bare and insulated conductors of several diameters are considered. Empirical expressions from which a reasonably accurate prediction of the corona inception voltage (CIV) may be made have been inferred from experimental data 相似文献
79.
80.
This paper deals with the application of modern estimation techniques to the problem of speech data rate reduction. It is desirable to adaptively identify and quantitize the parameters of the speech model. These paramaters cannot be identified and quantized exactly; the performance of the predictor is thereby degraded and this could prevent data reduction. In many cases it is desirable to emply a suboptimal predictor in order to simplify the algorithms, and predictor performance is again degraded. This paper develops sensitivity and error analysis as a potential method for determining quantitatively how speech data reduction system performance is degraded by imprecise parameter knowledge or suboptimal filtering. An intended use of the sensitivity and error analysis algorithms is to determine parameter identification and model structure requirements of configuration concepts for adaptive speech digitizers. First, sensitivity and error analysis algorithms are presented that form the basis for the remainder of the work. The algorithms are then used to determine how imprecise knowledge of vocal tract parameters degrades predictor performance in speech. Transversal filters have previously been proposed for this application. The sensitivity analysis algorithms are then used to determine when and by how much the transverse filter is suboptimal to the Kalman filter. In particular, the question of how effectively a higher order of all-pole model approximates a system with zeros is answered, as this question is of considerable importance in speech. Finally, the physical significance of the innovations process in speech data rate reduction is studied. 相似文献