全文获取类型
收费全文 | 8479篇 |
免费 | 18篇 |
国内免费 | 23篇 |
专业分类
航空 | 3770篇 |
航天技术 | 2852篇 |
综合类 | 31篇 |
航天 | 1867篇 |
出版年
2021年 | 89篇 |
2019年 | 50篇 |
2018年 | 246篇 |
2017年 | 172篇 |
2016年 | 154篇 |
2015年 | 64篇 |
2014年 | 218篇 |
2013年 | 271篇 |
2012年 | 264篇 |
2011年 | 408篇 |
2010年 | 304篇 |
2009年 | 431篇 |
2008年 | 455篇 |
2007年 | 305篇 |
2006年 | 188篇 |
2005年 | 243篇 |
2004年 | 219篇 |
2003年 | 258篇 |
2002年 | 184篇 |
2001年 | 270篇 |
2000年 | 142篇 |
1999年 | 181篇 |
1998年 | 220篇 |
1997年 | 136篇 |
1996年 | 188篇 |
1995年 | 243篇 |
1994年 | 222篇 |
1993年 | 130篇 |
1992年 | 170篇 |
1991年 | 54篇 |
1990年 | 61篇 |
1989年 | 159篇 |
1988年 | 65篇 |
1987年 | 66篇 |
1986年 | 75篇 |
1985年 | 221篇 |
1984年 | 177篇 |
1983年 | 135篇 |
1982年 | 141篇 |
1981年 | 247篇 |
1980年 | 65篇 |
1979年 | 55篇 |
1978年 | 58篇 |
1977年 | 51篇 |
1975年 | 58篇 |
1974年 | 46篇 |
1973年 | 37篇 |
1972年 | 46篇 |
1971年 | 44篇 |
1970年 | 42篇 |
排序方式: 共有8520条查询结果,搜索用时 15 毫秒
441.
Glaser J.S. Witulski A.F. Myers R.G. 《IEEE transactions on aerospace and electronic systems》1994,30(1):135-143
A constant-frequency diode-clamped series resonant converter (CFCSRC) is proposed as a solution to problems associated with frequency-controlled resonant converters. This converter has two resonant frequencies, and control is achieved by varying the relative time spent at each switching frequency. Two zero-current-switching (ZCS) modes are examined and plotted in the output plane. An equation is given for the boundary between the two ZCS modes, as well as an expression for the boundary between ZCS and non-ZCS operation; both are plotted in the output plane. The output equation for the main mode is shown to be hyperbolic. Converter peak voltages limited to the input voltages, and peak currents are less than those of the frequency-controlled clamped series resonant converter over a large operating range. Data from a prototype converter are compared with theoretical data and are shown to be in good agreement with the theoretical model 相似文献
442.
The paper by Koc and Chen (1994) includes new formulas for computational complexity, but the method itself is already known and has dubious numerical stability. A poorly conditioned example is shown 相似文献
443.
On July 5.–6. 1983, during the EXOSAT performance verification (PV) and calibration phase, a raster scan of Cygnus X-2 was performed. In contrast to the previously observed smooth intensity variations on timescales of hours, the source revealed a behaviour unknown until now: active periods with high energy flares recurring on time scales of 300–500 s were interrupted by quiet periods of several hours. At all intensity levels the source spectra clearly require a two component continuum (blackbody + thermal bremsstrahlung). In addition, a weak iron emission line with equivalent widths between 39 an 70 eV was detected. The source has a much harder spectrum during the flares than during quiet periods, indicating drastic temperature changes within the emission region, while the absolute iron line flux does not vary. From the spectral characteristics it becomes clear that self-comptonization of the thermal bremsstrahlung spectrum plays an important role. The time variability and spectral behaviour in this peculiar state allow Cyg X-2 to be classified as a Low Mass X-ray Binary System (LMXB) very similar to the prototype of this class, Sco X-1. 相似文献
444.
R. H. D. Corbet A. P. Smale J. W. Menzies G. Branduardi-Raymont P. A. Charles K. O. Mason 《Space Science Reviews》1985,40(3-4):415-419
We present optical spectroscopy and photometry of 4U 1258-61 obtained over the course of six years. Evidence for the evolution of a circumstellar envelope throughout the course of the observations is presented. 相似文献
445.
S. Mereghetti G. F. Bignami P. A. Caraveo A. Goldwurm G. G. C. Palumbo 《Space Science Reviews》1985,40(3-4):495-501
A new EXOSAT (LE/CMA) observation of the region in Crux (R.A. 11h 45m, Dec. -62°) where Markert et al. (1981) reported the existence of two x-ray SNR's is presented. After cleaning the CMA field from the point source component, due to the UV emission of the numerous stars in the field, the smoothed x-ray contours are compared to the 408 MHz radio map of Caswell et al. (1983). The existence of two, well-separate x-ray emission regions is confirmed by EXOSAT, and the current x-ray/radio picture is not sufficent to distinguish clearly between the assumption of one or two (possibly interacting) SNR's in the region. 相似文献
446.
The imaging capabilities of the Exosat and Einstein satellites at soft X-ray wavelengths have begun to show that suitable Galactic X-ray sources have extended ( 10 arcmin) haloes due to scattering of soft X-rays by interstellar dust. A simple argument suggests that similar haloes, due to scattering by intergalactic dust, should exist around distant (z 1) quasars and detailed analysis confirms this conclusion. A search for such haloes around suitable X-ray quasars could provide valuable, model-independent, constraints on the amount and origin of intergalactic dust. 相似文献
447.
The BL Lac object Mkn 421 was observed by EXOSAT four times over a period of six days in February 1984. Significant X-ray variability was apparent on a timescale of less than a day, but with no accompanying spectral change. The source exhibited a very soft power law X-ray spectrum with an extremely low intrinsic column density (NH1020 cm–2). There was no evidence for an additional hard component attributable to synchrotron self-Compton emission. The observations when combined with other published data imply that significant changes occur in the form of the broad-band UV/X-ray continuum of this source. 相似文献
448.
We present the results of four observations made by the European Space Agency's EXOSAT Observatory of the X-ray transient 4U1630-47 during its 1984 outburst. We observed marked spectral changes as the source decayed from a maximum observed intensity of 8×10–9 erg/cm2/sec (1.5–10 keV). The spectrum could be modelled by a soft thermal-like component with a high energy power-law tail. The relative contribution of the soft to hard component decreased as the total luminosity decreased. We compare these changes with those observed from the black hole candidate Cyg X-1 when it transitions from a high to a low state. In addition we report the discovery of short timescale intensity variations (down to 50 msec) with a characteristic timescale of 20 sec. We present a precise position for this unidentified source.on leave from Università di Roma Dipartimento di Fisica G. Marconi. 相似文献
449.
Presented here are Exosat LE1-CMA images of fields in the Vela SNR. Soft X-ray emission is observed in the north of the remnant indicating a filamentry structure. No X-ray emission was seen in two fields to the west and south-west which have optical filaments but were not studied by Einstein. The Vela pulsar is observed and is significantly broader than a point source indicating the presence of a small nebula as seen by the Einstein HRI. The flux seen from the point source in the CMA is consistent with blackbody radiation from a 106 degree neutron star of approx. 10 km radius. A bright ridge of emission is seen north of the pulsar which may be part of the extended synchroton nebula seen in the 2–10 keV range. 相似文献
450.
The results of a previous discrete-time model of the series resonant dc-dc converter are reviewed and from these a small signal dynamic model is derived. This model is valid for low frequencies and is based on the modulation of the diode conduction angle for control. The basic converter is modeled separately from its output filter to facilitate the use of these results for design purposes. Experimental results are presented. 相似文献