全文获取类型
收费全文 | 5647篇 |
免费 | 5篇 |
国内免费 | 13篇 |
专业分类
航空 | 2861篇 |
航天技术 | 1819篇 |
综合类 | 183篇 |
航天 | 802篇 |
出版年
2021年 | 48篇 |
2019年 | 31篇 |
2018年 | 144篇 |
2017年 | 76篇 |
2016年 | 52篇 |
2014年 | 102篇 |
2013年 | 137篇 |
2012年 | 136篇 |
2011年 | 196篇 |
2010年 | 134篇 |
2009年 | 229篇 |
2008年 | 238篇 |
2007年 | 134篇 |
2006年 | 103篇 |
2005年 | 114篇 |
2004年 | 129篇 |
2003年 | 158篇 |
2002年 | 196篇 |
2001年 | 226篇 |
2000年 | 103篇 |
1999年 | 131篇 |
1998年 | 173篇 |
1997年 | 106篇 |
1996年 | 149篇 |
1995年 | 181篇 |
1994年 | 167篇 |
1993年 | 102篇 |
1992年 | 138篇 |
1991年 | 60篇 |
1990年 | 65篇 |
1989年 | 134篇 |
1988年 | 65篇 |
1987年 | 68篇 |
1986年 | 53篇 |
1985年 | 172篇 |
1984年 | 133篇 |
1983年 | 115篇 |
1982年 | 128篇 |
1981年 | 167篇 |
1980年 | 60篇 |
1979年 | 47篇 |
1978年 | 59篇 |
1977年 | 38篇 |
1976年 | 43篇 |
1975年 | 53篇 |
1974年 | 46篇 |
1972年 | 53篇 |
1971年 | 49篇 |
1970年 | 32篇 |
1969年 | 35篇 |
排序方式: 共有5665条查询结果,搜索用时 15 毫秒
941.
Vadher V.V. Smith I.R. Fanthome B.A. 《IEEE transactions on aerospace and electronic systems》1986,(1):64-70
Permanent magnet generators are often used as the pilot exciters of three-stage generating units to provide an efficient and reliable source of electrical power. No external power supply is then necessary, and the problems associated with brushwear and with arcing at the rubbing contacts are eliminated. A technique is presented here by which the performance of a permanent magnet generator, when combined with a rectifier-fed d.c. load at its output terminals, may be accurately and efficiently computed. The machine model takes into account the magnetization characteristic of the permanent magnet rotor and involves only parameters which can be determined from terminal measurements on the machine at standstill. Both computed and measured characteristics for an experimental machine are presented, and a comparison of these characteristics shows that the model can predict accurately all the major characteristics of the machine, as well as its detailed internal performance, when supplying a typical resistiveinductive load. 相似文献
942.
M.E. Glicksman R.S. Sokolowski 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(5):129-134
Micro- and macrosegregation, the nonuniform redistribution of solute during solidification, are common to both casting and welding, processes of fundamental importance in materials engineering. In multicomponent crystal growth where solid/liquid density differences are appreciable gravity-induced separation can lead to significant spatial variations in resulting ingot composition. In fact, this phenomenon is also operative in liquid/liquid systems such as monotectic alloys exhibiting a liquid miscibility gap, where buoyancy-driven flows can result in a sometimes unwanted separation of phases upon cooling through the miscibility dome. 相似文献
943.
R.Shankar Subramanian 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1983,3(5):145-153
The migration of a droplet in a large liquid body possessing a uniform temperature gradient is analyzed in the creeping flow limit for small values of the Marangoni number (ε). The method of matched asymptotic expansions is used for the solution of the field equations. The migration velocity is calculated to 0(ε2) and the result reduces to that of Young, Goldstein and Block [10] in the limit of zero Marangoni number. 相似文献
944.
An experimental study of the turbulence generated by an enclosed, premixed, propane-air flame has been carried out in a combustion chamber of 25 × 20 cm cross section. Care was taken to reduce any effects of the axial pressure gradient. By suitable changes in the grid geometry, the turbulence intensity and scale of the approach flow were varied independently. The results of these experiments show that a strong link exists between the mechanisms of turbulent flame propagation and flame-generated turbulence. Thus three distinct regions may be identified, each having different characteristics in regard to the effects of turbulence scale on flame-generated turbulence. For each region, a physical mechanism for flame-generated turbulence is proposed. In particular, it is observed that over a wide range of intensity and scale of the approach turbulence, (a) the relative turbulence intensity in the flame zone varies in the range 1–2 times the relative turbulence intensity of the cold flow, (b) in the region of intermediate turbulence levels (
) the flame-generated turbulence intensity reaches a minimum value which is equal to the approach stream turbulence intensity, (c) the flame-generated turbulence intensity reaches a maximum value when the rate of production of turbulent vorticity is equal to about half its rate of viscous dissipation. 相似文献
945.
M.C. Flemings J. Szekely N. El-Kaddah Y. Shiohara R.T. Frost 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):99-103
An electromagnetic furnace is being prepared for flights aboard the space shuttle. This apparatus is capable of melting metals and alloys up to 1400°C melting point by induction heating with subsequent solidification of the freely levitated melt without contact with any container. The solidification can be carried out with greatly reduced fields resulting in minimal heating and stirring of the free melt. Sequential specimens can be processed during flight. Several experiments are planned for a series of flights, beginning in 1985 with an undercooling experiment on NiSn alloys. These will be interspersed with detailed studies of fluid flow caused by low and high field levels in order to quantify the corresponding effect upon the solidification process. 相似文献
946.
A numerical method is described for analyzing the performance of an acoustooptic receiver. The method provides output waveforms, probability density functions for samples of the output, and detection probabilities for output samples. The approach is general in that input pulse envelope shapes, pulse offset frequencies, and output rilter magnitude responses are arbitrary. The basic analysis is also independent of the shape of the optical beam and of the photodiode windows. 相似文献
947.
The steady-state weight vector derived by either the least mean square (LMS) or normalized least mean square (NLMS) algorithms has random deviations from the optimum values. These deviations increase the steady-state residue power. A previous paper derived the LMS weight noise effects for a multiple sidelobe canceller (MSLC) application. This paper describes the NLMS weight noise effects. It is shown that for a thermal noise environment, the weight noise effect for the LMS algorithm is insignificant but is quite significant for the NLMS algorithm. Calculations for example noise plus interference environments imply that the NLMS weight noise effects are always larger than that for LMS. 相似文献
948.
G.J. Sofko J.A. Koehler J. Gilmer A.G. McNamara D.R. McDiarmid 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(7):149-152
During August 1981, a 50 MHz c.w. radar system was operated in central Canada to measure auroral scatter amplitudes and Doppler spectra from a scattering region centered near 66° magnetic invariant latitude (L ≈ 6.0). Narrow beams from 3 transmitters, differing in frequency by 1 KHz, were directed to cover a common volume of the ionosphere over a ground location at 56.3°N, 103.5°W. The scattered signals were received on narrow beam antennas at two receiving sites, and recorded in analog form on magnetic tape under the control of an AIM65 microcomputer. The analog tapes were digitized later and FFT-processed to obtain Doppler spectra and amplitudes.The 6 transmission paths were designed to provide several magnetic aspect angles varying by 1.5°-7° from perpendicularity with the earth's field B and two streaming aspect angles differing by ~38°. The objective was to employ controlled geometric factors to study the functional dependency of signal amplitudes and Doppler shifts on magnetic and streaming aspect angles. Several hundred hours of excellent data were obtained in continuous operation during the month of August 1981. Preliminary results will be reported. 相似文献
949.
R. Stalio 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(9):283-292
With the advent of high resolution space observations with high sensitivity, stellar atmospheric research has entered a new phase of rapid development. All stars, and especially hot stars, are now recognized to have atmospheric characteristics that were not suspected before. All hot stars that we can observe with sufficient accuracy show chromospheres and coronae indicative of non-radiative energy fluxes as well as mass loss; these phenomena exhibit a very great range in magnitude among different stars and, in several cases, they are variable in time. These discoveries have pointed out the need for determining the atmospheric structures of hot stars and, ultimately, of determining the mechanisms responsible for the likely common origin of chromospheres-coronae and mass fluxes. This paper will focus on these observational aspects of hot stars -mainly Be stars and OB-normal stars will be treated here- and on the constraints that the observations impose upon models for these stellar atmospheres. 相似文献
950.
T.F. Tuan D. Papadopoulos A.W. Peterson R.M. Nadile 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):137-141
The effects of gravity waves on OH and 5577 0I emissions have been used to investigate (1) the possible production of “secondary” waves or “ripples” by a “primary” wave; (2) the possible application of such observed optical emissions for computing atmospheric parameters such as turbulence viscosity in the mesospheric regions. 相似文献