全文获取类型
收费全文 | 5178篇 |
免费 | 35篇 |
国内免费 | 16篇 |
专业分类
航空 | 2191篇 |
航天技术 | 1624篇 |
综合类 | 25篇 |
航天 | 1389篇 |
出版年
2022年 | 22篇 |
2021年 | 52篇 |
2019年 | 35篇 |
2018年 | 227篇 |
2017年 | 175篇 |
2016年 | 145篇 |
2015年 | 55篇 |
2014年 | 149篇 |
2013年 | 181篇 |
2012年 | 170篇 |
2011年 | 290篇 |
2010年 | 229篇 |
2009年 | 327篇 |
2008年 | 322篇 |
2007年 | 225篇 |
2006年 | 111篇 |
2005年 | 160篇 |
2004年 | 144篇 |
2003年 | 159篇 |
2002年 | 113篇 |
2001年 | 164篇 |
2000年 | 63篇 |
1999年 | 89篇 |
1998年 | 99篇 |
1997年 | 81篇 |
1996年 | 74篇 |
1995年 | 115篇 |
1994年 | 84篇 |
1993年 | 61篇 |
1992年 | 80篇 |
1991年 | 21篇 |
1990年 | 33篇 |
1989年 | 68篇 |
1988年 | 28篇 |
1987年 | 31篇 |
1986年 | 27篇 |
1985年 | 116篇 |
1984年 | 96篇 |
1983年 | 79篇 |
1982年 | 73篇 |
1981年 | 122篇 |
1980年 | 55篇 |
1979年 | 29篇 |
1978年 | 30篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 26篇 |
1974年 | 22篇 |
1972年 | 19篇 |
1971年 | 17篇 |
排序方式: 共有5229条查询结果,搜索用时 15 毫秒
971.
A. I. Tkachenko 《Cosmic Research》2011,49(2):150-159
A method to find and identify failures in a spacecraft’s set of sensitive elements that measure vector quantities of different
nature is discussed. 相似文献
972.
Vázquez-Mayagoitia Á Horton SR Sumpter BG Šponer J Šponer JE Fuentes-Cabrera M 《Astrobiology》2011,11(2):115-121
The RNA-world theory hypothesizes that early Earth life was based on the RNA molecule. However, the notion that ribose, the sugar in RNA, is unstable still casts a serious doubt over this theory. Recently, it has been found that the silicate-mediated formose reaction facilitates the stabilization of ribose. Using accurate quantum chemical calculations, we determined the relative stability of the silicate complexes of arabinose, lyxose, ribose, and xylose with the intent to determine which would form predominantly from a formose-like reaction. Five stereoisomers were investigated for each complex. The stereoisomers of 2:1 ribose-silicate are the more stable ones, to the extent that the least stable of these is even more stable than the most stable stereoisomer of the other 2:1 sugar-silicate complexes. Thus, thermodynamically, a formose-like reaction in the presence of silicate minerals should preferentially form the silicate complex of ribose over the silicate complex of arabinose, lyxose, and xylose. 相似文献
973.
E. P. Kontar J. C. Brown A. G. Emslie W. Hajdas G. D. Holman G. J. Hurford J. Ka?parov�� P. C. V. Mallik A. M. Massone M. L. McConnell M. Piana M. Prato E. J. Schmahl E. Suarez-Garcia 《Space Science Reviews》2011,159(1-4):301-355
X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future. 相似文献
974.
975.
Hugh S. Hudson 《Space Science Reviews》2011,158(1):5-41
This article broadly reviews our knowledge of solar flares. There is a particular focus on their global properties, as opposed
to the microphysics such as that needed for magnetic reconnection or particle acceleration as such. Indeed solar flares will
always remain in the domain of remote sensing, so we cannot observe the microscales directly and must understand the basic
physics entirely via the global properties plus theoretical inference. The global observables include the general energetics—radiation
in flares and mass loss in coronal mass ejections (CMEs)—and the formation of different kinds of ejection and global wave
disturbance: the type II radio-burst exciter, the Moreton wave, the EIT “wave”, and the “sunquake” acoustic waves in the solar
interior. Flare radiation and CME kinetic energy can have comparable magnitudes, of order 1032 erg each for an X-class event, with the bulk of the radiant energy in the visible-UV continuum. We argue that the impulsive
phase of the flare dominates the energetics of all of these manifestations, and also point out that energy and momentum in
this phase largely reside in the electromagnetic field, not in the observable plasma. 相似文献
976.
E. M. Kraeva 《Russian Aeronautics (Iz VUZ)》2011,54(2):154-158
Based on the jet-vortex model for flow passage hydrodynamics in a high-speed centrifugal pump with the open-type impeller,
we present the calculated relations for design and optimization of the experimentally verified pump design parameters. 相似文献
977.
In this paper, one of the most possible methods for calculations of ejector devices equipped with the cylindrical mixing chambers
is considered; the method is based on averaging the flow parameters at the chamber inlet. We present the calculation results
and their comparison with the earlier published data, as well as the quantitative assessment of this method and feasibilities
of its use for different technical devices embodying the concept of ejection. 相似文献
978.
Some methods of optimizing a cutting tool kit by the minimum labour content criterion in milling of an article are considered.
Their advantages and disadvantages are given. 相似文献
979.
The scheme variants of implementing the thermal protection against heat flows being generated by the body of a high-temperature
stationary gas turbine engine (GTE) are presented. The scheme of the experimental bench with a working section is given. Methodical
approaches to the heat transfer calculation at different variants of forced and natural convection organization and under
various operating conditions are described. The generalized results of the experiments carried out using a heat curtain being
generated by porous injection are presented. 相似文献
980.
B. I. Mamaev 《Russian Aeronautics (Iz VUZ)》2011,54(2):159-164
Some design and experimental data on three cascades with the same 12° bending angle for the primary nozzle block of a gas
turbine are presented. As is seen from the investigation the most efficient at the transonic velocities of the outward flow
are the blades, the back curvature of which in the bevel cut zone decreases to the trailing edge. Also presented is the fact
that the blade with the increasing back curvature has noticeably larger losses. 相似文献