全文获取类型
收费全文 | 5954篇 |
免费 | 51篇 |
国内免费 | 27篇 |
专业分类
航空 | 2820篇 |
航天技术 | 1852篇 |
综合类 | 16篇 |
航天 | 1344篇 |
出版年
2021年 | 51篇 |
2019年 | 44篇 |
2018年 | 213篇 |
2017年 | 173篇 |
2016年 | 106篇 |
2015年 | 56篇 |
2014年 | 129篇 |
2013年 | 173篇 |
2012年 | 173篇 |
2011年 | 321篇 |
2010年 | 233篇 |
2009年 | 337篇 |
2008年 | 329篇 |
2007年 | 265篇 |
2006年 | 132篇 |
2005年 | 187篇 |
2004年 | 155篇 |
2003年 | 165篇 |
2002年 | 115篇 |
2001年 | 170篇 |
2000年 | 96篇 |
1999年 | 105篇 |
1998年 | 126篇 |
1997年 | 102篇 |
1996年 | 120篇 |
1995年 | 153篇 |
1994年 | 139篇 |
1993年 | 80篇 |
1992年 | 113篇 |
1991年 | 46篇 |
1990年 | 46篇 |
1989年 | 108篇 |
1988年 | 45篇 |
1987年 | 41篇 |
1986年 | 46篇 |
1985年 | 137篇 |
1984年 | 134篇 |
1983年 | 95篇 |
1982年 | 103篇 |
1981年 | 147篇 |
1980年 | 53篇 |
1979年 | 37篇 |
1978年 | 45篇 |
1977年 | 37篇 |
1976年 | 29篇 |
1975年 | 43篇 |
1974年 | 34篇 |
1973年 | 27篇 |
1972年 | 25篇 |
1968年 | 24篇 |
排序方式: 共有6032条查询结果,搜索用时 343 毫秒
991.
B.V. Dementjev S.G. Kukin M.N. Markov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1982,2(10):107-110
The radiation spectra of the ionosphere in the 4–5 μm region have been determined from stratospheric balloons by means of the specially elaborated method of the registration of angular and spectral distribution of the radiation. The radiation bands of 4.4 μm and 4.8 μm which have 0.1–0.2 erg cm?2 sr?1s?1. brightness obtained in two flights, are identified with the vibration-rotation transitions of the ion NO+ (0 - 1) and the molecules N14N15 (0 - 1) and CO (0 - 1) and (3 - 2) from altitudes between 120 and 500 km. 相似文献
992.
Abraham D. Krikorian F.Ronald Dutcher Carol E. Quinn F.C. Steward 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(14):117-127
Morphogenetically competent proembryonic cells and well-developed somatic embryos of carrot at two levels of organization were exposed for 18.5 days to a hypogravity environment aboard the Soviet Biosatellite Cosmos 1129. It was confirmed that cultured totipotent cells of carrot can give rise to embryos with well-developed roots and minimally developed shoots. It was also shown that the space hypogravity environment could support the further growth of already-organized, later somatic embryonic stages and give rise to fully developed embryo-plantlets with roots and shoots. 相似文献
993.
A technique for suppressing the ambiguities arising in pulsed scanning systems is described. A system of multiple antennas is employed, and a coded sequence of pulses is radiated. 相似文献
994.
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
995.
These studies were designed and coordinated to evaluate specific aspects of man's immunologic and hematologic systems which might be altered by or respond to the space flight environment. The biochemical functions investigated included cytogenetic damage to blood cells, immune resistance to disease, regulation of plasma and red cell volumes, metabolic processes of the red blood cell, and physical chemical aspects of red blood cell functions. Only minor changes were observed in the functional capacity of erythrocytes as determined by measuring the concentrations of selected intracellular enzymes and metabolites. Tests of red cell osmotic regulation indicated some elevation in the activity of the metabolic dependent Na-K pump, with no significant alterations in the cellular Na and K concentrations or osmotic fragility. A transient shift in red cell specific-gravity profile was observed on recovery, possibly related to changes in cellular water content. Measurements of hemoconcentration (hematocrit, hemoglobin concentration, red cell count) indicated significant fluctuations postflight, reflecting observed changes in red cell mass and plasma volume. There was no apparent reticulocytosis during the 18 days following the first manned Skylab mission in spite of a significant loss in red cell mass. However, the reticulocyte count and index did increase significantly 5 to 7 days after completion of the second, longer duration, flight. There were no significant changes in either the while blood cell count or differential. However, the capacity of lymphocytes to respond to an in vitro mitogenic challenge was repressed postflight, and appeared to be related to mission duration. The cause of this repression is unknown at this time. Only minor differences were observed in plasma protein patterns. In the second mission there were changes in the proteins involved in the coagulation process which suggested a hypercoagulative condition. 相似文献
996.
Linear FM Signal Formats for Beacon and Communication Systems 总被引:2,自引:0,他引:2
This paper examines the capabilities of the class of linear FM spread-spectrum signals within the context of potential communications systems usage in order to establish some performance criteria and bounds that permit comparison with other spread-spectrum formats. A systematic basis is provided for parameter selection for this class of signals by examining the interaction a mong the frequency-modulation indices, time-bandwidth product, and cross-talk criteria that determine the number of effective linear FM signals (or channels) that can be used within the constraints of a bounded time-frequency region. A general expression is derived relating N, the number of useful signals, R2, a cross-talk parameter, ToWo, the mean time-bandwidth product, and ?max and ?min, the maximum and minimum FM rates of the signal set. Canonic signal processor structures are described for ensembles of linear FM signals that have either constant duration or constant bandwidth. It is then shown that the signal modulation format can be modified in accordance with classical paired-echo theory to expand the utility of this class of signals in both synchronous and nonsynchronous operations to yield the equivalent of time-division and code multiplexing. Possible applications for this signal format are discussed. 相似文献
997.
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
998.
E. Arijs D. Nevejans J. Ingels 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(6):19-28
Recent in situ measurements with balloon borne quadrupole mass spectrometers, between 20 and 45 km altitude, are reviewed and discussed.The major stratospheric positive ions observed are proton hydrates [H+(H2O)n] and non proton hydrates of the form H+Xm(H2O)2. The data analysis allows a derivation of the vertical mixing ratio profile of X (most probably CH3CN), which is compared with recent model calculations. From negative ion composition data, showing the presence of NO3? and HSO4? cluster ions, the density of sulfuric acid in the stratosphere is deduced. The implications of these findings on our understanding of the sulfur chemistry is briefly treated.Finally some other aspects such as contamination, cluster break up and the use of stratospheric ion mass spectra for determination of thermochemical data and other minor constituents are discussed. 相似文献
999.
Belter S.E. Williams C.R. Bass S.C. 《IEEE transactions on aerospace and electronic systems》1976,(6):698-705
The application of 4-bit microprocessors to the communication of the control and status signals found in an advanced aircraft instrument landing system is presented. The advantages as well as some of the peculiarities of such an approach are described. Among the benefits, one must include the possibility of a very convenient and understandable human interface. 相似文献
1000.
P.J. Sell E. Maisch J. Siekmann 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(5):49-52
The wetting kinetics of model tubes of different geometrical shape has been investigated theoretically and experimentally under conditions of simulated zero-gravity (earthbound laboratory experiments) and low gravity (sounding rocket experiments within the TEXUS-programme). The present note deals with the hydrodynamic forces (capillary-, friction-, inertia forces, etc.) resulting from capillary rise in cylindrical, conical and sinusoidal tubes. The rôle of these forces is studied numerically in some detail. 相似文献