全文获取类型
收费全文 | 17679篇 |
免费 | 33篇 |
国内免费 | 121篇 |
专业分类
航空 | 9826篇 |
航天技术 | 5157篇 |
综合类 | 239篇 |
航天 | 2611篇 |
出版年
2021年 | 154篇 |
2018年 | 202篇 |
2016年 | 152篇 |
2014年 | 433篇 |
2013年 | 514篇 |
2012年 | 408篇 |
2011年 | 565篇 |
2010年 | 408篇 |
2009年 | 760篇 |
2008年 | 799篇 |
2007年 | 360篇 |
2006年 | 420篇 |
2005年 | 367篇 |
2004年 | 420篇 |
2003年 | 496篇 |
2002年 | 463篇 |
2001年 | 539篇 |
2000年 | 350篇 |
1999年 | 447篇 |
1998年 | 405篇 |
1997年 | 316篇 |
1996年 | 356篇 |
1995年 | 418篇 |
1994年 | 398篇 |
1993年 | 354篇 |
1992年 | 289篇 |
1991年 | 247篇 |
1990年 | 233篇 |
1989年 | 380篇 |
1988年 | 207篇 |
1987年 | 234篇 |
1986年 | 232篇 |
1985年 | 636篇 |
1984年 | 514篇 |
1983年 | 398篇 |
1982年 | 487篇 |
1981年 | 608篇 |
1980年 | 245篇 |
1979年 | 182篇 |
1978年 | 189篇 |
1977年 | 144篇 |
1976年 | 156篇 |
1975年 | 183篇 |
1974年 | 181篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
911.
水泵内部流动实质上是复杂的三维非稳定流动 ,它对水泵性能及结构振动有重要影响。本文介绍了一种求解这种复杂内流动的数值方法。三维雷诺数平均的纳维斯托克斯方程 ( 3-DReynolds-averaged Navier-Stokes,RANS)以及标准 k-ε的方程用于描述水泵内非定常紊流流场。系统特性方程与水泵的 CFD模型相结合以考虑流体在管道中的加速 ;采用任意滑移网格界面模拟叶轮和静止部件之间的相互干涉 ;将整个叶轮作为分析对象 ,以考虑失速情况下流动的周向非对称。这些技术的结合包括了水泵内非稳定流动的物理实质。一台实验数据比较齐全的离心式 -扩压器水泵被用于验证所提出的数值方法 相似文献
912.
913.
Tracking multiple objects with particle filtering 总被引:8,自引:0,他引:8
Hue C. Le Cadre J.-P. Perez P. 《IEEE transactions on aerospace and electronic systems》2002,38(3):791-812
We address the problem of multitarget tracking (MTT) encountered in many situations in signal or image processing. We consider stochastic dynamic systems detected by observation processes. The difficulty lies in the fact that the estimation of the states requires the assignment of the observations to the multiple targets. We propose an extension of the classical particle filter where the stochastic vector of assignment is estimated by a Gibbs sampler. This algorithm is used to estimate the trajectories of multiple targets from their noisy bearings, thus showing its ability to solve the data association problem. Moreover this algorithm is easily extended to multireceiver observations where the receivers can produce measurements of various nature with different frequencies. 相似文献
914.
915.
Examination of the spatial distribution of CO intensity of Comet Halley indicates that a large fraction of CO originates from
the refractory organic component in the coma, rather than directly from the volatiles in the nucleus. Based on the fluffy
aggregate interstellar dust comet model, we have estimated the upper limits of the total amount of CO provided by coma dust.
The implications from the comparison of the predicted results with the observed value have been discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
916.
Until pristine samples can be returned from cometary nuclei, primitive meteorites represent our best source of information
about organic chemistry in the early solar system. However, this material has been affected by secondary processing on asteroidal
parent bodies which probably did not affect the material now present in cometary nuclei. Production of meteoritic organic
matter apparently involved the following sequence of events: Molecule formation by a variety of reaction pathways in dense
interstellar clouds; Condensation of those molecules onto refractory interstellar grains; Irradiation of organic-rich interstellar-grain
mantles producing a range of molecular fragments and free radicals; Inclusion of those interstellar grains into the protosolar
nebula with probable heating of at least some grain mantles during passage through the shock wave bounding the solar accretion
disc; Agglomeration of residual interstellar grains and locally produced nebular condensates into asteroid-sized planetesimals;
Heating of planetesimals by decay of extinct radionuclides; Melting of ice to produce liquid water within asteroidal bodies;
Reaction of interstellar molecules, fragments and radicals with each other and with the aqueous environment, possibly catalysed
by mineral grains; Loss of water and other volatiles to space yielding a partially hydrated lithology containing a complex
suite of organic molecules; Heating of some of this organic matter to generate a kerogen-like complex; Mixing of heated and
unheated material to yield the meteoritic material now observed. Properties of meteoritic organic matter believed to be consistent
with this scenario include: Systematic decrease of abundance with increasing C number in homologous series of characterisable
molecules; Complete structural diversity within homologous series; Predominance of branched-chain isomers; Considerable isotopic
variability among characterisable molecules and within kerogen-like material; Substantial deuterium enrichment in all organic
fractions; Some fractions significantly enriched in nitrogen-15; Modest excesses of L-enantiomers in some racemisation-resistant
molecules but no general enantiomeric preference. Despite much speculation about the possible role of Fischer-Tropsch catalytic
hydrogenation of CO in production of organic molecules in the solar nebula, no convincing evidence for such material has been
found in meteorites. A similarity between some meteoritic organics and those produced by Miller-Urey discharge synthesis may
reflect involvement of common intermediates rather than the operation of electric discharges in the early solar system. Meteoritic
organic matter constitutes a useful, but not exact, guide to what we shall find with in situ analytical and sample-return
missions to cometary nuclei.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
917.
J. C. Raymond 《Space Science Reviews》1999,87(1-2):55-66
Order of magnitude variations in relative elemental abundances are observed in the solar corona and solar wind. The instruments
aboard SOHO make it possible to explore these variations in detail to determine whether they arise near the solar surface
or higher in the corona. A substantial enhancement of low First Ionization Potential (FIP) elements relative to high FIP elements
is often seen in both the corona and the solar wind, and that must arise in the chromosphere. Several theoretical models have
been put forward to account for the FIP effect, but as yet even the basic physical mechanism responsible remains an open question.
Evidence for gravitational settling is also found at larger heights in quiescent streamers. The question is why the heavier
elements don't settle out completely.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
918.
R. H. Brown K. H. Baines G. Bellucci J.-P. Bibring B. J. Buratti F. Capaccioni P. Cerroni R. N. Clark A. Coradini D. P. Cruikshank P. Drossart V. Formisano R. Jaumann Y. Langevin D. L. Matson T. B. Mccord V. Mennella E. Miller R. M. Nelson P. D. Nicholson B. Sicardy C. Sotin 《Space Science Reviews》2004,115(1-4):111-168
The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth–Moon system.This revised version was published online in July 2005 with a corrected cover date. 相似文献
919.
R. Srama T. J. Ahrens N. Altobelli S. Auer J. G. Bradley M. Burton V. V. Dikarev T. Economou H. Fechtig M. Görlich M. Grande A. Graps E. Grün O. Havnes S. Helfert M. Horanyi E. Igenbergs E. K. Jessberger T. V. Johnson S. Kempf A. V. Krivov H. Krüger A. Mocker-Ahlreep G. Moragas-Klostermeyer P. Lamy M. Landgraf D. Linkert G. Linkert F. Lura J. A. M. McDonnell D. Möhlmann G. E. Morfill M. Müller M. Roy G. Schäfer G. Schlotzhauer G. H. Schwehm F. Spahn M. Stübig J. Svestka V. Tschernjawski A. J. Tuzzolino R. Wäsch H. A. Zook 《Space Science Reviews》2004,114(1-4):465-518
The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10−19 and 10−9 kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta from rings and satellites. Ring and satellites phenomena which might be effects of meteoroid impacts will be compared with the interplanetary dust environment. Electrical charges of particulate matter in the magnetosphere and its consequences will be studied, e.g. the effects of the ambient plasma and the magnetic field on the trajectories of dust particles as well as fragmentation of particles due to electrostatic disruption.The investigation will be performed with an instrument that measures the mass, composition, electric charge, speed, and flight direction of individual dust particles. It is a highly reliable and versatile instrument with a mass sensitivity 106 times higher than that of the Pioneer 10 and 11 dust detectors which measured dust in the saturnian system. The Cosmic Dust Analyzer has significant inheritance from former space instrumentation developed for the VEGA, Giotto, Galileo, and Ulysses missions. It will reliably measure impacts from as low as 1 impact per month up to 104 impacts per second. The instrument weighs 17 kg and consumes 12 W, the integrated time-of-flight mass spectrometer has a mass resolution of up to 50. The nominal data transmission rate is 524 bits/s and varies between 50 and 4192 bps.This revised version was published online in July 2005 with a corrected cover date. 相似文献
920.
Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn 总被引:1,自引:0,他引:1
Carolyn C. Porco Robert A. West Steven Squyres Alfred Mcewen Peter Thomas Carl D. Murray Anthony Delgenio Andrew P. Ingersoll Torrence V. Johnson Gerhard Neukum Joseph Veverka Luke Dones Andre Brahic Joseph A. Burns Vance Haemmerle Benjamin Knowles Douglas Dawson Thomas Roatsch Kevin Beurle William Owen 《Space Science Reviews》2004,115(1-4):363-497
The Cassini Imaging Science Subsystem (ISS) is the highest-resolution two-dimensional imaging device on the Cassini Orbiter and has been designed for investigations of the bodies and phenomena found within the Saturnian planetary system. It consists of two framing cameras: a narrow angle, reflecting telescope with a 2-m focal length and a square field of view (FOV) 0.35∘ across, and a wide-angle refractor with a 0.2-m focal length and a FOV 3.5∘ across. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 μ on a side. The data system allows many options for data collection, including choices for on-chip summing, rapid imaging and data compression. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 200 to 1100 nm. These were chosen to address a multitude of Saturn-system scientific objectives: sounding the three-dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, capturing lightning on both bodies, imaging the surfaces of Saturn’s many icy satellites, determining the structure of its enormous ring system, searching for previously undiscovered Saturnian moons (within and exterior to the rings), peering through the hazy Titan atmosphere to its yet-unexplored surface, and in general searching for temporal variability throughout the system on a variety of time scales. The ISS is also the optical navigation instrument for the Cassini mission. We describe here the capabilities and characteristics of the Cassini ISS, determined from both ground calibration data and in-flight data taken during cruise, and the Saturn-system investigations that will be conducted with it. At the time of writing, Cassini is approaching Saturn and the images returned to Earth thus far are both breathtaking and promising.This revised version was published online in July 2005 with a corrected cover date. 相似文献