全文获取类型
收费全文 | 6473篇 |
免费 | 4篇 |
国内免费 | 15篇 |
专业分类
航空 | 3290篇 |
航天技术 | 2234篇 |
综合类 | 186篇 |
航天 | 782篇 |
出版年
2021年 | 41篇 |
2019年 | 36篇 |
2018年 | 88篇 |
2017年 | 57篇 |
2016年 | 54篇 |
2014年 | 105篇 |
2013年 | 141篇 |
2012年 | 141篇 |
2011年 | 213篇 |
2010年 | 143篇 |
2009年 | 255篇 |
2008年 | 272篇 |
2007年 | 146篇 |
2006年 | 118篇 |
2005年 | 131篇 |
2004年 | 160篇 |
2003年 | 204篇 |
2002年 | 217篇 |
2001年 | 269篇 |
2000年 | 128篇 |
1999年 | 180篇 |
1998年 | 209篇 |
1997年 | 148篇 |
1996年 | 174篇 |
1995年 | 219篇 |
1994年 | 206篇 |
1993年 | 122篇 |
1992年 | 155篇 |
1991年 | 77篇 |
1990年 | 84篇 |
1989年 | 163篇 |
1988年 | 75篇 |
1987年 | 82篇 |
1986年 | 65篇 |
1985年 | 200篇 |
1984年 | 149篇 |
1983年 | 141篇 |
1982年 | 155篇 |
1981年 | 196篇 |
1980年 | 77篇 |
1979年 | 66篇 |
1978年 | 74篇 |
1977年 | 49篇 |
1976年 | 50篇 |
1975年 | 62篇 |
1974年 | 50篇 |
1972年 | 60篇 |
1971年 | 51篇 |
1970年 | 37篇 |
1969年 | 43篇 |
排序方式: 共有6492条查询结果,搜索用时 875 毫秒
661.
The roll manoeuvre of SOHO on September 3, 1997 provided the opportunity to study the northern coronal hole with SUMER slits
in east-west orientation, so that polar plumes and inter-plume lanes could be observed simultaneously. A preliminary analysis
of the observations shows that lines emitted by ions with the lowest formation temperatures (with the exceptions of Ne7+ and Ar7+) have the largest ratios of plume to lane radiances at heights between 35 000 km and 70 000 km above the photosphere. All
lines have narrower widths inside plumes than outside. Electron densities have been deduced in plumes and lanes from Si VIII
and Mg VIII line radiance ratios. The Mg IX pair was used to determine the corresponding electron temperatures. Neon (with
a high first-ionization potential) is found to be less abundant relative to magnesium (with low FIP) in a plume compared to
an inter-plume lane, but the variation is smaller than previously determined Ne/Mg abundance ratios in a plume relative to
the photosphere.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
662.
R.E. Hartle 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(4):321-332
The Voyager 1 measurements made during the Titan flyby reveal that Saturn's rotating magnetospheric plasma interacts directly with Titan's neutral atmosphere and ionosphere. This results from the lack of an intrinsic magnetic field at Titan. The interaction induces a magnetosphere which deflects the flowing plasma around Titan and forms a plasma wake downstream. Within the tail of the induced magnetosphere, ions of ionospheric origin flow away from Titan. Just outside Titan's magnetosphere, a substantial ion-exosphere forms from an extensive hydrogen-nitrogen exosphere. The exospheric ions are picked up and carried downstream into the wake by the plasma flowing around Titan. Mass loading produced by the addition of exospheric ions slows the wake plasma down considerably in the vicinity of the magnetopause. 相似文献
663.
The results of experimental and numerical investigations of resonant vibration frequencies and modes for rectangular cantilever plates of constant thickness are presented. The investigations are carried out using the methods of interference holography and finite elements, and the results may serve as basic references in determining resonant frequencies and modes of compressor and turbine blade vibrations. 相似文献
664.
An iteration method for airfoil shape completing using the pressure coefficient distribution specified on its sought part is proposed. The incompressible flow viscosity is taken into account by the boundary layer model, the iteration process is constructed with the use of G.Yu. Stepanov’s idea. The solution algorithm is compiled and a set of numerical calculations is carried out. It is shown that the method proposed offers advantages over the well-known numerical-analytical scheme of solving mixed inverse boundary-value problems of aerohydrodynamics in the case of viscous incompressible fluid. 相似文献
665.
F. Bocchino R. Bandiera Y. Gallant 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(3):407-410
The investigation of the general properties of non-thermal (NT) X-ray shell supernova remnants (SNRs), of which SN 1006 is the prototype, is important to understand how electrons are accelerated in SNR shocks and what is the origin of cosmic rays. Using the XMM-Newton satellite, we are carrying on a survey of putative non-thermal SNR candidates previously unknown or little studied in the X-ray band, in order to investigate the different manifestations of NT emission in SNR shells. The SNRs we have selected are likely to expand in a low density medium, and therefore to have a low thermal X-ray emission, that usually outshines the non-thermal one. We report here preliminary results obtained on the SNR shell DA 530. 相似文献
666.
L.L. Lazutin A.A. Khrushchinsky T.V. Kozelova A.O. Melnikov Ya. Sakharov G. Kremser W.K. Riedler K.M. Torkar K. Glassmeier J.-P. Treilhou A. Pedersen G. Gustafsson R. Pellinen J. Kangas 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(4):171-174
For investigations of the isolated magnetospheric substorm the ground-based and satellite geophysical data are discussed. The active phase of the substorm has two parts: an active-convective phase and a classical active phase. The analysed data show that the breakup of the substorm takes place at closed and not very much stretched magnetic field lines. 相似文献
667.
T C Yang L M Craise M T Mei C A Tobias 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1989,9(10):131-140
Experimental data on molecular mechanisms are essential for understanding the bioeffects of radiation and for developing biophysical models, which can help in determining the shape of dose-response curves at very low doses, e.g., doses less than 1 cGy. Although it has been shown that ionizing radiation can cause neoplastic cell transformation directly, that high-LET heavy ions in general can be more effective than photons in transforming cells, and that the radiogenic cell transformation is a multi-step process [correction of processes], we know very little about the molecular nature of lesions important for cell transformation, the relationship between lethal and transformational damages, and the evolution of initial damages into final chromosomal aberrations which alter the growth control of cells. Using cultured mouse embryo cells (C3H10T1/2) as a model system, we have collected quantitative data on dose-response curves for heavy ions with various charges and energies. An analysis of these quantitative data suggested that two DNA breaks formed within 80 angstroms may cause cell transformation and that two DNA breaks formed within 20 angstroms may be lethal. Through studies with restriction enzymes which produce DNA damages at specific sites, we have found that DNA double strand breaks, including both blunt- and cohesive-ended breaks, can cause cell transformation in vitro. These results indicate that DNA double strand breaks can be important primary lesions for radiogenic cell transformation and that blunt-ended double strand breaks can form lethal as well as transformational damages due to misrepair or incomplete repair in the cell. The RBE-LET relationship is similar for HGPRT gene mutation, chromosomal deletion, and cell transformation, suggesting common lesions may be involved in these radiation effects. The high RBE of high-LET radiation for cell killing and neoplastic cell transformation is most likely related to its effectiveness in producing DNA double strand breaks in mammalian cells. At present the role of oncogenes in radiation cell transformation is unclear. 相似文献
668.
Measuring temperatures, mechanical loads and derived quantities precisely and reliably play an important role in spaceflight. With spacecraft becoming increasingly complex, upscaling of present telemetry techniques can become cumbersome. Additionally, there are entirely new sensory requirements, resulting from emerging technologies such as smart structures, active vibration damping and composite material health monitoring. It has been demonstrated in preceding studies that these measurements can be advantageously and efficiently carried out by means of fiber-optic systems. The most prominent fiber-optic strain and temperature sensor is the fiber Bragg grating. Typically, multiple fiber Bragg gratings are used to translate entire temperature and strain fields into an optical wavelength information. For the interrogation of these sensors, a broadband or scanning light source is required. Additional requirements with respect to the light source are high intensity and unpolarized illumination of the gratings. These constraints can be met by a light source that is based on amplified spontaneous emission in a rare-earth-doped fiber. In the presented work, a compact light source, adapted for measurement applications and targeted towards space applications, has been developed. The design of this light source is presented, as well as its implementation. The light source has been designed and tested for selected core aspects of space robustness and the results of these tests are summarized. 相似文献
669.
While there is great potential for successful use of neural network (NN) algorithms in automatic target recognition (ATR) and other pattern identification/classification applications, significant barriers have been encountered that, to date, defy rigorous use of NNs within feedback control designs. The status of several problems and contradictions involving NNs relating to control and estimation theory applications (and to practical failure detection within INS/GPS navigation systems) are summarized here. To give a positive spin and for a balanced perspective, we also mention many novel laudable NN results obtained by invoking the techniques and results of control and estimation theory 相似文献
670.
Z Han H Suzuki F Suzuki M Suzuki Y Furusawa T Kato Jr. M Ikenaga 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(12):1725-1732
We have studied the induction of morphological transformation of Syrian hamster embryo cells by low doses of heavy ions with different linear energy transfer (LET), ranging from 13 to 400 keV/μm. Exponentially growing cells were irradiated with 12C or 28Si ion beams generated by the Heavy Ion Medical Accelerator in Chiba (HIMAC), inoculated to culture dishes, and transformed colonies were identified when the cells were densely stacked and showed a crisscross pattern. Over the LET range examined, the frequency of transformation induced by the heavy ions increased sharply at very low doses no greater than 5 cGy. The relative biological effectiveness (RBE) of the heavy ions relative to 250 kVp X-rays showed an initial increase with LET, reaching a maximum value of about 7 at 100 keV/μm, and then decreased with the further increase in LET. Thus, we confirmed that high LET heavy ions are significantly more effective than X-rays for the induction of in vitro cell transformation. 相似文献