全文获取类型
收费全文 | 5730篇 |
免费 | 9篇 |
国内免费 | 29篇 |
专业分类
航空 | 2579篇 |
航天技术 | 2189篇 |
综合类 | 19篇 |
航天 | 981篇 |
出版年
2021年 | 55篇 |
2019年 | 43篇 |
2018年 | 121篇 |
2017年 | 81篇 |
2016年 | 73篇 |
2015年 | 25篇 |
2014年 | 128篇 |
2013年 | 165篇 |
2012年 | 158篇 |
2011年 | 230篇 |
2010年 | 148篇 |
2009年 | 260篇 |
2008年 | 314篇 |
2007年 | 168篇 |
2006年 | 146篇 |
2005年 | 174篇 |
2004年 | 178篇 |
2003年 | 199篇 |
2002年 | 123篇 |
2001年 | 183篇 |
2000年 | 128篇 |
1999年 | 139篇 |
1998年 | 158篇 |
1997年 | 126篇 |
1996年 | 153篇 |
1995年 | 198篇 |
1994年 | 178篇 |
1993年 | 93篇 |
1992年 | 144篇 |
1991年 | 49篇 |
1990年 | 52篇 |
1989年 | 127篇 |
1988年 | 40篇 |
1987年 | 40篇 |
1986年 | 58篇 |
1985年 | 173篇 |
1984年 | 132篇 |
1983年 | 107篇 |
1982年 | 131篇 |
1981年 | 156篇 |
1980年 | 47篇 |
1979年 | 30篇 |
1978年 | 37篇 |
1977年 | 36篇 |
1976年 | 29篇 |
1975年 | 26篇 |
1974年 | 34篇 |
1973年 | 25篇 |
1970年 | 27篇 |
1969年 | 24篇 |
排序方式: 共有5768条查询结果,搜索用时 125 毫秒
751.
Spaceflight and clinorotation cause cytoskeleton and mitochondria changes and increases in apoptosis in cultured cells. 总被引:7,自引:0,他引:7
The cytoskeleton is a complex network of fibers that is sensitive to environmental factors including microgravity and altered gravitational forces. Cellular functions such as transport of cell organelles depend on cytoskeletal integrity; regulation of cytoskeletal activity plays a role in cell maintenance, cell division, and apoptosis. Here we report cytoskeletal and mitochondria alterations in cultured human lymphocyte (Jurkat) cells after exposure to spaceflight and in insect cells of Drosophila melanogaster (Schneider S-1) after exposure to conditions created by clinostat rotation. Jurkat cells were flown on the space shuttle in Biorack cassettes while Schneider S-1 cells were exposed to altered gravity forces as produced by clinostat rotation. The effects of both treatments were similar in the different cell types. Fifty percent of cells displayed effects on the microtubule network in both cell lines. Under these experimental conditions mitochondria clustering and morphological alterations of mitochondrial cristae was observed to various degrees after 4 and 48 hours of culture. Jurkat cells underwent cell divisions during exposure to spaceflight but a large number of apoptotic cells was also observed. Similar results were obtained in Schneider S-1 cells cultured under clinostat rotation. Both cell lines displayed mitochondria abnormalities and mitochondria clustering toward one side of the cells which is interpreted to be the result of microtubule disruption and failure of mitochondria transport along microtubules. The number of mitochondria was increased in cells exposed to altered gravity while cristae morphology was severely affected indicating altered mitochondria function. These results show that spaceflight as well as altered gravity produced by clinostat rotation affects microtubule and mitochondria organization and results in increases in apoptosis. Grant numbers: NAG 10-0224, NAG2-985. 相似文献
752.
To cancel clutter, both medium-PRF waveforms which are ambiguous in both range and Doppler and high-PRF waveforms which are ambiguous in range but unambiguous in ambiguities, a previous paper has shown that superior results for a single target can be achieved by using a clustering algorithm. Here, the problem of multiple targets is considered. A maximum likelihood (ML) technique which incorporates the clustering algorithm is developed for the multiple target problem. Simulation results show that four targets which have the same speed but are at different ranges can be resolved by using a medium-PRF waveform and employing the ML resolution technique 相似文献
753.
Y. Lin S.F. Martin O. Engvold L.H.M. Rouppe van der Voort M. van Noort 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
High resolution Hα images and magnetograms (0.2 arc s) of an active region were obtained in alternating time series at 42 s cadences using the Swedish 1-m Solar Telescope on 2004 August 21. The Hα filtergrams reveal an active region filament and surges consisting of thread-like structures which have widths similar to the widths of chromospheric fibrils, both recorded down to the resolution limit in the best images. All observed structures in the active region appear highly dynamic. Fibrils show counterstreaming strongly resembling the counterstreaming threads in filaments. 相似文献
754.
Jeng-Shing Chern Ching-Yew Yang Nguyen X. Vinh John M. Hanson 《Acta Astronautica》1985,12(10):819-829
With increasing frequency in shuttle operation, it is of interest to have more than one or two landing fields within the boundary of the reachable area of the reentry vehicle. This boundary, called the footprint, depends on the aerodynamic characteristics of the vehicle and is severely restricted by the deceleration and heating constraints imposed upon the atmospheric reentry trajectory. This paper gives a general assessment of the footprint as a function of various deceleration and heating constraints. The difficulties in the computation of the three-dimensional reentry trajectories with optimal modulation in both the angle-of-attack and the bank angle are alleviated by the following devices: (a) nondimensionalizing of the equations of motion and use of the density as the altitude variable; (b) use of the classical integrals of the motion; (c) transformation of the adjoint variables into physical variables; and (d) spherical rotation of the coordinates. 相似文献
755.
P. Brown M.W. Dunlop A. Balogh C. Carr J. Gloag E. Lucek T. Oddy 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(10):1571-1578
The Fluxgate Magnetometer experiments on-board the European Space Agency’s four spacecraft Cluster Mission have the capability to store sampled magnetic field vectors in the instrument memory, either as a full resolution ‘event capture’ or as spin-resolution vectors transformed into a non-spinning co-ordinate system (de-spun). The latter capability has ensured a dataset is available which extends the partial orbital coverage achieved during nominal operations in the first years of operation. The on-board de-spin is achieved using a Walsh function with Haar coefficients and allows for up to 27 h additional data per non-coverage interval. A number of commissioning orbits were used to verify the accuracy of data collected by the de-spin mode, whereby individual spacecraft were operated separately in a number of standard normal sampling and de-spin mode combinations. Up to the present time, this data has not been available to the Cluster community. We present results here comparing the performance of the on-board de-spin algorithm versus the normal sampling modes over a number of boundary layer crossings, describe the techniques used for calibration and timeline recovery, and outline the context in which the data may be usable in future studies. 相似文献
756.
We propose a method that makes it possible to obtain in the framework of linear approximation the exact formulas for the wave resistance of the channel walls with an arbitrary plane pattern in the first and subsequent interference zones. It is shown by a particular example of the sinusoidal pattern that the pressure wave interference may lead to the positive or negative resistance resonance. 相似文献
757.
S. Seetha M.C. Ramadevi V.C. Babu M.R. Sharma N.S.R. Murthy B.N. Ashoka K.C. Shyama R. Kulkarni G. Meena P. Sreekumar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2995-2998
The Scanning Sky Monitor is one of the experiments onboard the ASTROSAT, an Indian multiwavelength astronomy satellite mission. This experiment will detect and monitor X-ray transients in the energy band 2–10 keV. It is similar in design to the ASM on RXTE. It consists of position-sensitive proportional counters with one-dimensional mask. We describe the configuration of the experiment. We also discuss some of the results obtained using a detector which has already been fabricated and tested in our laboratory. 相似文献
758.
Spreading the signal spectrum is a widely used technique in telecommunication applications. Variable-frequency and spread spectrum modulation methods can also be utilized in power electronic applications to improve electro-magnetic compatibility (EMC). This paper gives an overview to variable-frequency techniques in power electronics with experimental results. 相似文献
759.
B Shukitt-Hale G Casadesus I Cantuti-Castelvetri B M Rabin J A Joseph 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,31(1):119-126
Exposing rats to particles of high energy and charge (e.g., 56Fe) disrupts neuronal systems and the behaviors mediated by them; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, and our previous study showed that radiation disrupted Morris water maze spatial learning and memory performance, the present study used an 8-arm radial maze (RAM) to further test the cognitive behavioral consequences of radiation exposure. Control rats or rats exposed to whole-body irradiation with 1.0 Gy of 1 GeV/n high-energy 56Fe particles (delivered at the alternating gradient synchrotron at Brookhaven National Laboratory) were tested nine months following exposure. Radiation adversely affected RAM performance, and the changes seen parallel those of aging. Irradiated animals entered baited arms during the first 4 choices significantly less than did controls, produced their first error sooner, and also tended to make more errors as measured by re-entries into non-baited arms. These results show that irradiation with high-energy particles produces age-like decrements in cognitive behavior that may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere. 相似文献
760.