首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   6840篇
  免费   42篇
  国内免费   11篇
航空   3073篇
航天技术   2406篇
综合类   21篇
航天   1393篇
  2021年   74篇
  2019年   45篇
  2018年   160篇
  2017年   107篇
  2016年   112篇
  2015年   51篇
  2014年   178篇
  2013年   222篇
  2012年   212篇
  2011年   322篇
  2010年   229篇
  2009年   329篇
  2008年   366篇
  2007年   220篇
  2006年   156篇
  2005年   185篇
  2004年   182篇
  2003年   205篇
  2002年   146篇
  2001年   218篇
  2000年   127篇
  1999年   162篇
  1998年   190篇
  1997年   109篇
  1996年   171篇
  1995年   203篇
  1994年   189篇
  1993年   120篇
  1992年   149篇
  1991年   47篇
  1990年   48篇
  1989年   139篇
  1988年   59篇
  1987年   56篇
  1986年   63篇
  1985年   183篇
  1984年   144篇
  1983年   107篇
  1982年   115篇
  1981年   214篇
  1980年   49篇
  1979年   45篇
  1978年   50篇
  1977年   42篇
  1975年   51篇
  1974年   38篇
  1973年   32篇
  1972年   35篇
  1971年   35篇
  1970年   37篇
排序方式: 共有6893条查询结果,搜索用时 0 毫秒
361.
362.
The interaction of inert foreign particles and a solid-liquid interface during solidification is considered. The viscous drag on the particle which produces a force on the interface is calculated under general assumptions as well as the disjoining force for small particles and the buoyancy force for large ones.  相似文献   
363.
The airborne synthetic-aperture radar (SAR) system developed for the Canada Centre for Remote Sensing (CCRS) is described. It consists of two radars, at C-band and X-band. Each radar incorporates the following features: dual-channel receivers and dual-polarized antennas; a high quality, 7-look, real-time processor; a sensitivity time control for range-dependent gain control; a motion-compensation system for antenna steering in azimuth and elevation; and baseband I and Q signal phase rotation. The system also uses a high-power transmitter with a low-power back-up. The SAR maps to either side of the aircraft, at high or low resolution, at incidence angles which in high resolution span 0° to 80°. Radar operating parameters, data products, key specifications and the motion compensation scheme used are presented. Properties of the real-time imagery are discussed and examples of C-band SAR data in the three operating modes are given  相似文献   
364.
Epidermal growth factor (EGF) activates a well characterized signal transduction cascade in human A431 epidermoid carcinoma cells. The influence of gravity on EGF-induced EGF-receptor clustering and early gene expression as well as on actin polymerization and actin organization have been investigated. Different signalling pathways induced by the agents TPA, forskolin and A23187 that activate gene expression were tested for sensitivity to gravity. EGF-induced c-fos and c-jun expression were decreased in microgravity. However, constitutive beta-2 microglobulin expression remained unaltered. Under simulated weightlessness conditions EGF- and TPA-induced c-fos expression was decreased, while forskolin- and A23187-induced c-fos expression was independent of the gravity conditions. These results suggest that gravity affects specific signalling pathways. Preliminary results indicate the EGF-induced EGF-receptor clustering remained unaltered irrespective of the gravity conditions. Furthermore, the relative filamentous actin content of steady state A431 cells was enhanced under microgravity conditions and actin filament organization was altered. Under simulated weightlessness actin filament organization in steady state cells as well as in EGF-treated cells was altered as compared to the 1 G reference experiment. Interestingly the microtubule and keratin organization in untreated cells showed no difference with the normal gravity samples. This indicates that gravity may affect specific components of the signal transduction circuitry.  相似文献   
365.
366.
Stratospheric air samples collected between 10 and 35 km altitude my means of a cryogenic sampler were analyzed by gaschromatography. Thus vertical profiles of source gases for halogen radicals were derived, such as CCl4, CCl3F, CCl2F2, CClF3, CF4, C2F3Cl3, C2F4Cl2, C2F5Cl, C2F6, CH3Cl, CH3CCl3, CHF2Cl, CH3Br, CBrF3, and CBrCl2F. Systematic discrepancies between measured and modelled halocarbon profiles point to deficiencies of present one- and two-dimensional models. Measurements of fully halogenated hydrocarbons provide a tool for systematically studying these deficiencies and thus improving the models.  相似文献   
367.
An applied intelligence program for ATE fault diagnosis shows promise as an effective method to reduce mean time to repair (MTTR). The types of knowledge required by an intelligent diagnostic for VLSI test systems, the resources needed to derive that knowledge, the approach implemented to organize it, and the final form of the knowledge representations which resulted from our work are discussed in this paper.  相似文献   
368.
Geological analysis and evaluation of digitally processed and enhanced LANDSAT MSS data products for three test areas of Singhbhum Shear Zone, India has led to certain significant results regarding optimisation and utility of such techniques for geological studies and obtaining additional/new information regarding lithology and structure. Three band-to-band ratio images are found to give best results. Lithologic formations could be subdivided into a number of units. The Westward and southeastward extension of the shear zone could be delineated thereby indicating scope of mineralisation in these parts.  相似文献   
369.
A concentrated binary system (Ge-Si) and a dilute one (Ge-Ga) have been solidified at 1 g, in a new vertical Bridgman furnace where radial thermal gradients have been minimized. Very different solutal boundary layer extents (δGe-Si = 3 cm, δGe-Ga = 2.5 mm) obtained for both systems solidified in the same thermal conditions are explained by simple analytical hydrodynamic models. It is demonstrated that :1. The convective transport of Ga in the Ge-Ga system is due to the unavoidable residual horizontal thermal gradients associated with discontinuity of thermal properties at the solid-liquid interface.2. The larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport is the result of the stabilizing effect of the longitudinal solutal gradient in this system.Thus, in the case of dilute systems, convective effects can only be cancelled through a reduction of the gravity level. On the opposite, pure diffusive solute transport can be achieved in the vertical configuration a 1 g in the case of concentrated systems where a stabilizing solutal effect is operating.  相似文献   
370.
During August 1981, a 50 MHz c.w. radar system was operated in central Canada to measure auroral scatter amplitudes and Doppler spectra from a scattering region centered near 66° magnetic invariant latitude (L ≈ 6.0). Narrow beams from 3 transmitters, differing in frequency by 1 KHz, were directed to cover a common volume of the ionosphere over a ground location at 56.3°N, 103.5°W. The scattered signals were received on narrow beam antennas at two receiving sites, and recorded in analog form on magnetic tape under the control of an AIM65 microcomputer. The analog tapes were digitized later and FFT-processed to obtain Doppler spectra and amplitudes.The 6 transmission paths were designed to provide several magnetic aspect angles varying by 1.5°-7° from perpendicularity with the earth's field B and two streaming aspect angles differing by ~38°. The objective was to employ controlled geometric factors to study the functional dependency of signal amplitudes and Doppler shifts on magnetic and streaming aspect angles. Several hundred hours of excellent data were obtained in continuous operation during the month of August 1981. Preliminary results will be reported.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号