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   4466篇
  免费   13篇
  国内免费   7篇
航空   2086篇
航天技术   1604篇
综合类   16篇
航天   780篇
  2021年   44篇
  2019年   25篇
  2018年   104篇
  2017年   73篇
  2016年   76篇
  2015年   39篇
  2014年   130篇
  2013年   138篇
  2012年   143篇
  2011年   193篇
  2010年   127篇
  2009年   216篇
  2008年   293篇
  2007年   128篇
  2006年   107篇
  2005年   138篇
  2004年   117篇
  2003年   153篇
  2002年   89篇
  2001年   153篇
  2000年   74篇
  1999年   99篇
  1998年   120篇
  1997年   87篇
  1996年   90篇
  1995年   124篇
  1994年   121篇
  1993年   64篇
  1992年   95篇
  1991年   53篇
  1990年   36篇
  1989年   88篇
  1988年   40篇
  1987年   42篇
  1986年   36篇
  1985年   96篇
  1984年   86篇
  1983年   75篇
  1982年   86篇
  1981年   118篇
  1980年   41篇
  1979年   36篇
  1978年   32篇
  1977年   25篇
  1976年   28篇
  1975年   22篇
  1974年   22篇
  1973年   23篇
  1972年   23篇
  1971年   21篇
排序方式: 共有4486条查询结果,搜索用时 15 毫秒
101.
102.
Performance of efficient single-person cardiopulmonary resuscitation (CPR) is vital to maintain cardiac and cerebral perfusion during the 2–4 min it takes for deployment of advanced life support during a space mission. The aim of the present study was to investigate potential differences in upper body muscle activity during CPR performance at terrestrial gravity (+1Gz) and in simulated microgravity (μG). Muscle activity of the triceps brachii, erector spinae, rectus abdominis and pectoralis major was measured via superficial electromyography in 20 healthy male volunteers. Four sets of 30 external chest compressions (ECCs) were performed on a mannequin. Microgravity was simulated using a body suspension device and harness; the Evetts–Russomano (ER) method was adopted for CPR performance in simulated microgravity. Heart rate and perceived exertion via Borg scores were also measured. While a significantly lower depth of ECCs was observed in simulated microgravity, compared with +1Gz, it was still within the target range of 40–50 mm. There was a 7.7% decrease of the mean (±SEM) ECC depth from 48 ± 0.3 mm at +1Gz, to 44.3 ± 0.5 mm during microgravity simulation (p < 0.001). No significant difference in number or rate of compressions was found between the two conditions. Heart rate displayed a significantly larger increase during CPR in simulated microgravity than at +1Gz, the former presenting a mean (±SEM) of 23.6 ± 2.91 bpm and the latter, 76.6 ± 3.8 bpm (p < 0.001). Borg scores were 70% higher post-microgravity compressions (17 ± 1) than post +1Gz compressions (10 ± 1) (p < 0.001). Intermuscular comparisons showed the triceps brachii to have significantly lower muscle activity than each of the other three tested muscles, in both +1Gz and microgravity. As shown by greater Borg scores and heart rate increases, CPR performance in simulated microgravity is more fatiguing than at +1Gz. Nevertheless, no significant difference in muscle activity between conditions was found, a result that is favourable for astronauts, given the inevitable muscular and cardiovascular deconditioning that occurs during space travel.  相似文献   
103.
Time-dependent thermal X-ray spectra are calculated from physically plausible conditions around GRB. It is shown that account for time-dependent ionization processes strongly affects the observed spectra of hot rarefied plasma. These calculations may provide an alternative explanation to the observed X-ray lines of early GRBs afterglows (such as GRB 011211). Our technique will allow one to obtain independent constraints on the GRB collimation angle and on the clumpiness of circumstellar matter.  相似文献   
104.
We discuss the detection of soft excess X-ray emission in a sample of 19 clusters of galaxies observed by XMM-Newton. In 6/19 clusters evidence for a soft X-ray excess is found. Four of these clusters show soft X-ray and O VII line emission from gas with a temperature of 0.2 keV. The centroid of this oxygen line is consistent with the redshift of the cluster. The intensity and spatial extend of the soft excess agrees with previous PSPC measurements. These observations are interpreted as emission from warm-hot intergalactic medium filaments, with density enhancements near the cluster centers, consistent with theoretical predictions. In the other two soft excess clusters a non-thermal origin is consistent with the data.  相似文献   
105.
The INTEGRAL satellite has been successfully launched in October 2002 and has recently started its operational phase. The INTEGRAL burst alert system (IBAS) will distribute in real time the coordinates of the gamma ray bursts (GRBs) detected with INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe the main IBAS characteristics and report on the initial results. During the initial performance and verification phase of the INTEGRAL mission, which lasted about two months, two GRBs have been localized with accuracy of 2–4 arcmin. These observations have allowed us to validate the IBAS software, which is now expected to provide quick (few seconds delay) and precise (few arcmin) localization for 10–15 GRBs per year.  相似文献   
106.
We present an analysis of seven clusters observed by XMM-Newton as part of our survey of 17 most X-ray luminous clusters of galaxies at z  0.2 selected for a comprehensive and unbiased study of the mass distribution in massive clusters. Using the public software FTOOLS and XMMSAS we have set up an automated pipeline to reduce the EPIC MOS and pn spectro-imaging data, optimized for extended sources analysis. We also developped a code to perform intensive spectral and imaging analysis particularly focussing on proper background estimate and removal. XMM-Newton deep spectro-imaging of these clusters allowed us to fit a standard β-model to their gas emission profiles as well as a standard MEKAL emission model to their extracted spectra, and test their inferred characteristics against already calibrated relations.  相似文献   
107.
Seasonal-to-interannual variability of the winter-spring bloom in the Gulf of Cádiz, eastern North Atlantic, has been investigated using chlorophyll-a remote sensing (CHL). These data have been obtained from the GlobColour project; the temporal coverage extends from September 1997 to December 2010. In this study we develop a generic quantitative approach for describing the temporal variability in the shape of the winter-spring bloom within a region. Variability in both the timing and magnitude of the bloom in the basin has been evaluated as a function of physical properties in the water column such as Mixed Layer Depth (MLD, GODAS model), sea surface temperature (SST, from AVHRR radiometers), photosynthetically-active radiation (PAR, from ocean color data) and euphotic depth (Zeu, from ocean color data). The analysis indicated that the timing, size and duration of the phytoplankton bloom in this area are largely controlled by both meteorological and oceanographic conditions at different scales; this means that it is likely to vary widely from one year to another.  相似文献   
108.
We present a novel instrument concept to measure the energy and mass spectra of ions incident on the lunar surface, based on the E-parallel–B or Thomson-parabola device used extensively as a diagnostic in the plasma fusion community. The Apollo-era Suprathermal Ion Detector Experiment (SIDE) was the first instrument package to perform in-situ measurements of ions incident on the lunar surface. The ions can originate from a variety of sources, including the solar wind, the Earth’s magnetotail, and photoionization of the thin lunar atmosphere. The species and energy distribution of ions arriving at the lunar surface depend in a complicated and poorly-understood fashion on the phase of the lunar day, the position of the Moon with respect to the Earth, and on the local plasma environment.  相似文献   
109.
In this paper an analysis of data coordinate systems from selenographic catalogues and space missions was carried out. The lunar macrorelief models were made on basis of the software package ASNI USTU using method of the spherical harmonic expansion. These models accurately describe the global features of the lunar figure. To construct these models the following sources of topographic information were used: “Clementine” and “KАGUYА” (Selena, Japan mission) missions, “KSC-1162” (Kazan selenocentric catalogue), “Kiev” (selenodesic catalogue), “SAI” (Chuikova (1975)), “Bills, Ferrari”, “ULCN” (The Unified Lunar Control Network 2005). Direct comparison hypsometric information “KSС-1162” catalogue data with “Clementine” mission was carried out. These researches confirmed a good agreement of the hypsometric information of compared systems. The normalized coefficients were obtained on basis of the hypsometric information expansion for eight sources. The displacement of the lunar center of mass (LCM) relatively to the lunar center of figure (LCF) was obtained by using topographic data selenodetical catalogues and space missions.  相似文献   
110.
Four soybean cultivars (‘Atlantic’, ‘Cresir’, ‘Pr91m10’ and ‘Regir’), selected through a theoretical procedure as suitable for cultivation in BLSS, were evaluated in terms of growth and production. Germination percentage and Mean Germination Time (MGT) were measured. Plants were cultivated in a growth chamber equipped with a recirculating hydroponic system (Nutrient Film Technique). Cultivation was performed under controlled environmental conditions (12 h photoperiod, light intensity 350 μmol m−2 s−1, temperature regime 26/20 °C light/dark, relative humidity 65–75%). Fertigation was performed with a standard Hoagland solution, modified for soybean specific requirements, and EC and pH were kept at 2.0 dS m−1 and 5.5 respectively.  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