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   6700篇
  免费   21篇
  国内免费   9篇
航空   3012篇
航天技术   2274篇
综合类   29篇
航天   1415篇
  2021年   70篇
  2019年   39篇
  2018年   235篇
  2017年   163篇
  2016年   143篇
  2015年   62篇
  2014年   194篇
  2013年   225篇
  2012年   214篇
  2011年   304篇
  2010年   224篇
  2009年   347篇
  2008年   401篇
  2007年   217篇
  2006年   153篇
  2005年   199篇
  2004年   171篇
  2003年   219篇
  2002年   150篇
  2001年   231篇
  2000年   98篇
  1999年   132篇
  1998年   159篇
  1997年   115篇
  1996年   119篇
  1995年   177篇
  1994年   172篇
  1993年   96篇
  1992年   123篇
  1991年   59篇
  1990年   53篇
  1989年   114篇
  1988年   50篇
  1987年   58篇
  1986年   50篇
  1985年   147篇
  1984年   125篇
  1983年   113篇
  1982年   116篇
  1981年   174篇
  1980年   61篇
  1979年   49篇
  1978年   40篇
  1977年   36篇
  1976年   36篇
  1975年   34篇
  1974年   33篇
  1973年   29篇
  1972年   36篇
  1971年   32篇
排序方式: 共有6730条查询结果,搜索用时 15 毫秒
171.
The authors provide an overview of expert systems and how they may effect the development of future defense applications. Military uses of computers are outlined, and expert-systems fundamentals are described. Artificial research and development efforts by the military are examined, and potential military applications are discussed. Expert systems efforts at NASA, by the US Air Force, and for the Strategic Defence Initiative are considered  相似文献   
172.
Circulation and thermal structure for the 90–95 km region is described on the basis of the joint analysis of direct and indirect sounding data. Mean season charts of the 0.001 mb surface were compiled using data from meteor trials, ionospheric drifts, rocket launchings and data of the rotational temperature of hydroxyl emission.  相似文献   
173.
Beyond their role in the cycling of the major elements, carbon, nitrogen, oxygen, etc., little is known about geobiological interactions involving plants with the elements of lower abundance. Plants influence the distribution of volcanic mercury by uptake from the atmosphere and soil fluids, transport to the shoot and re-release as Hg° into the atmosphere. Release rates as high as 2.5 × 10?4 g.h?1 per kg are known but even at more typical rates, fluxes far greater than the Environmental Protection Agency calculated U.S. average annual degassing rate of 130 × 10?6 g.m?2 should exist. On a global basis, the presence or absence of vegetative cover may be a significant regulator of Hg° transport between land surfaces and the atmosphere.  相似文献   
174.
Results are given of the study of active regions and flares by a high resolution Mg XI ion spectra obtained aboard rockets and a satellite. It is shown that there is a noticable similarity in the physical conditions in the plasma of active regions and flares. Plasma of both sources consists okf a thermal component with the temperature T ~ 2.?3.106K for active regions and T ~ 1.5?2.5.107K for flares and in both cases of a relatively small number (~ 1–5%) of suprathermal electrons with an energy E ~ 3–6 kT.  相似文献   
175.
Simultaneous measurements taken by instruments on the Atmosphere Explorer - C satellite were used to compare electron and proton particle energy deposition, Joule heating, and neutral density perturbations in the region of the cusp.Altitude profiles of Joule heating, electron energy deposition, and electron density are derived using measurements taken by the satellite as input to a computer model. Electric fields are calculated using ion drift measurements. Figures are presented for a representative orbital pass.A peak Joule heating rate of 0.059 Wm?2 occurred in the cusp region with a peak of 0.025 Wm?2 in the evening auroral electrojet. Peak volume heating rates corresponding to these regions were 1.4 × 10?6Wm?3 and 7.10?7 Wm?3, both occurring at an altitude of 115 km. Particle energy deposition was about an order of magnitude less than Joule heating. Large neutral density perturbations are related to regions of heating.  相似文献   
176.
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   
177.
Asymptotic stability characteristics with respect to the onset of Marangoni convection in a liquid layer under the simultaneous action of a large transverse magnetic field and a large rate of rotation are investigated. In certain parameter ranges there is a decrease in critical Marangoni number for increasing magnetic field and rotation depending on the coupling between interfacial perturbations and rotation.  相似文献   
178.
179.
The radiation spectra of the ionosphere in the 4–5 μm region have been determined from stratospheric balloons by means of the specially elaborated method of the registration of angular and spectral distribution of the radiation. The radiation bands of 4.4 μm and 4.8 μm which have 0.1–0.2 erg cm?2 sr?1s?1. brightness obtained in two flights, are identified with the vibration-rotation transitions of the ion NO+ (0 - 1) and the molecules N14N15 (0 - 1) and CO (0 - 1) and (3 - 2) from altitudes between 120 and 500 km.  相似文献   
180.
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   
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