全文获取类型
收费全文 | 18414篇 |
免费 | 39篇 |
国内免费 | 130篇 |
专业分类
航空 | 9955篇 |
航天技术 | 5509篇 |
综合类 | 242篇 |
航天 | 2877篇 |
出版年
2021年 | 156篇 |
2018年 | 218篇 |
2016年 | 161篇 |
2014年 | 435篇 |
2013年 | 519篇 |
2012年 | 416篇 |
2011年 | 611篇 |
2010年 | 422篇 |
2009年 | 777篇 |
2008年 | 811篇 |
2007年 | 389篇 |
2006年 | 429篇 |
2005年 | 404篇 |
2004年 | 454篇 |
2003年 | 536篇 |
2002年 | 479篇 |
2001年 | 577篇 |
2000年 | 365篇 |
1999年 | 468篇 |
1998年 | 446篇 |
1997年 | 326篇 |
1996年 | 393篇 |
1995年 | 458篇 |
1994年 | 454篇 |
1993年 | 357篇 |
1992年 | 330篇 |
1991年 | 251篇 |
1990年 | 237篇 |
1989年 | 417篇 |
1988年 | 205篇 |
1987年 | 243篇 |
1986年 | 238篇 |
1985年 | 646篇 |
1984年 | 524篇 |
1983年 | 416篇 |
1982年 | 489篇 |
1981年 | 612篇 |
1980年 | 246篇 |
1979年 | 185篇 |
1978年 | 189篇 |
1977年 | 144篇 |
1976年 | 156篇 |
1975年 | 189篇 |
1974年 | 182篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
E. Miúrov R.A. Tigranyan M. Prasli
ka 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(14):225-230
Changes of deoxyribonucleoprotein in the spleen, thymus and liver of rats exposed to wegithlessness or artifical gravity on board biosatellites Cosmos 782 and Cosmos 936 after 20 days of flight were investigated. The level of polydeoxyribonucleotides in the spleen and thymus of rats exposed during the flight to weightlessness increased 4 – 11 hours after landing, suggesting breakdown of a part of the deoxyribonucleoprotein present. The use of artifical gravity prevented this breakdown in the thymus but not in the spleen. The breakdown was accompanied in the majority of cases by a decrease in teh deoxyribonucleoprotein content. We believe the breakdown of deoxyribonucleoprotein is due to a nonspecific stress reaction to the change from the weightless state to that of terrestrial gravity during landing. The polydeoxyribonucleotide level and amount of deoxyribonucleoprotein in the majority of cases returned to normal values during the 25 days of readaptation. No substantial change of deoxyribonucleoprotein was found in the liver. The different findings in the three organs are due to the fact that breakdown of deoxyribonucleoprotein takes place in sensitive cells underlying pycnosis. These cells are found in the spleen and thymus, but not in the liver. 相似文献
132.
J.L Culhane 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,19(12):1839-1848
The Yohkoh soft X-ray telescope obtains several images every 90 minutes. Data from the declining phase of the solar cycle have been used to compare the X-ray signal with other indicators of activity and to study coronal heating. X-ray emission from a north polar coronal hole is found broadly consistent with results of previous EUV observations. In diffuse emission regions, temperature rises to around 2.2 MK and levels off in the height range 1.5 – 1.9 RO. Such emission underlies streamers and may be the source of the low-speed solar wind. X-ray signatures for Coronal Mass Ejection (CME) events which involve the detection of reduced X-ray intensities in the corona, have been developed with Yohkoh data. CME observations are described 相似文献
133.
A.H. Brown 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1984,4(12):299-303
Growth process generate plant form and relate to most physiological functions. The Earth's gravity force affects plant growth in both obvious and subtle ways. It is a major environmental influence on morphology and physiology of plants. Gravity is less important as an agent for plant stress than as an environmental signal to guide growth. The plant's bioaccelerometers are remarkably sensitive, especially in hypogravity. Simulation (clinostat) studies and experiments in satellite laboratories are needed to understand the sensing, transduction, and response characteristics of g related mechanisms. By examining how plants alter growth processes to accomplish developmental or physiological “objectives” we may find it pragmatically desirable to ask ourselves how we might design a plant to achieve such responses to environmental influences. Examples of this design engineering approach for gravity related effects are described as an aid to experimentation. 相似文献
134.
This paper describes a new experimental approach to acoustic liner characterization in the presence of a grazing flow. The traditional methods of measurement use microphones to determine liner impedance. The in situ method in particular requires the simultaneous use of two microphones. The first is mounted flush with the surface of the liner grazed by the flow and the second is flush-mounted to the rear face of the liner. However, this method is invasive and assumes the reaction of the liner to be independent of the incidence of the waves (locally-reacting liner). The approach suggested here is radically different since Laser Doppler Velocimetry (LDV) is used to measure the acoustic perturbation of velocity, or acoustic velocity. This latter allows us to determine the acoustic displacement, which is the key parameter in Galbrun's linear theory for assessing the perturbation of pressure and the field of active intensity. The wall impedance and the propagation paths of acoustic energy in the presence of the liner may be deduced without any assumption and non-invasively. This approach was applied for characterizing a resistive liner in a test bench specially designed for aeroacoustic measurements, with a 2D LDV system. The flow was turbulent and the measured nominal Mach number was 0.13. The impedance and field of active intensity were then obtained. A comparison was carried out between the new approach and the in situ method using microphones. According to previous theoretical works in the literature and the presented test results, one has to be cautious about the definition of the impedance when performing in-flow acoustic measurements. 相似文献
135.
136.
Purpose of the work is to analyze and to summarize the data of investigations into human hemodynamics performed over 20 years aboard orbital stations Salyut-7 and Mir with participation of 26 cosmonauts on space flights (SF) from 8 to 438 days in duration. The ultrasonic techniques and occlusive plethysmography demonstrated dynamics of changes in the cardiovascular system during SF of various durations. The parameters of general hemodynamics, the pumping function of the heart and arterial circulation in the brain remained stable in all the space flights; however, there were alterations in peripheral circulation associated with blood redistribution and hypovolemie in microgravity. The anti-gravity distribution of the vascular tone decayed gradually as unneeded. The most considerable changes were observed in leg vessels, equally in arteries (decrease in resistance) and veins (increase in maximum capacity). The lower body negative pressure test (LBNP) revealed deterioration of the gravity-dependent reactions that changed for the worse as SF duration extended. The cardiovascular deconditioning showed itself as loss of descent acceleration tolerance and orthostatic instability in the postflight period. 相似文献
137.
Nandu Goswami Helmut Karl Lackner Ilona Papousek Jean-Pierre Montani Daniela Jezova Helmut G. Hinghofer-Szalkay 《Acta Astronautica》2011,68(9-10):1589-1594
Passive head up tilt (HUT) and mental arithmetic (MA) are commonly used for providing mental and orthostatic challenges, respectively. In animal experiments, even a single exposure to a stressor has been shown to modify the response to subsequent stress stimulus. We investigated whether MA applied before HUT elicits synergistic responses in orthostatic heart rate (HR), cardiac output (CO), heart rate variability and arterial blood pressure. The 15 healthy young males were subjected to two randomized protocols: (a) HUT and (b) HUT preceded by MA, with sessions randomized and ≥2 weeks apart. Beat to beat continuous hemodynamic variables were measured and saliva samples taken for hormonal assay. HUT alone increased HR from 59±7 (baseline) to 80±10 bpm (mean±SD) and mean blood pressure (MBP) from 88±10 to 91±14 mmHg. HUT results after MA were not different from those with HUT alone. The activity of alpha amylase showed differences during the experiments irrespective of the protocols. We conclude that mental challenge does not affect orthostatic cardiovascular responses when applied before; the timing of mental loading seems to be critical if it is intended to alter cardiovascular responses to upright standing. 相似文献
138.
This correspondence reports measurements taken in both the 4- and 6-GHz frequency bands of the protection afforded a pit-shielded Earth-station antenna. The measurement program described is an expansion of a previous one and was designed to determine the effects of artificial site shielding for a range of satellite locations rather than a single position. 相似文献
139.
The two-target technique proposed by the author in an earlier paper [1] for reducing radar multipath angle tracking errors has been simulated on a digital computer assuming an actual closed-loop system. When tracking with noise, the technique provides angle error performance which compares quite favorably with the expected performance given in [1] Furthermore, the large bias errors usually encountered in normal monopulse systems at low elevation angles are removed. Results of typical tracks are given, both for the method of [1], and for a modified version of the method which applies primarily to shipboard radar systems. Some results on loss of lock are also presented. 相似文献
140.
This paper develops synthesis techniques for a particular type of single-sideband sinusoidal carrier which is phase modulated by a subcarrier. Mathematical expressions for signal efficiency, sensitivity of design to parameter variation, and ratio of peak to average power are derived and incorporated in a computer program. Given the desired power ratios for modulated signal components, the program solves for the corresponding modulation parameters and evaluates signal efficiency, design sensitivity, and peak to average power ratio. A sample signal design is presented for clarity. 相似文献