全文获取类型
收费全文 | 2834篇 |
免费 | 7篇 |
国内免费 | 8篇 |
专业分类
航空 | 1294篇 |
航天技术 | 1167篇 |
综合类 | 14篇 |
航天 | 374篇 |
出版年
2021年 | 23篇 |
2019年 | 27篇 |
2018年 | 58篇 |
2017年 | 26篇 |
2016年 | 40篇 |
2015年 | 12篇 |
2014年 | 62篇 |
2013年 | 77篇 |
2012年 | 68篇 |
2011年 | 92篇 |
2010年 | 66篇 |
2009年 | 120篇 |
2008年 | 190篇 |
2007年 | 83篇 |
2006年 | 70篇 |
2005年 | 68篇 |
2004年 | 76篇 |
2003年 | 95篇 |
2002年 | 64篇 |
2001年 | 87篇 |
2000年 | 55篇 |
1999年 | 60篇 |
1998年 | 77篇 |
1997年 | 52篇 |
1996年 | 60篇 |
1995年 | 85篇 |
1994年 | 86篇 |
1993年 | 51篇 |
1992年 | 70篇 |
1991年 | 36篇 |
1990年 | 30篇 |
1989年 | 68篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 28篇 |
1985年 | 102篇 |
1984年 | 73篇 |
1983年 | 66篇 |
1982年 | 77篇 |
1981年 | 88篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 25篇 |
1977年 | 17篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1972年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有2849条查询结果,搜索用时 0 毫秒
751.
A possibility of enhancing the energy efficiency of the capacitance ignition system by realizing the batch mode of discharge impulse generation is substantiated. Using the numerical simulation methods, a problem is solved to study the voltage-current characteristics of such discharge units as a spark discharger and a spark plug and peculiarities of discharge formation in them; the batch mode is simulated with proper account for the influence of discharge-recurrence rate upon dynamics of electric strength recovery in the discharge units. A possibility of optimizing the capacitance ignition system parameters is theoretically evaluated to provide the maximum possible quantity of discharge impulse packet energy as well as the value of their peak capacity at the specified values of supply source power, discharge-recurrence rate, and total voltage of the entire ignition system. 相似文献
752.
D. McComas F. Allegrini F. Bagenal P. Casey P. Delamere D. Demkee G. Dunn H. Elliott J. Hanley K. Johnson J. Langle G. Miller S. Pope M. Reno B. Rodriguez N. Schwadron P. Valek S. Weidner 《Space Science Reviews》2008,140(1-4):261-313
The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. To meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft. 相似文献
753.
Venkatraman R. Bhat A.K.S. Edmunds M. 《IEEE transactions on aerospace and electronic systems》2000,36(4):1190-1203
A 1 /spl phi/ high-frequency (HF) transformer isolated, soft-switching single-stage ac-dc converter with low line-current harmonic distortion is presented. Its operation is explained with equivalent circuits for the various intervals. The converter is analyzed and design curves are obtained. An optimization parameter is introduced and a systematic design procedure is illustrated with a design example. Detailed SPICE simulation and experimental results of a 500 W converter with load as well as line voltage variation are given to verify theory. The proposed converter employs a zero-voltage transition (ZVT) network to ensure zero-voltage switching (ZVS) at all loads, and natural power factor correction is ensured using a simple control circuit. 相似文献
754.
Abhishek Pandey R. Prasad V.P. Singh S.K. Jha K.K. Shukla 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Learning fuzzy rule based systems with microwave remote sensing can lead to very useful applications in solving several problems in the field of agriculture. Fuzzy logic provides a simple way to arrive at a definite conclusion based upon imprecise, ambiguous, vague, noisy or missing input information. In the present paper, a subtractive based fuzzy inference system is introduced to estimate the potato crop parameters like biomass, leaf area index, plant height and soil moisture. Scattering coefficient for HH- and VV-polarizations were used as an input in the Fuzzy network. The plant height, biomass, and leaf area index of potato crop and soil moisture measured at its various growth stages were used as the target variables during the training and validation of the network. The estimated values of crop/soil parameters by this methodology are much closer to the experimental values. The present work confirms the estimation abilities of fuzzy subtractive clustering in potato crop parameters estimation. This technique may be useful for the other crops cultivated over regional or continental level. 相似文献
755.
P.V.S. Rama Rao K. VenkateshD.S.V.V.D. Prasad K. Niranjan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The Indian sector encompasses the equatorial and low latitude regions where the ionosphere is highly dynamic and is characterized by the equatorial ionization anomaly (EIA) resulting in large latitudinal electron density gradients causing errors and uncertainties in the estimation of range delays in satellite based navigation systems. The diurnal and seasonal variations of standard deviations in the TEC data measured during the low sunspot period 2004–2005 at 10 different Indian stations located from equator to the anomaly crest region and beyond are examined and presented. The day-to-day variability in TEC is found to be lowest at the equatorial station and increases with latitude up to the crest region of EIA and decreases beyond. 相似文献
756.
Sofya A. Ushakova Alexander A. Tikhomirov Valentin N. Shikhov Jean-Bernard Gros Tamara K. Golovko Igor V. Dal’ke Ilya G. Zakhozhii 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The main objective of a life support system for space missions is to supply a crew with food, water and oxygen, and to eliminate their wastes. The ultimate goal is to achieve the highest degree of closure of the system using controlled processes offering a high level of reliability and flexibility. Enhancement of closure of a biological life support system (BLSS) that includes plants relies on increased regeneration of plant waste, and utilization of solid and liquid human wastes. Clearly, the robustness of a BLSS subjected to stress will be substantially determined by the robustness of the plant components of the phototrophic unit. The aim of the present work was to estimate the heat resistance of two plants (wheat and lettuce) grown on human wastes. Human exometabolites mineralized by hydrogen peroxide in an electromagnetic field were used to make a nutrient solution for the plants. We looked for a possible increase in the heat tolerance of the wheat plants using changes in photosynthetically active radiation (PAR) intensity during heat stress. At age 15 days, plants were subjected to a rise in air temperature (from 23 ± 1 °C to 44 ± 1 °С) under different PAR intensities for 4 h. The status of the photosynthetic apparatus of the plants was assessed by external СО2 gas exchange and fluorescence measurements. The increased irradiance of the plants during the high temperature period demonstrated its protective action for both the photosynthetic apparatus of the leaves and subsequent plant growth and development. The productivity of the plants subjected to temperature changes at 250 W m−2 of PAR did not differ from that of controls, whereas the productivity of the plants subjected to the same heat stress but in darkness was halved. 相似文献
757.
G.D. Aburjania K.Z. Chargazia O.A. Kharshiladze 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
The linear mechanism of generation, intensification and further nonlinear dynamics of internal gravity waves (IGW) in stably stratified dissipative ionosphere with non-uniform zonal wind (shear flow) is studied. In case of the shear flows the operators of linear problem are non-selfadjoint, and the corresponding Eigen functions – nonorthogonal. Thus, canonical – modal approach is of less use studying such motions. Non-modal mathematical analysis becomes more adequate for such problems. On the basis of non-modal approach, the equations of dynamics and the energy transfer of IGW disturbances in the ionosphere with a shear flow is obtained. Exact analytical solutions of the linear as well as the nonlinear dynamic equations of the problem are built. The increment of shear instability of IGW is defined. It is revealed that the transient amplification of IGW disturbances due time does not flow exponentially, but in algebraic – power law manner. The effectiveness of the linear amplification mechanism of IGW at interaction with non-uniform zonal wind is analyzed. It is shown that at initial linear stage of evolution IGW effectively temporarily draws energy from the shear flow significantly increasing (by an order of magnitude) own amplitude and energy. With amplitude growth the nonlinear mechanism turns on and the process ends with self-organization of nonlinear solitary, strongly localized IGW vortex structures (the monopole vortex, the transverse vortex chain or the longitudinal vortex street). Accumulation of these vortices in the ionospheric medium can create the strongly turbulent state. 相似文献
758.
A.S. Laxmiprasad V.L.N. Sridhar Raja Surya Menon Adwaita Goswami M.V.H. Rao K.A. Lohar 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
A miniaturized in situ laser induced breakdown spectroscope-LIBS is one of the two lunar rover payloads to be flown in India’s next lunar mission Chandrayaan-2, with an objective to carry-out a precise qualitative and quantitative elemental analyses of lunar regolith at the proximity of the landing region. As per the imposed mission constraints and the executed design optimization studies, a compact and light-weight LIBS prototype model is developed at our premises. This paper mainly concerns with the estimation of theoretical aspects; especially on evaluation of elemental ablation parameters and signal-to-noise ratio (SNR) calculations for the designed instrument. Theoretical estimations and simulations yielded an incident laser power density of the order of 5 × 1010 W/cm2 on the target surface at a defined lens-to-surface distance (LTSD) of 200 mm and revealed an SNR > 100 for most of the elements under consideration. This paper also addresses the impact of LTSD variation on detection capability. The estimation of plasma-temperatures was carried out utilizing the emission spectra obtained under high vacuum environments employing the LIBS laboratory model. Experimental investigations and the performed theoretical estimations asserted the successful operation of the configured LIBS instrument for in situ elemental analyses on lunar surface. 相似文献
759.
B. Xun P.G. Chen K. Li Z. Yin W.R. Hu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(12):2094-2100
A linear stability analysis is applied to determine the onset of oscillatory thermocapillary convection in cylindrical liquid bridges of large Prandtl numbers (4 Pr 50). We focus on the relationships between the critical Reynolds number Rec, the azimuthal wave number m, the aspect ratio Γ and the Prandtl number Pr. A detailed Rec–Pr stability diagram is given for liquid bridges with various Γ. In the region of Pr > 1, which has been less studied previously and where Rec has been usually believed to decrease with the increase of Pr, we found Rec exhibits an early increase for liquid bridges with Γ around one. From the computed surface temperature gradient, it is concluded that the boundary layers developed at both solid ends of liquid bridges strengthen the stability of basic axisymmetric thermocapillary convection at large Prandtl number, and that the stability property of the basic flow is determined by the “effective” part of liquid bridge. 相似文献
760.
S.N.V.S. Prasad P.V.S. Rama RaoD.S.V.V.D. Prasad K. VenkateshK. Niranjan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
The Total Electron Content (TEC) from four locations in the Indian sector namely, Trivandrum (8.47°N, 76.91°E, Geomag.0.63°S, 0.3° dip), Waltair (17.7° N, 83.3°E, Geomag. 6.4°N, 20° dip), Bhopal (23.28°N, 77.34°E, Geomag.14.26°N, 33.2° dip), and Delhi (28.58°N, 77.21°E, Geomag.19.2°N, 43.4° dip) during a low sunspot year of 2004 are used to study the variabilities of the TEC. The day time TEC values are higher over Waltair and Bhopal compared to those at Trivandrum and Delhi. Considerable day-to-day variations in the diurnal values of TEC are observed at the anomaly crest locations. The observed GPS-TEC has been compared with the IRI-2007 model derived TEC considering three different options (IRI-2001, IRI-2001 corrected and Ne-Quick) available in the model for the topside electron density. The TEC derived with Ne-Quick and IRI-01 corrected options show better agreement with GPS-TEC while the TEC from IRI-01 method shows larger deviations. From the correlation analysis carried out between TEC value at 1300 h LT and solar indices parameters namely sunspot number (SSN), F10.7 and EUV, it is observed that the correlation is more during equinoctial months and less during summer months. The correlation coefficients observed over the anomaly locations, Bhopal and Delhi are lower compared to those at Trivandrum and Waltair. 相似文献