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351.
提出一种新颖的零电流零电压开关PWM Boost全桥变换器。超前管串联电感,并利用输出滤波电容的能量,可以在很宽的负载范围内实现ZCS。滞后管利用其寄生电容和与变压器原边并联的辅助电感,可以在任何负载下实现ZVS。与ZCS PWM Boost全桥变换器相比,所提出的变换器没有电流占空比丢失的问题。本文详细分析了该变换器的工作原理,参数设计原则。通过一个480W的原理样机,验证了分析结果。 相似文献
352.
大功率晶体管深度饱和以降低功率损耗与管子快速关断之间的矛盾并非不可调和。文中针对通常的抗饱和驱动,提出了一种新型驱动电路,很好地解决了这个矛盾。一方面在功率管饱和导通时,比例驱动管子工作于深饱和状态,使功率管的损耗达到最小;另一方面在功率管关断时,驱动电路通过低阻抗抽流回路及高反压辅助抽流的引入,在功率管的基极提供很强的基极反抽电流,使管子快速关断。实验表明与通常的抗饱和驱动相比,本方案由于功率管深度饱和,从而使管子的通态饱和压降降低了0.5V,损耗亦降低了63.4W;另一方面由于关断时基极反抽电流增大了三倍,存储时间增加权0.1us,实现了大功率晶体管的最佳驱动。 相似文献
353.
研究了在不同的偏置电压和扫描速度下加工的Si氧化线的一致性和均匀性,得到了加工高质量的Si氧化线的实验条件为:偏置电压8V,扫描速度1 μm/s。 相似文献
354.
It is established that the large-scale and global magnetic fields in the Sun's atmosphere do not change smoothly, and long-lasting periods of gradual variations are superseded by fast structural changes of the global magnetic field. Periods of fast global changes on the Sun are accompanied by anomalous manifestations in the interplanetary space and in the geomagnetic field. There is a regular recurrence of these periods in each cycle of solar activity, and the periods are characterized by enhanced flaring activity that reflects fast changes in magnetic structures. Is demonstrated, that the fast changes have essential influencing on a condition of space weather, as most strong geophysical disturbances are connected to sporadic phenomena on the Sun. An explanation has been offered for the origin of anomalous geomagnetic disturbances that are unidentifiable in traditionally used solar activity indices. Is shown, main physical mechanism that leads to fast variations of the magnetic fields in the Sun's atmosphere is the reconnection process. 相似文献
355.
Robert F. Wimmer-Schweingruber Rudolf Von Steiger Johannes Geiss George Gloeckler Fred M. Ipavich Berend Wilken 《Space Science Reviews》1998,85(1-2):387-396
Recent observations with UVCS on SOHO of high outflow velocities of O5+ at low coronal heights have spurred much discussion about the dynamics of solar wind acceleration. On the other hand, O6+ is the most abundant oxygen charge state in the solar wind, but is not observed by UVCS or by SUMER because this helium-like
ion has no emission lines falling in the wave lengths observable by these instruments. Therefore, there is considerable interest
in observing O5+ in situ in order to understand the relative importance of O5+ with respect to the much more abundant O6+. High speed streams are the prime candidates for the search for O5+ because all elements exhibit lower freezing-in temperatures in high speed streams than in the slow solar wind. The Ulysses
spacecraft was exposed to long time periods of high speed streams during its passage over the polar regions of the Sun. The
Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is capable of resolving this rare oxygen charge state. We present
the first measurement of O5+ in the solar wind and compare these data with those of the more abundant oxygen species O6+ and O7+. We find that our observations of the oxygen charge states can be fitted with a single coronal electron temperature in the
range of 1.0 to 1.2 MK assuming collisional ionization/recombination equilibrium with an ambient Maxwellian electron gas.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
356.
Results from a series of SOHO/Coronal Diagnostic Spectrometer (CDS) observations of coronal holes and plumes are presented,
including analysis of a low-latitude plume observed in August 1996. Spectroscopic diagnostic techniques using the CHIANTI
atomic database are applied to derive the plasma parameters: electron density, temperature, and element abundances. The results
are compared with quiet sun values. Coronal electron densities in the holes are found to be about 2 × 108 cm-3, a factor of two to three lower than in the quiet sun. The plasma thermal distribution exhibits differences between coronal
holes, the quiet sun and plumes. For example, the peak of the emission in coronal holes is at a lower temperature (T ⋍ 8 ×
105 K) than in the quiet sun (T ⋍ 1 × 106 K), while plumes are cooler (T ⋍ 7.6 × 105 K) and show a different distribution, closer to an isothermal state.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
357.
Valery M. Nakariakov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2007,39(12):1804-1813
Wave and oscillatory activity is observed with modern imaging and spectral instruments in the visible light, EUV, X-ray and radio bands in all parts of the solar corona. Magnetohydrodynamic (MHD) wave theory gives satisfactory interpretation of these phenomena in terms of MHD modes of coronal structures. The paper reviews the current trends in the observational study of coronal oscillations, recent development of theoretical modelling of MHD wave interaction with plasma structures, and implementation of the theoretical results for the mode identification. Also the use of MHD waves for remote diagnostics of coronal plasmas is discussed. In particular, the applicability of this method to the estimation of the coronal magnetic field is demonstrated. 相似文献
358.
359.
360.
利用试验和数值模拟相结合的方法研究6 cm Kaufman离子推力器放电电压和屏栅电压的变化对其工作性能的影响。试验中,离子推力器使用氩气作为推进剂,测量了多组不同工况下的性能参数。此外,基于Goebel的理论模型模拟了放电电压对束流电流和推进剂利用率的影响;采用单元内粒子 蒙特卡罗碰撞(PIC-MCC方法模拟屏栅电压对束流电流、推进剂利用率和加速栅极电流的影响。试验和数值模拟结果一致,发现当放电电压逐渐增大时,引出的束流电流和推进剂利用率先增加然后趋于稳定;当屏栅电压逐渐增大时,引出的束流电流和推进剂利用率先增加然后趋于稳定,加速栅极电流先减小后趋于稳定。研究可以为提高多模式离子推力器的性能提供参考。 相似文献