全文获取类型
收费全文 | 5343篇 |
免费 | 25篇 |
国内免费 | 32篇 |
专业分类
航空 | 2611篇 |
航天技术 | 1963篇 |
综合类 | 14篇 |
航天 | 812篇 |
出版年
2021年 | 36篇 |
2019年 | 37篇 |
2018年 | 71篇 |
2017年 | 60篇 |
2016年 | 55篇 |
2014年 | 105篇 |
2013年 | 136篇 |
2012年 | 125篇 |
2011年 | 174篇 |
2010年 | 121篇 |
2009年 | 213篇 |
2008年 | 264篇 |
2007年 | 142篇 |
2006年 | 120篇 |
2005年 | 144篇 |
2004年 | 150篇 |
2003年 | 167篇 |
2002年 | 113篇 |
2001年 | 174篇 |
2000年 | 100篇 |
1999年 | 123篇 |
1998年 | 151篇 |
1997年 | 94篇 |
1996年 | 146篇 |
1995年 | 172篇 |
1994年 | 167篇 |
1993年 | 94篇 |
1992年 | 119篇 |
1991年 | 60篇 |
1990年 | 58篇 |
1989年 | 135篇 |
1988年 | 51篇 |
1987年 | 50篇 |
1986年 | 58篇 |
1985年 | 182篇 |
1984年 | 142篇 |
1983年 | 116篇 |
1982年 | 111篇 |
1981年 | 177篇 |
1980年 | 51篇 |
1979年 | 47篇 |
1978年 | 51篇 |
1977年 | 47篇 |
1976年 | 32篇 |
1975年 | 55篇 |
1974年 | 40篇 |
1973年 | 42篇 |
1972年 | 40篇 |
1970年 | 41篇 |
1969年 | 43篇 |
排序方式: 共有5400条查询结果,搜索用时 93 毫秒
631.
A mission template for exploration and damage mitigation of potential hazard of Near Earth Asteroids
D. C. Hyland H. A. Altwaijry R. Margulieux J. Doyle J. Sandberg B. Young N. Satak J. Lopez S. Ge X. Bai 《Cosmic Research》2010,48(5):437-442
The Apophis Exploratory and Mitigation Platform (AEMP) concept was developed as a prototype mission to explore and potentially
deflect the Near Earth Asteroid (NEA) 99942 Apophis. Deflection of the asteroid from the potential 2036 impact will be achieved
using a gravity tractor technique, while a permanent deflection, eliminating future threats, will be imparted using a novel
albedo manipulation technique. This mission will serve as an archetypal template for future missions to small NEAs and could
be adapted to mitigate the threat of collision with other potential Earth-crossing objects. 相似文献
632.
Optical fibre sensing is becoming increasingly attractive in both the smart structure sphere of interest as well as the medical and industrial concerns. Fibre-optic interferometric sensors can be designed as compact and robust transducers. In general, the transduction mechanism involves the phase modulation of coherent or monochromatic light propagating through a fibre-optic cable, and detecting the changes in the energy associated with this phase change. Sensors based on this technique can generally be configured to be quite sensitive and measurements of a wide variety of physical parameters are achievable. Intrinsic features of optical fibres, such as immunity to electromagnetic interference, flexibility, thinness, strength and weight, make this ideal for sensor technology. In the process of developing fibre-optic sensors for laboratory experimentation, as well as generalised research, an appropriate, easy to use opto-electronic drive and measurement system is required. This paper discusses the operation of a programmable opto-electronic drive and measurement system 相似文献
633.
Evidence suggests that changes of solar irradiance in recent centuries have provided a significant climate forcing and that the sun has been one of the principal causes of long-term climate change. During the past two decades the solar forcing has been much smaller than the climate forcing caused by increasing greenhouse gases. But it is incorrect to assume that the sun necessarily will be an insignificant player in climate change of the 21st century. Indeed, I argue that moderate success in curtailing the growth of anthropogenic climate forcings could leave the sun playing a pivotal role in future climate change. 相似文献
634.
Electro Energy Inc. (EEI) is developing high power, long life, bipolar nickel-metal hydride batteries for aerospace applications. Bipolar nickel-metal hydride designs allow for high energy and high power designs with a 25 percent reduction in both weight and volume as compared to prismatic and/or cylindrical Ni-MH designs. Utilizing a sealed wafer cell design EEI has demonstrated a 1.2 kW/kg power capability. Prototype designs have achieved 70 Wh/kg. Designs studies show 80 Wh/kg are achievable with EEI's state-of-the-art technology. The sealed wafer cell is the building block for EEI's high power and high voltage bipolar batteries making the assembly easy and significantly lower in cost. Satellite and aircraft batteries are being developed which provide high power and long life. Sealed cells now show excellent rate capability and life. Cells tested in a low earth orbit (LEO) cycle have reached 9000 cycles and continue on test. High power, bipolar battery designs are ideal in applications where using conventional aerospace battery technology would require excessive capacity; weight and volume, thereby reducing usable payload on the vehicle 相似文献
635.
L H Fraser 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(3):297-302
Since 1986, a series of microcosm experiments has been conducted at the Unit of Comparative Plant Ecology (UCPE) in an attempt to test our understanding of the principles controlling the structure and dynamics of plant communities and ecosystems. In each experiment microcosms have been seeded with a common pool of organisms, and systems have been allowed to assemble under replicated controlled conditions. Experiment variables have included mineral nutrient supply, temperature, moisture supply, soil depth, carbon dioxide concentration, mycorrhizas, rhizobia, herbivores and carnivores. Results from these experiments are presented to illustrate the value of synthesised ecosystems in ecological research. 相似文献
636.
637.
This article presents an innovative time-domain nonlinear mapping-based identification method. The method reported is applied to identify the unknown parameters of multivariable dynamic systems which are mapped by nonlinear differential equations. A systematic identification method is introduced, and a novel algorithm is developed using nonlinear error maps. An analysis of parameter convergence is provided and the regions of convergence can be found using the second method of Lyapunov. Innovative nonquadratic Lyapunov functions are designed and used. Analytical and numerical studies are performed to illustrate and validate the identification concept. The unsteady flight of a high-alpha aircraft in the longitudinal axis is chosen as a nonlinear case study. The unknown parameters are identified. Simulation results show that the model dynamics match the experimental data. The reported example demonstrates that the time-domain nonlinear mapping-based identification method ensures robustness and reduces major shortcomings in stability, convergence, and computational efficiency compared with other algorithms available 相似文献
638.
One of the more significant errors in the history of science occurred during the “marginal revolution” in economics near the end of the nineteenth century. Rather than explicitly recognize in basic theory that the happiness we seek to maximize in life originates within oneself, the early theorists formulated their equations as though human satisfaction resided in external things-as in so much utility (economist's term for satisfaction) per pound of sugar. Over the past 125 years, economists have developed mainstream mathematical economics on this incorrect basis resulting in the ill-founded neoclassical Equilibrium Theory. Because of this error economic theory is fundamentally timeless. The present theory, in its canonical form, corrects the early marginalist's error by identifying utility (time-integrated pleasure) exclusively with the stream-of-consciousness attending (expected) mental and physical activity. Time is now explicit in basic theory, thereby allowing, for the first time, the substantive computer-modeling of time-dependent, small- and large-scale economic systems. Furthermore, this new approach is methodologically compatible with mainstream sociology and institutional economics, allowing increased interdisciplinary cooperation that may influence policy and thereby affect industry and markets. And safety engineering stands to benefit from the accommodation of neuropsychology in understanding human error in the supervision and control of technology 相似文献
639.
640.
A possible variant presented for solving the equation of motion of a rotating flow is more general with respect to the variants known in the literature. On the basis of the simultaneous solution of the equations of motion and energy, the temperature fields are determined under changes of the rotation nature. 相似文献