全文获取类型
收费全文 | 2363篇 |
免费 | 40篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 30篇 |
综合类 | 1篇 |
化学工业 | 255篇 |
金属工艺 | 27篇 |
机械仪表 | 38篇 |
建筑科学 | 21篇 |
矿业工程 | 1篇 |
能源动力 | 66篇 |
轻工业 | 28篇 |
水利工程 | 4篇 |
石油天然气 | 35篇 |
无线电 | 444篇 |
一般工业技术 | 286篇 |
冶金工业 | 868篇 |
原子能技术 | 14篇 |
自动化技术 | 290篇 |
出版年
2023年 | 11篇 |
2022年 | 52篇 |
2021年 | 32篇 |
2020年 | 28篇 |
2019年 | 28篇 |
2018年 | 40篇 |
2017年 | 33篇 |
2016年 | 29篇 |
2015年 | 42篇 |
2014年 | 42篇 |
2013年 | 123篇 |
2012年 | 60篇 |
2011年 | 55篇 |
2010年 | 69篇 |
2009年 | 50篇 |
2008年 | 55篇 |
2007年 | 50篇 |
2006年 | 55篇 |
2005年 | 51篇 |
2004年 | 45篇 |
2003年 | 44篇 |
2002年 | 46篇 |
2001年 | 26篇 |
2000年 | 41篇 |
1999年 | 53篇 |
1998年 | 282篇 |
1997年 | 163篇 |
1996年 | 120篇 |
1995年 | 88篇 |
1994年 | 68篇 |
1993年 | 73篇 |
1992年 | 26篇 |
1991年 | 32篇 |
1990年 | 30篇 |
1989年 | 34篇 |
1988年 | 25篇 |
1987年 | 27篇 |
1986年 | 27篇 |
1985年 | 38篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 19篇 |
1979年 | 11篇 |
1978年 | 9篇 |
1977年 | 30篇 |
1976年 | 56篇 |
1974年 | 9篇 |
1973年 | 9篇 |
排序方式: 共有2408条查询结果,搜索用时 15 毫秒
91.
We present an iterative algorithm for robustly estimating the ego-motion and refining and updating a coarse depth map using parametric surface parallax models and brightness derivatives extracted from an image pair. Given a coarse depth map acquired by a range-finder or extracted from a digital elevation map (DEM), ego-motion is estimated by combining a global ego-motion constraint and a local brightness constancy constraint. Using the estimated camera motion and the available depth estimate, motion of the three-dimensional (3-D) points is compensated. We utilize the fact that the resulting surface parallax field is an epipolar field, and knowing its direction from the previous motion estimates, estimate its magnitude and use it to refine the depth map estimate. The parallax magnitude is estimated using a constant parallax model (CPM) which assumes a smooth parallax field and a depth based parallax model (DBPM), which models the parallax magnitude using the given depth map. We obtain confidence measures for determining the accuracy of the estimated depth values which are used to remove regions with potentially incorrect depth estimates for robustly estimating ego-motion in subsequent iterations. Experimental results using both synthetic and real data (both indoor and outdoor sequences) illustrate the effectiveness of the proposed algorithm. 相似文献
92.
Qing Yang Alvin Lim Shuang Li Jian Fang Prathima Agrawal 《Mobile Networks and Applications》2010,15(1):36-60
Multi-hop vehicle-to-vehicle communication is useful for supporting many vehicular applications that provide drivers with
safety and convenience. Developing multi-hop communication in vehicular ad hoc networks (VANET) is a challenging problem due
to the rapidly changing topology and frequent network disconnections, which cause failure or inefficiency in traditional ad
hoc routing protocols. We propose an adaptive connectivity aware routing (ACAR) protocol that addresses these problems by
adaptively selecting an optimal route with the best network transmission quality based on statistical and real-time density
data that are gathered through an on-the-fly density collection process. The protocol consists of two parts: 1) select an
optimal route, consisting of road segments, with the best estimated transmission quality, and 2) in each road segment of the
chosen route, select the most efficient multi-hop path that will improve the delivery ratio and throughput. The optimal route
is selected using our transmission quality model that takes into account vehicle densities and traffic light periods to estimate
the probability of network connectivity and data delivery ratio for transmitting packets. Our simulation results show that
the proposed ACAR protocol outperforms existing VANET routing protocols in terms of data delivery ratio, throughput and data
packet delay. Since the proposed model is not constrained by network densities, the ACAR protocol is suitable for both daytime
and nighttime city VANET scenarios. 相似文献
93.
Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52-μm mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems 相似文献
94.
Yogesh S. Kashyap Ashish Agrawal P.S. Sarkar Mayank Shukla Tushar Roy Amar Sinha 《NDT & E International》2011,44(1):41-46
We have used the contrast transfer function based X-ray phase retrieval technique for phase retrieval studies on a two-component system. Pyro-carbon coated alumina matrix was chosen as a two-component system for these studies. Simulations as well as experimental results are presented. This paper shows that X-ray phase contrast along with phase retrieval can become an alternative tool for non-destructive characterization of these materials. We have also attempted to retrieve the spatial distribution of the projected thickness map of the two different elements. 相似文献
95.
Communication services using higher altitude platforms have highlighted a need for estimating the effect of local obstructions on radio wave link. Vegetation and buildings are the major factors responsible for attenuation. The authors have calculated the radio wave attenuation because of vegetation and buildings. Here, the authors have used the fitted radio international telecommunication union model to calculate the vegetation attenuation. The attenuation because of buildings is calculated for an urban-site (residential area of Netaji Subhas Institute of Technology, NSIT, New Delhi, India) using geometrical optics and a uniform theory of diffraction. Further, channel capacity is calculated for vegetation and urban-site environment using Shannon?s channel capacity theorem. 相似文献
96.
S. L. Agrawal Markandey Singh Mridula Tripathi Mrigank Mauli Dwivedi Kamlesh Pandey 《Journal of Materials Science》2009,44(22):6060-6068
Present work deals with findings on dielectric relaxation behaviour and a.c. conduction in a SiO2-doped polymer nanocomposite electrolyte system, namely, [(100 − x)PEO + xSiO2]:yNH4SCN. The formation of nanocomposite has been ascertained by XRD measurements. The effect of salt and filler (SiO2) on conductivity response of PEO-based nanocomposite polymer electrolyte has been investigated by impedance spectroscopy.
The variation of dielectric permittivity, dielectric loss and modulus spectra with frequency and temperature was carried out
from impedance spectroscopy data. The a.c. conductivity seems to follow the universal power law. 相似文献
97.
98.
Understanding the erosion mechanism is a key to improve the performance of material subjected to erosive condition. Capability to predict the erosion mechanism could prove to be useful tool. In this work, a parameter named “erosion mechanism identifier,” ξ, is proposed to predict the erosion mechanism in materials. Suitability of ξ in predicting erosion mechanism of ductile and brittle materials was evaluated using the data reported in the literature. It was observed that ξ is able to predict the erosion mechanism for both categories of material. The predictability of ξ was not restrained by different operating conditions. 相似文献
99.
100.