全文获取类型
收费全文 | 83495篇 |
免费 | 942篇 |
国内免费 | 406篇 |
专业分类
电工技术 | 770篇 |
综合类 | 2316篇 |
化学工业 | 11357篇 |
金属工艺 | 4779篇 |
机械仪表 | 3012篇 |
建筑科学 | 2153篇 |
矿业工程 | 562篇 |
能源动力 | 1099篇 |
轻工业 | 3567篇 |
水利工程 | 1266篇 |
石油天然气 | 341篇 |
无线电 | 9219篇 |
一般工业技术 | 16243篇 |
冶金工业 | 2617篇 |
原子能技术 | 253篇 |
自动化技术 | 25289篇 |
出版年
2018年 | 14449篇 |
2017年 | 13374篇 |
2016年 | 9953篇 |
2015年 | 597篇 |
2014年 | 220篇 |
2013年 | 179篇 |
2012年 | 3124篇 |
2011年 | 9390篇 |
2010年 | 8265篇 |
2009年 | 5531篇 |
2008年 | 6759篇 |
2007年 | 7765篇 |
2006年 | 106篇 |
2005年 | 1202篇 |
2004年 | 1121篇 |
2003年 | 1165篇 |
2002年 | 531篇 |
2001年 | 93篇 |
2000年 | 174篇 |
1999年 | 54篇 |
1998年 | 49篇 |
1997年 | 26篇 |
1996年 | 43篇 |
1995年 | 8篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 22篇 |
1988年 | 9篇 |
1969年 | 24篇 |
1968年 | 43篇 |
1967年 | 33篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1964年 | 11篇 |
1963年 | 28篇 |
1962年 | 22篇 |
1961年 | 18篇 |
1960年 | 30篇 |
1959年 | 35篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
1953年 | 5篇 |
1952年 | 6篇 |
1951年 | 4篇 |
1950年 | 6篇 |
1949年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Magnifying micro-movements of natural videos that are undetectable by human eye has recently received considerable interests, due to its impact in numerous applications. In this paper, we use dual tree complex wavelet transform (DT-CWT), to analyze video frames in order to detect and magnify micro-movements to make them visible. We use DT-CWT, due to its excellent edge-preserving and nearly-shift invariant features. In order to detect any minor change in object’s spatial position, the paper proposes to modify the phases of the CWT coefficients decomposition of successive video frames. Furthermore, the paper applies Radon transform to track frame micro-movements without any temporal band-pass filtering. The paper starts by presenting a simple technique to design orthogonal filters that construct this CWT system. Next, it is shown that modifying the phase differences between the CWT coefficients of arbitrary frame and a reference one results in image spatial magnification. This in turn, makes these micro-movements seen and observable. Several simulation results are given, to show that the proposed technique competes very well to the existing micro-magnification approaches. In fact, as it manages to yield superior video quality in far less computation time. 相似文献
992.
993.
In this paper, we focus first on the time needed by a node to join a Time Slotted Channel Hopping (TSCH) network, this time is called joining time. Second, we are also interested in the network building time. Since the data generated by a sensor node remain unavailable as long as this node has not yet joined the wireless sensor network, these times are of prime importance for applications having strong latency requirements on data gathering. The joining time depends on the beacon advertising policy that has been left unspecified by the standard. The contribution of this paper is triple. First, we propose an Enhanced Deterministic Beacon Advertising algorithm, called EDBA, that ensures a collision-free advertising of beacons and minimizes the average joining time. Second, we model the behavior of a joining node by a Markov chain, validated by NS3 simulations, and compute the average joining time. Third, we compare the performance of EDBA with this of MBS, considered as the best beacon advertising algorithm in the literature. 相似文献
994.
Hui Li Erqing Xie Min Qiao Xiaojun Pan Yongzhe Zhang 《Journal of Electronic Materials》2007,36(9):1219-1223
Indium-doped zinc oxide (ZnO:In) films were prepared in an Ar:O2 plasma by reactive magnetron sputtering. The x-ray diffraction (XRD) patterns presented the crystal structures of ZnO:In
films, while transmission spectra and photoluminescence (PL) spectra showed the changed band gap and the visible emission
from defects, as compared to the PL spectra of undoped ZnO films. It was concluded that the increase of substrate temperature
enhanced the crystal quality of ZnO:In films; the incorporation of In made the c-axis constant of the samples larger than that of undoped ZnO films; the blue emission was due to the transition from an unknown
donor level by indium doping to the valance band; and the orange-green emission originated from acceptor defects (OZn) formed in the O-rich plasma. Meanwhile, the current- voltage characteristics and persistent photoconductivity phenomenon
also could be explained by the increased acceptor defects (OZn) that formed when the substrate temperature was increased. 相似文献
995.
Vladimir I. Shashkin Yuri A. Drjagin Vjacheslav R. Zakamov Sergei V. Krivov Lev M. Kukin Arcady V. Murel Yuri I. Chechenin 《Journal of Infrared, Millimeter and Terahertz Waves》2007,28(11):945-952
The principles of construction of millimeter wave detectors based on low-barrier Schottky diodes and planar antennas are discussed. The modified planar slot antenna with low beam spillover at the resonant frequency of 94 GHz has been developed. Experiments have been carried out to investigate detecting characteristics of the diodes with differential contact resistances \( R_{j} = 1 \div 1000\;{\text{k}}\Omega \) at zero bias. Experimental data are well correspond to calculations in a simple model of detector. At \( R_{j} = 20 \div 100\;{\text{k}}\,\OmegaΩ \) the maximum of rf-to-dc voltage sensitivity - more than 10000 V/W - is obtained. At lower values of \( R_{j} = 2 \div 6\;{\text{k}}\,\Omega Ω \) a better noise equivalent power (NEP), around 10?12 W Hz?1/2, is predicted. 相似文献
996.
We report a sol–gel method to deposit a high-k dielectric, zirconium oxide (ZrO2). This solution-based approach has advantages of easy processing and low fabrication cost. Effects of annealing temperatures
on dielectric properties, such as tunneling current density and capacitance density, are reported. Morphological and chemical
characterizations suggest that the process temperature can be kept at or below 300°C. We have employed the solution-processed
ZrO2 dielectric in a zinc tin oxide thin-film transistor. Saturation mobility of 4.0 cm2/V s at operating voltage of 2 V has been observed. The measured subthreshold swing is 74 mV/decade, which is the result of
the combination of an electronically clean dielectric/semiconductor interface and high insulator capacitance. 相似文献
997.
This paper is focused on the study of layered space-time-frequency (LSTF) architectures with channel coding for orthogonal
frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) multiplexing systems for high speed wireless
communications over a frequency-selective fading channel. In order to achieve the available spatial, temporal and frequency
diversities, and also make the system implementation feasible for high speed OFDM MIMO multiplexing, a novel LSTF architecture
with multiple channel encoders is proposed with each independently coded layer being threaded in the three-dimensional space-time-frequency
transmission resource array. Non-iterative receiver is adopted which consists of list sphere detector and irregular low-density
parity-check codes as the constituent codes. Simulation results show that the performance of the proposed multiple-encoder
LSTF architecture is very close to that of the conventional single-encoder LSTF where coding is applied across the whole information
stream. However, due to the use of multiple parallel encoders/decoders with a shorter codeword length, the proposed LSTF architecture
has much lower hardware processing speed and complexity than the conventional LSTF. 相似文献
998.
Automatic recognition of the communication signals plays an important role for various applications. This paper presents a
novel intelligent system for recognition of digital communication signals. This system includes three main modules: feature
extraction module, classifier module and optimization module. In the feature extraction module, multi-resolution wavelet analysis
is proposed for extraction the suitable features. In the classifier module, a multi-class support vector machine (SVM) based
classifier is proposed as the multi-class classifier. For optimization module, a particle swarm optimization algorithm is
proposed to improve the generalization performance of the recognizer. In this module, it is optimized the SVM classifier design
by searching for the best value of the parameters that tune its discriminant function, and upstream by looking for the best
subset of features that feed the classifier. Simulation results show that the proposed hybrid intelligent system has high
performance even at very low signal to noise ratios (SNRs). 相似文献
999.
Yuting Cao Zhigang Zeng Tingwen Huang Shiping Wen 《Circuits, Systems, and Signal Processing》2016,35(8):2810-2831
This paper investigates new criteria of the robust \(H_\infty \) stability for a class of uncertain stochastic fuzzy mixed-delay systems with nonlinear noise disturbances by employing an improved free-weighting matrix approach. The fuzzy system is based on the Takagi–Sugeno model that is often used to represent the complex nonlinear systems in terms of fuzzy sets and fuzzy reasoning. To reflect more realistic dynamical behaviors of the system, both the parameter uncertainties and stochastic disturbances are considered, the stochastic disturbances are given in the form of a Brownian motion. The mixed delays comprise both discrete and distributed time-varying delays. In terms of a stochastic fuzzy Lyapunov functional, a sufficient criterion is proposed to investigate dynamical behaviors of the system in the mean-square sense with an \(H_\infty \) performance index. 相似文献
1000.
Chaolong Zhang Yigang He Lifen Yuan Wei He Sheng Xiang Zhigang Li 《Journal of Electronic Testing》2016,32(5):531-540
This paper presents a novel analog circuit fault diagnosis approach using generalized multiple kernel learning-support vector machine (GMKL-SVM) method and particle swarm optimization (PSO) algorithm. First, the wavelet coefficients’ energies of impulse responses are generated as features. Then, a diagnosis model is constructed by using GMKL-SVM method based on features. Meanwhile, the PSO algorithm yields parameters for the GMKL-SVM method. Sallen-Key bandpass filter and two-stage four-op-amp biquad lowpass filter fault diagnosis simulations are given to demonstrate the proposed diagnose procedure, and the comparison simulations reveal that the proposed approach has higher diagnosis precision than the referenced methods. 相似文献