全文获取类型
收费全文 | 493916篇 |
免费 | 7149篇 |
国内免费 | 2969篇 |
专业分类
电工技术 | 9446篇 |
技术理论 | 5篇 |
综合类 | 1671篇 |
化学工业 | 75189篇 |
金属工艺 | 20194篇 |
机械仪表 | 14775篇 |
建筑科学 | 12094篇 |
矿业工程 | 3007篇 |
能源动力 | 11614篇 |
轻工业 | 43714篇 |
水利工程 | 5433篇 |
石油天然气 | 10658篇 |
武器工业 | 286篇 |
无线电 | 55742篇 |
一般工业技术 | 96495篇 |
冶金工业 | 90897篇 |
原子能技术 | 11639篇 |
自动化技术 | 41175篇 |
出版年
2021年 | 4268篇 |
2019年 | 3836篇 |
2018年 | 6385篇 |
2017年 | 6380篇 |
2016年 | 6909篇 |
2015年 | 4909篇 |
2014年 | 8140篇 |
2013年 | 21618篇 |
2012年 | 13423篇 |
2011年 | 18157篇 |
2010年 | 14376篇 |
2009年 | 15916篇 |
2008年 | 16913篇 |
2007年 | 17118篇 |
2006年 | 14866篇 |
2005年 | 13366篇 |
2004年 | 12637篇 |
2003年 | 12101篇 |
2002年 | 11878篇 |
2001年 | 11972篇 |
2000年 | 11218篇 |
1999年 | 11588篇 |
1998年 | 26742篇 |
1997年 | 19304篇 |
1996年 | 14922篇 |
1995年 | 11410篇 |
1994年 | 10218篇 |
1993年 | 10138篇 |
1992年 | 7634篇 |
1991年 | 7287篇 |
1990年 | 7233篇 |
1989年 | 7200篇 |
1988年 | 6686篇 |
1987年 | 5915篇 |
1986年 | 5929篇 |
1985年 | 6596篇 |
1984年 | 6264篇 |
1983年 | 5793篇 |
1982年 | 5236篇 |
1981年 | 5490篇 |
1980年 | 5211篇 |
1979年 | 5166篇 |
1978年 | 5144篇 |
1977年 | 5515篇 |
1976年 | 7230篇 |
1975年 | 4474篇 |
1974年 | 4267篇 |
1973年 | 4294篇 |
1972年 | 3662篇 |
1971年 | 3265篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
A new architecture for the Van Atta self-steering array is presented. Active antenna techniques are used to realise a compact self-steering array with enhanced retrodirective properties. The experimental performance of the array is demonstrated and a technique is described which compensates for the beam pointing error induced by antenna array element directivity 相似文献
983.
Corner detection by local histograms of contour chain code 总被引:4,自引:0,他引:4
984.
A series active power filter working as a sinusoidal current source, in-phase with the mains voltage, has been developed and tested. The amplitude of the fundamental current in the series filter is controlled through the error signal generated between the load voltage and a pre-established reference. The control allows an effective correction of power factor, harmonic distortion and load voltage regulation. Compared with previous methods of control developed for series active filters, this method is simpler to implement because it is only required to generate a sinusoidal current, in-phase with the mains voltage, the amplitude of which is controlled through the error in the load voltage. The proposed system has been studied analytically and tested using computer simulations and experiments. In the experiments, it has been verified that the filter keeps the line current almost sinusoidal and in-phase with the line voltage supply. It also responds very quickly under sudden changes in load conditions, reaching its steady-state in about two cycles of the fundamental 相似文献
985.
Quenching of Er3+ ions by homogeneous energy-transfer upconversion in high-concentration erbium-doped silica glasses has been theoretically investigated, The results indicate that at Er3+ concentrations of 1.0-2.0·1026 m-3 or below, the kinetic limit of strong migration is not reached, and hence the widely accepted quadratic upconversion model is not generally valid. Nevertheless, the results offer an explanation of the experimental observations of quadratic upconversion. Furthermore, it has been shown that at a given population inversion, the quenching rate depends on the rate of exchange of the excited Er3+ ions by emission and absorption 相似文献
986.
Knippels G.M.H. van der Meer A.F.G. Oepts D. van Amersfoort P.W. 《Quantum Electronics, IEEE Journal of》1997,33(1):10-17
We have investigated the optical output of the free-electron laser for infrared experiments (FELIX) when it is driven by an electron beam with a ramped energy. We show that the applied slow ramp on the electron beam energy leads to a frequency chirp on each picosecond optical pulse. Typical values for the chirp are 0.2% frequency sweep across a 1.5-ps-long optical pulse. The optical pulses were analyzed with a double-grating pair and with a second-order autocorrelator. The pulse duration was reduced in the double-grating pair by 20%. A linear dependence of the chirp on the cavity desynchronization was measured 相似文献
987.
Yuan Y. Jambunathan R. Singh J. Bhattacharya P. 《Quantum Electronics, IEEE Journal of》1997,33(3):408-415
The performance characteristics of a coupled cavity InGaAsP-InP MQW laser/active waveguide made by one-step epitaxy and well-controlled reactive ion etching (RIE) have been theoretically analyzed and experimentally determined. A theoretical model based on a finite-difference time-domain (FDTD) technique was used to simulate the propagation of an optical wave launched in the coupled system and determine the reflectivity of the facets created by RIE. The calculated effective reflectivity of the coupling region consisting of two facets and an air gap is between 0.45 and 0.55, which is in good agreement with the experimentally measured value of 0.5. The reflectivity of a single etched mirror derived from this value is estimated to be 0.3. A 120-μm-long monolithically integrated active waveguide when biased as a modulator and excited by the laser shows a maximum extinction ratio of 8 dB and a modulation bandwidth ⩾14 GHz at a dc bias of -0.5 V with a bias swing of 2 V 相似文献
988.
Rogers D.V. Ippolito L.J. Jr. Davarian F. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1997,85(6):810-820
Accurate estimates of the propagation impairments that affect link quality and availability and determine signal interference fields are essential for the reliable design of telecommunication systems and the efficient use of the electromagnetic spectrum. Recent announcements by commercial entities of their intent to use Ka-band spectrum to supply satellite services have heightened interest in propagation data and models for these frequencies. This paper provides a brief overview of Ka-band Earth-satellite systems and requirements in relation to the need for specific types of propagation data 相似文献
989.
Improved modulation techniques for PWM-VSI drives 总被引:2,自引:0,他引:2
Blaabjerg F. Pedersen J.K. Thoegersen P. 《Industrial Electronics, IEEE Transactions on》1997,44(1):87-95
PWM-VSI based AC motor drives have two main problems. The inverter is nonlinear which causes instability problems in some specific working points of the AC machine and it emits acoustic noise due to the switching frequency. Nonlinearities like dead-time in the inverter, load dependent DC-link voltage ripple and the voltage drop across the switches are modeled and compensated by improved modulation techniques in order to obtain an almost ideal inverter. Different feedback and feedforward techniques are proposed. The acoustic noise is reduced by using a random modulation strategy. Measurements show a significant improvement by using feedforward and feedback techniques for linearizing the inverter. An improvement in reduction of the acoustic noise emission is also achieved by using random modulation. It is concluded that a combination of a random modulation strategy and feedforward/feedback techniques gives an almost ideal AC motor drive system 相似文献
990.
Jenghwa Chang Graber H.L. Ping Chen Koo Aronson R. Barbour S.-L.S. Barbour R.L. 《IEEE transactions on medical imaging》1997,16(1):68-77
Presents a model suitable for computing images of absorption cross sections of thick tissue structures illuminated at near infrared (NIR) wavelengths from tomographic projection data. Image reconstruction is accomplished by solving a system of linear equations derived from transport theory. Reconstruction results using different algebraic solvers are shown for anatomical maps of the breast, derived from magnetic resonance imaging data, containing two simulated pathologies, in which case qualitatively good reconstructions were obtained. Evaluation of magnetic resonance (MR) data to optimize NIR optical tomographic imaging methods and to assess the feasibility of a combined MR-optical measurement scheme is discussed 相似文献