全文获取类型
收费全文 | 1572679篇 |
免费 | 27144篇 |
国内免费 | 7067篇 |
专业分类
电工技术 | 34731篇 |
综合类 | 6438篇 |
化学工业 | 276661篇 |
金属工艺 | 65669篇 |
机械仪表 | 44756篇 |
建筑科学 | 47426篇 |
矿业工程 | 11445篇 |
能源动力 | 51127篇 |
轻工业 | 120368篇 |
水利工程 | 15625篇 |
石油天然气 | 37685篇 |
武器工业 | 132篇 |
无线电 | 200191篇 |
一般工业技术 | 295768篇 |
冶金工业 | 191361篇 |
原子能技术 | 34280篇 |
自动化技术 | 173227篇 |
出版年
2021年 | 16212篇 |
2020年 | 12297篇 |
2019年 | 15112篇 |
2018年 | 16968篇 |
2017年 | 16304篇 |
2016年 | 22344篇 |
2015年 | 17900篇 |
2014年 | 29531篇 |
2013年 | 89201篇 |
2012年 | 37785篇 |
2011年 | 51087篇 |
2010年 | 44245篇 |
2009年 | 52414篇 |
2008年 | 47001篇 |
2007年 | 44078篇 |
2006年 | 46120篇 |
2005年 | 40699篇 |
2004年 | 42808篇 |
2003年 | 42621篇 |
2002年 | 41566篇 |
2001年 | 38783篇 |
2000年 | 37045篇 |
1999年 | 36626篇 |
1998年 | 54042篇 |
1997年 | 44499篇 |
1996年 | 38959篇 |
1995年 | 32932篇 |
1994年 | 30709篇 |
1993年 | 30417篇 |
1992年 | 26760篇 |
1991年 | 24026篇 |
1990年 | 24206篇 |
1989年 | 23303篇 |
1988年 | 21938篇 |
1987年 | 20046篇 |
1986年 | 19552篇 |
1985年 | 22819篇 |
1984年 | 22716篇 |
1983年 | 20722篇 |
1982年 | 19537篇 |
1981年 | 19675篇 |
1980年 | 18312篇 |
1979年 | 18820篇 |
1978年 | 17960篇 |
1977年 | 18310篇 |
1976年 | 20259篇 |
1975年 | 16210篇 |
1974年 | 15707篇 |
1973年 | 15772篇 |
1972年 | 13264篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
This letter focuses on the performance analysis of the decorrelating receiver in multipath Rician faded CDMA channels. M-ary QAM scheme is employed to improve the spectral efficiency. Approximate expressions are first derived for the two performance indexes: the average symbol error rate (SER) and the average bit error rate (BER) when the decorrelating-first receiver perfectly knows the channel information of the user of interest. To achieve desirable closed-form expressions of the SER and the BER, we exploit results in large system analysis and make assumptions of a high signal-to-interference ratio (SIR) and/or a small Rician K-factor. To measure the receiver performance in the practical scenario, we further derive expressions to approximate the average SER and BER of the decorrelating-first scheme with channel uncertainty. Simulation results demonstrate that the analytical results can also be employed to evaluate the performance of the combining-first receiver. 相似文献
992.
Solar cells based on SnO2/Cd0.4Zn0.6S/CdTe heterostructures are fabricated by electrochemical deposition, and the dependences of their electrical and photoelectric properties on the thermal annealing conditions are studied. It is shown that thermal annealing reduces the tunnel currents by almost two orders of magnitude. The best conditions of thermal annealing are determined (t = 300°C and τ = 9 min). These conditions provide the highest photosensitivity of the heterostructures under study (I sc ≈ 21.2 mA/cm2, U oc ≈ 813 mV, and η = 14.7%). 相似文献
993.
O. Yu. Alekseev V. S. Borisov M. V. Davidovich N. F. Popova 《Journal of Communications Technology and Electronics》2006,51(11):1240-1247
Direct and inverse problems are considered for diffraction by an open end of a rectangular waveguide (RW) with a flange that adjoins a piecewise inhomogeneous planar layered lossy medium. Also considered are similar diffraction problems for a junction of an RW and a rectangular resonator filled with a multilayer medium and a junction of two RWs, one of which contains a multilayer plate. Such open and shielded waveguide probe structures (WPSs) are used for determination and nondestructive testing of parameters of multilayer samples. The direct problem is formulated on the basis of admittance and impedance algorithms with consideration for losses existing in the medium, flange, and screens. In this case, the approximation of the given aperture field allows obtainment of explicit solutions for open and shielded WPSs in the form of integrals. Solution of the inverse problem that lies in determining thicknesses, permittivities, and permeabilities of the layers from measured values of the magnitude of the reflection coefficient is obtained by minimizing the corresponding least-squares error and by constructing artificial neural networks. In order to increase the accuracy, it is proposed to use a two-port of errors and perform measurements for several positions of the sample with respect to the flange and different impedance conditions behind the sample. 相似文献
994.
L. G. Gerchikov Yu. A. Mamaev A. V. Subashiev Yu. P. Yashin D. A. Vasil’ev V. V. Kuz’michev A. E. Zhukov E. S. Semenova A. P. Vasil’ev V. M. Ustinov 《Semiconductors》2006,40(11):1326-1332
Photoemission of polarized electrons from heterostructures based on InAlGaAs/GaAs superlattices with minimum conduction-band offsets is investigated. The comparison of the excitation energy dependence of the photoemission polarization degree with the calculated spectra make it possible to determine the polarization losses at different stages of the photoemission. A maximum polarization of P = 91% and a quantum yield of 0.14% are close to the best results obtained for photocathodes on the basis of strained semiconductor superlattices. 相似文献
995.
Towards intelligent dressing 总被引:1,自引:0,他引:1
Egana I. Mendikute A. Urionaguena X. Alberdi H. 《IEEE instrumentation & measurement magazine》2006,9(3):38-43
The aim of this article is to state the principles of an intelligent monitoring and control system for the grinding machine, comprising the dressing process as well as grinding stability. 相似文献
996.
Dobkin R. Ginosar R. Sotiriou C.P. 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》2006,14(10):1063-1074
Globally asynchronous, locally synchronous (GALS) systems-on-chip (SoCs) may be prone to synchronization failures if the delay of their locally-generated clock tree is not considered. This paper presents an in-depth analysis of the problem and proposes a novel solution. The problem is analyzed considering the magnitude of clock tree delays, the cycle times of the GALS module, and the complexity of the asynchronous interface controllers using a timed signal transition graph (STG) approach. In some cases, the problem can be solved by extracting all the delays and verifying whether the system is susceptible to metastability. In other cases, when high data bandwidth is not required, matched-delay asynchronous ports may be employed. A novel architecture for synchronizing inter-modular communications in GALS, based on locally delayed latching (LDL), is described. LDL synchronization does not require pausable clocking, is insensitive to clock tree delays, and supports high data rates. It replaces complex global timing constraints with simpler localized ones. Three different LDL ports are presented. The risk of metastability in the synchronizer is analyzed in a technology-independent manner 相似文献
997.
The variation of current density with bias or temperature is examined for DNA molecules of different configuration. To this end, the DNA molecule is represented as an equivalent electrical network whose behavior is then simulated with PSPICE. The results are found to be in close agreement with ones obtained within a physical model. It is established that the electrical response of a DNA molecule to an applied electric field depends on the boundary conditions and the potential profile along the molecule. This finding should contribute to the creation of a complete library of DNA-molecule configurations with prescribed electrical properties. 相似文献
998.
Self-induced effects in a passive polarization-independent vertical-cavity semiconductor gate are investigated numerically and experimentally. We demonstrate all-optical seed-pulse extraction for synchronization of differential phase-shift keying and ON-OFF keying packets at 10 Gb/s. Our results provide evidence that vertical-cavity gates, exploiting saturable absorption in semiconductor quantum-wells, exhibit attractive performances in terms of efficiency, power consumption, and polarization independency. 相似文献
999.
This paper provides an overview of the main aspects of modern fluorescence microscopy. It covers the principles of fluorescence and highlights the key discoveries in the history of fluorescence microscopy. The paper also discusses the optics of fluorescence microscopes and examines the various types of detectors. It also discusses the signal and image processing challenges in fluorescence microscopy and highlights some of the present developments and future trends in the field. 相似文献
1000.
V. I. Pipa 《Semiconductors》2006,40(6):665-667
Radiative lifetimes of nondegenerate electrons and holes distributed uniformly in a semiconductor layer either deposited on a substrate or bounded by two dielectric media are calculated. The obtained expression takes into account the radiation reabsorbtion and interference effects and determines the dependences of the radiative lifetimes on the refractive indices of the external media and on the layer thickness. 相似文献