全文获取类型
收费全文 | 263060篇 |
免费 | 4316篇 |
国内免费 | 1824篇 |
专业分类
电工技术 | 5453篇 |
技术理论 | 5篇 |
综合类 | 1167篇 |
化学工业 | 42399篇 |
金属工艺 | 11091篇 |
机械仪表 | 7791篇 |
建筑科学 | 6699篇 |
矿业工程 | 1670篇 |
能源动力 | 5635篇 |
轻工业 | 25935篇 |
水利工程 | 2911篇 |
石油天然气 | 6012篇 |
武器工业 | 243篇 |
无线电 | 27961篇 |
一般工业技术 | 49527篇 |
冶金工业 | 47779篇 |
原子能技术 | 5634篇 |
自动化技术 | 21288篇 |
出版年
2021年 | 2494篇 |
2019年 | 2140篇 |
2018年 | 3412篇 |
2017年 | 3386篇 |
2016年 | 3719篇 |
2015年 | 2829篇 |
2014年 | 4630篇 |
2013年 | 11493篇 |
2012年 | 7756篇 |
2011年 | 10299篇 |
2010年 | 8185篇 |
2009年 | 8737篇 |
2008年 | 9587篇 |
2007年 | 9664篇 |
2006年 | 8507篇 |
2005年 | 7454篇 |
2004年 | 6723篇 |
2003年 | 6260篇 |
2002年 | 6272篇 |
2001年 | 6355篇 |
2000年 | 5947篇 |
1999年 | 5921篇 |
1998年 | 12918篇 |
1997年 | 9570篇 |
1996年 | 7316篇 |
1995年 | 5565篇 |
1994年 | 5135篇 |
1993年 | 4976篇 |
1992年 | 3992篇 |
1991年 | 3781篇 |
1990年 | 3861篇 |
1989年 | 3802篇 |
1988年 | 3562篇 |
1987年 | 3046篇 |
1986年 | 3078篇 |
1985年 | 3436篇 |
1984年 | 3357篇 |
1983年 | 3104篇 |
1982年 | 2731篇 |
1981年 | 2926篇 |
1980年 | 2667篇 |
1979年 | 2862篇 |
1978年 | 2765篇 |
1977年 | 2864篇 |
1976年 | 3721篇 |
1975年 | 2477篇 |
1974年 | 2316篇 |
1973年 | 2343篇 |
1972年 | 1988篇 |
1971年 | 1796篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
We describe a simple way to achieve CW single-frequency laser operation with a grating as the sole tuning element. It is Shown, both experimentally and theoretically, that by proper choice of cavity parameters, the competing hole burning modes can be completely suppressed. Experiments to demonstrate the theoretical calculations were carried out in a CW color center laser using Tl0(1) centers. Linewidths of 0.01 cm-1were obtained and this figure can probably be much improved by proper cavity stabilization. The method can be readily extended to any compact gain medium. 相似文献
122.
The paper considers local-area computer networks with a protocol that supports carrier-sense multiple access with instantaneous conflict detection and is a generalization of slotted ALOHA. An approach based on recurrent processes is applied to analyze the asymptotic behavior of the number of waiting calls under conditions of low retrial rate.Translated from Kibernetika i Sistemnyi Analiz, No. 4, pp. 54–61, July–August, 1991. 相似文献
123.
124.
Shamansky H.T. Dominek A.K. Peters L. Jr. 《Antennas and Propagation, IEEE Transactions on》1989,37(8):1019-1025
The traveling-wave energy, which multiply diffracts on a straight thin wire, is represented as a sum of terms, each with a distinct physical meaning, that can be individually examined in the time domain. Expressions for each scattering mechanism on a straight thin wire are cast in the form of four basic electromagnetic wave concepts: diffraction, attachment, launch, and reflection. Using the basic mechanisms from P.Ya. Ufimtsev (1962), each of the scattering mechanisms is included into the total scattered field for the straight thin wire. Scattering as a function of angle and frequency is then compared to the moment-method solution. These analytic expressions are then extended to a lossy wire with a simple approximate modification using the propagation velocity on the wire as derived from the Sommerfeld wave on a straight lossy wire. Both the perfectly conducting and lossy wire solutions are compared to moment-method results, and excellent agreement is found. As is common with asymptotic solutions, when the electrical length of wire is smaller than 0.2 λ the results lose accuracy. The expressions modified to approximate the scattering for the lossy thin wire yield excellent agreement even for lossy wires where the wire radius is on the order of skin depth 相似文献
125.
Buttari D. Chini A. Meneghesso G. Zanoni E. Moran B. Heikman S. Zhang N.Q. Shen L. Coffie R. DenBaars S.P. Mishra U.K. 《Electron Device Letters, IEEE》2002,23(2):76-78
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs 相似文献
126.
Nicholas Biery Marc De Graef Jack Beuth Rafel Raban Andrew Elliott Tresa M. Pollock Curt Austin 《Metallurgical and Materials Transactions A》2002,33(10):3127-3136
Tensile experiments have been performed on specimens of four different investment-cast TiAl-based alloys with variations in
casting conditions. The average ductilities obtained in these experiments vary between approximately 0.8 to 2.0 pct plastic
strain to failure in tension. By using the three-parameter form of the Weibull relation, with the 0.2 pct offset yield strength
as the minimum failure strength, the resulting variability in the data can be quantified and is found to be similar for those
alloys with similar microstructural scale (grain size). Large variations in lamellar volume fraction, segregation, and phase
distribution have a minor influence on property variability, compared to changes in the scale of the grain structure caused
by either variations in cooling rate during casting or the addition of grain refiners. 相似文献
127.
A new technique for mixing optical waves to generate microwave-frequency signals using an injection laser diode and a fibre-optic interferometer is reported. A beat signal is produced by interfering light from the laser with light from the same laser emitted earlier at a different frequency. The interferometer consists of an evanescent-field fibre coupler joined to a length of single-mode optical fibre. The laser is tuned by pulsed or bipolar current waveforms superimposed on a DC bias current. Feedback from an external cavity stabilises the laser frequency and reduces its linewidth. Efficient mixing at difference frequencies from 250 MHz to 2 GHz is obtained, with spectral widths of less than 1% of the centre frequency. 相似文献
128.
129.
The dynamic linewidth of 1.5 ?m ridge waveguide DFB lasers is shown to be reduced by shaping the pulse of the laser modulating waveform. Pulse shaping is performed by a second-order network designed to cancel the small-signal laser resonance. Results demonstrate a dynamic linewidth reduction from 1.4 ? to 0.55 ? FWHM for a 500 ps pulse. 相似文献
130.
The pulse-stream technique, which represents neural states as sequences of pulses, is reviewed. Several general issues are raised, and generic methods appraised, for pulsed encoding, arithmetic, and intercommunication schemes. Two contrasting synapse designs are presented and compared. The first is based on a fully analog computational form in which the only digital component is the signaling mechanism itself-asynchronous, pulse-rate encoded digital voltage pulses. In this circuit, multiplication occurs in the voltage/current domain. The second design uses more conventional digital memory for weight storage, with synapse circuits based on pulse stretching. Integrated circuits implementing up to 15000 analog, fully programmable synaptic connections are described. A demonstrator project is described in which a small robot localization network is implemented using asynchronous, analog, pulse-stream devices. 相似文献