全文获取类型
收费全文 | 565321篇 |
免费 | 7343篇 |
国内免费 | 1852篇 |
专业分类
电工技术 | 10709篇 |
综合类 | 455篇 |
化学工业 | 82617篇 |
金属工艺 | 20064篇 |
机械仪表 | 15869篇 |
建筑科学 | 14728篇 |
矿业工程 | 1554篇 |
能源动力 | 14581篇 |
轻工业 | 53308篇 |
水利工程 | 4561篇 |
石油天然气 | 4538篇 |
武器工业 | 20篇 |
无线电 | 75110篇 |
一般工业技术 | 104423篇 |
冶金工业 | 113444篇 |
原子能技术 | 8288篇 |
自动化技术 | 50247篇 |
出版年
2021年 | 3622篇 |
2019年 | 3466篇 |
2018年 | 5501篇 |
2017年 | 5582篇 |
2016年 | 5866篇 |
2015年 | 4595篇 |
2014年 | 7629篇 |
2013年 | 25358篇 |
2012年 | 13420篇 |
2011年 | 19073篇 |
2010年 | 14909篇 |
2009年 | 17224篇 |
2008年 | 18295篇 |
2007年 | 18751篇 |
2006年 | 16913篇 |
2005年 | 15779篇 |
2004年 | 15369篇 |
2003年 | 14932篇 |
2002年 | 14470篇 |
2001年 | 14879篇 |
2000年 | 13881篇 |
1999年 | 14845篇 |
1998年 | 36755篇 |
1997年 | 25905篇 |
1996年 | 20208篇 |
1995年 | 15411篇 |
1994年 | 13697篇 |
1993年 | 13388篇 |
1992年 | 9615篇 |
1991年 | 9258篇 |
1990年 | 8835篇 |
1989年 | 8479篇 |
1988年 | 8052篇 |
1987年 | 6709篇 |
1986年 | 6695篇 |
1985年 | 8052篇 |
1984年 | 7404篇 |
1983年 | 6474篇 |
1982年 | 6014篇 |
1981年 | 6152篇 |
1980年 | 5625篇 |
1979年 | 5408篇 |
1978年 | 5132篇 |
1977年 | 6264篇 |
1976年 | 8462篇 |
1975年 | 4300篇 |
1974年 | 4127篇 |
1973年 | 3973篇 |
1972年 | 3262篇 |
1971年 | 2806篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Starting from a microscopic Hamiltonian defined on a semi-infinite cubic lattice, and employing a mean-field approximation, the surface parameters relevant for wetting in confined ternary mixtures are derived. These are found in terms of the microscopic coupling constants, and yield a physical interpretation of their origins. In comparison with the standard expression for the surface free-energy density, several new terms arising from the derivation are identified. The influence of the surface parameters on a predicted unbinding transition in a mixture of oil, water, and amphiphile demonstrate that existing results are robust to the addition of the extra surface terms. 相似文献
92.
Takauchi H. Tamura H. Matsubara S. Kibune M. Doi Y. Chiba T. Anbutsu H. Yamaguchi H. Mori T. Takatsu M. Gotoh K. Sakai T. Yamamura T. 《Solid-State Circuits, IEEE Journal of》2003,38(12):2094-2100
We describe a CMOS multichannel transceiver that transmits and receives 10 Gb/s per channel over balanced copper media. The transceiver consists of two identical 10-Gb/s modules. Each module operates off a single 1.2-V supply and has a single 5-GHz phase-locked loop to supply a reference clock to two transmitter (Tx) channels and two receiver (Rx) channels. To track the input-signal phase, the Rx channel has a clock recovery unit (CRU), which uses a phase-interpolator-based timing generator and digital loop filter. The CRU can adjust the recovered clock phase with a resolution of 1.56 ps. Two sets of two-channel transceiver units were fabricated in 0.11-/spl mu/m CMOS on a single test chip. The transceiver unit size was 1.6 mm /spl times/ 2.6 mm. The Rx sensitivity was 120-mVp-p differential with a 70-ps phase margin for a common-mode voltage ranging from 0.6 to 1.0 V. The evaluated jitter tolerance curve met the OC-192 specification. 相似文献
93.
94.
Sheldon H. Jacobson Julie L. Virta Jon M. Bowman John E. Kobza John J. Nestor 《IIE Transactions》2003,35(3):259-269
Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide. 相似文献
95.
Martin F. Falcone F. Bonache J. Marques R. Sorolla M. 《Microwave and Wireless Components Letters, IEEE》2003,13(12):511-513
A novel compact stop band filter consisting of a 50 /spl Omega/ coplanar waveguide (CPW) with split ring resonators (SRRs) etched in the back side of the substrate is presented. By aligning SRRs with the slots, a high inductive coupling between line and rings is achieved, with the result of a sharp and narrow rejection band in the vicinity of the resonant frequency of the rings. In order to widen the stop band of the filter, several ring pairs tuned at equally spaced frequencies within the desired gap are cascaded. The frequency response measured in the fabricated prototype device exhibits pronounced slopes at either side of the stop band and near 0 dBs insertion loss outside that band. Since SRR dimensions are much smaller than signal wavelength, the proposed filters are extremely compact and can be used to reject frequency parasitics in CPW structures by simply patterning properly tuned SRRs in the back side metal. Additional advantages are easy fabrication and compatibility with MMIC or PCB technology. 相似文献
96.
On the capacities of bipartite Hamiltonians and unitary gates 总被引:2,自引:0,他引:2
Bennett C.H. Harrow A.W. Leung D.W. Smolin J.A. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(8):1895-1911
We consider interactions as bidirectional channels. We investigate the capacities for interaction Hamiltonians and nonlocal unitary gates to generate entanglement and transmit classical information. We give analytic expressions for the entanglement generating capacity and entanglement-assisted one-way classical communication capacity of interactions, and show that these quantities are additive, so that the asymptotic capacities equal the corresponding 1-shot capacities. We give general bounds on other capacities, discuss some examples, and conclude with some open questions. 相似文献
97.
A.L. Pan H.G. ZhengZ.P. Yang F.X. Liu Z.J. Ding Y.T. Qian 《Materials Research Bulletin》2003,38(5):789-796
Small Ag particles or clusters dispersed mesoporous SiO2 composite films were prepared by a new method: First the matrix SiO2 films were prepared by sol-gel process combined with the dip-coating technique, then they were soaked in AgNO3 solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO2 films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements. 相似文献
98.
Membrane separation technology represents an alternative way to achieve biomass retention in anaerobic bioreactors for wastewater treatment. Due to high biomass concentrations of anaerobic reactors, cake formation is likely to represent a major cause of flux decline. In the presented research, experiments are performed on the effect of biomass concentration and level of gas sparging on the hydraulic capacity of a submerged anaerobic membrane bioreactor. Both parameters significantly affected the hydraulic capacity, with biomass exerting the most pronounced effect. After 50 days of continuous operation the critical flux remained virtually unchanged, despite an increase in membrane resistance, suggesting that biomass characteristics and hydraulic conditions determine the bio-layer formation rather than the membrane's fouling level. The concept of bio-layer management is introduced to describe the programmed combination of actions performed in order to control the formation of biomass layer over membranes. 相似文献
99.
Lind M.G.J. Weidong Xiao Dunford W.G. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(2):409-418
This paper presents a comprehensive nonlinear model of the controlled constant voltage transformer also known as the ferroresonant transformer. Saturation is a normal mode of operation for this device. This paper derives an equivalent electrical circuit that relates to the physical structure of a typical design. The level of detail includes winding resistances, continuously nonlinear magnetizing inductances, tapped windings, and leakage inductances. The paper describes methods to extract the winding resistances, leakage inductances, and hysteresis loops of the transformer and how to fit the latter into single-valued nonlinear functions. The paper compares computer simulation results of the model with those obtained analytically and experimentally. The results show that the derived circuit will be very useful for designers of the ferroresonant transformer, which is used in uninterruptible power supplies. 相似文献
100.
Some Insights into MIMO Mutual Information: The High SNR Case 总被引:1,自引:0,他引:1
Salo J. Suvikunnas P. El-Sallabi H.M. Vainikainen P. 《Wireless Communications, IEEE Transactions on》2006,5(11):2997-3001
We consider mutual information of multiple-input multiple-output (MIMO) wireless channels with complex isotropic Gaussian input in the case where the receiver has perfect channel, knowledge. For arbitrary fading statistics, a mutual information lower bound is decomposed in a sum of three terms involving: a) average SNR; b) channel fading; and c) a term characterizing the "effective rank", or eigenvalue dispersion, of the channel matrix. The decomposition suggests that spatial multiplexing efficiency of a MIMO channel can be characterized by the so-called ellipticity statistic. Distribution functions, means and variances of the random terms in the decomposition for the case of Rayleigh fading are also derived 相似文献