全文获取类型
收费全文 | 257827篇 |
免费 | 4527篇 |
国内免费 | 1786篇 |
专业分类
电工技术 | 5570篇 |
技术理论 | 1篇 |
综合类 | 1249篇 |
化学工业 | 37834篇 |
金属工艺 | 11577篇 |
机械仪表 | 7979篇 |
建筑科学 | 7691篇 |
矿业工程 | 923篇 |
能源动力 | 6704篇 |
轻工业 | 24984篇 |
水利工程 | 2077篇 |
石油天然气 | 2258篇 |
武器工业 | 77篇 |
无线电 | 32793篇 |
一般工业技术 | 46705篇 |
冶金工业 | 50793篇 |
原子能技术 | 4026篇 |
自动化技术 | 20899篇 |
出版年
2021年 | 2185篇 |
2020年 | 1520篇 |
2019年 | 1781篇 |
2018年 | 2723篇 |
2017年 | 2703篇 |
2016年 | 2839篇 |
2015年 | 2460篇 |
2014年 | 3840篇 |
2013年 | 11693篇 |
2012年 | 6339篇 |
2011年 | 8718篇 |
2010年 | 7153篇 |
2009年 | 7749篇 |
2008年 | 8391篇 |
2007年 | 8400篇 |
2006年 | 7808篇 |
2005年 | 7245篇 |
2004年 | 6706篇 |
2003年 | 6688篇 |
2002年 | 6698篇 |
2001年 | 6748篇 |
2000年 | 6203篇 |
1999年 | 6838篇 |
1998年 | 16897篇 |
1997年 | 11871篇 |
1996年 | 9215篇 |
1995年 | 6917篇 |
1994年 | 6088篇 |
1993年 | 5845篇 |
1992年 | 4152篇 |
1991年 | 4097篇 |
1990年 | 3648篇 |
1989年 | 3555篇 |
1988年 | 3468篇 |
1987年 | 2930篇 |
1986年 | 2855篇 |
1985年 | 3424篇 |
1984年 | 3056篇 |
1983年 | 2754篇 |
1982年 | 2552篇 |
1981年 | 2571篇 |
1980年 | 2435篇 |
1979年 | 2272篇 |
1978年 | 2236篇 |
1977年 | 2808篇 |
1976年 | 4171篇 |
1975年 | 1899篇 |
1974年 | 1795篇 |
1973年 | 1783篇 |
1972年 | 1460篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
Space-time block codes (STBCs) allow utilising the diversity provided by multiple-input-multiple-output (MIMO) communication channels, thereby decreasing the outage probability for a given communication rate. The contribution of this letter is the derivation of a closed-form expression of the outage probability of distributed STBCs deployed over Nakagami flat fading channels with different channel gains and fading parameters. 相似文献
172.
基于扩展分形和CFAR特征融合的SAR图像目标识别 总被引:3,自引:0,他引:3
研究了多信息融合技术在SAR图像目标识别中的应用。将扩展分形特征(Extended Fractal)与双参数恒虚警特征(Double Parameter CFAR)形成的多信息进行融合处理。运用Dempster-Shafer证据理论,在决策层对SAR图像中的像素进行识别分类。实验结果表明通过融合对像素分类的准确性明显好于单特征的检测结果,减少了虚警概率,提高了系统的识别能力。 相似文献
173.
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
174.
Superconducting qubits are solid state electrical circuits fabricated using techniques borrowed from conventional integrated circuits. They are based on the Josephson tunnel junction, the only non-dissipative, strongly non-linear circuit element available at low temperature. In contrast to microscopic entities such as spins or atoms, they tend to be well coupled to other circuits, which make them appealling from the point of view of readout and gate implementation. Very recently, new designs of superconducting qubits based on multi-junction circuits have solved the problem of isolation from unwanted extrinsic electromagnetic perturbations. We discuss in this review how qubit decoherence is affected by the intrinsic noise of the junction and what can be done to improve it.
PACS: 03.67.-a, 03.65.Yz, 85.25.-j, 85.35.Gv 相似文献
175.
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
176.
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
177.
Use of X-ray computed microtomography to understand why gels reduce relative permeability to water more than that to oil 总被引:1,自引:0,他引:1
R. S. Seright J. Liang W. Brent Lindquist John H. Dunsmuir 《Journal of Petroleum Science and Engineering》2003,39(3-4):217
X-ray computed microtomography (XMT) was used to investigate why gels reduce relative permeability to water more than that to oil in strongly water-wet Berea sandstone. XMT allows saturation differences to be monitored for individual pores during various stages of oil, water, and gelant flooding. The method also characterizes distributions of pore size, aspect ratio, and coordination number for the porous media. We studied a Cr(III) acetate–HPAM gel that reduced permeability to water (at Sor) by a factor 80–90 times more than that to oil (at Swr). In Berea, the gel caused disproportionate permeability reduction by trapping substantial volumes of oil that remained immobile during water flooding (i.e., 43.5% Sor before gel placement versus 78.7% Sor after gel placement). With this high trapped oil saturation, water was forced to flow through narrow films, through the smallest pores, and through the gel itself. In contrast, during oil flooding, oil pathways remained relatively free from constriction by the gel. 相似文献
178.
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
179.
A new topology for a photonic signal processor, which overcomes the basic recursive frequency response problem that limits the passband range, is presented. The structure is based on a new multiple-wavelength offset-cavity structure that is cascaded with a series of unbalanced delay line structures. This not only can synthesize a very narrow notch response with good shape factor but also permits a multifold extension of the free spectral range (FSR) and passband width. Results on the interference mitigation filter demonstrate a stopband of 1% of center frequency and a fourfold increase in the FSR and passband width, while also having a very small shape factor, in excellent agreement with predictions. 相似文献
180.