全文获取类型
收费全文 | 303656篇 |
免费 | 4654篇 |
国内免费 | 1335篇 |
专业分类
电工技术 | 4959篇 |
综合类 | 533篇 |
化学工业 | 46203篇 |
金属工艺 | 11467篇 |
机械仪表 | 8968篇 |
建筑科学 | 7232篇 |
矿业工程 | 1622篇 |
能源动力 | 7478篇 |
轻工业 | 27942篇 |
水利工程 | 3311篇 |
石油天然气 | 5891篇 |
武器工业 | 44篇 |
无线电 | 34494篇 |
一般工业技术 | 60452篇 |
冶金工业 | 55176篇 |
原子能技术 | 7541篇 |
自动化技术 | 26332篇 |
出版年
2021年 | 3050篇 |
2020年 | 2326篇 |
2019年 | 2777篇 |
2018年 | 4772篇 |
2017年 | 4833篇 |
2016年 | 5114篇 |
2015年 | 3429篇 |
2014年 | 5473篇 |
2013年 | 14190篇 |
2012年 | 8739篇 |
2011年 | 11251篇 |
2010年 | 8766篇 |
2009年 | 9819篇 |
2008年 | 10215篇 |
2007年 | 10159篇 |
2006年 | 8469篇 |
2005年 | 7805篇 |
2004年 | 7719篇 |
2003年 | 7440篇 |
2002年 | 7125篇 |
2001年 | 7128篇 |
2000年 | 6719篇 |
1999年 | 7042篇 |
1998年 | 17755篇 |
1997年 | 12518篇 |
1996年 | 9495篇 |
1995年 | 7169篇 |
1994年 | 6262篇 |
1993年 | 6296篇 |
1992年 | 4467篇 |
1991年 | 4248篇 |
1990年 | 4123篇 |
1989年 | 4108篇 |
1988年 | 3822篇 |
1987年 | 3452篇 |
1986年 | 3443篇 |
1985年 | 3800篇 |
1984年 | 3525篇 |
1983年 | 3252篇 |
1982年 | 3058篇 |
1981年 | 3093篇 |
1980年 | 3083篇 |
1979年 | 2855篇 |
1978年 | 2898篇 |
1977年 | 3227篇 |
1976年 | 4267篇 |
1975年 | 2443篇 |
1974年 | 2402篇 |
1973年 | 2429篇 |
1972年 | 2023篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Presents an obituary and information about the life and accomplishments of Josef Maria Bro?ek. Born August 14, 1913, in the ancient town of Melnik, in central Bohemia, today the Czech Republic, Josef spent his childhood in Poland (Warsaw, 1913- 1915) and in Siberia (1915-1920). His education in Czechoslovakia culminated with a thesis on "Memory, Its Measurement and Structure" and a PhD awarded in June 1937. In the history of psychology, his lifetime project bore the title "Historiography of Psychology Around the World," and it covered about 20 geographical areas. Extensive attention was devoted to institutional and organizational developments as journals, academic settings, archives, museums, research groups, conferences, and institutes. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
42.
The paper presents an improved statistical analysis of the least mean fourth (LMF) adaptive algorithm behavior for a stationary Gaussian input. The analysis improves previous results in that higher order moments of the weight error vector are not neglected and that it is not restricted to a specific noise distribution. The analysis is based on the independence theory and assumes reasonably slow learning and a large number of adaptive filter coefficients. A new analytical model is derived, which is able to predict the algorithm behavior accurately, both during transient and in steady-state, for small step sizes and long impulse responses. The new model is valid for any zero-mean symmetric noise density function and for any signal-to-noise ratio (SNR). Computer simulations illustrate the accuracy of the new model in predicting the algorithm behavior in several different situations. 相似文献
43.
2-D symmetry: theory and filter design applications 总被引:1,自引:0,他引:1
In this comprehensive review article, we present the theory of symmetry in two-dimensional (2-D) filter functions and in 2-D Fourier transforms. It is shown that when a filter frequency response possesses symmetry, the realization problem becomes relatively simple. Further, when the frequency response has no symmetry, there is a technique to decompose that frequency response into components each of which has the desired symmetry. This again reduces the complexity of two-dimensional filter design. A number of filter design examples are illustrated. 相似文献
44.
Hambleton P.J. Ng B.K. Plimmer S.A. David J.P.R. Rees G.J. 《Electron Devices, IEEE Transactions on》2003,50(2):347-351
The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions. 相似文献
45.
46.
47.
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
48.
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
49.
P. Frantzis 《Journal of Materials Science》2003,38(7):1397-1401
Bituminous binders are widely used in the construction of flexible pavements. However, in some applications, the performance of conventional binders is not considered to be satisfactory. Reinforcing these binders with selected polymers prevents premature failure of a pavement by improving the properties of the binder. Another source of reinforcement comes from crumb (ground) rubber produced from waste tyres. After they have been worn-out during their limited service life, millions of used tyres are discarded every year and are hauled to a dump. The fatigue resistance at temperatures below normal service temperatures (25°C), one of the key engineering properties of crumb rubber reinforced binders, has been found to be lower than that of neat binders. This paper is concerned with the development of a rubber reinforced binder. It was shown that the binder has the potential to be used as an all-weather wearing course in flexible roads, whilst at the same time recycling a considerable amount of waste rubber. 相似文献
50.
The Woodman Point Wastewater Treatment Plant (WWTP) in Western Australia has experienced two separate problems causing avoidable maintenance costs: the build-up of massive struvite (MgNH4PO4. 6H2O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H2S) levels in the digester gas to levels that compromised gas engine operation and caused high operating costs on the gas scrubber. As both problems hang together with a chemical imbalance in the anaerobic digester, we decided to investigate whether both problems could be (feasibly and economically) addressed by a common solution (such as dosing of iron solutions to precipitate both sulfide and phosphate), or by using separate approaches. Laboratory results showed that, the hydrogen sulfide emission in digesters could be effectively and economically controlled by the addition of iron dosing. Slightly higher than the theoretical value of 1.5 mol of FeCl3 was required to precipitate 1 mol of dissolved sulfide inside the digester. Due to the high concentration of PO4(3-) in the digested sludge liquor, significantly higher iron is required for struvite precipitation. Iron dosing did not appear an economic solution for struvite control via iron phosphate formation. By taking advantage of the natural tendency of struvite formation in the digester liquid, it is possible to reduce the risk of struvite precipitation in and around the sludge-dewatering centrifuge by increasing the pH to precipitate struvite out before passing through the centrifuge. However, as the Mg2+/PO4(3-) molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO4(3-) was limited by the amount of cations (Ca2+ and Mg2+) available in the sludge. Although this would reduce struvite precipitation in the centrifuge, it could not significantly reduce PO4(3-) recycling back to the plant. For long-term operation, maximum PO4(3-) reduction should be the ultimate aim to minimise PO4(3-) accumulation in the plant. Magnesium hydroxide liquid (MHL) was found to be the most cost-effective chemical to achieve this goal. It enhanced struvite precipitation from both, digested sludge and centrate to the point where more than 95% PO4(3-) reduction in the digested sludge was achieved. 相似文献