全文获取类型
收费全文 | 513300篇 |
免费 | 5681篇 |
国内免费 | 1985篇 |
专业分类
电工技术 | 8899篇 |
综合类 | 671篇 |
化学工业 | 77920篇 |
金属工艺 | 20773篇 |
机械仪表 | 15326篇 |
建筑科学 | 12245篇 |
矿业工程 | 3099篇 |
能源动力 | 12531篇 |
轻工业 | 43824篇 |
水利工程 | 5661篇 |
石油天然气 | 11453篇 |
武器工业 | 56篇 |
无线电 | 58492篇 |
一般工业技术 | 102318篇 |
冶金工业 | 93295篇 |
原子能技术 | 12998篇 |
自动化技术 | 41405篇 |
出版年
2021年 | 3875篇 |
2019年 | 3665篇 |
2018年 | 6530篇 |
2017年 | 6557篇 |
2016年 | 7047篇 |
2015年 | 4550篇 |
2014年 | 7720篇 |
2013年 | 21930篇 |
2012年 | 12536篇 |
2011年 | 17180篇 |
2010年 | 13740篇 |
2009年 | 15797篇 |
2008年 | 16286篇 |
2007年 | 16152篇 |
2006年 | 14156篇 |
2005年 | 13222篇 |
2004年 | 12739篇 |
2003年 | 12261篇 |
2002年 | 12182篇 |
2001年 | 12049篇 |
2000年 | 11450篇 |
1999年 | 11773篇 |
1998年 | 28131篇 |
1997年 | 20510篇 |
1996年 | 15889篇 |
1995年 | 12147篇 |
1994年 | 10819篇 |
1993年 | 10714篇 |
1992年 | 8040篇 |
1991年 | 7908篇 |
1990年 | 7610篇 |
1989年 | 7568篇 |
1988年 | 7257篇 |
1987年 | 6375篇 |
1986年 | 6345篇 |
1985年 | 7212篇 |
1984年 | 6759篇 |
1983年 | 6231篇 |
1982年 | 5856篇 |
1981年 | 5970篇 |
1980年 | 5849篇 |
1979年 | 5636篇 |
1978年 | 5668篇 |
1977年 | 6417篇 |
1976年 | 8401篇 |
1975年 | 5076篇 |
1974年 | 4868篇 |
1973年 | 4874篇 |
1972年 | 4201篇 |
1971年 | 3829篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
K. N. Subramanian J. G. Lee 《Journal of Materials Science: Materials in Electronics》2004,15(4):235-240
Properties of body centered tetragonal tin are highly anisotropic. As a consequence large stresses can develop at the tin grain boundaries due to coefficient of thermal expansion mismatch during temperature excursions. A modeling approach to evaluate the 3D stress states that develop at grain boundaries during thermomechanical fatigue in tin-based solder is presented. Development of significant amounts of stresses in the plane of the grain boundary can cause grain-boundary sliding and surface-relief effects, while those normal to the grain boundary can cause grain-boundary decohesion and cracking. 相似文献
82.
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
83.
84.
Lorenzetto G. Galtarossa A. Palmieri L. Santagiustina M. Someda C.G. Fiorone R. 《Lightwave Technology, Journal of》2003,21(2):424-431
First-order polarization-mode dispersion (PMD) compensation by means of a polarization controller and a differential delay line is not sufficient to guarantee error-free transmission for 40-Gb/s channels when higher order effects severely increase signal distortion. Higher order mitigation is possible by cascading more than one first-order block. However, only two-stage or three-stage devices remain simple enough to be actually controlled. The performance of such higher order PMD compensators is evaluated by means of numerical simulations. Two different feedback signals have been used, demonstrating that first-order and higher order PMD distortion of nonreturn-to-zero (NRZ) pulses at 40 Gb/s can be strongly mitigated for instantaneous values of the differential group delay (DGD) up to the bit slot, when the compensator is properly controlled. 相似文献
85.
Thomas S. Lowry John C. Bright Murray E. Close Christina A. Robb Paul A. White Stewart G. Cameron 《国际水资源开发杂志》2003,19(4):579-592
The primary objective of this project is to identify gaps, whether real or perceived, that hinder effective groundwater management in New Zealand. These gaps show as gaps in information, gaps in implementation, gaps in technological and management tools, and gaps in understanding of fundamental processes. The secondary objective is to propose a management strategy to close the identified gaps. Several methods are used to meet these objectives: surveys distributed to selected staff in each regional council; the review of various written reports; the analysis of land-use databases; and private consultation within each regional council. Results show that groundwater management in New Zealand is generally reactionary with the main gaps being in strategic planning and national guidelines. Most gaps appear to be predominantly information and implementation issues. In some cases there are gaps in the understanding of fundamental processes within an aquifer system, including the long-term effects of land-use on groundwater quality. An adaptive management approach is suggested as a means of closing these gaps. 相似文献
86.
87.
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
88.
P. CRISP T.J. WICKS D. BRUER E.S. SCOTT 《Australian Journal of Grape and Wine Research》2006,12(3):203-211
Grapevine powdery mildew ( Erysiphe necator) affects grape yield and fruit quality worldwide. Managers of conventional vineyards rely mainly on synthetic fungicides and sulfur to control powdery mildew, while in organic vineyards sulfur is the main control agent, often in rotation with canola-based oils, bicarbonates and biological control agents. The efficacy of those materials has not been evaluated critically under field conditions in Australia. Accordingly, a range of materials showing most promise in previous greenhouse trials (Crisp et al. 2006 Australian Journal of Grape and Wine Research 12 , pp. 192–202) were assessed via field trials in commercial vineyards. Applications of either milk or whey (alone, or mixed with a canola oil-based product), as well as applications of potassium bicarbonate (commercial formulation), all reduced the severity of powdery mildew compared with untreated vines. Eight applications of a 1:10 dilution of milk, 45 g/L whey powder or programs comprising rotations of potassium bicarbonate plus oil and whey, applied at 10–14 day intervals, reduced the severity of powdery mildew to levels not significantly different from that on vines sprayed with sulfur (wettable powder, 3–6 g/L). However, the relative control of powdery mildew by the test materials in field trials was dependent on the susceptibility of the grapevine cultivar and the extent of spray coverage achieved. In vineyards where highly susceptible cultivars were planted, and spray coverage was compromised, the resultant control of powdery mildew was reduced; and sometimes to commercially unacceptable levels. 相似文献
89.
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
90.
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