收费全文 | 411984篇 |
免费 | 4907篇 |
国内免费 | 1708篇 |
电工技术 | 6888篇 |
综合类 | 2869篇 |
化学工业 | 58095篇 |
金属工艺 | 20538篇 |
机械仪表 | 16141篇 |
建筑科学 | 9703篇 |
矿业工程 | 2734篇 |
能源动力 | 8749篇 |
轻工业 | 26484篇 |
水利工程 | 4854篇 |
石油天然气 | 7353篇 |
武器工业 | 43篇 |
无线电 | 47141篇 |
一般工业技术 | 86061篇 |
冶金工业 | 57740篇 |
原子能技术 | 8618篇 |
自动化技术 | 54588篇 |
2021年 | 2398篇 |
2020年 | 1812篇 |
2019年 | 2242篇 |
2018年 | 22696篇 |
2017年 | 22288篇 |
2016年 | 16174篇 |
2015年 | 3690篇 |
2014年 | 4948篇 |
2013年 | 12650篇 |
2012年 | 12056篇 |
2011年 | 24735篇 |
2010年 | 21157篇 |
2009年 | 19293篇 |
2008年 | 20261篇 |
2007年 | 22608篇 |
2006年 | 8223篇 |
2005年 | 11519篇 |
2004年 | 9853篇 |
2003年 | 9375篇 |
2002年 | 7815篇 |
2001年 | 7045篇 |
2000年 | 6831篇 |
1999年 | 6911篇 |
1998年 | 16422篇 |
1997年 | 11683篇 |
1996年 | 9045篇 |
1995年 | 6912篇 |
1994年 | 6091篇 |
1993年 | 6127篇 |
1992年 | 4460篇 |
1991年 | 4258篇 |
1990年 | 4091篇 |
1989年 | 4067篇 |
1988年 | 3775篇 |
1987年 | 3403篇 |
1986年 | 3404篇 |
1985年 | 3752篇 |
1984年 | 3467篇 |
1983年 | 3223篇 |
1982年 | 3028篇 |
1981年 | 3043篇 |
1980年 | 3039篇 |
1979年 | 2831篇 |
1978年 | 2871篇 |
1977年 | 3143篇 |
1976年 | 4099篇 |
1975年 | 2420篇 |
1974年 | 2381篇 |
1973年 | 2398篇 |
1972年 | 2022篇 |
Hydrograph clustering helps to identify dynamic patterns within aquifers systems, an important foundation of characterizing groundwater systems and their influences, which is necessary to effectively manage groundwater resources. We develope an unsupervised modeling approach to characterize and cluster hydrographs on regional scale according to their dynamics. We apply feature-based clustering to improve the exploitation of heterogeneous datasets, explore the usefulness of existing features and propose new features specifically useful to describe groundwater hydrographs. The clustering itself is based on a powerful combination of Self-Organizing Maps with a modified DS2L-Algorithm, which automatically derives the cluster number but also allows to influence the level of detail of the clustering. We further develop a framework that combines these methods with ensemble modeling, internal cluster validation indices, resampling and consensus voting to finally obtain a robust clustering result and remove arbitrariness from the feature selection process. Further we propose a measure to sort hydrographs within clusters, useful for both interpretability and visualization. We test the framework with weekly data from the Upper Rhine Graben System, using more than 1800 hydrographs from a period of 30 years (1986-2016). The results show that our approach is adaptively capable of identifying homogeneous groups of hydrograph dynamics. The resulting clusters show both spatially known and unknown patterns, some of which correspond clearly to external controlling factors, such as intensive groundwater management in the northern part of the test area. This framework is easily transferable to other regions and, by adapting the describing features, also to other time series-clustering applications.
相似文献We discuss the temperature dependence of a common low temperature local thermometer, a tunnel junction between a superconductor and a normal metal (NIS junction). Towards the lowest temperatures its characteristics tend to saturate, which is usually attributed to selfheating effects. In this technical note, we reanalyze this saturation and show that the temperature independent subgap current of the junction alone explains in some cases the low temperature behavior quantitatively.
相似文献In this work we analysed the stepwise charging technique to find the limits from which it is beneficial in terms of load capacitance and charge–discharge frequency. We included in the analysis practical limitations such as the consumption of auxiliary logic needed to implement the technique and the minimum size of auxiliary switches imposed by the technology. We proposed an ultra-low-power logic block to push these limits and to obtain benefits from this technique in small capacitances. Finally, we proposed to use a stepwise driver in the driving of the gate capacitance of power switches in switched-capacitor (SC) DC–DC converters. We designed and manufactured, in a 130 nm process, a SC DC–DC converter and measured a 29% energy reduction in the gate-drive losses of the converter. This accounts for an improvement of 4% (from 69 to 73%) in the overall converter efficiency.
相似文献