收费全文 | 56074篇 |
免费 | 3229篇 |
国内免费 | 47篇 |
电工技术 | 362篇 |
综合类 | 36篇 |
化学工业 | 10310篇 |
金属工艺 | 1009篇 |
机械仪表 | 1036篇 |
建筑科学 | 1661篇 |
矿业工程 | 104篇 |
能源动力 | 1100篇 |
轻工业 | 7986篇 |
水利工程 | 549篇 |
石油天然气 | 238篇 |
武器工业 | 6篇 |
无线电 | 2911篇 |
一般工业技术 | 9491篇 |
冶金工业 | 14898篇 |
原子能技术 | 271篇 |
自动化技术 | 7382篇 |
2023年 | 455篇 |
2022年 | 498篇 |
2021年 | 1218篇 |
2020年 | 1019篇 |
2019年 | 1022篇 |
2018年 | 2050篇 |
2017年 | 2011篇 |
2016年 | 2102篇 |
2015年 | 1529篇 |
2014年 | 1971篇 |
2013年 | 3852篇 |
2012年 | 3231篇 |
2011年 | 2939篇 |
2010年 | 2411篇 |
2009年 | 2175篇 |
2008年 | 2412篇 |
2007年 | 2162篇 |
2006年 | 1566篇 |
2005年 | 1281篇 |
2004年 | 1260篇 |
2003年 | 1113篇 |
2002年 | 979篇 |
2001年 | 673篇 |
2000年 | 646篇 |
1999年 | 874篇 |
1998年 | 4761篇 |
1997年 | 2907篇 |
1996年 | 1886篇 |
1995年 | 1061篇 |
1994年 | 919篇 |
1993年 | 926篇 |
1992年 | 296篇 |
1991年 | 308篇 |
1990年 | 297篇 |
1989年 | 304篇 |
1988年 | 280篇 |
1987年 | 233篇 |
1986年 | 258篇 |
1985年 | 294篇 |
1984年 | 226篇 |
1983年 | 174篇 |
1982年 | 210篇 |
1981年 | 234篇 |
1980年 | 215篇 |
1979年 | 151篇 |
1978年 | 144篇 |
1977年 | 318篇 |
1976年 | 660篇 |
1975年 | 102篇 |
1973年 | 93篇 |
Neonatal brain and cardiac imaging would benefit from the increased signal-to-noise ratio levels at 7 T compared to lower field. Optimal performance might be achieved using purpose designed RF coil arrays. In this study, we introduce an 8-channel dipole array and investigate, using simulations, its RF performances for neonatal applications at 7 T.
MethodsThe 8-channel dipole array was designed and evaluated for neonatal brain/cardiac configurations in terms of SAR efficiency (ratio between transmit-field and maximum specific-absorption-rate level) using adjusted dielectric properties for neonate. A birdcage coil operating in circularly polarized mode was simulated for comparison. Validation of the simulation model was performed on phantom for the coil array.
ResultsThe 8-channel dipole array demonstrated up to 46% higher SAR efficiency levels compared to the birdcage coil in neonatal configurations, as the specific-absorption-rate levels were alleviated. An averaged normalized root-mean-square-error of 6.7% was found between measured and simulated transmit field maps on phantom.
ConclusionThe 8-channel dipole array design integrated for neonatal brain and cardiac MR was successfully demonstrated, in simulation with coverage of the baby and increased SAR efficiency levels compared to the birdcage. We conclude that the 8Tx-dipole array promises safe operating procedures for MR imaging of neonatal brain and heart at 7 T.
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Probabilistic topic modeling algorithms like Latent Dirichlet Allocation (LDA) have become powerful tools for the analysis of large collections of documents (such as papers, projects, or funding applications) in science, technology an innovation (STI) policy design and monitoring. However, selecting an appropriate and stable topic model for a specific application (by adjusting the hyperparameters of the algorithm) is not a trivial problem. Common validation metrics like coherence or perplexity, which are focused on the quality of topics, are not a good fit in applications where the quality of the document similarity relations inferred from the topic model is especially relevant. Relying on graph analysis techniques, the aim of our work is to state a new methodology for the selection of hyperparameters which is specifically oriented to optimize the similarity metrics emanating from the topic model. In order to do this, we propose two graph metrics: the first measures the variability of the similarity graphs that result from different runs of the algorithm for a fixed value of the hyperparameters, while the second metric measures the alignment between the graph derived from the LDA model and another obtained using metadata available for the corresponding corpus. Through experiments on various corpora related to STI, it is shown that the proposed metrics provide relevant indicators to select the number of topics and build persistent topic models that are consistent with the metadata. Their use, which can be extended to other topic models beyond LDA, could facilitate the systematic adoption of this kind of techniques in STI policy analysis and design.
相似文献