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排序方式: 共有67条查询结果,搜索用时 15 毫秒
1.
van Wesenbeeck M.P.N. Klaasens J.B. von Stockhausen U. Munoz de Morales Anciola A. Valtchev S.S. 《Industrial Electronics, IEEE Transactions on》1997,44(6):780-787
Series connection of power devices has evolved into a mature technique and is widely applied in HV DC power systems. Static and dynamic voltage balance is ensured by shunting individual devices with dissipative snubbers. The snubber losses become pronounced for increased operating frequencies and adversely affect power density. Capacitive snubbers do not exhibit these disadvantages, but they require a zero-voltage switching mode. Super-resonant power converters facilitate the principle of zero-voltage switching. A high-voltage DC-DC power converter with multiple series-connected devices is proposed. It allows the application of nondissipating snubbers to assist the voltage sharing between the multiple series-connected devices and lowers turnoff losses. Simulation results obtained with a circuit simulator are validated in an experimental power converter operating with two series-connected devices. The behavior of the series connection is examined for MOSFETs and IGBTs by both experimental work with a 2 kW prototype and computer simulation. Applications can be found in traction and heavy industry, where the soft-switching power converter is directly powered from a high-voltage source 相似文献
2.
Éric Lunaud Ngoupé Clément Parisot Sylvan Stoesel Petko Valtchev Roger Villemaire Omar Cherkaoui Pierre Boucher Sylvain Hallé 《Journal of Network and Systems Management》2017,25(1):180-209
Configuration Logic (CL) is a formal language that allows a network engineer to express constraints in terms of the actual parameters found in the configuration of network devices. We present an efficient algorithm that can automatically check a pool of devices for conformance to a set of CL constraints; moreover, this algorithm can point to the part of the configuration responsible for the error when a constraint is violated. Contrary to other validation approaches that require dumping the configuration of the whole network to a central location in order to be verified, we also present an algorithm that analyzes the correct formulas and greatly helps reduce the amount of data that need to be transferred to that central location, pushing as much of the evaluation of the formula locally on each device. The procedure is also backwards-compatible, in such a way that a device that does not (or only partially) supports a local evaluation may simply return a subset or all of its configuration. These capabilities have been integrated into a network management tool called ValidMaker. 相似文献
3.
Laszlo Szathmary Petko Valtchev Amedeo Napoli Robert Godin Alix Boc Vladimir Makarenkov 《Annals of Mathematics and Artificial Intelligence》2014,70(1-2):81-105
In pattern mining and association rule mining, there is a variety of algorithms for mining frequent closed itemsets (FCIs) and frequent generators (FGs), whereas a smaller part further involves the precedence relation between FCIs. The interplay of these three constructs and their joint computation have been studied within the formal concept analysis (FCA) field yet none of the proposed algorithms is scalable. In frequent pattern mining, at least one suite of efficient algorithms has been designed that exploits basically the same ideas and follows the same overall computational schema. Based on an in-depth analysis of the aforementioned interplay that is rooted in a fundamental duality from hypergraph theory, we propose a new schema that should enable for a more parsimonious computation. We exemplify the new schema in the design of Snow-Touch, a concrete FCI/FG/precedence miner that reuses an existing algorithm, Charm, for mining FCIs, and completes it with two original methods for mining FGs and precedence, respectively. The performance of Snow-Touch and of its closest competitor, Charm-L, were experimentally compared using a large variety of datasets. The outcome of the experimental study suggests that our method outperforms Charm-L on dense data while on sparse one the trend is reversed. Furthermore, we demonstrate the usefulness of our method and the new schema through an application to the analysis of a genome dataset. The initial results reported here confirm the capacity of the method to focus on significant associations. 相似文献
4.
Laszlo Szathmary Petko Valtchev Amedeo Napoli 《International Journal of Software and Informatics》2010,4(3):219-238
Rare association rules correspond to rare, or infrequent, itemsets, as opposed
to frequent ones that are targeted by conventional pattern miners. Rare rules reflect regularities
of local, rather than global, scope that can nevertheless provide valuable insights
to an expert, especially in areas such as genetics and medical diagnosis where some specific
deviations/illnesses occur only in a small number of cases. The work presented here is motivated
by the long-standing open question of efficiently mining strong rare rules, i.e., rules
with high confidence and low support. We also propose an efficient solution for finding the
set of minimal rare itemsets. This set serves as a basis for generating rare association rules. 相似文献
5.
The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering. Single zeolite A crystals nucleated in amorphous gel particles of 40 to 80 nanometers within 3 days at room temperature. The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles. The gel particles were consumed during further crystal growth at room temperature, forming a colloidal suspension of zeolite A nanocrystals of 40 to 80 nanometers. On heating this suspension at 80 degrees C, solution-mediated transport resulted in additional substantial crystal growth. 相似文献
6.
Martin Tomas Valtchev Petko Roux Louis-Romain 《Knowledge and Information Systems》2023,65(8):3295-3335
Knowledge and Information Systems - Mining frequent itemsets (FIs) from data streams is a challenging task due to the limited resources available w.r.t. the typically large size of the result and... 相似文献
7.
8.
There is considerable interest in interleaving multiple phased array antennas into a single common aperture system. Current phased array antenna technology is limited to narrowband operation, leading to the appearance of grating lobes and strong mutual coupling effects when they are incorporated into the design of a common aperture system. To overcome this obstacle, a new class of arrays, called polyfractal arrays, has been introduced that possess natural wideband properties well suited for large-scale genetic algorithm optimizations. These arrays also possess recursive beamforming properties and an autopolyploidy-based chromosome expansion that can dramatically accelerate the convergence of a genetic algorithm. In addition, a robust Pareto optimization can be applied to reduce the peak sidelobe levels at several frequencies throughout the intended operating band, leading to ultrawideband antenna array designs. Because of their lack of grating lobes, these polyfractal arrays are ideal building blocks for interleaved antenna array systems. This paper develops these concepts, first creating ultrawideband array designs based on polyfractal geometries and then interleaving these designs into a common aperture system. Several examples of interleaved systems are discussed, with one two-array system possessing a peak sidelobe level of nearly -18 dB with no grating lobes over a 20:1 bandwidth with either of the component array mainbeams steered independently up to 60deg from broadside. 相似文献
9.
Variation of the Si/Al ratio in nanosized zeolite Beta crystals 总被引:3,自引:0,他引:3
S. Mintova V. Valtchev T. Onfroy C. Marichal H. Knzinger T. Bein 《Microporous and mesoporous materials》2006,90(1-3):237-245
Zeolite Beta nanocrystals were prepared from basic aluminosilicate precursor solutions upon hydrothermal treatment at 100 °C. The Si/Al ratio of the initial system was systematically changed from 25 to infinity in order to study the limits in the framework composition of BEA-type crystallites synthesized from clear basic solutions. Furthermore, the effect of the Si/Al ratio on the precursor species, ultimate crystal size, morphology and yield was investigated. The results revealed that the crystallization kinetics of nanosized Beta are dependent on the amount of Al in the precursor solutions, that is, the nucleation and growth processes are faster in Al-rich systems. The crystallization process of zeolite Beta with Si/Al ratios in the initial solutions of 14, 23 and 32 was accomplished within 72 h, whereas longer crystallization times, 140 and 264 h, were necessary to obtain crystalline products with Si/Al ratios of 42 and infinity, respectively. The intermediates and final products were investigated by complementary techniques such as XRD, HRTEM, DLS, IR, NMR spectroscopy and chemical analysis. Low temperature (77 K) CO adsorption infrared spectroscopy was used to study the Brønsted acidity of zeolite Beta samples with different Si/Al ratios. The properties of Beta nanocrystals important for the design of catalysts and selective separation materials are provided based on the results obtained from the detailed characterization. 相似文献
10.
Veselina Georgieva Richard Retoux Valerie Ruaux Valentin Valtchev Svetlana Mintova 《Frontiers of Chemical Science and Engineering》2018,12(1):94-102
Detection of oxygen and carbon dioxide is important in the field of chemical and biosensors for atmosphere and biosystem monitoring and fermentation processes. The present study reports on the preparation of zeolite films doped with iron nanoparticles for detection of CO2 and O2 in gas phase. Pure nanosized LTL type zeolite with monomodal particle size distribution loaded with iron (Fe-LTL) was prepared under hydrothermal condition from colloidal precursor suspensions. The zeolite was loaded with iron to different levels by ion exchange. The Fe-LTL suspensions were used for preparation of thin films on silicon wafers via spin coating method. The reduction of the iron in the zeolite films was carried out under H2 flow (50% H2 in Ar) at 300 °C. The presence of iron nanoparticles is proved by in situ ultra-violet-visible spectroscopy. The properties of the films including surface roughness, thickness, porosity, and mechanical stability were studied. In addition, the loading and distribution of iron in the zeolite films were investigated. The Fe-LTL zeolite films were used to detect O2 and CO2 in a concentration dependent mode, followed by IR spectroscopy. The changes in the IR bands at 855 and 642 cm–1 (Fe?O?H and Fe?O bending vibrations) and at 2363 and 2333 cm–1 (CO2 asymmetric stretching) corresponding to the presence of O2 and CO2, respectively, were evaluated. The response to O2 and CO2 was instant, which was attributed to great accessibility of the iron in the nanosized zeolite crystals. The saturation of the Fe-LTL films with CO2 and O2 at each concentration was reached within less than a minute. The Fe-LTL films detected both oxygen and carbon dioxide in contrast, to the pure LTL zeolite film. 相似文献