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Xavier Gelabert Oriol Sallent Jordi Pérez-Romero Ramon Agustí 《Computer Networks》2010,54(14):2519-2536
An efficient and utmost utilization of currently scarce and underutilized radio spectrum resources has stimulated the introduction of what has been coined Cognitive Radio (CR) access methodologies and implementations. While the long-established approach has been based on licensed (or primary) spectrum access, this new communication paradigm enables an opportunistic secondary access to shared spectrum resources provided mutual interference is kept below acceptable levels. In this paper we address the problem of primary-secondary spectrum sharing in cognitive radio access networks using a framework based on a Discrete Time Markov Chain (DTMC) model. Its applicability and advantages with respect to other approaches is explained and further justified. Spectrum awareness of primary activity by the secondary users is based on spectrum sensing techniques, which are modeled in order to capture sensing errors in the form of false-alarm and missed-detection. Model validation is successfully achieved by means of a system-level simulator which is able to capture the system behavior with high degree of accuracy. Parameter dependencies and potential tradeoffs are identified enabling an enhanced operation for both primary and secondary users. The suitability of the specified model is justified while allowing a wide range of extended implementations and enhanced capabilities to be considered. 相似文献
64.
Lluís Jofre Oriol Lehmkuhl Jordi Ventosa F. Xavier Trias 《Numerical Heat Transfer, Part B: Fundamentals》2013,63(1):53-79
The Navier-Stokes equations describe fluid flow by conserving mass and momentum. There are two main mesh discretizations for the computation of these equations, the collocated and staggered schemes. Collocated schemes locate the velocity field at the same grid points as the pressure one, while staggered discretizations locate variables at different points within the mesh. One of the most important characteristic of the discretization schemes, aside from accuracy, is their capacity to discretely conserve kinetic energy, specially when solving turbulent flow. Hence, this work analyzes the accuracy and conservation properties of two particular collocated and staggered schemes by solving various problems. 相似文献
65.
Effects of long-term pH elevation on the sulfate-reducing and methanogenic activities of anaerobic sewer biofilms 总被引:2,自引:0,他引:2
The dosage of alkali is often applied by the wastewater industry to reduce the transfer of hydrogen sulfide from wastewater to the sewer atmosphere. In this paper the activities of Sulfate Reducing Bacteria (SRB) and Methanogenic Archaea (MA) under elevated pH conditions (8.6 and 9.0) were evaluated in a laboratory scale anaerobic sewer reactor. Compared to those in a control reactor without pH control (pH 7.6 ± 0.1), the SRB activity was reduced by 30% and 50%, respectively, at pH 8.6 and pH 9.0. When normal pH was resumed, it took approximately 1 month for the SRB activity to fully recover. Methanogenic activities developed in the control reactor in 3 months after the reactor start-up, while no significant methanogenic activities were detected in the experimental reactor until normal pH was resumed. The results suggest that elevated pH at 8.6-9.0 suppressed the growth of methanogens. These experimental results clearly showed that, in addition to its well-known effect of reducing H2S transfer from the liquid to the gas phase, pH elevation considerably reduces sulfide and methane production by anaerobic sewer biofilms. These findings are significant for the optimal use of alkali addition to sewers for the control of H2S and CH4 emissions. A model-based study showed that, by adding the alkali at the beginning rather than towards the end of a rising main, substantial savings in chemicals can be achieved while achieving the same level of sulfide emission control, and complete methane emission control. 相似文献
66.
This paper presents a novel methodology for capturing the coupling between the different cells in both the uplink and downlink
directions in a Wideband Code Division Multiple Access (WCDMA) scenario. It is based on the definition and computation of
the gradient of the uplink cell load factor and the downlink transmitted power, which are the two main parameters that reflect
the actual cell load in the two link directions. The paper shows that the gradient is able to capture the relevant information
about the spatial distribution of traffic, which has an impact on cell performance. The proposed methodology is also used
as the basis for defining and evaluating new Radio Resource Management (RRM) strategies that operate at a multi-cell level.
相似文献
Ramon AgustíEmail: |
67.
68.
Lorenza Giupponi Ramon Agustí Jordi Pérez-Romero Oriol Sallent 《Mobile Networks and Applications》2006,11(6):825-846
Inter-working and convergence of heterogeneous wireless networks are paving the way to scenarios in which end users will be
capable of using simultaneously services through different Radio Access Technologies (RATs), by means of reconfigurable mobile
terminals and different network elements. In order to exploit the potential of these heterogeneous networks scenarios, optimal
RAT selection and resource utilization mechanisms are required. As a result, the heterogeneous networks are introducing a
new dimension to the Radio Resource Management (RRM) problem, so that new algorithms dealing with the dissimilarities and
complementarities of the multiple RATs from a joint perspective have to be considered. In this sense, this paper proposes
a Joint Radio Resource Management (JRRM) strategy in a multi-RAT, multicellular and multiservice scenario. An approach based
on Fuzzy Neural methodology is presented. Firstly, the way how the proposed Fuzzy Neural framework deals with the multiservice
allocation in a heterogeneous scenario is presented. A reinforcement learning algorithm based on neural networks allows guaranteeing
a multidimensional QoS focusing on those QoS requirements which mainly affect the user perception of the service. In addition
to this, the performances obtained by the Fuzzy Neural JRRM for both real-time and non real-time services, are compared to
the ones offered by alternative JRRM strategies. Secondly, special attention is paid to real-time services and to mechanisms
to improve their performances. An approach based on predicting future JRRM decisions and on accordingly reserving radio resources
for handoff calls is presented. Simulation results will show improvements in terms of both new connection blocking and handoff
call dropping probabilities. Finally, the full set of results provides the sufficient insight into the problem to allow stating
that the present Fuzzy Neural framework can be a firm candidate for JRRM.
相似文献
Oriol SallentEmail: |
69.
The speciation of Ga, Ge, Ni, V, S and Fe in fly ash from IGCC power plant were investigated for possible further extraction process by combining conventional mineral and chemical analysis, leaching tests, wet sequential extraction, Mössbauer and XAFS spectroscopies. The results shown that Ge occurs mainly as water-soluble species, GeS and/or GeS2 and hexagonal GeO2. Ga is present as an oxide, Ni occurs mainly as nickeline (NiAs), with minor proportions of Ni arsenates and vanadium as V(III) with minor amounts of V(IV) in the aluminosilicate glass matrix. Pyrrhotite and wurtzite-sphalerite are sulfide species containing Fe and Zn, but an important fraction of iron is also present in the aluminosilicate glass. These clear differences between the speciation of the above elements in this material and those reported for fly ash from conventional PC combustion. 相似文献
70.
Llorens O Perez JJ Palomer A Mauleon D 《Journal of molecular graphics & modelling》2002,20(5):359-371
Non-steroidal anti-inflammatory drugs (NSAIDs) are competitive inhibitors of cyclooxygenase (COX), the enzyme that mediates biosynthesis of prostaglandins and thromboxanes from arachidonic acid. There are at least two different isoforms of the enzyme known as COX-1 and -2. Site directed mutagenesis studies suggest that non-selective COX inhibitors of diverse chemical families exhibit differential binding modes to the two isozymes. These results cannot clearly be explained from the sole analysis of the crystal structures of COX available from X-ray diffraction studies. With the aim to elucidate the structural features governing the differential inhibitory binding behavior of these inhibitors, molecular modeling studies were undertaken to generate atomic models compatible with the experimental data available. Accordingly, docking of different COX inhibitors, including selective and non-selective ligands: rofecoxib, ketoprofen, suprofen, carprofen, zomepirac, indomethacin, diclofenac and meclofenamic acid were undertaken using the AMBER program. The results of the present study provide new insights into a better understanding of the differential binding mode of diverse families of COX inhibitors, and are expected to contribute to the design of new selective compounds. 相似文献