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Zhu J.,Rice University | Cao L.,Rice University | Cao L.,Chongqing University | Wu Y.,Rice University | And 10 more authors.
Nano Letters | Year: 2013

Various two-dimensional (2D) materials have recently attracted great attention owing to their unique properties and wide application potential in electronics, catalysis, energy storage, and conversion. However, large-scale production of ultrathin sheets and functional nanosheets remains a scientific and engineering challenge. Here we demonstrate an efficient approach for large-scale production of V2O5 nanosheets having a thickness of 4 nm and utilization as building blocks for constructing 3D architectures via a freeze-drying process. The resulting highly flexible V 2O5 structures possess a surface area of 133 m2 g-1, ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they are used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g-1 is achieved in a neutral aqueous Na2SO4 electrolyte as the 3D architectures are utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg-1 at a high power density of 9.4 kW kg -1. © 2013 American Chemical Society.

Steinhorst S.,TUM CREATE | Hedrich L.,Goethe University Frankfurt
IEEE/ACM International Conference on Computer-Aided Design, Digest of Technical Papers, ICCAD | Year: 2012

In this paper a novel approach to discrete state space modeling of nonlinear analog circuits is presented, based on the introduction of an underlying discrete analog transition structure (DATS) and the related optimization problem of accurately representing a nonlinear analog circuit with a DATS. Starting from a circuit netlist, a partitioning of the state space to the discrete model is generated parallel and orthogonal to the trajectories of the state space dynamics. Therefore, compared to previous approaches, a significantly higher accuracy of the model is achieved with a lower number of states. The mapping of the partitioning to a DATS enables the application of formal verification algorithms. Experimental validations show the soundness of the approach with an increase in accuracy between a factor of 4 to 10 compared to the state of the art. A model checking case study illustrates the application of the new discretization algorithm to identify a hidden circuit design error. © 2012 ACM.

Zhang G.,TUM CREATE | Zhang G.,Nanyang Technological University | Xia B.Y.,Nanyang Technological University | Xiao C.,Anhui University of Science and Technology | And 4 more authors.
Angewandte Chemie - International Edition | Year: 2013

Tube in a tube: A general strategy for the fabrication of novel complex tube-in-tube nanostructures for many metal oxides has been developed. The method involves coating carbon nanofibers with a layer of metal glycolate followed by calcination in air. The unique complex tubular structures of metal oxides are shown to exhibit promising properties for the title applications. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Maher K.,TUM CREATE | Yazami R.,Nanyang Technological University
Journal of Power Sources | Year: 2014

Lithium ion batteries (LiB) are cycled under a galvanostatic regime (∼C/2-rate) between 2.75 V and 4.2 V for up to 1000 cycles. After each completed 100 cycles, the discharge capacity, capacity loss, average discharge potential were determined under the same C/2 rate. Then cells undergo an additional charge and discharge cycle at C/6 rate followed by a thermodynamics measurements test. This enables open-circuit potential (OCP), entropy (ΔS) and enthalpy (ΔH) data to be assessed. It is found that with increasing cycle number, the entropy and enthalpy profiles show more significant changes than those observed in the discharge and the OCP curves especially at particular SOC and OCP values. These differences are attributed to the higher sensitivity of entropy and enthalpy state functions to changes in the crystal structure of the graphite anode and the lithiated cobalt oxide (LCO) induced by cycle aging compared to the free energy ΔG (OCP) alone. The thermodynamics data are supported by post-mortem X-ray diffractometry (XRD) and Raman scattering (RS) analyses on the electrode materials. The results show important LCO crystal structure degradation, whereas, surprisingly, the graphite anode remains almost unaffected by heavy cycling, if not improved. © 2013 Elsevier B.V. All rights reserved.

Riemann R.,TUM CREATE | Wang D.Z.W.,Nanyang Technological University | Busch F.,TU Munich
Transportation Research Part C: Emerging Technologies | Year: 2015

In this study, the optimal locations of a specific type of charging facilities for electric vehicles (EVs), wireless power transfer facilities, are investigated. A mathematical model has been developed to address this problem. The objective of the model is to locate a given number of wireless charging facilities for EVs out of a set of candidate facility locations for capturing the maximum traffic flow on a network. The interaction between traffic flow patterns and the location of the charging facilities is incorporated explicitly by applying the stochastic user equilibrium principle to describe electric vehicle drivers' routing choice behavior. Firstly, the problem is formulated into a mixed-integer nonlinear program, secondly a solution method is developed to obtain the global optimal solution of the linearized program. Numerical experiments are presented to demonstrate the model validity. © 2015 Elsevier Ltd.

Zhang G.,TUM CREATE | Zhang G.,Nanyang Technological University | Xia B.Y.,Nanyang Technological University | Wang X.,Nanyang Technological University | And 2 more authors.
Advanced Materials | Year: 2014

Strongly coupled NiCo2O4-rGO hybrid nanosheets are syntheiszed through a cost-effective two-step strategy involving a facile polyol process and subsequent thermal annealing treatment in air. The hybrid nanosheets exhibit impressive electrocatalytic performance for the oxygen reduction reaction (ORR) with a current density and onset potential comparable to those of commercial Pt/C catalyts, while having perfect tolerance to methanol. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zhang G.,TUM CREATE | Zhang G.,Nanyang Technological University | Wu H.B.,Nanyang Technological University | Song T.,Hanyang University | And 3 more authors.
Angewandte Chemie - International Edition | Year: 2014

While the synthesis of TiO2 hollow structures is wellestablished, in most cases it is particularly difficult to control the crystallization of TiO2 in solution or by calcination. As a result, TiO2 hollow structures do not really exhibit enhanced lithium storage properties. Herein, we report a simple and costeffective template-assisted method to synthesize anatase TiO2 hollow spheres composed of highly crystalline nanocrystals, in which carbonaceous (C) spheres are chosen as the removable template. The release of gaseous species from the combustion of C spheres may inhibit the growth of TiO2 crystallites so that instead small TiO2 nanocrystals are generated. The small size and high crystallinity of primary TiO2 nanoparticles and the high structural integrity of the hollow spheres gives rise to significant improvements in the cycling stability and rate performance of the TiO2 hollow spheres. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zhang G.,TUM CREATE | Zhang G.,Nanyang Technological University | Lou X.W.,TUM CREATE | Lou X.W.,Nanyang Technological University
Angewandte Chemie - International Edition | Year: 2014

Complex hollow structures of transition metal oxides, especially mixed metal oxides, could be promising for different applications such as lithium ion batteries. However, it remains a great challenge to fabricate well-defined hollow spheres with multiple shells for mixed transition metal oxides. Herein, we have developed a new "penetration-solidification-annealing" strategy which can realize the synthesis of various mixed metal oxide multi-shelled hollow spheres. Importantly, it is found that multi-shelled hollow spheres possess impressive lithium storage properties with both high specific capacity and excellent cycling stability. Specifically, the carbon-coated CoMn2O4 triple-shelled hollow spheres exhibit a specific capacity of 726.7 mAhg-1 and a nearly 100% capacity retention after 200 cycles. The present general strategy could represent a milestone in design and synthesis of mixed metal oxide complex hollow spheres and their promising uses in different areas. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Subiantoro A.,TUM CREATE | Ooi K.T.,Nanyang Technological University
International Journal of Refrigeration | Year: 2013

Expanders have been shown to improve the energy efficiencies of refrigeration systems. The current technology is also adequate to manufacture and integrate expanders to practical air-conditioners. In this paper, an economic analysis of the installation of expanders on to existing vapor compression cooling systems, particularly medium scale air-conditioners, is presented. Various refrigerants, including the established and the newly proposed varieties, are considered. From the investigations, it was found that when the expander efficiency is 50%, the payback periods of most conventional systems are below 3 years in high temperature countries with high electricity tariffs and are above 5 years in other countries. Expanders are especially attractive for the transcritical CO2 and the R404A systems. The payback periods are shorter for systems with highly efficient expanders, high cooling loads, high ambient temperatures and for low refrigerating temperature applications. © 2013 Elsevier Ltd and IIR. All rights reserved.

Lukasiewycz M.,TUM CREATE | Chakraborty S.,TU Munich
CODES+ISSS'12 - Proceedings of the 10th ACM International Conference on Hardware/Software-Codesign and System Synthesis, Co-located with ESWEEK | Year: 2012

This paper presents a methodology for the concurrent optimization of the architecture and scheduling of upcoming synchronous time-triggered automotive systems. A fully synchronous time-triggered system is highly predictable and therefore the best candidate for safety and drive-by-wire functions with strict real-time constraints. While the architecture of these systems has to be optimized in terms of resource allocation, task mapping, and message routing by taking multiple conflicting objectives into account, the scheduling has to be carried out such that application deadlines are satisfied. In case of stringent real-time constraints, available approaches that treat architecture optimization and scheduling as separate problems become inapplicable as most architectures do not permit a feasible schedule. As a remedy, a novel and efficient approach based on conflict refinement is presented. For a given architecture, either a schedule might be obtained or a conflict refinement is performed to determine and exclude the architecture decision that prevents a feasible schedule. In this paper, an extended architecture model is presented and the scheduling and refinement approaches are given for time-triggered architectures based on the FlexRay protocol. This approach can be extended to other protocols and scenarios. A case study of a realistic time-triggered system gives evidence of the efficiency of the proposed approach, solving a large design problem from the automotive domain. Copyright 2012 ACM.

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