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  • 121151.
    Zhang, Feng
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Ai, Yue-Jie
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Fang, Wei-Hai
    Nonadiabatic histidine dissociation of hexacoordinate heme in neuroglobin protein2010In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215Article in journal (Refereed)
    Abstract [en]

    In the present work, density functional theory and canonical nonadiabatic Monte Carlo transition state theory have been used to investigate histidine dissociation process from the hexacoordinate heme in Ngb protein. The potential energy surfaces (PES) of the lowest singlet, triplet, and quintet states are calculated by stepwise optimization along with the histidine dissociation pathway. Based on the calculated two-dimensional PES, the histidine dissociation rates for the spin-forbidden processes via singlet to triplet and singlet to quintet transitions have been calculated by the nonadiabatic Monte Carlo transition state theory in canonical ensemble. The present study provides a quantitative description on spin-forbidden histidine dissociation processes.

  • 121152.
    Zhang, Feng
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ai, Yue-Jie
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Fang, Wei-Hai
    Beijing Normal University.
    Nonradiative decay of the lowest excited singlet state of 2-aminopyridine is considerably faster than the radiative decay2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, no 14, p. 144315-Article in journal (Refereed)
    Abstract [en]

    Ab initio calculations reveal that radiative lifetime of the lowest excited singlet state of 2-aminopyridine (2AP) molecule should be around 20 ns, consistent with the molecules of the same type, but is about one order of magnitude larger than the claimed experimental fluorescent lifetime in recent years. A S1/S0 conical intersection close to the S1 state has been located which could be the possible nonradiative channel that is responsible for the fast decay observed in the experiment.

  • 121153.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gustavsson, Katarina
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Numerical Analysis, NA. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Near-Wall Convection in a Sedimenting Suspension of Fibers2014In: AIChE Journal, ISSN 0001-1541, E-ISSN 1547-5905, Vol. 60, no 12, p. 4253-4265Article in journal (Refereed)
    Abstract [en]

    The sedimentation of a fiber suspension near a vertical wall is investigated numerically. Initially, the near-wall convection is an upward backflow, which originates from the combined effects of the steric-depleted layer and a hydrodynamically depleted region near the wall. The formation of the hydrodynamically depleted region is elucidated by a convection-diffusion investigation, in which fibers are classified according to the different directions in which they drift. For fibers with sufficiently large aspect ratio, the initial near-wall backflow keeps growing. However, the backflow reverses to downward flow at later times if the aspect ratio is small. This is due to the fiber-wall interactions which rotate fibers to such angles that make fibers drift away from the wall, inducing a dense region and a correspondingly downward flow outside the initial backflow. Moreover, the steric-depleted boundary condition is of secondary importance in the generation and evolution of the near-wall convection.

  • 121154.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Evolution of streamers in sedimentation of fibre suspensions bounded by vertical wallsManuscript (preprint) (Other academic)
    Abstract [en]

    The simulation, based on the Navier-Stokes equations coupled to a transport equation for the PDF of fibres, shows that a series of alternating structures of risers and streamers emerge continuously from the walls until they meet in the middle of the domain. For moderate times, this agrees qualitatively with experimental and theoretical results. Moreover, our simulation in a vessel of infinite height obtained an increasing wavelength evolution due to the congregation of the streamers or risers. In the end, there is constantly only one streamer left, and it drifts randomly to one side of the container until the evolution reaches a steady state. It is also found that the perturbations added to the initial conditions can induce more high density regions which sizes and velocities are strongly linked to the initial perturbations of the number density or the flow field. In addition, the maximum number of streamers increases with Reynolds number, volumefraction and channel width.

  • 121155.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Non-linear disturbance growth during sedimentation in dilute fibre suspensions2013In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 719, p. 268<-294Article in journal (Refereed)
    Abstract [en]

    Disturbances in a dilute fibre suspension is studied with an Eulerian approach. Based on a linear stability analysis, it is shown that inertia and hydrodynamic diffusion damp perturbations at long wavelengths and short wavelengths, respectively, leading to a wavenumber selection. For small, but finite Reynolds number of the fluid bulk motion, the most unstable wavenumber is a finite value which increases with Reynolds number, and where the diffusion narrows the range of unstable wavenumbers. Numerical simulations of the full non-linear evolution in time of a normal mode perturbation show that the induced flow may either die or saturate on a finite amplitude. The character of this long time behaviour is dictated by the wavenumber and the presence or absence of the translational and rotational diffusivities.

  • 121156.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Dayarathn, Rasika
    Stockholm University.
    Is Your Email Box Safe?2010In: Journal of Information Privacy & Security, ISSN 1553-6548, Vol. 6, no 1, p. 28-52Article in journal (Refereed)
    Abstract [en]

    Electronic mail (email) is a widely adopted communication mechanism often used to communicate sensitive and confidential information. Therefore, safeguarding the security of email accounts and letters has become an important issue. There are frequent media reports pertaining to security problems with email accounts. Therefore, studies on strengths, limitations, and possible improvements about email security are essential. This paper examined security and privacy protection mechanisms of four leading email service providers: Gmail, Yahoo Mail, Hotmail, and AOL Mail. A number of observations and experiments were conducted in order to understand existing security and privacy protection mechanisms of these providers. After that, this paper proposes some recommended protection mechanisms, which can be implemented by service providers, system developers, and email users. This study also explores several research avenues for academia.

  • 121157.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gustavsson, Katarina
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Near-wall convection in a sedimenting suspension of fibresManuscript (preprint) (Other academic)
  • 121158.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Kondoro, Aron
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Muftic, Sead
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Location-based authentication and authorization using smart phones2012In: Proc. of the 11th IEEE Int. Conference on Trust, Security and Privacy in Computing and Communications, TrustCom-2012 - 11th IEEE Int. Conference on Ubiquitous Computing and Communications, IUCC-2012, IEEE , 2012, p. 1285-1292Conference paper (Refereed)
    Abstract [en]

    Authentication and authorization are two of the most important security features for mobile transaction systems. Most commonly, these schemes depend on three factors: what you know (secret), what you have (token), and what you are (biometrics). In this paper, we propose a location-based authentication and authorization scheme for mobile transactions using smart phones. The paper first describes the distinguished features and the architecture of our proposed solution. Second, the core of our design, including three parts: location registration, authentication and authorization as well as location verification, are described.

  • 121159.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Kounelis, Ioannis
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Muftic, Sead
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Generic, secure and modular (GSM) methodology for design and implementation of secure mobile applications2012In: SECURWARE 2012 - 6th International Conference on Emerging Security Information, Systems and Technologies, Rome, Italy: International Academy, Research and Industry Association (IARIA), 2012, p. 1-6Conference paper (Refereed)
    Abstract [en]

    The generic, secure and modular methodology, described in this paper, provides a generic approach for the design and development of secure mobile applications. It is applicable to multiple mobile phone platforms and mobile operating environments. This approach treats a mobile application in a holistic way and structures it into four groups of modules: user interface modules, communication modules, security modules, and business logic modules. These four groups of modules can be designed and implemented independently and finally be integrated together. This approach not only simplifies the process of design and development of mobile applications, but also improves the reusability and robustness of mobile applications. In addition, this paper proposes a trusted layer model for designing the security modules of mobile applications, which provides generic application interfaces and comprehensive data protection. The paper finally gives an example of a secure mobile application, called SAFE Mobile Wallet, which was designed and implemented using GSM methodology.

  • 121160.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Effects of walls and perturbations on the sedimentation in fibre suspensionsManuscript (preprint) (Other academic)
  • 121161.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Nonlinear interactions of multiple modes and spectral analysis of a suspension of settling fibresManuscript (preprint) (Other academic)
  • 121162.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Muftic, Sead
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Experiences on Mobile-ATM Deployment in a Developing Country2008In: Experiences on Mobile-ATM Deployment in a Developing Country, Karlstad: Karlstad University Studies , 2008, p. 178-Conference paper (Refereed)
    Abstract [en]

    Mobile-Commerce is the latest concept of enabling the financial transactions on mobile phones and hand-held devices. With the rapid development of the society, the M-Commerce applications play a vital role. Mobile-ATM is one such application, enabling the banking services on mobile phones. Even though users have a poor computer literacy, they will be able to use the Mobile-ATM system easily. This kind of application is very useful, especially in rural areas, where accessing financial and banking services is a critical issue due to the distance barriers. Hence this paper discusses the social, economical and technical impact of the Mobile-ATM system, which is developed by the authors. Moreover the paper points out the essential value added services provided by our system with respect to financial transactions services such as security and confidentiality. Although the Mobile-ATM is technically feasible and practically deployed, it is important to have community acceptance. This paper discusses the community acceptance of this system and related issues.

  • 121163.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Muftic, Sead
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    SAFE SYSTEM: SECURE APPLICATIONS FOR FINANCIAL ENVIRONMENTS USING MOBILE PHONES2009In: SAFE SYSTEM: SECURE APPLICATIONS FOR FINANCIAL ENVIRONMENTS USING MOBILE PHONES, Barcelona, Spain, 2009, p. 387-392Conference paper (Refereed)
    Abstract [en]

    Mobile financial transactions are used by more and more people due to a widespread proliferation of mobile phones and wireless technologies. One of the most important concerns with such transactions is their security. The reasons are based on weaknesses of wireless protocols and handling of financial data. These aspects make mobile financial applications even more vulnerable to fraud and illegal use than similar transactions performed over fixed networks. Therefore, one of the main prerequisites for successful, large-scale and broad deployment of mobile financial applications is their security. This paper introduces the concept of SAFE system (Secure Applications for Financial Environment) that represents a secure, convenient and reliable infrastructure for mobile financial transactions. The infrastructure comprises Mobile Wallet, three servers: Gateway, IDMS and Bank servers, security protocols, and messages between all components.

  • 121164.
    Zhang, Feng
    et al.
    KTH, School of Information and Communication Technology (ICT), Communication: Services and Infrastucture, Telecommunication Systems Laboratory, TSLab.
    Muftic, Sead
    KTH, School of Information and Communication Technology (ICT), Communication: Services and Infrastucture, Telecommunication Systems Laboratory, TSLab.
    Schmöelzer, Gernot
    Secure Service-Oriented Architecture for Mobile Transactions2011In: World Congress on Internet Security (WorldCIS-2011), London: IEEE Communications Society, 2011, p. 133-138Conference paper (Refereed)
    Abstract [en]

    The paper describes secure service-oriented architecture for mobile transactions. The architecture comprises components, protocols, applications and interfaces and it provides various security services to various mobile applications: registration, certification, authentication, and authorization of users, secure messaging at an application–level (end–to–end security), protection of data in databases, and security services for protection of its own components. The architecture is modular, integrated, extendible and scalable. The paper describes design of the architecture, the status of its current implementation, and future research and development plans.

  • 121165.
    Zhang, Feng
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    SARDINA, GAETANO
    Dahlkild, Anders A.
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Lundell, Fredrik
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Brandt, Luca
    Numerical simulations of the sedimentationin dilute fibre suspensions2013Conference paper (Other academic)
  • 121166.
    Zhang, Feng
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Wei-Hai, Fang
    Yi, Luo
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Ruo-Zhuang, Liu
    A multidimensional microcanonical Monte Carlo study of S0-T1 intersystem crossing of isocyanic acid2009In: Science in China Series B: Chemistry, ISSN 1006-9291, E-ISSN 1862-2771, Vol. 52, no 11, p. 1885-1891Article in journal (Refereed)
    Abstract [en]

    A general formula for multi-dimensional Monte Carlo microcanonical nonadiabatic rate constant expressed in configuration space is applied to calculate the rate of intersystem crossing (ISC) between the ground (S0) and first excited triplet (T1) states for isocyanic acid. One-, two- and three-dimensional potential energy surfaces are constructed by the CCSD calculations, which are used for Monte Carlo sampling. The calculated S0→T1 ISC rate is in good agreement with experimental findings, which gives us reason to expect that the multi-dimensional Monte Carlo microcanonical nonadiabatic rate theory is one of very effective methods for calculating nonadiabatic transition rate of a polyatomic molecule.

  • 121167.
    Zhang, Fuguo
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Organic chemistry.
    Cong, Jiayan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Li, Yuanyuan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Bergstrand, Jan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Liu, Haichun
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Cai, Bin
    State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT).
    Hajian, Alireza
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Yao, Zhaoyang
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Organic chemistry.
    Wang, Linqin
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Organic chemistry.
    Hao, Yan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Yang, Xichuan
    State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT).
    Gardner, James M.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Ågren, Hans
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Widengren, Jerker
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.
    Kloo, Lars
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Sun, Licheng
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry. State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT).
    A facile route to grain morphology controllable perovskite thin films towards highly efficient perovskite solar cells2018In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 53, p. 405-414Article in journal (Refereed)
    Abstract [en]

    Perovskite photovoltaics have recently attracted extensive attention due to their unprecedented high power conversion efficiencies (PCEs) in combination with primitive manufacturing conditions. However, the inherent polycrystalline nature of perovskite films renders an exceptional density of structural defects, especially at the grain boundaries (GBs) and film surfaces, representing a key challenge that impedes the further performance improvement of perovskite solar cells (PSCs) and large solar module ambitions towards commercialization. Here, a novel strategy is presented utilizing a simple ethylammonium chloride (EACl) additive in combination with a facile solvent bathing approach to achieve high quality methyammonium lead iodide (MAPbI3) films. Well-oriented, micron-sized grains were observed, which contribute to an extended carrier lifetime and reduced trap density. Further investigations unraveled the distinctively prominent effects of EACl in modulating the perovskite film quality. The EACl was found to promote the perovskite grain growing without undergoing the formation of intermediate phases. Moreover, the EACl was also revealed to deplete at relative low temperature to enhance the film quality without compromising the beneficial bandgap for solar cell applications. This new strategy boosts the power conversion efficiency (PCE) to 20.9% and 19.0% for devices with effective areas of 0.126 cm2 and 1.020 cm2, respectively, with negligible current hysteresis and enhanced stability. Besides, perovskite films with a size of 10 × 10 cm2, and an assembled 16 cm2(5 × 5 cm2 module) perovskite solar module with a PCE of over 11% were constructed.

  • 121168. Zhang, Fuguo
    et al.
    Yang, Xichuan
    Cheng, Ming
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry. KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
    Li, Jiajia
    Wang, Weihan
    Wang, Haoxin
    Sun, Licheng
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry. KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD. Dalian Univ Technol, China.
    Engineering of hole-selective contact for low temperature-processed carbon counter electrodebased perovskite solar cells2015In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 48, p. 24272-24280Article in journal (Refereed)
    Abstract [en]

    A cost-effective and solution processable hole transport material (HTM), TPDI (5,10,15-triphenyl-5H-diindolo[3,2-a:3',2'-c]carbazole), was synthesized and explored as a hole selective contact material in low temperature (100 degrees C) and printable processed carbon counter electrode based perovskite solar cells (PSCs) for the first time. This material demonstrated excellent thermal stability, high hole mobility and appropriate energy level alignment with CH3NH3PbI3 and carbon, which make it a potentially excellent alternative interfacial material for PSCs. By interfacial engineering with doped TPDI, the energy barrier at the CH3NH3PbI3/carbon interface was efficiently eliminated. Dramatically enhanced power conversion efficiency (PCE) of 15.5% was afforded, which is comparable to or even better than that of the reference device with 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) as HTM under equivalent conditions. Besides, TPDI can also function well in its pristine form although the efficiency (13.6%) obtained is slightly lower than that with the device containing doped TPDI as the HTM. Moreover, these newly integrated noble metal-free, vacuum-free and cost effective PSCs exhibited excellent durability during the long term stability measurements for 30 days. The remarkable performance as well as dramatically reduced fabrication cost demonstrated by integrating TPDI as the HTM and cost effective commercial carbon as the cathode revealed their great potential in the scalable and practical application of PSCs.

  • 121169.
    Zhang, Fuguo
    et al.
    DUT-KTH Joint Education and Research Center on Molecular Devices, China.
    Yang, Xichuan
    DUT-KTH Joint Education and Research Center on Molecular Devices, China.
    Wang, Haoxin
    DUT-KTH Joint Education and Research Center on Molecular Devices, China.
    Cheng, Ming
    DUT-KTH Joint Education and Research Center on Molecular Devices, China.
    Zhao, Jianghua
    DUT-KTH Joint Education and Research Center on Molecular Devices, China.
    Sun, Licheng
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Organic Chemistry. KTH, School of Chemical Science and Engineering (CHE), Centres, Centre of Molecular Devices, CMD.
    Structure Engineering of Hole-Conductor Free Perovskite-Based Solar Cells with Low-Temperature-Processed Commercial Carbon Paste As Cathode2014In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 6, no 18, p. 16140-16146Article in journal (Refereed)
    Abstract [en]

    Low-temperature-processed (100 degrees C) carbon paste was developed as counter electrode material in hole conductor free perovskite/TiO2 heterojunction solar cells to substitute noble metallic materials. Under optimized conditions, an impressive PCE value of 8.31% has been achieved with this carbon counter electrode fabricated by doctor-blading technique. Electrochemical impedance spectroscopy demonstrates good charge transport characteristics of low-temperature-processed carbon counter electrode. Moreover, this carbon counter electrode-based perovskite solar cell exhibits good stability over 800 h.

  • 121170. Zhang, G.
    et al.
    Klejsa, Janusz
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Optimal index assignment for multiple description scalar quantization with translated lattice codebooks2012In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 60, no 8, p. 4444-4451Article in journal (Refereed)
    Abstract [en]

    We design a K -description scalar quantizer, whose construction is based on a structure of translated scalar lattices and a lattice in K-1 dimensional space. The use of translated lattices provides a performance advantage by exploiting a so-called staggering gain. The use of the K-1 dimensional lattice facilitates analytic insight into the performance and significantly speeds up the computation of the index assignment compared to state-of-the-art methods. Using a common decoding method, the proposed index assignment is proven to be optimal for the K-description case. It is shown that the optimal index assignment is not unique. This is illustrated for the two-description case, where a periodic index assignment is selected from possible optimal assignments and described in detail. The performance of the proposed quantizer accurately matches theoretic analysis over the full range of operational redundancies. Moreover, the quantizer outperforms the state-of-the-art MD scheme as the redundancy among the description increases.

  • 121171.
    Zhang, Guoqiang
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    On the rate region of the vector gaussian one-helper distributed source-coding problem2011In: 2011 DATA COMPRESSION CONFERENCE (DCC), 2011, p. 263-272Conference paper (Refereed)
    Abstract [en]

    In this paper we consider the rate region of the vector Gaussian one-helper distributed source coding problem. In particular, we derive optimality conditions under which a weighted sum rate is minimum by using a contradiction-based argument. When the sources are specified to be scalar, the optimality conditions can always be constructed for any weighted sum rate. In the derivation of the optimality conditions, we introduce a new concept of "source enhancement", which can be viewed as a dual to the well-known "channel enhancement" technique. In particular, source enhancement refers to the operation of increasing the covariance matrix of a Gaussian source in a partial ordering sense. This new technique makes the derivation of the optimality conditions straightforward.

  • 121172.
    Zhang, Guoqiang
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Robust Multimedia Communications over Packet Networks2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Multimedia communications over packet networks, and in particular the voice over IP (VoIP) application, have become an integral part of society. However, the unreliable and heterogeneous nature of packet networks has led to a best-effort delivery of services. Delay, limitation of bandwidth, and packet-loss rate all affect the quality of service (QoS). In this thesis, we address two important network impairments in the design of robust multimedia communication systems: packet delay-variation and packet-loss.

    Paper A considers the mitigation of the effect of packet delay-variation for audio communications by introducing a buffer at the receiver side. A new adaptive playout scheduling approach is proposed to control the buffering length, or, equivalently, the packet playout deadlines, in response to varying network conditions. A Wiener process is used to model the fluctuation of the buffering length without any playout adjustment. The playout scheduling problem is then reformulated as a stochastic impulse control problem by taking the playout adjustment as the control signal. The proposed approach is shown to be the optimal solution to the new control problem. It is demonstrated experimentally that the proposed approach provides improved perceived conversional quality.

    Papers B, C and D address the packet-loss issue. Paper B focuses on the design of a low-complexity packet-loss concealment (PLC) method that is compatible with existing speech codecs for VoIP application. The new method is rigorously motivated based on the autoregressive (AR) speech model and the minimum mean squared error (MMSE) criterion. The effect of model estimation error on the prediction of the missing speech segment is also considered and an upper bound for the prediction error is derived. Both the theoretical and experimental results provide insight in the performance of the heuristically designed PLC methods. On the other hand, Paper C and D consider an active packet-loss-resilient coding scheme, namely multiple description coding (MDC). In general, MDC can be used for the transmission of any media data. Paper C derives a simple and accurate approximation of the rate-distortion lower bound of a particular multiple- description scenario and then demonstrates that the performance loss of some practical MD systems can be evaluated easily with the new approximation. Paper D studies the performance limit of a vector Gaussian multiple description scenario. An outer bound to the rate-distortion region is derived, and the outer bound is tight when the problem specializes to the scalar Gaussian case.

     

  • 121173.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    BOUNDING THE RATE REGION OF THE TWO-TERMINAL VECTOR GAUSSIAN CEO PROBLEM2011In: 2011 DATA COMPRESSION CONFERENCE (DCC), 2011, p. 488-488Conference paper (Refereed)
    Abstract [en]

    The rate region of the two-terminal vector Gaussian CEO problem is studied. A lower bound on the rate region is derived. It is obtained by lower-bounding a weighted sum rate for each supporting hyperplane of the rate region. The bound is in the form of a closed-form expression rather than the form of an optimization problem.

  • 121174.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Østergaard, Jan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Bounding the Rate Region of Vector Gaussian Multiple Descriptions with Individual and Central Receivers2010In: Data Compression Conference Proceedings, 2010, p. 13-19Conference paper (Refereed)
    Abstract [en]

    In this work, we study the rate region of the vector Gaussian multipledescription problem with individual and central quadratic distortion con-straints. In particular, we derive an outer bound to the rate region of theL-description problem. The bound is obtained by lower bounding a weightedsum rate for each supporting hyperplane of the rate region. The key ideais to introduce at most L-1 auxiliary random variables and further imposeupon the variables a Markov structure according to the ordering of the de-scription weights. This makes it possible to greatly simplify the derivationof the outer bound. In the scalar Gaussian case, the complete rate regionis fully characterized by showing that the outer bound is tight. In this case,the optimal weighted sum rate for each supporting hyperplane is obtained bysolving a single maximization problem. This contrasts with existing results,which require solving a min-max optimization problem.

  • 121175.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Klejsa, Janusz
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    ANALYSIS OF K-CHANNEL MULTIPLE DESCRIPTION QUANTIZATION: 2009 DATA COMPRESSION CONFERENCE, PROCEEDINGS2008In: DCC 2009: 2009 DATA COMPRESSION CONFERENCE, PROCEEDINGS / [ed] Storer JA, Marcellin MW, LOS ALAMITOS, CA: IEEE COMPUTER SOC , 2008, p. 53-62Conference paper (Refereed)
    Abstract [en]

    This paper studies the tight rate-distortion bound for K-channel symmetric multiple-description coding for a memoryless Gaussian source. We find that the product of a function of the individual side distortions (for single received descriptions) and the central distortion (for K received descriptions) is asymptotically independent of the redundancy among the descriptions. Using this property, we analyze the asymptotic behaviors of two different practical multiple-description lattice vector quantizers (MDLVQ). Our analysis includes the treatment of a MDLVQ system from a new geometric viewpoint, which results in an expression for the side distortions using the normalized second moment of a sphere of higher dimensionality than the quantization space. The expression of the distortion product derived from the lower bound is then applied as a criterion to assess the performance losses of the considered MDLVQ systems.

  • 121176.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lundin, Henrik
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Band control policy of playout scheduling for voice over IP2008In: Proceedings European Signal Processing Conference, 2008Conference paper (Refereed)
    Abstract [en]

    We study adaptive-playout scheduling for VoIP using the framework of stochastic impulse control theory. A Wiener process is introduced to model the fluctuation of the buffer length in the absence of control. In this context, the control signal consists of length units that correspond to inserting or dropping a pitch cycle. We define an optimality criterion that has an adjustable trade-off between average buffing delay and average control length (the length of the pitch cycles added plus the length of the pitch cycles dropped). The clock-drift effect is treated in a unified manner within this framework. A band control policy is shown to be optimal. The algorithm does not require knowledge of the clock drift. It maintains the buffer length within a band region by imposing impulse control (inserted or dropped pitch cycles) whenever the bounds of the band are reached. Our experiments show that the proposed method outperforms a popular reference method.

  • 121177. Zhang, Guoqiang
    et al.
    Yan, Jinyue
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Jin, Hongguang
    Dahlquist, Erik
    Integrated Black Liquor Gasification Polygeneration System with CO2 Capture in Pulp and Paper Mills to Produce Methanol and Electricity2011In: International Journal of Green Energy, ISSN 1543-5075, E-ISSN 1543-5083, Vol. 8, no 2, p. 275-293Article in journal (Refereed)
    Abstract [en]

    Based on KAMa pulp and paper mill, a polygeneration system integrated with a black liquor gasifier is proposed. The effects of CO2 captured by oxygen-fuel combustion and Selexol absorption on the performance of the polygeneration system are studied in terms of both thermodynamic performance and cost assessment. Using the Aspen Plus simulator, the performance of the studied polygeneration systems are analyzed from the perspectives of the first and second laws of thermodynamics. Compared with the reference system, the first law efficiency of the polygeneration system increased from 15.7% to 29.3%, with an investment increment of 17.9%. The investment incremental rates for CO2 capture by oxyfuel combustion and Selexol absorption are 15.1% and 16.7%, respectively. Energy penalty due to CO2 capture and compression is 0.61 MJ electricity/kg CO2, avoided in the oxygen-fuel method at a cost of $29.6/tonne CO2. However, energy penalty can reach 1.03 MJ product (electricity and methanol) per kg CO2, avoided in the Selexol absorption CO2 capture process at a cost of $46.0/tonne CO2.

  • 121178.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Østergaard, Jan
    Klejsa, Janusz
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    High-Rate Analysis of Symmetric L-Channel Multiple Description Coding2011In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 59, no 7, p. 1846-1856Article in journal (Refereed)
    Abstract [en]

    This paper studies the tight rate-distortion bound for L-channel sym-metric multiple-description coding of a vector Gaussian source with twolevels of receivers. Each of the first-level receivers obtains κ (κ < L) ofthe L descriptions. The second-level receiver obtains all L descriptions.We find that when the theory is applied to the scalar Gaussian source, theproduct of a function of the side distortions (corresponding to the first-level receivers) and the central distortion (corresponding to the second-levelreceiver) is asymptotically independent of the redundancy among the de-scriptions. Using this property, we analyze the asymptotic behavior of apractical multiple-description lattice vector quantizer (MDLVQ). Our anal-ysis includes the treatment of the MDLVQ system from a new geometricviewpoint, which results in an expression for the side distortions using thenormalized second moment of a sphere of higher dimensionality than thequantization space. The expression of the distortion product derived fromthe lower bound is then applied as a criterion to assess the performance lossof the considered MDLVQ system. In principle, the efficiency of other prac-tical MD systems can also be evaluated using the derived distortion product.

  • 121179. Zhang, Guowu
    et al.
    Zhang, Junwei
    Hong, Xuezhi
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Low-complexity frequency domain nonlinear compensation for OFDM based high-speed visible light communication systems with light emitting diodes2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 4, p. 3780-3794Article in journal (Refereed)
    Abstract [en]

    A novel frequency domain nonlinear compensation method, FD-NC, is proposed for orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. By tackling the memory nonlinear impairments from light emitting diodes (LEDs) in the frequency domain rather than in the time domain, the proposed method has much lower computational complexity than the conventional time domain Volterra nonlinear compensation method (TD-NC). Both theoretical derivation and experimental investigation of the proposed method in OFDM based VLC systems with four types of commercial LEDs are presented. The results of experiments show that the proposed low-complexity FD-NC method with a moderate truncation factor achieves a performance comparable to that of the TD-NC. The application of FD-NC method in the bit-power loading OFDM VLC system is also experimentally demonstrated. Compared with the linear equalization case, at a bit error rate (BER) of 3.8 x 10(-3) (a), the transmission distance of a 960 Mbps VLC system can be extended from 0.7 m to 1.8 m by the FD-NC, and (b) the achievable system capacity can be enhanced by 18.7%similar to 36.5% for transmission distance in the range of 0.5 m similar to 2 m with the FD-NC. The complexity analysis shows that the required number of real-valued multiplications (RNRM) of the FD-NC is independent of linear or nonlinear memory length. The reduction of RNRM achieved by the FD-NC over the TD-NC becomes more profound for a larger nonlinear memory length or a smaller truncation factor.

  • 121180. Zhang, H.
    et al.
    Gandin, C. -A
    He, J.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Prediction of solidification path and carbide precipitation in Fe-C-V-Cr-Mo-W high speed steels2012In: MCWASP XIII: International Conference On Modeling Of Casting, Welding And Advanced Solidification Processes, Institute of Physics (IOP), 2012, no 1, p. 012061-Conference paper (Refereed)
    Abstract [en]

    The solidification path and precipitation of carbides in the Fe-C-V-Cr-Mo-W high speed steel system are predicted with the help of thermodynamic equilibrium calculations. The Partial Equilibrium (PE) approximation is favoured. According to experimental data for high speed steel samples, the precipitating solidification sequence of carbides, including nature, composition and amount are discussed as a function of the nominal composition of C and V. The results show that the solidification path can be reasonably predicted by the Partial Equilibrium approximation for cooling rate lower than 10 K min -1. The experimental results suffer from the sensitivity limitation of the characterization methods used when the phase fraction becomes too small.

  • 121181. Zhang, H.
    et al.
    Gandin, C. -A
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    He, J.
    A multiphase segregation model for multicomponent alloys with a peritectic transformation2012In: MCWASP XIII: International Conference On Modeling Of Casting, Welding And Advanced Solidification Processes, 2012, no 1, p. 012063-Conference paper (Refereed)
    Abstract [en]

    A multiphase microsegregation model for the solidification of multicomponent alloys that experience a peritectic transformation is developed. It is based on a volume averaging method. Coupling is achieved among the conservation equations for total mass, solute mass and energy. The diffusion in extradendritic and several interdendritic liquid and solid phases, the growth kinetics of the dendritic and peritectic solidifying microstructures, as well as the velocities of the solid/liquid and solid/solid phase interfaces are considered in the model. The model is applied to the dendritic and peritectic solidification of a Fe-C-Cr alloy. Equilibrium between phases at the interface is taken into account and computed using a thermodynamic software. The occurrence of several recalescences due to the growth of the microstructure and the peritectic transformation are predicted and the solidification behaviors near recalescence are evaluated. By adjusting the parameters, the lever rule (LR), Gulliver-Scheil (GS) and partial equilibrium (PE) approximations are retrieved.

  • 121182. Zhang, H.
    et al.
    Hasler, J. -P
    Johansson, N.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Nee, Hans-Peter
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Frequency response improvement with synchronous condenser and power electronics converters2017In: 2017 IEEE 3rd International Future Energy Electronics Conference and ECCE Asia, IFEEC - ECCE Asia 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, p. 1002-1007, article id 7992178Conference paper (Refereed)
    Abstract [en]

    This paper gives an overall review of the working principle of the inertial response from the synchronous condenser for frequency support in the grid. In addition to this, four proposed frequency control strategies for improvement of network frequency response by static energy storage devices, such as the Static Synchronous Compensator (STATCOM) with suitable energy storage elements or the battery energy storage system (BESS), are presented. The proposed frequency control strategies are evaluated and benchmarked with the synchronous condenser in the same network. It is shown that a power electronic converter based system with energy storage can be designed with lower rating, still providing the same or better frequency support than the synchronous condenser.

  • 121183. Zhang, H.
    et al.
    He, Sailing
    Chen, P.
    Sun, W.
    Inverse problems for three-dimensional localization of an inhomogeneity in a stratified scattering medium by using a weighted Fourier transform2001In: Wuli xuebao, ISSN 1000-3290, Vol. 50, no 8, p. 1481-1485Article in journal (Refereed)
  • 121184. Zhang, H.
    et al.
    Li, X.
    Liu, X.
    Yan, Jinyue
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes.
    Enhancing fuel cell durability for fuel cell plug-in hybrid electric vehicles through strategic power management2019In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, p. 483-490Article in journal (Refereed)
    Abstract [en]

    Fuel cell plug-in hybrid electric vehicles (FC-PHEVs) can have extended range while utilizing cheap grid electricity, but has poor durability of onboard fuel cells due to dynamic loading. In this study, fuel cell durability is enhanced significantly for a novel configuration of FC-PHEVs with three fuel cell stacks through strategic power management by making each fuel cell stack work only at a fixed operating point (i.e., constant output power) and by shortening its active time (operation) via on-off switching control. A hysteresis control strategy of power management is designed to make the active time evenly distributed over the three fuel cell stacks and to reduce the number of on-off switching. The results indicate that the durability of the onboard fuel cells can be increased 11.8, 4.8 and 6.9 times, respectively, for an urban, highway and a combined urban-highway driving cycle. This enhanced fuel cell durability is derived from the fact that the average power demand of real-time driving cycles is only a fraction of the maximum power that FC-PHEVs could provide, and substantially increased durability can be used to reduce the over-design, hence the cost, of fuel cells.

  • 121185. Zhang, H.
    et al.
    Li, Z. C.
    Bergman, Bill
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
    Zou, X. D.
    Investigation of La9.33Si6O26 oxygen ionic conductor2007In: Journal of Materials Science & Technology, ISSN 1005-0302, Vol. 23, no 5, p. 629-632Article in journal (Refereed)
    Abstract [en]

    La9.33Si6O26 oxygen ionic conductor was synthesized by solid state reaction method. Its structure was determined by single-crystal X-ray diffraction analysis at room temperature. The results showed that La9.33Si6O26 oxide has the apatite structure with space group P6(3)/m. AC impedance measurements indicated that the oxides sintered in nitrogen have much higher conductivity than those sintered in air. The effects of grain boundaries on the conductivity were discussed.

  • 121186. Zhang, H.
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Su, M.
    Shibata, H.
    Hedström, Peter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Wang, W.
    Lei, H.
    Wang, Q.
    Jönsson, Pär
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Processing.
    He, J.
    Prediction of Influences of Co, Ni, and W Elements on Carbide Precipitation Behavior in Fe–C–V–Cr–Mo Based High Speed Steels2018In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 89, no 10, article id 1800172Article in journal (Refereed)
    Abstract [en]

    The effects of Co, Ni together with W addition on the precipitation sequence, amount, and composition of carbides and FCC matrix in Fe–C–V–Cr–Mo based alloys are investigated with the help of Partial Equilibrium (PE) approximation and thermodynamic calculations as well as differential scanning calorimetry (DSC) and electron backscatter diffraction (EBSD) - energy dispersive spectrometer (EDS) analyses. Results show that, individually, Co and Ni elements strengthen the matrix by their great solubility in FCC matrix; W element enlarges the hardness of the alloy through benefiting the formation of M6C carbide. Mutually, the addition of Co and Ni together with W increases the precipitation temperature of the eutectic carbides, although the addition of Co and Ni itself exerts little influence on the nature (type, amount, and composition) of the carbides. These predictions combined with the experimental verifications provide potentials for the alloy design and the property control in high speed steels.

  • 121187. Zhang, H.
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Wang, E.
    He, J.
    Modeling of macrosegregation and solidification microstructure for Al-Si alloy under unidirectional solidification by a coupled cellular automaton-finite volume model2012In: MCWASP XIII: International Conference On Modeling Of Casting, Welding And Advanced Solidification Processes, Institute of Physics (IOP), 2012, no 1, p. 012093-Conference paper (Refereed)
    Abstract [en]

    A coupled Cellular Automaton-Finite Volume (CA-FV) model is developed for the prediction of macrosegregation and solidification microstructure of an Al-Si binary alloy under unidirectional solidification. The enthalpy, concentration and velocities are calculated at FV scale following macroscopic fluid flow, heat and solute transfer equations. The temperature and solid fraction are updated at CA scale from both the energy balance and the nucleation, grain growth under fluid flow and entrapment procedure with the help of microsegregation approximation. The variables at FV and CA sites exchange information through topological relations. The recalescence and the inter- and intra-granular segregation during solidification are predicted. The effects of thermal/solutal buoyancy and the grain structure on molten natural convection and the segregation pattern are also analyzed. Verification of the model has been done using the previous experimental solidification morphologies by other researchers.

  • 121188. Zhang, H. -Q
    et al.
    Akram, N.
    Skog, P.
    Soroka, Inna
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.
    Trautmann, C.
    Schuch, R.
    Tailoring of keV-ion beams by image charge when transmitting through rhombic and rectangular shaped nanocapillaries2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 19, p. 193202-Article in journal (Refereed)
    Abstract [en]

    We report on an unexpected effect of tailoring transmission profiles of Ne7 + ions through nanocapillaries of rhombic and rectangular cross sections in mica. We find that capillaries of rhombic cross sections produce rectangular shaped ion transmission profiles and, vice versa, that capillaries of rectangular geometry give a rhombic beam shape. This shaping effect only occurs for transmitted ions and is absent for the small fraction of neutralized particles. The experimental findings and simulations of the projectile trajectories give clear evidence that the observed effect is due to the image forces experienced by the transmitting ions. This novel beam shaping mechanism suggests applications for the guiding, focusing, and shaping of ion beams.

  • 121189. Zhang, H. Q.
    et al.
    Akram, N.
    Soroka, Inna L.
    Physics Department, Stockholm University, Sweden.
    Trautmann, C.
    Schuch, R.
    Transmission of highly charged ions through mica nanocapillaries of rhombic cross section2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, no 2, p. 022901-Article in journal (Refereed)
    Abstract [en]

    The angular distributions of Ne7 + ions transmitted at various kinetic energies of 7-70 keV through muscovite mica nanocapillaries of rhombic cross section are measured. It is found that the transmitted ion beams form a rectangular shape at tilt angles that are small compared to those given by the aspect ratio, i.e., the capillary geometrical opening angle. This shape is retained for all ion energies, but its size changes. The time evolution of the transmitted angular distributions shows that the characteristic profile occurs instantaneously and remains in the stationary state of the transmission, whereas it shifts with the increase of the accumulated incident charge. At tilt angles of the capillaries larger than their aspect ratio, the shape in the transmission profile gets distorted from the rectangle by the deposited charge. Combined with trajectory simulations we show the observed shaping effect is due to the image force seen by the ions, interplaying with a deflection by the deposited charge on the capillary walls.

  • 121190. Zhang, H.
    et al.
    Qi, Y.
    Wu, Junfeng
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimal jamming power allocation against remote state estimation2017In: American Control Conference (ACC), 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, p. 1660-1665, article id 7963191Conference paper (Refereed)
    Abstract [en]

    This paper investigates a remote state estimation problem, where a smart sensor observes the state of a physical plant, locally estimates the state with Kalman filter, and then sends its local estimation data to a remote estimator through a communication network. There is a jamming attacker in the communication network who intentionally blocks the network with the purpose of deteriorating estimation quality subject to the jamming energy constraint. From the viewpoint of jamming attacker, in order to maximize attack effect, we focus on the questions 'when to jam the communication network', and 'how much power the attacker should adopt to jam the network'. After theoretic analysis, we point out that any grouped together attack schedule is optimal for the given jamming power. We provide a close form of jamming power when a sufficient condition holds, and the corresponding jamming effect on remote state estimation is explicitly presented. For a general case, we design a jamming power allocation algorithm and show the computational complexity of the proposed algorithm is not worse than O(T), where T is the length of the time horizon considered.

  • 121191. Zhang, H.
    et al.
    Soldati, P.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Time- and Channel-Efficient Link Scheduling for Convergecast in WirelessHART Networks2011In: 2011 IEEE 13th International Conference on Communication Technology (ICCT), 2011, p. 99-103Conference paper (Refereed)
    Abstract [en]

    Fast convergecast is a critical functionality for wireless networks deployed for industrial monitoring and control. We address the time- and channel-optimal link scheduling problem for convergecast in multi-line networks, operating according to the recent WirelessHART standard. We first establish the lower bound on convergecast time and the lower bound on the number of channels for time-optimal convergecast. Then we present optimal scheduling policy for time-optimal convergecast and near-optimal policy for jointly time- and channel-optimal convergecast. Numerical and simulation results show that our scheme can provide fast convergecast inWirelessHART networks.

  • 121192. Zhang, H. -W
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Wang, E. -G
    He, J. -C
    Simulation of macrosegregation and solidification microstructure evolution for Al-Si alloy by coupled cellular automaton-finite volume model2012In: Zhongguo Youse Jinshu Xuebao/Chinese Journal of Nonferrous Metals, ISSN 1004-0609, Vol. 22, no 7, p. 1883-1896Article in journal (Refereed)
    Abstract [en]

    A coupled cellular automaton-finite volume (CA-FV) model for macroscopic fluid flow, heat transfer, solute transport and microscopic nucleation and grain growth procedure was developed and applied in an Al-Si binary alloy solidification process. The model reflects the dendrite growth kinetics in the presence of fluid flow, the nucleation and growth in CA scale due to the increase of undercooling, and the feedback of solid fraction and temperature to FV nodes due to the nucleation and growth. The coupled CA-FV model can predict the recalescence and the intergranular segregation during alloy solidification process, which shows great advantages compared with the results by FV model and CA-FV model without fluid flow. The effects of fluid flow on the solute distribution and the solidification morphologies, as well as the influence of ingot size on the solidification structures, were discussed with the CA-FV model.

  • 121193. Zhang, H.
    et al.
    Wang, S.
    Gong, Y.
    Liu, T.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    Jia, D.
    Zhang, Y.
    A quantitative robustness evaluation model for optical fiber sensor networks2013In: Journal of Lightwave Technology, ISSN 0733-8724, E-ISSN 1558-2213, Vol. 31, no 8, p. 1240-1246Article in journal (Refereed)
    Abstract [en]

    Optical fiber sensor networks (OFSNs) are facing the problem of a lack of systematic evaluation criteria to assess network performance. In this paper, a universal quantitative robustness evaluation model for OFSNs is proposed. The model defines robustness as the mathematical expectation of the monitoring coverage ratio, which has taken into account the performance under all possible network states and the probability of each state. This model is applied to four basic network topologies including line, ring, star and bus topologies, and their mathematical expressions of robustness are derived by analyzing all possible states in detail. Further simulation gives a quantitative comparison among these topologies, proving that the ring and star topologies are optimal for the monitoring of strip-shaped and square regions, respectively. Finally, two influencing factors, the attenuation coefficient and the threshold, are investigated for their impact on the robustness of the network.

  • 121194. Zhang, H.
    et al.
    Wang, X.
    Harnefors, Lennart
    ABB, Västerås, Sweden.
    Gong, H.
    Hasler, J. -P
    Nee, Hans-Peter
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems.
    SISO Transfer Functions for Stability Analysis of Grid-Connected Voltage-Source Converters2019In: IEEE transactions on industry applications, ISSN 0093-9994, E-ISSN 1939-9367, Vol. 55, no 3, p. 2931-2941, article id 8640057Article in journal (Refereed)
    Abstract [en]

    Converter-grid interaction is of great interest in a weak-grid condition. This paper presents a single-input-single-output (SISO) open-loop transfer function for the stability analysis of grid-connected voltage-source converters. Differing from the conventional input impedance method and the eigenvalue analysis, an alternative multi-input-multi-output closed-loop system is developed in the paper and it eventually yields an SISO open-loop transfer function. This enables the application of a single Nyquist curve for analyzing the overall system stability. The model is validated against time-domain simulations as well as experimental results showing excellent accuracy for predicting the system stability.

  • 121195. Zhang, H.
    et al.
    Zhao, Z.
    Meng, Ziyang
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lin, Z.
    Experimental verification of a multi-robot distributed control algorithm with containment and group dispersion behaviors: The case of dynamic leaders ?2013In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2013, no PART 1, p. 153-158Conference paper (Refereed)
    Abstract [en]

    This paper studies the containment and group dispersion control for a multi-robot system in the presence of dynamic leaders. Each robot is represented by a double-integrator dynamic model and a distributed control algorithm is developed to drive the multi-robot system to follow a group of dynamic leaders with containment and group dispersion behaviors. The effectiveness of the algorithm is then verified on a multi-robot control platform.

  • 121196. Zhang, H.
    et al.
    Zhou, H. T.
    Bergman, Bill A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Z. C.
    Fabrication and conductivities of CeO2-based oxygen-ion conductors doped by M2O3 (M=La, Nd, Sm and/or Ga)2008In: Key Engineering Materials, 2008, p. 244-246Conference paper (Refereed)
    Abstract [en]

    Ceria oxygen-ion conductors doped with M2O3 (M=La, Nd, Sm and/or Ga) were prepared by a wet chemical method. Phases were analyzed by X-ray diffraction method and conductivities were measured by using AC impedance analyzer. The results showed that Ga2O3-CeO 2 solid solution with 17 nm grains and pure fluorite phase had been obtained even at calcination temperature of 800 °C for 5 hs. Conductivities of Ga2O3-La2O3 co-doped CeO2 were higher than that of Sm2O3-doped CeO2, which was reported having the highest conductivity among CeO2-based oxides.

  • 121197. Zhang, Haibo
    et al.
    Osterlind, Fredrik
    Soldati, Pablo
    Voigt, Thiemo
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Time-Optimal Convergecast With Separated Packet Copying: Scheduling Policies and Performance2015In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 64, no 2, p. 793-803Article in journal (Refereed)
    Abstract [en]

    Convergecast, in which packets originating from multiple sources are reported to a single sink, is a fundamental primitive for data collection in wireless sensor networks. This paper investigates the time-optimal link-scheduling problem for time-division multiple-access (TDMA)-based convergecast, aiming to minimize the amount of time required to complete convergecast. We observe that packet copying between the microcontroller and the radio transceiver in existing sensor platforms has a big impact on the packet forwarding delay, and we propose a novel model for convergecast in which packet copying is separated from packet transmission and reception. We establish tight lower bounds on the number of time slots required for convergecast in networks with line and tree routing topologies, and we present both centralized and distributed algorithms for constructing the time-optimal convergecast schedules. We evaluate our scheme in both simulations and experiments on hardware. The results show that our scheme can achieve a system throughput (defined as the number of data bits received by the sink per second) of 202.8 kb/s, which is 86.31% of the theoretical bound. In comparison with the traditional TDMA-based convergecast schemes, our scheme can achieve up to a 86.22% improvement on system throughput.

  • 121198.
    Zhang, Haibo
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Shen, Hong
    Energy-Efficient Beaconless Geographic Routing in Wireless Sensor Networks2010In: IEEE Transactions on Parallel and Distributed Systems, ISSN 1045-9219, E-ISSN 1558-2183, Vol. 21, no 6, p. 881-896Article in journal (Refereed)
    Abstract [en]

    Geographic routing is an attractive localized routing scheme for wireless sensor networks (WSNs) due to its desirable scalability and efficiency. Maintaining neighborhood information for packet forwarding can achieve a high efficiency in geographic routing, but may not be appropriate for WSNs in highly dynamic scenarios where network topology changes frequently due to nodes mobility and availability. We propose a novel online routing scheme, called Energy-efficient Beaconless Geographic Routing (EBGR), which can provide loop-free, fully stateless, energy-efficient sensor-to-sink routing at a low communication overhead without the help of prior neighborhood knowledge. In EBGR, each node first calculates its ideal next-hop relay position on the straight line toward the sink based on the energy-optimal forwarding distance, and each forwarder selects the neighbor closest to its ideal next-hop relay position as the next-hop relay using the Request-To-Send/Clear-To-Send (RTS/CTS) handshaking mechanism. We establish the lower and upper bounds on hop count and the upper bound on energy consumption under EBGR for sensor-to-sink routing, assuming no packet loss and no failures in greedy forwarding. Moreover, we demonstrate that the expected total energy consumption along a route toward the sink under EBGR approaches to the lower bound with the increase of node deployment density. We also extend EBGR to lossy sensor networks to provide energy-efficient routing in the presence of unreliable communication links. Simulation results show that our scheme significantly outperforms existing protocols in wireless sensor networks with highly dynamic network topologies.

  • 121199.
    Zhang, Haibo
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Optimal Link Scheduling and Channel Assignment for Convergecast in Linear Wireless HART Networks2009In: 2009 7TH INTERNATIONAL SYMPOSIUM ON MODELING AND OPTIMIZATION IN MOBILE, AD HOC, AND WIRELESS, NEW YORK: IEEE , 2009, p. 82-89Conference paper (Refereed)
    Abstract [en]

    Convergecast, in which data from a set of sources is routed toward one data sink, is a critical functionality for wireless networks deployed for industrial monitoring and control. We address the joint link scheduling and channel assignment problem for convergecast in networks operating according to the recent WirelessHART standard. For a linear network with N single-buffer devices, we demonstrate that the minimum time to complete convergecast is 2N - 1 time-slots, and that the minimum number of channels required for this operation is inverted right perpendicularN/2inverted left perpendicular. When the devices are allowed to buffer multiple packets, we prove that the optimal convergecast time remains the same while the number of required channels can be reduced to inverted right perpendicularN - root N-(N-1)/2inverted left perpendicular. For both cases, we present jointly time- and channel-optimal scheduling policies with complexity O(N-2). Numerical results demonstrate that our schemes are also efficient in terms of memory utilization.

  • 121200. Zhang, Haibo
    et al.
    Soldati, Pablo
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Performance Bounds and Latency-Optimal Scheduling for Convergecast in WirelessHART Networks2013In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 12, no 6, p. 2688-2696Article in journal (Refereed)
    Abstract [en]

    Convergecast, in which data from a set of source devices is delivered to a single data sink, is a critical functionality in networks deployed for industrial monitoring and control. We address the latency-optimal link scheduling problem for convergecast in networks operating according to the recent WirelessHART standard. When there is no restriction on the number of channels, we present a latency-optimal scheduling policy in which each routing node is required to buffer at most one packet at any point in time. For networks with a limited number of channels, we first establish a lower bound on the number of channels for latency-optimal convergecast and a lower bound on latency for convergecast using a fixed number of channels, and then present a heuristic scheme for channel-constrained latency-optimal convergecast scheduling. Simulation results confirm that, at much modest computational cost, our heuristic scheme can construct convergecast schedules with latency close to that of the optimal schedules.

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