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  • 121201.
    Zhang, Haibo
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Österlind, F.
    Soldati, Pablo
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Voigt, T.
    Johansson, Mikael
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Rapid convergecast on commodity hardware: Performance limits and optimal policies2010In: SECON 2010 - 2010 7th Annual IEEE Communications Society Conference on Sensor, 2010, Vol. Mesh and Ad Hoc Communications and NetworksConference paper (Refereed)
    Abstract [en]

    The increased industrial interest in wireless sensor networks demands a shift from optimizing protocols for reporting sporadic events, to solutions for high-rate data collection and dissemination. We study time-optimal convergecast under the communication constraints of commodity sensor network platforms. We propose a novel convergecast model in which packet copying between the microcontroller and the radio transceiver is separated from packet transmission, thereby improving channel utilization and system throughput. Based on this model, we establish the tight lower bound on the number of time slots for convergecast in networks with tree routing topology, and present both centralized and distributed algorithms for computing time-optimal convergecast schedules. Our scheme is memory-efficient as each node buffers at most one packet at any time. We evaluate our scheme in simulation and on real hardware, and show that our scheme can achieve a throughput of 203 kbit/s (86.4% of the theoretical upper bound): up to 86.24 % improvement compared with traditional TDMA-based convergecast. With an optimal routing tree and the maximum MAC layer payload, convergecast in a network with 20 sensor nodes can be completed in only 100ms.

  • 121202.
    Zhang, Han
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Global Analysis and Structural Performance of the Tubed Mega Frame2014Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The Tubed Mega Frame is a new structure concept for high-rise buildings which is developed by Tyréns. In order to study the structural performance as well as the efficiency of this new concept, a global analysis of the Tubed Mega Frame structure is performed using finite element analysis software ETABS. Besides, the lateral loads that should be applied on the structure according to different codes are also studied. From the design code study for wind loads and seismic design response spectrums, it can be seen that the calculation philosophies are different from code to code. The wind loads are approximately the same while the design response spectrums vary a lot from different codes.

    In the ETABS program, a 3D finite element model is built and analyzed for linear static, geometric non-linearity (P-Delta) and linear dynamic cases. The results from the analysis in the given scope show that the Tubed Mega Frame structural system is potentially feasible and has relatively high lateral stiffness and global stability. For the service limit state, the maximum story drift ratio is within the limitation of 1/400 and the maximum story acceleration is 0.011m/sec 2 which fulfill the comfort criteria.

  • 121203.
    Zhang, Han
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Optimizing Networked Systems and Inverse Optimal Control2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is concerned with the problems of optimizing networked systems, including designing a distributed energy optimal consensus controller for homogeneous networked linear systems, maximizing the algebraic connectivity of a network by projected saddle point dynamics. In addition, the inverse optimal control problems for discrete-time finite time-horizon Linear Quadratic Regulators (LQRs) are considered. The goal is to infer the Q matrix in the quadratic cost function using the observations (possibly noisy) either on the optimal state trajectories, optimal control input or the system output.

    In Paper A, an optimal energy cost controller design for identical networked linear systems asymptotic consensus is considered. It is assumed that the topology of the network is given and the controller can only depend on relative information of the agents. Since finding the control gain for such a controller is hard, we focus on finding an optimal controller among a classical family of controllers which is based on the Algebraic Riccati Equation (ARE) and guarantees asymptotic consensus. We find that the energy cost is bounded by an interval and hence we minimize the upper bound. Further, the minimization for the upper bound boils down to optimizing the control gain and the edge weights of the graph separately. A suboptimal control gain is obtained by choosing Q=0 in the ARE. Negative edge weights are allowed, meaning that "competitions" between the agents are allowed. The edge weight optimization problem is formulated as a Semi-Definite Programming (SDP) problem. We show that the lowest control energy cost is reached when the graph is complete and with equal edge weights. Furthermore, two sufficient conditions for the existence of negative optimal edge weights realization are given. In addition, we provide a distributed way of solving the SDP problem when the graph topology is regular.

    In Paper B, a projected primal-dual gradient flow of augmented Lagrangian is presented to solve convex optimization problems that are not necessarily strictly convex. The optimization variables are restricted by a convex set with computable projection operation on its tangent cone as well as equality constraints. We show that the projected dynamical system converges to one of the saddle points and hence finding an optimal solution. Moreover, the problem of distributedly maximizing the algebraic connectivity of an undirected network by optimizing the "port gains" of each nodes is considered. The original SDP problem is relaxed into a nonlinear programming (NP) problem that will be solved by the aforementioned projected dynamical system. Numerical examples show the convergence of the aforementioned algorithm to one of the optimal solutions. The effect of the relaxation is illustrated empirically with numerical examples. A methodology is presented so that the number of iterations needed to converge is reduced. Complexity per iteration of the algorithm is illustrated with numerical examples.

    In Paper C and D, the inverse optimal control problems over finite-time horizon for discrete-time LQRs are considered. The well-posedness of the inverse optimal control problem is first justified. In the noiseless case, when these observations of the optimal state trajectories or the optimal control input are exact, we analyze the identifiability of the problem and provide sufficient conditions for uniqueness of the solution. In the noisy case, when the observations are corrupted by additive zero-mean noise, we formulate the problem as an optimization problem and prove that the solution to this problem is statistically consistent. The following two scenarios are further considered: 1) the distributions of the initial state and the observation noise are unknown, yet the exact observations on the initial states and the noisy observations on the system output are available; 2) the exact observations on the initial states are not available, yet the observation noises are known to be white Gaussian and the distribution of the initial state is also Gaussian (with unknown mean and covariance). For the first scenario, we show statistical consistency for the estimation. For the second scenario, we fit the problem into the framework of maximum-likelihood and Expectation Maximization (EM) algorithm is used to solve this problem. The performance of the proposed method is illustrated through numerical examples.

  • 121204.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Consensus control for linear systems with optimal energy cost2018In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 93, p. 83-91Article in journal (Refereed)
    Abstract [en]

    In this paper, we design an optimal energy cost controller for linear systems asymptotic consensus given the topology of the graph. The controller depends only on relative information of the agents. Since finding the control gain for such controller is hard, we focus on finding an optimal controller among a classical family of controllers which is based on Algebraic Riccati Equation (ARE) and guarantees asymptotic consensus. Through analysis, we find that the energy cost is bounded by an interval and hence we minimize the upper bound. In order to do that, there are two classes of variables that need to be optimized: the control gain and the edge weights of the graph and are hence designed from two perspectives. A suboptimal control gain is obtained by choosing Q=0 in the ARE. Negative edge weights are allowed, and the problem is formulated as a Semi-definite Programming (SDP) problem. Having negative edge weights means that “competitions” between the agents are allowed. The motivation behind this setting is to have a better system performance. We provide a different proof compared to Thunberg and Hu (2016) from the angle of optimization and show that the lowest control energy cost is reached when the graph is complete and with equal edge weights. Furthermore, two sufficient conditions for the existence of negative optimal edge weights realization are given. In addition, we provide a distributed way of solving the SDP problem when the graph topology is regular.

  • 121205.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Inverse Optimal Control for Finite-Horizon Discrete-time Linear Quadratic Regulator Under Noisy OutputManuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, the problem of inverse optimal control for finite-horizon discrete-time Linear Quadratic Regulators (LQRs) is considered. The goal of the inverse optimal control problem is to recover the corresponding objective function by the noisy observations. We consider the problem of inverse optimal control in two scenarios: 1) the distributions of the initial state and the observation noise are unknown, yet the exact observations on the initial states and the noisy observations on system output are available; 2) the exact observations on the initial states are not available, yet the observation noises are known white Gaussian and the distribution of the initial state is also Gaussian (with unknown mean and covariance). For the first scenario, we formulate the problem as a risk minimization problem and show that its solution is statistically consistent. For the second scenario, we fit the problem into the framework of maximum-likelihood and Expectation Maximization (EM) algorithm is used to solve this problem. The performance for the estimations are shown by numerical examples.

  • 121206.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Inverse Optimal Control for Finite-Horizon Discrete-time Linear Quadratic Regulator Under Noisy OutputManuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, the problem of inverse optimal control for finite-horizon discrete-time Linear Quadratic Regulators (LQRs) is considered. The goal of the inverse optimal control problem is to recover the corresponding objective function by the noisy observations. We consider the problem of inverse optimal control in two scenarios: 1) the distributions of the initial state and the observation noise are unknown, yet the exact observations on the initial states and the noisy observations on system output are available; 2) the exact observations on the initial states are not available, yet the observation noises are known white Gaussian and the distribution of the initial state is also Gaussian (with unknown mean and covariance). For the first scenario, we formulate the problem as a risk minimization problem and show that its solution is statistically consistent. For the second scenario, we fit the problem into the framework of maximum-likelihood and Expectation Maximization (EM) algorithm is used to solve this problem. The performance for the estimations are shown by numerical examples.

  • 121207.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Optimal energy consensus control for linear multi-agent systems2017In: 2017 36th Chinese Control Conference, CCC, IEEE Computer Society, 2017, p. 2663-2668, article id 8027765Conference paper (Refereed)
    Abstract [en]

    In this paper, an optimal energy cost controller for linear multi-agent systems' consensus is proposed. It is assumed that the topology among the agents is fixed and the agents are connected through an edge-weighted graph. The controller only uses relative information between agents. Due to the difficulty of finding the controller gain, we focus on finding the optimal controller among a sub-family whose design is based on Algebraic Riccati Equation (ARE) and guarantee consensus. It is found that the energy cost for such controllers is bounded by an interval and hence we minimize the upper bound. To do that, the control gain and the edge weights are optimized separately. The control gain is optimized by choosing Q = 0 in the ARE; the edge weights are optimized under the assumption that there is limited communication resources in the network. Negative edge weights are allowed, and the problem is formulated as a Semi-definite Programming (SDP) problem. The controller coincides with the optimal control in [8] when the graph is complete. Furthermore, two sufficient conditions for the existence of negative optimal edge weights realization are given.

  • 121208.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Umenberger, Jack
    Department of Information Technology, Division of Systems and Control, Uppsala University.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Inverse Quadratic Optimal Control for Discrete-Time Linear SystemsManuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, we consider the inverse optimal control problem for discrete-time Linear Quadratic Regulators (LQRs), over finite-time horizons. Given observations of the optimal trajectories, or optimal control inputs, to a linear time-invariant system, the goal is to infer the parameters that define the quadratic cost function. The well-posedness of the inverse optimal control problem is first justied. In the noiseless case, when these observations are exact, we analyze the identiability of the problem and provide sufficient conditions for uniqueness of the solution. In the noisy case, when the observations are corrupted by additive zero-mean noise, we formulate the problem as an optimization problem and prove that the solution to this problem is statistically consistent. The performance of the proposed method is illustrated through numerical examples.

  • 121209.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Umenberger, Jack
    Department of Information Technology, Division of Systems and Control, Uppsala University.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Inverse Quadratic Optimal Control for Discrete-Time Linear SystemsManuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, we consider the inverse optimal control problem for discrete-time Linear Quadratic Regulators (LQRs), over finite-time horizons. Given observations of the optimal trajectories, or optimal control inputs, to a linear time-invariant system, the goal is to infer the parameters that define the quadratic cost function. The well-posedness of the inverse optimal control problem is first justied. In the noiseless case, when these observations are exact, we analyze the identiability of the problem and provide sufficient conditions for uniqueness of the solution. In the noisy case, when the observations are corrupted by additive zero-mean noise, we formulate the problem as an optimization problem and prove that the solution to this problem is statistically consistent. The performance of the proposed method is illustrated through numerical examples.

  • 121210.
    Zhang, Han
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Wei, Jieqiang
    KTH, School of Electrical Engineering and Computer Science (EECS), Automatic Control.
    Yi, Peng
    Washington Univ, Dept Elect & Syst Engn, St Louis, MO USA..
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Projected primal-dual gradient flow of augmented Lagrangian with application to distributed maximization of the algebraic connectivity of a network2018In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 98, p. 34-41Article in journal (Refereed)
    Abstract [en]

    In this paper, a projected primal-dual gradient flow of augmented Lagrangian is presented to solve convex optimization problems that are not necessarily strictly convex. The optimization variables are restricted by a convex set with computable projection operation on its tangent cone as well as equality constraints. As a supplement of the analysis in Niederlander and Cortes (2016), we show that the projected dynamical system converges to one of the saddle points and hence finding an optimal solution. Moreover, the problem of distributedly maximizing the algebraic connectivity of an undirected network by optimizing the port gains of each nodes (base stations) is considered. The original semi-definite programming (SDP) problem is relaxed into a nonlinear programming (NP) problem that will be solved by the aforementioned projected dynamical system. Numerical examples show the convergence of the aforementioned algorithm to one of the optimal solutions. The effect of the relaxation is illustrated empirically with numerical examples. A methodology is presented so that the number of iterations needed to reach the equilibrium is suppressed. Complexity per iteration of the algorithm is illustrated with numerical examples.

  • 121211.
    Zhang, He
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Non-Standard Neutrino Interactions and Non-Unitarity Effects from TeV Seesaw Models2010In: NEUTRINO FACTORIES, SUPERBEAMS, AND BETA BEAMS / [ed] Goodman MC; Kaplan DM; Sullivan Z, 2010, Vol. 1222, p. 145-149Conference paper (Refereed)
    Abstract [en]

    The motivations and phenomena of seesaw models at the TeV scale are briefly reviewed. We show that nonstandard neutrino interactions and non-unitary effects are two typical features of low-scale scalar and fermionic seesaw models, respectively. For scalar seesaw models, in principle, significant non-standard interaction effects can be accommodated in the type-II seesaw model. As for the low-scale fermionic seesaw models, the inverse seesaw model turns out to be the most natural one, and could be well tested at the future long-baseline neutrino oscillation experiments and the LHC.

  • 121212.
    Zhang, He
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Non-standard neutrino interactions in the type-II seesaw model2010In: TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS (TAUP2009), 2010, Vol. 203Conference paper (Other academic)
    Abstract [en]

    Within the framework of the type-II seesaw model, we investigate in detail the non-standard neutrino interactions (NSIs). Non-trivial correlations between NSI parameters and neutrino masses and mixing parameters are established. We show that sizable NSIs can be generated as a consequence of a nearly degenerate neutrino mass spectrum. Significant zero distance effects in the near detector of a future neutrino factory, as well as characteristic decays of the doubly charged Higgs at the Large Hadron Collider are discussed.

  • 121213.
    Zhang, He
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Zhou, Shun
    The minimal seesaw model at the TeV scale2010In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 685, no 4-5, p. 297-301Article in journal (Refereed)
    Abstract [en]

    We point out that the minimal seesaw model can provide a natural framework to accommodate tiny neutrino masses, while its experimental testability and notable predictiveness are still maintained. This possibility is based on the Observation that two heavy right-handed Majorana neutrinos in the minimal seesaw model may naturally emerge as a pseudo-Dirac fermion In a specific scenario, we show that the tri-bimaximal neutrino mixing can be produced, and only the inverted neutrino mass hierarchy is allowed The low-energy phenomena, including non-unitarity effects in neutrino oscillations, neutrinoless double-beta decays and rare lepton-flavor-violating decays of charged leptons l(alpha) -> l(beta)gamma, have been explored. The collider signatures of the heavy singlet neutrino are also briefly discussed (C) 2010 Elsevier B.V All rights reserved

  • 121214. Zhang, Hejin
    et al.
    Zhao, Zhiyun
    Meng, Ziyang
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lin, Zongli
    Experimental verification of a multi-robot distributed control algorithm with containment and group dispersion behaviors: The case of dynamic leaders2014In: IEEE/CAA Journal of Automatica Sinica, ISSN 2329-9266, Vol. 1, no 1, p. 54-60, article id 7004620Article in journal (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 doubleintegrator 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.

  • 121215.
    Zhang, Heng
    et al.
    Huaihai Inst Technol, Lianyungang, Peoples R China..
    Qi, Yifei
    Zhejiang Univ, State Key Lab Ind Control Technol, Hangzhou, Zhejiang, Peoples R China..
    Wu, Junfeng
    KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for Autonomous Systems, CAS. KTH, School of Electrical Engineering and Computer Science (EECS), Centres, ACCESS Linnaeus Centre.
    Fu, Lingkun
    Zhejiang Univ, State Key Lab Ind Control Technol, Hangzhou, Zhejiang, Peoples R China..
    He, Lidong
    Zhejiang Univ, State Key Lab Ind Control Technol, Hangzhou, Zhejiang, Peoples R China..
    DoS Attack Energy Management Against Remote State Estimation2018In: IEEE Transactions on Big Data, ISSN 2325-5870, E-ISSN 2168-6750, Vol. 5, no 1, p. 383-394Article in journal (Refereed)
    Abstract [en]

    This paper considers a remote state estimation problem, where a sensor measures the state of a linear discrete-time process and has computational capability to implement a local Kalman filter based on its own measurements. The sensor sends its local estimates to a remote estimator over a communication channel that is exposed to a Denial-of-Service (DoS) attacker. The DoS attacker, subject to limited energy budget, intentionally jams the communication channel by emitting interference noises with the purpose of deteriorating estimation performance. In order to maximize attack effect, following the existing answer to "when to attack the communication channel", in this paper we manage to solve the problem of "how much power the attacker should use to jam the channel in each time". For the static attack energy allocation problem, when the system matrix is normal, we derive a sufficient condition for when the maximum number of jamming operations should be used. The associated jamming power is explicitly provided. For a general system case, we propose an attack 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. When the attack can receive the real-time ACK information, we formulate a dynamic attack energy allocation problem, and transform it to a Markov Decision Process to find the optimal solution.

  • 121216.
    Zhang, Hengchong
    KTH, School of Computer Science and Communication (CSC).
    Microblog Marketing in China.2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Currently, microblog marketing has become an important tool for market competition among companies in many countries. People become aware of its unique commercial value and pay more attention on it. As a newly developed social media tool, although microblogging has just started in China, by the end of March in 2011, the number of registered users has drastically reached 230 million compared to 53 million one year ago and it is still increasing. As microblog becomes increasingly popular in China, it is a great platform for companies to promote their products and services, broaden their business network and build their brand. Microblogging has opened a new dimension of marketing and there is great significance for conducting research on the microblog marketing in China.

    The aim of this thesis work is to probe the strategies of microblog marketing for different companies in China which could be a guideline for those who plan to try microblog marketing.

    In order to achieve this goal and find out the useful strategies, case studies and SWOT analysis are used to analyze different situations. Specifically, three case studies are from Dell, Kogi BBQ, and VANCL, who have achieved great success via microblog marketing. Through case studies and SWOT analysis, related strategies are proposed and the trend is also predicted.

    In the end, we conclude that most companies may benefit from microblog marketing if they use it properly and take suitable strategies. However, microblog is a double-edged sword and improper message posts may bring negative effects to companies. Companies should treat microblog cautiously and make good use of it.

    Key words: Microblog, Microblog marketing, Strategy, Company

  • 121217.
    Zhang, Hengchong
    KTH, School of Information and Communication Technology (ICT), Communication Systems, CoS.
    Non-binary Authentication: Supplicant2009Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    There are a number of authentication methods for wireless local area networks. The IEEE 802.1x standard is one such method. This standard specifies a port-based access control protocol. There are three entities involved: a supplicant (a device that wishes to have network access and perhaps other services), an Access Point (AP) or other port to which access is to be controlled, and an Authentication Server (AS). The goal of this project was to design, implement, and evaluate a prototype of a non-binary alternative to IEEE 802.1x authentication. This report focuses on the supplicant. Specifically it describes the design, implementation, and evaluation of a supplicant program to test and stress the authenticator, in order to evaluate a non-binary authentication process.

    Following, a brief introduction is given to the problem that is to be solved, a number of existing IEEE 802.1x supplicants are described and compared. Following this, a number of potential non-binary authentication processes are analyzed. The ability of a supplicant to send and receive packets before and after authentication is also examined. Based upon our implementation and evaluation of a supplicant and an emulation of the non-binary authentication process, we conclude that non-binary authentication is both feasible and valuable. Furthermore, the thesis evaluates why and how non-binary authentication is valuable from the viewpoint of a supplicant. Additional future work is suggested at the end of this thesis.

  • 121218.
    Zhang, Hongqi
    et al.
    Chinese Acad Sci, Key Lab Solar Act, Natl Astron Observ, Beijing 100012, Peoples R China..
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Dept Astrophys & Planetary Sci, Boulder, CO 80303 USA.;Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, SE-10691 Stockholm, Sweden.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA..
    Solar Kinetic Energy and Cross Helicity Spectra2018In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 862, no 2, article id L17Article in journal (Refereed)
    Abstract [en]

    We develop a formalism that treats the calculation of solar kinetic energy and cross helicity spectra in an equal manner to that of magnetic energy and helicity spectra. The magnetic helicity spectrum is shown to be equal to the vertical part of the current helicity spectrum divided by the square of the wavenumber. For the cross helicity, we apply the recently developed two-scale approach globally over an entire active region to account for the sign change between the two polarities. Using vector magnetograms and Dopplergrams of NOAA. 11158 and 12266, we show that kinetic and magnetic energy spectra have similar slopes at intermediate wavenumbers, where the contribution from the granulation velocity has been removed. At wavenumbers around 0.3 Mm(-1), the magnetic helicity is found to be close to its maximal value. The cross helicity spectra are found to be within about 10% of the maximum possible value. Using the two-scale method for NOAA. 12266, the global cross helicity spectrum is found to be particularly steep, similarly to what has previously been found in theoretical models of spot generation. In the quiet Sun, by comparison, the cross helicity spectrum is found to be small.

  • 121219. Zhang, Hongqi
    et al.
    Brandenburg, Axel
    Nordita, Sweden; Stockholm University, Sweden; University of Colorado, United States;.
    Sokoloff, D. D.
    EVOLUTION OF MAGNETIC HELICITY AND ENERGY SPECTRA OF SOLAR ACTIVE REGIONS2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 819, no 2, article id 146Article in journal (Refereed)
    Abstract [en]

    We adopt an isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field to estimate the magnetic energy and helicity spectra as well as current helicity spectra of two individual active regions (NOAA. 11158 and NOAA. 11515) and the change of the spectral indices during their development as well as during the solar cycle. The departure of the spectral indices of magnetic energy and current helicity from 5/ 3 are analyzed, and it is found that it is lower than the spectral index of the magnetic energy spectrum. Furthermore, the fractional magnetic helicity tends to increase when the scale of the energy-carrying magnetic structures increases. The magnetic helicity of NOAA. 11515 violates the expected hemispheric sign rule, which is interpreted as an effect of enhanced field strengths at scales larger than 30-60Mm with opposite signs of helicity. This is consistent with the general cycle dependence, which shows that around the solar maximum the magnetic energy and helicity spectra are steeper, emphasizing the large-scale field.

  • 121220. Zhang, Hongqi
    et al.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Sokoloff, D. D.
    Magnetic helicity and energy spectra of a solar active region2014In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 784, no 2, p. L45-Article in journal (Refereed)
    Abstract [en]

    We compute for the first time the magnetic helicity and energy spectra of the solar active regionNOAA 11158 during 2011 February 11-15 at 20 degrees southern heliographic latitude using observational photospheric vector magnetograms. We adopt the isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field. The sign of the magnetic helicity turns out to be predominantly positive at all wavenumbers. This sign is consistent with what is theoretically expected for the southern hemisphere. The magnetic helicity normalized to its theoretical maximum value, here referred to as relative helicity, is around 4% and strongest at intermediate wavenumbers of k approximate to 0.4Mm(-1), corresponding to a scale of 2 pi/k approximate to 16 Mm. The same sign and a similar value are also found for the relative current helicity evaluated in real space based on the vertical components of magnetic field and current density. The modulus of the magnetic helicity spectrum shows a k(-11/3) power law at large wavenumbers, which implies a k(-5/3) spectrum for the modulus of the current helicity. A k(-5/3) spectrum is also obtained for the magnetic energy. The energy spectra evaluated separately from the horizontal and vertical fields agree for wavenumbers below 3 Mm(-1), corresponding to scales above 2 Mm. This gives some justification to our assumption of isotropy and places limits resulting from possible instrumental artifacts at small scales.

  • 121221.
    Zhang, Hongtao
    et al.
    Univ Waterloo, Dept Mech & Mechatron Engn, Lab Fuel Cell & Green Energy RD&D 20 20, Waterloo, ON N2L 3G1, Canada.;Univ Waterloo, Dept Appl Math, Waterloo, ON N2L 3G1, Canada..
    Li, Xianguo
    Univ Waterloo, Dept Mech & Mechatron Engn, Lab Fuel Cell & Green Energy RD&D 20 20, Waterloo, ON N2L 3G1, Canada..
    Liu, Xinzhi
    Univ Waterloo, Dept Appl Math, Waterloo, ON N2L 3G1, Canada..
    Yan, Jinyue
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes. Malardalen Univ, Sch Sustainable Dev Soc & Technol, S-72123 Vasteras, Sweden..
    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, Vol. 241, 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.

  • 121222. Zhang, Hongwei
    et al.
    Gandin, Charles-Andre
    Ben Hamouda, Haithem
    Tourret, Damien
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    He, Jicheng
    Prediction of Solidification Paths for Fe-C-Cr Alloys by a Multiphase Segregation Model Coupled to Thermodynamic Equilibrium Calculations2010In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 50, no 12, p. 1859-1866Article in journal (Refereed)
    Abstract [en]

    A microsegregation model for the solidification of multicomponent alloys is developed It couples the volume-averaged conservation equations for total mass solute mass and energy assuming a uniform temperature The diffusion in the liquid and solid phases the growth kinetics of the solidifying microstructures and the velocity of the solid/liquid and solid/solid phase interfaces are considered in the model Equilibrium between phases is taken into account and computed using dedicated thermodynamic software The thermodynamic properties and their evolutions during solidification are directly retrieved from a database Illustration is provided by the solidification of a Fe-C-Cr alloy The occurrence of the recalescence due to the growth of the microstructure and the progress of solidification are predicted The solidification behavior near to recalescence is evaluated By adjusting the cooling intensity and the solute diffusivities extreme approximations are retrieved The model shows potentials to be coupled with a macrosegregation model for application to the solidification of multicomponent alloys

  • 121223. Zhang, Hongwei
    et al.
    Liu, Qiang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Shibata, Hiroyuki
    Wang, Qiang
    Jönsson, Pär
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    He, Jicheng
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Partial Equilibrium Prediction of Solidification and Carbide Precipitation in Ti-added High Cr Cast Irons2014In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 54, no 2, p. 374-383Article in journal (Refereed)
    Abstract [en]

    Carbide precipitation and eutectic phase transformation during solidification of Fe-C-Cr-Ti-Mn-Mo-Ni-Si Ti-added high-chromium cast irons (HCCIs) were studied numerically and experimentally by the help of Partial Equilibrium approximation, DSC thermal analyses and EDX analyses. The main carbides formed during the solidification are distinguished as MC, primary M7C3 and eutectic M7C3 from their distinguished constitution, while other researchers didn't distinguish the primary and eutectic M7C3 carbide. Through comparing the prediction of Partial Equilibrium approximation with DSC thermal analysis measurement, the precipitation sequence of the eutectic structure in HCCIs is clarified to follow the sequence of FCC prior to the eutectic M7C3, although they were usually expected to precipitate simultaneously. The hardness index of the HCCIs is evaluated quantatively by summation of the contributions of the Vickers hardness of MC, primary M7C3 and eutectic M7C3 carbides with predicted precipitation amount and composition / constitution. The effects of C, Ti and Cr contents on the precipitation sequence, the amount and the composition of carbides as well as the hardness of the HCCIs are discussed deeply. Finally, the validity of Partial Equilibrium approximation is shown in prediction of the solidification in multicomponent system with large amount of precipitated carbides.

  • 121224. Zhang, Hongwei
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Gandin, Charles-Andre
    He, Jicheng
    Prediction of Carbide Precipitation Using Partial Equilibrium Approximation in Fe-C-V-W-Cr-Mo High Speed Steels2013In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 53, no 3, p. 493-501Article in journal (Refereed)
    Abstract [en]

    In Fe-C-V-W-Cr-Mo high speed steels, the nature of carbides during the solidification are discussed as a function of C, V and W content by the help of Partial Equilibrium (PE) approximation and thermodynamic calculations. The results show that the solidification path and carbide precipitation can be reasonably predicted by the Partial Equilibrium approximation for cooling rate lower than 10-13 K min(-1). From the viewpoint of hardness control by carbides, it is found that among the main carbides MC, M6C and M7C3, the increase of C favours the formation of MC, M7C3 but decreases the hardness of M7C3 by increasing the Fe content in it. Meanwhile, the increase of V only increases the amount of MC and V content therein, and the increase of W largely increases the amount of M6C and W content in it. As a result, the addition of V and W improves the hardness of MC and M6C carbides.

  • 121225. Zhang, Hongwei
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Lei, Hong
    He, Jicheng
    Restrictions of Physical Properties on Solidification Microstructures of Al-based Binary Alloys by Cellular Automaton2010In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 50, no 12, p. 1835-1842Article in journal (Refereed)
    Abstract [en]

    The solidification microstructure evolution and the Columnar to Equiaxed Transition (CET) during Al-Si and Al-Cu binary alloy solidification processes are analyzed by the help of Cellular Automaton-Finite Difference (CA-FD) model The effects of the physical properties except the effects of the nucleation parameters and the operation parameters on the cooling curves the dendrite growth the solidification morphologies and the CET of the Al-Si and Al-Cu binary alloys are emphatically discussed Results show that the solidification morphologies are internally influenced by the physical properties related to the dendrite tip growth kinetics Besides the solute diffusivity in liquid D-L and the growth restriction parameter Q=mC(o)(k(S/L)-1) the Gibbs-Thomson coefficient Gamma shows a great effect on dendrite tip growth rate Their effects on the dendrite tip kinetics are ordered as m(k(S/L)-1)>Gamma>D-L The growth rate can be predicted efficiently by the present simplified expression based on GGAN model combining several physical parameters and the local under-cooling which clearly shows the physical meaning of the constant coefficients in the simplified expression based on KGT model The equiaxed ratio related to the solidification morphologies can also be evaluated as a function of those physical properties

  • 121226. Zhang, Hongwei
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Wu, Rangqiong
    Wang, Qiang
    He, Jicheng
    Prediction of Solidification Microstructure and Columnar-to-equiaxed Transition of Al-Si Alloy by Two-dimensional Cellular Automaton with "Decentred Square" Growth Algorithm2009In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 49, no 7, p. 1000-1009Article in journal (Refereed)
    Abstract [en]

    A Cellular Automaton (CA)-Finite Difference (FD) coupling model was developed to analyze the evolution of solidification microstructure and the columnar-to-equiaxed transition (CET) in AI-Si alloy. Kobayashi's microsegregation equation was adopted to describe the solute diffusion in solid phase, and a "decentred square" growth algorithm with coordinate transformation was performed to describe the grain growth and the entrapment of neighbor cells. Through the examination on the effects of operation parameters and nucleation parameters on solidification morphologies, it was found that the length of columnar grains is controlled by the dendrite tip growth kinetics, and that the width of columnar grains is controlled by the implicit relationship between nucleation density and cooling rate at ingot surface. It was also found that the size of equiaxed grains is controlled by the competition of the nucleation and the grain growth. With the controllability of nucleation density in the bulk of liquid for equiaxed grain size, the nucleant and the nucleation density in actual AI-Si alloy were estimated. Both of the CET criteria based on the solidification path by CA-FD coupling model and the one based on the curves of critical temperature gradient conditions by Hunt's model were strongly dependent on nucleation undercooling and Si concentration. A good agreement was obtained between these two.

  • 121227. Zhang, Hongwei
    et al.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Xing, Wei
    Wang, Aili
    He, Jicheng
    Influences of Flow Intensity, Cooling Rate and Nucleation Density at Ingot Surface on Deflective Growth of Dendrites for Al-based Alloy2009In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 49, no 7, p. 1010-1018Article in journal (Refereed)
    Abstract [en]

    The dendrite tip growth kinetics in the flow field and the decentred quadrilateral growth algorithm for describing the evolution of grain growth are combined in Cellular Automaton model to predict the deflective growth of dendrites inclined toward upstream direction. The influences of flow intensity, cooling rate (or solidification rate), nucleation density at ingot surface on the deflective growth of dendrites are discussed. The increase of flow intensity dominantly forces the dendrites to grow in upstream direction; on the contrary, the increases of nucleation density at ingot surface and cooling rate suppress slightly this deflective growth. The relations predicted among deflection angle, flow intensity and solidification rate for AI-Si alloy and Al-Cu alloy show the same tendency as that in Okano et al.'s empirical expression deduced from experiments on steel. The deflection angle predicted for Al-Cu alloy fits well with previous experimental results.

  • 121228. Zhang, Hongxia
    et al.
    Chen, Xinwei
    Ye, Wenting
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Jia, Dagong
    Zhang, Yimo
    Mitigation of the birefringence dispersion on the polarization coupling measurement in a long-distance high-birefringence fiber2012In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 23, no 2, p. 025203-Article in journal (Refereed)
    Abstract [en]

    Due to the birefringence dispersion, the polarization coupling parameter measurement in high-birefringence fiber decreases obviously with the fiber length, especially for long-distance fibers. In this paper, two methods for mitigating the birefringence dispersion in a long-distance fiber are proposed. The first method is a spectral-domain measurement method. The experimental setup and results are described in detail. The other method is a time-domain numerical dispersion compensation algorithm to amend the coupling strength calculation equation. It is based on the fact that the interferogram envelope area is constant even with the existence of birefringence dispersion. The experimental result shows that the time-domain algorithm has high accuracy, and the absolute deviation is less than 1%. The two methods are validated to mitigate the birefringence dispersion in the long-distance high-birefringence fiber effectively.

  • 121229. Zhang, Hongxia
    et al.
    Liu, Kun
    Jia, Dagong
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Material Physics.
    Liu, Tiegen
    Peng, Gangding
    Jing, Wencai
    Zhang, Yimo
    Improved low concentration gas detection system based on intracavity fiber laser2011In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 82, no 2, p. 023104-Article in journal (Refereed)
    Abstract [en]

    The improvement of a low concentration gas detection system based on the intracavity fiber laser is proposed in this paper. The sensitivity of the system is deduced based on Lambert-Beer law. The optimized system was established with the gas cell made elaborately. In order to apply the wavelength sweeping technique, the fiber Bragg grating reflector was substituted by the wavelength independent Faraday rotation reflector. The sensitivity of the system for acetylene detection is reduced to less than 100 ppm by using the average of three absorption spectra. The acetylene detection coefficients of variation with different concentrations are measured. The gas measurement system is validated to detect low concentration gas effectively.

  • 121230. Zhang, Hongxia
    et al.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Jia, Dagong
    Jing, Wencai
    Liu, Kun
    Zhang, Yimo
    Effects of angular misalignment in interferometric detection of distributed polarization coupling2009In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 20, no 9, p. 095112-Article in journal (Refereed)
    Abstract [en]

    White light interferometry is used to measure the distributed polarization coupling (DPC) in polarization-maintaining fibers (PMFs). By using a scanning Michelson interferometer to compensate the optical path difference (OPD) induced by the modal birefringence of PMFs, both the coupling strength and positions of the coupling points can be acquired. In ideal DPC measurement, the two reflective mirrors on the fixed and scanning arms of the Michelson interferometer are normal to each other. But in practice, the movable reflective mirror cannot be aligned normally to the fixed mirror exactly, which leads to an angular misalignment. The angular misalignment causes a variation of the OPD, which will reduce the fringe visibility. The theoretical simulation is investigated correspondingly. Consequently, the angular error leads to the miscalculation of the polarization coupling intensity. Based on the experimental results, a revised coupling strength calculation equation is proposed to minimize the influence of angular misalignment.

  • 121231.
    Zhang, Hongyang
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems. ABB, Power Grids Div, S-72164 Vasteras, Sweden.
    Harnefors, Lennart
    KTH, School of Electrical Engineering and Computer Science (EECS), Electric Power and Energy Systems. ABB, Corp Res, S-72178 Vasteras, Sweden..
    Wang, Xiongfei
    Aalborg Univ, Dept Energy Technol, DK-9220 Aalborg, Denmark..
    Gong, Hong
    Aalborg Univ, Dept Energy Technol, DK-9220 Aalborg, Denmark..
    Hasler, Jean-Philippe
    ABB, Power Grids Div, S-72164 Vasteras, Sweden..
    Stability Analysis of Grid-Connected Voltage-Source Converters Using SISO Modeling2019In: IEEE transactions on power electronics, ISSN 0885-8993, E-ISSN 1941-0107, Vol. 34, no 8, p. 8104-8117Article in journal (Refereed)
    Abstract [en]

    The interaction of a grid-connected voltage-source converter with a weak grid is of significant interest. In this paper, the converter together with the grid impedance is modeled as a single-input single-output (SISO) system. Provided that certain assumptions hold, this allows us to apply the standard SISO Nyquist stability criterion for stability analysis and controller design. The derivedmodel is verified against time-domain simulations and experiments. Themethod facilitates the design of the converter control system with adequate stability margins.

  • 121232.
    Zhang, Hongyang
    et al.
    ABB, Power Grids Div, Vasteras, Sweden..
    Harnefors, Lennart
    ABB, Corp Res, Vasteras, Sweden..
    Wang, Xiongfei
    Aalborg Univ, Dept Energy Technol, Aalborg, Denmark..
    Hasler, Jean-Philippe
    ABB, Power Grids Div, Vasteras, Sweden..
    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 Converters2018In: 2018 International Power Electronics Conference (IPEC-NIIGATA 2018 -ECCE ASIA), Institute of Electrical and Electronics Engineers (IEEE), 2018, p. 3684-3691Conference paper (Refereed)
    Abstract [en]

    Converter grid interaction is of great interest in a weak-grid condition. In this paper, an alternative multi-input multi-output closed-loop system is developed for the stability analysis of grid-connected voltage-source converters. In contrast to the conventional dq-impedance model and the eigenvalue analysis, this model eventually yields a single-input single-output transfer function. This enables to apply a single Nyquist curve for analyzing the overall system stability. The model is validated against time domain simulations and it shows excellent accuracy for predicting the system stability.

  • 121233.
    Zhang, Hongyang
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Ängquist, Lennart
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Norrga, Staffan
    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.
    Östlund, Stefan
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Benchmark of high-power STATCOM topologies for flicker compensation2016In: 2016 18TH EUROPEAN CONFERENCE ON POWER ELECTRONICS AND APPLICATIONS (EPE'16 ECCE EUROPE), IEEE, 2016Conference paper (Refereed)
    Abstract [en]

    The current from an electrical arc furnace (EAF) in a steel plant is highly unsymmetrical, i.e. it comprises a large amount of negative sequence. This causes problems with the voltage stability in the capacitor submodules for a static synchronous compensator (STATCOM), when implemented using the modular multilevel converter (M2C) concept. In order to counteract the uncontrollability of the capacitor voltage, a common practice is to circulate a zero-sequence fundamental current or a direct current. The method brings the benefit of re-balancing the active power among the individual phases while it requires over-rating of the valve current or dc voltage of the STATCOM. This would in practice give a more expensive design. This paper presents a benchmark of various M2C-based STATCOM designs considering the valve overrating. The STATCOM topologies will be benchmarked with the EAF modeled for all possible combinations of positive-and negative-sequence current. The rating comparison considering the dc capacitor ripple and zero-sequence harmonic injection are discussed.

  • 121234.
    Zhang, Hongyu
    KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630).
    Significance of Nondetects in the Mapping of Soil contaminants.2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In the sample data of soil contaminants, the existence of nondetects is a common phenomenon. Due to their small values, they are always ignored. However, they form an essential part of the sample distribution and arbitrary changes of their values will affect the properties of the distribution, for example, the 95% upper confidence limit of the mean (95UCLM), which is an important index in risk assessment, is strongly related with the sample distribution. Statistical analysis methods for nondetects involve substitution by half of the detection limit (DL/2), maximum likelihood estimation (MLE), Kaplan-Meier and regression on ordered statistics (ROS). The significance of nondetects was examined in this study. Two large data sets of cadmium (Cd) and mercury (Hg) containing censored observations in Annedal’s park were used, where the censored observations were known from the laboratory. Large data sets were subsampled into small data sets with different sample sizes and censoring levels. The 95UCLM value of each data set was calculated by use of the statistical software ProUCL 4.1.00. Through comparison, it was found that in most cases the 95UCLM value calculated with lab values was lower than that of the censored observation for each data set. The difference in 95UCLM values between the data set with nondetects and the data set with lab values varied in each sample and was found to be related to sample size and to the censoring level. The higher the censoring level was, the bigger the 95UCLM value difference became. Either too small or too large a sample size would reduce the difference between the 95UCLM values. This result helps in certain cases, when the 95UCLM value of the sample data is a little lower than the threshold; using the lab values instead of nondetects to recalculate the 95UCLM value may supply a manageable and economic tool to classify the contaminated area.

  • 121235.
    Zhang, Huaeli
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic parameters of paramagnetic iron-based alloys from first-principles calculations2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 5, p. 054107-Article in journal (Refereed)
    Abstract [en]

    The elastic properties of paramagnetic (PM) Fe1-xMx (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) solid solutions in the body-centered-cubic (bcc) and face-centered-cubic (fcc) structures are investigated using the exact muffin-tin orbital density functional method in combination with the coherent-potential approximation and disordered local-magnetic-moment model. All impurities considered here enlarge or leave nearly constant the equilibrium volume of PM Fe but at the same time produce both positive and negative changes in the elastic parameters. Some of the alloying elements induce opposite effects on shear elastic parameters C' and C-44 of PM bcc and fcc Fe, which is discussed. With a few exceptions, we find that the alloying effects on PM bcc Fe are smaller than on PM fcc Fe. The trends in the tetragonal elastic constant C' show a general correlation with the trends obtained for the bcc-fcc lattice energy difference.

  • 121236.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM).
    Elastic properties of ferromagnetic BCC Fe alloys form first-principles theory2010Licentiate thesis, comprehensive summary (Other academic)
  • 121237.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic Properties of Iron Alloys from First-Principles Theory2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Accurate description of materials requires the most advanced atomic-scale techniques from both experimental and theoretical areas. In spite of numerous available techniques, however, the experimental study of the atomic-scale properties and phenomena even in simple solids is rather difficult. Iron and its alloys (including steels) are among the most important engineering materials due to their excellent mechanical properties. In these systems, the above challenges become more complex due to the interplay between the structural, chemical, andmagnetic effects. On the other hand, advanced computational methods based on density functional theory (DFT) ensure a proper platform for studying the fundamental properties of materials from first-principles theory. The present thesis belongs to the latter category. We use advanced theoretical tools to give a systematic description of Fe and a series of Fe-rich alloys in the ferromagnetic (FM) body-centered-cubic (bcc), paramagnetic (PM) bcc, and PM face-centered-cubic (fcc) structures. For solving the basic DFT equations for steel alloys, we adopt the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA) and the disordered local magnetic moment (DLM) model.

    We start by assessing our theoretical tools in the case of Fe. For the FM state, we find that there is a magnetic transition close to the ground state volume of bcc Fe, which is explained by the peculiarmagnetic band structure. We conclude that the common equation of state functions can not capture the physics of this magnetic transition, leading to serious underestimation of theoretical bulk modulus of Fe. When the above effect is properly taken into account, theory is shown to reproduce the low-temperature experimental bulk properties (equation of state and elastic parameters) of FM bcc Fe within ∼ 1% for the volume and ∼7% for the elastic constants.

    Using the EMTO-CPA-DLM picture, in contrast to previous theoretical predictions, we demonstrate that the competing high-temperature cubic phases of PM Fe correspond to two distinct total energy minima in the tetragonal (Bain) configurational space. Both fcc and bcc lattices are dynamically stable, and at static conditions the fcc structure is found to be the thermodynamically stable phase. When the thermal expansion is taken into account, our theoretical bulk properties calculated for PM Fe agree well with the available experimental data. Increasing temperature is predicted to stabilize the bcc (δ) phase against the fcc (γ) one because of the shallow energy minimum around the bcc structure.

    The calculated composition-dependent equilibriumlattice constants, single-crystal elastic constants Cij(c) (here c stands for the amount of alloying additions), and polycrystalline elastic parameters of FM bcc Fe show good agreement with former theoretical and available experimental data, implying that the employed theoretical approach is suitable to calculate the elastic properties of FM Fe alloys. For FM bcc Fe alloys, all impurities considered in this thesis (Al, Si, V, Cr, Mn, Co, Ni, and Rh) enlarge the equilibrium lattice parameter and accordingly decrease the C11(c), C12(c), and C′(c) elastic constants. However, a peculiar phenomenon appears for C44(c). Namely, in spite of increasing volume, Al, Si, V, Cr, and Mn are found to increase C44(c), whereas the alloying effects of Co, Ni, and Rh are small. The anomalous alloying effect in C44(c) isshown to originate from the particular electronic structure of FM bcc Fe. The complex composition dependence of C44(c) is reflected in the polycrystalline properties of FM Fe as well.

    Unlike for FM bcc Fe, both positive and negative alloying effects appear for the theoretical equilibrium lattice parameters, single-crystal and polycrystalline elastic properties of PM bcc and fcc Fe. For many elastic parameters and binary systems considered in this thesis, alloying element induces opposite effects in fcc and bcc phases. In other words, the alloying effects on the elastic properties of PM Fe-based alloys show strong structure dependence. While neither the volume nor the electronic effect can explain the calculated trends of C′(c), we find that there is a general correlation between alloying effects on the lattice stability and C′(c). With a few exceptions, alloying elements have much larger effects on FM bcc Fe than on PM fcc Fe. A slightly larger alloying effect appears on PM fcc Fe compared to PM bcc Fe.

    According to the calculated fundamental properties, we also estimate the relative hardness of Fe alloys via two phenomenological solid-solution strengthening mechanisms. In those caseswhere experimental data are available, the predicted solid-solution strengthening effects are in line with the observations. The metastable Mg-doped Fe alloys surpass all rival binaries in density and solid-solution strengthening effects. The Fe-Cr and Fe-Cr-Ni alloys containing a few percent of Mg are also predicted to possess unusually high solid-solution hardening and low density compared to the host alloys. These attributes make theMg-bearing stainless steels very promising candidates for many applications, such as the high-strength and light-weight designs desired by for example the automotive industry.

  • 121238.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Al-Zoubi, Noura
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Alloying effects on the elastic parameters of ferromagnetic and paramagnetic Fe from first-principles theory2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 7, p. 073707-Article in journal (Refereed)
    Abstract [en]

    The elastic properties of paramagnetic face-centered-cubic (fcc) Fe(1-x)M(x) (M = Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys are investigated using the exact muffin-tin orbitals density functional method in combination with the coherent-potential approximation. We find that the theoretical lattice parameter of fcc Fe is strongly enlarged by Al, V, and Rh and slightly reduced by Si, Cr, and Co, while it remains nearly constant with Mn and Ni. Both positive and negative alloying effects appear for the elastic constants C(ij)(x) of fcc Fe. These findings are in contrast to those obtained for ferromagnetic body-centered-cubic (bcc) Fe alloys, where all alloying elements considered here are predicted to enlarge the lattice parameter and decrease the C(11)(x) and C(12)(x) elastic constants of bcc Fe. With some exceptions, alloying has much larger effects on ferromagnetic bcc alloys than on paramagnetic fcc ones. Based on the theoretical elastic parameters of the paramagnetic fcc and ferromagnetic bcc phases, simple parameterizations in terms of chemical composition of the equilibrium lattice constants, single-crystal elastic constants, and polycrystalline elastic moduli of Fe-based alloys are presented.

  • 121239.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles study of solid-solution hardening in steel alloys2012In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 55, p. 269-272Article in journal (Refereed)
    Abstract [en]

    Materials with excellent mechanical properties, such as light mass combined with remarkable hardness and toughness, are technologically important not least for automotive and other transport applications. Solid solution strengthening, due to dislocation pinning by impurities, is an effective route to enhance the intrinsic hardness of alloys. In the present work, we use advanced quantum theory to reveal the mechanical characteristics of iron alloys within and beyond their thermodynamic stability fields. Among the considered alloying elements, magnesium strongly reduces the density of the host alloys and significantly enhances the hardness. Our findings suggest that stainless steel grades containing a few percent of magnesium are promising engineering materials for high-strength and light-weight designs.

  • 121240.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ab initio calculations of elastic properties of bcc Fe-Mg and Fe-Cr random alloys2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 22Article in journal (Refereed)
    Abstract [en]

    Using the ab initio exact muffin-tin orbitals method in combination with the coherent-potential approximation, we have calculated the elastic parameters of ferromagnetic Fe1-mMgm (0 < m < 0.1) and Fe1-cCrc (0 < c < 0.2) random alloys in the body-centered cubic (bcc) crystallographic phase. Results obtained for Fe1-cCrc demonstrate that the employed theoretical approach accurately describes the experimentally observed composition dependence of the polycrystalline elastic moduli of Fe-rich alloys encompassing maximum similar to 10% Cr. The elastic parameters of Fe-Cr alloys are found to exhibit anomalous composition dependence around 5% Cr. The immiscibility between Fe and Mg at ambient conditions is well reproduced by the present theory. The calculated lattice parameter for the Fe-Mg regular solid solution increases by similar to 1.95% when 10% Mg is introduced in Fe, which corresponds approximately to 11% decrease in the average alloy density, in perfect agreement with the experimental finding. At the same time, we find that all of the elastic parameters of bcc Fe-Mg alloys decrease almost linearly with increasing Mg content. The present results show a much stronger alloying effect for Mg on the elastic properties of alpha-Fe than that for Cr. Our results call for further experimental studies on the mechanical properties of the Fe-Mg system.

  • 121241.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Density-functional study of paramagnetic iron2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 14, p. 140411-Article in journal (Refereed)
    Abstract [en]

    By using density-functional theory in combination with the coherent-potential approximation and the disordered local magnetic moment picture, we demonstrate that the competing high-temperature cubic phases of paramagnetic Fe correspond to two distinct total energy minima in the tetragonal (Bain) configurational space. Both the face-centered-cubic (fcc) and the body-centered-cubic (bcc) lattices are dynamically stable, and at static conditions the fcc structure is found to be the thermodynamically stable phase. The theoretical bcc and fcc bulk parameters are in agreement with the experimental data. Due to the shallow energy minimum around the bcc structure, increasing temperature is predicted to stabilize the bcc (δ) phase against the fcc (γ) one.

  • 121242.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Jesperson, Henrik
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Anomalous elastic hardening in Fe-Co alloys at high temperature2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 18, p. 184107-Article in journal (Refereed)
    Abstract [en]

    The elastic moduli of Fe1-cCoc (c <= 0.2) alloys are found to decrease strongly with increasing temperature, but show very weak alloying effects for both low-temperature ferromagnetic and high-temperature paramagnetic states. For temperatures slightly below and around the Curie temperature of Fe, Co addition significantly increases the elastic moduli. The variation of the tetragonal shear elastic constant upon 20% Co addition increases from a small negative value to more than 135% as the temperature rises from 0 to 1200 K. The expected elastic softening in the case of Al doping is not confirmed. Both anomalous trends are ascribed to the interplay between intrinsic chemical effects, magnetism, and temperature.

  • 121243.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hu, Qing-Miao
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Static equation of state of bcc iron2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 13, p. 132409-Article in journal (Refereed)
    Abstract [en]

    Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.

  • 121244.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Center of Microstructure Science, Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'An Jiaotong University, Xi'an, China.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhou, Minna
    Punkkinen, Marko P. J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala Univ, Dept Phys & Mat Sci, Div Mat Theory, SE-75120 Uppsala, Sweden.
    Vitos, Levente
    Ab initio determination of the elastic properties of ferromagnetic body-centered cubic Fe-Mn-Al alloys2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 10, article id 103904Article in journal (Refereed)
    Abstract [en]

    The elastic properties of ferromagnetic Fe1-x-yMnyAlx (0 <= x <= 0.5, y = 0, 0.1, and 0.2) random solid solutions in the body-centered cubic (bcc) crystallographic phase have been investigated using the ab initio exact muffin-tin orbitals method in combination with the coherent-potential approximation. Comparison with the experimental data demonstrates that the employed theoretical approach accurately captures the observed composition dependence of the lattice parameter. The predicted elastic parameters follow complex composition dependence. The C-11, C-12, and C' = (C-11 - C-12)/2 single-crystal elastic constants, the bulk (B), shear (G), and Young's (E) moduli, and the Cauchy pressure (C-12 - C-44) mainly decrease with increasing Al content, whereas the Zener anisotropy ratio (C-44/C') strongly increases with x. C-44 exhibits a non-linear x dependence. The Poisson (v) and Pugh (B/G) ratios first decrease with x but show non-monotonous behavior in high-Al alloys. In terms of the Pugh criterion, these trends suggest an increased brittleness in Al-containing alloys. Manganese has a complex non-monotonous effect on B/G in low-Al alloys (below similar to 15 at. % Al) but enhances the brittleness of the bcc solid solution in large-Al regime. The peculiar Mn alloying effect is explained in terms of magneto-volume mechanisms.

  • 121245.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Punkkinen, Marko P. J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Theoretical elastic moduli of ferromagnetic bcc Fe alloys2010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 27, p. 275402-Article in journal (Refereed)
    Abstract [en]

    The polycrystalline elastic parameters of ferromagnetic Fe1-xMx (M = Al, Si, V, Cr, Mn, Co, Ni, Rh; 0 <= x <= 0.1) random alloys in the body centered cubic (bcc) crystallographic phase have been calculated using first-principles alloy theory in combination with statistical averaging methods. With a few exceptions, the agreement between the calculated and the available experimental data for the polycrystalline aggregates is satisfactory. All additions considered here decrease the bulk modulus (B) and Poisson's ratio (nu) of bcc Fe. The complex composition dependence of the C-44 single-crystal elastic constant is reflected in the polycrystalline shear modulus (G), Young's modulus (E), and Debye temperature (Theta). The polycrystalline anisotropy of bcc Fe is increased by all additions, and Al, Si, Ni, and Rh yield the largest alloying effects.

  • 121246.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hertzman, Staffan
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Single-crystal elastic constants of ferromagnetic bcc Fe-based random alloys from first-principles theory2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 18, p. 184105-Article in journal (Refereed)
    Abstract [en]

    The elastic properties of ferromagnetic Fe1-xMx (M=Al, Si, V, Cr, Mn, Co, Ni, and Rh; 0 <= x <= 0.1) random alloys in the body-centered-cubic (bcc) crystallographic phase have been studied using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The theoretical lattice parameters and the single-crystal elastic constants agree well with the available experimental data. The most significant alloying effects are found for Al, Si, and Ni additions. All elements enlarge the lattice parameter and decrease the C-11, C-12, and C' elastic constants and the bulk modulus of bcc Fe. At the same time, C-44 is found to increase with Al, Si, V, Cr, or Mn and remain nearly constant with Co, Ni, and Rh. Accordingly, the elastic anisotropy of bcc Fe increases with all alloying elements considered here. The calculated alloying effects on the single-crystal elastic constants are shown to originate from volume effects in combination with the peculiar electronic structure of bcc Fe.

  • 121247. Zhang, Hualei
    et al.
    Sun, Xun
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dong, Zhihua
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ding, Xiangdong
    Wang, Yunzhi
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 155, p. 12-22Article in journal (Refereed)
    Abstract [en]

    Using ab initio calculations, we investigate the elastic properties of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high -entropy alloys (HEAs) in both body-centered cubic (bcc) and face-centered cubic (fcc) structures. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli. The predicted lattice constants increase monotonously with Al addition, whereas the elastic parameters exhibit complex composition dependences. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is due to the relatively low bulk modulus and C-12 elastic constant, which in turn are consistent with the relatively low cohesive energy. The present findings in combination with the experimental data suggest anomalous metallic character for the HEAs system. 

  • 121248.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Wang, Guisheng
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko P. J.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Hertzman, Staffan
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic anomalies in Fe-Cr alloys2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 19, p. 195501-Article in journal (Refereed)
    Abstract [en]

    Using ab initio alloy theory, we determine the elastic parameters of ferromagnetic and paramagnetic Fe1-cCrc (0 <= c <= 1) alloys in the body centered cubic crystallographic phase. Comparison with the experimental data demonstrates that the employed theoretical approach accurately describes the observed composition dependence of the polycrystalline elastic moduli. The predicted single-crystal elastic constants follow complex anomalous trends, which are shown to originate from the interplay between magnetic and chemical effects. The nonmonotonic composition dependence of the elastic parameters has marked implications on the micro-mechanical properties of ferrite stainless steels.

  • 121249.
    Zhang, Huang
    KTH, School of Industrial Engineering and Management (ITM).
    Energy Modelling for Data CenterInfrastructure2017Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 121250.
    Zhang, Hui
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    On Electric Machinery for Integrated Motor Drives in Automotive Applications2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Compact, electric drives for automotive traction applications represent animportant enabler towards realizing tomorrow’s fossil free transport solutions.One attractive solution is to integrate the power electronic converter withits associated electric machinery into a single unit. This thesis, along withits appended papers, considers design and analysis of electric machinery forintegrated electric drives intended for automotive applications. Particular focusis put on permanent-magnet synchronous machines (PMSMs) with interiormountedpermanent magnets combined with modular converter topologies.In the first part of the thesis, different converter concepts and windingarrangements suitable for an integrated drive are reviewed. Compared to theconventional solution utilizing a three-phase two-level converter, a compactintegration can be implemented by physically splitting the converter and itsassociated dc-link capacitor into a number of converter submodules. Moreover,a modular concept also enables a certain level of fault tolerance.In the second part of the thesis, fractional-slot concentrated windings(FSCWs) are analyzed. First, a review for how to determine suitable slot, pole,and phase combinations is identified considering mainly the winding factor forthe main harmonic and the associated rotor losses. Then, integrated modularconverter concepts and associated winding configurations are considered andslot, pole and phase combinations that also comply with the consideredmodular converters are proposed. Further, two possible winding arrangementssuitable for the stacked polyphase bridges (SPB) and the parallel polyphasebridges (PPB) type converter are compared with respect to torque duringpost-fault operation in the event of failure of a single converter submodule.In the third part, an iterative process adopting both finite element analysisand analytical techniques is proposed for the design of PMSMs with interiormountedpermanent magnets and FSCWs. The resulting machine designsillustrate tradeoffs in terms of fault tolerance, power factor, torque density,and potential for field-weakening operation. From a given set of specifications,an experimental prototype is also designed and built.Finally, since a FSCW generally results in a large harmonic content ofthe resulting flux-density waveform, models for predicting eddy-current lossesin the permanent magnets are analyzed and compared. Particularly, modelsadopting different formulations to the Helmholtz equation to solve for the eddycurrents are compared to a simpler model relying on an assumed eddy-currentdistribution. Boundaries in terms of magnet dimensions and angular frequencyare also identified in order to aid the machine designer whether the mostsimple loss model is applicable or not. With a prediction of the eddy-currentlosses in the permanent magnets together with a corresponding thermal model,predicted volumetric loss densities exemplified for combinations of slot andpole numbers common in automotive applications are presented along withthe associated thermal impact.

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