[diss] Perustieteiden korkeakoulu / SCI

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Browsing [diss] Perustieteiden korkeakoulu / SCI by Department "Department of Engineering Physics"

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    Characterization of transport phenomena in small polymer electrolyte membrane fuel cells
    (Helsinki University of Technology, 2008-02-01) Himanen, Olli; Department of Engineering Physics; Teknillisen fysiikan laitos; Faculty of Information and Natural Sciences; Informaatio- ja luonnontieteiden tiedekunta
    In small fuel cell systems, energy consumption and size of auxiliary devices should be minimized. One option is to use passive controlling methods that rely on material and structural solutions. Therefore it is important to understand transport phenomena occurring in the cells. In this thesis, charge, mass, and heat transport phenomena related to small PEMFCs were studied experimentally and by modeling. A new method was developed for the characterization of water transport properties of polymer electrolyte membrane under realistic operating conditions. The method was used to evaluate the diffusion coefficient of water in the membrane. Due to channel-rib structure, cell components are inhomogeneously compressed. Charge and mass transport parameters were experimentally evaluated as a function of compression. The effect of inhomogeneous compression on cell operation was studied by modeling. Inhomogeneous compression does not significantly affect the polarization behavior of the cell, but it creates uneven current and temperature distributions inside the cell. This affects both cell performance and life-time and should not be ignored in cell design and modeling. The operation of a free-breathing PEMFC was studied at subzero temperatures. To be able to operate at low temperatures, current density must be high enough to avoid freezing of reactant product water inside the cell. Start-up at cold temperatures requires active heating. To maximize fuel efficiency, the operation of a free-breathing PEMFC in dead end mode was investigated. Dead ended operation with periodic purging enables high fuel utilization rate and the test cell operated without significant water management problems or performance loss.
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    Inhomogeneous compression of PEMFC gas diffusion layers
    (Helsinki University of Technology, 2008-03-14) Nitta, Iwao; Department of Engineering Physics; Teknillisen fysiikan laitos; Faculty of Information and Natural Sciences; Informaatio- ja luonnontieteiden tiedekunta
    Proton exchange membrane fuel cells (PEMFCs) are electrochemical devices which convert the chemical energy of reactants directly into electrical energy. This technology enables high efficiency and high energy density compared to internal combustion engines and current batteries, thereby making the technology attractive for broad range of applications. Furthermore, the only exhaust from PEMFCs is water, which makes them favorable from the environmental point of view. Gas diffusion layer (GDL) is one of the most important components in a fuel cell, whose functions cover a wide range of operations: to provide a passage for reactant access and product water removal, to conduct electricity and heat between adjacent components, and to provide mechanical support for the MEA. The properties of GDL are strongly dependent on compression pressure. Of particular importance is the fact that the compression pressure on GDL is inhomogeneous because of the rib/channel structure of bipolar plate. However, previous theoretical studies have typically neglected this effect, and thus they inherently contained errors in the modeled results. Therefore, the aim of this study is to obtain insight into the actual effects of the inhomogeneous compression of GDL using experimental and theoretical approaches. The experimentally evaluated properties are GDL mechanical properties, gas permeability, in-plane and through-plane electric conductivities, electric contact resistances between fuel cell components, thermal bulk conductivity and thermal contact resistance. All parameters are evaluated as a function of compressed GDL thickness. It was found that compression increases electric GDL conductivities but does not affect the thermal conductivity. Both electric and thermal contact resistances and gas permeability are decreased nonlinearly by the compression. The modeling study was performed by applying the experimentally evaluated parameters for the systematic investigation of the effect of inhomogeneous compression. It was found that the inhomogeneous compression does not significantly affect the polarization behavior and gas-phase mass transport. However, the effects on the current density distribution were evident. This was caused by the changes in the selective current path, which is determined by the combinations of conductivities of components and contact resistance between them. Despite the highly uneven current distribution and variation in material parameters by the inhomogeneous compression, the temperature profile was fairly even over the active area, contrary to the predictions in previous studies. This study suggests that high current density distribution caused by the inhomogeneous compression of GDL has a significant effect on the local cell performance and cell durability. The insight obtained from this study is highly beneficial for development and construction of fuel cells, as well as predicting their performance and life time.
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    Scanning force microscopy simulations of nanoparticles on insulating surfaces
    (Helsinki University of Technology, 2008-02-15) Pakarinen, Olli; Department of Engineering Physics; Teknillisen fysiikan laitos; Faculty of Information and Natural Sciences; Informaatio- ja luonnontieteiden tiedekunta
    Scanning (atomic) force microscopy (SFM/AFM) is a surface science method capable of imaging surfaces with atomic resolution. SFM is a local probe method, closely related to other scanning probe microscopy methods like scanning tunneling microscopy (STM). Dynamic SFM studied in this thesis utilizes a very sharp tip at the end of an oscillating cantilever, and forms images of surfaces by measuring the tip-sample interaction while scanning very close (typically less than 1 nm) above the surface. Computational work is typically needed for interpretation of experimental SFM results, as the output of the instrument depends strongly on the atomic structure of the tip apex, unknown in most experiments. Simulations also open a window to view the atomic scale processes which determine the outcome of the experiment, and can show new ways to optimise the use of SFM. This dissertation presents computational simulations of scanning force microscopy, focusing on imaging nanoscale particles on insulating surfaces. Numerical methods to calculate the tip-sample interactions are developed. Simulations of atomic resolution contrast in SFM imaging are performed utilizing density functional theory as well as semiclassical methods. Larger scale simulations focusing on the tip convolution problem are made possible with the development of a numerical code calculating van der Waals interaction between arbitrarily shaped objects. The effect of humidity on particle-surface interaction is studied by development of another numerical code modeling the capillary forces. The described work generates new understanding of image formation in SFM, and of the change of behavior of capillary forces at the nanoscale. A new application to utilize the constant height mode of SFM to greatly diminish the tip convolution effect is presented, and its success is explained with simulations.
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    Stability and dynamics of quantized vortices in gaseous Bose-Einstein condensates
    (Helsinki University of Technology, 2008-03-07) Huhtamäki, Jukka; Department of Engineering Physics; Teknillisen fysiikan laitos; Faculty of Information and Natural Sciences; Informaatio- ja luonnontieteiden tiedekunta
    Bose-Einstein condensation is a quantum statistical phase transition which was theoretically predicted almost a hundred years ago. After years of seminal research, physicists realized the first almost ideal Bose-Einstein condensates in ultracold dilute atomic gases in 1995. Since then, the theoretical and experimental methods concerning such systems have been developing rapidly, and many fascinating phenomena have been found in these novel quantum systems. Bose-Einstein condensation occurs in a system consisting of massive bosons when a single quantum state becomes macroscopically occupied as the temperature is lowered below the transition temperature. In general, condensates consisting of repulsively interacting bosons exhibit superfluidity: Particle currents can flow in the system without dissipation and viscosity. Moreover, the velocity fields of condensates have to be irrotational, which severely restricts the rotational characteristics of these systems. Apart from the center of mass motion, the system may carry angular momentum in the form of elementary excitations or so-called quantized vortices. This Thesis is a theoretical study of subjects related to stability and dynamics of quantized vortices in dilute atomic Bose-Einstein condensates. The precession and instability of off-centered vortices in trapped condensates is investigated both in the zero-temperature limit and at finite temperatures. Dynamical stability of multiply quantized vortices and vortex clusters is studied in axisymmetric trap geometries. Splitting of energetically and dynamically unstable multiply quantized vortices into singly quantized vortices is also studied. Finally, as a separate subject, tunneling of a condensate through a potential barrier is investigated. Majority of this work relies on numerical methods for solving the Gross-Pitaevskii and Bogoliubov equations, which are of central importance in the study of dilute atomic Bose-Einstein condensates.