A method is evaluated that determines the chemical oxygen surface exchange k δ and chemical bulk ... more A method is evaluated that determines the chemical oxygen surface exchange k δ and chemical bulk diffusion coefficient D δ of mixed ionic-electronic conducting La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ (LSCF) by using electrochemical impedance spectroscopy. Each measured spectrum contains the Gerischer impedance, which represents the polarization characteristics of a porous LSCF cathode structure. Firstly, it was separated from the impedance data by a well-established equivalent circuit model. Second, the specific values for k δ and D δ were calculated from the Gerischer impedance using the ALS (Adler, Lane, Steele) model. Third, the corresponding microstructure parameters, porosity, surface area and tortuosity, were quantified by focused ion beam (FIB) tomography. This allows a consideration of the actual sample characteristics. This approach was applied, for the first time, to follow the time-and temperature-dependent course of k δ and D δ values for porous LSCF cathode structures, from the very beginning of cell operation up to several hundred hours. The microscopic structure characteristics remain constant, as indicated by scanning electron microscope analysis and proven by FIB tomography before and after measurements. At T = 600°C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both k δ and D δ. At T = 750°C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for k δ but a distinct decrease for D δ. For T = 900°C the cathode polarization resistance increased only marginal, and so k δ and D δ remained constant.
The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resista... more The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resistance of the electrodes. Electrochemical impedance spectroscopy (EIS) enables a deconvolution of individual electrochemical processes. In case of mixed ionic-electronic conducting (MIEC-) cathodes the impedance spectra result from the coupling of gas diffusion, surface exchange and bulk diffusion of oxygen ions. In this paper we present a three-dimensional (3D) finite element method (FEM) model which allows the transient simulation of the underlying processes in a porous cathode structure. The developed model is validated with a well established homogenized 1D model by comparing the area specific resistance and the corresponding impedance spectra. In case of a homogeneous 3D microstructure the FEM simulation results show an excellent agreement with the homogenized 1D model. Furthermore, the 3D FEM model is applied for impedance simulations of a technical MIEC cathode which microstructure w...
Page 1. Octoba ]CJiO MCGOVERN AND BEROZA: JAPANESE BEETLE ATTRACTANTS 1479 al though useful for r... more Page 1. Octoba ]CJiO MCGOVERN AND BEROZA: JAPANESE BEETLE ATTRACTANTS 1479 al though useful for research purposes, is elaborate and expensive and therefore impractical for large-scale insect detection programs. ...
2008 IEEE 21st International Conference on Micro Electro Mechanical Systems, 2008
This paper reports on a novel four-degree-of-freedom solid state joystick. It consists of a rigid... more This paper reports on a novel four-degree-of-freedom solid state joystick. It consists of a rigid polymer cylinder with spherical dome on top of a CMOS chip. The sensor chip with a size of 2.0times2.5 mm2 contains 10 octagonal stress sensors distributed across the surface exploiting the shear piezoresistive effect in silicon. The joystick shows a linear response of its integrated stress sensors to forces Fx, Fy, and Fz up to 5 N and to a moment Mz up to 5 mNm. For calibration, the sensitivities towards these loads were measured individually. The Moore-Penrose inverse of the sensitivity matrix enables the extraction of these four loads from the vector of the piezoresistor signals.
We present a non-resonant light emitting diode with a novel concept of light outcoupling. Light i... more We present a non-resonant light emitting diode with a novel concept of light outcoupling. Light is generated in the center of a radially symmetric structure and propagates between two mirrors to a tapered region where outcoupling occurs. Different process routes are developed resulting in on-substrate as well as substrateless devices.
This paper reports a novel sensor concept to measure the six components of the forces and torques... more This paper reports a novel sensor concept to measure the six components of the forces and torques applied to smart orthodontic brackets. The concept is demonstrated using a packaged CMOS based sensor chip and a novel test setup to measure the six forces and moments applied to a smart bracket. By measuring the stresses in the plane of a sensor chip inside the bracket, it is possible to extract the forces and moments externally applied to the bracket. The sensor chip contains three CMOS piezoresistive stress sensors making it possible to separate one force and two moments with the range typical for orthodontic therapy.
Polarization losses within solid oxide fuel cell electrodes are strongly related to microstructur... more Polarization losses within solid oxide fuel cell electrodes are strongly related to microstructure characteristics and composition. High-resolution microstructure analysis is a sensible method to understand and improve electrode performance. This study presents the application of a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) for the three-dimensional (3D) reconstruction of a high performance LSCF-cathode. Error sources arising from FIB slicing and SEM sequencing are identified and methods of correction are presented. The corrected reconstruction data representing altogether a volume of ~ 2800µm3 are the basis for the calculation of the microstructure parameters (i) surface area, (ii) volume/porosity fraction and (iii) tortuosity. These parameters constitute the basis to calculate cathode performance via simplified microstructure models (1-3). Furthermore we present a detailed 3D finite element method (FEM) model to calculate the tortuosity from the accurately r...
A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting soli... more A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting solid oxide fuel cell (SOFC) cathodes that considers the complex coupling of gas diffusion, surface exchange, ionic bulk-diffusion and electrolyte conductivity is presented. By using the finite element method, this model enables the time-dependent and space-resolved simulation of the physicochemical processes in a porous cathode microstructure. The developed model is used for a detailed analysis of the formation of a 'Gerischer-type' impedance. It is detected that the low-frequency part is dominated by the surface exchange reaction, whereas the typical 45 • ramp of the Gerischer impedance is related to the ionic diffusion in the bulk. The capability of the time-dependent 3D impedance model is evaluated versus a well-established homogenized analytical model. For homogeneous 3D microstructures both models calculate impedance curves which are in excellent agreement. Further impedance simulations with microstructures containing features of high-performance SOFC cathodes clearly show that model separates and quantifies the contribution of the gas diffusion in a porous cathode layer. At an oxygen partial pressure of 0.21 atm the gas diffusion accounts for only 2% of the total polarization resistance, whereas a depletion of oxygen to 0.01 atm significantly increases this value to 38%.
ABSTRACT The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode fo... more ABSTRACT The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode for lithium-ion cells are analyzed by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. The spatial distributions of (a) carbon black as electronic conductor (b) LiFePO4 as active material and (c) pore volume are reconstructed by appropriate image processing methods. The global threshold segmentation procedure is replaced by a refined local threshold method, which accounts for gradients in luminosity even within very large imaged volumes. The precise analysis of the high-power cathode demands for reconstructing a very large volume of 18.15 x 17.75 x 27.8 mu m(3), caused by the dual length-scale design of LiFePO4, carbon black and pore phase. The microstructure features, (a) electrochemically active surface area and particle size distribution of LiFePO4, (b) shape and particle size distribution of carbon black and (c) porosity and tortuosity of the pore phase are compared between lab-scale and high-power cathode.
High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) ar... more High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode
A method is evaluated that determines the chemical oxygen surface exchange k δ and chemical bulk ... more A method is evaluated that determines the chemical oxygen surface exchange k δ and chemical bulk diffusion coefficient D δ of mixed ionic-electronic conducting La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3 − δ (LSCF) by using electrochemical impedance spectroscopy. Each measured spectrum contains the Gerischer impedance, which represents the polarization characteristics of a porous LSCF cathode structure. Firstly, it was separated from the impedance data by a well-established equivalent circuit model. Second, the specific values for k δ and D δ were calculated from the Gerischer impedance using the ALS (Adler, Lane, Steele) model. Third, the corresponding microstructure parameters, porosity, surface area and tortuosity, were quantified by focused ion beam (FIB) tomography. This allows a consideration of the actual sample characteristics. This approach was applied, for the first time, to follow the time-and temperature-dependent course of k δ and D δ values for porous LSCF cathode structures, from the very beginning of cell operation up to several hundred hours. The microscopic structure characteristics remain constant, as indicated by scanning electron microscope analysis and proven by FIB tomography before and after measurements. At T = 600°C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both k δ and D δ. At T = 750°C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for k δ but a distinct decrease for D δ. For T = 900°C the cathode polarization resistance increased only marginal, and so k δ and D δ remained constant.
The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resista... more The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resistance of the electrodes. Electrochemical impedance spectroscopy (EIS) enables a deconvolution of individual electrochemical processes. In case of mixed ionic-electronic conducting (MIEC-) cathodes the impedance spectra result from the coupling of gas diffusion, surface exchange and bulk diffusion of oxygen ions. In this paper we present a three-dimensional (3D) finite element method (FEM) model which allows the transient simulation of the underlying processes in a porous cathode structure. The developed model is validated with a well established homogenized 1D model by comparing the area specific resistance and the corresponding impedance spectra. In case of a homogeneous 3D microstructure the FEM simulation results show an excellent agreement with the homogenized 1D model. Furthermore, the 3D FEM model is applied for impedance simulations of a technical MIEC cathode which microstructure w...
Page 1. Octoba ]CJiO MCGOVERN AND BEROZA: JAPANESE BEETLE ATTRACTANTS 1479 al though useful for r... more Page 1. Octoba ]CJiO MCGOVERN AND BEROZA: JAPANESE BEETLE ATTRACTANTS 1479 al though useful for research purposes, is elaborate and expensive and therefore impractical for large-scale insect detection programs. ...
2008 IEEE 21st International Conference on Micro Electro Mechanical Systems, 2008
This paper reports on a novel four-degree-of-freedom solid state joystick. It consists of a rigid... more This paper reports on a novel four-degree-of-freedom solid state joystick. It consists of a rigid polymer cylinder with spherical dome on top of a CMOS chip. The sensor chip with a size of 2.0times2.5 mm2 contains 10 octagonal stress sensors distributed across the surface exploiting the shear piezoresistive effect in silicon. The joystick shows a linear response of its integrated stress sensors to forces Fx, Fy, and Fz up to 5 N and to a moment Mz up to 5 mNm. For calibration, the sensitivities towards these loads were measured individually. The Moore-Penrose inverse of the sensitivity matrix enables the extraction of these four loads from the vector of the piezoresistor signals.
We present a non-resonant light emitting diode with a novel concept of light outcoupling. Light i... more We present a non-resonant light emitting diode with a novel concept of light outcoupling. Light is generated in the center of a radially symmetric structure and propagates between two mirrors to a tapered region where outcoupling occurs. Different process routes are developed resulting in on-substrate as well as substrateless devices.
This paper reports a novel sensor concept to measure the six components of the forces and torques... more This paper reports a novel sensor concept to measure the six components of the forces and torques applied to smart orthodontic brackets. The concept is demonstrated using a packaged CMOS based sensor chip and a novel test setup to measure the six forces and moments applied to a smart bracket. By measuring the stresses in the plane of a sensor chip inside the bracket, it is possible to extract the forces and moments externally applied to the bracket. The sensor chip contains three CMOS piezoresistive stress sensors making it possible to separate one force and two moments with the range typical for orthodontic therapy.
Polarization losses within solid oxide fuel cell electrodes are strongly related to microstructur... more Polarization losses within solid oxide fuel cell electrodes are strongly related to microstructure characteristics and composition. High-resolution microstructure analysis is a sensible method to understand and improve electrode performance. This study presents the application of a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) for the three-dimensional (3D) reconstruction of a high performance LSCF-cathode. Error sources arising from FIB slicing and SEM sequencing are identified and methods of correction are presented. The corrected reconstruction data representing altogether a volume of ~ 2800µm3 are the basis for the calculation of the microstructure parameters (i) surface area, (ii) volume/porosity fraction and (iii) tortuosity. These parameters constitute the basis to calculate cathode performance via simplified microstructure models (1-3). Furthermore we present a detailed 3D finite element method (FEM) model to calculate the tortuosity from the accurately r...
A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting soli... more A time-dependent three-dimensional (3D) impedance model of mixed ionic electronic conducting solid oxide fuel cell (SOFC) cathodes that considers the complex coupling of gas diffusion, surface exchange, ionic bulk-diffusion and electrolyte conductivity is presented. By using the finite element method, this model enables the time-dependent and space-resolved simulation of the physicochemical processes in a porous cathode microstructure. The developed model is used for a detailed analysis of the formation of a 'Gerischer-type' impedance. It is detected that the low-frequency part is dominated by the surface exchange reaction, whereas the typical 45 • ramp of the Gerischer impedance is related to the ionic diffusion in the bulk. The capability of the time-dependent 3D impedance model is evaluated versus a well-established homogenized analytical model. For homogeneous 3D microstructures both models calculate impedance curves which are in excellent agreement. Further impedance simulations with microstructures containing features of high-performance SOFC cathodes clearly show that model separates and quantifies the contribution of the gas diffusion in a porous cathode layer. At an oxygen partial pressure of 0.21 atm the gas diffusion accounts for only 2% of the total polarization resistance, whereas a depletion of oxygen to 0.01 atm significantly increases this value to 38%.
ABSTRACT The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode fo... more ABSTRACT The three-dimensional microstructures of a lab-scale and a high-power LiFePO4 cathode for lithium-ion cells are analyzed by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. The spatial distributions of (a) carbon black as electronic conductor (b) LiFePO4 as active material and (c) pore volume are reconstructed by appropriate image processing methods. The global threshold segmentation procedure is replaced by a refined local threshold method, which accounts for gradients in luminosity even within very large imaged volumes. The precise analysis of the high-power cathode demands for reconstructing a very large volume of 18.15 x 17.75 x 27.8 mu m(3), caused by the dual length-scale design of LiFePO4, carbon black and pore phase. The microstructure features, (a) electrochemically active surface area and particle size distribution of LiFePO4, (b) shape and particle size distribution of carbon black and (c) porosity and tortuosity of the pore phase are compared between lab-scale and high-power cathode.
High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) ar... more High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode
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