Dissertations / Theses on the topic 'Dielectric boundary'

Microfluidics and its applications to Lab-on-a-Chip have attracted a lot of attention. Because of the small length scale, the flow is characterized by a low Re number. The governing equations become linear. Boundary element method (BEM) is a very good option for simulating the fluid flow with high accuracy. In this thesis, we present a 2D numerical simulation of the electrothermal flow using BEM. In electrothermal flow the volumetric force is caused by electric field and temperature gradient. The physics is mathematically modeled by (i) Laplace equation for the electrical potential, (ii) Poisson equation for the heat conduction caused by Joule heating, (iii) continuity and Stokes equation for the low Reynolds number flow. We begin by solving the electrical potential and electrical field. The heat conduction is caused by the Joule heating as the heat generation term. Superposition principle is used to solve for the temperature field. The Coulomb and dielectric forces are generated by the electrical field and temperature gradient of the system. The buoyancy force is caused by the non-uniform temperature distribution inside the system. We analyze the Stokes flow problem by superposition of fundamental solution for free-space velocity caused by body force and BEM for the corresponding homogeneous Stokes equation. It is well known that a singularity integral arises when the source point approaches the field point. To overcome this problem, we solve the free-space velocity analytically. For the BEM part, we also calculate all the integrals analytically. With this effort, our solution is more accurate. In addition, we improve the robustness of the matrix system by combining the velocity integral equation with the traction integral equation when we simulate the electrothermal pump. One of our purpose is to design a pump for the microfluidics system. Since the system is a long channel, the flow is fully developed in the area far away from the electrodes. With this assumption, the velocity profile is parabolic at the inlet and outlet of the channel. So we can get appropriate boundary conditions for the BEM part of Stokes equation. Consequently, we can simulate the electrothermal flow in an open channel. In this thesis, we will present the formulation and implementation of BEM to model electrothermal flow. Results of electrical potential, temperature field, Joule heating, electrothermal force, buoyancy force and velocity field will be presented.

This dissertation presents an experimentally supported multi-physics model of a dielectric barrier discharge boundary layer flow control actuator. The model is independent of empirical data about the specific behavior of the system. This model contributes to the understanding of the specific mechanisms that enable the actuator to induce flow control. The multi-physics numerical model couples a fluid model, a chemistry model, and an electrostatics model. The chemistry model has been experimentally validated against known spectroscopic techniques, and the fluid model has been experimentally validated against the time-resolved shadowgraphy. The model demonstrates the capability to replicate emergent flow structures near a wall. These structures contribute to momentum transport that enhance the boundary layer’s wall attachment and provide for better flow control. An experiment was designed to validate the model predictions. The spectroscopic results confirmed the model predictions of an electron temperature of 0.282eV and an electron number density of 65.5 × 10⁻¹²kmol/m³ matching to within a relative error of 12.4% and 14.8%, respectively. The shadowgraphic results also confirmed the model predicted velocities of flow structures of 3.75m/s with a relative error of 10.9%. The distribution of results from both experimental and model velocity calculations strongly overlap each other. This validated model provides new and useful information on the effect of Dielectric Barrier Discharge actuators on flow control and performance. This work was supported in part by NSF grant CNS-0960081 and the HokieSpeed supercomputer at Virginia Tech.
Ph. D.

This thesis describes a project in which algorithms are developed for the rapid and accurate solution of Poisson's equation in the presence of a dielectric boundary and multiple point charges. These algorithms are then used to perform Brownian dynamics simulations on realistic models of biological ion channels. An iterative method of solution, in which the dielectric boundary is tiled with variable sized surface charge sectors, provides the flexibility to deal with arbitrarily shaped boundaries, but is too slow to perform Brownian dynamics. An analytical solution is derived, which is faster and more accurate, but only works for a toroidal boundary. Finally, a method is developed of pre-calculating solutions to Poisson's equation and storing them in tables. The solution for a particular configuration of ions in the channel can then be assembled by interpolation from the tables and application of the principle of superposition. This algorithm combines the flexibility of the iterative method with greater speed even than the analytical method, and is fast enough that channel conductance can be predicted. The results of simulations for a model single-ion channel, based on the acetylcholine receptor channel, show that the narrow pore through the low dielectric strength medium of the protein creates an energy barrier which restricts the permeation of ions. They further show that this barrier can be removed by dipoles in the neck of the channel, but that the barrier is not removed by shielding by counter-ions. The results of simulations for a model multi-ion channel, based on a bacterial potassium channel, show that the model channel has conductance characteristics similar to those of real potassium channels. Ions appear to move through the model multi-ion channel via rapid transitions between a series of semi-stable states. This observation suggests a possible physical basis for the reaction rate theory of channel conductance, and opens up an avenue for future research.

On propose de preparer un materiau a haute constante dielectrique a couches d'arret aux joints de grains par un frittage a basse temperature (1100**(o)c) de srtio::(3) dope et reduit en presence d'un sel de lithium. La formation de la seconde phase isolante constituee de bi::(2)o::(3) est envisagee au cours de ce meme cycle thermique. La poudre de srtio::(3) est prealablement dopee par des composes de structure perovskite afin de faciliter l'incorporation du dopant, et reduite a 1350**(o)c sous atmosphere reductrice. La physico-chimie de frittage a 1100**(o)c en presence d'un sel de lithium et les proprietes dielectriques et microstructurales de srtio::(3) dope par srli::(1/4)nb::(3/4)o::(3) d'une part, la::(2/3)tio::(3-epsilon ) d'autre part, sont etudiees puis comparees a celles de srtio::(3) dope par la la::(2)o::(3) et srtio::(3) non dope

The finite difference time domain (FDTD) method has become a main stream analysis tool for engineers solving complex electromagnetic wave interaction problems. Its first principles approach affords it a wide range of applications from radar cross section (RCS) predictions of electrically large structures to molecular scale analysis of complex materials. This wide area of application may be attributed to the coupling of auxiliary differential equations with Maxwell's equations to describe the physical properties of a given problem. Previous extensions have included sub-cell models for describing lumped circuit elements within a single Yee cell, transformation of near-field information to the far-field for the analysis of antenna problems, dispersive material models and mesh truncation techniques. A review of these extensions is presented. What has not been previously developed is the ability to truncate lossy dielectric materials at the boundary of the simulation domain. Such outer boundary conditions (OBCs) are required in simulations dealing with ground penetrating radar, integrated circuits and many microwave devices such as stripline and microstrip structures. We have developed such an OBC by surrounding the exterior of the simulation domain with a lossy dispersive material based on a two time-derivative Lorentz model (L2TDLM). We present the development of the material as an absorber and ultimately as a full 3D OBC. Examples of microstrip, structures are presented to re-enforce the importance of modeling losses in dielectric structures. Finally, validation of the FDTD simulator and demonstration of the L2TDLM OBC's effectiveness is achieved by comparison with measured results from these microwave devices.

The work considers pulse wire-guided electromagnetic (EM) waves. The main contribution of the work is the original design of the isolation bushing for maximizing of the received wave (echo) thanks to the results of the numerical simulations, the acquirement of the new acknowledges of the reflections of the guided EM wave from the various boundaries, the construction of PML layers for FDTD simulations in rotational coordinates, the advances made in temporary excitation sources for FDTD and the formulation of the program code for FDTD in rotational coordinates in Matlab environment. The rightness of the numerical simulations was verified in practical experiments. The practical exploitation of the results is supposed in instrumentation and control technology - the level measurement in reservoirs.

La recherche présentée dans cette thèse se concentre sur la conception et l’utilisation d’actionneurs plasma à décharge à barrière diélectrique (DBD) de faible épaisseur et à géométrie complexe afin d’exercer un contrôle d’instabilité sur deux configurations d’écoulement dont la dynamique est régie par des mécanismes d’instabilités primaires et/ou secondaires. Le cas d’une couche limite tridimensionnelle telle que rencontrée sur une aile en flèche est étudié à l’aide de deux stratégies de forçage permettant de manipuler la transition induite par un phénomène d’instabilité stationnaire. Ici, un réseau d’éléments de rugosité discrets (DRE) est installé en amont du forçage par DBD afin de verrouiller l’origine et l’évolution des tourbillons stationnaires transversaux de la couche limite. La première approche de forçage consiste à modifier l’écoulement amont par déformation (UFD). Une seconde approche par modification directe de l’écoulement de base est également introduite (BFM). Un retard de transition est observé indépendamment du forçage réalisé. Cependant, comme les tourbillons transverses sont fortement amplifiés en raison de l’utilisation de DRE, l’action par approche UFD peut conduire à la fois à une atténuation directe des structures fluidiques transverses telle qu’envisagée mais aussi à une action non intentionnelle sur la nature inflectionnelle de l’écoulement de base. La méthode BFM résulte en une interaction directe sur les tourbillons transverses, interactions confirmées par une étude théorique de l’instabilité sous l’effet d’un modèle simplifié d’actionneur DBD. Il s’agit de la première démonstration expérimentale du retard de transition sur une aile en flèche grâce à l’effet d’un actionneur plasma et également à la première preuve de concept expérimentale de la stratégie BFM.Le sillage d’une couche de mélange plane à bord épais et les phénomènes d’instabilité primaire et secondaire responsables pour l’expansion spatio-temporelle du sillage sont également étudiés. Des conditions de forçage fréquentiel puis spatial sont successivement testées et analysées par approche spectrale (décomposition orthogonale spectrale, SPOD) sur des données expérimentales de PIV multi-champs résolues en temps. L’instabilité primaire est excitée par un forçage spatialement homogène pulsé à la fréquence naturellement la plus amplifiée. Ce forçage l’atténue les instabilités aux sous-harmoniques et inhibe l’appariement tourbillonnaires. A l’inverse, le forçage aux sous-harmoniques renforce les phénomènes d’appariement conduisant à un fort taux d’épanouissement de la couche de mélange. Enfin, l’effet d’un forçage modulé spatialement se traduit par un taux d’accroissement variant selon la position transverse et qui traduit à la fois le renforcement et la modulation spatiale des structures à grande échelle. La segmentation du forçage selonl’envergure de la couche de mélange permet toujours de modifier les structures transverses mais en sus, la coalescence des structures longitudinales et transversales est favorisée.Les travaux de recherche réalisés confirment la capacité des actionneurs plasma de type DBD à exercer un forçage modulé à la fois temporellement et spatialement. La large réduction de la puissance électrique consommée dans le cas d’un forçage modulé spatialement permet une amélioration notable de l’efficacité du système de contrôle
The research presented in this thesis focuses on the design and use of dielectric barrier discharge (DBD) plasma actuators with thin and complex geometry electrodes to exert instability control on two flow configurations whose dynamics are governed by primary and/or secondary instability mechanisms.The case of a three-dimensional boundary layer as encountered on a swept wing is studied using two forcing strategies to manipulate the transition induced by a stationary instability phenomenon. Here, an array of discrete roughness elements (DRE) is installed upstream of the DBD forcing in order to lock the origin and evolution of the stationary cross-flow (CF) vortices in the boundary layer. The first forcing approach is upstream flow deformation (UFD). The second approach based on direct modification of the base flow is also introduced (BFM). Independent of the forcing applied, a transition delay is observed. However, as the CF vortices are strongly amplified due to the use of DRE, the action by UFD approach can lead both to a direct attenuation of the CF vortices as envisaged but also to an unintentional action on the inflectional nature of the base flow. The BFM method results in a direct attenuation of the CF velocity component, which is also confirmed by a theoretical study of instability under the effect of the DBD actuator through a simplified model. This is not only the first experimental demonstration of transition delay on a swept wing using plasma actuators, but also the first experimental proof of concept of the BFM strategy.The wake of a plane mixed layer with a thick edge and the primary and secondary instability phenomena responsible for the spatio-temporal expansion of the wake are also studied. Frequency and then spatial forcing conditions are successively tested and analysed by spectral approach (spectral proper orthogonal decomposition, SPOD) on experimental data from multi-field time-resolved particle image velocimetry. The primary instability is excited by a spatially uniform forcing pulsed at the naturally most amplified frequency. It is shown that the mean component of the flow is not modified while the spectral content of the mixing layer is largely affected. This forcing leads, in particular, to the inhibition of the pairing of vortical structures due to the attenuation of sub-harmonic instabilities. Conversely, direct forcing of sub-harmonic instabilities results in a reinforcement of the pairing phenomena, leading to a higher growth rate of the mixing layer. Finally, spatially modulated forcing results in a growth that varies according to the spanwise position, which reflects both the reinforcement and the spatial modulation of large-scale spanwise structures. The modulation of the forcing according to the scale of the mixing layer always allows the modification of the spanwise structures but in addition, the coalescence of the streamwise and spanwise structures is favoured.The research work carried out confirms the ability of DBD plasma actuators to exert a forcing modulated both temporally and spatially. The proposed actuators allow only a partial control of the instability phenomena in the three-dimensional boundary layer while the high receptivity of the initial region of a mixing layer has led to significant results both on the dynamics of spanwise and streamwise coherent structures. Thanks to a large reduction of the electrical power consumed in the case of spatially modulated forcing, the efficiency of the control system is greatly improved

Conception et mise au point d'un appareil permettant de mesurer la différence de réflectivité entre un échantillon chauffé jusqu'à 600°C et un échantillon à température ambiante dans un domaine spectral allant de 1,5 à 10 microns. Etude des interactions électron-phonon et électron-électron dans l'argent et le cuivre à partir de leurs propriétés optiques dans l'infrarouge. Influence des défauts de volume et de surface. Détermination de l'indice complexe de couches minces peu absorbantes, dans l'infrarouge.

碩士
義守大學
電機工程學系碩士班
99
In the thesis, we analyze the metallic waveguide and optical dielectric waveguide for different structures, which includes the rectangle waveguide, circular waveguide, metal waveguide with two media, dielectric waveguides, and multi-layer dielectric waveguides. Based on the boundary element method (BEM), we derived the characteristic equations of different waveguide structures by matching the boundary condition, Moreover, in this thesis, we proposed a numerical technique to calculate a improper integral problems. Both the Gaussian and Simpson integral methods are used to calculate the problem. The numerical solutions of the metallic waveguide by BEM matched with the analytical solutions.

The present work combines experimental and numerical efforts to enhance the maturity of dielectric barrier discharge (DBD) plasma actuators as flow-control devices. In an attempt to increase the effectiveness of controlling laminar-turbulent transition, the understanding of a stabilizing effect of the actuator force field on laminar boundary-layer flow is fostered. Parametrical studies extend the Reynolds number range for effective transition control beyond the limits of earlier investigations. A numerical tool kit, consisting of a boundary-layer solver with implemented DBD force model and coupled stability analysis, is developed to predict the flow-control effectiveness. Validation experiments show considerable transition delay in a wind-tunnel setting. The excellent agreement of the experimental data with the numerical predictions renders the latter valuable for the design of flow-control applications. In order to realize an application of plasma actuators under non-laboratory conditions, a transition control experiment is designed for in-flight application on a full-sized motorized glider. The performed proof-of-concept experiment at a Reynolds number of 3000000 is the first to show a successful use of DBD transition control under atmospheric flight conditions. The discussion of the significantly delayed transition and resulting drag reduction concludes with an estimate for the flow-control efficiency.

A numerical investigation of active wave cancellation, using a plasma actuator in both continuous and pulsed operation modes, was carried out for a flat-plate boundary layer with an adverse pressure gradient at low Reynolds number. Pulsing was achieved by rectangular and sinusoidal modulation of the high-frequency plasma excitation voltage. A closed-loop control was developed and implemented using Large-Eddy Simulations into a CFD code (FASTEST). With this feed-back control algorithm it was found that the control can be limited to two operating parameters in order to significantly reduce Tollmien-Schlichting waves (TS-waves). The feed-back control algorithm was validated using two existing optimization methods which were also implemented in the code. The first method finds a local minimum of a function with several variables using a pattern search technique that compares function values at the three vertices of a triangle. The second method, known as the trust-region method, is based on quadratic models for derivative-free minimization. It was found that the developed feed-back control works efficiently and can be used to determine the optimum operating parameters of the plasma actuator for cancellation of TS-waves. The amplitude reduction of TS-waves is of interest since it allows for a delay of laminar-to-turbulent transition in the boundary layer, resulting in significant drag reduction.

The drag developed on an object as it moves through a fluid comprises of a number of components arising from various and differing fluid phenomena. For aerodynamic bodies such as aircraft, one of the most dominant components of the total drag force is that arising from shear interactions between the surface of the object and the fluid. In steady, cruise conditions this shear-induced skin friction drag can account for almost 50% of the total drag force on the body and hence this is the reason much interest surrounds the minimisation of this component. Laminar Flow Control (LFC) is the field of aerodynamics focused on minimising skin friction, or viscous drag. The viscosity of a fluid, and the shear interactions between the layers of fluid and the aerodynamic body give rise to a boundary layer, a region of fluid with diminished fluid velocity and momentum. Laminar Flow Control aims to minimise the momentum deficit within the boundary layer by manipulating the flow within and encouraging favourable flow conditions to exist and be maintained. In essence, Laminar Flow Control attempts to maintain laminar flow within the boundary layer, improving the stability of the flow, delaying the onset of turbulence and the formation of a turbulent boundary layer that develops significantly more drag than an equivalent laminar structure. A number of techniques exist for controlling and maintaining laminar flow within a boundary layer. Examples include compliant surfaces, acoustic arrays and suction, and all share the common trait of complexity, which to date has limited the application of such systems in the real world. In the search for simpler Laminar Flow Control technology, attention has been turned towards Dielectric Barrier Discharge (DBD) plasma actuators as a possible alternative. Through the formation of a small volume of plasma, these actuators are capable of producing an electrostatic body force that can couple with the surrounding air and bring about a jetting effect without the addition of mass. This jetting effect, if controlled effectively, can potentially favourably augment a boundary layer flow and lead to a delay in transition. The work discussed in this thesis represents a contribution to the field of DBD-based Laminar Flow Control. The aim was to further investigate the potential of plasma actuators for improving the hydrodynamic stability of a boundary layer and hence contribute to the limited published data pertaining to this field. The research involved the development of a DBD-based LFC system in which plasma actuators were used to augment the most fundamental of boundary layer flows, the flat plate, Blasius-type. By measuring the augmentation to the velocity profile of the boundary layer brought about by the LFC system, the stability of the flow was able to be investigated and hence the feasibility of the technology determined. The plasma actuators utilised in this research were designed such that control could be achieved over the shape of the induced jetting profile. To minimise adverse interactions with boundary layer flows, the plasma actuators were designed so that the magnitude and position of the maximal induced jetting velocity could be controlled. After consultation of the literature, novel actuators utilising orthogonally arranged electrodes were conceived and tested in a parametric study. Through variation of the distance to which the exposed electrode sat proud above the surface of the actuator, in addition to variation of the applied voltage, it was found that the desired control over the induced jet could be attained, leading to the identification of two mechanisms through which the DBD-based LFC system could be tuned. The details of the design and development of these orthogonal actuators and the effect of the electrode height on the jetting characteristics of the devices can be found in Gibson et al. (2009a) and Gibson et al. (2009b). After identifying suitable and novel actuator arrangements, a tuning strategy was conceived to hasten the development of the LFC system. Rather than implementing the actuators and measuring the response of the boundary layer to the plasma first, Linear Stability Theory was instead used to identify desirable boundary layer augmentation objectives for the LFC system. Linear Stability Analyses (LSAs) were performed on a number of idealised boundary layer flows, obtained from curve fitting analytical functions to published DBDaugmented boundary layer data, as well as from boundary layer theory. The LSAs were conducted using an Orr-Sommerfeld Equation solver developed as part of this research, which utilises a finite differencing scheme. The outcome of this comparison process was that the developed DBD-based LFC system was used to attempt to augment the boundary layer such that the flow attained an asymptotic suction velocity profile, which would give the boundary layer a limit of stability almost two orders of magnitude greater than that of the base flow, and hence significant robustness to transition. The conceived DBD-based LFC system was implemented into a Blasiusv type boundary layer which was formed over the Flat Plate Rig (FPR) designed and developed as part of this research. Initially a single actuator was utilised, positioned just upstream of the location of the critical Reynolds Number (limit of stability) of the flow. Due to the design of the FPR and the actuators utilised, it was possible to study the response of the layer to the plasma with and without a mild suction effect, introduced through a 5mm wide slot that was required for operation of the actuator. This mild suction effect was measured to be approximately 4Pa, and by itself was found to be insufficiently strong enough to augment the flow such that it attained the characteristics of a boundary layer with uniform wall suction. With the FPR, measurements of the velocity profile of the boundary layer with and without flow control were made around the critical Reynolds Number location of the flow (80000 < Rex < 120000), which allowed the changes to the stability of the flow to be studied. As discussed in Gibson et al. (2012) the initial results of the DBD-based LFC system showed that the plasma was adversely affecting the stability of the flow. Subsequent tuning of the system was therefore performed through variation of the applied voltage of the actuator. From this tuning it was found that an actuator operated with an applied voltage of 19.0kVpp (referred to as a low-voltage actuator) in conjunction with the mild suction effect, produced boundary layer characteristics akin to those of a flow exposed to uniform wall suction. In addition, an actuator operated with an applied voltage of 21.4kVpp (referred to as a high-voltage actuator) was found to adversely affect the stability, even more so in the absence of the mild suction effect. The single low-voltage actuator was found to be able to maintain uniform wall suction-like characteristics for 50mm beyond the trailing edge of the encapsulated electrode. This finding pertaining to the use of the low-voltage actuator highlighted the potential of a single DBD device to develop uniform wall suction-like characteristics with only a mild suction effect through a single slot, and hence in a less complex fashion than conventional suction systems. An attempt was made to maintain the favourable benefits of the single, low-voltage actuator by using two such actuators placed in series. However, the effect of this combined double-actuator/suction system differed only slightly from the suction-only system (with two slots instead of one), meaning that in this configuration, the use of the plasma was somewhat superfluous. Hence it could be concluded from the results of the research that a single low-voltage actuator operated in conjunction with a mild suction effect is more effective as a LFC system than a single mild-suction slot, but a combined double-low-voltage actuator/suction system is no better than a simpler and less energy consuming double-mild-suction slot system. It is, however, anticipated that through the undertaking of future works, utilising additional actuators that have undergone further tuning, a LFC even more effective than the double suction slot system tested in this research will ultimately be developed.
Thesis(Ph.D)-- University of Adelaide, School of Mechanical Engineering, 2012

The aim of this study is the characterization and quantification of dielectric barrier discharge plasma actuators when applied to the generation of stable vortices in laminar boundary layers for flow control. To this aim, existing measurement and analysis strategies were applied and extended. Using a specific actuator configuration the lower and upper operation limits have been identified. Between these limits, a constant and two-dimensional force distribution of the actuator can be expected. These operation limits were used to define the parameter range for the description of the induced body force in extent and magnitude. Alongside with this characterization the time resolved force production was analyzed. Based on these studies, the influencing parameters resulting from the actuator setup and the operating conditions were analyzed and quantified. Empirically determined scaling laws describing each influencing parameter were introduced. This enabled an improved prediction of the induced force for varying operating conditions. In addition to this direct scaling of the induced force by influencing the discharge, an indirect scaling method was presented by a non-continuous operation. This reduces the time-averaged force production and, therefore, allows decoupling of the force magnitude and its spatial extent. The analysis of the influencing parameters of the force production was used as a basis for parameter identification during the generation of longitudinal vortices by plasma actuator arrays. The qualification of plasma actuator vortex generators for transition control was demonstrated by the damping of Tollmien-Schlichting waves in laminar boundary layers in two different wind tunnels.

碩士
國立臺灣大學
應用力學研究所
99
Ferroelectric materials have been widely recognized as one of important smart materials due to their excellent dielectric and piezoelectric behavior, and are therefore often used in actuator and sensor applications. It has been widely known that the abnormally large dielectric and piezoelectric behavior can be observed by forming MPB (morphotropic phase boundary) due to chemical alloying in certain conventional lead-based ferroelectrics. The explanations vary due to different observations of experiment. One of the important observations is the appearance of fine scales of laminar domain patterns as the composition approaches MPB (Wang et al. 2003; Wang 2006; Schonau et al. 2007; Wang 2007). As a result, the convention models based on single domain state for explaining the enhancement of piezoelectricity are questionable. Here, we propose a model based on energy minimizing laminated domain pattern for studying the effective dielectric and piezoelectric properties at composition near MPB. It is found that the effective coefficients are not equivalent to the simple volume average of the distinct domain states. The results are consistent to those base on micromechanics calculations when the applied field is small. In addition, the effective dielectric and longitude piezoelectric coefficients at the poling direction are much larger than those calculated based on the single domain state.

博士
臺灣大學
電信工程學研究所
95
A novel hybrid numerical technique is proposed for analyzing general 2D electromagnetic problems with 1D gratings. The proposed method combines the conventional boundary integral-equation method (BIEM) with the plane-wave-expansion technique, taking the benefit from either method. In the framework of the proposed method, the BIEM is in charge of formulating the major part of the problem containing the grating structure, while the plane-wave expansion is for describing the nature of the outgoing wave and truncating the computation domain in the direction perpendicular to the grating extension. The free space Green’s function with the periodic boundary condition is used to replace the commonly used periodic Green’s function for dealing with the periodicity of the problem. This circumvents the convergence difficulty of the periodic Green’s function. The performance of the PW-BIEM with the non-uniform mesh is also verified, showing the capability of the PW-BIEM in modeling the grating structure with fine geometry. With the proposed method, the transmission behavior of the wavy layered structure with a substrate-metal-cover-air architecture is investigated. The enhanced transmission due to the mechanism of the coupled surface plasmon polariton (SPP) is discussed and compared with another enhancement mechanism, cavity resonance effect, of the slit grating. For slit gratings, in addition to that of the rectangular type, the modified slit gratings are also simulated. The transmission behavior is discussed and interpreted.

博士
國立臺灣大學
電機工程學研究所
91
The boundary integral method and finite element-boundary integral method are introduced and implemented in this dissertation. Both are used to analyze the scattering and radiation problems involving finite-sized conductor-dielectric electromagnetic structures. The numerical results of the examples demonstrate good consistence for the frequency response. Effects of the size of the substrate and ground plane of a microstrip antenna on the near field are studied. Applying the finite element-boundary integral method, the electric and magnetic fields radiated on the dielectric boundaries of finite microstrip structures can be solved numerically, without the assumption of an infinite substrate. The numerical results are compared with those obtained by an integral equation method and commercial electromagnetic simulator, and show good consistence with one another.

碩士
國立交通大學
電信工程研究所
107
In this thesis, the surface-wave propagating on the strip-grated dielectric coated metal rod is analyzed by using the asymptotic strip boundary conditions (ASBC). Starting from the vector potentials of the classical electromagnetism for the TE and TM modes to the system matrix equations, the characteristic equation and the electric and magnetic field expressions in closed form are derived. The dispersion diagram and the field distribution of the first three modes are obtained and compared with the results generated by the commercial full wave solver: CST Microwave Studio to validate the method. The comparison of the time cost to generate the dispersion diagrams by the ASBC method and the CST Microwave Studio have been shown. The results show that the method using ASBC has high efficiency and qualified accuracy. The basic parametric studies pertain to the dispersion diagrams change in response to the geometrical parameters are done. By comparing the dispersion diagrams of this strip-grated structure and the dielectric-covered metal rod, the property that the stronger confinement of the surface-wave on this strip-grated cylindrical structure is concluded. The surface-wave modal analysis with dielectric loss included is also investigated by this asymptotic method. The measured results of the dispersion and the energy level against the radial distance on the strip-grated rod have also been presented.

碩士
國立臺灣大學
數學研究所
105
In this thesis, we study the asymptotic behavior of charge conserving Poisson-Boltzmann equation with variable dielectric coefficient. As the parameter " goes to zero, if electric double layer(EDL) appears at boundary, then this kind of solutions can be related to the electric double layer model in electrochemistry. In this paper, we give the condition of appearance of EDL and interior potential value and boundary potential value satisfy the system (cf. Theorem 1). In addition, we also study the asymptotic pointwise estimate of equation in boundary layer. By this method, we can compute the thickness of boundary layer in this model and provide the formula of capacitance (1:7). Besides, the non-electroneutrality case is studied preliminarily. This study points out the potential behavior approaches at with leading order term log(1/eps^2) . Hence this shows that the electrolyte tends to electroneutrality in the physical bulk.

碩士
國立交通大學
電信工程研究所
102
Metallic strip grating on grounded dielectric slab is a canonical structure applied to many kinds of microwave components and antennas. However that there is no efficient method can be used to analyze the structure and obtain field distributions in closed form until now. This thesis applies ASBC on the strip-grating structure and obtains EM properties in an easy and fast way. Commercial software CST is used to confirm the ASBC analysis result. Nevertheless inconsistency of CST simulation result occurs in infinite long structure. The problem is also be discussed and solved in this work. By solving the characteristic matrix which result from ASBC analysis all directions of dispersion diagram can be acquired including OX, XM, and MO path. Setting proper incident wave (TE and TM), and solving nonhomogeneous matrix, ASBC also can be used to figure out refection problem even for oblique azimuth planes which CST cannot do. Finally, we find that accuracy of pure ASBC analysis is not so good when the width of metal strip become wider. Therefore we combine the concept of consistency of surface impedances with pure ASBC analysis. With this correction method, ASBC is accurate even for wider metal strip cases.

This thesis describes a project in which algorithms are developed for the rapid and accurate solution of Poisson's equation in the presence of a dielectric boundary and multiple point charges. These algorithms are then used to perform Brownian dynamics simulations on realistic models of biological ion channels. An iterative method of solution, in which the dielectric boundary is tiled with variable sized surface charge sectors, provides the flexibility to deal with arbitrarily shaped boundaries, but is too slow to perform Brownian dynamics. An analytical solution is derived, which is faster and more accurate, but only works for a toroidal boundary. Finally, a method is developed of pre-calculating solutions to Poisson's equation and storing them in tables. The solution for a particular configuration of ions in the channel can then be assembled by interpolation from the tables and application of the principle of superposition. This algorithm combines the flexibility of the iterative method with greater speed even than the analytical method, and is fast enough that channel conductance can be predicted. The results of simulations for a model single-ion channel, based on the acetylcholine receptor channel, show that the narrow pore through the low dielectric strength medium of the protein creates an energy barrier which restricts the permeation of ions. They further show that this barrier can be removed by dipoles in the neck of the channel, but that the barrier is not removed by shielding by counter-ions. The results of simulations for a model multi-ion channel, based on a bacterial potassium channel, show that the model channel has conductance characteristics similar to those of real potassium channels. Ions appear to move through the model multi-ion channel via rapid transitions between a series of semi-stable states. This observation suggests a possible physical basis for the reaction rate theory of channel conductance, and opens up an avenue for future research.

Piezoelectric materials find use as actuators and sensors in automotive, aerospace and other related industries. Automotive applications such as fuel injection nozzles and engine health monitoring systems require operating temperatures as high as 300-500 oC. The commercially used piezoelectric material PbZr1-xTixO3 (PZT) is limited to operating temperatures as low as 200 oC due to the temperature induced depolarization effects. PZT, in the undoped state exhibits a piezoelectric coefficient (d33) of 223 pC/N and ferroelectric-paraelectric transition temperature (Tc) of 386 oC. The enhanced properties of PZT occur at a region between the tetragonal and rhombohedral phases, called the Morphotropic Phase Boundary (MPB). Therefore, search for new materials with higher thermal stability and better sensing capabilities were focused on systems that exhibit a PZT-like MPB. This led to the discovery of (x)BiScO3-(1-x)PbTiO3 (BSPT), which exhibits an MPB with enhanced Tc (450 oC) and exceptionally high piezoelectric response (d33 = 460 pC/N). Theoretical studies have shown that the mechanism of enhanced piezoresponse in ferroelectric systems is related to the anisotropic flattening of the free energy profiles. An alternative view point attributes the anomalous piezoelectric response to the presence of high density of low energy domain walls near an inter-ferroelectric transition. Diffraction is a versatile tool to study the structural and microstructural changes of ferroelectric systems upon application of electric field. However, characterization of electric field induced structural and microstructural changes is not a trivial task, since in situ electric field dependent diffraction studies almost invariably give diffraction patterns laden with strong preferred orientation effects, due to the tendency of the ferroelectric/ferroelastic domains to align along the field direction. Additionally, diffraction profiles of MPB compositions exhibit severe overlap of Bragg peaks of the coexisting phases, and hence, it is difficult to ascertain with certainty, if the alteration in the intensity profiles upon application of electric field is due to change in phase fraction of the coexisting phases or due to preferred orientation induced in the different phases by the electric field. The characterization of electric field induced phase transformation in MPB systems, has therefore eluded researchers and has been considered of secondary importance, presumably due to the difficulties in unambiguously establishing the structural changes upon application of electric field. In fact, majority of the in situ electric field dependent diffraction studies have been carried out on compositions just outside the MPB range, i.e. on single phase compositions. In such studies, the focus has been mainly on explaining the piezoelectric response in terms of motions of the non-180° domain walls and field induced lattice strains. In this dissertation, the BSPT system has been systematically investigated with the view to understand the role of different contributing factors to the anomalous piezoelectric response of compositions close to the MPB. Using a comparative in situ electric field dependent diffraction study on a core MPB composition exhibiting highest piezoelectric response and a single phase monoclinic (pseudo-rhombohedral) composition just outside the MPB, it is demonstrated that, inspite of the significantly large domain switching and lattice strain (obtained from peak shifts) in the single phase composition, as compared to the MPB composition, the single phase composition shows considerably low piezoelectric response. This result clearly revealed that the anomalous piezoelectric response of the MPB composition is primarily associated with field induced inter-ferroelectric transformation and the corresponding field induced interphase boundary motion. A simple strategy has been employed to establish the field induced structural transformation for the MPB compositions, by overcoming the experimental limitation of in situ electric field dependent diffraction studies. The idea stemmed from the fact that, if the specimens for diffraction study can be used in powder form instead of pellet, the problems associated with preferred orientation effects can be eliminated, and the nature of field induced structural changes can be accurately determined. A comparative study of the diffraction profiles from poled (after subjecting the specimen to electric field) and unpoled (before subjecting the specimen to electric field) powders could precisely establish the nature of electric field induced phase transformation for the MPB compositions of BSPT and provided a direct correlation between the electric field induced structural changes and the enhanced piezoelectric response. A new ‘powder poling’ technique was devised, which involves application of electric field to powder form of the specimen. Using this technique, it was possible to study separately, the effect of stress and electric field on the nature of structural transformation. A unique outcome of this study was, it could demonstrate for the first time, analogous nature of the stress and electric field induced structural transformation. A comparative study of the dielectric response of poled and unpoled samples was used to show a counterintuitive phenomenon of field induced decrease in polarization coherence for the MPB compositions. This approach was used to suggest that the criticality associated with the MPB extends beyond the composition boundary conventionally reported in literature based on bulk diffraction techniques (x-ray and neutron powder diffraction). The layout of the dissertation is as follows: Chapter 1 gives a brief introduction of the fundamental concepts related to ferroelectric materials. The theories that explain the enhanced piezoresponse of MPB based ferroelectric systems have been outlined. Detailed information of the existing literature is presented in the relevant chapters. Chapter 2 presents the details of the solid state synthesis of BSPT compositions and structural analysis using diffraction studies. The dielectric measurements were used to establish the Tc for the different compositions. The enhanced ferroelectric and piezoelectric properties were observed for the MPB compositions, which were shown to exhibit coexistence of tetragonal and monoclinic phases from structural studies. The critical MPB composition exhibiting highest piezoelectric and ferroelectric properties was established to be x = 0.3725. The thermal stability of the critical MPB composition was established to be 400 oC using ex situ thermal depolarization studies. The common approach of structural analysis in the unpoled state failed to provide a unique relationship between the anomalous piezoelectric response and the structural factors at the MPB, emphasizing the need to characterize these system using electric field dependent structural studies. Chapter 3 presents the results of in situ electric field dependent diffraction measurements carried out at Argonne National Laboratory, USA. The quasi-static field measurements could successfully quantify the non-180o domain switching fractions and the field induced lattice strains. The changes in the integrated intensities were used to obtain the non-180o domain switching fraction and the shift in peak positions were used to quantify the field induced lattice strains. The in situ studies could successfully explain the macroscopic strain response for the single phase pseudo-rhombohedral (monoclinic) composition on the basis of domain switching mechanisms and field induced lattice strains. The MPB compositions were shown to have additional contributions from interphase boundary motion, resulting from change in phase fraction of the coexisting phases. The results emphasized the need to investigate the electric field induced transformation for MPB compositions, in order to give a comprehensive picture of the various contributions to the macroscopic piezoreponse. While Rietveld analysis could be used to investigate the phase transformation behaviour upon application of electric field, textured diffraction profiles obtained using in situ studies, in addition to the severely overlapping Bragg reflections of the coexisting phases for the MPB compositions hindered reliable estimation of the structural parameters. An alternate approach to investigate the field induced phase transformation is presented in Chapter 4. The stroboscopic measurements on the MPB composition showed evidence of non-180o domain wall motion even at sub-coercive field amplitudes as low as 0.1 kV/mm. Chapter 4 presents the results of the ex situ electric field dependent structural study, wherein the diffraction profiles collected from poled powders is compared to that of unpoled powders. The diffraction profiles from the poled powders did not exhibit any field induced crystallographic texture and could successfully be analyzed using Rietveld analysis. High resolution synchrotron diffraction studies (ESRF, France) carried out on closely spaced compositions revealed that, the composition exhibiting the highest piezoelectric response is the one, which exhibits significantly enhanced lattice polarizability of both the coexisting (monoclinic and tetragonal) phases. The enhanced lattice polarizability manifests as significant fraction of the monoclinic phase transforming irreversibly to the tetragonal phase after electric poling. The monoclinic to tetragonal transformation suggested the existence of a low energy polarization rotation pathway towards the [001]pc direction in the (1 1 0)pc pseudocubic plane of the monoclinic phase. The results are discussed on the basis of the existing theories that explain piezoresponse in MPB systems and are in support of the Polarization rotation model, in favor of a genuine monoclinic phase. Chapter 5 discusses the ferroelectric-ferroelectric stability of the MPB compositions in response to externally applied stress and electric field independently. Using the newly developed ‘powder poling’ technique, which is based on the concept of exploiting the irreversible structural changes that occur after application of electric field and stress independently, it was possible to ascertain that, both moderate stress and electric field induce identical structural transformation - a fraction of the monoclinic phase transforms irreversibly to the tetragonal phase. The powder poling technique was also used to demonstrate field induced inter-ferroelectric transformation at sub-coercive field amplitudes. In addition, the analysis of the dielectric response before and after poling revealed a counterintuitive phenomenon of poling induced decrease in the spatial coherence of polarization for compositions around the MPB and not so for compositions far away from the MPB range. Exploiting the greater sensitivity of this technique, it was demonstrated that, the criticality associated with the inter-ferroelectric transition spans a wider composition range than what is conventionally reported in the literature based on bulk x-ray/neutron powder diffraction techniques. Chapter 6 presents the closure and important conclusions from the present work and summarizes the key results, highlighting the proposed mechanism of enhanced piezoresponse in BSPT. The last part of the chapter deals with suggestions for future work from the ideas evolved in the present study. vi