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Thesis advisor: Willie J. Padilla / Realization of an ideal electromagnetic absorber has long been a goal of engineers and is highly desired for frequencies above the microwave regime. On the other hand, the desire to control the blackbody radiation has long been a research topic of interest for scientists--one particular theme being the construction of a selective emitter whose thermal radiation is much narrower than that of a blackbody at the same temperature. In this talk, I will present the computational and experimental work that was used to demonstrate infrared metamaterial absorbers and selective thermal emitters. Based on these work, we further demonstrate an electrically tunable infrared metamaterial absorber in the mid-infrared wavelength range. A voltage potential applied between the metallic portion of metamaterial array and the bottom ground plane layer permits adjustment of the distance between them thus altering the electromagnetic response from the array. Our device experimentally demonstrates absorption tunability of 46.2% at two operational wavelengths. Parts of this thesis are based on unpublished and published articles by me in collaboration with others. The dissertation author is the primary researcher and author in these publications. The text of chapter two, chapter five, and chapter seven is, in part, a reprint of manuscript being prepared for publication. The text of chapter three is, in part, a reprint of material as it appears in Physical review letters 104 (20), 207403. The text of chapter four is, in part, a reprint of material as it appears in Physical Review Letters 107 (4), 45901. The text of chapter six is, in part, a reprint of material as it appears in Applied Physics Letters 96, 011906 / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The aim of this thesis is to investigate the use of a Dynamic Vibration Absorber to control vibration in a beam. Traditional means of vibration control have involved the use of passive and more recently, active methods. This study is different in that it involves an adaptive component in the design of vibration absorber using two novel designs for the adaptive mechanism. The first design incorporates the use of an enclosed air volume to provide the variable stiffness component in the absorber. By adjusting the volume of compressible air within the absorber, the stiffness characteristics of the absorber can be altered, enabling the device to adapt to changing vibration frequencies. Work here includes a theoretical investigation of the device. Following this, two prototypes are constructed and tested, the second of which is the refined model used for further testing. The second design incorporates the use of two concentrated masses cantilevered from two rods. The adaptive solution is achieved by moving the two masses along the length of the rod, producing a changing natural frequency for the absorber device. An analytical model of this device is developed as well as a finite element model. Results from both are compared to those obtained experimentally. Finally, a tuning algorithm is derived for the second absorber, and a control system constructed to make the dynamic vibration absorber "adaptive". Experiments are undertaken to determine the effectiveness of the absorber on the beam subject to changing excitation frequencies. The outcome of this research is that an Adaptive Vibration Absorber has been constructed with a computer interface such that the device can be used "on line". / Thesis (M.Eng.Sc.)--Mechanical Engineering, 1999.

Tuned vibration absorbers (TVAs) are spring-mass-damper devices used to mini-mize energy in a vibrating body. TVAs decrease in efficiency when the vibrating body is subjected to variable, broadband, or random excitation. A state-switched absorber (SSA) can be used in these variable, broadband, or random excitation cases to more effectively reduce excitation. An SSA is a mass-spring-damper where one or more of these elements can instantaneously and discretely change. An SSA was designed, built and tested to fit the specifications for TVAs found on ATR 72 and 42 airplane fuselages. A magnetorheological elastomer (MRE), an elastomer with imbedded iron particles, was selected as a variable spring. Flux lines concentrated through all MREs in absorber configurations. MREs containing 35% iron by volume yielded the largest frequency shift. A 35% MRE based absorber had a frequency range of 45-183 Hz, which corresponds to a 360% frequency increase. Transient absorber behavior was observed by recording rise and drop times to step field intensity changes.The 35% MRE absorber yielded a 0.20 second rise time and a 0.03 second drop time. Future work will determine whether a modified input signal can generate an appropriate transient response.

This thesis describes the implementation of a parametric model of an automotive damper. The goal of this research was to create a damper model to predict accurately damping forces to be used as a design tool for the Formula SAE racecar team. This study pertains to monotube gas charged dampers appropriate to Formula SAE racecar applications. The model accounts for each individual flow path in the damper, and employs a flow resistance model for each flow path. The deflection of the shim stack was calculated from a force balance and linked to the flow resistance. These equations yield a system of nonlinear equations that was solved using Newton's iterative method. The goal of this model was to create accurately force vs. velocity and force vs. displacement plots for examination. A shock dynamometer was used to correlate the model to real damper data for verification of accuracy. With a working model, components including the bleed orifice, piston orifice, and compression and rebound shims which were varied to gain an understanding of effects on the damping force.

The ability to understand and control the propagation of electromagnetic radiation underpins a vast array of modern technologies, including: communication, navigation and information technology. Therefore, there has been much work to understand the interaction between electromagnetic waves and metal surfaces, and in particular to design materials the characteristics of which can be tailored to produce a desired response to microwave radiation. It is the objective of this thesis to demonstrate that patterning metal surfaces with sub-wavelength apertures can afford hitherto unrealised control over the reflection and transmission characteristics of materials which are an order of magnitude thinner than those employed historically. The work presented herein aims to establish ultra thin cavity structures as novel materials for the selective absorption and transmission of microwave radiation. Experimental and theoretical approaches are used to elucidate the mechanism that allows such structures to produce highly efficient absorption via the excitation of standing wave modes in structures that are two orders of magnitude thinner than the operating wavelength. Also considered is how this same mechanism mediates transmission of selected frequencies through similarly thin structures. Later chapters focus on ultra thin cavity structures which, through higher-order rotational symmetry, exhibit resonant absorption which is almost completely independent of incident and azimuthal angle and polarisation state. A detailed studied of the absorption bandwidth of these devices is also presented in the context of fundamental theoretical limitations arising from the thickness and magnetic permeability of the structure.

Secondary structures may have to endure severe vibration amplitudes under the influence of the primary structures on which they are mounted. A series of numerical case studies are presented in this thesis to investigate the effectiveness of a passive vibration controller which combines a conventional tuned absorber with an impact damper, to attenuate the excessive vibration amplitudes of light secondary structures. In addition, experimental measurements are reported for some selective cases and comparisons are made with numerical predictions. This suggested configuration seems to suit ideally as an add-on enhancer for existing conventional absorbers. Most of the Results are presented for random white noise excitation, and a few representative transient vibration cases are also studied. / Thesis / Master of Engineering (ME)

Tuned vibration absorbers (TVA) have been discussed in literature since the early twentieth century. These devices are implemented to suppress the system's vibration by transferring energy to the absorber mass. This research examines an electromagnetic tuned vibration absorber that can have its tuned frequency altered by gap and current variation. The advantage of an adjustable TVA is that the system can be tuned to various excitation frequencies to cancel vibration. This research examines a unique embodiment using permanent magnets and an electromagnetic absorber to alter the system dynamics. The focus is to allow changes in tuned frequency to cancel system vibrations. This research develops the electromagnetic theory, presents absorber system simulations, and tests the dynamic absorber's response. The electromagnetic field is investigated to determine the field between a stationary magnet and the absorber electromagnet. This field can be numerically calculated as the superposition of four constituent fields. With the electromagnetic field determined, the force to displacement relation between the stationary magnet and the absorber electromagnet is calculated. The best fit is determined to be an inverse square relationship. Once the spring force relation is determined, the damping mechanisms are discussed and experiments proposed to isolate the different damping mechanisms. In the simulations, it is found that by having an adjustable electromagnetic TVA the natural frequency can be adjusted 2-3% with a +10 amp input and over 50% for a variable gap. The advantage of the variable gap is that it may be adjusted once and then no additional energy is needed, while the advantage of the variable current is that the system may be rapidly altered. The experiments are undertaken to test the constructed absorber for the spring and damping force. The tests confirm the spring force relation and quantify the high damping present in the tested configuration. Then the absorber system transfer functions are recorded. The absorber is then applied to a single degree of freedom system to verify its cancellation results by a gap variation. / Ph. D.

Master of Science / Department of Chemical Engineering / Larry Erickson / Oxy-fired boilers are receiving increasing focus as a potential response to reduced boiler emissions limits and greenhouse gas legislation. Among the challenges in cleaning boiler gas for sequestration is attaining the necessary purity of the CO[subscript]2. A key component in the oxy-fired cleaning path is high purity SO[subscript]x and NO[subscript]x removal, often through absorption using the lead-chamber or similar process. Aerosol formation has been found to be a source of product contamination in many flue gas absorption processes. A number of authors presented simulation methods to determine the formation of aerosols in gas absorption. But these methods are numerically challenging and not suitable for day-to-day analysis of live processes in the field. The goal of this study is to devise a simple and practical method to predict the potential for and effect of aerosol formation in gas absorption using information from Aspen Plus, a commonly used process simulation tool. The NO[subscript]x absorber in an oxy-fired boiler CO[subscript]2 purification system is used as a basis for this investigation. A comprehensive review of available data suitable for simulating NO[subscript]x absorption in an oxy-fired boiler slipstream is presented. Reaction rates for eight reactions in both liquid and vapor phases are covered. These are entered into an Aspen Plus simulation using a RadFrac block for both rate-based and equilibrium reactions. A detailed description of the simulation format is given. The resulting simulation was compared to a previously published simulation and process data with good agreement. An overall description of the aerosol formation mechanism is presented, along with an estimate of expected aerosol nuclei reaching the NO[subscript]x absorption process. A method to estimate aerosol quantities produced based on inlet gas nuclei concentration and available condensable water vapor is presented. To estimate aerosol composition and emissions, an exit gas slipstream is used to equilibrate with a pure water aerosol using an Aspen Plus Equilibrium Reactor block. Changing the composition of the initial aerosol feed liquid suggests that the location of aerosol formation may influence the final composition and emissions.

Thesis advisor: Willie J. Padilla / This thesis will describe the implementation of novel imaging applications with electromagnetic metamaterials. Metamaterials have proven to be host to a multitude of interesting physical phenomena and give rich insight electromagnetic theory. This thesis will explore not only the physical theory that give them their interesting electromagnetic properties, but also the many applications of metamaterials. There is a strong need for efficient, low cost imaging solutions, specifically in the longer wavelength regime. While this technology has often been at a standstill due to the lack of natural materials that can effectively operate at these wavelengths, metamaterials have revolutionized the creation of devices to fit these needs. Their scalability has allowed them to access regimes of the electromagnetic spectrum previously unobtainable with natural materials. Along with metamaterials, mathematical techniques can be utilized to make these imaging systems streamlined and effective. Chapter 1 gives a background not only to metamaterials, but also details several parts of general electromagnetic theory that are important for the understanding of metamaterial theory. Chapter 2 discusses one of the most ubiquitous types of metamaterials, the metamaterial absorber, examining not only its physical mechanism, but also its role in metamaterial devices. Chapter 3 gives a theoretical background of imaging at longer wavelengths, specifically single pixel imaging. Chapter 3 also discusses the theory of Compressive Sensing, a mathematical construct that has allowed sampling rates that can exceed the Nyquist Limit. Chapter 4 discusses work that utilizes photoexcitation of a semiconductor to modulate THz radiation. These physical methods were used to create a dynamic THz spatial light modulator and implemented in a single pixel imaging system in the THz regime. Chapter 5 examines active metamaterial modulation through depletion of carriers in a doped semiconductor via application of a bias voltage and its implementation into a similar single pixel imaging system. Additionally, novel techniques are used to access masks generally unobtainable by traditional single pixel imagers. Chapter 6 discusses a completely novel way to encode spatial masks in frequency, rather than time, to create a completely passive millimeter wave imager. Chapter 7 details the use of telecommunication techniques in a novel way to reduce image acquisition time and further streamline the THz single pixel imager. Finally, Chapter 8 will discuss some future outlooks and draw some conclusions from the work that has been done. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.