In this work passive piezoelectric devices for vibration damping are studied. It is developed the basic idea of synthesizing electrical wave guides to obtain an optimal electro-mechanical energy exchange and therefore to dissipate the mechanical vibrational energy in the electric form. Modular PiezoElectroMechanical (PEM) structures are constituted by continuous elastic beams (or bars) coupled, by means of an array of PZT transducers, to lumped dissipative electric networks. Both refined and homogenized models of those periodic systems are derived by an energetic approach based on the principle of virtual powers. Weak and strong formulation of the dynamical problem are presented having in mind future studies involving the determination of numerical solutions. In this framework the effectiveness of the proposed devices for the suppression of mechanical vibrations is investigated by a wave approach, considering both the extensional and flexural oscillations. The optimal values of the electric parameters for a fixed network topology are derived analytically by a pole placement technique. Their sensitivities on the dimensions of the basic cell of the periodic system and on the design frequency are studied. Moreover the dependence of damping performances on the frequency is analyzed. Comparing the performances of different network topological configurations, the advantages of controlling a mechanical structure with an electric analog are shown. As a consequence of those results, new interconnections of PZT transducers are proposed. An experimental setup for the validation of the analytical and numerical results is proposed and tested. A classical experience on resonant shunted PZT is reproduced. Future experimental work is programmed. / Master of Science

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The synthesis and characterization of step-growth polymers with novel monomers provided materials with tailored properties for emerging technologies. Specifically, multiphase materials (i.e., microphase separated block copolymers) exploit the synergistic relationship of combining polymers with disparate thermal and mechanical properties. The introduction of intramolecular interactions such as hydrogen and ionic bonding into these polymers further tailored their properties for applications including elastomers, electromechanical transducers, and additive manufacturing (AM). A review of recent literature revealed the material properties required for polymeric materials in electromechanical transducers, which aided in the design of polymers for this application. An isocyanate-, catalyst-, and solvent-free approach facilitated the synthesis of segmented polyureas with tunable thermal and mechanical properties. These materials found use as high dielectric elastomers and water-soluble polymers for extrusion-based AM dependent on the backbone composition. Vat photopolymerization (VP) AM served as a technique to 3D printed novel unsaturated polyester resins (UPR). Incorporating a phosphonium ionic liquid as a reactive diluent replaced styrene and reduced the volatility of commonly used UPRs. VP successfully provided 3D structures from these UPRs that demonstrated limited ionic conductivities. An extensive review of the literature surrounding the structure-property relationships of charged block copolymers with varying architectures helped to inform the synthesis of novel, cationic step-growth polymers. The synthesis of a new phosphonium IL facilitated the synthesis of a segmented polyurethane containing a phosphonium-functionalized soft segment for the first time. This phosphonium polyurethane exhibited ionic conductivities comparable to literature examples of block copolymers used for ionic polymer transducers, which suggests that these materials may serve for this application as well. Carbonyldiimidazole provides a novel route towards synthesizing imidazolium ionenes with unique backbone structures. The coupling of poly(ethylene glycol) dibromides with a bis-carbonylimidazole monomer and a commercial aliphatic dibromide led to the formation of segmented imidazolium ionenes. These polymers exhibited significant atmospheric water uptake as well as water solubility. However, the physical properties of the materials suggested that the synthetic procedure resulted in low molecular weights. Suggested future work provides methods for circumventing this issue and proposes next steps for all the projects discussed herein. / Doctor of Philosophy / Emerging technologies require new polymeric materials with intentionally designed properties. Step-growth polymers such as polyesters, polyurethanes, and polyureas find use in many applications of our everyday lives. Although these materials have served mainly as commodity plastics historically, a reimagining of their syntheses and chemical structures makes them accessible for modern technologies. For example, applying green chemistry principles to the synthesis of polyureas resulted in a less toxic synthetic procedure. Polyureas synthesized through this method exhibited elastic properties comparable to classical polyureas and displayed high dielectric constants, which lend them towards use in dielectric elastomer actuators. This chemistry also allowed for the synthesis of water-soluble polyureas, which served as a material for low temperature extrusion additive manufacturing, colloquially known as 3D printing. Vat photopolymerization describes another type of 3D printing that involves the selective curing of liquid resins with light to form a 3D structure. Employing a reactive ionic liquid monomer with a commercially-relevant unsaturated polyester allowed for a nontoxic method of printing these materials, which also imparted ionic conductivity. Finally, the synthesis of positively charged polyurethanes and ionenes led to the production of ionically conductive materials that may find use in polymeric transducers.

A new gage which can measure skin friction and heat flux simultaneously was designed, constructed, and tested. This gage is the combination of a non-nulling type skin friction balance and a heat flux microsensor. By mounting the heat flux microsensor directly on the surface of the floating element of the skin friction balance, it was possible to perform simultaneous measurements of the skin friction and the heat flux. The total thickness of the heat flux microsensor is less than 2 μm, so the presence of this microsensor creates negligible disruption on the thermal and the mechanical characteristics of the air flow. Tests were conducted in the Virginia Tech supersonic wind tunnel. The nominal Mach number was 2.4, and Reynolds number per meter was 4.87 x 10⁷ with total pressure of 5.2 atm and total temperature of 300 °K. Results of the tests showed that this new gage was quite reliable and could be used repeatably in the supersonic flow. This gage also has an active heating system inside of the cantilever beam of the skin friction balance so that the surface temperature of the floating element can be controlled as desired. With these features, the effects of a temperature mismatch between the gage surface and the surrounding wall on the measurements of the skin friction and the heat flux were investigated. An infrared radiometer was used to measure the surface temperature distributions. Without the active heating, the amount of temperature mismatch generated by the gage itself was from 2.5 °K to 4.5 °K. The active heating produced the temperature mismatch of 18.7 °K. The largest temperature mismatch corresponds to the levels typically found in high heat flux cases when it is expressed in dimensionless terms. This temperature mismatch made sizable effects — a 24 % increase in the skin friction measurement and a 580 % increase in the heat flux measurements. These experimental results were compared with the computational results using the Computational Fluid Dynamics code GASP. The input flow conditions were obtained from the boundary layer measurements. The temperature mismatch was input by specifying the density and the pressure at each grid point on the wall. The Baldwin-Lomax algebraic turbulence model was used with the thin layer approximations. The comparison showed that the difference in the skin friction and heat flux was less than 10 % of the measured data when the temperature mismatch was less than 8.5 °K, but the difference was increased as the amount of the temperature mismatch increased. It is presumed that the disagreement between the measurements and the calculations was caused mainly by deficiencies in the turbulence model for this complex, developing viscous flow, because the Baldwin-Lomax model cannot account for the multiple length scale in this flow. / Ph. D.

The imidazole ring offers great potential for a variety of applications including gene delivery vectors, ionic liquids, electromechanical actuators, and novel monomers and polymers. The imidazole ring provides a unique building block for these applications due to its thermal stability, aromatic nature, ability to form ionic salts, and ease of functionalization. Free radical polymerization of 1-vinylimidazole (1-VIm) and free radical copolymerizations with methyl methacrylate (MMA) and n-butyl acrylate (nBA) afforded homopolymers and copolymers with tunable solution and thermal properties. Aqueous SEC provided reproducible and reliable molecular weights for poly(1-VIm) in the absence of polymer aggregates. Analysis of the thermal properties revealed ideal random copolymers with MMA and non-ideal copolymers with nBA. Small angle X-ray scattering determined that the spacing between ionic groups remained constant with increased nonionic comonomer incorporation while the spacing between adjacent polymer backbones increased. Functionalization of 1-VIm with varying length alkyl halides and polymerization prepared a series of imidazolium homopolymers. Anion exchange reactions controlled the thermal and solution properties, and the bromide counteranion quantitatively exchanged to tetrafluoroborate (BF4), trifluoromethanesulfonate (TfO), and bis(trifluoromethanesulfonyl)imide (Tf2N). Thermogravimetric analysis revealed that thermal stability increased with decreased alkyl substituent length and larger counteranion size, and differential scanning calorimetry determined that glass transition temperature (Tg) decreased with increased alkyl substituent length and larger counteranion size. Electrochemical impedance spectroscopy determined the ionic conductivities of the imidazolium homopolymers, and analysis using the Vogel-Fulcher-Tammann equation revealed that the activation energy of ion conduction increased as alkyl substituent length increased. Polymer morphology determined using X-ray scattering also influenced the ionic conductivity. As the alkyl substituent length increased, the spacing between adjacent polymer backbones increased, which decreased the ionic conductivity due to the ion-hopping mechanism of ion conduction. Unsuccessful attempts to control the radical polymerization of 1-VIm led to the investigation of 1-(4-vinylbenzyl)imidazole (VBIm), which is a styrenic-based monomer with excellent propagating radical stability. Triblock copolymers incorporating VBIm monomer into a soft random copolymer center block and reinforcing, hard segment outer blocks provided a template for tuning the properties of the ionomer membranes for electroactive devices. Analysis of the morphology and mechanical properties using small angle X-ray scattering and dynamic mechanical analysis determined microphase separation and optimal mechanical properties for electromechanical transducer fabrication. Testing electromechanical transducers revealed superior performance relative to the benchmark Nafion®. Optimization of triblock copolymer design criteria through varying the comonomer ratio of VBIm and nBA in the soft center block, quaternization reactions, and ionic liquid introduction influenced mechanical properties and ionic conductivity. Higher percentages of VBIm and quaternization of VBIm in the random central block increased Tg and ionic conductivity. IL selectively incorporated into the imidazole-containing phases with no leakage observed for ionic systems, reduced the center block Tg, and increased ionic conductivity. Controlling charge density along poly(1-VIm) through well-defined alkylation reactions with 1-bromobutane provided a potential vector for nonviral gene delivery and polyanion binding. Analysis of DNA and heparin binding using gel electrophoresis revealed a decrease in N/P ratio with increased alkylation percentage. Dynamic light scattering indicated an increase in zeta potential with increasing alkylation percentages, and relatively uniform polyplex sizes in aqueous media. The MTT assay developed cytotoxicity profiles with little toxicity prior to 83% alkylation. Finally, the luciferase expression assay revealed inefficient nucleic acid delivery to multiple cell types. Synthesis of poly(1-VIm) vectors with glutathione conjugates provided an avenue for simultaneous therapeutic gene and anti-oxidant delivery in vitro. Cytotoxicity assays of cells pretreated with glutathione-conjugated poly(1-VIm) prior to oxidative stress showed that higher glutathione conjugation levels improved cell viability. / Ph. D.

Instrumentation has been designed to process the signals from two types of skin friction gages and a microfabricated heat flux gage. Design changes for the skin friction gages are presented which will improve the performance of the two transducers. The instrumentation is simple in design and use and has been designed to maximize the performance of the skin friction and heat flux gages. The instrumentation is battery powered to minimize noise levels and to maintain instrumentation portability. A high-quality instrumentation amplifier, a voltage regulator, and a custom-designed circuit board have been combined to produce an instrumentation package which is stable and durable. The instrumentation has been specifically designed to handle low-level signals and can operate over a wide range of frequencies. Problems commonly associated with low-level signal conditioning like electrical noise, nonlinearities, and output drift are addressed. The performance specifications of the instrumentation are presented along with sample gage measurements. / Master of Science

A fast response thin-film heat flux sensor was used to measure the time-resolved surface heat flux and temperature from a turbulent combustion flame impinging on a surface. Using the analytical semi-infinite conduction model, the unsteady surface heat flux was calculated from the transient temperature measurements and the surface temperature was calculated from the unsteady surface heat flux measurements. Methods of comparing time-resolved heat flux and temperature data were presented and discussed. The standard analytical method for converting surface temperature to heat flux was used. Two new analytical methods were developed for converting heat flux to surface temperature. The study is the first demonstration of time-resolved temperature signals generated from time-resolved heat flux measurements. The results graphically illustrate the effects of data processing on electrical noise present in the actual signal. The effect of flame unsteadiness is also shown, especially in the time-resolved heat flux measurements, which gives insight into the behavior of a propane torch. One application is for development of feed-forward control systems in industrial processes with fast transients. / Master of Science

A local heat-flux measurement system was built, calibrated and tested for use in unsteady flows. The system was designed to maintain constant wall temperature boundary conditions. The measuring element is a thin-film heat flux gage made by sputter-coating gold on a substrate. A constant-temperature anemometer is used to maintain the thin-film gage at a specified temperature under fluctuating conditions. A separate temperature control system maintains the surrounding boundary at the gage temperature. The system was calibrated for both steady and unsteady flows using a specially designed calibrator for local heat flux gages. The steady calibration was done with predominantly convective heat transfer . The unsteady calibration was achieved by adding oscillating radiant energy to the surface. Consequently, quantitative results can be obtained for both mean and fluctuating components of the heat transfer. The frequency response was good to 92 hertz. Sample results are presented for unsteady heat transfer caused by the vortex shedding from a cylinder in a steady crossflow. The shedding frequency was 82 hertz. / M.S.