<div>The work discussed in this document seeks to integrate conductive additives with energetic material systems to offer an alternative source of ignition for the energetic material. By utilizing the conductive properties of the additives, ohmic heating may serve as a method for preheating and igniting an energetic material. This would allow for controlled ignition of the energetic material without the use of a traditional ignition source, and could also result in easier system fabrication.</div><div>For ohmic heating to be a viable method of preheating or igniting these conductive energetic materials, there cannot be significant impact on the energetic properties of the energetic materials. Various mass solids loadings of graphene nanoplatelets (GNPs) were mixed with a reactive mixture of aluminum (Al)/polyvinylidene fluoride (PVDF) to test if ohmic heating ignition was feasible and to inspect the impact that these loadings had on the energetic properties of the Al/PVDF. Results showed that while ohmic heating was a plausible method for igniting the conductive energetic samples, the addition of GNPs degraded the energetic properties of the Al/PVDF. The severity of this degradation was minimized at lower solids loadings of GNPs, but this consequently resulted in larger voltage input requirements to ignite the conductive energetic material. This was attributable to the decreased conductivities of the samples at lower solids loading of GNPs.</div><div>In hopes of conserving the energetic properties of the Al/PVDF while integrating the conductive additives, additive manufacturing techniques, more specifically fused filament fabrication, was used to print two distinct materials, Al/PVDF and a conductive composite, into singular parts. A CraftBot 3 was used to selectively deposit Conductive Graphene PLA (Black Magic) filament with a reactive filament comprised of a PVDF binder with 20% mass solids loadings of aluminum. Various amounts of voltage were applied to these conductive energetic samples to quantify the time to ignition of the Al/PVDF as the applied voltage increased. A negative correlation was discovered between the applied voltage and time to ignition. This result was imperative for demonstrating that the reaction rate could be influenced with the application of higher applied voltages.</div><div>Fused filament fabrication was also used to demonstrate the scalability of the dual printed conductive energetic materials. A flexural test specimen made of the Al/PVDF was printed with an embedded strain gauge made of the Black Magic filament. This printed strain gauge was tested for dual purposes: as an igniter and as a strain sensor, demonstrating the multi-functional use of integrating conductive additives with energetic materials.</div><div>In all, the experiments in this document lay a foundation for utilizing conductive additives with energetic materials to offer an alternative form of ignition. Going forward, ohmic heating ignition may serve as a replacement to current, outdated methods of ignition for heat sensitive energetic materials.</div>

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In this thesis, the results pertaining to various investigations carried out on Triglycine sulphate (TGS) single crystals, polyvinylidene fluoride (PVDF) films, lithium tantalate (LT)/PVDF nanocomposites and LT thin films are presented with emphasis on the characteristics that are crucial for their use in pyroelectric sensors. TGS single crystals (size 68 x 45 x 42 mm3), which have high pyroelectric coefficients, were grown by slow cooling method using newly designed platform technique based crystal growth work stations. The problem of slow growth rate along c-direction was overcome by placing (010) oriented seeds on the platform. The grown TGS crystals were used for the fabrication of the laser energy meter and temperature sensor. One drawback of TGS is its low Curie temperature (490C). As a consequence when the operating temperature approaches the Curie temperature, the crystals start depolarizing owing to the movement of domains. As a result the linearity of the devices gets affected and restricts the use of TGS. Therefore pyroelectric materials possessing higher Curie temperatures and larger pyroelectric coefficients than that of TGS are desirable. LT in single crystalline form having Curie temperature of ≈6000C has already been in use for pyroelectric device applications. However, growing stoichiometric LT single crystal is very difficult. On the other hand PVDF polymer films (Tc≈1800C) have low pyrolectric coefficients and difficult to pole electrically. Therefore efforts were made to prepare LT/PVDF nanocrystal composites to increase the pyroelectric coefficient of PVDF and to reduce the poling field. Nanoparticles of LT were prepared using sol-gel route. Spherical nanoparticles of size 20-40nm were prepared from sol by adding oleic acid to it. These nanoparticles were characterized using XRD, TEM, DSC and Raman spectroscopy. PVDF films with large percentage of β-phase (ferroelectric phase) were fabricated from solutions prepared using dimethylsulphoxide (DMSO) solvent. PVDF films (30µm thick), embedded with 20-40nm sized nanocrystallites of LT were fabricated to utilize them for pyroelectric sensor applications. The ferroelectric and pyrolectric properties of nano composite films were studied for sensor applications point of view. As a replacement for the single crystals of LT in pyroelectric sensors, investigations were carried out on oriented LT thin films. The studies on LT thin films yielded promising results which could be exploited for pyroelectric sensor applications.

Les travaux de cette thèse ont permis de contribuer à la compréhension et au développement de la physique et de la chimie des matériaux. Dans cette étude, des nouveaux nanocomposites polymères de hautes performances structurales, thermiques et mécaniques ont été fabriqués en utilisant le graphène, l'oxyde de graphène et les nanotubes de carbone modifiés comme nanocharges de renforcement ou des agents de nucléation. L'étude a porté, d'une part, sur des échantillons nanocomposites à matrices polypropylène (PP), polyéthylène haut densité (PEHD) et polyfluorure de vinylidène (PVDF) fabriqués par un procédé d'extrusion et d'autre part, sur des films nanocomposites à matrice PVDF fabriqués par un procédé de mélange en solution suivi par l'approche coulée-évaporation. Les nanofeuillets de graphène (NFG) et ceux d'oxyde de graphène (NFOG) ont été obtenus via l'exfoliation du graphite naturel en utilisant une méthode chimique. Des techniques de caractérisation expérimentales ont confirmé que les NFG et NFOG ont été bien formés en large quantité avec une haute qualité structurale, une épaisseur entre 0,95-1nm et des dimensions latérales entre 0,1 et 1μm. Les nanotubes de carbone (NTC) ont été fonctionnalisés par un surfactant polymérique, la polyvinylpyrrolidone (PVP), via le mécanisme d'adsorption physique, afin d'augmenter leur dispersion dans des solvants organiques et des matrices polymères. Les propriétés structurales (morphologie et propriétés rhéologiques), thermiques (stabilité thermique, comportement de cristallisation et de fusion), mécaniques (traction, flexion) des matrices sélectionnées (PP, HDPE et PVDF) ont été largement améliorées par l'addition de faibles fractions massiques des NFG (< 3%). Les améliorations obtenues au niveau des propriétés sélectionnées des nanocomposites peuvent étre élargir le champ d'application des polymères thermoplastiques. Des approches efficaces ont été développées afin de produire des films nanocomposites à matrice PVDF chargés par les nanofeuillets d'oxyde de graphène (NFOG) et les nanotubes de carbone modifiés (NTC/PVP). Ces films ont été fabriqués dans le cadre de contrôler la structure cristalline du PVDF et ce en terme de phase β (responsable de la piézoélectricité). Ainsi que des améliorations très significatives des propriétés mécaniques et thermiques ont été réalisées par l'addition de faibles fractions massiques de NFOG et NTC (< 2 %).

Structural Health Monitoring (SHM) systems for future structures and vehicles would involve a process of damage identification and prediction of certain quantities of interest that concerns the function and safety. This process provides SHM systems the ability to not only save cost but also enhance the service life, safety and reliability of the structures and vehicles. Integrated SHM system (ISHM) is an advancement of SHM system that has additional capability of predicting the component life/failure. ISHM system development involves detailed understanding of diagnostic waves, hardware components, signal processing paradigms and intelligent use of algorithms. Diagnostic waves like the guided waves are the elastic waves that propagate in a direction defined by the material boundaries. These waves have the capability of traveling large distance probing the entire thickness in plates/shells. Thus, they are widely used by SHM systems in monitoring the plate structures. Piezoelectric transducers are often employed in the interrogation using guided waves. Most SHM systems employing guided waves are designed for specific structures. Current paradigms of SHM systems are unable to enable the transition from simple or ideal structures to realistic and complicated structures. This is due to the challenges at the fundamental level involving transducer, wave propagation and phenomena of guided wave scattering with damages to evaluate the possible solutions through mathematical modeling and signal analysis capability required by ISHM systems. This thesis aims to develop understanding of these problems at a fundamental level. Complex system level understanding is still needed which is left out as open problem. A primary requirement in designing SHM system is the proper understanding of wave characteristics such as number of modes, wavelength and dispersiveness. Although three-dimensional elasticity solution and simplified theories are available to understand them, their applicability in SHM problem requires a much more detailed look. Effort toward this direction has led to the development of simpler models. However, mathematical models are not available for understanding the wave characteristics in complex structures involving stiffeners and adhesive joints. This problem is addressed in this thesis. There is a fair amount of understanding developed regarding transducer characteristics. This is accomplished by analytical and finite element models of transducers in the past. However, simplified transducer model that are computationally fast to suit SHM system requirements needs to be developed. The development of such model is presented in this thesis. Apart from modeling the transducers and wave scattering due to damage, signal correlation and calibration are needed for practical implementation in SHM. Characterization studies reported in published literature are limited to quasi-static and low frequencies applications. However, SHM of aerospace structures employ guided waves typically in the frequency range of 100-500 kHz. Methods to characterize the transducers at this frequency range needs to be developed, which is addressed in this thesis. Another major requirement of SHM system is the design and development of sensor-actuator network and appropriate algorithm. Techniques developed earlier involving transducer arrays in this regard have limitation due to complexity of geometry and signal interpretation that needs to be addressed. The network with suitable algorithm should ideally monitor large area including the critical areas of failure with minimum number of transducers. ISHM systems further require some capability to estimate the useful life of the damaged structure in order to take suitable decisions. Efficient techniques to achieve these are not developed. Overall, there is a need to improve highly interdisciplinary areas involving mathematical modeling, transducer design, fabrication and characterization, damage detection and monitoring strategies. In this thesis, various novel techniques to combine mathematical model with experimental signals to enhance the damage detection capability are presented. In this thesis, developments in the three main aspects of SHM systems are focused upon. They are (1) development of mathematical models of sensors/actuators, wave propagation and scattering due to damage (2) characterization and calibration of transducers and (3) development of technique to monitor wide variety of damages within the scope of ultrasonic guided wave based SHM. The thesis comprises of ten chapters. First chapter is devoted to the background and motivation for the problem addressed in this thesis. In second chapter, brief overview of available mathematical models and conventional damage monitoring strategy is presented. The significant contributions reported in the subsequent chapters in this thesis are outlined below In chapter 3, a reduced-order model of guided wave propagation in thick structures with reduced-order approximation of higher-order elasto-dynamic field is formulated. The surface normal and shear tractions of the thick structure are satisfied in a closed form. The time-frequency Fourier spectral finite element is developed and is validated using detailed and computationally intensive finite element simulations. Natural frequencies obtained from the developed spectral finite element and the detailed finite element simulations are compared. Transient response due to broad frequency band and narrow frequency band excitations given in the form of surface tractions are validated by comparing with the detailed finite element simulations. Using the developed spectral finite element, wave scattering from a free edge and a notch are simulated and validated by comparing with the detailed finite element simulations. In chapter 4, two-dimensional plane wave and flexural wave scattering models for more complicated features such as stiffener with delamination and stiffener with bolt failures in a stiffened panel are derived using ultrasonic ray tracing based approach combined with wave-field representation. Dispersion relations are reformulated for the base plate where it is bolted with the stiffener. Surface conditions due to contact stiffness and contact damping are modeled by introducing springs and dampers. Scattering coefficients for the bonded and bolted stiffeners are derived. The scattering coefficients are evaluated for various different frequencies. Results are compared for different stiffener parameters. In chapter 5, a simplified analytical model of a piezoelectric actuator with uniform electrodes is modeled. The problem is to determine the launched guided wave characteristics in the structure. The analytical model is derived considering two-dimensional elasticity based approach and Airy’s stress function. The actuator model is used to specify the displacement boundary conditions in the detailed finite element model. The radiated wave patterns in a plate due to actuation from transducers of different shapes are obtained and validated with experiments. Phased array actuators are modeled in the detailed finite element model using the displacements estimated from the actuator model. The radiated wave pattern from the detailed finite element simulations are validated with experiments. Chapter 6 is devoted to the design and characterization of transducers for ultrasonic guided wave applications. The characterization techniques involve the estimation of voltage response for the induced strain by the guided wave at various different frequencies. First, a novel removable bonding technique and a calibration technique are demonstrated and related advantages are discussed. Performance of the piezoelectric thin film under quasi-static, dynamic and transient impact loadings are analyzed first. Next, a guided wave technique is developed to characterize piezoelectric thin film sensors and actuators at ultrasonic frequencies. The transducers with inter digital electrodes are characterized for frequency tuning and directional sensitivity. This characterization study enables in the selection of optimal frequency bands for interrogation. Further, the characterization of transducers with thermal degradation is presented. In chapter 7, a novel guided wave technique to calibrate the thin film sensors for ultrasonic applications is presented. Calibration procedure involves the estimation of the piezoelectric coefficient at ultrasonic range of frequencies. Calibration is done by the measurement of voltage generated across thin films when guided waves are induced on them. With the proposed technique, piezoelectric coefficient can be estimated accurately at any frequency of the propagating wave. Similarly, the measurement of piezoelectric coefficient of thin films with inter digital electrodes is presented. The estimation of piezoelectric coefficient at various different directions using laser Doppler vibrometer is presented. Lastly, the degradation of piezoelectric coefficient is studied for increasing thermal fatigue. In chapter 8, toward SHM methodology development, a guided wave based technique to detect and monitor cracks in a structure is presented. To establish the methodology, a detailed study is carried out on the effect of crack and specimen size on the guided wave propagation characteristics. Using the wave characteristics, an analytical way of modeling Lamb wave propagation in the specimen with plastic zone is proposed. The feasibility to determine plastic zone and fatigue crack propagation with integrated piezoelectric transducers is demonstrated experimentally and the results are verified analytically. A method is further established to detect damage at initial stage and crack-tip plastic zone size along with crack length for a given stress amplitude or vice-versa. An approach to estimate fatigue life from the transducer signals is also proposed. In chapter 9, a compact circular array of sensor-actuator network and an algorithm is presented to monitor large plate structures. A method based on the wavelet transforms of transducer signals is established to localize and estimate the severity of damages. Experiments are conducted to demonstrate the capability of the circular array based method in the localization and quantification of various types of damages like debonding of stiffeners, failure of bolted joints, corrosion and hole-enlargement. A damage index is then computed from wavelet time-frequency map that indicates the severity of damage. Chapter 10 ends with the concluding remarks on the work done with simultaneous discussion on the future scope. The work reported in this thesis is interdisciplinary in nature and it aims to combine the modeling and simulation techniques with realistic data in SHM to impart higher confidence levels in the prediction of damages and its prognosis. The work also aims in incorporating various mathematical models of wave propagation and ray tracing based algorithm to optimize the detection scheme employed in SHM. The future direction based on this study could be aimed at developing intelligent SHM systems with high confidence levels so that statistical machine learning would be possible to deal with complex real-world SHM problems.

Zur Messung pyrelektrischer Koeffzienten wurde ein Messplatz nach einem erweiterten SHARP-GARN-Verfahren entwickelt und zur Untersuchung von Phasenumwandlungen in Pyroelektrika eingesetzt. Einerseits konnten pyroelektrische Messungen im elektrischen Feld die Pyroelektrizität einer neuen durch elektrisch angetriebene Defektmigration erzeugten Phase in Strontiumtitanat nachweisen. Andererseits gelang es, Ferroelektrizität in der Hochtemperaturphase von Poly(Vinylidenfluorid-Trifluorethylen), mittels phasenreiner Präparation der Hochtemperaturphase unterhalb der CURIEtemperatur und anschließender Polarisierung, nachzuweisen. Ferner ließen sich mittels thermisch angeregter Pyroelektrika Redoxprozesse antreiben, was durch Desinfektion von Escherichia coli Bakterien in wässriger Lösung mittels Lithiumniobat und -tantalat gezeigt wurde. Die Hypothese der Desinfektion durch reaktive Sauerstoffspezies konnte durch spektroskopisch nachgewiesene OH-Radikale - erzeugt mittels thermisch angeregter Bariumtitanatnanopartikel - belegt werden.

Membrane filtration processes are widely utilized across different industrial sectors for biological and environmental separations. Examples of the former are sterile filtration and protein fractionation via microfiltration (MF) and ultrafiltration (UF) while drinking water treatment, tertiary treatment of wastewater, water reuse and desalination via MF, UF, nanofiltration (NF) and reverse-osmosis (RO) are examples of the latter. A common misconception is that the performance of membrane separation is solely dependent on the membrane pore size, whereas a multitude of parameters including solution conditions, solute concentration, presence of specific ions, hydrodynamic conditions, membrane structure and surface properties can significantly influence the separation performance and the membrane’s fouling propensity. The conventional approach for studying filtration performance is to use a single lab- or pilot-scale module and perform numerous experiments in a sequential manner which is both time-consuming and requires large amounts of material. Alternatively, high-throughput (HT) techniques, defined as the miniaturized version of conventional unit operations which allow for multiple experiments to be run in parallel and require a small amount of sample, can be employed. There is a growing interest in the use of HT techniques to speed up the testing and optimization of membrane-based separations. In this work, different HT screening approaches are developed and utilized for the evaluation and optimization of filtration performance using flat-sheet and hollow-fiber (HF) membranes used in biological and environmental separations. The effects of various process factors were evaluated on the separation of different biomolecules by combining a HT filtration method using flat-sheet UF membranes and design-of-experiments methods. Additionally, a novel HT platform was introduced for multi-modal (constant transmembrane pressure vs. constant flux) testing of flat-sheet membranes used in bio-separations. Furthermore, the first-ever HT modules for parallel testing of HF membranes were developed for rapid fouling tests as well as extended filtration evaluation experiments. The usefulness of the modules was demonstrated by evaluating the filtration performance of different foulants under various operating conditions as well as running surface modification experiments. The techniques described herein can be employed for rapid determination of the optimal combination of conditions that result in the best filtration performance for different membrane separation applications and thus eliminate the need to perform numerous conventional lab-scale tests. Overall, more than 250 filtration tests and 350 hydraulic permeability measurements were performed and analyzed using the HT platforms developed in this thesis. / Thesis / Doctor of Philosophy (PhD) / Membrane filtration is widely used as a key separation process in different industries. For example, microfiltration (MF) and ultrafiltration (UF) are used for sterilization and purification of bio-products. Furthermore, MF, UF and reverse-osmosis (RO) are used for drinking water and wastewater treatment. A common misconception is that membrane filtration is a process solely based on the pore size of the membrane whereas numerous factors can significantly affect the performance. Conventionally, a large number of lab- or full-scale experiments are performed to find the optimum operating conditions for each filtration process. High-throughput (HT) techniques are powerful methods to accelerate the pace of process optimization—they allow for multiple experiments to be run in parallel and require smaller amounts of sample. This thesis focuses on the development of different HT techniques that require a minimal amount of sample for parallel testing and optimization of membrane filtration processes with applications in environmental and biological separations. The introduced techniques can reduce the amount of sample used in each test between 10-50 times and accelerate process development and optimization by running parallel tests.