Lead zirconate titanate (PZT), in the form of both bulk and thin films, is used in most piezoelectric applications due to its high piezoelectric response coefficients. Strontium-doped lead zirconate titanate (PSZT) has shown improved piezoelectric response characteristics in bulk form. This work investigates the deposition and characterisation of PSZT in the form of thin films, and reports on results from the estimation of the piezoelectric response of these thin films using two new techniques. The influence of RF magnetron sputter deposition parameters on the composition and orientation of PSZT thin films has been studied. Investigation of the consequence of varying the oxygen partial pressure during deposition on thin film stoichiometry, the influence of the choice of metal-coated silicon substrates on thin film orientation, and the effect of post-deposition cooling rate have been used to identify optimal deposition conditions. The existence of a modified unit cell resulting from these deposition parameters has been verified, and the resulting lattice parameters were estimated. Extensive materials characterisation (using microscopy, diffraction, and spectroscopy) of the PSZT thin films deposited on gold and platinum coated silicon substrates is reported. The limited techniques available for quantitative estimation of d33 for piezoelectric thin films initiated an investigation into alternative possibilities, as a consequence of which two new techniques for piezoelectric coefficient estimation, under the inverse piezoelectric effect, have been developed. One technique capitalises on the measurement accuracy of the nanoindenter in following thin film displacement, while the other uses a standard atomic force microscope in contact imaging mode to estimate d33. The development, scope, and limitations of both techniques are discussed. The techniques developed have been used to estimate the piezoelectric response of PSZT thin films. Depending on the thin film deposition parameters and the analysis frequency, values of piezoelectric response higher than any measured for thin films on silicon have been estimated. PSZT thin films deposited on gold-coated silicon at low deposition temperatures resulted in d33 values up to 892 pm/V. The study of the piezoelectric response in the millihertz frequency regime resulted in colossal values (ranging in tens of thousands of pm/V) for PSZT thin films deposited at high temperatures on platinum-coated silicon. This was hypothesised to be a result of enhanced ferroelastic domain switching. This hypothesis was verified by reducing the clamping on domains by synthesising island-structured PSZT films and obtaining an increased piezoelectric response.

No description available.

No description available.

The behaviour of a free-standing thin film differs from that of a film surface-bonded or embedded due to the boundary constraints. A general dearth of analytical models, in regard to prediction of the operational competence of a constrained Piezoelectric thin film, prevails. In conventional design of miniaturized thin film devices, several non classical effects, for instance the effect of boundary constraints, are not considered. To warrant the design and performance optimisation of thin film sensors, such effect must be taken into account in a forethoughtful manner. This thesis is an attempt to achieve such optimisation through modeling of thin films. The coupled problem of a film on a substrate is solved semi-analytically in theoretical cases; and by finite element analysis in realistic cases for damage identification in the host structure. We first propose a two-dimensional analytical model of a constrained Piezoelectric thin film embedded in a host. Analytical expressions of capacitance and voltage across the electrodes are obtained by assuming first order shear deformation across the film thickness. The bonding layer between the film and the substrate, which is assumed to be an equivalent single layer including electrodes, insulation layer, adhesive layer etc., is modeled by taking into account its viscoelastic property. Residual stress is incorporated in the constitutive model through equivalent residual strain. Simulations on 10 m thick PVDF and 100 mPZT films are conducted. They illustrate the dependence of voltage response and capacitance on the applied stress, as well as on the residual stress. A maximum percentage variation in capacitance, as compared to the conventional estimate, is about 2% in a PVDF film and +75% to-65% in a PZT film for various combinations of tensile stresses applied at the ends of the film. Effect of residual stress is also exemplified via comparative response of a 1 m PZT film deposited on Pt/Ti/Si(0 0 1), with and without residual stress. For this case, an almost +50% increase in the voltage and an equivalent drop in the capacitance is observed. Next, we look into the voltage response profile of this model by employing it as a sensor to identify a finite mode I and mode II sub-surface cracks in a finite size host. To model the embedded crack, additional perturbation functions in the displacement field due to linear elastic crack tips in an infinite solid under plane strain condition are introduced to accommodate the stress free conditions at its surfaces. The film model requires the interfacial displacement and traction conditions, which are obtained from the analysis of the host. The combined analysis of the film and crack models brings forth the voltage gradient along the film span as a direct indicator of the location of crack in the axial direction, whereas the voltage magnitude represents the size of the crack. Following this analysis, a quasi three-dimensional(3-D) model of a Piezoelectric thin film surface-bonded to the host structure is proposed. With due consideration of restriction on the thickness of the film, here the model is based on a reduced 3-D continuum mechanics approach. The displacement field in the film is assumed to vary according to the third-order shear deformation theory; and the electrical and mechanical boundary conditions on the surfaces of the film are accommodated in a consistent manner. The formulation yields a governing inhomogeneous system of second-order Partial Differential Equations(PDEs), which is dependent on the displacement field at the film-host interface through force terms. Semi-analytical expressions of potential difference and capacitance are obtained. This system is solved numerically for two unknown rotations about X and Y axes of the film by finite element method. A maximum variation of about 2.5% is obtained in the capacitance of a 10 m PVDF film, as compared to its conventional estimate. The operational performance of this model is assessed in terms of its voltage response over the film area for various displacement fields. Conformation of this response to the input displacement field attests to its mathematical integrity. Next, we ascertain the versatility of this model in its role as a sensor for Structural Health Monitoring. To deal with cracks in the host plate, finite size rectangular surfaces are introduced as crack faces. The film domain and the host domain are discretized with an a posteriori h-refinement strategy and compatible interfacial nodes at the film-host interface via finite element interpolation. The resulting coupled problem is solved by static finite element analysis. The nature of the voltage pattern over the film surface is peculiar to the mode of crack, and is a qualitative portrayal of its presence. To correlate the electric potential(voltage) –a distributed parameter – to the geometry and orientation of the crack, as well as to quantify it, electrostatic measures in terms of integrated potential difference and its spatial gradients on the film surface are proffered. The numerical implications of these measures are elicited through simulation results of various crack sizes in damaged and healthy hosts under identical conditions of stress and boundary. The pattern of these measures in a damaged host becomes oscillatory as compared to straight lines observed in a healthy host. Furthermore, the reduced 3-D model is extended to perform dynamic analysis with the inclusion of inertial terms in the governing equilibrium equations. Subsequently, the acceleration terms appear in the governing inhomogeneous system of PDEs in the force terms. Finite element analyses of this extended film model on an isotropic beam with surface and sub-surface cracks, and on a composite plate with delamination, are then performed in the time domain. In all cases, an excellent conformation of the voltage profile at any point in the film domain to the velocity profile at the corresponding point in the film-host interface is observed. Again, to quantify the extent of damage in the host, we proffer electrical measures based on the Lpnorm, of second order, of the voltage and its directional derivatives. We exemplify the numerical implications of these measures in the time domain through sensitivity analysis in regard to the defected areas, and their region of occurrence relative to the film sensor. The response of the film model educes that the relatively flat curves after the first incident pulse in a healthy structure shoots off to a monotonic pattern in damaged hosts. The measures depict high degree of sensitivity in regard to the variation in the area of damage of any nature. An apposition of the static and dynamic analyses is elaborated towards the end of this dissertation. It proves to be very insightful in the damage assessment of the host structure, for it shows the utility of the dynamic model to sense the location of the damage occurrence, whereas a more in-depth assessment on its nature and mode of the crack would demand a static analysis in its proximal regions. To sum up, in light of these models and the proposed measures, this thesis establishes salient justifications pertaining to their pragmatic significance. We believe that these results represent an important contribution towards the ongoing research on understanding the role of boundary constraints in mechanically thin Piezoelectric films.

Les MEMS et NEMS résonants sont d'excellents candidats pour la réalisation de systèmes de détection de gaz haute résolution et faible couts ayant des applications dans les domaines de la sécurité, la défense, l'environnement et la santé. Cependant, la question du choix des techniques de transduction est toujours largement débattue. La transduction piézoélectrique pourrait être avantageusement exploitée mais elle est encore peu connue à l'échelle nanométrique. L'objectif de cette thèse est donc de progresser vers la réalisation de capteur de gaz à haute résolution à l'aide résonateurs à base de micro / nano poutres piézoélectriques en couvrant la chaîne de prototypage complète depuis les techniques de dépôt des matériaux jusqu'à l'expérience de preuve de principe de mesure de gaz. Pour cela, notre première contribution concerne la modélisation analytique des performances et l'optimisation, design et système, d'un capteur de gaz à base de poutres résonantes piézoélectriques. En particulier, nous démontrons que la diminution de l'épaisseur du film piézoélectrique actif sous la barre des 100 nm permet d'atteindre les meilleures performances. La deuxième contribution concerne la fabrication, la caractérisation et la démonstration des performances capteur de poutres résonantes de 80 μm de long exploitant un film piézoélectrique en AlN de 50 nm d'épais. Ainsi nous avons démontré expérimentalement la stabilité fréquentielle exceptionnelle de ces dispositifs atteignant des déviations standard de l'ordre de 〖10〗^(-8), au niveau de l’état de l'art. Ainsi, ils permettent la détection de vapeurs Di -Methyl -méthyl- phosphonates, un simulateur de gaz sarin, avec des concentrations aussi faibles que 10 ppb. Bien que le niveau d'intégration de notre système de détection ne soit pas suffisant, ces résultats prouvent le fort potentiel de ces résonateurs cantilever piézoélectriques pour un développement industriel futur. / Resonant MEMS and NEMS are excellent candidate for the realization of low cost and high resolution gas sensing systems that have several applications in security, defense, and environment and health care domains. However, the question of the transduction technique used to couple micro or nano scale signals to the macro scale is still a key issue. Piezoelectric transduction can be advantageously exploited but has been rarely studied at the nano-scale. The objective of this PhD is thus to progress toward the realization of high-resolution gas sensor using piezoelectric micro/nano cantilevers resonators and cover the whole prototyping chain from device fabrication to proof of principle experiment. Our first contribution in this research relates the analytical modeling of the sensing performance and the system and design optimization. In particular we demonstrate that decreasing the piezoelectric active film thickness below 100 nm is particularly beneficial. The second contribution relates the fabrication, characterization and demonstration of the high sensing performances of 80 μm long cantilevers embedding a 50 nm thick piezoelectric AlN film for transduction. These devices exhibit state of the art performances in terms of resonance frequency deviation down to the 〖10〗^(-8) range. They allow thus the detection of Di-Methyl-Methyl-Phosphonate vapors, a sarin gas simulant, with concentration as low as 10 ppb. Although the level of integration of our sensing system is not sufficient for real life application, these results prove the high potential of these piezoelectric cantilever resonators for future industrial development.

博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

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.

Man’s desire to replicate/mimic the nature’s creation provided an impetus and inspiration to the rapid advancements and progress made in the sensors and actuators technology. A normal human being has five basic sensory organs, which helps and guides him in performing the routine tasks. This underlines the importance of basic sensory organs in a human life. In a similar fashion, sensors and actuators are of paramount importance for most of the science and engineering applications. The aim of the present thesis work is to explore the application of piezoelectric ZnO thin films deposited on a flexible substrate for the development of sensors and actuators. Detailed study was performed on the suitability of three different flexible substrates namely Phynox, Kapton and Mylar. However, Phynox alloy substrate was found to be a suitable substrate material for the above mentioned applications. Sputtering technique was chosen for the deposition of ZnO thin films on to Phynox substrate. The necessary process parameters were optimized to achieve good quality piezoelectric thin films. In the present work, sensors have been developed by utilizing the direct piezoelectric effect of ZnO thin films deposited on Phynox alloy substrate. These includes a flow sensor for gas flow rate measurement, impact sensor for non-destructive material discrimination study and a Thin Film Sensor Array (TFSA) for monitoring the impact events. On the other hand, using the converse piezoelectric effect of ZnO thin films, actuators have also been developed. These include a thin film micro actuator and a Thin Film Micro Vibrator (TFMV) for vibration testing of micro devices. The thesis is divided into following seven chapters. Chapter 1: This chapter gives a general introduction about sensors and actuators, piezoelectric thin films, flexible substrates, thin film deposition processes and characterization techniques. A brief literature survey of different applications of piezoelectric thin films deposited on various flexible substrates in device development is presented. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. ZnO thin films were deposited on Phynox substrate by Rf reactive magnetron sputtering technique. The sputtering process parameters such as: Ar:O2 gas ratio, substrate temperature and RF power were optimized for the deposition of good quality piezoelectric ZnO thin films. The deposited ZnO thin films were characterized using XRD, SEM, AFM and d31 coefficient measurement techniques. Chapter 3: It reports on the comparative study of properties of piezoelectric ZnO thin films deposited on three different types of flexible substrates. The substrate materials employed were a metal alloy (Phynox), polyimide (Kapton), and polyester (Mylar). Piezoelectric ZnO thin films deposited on these flexible substrates were characterized by XRD, SEM, AFM and d31 coefficient measurement techniques. A vibration sensing test was also performed for the confirmation of good piezoelectric property. Compared to the polymer flexible substrates, the metal alloy flexible substrate (Phynox) was found to be more suitable for integrating ZnO thin film for sensing applications. Chapter 4: The development of a novel gas flow sensor for the flow rate measurement in the range of L min-1 is presented in this chapter. The sensing element is a Phynox alloy cantilever integrated with piezoelectric ZnO thin film. A detailed theoretical analysis of the experimental set–up showing the relationship between output voltage generated and force at a particular flow rate has been discussed. The flow sensor is calibrated using an in-house developed testing set-up. Chapter 5: This chapter is divided into two sections. Section 5.1 reports on the development of a novel packaged piezoelectric thin film impact sensor and its application in non-destructive material discrimination studies. Different materials (Iron, Glass, Wood and Plastic) were successfully discriminated by using the developed impact sensor. The output response of impact sensor showed good linearity and repeatability. The impact sensor is sensitive, reliable and cost-effective. Section 5.2 reports on the development of a Thin Film Senor Array (TFSA) for monitoring the location and magnitude of the impact force. The fabricated TFSA consists of evenly distributed ZnO thin film sensor array. Chapter 6: It consists of two sections. Section 6.1 reports on the fabrication of micro actuator using piezoelectric ZnO thin film integrated with flexible Phynox substrate. A suitable concave Perspex mounting was designed for the actuator element. The actuator element was excited at different frequencies for the supply voltages of 2V, 5V and 8V. The developed micro actuator has the potential to be used as a micro pump for pumping nano liters to micro liters of fluids. Section 6.2 reports the design and development of a portable ready to use Thin Film Micro Vibrator (TFMV). The TFMV is capable of providing the vibration amplitude in the range of nanometer to micrometer. A thin silicon diaphragm was used as a test specimen for its vibration testing studies using the developed TFMV. The TFMV is light-weight and have internal battery, hence no external power supply is required for its functioning. Chapter 7: The first section summarizes the salient features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.