Des céramiques piézoélectriques sans-plomb de composition (CazBa1-z)(Ti1-x-wCoxNbw)O3- ont étésynthétisées e Li2O ou de Li2CO3des matériaux denses à une température de 1100 °C au lieu des 1300 °C usuellement requis pour fritter leBaTiO3.fonctionnelles : le coefficient piézoélectrique d33 et le coefficient de couplage kp atteignent respectivement255 pC/N et 43,5%. De plus, un recuit sous O2 des échantillons dopés au cobalt mène à un d33 = 265 pC/Net à un kp = 42,8%.En fonction de la nature des dopants, un caractère « doux » ou « dur » a été observé dans les céramiquespiézoélectriques. Le dopage par les ions Co/Li mène à un comportement « dur » et provoque desphénomènes de vieillissement. On observe ainsi pour un BT:Co,Li vieilli, un cycle P = f(E) àdouble boucle ou déformé pour des échantillons respectivement non-polarisés ou polarisés. Les cyclescours du temps. Ce champ inCes phénomènes particuliers sont dus à la formation de dipôle de défaut (MTi VO )x causée parin, les grandes valeurs ducoefficient de qualité mécanique (Qmsynthétisées vis-à-vis de fortes contraintes, mécanique ou électrique. Cela rend ces matériaux compétitifscomparés au PZT 4 pour des applications de type transduction acoustique. / (CazBa1-z)(Ti1-x-wCoxNbw)O3- lead-free piezoelectric ceramics were prepared using solid-state reaction. Theuse of a Li2O or Li2CO3 sintering aid enables one to obtain dense ceramics at a temperature of 1100 °Cinstead of the 1300°C used for BaTiO3 in conventional sintering. Insertion of Li/Ca/Co/Nb in the perovskitestructure improves functional properties: for micrograin-size ceramics, a piezoelectric charge constant andelectromechanical coupling factor of d33 = 255 pC/N and kp = 43,5% were reached, respectively.Furthermore, a thermal annealing of the cobalt doped sample under O2 atmosphere led to d33 = 265 pC/Nand kp = 42,8%.Soft/hard characteristics of the piezoelectric ceramics are observed depending on the dopant ions. The Co/Liacceptor dopants lead to hard piezoelectric ceramics and aging phenomena. The aged BT:Co,Li exhibitsdouble loops and a distorted hysteresis cycle for non-poled and poled ceramics, respectively. Distortedhysteresis loops for BT:Co,Li show an increased internal bias field with aging time. Insertion of donordopants such as niobium ions significantly reduces the internal field. These behaviors are related to thepresence of defect dipoles (MTi VO )x due to the insertion of acceptor dopants in the B sites following theoxygen vacancies to equilibrate charge compensation. The high mechanical quality factors (Qm > 1000)obtained for the doped BaTiO3 ceramics affords stability against mechanical stress and electrical stress of upto 400 VRMS/mm, which makes these materials competitive with PZT4 for acoustic transducer applications.

By focusing on the cooling requirement of high power LED, the study aims to explore the spray cooling method and analyze its cooling performance. The ultrasonic micro-nozzle plate made of piezoelectric ceramic material was used in this experiment in order to establish a spray cooling system. The nozzle plate array (3 ¡Ñ 2) was used to carry out a cooling test for 24 LEDs with high power (6 ¡Ñ 4). Three different watts (1 W, 3 W, 5 W) of LED were tested, the total input power was 24W, 72W and 120 W, respectively, and the working medium was DI water. The goal is to understand the variance in performance caused by nozzle plates of different nozzle diameters (dj = 7, 35 £gm) in varied nozzle distances (z = 10, 20, 30, 40, 50 mm). The experiment used thermocouples to measure the slug temperature of LED. By applying thermal resistnace to the LED to calculate its chip temperature, and using micrometer resolution particle image velocimetry (£gPIV) to observe the spray flowfield inside the LED chamber, this study analyzes the influence of flowfield change on cooling performance.

This study aims to explore the use of an ultrasonic micro-nozzle plate, made of piezoelectric ceramic material, as a core material to establish a set of spray cooling system for high power LED. The system uses a single nozzle plate to implement a cooling test for 4 high power LEDs (2 ¡Ñ 2). The total input power was 4 W, 12 W and 20 W, and working medium was DI water. In order to understand the performance variance introduced by utilizing nozzle plates with differing nozzle diameters (dj = 7, 35 £gm) across various nozzle exit to test distance (z = 10, 20, 30, 40, 50 mm). By using micrometer resolution particle image velocimetry (£gPIV) to observe the spray flowfield inside the chamber, and using thermocouples to measure the temperature of LED slug and thermal resistance was used to calculate the LED junction temperature , Tj, for analyzing the influence of flowfield change spread in chamber on its cooling performance. The possibility of an LED spray cooling system is also explored.

In the mammalian auditory system, specifically in the cochlea of the inner ear, the Basilar Membrane (BM) and hair cells are responsible for transducing incoming acoustic waves into electrical signals. These acoustic signals are carried as traveling waves by the BM and propagate from the base of the cochlea toward its apex where the helicotrema is located. An impressive feature of the mammalian auditory system is to prevent the propagated waves from reflecting which allows mammals to hear sounds without any reflection or overlap. This extraordinary characteristic of the inner ear is the main inspiration for this work. In the present study, the dynamic behavior of a beam structure with one or more attached spring-damper (SD) systems as passive absorbers is studied when excited by a harmonic force. The location of the spring-damper system divides the beam into two dynamic regions: one which exhibits non-reflecting traveling waves and the other with standing waves. In this work, the separation of traveling and standing waves is studied analytically, numerically, and experimentally. To the best of the author's knowledge, this is the first time in the literature that traveling and standing wave separation in a beam is realized experimentally using a single-point excitation and a spring-damper. Experimental results are used to validate the models of the system. Moreover, a parametric study is performed to gain a better understanding of the effect of different parameters on the quality of the generated waves in the structure. Furthermore, the effect of attaching the second spring-damper to the system is presented. Adding the secondary SD system results in increasing the excitation frequency range so that wave separation can be achieved. The results of this work can be used in various applications such as vibration suppression, energy absorption, particle transportation, and in exploring possible explanations for the BM and helicotrema functions in the cochlea. / Doctor of Philosophy / In the inner ear of the mammalian auditory system, the sound waves travel inside the cochlea where they are converted to electrical signals sent to the brain. A fascinating characteristic of the mammalian auditory system is that the sound waves traveling in the cochlea do not reflect when they reach its apex where the helicotrema is located. Therefore, we are able to hear sounds without any reflection or overlap. This work is inspired by the biological behavior of the inner ear and studies the dynamic behavior of a simple structure such as a beam with one (or two) attached spring-damper(s). In this work, the attached spring-damper system(s) prevents the waves traveling from the source to the beam's boundary from reflecting. This is similar to what happens in the inner ear. The location of the spring-damper divides the beam into two dynamic regions, one which exhibits non-reflecting traveling waves and the other with standing waves. The wave separation and parameters affecting the wave quality and its reflective or non-reflective features are studied analytically, numerically, and experimentally. To the best of the author's knowledge, the experiments carried out to generate the aforementioned wave types coexisting with each other on the beam are one of a kind. Furthermore, the results of this study showed a very good agreement between the experimental and theoretical results. The outcomes of this work can potentially be used in exploring possible explanations for the function of the cochlea and helicotrema and various applications such as particle transportation and suppression of unwanted vibrations.

Maintenance of the structural integrity and damage detection are critical for all massive and complicated new and aging structures. A structural health monitoring (SHM) system intends to identify damage on the structure under monitoring, so that necessary action can be taken in advance to avoid catastrophic results. Impedance-based SHM utilizes a piezoelectric ceramic as a collocated actuator and sensor, which measures the electrical impedance of the piezoelectric ceramic over a certain frequency range. The impedance profile of a structure under monitoring is compared against a reference profile obtained from the healthy structure. An existing approach called the sinc method adopts a sinc wave excitation and performs traditional discrete Fourier transform (DFT) based structural condition assessment. The sinc method requires rather intensive computing and a digital-to-analog converter (DAC) to generate a sinc excitation signal. It also needs an analog-to-digital converter (ADC) to measure the response voltage, from which impedance profile is obtained through a DFT. This dissertation investigates system design approaches for impedance-based structural health monitoring (SHM), in which a primary goal is low power dissipation. First, we investigated behaviors of piezoelectric ceramics and proposed an electrical model in order to enable us to conduct system level analysis and evaluation of an SHM system. Unloaded and loaded piezoelectric ceramics were electrically modeled with lumped linear circuit components, which allowed us to perform system level simulations for various environmental conditions. Next, we explored a signaling method called the wideband method, which uses a pseudorandom noise (PN) sequence for excitation of the structure rather than a signal with a particular waveform. The wideband method simplifies generation of the excitation signal and eliminates a digital-to-analog converter (DAC). The system form factor and power dissipation is decreased compared to the previously existing system based on a sinc signal. A prototype system was implemented on a digital signal processor (DSP) board to validate its approach. Third, we studied another low-power design approach which employs binary signals for structural excitation and structural response measurement was proposed. The binary method measures only the polarity of a response signal to acquire the admittance phase, and compares the measured phase against that of a healthy structure. The binary method eliminates the need for a DAC and an ADC. Two prototypes were developed: one with a DSP board and the other with a microcontroller board. Both prototypes demonstrated reduction of power dissipation compared with those for the sinc method and for the wideband method. The microcontroller based prototype achieved an on-board SHM system. Finally, we proposed an analytical method to assess the quality of the damage detection for the binary method. Using our method, one can obtain the confidence level of a damage detection for a given damage distance. / Ph. D.

No description available.

Ferroelectric ceramics are very promising materials for a variety of piezoelectric applications such as high permittivity dielectrics, piezoelectric sensors, piezoelectric/electrostrictive transducers, actuators, electro-optic devices, etc. Among the commercially viable ferroelectric ceramics, the lead-zircon ate-titivate Pb(Zr1-xTix)O3 (PZT) based ceramics have dominated the market due to their superior piezoelectric and dielectric property along with other advantages like high electromechanical coupling, ease of processing and low cost. However, the toxicity of lead based materials, and its volatility at processing temperatures is a serious health and environmental concern. Several legislations against lead-based products have been passed all over the world in order to encourage identification of alternative lead-free materials for these applications. As a consequence, there has been a remarkable surge in efforts by researchers on identifying lead-free alternatives for piezoelectric applications. A larger emphasis has been placed on perovskite based ceramics since in addition to possessing excellent properties, their relatively simple structure facilitates understanding structure-property relationships which are important for developing novel high performance materials. Na1/2Bi1/2TiO3 (NBT) and its solid solutions are one of the leading classes of perovskite ceramics, which show promising ferroelectric, piezoelectric and dielectric property thereby having the potential to replace PZT based ferroelectrics. The parent compound NBT is ferroelectric with large ferroelectric polarization (~40 C/cm2), promising piezoelectric properties with 0.08% maximum strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. Though NBT was discovered nearly six decades ago, a clear understanding of its structure remained elusive for a long time since different characterization techniques yielded contradicting reports on its structure and nature of phase transformation. However, rapid advances in characterization techniques in recent years have led to uncovering of new results, thereby shedding light on the true structure of NBT. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) structure at room temperature, which undergoes a gradual transformation into tetragonal (P4bm) structure at ~230oC. However, recent characterization of both single crystal and powder of NBT using high resolution x-ray diffraction showed that the room temperature structure is not purely rhombohedral and the structure can be better modeled with a monoclinic (Cc) structure. In contrast to x-ray and neutron diffraction, electron diffraction studies have shown evidence for the presence of planar disorders, corresponding to in-phase octahedral tilts in the sample which cannot be accounted for by either R3c or Cc structure. In addition, EXAFS, x-ray and neutron total scattering studies, diffuse scattering studies, etc. have shown that the structural parameters obtained from bulk diffraction techniques are significantly different from the local structure of the material. Similar ambiguities have been observed even in NBT based solid solutions like BaTiO3, K1/2Bi1/2TiO3, etc. which show enhanced properties at the morphotropic phase boundary (MPB). A major breakthrough in understanding the structural complexity involved in NBT based solid solutions was achieved when it was found that the structure of the MPB compositions were sensitive to electric field. This led to solving the mystery behind the appearance of cubic-like phase at some of the MPB compositions where the application of electric-field resulted in the transformation of the structure into a co-existence of rhombohedral and tetragonal phases. Observation of an electric-field-induced structural transition at the MPB compositions was expected, because the MPB signifies instability in the system and even a minor external force can significantly influence the system. However, we have found that the structure of even pure NBT is significantly influenced by electric field and the framework of this thesis is based on this particularly important result. The intrinsic tendency of the electric field to affect the structure of NBT is a major factor which needs to be considered when studying similar phase transitions in the MPB compositions of NBT-substituted systems also. This was not taken into account by other research groups, and they assumed that the instability associated with the MPB was the only major factor involved in the electric-field induced transitions. A simple but highly effective strategy of grinding the electrically poled pellet into fine powder and then collecting x-ray diffraction patterns, facilitated elimination of preferred orientation in the sample. Thus, structural analysis by Rietveld refinement was possible even on the poled sample, which has not been carried out by any other groups on any ferroelectric ceramics so far. The initial part of the thesis deals with addressing the structural complexity of pure NBT, wherein the effect of electric field on the bulk structure as well as the local structure was studied in great detail. Later on, similar concepts are used to investigate BaTiO3 and K1/2Bi1/2TiO3 substituted NBT also. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. An exposure to concepts like relaxor ferroelectrics, morphotrophic phase boundary, octahedral tilting, etc. has been provided. Then, a detailed overview of the existing literature related to the structure of NBT and its phase transition studies with temperature has been discussed. The details of the experimental procedures, characterization techniques used, and some theory behind these techniques have been provided in chapter 2. The third chapter deals with the room temperature structural characterization of pure NBT using techniques like x-ray diffraction, neutron diffraction, electron diffraction and EXAFS analysis. The results of these structural characterizations are also complemented with first-principles calculation study of the ground state structure of NBT, dielectric and ferroelectric characterization, and ageing studies. While x-ray and neutron diffraction clearly established electric-field induced structural transition from a monoclinic (Cc) to rhombohedral (R3c) structural transition, first principles calculation showed that the monoclinic phase is not stable and hence cannot be the ground state structure of NBT. Also, the possibility of the monoclinic features appearing in the x-ray diffraction solely due to micro structural effects by nano-sized domains was discussed. Comparison of electron diffraction of poled and unpoled samples of NBT showed that the in-phase tilted regions were greatly suppressed in the poled samples. Even HRTEM images showed that the unpoled samples had a very high concentration of strain heterogeneity in them, which was absent in the poled samples. This gave a direct evidence of correlation between observation of monoclinic phase and the presence of in-phase tilted regions in the unpoled samples. It was proposed that the strain caused by these in-phase tilted disorders caused distortion in the original rhombohedral lattice thereby making the structure appear monoclinic. Application of electric field causes the in-phase octahedral tilt disorders to vanish, thereby even the monoclinic features observed in the XRD also disappear. The fourth chapter discusses the consequences of poling on the high temperature phase transition behavior of NBT. We have used temperature dependent x-ray and neutron diffraction, Raman spectroscopy and EXAFS analysis whose results were correlated with the anomalies observed in temperature dependent dielectric and polarization studies. We found that the poled sample shows a sharp anomaly at the depolarization temperature (Td) in all the characterization techniques used, in contrast to a diffuse or negligible effect seen in the unpoled sample. The depolarization temperature was found to be associated with introduction of structural disorder in the sample in the form of in-phase octahedral tilts. This also gave rise to a normal to relaxor ferroelectric transition at Td, and this relaxor behavior persisted even after cooling the sample to room temperature. An intermediate cubiclike phase was observed from x-ray diffraction at around 300C wherein the rhombohedral phase vanishes and the tetragonal phase begins to appear. Even single crystal study of NBT in the past showed sudden disappearance of the domains at 300C, even though they were visible at both above and below this temperature. This effect was called isotropization, and was postulated to arise due to extremely small domains which made the system isotropic. However, our neutron diffraction pattern showed that in-phase tilted superlattice reflections persisted at this temperature which meant that the structure was not truly cubic at this temperature. Further, a neutron diffraction study at higher temperatures showed that the in-phase tilted superlattice reflections persisted even above the cubic phase transition temperature, in corroboration with similar reports from high temperature electron diffraction. Chapter five deals with the BaTiO3 substituted NBT system, which has gained tremendous interest in the last decade as a viable piezoelectric ceramic for commercial applications. Though a large number of groups have already carried out exhaustive studies on this system, most of the work concentrated mainly on the MPB compositions which showed enhanced piezoelectric properties. In this chapter, we highlight some important findings in the pre-MPB and post-MPB compositions. Using room-temperature and high temperature x-ray diffraction, we show that there exists a subtle compositional phase boundary at x = 0.03, which disappears upon poling the sample. While the monoclinic phase in pure NBT becomes cubiclike at this composition, the depolarization temperature (Td) also slightly increases up to this composition and then decreases upon further Ba substitution. Similar studies were also carried out with compositions containing slightly excess bismuth in them (0.1 mol %), whose purpose was to negate the effects of Bi-vaporization during sintering. It was found that while the compositional phase boundary remained essentially unchanged, there was a decrease in Td for all the compositions. It was also realized that the addition of excess bismuth improved the overall piezoelectric property of the system. Studies on higher compositions of Ba in the post-MPB regions further revealed two additional compositional phase boundaries. A criticality in the coercive field and spontaneous tetragonal strain was observed at x = 0.2, which was found to be associated with crossover from a long-period modulated tetragonal phase (for x < 0.2) to a no modulated tetragonal phase (for x > 0.2). Near the BT rich end (x ~ 0.7), the system exhibits a crossover from normal to a diffuse/relaxor ferroelectric transition with increasing Na1/2Bi1/2 substitution. The onset of relaxor state by Na1/2Bi1/2 substitution on the Ba-site, was shown to increase the spontaneous tetragonal strain in the system. This was because of the enhancement in the covalent character of the A-O bond by virtue of the Bi+3 6s2 lone pair effect, and it also led to a sudden increase in the tetragonal-to-cubic transition temperature. This was in contrast to other chemical modifications of BT reported in the past (like Zr, Sn, Sr, etc.) where the relaxor state is accompanied by a weakening of the ferroelectric distortion and a decrease in the critical temperature. Finally, chapter six covers the effect of electric field induced phase transition in K1/2Bi1/2TiO3 substituted NBT. Visual observation showed that while the compositions (x < 0.2) behaved similar to pure NBT, wherein the structure became purely rhombohedral upon poling, the effect of electric field was negligible in the post-MPB compositions (x > 0.2). In addition, the peaks in the annealed samples were considerably overlapping which made identifying the structural transitions at the MPB compositions difficult using Rietveld analysis. However, comparison of the FWHM of the {200}pc peaks of compositions x < 0.2 showed that the FWHM began to increase suddenly for x > 0.15 indicating emergence of the tetragonal phase. Also, all the compositions showed an increase in the {200}pc peak FWHM by 0.03 after poling. The depolarization temperature showed only slight variation in the pre-MPB compositions, but showed a minimum at the MPB compositions. Temperature dependent dielectric studies further showed that for the post-MPB compositions, the system remains relaxor even after poling.