Comprehensive Study of External Field and Stress Bias Effect on Piezoelectric Properties and Loss Factors.

다국어 초록 (Multilingual Abstract)

This study seeks to clarify the effect of external stress and electric field DC biases on piezoelectric material properties, specifically on piezoelectric loss factors. In order to improve the reliability, piezoelectric devices are often subjected to external electrical and mechanical biases, in many applications such as underwater transducers. While the performance and property changes under these biases have been widely investigated from a practical viewpoint, there is a significant lack of fundamental research on effect of external electric and stress bias on the loss mechanism. The piezoelectric material performance in high power applications is highly dependent on material’s mechanical quality factor, which is originated from the elastic, dielectric, and piezoelectric loss factors. The study of electrical and stress bias effect on the loss parameters is expected to provide a better understanding of the microscopic origin of the loss mechanism in piezoelectric materials, in terms of domain wall dynamics. Also in practice, it can improve the finite element analysis methods used for device designing.In order to study the fundamental origin of loss mechanism in piezoelectric materials, studies of their behavior under external field and stress biases and different driving conditions are very helpful. The conventional loss characterization studies are mainly performed at low power condition and have considerable assumptions that requires to be verified. Therefore, loss characterization at high power at resonance is required to verify these methods and to distinguish the high AC drive condition effect on material performance (Chapter III). Therefore, in this study we aim to develop new methodologies to improve loss characterization methods under high AC drive condition under external DC electric field and stress biases. Accordingly, we have proposed a new methodology for high power measurement based on burst mode, for eliminating the temperature rise effect. We succeeded to measure the dielectric loss and dielectric permittivity around the resonance/antiresonance frequencies, for the first time in the world. The characterization methods used for studying DC stress and DC electric field effect on piezoelectric material properties, include various difficulties, such as clamping effect, relaxation, and creep effects. In this project, we have addressed these issues and developed more reliable characterization methodologies to provide a better examination of the effect of external biases on piezoelectric material properties and loss mechanism. Accordingly, a new methodology based on bolt clamped Langevin transducer design was proposed to study the effect of DC stress bias (Chapter IV), which succeeded to improve the measurement reliability and accuracy under external DC stress bias. Chapters V and VI describe the experimental results, where we discovered that electric field bias along positive polarization direction exhibits a decrease in both dielectric and elastic losses in k31 mode, as well as increasing elastic and dielectric losses perpendicular to spontaneous polarization direction (for both positive and negative electric field), while the compressive stress bias along polarization direction (33-direction), exhibits a decrease in the elastic loss, and dielectric loss with slight increase of piezoelectric loss.While the piezoelectric thermodynamic phenomenology has been widely studied, there is a lack of comprehensive studies on the behavior of material properties under external biases, which implies the non-linear behavior inclusion. Therefore, this project also aims to develop a comprehensive phenomenological model to clarify the nature of nonlinear material properties change under external biases (Chapter VII). Developing a comprehensive model can enhance our understanding from effects of DC electric field and stress on material properties, “real parameters”, and can help the study of loss parameters, “imaginary parameters”, under these effects. Therefore, we have developed a thermodynamic model based on Gibb’s elastic free energy. In this study we have shown that in order to explain elastic nonlinear material changes under external stress and electric field, it is necessary to introduce higher order elastic and electrostrictive energy expansion terms (sijk, and Qijk dependent energy expansion terms, respectively). We have developed the model based on proposed energy expansion terms and verified the reliability of the model by experimental data we have accumulated during this project. The proposed model has succeeded to predict the material properties change’s tendency under external biases.Also, this thesis aims to propose an intuitive model to explain the loss mechanism and specially the loss parameters behavior under external biases, based on domain wall dynamics. Though the approach is very primitive, we could correlate the domain wall damping factor with the dielectric loss parameter. The studies of loss mechanism can be very beneficial for design and production of high-power piezoelectric applications and also it can build a better understanding from heat generation due to loss mechanism.Finally, in this research we succeeded to contribute numerous achievements to study of loss mechanism in piezoelectric materials. We have succeeded to measure the effect of external electric field and external compressive stress on piezoelectric material properties and loss parameters. For the first time, we have introduced a new methodology for measurement of dielectric properties and dielectric loss at resonance condition and we have shown the effect of high power driving conditions on loss parameters for the first time. Our findings demonstrate that while conventional methods based on IEEE standard can be used at low power characterization conditions, at high power characterization systems these methods show considerable unreliability. Furthermore, we have studied the piezoelectric material properties change under external biases from phenomenological point of view and we have shown that conventional phenomenological approaches are not capable of explaining the nonlinear material change under external biases. Therefore, we have introduced both higher order elastic expansion and higher order electromechanical coupling energy expansions to explain the nonlinear material change under external biases. Our proposed model was capable of explaining all property changes under both external field and external mechanical stress. All of our achievements and industrial/scientific contributions in this research are listed and explained in detail in Chapter IX.