LOSS MECHANISMS IN PIEZOELECTRIC PZT CERAMICS AND SINGLE CRYSTALS

Open Access
Author:
Rajapurkar, Aditya Mohan
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 02, 2008
Committee Members:
  • Kenji Uchino, Thesis Advisor
  • Leslie Eric Cross, Thesis Advisor
Keywords:
  • mechanical quality factor
  • loss mechanisms
  • piezoelectric
  • ceramics
  • single crystals
Abstract:
This thesis aims to clarify the loss mechanisms in piezoelectric ceramics and single crystals in the view of the recent developments in high power piezoelectric devices such as piezoelectric actuators, ultrasonic motors and piezoelectric transformers in commercial applications. In these devices the piezoelectric materials are driven under high voltages and currents. At high power conditions, the materials deviate from their linear constitutive equations due to non-linearity and hysteresis in physical parameters. The non-linearity and hysteresis manifests in terms of loss and consequently, heat generation in piezoelectric materials. Therefore, analysis of the fundamental principles of loss mechanisms is essential in the determination of heat generation. The loss in piezoelectric materials is directly related to the mechanical quality factor which is defined as the inverse of the loss tangent factor. The quality factor is experimentally calculated following IEEE standards using the impedance curves, Qm as well as near resonance and anti-resonance frequencies and defined as QA and QB. The low power measurement setup in terms of sample holders is reexamined in order to obtain reproducible results. Loss in the 33- and 31- vibration modes is clarified for ‘hard’ and ‘soft’ piezoelectric PZT ceramics. Further loss anisotropy in PMN-PT based single crystals as a function of crystal orientation dependence, vibration mode and doping is investigated. Losses associated with resonance and anti-resonance modes are discussed in the above materials. Significant difference (20-40%) between Q values at resonance (QA) and anti-resonance (QB) with Q being higher for anti-resonance mode. This result is verified analytically and dependence of the quality factor on k31 (electromechanical coupling coefficient), tanδ’ (dielectric loss), tanϕ’(mechanical loss) and tanθ’ (piezoelectric loss) is observed. Quality factors (QA and QB) are also calculated at high power near resonance and anti-resonance while maintaining the vibration velocity constant in order to observe the difference at ‘in-service’ conditions. At high power the quality factors drop by almost half as compared to the low power characterization while maintaining the difference in QA and QB seen at low power. Based on these results it is recommended to drive piezoelectric devices at anti-resonance mode in order to get better efficiency.