Low loss 1-3 piezocomposites for high power ultrasonic transducers.

摘要

The goal of this dissertation was to improve the high power characteristics of 1-3 piezoelectric / polymer composites. A 1-3 composite has proven to be an effective material design for transducer applications due to several benefits offered by this structure, such as high electromechanical coupling, low parasitic vibrations and low acoustic impedance. However, a conventional 1-3 composite has been limited for use in high power ultrasonic transducers due to its inherently low mechanical quality factor, Qm, giving rise to power loss and internal heating under high power operation.;Various piezoelectrics and polymers were explored for the design of high Qm (low loss) 1-3 composites. "Hard" lead zirconate titanates (PZT4 and PZT8), acceptor modified (Bi,Na)TiO3 (BNT) and BaTiO3 (BT) based ceramics were found to be promising active piezoelectric components owing to their high Qms. The passive polymers were selected based on the desired properties for high power composites - low elastic loss, low elastic modulus and high thermal conductivity. Various piezoelectric ceramics and 1-3 composites were characterized using various measurement techniques, including constant vibration velocity, linear frequency modulation, isothermal testing, pulse-echo response, and radiated output power measurements.;The results showed that although the overall electromechanical properties of BNT based ceramics were lower than those of BT and PZT based ceramics, the higher stability in Qm under high drive conditions allowed for comparable dynamic strain to that of PZTs at high fields. BT based ceramics, on the other hand, showed comparable small signal properties to PZT8 ceramics, being on the order of ∼190 pC/N of d33 and ∼1000 of Qm, but showed a significant performance degradation at high fields. The origin of this difference between BNT and BT based ceramics is believed to be related to the domain stability under high drive conditions, evidenced by high coercive field levels, being >35 kV/cm and ∼7 kV/cm, respectively.;For 1-3 composites, a low loss polymer (Spurr resin) offered an improved Qm, being on the order of ∼400, ∼200, and ∼150 for PZT8, PZT4, and BNT based composites, respectively. In contrast, the Qm of composites with high thermal conductivity polymers (>1 W/m.K) were found to be lower (Qm<100) due to the effects of high elastic modulus and loss factor of the passive components. Analysis of 1-3 composites under high field and isothermal conditions in air revealed that 1-3 composites with optimized composite components, (i.e., high Qm composites), improved the electromechanical efficiency and thermal stability, particularly under low duty cycle conditions, <20%. The high Qm 1-3 composites also showed improved acoustic output power and power efficiency in water with broader bandwidth compared to monolithic ceramics, demonstrating great potential for high power ultrasonic transducers.