HIGH PERMITTIVITY RELAXOR-LEAD TITANATE PIEZOELECTRICS FOR HIGH FREQUENCY ULTRASONIC TRANSDUCERS

Open Access
Author:
Lee, Hyeong Jae
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 09, 2009
Committee Members:
  • Shujun Zhang, Thesis Advisor
  • Thomas R Shrout, Thesis Advisor
Keywords:
  • pmn-pt
  • composite
  • high frequency
  • transducer
  • piezoelectric properties
  • thickness
Abstract:
Relaxor Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) based ferroelectric materials have been significant interest for active transducer materials because of their superior dielectric and electromechanical properties. However, there are some limitations for these kinds of materials to implement into high frequency (20-100 MHz) ultrasound transducers, arising from the inability to fabricate fine scale features and degraded electrical properties with increasing frequency. Therefore, this thesis deals with two important topics. One is the improvement of the mechanical properties of relaxor-PTs to enable the fabrication of high frequency transducers using these materials. The second is to understand the mechanism of degraded electrical properties of these materials with increasing frequency. This thesis first presents the synthesis and characterization of fine (~1 microns) grained PMN-PT polycrystalline ceramics. Fine grain versions of piezoelectric materials have proven to be a great promise for high frequency transducers because of their benefits, such as improved mechanical strength and property stability at high frequency range. This thesis also includes the investigation of size and/or frequency dependent properties of various relaxor-PTs to study their applicability to high frequency ultrasound transducers. The compositions studied in this work include binary PMN-PT polycrystalline ceramics and single crystals, as well as modified ternary based PMN-PT single crystals. The characterization of electrical properties of relaxor-PTs as a function of thickness and/or frequency showed that fine-grained PMN-PT ceramics provide greater mechanical strength and improved property stability at small thicknesses less than 50 microns, which correspond to >40 MHz ultrasound frequencies. For single crystals, binary PMN-PTs were found to exhibit significant property degradation in terms of dielectric and piezoelectric properties with decreasing sample thicknesses. In contrast, modified ternary based PMN-PT single crystals showed promise for a use in high frequency ultrasound transducers, with no performance degradation with decreasing sample thicknesses down to 40 microns. Analyses for thickness dependent permittivity at high temperatures and domain observations revealed that the thickness dependent properties of relaxor-PTs are closely related to the domain size with respect to the associated macroscopic scale of the samples. The influence of dicing damage on the properties was further studied and discussed through the investigations of 1-3 relaxor-PT composites, which were fabricated by dicing-fill technique. It was found that 1-3 composites showed further property degradation compared to monolithic relaxor-PTs at the same thickness level. X-ray diffraction pattern revealed that diced surface was covered with heavily stressed and damaged surface layer, which implies that surface damage created by dicing could be the origin of property degradation of 1-3 composites.