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Materials Science at Oregon State University

Hardening in A-site Non-stoichiometry (1-x)Bi0.5Na0.5TiO3-xBaTiO3 Lead Free Ceramics Near Morphotropic Phase Boundary

Date: Thursday, Apr 9th
Presenter: Noon Prasertpalichat, OSU Materials Science

Abstract


Piezoelectric ceramics with hardening characteristic are very desirable for high-power applications such as transformer, ultrasonic motor and high intensity focused ultrasound (HIFU). For lead zirconate titanate (Pb[Zrx,Ti1-x]O3 , PZT), which is currently the most widely used piezoelectric materials, hardening is typically done by acceptor doping or doping with low valence ions on either A- or B- site (e.g. Fe3+ replacing Ti4+/Zr4+ ). However, due to an environmental concern regarding the toxicity of lead, a search for lead-free piezoelectric ceramics with comparable properties to that of PZT is needed. A solid solution of (1-x)(Bi0.5Na0.5)TiO3 - xBaTiO3 ,(BNT - xBT), is one of the most promising candidate due to the excellent piezoelectric and electromechanical properties at the morphotropic phase boundary (MPB) around x=6-7 mol%. Many studies have investigated whether or not the hardening characteristics, which have been successfully obtained in PZT, can be achieved in BNT-based system. However, unlike PZT, acceptor doping innBNT-based solid solution results in a slight hardening, which was ascribed to a relaxor nature of the compound. Therefore, this study will investigate the alternative A-site acceptor-doping (through Na excess) and donor-doping (through Bi excess) in BNT-xBT near the MPB.
A significant improvement of dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (Ec) which is an indication of domain wall pinning as found in hard Pb(ZrxTi1-x)O3. This result was further confirmed via polarization hysteresis studies including PUND tests and remanent P-E hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Qm) as well as a decrease in piezoelectric coefficient (d33), dielectric loss, remanent polarization (Pr) and dielectric permittivity, which are all the typical characteristics of the effects of “hardening”. The mechanism for the observed hardening in A-site acceptor doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry.