Mechanism for the Electromechanical Properties of Relaxor-PbTiO3 Piezoelectric Single Crystals

Abstract

Piezoelectricity, i.e, electromechanical coupling, has been widely used for sensors, actuators and transducers. Lead-based relaxor-PbTiO3 single crystals such as Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) have received considerable attention due to their unprecedented piezoelectric responses in the vicinity of morphotropic phase boundary (MPB). However, the origin of the ultrahigh piezoelectric properties is still controversial due to the lack of direct evidences.

The goal of the part 1 is to understand the property-enhancing mechanism at the morphotropic phase boundary of the relaxor-PbTiO3 single crystals. The electric-field-induced volume changes and Poisson ratios of PMN-PT single crystals were measured in situ to resolve phase-related issues. The results proposed that the so-called electric-field-induced phase transformation should not be the reason for the enhanced piezoelectric properties in contrast to the common belief. To propose a new model explaining the MPB-assisted property enhancement, various techniques such as polarization mapping at an atomistic scale and spectroscopic techniques have been applied.

As a verification of the proposed mechanism in the part 1, the correlation between the presence of MPB and the domain characteristics have been further explored through domain engineering techniques such as poling. For fascinating ferroelectric materials to be macroscopically piezoelectric, a conventional post-treatment, i.e. poling, is vitally required. Poling is usually conducted by sufficiently high electric field at an elevated temperature for 30 ~ 60 min, which is apparently time-consuming and cost-ineffective. Recently, a newly-developed poling method, so-called alternating-current (AC) poling where several alternating current (ac) signals are applied, has been inspired by recent reports on the enhancements in piezoelectric and dielectric performances than customarily-implemented direct-current (DC) poling.

In the part 2, three works relevant to AC poling will be primarily discussed. 1) We carried out AC poling on relaxor-based single crystals grown by a solid state crystal growth (SSCG) method and a Bridgman method. The feasibility of AC poling was verified only for Bridgman crystals whereas SSCG crystals did not show improvements due to an unavoidable remanent poled state. Such memory effect would hinder spontaneous polarizations from switching, which highly affects the performance of piezoelectric single crystals. 2) We intentionally incorporated an excessive amount of dopants into relaxor-based single crystals to intensify the memory effect. As a result, a unique thermally-resistive poled state was obtained and such state is able to be maintained even after temperature increased up to its Curie temperature (Tc). The reason for such unusual phenomenon is attributed to thermally-endurable internal bias field due to space charge accumulations around pores. Unfortunately, the feasibility of AC-poling with sequent ac signals in the excessively-doped crystals was ineffective due to the relatively-large internal bias field. To solve the problem, 3) we found that an AC-poling like effect on SSCG crystals can be achieved by simply applying DC-poling opposite to the direction of the internal bias field. Such alternative method showed an enhancement in piezoelectric properties which are comparable to the conventional AC-poling without successive ac signals. On the basis of the aforementioned works, two issues will be primarily addressed; 1) how to apply AC-poling on SSCG crystals, and 2) revisiting the effect of dopants in relaxor-based single crystals. This part will shed light on designing the composition and the functionality of relaxor-based piezoelectric single crystals grown by SSCG methods.