Transmission electron microscopy investigation of the microstructural mechanisms for the piezoelectricity in lead-free perovskite ceramics
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Lead-free materials with superior piezoelectricity are in increasingly urgent demand in the current century, because the industrial standard Pb(Zr,Ti)O3-based piezoelectrics, which contain over 60 weight% of the toxic element lead, pose severe environmental hazards. Although significant research efforts have been devoted in the past decade, no effective lead-free substitute for Pb(Zr,Ti)O3 has been identified yet. One of the primary hindrances to the development of lead-free piezoelectrics lies in the ignorance of the microstructural mechanism for the electric-field-induced strains in the currently existing compositions.
In this dissertation, the microstructural origin for the high piezoelectricity in (1-x)(Bi1/2Na1/2)TiO3-xBaTiO3 [(1-x)BNT-xBT], the most widely studied lead-free piezoelectric system, has been elucidated. The combination of dielectric characterization and conventional transmission electron microscopy (TEM) demonstrates that the phase relationship for unpoled ceramics is different from that for poled ceramics. This discovery leads to the update of the BNT-BT phase diagram that has been widely accepted for 20 years. The hot-stage TEM study further extends the updated phase diagram to 600 °C and clarifies the complicated relationship between the crystal/domain structures and dielectric properties. Based on these findings, the electric-field in-situ TEM investigation reveals that the morphotropic phase boundary (MPB) in this system, along with the associated piezoelectricity enhancement, could be created, destroyed, or even replaced by another MPB during electrical poling, an indispensable process for ferroelectric materials to exhibit usable piezoelectricity. The optimal piezoelectric response in the originally single-phase composition x = 7% is found to be the result of a stable MPB induced by electric fields during poling.
The discovery summarized in this dissertation not only elucidates the microstructural mechanism for the high piezoelectricity in BNT-BT, but also fundamentally alters the long-standing guiding rule for designing superior piezoelectric materials. It suggests that even single-phase compositions, which were largely excluded before, may also exhibit strong piezoelectricity if stable MPBs form during poling. Such a paradigm shift adds a new dimension to the development of next generation high-performance piezoelectrics.