Self-Polarization in Epitaxial Fully Matched Lead-Free Bismuth Sodium Titanate Based Ferroelectric Thin Films

The Bi_0.5Na_0.5TiO₃-based ferroelectric is one of the most promising candidates for environment-friendly lead-free ferroelectric/piezoelectric materials for various applications such as actuators and micro-electromechanical systems. The understanding and tailoring of the ferro-(piezo-)­electric properties of thin films, however, are strongly hindered by the formation of the defects such as dislocations, ion vacancies in the film, as well as by the complexity of the interface between the film and the substrate. An ideal system for the study of the polarization behavior in the ferro-(piezo-)­electric film would be a fully matched system. In this work, monocrystalline 0.89Bi_0.5Na_0.5TiO₃–0.11BaTiO₃ thin films were epitaxially grown on (001)-oriented Nb-doped SrTiO₃ substrates using a sol–gel technique. The films were almost fully lattice- and thermally matched with the substrate, thus avoiding the impact of dislocations and thermal stress. The films were self-poled by a built-in electric field, originating from the sedimentation of heavier atoms during the film preparation. As a consequence, an upward self-polarization was introduced into the films, giving rise to asymmetric phase hysteresis loops and domain switching current responses. These results highlight the importance of the interface complexity for the self-polarization of piezoelectric thin films, even for fully matched films, which will therefore facilitate the control of properties of piezoelectric films and their applications for various functional devices

Recoverable Self-Polarization in Lead-Free Bismuth Sodium Titanate Piezoelectric Thin Films

Bismuth sodium titanate, Bi_0.5Na_0.5TiO₃ (BNT), is a promising lead-free ferroelectric material. However, its potential applications have not been fully explored, mainly because of the complex domain structure arising from its intricate phase transitions. A deep and thorough study of its domain structure and polarization switching behavior will greatly help with understanding the polarization nature and with promoting future applications. In this work, we demonstrate that BNT polycrystalline films possess a highly ordered out-of-plane polarization (self-polarization) and randomly oriented in-plane polarizations. Interestingly, the inherent nature of polarization in the BNT films does not allow for the nonvolatile domain writing, as the switched polarization spontaneously and rapidly reverses to the initial orientation state once the external poling electric field is removed, making the self-polarization recoverable. Such a stable self-polarization vanishes gradually with temperature increasing over 150 °C but starts to recover to its initial state upon cooling down to 250 °C, and entirely recovers once the temperature is reduced to below 200 °C. Such interesting properties of BNT films are attributed to the combined effects of the free charges at the Pt electrode, (detected) cation vacancies at the oxide/Pt interface and the defects in oxide lattices. Our results make a step closer to fully understand the nature of polarization and related piezoelectricity in BNT. Such films with recoverable self-polarization are of great interest for applications as sensors, actuators, and transducers that can operate particularly under high temperatures and high electric field conditions

Large Piezoelectric Strain with Superior Thermal Stability and Excellent Fatigue Resistance of Lead-Free Potassium Sodium Niobate-Based Grain Orientation-Controlled Ceramics

Environment-friendly lead-free piezoelectric materials with high piezoelectric response and high stability in a wide temperature range are urgently needed for various applications. In this work, grain orientation-controlled (with a 90% ⟨001⟩_c-oriented texture) (K,Na)­NbO₃-based ceramics with a large piezoelectric response (<i>d</i>₃₃*) = 505 pm V^–1 and a high Curie temperature (<i>T</i>_C) of 247 °C have been developed. Such a high <i>d</i>₃₃* value varies by less than 5% from 30 to 180 °C, showing a superior thermal stability. Furthermore, the high piezoelectricity exhibits an excellent fatigue resistance with the <i>d</i>₃₃* value decreasing within only by 6% at a field of 20 kV cm^–1 up to 10⁷ cycles. These exceptional properties can be attributed to the vertical morphotropic phase boundary and the highly ⟨001⟩_c-oriented textured ceramic microstructure. These results open a pathway to promote lead-free piezoelectric ceramics as a viable alternative to lead-based piezoceramics for various practical applications, such as actuators, transducers, sensors, and acoustic devices, in a wide temperature range