报告题目：Field Induced Phase Transitions in Bi0.5Na0.5TiO3 Based Ferroelectrics
报告人：Dr Haixue Yan (School of Engineering and Materials Science, Queen Mary University of London)
Lead-free Bi0.5Na0.5TiO3 is a potential ferroelectic candidate to replace lead-based PZT ceramics for piezoelectric and/or dielectric applications. For piezoelectric application, 0.94(Bi0.5Na0.5TiO3)–0.06(BaTiO3) (BNTBT) is the most important compostion in BNT system due to their high piezoelectric constant. The thermal depoling temperature sets the upper limit for the high temperature application of piezoelectric materials. Recently, an interface model was proposed to explain the good resistance to thermal depoling of BNTBT-ZnO composite. However, we found that the presence of ZnO was not limited to the interface, but contributed intrinsically to the BNTBT lattice. This played a critical role in the structural changes of BNTBT, confirmed by a unit volume change supported by XRD, which was further proved by Raman, EDS, and dielectric characterization at different temperatures. The previous interface model is not correct because BNTBT shows thermally stable piezoelectric properties, even though there is no interface between BNTBT and ZnO. The thermal depoling behavior of BNTBT-based materials is directly related to the transition temperature from the rhombohedral phase to the tetragonal phase in our phase transition model, which is consistent with four current peaks in their ferroelectric loops close to the depoling temperature. For dielectric applications, Sr-substituted Bi0.5Na0.5TiO3 shows relaxor behaviour which are good for energy storage. In this work, lead-free single-phase perovskite Srx(Bi1?xNa0.97?xLi0.03)0.5TiO3 (x = 0.30 and 0.38) bulk ceramics, prepared using solid-state reaction method, were carefully studied for the dielectric capacitor application. Polar nano regions (PNRs) were created in this material using co-substitution at A-site to enable relaxor behaviour with low remnant polarization (Pr) and high maximum polarization (Pmax). Moreover, Pmax was further increased due to the electric field induced reversible phase transitions in nano regions. Comprehensive structural and electrical studies were performed to confirm the PNRs and reversible phase transitions. And finally a high energy density (1.70 J/cm3) with an excellent efficiency (87.2%) was achieved using the contribution of field-induced rotations of PNRs and PNR-related reversible transitions in this material.
Haixue Yan received his Ph.D. in Materials Science and Technology from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2001. He is a Senior Lecturer in functional materials at Queen Mary University of London (QMUL). High temperature ferroelectrics, lead-free piezoelectric, energy materials and Spark Plasma Sintering (SPS) are important topics of current fundamental research and industrial applications. His research includes these topics with textured, nano- and metastable structures. He found contradictory evidence on the Tc of ferroelectric CaBa2Nb2O9. Using two independent experiments he measured the Tc of CaBi2Nb2O9. He then obtained ceramics with the highest thermal depoling temperature (800 °C) in polycrystalline ferroelectric ceramics. The material can be operated up to 800 °C without degradation of its properties; this is 200 °C above the operating temperature for currently used ceramics materials. His work on the texturing of ferroelectric ceramics has solved a thirty year problem in this field. In 2009 he reported for the first time ferroelectricity and piezoelectricity in layer-structured A2B2O7 compounds with super-high Curie points (>1,450 °C). This work has opened up this field, with the prospect of producing a step change in the operating temperature of piezoelectric sensors. His work on nanotechnology demonstrated that nano particles can work as building blocks during sintering to improve ferroelectric polarization. Recently his work on ferroelectric characterization provided evidences to answer an open question on phase transitions in lead-free (BiNa)0.5TiO3 and AgNbO3-based materials for high power energy storage and/or actuators. He has 146 publication (including Adv Mater, Nano Energy, JMCA, JMCC, JPCC and Acta Mater) with H-index=38. He has been serving as an editorial board member of Advance in Applied Ceramics and Materials Research Bulletin.