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Linear-like lead-free relaxor antiferroelectric (Bi0.5Na0.5)TiO3-NaNbO3 with giant energy-storage density/efficiency and super stability against temperature and frequency
摘要： A novel lead-free polar dielectric ceramic with linear-like polarization responses was found out in (1-x)(Bi0.5Na0.5)TiO3-xNaNbO3 ((1-x)BNT-xNN) solid solutions, exhibiting giant energy storage density/efficiency and super stability against temperature and frequency. High-resolution transmission electron microscopy, Raman scattering and Rietveld refinements of x-ray diffraction data suggest that these property characteristics be derived from temperature and electric field insensitive large permittivity as a result of relaxor antiferroelectricity (AFE) with polar nanoregions. Additionally, this feature intrinsically requires high driving field for AFE to ferroelectric (FE) phase transitions due to large random fields. Measurements of temperature-dependent permittivity and polarization versus electric field hysteresis loops indicate that high-temperature AFE P4bm phase in BNT was gradually stabilized close to room temperature, accompanying a phase transition from relaxor rhombohedral FEs to relaxor tetragonal AFEs approximately at x=0.15-0.2. A record high for recoverable energy-storage density W~7.02 J/cm3 as well as a high efficiency η~85% was simultaneously achieved in the x=0.22 bulk ceramic, which challenges the existing fact that W and η must be seriously compromised. Furthermore, desirable W (?3.5 J/cm3) and η (?88%) with a variation of less than 10% can be accordingly obtained in the temperature range of 25-250 oC and frequency range of 0.1-100 Hz. These excellent energy-storage properties would make BNT-based lead-free AFE ceramic systems be a potential candidate applied for pulsed power systems.
关键词： Relaxor antiferroelectric,Dielectric,Energy storage,Polar nanoregions,Lead-free ceramics
更新于2025-09-19 17:15:36
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Origin of polar nanoregions and relaxor properties of ferroelectrics
摘要： In spite of more than 60 years of research, hundreds of publications, and dramatic influence on properties, the driving force of the formation of the polar nanoregion (PNR) in the paraelectric phase of perovskite ferroelectrics and consequent relaxor properties remains uncertain. We show that these peculiar features follow directly from the vibronic, pseudo-Jahn-Teller (PJT) theory of ferroelectricity. Due to the higher disorder (and entropy) in the paraelectric phase (created by the local PJT dynamics), as compared with the polarized phase (where the PJT dynamics is partially quenched), a small PNR of the latter with n unit cells is formed in a dipole-alignment self-assembly process. It emerges encapsulated by a border layer with intermedium ordering that produces “surface tension” and limits its size. The thermodynamic equilibrium between the PNR and the bulk cubic phase at temperatures Tn, well above the phase transition TC, is reached by compensation of the excessive entropy contribution Tn(cid:2)S with ordering energy and the work against this surface tension. The calculations based on the vibronic theory, including the PJT induced local dipolar dynamics and intercell interactions, yield the size of PNR as a function of the temperature increments Tn ? TC and some crystal parameters. In accordance with experimental data, the size of the emerging PNR decreases with temperature, n ～ (Tn ? TC)?3, becoming undetectable at the Burns temperature TB. At temperatures Tf , nearer to the phase transition, the sizes of PNRs grow rapidly, and their interaction leads to the formation of the nonergodic (glasslike) phase.
关键词： polar nanoregions,pseudo-Jahn-Teller effect,relaxor properties,ferroelectrics,vibronic theory
更新于2025-09-09 09:28:46