Yingying Zhao

Rapid stability of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO3 ceramics.

In this work, we report a rapid stability phenomenon of ferroelectric polarization in the Ca, Ce hybrid doped BaTiO3 ceramics (BCaxT+BTCe8) (x = 10, 20, 24, 30 mol%) prepared by separate doping Ca2+ and Ce4+ ions. Double hysteresis loops are identified in the aged BCaxT+BTCe8 samples; meanwhile, the polarization of these loops present a rapid decrease within very short aging time (about 1 h), and then the polarization remains almost unchanged over the followed ~1000 h. This phenomenon is not reported in previous researches. Raman scattering spectrum indicates that oxygen vacancies are generated because of Ca2+ ions entering into Ti sites partly in the BCaxT+BTCe8 samples, and then the oxygen vacancies are quantitatively characterized by half of the Ce3+ content through the XPS test. The emergence of the aging phenomenon is explained through the defect dipole reorientation mechanism. The larger radius of Ca2+ ions is further discussed as a possible reason for the rapid stability phenomenon of ferroelectric polarization. It may provide an effective design method from the viewpoint of the ionic radius to accelerate polarization stability, and thus to facilitate the possible practical applications of the aging effect.

Large activation energy in aged Mn-doped Sr0.4Ba0.6Nb2O6 ferroelectric ceramics

The properties of ferroelectrics can be significantly enhanced through appropriate doping, and also can be strongly influenced by aging treatment. In this paper, we report a slow aging effect with a large activation energy and an enhanced energy storage density in the aged Mn-doped Sr0.4Ba0.6Nb2O6 ferroelectric ceramics. Mn-doped Sr0.4Ba0.6Nb2O6 (SBN40-xMn) (x = 0.0, 1.0, 2.0, 4.0, 6.0, 8.0 mol%) ferroelectric ceramics were prepared by the conventional solid state reaction method. The dielectric constant of Mn-doped Sr0.4Ba0.6Nb2O6 ceramics shows a decrease after aging in the ferroelectric state. The activation energy derived from the dielectric aging curves shows an obviously large value of 1.87 eV and 2.01 eV for SBN40-4Mn and SBN40-6Mn sample, respectively, compared with the value of 0.55 eV in SBN40-2Mn sample. Besides, the large activation energy is much higher than those values reported in previous literatures for perovskite ferroelectrics such as BaTiO3 and Pb(Ti0.42Zr0.58)O3 based system. The relaxation time (reflecting the aging rate) is also much longer than the cited perovskite systems. The large relaxation time and activation energy both indicate the slow aging process. Moreover, the aging is related to the migration of oxygen vacancies, and the existence of oxygen vacancies in the Sr0.4Ba0.6Nb2O6 ceramics is proved by the dielectric relaxation at high temperature and the XPS spectra investigation. Based on the longer relaxation time and larger activation energy, a different diffusion path of oxygen vacancy in our tetragonal tungsten bronze structure high Mn doped SBN is proposed: a long distance migration path of oxygen vacancies among neighbor oxygen octahedron, in contrasting to the short migration distance of oxygen vacancies within perovskite unit cells. In consequence of the long path oxygen vacancy diffusion, a typical double hysteresis loop appeared and the maximum energy storage density is achieved in aged SBN40-4Mn sample. These results can provide guidance for understanding the dynamic of aging process and improving the energy storage properties of doped ferroelectric materials with the tungsten bronze structure.