Dielectric, ferroelectric and high energy storage behavior of (1–x)K0.5Na0.5NbO3–xBi(Mg0.5Ti0.5)O3 lead free relaxor ferroelectric ceramics

Abstract: Lead-free dielectric ceramics with high energy-storage density and efficiency are ideal energy materials for sustainable development of the enery resource. In this paper, (1–x)K0.5Na0.5NbO3–xBi(Mg0.5Ti0.5)O3 ((1–x)KNN-xBMT, x = 0.05, 0.10, 0.15, 0.20) lead-free relaxor ferroelectric ceramics are prepared by the traditional solid-state  method.  The  effects  of  BMT  on  the  phase  structure,  microstructure,  dielectric  properties  and  energy storage behavior of KNN based ceramics are studied. With the increase of BMT content, the crystal structures of  (1 –x)KNN-xBMT  ceramics  gradually  change  from  orthorhombic  to  pseudo-cubic  phase,  and  transform  into cubic  phase  finally.  The  addition  of  BMT  can  suppress  grain  growth  of  the  ceramics,  resulting  in  the  average grain size decreasing from 850 to 195 nm when x increases from 0.05 to 0.20. Dielectric properties exhibit that the  Curie  temperature  decreases  with  BMT  content  increasing,  and  dielectric  peak  at  Curie  temperature  is broadened  due  to  the  addition  of  BMT.  In  addition,  ferroelectric  properties  demonstrate  that  the  addition  of BMT  reduces  the  remnant  polarization  (Pr)  and  coercive  field  (Ec)  of  the  ceramics.  The  results  indicate  that (1–x)KNN-xBMT ceramics transform from ferroelectric to relaxor ferroelectric phase. Based on the calculation of hysteresis loop, the best energy storage performance is obtained at x = 0.15, of which the recoverable energy storage density (Wrec) and the energy storage efficiency (h) are 2.25 J·cm–3 and 84% at its dielectric breakdown strength of 275 kV·cm–1. Meanwhile, the ceramic with x = 0.15 exhibits good stability in a frequency range of 1–50 Hz, with an energy density variation of less than 5%, and temperature stability in a range of 25–125 ℃ with change of less than 8%. Moreover, based on direct measurement, the energy storage density (Wdis) of the ceramic with x = 0.15 is 1.54 J·cm–3, and the discharge time is only 88 ns. The research shows that (1–x)KNN-xBMT  ceramics  have  a  wide  application  prospect  in  the  field  of  environmentally  friendly  capacitors  with  high energy storage density.

Keywords: K0.5Na0.5NbO3-base, energy storage, lead-free, relaxor ferroelectric, direct measurement 

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