Achieving a high energy storage density in Ag(Nb,Ta)O_(3) antiferroelectric films via nanograin engineering

Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization(Pmax)and a small remnant polarization(Pr),AgNbO_(3)-based antiferroelectrics(AFEs)have attracted extensive research interest for electric energy storage applications.However,a low dielectric breakdown field(Eb)limits an energy density and its further development.In this work,a highly efficient method was proposed to fabricate high-energy-density Ag(Nb,Ta)O_(3) capacitor films on Si substrates,using a two-step process combining radio frequency(RF)-magnetron sputtering at 450℃and post-deposition rapid thermal annealing(RTA).The RTA process at 700℃led to sufficient crystallization of nanograins in the film,hindering their lateral growth by employing short annealing time of 5 min.The obtained Ag(Nb,Ta)O_(3) films showed an average grain size(D)of~14 nm(obtained by Debye-Scherrer formula)and a slender room temperature(RT)polarization-electric field(P-E)loop(Pr≈3.8 mC·cm^(−2) and P_(max)≈38 mC·cm^(−2) under an electric field of~3.3 MV·cm^(−1)),the P-E loop corresponding to a high recoverable energy density(W_(rec))of~46.4 J·cm^(−3) and an energy efficiency(η)of~80.3%.Additionally,by analyzing temperature-dependent dielectric property of the film,a significant downshift of the diffused phase transition temperature(T_(M2-M3))was revealed,which indicated the existence of a stable relaxor-like AFE phase near the RT.The downshift of the T_(M2-M3) could be attributed to a nanograin size and residual tensile strain of the film,and it led to excellent temperature stability(20-240℃)of the energy storage performance of the film.Our results indicate that the Ag(Nb,Ta)O_(3) film is a promising candidate for electrical energy storage applications.

Achieving a high dielectric tunability in strain-engineered tetragonal K_(0.5)Na_(0.5)NbO_(3) films

Using a modified Landau-Devonshire type thermodynamic potential,we show that dielectric tunabilityηof a tetragonal ferroelectric film can be analytically solved.At a given electric field E,ηis a function of the remnant polarization(P_(0)^(f))and the smallfield relative dielectric permittivity(χ_(0)^(f)),which are commonly measured material properties.After a survey of materials,a largeη~80%is predicted to be achievable in a(001)-oriented tetragonal(K_(0.5),Na_(0.5))NbO_(3)film.This strain-stabilized tetragonal phase is verified by density functional theory(DFT)calculations.(K_(0.5),Na_(0.5))NbO_(3)films based on this design were successfully prepared via a sputtering deposition process on SrRuO_(3)-buffered(100)SrTiO_(3) substrates.The resulted epitaxial films showed a sizable Pf0(~0.21C m^(−2))and a largeχ_(0)^(f)(~830–860),as well as a largeηclose to the theoretical value.The measured dielectric tunabilities as functions of E are well described by the theoreticalη(E)curves,validating our integrated approach rooted in a theoretical understanding.