Enhanced energy storage and breakdown strength in barium titanate zirconate solid solutions with niobates and tantalates

Lead-free relaxor compounds are considered as promising materials for the development of dielectric capacitors with a high energy storage density. This has stimulated many research groups around the world to search for the optimal composition and microstructure. In this paper, we report about two novel lead-free ceramic systems, (1-x)Ba(Ti0.85Zr0.15)O3–(x)Bi(Zn2/3Nb1/3)O3 (BTZ-BZNb) and (1-x)Ba(Ti0.85Zr0.15)O3–(x)Bi(Zn2/3Ta1/3)O3 (BTZ-BZTa), prepared by the solid-state method. Ba(Ti0.85Zr0.15)O3 was chosen as starting compositions due to its ferroelectric-relaxor crossover behavior which combines slim hysteresis loops with high a maximum polarization. By substituting with niobates and tantalates, we have investigated their influence on the polarization, dielectric properties and energy storage performance. Moderate doping enhances the relaxor behavior of BTZ while maintaining a large polarizability, which is beneficial for a high recoverable energy density. The substitution with Nb seems to affect the lattice distortion more than Ta, which can be related to a lower degree of covalency of the Nb-O bonds. This effect does not affect the polarization behavior, and the niobate-doped materials achieve higher breakdown strength than the tantalate-doped materials. The breakdown strength is more influenced by the grain size distribution of the ceramics than by the substitution content and the band gap. Overall, the 6BZNb ceramic shows the most favorable energy storage properties with a high energy density of 1.53 J/cm3 at 244 kV/cm, a high energy efficiency of 91%, and a high breakdown strength of 239 kV/cm.