Regulation of energy density and efficiency in transparent ceramics by grain refinement

Seeking for lead-free transparent ferroelectric ceramics with excellent recoverable energy storage density (W_rec) and high energy storage efficiency (η) is conducive to the development of transparent pulse capacitors for use in energy storage. In this work, we realized an effective grain size engineering via introducing a second component, namely, SrZrO₃, into the K_0.5Na_0.5NbO₃ (KNN) ceramics. Interestingly, the obtained submicron grains not only improve the transparency of the ceramics but also enlarge the breakdown field strength (E_b) and thus the W_rec. A good transparency of up to ~68% in the near-infrared region, an excellent W_rec of ~2.81 J·cm⁻³ and an extremely high η of ~80% were simultaneously achieved in the 0.91 K_0.5Na_0.5NbO₃-0.09SrZrO₃ (0.91KNN-0.09SZ) ceramics. The reduced grain and domain sizes, as well as the increased cation disorder, are believed to be responsible for the obtained narrow P-E loops and calculated large diffusion factor value (γ). This work provides a useful reference for developing KNN-based lead-free relaxor ferroelectric materials with both high energy storage properties and a good transparency.