Lead-free Bi_0.5(Na_1-xK_x)_0.5TiO₃ relaxor ferroelectric ceramics for a wearable energy harvester

We fabricated Bi_0.5(Na_1-xK_x)_0.5TiO₃ (BNKT) [x = 0.16–0.24] relaxor ferroelectric ceramics that undergo a transition from the non-ergodic state (NR) to the ergodic state (ER) using a conventional solid-state reaction method. Structural analysis was conducted using X-ray diffraction (XRD), and the electrical properties related to the polarization (P) and strain (S) resulting from the external electric field were analyzed. With an increase in the component ratio x, from 0.16 to 0.24, the remnant polarization (P_r) reduced continuously from ∼18 to ∼3 μC/cm², respectively. Concomitantly, the form of the S-E loops changed from a butterfly-like shape to a sprout shape (in which the negative strain value (S_neg) that approaches 0 appears as a special feature), suggesting that the NR of the BNKT relaxor ferroelectric ceramic at x = 0.16 transitions to the ER as the composition ratio x increases. In particular, for x = 0.20, the inverse piezoelectric coefficient (d₃₃*) achieved a maximum value of 386 pm/V, while the value of the P_r was relatively low at ∼ 6 μC/cm². To investigate the possible application as a wearable energy harvester, BNKT ceramic powders of various compositions were mixed with polydimethylsiloxane (PDMS) to fabricate flexible generator devices. The change in the output voltage and current as a function of the composition ratio x had a similar tendency as the change in the value of d₃₃*, suggesting that the ER of relaxor ferroelectric ceramics with a large strain value is suitable for application in wearable energy harvesters.