Hydrothermal growth of c-axis oriented ferroelectric (Bi_1/2K_1/2)TiO₃ films on metal substrates

Ferroelectric films fabricated on metal substrates are desirable for use in piezoelectric vibration energy harvesters. In this report, we demonstrate that dense films of lead-free ferroelectric (Bi_1/2K_1/2)TiO₃ (BKT) with preferred c-axis orientation can be grown on nickel-based alloy substrates by utilizing the hydrothermal synthesis method. A buffer layer of LaNiO₃ (LN) was formed on the substrates by the sol-gel spin coating method, and then BKT was grown by a hydrothermal reaction at a low temperature of 150 °C. The surface structure and thickness of the final BKT films were found to strongly depend on the numbers of the deposition cycles for LN and BKT, respectively. As a result, highly c-axis oriented BKT films with a smooth surface and a thickness up to 830 nm were obtained. Although the as-deposited BKT films were suggested to contain hydroxyl groups in their crystal lattice, a post-annealing process at 650 °C in air was found to be effective to remove them. The polarization response of the annealed BKT film proved the ferroelectric polarization switching in the film.