BiFeO₃/BaTiO₃ bi-layer thick films (~1 μm) were deposited on Pt/Ti/SiO₂/(100) Si substrates with LaNiO₃ buffer layers at 500 °C via a rf magnetron sputtering process. X-ray diffraction (XRD) analysis revealed that both BiFeO₃ and BaTiO₃ layers have a (00l) preferred orientation. The films showed a small remnant polarization (P_r ~ 7.8 μC/cm²) and a large saturated polarization (Ps ~ 65 μC/cm²), resulting in a slim polarization-electric field (P-E) hysteresis loop with improved energy storage characteristics (W_c = 71 J/cm³, η = 61%). The successful “slim-down” of the P-E loop from that of the pure BiFeO₃ film can be attributed to the competing effects of space charges and the interlayer charge coupling on charge transport of the bi-layer film. The accompanying electrical properties of the bi-layer films were measured and the results confirmed their good quality.

Achieving an excellent energy storage performance, together with a high cycling reliability, is desirable for expanding the technological applications of ferroelectric dielectrics. However, in a well-crystallized ferroelectric material, the concomitant high polarizability and low polarization-saturation field have led to a square-shaped polarization-electric field loop, fatally impairing both recoverable energy density (W_rec) and efficiency (η). Nanocrystalline ferroelectric films with a macroscopically amorphous structure have shown an improved W_rec and η, but their much-lower polarizability demands an extremely high electric field to achieve such performances, which is undesirable from economical viewpoints. Here, we propose a strategy to boost the energy storage performances and stabilities of ferroelectric capacitors simultaneously by constructing a tri-layer film in which a well-crystallized ferroelectric layer was sandwiched by two pseudo-linear dielectric layers with a dominant amorphous structure. In sol-gel-derived BaTiO₃/(Pb,La,Ca)TiO₃/BaTiO₃ (BTO/PLCT/BTO) tri-layer films, we show that the above design is realized via a rapid thermal annealing which fully crystallized the middle PLCT layer while left the top/bottom BTO cap layers in a poor crystallization status. This sandwiched structure is endowed with an enhanced maximum polarization while a small remnant one, and a much-delayed polarization saturation, which corresponds to a large W_rec~80 J/cm³ and a high η~86%. Furthermore, the film showed an outstanding cycling-stability: its W_rec and η remain essentially unchanged after 10⁹ electric cycles (DW/W<4%, Dη/η<2%). These good energy storage characteristics have proved the effectiveness of our proposed strategy, paving a way for the utilization of sandwiched films in applications of electric power systems and advanced pulsed-discharge devices.

Dielectric films with a high energy storage density and a large breakdown strength are promising material candidates for pulsed power electrical and electronic applications. Perovskite-type dielectric SrTiO₃ (STO) has demonstrated interesting properties desirable for capacitive energy storage, including a high dielectric constant, a wide bandgap and a size-induced paraelectric-to-ferroelectric transition. To pave a way toward large-scale production, STO film capacitors were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by the sol–gel method in this paper, and their electrical properties including the energy storage performance were studied as a function of the annealing temperature in the postgrowth rapid thermal annealing (RTA) process. The appearance of a ferroelectric phase at a high annealing temperature of 750 °C was revealed by X-ray diffraction and electrical characterizations (ferroelectric P-E loop). However, this high dielectric constant phase came at the cost of a low breakdown strength and a large hysteresis loss, which are not desirable for the energy storage application. On the other hand, when the RTA process was performed at a low temperature of 550 °C, a poorly crystallized perovskite phase together with a substantial amount of impurity phases appeared, resulting in a low breakdown strength as well as a very low dielectric constant. It is revealed that the best energy storage performance, which corresponds to a large breakdown strength and a medium dielectric constant, is achieved in STO films annealed at 650 °C, which showed a large energy density of 55 J/cm³ and an outstanding energy efficiency of 94.7% (@ 6.5 MV/cm). These findings lay out the foundation for processing high-quality STO film capacitors via the manufacturing-friendly sol–gel method.