In this work, thick BiFeO3 films (~1 μm) were prepared on LaNiO3-buffered (111)Pt/Ti/SiO2/(100)Si substrates via radio-frequency magnetron sputtering without post-growth annealing. The effects of the substrate temperature on the film’s crystallinity, defect chemistry, and associated electrical properties were investigated. In contrast to the poorly crystallized BiFeO3 film deposited at 300 °C and the randomly-oriented and (111)-textured films deposited at 500 and 650 °C, respectively, a (001)-preferred orientation was achieved in the BiFeO3 film deposited at 350 °C. This film not only showed a dense, fine-grained morphology but also displayed enhanced electrical properties due to the (001) texture and improved defect chemistry. These properties include a reduced leakage current (J ≈ 2.4×10−5 A/cm2@200 kV/cm), a small dielectric constant (εr ≈ 243–217) with a low loss (tanδ ≤ 0.086) measured from 100 Hz to 1 MHz, and a nearly intrinsic remnant polarization (Pr) of ~60 μC/cm2. A detailed TEM analysis confirmed the R3c symmetry of the BFO films and hence ensured good stability of their electrical properties. In particular, single-beam cantilevers fabricated from BiFeO3/LaNiO3/Pt/Ti/SiO2/Si heterostructures showed excellent electromechanical performance, including a large transverse piezoelectric coefficient (e31,f) of ~−2.8 C/m2, a high figure of merit (FOM) parameter of ~4.0 GPa, and a large signal-to-noise ratio of ~1.5 C/m2. An in-depth analysis revealed the intrinsic nature of the e31,f piezoelectric coefficient, which is well fitted along a straight line of e31,f ratio = (εrPr) ratio with the reported representative results. These high-quality lead-free piezoelectric films processed with a reduced thermal budget can open many possibilities for the integration of piezoelectricity into Si-based micro-electro–mechanical systems (MEMSs).
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Antiferroelectric PbZrO3 (AFE PZO) films have great potential to be used as the energy storage dielectrics due to the unique electric field (E)-induced phase transition character. However, the phase transition process always accompanies a polarization (P) hysteresis effect that induces the large energy loss (Wloss) and lowers the breakdown strength (EBDS), leading to the inferior energy storage density (Wrec) as well as low efficiency. In this work, the synergistic strategies by doping smaller ions of Li+–Al3+ to substitute Pb2+ and lowering the annealing temperature (T) from 700 to 550 ℃ are proposed to change the microstructures and tune the polarization characters of PZO films, except to dramatically improve the energy storage performances. The prepared Pb(1−x)(Li0.5Al0.5)xZrO3 (P(1−x)(L0.5A0.5)xZO) films exhibit ferroelectric (FE)-like rather than AFE character once the doping content of Li+–Al3+ ions reaches 6 mol%, accompanying a significant improvement of Wrec of 49.09 J/cm3, but the energy storage efficiency (η) is only 47.94% due to the long-correlation of FE domains. Accordingly, the low-temperature annealing is carried out to reduce the crystalline degree and the P loss. P0.94(L0.5A0.5)0.06ZO films annealed at 550 ℃ deliver a linear-like polarization behavior rather than FE-like behavior annealed at 700 ℃, and the lowered remanent polarization (Pr) as well as improved EBDS (4814 kV/cm) results in the superior Wrec of 58.7 J/cm3 and efficiency of 79.16%, simultaneously possessing excellent frequency and temperature stability and good electric fatigue tolerance.

Optimizing the high-temperature energy storage characteristics of energy storage dielectrics is of great significance for the development of pulsed power devices and power control systems. Selecting a polymer with a higher glass transition temperature (Tg) as the matrix is one of the effective ways to increase the upper limit of the polymer operating temperature. However, current high-Tg polymers have limitations, and it is difficult to meet the demand for high-temperature energy storage dielectrics with only one polymer. For example, polyetherimide has high-energy storage efficiency, but low breakdown strength at high temperatures. Polyimide has high corona resistance, but low high-temperature energy storage efficiency. In this work, combining the advantages of two polymer, a novel high-Tg polymer fiber-reinforced microstructure is designed. Polyimide is designed as extremely fine fibers distributed in the composite dielectric, which will facilitate the reduction of high-temperature conductivity loss for polyimide. At the same time, due to the high-temperature resistance and corona resistance of polyimide, the high-temperature breakdown strength of the composite dielectric is enhanced. After the polyimide content with the best high-temperature energy storage characteristics is determined, molecular semiconductors (ITIC) are blended into the polyimide fibers to further improve the high-temperature efficiency. Ultimately, excellent high-temperature energy storage properties are obtained. The 0.25 vol% ITIC-polyimide/polyetherimide composite exhibits high-energy density and high discharge efficiency at 150 ℃ (2.9 J cm−3, 90%) and 180 ℃ (2.16 J cm−3, 90%). This work provides a scalable design idea for high-performance all-organic high-temperature energy storage dielectrics.