With the rapid development of new energy, pulsed power systems and other fields, there is an urgent demand for energy storage devices with a high power density, a fast charge-discharge rate and a long service life. Inorganic thin-film dielectric capacitors, featuring microsecond-level discharge speed and high safety, face a challenge of synergistically improving energy density and efficiency (e.g., the existing PZT thin films have an efficiency of <50% due to the large remanent polarization). As a widely studied material, PZT has an excellent polarization but a large leakage current. The existing optimizations include doping and heterostructure construction. This paper was to fabricate a "PZT/MgO" multilayer heterostructure by a sol-gel method. MgO was introduced to obtain a slim hysteresis loop, and the effect of annealing temperature on the phase structure, microstructure, dielectric and energy storage properties was investigated.

0.5 mol/L PbZr_0.52Ti_0.48O₃ (PZT) and MgO thin film precursor solutions were prepared by a sol-gel method, aged at room temperature in dry darkness for 24 h, and then spin-coated on Pt/Ti/SiO₂/Si substrates at 4000 r/min for 30 s. The wet films were dried at 200 ℃ for 5 min and pyrolyzed at 450 ℃ for 10 min. The process was repeated to obtain "PZT/MgO" multilayer films, which were annealed in O₂ at 500-600 ℃ for 10 min.

The crystal structure was analyzed by X-ray diffraction (XRD, Rigaku Co., Japan). The surface/cross-sectional morphologies were determined by scanning electron microscopy (SEM, Thermo Scientific Co., USA). The leakage current was characterized by a Keithley 2400 analyzer (Keithley Co., USA). Au electrodes with the diameter of 200 μm were sputtered, and the P-E loops, breakdown strength (E_b), dielectric properties, and stability were determined by an aixACCT TF Analyzer 3000 (TF Co., Germany). Results and discussion The results show that the characteristic peaks of the perovskite phase of PZT and MgO phase are intensified, the full width at half maximum (FWHM) is narrowed, and the crystallinity is improved with the increase of temperature when the PZT/MgO multilayer heterostructure thin films are annealed at 500-600 ℃ for 10 min. The SEM images show that the thin film annealed at 550 ℃ for 10 min has a dense and defect-free surface, a tight cross-sectional structure without delamination, and a total thickness of approximately 250 nm. In terms of dielectric properties, the dielectric constant and dielectric loss change slightly when annealed at 500-550 ℃, but the both increases when the temperature exceeds 550 ℃. For energy storage performance, the optimal annealing temperature is 550 ℃. The recoverable energy density is 66.0 J/cm³ and the energy storage efficiency is 68.9% in an electric field of 6.77 MV/cm. At 550 ℃, the leakage current of the thin film is ≤ 10^-7 A/cm² in an electric field of 0-2.4 MV/cm, with the Weibull coefficient β of 19.93, indicating a high breakdown stability. Regarding the stability, the energy storage performance fluctuates slightly at 1-30 kHz in an electric field of 4 MV/cm. After 10⁸ charge-discharge cycles, the attenuation of W_rec (recoverable energy density) and η (energy storage efficiency) is extremely small. W_rec increases with the electric field ranging from 1 MV/cm to 6 MV/cm, and the fitting result shows that n ≈ 1.86, presenting the quasi-linear polarization characteristics.

The surface of PZT/MgO multilayer film annealed at 550 ℃ for 10 min was dense without obvious delamination, and the thickness was approximately 250 nm, exhibiting good dielectric properties. In an electric field of 6.77 MV/cm, this film had a recoverable energy density of 66.0 J/cm³ and an energy storage efficiency of 68.9%, with a leakage current ≤ 10^-7 A/cm² (the Weibull coefficient β of 19.93). In addition, the film also had excellent frequency and cycling stability. W_rec increased with the electric field, and the fitting result showed that n ≈ 1.86, presenting the quasi-linear polarization characteristics.