*3.5. Dielectric Properties of the BPA Film with BaTiO3 Nanoparticles*

The spherical BaTiO3 nanoparticles with a size range of 91 ± 14 nm (S8 in Table 1) obtained under the optimum conditions ([NaOH] = 2.0 mol L−1, *R*Ba/Ti = 2.0, *T* = 210 ◦C and *t* = 8 h) were used to prepare BaTiO3/polymer/Al (BPA) composite films to verify the feasibility of the BaTiO3 sample in capacitor energy-storage applications.

The typical XRD patterns, SEM image and dielectric properties of the typical BPA films with the BaTiO3 sample (S8) are shown in Figure 8. Figure 8a shows the XRD patterns of the BaTiO3 sample, polymer/Al foil, and BPA film. According to the JCPDS card (No. 99-0005), the diffraction peaks at

2θ = 38.47◦, 44.72◦, and 65.09◦ correspond to the (111), (200), and (220) of the Al foil, respectively. The XRD pattern of the BPA film is a superposition of the BaTiO3 sample and Al foil, and no other impurities are found in the BPA film. Figure 8b shows a typical SEM image of the BPA film. The film exhibits a uniform distribution of BaTiO3 nanoparticles. Figure 8c gives the dielectric properties of the BPA films with spherical BaTiO3 nanoparticles. As the statistical results show, the average dielectric constant of the BPA films reaches 59, the average dielectric loss reaches 0.008, and the average breakdown strength reaches 102 kV mm<sup>−</sup>1. These electrical properties are much higher than those of the previous reports [44–49]. The TiO2-seeded hydrothermal process is an efficient process to synthesize spherical BaTiO3 nanoparticles for potential capacitor energy-storage applications.

**Figure 8.** XRD, SEM, and dielectric properties of the typical BaTiO3/polymer/Al films (BPA) films with BaTiO3 nanocrystals (S8): (**a**) XRD patterns (A-BaTiO3 nanocrystals, B-Polymer/Al film and (C)-BaTiO3/polymer/Al (BPA) film); (**b**) the typical SEM image of the BPA film; (**c**) typical electric properties of the BPA films.

We compared the dielectric constant, dielectric loss, and breakdown strength of the BPA films with those of the literature reports [25,31,44,46,49,50], and the results are shown in Table 2. One can find that the BPA films with the TiO2-seeded BaTiO3 nanocrystals exhibit an excellent balanced dielectric performance.


**Table 2.** Comparisons of dielectric constant, dielectric loss and breakdown strength of the composites containing BT particles.
