*2.4. Characterization of BaTiO3 Nanocrystals and BPA Films*

The X-ray diffraction (XRD) patterns of the BPA composite films and BaTiO3 powders were recorded by a DX-2700BH X-ray diffractometer (Dandong, China) using Cu Kα irradiation. The morphologies and particle sizes of the BaTiO3 samples were measured using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The particle-size distribution was statistically analyzed according to the SEM images. The pH values of the suspensions were measured using a pH meter (PHS-2C). The yields of the BaTiO3 samples were calculated according to the ratios of experimental BaTiO3 mass to its theoretical mass on the basis of Ba conservation. Fourier-transform infrared (FT-IR) spectra were recorded on a Bruker–Equinox 55 spectrometer in a wavenumber range of 4000–400 cm−<sup>1</sup> using the KBr technique. The dielectric constant (ε) and loss (tanδ) of the BPA films were measured using a high-precision high-voltage capacitor bridge (QS89, Shanghai Yanggao Capacitor Co., Ltd., Shanghai, China), and the frequency during dielectric performance test was kept at 10 Hz. The breakdown strengths of the BPA films were measured using a withstand voltage tester (GY2670A, Guangzhou Zhizhibao Electronic Instrument Co., Ltd., Guangzhou, China).
