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Page 8 of 13 Liu et al. Microstructures 2023;3:2023008 https://dx.doi.org/10.20517/microstructures.2022.31

most significant enhancement was observed at 8 wt% monolayer BNT-BST/PVDF nanocomposites, i.e.,
from 9.53 for pure PVDF to 12.12 at 1 kHz, which is a 27% improvement. It is important to note that the
dielectric loss of monolayer BNT-BST/PVDF nanocomposites was consistently larger than that of trilayer
nanocomposites at the same mass fractions. The dielectric loss of 1 wt%, 2 wt%, 4 wt%, 6 wt%, and 8 wt%
monolayer BNT-BST/PVDF nanocomposites was 0.0199, 0.0201, 0.0228, 0.0253, and 0.0288, respectively, as
shown in Figure 5E. Figure 5F shows the dielectric loss at 1 kHz for 0-1-0, 0-2-0, 0-4-0, 0-6-0, and 0-8-0
trilayer BNT-BST nanocomposites to be 0.0165, 0.0172, 0.0206, 0.0243, and 0.0261, respectively. This is
primarily attributable to the trilayer structure. Compared to the BNT-BST/PVDF nanocomposite layer, the
pure PVDF outer layer in the symmetric trilayer nanocomposites had lower electron mobility and greater
insulation, as well as limited charge injection at the dielectric/dielectric interface. In addition, a high number
of deep traps existed at the interlayer interface of the trilayer structure, thereby impeding the long-distance
migration of electrons and reducing the leakage current considerably [15,29,38] .

Figure 6A and B show the corresponding P-E loops for each nanocomposite at the maximum breakdown
electric field. The BNT-BST nanofibers can enhance saturation polarization at high electric fields due to the
wide enclosed area between the P-E loop and the vertical P axis, which is advantageous for attaining a larger
discharge energy density in the nanocomposite. In addition to high saturation polarization, achieving high
breakdown strength (E ) is crucial for obtaining a high discharge energy density. The effect of BNT-BST
b
nanofibers with a high aspect ratio on the breakdown strength of nanocomposites is effectively illustrated
here using the Weibull statistical Equation (1):

where P(E), E, E , and β are the cumulative failure probability, breakdown electric field of the experimental
b
test sample, breakdown strength with a nanocomposite breakdown probability of 63.2%, and shape
parameter or slope obtained by fitting, respectively. As shown in Figure 6C and D, the breakdown electric
field was measured at least nine times for each nanocomposite, and the results were calculated using
Weibull statistics. The Weibull distribution breakdown electric field for each nanocomposite sample is
displayed in Figure 6E and F. For example, the E of pure PVDF, 1 wt%, 2 wt%, 4 wt%, 6 wt%, and 8 wt%
b
monolayer BNT-BST/PVDF nanocomposites was 431.3 kV/mm, 432.5 kV/mm, 445.7 kV/mm,
401.1 kV/mm, 344.6 kV/mm, and 332.5 kV/mm, respectively. In monolayer BNT-BST/PVDF
nanocomposites, the BNT-BST nanofibers with a high aspect ratio and small specific surface area were
easily dispersed and distributed along the plane during the solution-casting process. When oriented
perpendicular to the direction of the electric field, BNT-BST nanofibers can provide an ordered electron
[39]
scattering center . The BNT-BST nanofibers extended a tortuous breakdown path in the growth of the
electrical tree over the breakdown process, thereby limiting the transfer of charges to the electrode,
hindering the extension of the electrical tree, and resulting in an increase in E b [39,40] . However, the
overlapping interfaces between PVDF and BNT-BST nanofibers, particularly when the overloaded BNT-
BST nanofibers aggregate in the PVDF matrix, result in an uneven distribution of the electric field, thus
providing conducting routes for carriers. In addition, the incorporation of BNT-BST nanofibers leads to
defects such as air porosity and inorganic-organic interface, which increases the leakage current of BNT-
BST/PVDF nanocomposites and decreases E . Contrary to monolayer nanocomposites, trilayer
b
nanocomposites may spatially modify the distribution of the electric field and have a higher E despite a
b
high filler loading. Specifically, the E and β of pure PVDF, 0-1-0, 0-2-0, 0-4-0, 0-6-0, and 0-8-0 samples are
b
431.3 kV/mm (β~20.7), 535.5 kV/mm (β~25.3), 568.0 kV/mm (β~33.3), 524.0 kV/mm (β~28.8),
516.7 kV/mm (β~20.5), and 494.7 kV/mm (β~16.1), respectively. The justifications are as follows: first, the
BNT-BST nanofibers aligned perpendicular to the direction of the electric field increase the electron