The Effect of 2D $\gamma-\text{Al}_{2} \mathrm{O}_{3}$ on the Energy Storage Density Improvement of Biaxially Oriented Polypropylene Nanocomposite Films

Biaxially oriented polypropylene (BOPP) is widely used in capacitors, due to the combination of the high breakdown strength, low dielectric loss and the possibility of large-scale manufacturing. However, the fast development of the power system requires higher energy storage density of the dielectric material. In the present study, two-dimensional (2D) alumina nanosheets were introduced into polypropylene (PP) matrix, producing BOPP nanocomposite films. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}_{2} \mathrm{O}_{3}$</tex> nanosheets brought deep traps at the interface region between nanosheets and PP matrix, preventing the transportation of charge carriers. Meanwhile, the stretching process resulted in the regular orientation and dispersion of the nanosheets in the composite film, which benefited the uniform electric field distribution. In consequence, when the filling concentration of 2D alumina was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.5 \text{wt} \%$</tex>, the breakdown strength reached 625 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{kV} / \text{mm}$</tex>, which was 8.89 % higher than the neat PP. The relative permittivity was also increased to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{2. 3 1}$</tex>. The energy storage density was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.39 ~\mathrm{J} / \text{cm}^{3}$</tex> under <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$500 \text{kV} / \text{mm}$</tex>, with the increment about <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{2 8 \%}$</tex>.