Film capacitors have been widely used in pulsed power systems due to their high breakdown strength, high power density, low dielectric loss, and good self-healing properties. However, due to the low dielectric constant (about 2.2), the energy storage density of commercial polypropylene dielectrics is less than 2 J cm³, which seriously limits the miniaturization and large-scale application of film capacitors. Filling inorganic nanoparticles with high dielectric constant into polypropylene matrix can improve the dielectric constant of polypropylene composite dielectrics, but due to the poor interfacial compatibility between matrix and filler, the nanofiller is prone to agglomeration, which results in the increase of dielectric constant often accompanied by the decrease of breakdown field strength and the limited increase of energy storage density.

To address the above problems, we propose for the first time to prepare polypropylene composite dielectrics by using nonpolar sub-10 nm TiO₂ nanoparticles as fillers, and this innovative approach provides multiple advantages for improving the performance of polypropylene composite dielectrics. First, the TiO₂ nanoparticles were synthesized in a nonpolar solvent, and trioctylphosphine oxide was covalently adsorbed on the surface of TiO₂ during the initial nucleation stage, and these molecular ligands were able to improve the compatibility of the filler with the polypropylene matrix on one hand, and inhibit the agglomeration of fillers on the other hand; second, trioctylphosphine oxide has a much lower minimum unoccupied molecular orbital and a much higher maximum occupied molecular orbital, as compared with polypropylene. This property enables trioctylphosphine oxide to act as a potential trap to capture charge, thereby increasing the breakdown field strength and suppressing dielectric loss in polypropylene composites; in addition, this sub-10 nm nanofiller has a great specific surface area, which can increase the interfacial polarization and improve the dielectric constant. The final results show that the polypropylene composites prepared by incorporating 1 wt% of these non-polar sub-10 nm TiO₂ nanoparticles have a discharge energy density of 4.19 J cm^-3 at a field strength of 600 MV m^-1, which is an increase of up to 74.9% when compared with pure polypropylene films.

This work provides a new strategy for the construction of dielectric homogeneous polypropylene composite dielectric materials, and solves the difficult problem of simultaneous improvement of the dielectric constant and breakdown field strength of polypropylene composites, and the research results are expected to be extended to other dielectric polymer systems.

This work was published in Nano Energy under the title of “Nonpolar sub-10 nm TiO₂ nanocrystal for high energy density polypropylene nanocomposites”.

Fig. 1. (a) Design schematic; (b) SEM; (c) TEM; (d) TEM localized magnification; (e) dielectric constant; (f) breakdown field strength and (g) energy storage density of polypropylene composites.

论文链接：https://doi.org/10.1016/j.nanoen.2023.109237