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Figure 1.6 (a) Schematic of film blowing process to align the MMT nanofillers along the flow direction, with MMT surface parallel to film surface; (b) orientation factor of the MMT nanofillers in the composite films prepared by different shear rate in film blowing; (c) Weibull distribution and characteristic dielectric breakdown strength; and (d) electrical conductivity under 55 MV/m for the composite films with different MMT nanofiller orientation factor.

      Source: Li et al. [51]. Reproduced with permission of American Chemical Society.

      Xie et al. proposed a physical‐assisted casting method to fabricate the P(VDF‐CTFE)/BT nanowire composites with the BT nanowires aligned in the Z direction and X–Y direction [52]. The Z‐aligned nanowires are more efficient in improving the dielectric constant of the polymer composites. However, the composite with Z‐aligned nanowires shows the lowest electrical breakdown strength. In the Z‐aligned composite, the nanowires are aligned parallel to the direction of the applied electric field, so the inter‐filler distance is relatively small, which makes the breakdown channels traveling along the nanowires at relatively low electric field. Whereas, the X–Y aligned nanowires can act as barriers for the high‐energy charge carriers, which increase the electrical breakdown strength of the polymer composites.

1.5 Surface Modification of Nanofillers

      Well dispersion of the nanofillers is critical in achieving high performance polymer composites [53–56]. The inorganic nanofillers are not well compatible with the polymer matrix because of their high tendency to aggregate due to the high surface energy. So the nanofillers are usually surface modified in order to improve the compatibility with the polymer matrix and achieve homogenous nanofiller dispersion. The surface modification of the nanofillers is beneficial in three aspects: (i) removal of the impurities such as water molecules and ions absorbed on the surface of the nanofillers, (ii) elimination of the filler aggregation and getting rid of the defects at the nanofiller/polymer interface such as voids and pores, and (iii) achieving uniform dispersion of the nanofiller and minimizing the local electric field distortion at the nanofiller/polymer interface.

      The surface modification of the nanofillers can be accomplished by the reaction between the surface groups on the nanofillers and the small molecular coupling agent, such as the silane coupling agent, aluminate coupling agent, zirconate coupling agent, phosphonic acid, and dopamine. In order to improve the electrical breakdown strength of the high‐dielectric‐constant PVDF/BT composites, Hu et al. adopted the titanate coupling agent to modify the surface of BT nanoparticles [57]. It is shown that the surface modification of the BT nanoparticles can efficiently improve the compatibility between the BT nanoparticles and PVDF matrix, which ensures the uniform distribution of the BT nanoparticles. Benefited from the uniformly distributed BT nanoparticles, the electrical breakdown strength of the PVDF/BT composite can reach to 517 MV/m, which is even higher than that of the pristine PVDF polymer. Because of the concurrently increased dielectric constant and breakdown strength in the PVDF composite with surface‐modified BT nanoparticles, the discharged energy density reaches 11.27 J/cm³, which is nearly the double of that of the composite with non‐surface‐modified BT nanoparticles.

      To further improve the compatibility between the inorganic nanofillers and polymer matrix, core‐shell structured nanoparticles can be prepared by various “grafting from” and “grafting to” methods [58–62]. The core‐shell structured nanoparticles can provide unique merits and flexibility in optimizing the dielectric properties of the shell layer. The mismatch of the electrical parameters (i.e. dielectric constant and electrical resistivity) of the nanofillers and polymer matrix would result in local electric field concentration at the nanofiller/polymer interface, which largely decreases the electrical breakdown strength of the polymer composites. In order to mitigate the electric field concentration, the shell layer of the core‐shell structured nanofillers can act as a buffer layer by carefully controlling the electrical parameters of the shell layer. For example, the shell layer with medium dielectric constant between the high‐dielectric‐constant nanofillers and the low‐dielectric‐constant polymer matrix can obviously mitigate the local electric field distortion.

Zhu et al. prepared a series of BT‐based core‐shell structured nanoparticles with various polymer shells, i.e. poly(methyl methacrylate) (PMMA), poly(hydroxyethyl methacrylate) (PHEMA), and poly(glycidyl methacrylate) (PGMA), by surface‐initiated reversible‐addition‐fragmentation chain transfer (RAFT) polymerization [63]. The different polymer shells have different electrical parameters, for example, the order of the dielectric constants of the shell layers is PHEMA>PGMA>PMMA and the order of the electrical conductivities of the shell layers is PHEMA>PGMA>PMMA. The experimental results show that the core‐shell structured nanoparticles with high‐dielectric‐constant shell layer (i.e. PHEMA) can result in high dielectric constant and high discharged energy density of the polymer composite (Figure 1.7). However, due to the high electrical conductivity of PHEMA, the corresponding composite also shows the lowest breakdown strength and charge/discharge efficiency because of the high electrical conduction in the polymer composite. In contrast to PHEMA shell layer, the PMMA shell layer with low dielectric constant but high electrical resistivity can effectively improve the breakdown strength and charge/discharge efficiency of the polymer composite. So the optimized shell layer should simultaneously possess high dielectric constant and high electrical resistivity. However, it is sometimes difficult to find one material with both high dielectric constant and high electrical resistivity. To solve this problem, core‐double‐shell structured nanoparticles have been proposed to combine the advantages of the two shell layers [64].