different in the polymer composites with different nanofiller morphologies and their alignment (Figure 1.5). The polymer composite with vertically aligned nanofibers shows the lowest breakdown strength owing to the condensed electric field distribution that promotes the growth of the breakdown phase. Among the five simulated structures, only the polymer composite with parallel nanosheets exhibits the higher electrical breakdown strength than that of the pure polymer because of the dispersed electric field. These results indicate that the volume fraction of the nanofillers should not be higher than 10 vol% where high electrical breakdown strength is desired.
Figure 1.4 (a) TEM image of BNNSs, (b) cross‐section scanning electron microscopy (SEM) image of the P(VDF‐TrFE‐CFE)/BNNS composite, (c) Weibull breakdown strength and shape parameter as functions of BNNS fraction, (d) electrical resistivity of P(VDF‐TrFE‐CFE)/BNNS composite with different contents of BNNSs under 10 MV/m, (e) D‐E loops of P(VDF‐TrFE‐CFE)/BNNS composite with 12 wt% of BNNSs, and (f) discharged energy density and charge/discharge efficiency of pristine P(VDF‐TrFE‐CFE) and P(VDF‐TrFE‐CFE)/BNNS composite with 12 wt% of BNNSs.
Source: Li et al. [23]. Reproduced with permission of The Royal Society of Chemistry.
Figure 1.5 3D simulations of microstructure effects on breakdown, (a) breakdown phase morphology in the nanocomposites with different microstructures; (b) evolutions of the breakdown phase volume fraction under applied electric fields; (c) extracted breakdown strengths for corresponding nanocomposites.
Source: Shen et al. [36]. Reproduced with permission of John Wiley & Sons.
Various material processing methods have been proposed to align the nanofillers in polymer composites, including dielectrophoresis, film stretching assembly, electrospinning, template‐assisted fabrication, using nanorod array, and freeze casting [45–50].Li et al. prepared polymer composites with 2D MMT nanofillers aligned in the polyethylene (PE) matrix by film blowing [51]. The film blowing process can effectively align the 2D MMT nanofillers parallel to the film surface, and the orientation degree of the nanofillers can be readily controlled by changing the shear rate of the film blowing (Figure 1.6). By changing the shear rate, three PE/MMT composites with different MMT nanofiller orientation factors are obtained. The orientation of the MMT nanofillers is observed by transmission electron microscopy (TEM) and further quantified by two‐dimensional wide angle X‐ray diffraction (WAXD). The orientation factors for the three composites are 0.70, 0.78, and 0.83, respectively. It is shown that the electrical breakdown strength of the PE/MMT composites significantly increases with the orientation of the 2D MMT nanofillers. The electrical breakdown strength for the composites with low, medium, and high filler orientation factors are 440, 490, and 560 MV/m, respectively, which is substantially higher than that of the pristine PE polymer of 300 MV/m. The oriented 2D MMT nanofillers can act as barriers to add the tortuosity of the electrical breakdown pathway and thus impede the propagation of electrical breakdown. The orientation of the 2D MMT nanofillers is also beneficial to suppress the conduction current and increase the electrical resistivity of the polymer composites. The electrical resistivity of the composite with the highly oriented MMT is four times higher than that of the composite with the lowest filler orientation factor.
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
