**4.1 Simple compound**

Modification of the molecular structure of the polymer can improve its dielectric properties although the effect can be small. Using a simple compounding method with a high ε<sup>r</sup> filler (e.g., ceramic filler, conductive filler), a polymer with a high breakdown field strength can be obtained. This procedure has gained acceptance due to its simple preparation method.

The conductive filler polymer-based composite material can attain a high ε<sup>r</sup> for relatively small additions of filler, and the large increase in ε<sup>r</sup> can be explained by the percolation theory. Adding filler at the percolation threshold will greatly increase the electrical conductivity and ε<sup>r</sup> of the composite material, thereby improving the transition layer between the filler and the matrix. Carbon materials such as carbon nanofibers (CNFs), carbon black, carbon nanotubes (CNTs), graphene, and graphite flakes are most commonly used in recent research. Among these conductive fillers, CNTs are a good choice due to their high electrical and thermal conductivity and high aspect ratios. Wu et al. [14] functionalized multiwalled carbon nanotubes (MWCNTs) with carboxyl groups prior to dispersing into PI nanofibers using electrospinning technology. Hot pressing was then performed to produce high-performance PI/MWCNT composites with a high εr, good mechanical flexibility, and excellent thermal stability. When the concentration of MWCNT was close to the percolation threshold of 12–14 vol%, the material showed a high εr, low breakdown strength, and maximum *Ue*. When the MWCNT content was 12 vol%, the maximum *Ue* was 1.957 J cm<sup>3</sup> , which was 4.8 times that of pure PI (0.404 J cm<sup>3</sup> ), and the dielectric loss was less than 0.1. As a two-dimensional nanomaterial, graphene has great potential in the future because it can improve the mechanical, thermal, and electrical properties of polymers. Among these materials, graphene oxide (GO) has also been reported in some articles to improve the mechanical properties and thermal stability of polymer-based composites. Chen et al. [15] prepared pure PI, PI/GO, and PI/reduced GO (rGO) films by in situ

*High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

**Figure 7.** *Schematic illustration of the film preparation procedure for GO and PI/GO composites [15].*

polymerization, as shown in **Figure 7**. Among them, PI/GO and PI/rGO films both demonstrated improved thermal stability compared to pure PI films. Furthermore, at 100 Hz, when the mass fractions of GO and rGO were 2 wt%, ε<sup>r</sup> were also improved (4.9 and 5.8, respectively).

Adding conductive particles as a filler to the polymer matrix can improve the ε<sup>r</sup> of the polymer composite. When the added amount is close to the percolation threshold, the ε<sup>r</sup> can be significantly increased. However, as the amount of addition increases, a conductive network is formed, and their dielectric loss will increase sharply. In general, nanostructured BT fillers and BT-based nanocrystals are the more promising materials due to their excellent dielectric and ferroelectric properties [16].

Fan et al. [17] studied the relationship between the ε<sup>r</sup> and the temperature for thermosetting PI matrix nanocomposite films containing BT nanoparticles at 103 Hz. Two temperature changes were reported, namely heating from 50 to 150°C and cooling from 150 to 50°C to investigate the effects of the transition of the BT crystal phase and the free volume change in PI on the ε<sup>r</sup> for BT/PI nanocomposite membranes. Theoretical models were also used to predict the ε<sup>r</sup> of composite materials to study the role of the diameter and shape of the nanoparticles. Rajib et al. [18] prepared BT/PI nanocomposites and increased their energy density at high temperatures using different volume fractions to analyze their effect on the dielectric properties. All samples were tested at high temperatures to evaluate their energy storage capacity. The highest Ue was found when the volume fraction of BT was 20% reaching 9.63 J cm<sup>3</sup> at 20°C and 6.79 J cm<sup>3</sup> at 120°C. As a dielectric material, it is expected to maintain a high energy density value at a temperature of 120°C. A pure PI film prepared by Sun et al. [19] showed high breakdown strength (451 kV mm<sup>1</sup> ) and high energy density (5.2 J cm<sup>3</sup> ). The introduction of BT nanoparticles increased the ε<sup>r</sup> of the nanocomposite to 6.8, while the dielectric loss was still relatively low (0.012 at 104 Hz). However, a small amount of (3 vol%) BT nanoparticles also caused a significant decrease in the breakdown field strength (275 kV mm<sup>1</sup> ), which greatly reduced the energy density (1.7 J cm<sup>3</sup> ) of the BT/PI nanocomposite.

Therefore, for BT/PI nanocomposites, future research may concern improvements in the thermal conductivity of nanocomposites and the formation of interpenetrating networks throughout the polymer matrix. Improvements in this area will make nanocomposites less susceptible to breakdown [19]. Wang et al. [20] successfully prepared BT nanowire/PI (BT-NW/PI) and BT nanoparticle/PI (BT-NP/PI) composites with low volume fractions. Due to strong interfacial polarization, the ε<sup>r</sup> of BT-NW filled composites was greater than that of BT-NP/PI. The ε<sup>r</sup> of the composite containing 5 vol% BT-NW was 6.6 at 100 Hz, which was 94% higher than pure PI (ε<sup>r</sup> = 3.4 at 100 Hz) and 22% higher than that of composite containing 10 vol% BT-NP (ε<sup>r</sup> = 5.4 at 100 Hz). In addition, BT-NW also significantly improved the *Ue* of the composite. When the content of BT-NW was 2 vol%, the *Ue* obtained at 2200 kV cm<sup>1</sup> was 1.06 J cm<sup>3</sup> , which was 37% greater than pure PI. Therefore, it could be shown that the introduced linear ceramic filler had a positive effect on the dielectric properties and Ue of the composite material [20]. Hu et al. [1] prepared and studied the dielectric properties of a BT nanofiber/PI (BT-NF/PI) composite membrane over the temperature range 20–200°C. The introduction of BT-NF at 9 vol% increased the ε<sup>r</sup> for BT-NF/PI to 8.3 while the dielectric loss increased only slightly; these effects could be attributed to dipolar polarization and interfacial displacement of the nanocomposites. The breakdown strength of BT/PI composites containing 1 vol% BT-NF reached 550 kV mm<sup>1</sup> , and the discharge energy density reached 5.82 J cm<sup>3</sup> . Additionally, the introduction of BT-NF reduced the leakage current and improved the heat conduction. At 1 vol% BT-NF, the PI nanocomposites also exhibited high energy utilization efficiency and good thermal stability. At 150 and 100°C, when the efficiency was greater than 90%, the discharge energy density values were >2.1 J cm<sup>3</sup> and ≈4 J cm<sup>3</sup> , respectively [1]. The authors used electrospinning to prepare BT-NF while the PI composite membranes were prepared by in situ dispersion polymerization. The dielectric properties of BT-NF/PI composite films in the frequency range of 102 –10<sup>6</sup> Hz at a temperature of 20–150°C were studied. The results showed that the ε<sup>r</sup> of the PI nanocomposite film with 30 vol% BT-NF at 100 Hz increased to ≈27 while the dielectric loss was only 0.015.

Furthermore, the calcination temperature of BT has a significant influence on the ε<sup>r</sup> of the PI/BT-nanocomposite film as shown in **Figure 8**. The ε<sup>r</sup> of the PI composite film calcined at 1000°C was higher than the PI composite films calcined at 600 and 800°C; when the BT-NF content was 30 vol%, the ε<sup>r</sup> of the BT-NF/PI composite film increased to 26.6 [16]. Beier et al. [21] added Ba0.7Sr0.3TiO3 (BST)

### **Figure 8.**

*Frequency dependence of dielectric property of 30 vol% BT nanoparticles (a) dielectric permittivity and (b) dielectric loss measured at room temperature [16].*
