**2.1 Thermal conductivity**

For reliable operation at elevated temperatures, efficient heat removal from the interior of capacitors is essential but often challenging, especially in metallized polymer film designs due to the low thermal conductivity of both the polymer dielectric and the thin metallization [33]. As electrical insulators, polymer dielectrics are poor thermal conductors with thermal conductivities ranging from 0.1 W/(m·K) for amorphous polymers [34] to about 0.6 W/(m·K) for the present state-of-the-art BOPP capacitor films [35]. As for the electrodes, while the thermal conductivity of metals are typically three orders of magnitude higher than those of polymers, the metallization is also ~1000 times thinner than the dielectric and may not have the thermal conductivity of the bulk metal from which it is deposited. One approach to increase thermal conductivity of the polymer is by blending in ceramic nano-fillers. Researchers have shown that mixing 10 vol% boron-nitride nano-sheets (BNNS) with an amorphous polymer resulted in a six-fold increase in thermal conductivity from 0.3 to 1.8 W/(K·m) [36]. The resulting BNNS/polymer nanocomposite also exhibited an increase in dielectric constant, breakdown strength, and charge-discharge efficiency, the latter of which was attributed to reduction of the conduction current. A similar trend has also been observed in other polymer systems including polyimides [37, 38].
