Polymer nanocomposites for high-energy-density capacitor dielectrics: Fundamentals and recent progress

Polymer dielectrics are the key component in film capacitors, which are one of the most fundamental elements in modern electronics and power systems [1-3]. Film capacitors are capable of storing energy when voltage is applied, in the form of electric charges separated by a dielectric material sandwiched by a pair of metal electrodes. Film capacitors possess the advantages of high breakdown strength, low power loss and processing flexibility compared with their counterparts in competition such as electrolytic capacitors and ceramic capacitors [4], meaning they can sustain high voltage, have great efficiency in electrical energy storage and can be manufactured in low-cost and efficient methods. Moreover, the unique self-clearing (sometimes referred to as self-healing) characteristic of polymer dielectrics ensures reliable operation of film capacitors [5], which is of paramount significance since capacitors storing a large amount of electrical energy can explode when catastrophic failure occurs. To be specific, the self-clearing is a process that when localized breakdown occurs at a spot of metallized film capacitors (i.e., a film capacitor using dielectric polymer films with metallic electrodes deposited on the surface [5], [6]), the energy released in the breakdown process can "clear" the breakdown-site, forming an open circuit. This process protects the capacitor from catastrophic failure (Figure 1) [7]. Serving as a device for charge storage and control, film capacitors are commonly used in many types of circuits such as snubbers, phase shifters, filters and AC/DC converters/inverters. In particular, film capacitors are capable of releasing the stored energy in an extremely short period of time to supply a high power density [8-10], which is desirable for the pulsed power application [1]. However, sizeable energy storage has not yet been achievable with film capacitors due to the limited volumetric energy density (usually measured in Joule per cubic centimeter at the material level). This shortcoming has caused a major issue in the electrical apparatus and electrified systems where capacitors constitute a large volume and weight fraction, i.e., a bulky, and sometimes even cumbersome size. For instance, in the 800 kV voltage source converter-based high voltage direct current transmission system, each converter tower has a volume of approximately 480 m ³ , where capacitors are ~60% of the converter weight, and ~50% of the volume.