**2.2 Self-clearing capability: implications of chemical composition**

For a self-clearing event to occur successfully, the metallization needs to be thin enough to be vaporized/oxidized by the energy dissipated during the dielectric breakdown. In addition, the polymer also needs to be oxidized completely, leaving no conductive paths from free carbon in the region of cleared metallization around the breakdown site, as shown in **Figure 1** [8]. Based on an in-depth study of the physics and chemistry of clearing phenomena, polymers with low ratios of carbon to (hydrogen + oxygen) in the repeating units, such as cellulose, often exhibit excellent self-clearing. Conversely, polymers with high carbon to (hydrogen + oxygen) ratios, like polystyrene, tend to form greater amounts of free carbon for a given clearing energy and clear poorly [7]. This trend is illustrated in **Table 1**,


*b*

*Data taken from [7]. c*

*Observation taken from [5, 7].*

### **Table 1.**

*Chemical composition of various polymer dielectrics including ratios of (carbon + nitrogen + sulfur) to (hydrogen + oxygen), amount of residual carbon, and the observation of self-clearing behavior.*

*Polyimides as High Temperature Capacitor Dielectrics DOI: http://dx.doi.org/10.5772/intechopen.92643*

in which the chemical composition of various polymer dielectrics is correlated with the amount of carbon residue, both measured and calculated, based on comparison between amount of carbon in the gaseous by-products and composition of the dielectrics [5, 7]. Kapton® polyimide, also shown in **Table 1**, has a high degree of aromaticity in the structure and has a tendency toward carbonization (arc-tracking), as observed in wire insulation in commercial aircraft before fluoropolymers were applied as thin coatings to resist arc-tracking [39, 40]. Poor self-clearing can be mitigated to some degree by applying a thin coating of acrylate over the dielectric to increase the oxygen content. The clearing energy can also be reduced by increasing the metallization resistance in order to avoid damaging adjacent layers of dielectric during clearing (**Figure 2**), given that a metallized film capacitor is typically wound from two layers of single-side-metallized dielectric film as illustrated in **Figure 3** [8]. However, the trade-offs for increasing metallization resistance include higher equivalent series resistance (ESR), lower thermal conductivity, and lower ripple current handling capability. Thus, achieving an optimum balance between capacitor ESR and clearing energy depends on the application requirements.
