High energy density, high temperature, and low loss polymer dielectrics are highly desirable for electric energy storage, e.g., film capacitors in the power electronics of electric vehicles and high-speed trains. Fundamentally, high polarization and low dielectric loss are two conflicting physical properties, because more polarization processes will involve more loss mechanisms. As such, we can only achieve a delicate balance between high dielectric constant and reasonably low loss. This review focuses on achieving low dielectric loss while trying to enhance dielectric constants for dielectric polymers, which can be divided into two categories: extrinsic and intrinsic. For extrinsic dielectric systems, the working mechanisms include dipolar (e.g., nanodielectrics) and space charge (e.g., ion gels) interfacial polarizations. These polarizations do not increase the intrinsic dielectric constants, but cause decreased breakdown strength and increased dielectric loss for polymers. For intrinsic dielectric polymers, the dielectric constant originates from electronic, atomic (or vibrational), and orientational polarizations, which are intrinsic to the polymers themselves. Because of the nature of molecular bonding for organic polymers, the dielectric constant from electronic and atomic polarizations is limited to 2-5 for hydrocarbon-based insulators (i.e., band gap > 4 eV). It is possible to use orientational polarization to enhance intrinsic dielectric constantmore »
Effects of Rigid Amorphous Fraction and Lamellar Crystal Orientation on Electrical Insulation of Poly(ethylene terephthalate) Films
In response to the stringent requirements for future DC-link capacitors in electric vehicles (EVs), it is desirable to develop dielectric polymer films with high-temperature tolerance (at least 105 °C) and low loss (dissipation factor, tan δ < 0.003). Although the biaxially oriented poly(ethylene terephthalate) (BOPET) film has an alleged temperature rating of 120 °C, its dielectric performance in terms of breakdown strength and lifetime cannot satisfy the stringent requirements for power electronics in EVs. In this work, we carried out a structure–electrical insulation property relationship study to understand the working mechanism for various PET films, including a commercial BOPET film, an amorphous PET (AmPET) film, and two annealed PET films (AnPET, i.e., cold-crystallized from AmPET). Structural analyses revealed a uniform edge-on crystalline orientation in BOPET with the a* axis in the film normal direction. Meanwhile, a high content of the rigid amorphous fraction (RAF) was identified for BOPET, which resulted from biaxial stretching during processing. On the contrary, AnPET films had a random crystal orientation with lower RAF contents. From dielectric breakdown and lifetime studies, the high-crystallinity AnPET film exhibited better electrical insulation than BOPET, and AmPET had the worst electrical insulation. Electrical conductivity results revealed that the high RAF content more »
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