NANOSTRUCTURED DIELECTRIC FILMS FOR NEXT GENERATION OF ENERGY STORAGE CAPACITORS
Open Access
- Author:
- Thakur, Yash
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2017
- Committee Members:
- Qiming Zhang, Dissertation Advisor/Co-Advisor
Qiming Zhang, Committee Chair/Co-Chair
Jerzy Ruzyllo, Committee Member
Noel Chris Giebink, Committee Member
James Runt, Outside Member
Michael T Lanagan, Outside Member - Keywords:
- dielectrics
capacitors
nanoparticles
energy storage
polymers - Abstract:
- Advances in modern electronics require the development of polymer-based dielectric materials with high dielectric constant, low dielectric loss, and high thermal stability. The dielectric theory suggests that weakly-coupled and strongly-dipolar polymers have the potential to realize a high dielectric constant. The high dipole moment functional groups and amorphous structure provides strong scattering to the charge carriers, resulting in low losses even at high electric fields. These polymers also possess a high glass transition temperature which makes them suitable for high temperature operation. In this dissertation, the fundamental understanding has been carried forward to design and develop next generation of capacitors based on nanostructured materials for compact, light-weight, and reliable electric power systems to address the commercial, consumer, and military requirements. We show through combined theoretical and experimental investigations that nanostructure engineering of a weakly-coupled and strongly-dipolar polymer can result in a high-energy density polymer with low loss and high operating temperature. Our studies reveal that disorder in dipolar polymers creates a significantly larger free volume at temperatures far below the glass transition (Tg), enabling easier reorientation of dipoles in response to an electric field. The net result is a substantial enhancement in the dielectric constant while preserving low dielectric loss and very high breakdown field. It is the free volume effect that leads to a high dielectric constant (K > 5.6) at temperatures below Tg (> 200°C) in meta-phenylene polyurea (meta-PU). It possesses very low loss (high charge/discharge efficiency) even at high electric fields (> 600 MV/m). To extend the idea of free volume, we propose a blending approach where two glassy state dipolar polymers, poly(arylene ether urea) (PEEU, K=4.7) and an aromatic polythiourea (ArPTU, K=4.4), are combined. The resulting blend exhibits a very high dielectric constant(K=7.5) while maintaining low dielectric loss (< 1%). The experimental and simulation results demonstrate that blending these dissimilar dipolar polymers causes a slight increase in the interchain spacing of the blend in its glassy state. This reduces the barriers for the reorientation of dipoles in the polymer chains and generates a much higher dielectric response than the neat polymers. In addition to designing new dielectric materials with excellent dielectric properties, it is crucial that we continue to improve the electrical properties of the state-of-the-art materials. This allows us to utilize the existential large-scale manufacturing facilities of these polymers. Polyetherimide (PEI), a high glass transition amorphous polymer, is seen as the material of choice for high temperature capacitors. But it possesses a moderate dielectric constant of 3.2, which limits its energy density. We present a nanocomposite approach, where addition of small amounts of inorganic nanoparticles in PEI can improve the dielectric constant by 60% while maintaining the breakdown strength, thereby increasing the discharged energy density by 50%. This is a very promising approach and a breakthrough experimental discovery for engineering nanostructures by introducing low volume content of nanofillers with dielectric constant similar to that of the matrix, to achieve markedly enhanced dielectric response. The results are extremely intriguing and eliminate many undesirable features, primarily being low breakdown strength of traditional dielectric nanocomposites containing high dielectric constant fillers, which have been a focal point of study for the past 20 years in this area of research. For practical applications, it is critical that the dielectric material possesses low loss, especially the conduction loss, which could become significant at high temperatures and high electric fields. In this pursuit, we developed a strongly dipolar polymer, poly (ether methyl ether urea) (PEMEU) that exhibits a dielectric constant of 4 and is thermally stable up to 150°C. The experimental results show that the ether units are effective in softening the rigid polymer and making it thermally processable, while the high dipole moment of urea units and glass structure of the polymer leads to a low dielectric loss and low conduction loss. As a result, PEMEU high quality thin films exhibit exceptionally high breakdown field of >1.5 GV/m, and a low conduction loss at fields leading up to the breakdown. Consequently, the PEMEU films exhibit a high charge–discharge efficiency of 90% and a high discharged energy density of 36 J/cm3. Another key aspect is mitigating losses in available dielectric materials that show promise for scalability and are attractive for high energy density capacitors. The conduction at high fields and high temperatures of a semi-crystalline poly(tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride) terpolymer was investigated. Experimental results show that the insulating nanofillers are very effective in reducing the conduction current, i.e., more than two orders of magnitude reduction in conduction can be achieved with less than 1 wt.% (<0.5 vol.%) of Al2O3 nanofillers. Experimental measurements are compared with multiscale simulations, which provide insights into the dominant conduction mechanism, i.e., the carrier hopping in the polymer. The conduction is markedly reduced owing to a large decrease in the mobile carrier concentrations and increased trap depth, caused by the nanofillers. In summary, this dissertation focusses on the development of next generation capacitors by innovation in materials which possess high dielectric constant, low loss, high breakdown strength, and high temperature thermal stability. We believe that the insightful results and approaches shown by the introduction of localized free volume and low volume content of nanoparticles may unravel new directions for future research in advanced dielectrics.