Investigations of Novel High Dielectric Materials and New Mechanisms.

Abstract

A high dielectric constant material with excellent dielectric properties is highly desirable for a wide range of applications, such as high energy density capacitors and optical limiting materials. High dielectric constant materials used for embedded capacitors require characteristics such as a high dielectric constant (>7), a low dielectric loss (<0.01) as well as good thermal stability. Some success has been made in ceramics, polymers and polymeric composites, where a large dielectric constant was obtained at low frequency. However, many of these materials possess relatively large dielectric loss and their performance is limited by their percolative nature. Organic molecules have been widely investigated for various applications. However, the use of organic molecules toward obtaining large dielectric constant at high operational frequencies is a relatively new approach. Hyperelectronic polarization has been suggested as the main contributor to the high dielectric constant found in polyacene quinone radical (PAQR) polymers (e.g. 14000 at 100Hz for a PAQR polymer) by Pohl and his co-workers. However, the physics underlying this polarization mechanism is not well understood so far. In addition, this polarization mechanism hasn’t been explored in other organic systems, such as hyperbranched polymers and dendrimers yet. In my Ph.D investigations, I studied a novel strategy of creating a high dielectric constant material by utilizing the long-range delocalization in a controllable organic structure to produce hyperelectronic polarization. My studies initiated the investigation with the hyperbranched polyaniline and dendritic triarylamine. A remarkable enhancement in the dielectric response at higher frequency was obtained in comparison to linear polymer systems. For example, a dielectric constant ~ 200 was obtained in hyperbranched polyaniline at 1MHz, which is 45 times that of linear polyaniline base (4.4±0.05). The enhancement is due to the extended delocalization over several molecular units and a result of a hyperelectronic polarization. A large dielectric response with low loss is still a major obstacle.