Patricia Chapman Irwin

The future of nanodielectrics in the electrical power industry

While specialty applications of nanotechnology in the photonics and electronics areas have seen a tremendous growth in the past several years, the use of nanodielectrics in the electrical industry (high power density and high voltage) has not shown the same level of activity. In addition to a review of nanodielectrics, we discuss in this paper, our perspective on the current status, development needs and future potential to build or engineer nanostructured materials for dielectric applications in the electrical power industry. Short and long-term future research and development needs are considered from the point of view of industrial applications.

Thermal and mechanical properties of polyimide nanocomposites

Interest has grown in recent years on the effects of nano-sized fillers on the thermal, mechanical, and electrical properties of polymeric systems. In particular, we are interested in studying the changes in mechanical and thermal properties of thermosetting polyimides as nano-sized fillers are added in increasing levels of concentration. The results are compared to micron-sized filled polyimides of similar compositions. We have observed dramatic increases in elongation to failure, scratch hardness and thermal conductivity while the tensile modulus does not change significantly. A change in stress state around nanoparticles during deformation is the possible reason for the observed improvement in tensile properties. Thermal conductivity of the filled polymer systems seemed to be most affected by the surface treatment of the nano-fillers. Improved interactions between the filler and the matrix is suggested as a possible explanation for these conductivity differences.

The electrical conduction in polyimide nanocomposites

The electrical properties of polyimide based nanocomposites were fully investigated by isothermal steady-state current, dielectric spectroscopy, and thermally stimulated current measurements. The effect of nano fillers on the dielectric properties was evaluated via the interfacial polarization as well as the electrical conduction mechanism study. The electrical conduction of polyimide nanocomposites is found to be thermally assisted ionic in nature. The shift of the main thermally stimulated current peak to higher temperatures indicates deeper trapping by incorporating nano fillers. As a result, the nanocomposites will at least maintain the electrical breakdown strength because of the reduced electrical conductivity at elevated temperatures.

High-Temperature Capacitor Polymer Films

Film capacitor technology has been under development for over half a century to meet various applications such as direct-current link capacitors for transportation, converters/inverters for power electronics, controls for deep well drilling of oil and gas, direct energy weapons for military use, and high-frequency coupling circuitry. The biaxially oriented polypropylene film capacitor remains the state-of-the-art technology; however, it is not able to meet increasing demand for high-temperature (>125°C) applications. A number of dielectric materials capable of operating at high temperatures (>140°C) have attracted investigation, and their modifications are being pursued to achieve higher volumetric efficiency as well. This paper highlights the status of polymer dielectric film development and its feasibility for capacitor applications. High-temperature polymers such as polyetherimide (PEI), polyimide, and polyetheretherketone were the focus of our studies. PEI film was found to be the preferred choice for high-temperature film capacitor development due to its thermal stability, dielectric properties, and scalability.

Dielectric Breakdown of Polymeric Insulations Aged at High Temperatures

Most of electric insulation materials used in power generation and energy storage are polymeric dielectric materials: winding insulations in power transformers, electric starter/generators, electric actuators, high frequency resonators, and power converters; dielectric films in power capacitors or energy storage devices; encapsulation in electronic components and devices. These polymeric dielectric materials have limited continuous use temperatures because of their electrical properties deteriorating at high temperatures. The temperature dependence and thermal aging effect on their dielectric breakdown are of prime concern for analyzing their failure mechanism to prevent catastrophic breakdown of the power system in aviation and military platforms. In this paper, the thermal aging effect on breakdown strength, mechanical properties and physical features of various polymeric insulations, such as polyimide (PI), polyetherimide (PEI), polyetheretherketones (PEEK), and perfluoroalkoxy (PFA) films, will be presented and discussed.

Effect of polar particles on polymer composite dielectrics

Incorporated with polar ceramic particles, polymers were shown to exhibit improved dielectric properties. This work focused on the interaction of polar ceramic particles with the hosting polymers so as to improve the distribution of particles and the dielectric properties of polymer composites. Both computation and experimental observation were conducted toward the understanding of composite dielectric engineering. The electric field was found to be localized at the particle-polymer interface likely contributing to the polarization of polar particles.

DC Breakdown in Polyetherimide Composites and Implication for Structural Engineering

Recent advances in nanomaterials permit the improvement of dielectric characteristics of ceramic / polymers nanocomposites. In addition, it is recognized that incorporation of nanofillers improves the mechanical, chemical, optical, acoustic, and thermal properties of polymers. Voltage endurance and electrical breakdown strength of dielectrics were even enhanced with some nanometric particles. Nevertheless, the preferred filler type and consistent improvement in the breakdown strength in the resulting nanocomposites are not well understood. In this paper, the effect of various fillers (metal, semiconductor and insulator) was studied showing the weak relationship of breakdown strength with the band gap of the nanoceramic fillers. Published data on dielectric constant and breakdown strength of various dielectrics was analyzed from the viewpoint of energy band gap. The results show that interfacial interaction of the ceramic particle-polymer appears to be very important to the electrical breakdown of the nanocomposites. An engineered nanodielectric structure is proposed for higher permittivity and dielectric breakdown strength. Lacking of control of this interface and structural order at mesoscopic level is subjected to fundamental limitations of the nanodielectric engineering.

Nano-enabled metal oxide varistors

Zinc oxide based metal oxide varistors (MOV) are widely used electrical surge protection components. The design of modern high power, high-density electronic systems necessitate the need for smaller footprint, higher current density and higher nonlinearity MOVs. Such requirements can no longer be satisfied by commercially available MOVs due to their limited voltage capability, high leakage current and mechanical cracking related reliability issues, most of which are associated with the presence of defects and coarse granularity and lack of uniformity in their microstructures. New formulations and processes have been developed to overcome such limitations. This work has developed nano-enabled MOV compositions that can be sintered at relatively lower temperatures than typical commercial MOVs, but with largely improved I-V characteristics due to refined and uniform sub-micron structures. These nano-enabled MOVs show not only high breakdown strength (1.5 kV/mm) with low leakage current, but also a large nonlinear alpha coefficient > 50 at high fields, a measure of the speed of the transition from the insulating to conducting state and the effectiveness of over-voltage protection. A > 10x increase in breakdown strength compared to commercial MOVs, along with much higher nonlinearity, will enable MOV miniaturization, high voltage surge protection, and open up new areas of application.

Nanostructured dielectric materials

This paper presents a progress update with the development of nanostructured dielectric materials for various high energy, high power density electrical applications. It is demonstrated that novel electrical/dielectric properties can be achieved via the nanostructure and interface engineering. We present a high level summary of the progress achieved as well as challenges remaining in nanostructural engineering towards high energy density capacitor for energy storage and conversion, high voltage varistor for surge suppression and high proton conductivity membrane for fuel cell applications.

Tunable Nanodielectric Composites

This paper presents a progress update with the development of nanodielectric composites with electric field tunability for various high energy, high power electrical applications. It is demonstrated that nonlinear electrical/dielectric properties can be achieved via the nanostructure and interface engineering. A high level summary was given on the progress achieved as well as challenges remaining in nanodielectric engineering towards high energy density capacitors for energy storage and conversion, nonlinear dielectrics for tunable device, and high voltage varistor for surge suppression.