Improving Capacitors and High-fluence Mirrors with Nanoscale Interface Engineering

Project Overview

High-quality dielectric coatings, such as insulating oxides, are used in several Lawrence Livermore National Laboratory (LLNL) mission areas, such as thin-film capacitors and multilayer dielectric (MLD) laser mirrors. Performance is ultimately limited by how much energy can be stored as polarization of the dielectric before breakdown occurs, for example the breakdown voltage in a capacitor. Recent work in the literature has shown progress of 2-40x in energy storage, by engineering the interfaces and structure of the dielectric, and by suppression of defects. This project is a feasibility study to show that  LLNL's expertise and existing coating capabilities in precision, ultra-clean coatings for optics can be applied to achieving high energy storage in capacitors, and therefore that a pathway exists to make capacitors that industry does not have incentive to supply. Three-layer capacitors were made and tested on-site with existing equipment. This study made 55 capacitors out of three different dielectric stacks. We achieved breakdown voltage comparable to intrinsic material limits for dielectric consisting of a multilayer of HfO2/SiO2, and energy storage about 25-50% of the best reported values for comparable materials. Several paths to improvement are easily feasible with current equipment and tests, such as extending the dielectric stack to a greater number of thinner layers, including thin barrier layers, and extending to materials with greater energy-storage capacity such as mixed or doped oxides, for example bismuth zinc niobates. The results show that a more detailed development effort is both feasible and promising.

Mission Impact

This study has shown that existing fabrication and testing capabilities can produce capacitor materials approaching best-of-class for the simple oxides tested, even without an effort to engineer the best those materials can do. If these results can be extended to ~100X bigger capacitor area, a device storing ~0.5J at ~2.5kV could be produced in a very flat form factor (150mm diameter x 2mm thick), using the methods tested here. While there would be several additional steps to producing usable devices, this suggests these capabilities can be used to develop devices to address unmet LLNL needs. Progress in higher-storage, lighter capacitors, that increase portability in DoDT defense lasers, contributes to our Mission Area of Integrated Deterrence in the area of directed energy. Improved dielectrics with high breakdown are also needed for other applications such as pulse power and WCI devices, so their development adds to our Core Competency in Advanced Materials and Manufacturing. A deeper research effort, involving tests of basic breakdown mechanisms, relationship to observable defects, and the use/development of further breakdown theory could enable us to also apply these results to higher-fluence laser mirrors support our Core Competency in Lasers & Optical S&T.