Project Number: C96-22

Physics of Failure of High Temperature Capacitors

Point of Contact: Dr. Patrick McCluskey email: mcclupa@calce.umd.edu Phone: (301) 405-5323 Fax: (301) 314-9269

Objective

Background

Approach

Work Accomplished

Objective

Characterize and document the current state of development of high energy density capacitors designed for use at elevated temperatures, evaluate promising new devices both analytically and experimentally, and determine and describe the key technology drivers for further development.

Background

The development of electronics which can operate at highly elevated temperatures has been identified as a critical technology for the next century. Initiatives in this area are currently being pursued by the Army, the Air Force, and many commercial avionics and automotive electronics companies and their suppliers. Limiting temperatures to below 125° C has become a severe design constraint. It hinders the development of distributed control systems, smart sensors, and remote actuators, and it increases the cost of electronic systems used to monitor such environments as automotive underhood, anti-lock brakes, aircraft engine, aerospace propulsion, chemical process, and well logging.

This past year, a report was generated describing the generic, materials-related limitations to the use of electronics at elevated temperatures to 200° C. This information was then used to identify the key technical challenges to the use of six modules at elevated temperatures. One of the challenges identified in all modules was the lack of availability of high temperature capacitors with high values of capacitance, which would operate reliably at elevated temperatures. This project will address this issue by providing a critical analysis of the current state of development of high capacity capacitors for use at elevated temperatures and identifying the key technologies which must be developed for high temperature, high value capacitors to become a reality.

Approach

The current state of development of high temperature, high value capacitors will be documented. This document will include information from handbooks, technical articles, vendor literature and supplier interviews, and will provide information on their commercial availability.

Representative available capacitors will be selected and experimentally characterized for insulation resistance and capacitance at elevated temperature.

Analysis of this data will lead to identification of the technology drivers which limit the further development of high temperature, high value capacitors, including a determination of the fundamental limitations resulting from the choice of particular materials or design technologies.

Work Accomplished

High energy density capacitors were evaluated for performance at high temperature evaluation. These capacitors included:

Vitramons 0.2 µF X8R chip capacitors which performed adequately to 175°C
Olean Advanced Products 1.0 µF X7R capacitors which performed adequately to 150°C
Olean Advanced Products 0.2 µF NPO capacitors which performed adequately to 200°C

An analysis has been performed of the suitability of commercially available polymer film and ceramic dielectric capacitors for use in electronic systems at elevated temperatures to 200°C. The results of this analysis are presented in the related report below. This document outlines the current state of development of high temperature capacitors, along with the fundamental principles of materials science which force a tradeoff between high values of capacitance (i.e. dielectric constant) and high temperature stability. Also provided in the document is a critical analysis of the potential of new technologies to surmount this barrier and produce high value, high temperature capacitors, along with a list of potential suppliers. Such technologies as metallized fluoropolymer and polyimide films; high value, thermally stable X7R and X8R ceramic capacitors; and electrostatic supercapacitors are evaluated.
We have developed capabilities for measuring the performance of capacitors to 200°C, including the capacitance, dissipation factor, and insulation resistance. This effort has involved the development of a measurement fixture, which allows the simultaneous evaluation of five capacitors, each of which is held independently by a spring loaded plunger. Each capacitor is contacted by an independent power and ground contact which consists of a nickel plated copper strip that can be replaced after each test to minimize corrosion and oxidation. Two teflon insulated wires are placed in contact with each strip allowing four point Kelvin measurement from the fixture all the way to the measurement unit. The fixture is machined from a block of Teflon, which can stand up to the elevated temperatures and still provide adequate insulation between capacitors. Equipment used to conduct the measurements includes an electrometer (to measure insulation resistance), a source-measure unit (to supply power), a LCZ meter (to measure the capacitance and dissipation factor) and a switching matrix to control the application of the power to the five capacitors.
Measurements were conducted on Vitramon's 0.2 µF X8R chip capacitors which are rated to 175°C and which have been characterized by the manufacturer to 150°C. These measurements were conducted in collaboration with United Technologies Research Center, a CALCE EPRC member company. These capacitors were chosen because of their combination of high temperature rating and high energy density.