Experimentally Derived Resistivity for Dielectric Samples From the CRRES Internal Discharge Monitor

Resistivity values were experimentally determined using charge-storage methods for six samples remaining from the construction of the internal discharge monitor flown on the Combined Release and Radiation Effects Satellite (CRRES). Three tests were performed over a period of three to five weeks each in a vacuum of ~5times10 -6 torr with an average temperature of ~25degC to simulate a space environment. Samples tested included FR4, polytetrafluoroethylene (PTFE), and alumina with copper electrodes attached to one or more of the sample surfaces. FR4 circuit-board material was found to have a dark-current resistivity of ~1times10 18 Omegamiddotcm and a moderately high polarization current. Fiber-filled PTFE exhibited little polarization current and a dark-current resistivity of ~3times10 20 Omegamiddotcm. Alumina had a measured dark-current resistivity of ~3middot10 17 Omegamiddotcm, with a very large and more rapid polarization. Experimentally determined resistivity values were two to three orders of magnitude more than found using standard American Society for Testing and Materials (ASTM) test methods. The 1-min wait time suggested for the standard ASTM tests is much shorter than the measured polarization current-decay times for each sample indicating that the primary currents used to determine ASTM resistivity are caused by the polarization of molecules in the applied electric field rather than charge transport through the bulk of the dielectric. Testing over much longer periods of time in vacuum is required to allow this polarization current to decay away and to allow the observation of charged-particle transport through a dielectric material. Application of a simple physics-based model allows separation of the polarization current and dark-current components from long-duration measurements of resistivity over day- to month-long time scales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport