Methods for High Resistivity Measurements Related to Spacecraft-Charging

A key parameter in modeling differential spacecraft-charging is the resistivity of insulating materials. This parameter determines how charge will accumulate and redistribute across the spacecraft, as well as the timescale for charge transport and dissipation. American Society for Testing and Materials constant-voltage methods are shown to provide inaccurate resistivity measurements for materials with resistivities greater than
$sim!10^17 Omegacdothboxcm$
or with long polarization decay times such as are found in many polymers. These data have been shown to often be inappropriate for spacecraft-charging applications and have been found to underestimate charging effects by one to four orders of magnitude for many materials. The charge storage decay method is shown to be the preferred method to determine the resistivities of such highly insulating materials. A review is presented of methods to measure the resistivity of highly insulating materials—including the electrometer in resistance method, the electrometer in constant-voltage method, and the charge storage method. The different methods are found to be appropriate for different resistivity ranges and for different charging circumstances. A simple macroscopic physics-based model of these methods allows separation of the polarization current and dark current components from long-duration measurements of resistivity over day- to month-long timescales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport. The model largely explains the observed differences in resistivity found using the different methods and provides a framework for recommendations for the appropriate test method for spacecraft materials with different resistivities and applications.