Time-Evolved Constant Voltage Conductivity Measurements of Common Spaceborne Polymeric Materials

Long-duration current measurements were made for low-density polyethylene (LDPE), polyimide (PI), polyether ether ketone (PEEK), and biaxially oriented polypropylene (BOPP) to determine their bulk conductivity near room temperature as a function of time. These common thin-film spacecraft material samples were vacuum baked to remove moisture and volatile contaminants to better simulate space conditions. The constant voltage conductivity (CVC) method used a very stable, low-noise dc voltage source and measured the resulting current in a parallel plate geometry. Due to the extremely low conductivity of these four polymeric materials, extended experiments of up to ten days were necessary to establish equilibrium current flow and determine the saturated conductivity. The lower instrumental limit of conductivity measurements with this setup is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\cdot {10}^{-21} (\Omega \cdot \mathrm {cm})^{-1}$</tex-math> </inline-formula>. Changes in conductivity due to field-enhanced conductivity and radiation induced conductivity (RIC) as well as varying voltages, temperatures and dose rates are also considered. Current data for each material are fit to a multiterm model to account for the different electric and displacement charge transport contributions within highly disordered insulating materials (HDIMs), including polarization, dispersive transport, and saturated current. The regimes of disorder-induced dispersive transport demarcated by the transit time are observed in all materials, indicative of hopping transport between trap states in HDIM. Information on the energy distribution of localized trap states responsible for electron conduction in HDIM is extracted from the fitting parameters. Magnitudes and time-dependence of conductivity are reported, along with estimated polarization, transit, equilibrium, and decay times; these are compared favorably with previous measurements using constant voltage and charge storage decay conductivity test methods. These fits can be used with model simulations of time-dependent spacecraft charging to determine nonequilibrium transient conductivity for specific applications.