The conductivity of the material is a key transport parameter in spacecraft charging that determines how deposited charge will redistribute throughout the system, how rapidly charge imbalances will dissipate, and what equilibrium potential will be established under given environmental conditions. As the requirements for space missions extend to new regions of space and more stringent requirements are placed on spacecraft performance, it becomes necessary to better understand the underlying conduction mechanisms that determine the dynamic response of insulators to temperature, electric field dose rate, and sample conditioning and history. This study performed detailed measurements of the transient conductivity of representative highly disordered insulating materials using the constant voltage method and analyzed the data with dynamic models for the time, temperature, and electric field dependant conductivity.
We describe substantial upgrades to an existing Constant Voltage Chamber (CVC), which improved the precision of conductivity measurements by more than an order of magnitude. A battery operated voltage source supplied a highly stable applied voltage. Data acquisition and analysis algorithms and the interfaces between electronics and the data acquisition system were optimized for higher precision and accuracy. Painstaking attention to ground loops, shielding, filtering and other associated issues greatly reduced electrical noise in the extremely low (50 MV/m), the ultimate instrument conductivity resolution can increase to ≈4·10-22 (Ω-cm)-1 corresponding to decay times of more than a decade; this is comparable to both the thermal Johnson noise of the sample resistance and the radiation induced conductivity from the natural terrestrial background radiation dose from the cosmic ray background.
A theoretical model is presented to predict CVC conductivity measurements of charge injected at two metal-insulator interfaces at the electrodes. The dynamic bulk charge transport equations developed for electron charge carriers predict the time, temperature, and electric field dependence of the current measured at the rear electrode of the CVC. The model includes space charge limited effects for electron drift, diffusion, displacement, and polarization. The model makes direct ties to fundamental properties of the interactions of the injected electrons with the trap states in highly disordered insulating material, including the magnitude and energy dependence of the density of trap states within the gap, the carrier mobility, and the carrier trapping and de-trapping rates. Measured values of the conductivity of LDPE and polyimide (Kapton HN™) are compared with this theoretical model. The fits are excellent over more than ten orders of magnitude in current and more than five orders of magnitude in time. Residuals are typically in the range of zeptoamps per cm2 (10-18 A/cm2), and appear to be instrumentation resolution limited. The good agreement between the fitting parameters of the model and the corresponding physical parameters determined from the literature and measurements by related techniques is discussed.
Available at: http://works.bepress.com/justin_dekany/1/