Submitted for the 4CF12 Meeting of The American Physical Society Electrostatic Discharge Properties of Fused Silica Coatings1 ALLEN ANDERSEN, CHARLES SIM, J.R. DENNISON, USU Materials Physics Group — The electric field value at which electrostatic discharge (ESD) occurs was studied for thin coatings of fused silica (highly disordered SiO2/SiOx) on conductive substrates, such as those encountered as optical coatings and in Si microfabrication. The electrostatic breakdown field was determined using an increasing voltage, while monitoring the leakage current. A simple parallel-plate capacitor geometry was used, under medium vacuum and at temperatures down to ∼150 K using a liquid N2 reservoir. The breakdown field, pre-breakdown arcing and I-V curves for fused silica samples are compared for ∼60 nm and ∼80 μm thick, room and low temperature, and untreated and irradiated samples. Unlike typical I-V results for polymeric insulators, the thin film silica samples did not exhibit pre-breakdown arcing, displayed transitional resistivity after initial breakdown, and in many cases showed evidence of a second discontinuity in the I-V curves. This diversity of observed discharge phenomena is discussed in terms of breakdown modes and defect generation on a microscopic scale. 1This work was supported through funding from NASA Goddard Space Flight Center. Allen Andersen USU Materials Physics Group Date submitted: 20 Sep 2012 Electronic form version 1.4

An engineering tool has been developed to predict the equilibrium conductivity of common spacecraft insulating materials as a function of electric field, temperature, and adsorbed dose rate based on parameterized, analytic functions derived from physics-based theories. The USU Resistivity Calculator Engineering Tool calculates the total conductivity as the sum of three independent conductivity mechanisms: a thermally activated hopping conductivity, a variable range hopping conductivity, and a radiation induced conductivity using a total of nine independent fitting parameters determined from fits to an extensive data set taken by the Utah State University Materials Physics Group. It also provides a fit for the temperature dependence of the electrostatic breakdown field strength, in terms of a tenth independent fitting parameter related to an interchain bond strength. The extent of F, T and measured in the experiments were designed to cover as much of the ranges typically encountered in space environments as possible. This Mathcad worksheet calculates the total conductivity and the individual contributions from each conductivity mechanism based on user inputs for F, T and D . It also plots 2D and 3D graphs of the conductivities over the appropriate full ranges of F, T and

Electrostatic breakdown can be thought of as the point at which a buildup of local defects in insulators leads to a catastrophic change in electrical conductivity. Defects can be produced by temperature, radiation, or a prolonged exposure to constant electric fields. The endurance time is the time it takes to generate enough defects to create a conduction path for electric current to flow more readily. The literature for electrostatic breakdown in polymeric highly disordered insulating materials discusses two competing theories for electrostatic breakdown, based on generation of either recoverable defects or irrecoverable defects. Such defects in the polymer chains can be produced by the electric field and result in localized trapped states for the conduction electrons. Both mechanisms are characterized by the density of electron traps and the corresponding energy to create such defects. We propose a hybrid thermodynamic model for the electric field aging process that predicts the mean time to failure (the endurance time) as a function of applied electric field and temperature. The hybrid model incorporates both types of defects, and proposes an interdependence of the two production mechanisms. Measurements of the dependence of endurance time on electric field in the insulating polymer Low Density Polyethylene (LDPE) were fit against this hybrid model. Higher electric fields produced breakdown times of 4 s to 1 hr and were associated with creation of irrecoverable defects. Lower electric fields resulted in breakdown times on the order of 2 hours to several months; these were associated with recoverable defect generation. Intermediate range electric fields produced interesting results that illustrate the interdependence of the two types of defects. We end with consideration of an important application of the research. Charge buildup on insulating materials in the space environment can produce long exposure to electric fields, which can lead to breakdown at lower fields. This charge buildup is the leading cause of spacecraft failure due to space environment interactions. Understanding the electric field dependence of the time to electrostatic breakdown can assist designers in selecting appropriate materials for spacecraft construction and in mitigating these destructive processes.