Materials outgassing rate decay in vacuum at isothermal conditions
As a laboratory for scientific research, the International Space Station has been in Low Earth Orbit for over 17 years and is planned to be on-orbit for another 10 years. The ISS has been maintaining a relatively pristine contamination environment for science payloads. Materials outgassing induced contamination is currently the dominant source for sensitive surfaces on ISS and modelling the outgassing rate decay over a 20 to 30 year period is challenging. Using ASTM E 1559 rate data, materials outgassing is described herein as a diffusion-reaction process with the interface playing a key role. The observation of -1/2 (diffusion) or non-integers (reaction limited) as rate decay exponents for common ISS materials indicate classical reaction kinetics is unsatisfactory in modelling materials outgassing. Nonrandomness of reactant concentrations at the interface is the source of this deviation from classical reaction kinetics. A t-1/2 decay is adopted as the result of the correlation of the contaminant layer thicknesses and composition on returned ISS hardware, the existence of high outgassing silicone exhibiting near diffusion limited decay, the confirmation of nondepleted material after ten years in Low Earth Orbit, and a potential slowdown of long term materials outgassing kinetics due to silicone contaminants at the interface.
A + B → 0 Reaction with Different Initial Patterns
We present a detailed study of the chemical reaction A + B {yields} 0 in two dimensions for different initial distributions of reactants as characterized by the structure function. We argue that the long-wavelength components of the initial fluctuations determine the long-time delay of the reactant density. Numerical simulations of the coarse grained reaction-diffusion dynamics agree with analytical predictions. 27 refs., 8 figs.
Algorithm for calculating nonisothermal diffusion-limited outgassing rates
Formulas exist for prediction of diffusion limited outgassing rates under isothermal conditions. An algorithm has now been developed that predicts outgassing rates for material species that outgas via diffusion mechanisms for non-isothermal conditions. The algorithm uses initial species mass, diffusion coefficient, activation energy and a temperature history to determine outgassing rates at prescribed times. The algorithm accounts for bulk material temperature variations through time but does not account for thermal gradients through the material thickness. The non-isothermal mass loss equation is derived from a basic isothermal mass loss equation with the term that indicates species molecular propagation for the non-isothermal conditions. For isothermal conditions, the product of D(T) (where temperature T, is constant) and time is a measure of the species molecular propagation. This term in the isothermal mass loss equation is replaced with the term for nonisothermal molecular propagation, which is the integral over time of D(T) (where temperature, T, is time dependent). Temperature dependent outgassing rates are calculated by numerically differentiating the time and temperature dependent mass loss predictions. Mass loss rates for the non-isothermal conditions are compared to isothermal mass loss rates.