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.
Method of conservatively predicting long-term outgassing and deposition levels based on diffusion-limited outgassing theory
A method is presented that conservatively predicts long term outgassing for materials outgassing diffusely using short term test data. Accurate long term predictions require knowledge of the diffusion coefficient and initial concentrations of each species outgassed from a material which are obtainable only from often prohibitively long term testing. The method presented demonstrates how one can utilize standard 24 hour TML/CVCM data or short term outgassing rate testing data to conservatively predict long term outgassing. The method is based upon diffusion theory which predicts that the amount of material outgassing is proportional to the product of a constant, the square root of time, and a function that decays with time with maximum value of 1. The conservative approach taken is to assume that outgassing is proportional to a constant times the square root of time, replacing the decaying function with its maximum value of 1.
Surface accommodation of molecular contaminants
The transport of molecules, under vacuum conditions, from a source surface to a receiving surface is of major concern from the perspective of spacecraft contamination control. The transport phenomena involved is a complex mechanism comprising the physical characteristics of each surface, the properties of contaminant species participating, and the temperatures of both surfaces. Because of both the complex nature and the limited data available to describe such a phenomena, contamination modeling usually requires that a highly simplified engineering approach be undertaken. One area where this is particularly true is in the representation of the surface accommodation of incident molecules. When a molecule in the gas phase collides with the surface of a receiver it can either "stick" to that surface or be scattered away. Molecules accommodated by this surface become thermally equilibrated to the receiver temperature while the material that is not accommodated retains its original energy and undergoes specular reflection. The ratio of this thermally accommodated mass to the total incident mass is known as the "accommodation" or "sticking" coefficient. Most of the current theory and experimental work performed to date has been restricted to the accommodation coefficients of the rare gases in contact with metal surfaces3'10"1. UnfortUnately, the results generated by these studies cannot be made very useful to spacecraft contamination engineers who are predominantly interested in environments where contaminants are typically limited only to water and long-chain hydrocarbons. Because of this deficiency most current spacecraft contamination analyses are forced to rely on general mathematical expressions that consider the sticking coefficient to be only a direct function of the temperature gradient between the emitting and receiving surfaces. The major shortcoming of the simplified method presently in use is that it may provide an inadequate representation of the actual molecular transport occurring between surfaces. The purpose of this paper is, therefore, to study the nature of the transport mechanisms involved in the adsorption of high molecular weight gases on typical spacecraft surfaces, the overall concept of the sticking coefficient, and the quantitative and qualitative theory involved. In addition, this paper will examine some of the existing molecular accommodation data as it relates to spacecraft applications, as well as present new experimental data gathered by the Contamination Control Section of the Goddard Space Flight Center (GSFC). All this information will then be correlated and used to verify the accuracy of the most common sticking coefficient equations in use by contamination analyses.