Progress on the Physical Approach to Molecular Contamination Modeling

Abstract

A review of the contamination physics and of the most widespread engineering approaches to contamination assessment was carried out. The two main approaches are the physical and the empirical one. The main questions still open to validate the physical approach to outgassing and deposit physics were then studied. Among others, special attention was paid to the important point of a realistic separation of chemical species, probably a prerequisite for a physical modeling. Several original results were obtained. Some lead to a quite clear conclusion, like the preeminence of the limitation by desorption over the limitation by diffusion for outgassing. This observed trend needs yet to be validated on other materials. Other major results are progress on the validation of the physical approach and on the ambitious species separation program. Nomenclature D = Fick's law diffusion coefficient, cm 2 :s 1 E A = activation energy, J:K:mol 1 f = contaminant flux density, g:cm 2 :s 1 k = outgassing kinetic constant, s 1 L = effective diffusion length of the sample, m M = contaminant molar mass, g=mol m, m evap = deposited, evaporated or outgassed surfacic mass, g:cm 2 m 0 = contaminant monolayer mass, g:cm 2 n vol = contaminant volume density, g:cm 3 n surf = contaminant surface density, g:cm 2 P S = saturation vapor pressure, mbar R = gas constant, 8:314 J:K:mol 1 T = temperature, K t = time, s W = relative contaminant mass, % = outgassing or deposit evaporation characteristic time, s

Mixture Effects in Contaminant Reemission

“…Introduction I N-FLIGHT spacecraft contamination is driven by four main phenomena: contaminant generation (outgassing, propulsion, etc. ), transport, reemission, and fixation by space radiation [1,2]. No correct in-flight contamination assessment can be produced if one of these phenomena is neglected.…”

“…Temperature dependence for pure contaminant evaporation is given by the Clausius-Clapeyron law. As a result, the evaporation rate of a pure contaminant is almost an Arrhenius law characterized by an activation energy and a preexponential factor [1,2]. We previously showed that this simple description can be insufficient to describe reemission of a deposit generated by vacuum deposition, for two main reasons.…”

Twin-CQCM and Twin-TQCM Sensors With Wide Operating Temperature Range for Outgassing and Atomic Oxygen Measurement

“…Whereas the temperature of a spacecraft surface facing away from the sun can fall below -20 • C, outgassed deposits have a high probability of sticking. To prevent this contamination, materials for use in space are screened in accordance with established standards [9], [10] and simulation tools that emulate both outgassing and deposition kinetics to assess the amount of contaminant deposition in orbit [11], [12]. The QCM sensor is widely used to assess the deposition/desorption amounts of contaminants in a vacuum chamber, which simulates conditions in space such as extreme temperatures [3].…”

Modeling contamination at molecular level from elementary tests processing to flight contamination assessment

“…Extrapolating such 7-day isothermal ASTM-1559 data to a 15-year mission through an arbitrary power-law curve, or fitting 5-day temperature-step VBQC data through the sum of unjustified laws for 6 species is equally unsatisfactory. We have been convinced for many year now that the only way forward to really improve this state of the art is to perform a characterization at chemical species level, separating the contribution of each species to outgassing or deposit 1,2,3 . Whereas a total mass can easily be fitted by the sum of several laws regardless of their physical representativeness (exponentials for desorption or diffusion laws can usually equally do the job), a long-hoped-for independent measurement of each chemical species can probably only be discriminatorily fitted by the real physical law, the only one promising an accurate mission time extrapolation.…”