Kenneth T. Luey

Photochemical deposition of spacecraft material outgassing products

Laboratory measurements of photochemical deposition rates of outgassing products from Tefzel insulation have been conducted. We show that outgassing products from Tefzel insulation photodeposit under conditions of surface temperature and arrival rates for which bulk condensation will not occur. Normalized to the sample size, the photodeposition rate exceeds the reported condensable material outgassing rate. The result reported here strongly support the conclusion that photochemical deposition of contaminants from Tefzel is potentially a significant mechanism for degradation of thermal control surfaces on spacecraft.

Lifetimes and Failure Mechanisms of W/Re Hairpin Filaments

A life test of tungsten hairpin filaments used in a high-reliability electron beam instrument has been carried out to improve knowledge of filament life and failure mechanisms as a function of temperature. These filaments are made of nonsag tungsten/3 pct rhenium (W/3 pct Re) wire. A steep reduction in filament life is observed at 2760 K, which is not predicted by models which assume thermal evaporation as the principal failure mechanism. Failure analysis of the filaments shows that the sudden loss of life at 2760 K is the result of localized hot-spot formation caused by the accumulation of voids at grain boundaries. Examination of the crystal growth and growth rates indicates that the recrystallization temperature, TRec, occurs near 2760 K for the nonsag W/3 pct Re wire used in these filaments. This suggests that void accumulation acts as the principal, life-limiting failure mechanism; spontaneous recrystallization at 2760 K increases the rate of void growth and causes a severe reduction of filament life.

Contaminant film deposition on VUV-modified surfaces

Mechanisms for molecular contaminant droplet formation are investigated. The tendency for droplet formation is evaluated in terms of the surface tension of the liquid-like outgassed species and the surface energy of the collector. Results are presented indicating that VUV irradiation of the surface prior to contaminant deposition eliminates some droplet formation completely. This finding is discussed in terms of the removal of hydrocarbon and carbonyl-structured compounds from oxidized silicon surfaces.

Photochemical processes in a two-component molecular contaminant film

Previously, significant laboratory work has been performed on the photochemical deposition and darkening of molecular contaminant films. Much of this work addresses single, purified molecular species to understand fundamental photochemical processes. However, some of this work disagrees with other studies involving mixed, real spacecraft materials. There are also points of disagreement with contaminated returned optics from the Hubble Space Telescope where mixed contaminants were found. In this paper, we describe a method for vacuum depositing a controlled, reproducible contaminant film containing two molecular species: tetramethyl-tetraphenyl trisiloxane (DC 704) and dioctyl phthalate (DOP). We use this film to show differences in photochemical processes compared to a pure film of DC 704. We show that some photopolymerization processes occur more slowly in a two-component, mixed film during accelerated exposure to vacuum ultraviolet (VUV) radiation.

Digital Imaging of Particulate Contamination

The use of digital cameras and digital imaging software for the measurement of particle obscuration is discussed. Novel calibration standards are used to evaluate the sensitivity and accuracy of commercially available digital cameras for detecting microscopic dust particles and other contaminant features on surfaces. Lighting and illumination effects are also illustrated and discussed. The digital image histogram of particles on a surface is shown to give good results for the percent area coverage.

Optical properties of contaminant droplets

Novel light scattering properties of molecular films in a “droplet” configuration are presented and discussed. The illuminated films are shown to disappear when viewed at particular angles. The phenomenon is discussed in the context of Germer’s analysis of out-of-plane scattering from particles and surface micro-roughness.

Separation of volatile and nonvolatile deposits on real-time SAW NVR detectors

Real-time instruments based on surface acoustic wave (SAW) resonators are now seeing greater application for measuring the accumulation of nonvolatile residues (NVRs) on contamination sensitive surfaces. In this paper, we study the use of a desiccant, or dry GN+-2) to remove volatile films from the SAW sensing surfaces, with the intent of leaving the NVR behind. Using water as moderately volatile model material, the SAW device was capable of indicating monolayer growth in agreement with the expected frequency change. The drying agent was successful in removing all water from the SAW device. Additionally, the SAW device was capable of detecting different regimes of desorption kinetics. In trials of several candidates, only one example of NVR could be deposited, most likely a phthalate from flexible tubing heated beyond its working temperature. The deposit was so large that it overwhelmed subsequent observations of water desorption.

Contamination studies of the Pegasus fairing separation

A series of vacuum simulation tests were conducted to characterize the possible contamination of payload surfaces due to separation of the Pegasus fairing. This paper presents the test philosophy, experimental approach and the contaminant levels found, compared to MIL-STD- 1246B. These tests show that the Pegasus fairing separation using the frangible base joint and nose bolt nut results in Level 500 payload surfaces. The source of Zn particles near the bottom of the test payload needs to be found.

VUV modification of surfaces to induce film formation

This work presents further evaluation of the mechanisms driving the formation of molecular contaminant films and arrays of droplets on silicon and other types of space system optical surfaces. A simple model is presented describing a competition between the self-cohesive forces of a liquid-like droplet and the adhesive forces between the droplet and surface. We show in this work that irradiation of the silicon surface prior to contaminant deposition increases the adhesive forces, enhancing film formation. However, the surface states achieved by the VUV exposure cannot be reproduced by simple approaches such as solvent wiping. Higher intensity VUV exposure produces a silicon surface that allows film formation for even very pure contaminant analogs with high self-cohesion.

Infiltration of supermicron aerosols into a simulated space telescope

Purging is a common scheme to protect sensitive surfaces of payloads and spacecraft from airborne contaminant intrusion during ground assembly, integration, and launch vehicle encapsulation. However, the purge for space volumes must be occasionally interrupted. Thus it is important to gain insights into the transport of ambient particles penetrating through vent holes and entering the interior of a confined space system, such as a space telescope, during a purge outage. This study presents experimental work performed to measure time-dependent aerosol concentration changes during a purge outage. The laboratory results from the aerosol experiments were compared with a mass balance based mechanistic model which had been experimentally validated for aerosols ranging from 0.5 to 2 μm. The experimental data show that the steady-state aerosol concentration inside a simulated space telescope (SST) is governed by the surrounding particle concentration, SST air exchange rate, and the particle deposition rate.