Polymer photonic modulators and switches offer several attractive features for space systems, including rf bandwidth of 50 GHz or higher, and the potential for high sensitivity and low cost. We have shown that polymer modulators experience little if any degradation (less than 2%) in the half-wave voltage due to total dose exposure of up to 5 MRad from a Co-60 source. An outgassing test at 125 degrees Celsius in vacuum for 24 hr resulted in a mass loss of less than 0.2% of the polymer material.
The mass sensitivity of a 200 MHz surface acoustic wave (SAW) quartz crystal microbalance (QCM) has been determined by comparison with a 10 MHz shear wave (SW) QCM. The calibration technique consisted of exposing the SW QCM and SAW QCM to the same impinging molecular flux, assuming the same rate of deposition on both surfaces, and comparing the change of frequency for the two QCMs. Although the major objective was to determine mass sensitivity at ambient temperature sensitivity was also measured for a range of temperatures between 130 K and 300 K. The materials used to provide the molecular flux were Coray 100 mineral oil, DC 704 silicone oil, and Rheolube R2000 synthetic grease. The mass sensitivity of the SAW QCM measured at 300 K is on the order of 50 Hz.cm2/ng. Mass sensitivity was observed to decrease with temperature, with the lowest value measured being about 39 Hz.cm2/ng at 130 K for Coray 100. The theoretical expression describing the mass sensitivity of the SAW QCM contains two terms, for mass and material elasticity, respectively. The observed decrease in response at lower temperatures (i.e., reduced elasticity) confirm that the material elasticity term, typically ignored at cryogenic temperatures, must be incorporated cryogenic temperatures.
Spacecraft surface coatings will outgas various molecular components when the spacecraft is subjected to the vacuum of space. Some of these outgassing molecules have the potential to contaminate sensitive, critical optical surfaces on the spacecraft. Some of this outgassing can be mitigated through a thermal vacuum bakeout of the coated structures. The residual outgassing, the return flux, and the deposition of outgassing molecules for the as-flown spacecraft environment needs to be modeled to assess the vulnerability of the spacecraft to this contamination. Laboratory outgassing measurements provide the data that can be used to set the protocols for thermal vacuum bakeout procedures and to develop reasonable models of the in-flight contamination process. Several questions arise: How is the outgassing information affected by the outgassing measurement procedure? After a thermal vacuum bakeout, what happens to the outgassing properties of a sample, especially after long term interim storage? Is the rate of outgassing affected by temperature cycling? How does exposure to ultraviolet radiation affect the rate of outgassing of molecules from coatings and the rate and nature of deposition onto receptor surfaces? What is the influence of primers when used as a conditioner of the metal surface to be coated? How can laboratory outgassing information be related to the to-be-flown temperature of spacecraft outgassing? The purpose of this paper is to describe these issues and to discuss some of them with examples taken from outgassing measurements performed on a black optical paint (Chemglaze Z306).
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