Changes in the directional reflectance properties of pressed and sintered polytetrafluoroethylene (PTFE) diffusers induced by exposure to vacuum ultraviolet (VUV) irradiation before and after controlled contamination with Pennzane are presented in this paper. A set of 99% reflective, white, optical grade diffuse calibration standards were irradiated with a VUV source positioned at 60° to the diffuser normal. The bidirectional reflectance distribution functions before and after contamination and VUV irradiation were measured and compared at a number of scatter geometries and wavelengths in the UV, VIS and IR spectral ranges. The 8° directional hemispherical reflectance’s were also measured and compared from 200 nm to 2500 nm. Our results indicate a measureable impact of VUV irradiation on pressed and sintered PTFE diffusers as manifested by a directional dependent change in their reflectance. Such an effect needs to be considered in the on-orbit deployment of PTFE diffusers.
In the harsh vacuum environment of deep space, surfaces shielded from the Sun may easily develop temperatures low enough to condense water vapor for extended periods of time. The condensed vapor will subsequently desorb at rates consistent with its temperature-sensitive equilibrium vapor pressure, and under certain circumstances it is important to predict this release rate. A review of available scientific literature to confirm model predictions indicated no such measurements had been reported below 131 K. Contamination control personnel at NASA Goddard Space Flight Center recognized the possibility they readily possessed the means to collect such measurements at lower temperatures with an existing apparatus commonly used for making outgassing observations. This paper will describe how the ASTM E-1559 "MOLEKIT" apparatus was used without modification to measure water vapor sublimation down to 120 K and compare this data to existing equilibrium vapor pressure models. In addition, an in-depth analysis of theoretical formulations for vapor pressure gives insight into the physical basis underlying characteristics associated with high-fidelity models.
This paper reports on experiments in projection ion lithography in which multiple demagnified images have been etched directly into a substrate through an aperture lens array, without using a resist or proximity mask. The ion source is an 8-cm-beam-diam, Kaufman-type mercury ion thruster, which accelerates ions through a grid of perforated plates, and then neutralizes the resultant beamlets by injecting electrons at ground potential. The neutralized ion beam is allowed to pass through a suitable mask and thence onto an array of apertures of a few tens of microns in diameter. Ions impinging on the apertures are accelerated through a field produced by imposing a negative potential on a substrate placed just downstream of the aperture array. An image of the mask is etched in the substrate directly below each aperture. The substrate is a polished metal disk which has been sputter coated with a number of alternating layers of chromium and copper, each about 500 Å thick. Etching of these layers quickly produces an easily visible contour map which accurately depicts variations in the etch rate over the substrate surface. It is necessary to generate random motion of the substrate relative to the ion source to prevent projection of a superimposed image of the ion source grid system. To date, mask elements of the order of 2–3 mm in diameter have been used to produce multiple images demagnified by about 20–30× with resolution better than 10 μm. Suitable equipment is now being fabricated to permit exploration of the submicron region.
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