Randy M. Villahermosa

Cobalt-containing polyimides for moisture sensing and absorption.

A modified polyimide has been created that both senses and absorbs moisture. The material was prepared by introducing cobalt(II) chloride, a well-known moisture sensing and absorbing metal salt, into a polyimide. The final polymer was found to be fully cured, retaining the metal without any leeching, and was thermally stable. Furthermore, the material underwent a visible color change when exposed to moisture and reverted back to the original color when subsequently dried. The total moisture absorbed was measured with a quartz crystal microbalance. The amount of moisture absorption was found to be proportional to the relative humidity, and the modified polymer was found to have up to 3 times greater moisture absorption compared to the unmodified polymer.

Managing contamination-enhanced Laser Induced Damage (CLID)

Contamination-enhanced Laser Induced Damage (CLID) occurs when molecular or particulate contamination, present on or in the vicinity of an optical material, leads to accelerated laser power degradation and premature failure. The physical mechanisms that cause CLID are not sufficiently understood to predict the extent to which a contaminant will cause damage. Although standard computational methods can be used to predict the amount of contamination on an optic, the effects of those molecules or particles on laser performance has not been sufficiently quantified. This paper will describe an approach for managing CLID that relies on laboratory studies to understand the relationship between contaminant type or quantity and CLID thresholds. That insight can then be used to guide the definition of cleanliness requirements and the design of material screening tests. Initial efforts to study how mass transport, the movement of contaminants in and out of the laser beam, affects damage rates will be discussed as well.

Chemical analysis of silicone outgassing

Silicone materials pose a contamination control challenge because of their ubiquity in satellite hardware, the tendency of the material and its outgassed contaminants to migrate along surfaces, and the difficulty in cleaning away the residue. To devise effective mitigation strategies, accurate knowledge of the chemical identity and properties of the outgassed species is needed. This information is critical for modeling silicone outgassing deposition processes and for developing effective cleaning methods. To this end, a chemical analysis study of several common silicone materials was conducted to identify and characterize the outgassed contaminants. Gas Chromatography-Mass Spectrometry (GC-MS) and other laboratory techniques were used to identify and characterize the outgassed species. In this report, the results of this study will be discussed with a particular emphasis on comparing the outgassing properties of the species collected from these materials to DC704, which is typically used to model silicone outgassing.

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.

Contamination resistant coatings for enhanced laser damage thresholds

This paper describes a novel approach for the suppression of contamination enhanced laser damage to optical components by the use of fluorinated coatings that repel organic contaminates. In prior work we studied laser damage thresholds induced by ppm levels of toluene under nanosecond 1.064 μm irradiation of fused silica optics. That work showed that moderate vapor-phase concentrations (< 15%) of water and alcohols dramatically increased the laser damage threshold. The data are consistent with the hypothesis that water and alcohols interact more favorably with the hydroxylated silica surface thereby displacing toluene from the surface. In this work, preliminary results show that fluorinated self assembled monolayer coatings can be used to accomplish the same effect. Optics coated with fluorinated films have much higher survival rates compared with uncoated optics under the same conditions. In addition to enhancing survival of laser optics, these coatings have implications for protecting spacecraft imaging optics from organic contamination.

Comparative Evaluation of the Moisture Permeation of Polyurethane, Polyethylene, and Fluoropolymer Tubing

Polymer tubing is used for a variety of applications such as the transport of dry nitrogen and other gases in order to purge systems of moisture and other contaminants. While some polymer tubing are marketed for low-moisture applications, permeation data are often not reported to aid in material selection. Tests were performed to compare moisture permeation for a variety of tubing materials that could be used in dry gas purged systems. The results demonstrated that polyethylene and fluoropolymer tubing exhibited practically no moisture permeation, while polyurethane tubing exhibited a noticeable amount of moisture permeation. The results were compared with the moisture permeation of thin films. The permeation was found to be influenced by many factors as dictated by the various results derived from different brands and different durometers within the same brand. The effect of external relative humidity on permeation was also measured. Approximations can also be made on the internal relative humidity based on the external relative humidity. These approximations can be used as well to determine the maximum possible moisture exposure level outside the tubing or the purge time required for a specific application. The results could assist in material selection based on the desired application.

Inhibition of contamination laser induced damage to optical substrates

We have characterized the thresholds for contamination laser induced damage (C-LID) process using toluene as a model contaminant by varying oxygen and toluene concentrations. In the presence of 300 ppm toluene and nitrogen, the damage threshold is (7.8 ± 1.9) × 103 laser pulses, in synthetic air the damage threshold is (18.0 ± 2.1) × 103 laser pulses. We have found several high vapor pressure molecules that effectively inhibit the (C-LID) process and greatly extend the lifetime of fused silica optics under high power laser irradiation. With the addition of ~4000 ppm of water, methanol or ethanol, the lifetime exceeds 1 × 106 laser pulses with no damage observed. Possible mechanisms are discussed.

Calibration of on-orbit IR sensors by off-board illumination of neighboring satellites

Space borne overhead non-imaging (non-focusing) infrared (ONIR) sensors require on-orbit calibration to validate performance of sensor payloads. Typically this is accomplished by the use of ground based observations including laser illumination of the calibrated sensor. This provides a-priori knowledge of the laser characteristics and atmospheric propagation thereby providing the sensor operators a method for deducing the true system level performance. Of concern is the need to avoid laser illumination of other satellites to prevent inadvertent damage or temporary mission degradation. The complex predictive avoidance process is necessarily bureaucratic and time-consuming due to the need to entertain the interests of multiple stakeholders. Herein is described a method for mutual calibration of co-orbital ONIR sensors by use of incoherent off-board illumination of a sample with known spectral reflectivity. The method will not involve laser illumination, will be less threatening to neighboring spacecraft, and will not require predictive avoidance processes.

Modified polymers for contamination sensing and prevention of optical and space systems

The ability of a space-grade material to both sense and absorb molecular contamination is of great use. A series of polymer materials that both sense and absorb moisture, a contaminant detrimental to optical and space systems, has been created. The materials were prepared by introducing additives into polymers while still retaining the original properties of both the polymers and the additives. The additive acts as the contaminant absorber and sensor. Measurements to determine the amount of moisture absorption compared to other materials have been executed. The final materials were easy to fabricate and could be produced on a large scale. The materials also could easily be regenerated again for multiple uses. Development for practical applications such as a desiccant material has been carried out.

Characterization of outgassed contaminants from polymeric spacecraft materials

Silicones and polyolefins are versatile polymeric materials that are often used for spacecraft applications but can produce considerable amounts of non-volatile residue (NVR) contamination. Outgassing properties of a commercial off-the-shelf (COTS) polyolefin tubing and GE RTV615 silicone potting, both of which are known to outgas at high levels, were characterized using ASTM E595 testing and infrared (IR) absorption spectroscopy. The total mass loss (TML) values for the polyolefin tubing varied between 1.8 and 2.5%, while the collected volatile condensable material (CVCM) was between 0.7 and 1.2%. The silicone potting had somewhat lower outgassing levels, with TML values between 1.0 and 1.7% and CVCM ranging from 0.7 to 1.3%. IR analysis of the outgassed residue indicates the materials produce NVR contamination through different mechanisms. The polyolefin tubing, which was composed of a hydrocarbon co-polymer mixed with additives, disproportionately outgassed low-weight molecular compounds containing ester functional groups. In contrast, RTV615 outgassing appeared to proceed through the release of shorter chain silicone polymers or oligomers. Combining outgassing test data with the chemical characterization of NVR residue provides a better understanding of contamination processes and will contribute to the development of more efficient mitigation strategies.