A series of experiments has been performed to allow observation of the foaming process and the collection of temperature, rise rate, and microstructural data. Microfocus video is used in conjunction with particle image velocimetry (PIV) to elucidate the boundary condition at the wall. Rheology, reaction kinetics and density measurements complement the flow visualization. X-ray computed tomography (CT) is used to examine the cured foams to determine density gradients. These data provide input to a continuum level finite element model of the blowing process.
Conformal coatings are used in space applications on printed circuit board (PCB) assemblies primarily as a protective barrier against environmental contaminants. Such coatings have been used at Sandia for decades in satellite applications including the GPS satellite program. Recently, the value of conformal coating has been questioned because it is time consuming (requiring a 5-6 week schedule allowance) and delays due to difficulty of repairs and rework performed afterward are troublesome. In an effort to find opportunities where assembly time can be reduced, a review of the literature as well as discussions with satellite engineers both within and external to Sandia regarding the value of conformal coating was performed. Several sources on the value of conformal coating, the functions it performs, and on whether coatings are necessary and should be used at all were found, though nearly all were based on anecdotal information. The first section of this report, titled 'Conformal Coating for Space Applications', summarizes the results of an initial risk-value assessment of the conformal coating process for Sandia satellite programs based on information gathered. In the process of collecting information to perform the assessment, it was necessary to obtain a comprehensive understanding of the entire satellite box assembly process. A production time-line was constructed and is presented in the second section of this report, titled 'Satellite Box Assembly', specifically to identify potential sources of time delays, manufacturing issues, and component failures related to the conformal coating process in relation to the box assembly. The time-line also allows for identification of production issues that were anecdotally attributed to the conformal coating but actually were associated with other production steps in the box assembly process. It was constructed largely in consultation with GPS program engineers with empirical knowledge of times required to complete the production steps, and who are familiar with associated risks from activities such as handling, assembly, transportation, testing, and integration into a space vehicle (SV) system. Finally, section three titled, 'Summary and Recommendations for Future Work', briefly summarizes what we have learned and describes proposed future work.
Foam encapsulants are used to encapsulate electromechanical assemblies for reasons such as shock mitigation, structural support, and voltage breakdown protection. Characterization of electrical properties of polymer encapsulants is important in situations where potting materials are in intimate contact with electrical components (e.g., printed wiring boards). REF308, REF320, RSF200, and EF-AR20 foams were developed for encapsulation in some potting applications at Sandia. Select electrical properties were measured for these Sandia encapsulants to characterize them for use in electromechanical potting applications. Dielectric constant with dissipation factors, volume resistivity, and dielectric strength were measured for REF308, REF320, RSF200, and EF-AR20 encapsulants. Fabrication of foam test specimens and the electrical test procedures will be discussed, and electrical testing results will be reported.
A new epoxy foam encapsulant, EF-ARIO/20, has been developed at Sandia National Laboratories (SNL) as a replacement for Ablefoam", an epoxy foam encapsulant used in the W76 Arming, Fusing, and Firing (Al%@) system. Since it contained toxic ingredients including a known carcinogen, Ablefoarn" is no longer commercially available. It has been demonstrated by scanning electron microscopy (SEM) that the microstructure of the new epoxy foam is similar to that of Ablefoam@. Mechanical properties of tensile and compressive strength, and tensile and compressive modulus, and thermal properties of glass transition temperature (.TJ, and coefficient of thermal expansion (CTE) have been measured for the new foam. Electrical properties of dielectric constant, dissipation factors, volume resistivity, and dielectric strength were also measured. These property measurements are comparable to those of Ablefoam@. Development and characterization of the new foam will be discusse~ and a comparison of mechanical, thermal, and electrical properties for the new epoxy foam and Ablefoam@ will be reported.
With the eventual phase-out of chlorofluorocarbons and hydrochlorofluorocarbons, and restrictive regulations concerning the use of other volatile organic compounds as cleaning solvents, it is essential to seek new, environmentally acceptable cleaning processes. We are investigating supercritical carbon dioxide (CO2) as an alternative solvent for precision cleaning of machined metal parts in governmental and industrial cleaning processes. The compatibility of metals in supercritical-fluid cleaning media with respect to corrosion must be addressed. In this work, a screening study of the corrosive effects of supercritical CO2 and several supercritical cosolvents on selected metals was conducted. Sample coupons of stainless steel (grades 304LSS, 316SS), aluminum (grades 2024, 6061, 7075), carbon steel (1018), and copper (CDA 101) were statically exposed to pure supercritical CO2, water-saturated supercritical CO2, 10 wt % methanol/CO2 cosolvent, and 4 wt % tetrahydrofurfuryl alcohol (THFA)/CO2 at 24,138 kPa (3500 psig) and 323 K (50 °C) for 24 h. Gravimetric analysis and magnified visual inspection of the coupons were performed before and after the exposure tests. Surface analyses including electron microprobe analysis (EMPA), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) were done where visual and gravimetric changes were indicative of corrosive attack. The metal alloys were found to be compatible with the supercritical test media barring a few exceptions. Corrosive attack was observed on 1018 carbon steel in the water-saturated CO2 environment, and also on 2024 aluminum and CDA 101 copper, both in the 10 wt % methanol-CO2 cosolvent. The results of all compatibility testing are reported, and hypotheses are formed in an attempt to explain possible corrosion mechanisms.
Supercritical fluid technology has been used in industry for the extraction and separation of materials in a number of applications such as food preparation and petroleum processing. Gel-derived microcellular polymer foam technology, developed at Sandia, requires the extraction of organic solvents from thermally-induced phase separated gels to yield microcellular foams. We have applied supercritical fluid extraction technology to these specialized materials by using supercritical carbon dioxide to extract a variety of organic solvents from gels to produce foams. Our supercritical extraction process will be described, as well as high pressure equipment that is used to perform the extractions. The results of gel extraction trails and qualitative supercritical carbon dioxide/solvent miscibility experiments will also be presented. We plan to pursue other related areas in this field, including supercritical fluid cleaning, quantitative solubility studies, and supercritical fluid chromatography.