Because of their low density and high mesoporosity, sol-gel-derived silica aerogels are attractive candidates for many unique thermal, optical, catalytic, and chemical applications (ref. 1). However, their inherent fragility has restricted their use. Researchers at the NASA Glenn Research Center previously reported crosslinking the mesoporous silica structure of an aerogel with polymers such as polyureas (refs. 2 to 4) and epoxies (ref. 5). These materials have very high specific strength in comparison to that of uncrosslinked aerogels with only a small effect on density. Thus, they may be enabling for future space exploration missions as well as advanced aeropropulsion systems that demand lighter weight, robust, dual-purpose materials for insulation, radiation protection, and/or the structural elements of habitats, rovers, astronaut suits, and cryotanks.
Although density is a prime predictor of the strength and thermal conductivity of aerogels, previous studies indicated that varying the silica backbone and size of the polymer crosslink independently can give rise to combinations of properties unpredictable from density alone. For use as a multifunctional insulation/structural material, such as in a cryotank, Glenn researchers wished to optimize the strength of aerogels while reducing density and thermal conductivity as much as possible. The effects of four processing parameters for producing di-isocyanate crosslinked aerogel were examined using statistical experimental design methodology to reduce the number of experiments and to allow computation of empirical models describing the relationship between the four variables and resulting properties.
Scanning electron micrographs of aerogels comparing no washings, (a) and (c); four washings, (b) and (d); low concentrations of silane, water, and polymer,(a) and (b); and low concentration of silane but high concentrations of water and polymer, (c) and (d).
The preceding photomicrographs show aerogel monoliths from the study at some of the extreme values of the four variables. It is evident, by comparing an aerogel with no washings (a) to that with four washings (b), that the number of washings is less critical when low values of polymer and water are employed: (a) and (b) are very similar. Other properties of the monoliths in (a) and (b) are also very similar (8 to 10 repeat units of isocyanate in the crosslink, porosity of >94 percent, and mechanical properties). However, at high values of water and polymer, no washings (c) produced a monolith with very low porosity, whereas four washings (d) resulted in a porosity of 90 percent. Note that the difference in properties and morphology of (b) and (d) are due entirely to the increased size of the crosslink in (d) (7 times as many repeat units) since both start with the same silane concentration.
Slices of the response surface models plotted versus total silane and water concentration with di-isocyanate held constant at the lowest concentration. (a) Density. (b) Polymer repeat units. (c) Average pore diameter. (d) Offset yield strength.
Some of the properties measured and modeled in this study are plotted in the preceding graphs, where di-isocyanate concentration is held constant at the lowest value studied. In general, mechanical properties, such as offset yield strength at 0.2-percent strain (d) increase as density (a) increases. Other properties, such as size of the crosslink (b) or mean pore size (c), are at a maximum when density is minimized. These types of models provide the ability to dial in a property for a particular application, for example, the minimum density aerogel with a desired mechanical strength.
To assess the validity of the models and test their ability to accurately predict aerogel properties, seven additional monoliths were produced, corresponding to model-generated optima for certain responses, and their properties were measured and compared with the predicted values. Some of these are shown in the bar charts for the same properties as in the preceding graphs. More of the results from this study are reported in reference 6.
Predicted and measured properties of polymer crosslinked aerogel optima from the experimental design. New runs were added to the original data to refine the models. HDI indicates hexa-methylene di-isocyanate, the basic unit in the polymer crosslink.
Glenn contacts:
Dr. Mary Ann B. Meador, 216-433-3221, Maryann.Meador@nasa.gov
Dr. Lynn A. Capadona; 216-433-5013, Lynn.A.Capadona@nasa.gov
Authors:
Dr. Mary Ann B. Meador, Dr. Lynn A. Capadona, and Dr. Nicholas Leventis
Headquarters program office:
Aeronautics Research Mission Directorate
Programs/projects:
Fundamental Aeronautics (subsonics fixed wing and subsonics rotary wing), Exploration Systems Research & Technology, Advanced Extravehicular Activity Project
Last updated: December 14, 2007
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