Because of their low density and insulating properties, silica aerogels are attractive candidates for many thermal, optical, electrical, and chemical applications. However, their inherent fragility has restricted the use of such materials to, for example, insulation in only extreme temperature environments, such as Mars. Future space exploration missions demand lighter-weight, robust, dual-purpose materials for insulation, radiation protection, and structural members of habitats, rovers, astronaut suits, and other items. Previous work at the NASA Glenn Research Center showed that covalently bonding a polymer coating to the surface of a silica framework before supercritical drying occurred would reinforce the aerogels and increase their strength in comparison to uncrosslinked aerogels with a similar silica framework. Previously, polymer crosslinked aerogels had been reported with density ranges of 0.2 to 0.5 g/cm3.
In 2005, Glenn carried out a study to systematically adjust processing variables to control the macroscopic properties of polymer crosslinked aerogels with much lower densities. These materials may not be suitable as structural materials, but they maintain some mechanical integrity over uncrosslinked silica aerogels of similar density and are superior insulating materials.
Top: Response surface models for density plotted versus di-isocyanate and silane concentration. Bottom: Maximum stress at break of polymer crosslinked aerogels plotted versus di-isocyanate and silane concentration. The transparent surface shows predictions for 72 °C-cured aerogels, and the shaded surface shows predictions for room-temperature-cured aerogels. Data points from the study also are shown.
In the study, researchers varied the concentration of the silanes in the starting gel, the concentration of polymer crosslink (di-isocyanate) that the gels were exposed to, and the reaction temperature. Then, the resulting gels were characterized for a variety of properties. Empirical models were derived from these measurements so that significant effects of the variables on the measured properties could be discerned. The preceding graphs show the empirical models for density and stress at break. As evidenced from these graphs, temperature has a pronounced effect on both measured responses. Density and maximum stress were higher for all combinations of both polymer concentration and silane concentration when the materials were cured at the higher temperature. Density and maximum stress at break both increased with increasing silane concentration as well. As is also evident from these graphs, increasing the di-isocyanate concentration had a pronounced effect on stress, strongly increasing the maximum stress at break. Although its effect on density was modest, it was still statistically significant.
Within the bounds of the present study, the highest density crosslinked aerogel produced the highest maximum stress at break. Stress at break for these aerogels was 350 times stronger than for the corresponding uncrosslinked aerogel, whereas density only increased by a factor of 2. The lowest density crosslinked aerogels still exhibited a fortyfold increase in stress at break over the corresponding uncrosslinked aerogels with the same twofold increase in density. These aerogels boasted a nominal density of 0.036 g/cm3 and were flexible.
The models can be used to predict properties of monoliths prepared using other polymer and silane combinations. This is shown in the following graphs. The graphs show slices from the three-dimensional surfaces of monoliths that were density and maximum stress cured at 71 °C and of monoliths that were made using no polymer, 6 wt% polymer, and 13 wt% polymer. For example, it may be desirable for a particular application to make a polymer crosslinked aerogel with the lowest possible density and a maximum stress at break of at least 1×106 N/m2. The point at position A shows that this aerogel would have to be made using about 24.5 wt% silane and 13 wt% polymer, and that the density of this aerogel would be a little above 0.4 g/cm3. If a lower strength could be tolerated (1×106 N/m2, for example), aerogel B, which can be made from 7.5 wt% silane and 6 wt% polymer is predicted to have a density of a little over 0.1 g/cm3.
Slices of the response surface models for maximum stress at break and density plotted versus total silane concentration.
This method of picking and choosing the desired aerogel properties for a given application can be expanded upon by quantifying for a broader range of processing conditions (for example, varying the amount of water, catalyst, washings, etc.) and for a host of other properties such as thermal conductivity, compressive strength, density, and modulus and stress at break. This would enable researchers to tailor the materials for a wide variety of applications.
Find out more about the research of Glenn’s Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Glenn contacts:
Dr. Mary Ann B. Meador, 216-433-3221, Maryann.Meador-1@nasa.gov; Dr. Nicholas Leventis, 216-433-3202, Nicholas.Leventis-1@nasa.gov; and Dr. Lynn A. Capadona, 216-433-5013, Lynn.A.Capadona@nasa.gov
Authors:
Dr. Mary Ann B. Meador, Dr. Nicholas Leventis, and Dr. Lynn A. Capadona
Headquarters program office:
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Programs/Projects:
Advanced EVA, Vehicle Systems
Last updated: October 16, 2006
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