Silica aerogels are attractive candidates for a variety of NASA applications because of their lightweight structure, high surface area, and low thermal conductivity. Because silica aerogels are fragile, their use has been limited to protected, temperature-extreme environments, such as battery insulation for Mars surface rovers. However, advances over the last several years at the NASA Glenn Research Center have led to significant enhancements in mechanical durability as a result of polymer crosslinking the silica aerogel network (refs. 1 to 3). Recently, the incorporation of nanoscale fillers, such as carbon nanofibers, into the crosslinked aerogel matrix was demonstrated. The addition of these fill-ers can be advantageous since they may enhance not only the mechanical properties of the aerogel but other material properties, such aselectrical conductivity or catalytic activity.
The Pyrograf-III (Applied Sciences, Inc.) nanofiber (top micrograph) was chosen for several reasons, primarily because the surface functionalization with alcohol provided good chemical compatibility with the existing processes. In addition, other properties unique to carbon nanotubes can be realized by substituting nanofibers that are easier to produce and are more cost effective. For certain applications, nanofiber-integrated crosslinked aerogels can simultaneously enhance the electrical conductivity and the mechanical reinforcement of the silica matrix. Other benefits include improved high-energy storage and decreased radiative heat transfer (due to the black color), both of which are desirable properties for efficient solar energy collection material.
Scanning electron micrographs of carbon nanofibers. Top: As-received. Bottom: As integrated into the crosslinked aerogel matrix.
Using a statistical design methodology, Glenn researchers produced 19 samples, with approximately two-thirds containing fiber in varying amounts according to the total silane component of the gel mixture. The bottom micrograph shows the successful integration of carbon fiber into the silica-polymer matrix on the nanoscale. Empirical models were derived for several macroscopic properties from data from the 19 samples using multiple linear regression and incorporating only highly statistically significant terms. Interestingly, the response surface model for density shown in the left plot demonstrates that the addition of carbon fiber has no significant effect. In fact, the only drivers for density at the levels studied are total silane and polymer concentrations. The lack of effect on density with fiber addition could be ideal, since properties may be changed without the addition of weight, a premium for most NASA missions.
Response surface models. All raw data points for the 19 samples measured are also displayed on the surfaces. Left: Density. Right: Yield stress for low and high polymer models. There is no clear effect resulting from carbon fiber addition.
The effect of carbon fiber incorporation on the mechanical properties of the crosslinked aerogels is still under investigation; however, initial results show that it has little effect on the yield stress. The right plot shows the model for the load at 0.2-percent strain for both the low and high polymer cases, noting that the other significant variables are fiber content (percent) and total silane. The load values through the fiber range studied are very similar. However, before crosslinking, the fiber-containing gels are easier to handle without damaging them, especially at low density, indicating that the materials’ green strength is improved by the fiber addition. Further tests of this new material are being developed to accurately capture the perceived improvements. We also plan to look in greater detail at the dispersion of the fibers into the matrix, as this is a common problem with carbon nanotubes (ref. 4) and with nanofibers by extension. This problem often requires purification and functionalization to avoid fiber agglomeration. Other properties being examined are specific compressive strength, Young’s modulus, and electrical and thermal conductivities.
Last updated: December 14, 2007
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