Optimizing solution chemistry and the addition of titania and fumed silica powder reduces shrinkage. These materials would serve to increase thermal efficiency by providing thermal insulation to suppress lateral heat leaks. They would also serve to prolong operational lifetime by suppressing sublimation of certain constituents of thermoelectric materials (e.g., sublimation of Sb from CoSb3) at typical high operating temperatures. [The use of pure silica aerogels as cast-in-place thermal-insulation and sublimation-suppression materials was described in “Aerogels for Thermal Insulation of Thermoelectric Devices” (NPO-40630), NASA Tech Briefs, Vol. 30, No. 7 (July 2006), page 50.]

Shrinkages of Representative Aerogels of various densities and of aerogel/titania composites were measured.

A silica aerogel is synthesized in a solgel process that includes preparation of a silica sol, gelation of the sol, and drying of the gel in a solvent at a supercritical temperature and pressure. The utility of pure silica aerogel is diminished by a tendency to shrink (and, therefore, also to crack) during the gelation and supercritical-drying stages. Moreover, to increase suppression of sublimation, it is advantageous to make an aerogel having greater density, but shrinkage and cracking tend to increase with density.

A composite material of the type under investigation consists mostly of titania oxide powder particles and a small addition of fumed silica powder, which are mixed into the sol along with other ingredients prior to the gelation stage of processing. The silica aerogel and fumed silica act as a binder, gluing the titania particles together. It is believed that the addition of fumed silica stiffens the aerogel network and reduces shrinkage during the supercritical-drying stage. Minimization of shrinkage enables establishment of intimate contact between thermoelectric legs and the composite material, thereby maximizing the effectiveness of the material for thermal insulation and suppression of sublimation.

To some extent, the properties of the composite can be tailored via the proportions of titania and other ingredients. In particular (see figure), the addition of a suitably large proportion of titania (e.g., 0.6 g/cm3) along with a 10-percent increase in the amount of tetraethylorthosilicate [TEOS (an ingredient of the sol)] to an aerogel component having a density 40 mg/cm3 makes it possible to cast a high-average-density (>0.1 g/cm3) aerogel/particle composite having low shrinkage (2.3 percent).

This work was done by Jong-Ah Paik, Jeffrey Sakamoto, and Steven Jones of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention.

Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena
CA 91109-8099 (818) 354-2240 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-42031, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Adhesives and sealants Composite materials Drying Emissions Exhaust emissions Insulation Manufacturing processes Materials Particulate matter (PM) Pressure
This Brief includes a Technical Support Package (TSP).
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Aerogel/Particle Composites for Thermoelectric Devices

(reference NPO-42031) is currently available for download from the TSP library.

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    NASA Tech Briefs Magazine

    This article first appeared in the September, 2006 issue of NASA Tech Briefs Magazine (Vol. 30 No. 9).

    Read more articles from the archives here.

    Overview

    The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on Aerogel/Particle Composites for Thermoelectric Devices. It outlines the synthesis, properties, and applications of aerogels, particularly in enhancing thermoelectric materials, which are crucial for energy conversion technologies.

    Aerogels are highly porous materials known for their low density and high surface area. The synthesis process described involves a two-step sol-gel method. Initially, a silica sol is created using tetraethylorthosilicate (TEOS), ethanol, and nitric acid through refluxing. This sol is then combined with other components, such as fumed silica and titania powder, to form composite aerogels. The addition of titania serves as an opacifying agent and helps reduce the shrinkage of the aerogel during the drying process.

    The document highlights the importance of controlling the density of the aerogel, which can significantly affect its properties. For instance, a lower density (around 40 mg/cc) minimizes shrinkage, while increasing the density can lead to higher shrinkage rates. The research indicates that incorporating titania powder can effectively reduce shrinkage from 6.9% to 2.3%, even at higher densities.

    Thermogravimetric analysis (TGA) is employed to study the sublimation of antimony (Sb) through the aerogel, as Sb is a key subliming species in thermoelectric materials. The results show a significant difference in weight loss between antimony encapsulated with aerogel and without it, indicating that the aerogel can effectively mitigate the sublimation process, which is a major degradation pathway for thermoelectric materials.

    The document also includes graphical data comparing the weight loss of antimony with and without aerogel, demonstrating the effectiveness of the composite aerogel in preserving the integrity of thermoelectric materials. The findings suggest that the use of aerogel composites can enhance the performance and longevity of thermoelectric devices, making them more viable for various applications, including aerospace technologies.

    Overall, this Technical Support Package provides valuable insights into the synthesis and application of aerogel/particle composites, emphasizing their potential to improve thermoelectric materials and contribute to advancements in energy conversion technologies.