Silica aerogels have been shown to be attractive for use as thermal insulation materials for thermoelectric devices. It is desirable to thermally insulate the legs of thermoelectric devices to suppress lateral heat leaks that degrade thermal efficiency. Aerogels offer not only high thermal insulation effectiveness, but also a combination of other properties that are especially advantageous in thermoelectric device applications.
Aerogels are synthesized by means of solgel chemistry, which is ideal for casting insulation into place. As the scale of the devices to be insulated decreases, the castability from liquid solutions becomes increasingly advantageous: By virtue of castability, aerogel insulation can be made to encapsulate devices having any size from macroscopic down to nanoscopic and possibly having complex, three-dimensional shapes. Castable aerogels can permeate voids having characteristic dimensions as small as nanometers. Hence, practically all the void space surrounding the legs of thermoelectric devices could be filled with aerogel insulation, making the insulation highly effective. Because aerogels have the lowest densities of any known solid materials, they would add very little mass to the encapsulated devices.
The thermal-conductivity values of aerogels are among the lowest reported for any material, even after taking account of the contributions of convection and radiation (in addition to true thermal conduction) to overall effective thermal conductivities. Even in ambient air, the contribution of convection to effective overall thermal conductivity of an aerogel is extremely low because of the highly tortuous nature of the flow paths through the porous aerogel structure. For applications that involve operating temperatures high enough to give rise to significant amounts of infrared radiation, opacifiers could be added to aerogels to reduce the radiative contributions to overall effective thermal conductivities. One example of an opacifier is carbon black, which absorbs infrared radiation. Another example of an opacifier is micron-sized metal flakes, which reflect infrared radiation.
Encapsulation in cast aerogel insulation also can help prolong the operational lifetimes of thermoelectric devices that must operate in vacuum and that contain SiGe or such advanced skutterudite thermoelectric materials as CoSb3 and CeFe3.5Co0.5Sb12. The primary cause of deterioration of most thermoelectric materials is thermal decomposition or sublimation (e.g., sublimation of Sb from CoSb3) at typical high operating temperatures. Aerogel present near the surface of CoSb3 can impede the outward transportof Sb vapor by establishing a highly localized, equilibrium Sb-vapor atmosphere at the surface of the CoSb3.
This work was done by Jeffrey Sakamoto, Jean-Pierre Fleurial, Jeffrey Snyder, Steven Jones, and Thierry Caillat 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
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Refer to NPO-40630, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Aerogels for Thermal Insulation of Thermoelectric Devices
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Overview
The document titled "Aerogels for Thermal Insulation of Thermoelectric Devices" is a technical support package from NASA's Jet Propulsion Laboratory, detailing the development and application of aerogels in enhancing the performance of thermoelectric devices. Aerogels are highly porous materials known for their exceptional thermal insulation properties, making them ideal for aerospace applications where weight and thermal management are critical.
The document outlines the standard two-step sol-gel process used to prepare the aerogel. This involves combining tetraethylorthosilicate, ethanol, and nitric acid, followed by the addition of water, ammonia hydroxide, and acetonitrile. The resulting sol is poured into molds containing CoSb3 samples, which are then allowed to gel. After gelation, the samples undergo a critical point drying process using acetonitrile to achieve supercritical conditions, which is essential for maintaining the aerogel's structure.
Experimental results demonstrate that aerogel coatings significantly reduce the sublimation rate of antimony (Sb) from CoSb3 samples when subjected to high temperatures (700°C) under vacuum conditions. Uncoated samples exhibited substantial mass loss and noticeable depletion layers, while aerogel-coated samples showed minimal decomposition and only a 2.6 wt. % loss over 29 hours, indicating that the aerogel effectively suppresses sublimation by creating a localized equilibrium environment that reflects Sb vapor back to the surface.
The document also discusses the envisioned use of aerogel in thermoelectric modules, where it will form an interpenetrating network around individual thermoelectric legs. This configuration not only provides thermal insulation but also adds negligible mass to the overall module, enhancing its efficiency.
Figures included in the document illustrate the experimental setup, the sol-gel process, and the effects of aerogel on sublimation behavior. The findings suggest that further research is needed to characterize mass loss over extended periods, potentially leading to long-term applications in thermoelectric devices.
Overall, this technical support package highlights the innovative use of aerogels in thermal insulation, showcasing their potential to improve the performance and longevity of thermoelectric devices in various applications, particularly in aerospace technology.
