A process for fabricating thermoelectric modules with vacuum gaps separating the thermoelectric legs has been conceived, and the feasibility of some essential parts of the process has been demonstrated. The vacuum gaps are needed to electrically insulate the legs from each other. The process involves the use of scaffolding in the form of sheets of a polymer to temporarily separate the legs by the desired distance, which is typically about 0.5 mm. During a bonding subprocess that would take place in a partial vacuum at an elevated temperature, the polymer would be vaporized, thereby creating the vacuum gaps. If desired, the gaps could later be filled with an aerogel for thermal insulation and to suppress sublimation of thermoelectric material, as described in “Aerogels for Thermal Insulation of Thermoelectric Devices” (NPO-40630), NASA Tech Briefs, Vol. 30, No. 7 (July, 2006), page 50.
A simple thermoelectric module would typically include thermoelectric legs stacked perpendicularly between metal contact pads on two ceramic substrates (see figure). As the design of the thermoelectric module and the fabrication process are now envisioned, the metal contact pads on the ceramic substrates would be coated with a suitable bonding metal (most likely, titanium), and the thermoelectric legs would be terminated in a possibly different bonding metal (most likely, molybdenum). Prior to stacking of the thermoelectric pads between the metal pads on the ceramic substrates, the polymer sheets would be bonded to the appropriate sides of the thermoelectric legs. After stacking, the resulting sandwich structure would be subjected to uniaxial pressure during heating in a partial vacuum to a temperature greater than 700 °C. The heating would bond the metal pads on the legs to the metal pads on the substrates and would vaporize the polymer sheets. The uniaxial pressure would hold the legs in place until bonding and vaporization were complete.
Ideally, the polymer chosen for use in this process should be sufficiently rigid to enforce dimensional stability of the gaps and should vaporize at a temperature low enough that it does not undergo pyrolysis. (Pyrolysis would create an undesired electrically conductive carbonaceous residue.) Poly(a-methylstyrene) [PAMS] has been selected as a promising candidate. PAMS is considered to be rigid, and, in a partial vacuum of 10–6 torr (˜1.3 × 10–4 Pa), it vaporizes in the temperature range of 250 to 400 °C, without pyrolizing.
Initially, the primary concern raised by vaporization of polymer scaffolding was that polymer-vapor residue might interfere with the bonding of the thermoelectric legs to the metal pads on the ceramic substrates. In an experiment to investigate the likelihood of such interference, a mockup comprising two molybdenum legs separated by a PAMS sheet in contact with a titanium plate was placed under uniaxial pressure and heated to a temperature of 950 °C in a partial vacuum of 10–6 torr. Strong, uniform bonds were made between the molybdenum legs and the titanium plate, demonstrating that the PAMS vapor did not interfere with bonding.
This work was done by Jeffrey Sakamoto, Shiao-pin Yen, Jean-Pierre Fleurial, and Jong- Ah Paik 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.
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Refer to NPO-41248 volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Vaporizable Scaffolds for Fabricating Thermoelectric Modules
(reference NPO-41248) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on "Vaporizable Scaffolds for Fabricating Thermoelectric Modules." It outlines advancements in the fabrication of thermoelectric modules using innovative materials and techniques, particularly emphasizing the use of vaporizable scaffolds.
Thermoelectric modules are devices that convert temperature differences into electrical energy, making them valuable for various applications, including power generation and cooling systems. The document discusses the significance of using vaporizable materials, which can be easily removed after the desired structure is formed, allowing for the creation of complex geometries and enhancing the efficiency of the thermoelectric modules.
Key findings include the complete vaporization of Poly-α−methylstyrene, a material used in the scaffolding process, which occurs between 250°C and 400°C. This temperature range is critical for ensuring that the scaffolds can be effectively removed without interfering with the bonding of other materials, such as molybdenum legs to titanium plates, as demonstrated in the experiments referenced in the document.
The document also highlights the importance of the vaporization process in maintaining the integrity of the bonding between different materials. For instance, it notes that the vapor from the scaffolds did not interfere with the bonding process, which is crucial for the performance and reliability of the thermoelectric modules.
Additionally, the Technical Support Package serves as a resource for further exploration of the research and technology in this area, providing contact information for the NASA Scientific and Technical Information (STI) Program Office for those interested in more detailed inquiries or related publications.
Overall, this document encapsulates NASA's efforts to innovate in the field of thermoelectric technology through the use of vaporizable scaffolds, showcasing the potential for improved manufacturing processes and enhanced performance of thermoelectric devices. The findings and methodologies presented could have broader implications for various technological and commercial applications, reflecting NASA's commitment to advancing aerospace-related developments for wider societal benefits.
