The development of more efficient thermoelectric couple technology capable of operating with high-grade heat sources up to 1,275 K is key to improving the performance of radioisotope theroelectric generators. Lanthanum telluride La3–xTe4 and 14-1-11 Zintls (Yb14MnSb11) have been identified as very promising materials.
Differences in the physical, mechanical, and chemical properties of the materials that make up the thermoelectric couple, especially differences in the coefficients of thermal expansion (CTE), result in undesirable interfacial stresses that can lead to mechanical failure of the device. The problem is further complicated by the fact that the thermoelectric materials under consideration have large CTE values, are brittle, and cracks can propagate through them with minimal resistance.
The inherent challenge of bonding brittle, high-thermal-expansion thermoelectric materials to a hot shoe material that is thick enough to carry the requisite electrical current was overcome. A critical advantage over prior art is that this device was constructed using all diffusion bonds and a minimum number of assembly steps.
The fabrication process and the materials used are described in the following steps:
- Applying a thin refractory metal foil to both sides of lanthanum telluride. To fabricate the n-type leg of the advanced thermoelectric couple, the pre-synthesized lanthanum telluride coupon was diffusion bonded to the metal foil using a thin adhesion layer.
- Repeating a similar process for the 14-1-11 Zintl p-type leg of the advanced thermoelectric couple.
- Bonding thick CTE-matched metal plates on the metallized lanthanum telluride and Yb14MnSb11 to form the hot and cold sides of the thermoelectric couple.
The calculated conversion efficiency of such an advanced couple would be about 10.5 percent, about 35 percent better than heritage radioisotope thermoelectric technology that relies on SiGe alloys. In addition, unlike Si-Ge alloys, these materials can be combined with many other thermoelectric materials optimized for operation at lower temperatures to achieve conversion efficiency in excess of 15 percent (a factor of 2 increase over heritage technology).
This work was done by Vilupanur A. Ravi, Billy Chun-Yip Li, and Jean-Pierre Fleurial of Caltech and Kurt Star of UCLA for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) 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:
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Refer to NPO-46655, volume and number of this NASA Tech Briefs issue, and the page number.
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Fabrication of Lanthanum Telluride 14-1-11 Zintl High-Temperature Thermoelectric Couple
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Overview
The document discusses the fabrication of advanced high-temperature thermoelectric couples using Lanthanum Telluride and 14-1-11 Zintl materials, aimed at improving the efficiency of radioisotope thermoelectric generators (RTGs) for NASA's deep space missions. The motivation behind this research is to develop thermoelectric technology that can operate at high temperatures (up to 1275 K) and recover waste heat from energy-intensive industrial processes.
The document outlines the challenges associated with fabricating thermoelectric couples, which typically involve joining dissimilar materials through diffusion bonding and brazing. These materials often have different coefficients of thermal expansion (CTE), leading to interfacial stresses that can cause mechanical failure. The brittleness of the thermoelectric materials further complicates the fabrication process, as cracks can easily propagate through them.
To address these challenges, the researchers developed a streamlined fabrication process that minimizes the number of steps and thermal cycles. Key strategies included selecting appropriate hot shoe materials and metallic interconnects, using transition materials for bonding, and ensuring long-term stability of diffusion bonds compatible with both Lanthanum Telluride and Zintl materials.
The novelty of this work lies in the first demonstration of a high-temperature thermoelectric couple that utilizes these advanced materials, achieving a calculated conversion efficiency of approximately 10.5%. This represents a 35% improvement over traditional Si-Ge alloy-based RTGs. Additionally, the new materials can be combined with other thermoelectric materials optimized for lower temperatures, potentially achieving conversion efficiencies exceeding 15%, which is double that of heritage technologies.
The document emphasizes the critical advantage of using all diffusion bonds and a minimal number of fabrication steps, which enhances the reliability and performance of the thermoelectric couple. The successful integration of these materials and innovative bonding techniques marks a significant advancement in thermoelectric technology, with implications for both space exploration and industrial applications.
Overall, this research represents a promising step forward in the development of efficient thermoelectric systems, capable of harnessing high-grade heat sources and improving energy recovery processes in various fields.
