An all-solid-state diffusion bonding process that exploits the eutectoid reaction between molybdenum and titanium has been developed for use in fabricating thermoelectric devices based on skutterudite compounds. In essence, the process is one of heating a flat piece of pure titanium in contact with a flat piece of pure molybdenum to a temperature of about 700 °C while pushing the pieces together with a slight pressure [a few psi (of the order of 10 kPa)]. The process exploits the energy of mixing of these two metals to form a strong bond between them. These two metals were selected partly because the bonds formed between them are free of brittle intermetallic phases and are mechanically and chemically stable at high temperatures.
The process is a solution of the problem of bonding hot-side metallic interconnections (denoted "hot shoes" in thermoelectric jargon) to titanium-terminated skutterudite n and p legs during the course of fabrication of a unicouple, which is the basic unit cell of a thermoelectric device (see figure). The hot-side operating temperature required for a skutterudite thermoelectric device is 700 °C. This temperature precludes the use of brazing to attach the hot shoe; because brazing compounds melt at lower temperatures, the hot shoe would become detached during operation. Moreover, the decomposition temperature of one of the skutterudite compounds is 762 °C; this places an upper limit on the temperature used in bonding the hot shoe.
Molybdenum was selected as the interconnection metal because the eutectoid reaction between it and the titanium at the ends of the p and n legs has characteristics that are well suited for this application. In addition to being suitable for use in the present bonding process, molybdenum has high electrical and thermal conductivity and excellent thermal stability — characteristics that are desired for hot shoes of thermoelectric devices.
The process takes advantage of the chemical potential energy of mixing between molybdenum and titanium. These metals have a strong affinity for each other. They are almost completely soluble in each other and remain in the solid state at temperatures above the eutectoid temperature of 695 °C. As a result, bonds formed by interdiffusion of molybdenum and titanium are mechanically stable at and well above the original bonding temperature of about 700 °C. Inasmuch as the bonds are made at approximately the operating temperature, thermomechanical stresses associated with differences in thermal expansion are minimized.
This work was done by Jeffrey Sakamoto, Adam Kisor, Thierry Caillat, Liana Lara, Vilupanur Ravi, Samad Firdosy, and Jean-Pierre Fleurial of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers 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-40883, volume and number of this NASA Tech Briefs issue, and the page number
This Brief includes a Technical Support Package (TSP).
Mo/Ti Diffusion Bonding for Making Thermoelectric Devices
(reference NPO-40883) 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 (JPL) detailing the process of Mo/Ti diffusion bonding for the fabrication of thermoelectric devices. It focuses on the bonding of Molybdenum interconnects to thermoelectric legs, particularly under high-temperature conditions exceeding 700°C.
The initial experiments described involve the Titanium/Molybdenum Eutectoid Reaction (TMER), which is critical for achieving strong and reliable bonds in thermoelectric applications. The bonding process utilizes a hot press capable of applying 100 MPa of pressure and maintaining high temperatures in an inert argon atmosphere to prevent oxidation.
To prepare the Molybdenum plates for bonding, they are subjected to an annealing process at 1200°C for one hour to relieve residual stresses that could lead to embrittlement. The plates are then roughened with 600 grit sandpaper and cleaned with acetone to ensure a proper bond. The bonding setup involves placing the Molybdenum plate in a specially designed graphite die, where Titanium powder is loaded on top. A plunger is used to apply pressure and heat according to a specific Time-Temperature-Pressure profile.
The document also includes figures illustrating the bonding apparatus and micrographs of the bonded coupons. For instance, a scanning electron micrograph shows a Molybdenum-Titanium coupon bonded at 1 MPa and 740°C for 100 minutes under a vacuum of 10^-6 torr, highlighting the effectiveness of the bonding process.
Additionally, the document discusses the growth of the TMSS (Titanium-Molybdenum-Silicon-Sulfur) interphase over time at 700°C, noting that the growth rate slows with time, which is significant for understanding the long-term stability of the bonds.
Overall, this Technical Support Package serves as a comprehensive guide for researchers and engineers involved in the development of thermoelectric devices, providing insights into the materials, processes, and conditions necessary for successful bonding. It emphasizes the importance of precision and control in high-temperature applications, contributing to advancements in aerospace technology and its potential commercial applications. Further assistance and information can be obtained from JPL's Innovative Technology Assets Management.
