Patent Application
20100307551
The present invention relates to a thermoelectric couple and, more particularly, to a method for fabricating a Lanthanum Telluride-14-1-11 Zintl High-Temperature Thermoelectric Couple.
There are several applications in which thermoelectric couples are used at high temperatures. The fabrication of advanced high temperature thermoelectric couples requires the joining of several dissimilar materials, typically including a number of diffusion bonding and brazing steps, to achieve a device capable of operating at elevated temperatures across a large temperature differential (for example, 900 Kelvin). A thermoelectric couple typically comprises a heat collector/exchanger, metallic interconnects on both hot and cold sides, n-type and p-type conductivity thermoelectric elements, and cold side hardware to connect to the cold side heat rejection and provide electrical connections.
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. Therefore, fabrication of devices utilizing these materials requires the development of innovative processes.
Additionally, the development of more efficient thermoelectric couple technology capable of operating with high-grade heat sources (for example, 1,275 Kelvin) is key to improving the performance of radioisotope thermoelectric generators that support some of the National Aeronautics and Space Administration's (NASA) deep space exploration science missions. In addition, there has been increased interest in the potential of thermoelectric technology to recover waste heat from large scale energy intensive industrial processes and machinery.
Thus, a continuing need exists for a thermoelectric couple that is capable of operating with high-grade heat sources.
While considering the failure of others to make use of all of the above components in this technology space, the inventors realized that a Lanthanum Telluride-14-1-11 Zintl High Temperature Thermoelectric Couple would provide a conversion efficiency of about 10.5 percent, which is about 35 percent better than the thermoelectric coupling technology of the prior art and also could be accomplished fairly simply by benefiting from the similar mechanical properties of both materials (CTE in particular) and matching it with widely available high CTE, highly conductive materials. Thus, the present invention is directed to a method for fabricating such a thermoelectric couple.
The method includes an act of fabricating an n-type leg that, in a stacked configuration, includes a refractory metal foil (e.g., molybdenum foil) that is connected to each of the two sides of Lanthanum Telluride via an adhesion layer (e.g., titanium foil). A p-type leg is fabricated such that in a stacked configuration it includes a refractory metal foil (e.g., molybdenum foil) that is connected to each of the two sides of 14-1-11 Zintl. Further, a thick metal plate (e.g., Nickel) serves as an interconnect between the two legs on the hot side. Finally, separate thick metal plates (e.g., Nickel) are connected to each of the cold ends of the legs for connection with an external device on the cold side.
As can be appreciated by one skilled in the art, the present invention also comprises a thermoelectric couple that is formed according to the fabrication method described herein. Further, the thermoelectric couple is not limited to the particular fabrication method as it can be conceived by any suitable method that results in the end stacked configuration as illustrated and described.
The objects, features and advantages of the present invention will be apparent from the following detailed descriptions of the various aspects of the invention in conjunction with reference to the following drawings, where:
The present invention relates to a thermoelectric couple and, more particularly, to a method for fabricating a Lanthanum Telluride (La3−x Te4)-14-1-11 Zintl High-Temperature Thermoelectric Couple. The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
The reader's attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
Furthermore, any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6. In particular, the use of “step of” or “act of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.
Before describing the invention in detail, an introduction provides the reader with a general understanding of the present invention. Next, details of the present invention are provided to give an understanding of the specific aspects.
(1) Introduction
As noted above, the development of more efficient thermoelectric couple technology capable of operating with high-grade heat sources (for example, up to 1,275 Kelvin) is key to improving the performance of a variety of technologies.
The solution to the fabrication challenges described above was to develop a highly streamlined process that minimized the number of fabrication steps and thermal cycles, including:
Lanthanum telluride, based on La3−x Te4 and 14-1-11 Zintls, based on Yb14Mn1Sb11, have been identified as materials that fulfill this need. 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 Si-Ge alloys. In addition, unlike Si-Ge alloys, these materials have favorable thermoelectric and mechanical properties allowing them to 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 the prior art).
(2) Specific Details of the Invention
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 by the present invention. A critical advantage over prior art is that the present invention was constructed using all diffusion bonds and a minimum number of assembly steps. Generally speaking, the method includes fabricating an n-type leg, fabricating a p-type leg, and bonding a metal interconnect to each of the legs. The particular fabrication process and the materials used are described in further detail below.
(2.1) Fabricating the N-Type Leg
As shown in
It should be understood that the refractory metal foil 102 is formed of any suitably refractive material which leads to low electrical contact resistance, a non-limiting example of which includes a Molybdenum foil. Further, any suitable material can be used as the adhesion layer 106, a non-limiting example of which includes a Titanium foil.
As can be appreciated by one skilled in the art, release layers 108 during couple fabrication can be used to act as a barrier between the thermocouple and the hot press. In other words, when using a hot press, the components may have a tendency to stick to the hot press. Thus, the release layers 108 can be employed to allow a user to remove the components from the hot press without the assembly sticking to the hot press. Any suitable material can be used as a release layer 108, a non-limiting example of which includes Grafoil; however, straight Grafoil can create fabrication problems. Thus, a hard substrate layer 110 can be used to solve problems as presented by straight Grafoil. In this aspect, the hard substrate layer 110 is positioned between the release layer 108 and the rest of the assembly. Any suitable material can be used as a hard substrate layer 110, a non-limiting example of which includes Sapphire. It was found that Sapphire solves problems as presented by straight Grafoil and is operable as a desired hard substrate layer 110.
(2.2) Fabricating the P-Type Leg
As shown in
Again and as was the case above, release layers 108 (e.g., Grafoil) are used to act as a barrier between the hot press and the thermoelectric couple components, with a hard substrate layer 110 (e.g., Sapphire) being positioned between the release layer 108 and the rest of the assembly.
(2.3) Bonding a Metal Interconnect to the Legs
To operate as a thermoelectric couple, the device needs metallic interconnects.
Thus, as shown in
The hot shoe 300 and cold shoes 301 are formed of any suitably conductive material. Desirably, the hot shoe 300 and cold shoes 301 are formed of a metallic material with a coefficient of thermal expansion (CTE) that is matched (i.e., to the thermoelectric) to each of the legs, a non-limiting example of which includes being formed of nickel. A CTE matched electrical interconnect, for example, nickel, can be used to minimize interfacial stresses. The interconnect material (e.g., nickel) can also be used as a heat collector or as a thermal interface to the heat source. In addition to being CTE matched, the interconnect material needs to have high electrical and thermal conductivity as well to effectively operate as an interconnect and resulting shoe.
For further understanding,
Alternatively, the p-type leg 200 includes 14-1-11 Zintl 202 that is metallized with a refractory metal 204. Finally, upon the p-type leg 200 is bonded another cold shoe 301 (e.g., Nickel cold shoe) that allows the p-type leg 200 to be interconnected with an external device, etc.
In summary, the fabrication method of the present invention results in a thermoelectric couple to provide a conversion efficiency that is about 35 percent better than that of the prior art. As can be appreciated by one skilled in the art, such an increased conversion efficiency can be incorporated into a variety of technologies to increase performance of attached systems.
The present application is a non-provisional patent application, claiming the benefit of priority of U.S. Provisional Application No. 61/184,252, filed on Jun. 04, 2009, entitled, “Fabrication of Lanthanum Telluride-14-1-11 Zintl High Temperature Thermoelectric Couple.”
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
| Number | Date | Country | |
|---|---|---|---|
| 61184252 | Jun 2009 | US |
