1. Field of the Invention
This invention relates to thermoelectric devices. Particularly, this invention relates to interfacing materials for thermoelectric materials in thermoelectric power generation devices, particularly those using Zintl, e.g. Yb14MnSb11 (YMS) and skutterudite.
2. Description of the Related Art
Thermoelectric materials exhibit the property of producing an electric voltage from an applied temperature differential across the material, the so-called thermoelectric effect or Seebeck effect. Accordingly, such materials may be used in thermoelectric devices to generate electrical power from a temperature differential. Such thermoelectric generators have been used to convert heat directly to electrical power for applications including isolated facilities or space applications. Depending upon the application, the applied heat may be naturally available or generated, e.g. by burning fuel or from a decaying radioisotope.
As mentioned, thermoelectric materials are known to provide a means for directly converting heat into electrical energy in a fully solid state device. Due to the nature of thermoelectric materials, power generating devices require a pairing of two different materials, typically comprised of highly doped narrow band gap semiconductors (one with an excess of n-type charge carriers, the other with an excess of p-type carriers) connected in a junction.
Prior art thermoelectric devices have featured materials such as silicon germanium, lead telluride, bismuth telluride or other related materials. To achieve greater device efficiency and greater specific power, however, new thermoelectric materials, are required in more complex combinations. One suitable material is found in the class of Zintl materials, particularly the compound p-type semiconductor Yb14MnSb11 (YMS), which has been demonstrated to have one of the highest zT values at 1000° C., a typical operational temperature of space-based radioisotope thermoelectric generators (RTGs).
For example, some thermoelectric power generation for deep space applications have employed SiGe thermoelectric materials generating electric power using a decaying radioisotope, e.g. plutonium 238, as a heat source, in an RTG. The fuel source and solid state nature of the devices afford exceptional service life and reliability, paramount considerations in space applications which offset the relatively low efficiency of such devices. Many working RTG devices for space applications have been developed and successfully employed. See e.g. Winter et al., “The Design of a Nuclear Power Supply with a 50 Year Life Expectancy: The JPL Voyager's SiGe MHW RTG,” IEEE AES Systems Magazine, April 2000, pp. 5-12; and U.S. Pat. No. 3,822,152, issued Jul. 2, 1974 to Kot, which are incorporated by reference herein.
Recent focus on renewable energy and increased energy efficiency has resulted in increased interest in thermoelectric materials and devices for applications such as automotive and industrial waste heat recovery. Zintl materials in particular have been studied for thermoelectric applications. A particular Zintl compound, Yb14MnSb11, has shown exceptional promise for thermoelectric power generation applications. See e.g. Brown et al., “Yb14MnSb11: New High Efficiency Thermoelectric Materials for Power Generation,” Chem. Mater., 18, 2006, 1873-1877, which is incorporated by reference herein. However, defining the properties of a particular material are only a first step in the development of a practical thermoelectric power generation device using that material.
SiGe has been well studied as a thermoelectric material as a result of previous RTG development. See e.g., Rowe, “Recent Advanced in Silicon-Germanium Alloy Technology and an Assessment of the Problems of Building the Modules for a Radioisotope Thermoelectric Generator,” Journal of Power Sources, 19 (1987), pp. 247-259; and “Silicon Germanium Thermoelectric Materials and Module Development Program,” ALO (2510)-T1, AEC Research and Development Rep, Cat. UC33, TID 4500, which are incorporated by reference herein. However, although the general configurations of previously developed SiGe thermoelectric power generation devices may be applicable, there are differences in the physical properties of Zintl materials and SiGe that demand new solutions in the development of a practical thermoelectric power generation devices using Zintl materials; the solutions for SiGe thermoelectric materials cannot be readily applied to Zintl thermoelectric materials.
To achieve high thermal-to-electric energy conversion efficiency (“Carnot” efficiency) operating across large temperature differentials is required. When using solid state thermoelectric devices for power generation using high grade heat sources, best conversion efficiencies are achieved by combining the highest performance materials in their respective optimum operating temperature ranges into multi-stage cascaded or segmented device architectures. Such segmented architectures have been used primarily for long life thermoelectric generators on board space science and exploration missions, operating across temperature differentials in excess of 700 K with maximum hot side operating temperatures of up to 1273 K.
Next generation high temperature thermoelectric power generating devices will employ segmented architectures and will have to reliably withstand thermally induced mechanical stresses produced during component fabrication, device assembly and operation. Thermoelectric materials have typically poor mechanical strength, can exhibit brittle behavior, and possess a wide range of coefficient of thermal expansion (CTE) values. As a result, the direct bonding at elevated temperatures of these materials to each other to produce segmented leg components is difficult and often results in localized microcracking at interfaces and mechanical failure due to the stresses that arise from CTE mismatch between the various materials. Even in the absence of full mechanical failure, the degraded interfaces can lead to increased electrical and thermal resistances which adversely impact conversion efficiency and power output.
For example, cracking can occur in a segmented p-type thermoelectric leg made of high CTE (e.g. approximately 18 ppm) Yb14MnSb11 Zintl material (high temperature segment) bonded to an intermediate CTE (e.g. approximately 13 ppm) CefFe4-xCoxSb12 skutterudite material (low temperature segment) using a direct brazing method on pre-metallized thermoelectric material segments. In one test a large crack has been observed post fabrication in the Zintl segment, presumed to have occurred as a consequence of the bonding process.
In view of the foregoing, there is a need in the art for apparatuses and methods for mechanically compliant interface materials, particularly to accommodate induced stresses from very high temperature gradients, e.g. above 700 K. There is a need for such apparatuses and methods to have high thermal and electrical conductivity. In addition, there is a particular need for such apparatuses and method to operate with Zintl thermoelectric materials such as Yb14MnSb11. There is also a need for such apparatuses and methods in Zintl-based thermoelectric devices operating at high temperatures, e.g. around or above 1,000 K and up to 1273 K. There is a need for such apparatuses and methods to operate for such thermoelectric devices in space applications. These and other needs are met by embodiments of the present invention as detailed hereafter.