Development of a Bi 2 Te 3 -based thermoelectric generator with high-aspect ratio, free-standing legs
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A bismuth telluride alloy-based thermoelectric generator with high-aspect ratio, free-standing legs was fabricated. Such legs are desirable for efficient generator performance from low-grade heat sources but are difficult to assemble because they are fragile and difficult to handle and position. Plunge and wire electro-discharge machining (EDM) were used to produce 150 lm 300 lm legs, approximately 6-mm long, with high fidelity. Removal of recast material from EDM was necessary for good adhesion of metallization, but sputter etching was found to deteriorate the mechanical strength of the contacts. A wet chemical cleaning process was developed instead that resulted in good adhesion under test conditions. Au was preferred for designs where interconnects could be patterned directly on the module. Module figure of merit (ZT) was 0.72, close to the 0.85 value expected from bulk material property measurements. Impedance spectroscopy and the Harman technique were shown to significantly underestimate module ZT in the present test configuration. Shear and fatigue testing were performed on arrays of high-aspect ratio legs. Legs survived over 104 cycles of shear loading at 90% of the load to failure.
I. INTRODUCTION
Bismuth telluride alloys have the highest thermoelectric (TE) figure of merit (ZT) near-room temperature for a commercially available bulk material.1 It is the preferred choice for cooler and generator applications with mean temperatures up to 200 °C when lead telluride and the TAGS alloy (GeTe)0.85(AgSbTe2)0.15 become a more efficient option. Legs with long length-to-area aspect ratios (.104 m1) are advantageous for generator applications with low grade heat sources (,1 W/cm2), but are difficult to assemble because leg dimensions may be as fine as 80 lm.2 It is difficult to keep the legs parallel and properly registered when assembling such fine legs in a jig, as is often done for thermoelectric modules (TEMs) with lower aspect ratios. One technique for fabrication of such modules is to solder a wafer of material (P and N type) to a carrier plate containing the interconnects. A machining process such as wire saw, dicing saw, or electro-discharge machining (EDM) is used to define pillars of material.2,3 The two plates are then flipped such that the pillars interleave and a soldering operation forms the final module. Diffusion barriers plated on the Bi2Te3 can be used to make modules practical for high-temperature (.150 °C) use.4 However, the soldering operation to connect the top interconnect plate is challenging because of the poor mechanical strength of bismuth telluride. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.27
High-temperature compatible TE generator modules often use a filler material, such as epoxy or polyimide, between the legs.5,6 This allows for the module to be fabricated as a monolithic structure without the need for interconnect plates. The composite structure is mechanically robust due to the presence of the filler material, facilitating the construction o
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