Scientists have long attempted to convert the enormous amounts of waste heat generated by power plants, data centers, and cars into electricity via thermoelectric power generators (TEPGs). Scientists know how to convert waste heat into electricity. The problem is how to do so in an efficient manner using a material that justifies the cost of the conversion process.

Schematic of the high-efficiency thermoelectric module.

NASA engineers were looking to save fuel for spacecraft missions using a successful thermoelectric conversion process. As a result, NASA’s Langley Research Center in Hampton, VA, awarded a Phase II Small Business Technology Transfer (STTR) contract to Brimrose Technology Corp. to further develop a promising thermoelectric material and process the company has embarked upon. Brimrose estimates that the process will increase thermoelectric efficiency to 15-20%, compared with the current 5-8% level.

Lead telluride (PbTe) was selected because the compound is highly suitable for the temperature range of 400-1000K, where substantial amounts of waste heat are produced. Doped single crystalline PbTe structures were grown, and homogeneous crystals were achieved. The compound was developed into nanometer-sized powders via ball milling. The powders were sent to Penn State University’s Applied Research Laboratory, where they were consolidated using field assisted spark plasma sintering technology (FAST). Using FAST, the powder is simultaneously compacted and sintered in a graphite die under concurrent load and temperature, as well as high-density current.

The thermoelectric devices were created using the treated compound by developing p-type and n-type compounds joined together by a copper strip on a ceramic plate — a process known as thermocoupling. The assembly was based on a device fabrication technology called flip-chip assembly. Electrical contact technology having low electrical resistance that could withstand significantly elevated temperatures also was developed.

Researchers also engineered a novel heat sink design in which heat can be stored prior to its being turned into electricity, rather than being released into the atmosphere. The heat sink improved heat transport efficiency by up to 250-300%. The result was a TEPG component that researchers estimated was close to reaching theoretical limits of performance.

The second phase of the STTR will focus on designing a mold to allow for the production of a predetermined size of ntype and p-type legs with electrical contacts. Researchers hope to build a TEPG module to demonstrate the generation of 1kW of power.

Thermoelectric material and process
Brimrose Technology Corp.
Sparks, MD

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