Non-toxic thermoelectric energy harvester could help enable the IoT

Energy scavenging electronics has long used the thermoelectric Seebeck effect to convert temperature differences into electric voltage. A Seebeck device creates voltage when there is a different temperature on each side. One bugaboo with using thermoelectrics, however, is that most employ toxic materials. But in a development that could help enable a variety of internet-of-things applications, University of Utah engineering professor Ashutosh Tiwari has found that a cobalt-calcium compound laced with terbium can create an efficient, inexpensive and bio-friendly material that exhibits a thermoelectric effect.

Writing in the March 20 issue of Scientific Reports, Tiwari and engineering postdoctoral researcher Shrikant Saini say they use a base material made of calcium, cobalt, and oxygen. Single crystals of Ca₃Co₄O₉ can exhibit a big thermoelectric effect. But large batches of the stuff exhibit much weaker thermoelectric response than when in single-crystal form. The researchers were able to enhance the thermoelectric response in polycrystalline Ca₃Co₄O₉ by doping it with terbium ions.

Terbium is a relatively common rare-earth material. One use for it is as a dopant for materials used in solid-state devices. It is also a component of Terfenol-D, an alloy that exhibits a huge piezoelectric effect and which is used in actuators, naval sonar generators, and in sensors.

The work at Utah is seen as an advance because most thermoelectrics are cadmium-, telluride- or mercury-based materials which are toxic to humans. Tiwari says the new material is inexpensive to produce while being bio- and eco-friendly.

The researchers say the material they devised has a thermoelectric figure of merit ZT of about 0.74 at 800 K which is the highest value observed to date for such material. For comparison, researchers at Northwestern and Michigan State Universities a few years ago created a laboratory material exhibiting a ZT of 2.2. But it was based on telluride. The Mars rover Curiosity also carries lead-telluride thermoelectrics having a ZT of 1.

ZT is related to a material’s Seebeck coefficient, electrical conductivity, and thermal conductivity. Utah researchers say one of the major challenges faced today by the thermoelectric community is to practically find stable material whose ZT is close to 1. Materials with a ZT this large have limitations due to issues related to their chemical stability, researchers say. At high temperatures, these materials degrade/evaporate limiting their application to low working temperatures.

To give a high ZT, thermoelectrics must possess not only a high Seebeck coefficient but also high electrical conductivity and low thermal conductivity. A high electrical conductivity reduces the joule heating, and a low thermal conductivity is necessary to maintain a high temperature difference between the hot and cold ends of the thermoelectric device, Utah researchers say. These are the contradictory requirements and few materials satisfy them.

Layered cobalt oxides comprise one such material. Their crystal structure consists of two alternate CoO₂ layers sandwiching a block layer comprising of insulating rock salt structure.

Tiwari envisions the new non-toxic thermoelectric material being built into jewelry that uses body heat to power implantable medical devices such as blood-glucose monitors or heart monitors. He thinks it could be used to charge mobile devices through cooking pans, or in cars where it draws from the heat of the engine.

Finally, Tiwari says it could be used in developing countries where electricity is scarce and the only source of energy is the fire in stoves.

The Technology & Venture Commercialization Office of the University of Utah has filed a U.S. patent for the material, and the team will initially develop it for use in cars and for biosensors, Tiwari says.

In addition to Tiwari and Saini, co-authors on the paper include graduate students Haritha Sree Yaddanapudi, Kun Tian, Yinong Yin and David Magginetti.