Scientists find an 'ugly duckling' to convert waste heat to electricity

Researchers looking for better ways to convert waste heat into electricity have stumbled across a simple material that is smashing records for making that conversion efficiently.

This new material – a semiconductor made by blending tin and selenium – promises to convert heat to energy more efficiently than current technologies and with relatively accessible, inexpensive elements.

More than 90 percent of the energy produced to generate electricity, propel vehicles, or dry bricks requires a heat source, researchers say. Yet only 30 to 40 percent of the heat produced actually does the work. Most of the heat is wasted.

In principle, much of that heat could be recovered, using thermoelectric generators made from materials capable of turning a difference in temperature into electricity. 

Indeed, the "holy grail" in this field is to find materials that can act as efficient thermoelectric generators from room temperature up to perhaps 1,000 degrees F. or more – a span that would significantly broaden the range of sources from which they could draw heat.

Such a development could have as big an impact on energy use as another "holy grail" – the quest for materials that conduct electricity with no resistance at room temperatures, says Mercouri Kanatzidis, a solid-state chemist at Northwestern University in Evanston, Ill., and a lead member of the research team that is reporting the results in the current issue of the journal Nature. The nine-member team also included scientists from the University of Michigan in Ann Arbor.

 In its simplest form, a thermoelectric generator consists of two slabs of semiconductors with different electrical properties and joined at one end by a heated plate that can conduct electricity. At the other end, which must be kept colder, the semiconductor slabs are not connected. The temperature difference between the hot and cold ends of the semiconductors allows voltage to build up at the unconnected ends.

In effect, the unconnected ends act like the terminals of a battery. As long as the temperature difference is maintained, the generator continues to produce the voltage needed to allow current to flow through whatever is connected to the terminals.

The challenge: These semiconductors must be inefficient conductors of heat, to maintain the temperature difference, while at the same time being efficient electrical conductors. Typically, the less heat a semiconductor can conduct, the less efficient it is at conducting electricity, researchers say.

The tin selenide semiconductor must reach temperatures of just over 1,660 degrees F. in order to achieve its maximum efficiency. Even so, it can be used immediately, Dr. Kanatzidis says.

That is quite a feat for a material that researchers once dismissed as useless.

For years, commercial devices and the hunt for more-efficient thermoelectric materials have focused on recipes that have made semiconductors using lead-based alloys or relatively expensive rare-earth minerals. To increase thermoelectric efficiency, researchers have introduced additional crystals, themselves on the scale of billionths of an inch, into these semiconductors.

Two years ago, Kanatzidis and colleagues published a paper in Nature reporting what then was a record increase in efficiency. The team added nanocrystals of blended strontium and tellurium to lead telluride. Using a rating scheme that researchers have devised for quantifying a material's overall efficiency, this material achieved a rating of 2.2, with 2 widely seen as the number to beat.

As a sidelight to the team's main work, it opted to explore the properties of tin selenide.

That line of inquiry alone was a surprise, according to Joseph Heremans, a material scientist at Ohio State University in Columbus. The semiconductor was widely seen by researchers in the field as an unlikely candidate for a thermoelectric generator.

On the plus side, it has the thermal conductivity of a fence post. But at room temperature, its crystal structure showed little evidence that it would conduct electricity nearly as effectively as the materials that scientists – including Kanatzidis and his team – were working with.

Indeed, Dr. Heremans termed the material an ugly duckling in a companion article that accompanies the research results appearing in Nature.

"This was something that was so obvious that it was not going to be good that you don't even bother to work with it," Kanatzidis agrees.

But when the team, led by Northwestern University researcher Li-Dong Zhao, heated the material to 1,660 degrees F., the temperature altered its crystal structure in ways that allowed the material to retain its low thermal conductivity while substantially improving its ability to conduct electricity. Its efficiency rating at that temperature is 2.6. It maintains that rating over a span of about 200 to 300 degrees before its efficiency begins to fall off.

Now that scientists have shown the ugly duckling to be a swan, this discovery "opens our eyes to moving in other directions as well," he says – hunting for, then testing additional materials whose structure and behavior is similar to tin selenide's.