Exploring an area overlooked by other scientists, physicists at the Florida State University-headquartered National High Magnetic Field Laboratory have discovered that a class of materials called “1-2-20s” have very promising thermoelectric properties, opening the floodgates for further research into these fascinating materials.
The study was published in Science Advances.
Thermoelectric devices can produce electricity if there is a temperature difference between the two ends. They can also do the opposite: use electricity to absorb or release heat. This property has many potential applications, from compressor-free refrigeration to power generation in space to recouping all the energy wasted by car engines (about 40 percent) that escapes through heat.
“It’s not free energy,” said MagLab physicist Ryan Baumbach, corresponding author on the paper, “but it’s the next best thing.”
Most materials have very little thermoelectric effect. That’s because the transfer of electricity across material and the transfer of heat usually go hand in hand. In general, nature wants to keep heat and electrical conductivity linked, but to have good thermoelectric performance, these two properties need to be decoupled.
About two years ago, Baumbach suggested that Kaya Wei, the MagLab’s Jack Crow postdoctoral fellow and a member of Baumbach’s research group, study a “1-2-20” material that seemed like a good candidate for thermoelectricity.
The specific material Baumbach proposed featured three basic ingredients in a “1-2-20” ratio: the element ytterbium; a transition metal (either cobalt, rhodium or iridium); and the element zinc. Baumbach had a hunch this compound had what it takes if manipulated properly in his lab, to thumb its nose at nature and unlink thermal conductivity from heat conductivity.
Using high-temperature furnaces in Baumbach’s lab, Wei synthesized the compound in crystal form and subjected the samples to a gauntlet of measurements. The results confirmed that, at low temperatures, the material was, in fact, a promising thermoelectric material.
Then it was time to start playing around with the variables to see what else they could discover.
“Different compositions promote quite different physical properties,” said Wei, the paper’s lead author.
Building a better thermoelectric
The researchers wanted to make a material as thermoelectrically optimized as they could, a property represented by a parameter called the thermoelectric figure of merit (or ZT). To do that, they needed to tweak their crystal to: 1. Maximize its electrical conductivity; 2. Minimize its heat conductivity; and 3. Develop a large voltage when a small temperature gradient is applied (i.e., when one end is slightly warmer than the other), a property measured by a value called the Seebeck coefficient.
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