Thermoelectrics are slabs of semiconductor with a strange and useful property: heating them on one side generates an electric voltage that can be used to drive a current and power devices. To obtain that voltage, thermoelectrics must be good electrical conductors but poor conductors of heat, which saps the effect. Unfortunately, because a material’s electrical and heat conductivity tend to go hand in hand, it has proven difficult to create materials that have high thermoelectric efficiency—a property scientists represent with the symbol ZT.
(H/T Talk Polywell)
Researchers at Northwestern and the University of Michigan, Ann Arbor, decided to take another look at tin selenide. The researchers synthesized a bullet-sized sample of SnSe and cleaved pieces of it along three different orientations of the crystal’s atomic planes, known as the a-, b-, and c-axes—a standard technique for analyzing the properties of materials. They then charted the thermal and electrical conductivity of each sample across a wide temperature range. The b-axis sample turned out to have a better-than-expected electrical conductivity and a very low thermal conductivity to boot. Those properties gave the material a ZT of 2.6, the best value ever measured. The key to the ultralow thermal conductivity, Kanatzidis says, appears to be the pleated arrangement of tin and selenium atoms in the material, which looks like an accordion. The pattern seems to help the atoms flex when hit by heat-transmitting vibrations called phonons, thus dampening SbSe’s ability to conduct heat.
In addition to marking a big step toward thermoelectrics with a ZT of 3, Heremans says, the new material offers lessons on how to get there. Most likely, he says, researchers will try to boost the semiconductor’s electrical conductivity by spiking it with trace amounts of “dopant” atoms, while preserving the key accordionlike atomic arrangement. If anyone succeeds in producing a high-ZT material, Heremans says, it could lead to new, cheaper hybrid car engines in which the internal combustion engine doesn’t power the car, but rather generates heat that thermoelectric devices convert into electricity to power an electric motor.
The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m−1 K−1 at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.
Thermoelectrics at ZT 3 or higher could replace current methods of refrigeration and automotive engines and turbines at power plants.
The actual range of replacement depends upon the price and volume of thermoelectric material that can be produced and the operating temperatures.