A joint South Korean and American research group has developed a scalable production method for a state of the art alloy for the use in solid state thermoelectric devices. This new alloy is nearly twice as efficient as existing materials and may lead to a new host of applications. Uses include refrigeration, consumer electronics, transportation as well as novel devices which have not been produced yet do to the inefficiencies of existing materials.
In the 1960’s, Peltier devices were primarily made from Bismuth-Telluride (Bi2Te3) or Antimony-Telluride (Sb2Te3) alloys and had a peak efficiency (zT, figure of merit) of 1.1, meaning the electricity going in was only slightly less than the heat coming out.
The joint team, including IBS researchers, used a process called liquid-flow assisted sintering which combined all three antimony, bismuth and telluride granules into one alloy (Bi0.5Sb1.5Te3). Additional melted tellurium was used as the liquid between the Bi0.5Sb1.5Te3 granules to help fuse them into a solid alloy, and excess Te is expelled in the process.
By creating the alloy this way, the joints between the fused grains, also known as the grain boundaries, took on a special property. Traditionally sintered Bi0.5Sb1.5Te3 have thick, coarse joints which have led to a decrease in both thermal and electrical conductivity. The new liquid-phase sintering creates grain boundaries which are organized and aligned in seams called dislocation arrays. These dislocation arrays greatly reduce their thermal conduction, leading to an enhancement of their thermoelectric conversion efficiency.
In tests, the efficiency (zT) reached 2.01 at 320 K within the range of 1.86 ±0.15 at 320 K (46.85° C) for 30 samples, nearly doubling the industry standard.
The widespread use of thermoelectric technology is constrained by a relatively low conversion efficiency of the bulk alloys, which is evaluated in terms of a dimensionless figure of merit (zT). The zT of bulk alloys can be improved by reducing lattice thermal conductivity through grain boundary and point-defect scattering, which target low- and high-frequency phonons. Dense dislocation arrays formed at low-energy grain boundaries by liquid-phase compaction in Bi0.5Sb1.5Te3 (bismuth antimony telluride) effectively scatter midfrequency phonons, leading to a substantially lower lattice thermal conductivity. Full-spectrum phonon scattering with minimal charge-carrier scattering dramatically improved the zT to 1.86 ± 0.15 at 320 kelvin (K). Further, a thermoelectric cooler confirmed the performance with a maximum temperature difference of 81 K, which is much higher than current commercial Peltier cooling devices.
Squeezing out efficient thermoelectrics
Thermoelectric materials hold the promise of converting waste heat into electricity. The challenge is to develop high-efficiency materials that are not too expensive. Kim et al. suggest a pathway for developing inexpensive thermoelectrics. They show a dramatic improvement of efficiency in bismuth telluride samples by quickly squeezing out excess liquid during compaction. This method introduces grain boundary dislocations in a way that avoids degrading electrical conductivity, which makes a better thermoelectric material. With the potential for scale-up and application to cheaper materials, this discovery presents an attractive path forward for thermoelectrics.
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.
SOURCES – alphagalileo, journal Science, DOE
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