February 15, 2011

Thermoelectric TAGS 85 improved by 25% to about ZT 1.05


Ames Laboratory researchers found that adding just one percent of the rare-earth elements cerium or ytterbium to a TAGS (group of thermoelectric materials is composed of tellurium, antimony, germanium and silver) material was sufficient to boost its performance by 25%.

Previous work at Harvard showed that Melt spinning TAGS 85 achieved ZT of 1.48. The Ames workd is a 25% improvement would be a ZT score from 0.85 to 1.05.

High-Performance (Ag x SbTe x/2+1.5)15(GeTe)85 Thermoelectric Materials Prepared by Melt Spinning

A new preparation process combining melt spinning and hot pressing has been developed for the (Ag x SbTe x/2+1.5)15(GeTe)85 (TAGS-85) system. Compared with samples prepared by the traditional air-quenching and hot-pressing method, electrical conductivity and thermal conductivity are lowered. The thermoelectric performance of the TAGS-85 samples varied with changing Ag content and reached the highest ZT of 1.48 when x was 0.8 for the melt-spun sample, compared with the maximum ZT of 1.36 for the air-quenched sample. The Seebeck coefficient of the melt-spun TAGS-85 alloys was improved, while both the electrical conductivity and thermal conductivity were decreased. The net result of this process is to effectively enlarge the temperature span of ZT greater than 1, which will benefit industrial application.

A ZT of 1.85 for 700K is about 15-20% heat to electricity conversion efficiency



Analysis of Ce- and Yb-Doped TAGS-85 Materials with Enhanced Thermoelectric Figure of Merit

Doping of TAGS-85 with 1 at% Ce or Yb forms a dilute magnetic semiconductor system with non-interacting localized magnetic moments that obey the Curie law. X-ray diffraction patterns and slight broadening in 125Te NMR, attributed to paramagnetic effects, suggest that Ce and Yb atoms are incorporated into the lattice. 125Te NMR spin-lattice relaxation and Hall effect show similar hole concentrations of ≈10^21 cm−3. At 700 K, the electric conductivity of the Ce- and Yb-doped samples is similar to that of neat TAGS-85, while the thermal conductivity and the Seebeck coefficient are larger by 6% and 16%, respectively. Possible mechanisms responsible for the observed increase in thermopower may include i) formation of resonance states near the Fermi level and ii) carrier scattering by lattice distortions and/or by paramagnetic ions. Due to the increase in the Seebeck coefficient up to 205 μV K−1, the thermoelectric power factor of Ce- and Yb-doped samples reaches 36 μW cm−1 K−2, which is larger than that measured for neat TAGS-85, 27 μW cm−1 K−2. The increase in the Seebeck coefficient overcomes the increase in the thermal conductivity, resulting in a total increase of the figure of merit by ≈25% at 700 K compared to that observed for neat TAGS-85.

Thermoelectric summary at caltech

Old ZT scores for different materials

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