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ad telluride (Tl-PbTe), has a zT of 1.5.
The new material is most effective between 450° and 950°F—a typical
temperature range for power systems such as automobile engines. The application
of TE material to automotive waste heat recovery systems is of interest to the
research team, and to one of the project funders, BSST Corporation. (
Earlier
post
.)
The dimensionless zT for thermoelectric materials is calculated by the
formula zT= T*(S2σ)/κ), where S is the thermoelectric power or
Seebeck coefficient of the TE material, σ and κ are the electrical and thermal
conductivities, respectively, and T is the absolute temperature.
Recent progress in increasing the efficiency of thermoelectric materials has
primarily involved decreasing κ by using nanomaterials to lower the thermal
conductivity by scattering phonons.
Quantum-dot superlattices have reported values of zT >2, and silicon
nanowires have such a reduced κ that zT approaches that of commercial materials.
Although this certainly provides the evidence that high-zT material can be
prepared, the results were obtained on thin films or nanowires that are
challenging for high-volume applications that normally rely on bulk materials.
Structural complexity on various length scales has successfully reduced κ in
bulk TE materials, also yielding zT >1.
Unfortunately, in bulk material at least, there is a lower limit to the
lattice thermal conductivity imposed by wave mechanics: The phonon mean free
path cannot become shorter than the interatomic distance. The minimum thermal
conductivity of PbTe is about 0.35 W/mK at 300 K, a value measured on
quantum-dot superlattices. Although lower values have been seen for interfacial
heat transfer, progress beyond this point in bulk materials must come from the
numerator [of the equation] and in particular the Seebeck coefficient; we
describe here a successful approach in this direction for bulk
materials.
—Heremans 2008
For the new material, the researchers left
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