Bulk Nanostructured thermoelectrics: Is ZT ready?
Jonathan Eller
Literature Seminar
November 10, 2011
Thermoelectrics (TEs) are a class of materials that are used as thermoelectric
power generators (TEGs), as well as in heating/cooling applications.1 Due to their solidstate nature, TE devices have no moving parts, are quiet, and last decades without need of
maintenance.2 These unique qualities have made them useful in such applications as
deep space exploration and powering terrestrial remote data stations.3,4 It has been
proposed that thermoelectrics have the potential to impact the environment in a number
of ways. One is by the recovery of waste heat, from both automobiles and industrial
processes. Also, the use of TE coolers in the place of conventional refrigeration would
reduce greenhouse gas emission.
TE operation can be explained by the Seebeck effect for power generation and the
Peltier effect for heating and cooling, two complimentary phenomena that involve charge
carriers migration in a thermal gradient. The primary drawback of thermoelectric devices
is their low efficiency. Efficiency is characterized for TEGs and Peltier devices alike by
the dimensionless figure of merit, ZT.5 Between the 1950’s and early 2000’s the highest
reported ZT value was approximately 1, equating to a TEG efficiency of about 10%.4 For
use as waste heat TEGs and large-scale TE coolers, ZT values of at least 2-3 are required.
The reason for low efficiencies of available TE materials involves coupling of variables
in the ZT equation. Among these, thermal conductivity should be reduced without
significant loss in electrical conductivity.
In 1993 Hicks and Dresselhaus theoretically predicted that certain quantum well
structures of Bi2Te3 could allow independent optimization of thermoelectric parameters. 6
Thin film materials were soon fabricated that showed ZT as high as 2.4.7 Although
theoretically intriguing, these thin film materials are low-throughput and difficult to
scale.8
Armed with proof of concept and a desire to make robust, scalable TE devices
with the benefits of nanostructure, many researchers have begun investigating the
possibility of using “bulk nanostructured” thermoelectric devices.
These are
nanostructured materials that can be fabricated using common high-throughput
metallurgical methods.9,11-12 The bulk nanostructured systems highlighted here are lead
tellurides and bismuth tellurides.
AgPbmSbTe2+m (SALTs) are a class of compounds that have proven thermoelectric
properties. The Kanatzidis group has made various SALTS, and toying with the values
of m, fabricated materials with a calculated ZT of 2.2 at 800 K. The details of the
structure-property relationship was unclear to them, though they suspected structural
modulation and observed “nanodots” were important.9
The same group has recently conducted a study on a PbTe/SrTe system where
SrTe precipitates may be forming within a PbTe matrix, showing a ZT of 1.7 at 800 K,
the highest yet reported for a p-type PbTe thermoelectric material.10 Epitaxial alignment
is thought to reduce charge scattering, while phonon scattering can occur over a wide
range due to various mechanisms.
Figure 1: Boundaries enriched in Bi are thought to lower thermal conductivity and raise
electrical conductivity in BixSb2-xTe3 compounds
Improved thermoelectric performance has also been demonstrated by bulk
nanostructured BixSb2-xTe3.11. Structural analysis12 revealed that the material contained a
hierarchy of grains and nano-precipitates ranging from 5 nm to 10μm, which allow
phonon scattering over a wide range of frequencies. Additionally, Bismuth-rich
boundary regions (see Figure 1) are thought to increase overall hole concentration, lower
thermal conductivity with little concomitant scattering of charge carriers. As a result, ZT
reaches 1.2 at room temperature and 1.4 at 100 oC and is as high as 0.8 at 250 oC,
showing a 40-200% improvement over the bulk.
The interdependence of parameters determining TE efficiency presents a
significant challenge in improving their performance. Researchers have discovered that
nanoscale features, such as epitaxially embedded nanodots and other structural
modulations allow selective scattering of phonons and one type of charge carrier over
another. Future investigations should include using cheaper, more abundant, less toxic
material as potential TE devices.
References
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2.
Shakouri, A. Recent Developments in Semiconductor Thermoelectric Physics
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