Content uploaded by T.M. Tritt
Author content
All content in this area was uploaded by T.M. Tritt on Jan 31, 2017
Content may be subject to copyright.
Content uploaded by T.M. Tritt
Author content
All content in this area was uploaded by T.M. Tritt
Content may be subject to copyright.
188 MRS BULLETIN • VOLUME 31 • MARCH 2006
Introduction
Thermoelectric Phenomena:
Background and Applications
Over the past decade, there has been
heightened interest in the field of thermo-
electrics, driven by the need for more effi-
cient materials for electronic refrigeration
and power generation.1,2 Some of the re-
search efforts focus on minimizing the lat-
tice thermal conductivity, while other
efforts focus on materials that exhibit large
power factors. Proposed industrial and
military applications of thermoelectric
(TE) materials are generating increased
activity in this field by demanding higher-
performance high-temperature TE mate-
rials than those that are currently in use.
The demand for alternative energy tech-
nologies to reduce our dependence on
fossil fuels leads to important regimes
of research, including that of high-
temperature energy harvesting via the
direct recovery of waste heat and its con-
version into useful electrical energy. Thus,
the development of higher-performance
TE materials is becoming ever more im-
portant. Power-generation applications
are currently being investigated by the au-
tomotive industry as a means to develop
electrical power from waste engine heat
from the radiator and exhaust systems for
use in next-generation vehicles. In addi-
tion, TE refrigeration applications include
seat coolers for comfort and electronic
component cooling. Of course, the deep-
space applications of NASA’s Voyager
and Cassini missions using radioisotope
thermoelectric generators (RTGs) are well
established (see Reference 3 and the article
by Yang and Caillat in this issue). A key
factor in developing these technologies is
the development of higher-performance
TE materials, either completely new mate-
rials or through more ingenious materials
engineering of existing materials.
Thermoelectric refrigeration is an envi-
ronmentally “green” method of small-
scale, localized cooling in computers,
infrared detectors, electronics, and opto-
electronics as well as many other applica-
tions. However, most of the electronics
and optoelectronics technologies typically
require only small-scale or localized spot
cooling of small components that do not
impose a large heat load. If significant eco-
nomical cooling can be achieved, the re-
sulting “cold computing” could produce
speed gains of 30–200% in some computer
processors based on complementary
metal oxide semiconductor (CMOS) tech-
nology. Cooling of the processors is per-
ceived by many to be the fundamental
limit to electronic system performance.4
Thus, the potential payoff for the develop-
ment of low-temperature TE refrigeration
devices is great, and the requirement for
compounds with properties optimized
over wide temperature ranges has led to a
much expanded interest in new TE mate-
rials. Recent utilization of Peltier coolers
(see next section) for the refrigeration of
biological specimens and samples is an
emerging TE application.
The development and potential of bulk
materials for TE applications is an active
area of research. High-temperature bulk
materials such as skutterudites, clathrates,
half-Heusler alloys, and complex chalco-
genides are being investigated (see the ar-
ticle by Nolas et al. in this issue). These
materials possess complex crystal struc-
tures and exhibit properties that are favor-
able for potential thermoelectric materials.
For example, skutterudites and clathrates
are cage-like materials that have voids in
which “rattler” atoms are inserted to sig-
nificantly lower the thermal conductivity
due to the rattling atoms’ ability to scatter
phonons. Recently, ceramic oxide mate-
rials have also shown potential as high-
Thermoelectric
Materials,
Phenomena, and
Applications: A
Bird’s Eye View
Terry M.Tritt and M.A. Subramanian,
Guest Editors
Abstract
High-efficiency thermoelectric (TE) materials are important for power-generation
devices that are designed to convert waste heat into electrical energy.They can also be
used in solid-state refrigeration devices.The conversion of waste heat into electrical
energy may play an important role in our current challenge to develop alternative energy
technologies to reduce our dependence on fossil fuels and reduce greenhouse gas
emissions.
An overview of various TE phenomena and materials is provided in this issue of MRS
Bulletin.Several of the current applications and key parameters are defined and
discussed. Novel applications of TE materials include biothermal batteries to power
heart pacemakers, enhanced performance of optoelectronics coupled with solid-state TE
cooling, and power generation for deep-space probes via radioisotope TE generators. A
number of different systems of potential TE materials are currently under investigation by
various research groups around the world, and many of these materials are reviewed in
the articles in this issue. These range from thin-film superlattice materials to large single-
crystal or polycrystalline bulk materials, and from semiconductors and semimetals to
ceramic oxides.The phonon-glass/electron-crystal approach to new TE materials is
presented, along with the role of solid-state crystal chemistry.Research criteria for
developing new materials are highlighted.
Keywords: energy, thermal conductivity, thermoelectricity.
www.mrs.org/bulletin
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
MRS BULLETIN • VOLUME 31 • MARCH 2006 189
temperature TE materials (see Koumoto
et al. in this issue). The potential of
nanomaterials and their role in TE re-
search are an emerging area of interest
(see Rao et al. in this issue). Bulk material
applications are demanding new break-
throughs in both materials and device en-
gineering (see Yang and Caillat in this
issue). The role of thin-film properties, ap-
plications, and recent results is also very
important (see Böttner et al. in this issue).
Amore complete overview of state-of-the-
art materials, a theoretical and experimen-
tal discussion of the basic principles, and
an overview of some of the recent devel-
opments and materials are given in texts
by Tritt2and Nolas.5
Seebeck and Peltier Effects
Adiscussion of thermoelectric effects
and devices should start with one of the
most fundamental TE phenomena, the
Seebeck effect, or thermopower.6–8 In the
early 1800s, Seebeck observed that when
two dissimilar materials are joined to-
gether and the junctions are held at differ-
ent temperatures (Tand T∆T), a voltage
difference (∆V) develops that is propor-
tional to the temperature difference (∆T).6
The ratio of the voltage developed to the
temperature gradient (∆V/∆T) is related
to an intrinsic property of the materials
called the Seebeck coefficient, α. The
Seebeck coefficient is very low for metals
(only a few V/K) and much larger
for semiconductors (typically a few hun-
dred V/K).9Arelated effect—the Peltier
effect—was discovered a few years later
by Peltier,10 who observed that when an
electrical current is passed through the
junction of two dissimilar materials, heat
is either absorbed or rejected at the junc-
tion, depending on the direction of the
current. This effect is due largely to the
difference in Fermi energies of the two
materials. These two effects are related to
each other, as shown in the definition of
the Peltier coefficient, Π αT. The rate at
which the Peltier heat is liberated or re-
jected at the junction (QP) is given by QP
αIT, where Iis the current through the
junction and Tis the temperature in
kelvin. There are also a number of ther-
momagnetic effects such as the Hall,
Ettingshausen, and Nernst effects that are
beyond the scope of this article. The
reader is referred to the text by Nolas
et al.5for a discussion of these effects.
Definition and Description of the
Figure of Merit and Thermoelectric
Performance
The potential of a material for TE appli-
cations is determined in large part by a
measure of the material’s figure of merit,
ZT:*
(1)
where αis the Seebeck coefficient, σis the
electrical conductivity, ρ is the electrical
resistivity, and κis the total thermal con-
ductivity (κ κLκE, the lattice and elec-
tronic contributions, respectively). The
power factor, α2σT(or α2
Τ
/ρ), is typically
optimized in narrow-gap semiconducting
materials as a function of carrier concen-
tration (typically 1019 carriers/cm3),
through doping, to give the largest ZT.9
High-mobility carriers are most desirable,
in order to have the highest electrical con-
ductivity for a given carrier concentration.
The ZT for a single material is somewhat
meaningless, since an array of TE couples
is utilized in a device or module.
There are two materials in the TE cou-
ple, which is shown in Figure 1, an n-type
and a p-type. Ignoring parasitic contribu-
tions that reduce the device performance,
such as contact resistance and radiation ef-
fects, the resulting figure of merit for the
couple (based solely on the TE materials)
is given by
(2)
The coefficient of performance φ(refriger-
ation mode) and the efficiency η(power-
generation mode) of the TE couple are
directly related to the figure of merit
shown in Equation 3 for the efficiency. The
efficiency (η) of the TE couple is given by
the power input to the load (W) over the
net heat flow rate (QH), where QHis posi-
tive for heat flow from the source to the
sink:
(3)
where THis the hot-side temperature, TCis
the cold-side temperature, and TMis the
average temperature. Thus, one can see
THTC
TH1ZTM1/2 1
1ZTM1/2 TCTH,
η
W
QH
ZT
αpαn2T
ρnκn1/2 ρpκp1/2.
ZT α2σT
κα2T
ρκ ,
that ηis proportional to (1 ZTM)1/2 and
that the efficiency would approach the
Carnot efficiency if ZT were to approach
infinity.
Thermoelectric Modules: Devices
The Peltier effect is the basis for many
modern-day TE refrigeration devices, and
the Seebeck effect is the basis for TE
power-generation devices. The versatility
of TE materials is illustrated in Figure 1,
which shows a TE couple composed of
an n-type (negative thermopower and
electron carriers) and a p-type (positive
thermopower and hole carriers) semicon-
ductor material connected through
metallic electrical contact pads. Both re-
frigeration and power generation may be
accomplished using the same module, as
shown in Figure 1. ATE module or device
is built up of an array of these couples,
arranged electrically in series and ther-
mally in parallel. Thermoelectric energy
conversion utilizes the Seebeck effect,
wherein a temperature gradient is im-
posed across the device, resulting in a
voltage that can be used to drive a current
through a load resistance or device. This is
the direct conversion of heat into electric-
ity. Conversely, the Peltier heat generated
when an electric current is passed through
a TE material provides a temperature gra-
dient, with heat being absorbed on the
cold side, transferred through (or pumped
by) the TE materials, and rejected at the
sink, thus providing a refrigeration capa-
bility. The advantages of TE solid-state en-
ergy conversion are compactness,
quietness (no moving parts), and localized
heating or cooling. In addition, energy in
the form of waste heat (0% efficiency) that
would normally be lost may be converted
into useful electrical energy (7–8% effi-
ciency) using a TE power-generation de-
vice.
The best TE materials currently used in
devices have ZT 1. This value has been
a practical upper limit for more than 30
years, yet there are no theoretical or ther-
modynamic reasons for ZT 1 as an
upper barrier. As seen from Equation 1,
ZT may be increased by decreasing κLor
by increasing either αor σ. However, σis
tied to the electronic thermal conductivity,
κE, through the Wiedemann–Franz rela-
tionship, and the ratio is essentially con-
stant at a given temperature.
Some of the goals of current research ef-
forts are to find new materials that either
raise the current efficiency of TE devices
(i.e., increase ZT) or have the capability of
operating in new and broader tempera-
ture regimes, especially at lower tempera-
tures (T250 K) and higher temperatures
(T400 K). Over the past 30 years,
*The expressions for figure of merit, Zand ZT,
are used interchangeably in the field of thermo-
electrics. Zis the figure of merit with units of
1/K (1/T), and ZT is the dimensionless (unit-
less) figure of merit. Both must specify the tem-
perature at which the quoted value was
obtained.
190 MRS BULLETIN • VOLUME 31 • MARCH 2006
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
alloys based on the Bi2Te3system
[(Bi1–xSbx)2(Te1–xSex)3] and the Si1–yGeysys-
tem have been extensively studied and
optimized for their use as TE materials to
perform in a variety of solid-state TE re-
frigeration and power-generation applica-
tions.11,12 These traditional TE materials
have undergone extensive investigation,
and there appears to be little room for fu-
ture improvement in the common bulk
structures. However, recent results on
nanostructures of traditional TE materials
have shown a promising new direction for
these materials. In addition, entirely new
classes of compounds will have to be in-
vestigated. Figure 2 shows ZT as a func-
tion of temperature for the Bi2Te3 and
Si1–yGeymaterials as well as many of the
more recent bulk materials that have been
developed over the last decade. The ZT of
more exotic structures such as superlat-
tices and quantum dot structures are not
shown here but are addressed in the ar-
ticle by Böttner et al. in this issue.
Transport Properties
The thermopower, or Seebeck coeffi-
cient, can be thought of as the heat per car-
rier over temperature or, more simply, the
entropy per carrier, αC/q, where Cis
the specific heat and qis the charge of the
carrier.7For the case of a classical gas, each
particle has an energy of 3/2(kBT), where
kBis the Boltzmann constant. The ther-
mopower is thus approximately kB/e,
where eis the charge of the electron. For
metals, the heat per carrier is essentially a
product of the electronic specific heat and
the temperature divided by the number of
carriers (N), that is, αCelT/N, and then
α is approximately
(4)
where EFis the Fermi energy (related to
the chemical potential of the material).
αCel
qkB
ekBT
EF
,
The Fermi energy is basically the energy
such that at T0, all the states above
this energy are vacant and all the
states below are occupied. The quantity
kB/e87 V/K is a constant that repre-
sents the thermopower of a classical elec-
tron gas. Metals have thermopower values
of much less than 87 V/K (on the order
of 1–10 V/K) and decrease with decreas-
ing temperature, that is, EF kBT).
In a semiconductor, a charged particle
must first be excited across an energy gap
Eg. In this case, the thermopower is ap-
proximated by
(5)
Thus, the thermopower is larger than the
characteristic value of 87 V/K and in-
creases with decreasing temperature.
Semiconductors can exhibit either electron
conduction (negative thermopower) or
hole conduction (positive thermopower).
The thermopower for different carrier
types is given by a weighted average
of their electrical conductivity values
(σnand σp):
(6)
It is necessary to dope the semiconductors
with either donor or acceptor states to
ααnσnαpσp
σnσp.
αCel
qkB
eEg
kBT.
Figure 1. Diagram of a Peltier thermoelectric couple made of an n-type and a p-type
thermoelectric material. Refrigeration or power-generation modes are possible, depending
on the configuration. Iis current.
Figure 2. Figure of merit ZT shown as a function of temperature for several bulk
thermoelectric materials.
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
MRS BULLETIN • VOLUME 31 • MARCH 2006 191
allow extrinsic conduction of the appro-
priate carrier type, electrons or holes, re-
spectively. It is apparent that the total
thermopower will be lower than that of ei-
ther of the individual contributions, unless
the direct bandgap is large enough—
typically on the order of 10(kBT)—to
effectively minimize minority carrier con-
tributions. Typical thermopower values
required for good TE performance are on
the order of 150–250 V/K or greater.
For high-temperature applications, it is
important to minimize the contribution of
minority carriers in order to maintain a
high thermopower. In addition, the ther-
mal stability of the materials is an essential
aspect. Atomic diffusion within the mate-
rials and interdiffusion of contacts can se-
riously deteriorate the properties of a
given material at high temperatures. As-
pects of this are discussed elsewhere.2,5
These materials and devices are expected
to operate at elevated temperatures for
long periods of time without deterioration
of their properties or performance. The ef-
fects of diffusion and thermal annealing
are important to thoroughly investigate
and understand in any set of potential TE
materials over the expected operating
temperature range of the materials.
The description of electrical conductiv-
ity for metals and semiconductors has
been covered extensively in many texts on
solid-state physics, and the reader is re-
ferred there.13 There are a significant num-
ber of carriers and states available for
conduction in metals, typically n1022
carriers/cm3. The electrical conductivity is
then very large for metals, on the order of
106(Ωcm)–1. Again, for semiconductors,
the carriers must be thermally excited
across a gap for conduction to occur, as
shown from the activated behavior that is
derived for the temperature-dependence
of the electrical conductivity [σ
σ0exp(–Eg/kBT)]. There are two primary
ways to achieve a high conductivity in a
semiconductor, either by having a very
small gap to excite across (Eg/kBT) or by
having very high-mobility carriers, as dis-
cussed later. Typical values of the electri-
cal conductivity for a good TE material are
on the order of about 103(Ωcm)–1.
The thermal conductivity κis related to
the transfer of heat through a material, ei-
ther by the electrons or by quantized vi-
brations of the lattice, called phonons,
such that κ κLκE, as mentioned ear-
lier. The electrical conductivity and the
thermal conductivity are interrelated, in
that σ is tied to κEthrough the Wiedemann–
Franz relationship: κEL0σT, where the
Lorentz number L02.45 ×10–8 W Ω/K2
[or L0 2.45 ×10–8(V2/K2)]. The lattice
thermal conductivity is discussed later
in this article, in the section on minimum
thermal conductivity. Typical thermal
conductivity values for a good TE mate-
rial are κ 2 W m–1 K–1, and typically,
κLκE.
Investigating New Thermoelectric
Materials
The “Phonon-Glass/Electron-
Crystal” Approach
Slack has described the chemical char-
acteristics of candidates for a good TE ma-
terial.14 He states that the candidates
should be narrow-bandgap semiconduc-
tors with high-mobility carriers. Mahan
has also described the characteristics of
good TE materials,15,16 agreeing with Slack
that the candidate material is typically a
narrow-bandgap semiconductor [Eg
10(kBT), or 0.25 eV at 300 K]. Also, the
mobility of the carriers must remain high
(2000 cm2/V s), while the lattice ther-
mal conductivity must be minimized. In
semiconductors, the Seebeck coefficient
and electrical conductivity (both in the nu-
merator of ZT) are strong functions of the
doping level and chemical composition.
These quantities must therefore be opti-
mized for good TE performance. The ther-
mal conductivity of complex materials can
often be modified by chemical substitu-
tions, and the lattice thermal conductivity
needs to be as low as possible. Under-
standing these various effects and select-
ing optimization strategies can be an
exceedingly difficult problem, because in
complex materials there are often many
possible degrees of freedom. Slack sug-
gested that the best TE material would be-
have as a “phonon-glass/electron-crystal”
(PGEC); that is, it would have the electri-
cal properties of a crystalline material and
the thermal properties of an amorphous
or glass-like material. Materials engineer-
ing and the crystal chemistry approach to
good TE materials are discussed later.
Minimum Thermal Conductivity
In many areas of research related to new
TE materials, attempts are being made to
reduce the lattice part of the thermal con-
ductivity to essentially its minimum
value, that is, where a minimum lattice
thermal conductivity is achieved (when
all the phonons have a mean free path es-
sentially equal to the interatomic spacing
of the constituent atoms). This is being at-
tempted by scattering phonons in differ-
ent frequency ranges using a variety of
methods such as mass fluctuation scatter-
ing (a mixed crystal, in ternary and qua-
ternary compounds), “rattling” scattering,
grain-boundary scattering (due to the size
of the grains), and interface scattering in
thin films or multilayer systems.
The lattice thermal conductivity is given
by κL(1/3)(vsCLph), where vsis the ve-
locity of sound, Cis the heat capacity, and
Lph is the mean free path of the phonons.
At high temperatures (T300 K), the
sound velocity and the heat capacity are
essentially temperature-independent in
typical materials. Therefore, the magnitude
and the temperature-dependence of κLare
basically determined by the mean free path
of the phonons. Slack defined the minimum
thermal conductivity (κmin) as the thermal
conductivity when the mean free path is
essentially limited by the interatomic dis-
tance between the atoms within the crys-
tal.17 Typical analysis of κmin results in
values of κmin 0.25–0.5 W m–1 K–1.14,17
Minimum Thermopower
There are certain practical limits for
each of the parameters used to calculate
ZT. These practical limits must be possible
in order to achieve a material viable for
thermoelectric applications. For example,
in Bi2Te3, in order to achieve a ZT 1 at T
320 K, σ1 mΩcm, α225 V/K,
and κ1.5 W m–1 K–1. We have already
discussed the ZT “barrier,” which in effect
is given by minimizing the thermal con-
ductivity. It is practical to investigate ma-
terials where the electronic and lattice
terms are comparable, on the order of
0.75–1 W m–1 K–1. Let us look at the hypo-
thetical situation of a material in which
the lattice thermal conductivity is zero
(κL0). We will also assume the scatter-
ing in this system is elastic and that the
Wiedemann–Franz relationship, slightly
rearranged [κE/σ L0T], is well behaved
in this material. Then we can rewrite
Equation 1 as
ZT α2T/ρκEα2/L0.(7)
Therefore, for a material to be a viable TE
material, it must possess a minimum ther-
mopower that is directly related to the
value of ZT and L0. Given this description,
in order to achieve a certain value of ZT,
the material would require that α(L0)0.5
157 V/K for ZT 1, and α(2L0)0.5
225 V/K for ZT 2. Of course, any
“real” material will possess a finite κL, and
these values for the thermopower will
have to be higher to achieve the projected
values of ZT.
Solid-State Crystal Chemistry
Approaches to Advanced
Thermoelectric Materials
Thermoelectrics has always been a ma-
terials design problem involving intricate
tuning of structure–property relationships
in complex solids through principles of
solid-state chemistry and physics. The dis-
192 MRS BULLETIN • VOLUME 31 • MARCH 2006
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
cussion thus far indicates that new mate-
rials must be able to eventually achieve
certain minimum values of important pa-
rameters in order to be considered as a po-
tential TE material. It does not matter if a
material has a κLκmin; if it cannot be
“tuned” or doped in order to attain a min-
imum thermopower (150 V/K), it will
not be able to achieve ZT 1.
Classical Approach: Bulk Binary
Semiconductors
Within the framework of simple elec-
tronic band structure of solids, in general,
metals are poor TE materials. Hence, most
of the early TE work put much emphasis
on semiconductors.18 As stated earlier, in
order to have a maximum ratio of electri-
cal to thermal conductivity, the material
should have a low carrier concentration,
on the order of 1018 –1019 cm3, with very
high mobilities. Crystal structure and
bonding strongly influence the mobility.
Materials with diamond or zinc-blende
structures with a high degree of covalent
bonding frequently have high mobilities
(e.g., Si, Ge, InSb), but also exhibit high
thermal conductivity values. On the other
hand, low lattice thermal conductivities
are found in conjunction with low Debye
temperatures and high anharmonic lattice
vibrations. These conditions are best satis-
fied by highly covalent intermetallic com-
pounds and alloys of the heavy elements
such as Pb, Hg, Bi, Tl, or Sb, and S, Se, or Te.
Once a material system has been selected
with a favorable electrical-to-thermal con-
ductivity ratio, one optimizes the compo-
sition to further enhance the ZT by doping
the material to maximize the density of
states at the Fermi level and achieve a high
Seebeck coefficient.
The most studied TE material, Bi2Te3,
crystallizes in a layer structure (Figure 3)
with rhombohedral–hexagonal symmetry
with space group Rm(D53d). The hexa-
gonal unit cell dimensions at room tem-
perature are a3.8 Å and c30.5 Å. The
layers stacked along the c-axis are
··· Te–Bi–Te–Bi–Te ··· Te –Bi–Te–Bi–Te ···.
The Bi and Te layers are held together by
strong covalent bonds, whereas the bond-
ing between adjacent Te layers is of the
van der Waals type. This weak binding be-
tween the Te layers accounts for the ease
of cleavage along the plane perpendicular
to the c-axis and the anisotropic thermal
and electrical transport properties of
Bi2Te3. For example, the thermal conduc-
tivity along the plane perpendicular to the
c-axis (1.5 W m–1 K–1) is nearly twice that of
the value along the c-axis direction (0.7 W
3
m–1 K–1). When grown from a melt or by
zone refining, the Bi2Te3crystals are al-
ways nonstoichiometric and show p-type
behavior. On the other hand, n-type mate-
rials could be grown from the melt con-
taining excess Te, iodine, or bromine. The
thermal conductivity values of both p- and
n-type Bi2Te3are 1.9 W m–1 K–1, giving a
ZT of about 0.6 near room temperature.
Ioffe9suggested that alloying could fur-
ther reduce the lattice thermal conductiv-
ity of Bi2Te3through the scattering of
short-wavelength acoustic phonons. The
optimum compositions for thermoelectric
cooling devices are normally Bi2Te2.7Se0.3
(n-type) and Bi0.5Sb1.5Te3(p-type) with
ZT 1 near room temperature.
In contrast to Bi2Te3, PbTe crystallizes in
a cubic NaCl-type crystal structure, and
the TE properties are isotropic. Both p-
type and n-type thermoelements can be
produced by doping of acceptors (e.g.,
Na2Te o r K 2Te) or donors (PbI2, PbBr2, or
Ge2Te3). In analogy with the Bi2Te3, the
solid-solution compositions (e.g., PbTe-
SnTe) have been made to lower the lattice
thermal conductivity.19 The ZT value of
PbTe solid solutions is low near room tem-
perature but rises to ZT 0.7 at 700 K,
making PbTe a prime candidate for power
generation in that temperature range.
It is possible to achieve ZT in excess of
unity at 700 K in structurally related solid-
solution compositions, AgSbTe2 (80%)-
GeTe(20%), known as TAGS (alloys
containing Te, Ag, Ge, Sb). However, due
to high-temperature stability issues, these
compositions are not currently favored in
TE devices.
Neither Si nor Ge is a good TE material,
as the lattice thermal conductivity is very
large (150 W m–1 K–1 for Si and 63 W m–1
K–1 for Ge). The lattice thermal conductiv-
ity can be substantially reduced by alloy
formation between the two elements. The
best alloy composition is Si0.7Ge0.3; its ther-
mal conductivity is about 10 W m–1 K–1,
and the reduction relative to Si and Ge
is apparently due to the increased
phonon–phonon and phonon–electron
scattering.19 Remarkably, such a large re-
duction does not unduly reduce the car-
rier mobility, and ZT 0.6–0.7 could be
realized at elevated temperatures. Due to
their exceptional stability at high temper-
atures (1200 K), these alloys are of inter-
est to NASA for use in RTGs in deep-space
probes.
Figure 3. Crystal structure of the state-of-the-art thermoelectric material, Bi2Te 3.The blue
atoms are Bi and the pink atoms are Te.
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
MRS BULLETIN • VOLUME 31 • MARCH 2006 193
Modern Solid-State Chemistry
Design Concepts for High-ZT
Materials
Complex Inorganic Structures. Most of
the earlier investigations mentioned so far
focused on binary intermetallic semicon-
ductor systems. Recent approaches to
high-performance bulk TE materials focus
on ternary and quaternary chalcogenides
containing heavy atoms with low-
dimensional or isotropic complex struc-
tures to take advantage of the large carrier
effective masses and low lattice thermal
conductivity associated with such sys-
tems.20 Along these lines, CsBi4Te6pos-
sessing the layered structure has been
identified as a material showing a ZT of
0.8 at 225 K, which is 40% greater than that
of the Bi2–xSbxTe3–ySeyalloys.21 Other po-
tential low-temperature TE materials
currently under investigation are low-
dimensional semiconducting or semimetal-
lic doped layered pentatellurides (ZrTe5
and HfTe5).22 These compounds have a
structure similar to Bi2Te3, with van der
Waals gaps between the individual layers.
Although doped pentatellurides exhibit
very high power factors (exceeding the
optimally doped Bi2Te3solid solutions) in
the low-temperature range (250 K), their
thermal conductivity is relatively high
(4–8 W m–1 K–1), and the materials need
to be compositionally tuned further to
make them useful as thermoelectrics.
Recently, cubic quaternary compounds
with a complex formula AgnPbmMnTem+2n
(M Sb, Bi), crystallizing in the PbTe
structure, have been reported.23 The com-
position AgPb10SbTe12 shows an excep-
tionally high ZT value (2) at elevated
temperature (shown in Figure 2). This is
due to the very low total thermal conduc-
tivity of the bulk material, possibly arising
from compositional modulations (seen as
“nanodots”) similar to the one found in
superlattices. If this is verified, it provides
an additional “knob” to turn to achieve
high ZT in bulk materials. Another
group of materials under investigation
are half-Heusler alloys, with the general
formula MNiSn (M Zr, Hf, Ti). A
complex composition of the type
Zr0.5Hf0.5Ni0.5Pd0.5Sn0.99Sb0.01 shows a ZT of
0.7 at T800 K, highlighting the intricate
balance in structure, composition, and prop-
erty relationships in these compounds.24
The β-Zn4Sb3system has been reinvesti-
gated for TE power-generation applica-
tions at the Jet Propulsion Laboratory.25
Crystal Structures with “Rattlers.” The
method of lowering the lattice thermal
conductivity through mixed-crystal or
solid-solution formation does not always
produce enough phonon scattering to
lower the lattice thermal conductivity to
κmin. Slack’s concept of a “phonon-
glass/electron-crystal,” described earlier,
avoids this limitation. The concept of κmin
is successfully verified in crystal struc-
tures with large empty cages or voids
where atoms can be partially or com-
pletely filled in such a way that they “rat-
tle,” resulting in the scattering of the
acoustic phonons. This approach espe-
cially works well in highly covalent semi-
conductor materials based on clathrates
(Si, Ge, or Sn) and void structures formed
by heavy elements of low electronegativ-
ity differences (e.g., CoSb3-based skutteru-
dites). Some doped skutterudites show
exceptionally high ZT values at elevated
temperatures (ZT 1.5 at 600–800 K).
The structure–property relationships of
these materials are discussed in the article
by Nolas et al. in this issue.
Oxide Thermoelectrics. There are nu-
merous oxides with metal atoms in their
common oxidation states that are stable at
elevated temperatures and show electrical
properties ranging from insulating to su-
perconducting. Nevertheless, oxides have
received very little attention for TE appli-
cations. This is due to their strong ionic
character, with narrow conduction band-
widths arising from weak orbital overlap,
leading to localized electrons with low
carrier mobilities. This situation changed
with the unexpected discovery of good TE
properties in a strongly correlated layered
oxide, NaCo2O4.26 This oxide attains ZT
0.7–0.8 at 1000 K. Inspired by the striking
TE performance of NaCo2O4, most of the
current studies are focused on Co-based
layered oxides, such as Ca3Co4O9and
Bi2Sr3Co2Oy,crystallizing in “misfit”
(lattice-mismatched) layered structures.26
Among the n-type oxides, Al-doped ZnO
(Al0.02Zn0.98O) shows reasonably good TE
performance (ZT 0.3 at 1000 K).27
Rare-Earth Intermetallics with High
Power Factors. As mentioned earlier,
metallic compounds are not suitable for
TE applications. The exceptions to this
rule are intermetallic compounds contain-
ing rare-earth elements (e.g., Ce and Yb),
with localized magnetic moments where
the Seebeck coefficient can approach
100 V/K with metal-like conductivi-
ties.28,29 In these compounds, the 4flevels
lie near the Fermi energy and form nar-
row non-parabolic bands, resulting in a
large density of states at the Fermi level
and large Seebeck coefficient values. The
highest Seebeck values are found in cubic
YbAl3(n-type) and CePd3(p-type). YbAl3
shows a very high power factor
(120–180 W/cm K2, or 3.6–5.4 W m–1 K–1)
at room temperature (300 K), which is
nearly 4–5 times larger than that observed
in optimized Bi2Te3-based thermo-
electrics.30 Unfortunately, the large ther-
mal conductivity (15–22 W m–1 K–1)
lowers the ZT to about 0.2 at room tem-
perature. Recently, the lattice thermal con-
ductivity of this system has been lowered
by doping Mn in the interstitial positions,
resulting in the increase of ZT to about 0.4
at room temperature.31 As mentioned ear-
lier, ZT 1 requires a minimum Seebeck
coefficient value of 156 µV/K. The corre-
lated metal with the highest Seebeck coef-
ficient is CePd3, which has a maximum of
125 V/K at 140 K.29 Future investigations
should focus on increasing the Seebeck co-
efficients of these materials above
150 V/K through compositional and
structural tuning.
Engineered Crystal Lattices. The ap-
proaches in bulk materials research rely
heavily on the thermodynamic stability of
the phases at a given condition, whereas
thin-film deposition can yield metastable
“designer” phases with unique properties.
Quantum well systems (0D, 1D, and 2D)
take advantage of their low-dimensional
character through physical confinements
in quantum dots, nanowires, and thin-film
structures to enhance the electronic prop-
erties of a given material.32 In addition,
nanostructured semiconductor materials
could scatter mid- to long-wavelength
phonons and thereby reduce the lattice
thermal conductivity to κmin.
Researchers at the Research Triangle In-
stitute (RTI) have demonstrated a signifi-
cant enhancement in ZT through the
construction of Bi2Te3/Sb2Te3superlat-
tices.33 These materials exhibited ZT 2.4
at T330 K. The enhancement is attrib-
uted to creating a “nanoengineered” ma-
terial that is efficient in thermal insulation
while remaining a good electrical conduc-
tor. The thermal insulation arises from a
complex localized behavior for phonons,
while the electron transmission is facilitated
by optimal choice of band offsets in these
semiconductor heterostructures. Also, there
have been reports on PbTe/PbTeSe quantum
dot structures that yield ZT 1.3–1.6.34
These materials have been grown as thick
films that are then “floated off” the sub-
strate to yield freestanding films, which
were measured to yield these results. The
enhancement in ZT in the superlattice ma-
terials appears to be more from a reduc-
tion in the lattice thermal conductivity
than an increase in power factor.
Summary
Currently, there are no theoretical or
thermodynamic limits to the possible
194 MRS BULLETIN • VOLUME 31 • MARCH 2006
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
values of ZT. Given the current need for
alternative energy technologies and mate-
rials to replace the shrinking supply of
fossil fuels, the effort is becoming more ur-
gent. Energy-related research will grow
rapidly over the next few years, and
higher-performance thermoelectric mate-
rials and devices are direly needed. Slack
estimated that an optimized phonon-
glass/electron-crystal material could pos-
sibly exhibit values of ZT 4.14 This gives
encouragement that such materials may
be possible and could address many of
our energy-related problems. Thus, a
systematic search and subsequent thor-
ough investigation may eventually yield
these much-needed materials for the next
generation of TE devices.
Although many strategies are being em-
ployed in hopes of identifying novel TE
materials, the PGEC approach appears to
be the best, as will become apparent in the
following articles. One has to decide
whether “holey” semiconductors (mate-
rials with cages, such as skutterudites or
clathrates) or “unholey” semiconductors
(such as SiGe or PbTe) are the best to pur-
sue, and which tuning parameters are
available to improve these materials.35 To
date, none of the new materials has dis-
placed the current state-of-the-art mate-
rials (Bi2Te3, PbTe, or SiGe)in acommercial
TE device. These materials have held that
distinction for more than 30 years.
However, given the many materials yet
to be investigated, there is certainly much
more work ahead and promise for devel-
oping higher-efficiency thermoelectric
materials and devices. While the results
are very exciting, thin films may be most
appropriate for small-scale electronic and
optoelectronics applications where small
heat loads or low levels of power genera-
tion are more appropriate. To address
large-scale refrigeration (home refrigerators)
or power-generation (automotive or in-
dustrial) requirements, higher-perform-
ance bulk materials will have to be
developed.
Certainly, theoretical guidance, in terms
of band structure calculations and model-
ing, will be essential to identifying the
most promising TE materials. In addition,
rapid yet accurate characterization of ma-
terials and verification of results are also
essential in order to effectively advance
this field of research. A multidisciplinary
approach will be required to develop
higher-efficiency thermoelectric materials
and devices. The techniques used to de-
velop “designer materials” needed for
thermoelectrics will most likely prove im-
portant in other areas of materials re-
search as well.
References
1. The reader is referred to the many MRS
Symposium Proceedings volumes on the topic
of thermoelectric materials and energy-
conversion technologies: Thermoelectric Materi-
als—New Directions and Approaches (Mater. Res.
Soc. Symp. Proc. 478, 1997); Thermoelectric Mate-
rials 1998—The Next Generation Materials for
Small-Scale Refrigeration and Power Generation
Applications (Mater. Res. Soc. Symp. Proc. 545,
1999); Thermoelectric Materials 2000—The Next
Generation Materials for Small-Scale Refrigeration
and Power Generation Applications (Mater. Res.
Soc. Symp. Proc. 626, 2001); Thermoelectric Mate-
rials 2001—Research and Applications (Mater.
Res. Soc. Symp. Proc. 691, 2002); Thermoelectric
Materials 2003—Research and Applications
(Mater. Res. Soc. Symp. Proc. 793, 2004); and
Materials and Technologies for Direct Thermal-to-
Electric Energy Conversion (Mater. Res. Soc.
Symp. Proc. 886, 2006) in press.
2. T.M. Tritt, ed., “Recent Trends in Thermo-
electric Materials Research,” Semiconductors and
Semimetals, Vols. 69–71, treatise editors, R.K.
Willardson and E. Weber (Academic Press,
New York, 2000).
3. Jet Propulsion Laboratory Thermoelectric
Science and Engineering Web site,
http://www.its.caltech.edu/jsnyder/ther-
moelectrics/ (accessed February 2006).
4. A.W. Allen, Laser Focus World 33 (March
1997) p. S15.
5. G.S. Nolas, J. Sharp, and H.J. Goldsmid,
Thermoelectrics: Basic Principles and New Materi-
als Developments (Springer, New York, 2001).
6. T.J. Seebeck, Abh. K. Akad. Wiss. (Berlin, 1823)
p. 265.
7. D.T. Morelli, in Encyclopedia of Applied
Physics, Vol. 21 (1997) p. 339.
8. P.M. Chaiken, in Organic Superconductors, ed-
ited by V.Z. Kresin and W.A. Little (Plenum
Press, New York, 1990) p. 101.
9. A.F. Ioffe, Semiconductor Thermoelements and
Thermoelectric Cooling (Infosearch, London,
1957).
10. J.C. Peltier, Ann. Chem. LVI (1834) p. 371.
11. H.J. Goldsmid, Electronic Refrigeration (Pion
Limited, London, 1986).
12. D.M. Rowe, ed., CRC Handbook of Thermo-
electrics (CRC Press, Boca Raton, FL, 1995).
13. P.L. Rossiter, The Electrical Resistivity of Met-
als & Alloys (Cambridge Press, New York, 1987);
F.J. Blatt, Physics of Electronic Conduction in Solids
(McGraw-Hill, New York, 1968); L. Solymar
and D. Walsh, Electrical Properties of Materials,
6th Ed. (Oxford Press, New York, 1998).
14. G.A. Slack, in CRC Handbook of Thermo-
electrics, ed. by D.M. Rowe (CRC Press, Boca
Raton, FL, 1995) p. 407.
15. G.D. Mahan and J.O. Sofo, Proc. Natl. Acad.
Sci. USA 93 (1996) p. 7436.
16. G.D. Mahan, B. Sales, and J. Sharp, Phys.
Today 50 (3) (1997) p. 42.
17. G.A. Slack, in Solid State Physics, Vol. 34, ed-
ited by F. Seitz, D. Turnbull, and H. Ehrenreich
(Academic Press, New York, 1979) p. 1.
18. R.R. Heikes and R.W. Ure Jr., Thermoelectric-
ity: Science and Engineering (Wiley Interscience,
New York, 1961) p. 405.
19. C. Wood, Rep. Prog. Phys. 51 (1988) p. 459.
20. M.G. Kanatzidis, S.D. Mahanti, and T.P.
Hogan, eds., Chemistry, Physics and Materials
Science of Thermoelectric Materials: Beyond
Bismuth Telluride (Plenum, New York, 2003)
p. 35.
21. D.Y. Chung, T. Hogan, P. Brazis, M.
Rocci-Lane, C. Kannewurf, M. Bastea, C. Uher,
and M.G. Kanatzidis, Science 287 (2000)
p. 1024.
22. R.T. Littleton IV, T.M. Tritt, M. Korzenski, D.
Ketchum, and J.W. Kolis, Phys. Rev. B. Rap. Com-
mun. 64 121104 (2001).
23. K.F. Hsu, S. Loo, F. Gao, W. Chen, J.S. Dyck,
C. Uher, T. Hogan, E.K. Polychroniadis, and M.
Kanatzidis, Science 303 (2004) p. 8181.
24. Q. Shen, L. Chen, T. Goto, T. Hirai, J. Yang,
G.P. Meisner, and C. Uher, Appl. Phys. Lett. 79
(2001) p. 4165.
25. T. Caillat, J.P. Fleurial, and A. Borshchevsky,
J. Phys. Chem. Solids 58 (1997) p. 1119.
26. I. Terasaki and N. Murayama, eds., Oxide
Thermoelectrics (Research Signpost, Trivan-
drum, India, 2002).
27. M. Ohtaki, T. Tsubota, K. Eguchi, and H.
Arai, J. Appl. Phys. 79 (1996) p. 1816.
28. R.J. Gambino, W.D. Grobman, and A.M.
Toxen, Appl. Phys. Lett. 22 (1973) p. 506.
29. G.D. Mahan, in Solid State Physics, edited by
F. Seitz, H. Ehrenreich, and F. Spaepen (Acade-
mic Press, New York, 1997) p. 51.
30. D.M. Rowe, G. Min, and V.L. Kuznetsov,
Philos. Mag. Lett. 77 (1998) p. 105; D.M. Rowe,
V.L. Kuznetsov, L.A. Kuznetsova, and G. Min,
J. Phys. D: Appl. Phys. 35 (2002) p. 2183.
31. T. He, T.G. Calvarese, J.-Z. Chen, H.D.
Rosenfeld, R.J. Small, J.J. Krajewski, and M.A.
Subramanian, Proc. 24th Int. Conf. on Thermo-
electrics, edited by T.M. Tritt (IEEE, Piscataway,
NJ, 2005) p. 434.
32. L.D. Hicks and M.S. Dresselhaus, Phys. Rev.
B47 (1993) p. 12727.
33. R. Venkatasubramanian, E. Siivola, T.
Colpitts, and B. O’Quinn, Nature 13 (2001)
p. 597.
34. T.C. Harman, P.J. Taylor, M.P. Walsh, and
B.E. LaForge, Science 297 (2002) p. 2229.
35. T.M. Tritt, Science 283 (1999) p. 804. ■■
MRS BULLETIN • VOLUME 31 • MARCH 2006 195
Terry M. Tritt, Guest
Editor for this issue of
MRS Bulletin, is a pro-
fessor of physics at
Clemson University. He
received both his BA de-
gree (1980) and his PhD
degree (1985) in physics
from Clemson Univer-
sity and then went to the
Naval Research Labora-
tory (NRL) under a Na-
tional Research Council
postdoctoral fellowship.
He subsequently became
a staff scientist at NRL,
where he remained for
11 years before joining
the faculty at Clemson
in 1996. His primary re-
search expertise lies in
electrical and thermal
transport properties and
phenomena, and espe-
cially in measurement
and characterization
techniques in novel ma-
terials. He has extensive
expertise in thermoelec-
tric materials and meas-
urement science and has
built an internationally
known laboratory for
the measurement and
characterization of ther-
moelectric material
parameters, particularly
thermal conductivity. He
has recently become in-
volved in the synthesis
and characterization
of thermoelectric
nanomaterials.
Tritt has served as
lead organizer of three
Materials Research Soci-
ety symposia on ther-
moelectric materials,
edited the three-volume
Recent Trends in Thermo-
electric Materials Research
(Academic Press, 2000),
and also recently edited
a book published by
Kluwer Press on ther-
mal conductivity. He
has been a member of
the executive board of
the International Ther-
moelectric Society (ITS)
since 1999 and served as
chair and host of the
24th International
Conference on Thermo-
electrics (ICT-2005)
at Clemson in June
2005. He has written
more than 140 journal
publications and regu-
larly gives invited pre-
sentations at national
and international
meetings.
Tritt can be reached at
Clemson University, De-
partment of Physics, 103
Kinard Laboratory,
Clemson, SC 29634,
USA; tel. 864-656-5319
and e-mail ttritt@
clemson.edu.
M.A. Subramanian,
Guest Editor for this
issue of MRS Bulletin, is
a research fellow at
DuPont Central Re-
search and Develop-
ment. He holds BS and
MS degrees in chemistry
from the University of
Madras in India and re-
ceived his PhD degree
in solid-state chemistry
in 1982 from the Indian
Institute of Technology
in Madras, where he fo-
cused on synthesis and
solid-state studies of ox-
ides with pyrochlore
and perovskite struc-
tures. He subsequently
joined the Department
of Chemistry at Texas
A&M University as an
NSF postdoctoral fellow,
where he worked on de-
signing new fast ion
conductors for solid-
state batteries. He joined
DuPont in 1985 as a sci-
entist and was recently
appointed to research
fellow. Subramanian’s
current interests include
the design and under-
standing of structure–
property relationships in
new solid-state inorganic
functional materials re-
lated to superconductivity,
colossal magnetoresistive
materials, high-κand
low-κdielectrics, ferro-
electrics, multiferroics,
oxyfluorination, and
thermoelectrics.
Subramanian is a vis-
iting professor at the
Institut de Chimie de la
Matière Condensée de
Bordeaux (ICMCB),
University of Bordeaux,
France. He serves as edi-
tor for Solid State Sciences
and Progress in Solid
State Chemistry, and
serves or has served on
the editorial boards of
the Materials Research
Bulletin, Chemistry of Ma-
terials, and the Journal of
Materials Chemistry. He
was awarded the
Charles Pedersen Medal
by DuPont in 2004 for
his outstanding scien-
tific, technological, and
business contributions
to the company. He has
authored more than
200 publications and
holds 42 U.S. patents,
with 10 applications
pending.
Subramanian can be
reached at DuPont Cen-
tral Research and Devel-
opment, Experimental
Station, E328/219,
Wilmington, DE 19880-
0328, USA; tel. 302-695-
2966 and e-mail
mas.subramanian@usa.
dupont.com.
Harald Böttner is
deputy head of the
Components and Micro-
systems Department of
the Fraunhofer Institute
for Physical Measure-
ment Techniques in
Freiburg, Germany, and
is currently responsible
for its thermoelectric ac-
tivities. He graduated
with a diploma degree
in chemistry from the
University of Münster
in 1974 and received his
PhD degree in 1977 at
the same university for
his thesis on diffusion
and solid-state reaction
in the quaternary
semiconductor
II–VI/IV–VI material
system. He joined the
Fraunhofer Institut für
Silicatforschung in 1978
and accepted his present
appointment in 1980. He
developed IV–VI in-
frared semiconductor
lasers in parallel with
activities in thermo-
electrics until 1995,
when he became re-
sponsible for the
development of semi-
conductor gas sensors
and thermoelectric
materials until 2003.
Böttner’s current re-
search activities focus
on thin-film and
nanoscale thermo-
electrics and microelec-
tronics-related device
technology. He is the
author or co-author of
more than 100 publica-
tions, holds more than
10 patents, and is a
member of the board
of the International
Thermoelectric Society.
Böttner can be
reached at Fraunhofer
IPM, Research Field
Thermoelectrics, Hei-
denhofstrasse 8, 79110
Freiburg, Germany; tel.
49-761-8857121 and
e-mail harald.boettner@
ipm.fraunhofer.de.
Thierry Caillat is a sen-
ior member of technical
staff with the Jet Propul-
sion Laboratory at the
California Institute of
Technology. He received
his PhD degree in mate-
rials science from the
National Polytechnique
Institute of Lorraine,
France, in 1991. He then
received a National Re-
search Council fellow-
ship to study new
materials for thermo-
electric applications at
JPL. He joined the per-
manent staff at JPL in
1994. Caillat’s primary
research interests have
focused on the identifi-
cation and development
of new thermoelectric
materials and devices.
In the last ten years, he
has played a key role
in identifying several
families of promising
compounds for thermo-
electric applications,
including skutterudites
and β-Zn4Sb3–based
materials. More recently,
he has been involved
in the development
of advanced thermoelec-
tric power-generation
devices for both
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
Terry M. Tritt M.A. Subramanian Harald Böttner Thierry Caillat
196 MRS BULLETIN • VOLUME 31 • MARCH 2006
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
space and terrestrial
applications.
Caillat has authored
or co-authored more
than 100 publications,
given more than 40 in-
vited presentations, and
served on numerous na-
tional and international
organization committees
and panels. He was also
a board member of the
International Thermo-
electric Society from
1996 to 2005.
Caillat can be reached
at the California Insti-
tute of Technology, Jet
Propulsion Laboratory,
Mail Stop 277/207, 4800
Oak Grove Drive,
Pasadena, CA 91109,
USA; tel. 818-354-0407
and e-mail thierry.
caillat@jpl.nasa.gov.
Gang Chen is a profes-
sor of mechanical
engineering at the
Massachusetts Institute
of Technology. He re-
ceived his BS (1984) and
MS (1987) degrees from
the Power Engineering
Department at
Huazhong University of
Science and Technology,
China, in 1984 and 1987,
respectively, and his
PhD degree in mechani-
cal engineering from the
University of California,
Berkeley, in 1993. He
was an assistant profes-
sor at Duke University
from 1993 to 1997 and
an associate professor at
the University of Cali-
fornia, Los Angeles,
from 1997 to 2001. His
research interests center
on microelectronics ther-
mal management and
nanoscale transport
phenomena, particularly
nanoscale heat transfer,
and their applications
in energy storage and
conversion.
Chen is a recipient of
a K.C. Wong Education
Foundation fellowship,
an NSF Young Investi-
gator Award, and a John
Simon Guggenheim
Foundation fellowship.
He serves on the edito-
rial boards of Annual Re-
view of Heat Transfer, the
Journal of Computational
and Theoretical
Nanoscience, the Journal
of Nanomaterials, and
Microscale Thermophysical
Engineering. He also
serves as the chair of the
Advisory Board of the
ASME Nanotechnology
Institute and on the ad-
visory boards of several
organizations.
Chen can be reached
at the Massachusetts
Institute of Technology,
Department of Mechani-
cal Engineering, Room
3-158, 77 Massachusetts
Avenue, Cambridge,
MA 02139-4307, USA;
tel. 617-253-0006
and e-mail gchen2@
mit.edu.
Ryoji Funahashi has
been a senior researcher
at the National Institute
of Advanced Industrial
Science and Technology
in Japan since 1992. He
received his BS (1989)
and MS (1992) degrees
in physical chemistry
and his PhD degree
(1998) in crystalline
material science from
Nagoya University.
His research interests
include novel synthetic
techniques for high-
performance
superconducting oxide
materials and, recently,
the exploration of new
thermoelectric oxide
materials.
Funahashi is a recipi-
ent of the Thermoelec-
tric Conference of
Japan’s Best Paper
Award, the Japan Journal
of Applied Physics Paper
Award, and a NEDO
Industrial Technology
Research and Develop-
ment Project grant. He
has authored or co-
authored more than 150
papers, conference pro-
ceedings, invited talks,
and reviews. He is also
a board member of the
International Thermo-
electric Society and the
Thermoelectric
Society of Japan.
Funahashi can be
reached at AIST,
UBIQEN, Molecular
Materials and Devices,
1-8-31 Midorigaoka,
Ikeda, Osaka 563-8577,
Japan; tel. 81-727-51-
9485 and e-mail
funahashi-r@aist.go.jp.
Xiaohua Ji is a postdoc-
toral researcher in the
Department of Physics
and Astronomy at
Clemson University. In
2005, she received her
PhD degree in materials
physics and chemistry
from Zhejiang Univer-
sity in China under the
guidance of Xinbing
Zhao. For her graduate
work, she developed
solvothermal and
hydrothermal methods
for synthesizing nano-
structured thermoelec-
tric materials. She
received the Best Scien-
tific Paper Award at the
2004 ICT meeting in
Adelaide, Australia.
Under the guidance of
Terry M. Tritt at Clem-
son, her current research
involves the synthesis
and characterization of
nanostructured thermo-
electric materials and
nanocomposite bulk
thermoelectric materials.
Ji can be reached at
Clemson University, De-
partment of Physics and
Astronomy, Clemson,
SC 29634, USA; tel. 864-
656-4596 and e-mail
xiaohji@clemson.edu.
Mercouri Kanatzidis is a
University Distinguished
Professor of Chemistry at
Michigan State Univer-
sity, where he has served
on the faculty since 1987.
He received his BS
degree from Aristotle
University in Greece,
followed by a PhD de-
gree in chemistry from
the University of Iowa
in 1984. He was then a
postdoctoral research as-
sociate at Northwestern
University from 1985 to
1987. He has generated
seminal work in metal
chalcogenide chemistry
through the develop-
ment of novel synthetic
approaches aimed at
new materials discovery.
His research interests
include novel chalco-
genides, thermoelectric
materials, and the design
of framework solids, in-
termetallic phases, and
organic– inorganic
nanocomposites.
Kanatzidis is a recipi-
ent of the Presidential
Young Investigator
Award, the ACS Morley
Medal, the ACS Exxon
Solid State Chemistry
Award, and the Hum-
boldt Prize. He has been
a Guggenheim fellow as
well as a visiting profes-
sor at the University of
Nantes, the University
of Münster, and the
University of Munich.
The bulk of his work
is described in more
than 450 research
publications. He holds
six patents and is editor
in chief of the Journal of
Solid State Chemistry.
Kanatzidis can be
reached at Michigan
State University, Depart-
ment of Chemistry, 406
Chemistry Building,
East Lansing, MI 48824-
1322, USA; tel. 517-355-
9715 and e-mail
kanatzid@cem.msu.edu.
Kunihito Koumoto is a
professor in the Gradu-
ate School of Engineer-
ing at Nagoya
University, Japan. He re-
ceived BS, MS, and PhD
degrees in applied
chemistry from the Uni-
versity of Tokyo in 1974,
1976, and 1979, respec-
tively. He served as an
assistant professor and
associate professor at
the University of Tokyo
before joining Nagoya
University as a full pro-
fessor in 1992. His re-
search focuses on
thermoelectric materials
Gang Chen Ryoji Funahashi Xiaohua Ji Mercouri Kanatzidis
MRS BULLETIN • VOLUME 31 • MARCH 2006 197
and bio-inspired pro-
cessing of inorganic
materials.
Koumoto received the
Richard M. Fulrath
Award in 1993 and the
Academic Achievement
Award of the Ceramic
Society of Japan in 2000.
He became a fellow of
the American Ceramic
Society and received the
Chinese Ceramic Society
Award in 2005. He also
served the International
Thermoelectric Society
as president from 2003
to 2005 and is the author
or co-author of more
than 320 scientific
papers and 38 books.
Koumoto can be
reached at Nagoya Uni-
versity, Graduate School
of Engineering,
Furo-cho, Chikusa-ku,
Nagoya, 464-8603,
Japan; tel. 81-52-789-
3327 and e-mail
koumoto@apchem.
nagoya-u.ac.jp.
George S. Nolas is an
associate professor of
physics at the Univer-
sity of South Florida,
where he has been since
2001. He received his
PhD degree from
Stevens Institute of
Technology in 1994 and
conducted pioneering
studies on the synthesis
and thermal properties
of filled skutterudites
as a postdoctoral associ-
ate with Glen Slack at
Rensselaer Polytechnic
Institute. Before accept-
ing his current position,
he spent five years as a
physicist and senior
member of the technical
staff at Marlow Industries
Inc., a thermoelectrics
manufacturer in Dallas,
Texas. His research inter-
ests are in the synthesis
and structure– property
relationships of new
materials, and his cur-
rent research focuses on
new materials for power
conversion and alterna-
tive energy applications,
including thermo-
electrics, photovoltaics,
and hydrogen storage.
Nolas is author of
Thermoelectrics: Basic
Principles and New
Materials Developments,
published by Springer
with co-authors Jeffrey
Sharp and Julian
Goldsmid. He holds
three patents, with
another two pending,
on new materials for
power-conversion appli-
cations. He has edited
four Materials Research
Society proceedings vol-
umes (two as lead edi-
tor) on thermoelectric
materials research, or-
ganized symposia for
the American Physical
Society and the Ameri-
can Ceramic Society,
given numerous invited
conference presentations
and seminars, and is
currently in his second
term as a board member
of the International
Thermoelectric Society.
Nolas can be reached
at the University of
South Florida, Depart-
ment of Physics, PHY
114, 4202 E. Fowler Av-
enue, Tampa, FL 33620-
5700, USA; tel.
813-974-2233 and e-mail
gnolas@cas.usf.edu.
Joseph Poon is the
William Barton Rogers
Professor of Physics at
the University of Vir-
ginia, where he joined
the Physics Department
in 1980. He received
both his BS and PhD de-
grees from the Califor-
nia Institute of
Technology and was a
research associate in ap-
plied physics at Stan-
ford University. His
areas of research have
included amorphous
superconductors, quasi-
crystals, bulk amorphous
metals, and thermoelec-
tric alloys. He is a fellow
of the American Physi-
cal Society and was
named one of the “Sci-
entific American 50” in
2004 for the creation of
amorphous steel. He has
published more than
200 papers.
Poon can be reached
at the University of
Virginia, Department of
Astronomy-Physics, PO
Box 400714, Jesse Beams
Lab, Room 167,
Charlottesville, VA
22901, USA; tel. 434-924-
6792 and e-mail
sjp9x@virginia.edu.
Apparao M. Rao is a
professor in condensed-
matter physics at
Clemson University. He
obtained his PhD degree
in physics from the
University of Kentucky
in 1989 and held a
postdoctoral appoint-
ment with Mildred S.
Dresselhaus at MIT
until 1991. His current
research focuses on
understanding and con-
trolling the synthesis of
1D nanostructured
organic and inorganic
materials. He has pub-
lished extensively on the
synthesis, characteriza-
tion, and applications of
carbon nanotubes.
Rao can be reached at
Clemson University, De-
partment of Physics and
Astronomy, 107 Kinard
Laboratory, Clemson,
SC 29634-0978, USA; tel.
864-656-6758 and e-mail
arao@clemson.edu.
Ichiro Terasaki is a pro-
fessor in the Applied
Physics Department at
Wase da University in
Tokyo. He received his
BE, ME, and PhD de-
grees in applied physics
from the University of
Tokyo in 1986, 1988, and
1992, respectively. He
began his research ca-
reer as a research associ-
ate at the University of
Tokyo, moving on to be-
come chief researcher at
the International Super-
conductivity Technology
Center. He became an
associate professor at
Waseda in 1997 and ac-
cepted his current posi-
tion as a full professor
in 2004. His research in-
terests include experi-
mental studies in con-
densed-matter physics,
especially charge trans-
port properties of transi-
tion-metal oxides,
organic conductors, and
intermetallic com-
pounds.
Terasaki is the recipi-
ent of the Sir Martin
Wood Prize and ICT’s
Best Scientific Paper
Award. He has pub-
lished more than 150
papers, conference pro-
ceedings, invited talks,
books, and reviews.
Terasaki can be
reached at Waseda Uni-
versity, Department of
Applied Physics, 3-4-1
Okubo, Shinjuku-ku,
Tokyo 169-8555, Japan;
tel. 81-3-5286-3854 and
e-mail terra@waseda.jp.
Rama Venkatasubra-
manian is director of the
Center for Thermoelec-
tric Research at RTI
International in North
Carolina and the
founder and chief tech-
nology officer of Nex-
treme Thermal Solutions
Inc., a company spun off
from RTI to commercial-
ize its unique thin-film
superlattice thermoelec-
tric technology. He
earned his PhD degree
in electrical engineering
from Rensselaer Poly-
technic Institute and is a
National Talent Scholar
and a graduate of the
Indian Institute of Tech-
nology in Madras. His
research interests in-
clude photovoltaics,
heteroepitaxy of novel
materials, photonic
materials, the study of
nanoscale thermal
physics, thermal
management in high-
performance electronics,
and direct thermal-to-
electric conversion de-
vices. He is well known
for pioneering thermo-
electric superlattice
materials and devices.
Venkatasubramanian’s
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
Kunihito Koumoto George S. Nolas Joseph Poon Apparao M. Rao
198 MRS BULLETIN • VOLUME 31 • MARCH 2006
Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View
work on superlattices
has been recognized as a
significant breakthrough
in thermoelectrics using
nanoscale engineered
materials. This technol-
ogy has won an R&D
100 Award (2002) and
the Technology of the
Year Award (2005)
from the Council for
Entrepreneurial Devel-
opment in North
Carolina.
Venkatasubramanian
is a recipient of the
Allen B. Dumont Prize
from Rensselaer, RTI’s
Margaret Knox Excel-
lence Award in Research
in 2002, and the IEEE
Eastern North Carolina
Inventor of the Year in
2003. He has several
patents issued in
thermoelectrics, has
authored more than 100
refereed publications,
and has contributed to
two book chapters.
Venkatasubramanian
can be reached at RTI
International, Center for
Thermoelectric
Research, PO Box 12194,
Research Triangle Park,
NC 27709-2194, USA;
tel. 919-541-6889 and
e-mail rama@rti.org.
Jihui Yang is a staff re-
search scientist in the
Materials and Processes
Laboratory at General
Motors Research and
Development Center.
He received a BS degree
in physics from Fudan
University of China in
1989, an MS degree
in physics from the
University of Oregon in
1991, an MS degree in
radiological physics
from Wayne State
University in 1994, and
a PhD degree in physics
from the University of
Michigan in 2000. His
research interests in-
clude low-temperature
transport properties of
intermetallic compounds
and semiconductors,
magnetism, thermoelec-
tric materials, and the
development of thermo-
electric technology for
automotive applications.
Yang has published
several book chapters
and more than 30
papers. He was the re-
cipient of the GM DEGS
fellowship in 1997 and
the Kent M. Terwilliger
Prize for Best Doctoral
Thesis from the Physics
Department of the Uni-
versity of Michigan in
2001. He has served on
various committees for
APS and the Materials
Research Society and or-
ganized several sym-
posia for MRS and
ACerS. He was also
elected to the board of
directors of the Interna-
tional Thermoelectric
Society in 2005.
Yang can be reached
at General Motors
Research and Develop-
ment Center, Mail Code
480-106-224, 30500
Mound Road, Warren,
MI 48090, USA; tel. 586-
986-9789 and e-mail
jihui.yang@gm.com. ■■
Rama
Venkatasubramanian
Jihui YangIchiro Terasaki
For more information, see http://www.mrs.org/bulletin_ads
www.mrs.org/bulletin
