Thermoelectric TE materials is ... · High-efficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical

188 MRS BULLETIN • VOLUME 31 • MARCH 2006

IntroductionThermoelectric Phenomena:Background and Applications

Over the past decade, there has beenheightened interest in the field of thermo-electrics, driven by the need for more effi-cient materials for electronic refrigerationand power generation.1,2 Some of the re-search efforts focus on minimizing the lat-tice thermal conductivity, while otherefforts focus on materials that exhibit largepower factors. Proposed industrial andmilitary applications of thermoelectric

(TE) materials are generating increasedactivity 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 thedirect 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 applicationsare currently being investigated by the au-tomotive industry as a means to developelectrical power from waste engine heatfrom the radiator and exhaust systems foruse in next-generation vehicles. In addi-tion, TE refrigeration applications includeseat coolers for comfort and electroniccomponent cooling. Of course, the deep-space applications of NASA’s Voyagerand Cassini missions using radioisotopethermoelectric generators (RTGs) are wellestablished (see Reference 3 and the articleby Yang and Caillat in this issue). A keyfactor in developing these technologies isthe development of higher-performanceTE materials, either completely new mate-rials or through more ingenious materialsengineering 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 electronicsand optoelectronics technologies typicallyrequire only small-scale or localized spotcooling of small components that do notimpose a large heat load. If significant eco-nomical cooling can be achieved, the re-sulting “cold computing” could producespeed gains of 30–200% in some computerprocessors based on complementarymetal oxide semiconductor (CMOS) tech-nology. Cooling of the processors is per-ceived by many to be the fundamentallimit to electronic system performance.4Thus, the potential payoff for the develop-ment of low-temperature TE refrigerationdevices is great, and the requirement forcompounds with properties optimizedover wide temperature ranges has led to amuch expanded interest in new TE mate-rials. Recent utilization of Peltier coolers(see next section) for the refrigeration ofbiological specimens and samples is anemerging TE application.

The development and potential of bulkmaterials for TE applications is an activearea of research. High-temperature bulkmaterials 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). Thesematerials possess complex crystal struc-tures and exhibit properties that are favor-able for potential thermoelectric materials.For example, skutterudites and clathratesare cage-like materials that have voids inwhich “rattler” atoms are inserted to sig-nificantly lower the thermal conductivitydue to the rattling atoms’ ability to scatterphonons. Recently, ceramic oxide mate-rials have also shown potential as high-

ThermoelectricMaterials,Phenomena, andApplications: ABird’s Eye View

Terry M.Tritt and M.A. Subramanian,Guest Editors

AbstractHigh-efficiency thermoelectric (TE) materials are important for power-generation

devices that are designed to convert waste heat into electrical energy.They can also beused in solid-state refrigeration devices.The conversion of waste heat into electricalenergy may play an important role in our current challenge to develop alternative energytechnologies to reduce our dependence on fossil fuels and reduce greenhouse gasemissions.

An overview of various TE phenomena and materials is provided in this issue of MRSBulletin. Several of the current applications and key parameters are defined anddiscussed. Novel applications of TE materials include biothermal batteries to powerheart pacemakers, enhanced performance of optoelectronics coupled with solid-state TEcooling, and power generation for deep-space probes via radioisotope TE generators. Anumber of different systems of potential TE materials are currently under investigation byvarious research groups around the world, and many of these materials are reviewed inthe articles in this issue.These range from thin-film superlattice materials to large single-crystal or polycrystalline bulk materials, and from semiconductors and semimetals toceramic oxides.The phonon-glass/electron-crystal approach to new TE materials ispresented, along with the role of solid-state crystal chemistry. Research criteria fordeveloping new materials are highlighted.

Keywords: energy, thermal conductivity, thermoelectricity.

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Thermoelectric Materials, Phenomena, and Applications: A Bird’s Eye View

MRS BULLETIN • VOLUME 31 • MARCH 2006 189

temperature TE materials (see Koumotoet al. in this issue). The potential ofnanomaterials and their role in TE re-search are an emerging area of interest(see Rao et al. in this issue). Bulk materialapplications are demanding new break-throughs in both materials and device en-gineering (see Yang and Caillat in thisissue). The role of thin-film properties, ap-plications, and recent results is also veryimportant (see Böttner et al. in this issue).A more complete overview of state-of-the-art materials, a theoretical and experimen-tal discussion of the basic principles, andan overview of some of the recent devel-opments and materials are given in textsby Tritt2 and Nolas.5

Seebeck and Peltier EffectsA discussion of thermoelectric effects

and devices should start with one of themost fundamental TE phenomena, theSeebeck effect, or thermopower.6–8 In theearly 1800s, Seebeck observed that whentwo dissimilar materials are joined to-gether and the junctions are held at differ-ent temperatures (T and T ∆T), a voltagedifference (∆V) develops that is propor-tional to the temperature difference (∆T).6The ratio of the voltage developed to thetemperature gradient (∆V/∆T) is relatedto an intrinsic property of the materialscalled 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).9 A related effect—the Peltiereffect—was discovered a few years laterby Peltier,10 who observed that when anelectrical current is passed through thejunction of two dissimilar materials, heatis either absorbed or rejected at the junc-tion, depending on the direction of thecurrent. This effect is due largely to thedifference in Fermi energies of the twomaterials. These two effects are related toeach other, as shown in the definition ofthe Peltier coefficient, Π αT. The rate atwhich the Peltier heat is liberated or re-jected at the junction (QP) is given by QP αIT, where I is the current through thejunction and T is the temperature inkelvin. There are also a number of ther-momagnetic effects such as the Hall,Ettingshausen, and Nernst effects that arebeyond the scope of this article. Thereader is referred to the text by Nolas et al.5 for a discussion of these effects.

Definition and Description of theFigure of Merit and ThermoelectricPerformance

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 α is the Seebeck coefficient, σ is theelectrical conductivity, ρ is the electricalresistivity, and κ is the total thermal con-ductivity (κ κL κE, the lattice and elec-tronic contributions, respectively). Thepower factor, α2σT (or α2Τ/ρ), is typicallyoptimized in narrow-gap semiconductingmaterials as a function of carrier concen-tration (typically 1019 carriers/cm3),through doping, to give the largest ZT.9High-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 somewhatmeaningless, since an array of TE couplesis utilized in a device or module.

There are two materials in the TE cou-ple, which is shown in Figure 1, an n-typeand 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 thecouple (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 aredirectly related to the figure of meritshown in Equation 3 for the efficiency. Theefficiency (η) of the TE couple is given bythe power input to the load (W) over thenet heat flow rate (QH), where QH is posi-tive for heat flow from the source to thesink:

(3)

where TH is the hot-side temperature, TC isthe cold-side temperature, and TM is theaverage temperature. Thus, one can see

TH TC

TH 1 ZTM1/2 1

1 ZTM1/2 TCTH,

η W

QH

ZT αp αn2T

ρnκn1/2 ρpκp1/2.

ZT α2σT

κ

α2Tρκ

,

that η is proportional to (1 ZTM)1/2 andthat the efficiency would approach theCarnot efficiency if ZT were to approachinfinity.

Thermoelectric Modules: DevicesThe Peltier effect is the basis for many

modern-day TE refrigeration devices, andthe Seebeck effect is the basis for TEpower-generation devices. The versatilityof TE materials is illustrated in Figure 1,which shows a TE couple composed of an n-type (negative thermopower andelectron carriers) and a p-type (positivethermopower and hole carriers) semicon-ductor material connected through metallic electrical contact pads. Both re-frigeration and power generation may beaccomplished using the same module, asshown in Figure 1. A TE module or deviceis built up of an array of these couples,arranged electrically in series and ther-mally in parallel. Thermoelectric energyconversion utilizes the Seebeck effect,wherein a temperature gradient is im-posed across the device, resulting in avoltage that can be used to drive a currentthrough a load resistance or device. This isthe direct conversion of heat into electric-ity. Conversely, the Peltier heat generatedwhen an electric current is passed througha TE material provides a temperature gra-dient, with heat being absorbed on thecold side, transferred through (or pumpedby) the TE materials, and rejected at thesink, thus providing a refrigeration capa-bility. The advantages of TE solid-state en-ergy conversion are compactness,quietness (no moving parts), and localizedheating or cooling. In addition, energy inthe form of waste heat (0% efficiency) thatwould normally be lost may be convertedinto useful electrical energy (7–8% effi-ciency) using a TE power-generation de-vice.

The best TE materials currently used indevices have ZT 1. This value has beena practical upper limit for more than 30years, yet there are no theoretical or ther-modynamic reasons for ZT 1 as anupper barrier. As seen from Equation 1,ZT may be increased by decreasing κL orby increasing either α or σ. However, σ istied 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 eitherraise the current efficiency of TE devices(i.e., increase ZT) or have the capability ofoperating in new and broader tempera-ture regimes, especially at lower tempera-tures (T 250 K) and higher temperatures(T 400 K). Over the past 30 years,

*The expressions for figure of merit, Z and ZT,are used interchangeably in the field of thermo-electrics. Z is the figure of merit with units of1/K (1/T), and ZT is the dimensionless (unit-less) figure of merit. Both must specify the tem-perature at which the quoted value wasobtained.







alloys based on the Bi2Te3 system[(Bi1–xSbx)2(Te1–xSex)3] and the Si1–yGey sys-tem have been extensively studied andoptimized for their use as TE materials toperform in a variety of solid-state TE re-frigeration and power-generation applica-tions.11,12 These traditional TE materialshave undergone extensive investigation,and there appears to be little room for fu-ture improvement in the common bulkstructures. However, recent results onnanostructures of traditional TE materialshave shown a promising new direction forthese materials. In addition, entirely newclasses of compounds will have to be in-vestigated. Figure 2 shows ZT as a func-tion of temperature for the Bi2Te3 andSi1–yGey materials as well as many of themore recent bulk materials that have beendeveloped over the last decade. The ZT ofmore exotic structures such as superlat-tices and quantum dot structures are notshown here but are addressed in the ar-ticle by Böttner et al. in this issue.

Transport PropertiesThe thermopower, or Seebeck coeffi-

cient, can be thought of as the heat per car-rier over temperature or, more simply, theentropy per carrier, α C/q, where C isthe specific heat and q is the charge of thecarrier.7 For the case of a classical gas, eachparticle has an energy of 3/2(kBT), wherekB is the Boltzmann constant. The ther-mopower is thus approximately kB/e,where e is the charge of the electron. Formetals, the heat per carrier is essentially aproduct of the electronic specific heat andthe temperature divided by the number of

carriers (N), that is, α CelT/N, and thenα is approximately

(4)

where EF is the Fermi energy (related tothe chemical potential of the material).

α Cel

q kB

e kBTEF

,

The Fermi energy is basically the energysuch that at T 0, all the states above this energy are vacant and all the states below are occupied. The quantitykB/e 87 V/K is a constant that repre-sents the thermopower of a classical elec-tron gas. Metals have thermopower valuesof much less than 87 V/K (on the orderof 1–10 V/K) and decrease with decreas-ing temperature, that is, EF kBT).

In a semiconductor, a charged particlemust first be excited across an energy gapEg. In this case, the thermopower is ap-proximated by

(5)

Thus, the thermopower is larger than thecharacteristic value of 87 V/K and in-creases with decreasing temperature.Semiconductors can exhibit either electronconduction (negative thermopower) orhole conduction (positive thermopower).The thermopower for different carriertypes is given by a weighted average of their electrical conductivity values (σn and σp):

(6)

It is necessary to dope the semiconductorswith either donor or acceptor states to

α αnσn αpσp

σn σp.

α Cel

q kB

e Eg

kBT.

Figure 1. Diagram of a Peltier thermoelectric couple made of an n-type and a p-typethermoelectric material. Refrigeration or power-generation modes are possible, dependingon the configuration. I is current.

Figure 2. Figure of merit ZT shown as a function of temperature for several bulkthermoelectric materials.







allow extrinsic conduction of the appro-priate carrier type, electrons or holes, re-spectively. It is apparent that the totalthermopower will be lower than that of ei-ther of the individual contributions, unlessthe direct bandgap is large enough—typically on the order of 10(kBT)—to effectively minimize minority carrier con-tributions. Typical thermopower valuesrequired for good TE performance are onthe order of 150–250 V/K or greater.

For high-temperature applications, it isimportant to minimize the contribution ofminority carriers in order to maintain ahigh thermopower. In addition, the ther-mal stability of the materials is an essentialaspect. Atomic diffusion within the mate-rials and interdiffusion of contacts can se-riously deteriorate the properties of agiven material at high temperatures. As-pects of this are discussed elsewhere.2,5

These materials and devices are expectedto operate at elevated temperatures forlong periods of time without deteriorationof their properties or performance. The ef-fects of diffusion and thermal annealingare important to thoroughly investigateand understand in any set of potential TEmaterials over the expected operatingtemperature range of the materials.

The description of electrical conductiv-ity for metals and semiconductors hasbeen covered extensively in many texts onsolid-state physics, and the reader is re-ferred there.13 There are a significant num-ber of carriers and states available forconduction in metals, typically n 1022

carriers/cm3. The electrical conductivity isthen very large for metals, on the order of106(Ω cm)–1. Again, for semiconductors,the carriers must be thermally excitedacross a gap for conduction to occur, asshown from the activated behavior that isderived for the temperature-dependenceof the electrical conductivity [σσ0exp(–Eg/kBT)]. There are two primaryways to achieve a high conductivity in asemiconductor, either by having a verysmall gap to excite across (Eg/kBT) or byhaving very high-mobility carriers, as dis-cussed later. Typical values of the electri-cal conductivity for a good TE material areon the order of about 103(Ω cm)–1.

The thermal conductivity κ is related tothe transfer of heat through a material, ei-ther by the electrons or by quantized vi-brations of the lattice, called phonons,such that κ κL κE, as mentioned ear-lier. The electrical conductivity and thethermal conductivity are interrelated, inthat σ is tied to κE through the Wiedemann–Franz relationship: κE L0σT, where theLorentz number L0 2.45 × 10–8 W Ω/K2

[or L0 2.45 × 10–8(V2/K2)]. The latticethermal conductivity is discussed later

in this article, in the section on minimumthermal conductivity. Typical thermalconductivity values for a good TE mate-rial are κ 2 W m–1 K–1, and typically,κL κE.

Investigating New ThermoelectricMaterialsThe “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 candidatesshould be narrow-bandgap semiconduc-tors with high-mobility carriers. Mahanhas also described the characteristics ofgood TE materials,15,16 agreeing with Slackthat the candidate material is typically anarrow-bandgap semiconductor [Eg10(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. Insemiconductors, the Seebeck coefficientand electrical conductivity (both in the nu-merator of ZT) are strong functions of thedoping level and chemical composition.These quantities must therefore be opti-mized for good TE performance. The ther-mal conductivity of complex materials canoften be modified by chemical substitu-tions, and the lattice thermal conductivityneeds to be as low as possible. Under-standing these various effects and select-ing optimization strategies can be anexceedingly difficult problem, because incomplex materials there are often manypossible 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 andthe thermal properties of an amorphousor glass-like material. Materials engineer-ing and the crystal chemistry approach togood TE materials are discussed later.

Minimum Thermal ConductivityIn many areas of research related to new

TE materials, attempts are being made toreduce the lattice part of the thermal con-ductivity to essentially its minimumvalue, that is, where a minimum latticethermal conductivity is achieved (whenall the phonons have a mean free path es-sentially equal to the interatomic spacingof the constituent atoms). This is being at-tempted by scattering phonons in differ-ent frequency ranges using a variety ofmethods such as mass fluctuation scatter-ing (a mixed crystal, in ternary and qua-ternary compounds), “rattling” scattering,grain-boundary scattering (due to the sizeof the grains), and interface scattering inthin films or multilayer systems.

The lattice thermal conductivity is givenby κL (1/3)(vsCLph), where vs is the ve-locity of sound, C is the heat capacity, andLph is the mean free path of the phonons.At high temperatures (T 300 K), thesound velocity and the heat capacity areessentially temperature-independent intypical materials. Therefore, the magnitudeand the temperature-dependence of κL arebasically determined by the mean free pathof the phonons. Slack defined the minimumthermal conductivity (κmin) as the thermalconductivity when the mean free path isessentially limited by the interatomic dis-tance between the atoms within the crys-tal.17 Typical analysis of κmin results invalues of κmin 0.25–0.5 W m–1 K–1.14,17

Minimum ThermopowerThere are certain practical limits for

each of the parameters used to calculateZT. These practical limits must be possiblein order to achieve a material viable forthermoelectric applications. For example,in Bi2Te3, in order to achieve a ZT 1 at T 320 K, σ 1 mΩ cm, α 225 V/K,and κ 1.5 W m–1 K–1. We have alreadydiscussed the ZT “barrier,” which in effectis given by minimizing the thermal con-ductivity. It is practical to investigate ma-terials where the electronic and latticeterms are comparable, on the order of0.75–1 W m–1 K–1. Let us look at the hypo-thetical situation of a material in which the lattice thermal conductivity is zero (κL 0). We will also assume the scatter-ing in this system is elastic and that theWiedemann–Franz relationship, slightlyrearranged [κE/σ L0T], is well behavedin this material. Then we can rewriteEquation 1 as

ZT α2T/ρκE α2/L0. (7)

Therefore, for a material to be a viable TEmaterial, it must possess a minimum ther-mopower that is directly related to thevalue 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, andthese values for the thermopower willhave to be higher to achieve the projectedvalues of ZT.

Solid-State Crystal ChemistryApproaches to AdvancedThermoelectric Materials

Thermoelectrics has always been a ma-terials design problem involving intricatetuning of structure–property relationshipsin complex solids through principles ofsolid-state chemistry and physics. The dis-







cussion thus far indicates that new mate-rials must be able to eventually achievecertain minimum values of important pa-rameters in order to be considered as a po-tential TE material. It does not matter if amaterial has a κL κmin; if it cannot be“tuned” or doped in order to attain a min-imum thermopower (150 V/K), it willnot be able to achieve ZT 1.

Classical Approach: Bulk BinarySemiconductors

Within the framework of simple elec-tronic band structure of solids, in general,metals are poor TE materials. Hence, mostof the early TE work put much emphasison semiconductors.18 As stated earlier, inorder to have a maximum ratio of electri-cal to thermal conductivity, the materialshould have a low carrier concentration,on the order of 1018–1019 cm3, with veryhigh mobilities. Crystal structure andbonding strongly influence the mobility.Materials with diamond or zinc-blendestructures with a high degree of covalentbonding frequently have high mobilities(e.g., Si, Ge, InSb), but also exhibit highthermal conductivity values. On the otherhand, low lattice thermal conductivitiesare found in conjunction with low Debyetemperatures and high anharmonic latticevibrations. These conditions are best satis-fied by highly covalent intermetallic com-pounds and alloys of the heavy elementssuch as Pb, Hg, Bi, Tl, or Sb, and S, Se, or Te.Once a material system has been selectedwith a favorable electrical-to-thermal con-ductivity ratio, one optimizes the compo-sition to further enhance the ZT by dopingthe material to maximize the density ofstates at the Fermi level and achieve a highSeebeck coefficient.

The most studied TE material, Bi2Te3,crystallizes in a layer structure (Figure 3)with rhombohedral–hexagonal symmetrywith space group R m (D5

3d). The hexa-gonal unit cell dimensions at room tem-perature are a 3.8 Å and c 30.5 Å. Thelayers stacked along the c-axis are

··· Te–Bi–Te–Bi–Te ··· Te–Bi–Te–Bi–Te ···.

The Bi and Te layers are held together bystrong covalent bonds, whereas the bond-ing between adjacent Te layers is of thevan der Waals type. This weak binding be-tween the Te layers accounts for the easeof cleavage along the plane perpendicularto the c-axis and the anisotropic thermaland electrical transport properties ofBi2Te3. For example, the thermal conduc-tivity along the plane perpendicular to thec-axis (1.5 W m–1 K–1) is nearly twice that ofthe value along the c-axis direction (0.7 W

3

m–1 K–1). When grown from a melt or byzone refining, the Bi2Te3 crystals are al-ways nonstoichiometric and show p-typebehavior. On the other hand, n-type mate-rials could be grown from the melt con-taining excess Te, iodine, or bromine. Thethermal conductivity values of both p- andn-type Bi2Te3 are 1.9 W m–1 K–1, giving aZT of about 0.6 near room temperature.Ioffe9 suggested that alloying could fur-ther reduce the lattice thermal conductiv-ity of Bi2Te3 through the scattering ofshort-wavelength acoustic phonons. Theoptimum compositions for thermoelectriccooling 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 ina cubic NaCl-type crystal structure, andthe TE properties are isotropic. Both p-type and n-type thermoelements can beproduced by doping of acceptors (e.g.,Na2Te or K2Te) or donors (PbI2, PbBr2, orGe2Te3). In analogy with the Bi2Te3, thesolid-solution compositions (e.g., PbTe-SnTe) have been made to lower the latticethermal conductivity.19 The ZT value ofPbTe solid solutions is low near room tem-perature but rises to ZT 0.7 at 700 K,

making PbTe a prime candidate for powergeneration in that temperature range. It is possible to achieve ZT in excess ofunity at 700 K in structurally related solid-solution compositions, AgSbTe2 (80%)-GeTe(20%), known as TAGS (alloyscontaining Te, Ag, Ge, Sb). However, dueto high-temperature stability issues, thesecompositions are not currently favored inTE devices.

Neither Si nor Ge is a good TE material,as the lattice thermal conductivity is verylarge (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 alloyformation between the two elements. Thebest 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 increasedphonon–phonon and phonon–electronscattering.19 Remarkably, such a large re-duction does not unduly reduce the car-rier mobility, and ZT 0.6–0.7 could berealized at elevated temperatures. Due totheir exceptional stability at high temper-atures (1200 K), these alloys are of inter-est to NASA for use in RTGs in deep-spaceprobes.

Figure 3. Crystal structure of the state-of-the-art thermoelectric material, Bi2Te3.The blueatoms are Bi and the pink atoms are Te.







Modern Solid-State ChemistryDesign Concepts for High-ZTMaterialsComplex Inorganic Structures. Most ofthe earlier investigations mentioned so farfocused on binary intermetallic semicon-ductor systems. Recent approaches tohigh-performance bulk TE materials focuson ternary and quaternary chalcogenidescontaining heavy atoms with low-dimensional or isotropic complex struc-tures to take advantage of the large carriereffective masses and low lattice thermalconductivity associated with such sys-tems.20 Along these lines, CsBi4Te6 pos-sessing the layered structure has beenidentified as a material showing a ZT of0.8 at 225 K, which is 40% greater than thatof the Bi2–xSbxTe3–ySey alloys.21 Other po-tential low-temperature TE materials currently under investigation are low-dimensional semiconducting or semimetal-lic doped layered pentatellurides (ZrTe5and HfTe5).22 These compounds have astructure similar to Bi2Te3, with van derWaals gaps between the individual layers.Although doped pentatellurides exhibitvery high power factors (exceeding theoptimally doped Bi2Te3 solid solutions) inthe low-temperature range (250 K), theirthermal conductivity is relatively high(4–8 W m–1 K–1), and the materials needto be compositionally tuned further tomake them useful as thermoelectrics.

Recently, cubic quaternary compoundswith a complex formula AgnPbmMnTem+2n

(M Sb, Bi), crystallizing in the PbTestructure, have been reported.23 The com-position AgPb10SbTe12 shows an excep-tionally high ZT value (2) at elevatedtemperature (shown in Figure 2). This isdue to the very low total thermal conduc-tivity of the bulk material, possibly arisingfrom compositional modulations (seen as“nanodots”) similar to the one found insuperlattices. If this is verified, it providesan additional “knob” to turn to achievehigh ZT in bulk materials. Another group of materials under investigation are half-Heusler alloys, with the generalformula MNiSn (M Zr, Hf, Ti). Acomplex composition of the typeZr0.5Hf0.5Ni0.5Pd0.5Sn0.99Sb0.01 shows a ZT of0.7 at T 800 K, highlighting the intricatebalance in structure, composition, and prop-erty relationships in these compounds.24

The β-Zn4Sb3 system has been reinvesti-gated for TE power-generation applica-tions at the Jet Propulsion Laboratory.25

Crystal Structures with “Rattlers.” Themethod of lowering the lattice thermalconductivity through mixed-crystal orsolid-solution formation does not alwaysproduce enough phonon scattering to

lower the lattice thermal conductivity toκmin. Slack’s concept of a “phonon-glass/electron-crystal,” described earlier,avoids this limitation. The concept of κminis successfully verified in crystal struc-tures with large empty cages or voidswhere atoms can be partially or com-pletely filled in such a way that they “rat-tle,” resulting in the scattering of theacoustic phonons. This approach espe-cially works well in highly covalent semi-conductor materials based on clathrates(Si, Ge, or Sn) and void structures formedby heavy elements of low electronegativ-ity differences (e.g., CoSb3-based skutteru-dites). Some doped skutterudites showexceptionally high ZT values at elevatedtemperatures (ZT 1.5 at 600–800 K).The structure–property relationships ofthese materials are discussed in the articleby Nolas et al. in this issue.

Oxide Thermoelectrics. There are nu-merous oxides with metal atoms in theircommon oxidation states that are stable atelevated temperatures and show electricalproperties ranging from insulating to su-perconducting. Nevertheless, oxides havereceived very little attention for TE appli-cations. This is due to their strong ioniccharacter, with narrow conduction band-widths arising from weak orbital overlap,leading to localized electrons with lowcarrier mobilities. This situation changedwith the unexpected discovery of good TEproperties in a strongly correlated layeredoxide, NaCo2O4.26 This oxide attains ZT 0.7–0.8 at 1000 K. Inspired by the strikingTE performance of NaCo2O4, most of thecurrent studies are focused on Co-basedlayered oxides, such as Ca3Co4O9 andBi2Sr3Co2Oy, crystallizing in “misfit”(lattice-mismatched) layered structures.26

Among the n-type oxides, Al-doped ZnO(Al0.02Zn0.98O) shows reasonably good TEperformance (ZT 0.3 at 1000 K).27

Rare-Earth Intermetallics with HighPower Factors. As mentioned earlier,metallic compounds are not suitable forTE applications. The exceptions to thisrule are intermetallic compounds contain-ing rare-earth elements (e.g., Ce and Yb),with localized magnetic moments wherethe Seebeck coefficient can approach100 V/K with metal-like conductivi-ties.28,29 In these compounds, the 4f levelslie near the Fermi energy and form nar-row non-parabolic bands, resulting in alarge density of states at the Fermi leveland large Seebeck coefficient values. Thehighest Seebeck values are found in cubicYbAl3 (n-type) and CePd3 (p-type). YbAl3shows 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 isnearly 4–5 times larger than that observedin 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 loweredby doping Mn in the interstitial positions,resulting in the increase of ZT to about 0.4at room temperature.31 As mentioned ear-lier, ZT 1 requires a minimum Seebeckcoefficient value of 156 µV/K. The corre-lated metal with the highest Seebeck coef-ficient is CePd3, which has a maximum of125 V/K at 140 K.29 Future investigationsshould focus on increasing the Seebeck co-efficients of these materials above150 V/K through compositional andstructural tuning.

Engineered Crystal Lattices. The ap-proaches in bulk materials research relyheavily on the thermodynamic stability ofthe phases at a given condition, whereasthin-film deposition can yield metastable“designer” phases with unique properties.Quantum well systems (0D, 1D, and 2D)take advantage of their low-dimensionalcharacter through physical confinementsin quantum dots, nanowires, and thin-filmstructures to enhance the electronic prop-erties of a given material.32 In addition,nanostructured semiconductor materialscould scatter mid- to long-wavelengthphonons and thereby reduce the latticethermal conductivity to κmin.

Researchers at the Research Triangle In-stitute (RTI) have demonstrated a signifi-cant enhancement in ZT through theconstruction of Bi2Te3/Sb2Te3 superlat-tices.33 These materials exhibited ZT 2.4at T 330 K. The enhancement is attrib-uted to creating a “nanoengineered” ma-terial that is efficient in thermal insulationwhile remaining a good electrical conduc-tor. The thermal insulation arises from acomplex localized behavior for phonons,while the electron transmission is facilitatedby optimal choice of band offsets in thesesemiconductor heterostructures. Also, therehave been reports on PbTe/PbTeSe quantumdot structures that yield ZT 1.3–1.6.34

These materials have been grown as thickfilms that are then “floated off” the sub-strate to yield freestanding films, whichwere measured to yield these results. Theenhancement in ZT in the superlattice ma-terials appears to be more from a reduc-tion in the lattice thermal conductivitythan an increase in power factor.

SummaryCurrently, there are no theoretical or

thermodynamic limits to the possible







values of ZT. Given the current need foralternative energy technologies and mate-rials to replace the shrinking supply offossil fuels, the effort is becoming more ur-gent. Energy-related research will growrapidly over the next few years, andhigher-performance thermoelectric mate-rials and devices are direly needed. Slackestimated that an optimized phonon-glass/electron-crystal material could pos-sibly exhibit values of ZT 4.14 This givesencouragement that such materials maybe possible and could address many ofour energy-related problems. Thus, a systematic search and subsequent thor-ough investigation may eventually yieldthese much-needed materials for the nextgeneration of TE devices.

Although many strategies are being em-ployed in hopes of identifying novel TEmaterials, the PGEC approach appears tobe the best, as will become apparent in thefollowing articles. One has to decidewhether “holey” semiconductors (mate-rials with cages, such as skutterudites orclathrates) or “unholey” semiconductors(such as SiGe or PbTe) are the best to pur-sue, and which tuning parameters areavailable to improve these materials.35 Todate, none of the new materials has dis-placed the current state-of-the-art mate-rials (Bi2Te3, PbTe, or SiGe) in acommercialTE device. These materials have held thatdistinction for more than 30 years.

However, given the many materials yetto be investigated, there is certainly muchmore work ahead and promise for devel-oping higher-efficiency thermoelectricmaterials and devices. While the resultsare very exciting, thin films may be mostappropriate for small-scale electronic andoptoelectronics applications where smallheat loads or low levels of power genera-tion are more appropriate. To addresslarge-scale refrigeration (home refrigerators)or power-generation (automotive or in-dustrial) requirements, higher-perform-ance bulk materials will have to bedeveloped.

Certainly, theoretical guidance, in termsof band structure calculations and model-ing, will be essential to identifying themost promising TE materials. In addition,rapid yet accurate characterization of ma-terials and verification of results are alsoessential in order to effectively advancethis field of research. A multidisciplinary

approach will be required to develophigher-efficiency thermoelectric materialsand devices. The techniques used to de-velop “designer materials” needed forthermoelectrics will most likely prove im-portant in other areas of materials re-search as well.

References1. The reader is referred to the many MRS Symposium Proceedings volumes on the topicof 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 forSmall-Scale Refrigeration and Power GenerationApplications (Mater. Res. Soc. Symp. Proc. 545,1999); Thermoelectric Materials 2000—The NextGeneration Materials for Small-Scale Refrigerationand Power Generation Applications (Mater. Res.Soc. Symp. Proc. 626, 2001); Thermoelectric Mate-rials 2001—Research and Applications (Mater.Res. Soc. Symp. Proc. 691, 2002); ThermoelectricMaterials 2003—Research and Applications(Mater. Res. Soc. Symp. Proc. 793, 2004); andMaterials 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 andSemimetals, Vols. 69–71, treatise editors, R.K.Willardson and E. Weber (Academic Press,New York, 2000).3. Jet Propulsion Laboratory ThermoelectricScience and Engineering Web site,http://www.its.caltech.edu/jsnyder/ther-moelectrics/ (accessed February 2006).4. A.W. Allen, Laser Focus World 33 (March1997) 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 AppliedPhysics, Vol. 21 (1997) p. 339.8. P.M. Chaiken, in Organic Superconductors, ed-ited by V.Z. Kresin and W.A. Little (PlenumPress, New York, 1990) p. 101.9. A.F. Ioffe, Semiconductor Thermoelements andThermoelectric Cooling (Infosearch, London,1957).10. J.C. Peltier, Ann. Chem. LVI (1834) p. 371.11. H.J. Goldsmid, Electronic Refrigeration (PionLimited, 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, BocaRaton, 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 MaterialsScience 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., OxideThermoelectrics (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 byF. 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.B 47 (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, andB.E. LaForge, Science 297 (2002) p. 2229.35. T.M. Tritt, Science 283 (1999) p. 804.






Terry M. Tritt, GuestEditor for this issue ofMRS Bulletin, is a pro-fessor of physics atClemson University. Hereceived both his BA de-gree (1980) and his PhDdegree (1985) in physicsfrom Clemson Univer-sity and then went to theNaval Research Labora-tory (NRL) under a Na-tional Research Councilpostdoctoral fellowship.He subsequently becamea staff scientist at NRL,where he remained for11 years before joiningthe faculty at Clemsonin 1996. His primary re-search expertise lies inelectrical and thermaltransport properties andphenomena, and espe-cially in measurementand characterizationtechniques in novel ma-terials. He has extensiveexpertise in thermoelec-tric materials and meas-urement science and hasbuilt an internationallyknown laboratory for the measurement andcharacterization of ther-moelectric materialparameters, particularlythermal conductivity. Hehas recently become in-volved in the synthesisand characterizationof thermoelectricnanomaterials.

Tritt has served aslead organizer of threeMaterials Research Soci-ety symposia on ther-moelectric materials,edited the three-volumeRecent Trends in Thermo-electric Materials Research(Academic Press, 2000),and also recently editeda book published byKluwer Press on ther-mal conductivity. Hehas been a member ofthe executive board ofthe International Ther-moelectric Society (ITS)since 1999 and served aschair and host of the24th International Conference on Thermo-electrics (ICT-2005)

at Clemson in June 2005. He has writtenmore than 140 journalpublications and regu-larly gives invited pre-sentations at nationaland international meetings.

Tritt can be reached atClemson University, De-partment of Physics, 103Kinard Laboratory,Clemson, SC 29634,USA; tel. 864-656-5319and e-mail [email protected].

M.A. Subramanian,Guest Editor for thisissue of MRS Bulletin, isa research fellow atDuPont Central Re-search and Develop-ment. He holds BS andMS degrees in chemistryfrom the University ofMadras in India and re-ceived his PhD degreein solid-state chemistryin 1982 from the IndianInstitute of Technologyin Madras, where he fo-cused on synthesis andsolid-state studies of ox-ides with pyrochloreand perovskite struc-tures. He subsequentlyjoined the Departmentof Chemistry at TexasA&M University as anNSF postdoctoral fellow,where he worked on de-signing new fast ionconductors for solid-state batteries. He joinedDuPont in 1985 as a sci-entist and was recentlyappointed to researchfellow. Subramanian’scurrent interests include

the design and under-standing of structure–property relationships innew solid-state inorganicfunctional materials re-lated to superconductivity,colossal magnetoresistivematerials, high-κ andlow-κ dielectrics, ferro-electrics, multiferroics,oxyfluorination, andthermoelectrics.

Subramanian is a vis-iting professor at the Institut de Chimie de laMatière Condensée deBordeaux (ICMCB),University of Bordeaux,France. He serves as edi-tor for Solid State Sciencesand Progress in SolidState Chemistry, andserves or has served onthe editorial boards ofthe Materials ResearchBulletin, Chemistry of Ma-terials, and the Journal ofMaterials Chemistry. Hewas awarded theCharles Pedersen Medalby DuPont in 2004 forhis outstanding scien-tific, technological, andbusiness contributionsto the company. He hasauthored more than 200 publications andholds 42 U.S. patents,with 10 applicationspending.

Subramanian can bereached at DuPont Cen-tral Research and Devel-opment, ExperimentalStation, E328/219,Wilmington, DE 19880-0328, USA; tel. 302-695-2966 and [email protected].

Harald Böttner isdeputy head of theComponents and Micro-systems Department ofthe Fraunhofer Institutefor Physical Measure-ment Techniques inFreiburg, Germany, andis currently responsiblefor its thermoelectric ac-tivities. He graduatedwith a diploma degreein chemistry from theUniversity of Münsterin 1974 and received hisPhD degree in 1977 atthe same university forhis thesis on diffusionand solid-state reactionin the quaternary semiconductorII–VI/IV–VI materialsystem. He joined theFraunhofer Institut fürSilicatforschung in 1978and accepted his presentappointment in 1980. Hedeveloped IV–VI in-frared semiconductorlasers in parallel withactivities in thermo-electrics until 1995,when he became re-sponsible for the development of semi-conductor gas sensorsand thermoelectric materials until 2003.Böttner’s current re-search activities focuson thin-film andnanoscale thermo-electrics and microelec-tronics-related devicetechnology. He is the author or co-author ofmore than 100 publica-tions, holds more than10 patents, and is amember of the board

of the International Thermoelectric Society.

Böttner can bereached at FraunhoferIPM, Research FieldThermoelectrics, Hei-denhofstrasse 8, 79110Freiburg, Germany; tel.49-761-8857121 and e-mail [email protected].

Thierry Caillat is a sen-ior member of technicalstaff with the Jet Propul-sion Laboratory at theCalifornia Institute ofTechnology. He receivedhis PhD degree in mate-rials science from theNational PolytechniqueInstitute of Lorraine,France, in 1991. He thenreceived a National Re-search Council fellow-ship to study newmaterials for thermo-electric applications atJPL. He joined the per-manent staff at JPL in1994. Caillat’s primaryresearch interests havefocused on the identifi-cation and developmentof new thermoelectricmaterials and devices.In the last ten years, hehas played a key role in identifying severalfamilies of promising compounds for thermo-electric applications, including skutteruditesand β-Zn4Sb3–based materials. More recently,he has been involved in the development of advanced thermoelec-tric power-generationdevices for both


Terry M. Tritt M.A. Subramanian Harald Böttner Thierry Caillat







space and terrestrial applications.

Caillat has authoredor co-authored morethan 100 publications,given more than 40 in-vited presentations, andserved on numerous na-tional and internationalorganization committeesand panels. He was alsoa board member of theInternational Thermo-electric Society from1996 to 2005.

Caillat can be reachedat the California Insti-tute of Technology, JetPropulsion Laboratory,Mail Stop 277/207, 4800Oak Grove Drive,Pasadena, CA 91109,USA; tel. 818-354-0407and e-mail [email protected].

Gang Chen is a profes-sor of mechanical engineering at the Massachusetts Instituteof Technology. He re-ceived his BS (1984) andMS (1987) degrees fromthe Power EngineeringDepartment atHuazhong University ofScience and Technology,China, in 1984 and 1987,respectively, and hisPhD degree in mechani-cal engineering from theUniversity of California,Berkeley, in 1993. Hewas an assistant profes-sor at Duke Universityfrom 1993 to 1997 andan associate professor atthe University of Cali-fornia, Los Angeles,from 1997 to 2001. Hisresearch interests centeron microelectronics ther-mal management andnanoscale transport phenomena, particularlynanoscale heat transfer,and their applications in energy storage andconversion.

Chen is a recipient ofa K.C. Wong EducationFoundation fellowship,an NSF Young Investi-gator Award, and a JohnSimon Guggenheim

Foundation fellowship.He serves on the edito-rial boards of Annual Re-view of Heat Transfer, theJournal of Computationaland TheoreticalNanoscience, the Journalof Nanomaterials, and Microscale ThermophysicalEngineering. He alsoserves as the chair of theAdvisory Board of theASME NanotechnologyInstitute and on the ad-visory boards of several organizations.

Chen can be reachedat the Massachusetts Institute of Technology,Department of Mechani-cal Engineering, Room3-158, 77 MassachusettsAvenue, Cambridge,MA 02139-4307, USA;tel. 617-253-0006 and e-mail [email protected].

Ryoji Funahashi hasbeen a senior researcherat the National Instituteof Advanced IndustrialScience and Technologyin Japan since 1992. Hereceived his BS (1989)and MS (1992) degreesin physical chemistryand his PhD degree(1998) in crystallinematerial science fromNagoya University. His research interestsinclude novel synthetictechniques for high-performance superconducting oxidematerials and, recently,the exploration of newthermoelectric oxidematerials.

Funahashi is a recipi-ent of the Thermoelec-tric Conference ofJapan’s Best PaperAward, the Japan Journalof Applied Physics PaperAward, and a NEDO Industrial TechnologyResearch and Develop-ment Project grant. Hehas authored or co-authored more than 150papers, conference pro-ceedings, invited talks,and reviews. He is alsoa board member of theInternational Thermo-electric Society and theThermoelectric Society of Japan.

Funahashi can bereached at AIST,UBIQEN, MolecularMaterials and Devices,1-8-31 Midorigaoka,Ikeda, Osaka 563-8577,Japan; tel. 81-727-51-9485 and [email protected].

Xiaohua Ji is a postdoc-toral researcher in theDepartment of Physicsand Astronomy atClemson University. In2005, she received herPhD degree in materialsphysics and chemistryfrom Zhejiang Univer-sity in China under theguidance of XinbingZhao. For her graduatework, she developedsolvothermal and hydrothermal methodsfor synthesizing nano-structured thermoelec-tric materials. Shereceived the Best Scien-tific Paper Award at the

2004 ICT meeting inAdelaide, Australia.Under the guidance ofTerry M. Tritt at Clem-son, her current researchinvolves the synthesisand characterization ofnanostructured thermo-electric materials andnanocomposite bulkthermoelectric materials.

Ji can be reached atClemson University, De-partment of Physics andAstronomy, Clemson,SC 29634, USA; tel. 864-656-4596 and e-mail [email protected].

Mercouri Kanatzidis is aUniversity DistinguishedProfessor of Chemistry atMichigan State Univer-sity, where he has servedon the faculty since 1987.He received his BSdegree from AristotleUniversity in Greece,followed by a PhD de-gree in chemistry fromthe University of Iowa in 1984. He was then apostdoctoral research as-sociate at NorthwesternUniversity from 1985 to1987. He has generatedseminal work in metalchalcogenide chemistrythrough the develop-ment of novel syntheticapproaches aimed atnew materials discovery.His research interests include novel chalco-genides, thermoelectricmaterials, and the designof framework solids, in-termetallic phases, andorganic–inorganicnanocomposites.

Kanatzidis is a recipi-ent of the PresidentialYoung InvestigatorAward, the ACS MorleyMedal, the ACS ExxonSolid State ChemistryAward, and the Hum-boldt Prize. He has beena Guggenheim fellow aswell as a visiting profes-sor at the University ofNantes, the Universityof Münster, and the University of Munich.The bulk of his work is described in more than 450 research publications. He holdssix patents and is editorin chief of the Journal ofSolid State Chemistry.

Kanatzidis can bereached at MichiganState University, Depart-ment of Chemistry, 406Chemistry Building,East Lansing, MI 48824-1322, USA; tel. 517-355-9715 and [email protected].

Kunihito Koumoto is aprofessor in the Gradu-ate School of Engineer-ing at NagoyaUniversity, Japan. He re-ceived BS, MS, and PhDdegrees in appliedchemistry from the Uni-versity of Tokyo in 1974,1976, and 1979, respec-tively. He served as anassistant professor andassociate professor atthe University of Tokyobefore joining NagoyaUniversity as a full pro-fessor in 1992. His re-search focuses onthermoelectric materials

Gang Chen Ryoji Funahashi Xiaohua Ji Mercouri Kanatzidis






and bio-inspired pro-cessing of inorganic materials.

Koumoto received theRichard M. FulrathAward in 1993 and theAcademic AchievementAward of the CeramicSociety of Japan in 2000.He became a fellow ofthe American CeramicSociety and received theChinese Ceramic SocietyAward in 2005. He alsoserved the InternationalThermoelectric Societyas president from 2003to 2005 and is the authoror co-author of morethan 320 scientific papers and 38 books.

Koumoto can bereached at Nagoya Uni-versity, Graduate Schoolof Engineering, Furo-cho, Chikusa-ku,Nagoya, 464-8603,Japan; tel. 81-52-789-3327 and [email protected].

George S. Nolas is anassociate professor ofphysics at the Univer-sity of South Florida,where he has been since2001. He received hisPhD degree fromStevens Institute ofTechnology in 1994 andconducted pioneeringstudies on the synthesisand thermal propertiesof filled skutterudites as a postdoctoral associ-ate with Glen Slack atRensselaer PolytechnicInstitute. Before accept-ing his current position,

he spent five years as aphysicist and seniormember of the technicalstaff at Marlow IndustriesInc., a thermoelectricsmanufacturer in Dallas,Texas. His research inter-ests are in the synthesisand structure–propertyrelationships of newmaterials, and his cur-rent research focuses onnew materials for powerconversion and alterna-tive energy applications,including thermo-electrics, photovoltaics,and hydrogen storage.

Nolas is author ofThermoelectrics: BasicPrinciples and New Materials Developments,published by Springerwith co-authors JeffreySharp and JulianGoldsmid. He holdsthree patents, withanother two pending,on new materials forpower-conversion appli-cations. He has editedfour Materials ResearchSociety proceedings vol-umes (two as lead edi-tor) on thermoelectricmaterials research, or-ganized symposia forthe American PhysicalSociety and the Ameri-can Ceramic Society,given numerous invitedconference presentationsand seminars, and iscurrently in his secondterm as a board memberof the InternationalThermoelectric Society.

Nolas can be reachedat the University ofSouth Florida, Depart-

ment of Physics, PHY114, 4202 E. Fowler Av-enue, Tampa, FL 33620-5700, USA; tel.813-974-2233 and [email protected].

Joseph Poon is theWilliam Barton RogersProfessor of Physics atthe University of Vir-ginia, where he joinedthe Physics Departmentin 1980. He receivedboth his BS and PhD de-grees from the Califor-nia Institute ofTechnology and was aresearch associate in ap-plied physics at Stan-ford University. Hisareas of research haveincluded amorphoussuperconductors, quasi-crystals, bulk amorphousmetals, and thermoelec-tric alloys. He is a fellowof the American Physi-cal Society and wasnamed one of the “Sci-entific American 50” in2004 for the creation ofamorphous steel. He haspublished more than200 papers.

Poon can be reachedat the University of Virginia, Department ofAstronomy-Physics, POBox 400714, Jesse BeamsLab, Room 167, Charlottesville, VA22901, USA; tel. 434-924-6792 and [email protected].

Apparao M. Rao is aprofessor in condensed-matter physics at Clemson University. He

obtained his PhD degreein physics from the University of Kentuckyin 1989 and held a postdoctoral appoint-ment with Mildred S.Dresselhaus at MIT until 1991. His currentresearch focuses on understanding and con-trolling the synthesis of1D nanostructured organic and inorganicmaterials. He has pub-lished extensively on thesynthesis, characteriza-tion, and applications ofcarbon nanotubes.

Rao can be reached atClemson University, De-partment of Physics andAstronomy, 107 KinardLaboratory, Clemson,SC 29634-0978, USA; tel.864-656-6758 and [email protected].

Ichiro Terasaki is a pro-fessor in the AppliedPhysics Department atWaseda University inTokyo. He received hisBE, ME, and PhD de-grees in applied physicsfrom the University ofTokyo in 1986, 1988, and1992, respectively. Hebegan his research ca-reer as a research associ-ate at the University ofTokyo, moving on to be-come chief researcher atthe International Super-conductivity TechnologyCenter. He became anassociate professor atWaseda in 1997 and ac-cepted his current posi-tion as a full professorin 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, andintermetallic com-pounds.

Terasaki is the recipi-ent of the Sir MartinWood Prize and ICT’sBest Scientific PaperAward. He has pub-lished more than 150 papers, conference pro-ceedings, invited talks,books, and reviews.

Terasaki can bereached at Waseda Uni-versity, Department ofApplied Physics, 3-4-1Okubo, Shinjuku-ku,Tokyo 169-8555, Japan;tel. 81-3-5286-3854 ande-mail [email protected].

Rama Venkatasubra-manian is director of theCenter for Thermoelec-tric Research at RTI International in NorthCarolina and thefounder and chief tech-nology officer of Nex-treme Thermal SolutionsInc., a company spun offfrom RTI to commercial-ize its unique thin-filmsuperlattice thermoelec-tric technology. Heearned his PhD degreein electrical engineeringfrom Rensselaer Poly-technic Institute and is aNational Talent Scholarand a graduate of theIndian Institute of Tech-nology in Madras. Hisresearch interests in-clude photovoltaics, heteroepitaxy of novelmaterials, photonic materials, the study ofnanoscale thermalphysics, thermal management in high-performance electronics,and direct thermal-to-electric conversion de-vices. He is well knownfor pioneering thermo-electric superlattice materials and devices.Venkatasubramanian’s


Kunihito Koumoto George S. Nolas Joseph Poon Apparao M. Rao







work on superlatticeshas been recognized as asignificant breakthroughin thermoelectrics usingnanoscale engineeredmaterials. This technol-ogy has won an R&D100 Award (2002) andthe Technology of theYear Award (2005) from the Council for Entrepreneurial Devel-opment in North Carolina.

Venkatasubramanianis a recipient of theAllen B. Dumont Prizefrom Rensselaer, RTI’sMargaret Knox Excel-lence Award in Researchin 2002, and the IEEEEastern North CarolinaInventor of the Year in2003. He has severalpatents issued in thermoelectrics, has authored more than 100refereed publications,

and has contributed totwo book chapters.

Venkatasubramaniancan be reached at RTIInternational, Center forThermoelectricResearch, PO Box 12194,Research Triangle Park,NC 27709-2194, USA;tel. 919-541-6889 and e-mail [email protected].

Jihui Yang is a staff re-search scientist in the

Materials and ProcessesLaboratory at GeneralMotors Research andDevelopment Center.He received a BS degreein physics from FudanUniversity of China in1989, an MS degree in physics from the University of Oregon in1991, an MS degree inradiological physicsfrom Wayne State University in 1994, anda PhD degree in physicsfrom the University ofMichigan in 2000. Hisresearch interests in-clude low-temperaturetransport properties ofintermetallic compoundsand semiconductors,magnetism, thermoelec-tric materials, and thedevelopment of thermo-electric technology forautomotive applications.

Yang has publishedseveral book chapters

and more than 30 papers. He was the re-cipient of the GM DEGSfellowship in 1997 andthe Kent M. TerwilligerPrize for Best DoctoralThesis from the PhysicsDepartment of the Uni-versity of Michigan in2001. He has served onvarious committees forAPS and the MaterialsResearch Society and or-ganized several sym-posia for MRS andACerS. He was alsoelected to the board ofdirectors of the Interna-tional ThermoelectricSociety in 2005.

Yang can be reachedat General Motors Research and Develop-ment Center, Mail Code480-106-224, 30500Mound Road, Warren,MI 48090, USA; tel. 586-986-9789 and [email protected].

Rama Venkatasubramanian

Jihui YangIchiro Terasaki

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Thermoelectric TE materials is ... · High-efficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical