04 Thermoelectric Materials

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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 onfossil fuels leads to important regimesof 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 important. Power-generation applicationsare currently being investigated by the automotive industry as a means to developelectrical power from waste engine heafrom the radiator and exhaust systems foruse in next-generation vehicles. In addition, TE refrigeration applications includseat coolers for comfort and electronicomponent cooling. Of course, the deepspace applications of NASA’s Voyageand Cassini missions using radioisotopethermoelectric generators (RTGs) are weestablished (see Reference 3 and the artic by Yang and Caillat in this issue). A kefactor in developing these technologies ithe development of higher-performanceTE materials, either completely new materials or through more ingenious materialengineering of existing materials.

Thermoelectric refrigeration is an environmentally “green” method of smallscale, localized cooling in computersinfrared detectors, electronics, and optoelectronics as well as many other applications. However, most of the electronicand optoelectronics technologies typicallrequire only small-scale or localized spocooling of small components that do noimpose a large heat load. If significant economical cooling can be achieved, the resulting “cold computing” could producespeed gains of 30–200% in some computprocessors based on complementarymetal oxide semiconductor (CMOS) technology. Cooling of the processors is perceived by many to be the fundamental

limit to electronic system performance4

Thus, the potential payoff for the development of low-temperature TE refrigeratiodevices is great, and the requirement focompounds with properties optimizedover wide temperature ranges has led to amuch expanded interest in new TE materials. Recent utilization of Peltier coole(see next section) for the refrigeration o biological specimens and samples is aemerging TE application.

The development and potential of bulkmaterials for TE applications is an activarea of research. High-temperature bulkmaterials such as skutterudites, clathrateshalf-Heusler alloys, and complex chalcogenides are being investigated (see the article by Nolas et al. in this issue). Thesmaterials possess complex crystal structures and exhibit properties that are favorable for potential thermoelectric materialFor example, skutterudites and clathratesare cage-like materials that have voids iwhich “rattler” atoms are inserted to significantly lower the thermal conductivitydue to the rattling atoms’ ability to scattephonons. Recently, ceramic oxide materials 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 MRS Bulletin . Several of the current applications and key parameters are defined anddiscussed. Novel applications of TE materials include biothermal batteries to power

heart 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 of nanomaterials 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).Amore 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 texts by Tritt2 and Nolas.5

Seebeck and Peltier Effects A 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 andT ∆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 . TheSeebeck coefficient is very low for metals(only a few V/K) and much largerfor semiconductors (typically a few hun-dred V/K).9 A related effect—the Peltiereffect—was discovered a few years later by Peltier,10 who observed that when anelectrical current is passed through the junction 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 of the Peltier coefficient,Π αT . The rate atwhich the Peltier heat is liberated or rejected at the junction (QP) is given byQPαIT , where I is the current through the junction andT is 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. Thereader is referred to the text by Nolaset al.5 for 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)

whereα is the Seebeck coefficient,σ is theelectrical conductivity,ρ is the electricalresistivity, andκ is the total thermal con-ductivity (κ κ L κ E, the lattice and elec-tronic contributions, respectively). Thepower factor,α 2σT (or α 2Τ /ρ), is typicallyoptimized in narrow-gap semiconductingmaterials as a function of carrier concen-tration (typically 1019 carriers/cm3),through doping, to give the largestZT .9High-mobility carriers are most desirable,in order to have the highest electrical con-ductivity for a given carrier concentration.TheZT 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, ann-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 performanceφ (refriger-ation mode) and the efficiencyη (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), whereQH is posi-tive for heat flow from the source to thesink:

(3)

whereT H is the hot-side temperature,T C isthe cold-side temperature, andT M is theaverage temperature. Thus, one can see

T H T CT H

1 ZT M 1/2 11 ZT M 1/2 T C T H

,

ηW

QH

ZT α p α n 2T

ρnκ n 1/2 ρ pκ p 1/2.

ZT α 2σT

κ

α 2T ρκ

,

that η is proportional to (1 ZT M)1/2 andthat the efficiency would approach thCarnot efficiency ifZT were to approachinfinity.

Thermoelectric Modules: Devices The Peltier effect is the basis for man

modern-day TE refrigeration devices, anthe Seebeck effect is the basis for Tpower-generation devices. The versatilitof TE materials is illustrated in Figurewhich shows a TE couple composed oan n-type (negative thermopower andelectron carriers) and a p-type (positivethermopower and hole carriers) semiconductor material connected throughmetallic electrical contact pads. Both rfrigeration and power generation may baccomplished using the same module, ashown in Figure 1. ATE module or deviis built up of an array of these couplearranged electrically in series and the

mally in parallel. Thermoelectric energconversion utilizes the Seebeck effecwherein a temperature gradient is imposed across the device, resulting in voltage that can be used to drive a currenthrough a load resistance or device. Thisthe direct conversion of heat into electriity. Conversely, the Peltier heat generatewhen an electric current is passed througa TE material provides a temperature gradient, with heat being absorbed on thcold side, transferred through (or pumpe by) the TE materials, and rejected at thsink, thus providing a refrigeration capa bility. The advantages of TE solid-state e

ergy conversion are compactnessquietness (no moving parts), and localizeheating or cooling. In addition, energy ithe form of waste heat (0% efficiency) thwould normally be lost may be converteinto useful electrical energy (7–8% effi-ciency) using a TE power-generation dvice.

The best TE materials currently used devices haveZT 1. This value has beena practical upper limit for more than 3years, yet there are no theoretical or themodynamic reasons forZT 1 as anupper barrier. As seen from EquationZT may be increased by decreasingκ L or by increasing eitherα or σ. However,σ istied to the electronic thermal conductivitκ E, through the Wiedemann–Franz relationship, and the ratio is essentially constant at a given temperature.

Some of the goals of current research eforts are to find new materials that eitheraise the current efficiency of TE devic(i.e., increaseZT ) or have the capability ofoperating in new and broader temperature regimes, especially at lower tempertures (T 250 K) and higher temperature(T 400 K). Over the past 30 year

*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 of 1/K (1/T ), andZT is the dimensionless (unit-less) figure of merit. Both must specify the tem-perature at which the quoted value wasobtained.


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alloys based on the Bi2Te3 system[(Bi1–xSbx)2(Te1–xSex)3] and the Si1– yGe y 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 showsZT as a func-tion of temperature for the Bi2Te3 andSi1– yGe y materials as well as many of themore recent bulk materials that have beendeveloped over the last decade. TheZT of more 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 Properties The 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, whereC isthe specific heat andq is the charge of thecarrier.7 For the case of a classical gas, eachparticle has an energy of 3/2(k BT), wherek B is the Boltzmann constant. The ther-mopower is thus approximatelyk B/ e,wheree 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 Celq

k Be

k BT EF

,

The Fermi energy is basically the energsuch that atT 0, all the states abovethis energy are vacant and all thestates below are occupied. The quantityk B/ e 87 V/K is a constant that repre-sents the thermopower of a classical electron gas. Metals have thermopower valueof much less than 87V/K (on the orderof 1–10 V/K) and decrease with decreas-ing temperature, that is,EF k BT ).

In a semiconductor, a charged particlemust first be excited across an energy gaEg. 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 temperatureSemiconductors can exhibit either electroconduction (negative thermopower) orhole conduction (positive thermopower)The thermopower for different carriertypes is given by a weighted averageof their electrical conductivity value(σn and σ p):

(6)

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

α α nσn α pσ p

σn σ p.

α Celq

k Be

Egk BT

.

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. I is current.

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


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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(k BT )—toeffectively minimize minority carrier con-tributions. Typical thermopower valuesrequired for good TE performance are onthe order of 150–250V/K or greater.

For high-temperature applications, it isimportant to minimize the contribution of minority 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.2,5These materials and devices are expected

to 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 has been covered extensively in many texts onsolid-state physics, and the reader is re-ferred there.13There are a significant number of carriers and states available forconduction in metals, typicallyn 1022

carriers/cm3

. The electrical conductivity isthen very large for metals, on the order of 106(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/ k BT )]. There are two primaryways to achieve a high conductivity in asemiconductor, either by having a verysmall gap to excite across (Eg/ k BT ) 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 conductivityκ 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 thatκ κ L κ E, as mentioned ear-lier. The electrical conductivity and thethermal conductivity are interrelated, inthatσ is tied toκ Ethrough the Wiedemann–Franz relationship:κ E L0σT , where theLorentz numberL0 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 areκ 2 W m–1 K–1, and typically,κ L κ E.

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 of good TE materials,15,16agreeing with Slackthat the candidate material is typically anarrow-bandgap semiconductor [Eg10(k BT ), or 0.25 eV at 300 K]. Also, themobility 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 ofZT ) 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 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 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 Conductivity In 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 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 sizeof the grains), and interface scattering inthin films or multilayer systems.

The lattice thermal conductivity is give by κ L (1/3)(vsCLph), wherevs is the ve-locity of sound,C is the heat capacity, andLph is the mean free path of the phononsAt high temperatures (T 300 K), thesound velocity and the heat capacity aressentially temperature-independent intypical materials. Therefore, the magnitudand the temperature-dependence ofκ L are basically determined by the mean free paof the phonons. Slack defined the minimuthermal conductivity(κ min) as the thermalconductivity when the mean free path iessentially limited by the interatomic ditance between the atoms within the crystal.17 Typical analysis ofκ min results invalues ofκ min 0.25–0.5 W m–1K–1.14,17

Minimum Thermopower There are certain practical limits fo

each of the parameters used to calculatZT . These practical limits must be possib

in order to achieve a material viable fothermoelectric applications. For examplin Bi2Te3, in order to achieve aZT 1 atT

320 K,σ 1 mΩ cm, α 225 V/K,and κ 1.5 W m–1 K–1. We have alreadydiscussed theZT “barrier,” which in effectis given by minimizing the thermal conductivity. It is practical to investigate mterials where the electronic and latticterms are comparable, on the order o0.75–1 W m–1 K–1. Let us look at thehypo-thetical situation of a material in whichthe lattice thermal conductivity is zer(κ L 0). We will also assume the scatteing in this system is elastic and that th

Wiedemann–Franz relationship, slightlrearranged [κ E/ σ L0T ], is well behavedin this material. Then we can rewritEquation 1 as

ZT α 2T / ρκ E α 2/ L0. (7)

Therefore, for a material to be a viable Tmaterial, it must possess a minimum themopower that is directly related to thvalue ofZT andL0. Given this description,in order to achieve a certain value ofZT ,the material would require thatα (L0)0.5

157 V/K forZT 1, andα (2L0)0.5225 V/K for ZT 2. Of course, any“real” material will possess a finiteκ L, andthese values for the thermopower wihave to be higher to achieve the projectevalues ofZT .

Solid-State Crystal ChemistryApproaches to AdvancedThermoelectric Materials

Thermoelectrics has always been a mterials design problem involving intricatuning of structure–property relationshipin complex solids through principles osolid-state chemistry and physics. The di


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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 aκ L κ min; if it cannot be“tuned” or doped in order to attain a min-imum thermopower (150 V/K), it willnot be able to achieveZT 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, 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 and bonding strongly influence the mobility.Materials with diamond or zinc-blendestructures with a high degree of covalent bonding 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 theZT by dopingthe material to maximize the density of states 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 groupR m (D53d). The hexa-gonal unit cell dimensions at room tem-perature area 3.8 Å andc 30.5 Å. Thelayers stacked along thec-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 of Bi2Te3. For example, the thermal conduc-tivity along the plane perpendicular to thec-axis (1.5 W m–1K–1) is nearly twice that of the value along thec-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-type behavior. 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–1K–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 of short-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 of PbTe solid solutions is low near room tem-perature but rises toZT 0.7 at 700 K,

making PbTe a prime candidate for powe

generation in that temperature range.It is possible to achieveZT in excess of unity at 700 K in structurally related solidsolution compositions, AgSbTe2 (80%)-GeTe(20%), known as TAGS (alloycontaining Te, Ag, Ge, Sb). However, duto high-temperature stability issues, thescompositions are not currently favored inTE devices.

Neither Si nor Ge is a good TE materiaas the lattice thermal conductivity is verylarge (150 W m–1 K–1 for Si and 63 W m–1K–1for Ge). The lattice thermal conductivity can be substantially reduced by alloyformation between the two elements. Th

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 Geis apparently due to the increasedphonon–phonon and phonon–electronscattering.19 Remarkably, such a large re-duction does not unduly reduce the car-rier mobility, andZT 0.6–0.7 could berealized at elevated temperatures. Due totheir exceptional stability at high temperatures ( 1200 K), these alloys are of inteest to NASAfor use in RTGs in deep-spacprobes.

Figure 3. Crystal structure of the state-of-the-art thermoelectric material, Bi 2 Te 3 . The blue atoms are Bi and the pink atoms are Te.


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Modern Solid-State Chemistry Design Concepts for High- ZTMaterials Complex Inorganic Structures. Most of the 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 aZT of 0.8 at 225 K, which is 40% greater than thatof the Bi2–xSbxTe3– ySe y alloys.21 Other po-tential low-temperature TE materialscurrently 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–1K–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 PbTe

structure, have been reported.23

The com-position AgPb10SbTe12 shows an excep-tionally highZT 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. Anothergroup of materials under investigationare half-Heusler alloys, with the generalformula MNiSn (M Zr, Hf, Ti). Acomplex composition of the typeZr0.5Hf 0.5Ni0.5Pd0.5Sn0.99Sb0.01shows aZT of 0.7 atT 800 K, highlighting the intricate balance in structure, composition, and prop-erty relationships in these compounds.24The 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 toκ min. Slack’s concept of a “phonon-glass/electron-crystal,” described earlier,avoids this limitation. The concept ofκ minis 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 formed by heavy elements of low electronegativ-ity differences (e.g., CoSb3-based skutteru-dites). Some doped skutterudites showexceptionally highZT values at elevatedtemperatures (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 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, NaCo2O4.26This oxide attainsZT

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 andBi2Sr3Co2O y, crystallizing in “misfit”(lattice-mismatched) layered structures.26Among then-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 approach

100 V/K with metal-like conductivi-ties.28,29In these compounds, the 4 f 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(120–180W/cm K2, or 3.6–5.4 W m–1K–1)

at room temperature (300 K), which nearly 4–5 times larger than that observein optimized Bi2Te3-based thermo-electrics.30 Unfortunately, the large ther-mal conductivity (15–22 W m–1 K–1)lowers theZT to about 0.2 at room tem-perature. Recently, the lattice thermal coductivity of this system has been lowere by doping Mn in the interstitial positionresulting in the increase ofZT to about 0.4at room temperature.31As mentioned ear-lier,ZT 1 requires a minimum Seebeccoefficient value of 156µV/K. The corre-lated metal with the highest Seebeck coeficient is CePd3, which has a maximum of125 V/K at 140K.29Future investigationsshould focus on increasing the Seebeck cefficients of these materials abov

150 V/K through compositional andstructural tuning.

Engineered Crystal Lattices. The ap-proaches in bulk materials research relheavily on the thermodynamic stability othe phases at a given condition, whereathin-film deposition can yield metastab“designer” phases with unique propertiesQuantum well systems (0D, 1D, and 2Dtake advantage of their low-dimensionacharacter through physical confinemenin quantum dots, nanowires, and thin-filmstructures to enhance the electronic properties of a given material.32 In addition,nanostructured semiconductor materialcould scatter mid- to long-wavelengtphonons and thereby reduce the latticthermal conductivity toκ min.

Researchers at the Research Triangle Istitute (RTI) have demonstrated a signifcant enhancement inZT through theconstruction of Bi2Te3/Sb2Te3 superlat-tices.33These materials exhibitedZT 2.4at T 330 K. The enhancement is attributed to creating a “nanoengineered” material that is efficient in thermal insulatiowhile remaining a good electrical condutor. The thermal insulation arises from complex localized behavior for phononwhile the electron transmission is facilitat by optimal choice of band offsets in thesemiconductor heterostructures. Also, therhave been reports on PbTe/PbTeSe quantumdot structures that yieldZT 1.3–1.6.34These materials have been grown as thicfilms that are then “floated off” the substrate to yield freestanding films, whicwere measured to yield these results. Thenhancement inZT in the superlattice ma-terials appears to be more from a reduction in the lattice thermal conductivitthan an increase in power factor.

SummaryCurrently, there are no theoretical o

thermodynamic limits to the possibl


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values ofZT . Given the current need foralternative energy technologies and mate-rials to replace the shrinking supply of fossil 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 ofZT 4.14This givesencouragement that such materials may be possible and could address many of our energy-related problems. Thus, asystematic 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 to be the best, as will become apparent in the

following 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 MRSSymposium 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 Generation Applications (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);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 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, inEncyclopedia of AppliedPhysics, Vol. 21 (1997) p.339.8. P.M. Chaiken, inOrganic 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, inCRC Handbook of Thermo-electrics, ed. by D.M. Rowe (CRC Press, BoRaton, 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, inSolid State Physics, Vol. 34, ed-ited by F. Seitz, D. Turnbull, and H. Ehrenreic(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 THogan, eds.,Chemistry, Physics and MaterialsScience of Thermoelectric Materials: BeyondBismuth Telluride (Plenum, New York, 2003)p. 35.21. D.Y. Chung, T. Hogan, P. Brazis, MRocci-Lane, C. Kannewurf, M. Bastea, C. Uhand M.G. Kanatzidis,Science 287 (2000)p. 1024.22. R.T. Littleton IV, T.M. Tritt, M.KorzenskiKetchum, 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. DC. Uher, T. Hogan, E.K. Polychroniadis, and MKanatzidis,Science303 (2004) p. 8181.24. Q. Shen, L. Chen, T. Goto, T. Hirai, J.YaG.P. Meisner, and C. Uher, Appl. Phys. Lett. 79(2001) p. 4165.25. T. Caillat, J.P. Fleurial, and A. Borshchevs J. Phys. Chem. Solids58 (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 HArai, J. Appl. Phys. 79 (1996) p. 1816.28. R.J. Gambino, W.D. Grobman, and A.MToxen, Appl. Phys. Lett. 22 (1973) p. 506.29. G.D. Mahan, inSolid State Physics, edited byF. Seitz, H. Ehrenreich, and F. Spaepen (Acadmic Press, New York, 1997) p. 51.30. D.M. Rowe, G. Min, and V.L. KuznetsoPhilos. Mag. Lett. 77 (1998) p. 105; D.M. RoweV.L. Kuznetsov, L.A. Kuznetsova, and G. Mi J. Phys. D: Appl. Phys. 35 (2002) p. 2183.31. T. He, T.G. Calvarese, J.-Z. Chen, H.Rosenfeld, R.J. Small, J.J. Krajewski, and MSubramanian,Proc. 24th Int. Conf. on Thermo-electrics, edited by T.M. Tritt (IEEE, PiscatawayNJ, 2005) p. 434.32. L.D. Hicks and M.S. Dresselhaus,Phys. Rev.B 47 (1993) p. 12727.33. R. Venkatasubramanian, E. Siivola, Colpitts, and B. O’Quinn,Nature 13 (2001)p. 597.34. T.C. Harman, P.J. Taylor, M.P. Walsh, anB.E. LaForge,Science297 (2002) p. 2229.35. T.M. Tritt,Science283 (1999) p. 804. ■


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Terry M. Tritt, GuestEditor for this issue of MRS Bulletin, is a pro-fessor of physics atClemson University. Hereceived both his BAde-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 thermal

transport properties andphenomena, and espe-cially in measurementand characterizationtechniques in novel ma-terials. He has extensiveexpertise in thermoelec-tric materials and meas-urement science and has built an internationallyknown laboratory forthe 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 of the executive board of the International Ther-moelectric Society (ITS)since 1999 and served aschair and host of the24th InternationalConference on Thermo-electrics (ICT-2005)

at Clemson in June2005. 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 of Madras 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 subsequently joined 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 magnetoresistive

materials, high-κ andlow-κ dielectrics, ferro-electrics, multiferroics,oxyfluorination, andthermoelectrics.

Subramanian is a vis-iting professor at theInstitut de Chimie de laMatière Condensée deBordeaux (ICMCB),University of Bordeaux,France. He serves as edi-tor forSolid State Sciencesand Progress in SolidState Chemistry, andserves or has served onthe editorial boards of the Materials ResearchBulletin, Chemistry of Ma-terials, and the Journal of

Materials Chemistry . Hewas awarded theCharles Pedersen Medal by DuPont in 2004 forhis outstanding scien-tific, technological, and business contributionsto the company. He hasauthored more than200 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 of the Fraunhofer Institutefor Physical Measure-ment Techniques in

Freiburg, 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 quaternarysemiconductorII–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 thedevelopment of semi-conductor gas sensorsand thermoelectricmaterials until 2003.Böttner’s current re-search activities focuson thin-film andnanoscale thermo-electrics and microelec-tronics-related devicetechnology. He is theauthor or co-author of more than 100 publica-tions, holds more than10 patents, and is amember of the board

of the InternationalThermoelectric Society.

Böttner can bereached at FraunhoferIPM, Research FieldThermoelectrics, Hei-denhofstrasse 8, 79110

Freiburg, Germany; tel.49-761-8857121 ande-mail [email protected].

Thierry Caillat is a sen-ior member of technicalstaff with the Jet Propul-sion Laboratory at theCalifornia Institute of Technology. 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 at JPL. He joined the per-manent staff at JPLin1994. Caillat’s primaryresearch interests havefocused on the identifi-cation and developmentof new thermoelectricmaterials and devices.In the last ten years, hehas played a key rolein identifying severalfamilies of promisingcompounds for thermo-electric applications,including skutteruditesand β-Zn4Sb3–basedmaterials. More recently,he has been involvedin the developmentof advanced thermoelec-tric power-generationdevices for both


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


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space and terrestrialapplications.

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 mechanicalengineering at theMassachusetts Instituteof Technology. He re-ceived his BS (1984) andMS (1987) degrees fromthe Power EngineeringDepartment atHuazhong University of Science 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 transportphenomena, particularlynanoscale heat transfer,and their applicationsin energy storage andconversion.

Chen is a recipient of a 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, the

Journal 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 severalorganizations.

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

Ryoji Funahashi has been 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-performancesuperconducting oxidematerials and, recently,the exploration of newthermoelectric oxidematerials.

Funahashi is a recipi-ent of the Thermoelec-tric Conference of Japan’s Best PaperAward, the Japan Journalof Applied PhysicsPaperAward, 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 theThermoelectricSociety 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 andhydrothermal methodsfor synthesizing nano-structured thermoelec-tric materials. Shereceived the Best Scien-tific Paper Award at the

2004 ICT meeting inAdelaide, Australia.Under the guidance of Terry M. Tritt at Clem-son, her current researchinvolves the synthesisand characterization of

nanostructured 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 [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 Iowain 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 interestsinclude 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 of Nantes, the Universityof Münster, and theUniversity of Munich.The bulk of his workis described in morethan 450 researchpublications. He holdssix patents and is editorin chief of the Journal of Solid 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 Tokyo before joining NagoyaUniversity as a full pro-fessor in 1992. His re-search focuses onthermoelectric materials

Gang Chen Ryoji Funahashi Xiaohua Ji Mercouri Kanatzidis


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and bio-inspired pro-cessing of inorganicmaterials.

Koumoto received theRichard M. FulrathAward in 1993 and theAcademic Achievement

Award of the CeramicSociety of Japan in 2000.He became a fellow of the 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 scientificpapers 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 of physics at the Univer-sity of South Florida,where he has been since2001. He received hisPhD degree fromStevens Institute of Technology in 1994 andconducted pioneeringstudies on the synthesisand thermal propertiesof filled skutteruditesas 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 of Thermoelectrics: 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 of South 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 received both his BS and PhD de-grees from the Califor-nia Institute of Technology 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 of amorphous steel. He haspublished more than200 papers.

Poon can be reachedat the University ofVirginia, Department of Astronomy-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 atClemson University. He

obtained his PhD degreein physics from theUniversity of Kentuckyin 1989 and held apostdoctoral appoint-ment with Mildred S.Dresselhaus at MIT

until 1991. His currentresearch focuses onunderstanding and con-trolling the synthesis of 1D nanostructuredorganic and inorganicmaterials. He has pub-lished extensively on thesynthesis, characteriza-tion, and applications of carbon nanotubes.

Rao can be reached atClemson University, De-partment of Physics andAstronomy, 107 Kinard

Laboratory, 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 of Tokyo in 1986, 1988, and1992, respectively. He began his research ca-reer as a research associ-ate at the University of Tokyo, 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 150papers, conference pro-ceedings, invited talks, books, and reviews.

Terasaki can bereached at Waseda Uni-versity, Department of Applied Physics, 3-4-1

Okubo, 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 RTIInternational 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, photonicmaterials, the study of nanoscale thermalphysics, thermalmanagement in high-performance electronics,and direct thermal-to-electric conversion de-vices. He is well knownfor pioneering thermo-electric superlatticematerials and devices.Venkatasubramanian’s


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


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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 forEntrepreneurial Devel-opment in NorthCarolina.

Venkatasubramanianis a recipient of theAllen B. Dumont Prizefrom Rensselaer, RTI’sMargaret Knox Excel-lence Award in Research

in 2002, and the IEEEEastern North CarolinaInventor of the Year in2003. He has severalpatents issued inthermoelectrics, hasauthored more than 100refereed publications,

and has contributed totwo book chapters.

Venkatasubramaniancan be reached at RTIInternational, Center forThermoelectric

Research, PO Box 12194,Research Triangle Park,NC 27709-2194, USA;tel. 919-541-6889 ande-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 degreein physics from theUniversity of Oregon in1991, an MS degree inradiological physicsfrom Wayne StateUniversity in 1994, anda PhD degree in physicsfrom the University of Michigan in 2000. Hisresearch interests in-clude low-temperaturetransport properties of intermetallic compounds

and semiconductors,magnetism, thermoelec-tric materials, and thedevelopment of thermo-electric technology forautomotive applications.

Yang has publishedseveral book chapters

and more than 30papers. 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 of directors of the Interna-tional ThermoelectricSociety in 2005.

Yang can be reached

at General MotorsResearch and Develop-ment Center, Mail Code480-106-224, 30500Mound Road, Warren,MI 48090, USA; tel. 586-986-9789 and e-mail [email protected]. ■

RamaVenkatasubramanian

Jihui YangIchiro Terasaki

For more information, see http://www.mrs.org/bulletin_ads



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Introduction

As the search for promising bulk ther-moelectric materials intensifies, certainmaterial systems stand out as possessingthe highest potential for achieving ther-moelectric figures of merit well above unity.These materials possess relatively goodelectrical properties while maintainingvery low thermal conductivities. In somecases, enhancements in the electrical prop-erties have been realized. These resultshave attracted great attention, and manyresearch laboratories worldwide are nowworking on one or more of these materialsystems in order to achieve further im-provements in their thermoelectric prop-erties. We discuss some of the bulkthermoelectric materials of primary inter-est in this article.

SkutteruditesThe physical properties of skutterudites

depend sensitively on their compositions.This compositional dependence not onlyprovides a means to investigate the struc-ture–property relationships in this mate-rial system but also allows the optimizationof transport properties for thermoelectric(TE) applications. The diversity of poten-

tial compositional variants allows for richvariation in physical properties and is oneof the key reasons this material systemcontinues to be investigated by many re-search groups. One approach for optimiz-ing these materials is void-filling.

The skutterudite-type (CoAs3-type)structure is a cubic structure with thespace group composed of eight corner-shared TX6 (T Co, Rh, Ir; X P, As, Sb)octahedra. As depicted in Figure 1, thelinked octahedra produce a void, or va-cant site, at the center of the (TX6)8 cluster,occupying a body-centered position in thecubic lattice. This is a large void that canaccommodate relatively large metalatoms, resulting in the formation of filledskutterudites. Many different elementshave been introduced into the voids of skutterudites, including lanthanide, ac-tinide, alkaline-earth, alkali, thallium, andGroup IV elements.1–3 The compositioncan be written as■ 2X8Y24 (typically XCo, Rh, or Ir; Y P, As, or Sb), with thesymbol■ illustrating the two voids percubic unit cell.

The concept of introducing “guest”atomsinto these voids to act as strong phonon-

scattering centers, thus greatly reducing thlattice thermal conductivity of these compounds, has resulted in improvements inthe TE properties of skutterudites.1,2 Thesmaller and heavier the ion in the voids, thlarger the disorder that is produced andthus the larger the reduction in the latticthermal conductivity. Skutterudite antimonides possess the largest voids and artherefore of particular interest for TE appcations. Results from Sales et al.4 andFleurial et al.5 show highZT values (thecommon figure of merit for comparing diferent TE materials) at elevated tempertures in LaFe3CoSb12 and CeFe3CoSb12 for both p-type andn-type specimens.ZT val-ues approaching 1.4 above 900°C for thesematerials have been reported,5 indicatingtheir successful optimization for TE poweconversion applications.

It should be noted, however, that asmall concentration of void-fillers resul

in a large reduction in thermal conductivity. Five percent of La6 or Ce,7 for example,in the voids of CoSb3 results in a thermal-conductivity reduction of 50%, as com-pared with CoSb3. In certain cases, higherpower factors have also been obtainewith partial filling, as compared witmore fully filled, charge-compensatecompositions. The aim in investigatinpartially filled skutterudites is realizing aoptimum electron concentration while re


Recent Developmentsin BulkThermoelectricMaterials

George S. Nolas, Joe Poon,and Mercouri Kanatzidis

AbstractGood thermoelectric materials possess low thermal conductivity while maximizing

electric carrier transport. This article looks at various classes of materials to understandtheir behavior and determine methods to modify or “tune” them to optimize theirthermoelectric properties. Whether it is the use of “rattlers” in cage structures such asskutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the abilityto manipulate the thermal conductivity of a material is essential in optimizing itsproperties for thermoelectric applications.

Keywords: alloy, compound, thermal conductivity, thermoelectricity.

Figure 1. Schematic illustration of a skutterudite crystal, where the guest atom is inside a 12-coordinated “cage” (green) surrounded by yellow pnicogen (family of Bi, Sb, As, P, or N) atoms. The metal sites are depicted in blue.The octahedral environment surrounding the metal sites is shown in the lower portion of the figure, also in blue.



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ducing the thermal conductivity, therebymaximizingZT . This is possible, since par-tially filled, uncompensated skutteruditesobey a rigid-band model, with the voidfillers being the electron donors. Thus, im-proved TE properties in partially filledskutterudites are achievable, owing to thecombination of a low thermal conductiv-ity with higher power factors as comparedwith those of nearly fully filled skutteru-dites. Figure 2 in the introductory article by Tritt and Subramanian in this issueshows one example8 (Yb0.19Co4Sb12) of how partial filling can result in higherZT values than full filling. Ba0.24Co4Sb12 hasalso demonstratedZT 1 above 800 K.9Although these results are encouraging,and of technological interest for waste-heat recovery, still higherZT values can beachieved by further optimization. Most re-cently, for example, a small amount of Nidoping for Co in Ba10 and Ca11 partially

filled skutterudites has resulted in higherZT values than the ZT values of BaxCo4Sb129 and CaxCo4Sb12.12

The success of Slack and Tsoukala’s13“rattling-atom” approach to skutteruditeresearch has lead toZT 1 in a bulk TEmaterial. The research into skutteruditecompounds has also led to a greater un-derstanding of thermal transport proc-esses, revealed novel phonon-scatteringmechanisms, and resulted in efforts to ex-plore other materials with similar proper-

ties to obtain low thermal conductivitieswhile maintaining high power factors. Infact, the field is now focusing on low-thermal-conductivity compounds as amajor component in high-ZT thermoelec-tric materials, as we will see in several ex-amples to follow.

ClathratesClathrates are a class of novel materials

that can be thought of as periodic solids inwhich tetrahedrally bonded atoms (typi-cally Ge, Si, or Sn) form a framework of cages that enclose relatively large metalatoms. Clathrates have demonstrated in-teresting properties that are rare incondensed-matter physics. One of themore interesting of these properties for TEapplications is a very low “glass-like”thermal conductivity in Type I clathrates.14The Type I structure (Figure 2) can berepresented by the general formula

X2Y6E46, where X and Y are guest atomsencapsulated in two different polyhedra,while E represents the elements Si, Ge,or Sn.

There have been little data reported onthe high-temperature TE potential of thesematerials, and even less on the optimiza-tion of the TE properties of these materialsfor power-conversion applications. Blakeetal.15used density functional calculationsto theoretically investigate the transportproperties of Type I clathrates. Their find-

ings showed room-temperatureZT valuesof up to 0.5 for optimized Sr8Ga16Ge30andBa8In16Sn30 Type I clathrates, whileZT 1.7 at 800 K was predicted for optimizecompositions. Kuznetsov et al.16were firstto measure the Seebeck coefficient and rsistivity above room temperature for theType I clathrates. Estimating the hightemperature thermal conductivity frompublished results of low-temperaturethermal transport in Type I clathrates re-sulted inZT 0.7 at 700 K for Ba8Ga16Ge30and ZT 0.87 at 870 K for Ba8Ga16Si30.

Recently, research was initiated towardoptimizing the TE properties of Type clathrates above room temperature.17Power factors approaching 1 W m–1K–1forGe clathrates were obtained at 650 K. Tgether with the low thermal conductivityachieved in these materials, these data indicate the potential for highZT values atelevated temperatures. As outlined earlier

attempts at optimizing the TE propertiesof these interesting materials have only recently begun. Future studies, howevershould also focus on other clathrate structure types—Type VIII clathrates, for example. Inn-type clathrates, optimizationhas reachedZT 0.4 at room tempera-ture,18 with theoretical predictions for p-type clathrates to beZT 1.2 at 400 K.19

The Type II clathrate structure (Figure 3) is particularly interesting, as it allows for partial filling of the polyhedra, icontrast to the totally filled Type I structure. Thus, the electrical properties of TypII clathrates can be more readily “tuned,


Recent Developments in Bulk Thermoelectric Materials

Figure 2. Crystal structure of the Type I clathrate. Framework atoms are shown in blue, guest atoms inside the tetrakaidecahedra are orange, and guest atoms inside the pentagonal dodecahedra are purple.

Figure 3. Crystal structure of the Type II clathrate. Framework atoms are shown in blue, guest atoms inside the tetrakaidecahedra are orange, and guest atoms inside the pentagonal dodecahedra are purple.


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allowing for better control of the dopinglevel.17 In compounds with the Type IIstructure, the interstitial “guest” atomsthat reside inside the polyhedra simulta-neously act as electrical dopants andphonon-scattering centers. These proper-ties allow an investigation into the interre-lationship between the guest atoms andtheir atomic cages and how this relation-ship affects their TE properties.

Half-Heusler AlloysHalf-Heusler (HH) intermetallic alloys

have recently received increasing atten-tion as potential TE materials for high-temperature applications. HH phases havethe MgAgAs (space groupF 3m),20 or“half-stuffed GaAs,” crystal structure,consisting of three interpenetrating fccsublattices. Their chemical formula isXYZ, where X, Y, and Z can be selectedfrom many different elemental groups.21

Figure 4 shows the unit cell of TiNiSn, inwhich Ti and Sn occupy a NaCl lattice andNi occupies an fcc sublattice. HH phasesare semiconductors21–24when the valencecount per formula unit is 8 (e.g., LiMgPhas a bandgap of 2.4 eV) or 18 (e.g.,TiNiSn has a bandgap of 0.3 eV). SomeHH alloys are half-metallic ferromagnetsand heavy-fermion metals.22,25The narrow bands give rise to a large effective massthat in turn leads to a large thermo-power.26Because of the large thermopowerand spin-polarized band structure, HH al-loys have been investigated as thermo-electric and spintronic materials.21,25–28

The chemistries of the three sublatticescan be tuned independently. For example,in TiNiSn, doping the Sn site provides thecharge carriers, while doping the Ti andNi sites causes mass fluctuations that canlead to the reduction of thermal conduc-tivity. Many of the refractory metal26,27,29,30and lanthanide metal28thermoelectric HHalloys exhibit large room-temperatureSeebeck coefficients of100µV K–1 andmoderate electrical resistivities of1–10µΩ m. The alloys reported are attractive ashigh-temperature TE materials, becausethey are relatively easily synthesized as100% dense samples. In particular, therefractory-based alloys exhibit high melt-ing points of 1100–1300°C as well aschemical stability and essentially zerosublimation at temperatures near 1000°C.

The effectiveness of doping in achievingimproved TE properties has been demon-strated in several HH alloys.26–32Sb-dopedTiNiSn alloys exhibit power factors ashigh as 4.6 W m–1K–1at 650 K (380°C).29Despite the large power factor values,there exists the challenge of reducing therelatively high thermal conductivity (κ 10 W m–1K–1) that is evident in the HH al-

4

loys. The effect of disorder induced by dif-ferences in mass and atomic size is evidentin the reduction ofκ .26–32

Notable progress was made when Shen

et al. reported a maximumZT 0.7 at800 K inn-type Sb-doped ZrNiSn alloyspartially substituted with the heavier Pdatoms in the Ni sublattice.31More recently,Sakurada and Shutoh reported maximumZT values near 1.4 at 700 K inn-type(Zr0.5Hf 0.5)0.5Ti0.5NiSn1– ySb.32 Despite failedattempts33 to reproduce the results of Sakurada and Shutoh, their report has al-ready generated considerable interest inHH alloys, as evidenced by the number of papers on HH alloys presented by re-searchers at the 2005 International Confer-ence on Thermoelectrics.

Meanwhile, recent approaches haveprovided new impetus for evaluating theZT of thermally stable, multicomponentHH alloys measured up to 1100 K. Indeed,the latest research has raised the prospectof achieving aZT value near 1. In recentstudies33performed jointly by two groups(University of Virginia and Clemson Uni-versity), the HH compositions were dopedwith Sb contents (up to 4%), which is sig-nificantly higher than previously reported.This is because it is recognized that higherdopant content can partly mitigate thecompensated behavior characterized by a

rollover in the Seebeck coefficientα ob-served inn-type alloys at high tempera-tures. It is believed that the shift ofα(T )can lead to a larger power factor and

therefore higherZT . The idea has beenverified in some (Ti,Zr,Hf)NiSn quinaalloys when indeed it was found that ahigher Sb doping content, the maximumin α 2σT , whereσ is the electrical conduc-tivity, is shifted to a higher temperature. Iaddition to doping the Sn sites with Sbthe refractory-metal sublattices are simutaneously and randomly occupied by HZr, and Ti, providing additional tuning fooptimizing the power factor as well afluctuations in the atomic mass and inteatomic force for reducing the lattice themal conductivity.34

Using the laser-flash method,κ (T )for three complex alloys have been meaured up to 1160 K by Tritt. The resulare shown in the lower-right inset in Figure 4.ZT for the quinary HH compositionHf 0.75Zr0.25NiSn0.975Sb0.025is also shown inFigure 4. It is noted that theZT values of HH alloys are already comparable witthose of state-of-the-art SiGe alloys ne800°C.

While the prospect exists of achievineven higherZT values in complex, ther-mally stable,n-type HH alloys, p-type HHalloys such as MM’X (MTi, Zr, and Hf;



Figure 4. Figure of merit ZT versus T for Hf 0.75 Zr 0.25 NiSn 0.975 Sb 0.025 obtained from measured ( ) and extrapolated ( ) thermal conductivities. (inset, upper left) Unit cell of TiNiSn half-Heusler phase. (inset, lower right) Thermal conductivities obtained from laser-flash thermal diffusivity and specific heat, with data points starting at 300 K: Hf 0.75 Zr 0.25 NiSn 0.975 Sb 0.025 ( ), Hf 0.6 Zr 0.25 Ti 0.15 NiSn 0.975 Sb 0.025 ( ), and Hf 0.75 Zr 0.25 Ni 0.9 Pd 0.1Sn 0.975 Sb 0.025 ( ).


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M’ Co, Ni, and Pt; X Sb and Sn, for ex-ample) should also be investigated. In-deed, TiCoSb exhibits a high Seebeckcoefficient of 400µV K–1 at 300 K. Fur-thermore, one can exploit grain- boundaryscattering effects35 through grain-size re-finement. A bottom-up approach has also been used to construct bulk nanostruc-tured samples that result in a lower latticethermal conductivity.36

β-Zn 4Sb 3 AlloysA p-type intermetallic compound most

suitable for use as a high-ZT material atmoderate temperatures is one of the threemodifications of the Zn4Sb3phase, namely,β-Zn4Sb3 (the other two modifications arethe α and γ phases).37Theβ phase has thehexagonal rhombohedric crystal structure(space groupR C, unit cell dimensionsa 12.231 Å andc 12.428 Å) and is stable between 263 K and 765 K. The highestZT value reported forβ-Zn4Sb3 is 1.4 at400°C.38 Apparently, β-Zn4Sb3 starts todecompose into ZnSb and Zn phasesabove 400°C. Single-phase, polycrystallinesamples were made by melting andhomogenizing Zn and Sb in sealed quartzampoules. The obtained ingots wereground into powders. The alloy powderswere then hot-pressed to form crack-freesamples. In other reports, direct synthesismethods involving hot pressing or sparkplasma sintering were also reported.39–41

Both single-crystal and polycrystallinesamples have been investigated. Thepower factor in Zn4Sb3 is rather low( 0.87 W m–1K–1 at 400°C) for a high-ZT material. For comparison, HH alloys havepower factors of 3–5 W m–1K–1 at 400°C.What makesβ-Zn4Sb3 a remarkable TEmaterial is its “phonon-glass” behavior,characterized by an unusually low ther-mal conductivity of 0.9 W m–1 K–1 at300 K.42The already low thermal conduc-tivity in Zn4Sb3 can be further reduced bydoping the Zn and Sn sites.42,43However,the doping concentrations are found to bequite limited; only a few percent of In orSn can be added to substitute Sb withoutresulting in a two-phase material. Even inthe case of substitution with the isostruc-tural compound Cd4Sb3, the solid-solutionrange of (Zn,Cd)4Sb3 is limited to only6 mol% Cd4Sb3 at 400°C.39 However, noimprovement inZT above the highestrecorded value of 1.4 at 400°C has beenreported for the doped samples.

The detailed crystal structure ofβ-Zn4Sb3has been determined by employing bothsingle-crystal and powder x-ray diffractionmethods coupled with maximum entropyanalysis.42,44 The study has enabled theidentification of valence states of Sb atomsthat fully occupy the nonequivalent

==3

Sb(1) and Sb(2) sites as Sb3– and Sb24–dimers, as well as the valence states of Znatoms that occupy 90% of the Zn(1) sitesas Zn2+. Overall,β-Zn4Sb3 is a valencecompound belonging to a similar class of compound semiconductors such as Bi2Te3and PbTe. The study has also uncoveredthree different interstitial sites for Znatoms in addition to the main Zn(1) sitesthat are only 90% occupied. Refinement of the structure based on this discovery ofinterstitial sites reveals a composition of Zn12.8Sb10. The mass density and composi-tion of the new crystal structure arereported to be in agreement with meas-urements. The Zn interstitials exhibit largethermal displacements. The glass-like in-terstitials are largely responsible for thephonon damping that suppresses the lat-tice thermal conductivity. There is appar-ently also Sb disorder along thec-axis thatcan contribute to the glass-like thermalconductivity. Electronic structure andtransport calculations using the crystalstructure obtained have identified thecompound as a p-type semiconductor,44inagreement with experimental findings of the doping trend. It was demonstratedthat the Zn interstitial atoms play a dualrole as electron donors and thermopowerenhancers. First-principles calculations of electronic structure and thermoelectricproperties have also been performed.45The band structure results reveal the cova-lent tendency of the compound, consistentwith the good carrier mobility measured.The high Seebeck coefficient can be attrib-

uted to the strong energy-dependence of theFermi surface topology near band-filling.Recently, study of the low-temperaturephase ofβ known asα-Zn4Sb3 (composi-tion Zn13Sb10) reveals an electronic struc-ture similar to that of theβ phase with a bandgap of 0.3 eV.46Further detailed in-vestigation of the structural and transportproperty changes across theα ↔ β transi-tion will shed light on the nature of order–disorder transition in this complexcrystal system.

ChalcogenidesChalcogenide compounds comprise a

large class of materials that are predomi-nantly semiconductors. Most of the com-pounds are environmentally stable andcan have high melting points. They com- bine a set of properties that make themsuitable for practical thermoelectric appli-cations. Because of the versatility in thechoice of element and chalcogen and thesuitable electronegativity of the chalcogenelements, which varies little from sulfur totellurium, it is possible to obtain semicon-ductors with energy gaps appropriate forTE applications over a wide range of tem-

perature (typically 100–1400 K). Chalcgenide materials have had a prominentposition in the field of thermoelectricitygoing back to its early stages. For example, the cornerstone of today’s TE cooing has been the compound Bi2Te3 and itssolid solutions Bi2–xSbxTe3 and Bi2Te3–xSex.47

Another chalcogenide of importance iPbTe, which has a maximumZT 0.8 at500°C; it has been suitable for power

generation at intermediate temperaturesThe germanium-based TAGS (Te-Ag-GSb) is more efficient than PbTe but hafound little use, due to its high sublimation rate and low-temperature phasetransition.48

The past decade has seen a strong resurgence of interest in achieving superior Tperformance, and these “old” chalcogenides now have found new forms thathave led to more than incremental in-creases inZT .49–51Recent reports indicate

that nanostructured thin-film superlat-tices of Bi2Te3 and Sb2Te3 haveZT 2.4 atroom temperature,50 whereas PbSe0.98Te0.02/PbTe quantum dot superlattices grown bymolecular-beam epitaxy haveZT 1.6.50These thin-film PbSe0.98Te0.02/PbTe sys-tems feature nanodots of PbSe embeddedin a PbTe material. At 550 K, thesesamples were reported to exhibitZT 2.5.The primary reason for the highZT valuesappears to be a very marked depression othe thermal conductivity, presumably bythe strong phonon scattering imposed bythe nanodots of PbSe.

One approach to searching for new

thermoelectrics is to build exotic newstructures from scratch through solid-stateexploratory synthesis. This has affordedthe promising compounds BaBiTe352 andK2Bi8Q13 (Q Se,53,54S55). These possesshighly anisotropic structures, low symmetry, and large unit cells, with “loosely” bound electropositive cations in channelformed by extended Bi/Q frameworks, resulting in a low lattice thermal conductivity

CsBi 4Te 6One noteworthy tellurium compound

is CsBi4Te6, which is in fact mixed-valentin Bi and can be viewed as containing Bi2+ions that form Bi–Bi bonds. This compound features a strongly anisotropicstructure with one of the directions beingprominent in terms of its charge transporproperties. It possesses a lamellar structure with slabs of (Bi4Te6)1– alternatingwith layers of Cs ions (Figure 5a).CsBi4Te6 is very responsive to the type andlevel of doping agent used. Low dopinglevels significantly affect the charge tranport properties of CsBi4Te6. Appropriate p-type doping of CsBi4Te6 with SbI3 or Sbgives rise to a highZT max of 0.8 at 225 K,




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which is among the highest values everreported below room temperature.56 Atthis temperature, CsBi4Te6 is the best-performing TE material and raises newhope for extending the use of TE materialsto lower temperatures than ever before.Different dopants can allow theZT max to be shifted to even lower temperatures, inthe neighborhood of 180 K.57 Band calcu-lations suggest that CsBi4Te6 has very ad-vantageous electronic structural featuresfor a promising TE material, characterized by a great deal of anisotropic effectivemass. The electrical conductivity and ther-mopower are directly attributed to theelaborate electronic structure of a materialnear the Fermi level. The presence of Bi–Bi bonds in the structure is responsible forthe material having a very narrow energygap, nearly half that of Bi2Te3. The nar-rower bandgap is related to the fact thatthe ZT maximum in CsBi4Te6 is achievedat lower temperatures than in Bi2Te3.

Tl9BiTe 6 and Tl 2SnTe 5In the last decade, it has been more fully

appreciated that thallium chalcogenides

tend to possess very low thermal conduc-tivities. Tl9BiTe6 and Tl2SnTe5 are two suchnoteworthy phases. Tl9BiTe6 belongs to alarge group of ternary compounds whichcan be derived from the isostructuralTl5Te3 (Figure 5b).58 Tl9BiTe6 can be opti-mized to exhibitZT 1.2 at 500K, mainlydue to its extremely low lattice thermalconductivity of 0.39 W m–1K–1 at 300 K.This value is nearly the same as the onereported for the PbSe0.98Te0.02/PbTe quan-tum dot superlattices grown by molecular- beam epitaxy.

The Tl2SnTe5 is a tetragonal phase, withchains of (SnTe5)2–running parallel to eachother and charge-balancing Tlions situ-ated in between (Figure 5c). The Tlionsare in an eightfold-coordinated site withrelatively long Tl–Te bonds. These long bonds produce very low-frequency phonons,which is one of the main reasons that thecompound has a very low thermal con-ductivity (0.5 W m–1K–1). This compoundcan be optimized to aZT of 1 at 500 K.59

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04 Thermoelectric Materials