journal homepage: www.elsevier.com/locate/nanoenergy

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nle

bFREng, FNAE, National University

Received 22 August 2012; receivedAvailable online 27 October 2012

concepts and explains briey the challenges in enhancing the gure of merits. It also reports the

future trends of nanocomposite materials and its underlying challenges of fabrication.

dense packaging, the heat generation in chips is nearly

showed that forlity of the siliconstudy indicatedic failures areoblem has, thusf scaling of CMOSement and logic-r system level

cool ambience without using harmful chemicals like CFC and

Nano Energy (2013) 2, 1902122211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.nanoen.2012.10.005

Abbreviations: CMOS, complementary metal oxide semiconductor; ZT, gure of merits; TE, thermoelectricnCorrespondence to: 200, Jalan Sultan # 13-10, 199018 Singapore, Singapore. Tel.: +65 83751070; fax: +65 6281 7761.

E-mail addresses: hilaal@qtechnanosystems.com, alam.hilaal@gmail.com (H. Alam).example, can perform reliably well for many years whenthey are operating at or near the ambient temperature. Dueto the advancement of semiconductor lithography and

approaches [4].The thermoelectric is a promising technique to convert

the waste thermal energies into useful power as well as toIntroduction

Researchers and industries are throwing their efforts andresources to nd solutions for the escalating energy crisisand the threatening global warming. One of the ways totackle these issues lies in achieving higher efcient thermalmanagement which solves several other issues in tandem.A solid state or semiconductor electronics component, for

touching 100 W/cm2 [1]. Bar-Cohen et al.every 21 K rise of temperature, the reliabichip decreases by 10% [2]. A U.S. Air Forcethat more than 50% of the electrontemperature-related [3]. The thermal prbecome one of the major limiting factors oand hence it is essential for thermal managow to be considered concurrently fo& 2012 Elsevier Ltd. All rights reserved.various approaches adopted in bulk materials, complex structures and the recent nanostructures tocircumvent the interdependency of parameters in achieving higher ZT. It ends with discussion of theKEYWORDSSeebeck;Thermoelectric;Nanocomposites;Microelectronics;Power generation;Thermal managementgaporeof Singapore, Singapore

in revised form 12 October 2012; accepted 13 October 2012

AbstractThermal management and energy crisis have been two major problems in this 21st century.The thermoelectric concept is seen as a perfect solution for the both issues provided its gure ofmerit is large enough to compete with the traditional techniques. Since the use of semiconductormaterials for thermoelectric applications, there has been a huge quest for improving its gure ofmerits (ZT) to cross 3 in order to make it commercially viable. This review starts with thermoelectricaQtech Nanosystems Pte. Ltd, SinREVIEW

A review on the enhancemefrom bulk to nano-thermoe

Hilaal Alama,n, Seeram Ramakrishnabt of gure of meritctric materials

kb Boltzmann constanth Plank constantm effective mass of charge carriersko electronic thermal conductivity electrochemical

potential gradient is zeroE energy

191Enhancement of gure of merit from bulk to nano-thermoelectric materialsmoving parts. It is, however, already in use to cool diodelasers [5], critical electronics [6], portable refrigeration [7],cool/ heat car seats and even railway passenger cars inFrance [8] where the cost is not a primary concern.Thermoelectric cooling is also used to produce 80 1Ctemperature to operate the sensors in infrared imagingsystems for heat-seeking missiles and night-vision sys-tems [9]. In life science biological assaying has beenrevolutionized with the development of polymerase chainreaction (PCR) systems which use thermoelectric to ther-mally cycle microliter quantities of enzymatic reactionsthrough the exact series of temperature cycles. The processis used to multiply specic sequence of DNA materials foranalytical purposes [10].

In the other direction, thermoelectric generators arechosen for generating power because of its reliable, main-tenance free operations. It is also used when the durabilitywith longer operating life under extreme conditions arerequired. The silicon germaniumthe high temperaturepower generation materialsled directly to the productionof heat engines with no moving part for space applicationthat could operate even in the absence of the sun. Althoughthe solar cells are used in spacecraft, they are useful onlyfor the short distance up to the Mars and beyond where thesolar radiant ux is not adequate, thermoelectric takes overto generate the power. All the power sources for US and

Nomenclature

S Seebeck coefcientE chargecoulombT temperatureKelvinI currentampQ HeatZ gure of merit -/KZT gure of merit (dimensionless)K thermal conductivityKe electronic thermal conductivityKth Lattice Thermal conductivityL Lorenz factor 2.4 108 J2/K2C2N number of charge carriersformer USSR deep-space probes have used thermoelec-tric heat engines to convert heat generated by nuclearssile materials to electricity [11,12] Micro-thermoelectricgenerator are used in many low power devices such ashearing aids and wrist watches. Recently Seiko and Citizenintroduced commercialized thermoelectrically driven lowpower wrist watches [12].

A report informs that 69% of energy demand is forheating/ cooling and electricity. 191 million vehicles strongUS, dissipate 66% of their energy as heat through emissionand about 36 TWh of waste process heat per year in USA[13]. Thus a huge amount of waste heat from automotive,industry operations, and mankind activities can be con-verted into the useful energy with TE. However, the gureof merit ZT of the thermoelectric materials should reachwell above 3 from the current1.5 to 2, in order to competewith the traditional techniques [14] such as vapor compres-sion machines.Law of thermoelectric effects

The phenomenon involving a direct energy conversion fromheat into electricity (or vice versa) is known as thermo-electric effect. In 1821 Thomas Seebeck discovered thegeneration of voltage when a conductor was subjected to atemperature gradient and converted the thermal energyinto electricity with the efciency of about 3% [15].An electromotive force is generated due to the chargecarrier diffusion and phonon drag between two extremetemperatures (Fig. 1). Actually, the whole systems is insemi-equilibrium i.e. chemical potential (m) due to theconcentration is balanced by the built-in electrostaticpotential which is known as Seebeck voltage [16]. Thechange in electrochemical potential (qxm) in x directionper unit charge e per unit temperature (qT) denes theSeebeck coefcient (S) [17],

S 1e@xm@T

i

The reversal of this phenomenon is known as Peltiereffect (

Q) which is a measure of the amount of heat

(Q) carried by electrons or holes and is proportional to theelectrical current (I) owing in the circuit [18].

P Q ii

s electric conductivityt0 relaxation timeP electric resistivitym carrier mobilityQ

Peltier coefcientt Thomson coefcientL scattering parametert0 energy-independent scaling co-efcientI

The Seebeck effect and the Peltier effects can bedescribed with what is known as the Thomson effect thatexplains the heating or cooling of a current carryingmaterial subject to a temperature gradient. More speci-cally heat is liberated if an electric current ows in thesame direction as the heat ows; otherwise it is absorbed.The Thomson coefcient t can be dened as [18]

dQdx

tI dTdx

iii

Figure 1 Charge transportation from the hot to cold end fornegative Seebeck co-efcient materials.

possibly all other competing concepts when temperature lift isvery small such as 5 1C [20]. Moreover, it is encouraging to know

H. Alam, S. Ramakrishna192that the second law of thermodynamics does not impose anyrestriction on increasing the value of ZT and yet, it has beendifcult to cross ZT beyond certain point [21].

Challenges in enhancing ZT

The best thermoelectric materials were succinctly denedas phonon-glass electron-crystal (or PGEC in short),which means that the materials should have a low latticethermal conductivity as in a glass, and a high electricalconductivity as in a crystal [22]. The interdependency of theTE parameters makes the enhancement efforts of ZT verychallenging. The normal ways of optimizing TE materials areto increase the power factor S2s by optimizing the carrierconcentration n, and/ or to reduce the lattice thermalconductivity Kth by introducing the scattering centers.These parameters are the function of scattering factor r,carrier effective mass mn and carrier mobility m and theirinterconnectivity limit ZT to about 1 in large bulk materials.

According to the kinetic denition S is the energy differ-ence between the average energy of mobile carrier and theFermi energy [23]. If the carrier concentration n is increased,the Fermi energy as well as the average energy increases.However, the Fermi energy increases more rapidly than theaverage energy when n is increased. As a result S decreases,dragging the power factor (S2n) down rapidly. Thus inattempting to increase ZT for most of the homogeneousmaterials, the carrier concentration (n) increases electricalconductivity (s) but reduces the Seebeck coefcient (S).For this reason, in metals and degenerate semiconductors(energy-independent scattering approximation), the Seebeckwhere dQ/ dx is the rate of heating (or cooling) per unit length,I is the electric current and dT/dx is the temperature gradient.The Seebeck coefcient becomes negative when electronsdiffuse from hot end to populate the cold end with electrons.The performance of the thermoelectric materials is oftendenoted as gure of merit Z whose unit is K1, or ZT thedimensionless unit [19].

ZT sS2

KeKthT iv

where s is electric conductivity and K=Ke+Kth+Kbi. Ke isthermal conductivity due to electron transport, Kth is thethermal conductivity due to the lattice phonon. The thermalconductivity is also contributed by the bipolar effect Kbi. Fromthe Eq. (iv), the absence of the thermal conductivity causes ZTto reach innity and the thermoelectric devices to approachthe Carnot cycle efciency. In ZT, the energy carriers such aselectrons and holes, contribute to electric conductivity (s) andelectronic thermal conductivity (ke) while the phonons con-tribute to the lattice thermal conductivity(Kth). It is also foundthat commercially available materials those have ZT1 cannotcompete with the traditional vapor compression systems whenoperating at a relatively larger temperature lift or difference(THTC), e.g. 30 1C. However, decreasing the temperature lift,the efciency of thermoelectric modules increases rapidly. It isexpected that the efciency of thermoelectric modules couldeventually surpass that of traditional vapor compression andcoefcient can be expressed as [24]

S 8p2k2B

3eh2mnT

p3n

23 v

The parameter mn is density of states effective mass inthe Eq. (v). The high mn inuence the power factor to raiseaccording to the Eq. (v). Most materials having high mn havegenerally low m which limits the power factor by a weightedmobility with the relationship of power factor proportional to(mn)3/2m. Also, there is no such thing as an optimal effectivemass. There are low mobility high effective mass polaronconductors (oxides, chalcogenides) as well as high mobilitylow effective mass semiconductors (SiGe, GaAs) [25].

It should also be noted that the defects scatter not onlythe phonons but also the electrons. Hence there are sometrade-offs carried out in carrier mobility when designing forreducing lattice thermal conductivity. The ratio of m/Kthdetermines the improvement of ZT [15]. Although theincrease in the ratio is usually experimentally achievedthrough a more reduction in Kth rather than that in m, somefundamental issues in this mechanism are not understoodwell[?].

WiedemannFrenz Law states that [25] the electroniccontribution to the thermal conductivity is proportional tothe electrical conductivity of the materials and the relation-ship is

Ke=s LT viwhere L is Lorenz factor 2.4 108 J2/K2C2 for free elec-trons and this can vary particularly with carrier concentra-tion. The electrical resistivity (r) is related to the carrierconcentration n, electron charge e and chemical poten-tial m as [25]

Ir s mne vii

The electronic thermal conductivity can thus beexpressed as

Ke sLT mneLT viiiThis relationship shows that the low carrier concentration

will result into the lower electrical conductivity decreasingZT. High mobility carriers are most important for high valueof electrical conductivity. Again from the Eqs. (v), it isshown that increasing the effective mass of the carrierincreases S but reduces the carrier mobility and hence theelectric conductivity s according to the Eq. (viii). Thethermal excitation of carrier from valence band to conduc-tion band creates holes and electrons in case of the narrowsemiconductor. However, the concentration of the majorcarrier does not vary much. Bipolar effects takes placewhen two types of carriers are present [26] and this isnotorious to achieve effective thermoelectrics. The impor-tant effect is conduction of the heat from hot side to coldside even during the absent of the net current, the origin ofKbi. The next is the suppression of Seebeck coefcient bythe presence of both carriers with opposite signs of electro-nics charge. The Kbi for n type materials can be expressedas [27]

Kbipolar seTkBe

2 2r5Zg 21eZf eZg mne=mnh

3=2 me=mh 3=2 !

ix

(of a given energy in the transport process) with zero

193Enhancement of gure of merit from bulk to nano-thermoelectric materialsvariance or no spread around the mean value is called Diracdelta distribution. At this narrow band, Ke can be minimizedwithout decreasing the electric conductivity s. This narrowbands produce high effective masses and hence the largeSeebeck coefcient and hence the enhanced power factor[29]. In addition to lowering the thermal conductivity,another criteria to achieve the ZT1, the material shouldbe able to attain some minimum values of importantparameters such as S150 mV/K; no matter if the materialpossess the lowest thermal lattice conductivity Kth=Kmin[30]. Based on the above explanations, the ideal thermo-electric materials are desired to have the distribution ofenergy carriers as narrow as possible, the high carriervelocity in the direction of the applied eld, and thedefects to scatter phonons in order to enhance ZT. However,Pichanusakorn et al. postulates that it is the sheer magni-tude not the shape of the DOS that is important [23].

The thermopower (or Seebeck coefcient) is less than thecharacteristic value 87 mV/K in case of metals. It is very lowin the order of 110 mV/K for metals and much larger in theorder of 102 to 103 mV/K for semiconductors. The heavilydoped semiconductors were found to have comparatively agood ZT. The thermoelectric materials thus comprise of ahuge family, including different materials from semimetals,semiconductor to ceramic, containing various crystallineforms from monocrystals, polycrystals to nanocompositeand covering varying dimensions from bulk, lm, wire tocluster. Some polymers are also showing interesting thermo-electric material properties recently [31].

Enhancing ZT in bulk materialsalloying anddoping approach

Ewing in 1881, studied the effects of mechanical stress onthe thermoelectric quality of metals and found that thecyclic phenomenon so conspicuous is not peculiar to thethermoelectric effects of stress, but is probably present inother effects of stress and other parameters such astemperature when this is subjected to increment anddecrement [32]. In 1904, there were attempts to studythe change of quality of thermoelectric power by magneti-zation with iron, nickel and cobalt [33]. Anisotropic thermo-electric power in different crystals with deliberatelyintroduced crystal defects with graphites was also explained[34]. Allnatt and Jacobs studied the single crystal pureIn short, any attempt to increase s, will increase Ke whichcontributes to thermal conductivity (K). In order to counterthe increment of Ke, Kth can be decreased by variousapproaches. However, decreasing Kth with phonon scatter-ing by adding defects results in decrease in carrier mobilityand electrical conductivity. These are the major conicts inthe bulk materials properties which were addressed in theresearches for more than a half century. Most of the effortshave been spent on reducing the lattice thermal conductiv-ity by phonon engineering in bulk materials and lowdimensional materials [28]. Spitzer reports that for semi-conductors, the lower limit for the lattice thermal con-ductivity is 0.2 W/mK [29]. Due to the lower limit of the Kth,there are suggestions to exploit the electronic thermalconductivity rather than Kth. The distribution of carrierspotassium chloride under 561963 1C. They also proposed atheory of thermoelectric in 1961[35] and conducted manyexperiments on some ionic crystals to show potentialproduced even in the absence of temperature gradient at470570 1C [36].

In the other direction, in 1930s, the researchers, gainedinterests in the synthetic semiconductor based thermo-electric materials having Seebeck co-efcient in excess of100 mV/K [15]. It was Ioffe who developed the theory ofsemiconductor, initiated wide research in semiconductorsbased thermoelectric materials [37]. In the semiconductors,the ratio of thermal conductivity to electrical conductivity(K/s) was greater than in metals owing to their poorerelectrical conductivity. In 1956, Ioffe and coworkers demon-strated that the ratio could be decreased if the thermo-electric material is alloyed with an isomorphous element orcompound [38]. The major approach to enhance the thermo-electric properties in bulk materials was carried out by tuningor doping techniques to vary the lattice thermal conductivitywithout affecting the electrical conductivity. A good thermo-electric material with very high ZT is typically heavily dopedsemiconductors with low thermal conductivity.

The simple and commonly available thermoelectric mate-rial is Bismuth Telluride (Bi2Te3) with interesting features ofits Seebeck coefcient depending on its composition. Anundoped Bi2Te3 corresponding to 60 atomic present Te-isp-type with a Seebeck coefcient of about 230 mV/K. As theconcentration of Te increases, the Seebeck coefcientgradually falls to zero then changes its sign to becomenegative (n-type). The maximum ZT of p-type and n-typeBi2Te3 crystalline materials (not alloys) at room tempera-ture are about 0.75 and 0.86, respectively [16].

The alloying allows ne tuning of the carrier concentrationof the lattice thermal conduction. This does not decreaseelectric conductivity but lowers only the thermal conductivity.As there is a complete solid solubility among Bi2Te3, Sb2Te3 andBi2Se3, the addition of Sb2Te3 and Bi2Se3 to Bi2Te3 improves theZT by reducing lattice thermal conductivity without causingtoo much degradation of electronic properties. In the lowtemperature regions, up to 300 K, Bi2Te3 and Sb2Te3 alloyswere found working ne [16,25]. Doping Bi2Te3 shifts itsproperties accordingly. Cs, for example in Bi2Te3 material(CsBi4Te6) exhibits a high thermoelectric ZT below roomtemperature ( 0.8 at 225 K). At cryogenic temperatures,the thermoelectric properties of CsBi4Te6 are matching with orexceeds that of Bi2Te3 [39].

The group IV tellurides (PbTe, GeTe, SiTe) based thermo-electric materials are used at higher temperature around 500900 K. Both n and p-type PbTe samples can be produced bymaking PbTe departure from its stoichiometry and by addingsome impurities such as chalogenides (PbCl2, PbBr2, PbI2 etc.)and chalcogenides as donors and alkali metals (Na2Te, K2Teetc.) as acceptors. The ZT of PbTe is about 0.001 at roomtemperature but found surpassing Bi2Te3 at higher tempera-tures. PbTe are usually used by substituting Sn for Pb to formPb1xSnxTe or Se for Te to form PbTexSe1x. While it is possibleto obtain a value of ZTslightly above 1 for n-type PbTe alloys ata temperature of about 600 K, the ZT for the p-type PbTe alloysis only about 0.7 [16]. Alloys with AgSbTe2 have stimulatedinterests in researches to increase ZT41 for both p and n typematerials. The p type alloy (GeTe)0.85(AgSbTe2)0.15 known asTAGS, has ZTabout 1.2 at around 400 K for long life applications

be achieved by scattering phonos in different frequency ranges

H. Alam, S. Ramakrishna194[40]. Since telluride is a rare in the Earths crust (0.001 ppm)even compared to Pt (0.005 ppm) and Au (0.004 ppm), PbSeand PbS were found as an alternative to Te [27]. The traditionalsemiconductor materials like SiGe alloys (used as high tem-perature thermoelectric materials) were reported to have lowZT in4900 K due to high thermal conductivity because of theirdiamond structure [25].

The coexistence of a large thermopower (100 mV/K at300 K) and a low resistivity was found in the transition-metaloxide NaCo2O4 which made this compound an attractivecandidate for thermoelectric (TE) application. This compoundhas two orders of magnitude larger carrier density (1021 cm3)but showing thermopower comparable to that of the usual lowcarrier density TE materials [41]. With this the partial replace-ment of Na with Ca systematically increases the thermopower.This enhancement showed the strong correlation with theelectronic specic heat (g). Though the Ca substitution doesnot reduce g, value appreciably, it reduces the carrier concen-tration and increases thermopower. The Na with Ca substitution(Na, Ca)Co2O4 also led a crucial conclusion on low dimension-ality of materials (which will be discussed later part thisreview). The high thermoelectric properties of this materialare attributed to the enhanced effective mass coming fromentropy of Co4+ in the low state spin [42]. Later Fujita et al.reported that ZTof single crystal NaCo2O4 exceeding 1 at about1000 K [43] and Ohtaki et al. of ZT 0.8 at 1000 K withpolycrystalline NaCo2O4 [44]. Sugiyama et al. reported thatfor both NaxCoO2 and 3 and 4 layer cobaltides, a common lowtemperature magnetic state form in the CoO2 [45].

The above cobalt based oxide semiconductors and theirderivatives are p-type semiconductors which have potentialadvantages over conventional high temperature thermo-electric semiconductors such as SiGe based alloy (ZT1) andb-FeSi2 (ZT0.3) in terms of chemical and thermal resis-tance at around 1000 K. On the other hand n-type semi-conductors which are inevitably required as a partner of thep-type oxide semiconductors to develop thermoelectricpower generators, have exhibited rather a low ZT valuesof0.3 for Al-doped ZnO at 1273 K and 0.31 for In2O3(ZnO)m (m=integer) at 1073 K. Shingo Ohta et al. developedheavily doped SrTiO3 as a promising candidate for p typesemiconductors due its large effective mass (m=610 m0).Also bulk single crystals of heavily La-doped SrTiO3 havebeen found to have large power factor of 3.6 103 W/mK2at the room temperature which is comparable to that ofpractical Peltier material Bi2Te3 [46]. This research group,in addition to La, used Nb as dopant after solving the issueof solubility of Nb in SrTiO3 lattice.

The compound Zn4Sb3 was reported as one of the mostefcient thermoelectric materials. Its high efciency is duethe extraordinary low thermal conductivity comparable tothat of glass in conjunction with the electronic structure of aheavily doped semiconductor. The thermopower of Zn4Sb3 is13 mW/cmK2 at 400C which is high but still less than half thatof the next best thermoelectric material (AgSbTe2)0.15(GeTe)0.85 (TAGS) [40]. Jeffrey et al. reported that Zn4Sb3contains signicantly glass like interstitial sites, with zincatoms distributed over multiple positions. This acts as aneffective mechanism for lowering thermal conductivity [47].

b-Zn4Sb3 has been found to be having thermal conductiv-ity that is nearly independent of temperature between300 K and 650 K [48]. This is unusual since Kth is inverselyutilizing a variety of methods such as mass uctuation scatter-ing (a mixed crystal in ternary and quaternary compounds),grain boundary scattering due to the size of the grains andinterface scattering in thin lms or multilayer systems.

In the second approach to reduce thermal conductivity,complex crystal structures were used to separate theelectron crystal from the phonon glass. In the third method,multiphase composites were mixed on the low dimensionalmaterials to increase scattering [50]. By using heavymasses, with low spring constants and the larger unit cells,lattice thermal conductivity Kth was found reduced. Inaddition to it, Kth was reduced by means of solid solutionalloying. Complex bulk materials like skutterudites, clath-rates and Zintl phases have been explored and reported asone of the several ways to obtain high gure of merits. Manyresearchers reduced thermal conductivity by creating dis-order in unit cells like interstitial sites or partial occupan-cies in the alloying. For example, rare earth chalcogenides(La3-xTe4) with Th3P4 structure have relatively low thermalconductivity due to large number of random vacancies [51].As phonon scattering by alloying is depending on the massratio of the alloy constituent, the random vacancies arefound to be ideal scattering sites. The developments innano-sized structures give new possibilities apart from usingcomplex structures. The transport properties which areproportional to T. This also exhibits the value of Kth=0.6 W/mK at 300 K which is nearly two fold lower than the Bi2Te3alloys. The Seebeck coefcient too increases with increas-ing temperature and peaks at 675 K to about 200 mV/K thatcan be considered to be the best among semimetals. This isone of the best ZT obtained with polycrystalline sample ofabout 1.3 at 675 K as shown in [48] by 1997. Similar tob-Zn4Sb3, CsFe4-xCoxSb12 gives ZT of 1.3 but at 900 K [30].Further reduction in Kth was found with Zn4-xCdxSb3 mixedcrystals but it is less temperature stable than the b-Zn4Sb3.More optimization with these materials seems to be limitedconsidering the doping techniques and the restricted com-positional variation possible. However, since 1960s therewas no much enhancement of ZT and in the next fourdecades the research community lost its quest to nd newmaterials. However, there were military and space applica-tions those were giving sufcient but slow momentum.

Cu2Se is a potentially interesting thermoelectric materialwhich can compete with other conventional TE materials.The ZT of 1.6 was obtained at 973 K with its b-phase madeby ball milling and hot pressing. The disordered Cu atomswere observed to have random motion at high temperaturethat causes its specic heat abnormally [49]. The resultingthermal conductivity is 0.40.5 W/m/K (Fig. 2).

Strategies to enhance ZT with novelapproaches

A renewed interest in thermoelectrics began in 1990s after thefour decade of slowdown in the research. The novel approachesopened several possibilities to enhance ZT. One of the widelyadopted methods to enhance ZT is to reduce the lattice thermalconductivity. Three general approaches are mainly used toreduce the thermal conductivity. In one approach, phononscattering is used to reduce the thermal conductivity. It can

195Enhancement of gure of merit from bulk to nano-thermoelectric materialsinterrelated in the bulk materials become relatively simplerto handle independently in nanoscale structures. Thisspurred new interests on the thermoelectric among theresearchers to the new height.

Half Heusler materialsmass uctuation strategy

One of the general strategies to reduce the thermalconductivity is, to introduce the mass uctuations in thehalf Heusler materials. The half Heusler materials get theirreputation for being high temperature thermoelectric mate-rials owing to their high temperature stability. The structureof half Heusler, generally denoted as ABX, is a simple rocksalt structure formed by A and X and lled with element B atone of the two body diagonal positions (1/4, 1/4, 1/4) in thecell, leaving the other one (3/4, 3/4, 3/4) unoccupied [27].

Figure 2 Temperature dependent thermoelectric properties oftemperatures. (a) Electrical resistivity, (b) Seebeck coefcient, (c) pobulk), (e) Total thermal conductivity (lled symbols) and lattice thermHalf Heusler alloys having MgAgAs type crystal structure areforming three interpenetrating face-centered-cubic (fcc)sublattices with one Ni sub-lattice vacant. Despite of havingsmall band gap semiconductors with a gap of 0.10.5 eV, highthermopower (S 300 mV/K) and high electric conductivityyielding very high power factors at room temperature, theirhigh thermal conductivity (10 W/mK) impose challenges inattaining high ZT [52]. These issues were solved with series ofresearches by increasing the phonon scattering via chemicalsubstitutions and nanocomposite inclusions.

ZrNiSn being a subset of a much larger family of MgAgAstype structure has a band gaps of about 0.210.24 eV [53].and are known to have a band gap inter-metallic compoundswith large thermopower and semi-metallic to semiconduct-ing transport properties. Shen et al. introduced the phononmass uctuation scattering by alloying Hf on Zr site that ledto an overall reduction of the total thermal conductivity K.

Cu2Se1.01 bulk samples prepared with different hot pressingwer factor, (d) specic heat (Cp), and thermal diffusivity (HP700al conductivity (open symbols) and (f) gure-of-merit, ZT [49].

siderably because the point defect induced by Ge substitu-

H. Alam, S. Ramakrishna196This group explored the effect of less than 1 at% Sb dopingon the Sn site, which results in several orders of magnitudelowering of the resistivity without signicantly reducing theSeebeck effect and leading to large power factors comparedto the state-of-the-art materials. Later Shen et al. exploredthe effect of isoelectronic alloying of Pd on Ni site, trying toreduce K of these compounds further. As a result of thesetwo, in Hf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01 compound, a powerfactor of 22.1 mW/K2cm and a thermal conductivity as lowas 4.5 W/mK were achieved at the room temperature. TheZT increased to 0.7 with the increment of temperature to800 K.

Browning et al. reported that even though the propertiesof ZrNiSn compound varied greatly with various chemicalsubstitutions, no net increase in ZTwas achieved. However,Uher et al. suggested the substitution of Indium samples inZrNiSn compounds to enhance ZT. The preliminary measure-ment of chemically substituted TiNiSn suggested that thissystem may hold promise for enhanced ZT values inchemically disordered materials [54].

Hiroaki Muta et al. applied substitutions and additions ofthe excess nickel to ZrNiSn compounds and conductedexperiments from the room temperature to 1000 K. Thelattice thermal conductivity of Zr0.7 0.3NiSn (X=Ti, Hf),ZrNi0.7Y0.3Sn (Y=Pd, Pt) and ZrNi1.05Sn compounds weredetermined and found that the excess amount of nickelcaused a drastic reduction of the lattice thermal conduc-tivity up to 700 K. Beyond that limit, it started increasingdue to ambipolar effect which depends on the hole trans-port properties. It indicates the necessity to disturb thehole conduction (or electronics conduction as a p typematerials) for reduction of the additional electronic thermalconduction. Ti0.5Zr0.25Hf0.25NiSn was reported to have verylarge ZT=1.5 at 700 K. The alloying Hafnium together withtitanium resulted in a strong reduction of the thermalconductivity [55]. Thermoelectric properties of ZrNiSn wasattempted to improve by changing the degree of antisitedefects of Zr and Sn atoms Qiu et al. [56]. These antisitesare found to shrink the band gap combined with theenhanced DOS slope contributing to the improved electricaltransport properties. As a result, a high ZT of 0.64 at 800 Kwas attained without doping or isoelectronic alloying. Whilemany n type half Heusler compounds have been investigatedon their high temperature thermoelectric properties, only avery few high temperature p type half Heusler compoundssuch as ZrPtSn, HfPtSn, ErPdSb and ErPdBi have beenreported.

Another interesting half heusler material TiCoSb is sensi-tive to the Ti and Co sites due to the conduction band andthe valence band of TiCoSb compound that consist of 3dorbitals of Ti and co atoms. TiCoSb has an energy band gapof 0.95 eV which is the highest among the half Heuslercompounds. Sekimoto et al. prepared (Ti, Zr, Hf)CoSbsystems by arc melting method and obtained ZT of 0.025on TiCoSb at 921 K [57]. Including Fe, in p type TiFexCo1xSb, with randomly distributed TiO2 increases theelectrical conductivity. It causes the increment of theSeebeck Coefcient to 300 mV/K at 850 K for x=0.15. Withthe inclusion of a small amount of Fe, TiCoSb shifts from ntype to p type conduction. Increasing Fe causes the drasticreduction of thermal conductivity due to the presence ofTiO2. The mass uctuation and strain eld uctuation due totion for Sb intensively scatters the phonons. The reductionof the lattice thermal conductivity is mainly attributed tothe strain eld uctuation. However, the thermal conduc-tivity was still higher than the state of the art materials.The ZT of TiCoGe0.15Sb0.85 was found to be 0.16 at 850 K.

Qiu et al. [60] achieved a decent ZT in TiCoSb compoundsby alloying Zr on the Ti site. In this alloy, the electronicsconductivity and thermal conductivity were suppressedwhile the Seebeck coefcient was improved dramaticallywith the highest value of 420 uV/K for Ti0.5Zr0.5CoSb at600 K. This provides a large space for optimizing thethermoelectric performance. Also by doping Ni on the Cosite resulted in increasing the electrical conductivityremarkably with the still large Seebeck coefcient. TheZT of 0.7 at 900 K was achieved for Ti0.6Hf0.4Co0.87Ni0.13Sb.

Sekimoto et al. studied the properties of polycrystallinesample of XPdBi (X=La, Gd). The band gap of LaPdBiand GdPdBi were found to be 0.05 and 0.07 eV, respectively,and showed the semiconductor like behavior with theelectrical resistivity of 106O m. Increasing the tempera-ture increases the carrier contribution on the electronicthermal conductivity and decreases its positive thermo-power. The maximum ZT for LdPdBi at 615 K was just 0.085and for GdPdBi at 469 K was just 0.084 [61] (Table 1).

XCoSb as p type materials and XNiSn as n-type materialshave been used in most of the researches (X=Ti, Zr and Hf)[62]. While XNiSn compounds have high ZT, the ZT of theACoSb compounds were less comparing its counterpart.Wang et al. reported the computational model with theelectronics properties with Pseudopotential and KorringaKohnRostoker methods to explore the ways to enhance thethermoelectric properties of half Heusler materials [62].Apart from (Ti, Zr, Hf) CoSb, (Ti, Zr, Hf) NiSn, (V, Nb,Ta)CoSn there are less investigated half Heusler materialssuch as NbCoSn, NbRhSn, ZrCoBi, VFeSb, NbFeSb for p typeand LaPdBi, NdCoSn, YNiSb, ZrCoBi for n type materials[27]. Generally arc melting and spark plasma sinteringmethods are widely adopted for preparation of half Heuslermaterials.

Complex cell structuresclathrates, skutteruditesand Zintl phase with rattling strategy

Slack suggested a new criterion for nding better thermo-electrics in which the cage compounds with a large unitcells containing encapsulated atoms that can rattleinside the voids will have a low thermal conductivity [63].The problem in skutterudites or clathrates is high thermalconductivity due to strong bonding and simple order, andthis was solved by doping carrier concentration for electronthe substitution of Fe to Co site too contributes thereduction in thermal lattice conduction. Comparing to thestate of the art materials, the thermal conductivity is stillhigh to 3.7 W/mK at 850 K. The ZT is 0.45 for TiFe0.15Co0.85Sb which is relatively higher among p type half Heusler[58]. Wu et al. [59] doped Ge in TiCoSb compounds andfound the increment of Seebeck Coefcient, electricalconductivity with the increasing Ge. The Seebeck coef-cient changed from negative to positive with Ge doping at850 K. Interestingly, the thermal conductivity reduced con-

Figure 3 Crystal structure of type-I clathrate compounds. Adodecahedron (X20) centered at the body center of the unit celland one of its neighboring tetrakaidekahedra (X24) are indi-cated in the gure. The atoms in the 6c sites are indicated withshaded circles.

CoSHf s

ion

197Enhancement of gure of merit from bulk to nano-thermoelectric materialsphonon interaction. In addition to it, alloying with transitionmetals or antimony sites reduces thermal conductivity.The size and vibrational motion of the lling atoms andthe thermal conductivity leads ZT to as high as 1. Also rareearth or other heavy atoms can ll the voids leading tolower the thermal conductivity. Large space for the llingatoms in the skutterudites or clathrates cause soft phononmodes and rattling mode that reduces thermal conductivity.

Bulk materials such as Bi2Te3 can be further modied withcomplex variants. CsBi4Te6 is for example a complex variantof commonly used Bi2Te3 and it has lower thermal con-ductivity than Bi2Te3. Cs adds complexity with Cs layers andBiBi bonds. The BiBi bonds lowers the band gap and thisshifts the peak ZT of 0.8 to lower temperature than that ofBi2Te3. CsBi4Te6 has also anisotropic effective mass whichimprove Seebeck co-efcient with slight detriment ofmobility [64]. Many ordered MTe/ Bi2Te3 type variants(M=Ge, Sn or Pb) are known making up a large homologousseries of compounds with Zt=0.6. Many of these materialsare still not explored well with doping. Low thermal

Table 1 Figure of merits of half Heuslers materials of NiSn,Gd. NiSn groups shows promising performance with Ti, Zr andhigh temperature applications.

Compounds Substitut

XNiSn (X=Ti, Zr and Hf)Ti0.5Zr0.25Hf0.25NiSn [55] Ti/ HfZrNiSn [56] AntisitesHf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01 [53] Pd on NiXCoSb (X=Ti, Zr and Hf)TiCoSb [57] TiTiFe0.15Co0.85Sb [58] Fe on CoTiCoGe0.15Sb0.85 [59] Ge on SbTi0.5Zr0.5CoSb [60] Zr on TiTi0.6Hf0.4Co0.87Ni0.13Sb [61] Ni on CoXPdBi (X=La, Gd)LdPdBi [61] GdPdBi [61] conductivity can be obtained with thallium based thermo-electric materials such as Ag3TlTe3 and Tl9BiTe6. Apart fromcomplex unit cell, the unique property of thallium of beingextremely soft bonding leads to the low thermal conductiv-ity (0.23 W/mK at room temperature) [65].

Clathrates are one of the new classes of candidates withtetrahedrally coordinated atoms forming a cage around ametal atom. The general formula of type I clathrates isAxByC46y. The B and C are tetrahedrally bonded to make aframework that forms cages around the guest atom A. A8C46(with A=Na, K, Rb; C=Si, Ge, Sn), A8B8C38 (with A=Na, K,Rb; B=Al, Ga, In; C=Si, Ge, Sn) and A8B16C30 (with A=Sr, Ba,Ca; B=Al, Ga, In; C=Si, Ge, Sn) are example of clathratescandidates. In these compounds the anionic frameworks arecovalently bonded providing them a medium for high carriermobility [66]. Usually type I clathrates compounds consist oftwo pentagonal dodecahedra (X20) and six tetrakaidecahe-dra (X24) in the cubic unit cell of the space group of Pm3n[67] as shown in the Fig. 3.

Sr8Ga16Ge30 is one of the most interesting clathratesexhibiting n-type semiconducting behavior with fourb and PdBi groups with substitutions such as Ti, Zr, Hf, La andubstitutions. Most of the half Heuslers materials are used for

s ZT Temperature (K)

1.5 7000.64 8000.7 800

0.025 9210.45 8500.16 850

NA 6000.7 900

0.085 6150.084 496cornered diamond lattice structure of Ge with Sr insidethe cage. It is common to assume that the metal atomsinside the cage donate electrons to the frame which causesthe rattling ions scattering conduction electrons, loweringthe conductivity. This would be a harmful effect if theelectrical conductivity were to take place through Sr. If Sr isionized, the Sr8Ga16Ge30 is not a PGEC material. However,the experimental result of Sr8Ga16Ge30 shows the highSeebeck coefcient S=320 mV/K with low thermal con-ductivity K=0.9 W/mK at the room temperature [68]. Thisshows Sr8Ga16Ge30 closer to being PGEC material than thecommon opinions. Bo B Iversen showed that Sr atoms in theclathrates are practically neutral, that Sr is weakly bound tothe cages and rattle around, that the electrical conductivitytakes place through the frame, and that Sr-based bands donot contribute to the transport coefcient [69]. Eventhough Sr8Ga16Ge30 is a metal, it has high Seebeck coef-cient because of its sharp peak in the density of states atthe Fermi level, which is known to lead to a large Seebeckcoefcient. The high density of states leads to an increase

H. Alam, S. Ramakrishna198of the electronics contribution to the thermal conductivity.The carrier concentration is reported to be around10171019 cm3 [70].

Some type I clathrates such as Ba8Ge43, K8Ge44,(K,Cs)8Sn44 and Rb8Sn44 are reported to have chemicalcomposition deviated from the general formula A8C46. Whileone-third of the 6C sites for X atoms are found to berandomly distributed for K8Ge44, (K,Cs)8Sn44 and Rb8Sn44,half the 6C sites for Ge atoms are occupied in an orderedmanner in Ba8Ge43 [67]. Hermann attributed the additionalreection to the ordering of Ge vacancies in the 6C sites.However, its ZTwas measured to be very low value of 0.057[71]. It changes from Ba8Ge43 superlattice structure tonormal BaGaGe type I clathrates structure when Gacontent is increased. The density of Ge vacancies decreaseswith the Ga contents and this results in increasing Seebeckcoefcient, electrical conductivity while reduction of lat-tice thermal conductivity. [72].

Beintien et al. showed with Ba8Ga16Ge30 the singlecrystalline samples that the excess of Ga shifted it to p-type semiconductor and thermal conductivity similar to isostructural Eu8Ga16Ge30 and Sr8Ga16Ge30 [73]. Hou et al.grew Ba8Ga16Ge30 clathrates by the Czochralski methodsand investigated with varied ratios of Ga/Ge [74]. The lowerpulling and higher pressure resulted in larger Ga/Ge ratioand crystal was found homogeneous. They showed thelarger electrical conductivity and lower thermal conductiv-ity with higher Ga/Ge ratio which does not affect theSeebeck coefcient. They obtained ZT Ba8Ga16Ge30 0.93 at850 K and 1.3 at 1000 K. The loss of Ga during heattreatment above certain temperature like 600 1C resultedin shifting from p type to n type extrinsic semiconductor andshowed high thermal power and reduced electrical conduc-tivity. After heat treating at 800C, the crystal structureseemingly loses less Ga and has negative impact on ZT [75].However, the change of Ga/Ge ratios in Sr8Ga16Ge30 wasreported to be working on the other way. The electricalconductivity was found to be increasing and the Seebeckcoefcient decreasing with decreasing Ga/Ge ratio. Thethermal conductivity was maintained at the same orderlower than 1 W/mK at the room temperature and ZT wasobtained as 0.85 at 650 K for the lowest Ga/Ge ratio sampleof Sr16.86Ga27.01Ge56.13 [76].

Hokazono et al. attempted to increase the Hall mobility inthe Ba8Ga16Ge30 material by substituting Ga with Cu. Heobtained the Hall concentration in n type Ba8CuxGayGe46xyin the order of 1020/cm that is comparable with type VIIIBa8Ga16Ge30 stoichiometric compounds. The Seebeck coef-cient was maintained to the higher value due to the highereffective mass of the conduction band similar to the value of ntype Ba8Ga16Ge30. The inverse temperature dependance of theHall mobility indicates the main scattering mechanism ofconduction electrons is alloy disorder scattering in the tem-perature range of 80 to 300 K [77]. Eto et al. took on extensivestudies on the band gap of intrinsic semiconductor Ba8ZnxGe46-xwhich possess indirect band gap and compared with thatof Ba8Zn8Ge38 the n type degenerate semiconductor andBa8Zn10Ge36 the p type degenerate semiconductor by substitut-ing Zn with Ga. Ba8Zn6Ga4Ge36 was reported to have Eg=0.9 eVwhich is wider than that of Ba8Zn8Ge38 (Eg=0.4 eV) and betterthermoelectric characteristics than Ba8Zn8Ge38 at high tem-perature [78].The p-type group IV clathrates Ba8MGe40 (M=Cu, Ag, Au)were found to have narrow band gap because of noble metaldoping. The thermoelectric power increases with tempera-tures monotonically and magnitude was found to be smallerthan 100 mV/K at 1000 K [79]. The detailed study was madeby Falmbigl et al. with Ba8NxGe46-x and Ba8NxSi46-x (N=Cu,Zn, Pd, Ag, Cd, Pt and Au) and listed in [80].

Another impressive work from S Deng et al. shows thesubstitution of Al for Ga in Ba8Ga16Sn30 material increasesthe negative Seebeck coefcient (300 uV/K at 600 K) due toits metallic behavior in electrical resistivity. Known as typeVIII clathrates Ba8AlxGa16xSn30 its ZT reaches 1.2 at 500 Kwhen x reaches 6 [81]. The authors suggest the possiblefurther improvements by controlling the starting composi-tions of Al and Sn. The recent similar work on Ba8Ga16Sn30 inwhich Cu replaces Ga shows that the ZT for x=0.033 reachesthe maximum of 1.35 at 540 K [82].

Mudryk et al. prepared a novel europium substitutedclathrates, Eu2-x(Sr,Ba)6-x My Si46-x (M=Al, Ga) that areconsistent with type I clathrates. Eu atoms occupy Bacompounds preferentially the 2a position and thus form anew quaternary version of the Ba8Al16Ge30 structure type.The researchers found that Eu2Ba6Al8Si36 and Eu2Ba6Ga8Si36exhibit long-range magnetic order below 32 and 38 K ofpresumably ferromagnetic type. Because of the low thermo-power in the order of 100uV/K, the ZT was achieved as0.025 only for Ba8Al16Si30 [83].

Nolas et al. investigated the transport properties of severalSn clathrates with type I hydrate crystal structures such asCs8Sn44, Cs8Zn4Sn42, Rb8Zn4Sn42, Rb8Ga8Sn38 and found the lowthermal conductivity attributed to the localized disorderassociated with the dynamics motion of the alkali metal atoms[84]. Chen et al. investigated the properties of Sn clathratesunder hydrostatic pressure and reported some interestingfacts of temperature dependence of electrical resistivity andSeebeck coefcient [85]. The theoretical calculation of Sn46shows that the energy gap closes under pressure. The similareffect in Cs8Sn44 and change of semiconductor to semimetaltransition are expected. An irreversible increase in Seebeckcoefcient was observed in both Cs8Sn44 and Cs8Zn4Sn42 afterapplication of pressure. This result demonstrates that thepower factor can be increased permanently by application ofhigh pressure and suggests the possibility of higher efciencythermoelectric device. Zaikina et al. investigated thermo-electric properties on Sn24P19.3(2)BrxI8x and Sn24P19.3(2)ClyI8y and found Sn24P19.3(2)BrxI8-x (x=08) to be with lowband gap value of 0.03 eV to 0.14 eV with linear function ofthe guest halogen atom ratios. The one of the lowest value ofthermal conductivity 0.5 W/mK at the room temperature hasbeen observed for Sn24P19.3Br2I6. The authors report the lowestdegree of ordering of the guest ions leads the signicant lowthermal conductivity not the rattling of guest atom [86].

Only type I clathrates was relatively well studied whileclathrates type II generally ignored due to the difcultyto synthesize it as pure phase with dened componentsand in a rational and reproducible way [87]. (Cs or Rb)8Na16(Si or Ge)136 was prepared in the pure form in 1999.Cs8Na16Si136, Rb8Na16Si136, Cs8Na16Ge136 and Cs8Na16Ge136were analyzed and reported along with electrical andthermal transport properties. Type I clathrates are foundto have edge over type II clathrates in terms of thermalconductivity especially [88].

Skutterudites is a cubic structure containing generally 32atoms per unit cubic cell (Fig. 4). It belongs to the binarysemiconducting compounds forming the skutterudites struc-tures consists of CoP3, CoAs3, CoSb3, RhP3, RhAs3, RhSb3,IrP3, IrAs3 and IrSb3. Unlike clathrates, it possesses voids inthe structure whether or not guest atoms are present. Withthe low electronegativity differences such as CoSb3 and IrSb3, there is a high degree of covalent bonding enablinghigh carrier mobilities and hence good electron-crystalproperties. The strong bonding and simple order cause tohigh thermal conductivity and hence the challenge withskutterudites is in reducing the lattice thermal conductivity.Doping the materials to cause rattle reduces the thermalconductivity. In order to maintain the structural stability,lling the skutterudites with electropositive ions in the voidwith the replacement of the electron decient neighbor isimportant [89].

The atomic displacement of La in lanthanum triironcobalt dodecaantimonide was studied and found that La

are scattered at phonon glass regions. This thin electron

199Enhancement of gure of merit from bulk to nano-thermoelectric materialswas too small to ll in the cage. Site occupancy renementsshow about 25% vacancies on the La site and an actual Fe:Coratio of 2.17:1 in LaFe3CoSb12. La atoms are linked to thedramatic decrease of the lattice contribution to the thermalconductivity [90]. Since Ce was thermodynamically stable itwas included in CoSb3. Ce and Fe were used in CoSb3 to formCeFe3CoSb12 and found the increased Seebeck coefcient,decreased thermal conductivity in the range of 300 850 K.The highest ZT of 0.63 was obtained at 700 K [91]. With Ruand Rh, an n type Ce(Ru0.67Rh0.33)4Sb12 was prepared underpressure and various temperature in pursuit of excellent ZTand low thermal conductivity. It transformed into p type at6 GPa and about 700C [92]. The broad range of resonantphonon scattering can be obtained with double lled skutter-udites in CoSb3. Ba and Yb ll to form BaxYbyCo4Sb12. The ZTofn type BaxYbyCo4Sb12 was achieved to be 1.36 at 800 K[93],[94]. Bai et al. investigated further with other rare earthmaterials such as Ce, Yb and Eu in addition to Yb [95].

SnzCo4Sb11.2Te0.8 was examined with varied temperaturefrom 300 K to 700 K. Sn lled and Te doped CoSb3 showed ntype conductivity at all temperature. In this case Te acts aselectron donor by substituting Sb atoms. The ZT was foundto be increasing to 0.6 at 700 K and when ZT reached above

Figure 4 Schematic illustrating the unit cell of the unlledskutterudite crystal structure.crystal region takes advantage of quantum connementand/ or electron ltering to enhance Seebeck co-efcient.The skutterudites and clathrates are 0 dimensional version ofsubstructures. Another widely known materials that uses sub-structure is cobaltite oxides (NaxCoO2) and CaCoO[41]. YangWang et al. showed the noticeable inuences of Ln ion in[Ca2CoO3]0.62[CoO2]. The Ln

3+ changes the hole concentrationsand introduces a strong point defect phonon scattering [99].

The CaYbZnSb the Zintl phase compound is similar toNaxCo2O with sheets of disordered cations between thelayers of covalently bound ZnSb and the substructureapproach can be clearly seen in it [41,42]. CaxYb1xZn2Sb2is found as promising materials for the thermoelectricapplications. It is simple tunability of their transportproperties lead to the low carrier concentration from 15to 3.1 1019/cm3 [100]. However its thermal conductivity isvery high to 1.5 W/mK [25]. With the broad range ofphonons involved in the heat transportation, long wave-length phonons need disorder on longer length-scales,leading to a need for hierarchal complexity. Combing thesubstructure approach with nanostructured materials seemsto be promising in enhancing the gure of merits.2 the ZT started declining due to higher thermal conductiv-ity and electrical resistivity [96]. In lled and Ni dopedCoSb3 was synthesized and examined in the range of 300700 K. The in lling and Ni doping lowered the thermalconductivity and increased optimized carrier concentration.The ZT of In0.25Co3.9Ni0.1Sb12 was obtained as 0.9 at 700 K[97]. Multiple lled skutterudites were prepared with Ba, Laand Yb CoSb3 and found have achieved ZT of 1.7 at 850 Kwith Ba0.08La0.05Yb0.04Co4Sb12 [98].

In addition to the voids, adding ions increases electronsto compensate cations elsewhere in the structure for chargebalance and create additional source of lattice disorder. Forthe case of CoSb3, Fe

2+ can replace Co3+ partially. Similarcharge balance can be introduced for clathrates wherelling needs replacing group IV (Si, Ge) with group III (Al,Ga) [25]. A Zintl Phase compound contains a valance-balanced combination of both ionically and covalentlybonded atoms. The ionic cations donate electrons to thecovalently bound anionic species. Yb14MnSb11 is one suchexample for the complex Zintl compounds with ZT=1 at900 K [25].

Enhancing ZT-substructure approach

Substructure approach where distinct regions provide dif-ferent functions is one of the effective approaches tocircumvent the PGEC conicts. For example, free chargecarriers conned to planar CuO sheets, are separated byinsulating oxide layers. The insulating layers act as chargereservoir that accommodates dopant atoms those donatecharge carriers to the CuO layers. This separation of thedoping and conducting regions keep the charge carrierssufciently screened from the dopant atoms so as not totrap carriers which will lead to low mobility. The phononglass regions house the dopants and disordered structureswithout disturbing the electron crystal regions. The elec-tron crystal regions are needed to be thin on the nanometeror angstrom scale so that phonon of short mean-free path

H. Alam, S. Ramakrishna200Nanostructured thermoelectric materials

Low dimensional materials were used either to enhance thepower factor or to reduce the lattice thermal conductivity.In one approach the nanoscale constituents are used tointroduce the quantum connement effects to increasepower factors. In the second approach the nanostructuresintroduce many internal interfaces to scatter phonons. Theenhancement of the density of states near Ef leads to thehigher Seebeck coefcient. With low dimensional materialsit is advantageous with anisotropic Fermi surfaces in multivalley cubic semiconductor and increased mobilities at agiven carrier concentration when the quantum conne-ments are satised so that modulation doping and deltadoping can be utilized. Thus they give us freehand tomanipulate the thermoelectric parameters. The anotheraspect is that the low dimensional particles are promisingcandidates in increasing ZT also because Wiedmann Franzlaw is not applicable to nanomaterials with delta like DOS[101]. The carrier mobility m is independent of electronphonon coupling under the normal condition such as nearand above the room temperature [102]. Hence phonon draghas been ignored in the most of the calculation in thesetemperature regions [103].

The theoretical predictions on the strong enhancement ofZTwas based on the modication of Ke and power factor dueto the spatial connement of carriers and correspondingchange in carrier density of states. These predictionsgenerally ignored spatial connement of phonon and usedbulk values of Kph. It should be noted that even in thesuperlattices of similar materials, the phonon transport canbe modied due to mini-band formation and the emergenceof mini-Umklapp processes. The phonon connement affectsthe entire phonon relaxation rate and this makes differencein the thermal transport properties of nanostructures fromthe bulk structures. The simulation results made on Bi2Te3sandwiched between air and soft polymer shows the effectof phonon spatial connement on thermoelectric proper-ties. The change of phonon group velocities and dispersiondue to the spatial connement leads to the increase of thephonon relaxation rate and strong drop in lattice thermalconductivity. The ZT was simulated to show more increasethan its bulk/ ingot counterpart at this condition [103].

2D multiple quantum well (MQW) structures were con-sidered to increase ZTover their bulk counterparts when thequantum well width is reduced. It is mainly because of thedensity of electron states per unit volume that occurs in thequantum well. PbTe/ Pb1xEuxTe were investigated becauseof its well established fabrication techniques and its goodthermoelectric properties. PbTe/ Pb1xEuxTe MQW sampleswere grown by Molecular Beam Epitaxy(MBE) with widthsvarying between 17 and 55 A for x=0.073. The increment ofZT of 2.0 at 300 K occurred only in quantum well due to theenhanced power factor. The lattice thermal conductivitywas assumed to be unchanged [104] from its bulk counter-parts whose value is below 2 mV2/K2cm3. Similarly Si/SiGesuperlattice systems were studied and increase in powerfactor was predicted [105]. Ga, In and Tl create additionalenergy level known as resonant levels in the classicalsemiconductor PbTe. The ZT has been reported to bedoubled in diluted alloys of p type PbTe to 1.5 at 773 Kfrom 1 to 2 atomic % Tl (Tl-PbTe) [106]. The origin of the Tlinduced state is still under investigation but ascribed toeither valance uctuation or a hybridization betweenexcited state of group III atom and neighboring Te p-statesor additional piece of Fermi level.

While one research group studied 2D superlattice systemsin more details to enhance ZT, there were other groupsworking on the development of the higher whole super-lattice sample (denoted as Z3DT). The quantum well widthand barrier width of GaAs/ AlAs whole superlattice waspredicted to have higher ZT than its GaAs bulk values [105].

In addition, the carrier pocket engineering (CPE) canimprove power factor while the band discontinuities can trapcold electrons in wells to increase the Seebeck coefcient[50]. With Carrier Pocket Engineering, it was calculated thatwidth of quantum well and barrier can be adjusted to raise theenergy of lowest G point sub-band to higher than the L and Xpoint sub-bands. This increases the density of state for L and Xpoint sub-bands and thereby enhancing Z3DT. The calculatedvalue of ZT of GaAs/ AlAs was 0.41 (at the room temperature)which was 50 times higher than that of GaAs value [107].While the experimental results showed the enhancement ofZ3DT, the power factor of several superlattices did not showthe enhancement of quantum carrier connement effectsincluding Bi2Te3/ Bi2Se3, PbTe/ Pb1xEuxTe and Si/SiGe [105].

Energy barrier ltering and high electron density in metalproduce large moment of differential conductivity about theFermi level. Also high interface density, large mismatch inphonon density of states and electronphonon interface resis-tance suppress thermal conductivity. If nanoscale roughness canbe designed to overcome the constraints of parallel momentumconservation, very high ZT may be possible [108].

The large anisotropy of three ellipsoidal constant energysurfaces for electrons at the L point in the rhombohedralBrillouin zone, the long mean free path of L point electronsand high mobility of the carriers make Bismuth attractivefor low dimensional thermoelectric behavior. The decreasedquantum well width raise the lowest bound state to thehighest bound state in the valence band leading to semi-metal to semiconductor transition (Fig. 5). The reduction ofwell width in 2D (quantum dots) and 1D (nanowire)enhances the ZT to above 2 and 6 theoretically [105].

One anisotropic hole pocket at the T point of the Brillouinzone and three highly anisotropic and non-parabolic electronellipsoids at the L points of Bi nanowire were theoreticallystudied and attributed that its small effective mass andanisotropic Fermi surface with its peaked DOS. It was foundthe Z1DT can be achieved if the T point hole can be removed orsuppressed to lie below the L point holes. This can be achievedby alloying Bi with Sb to form Bi1xSbx (0.07oxo0.18) [109].The size effect (Fig. 5) was intensively studied on the bismuthnanowires prepared in porous templates and was suggested thesmaller nanowires for the enhanced transport properties [110].(Note: Bi nanowires are chemically unstable with have lowmelting point of 271 1C and hence fabrication and electricalmeasurements are very challenging)

In the second approach the lattice thermal conductivity isreduced. The nite acoustic mismatch between structureand barrier materials in low dimensional structures leads tothe acoustic connement and corresponding reduction oflattice thermal conductivity in-plane. [111].

BixTe1-x nanowires deposited in the nanopores of anodizedalumina membranes using an electrochemical deposition

method were studied for various ratios. The Seebeck coef-cient was found to increase by 15%60% more than the bulk at300 K when x=0.46. For higher value of x=(0.54), Sdecreased smaller than the bulk. The surface roughness (r)on order of 1 nm of the nanowire played a crucial role inreducing lattice thermal conductivity without affecting theelectron conductivity much as the wavelength of phonon isbelow 1 nm while the electrons is around 10 nm [112]. Yet thefundamental reasons why and how nano-structuring reducesthermal conductivity in crystalline materials are not fullyunderstood. Kesker et al. recently presented evidence of asignicant enhancement of ZT with Bismuth nanotubes andnanowires made by aqueous chemical method and found thethermal conductivity being 5 times less than that of Bismuthpowder. The low thermal conductivity is attributed to thereduced lattice thermal conductivity of nanostructured bis-muth networks via interface scattering [113]. The ZTincreases dramatically to 0.2 at 280 K which is much higherthat its powder form as shown in the Fig. 6.

The thermal conductivity of pressure sintered Si0.8Ge0.2 alloyis less than that of crystalline alloy because of the heavy point

defects. However ZTwas not increased due to the proportionalreduction of electrical conductivity. Similar results wereobtained with Si/Ge superlattice, SiyGe1-y/SixGe1-x superlatticewhere the reduction of thermal conductivity did not increasethe ZT [114]. In GaAs/ AlAs superlattice of the order of 14 nmperiod thickness, the cross-plane thermal conductivity wasfound less than that of Al0.5Ga0.5As alloy. Bi2Te3/ Sb2Te3superlattices of 15 nm was reported to have less cross-planethermal conductivity than that of a solid solution alloy [115].Taking cue from this, Kim et al. embedded ErAs nanoparticlesepitaxially into In0.53Ga0.47As as matrix and random manner tocompare the results. The ErAs in In0.53Ga0.47As was reported tohave increased the electronic thermal conductivity butdecreased the phonon contribution due to electronphononscattering and phonon scattering [116]. ErAs was reported togive an additional scattering mechanism for mid to longwavelength phonon.

Yan et al. [117] decreased the average grain size to 1 mm inp-type half Heusler samples and obtained a slightly enhancedZT. By reducing the average grain size further to 200 nmZr0.5Hf0.5CoSb0.8Sn0.2 by ball milling the alloyed ingot into

foructured

Figure 5 Schematic energy band diagram showing the energies of the sub-band edges for the heavy and light electrons and for thetrixg th

201Enhancement of gure of merit from bulk to nano-thermoelectric materialsFigure 6 Thermal conductivity as a function of temperaturereference. The arrow indicates signicant reduction in thermal condTemperature dependence of gure of merit (ZT) for Bi nanostruct

holes for: bulk Bi, 100 nm diameter Bi nanowires along the bisecdirection. Calculated Z1DT for n-type Bi nanowires oriented alonthree different wire diameters [109].Bi nanostructured network samples in comparison with the Biivity of nanostructured Bi network samples compared to Ref. [113]network samples evaluated against Bi reference [113].

direction, and 50 nm diameter Bi nanowires along the bisectrixe trigonal axis at 77 K as a function of donor concentration for

nano-powders and then hot pressing them into dense bulksample, 60% improvement in the ZT was achieved. The peakZT from 0.5 to 0.8 at 700 1C was attained with nanostructuredgrains with the reduced thermal conductivity. Normally thegrain boundaries scatter not only the phonons but also theelectrons. However, in ZrxHf1-xCoSb and in ZrxHf1-xNiSn, nodecrease in carrier mobility was observed when the grain sizegoes from micron to nanometer. This suggests that by reducingthe average grains size further down to 3050 nm, higher ZTcan be accomplished. Joshi et at studied the effect of Tisubstitution for Hf in Hf0.75xTi

xZr0.25NiSn0.99Sb0.01 and foundthat Ti of 0.25 caused a slight increase in carrier concentrationand low thermal conductivity. The ZT of about 1 at 500 K wasachieved with the samples prepared with arc melting followedby ball milling and hot pressing to obtain nanostructures [118].

The nanostructures are generally obtained by ball milling thesamples and nanostructured bulk materials are achieved withdc current hot pressing the (ball milled) samples (Fig. 7). Themajor issue with the ball milling approach is the grains gettingagglomerated to increased grains size.

Nanostructured skutterudites Co1xNixSb3 was prepared toobtain high concentration of grain boundaries to lower thethermal conductivity. The highest ZTof 0.065 has been obtainedat 450 K for the conventionally prepared unlled samples. Thedoping concentration and compaction have to be optimized forfurther enhancement [119]. Jian et al. successfully synthesized

H. Alam, S. Ramakrishna202Figure 7 Temperature-dependent ZT of ball-milled and hot-pressed sample in comparison with that of the ingot [117].Temperature-dependent lattice part of thermal conductivity ofball-milled and hot-pressed sample in comparison with that ofthe ingot [117].nanostructured CoSb3 in particle form of 20 nm and sheet formof 80 nm in an attempt to improve thermoelectric properties byincreased phonon scattering [120]. He et al. obtained the ZTof0.7 at 525C with Co0.9Ni0.09Sb3 [121].

Some researchers were combined these two approaches suchas enhancing the transport properties and reducing latticethermal conductivity together. Quantum dot Superlattices(QDS) have been proposed as solution for the combination ofdecreased lattice thermal conductivity and quantum conne-ment of carriers. In QDS, the electrical conductivity is occurringdue to hopping between dots. The hopping conductivity ischaracterized by very low mobility. Hence while using QDS, it isnot worth relying on low lattice thermal conductivity but alsoon enhanced carrier transport. The strong coupling and regi-mentation was proposed to form the 3D extended mini-bandsinstead of localized quantum dot states. Ge quantum dotsgrown on Si was proposed which can be synthesized as 3Dregimented QDS [122]. The strong modication in phonon groupvelocity results in the enhanced phonon relaxation ratesbecause of the spatial connements [123]. The thermal con-ductivity in Ge/ Si quantum dot superlattices was investigatedtheoretically and experimentally from the range of 10400 K byincreasing the phonon group velocity and reported the reduc-tion of kph compared to the bulk materials [124126].

Superlattice nanowire (SLNW) which consists of series ofinterlaced two different 0D QDS forming a nanowire. Theelectronic transport along the wire axis is made possible bythe tunneling between adjacent QDs while uniqueness each QDsare maintained by the energy difference of the conduction orvalance bands between different materials. Also the nanodotsblock the phonon conduction along the wire axis. Based on this,several materials such as Co/Cu with electrochemical deposi-tion method and GaAs/GaP, Si/SiGe and InP/InAs with vaporliquid solid growth mechanism were synthesized. The lead saltswere investigated with the theoretical model for improving thethermoelectric properties based on this concept as the leadsalts such as PbTe, PbSe and PbS and their alloys are narrow-gapsemiconductors that have been widely studied. The studiessuggest possible enhancement of ZT of 3.3 at 77 K with PbSeof 2 nm and PbS of 8 nm [127].

Device fabrication with nanostructures

Although superlattices and nanowires give very high ZT, in orderto use in large scale applications, either thin lm materials ornanostructured bulk materials are adopted depending on theapplications. For example, on chip integration of Peltierdevices, thin lms are considered more appropriate whereasfor stand-alone devices, nanostructured bulk materials can besuggested.

Thin lm with embedded quantum dots

Thin-lm thermoelectric materials are found to offer tremen-dous scope for the ZT enhancement [128]. There are threegeneric approaches have been proposed to date. The rstmethod uses quantum connement effects to obtain theenhanced density of states near Fermi level [91]. With thisapproach, ZT of 0.9 at 300 k and 2.0 at 550 K have beenachieved in PbSe0.98Te0.02/ PbTe quantum dot structures. Thesecond approach is phonon blocking/ electron transmitting

superlattices. These structures utilize the acoustic mismatchbetween superlattices components to reduce Kph. The thirdapproach is based on thermionic effects in hetero-structures[100,109].

The nanostructured lm materials made up of n typePbSeTe/PbTe QDSL were synthesized with self-assembly tech-nique from MBE growth. The metal wire was used as a p typematerial to build the TE device with ZT of 1.31.6 at roomtemperature against the conservatively estimated value of 2 at300 K[129]. In addition to lead tellurides, Bi2Te3 also wasstudied extensively and were found that none of the telluridecompounds are suitable for integrated devices as these materi-als are not compatible with the standard IC fabricationtechnology [130]. Although, the polycrystalline SiGe and poly-crystalline Si were found as the suitable material based on theirfabrication compatibility, they are inferior to the telluridecompounds as shown in Table 2. The other fabrication techni-ques like MOCVD and ash evaporation are still under develop-ment [130]. The telluride compounds are mostly fabricatedwith MEMS like technologies such as electrochemical process[131].

There was an attempt to develop thermo-ionic coolerscomprising of superlattices within SiGe materials comple-tely based on CMOS technologies as discussed above [132].Since the thermoelectric properties of SiGe decreases at theroom temperature, the devices are not suitable at certainpoint. Non CMOS techniques were used to develop p typeB4C and B9C but it was limited to small place like 1 cm

2. Thesuperlattices such as p type Bi2Te3/Sb2Te3 and the n typeBi2Te3/Bi2(Te,Sb) were synthesized with non CMOS technol-

conductivity. The ZT was obtained as 2.4 at 300 K [100]simulated remarkable interest in this method. In generalthere are two types of congurations based on transportdirections i.e. in plane and cross plane superlattices withvaried characteristics (Fig. 8 a and b). In the cross-planedirection, phonons are reected while electrons are trans-mitted together with other mechanisms such as electronenergy ltering and thermionic emission.

The thermoelectric devices based on VIV compoundsBi2Te3 and (Bi, Sb)2Te3 were manufactured by thin lm tech-nology and microsystem technology [133]. The materialgrowth c axis perpendicular to the heat ux was found togive the best device performance as it is evident from theTable 3 [134].

The membrane materials should have both thermal isolationand strength enough to support itself. Other materials such asSiO2, SiN, SiC were found to have some attractive properties.Compared to SiO2, the mechanical properties of low stressLPCVD SiN are more suitable. However, SiO2 is superior whenthermal properties are concerned. Besides SiO2/ SiN, SiC/ SiNare showing good potential. It has been also observed that thethermal conductivity of polySi is so high that cooling isrestricted to just below the a few kelvins than the roomtemperature [130]. Unlike Si/Ge superlattice, in MEMS basedthermoelectric, the thermal contact resistance becomes rela-tively as important as the resistance of thermoelectric pairswhich decreases with increasing thickness of the thin lm. Thevertical and horizontal architecture are made with thin lmtechnology for power generation and cooling. Using Si/SiO2 as asubstrate, a common vertical architectures were made with

ilee3 a

ity

in

203Enhancement of gure of merit from bulk to nano-thermoelectric materialsogies where cross plane transport reduced phonon thermal

Table 2 Comparison list of Bi2Te3, Poly SiGe and Poly Si. WhSiGe and poly Si, the silicon germanium scores above Bi2Tfabrication technologies.

Materials[126,130]

Type Seebeckcoefcient(lV/K)

Resistiv(lXm)

Bi2Te3 n type 240 10p type 162 5.5

Poly SiGe n type 136 10.1p type 144 13.2

Poly Si n type 120 8.5p type 190 58

Figure 8 Quantum connementthin-lm sputtering deposition techniques.

Bi2Te3 are showing outstanding performance comparing polys far as fabrication is concerned due their established IC

Conductivity(W/mK)

ZT(103/K)

Dopingconcentration(1020/cm3)

2.02 2.89 0.232.06 2.32 0.234.45 0.328 134.80 0.413 24

24 0.071 3.417 0.037 1.6

plane and cross plane transport.

nanocrystalline bulk alloys were synthesized with ballmilling, followed by DC hot pressing. The ZT was measuredto be 1.4 at 100 1C and decreased to 0.85 at 250 1C. The ZTenhancement comes not only from phonon thermal reduc-tion but also from the reduction of bi-polar contributions tothe electronics thermal conduction at high temperature[137]. Grain growth is a major issue during the DC hotpressing the nano-powder into bulk samples. Organic agentssuch as oleic acid-OA solve this issue. The OA at thebeginning of the ball milling reported to have suppressedthe grain growth [138].

Another interesting strategy is reported by dispersing Cunano-rods in Te as Cu7Te4 in Bi0.4Sb1.6Te3 matrix. The ZT of1.14 at 444 K was found with Cu7Te4Bi0.4Sb1.6Te3 which is27% higher than Bi0.4Sb1.6Te3 [139]. While Cu based collidal

erpbecou

en

52350

130

H. Alam, S. Ramakrishna2048.2 Ordered & random nanocomposites based bulk TE

Although superlattices and nanowires give very high ZT,in order to use in large scale applications, nanostructuredbulk materials were proposed as an alternative to thinlm materials. Since the reduced phonon thermalconductivity comes from the sequential interfacescattering of phonon, using nanocomposites becomespotential and the cheap alternative to the expensivesuperlattices.

The main goal of designing materials for such applicationsis to introduce many interfaces for reducing the thermalconductivity more than the electrical conductivity by inter-facial scattering and increasing S by carrier-energy lteringor by quantum connement, more than decreasing theelectrical conductivity. This increases power factor andthere by ZT. The bulk materials act as host materials andnanomaterials become matrix materials or nanoscale mate-rials are assembled as nanocomposites containing a coupledassembly of nano-clusters (Fig.9). The random assemblageof two kinds of nanoparticles in the heterogeneous compo-sites or nanoparticles in host materials of the bulk can yieldenhanced ZT.

Since nanocomposites materials can be handled easily fromthe properties-measurement/ materials-characterization pointof view, they can be assembled into desired shapes and scaledup for commercial use.

However, there are challenges posed by electrons which arescattered by the randomly oriented grains and this leads toreduction of both electrical and thermal conductivities in thesame material. In this case, there would be less or no change inZT. The inclusion of BN or B4C nanoparticles into SiGe alloy

Table 3 Comparison of n Bi2Te3 in perpendicular c axis and pperpendicular c axis is twice that of parallel c axis while Seeconductivity is much higher in perpendicular c axis than its c

Property of n Bi2Te3 Perp

Seebeck coefcient (mV/K) 11Specic conductivity (O cm)1 600Conductivity (W/mK) 23Linear thermal expansion coefcient (K) 22.2ZT 1.5Power factor (mW/cm K) 205reduced both thermal and electrical conductivity contributingto nothing. Hence it is essential to reduce thermal conductivitywithout compromising electrical conductivity. This can beachieved by properly choosing the mismatch in the electronicproperties, the electron transport properties can be maintainedto the required level [135]

25% increase of ZT was demonstrated with tubular Bi2Te3nanowire inclusion in Bi2Te3 nanocomposites since nano-tubes have the structural features of holey and low dimen-sional materials [136]. The holey structures scatter phononsstrongly while the low dimensionality inuences the powerfactor. The nanotubes cause the reduction of the thermalconductivity without affecting the electrical conductancemuch while increases the gure of merit of the Bi2Te3 basedmaterials. The ZT of p type bismuth antimony tellurideendicular/parallel c axis ratio shows that ZT performance ofk coefcient of the both are almost the same. The thermalnterpart.

dicular c axis Perpendicular/parallel c axis

40 10 46

2.02.506 2103 2

46

Figure 9 Nanocomposite thermoelectric material.syntheses of monodispersed ternary and quaternary chalco-genide nanocrystals (CuInS2, CuInSe2 and CuZnSnS4) are hotissues as inorganic solar absorbers. Fan et al. studied theimpact of Cu2CdSnSe4 (CCTSe) nanoparticles of densematerials compacted from CCTSe nanoparticles-the lessstudied TE materials (Fig. 10). They reported the ZT of0.65 at 450 1C [140].

Grimm et al. used micro-wave plasma induced thermaldecomposition of gaseous precursors (GeH4 and SiH4) tosynthesize semiconductor nanoparticles and mixed themwith B2H4 and PH3 respectively. By adjusting the fraction ofthe gas phase to control the dopant concentration, nano-sized SiGe materials were prepared [141]. Sheele et al.reported 60% reduction in thermal conductivity with1520 nm thick Sb doped Sb(2x)BixTe3 nanoplatelet. The ZT

205Enhancement of gure of merit from bulk to nano-thermoelectric materialswas increased by 15% compared to its bulk counterpart [142].Yang et al. demonstrated a thermoelectric nano-generator of1.94 nW made with non-toxic and low cost oxides Sb dopedZnO. The single Sb doped ZnO showed the Seebeck coef-cient of 350 mV/K and a high power factor of 3.2 104 W/mK2 [142]. ZnO has excellent charge carrier transport whichare tunable via doping [143]. Zhou et al. grew Ga doped ZnOnanowires array which were well aligned and uniform, with asimple chemical vapor deposition method [144]. Theseprocesses made ordered nanocomposites possible to obtain

Figure 10 Temperature dependence of (a) thermal conductiv-ity and (b) dimensionless gure-of-merit parallel and perpendi-cular to the pressing direction. The arrows in gures indicatethe directions of the property being measured, the direction ofthe hot pressing is parallel to black arrows [140].many advantages over random nanocomposites.About 50% improvement of ZT from 0.65 to 0.95 at 800

900 K was reported with an atomic ratio of Si80Ge20 dopedwith boron. The reason for the high ZT was attributed withthe reduction of thermal conductivity due to the increasedphonon scattering form the high density nano-grain inter-faces in the nanocomposite. The theoretical study with Sinanocomposite with 10 nm was also found to have ZT above1 at 1000 K. Comparing SiGe alloy Si nanocomposite is moreadvantageous than expensive germanium [145].

Soni et al. demonstrated that Se doped Bi2Te3xSexnanoplatelet composites enhanced ZT by nearly a factorof 4 to ZT=0.54 at 300 K where x=0.3 in comparison withBi2Te3 nanocomposites (Fig. 11). This group achieved it bysynthesizing by polyol method [146]. The enhancement isattributed to the energy ltering of low energy electrons byordered nanocomposites with plenty of grains.

Fabrication of nanocomposites

Apart from growing nanoscale materials by epitaxial meth-ods, cheaper bulk materials with a nanoscale structures aremore attractive. One is based on mechanical alloying andsintering of the mixtures [147] and the next is based onrapid cooling of homogeneous multi component meltsexhibiting large miscibility gaps in solid state in therespective phase diagram [148]. The nanocomposites areproduced by rst synthesizing thermoelectric nanoparticlesand then assembling into dense bulk solids. The thermo-electric nanoparticles are prepared by wet chemical reac-tion, the hydrothermal method and high energy ball milling[149]. The high energy ball milling is an effective top-downapproach to make thermoelectric nanomaterials. The nano-composites are assembled into bulk solid by the sparkplasma sintering, cold pressing, sintering and extrusionmethods [150]. The combination of high energy ball millingand hot pressing is attractive combination from commercialpoint of view due to its mass scale production.

The nanostructured p type SiGe bulk alloys were pre-pared by the low cost ball milling and DC induced hot presscompaction process [151], not by the expensive and slowatomic layer deposition process [114]. The Bi2Te3, SiGealloys and skutterudite CoSb3can be produced with highenergy ball milling and hot pressing method [152]. Thematerials of VVI and IVVI groups such as Bi2Te3 and PbTeare fabricated by sintering with spark plasma sinteringarelevant technique for compaction of nanostructured pow-ders. Yucheng et al. gives a summarized table of thermo-electric materials prepared by hot pressing and ball millingmethod in Ref. [152]. The chemical synthesis and sparkplasma sintering method are also employed to producethermoelectric nanoparticles and nanocomposite respec-tively [153]. Alexis et al. fabricated a device comprised ofSi nanowires embedded in a polymer matrix. They usedvaporliquidsolid (VLS) growth mechanism because single-crystalline nanowires comprised of a wide range of materi-als can be synthesized with control of doping, diameter andgrowth orientation [154]. Zhu et al. prepared NaxCoO2/Co3O4 layered nanocomposite through exfoliation, stackingand sintering processes and observed the electrical con-ductivity to be 120 S/cm at 1200 K which is comparable withNa0.7CoO2 [155].

Despite of several recent reports on wet chemical synthesispreparation of thermoelectric nanoparticles such as bismuthtellurides, CZTSe and its analogs, only a few reports show asignicant progress in thermoelectric properties. Compactingor annealing procedures from several batch processes due thelow scale production, in the range of ten to hundreds ofmilligrams, cause the variations in the chemical compositionand particle sizes in the nanocomposite bulk TE materials[140]. Feng-Jia Fan very recently demonstrated large scalecolloidal synthesis of CSTSe nanocrystals which have attractedintensive attention due its good thermoelectric propertiesafter being doped. They achieved the output of 10 g ofCu2ZnSnSe4 in a single batch. The dense materials wereobtained by hot-pressing at different temperatures andobtained ZT of 0.44 at 450C which is comparable to the stateof the art ZT values of CZTSe or CTSe based materials at thesimilar temperature (Fig. 12) [156]. They further report thatthis synthesis procedure is highly extendable, in principle,therefore other binary, tertiary and quaternary selenidenanocrystals could be possible through this method.

Model calculations provide an important guide for the designand choice of processing parameters in the preparation of thenanocomposite structures. In an aligned-nanoparticles withperiodic boundary conditions and temperature difference,solution of the Boltzmann transport eq. is used whereas for

ivitaw;r n

H. Alam, S. Ramakrishna206Figure 11 Temperature dependence of the (a) electrical resistthermal conductivities (estimated from the WiedemannFranz lBi2Te3xSex NP composites. All the gures share the same colorandom unit cells, Monte Carlo methods are more suitable.Though Boltzmann transport equation and Monte Carlo methodshave been used for modeling the nanocomposites, there arestill challenges in optimizing their process conditions and dopingspecies. The wavelength (l) and mean free path (MFP) forphonons (or electrons) at certain energy are unknown for mostmaterials system and this makes the theoretical predication ofthermal conductivity (electronic conductivity) extremely dif-cult for nanocomposite with varying-size inclusions [157].

Future road map of nanocomposites

Si being abundant and widely used semiconductor, nanocompo-sites in thin lm is expected to grow and contribute to improvethe thermal management in microelectronics industries. Themain advantage of using Si nanowires for thermoelectricapplications lies in the large difference in mean free pathlengths between electrons and phonons at room temperature[158]. By using roughened silicon nanowires, the thermalconductivity was reduced to1.6 W/mK, with the phononcontribution close to the amorphous limit, with no muchcompromise on power factor so that ZT was achieved to nearunit at the room temperature. Instead of random nanocompo-site, ordered nanocomposites have proven better and hencehave good potential. It is not still understood clearly whichcarriers are dominant heat/ charge carrier, what the optimalsize distribution is and the type of interfaces that lead strongphonon scattering and the weak electrons [157].

The energy ltering techniques are used to lter phononsand allow high energy electrons to pass through. This increasesy, (b) thermoelectric power, (c) lattice (kl) and electronic (ke)see text for details) and (d) thermoelectric gure of merit ofotations as shown in (d) [146].the Seebeck coefcient due to the negative Seebeck distribu-tion. However, it will not be simple to lter energy by merecontrolling the grain boundaries as it is related to the electronmobility. The mobility of electrons through the grain in n typeSi80Ge20 for example, decreases by 40% in the experimentalresults comparing the theoretical calculations. The density ofstates can be increased by introducing energy level created byimpurities. Due to these impurities, the energy levels lie in theconduction or valence bands creating the resonant level and alocal maximum in the electronic density of states. It wasexperimentally tested with PbTe by introducing Tl in it.Another challenge is to reduce electronic thermal conductivitycompletely without disturbing its conductivity and predictedthat materials with such characteristics are not likely in nearfuture [159]. Apart from improving the intrinsic energyconversion efciency of the materials, there are attemptsfrom the perspective of systems architectures. There arethree general approaches proposed. The rst approach is tolook for design optimization and reduce parasitic losses byusing non-traditional materials in device fabrication. Thesecond approach is to determine if alternative thermodynamiccycle could be used to improve efciency. The third proposedapproach comes from the realization that there is no lengthscale in the equations that determines the efciency of athermoelectric engine [160].

Gaining the greater insights of electron and phononscattering at the grain boundary and adopting the suitablefabrication techniques to produce it will determine thefuture course of action. This will give the opportunities ofadopting the ordered nanocomposites instead of randomnanocomposite for high ZT thermoelectric materials [117].

207Enhancement of gure of merit from bulk to nano-thermoelectric materialsFuture potential applications

There are signicant commercial values in developing nextgeneration integrated cooling systems and power generationfrom the waste energies. Many of the existing micro-sensorsand micro-actuators can be modied based on the thermo-electric principle. They use thin-lm approach to achievesensing and actuator functionality, fabricated with micro-machining techniques to achieve device optimization. How-ever, the development of thermoelectric devices compatiblewith standard semiconductor process has huge potential toaddress many applications in microelectronics, made withMEMS technology, along with nanotechnology [117].

Micro-thermoelectric devices fabricated with thin-lmtechnology, have achieved sufcient miniaturization thatcan be integrated with microelectronics or semicondu-ctor devices instead of mounting them externally. As semi-conductor industry further reduces the size of transistors inICs, a trend is to fabricate more of the external circuitryinside the semiconductor packaging. The next generation ICswill thus be possible if the thermal management is appro-priately executed since the performance is directly propor-tional to power. If the failure rate is targeted to 10005000

Figure 12 Temperature dependence of: (a) thermal conductivFITs (failures in Terra hours), the successful thermal packa-ging should rely on a judicious combination of materials andheat transfer mechanisms those stabilize the componenttemperature at an acceptable level.