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New trends, strategies and opportunities in thermoelectric materials:A perspective

Weishu Liu a, *, Jizhen Hu a, Shuangmeng Zhang a, Manjiao Deng a, Cheng-Gong Han a,Yong Liu a, b

a Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, PR Chinab Beijing Institute of Aeronautical Materials, AECC, Beijing 100095, PR China

a r t i c l e i n f o

Article history:Received 30 May 2017Received in revised form2 June 2017Accepted 2 June 2017Available online 10 June 2017

* Corresponding author.E-mail address: wsliu@sustc.edu.cn (W. Liu).

http://dx.doi.org/10.1016/j.mtphys.2017.06.0012542-5293/© 2017 Elsevier Ltd. All rights reserved.

a b s t r a c t

Thermoelectric energy conversion system has great appeal in term of its silence, simplicity and reliabilityas compared with traditional power generator and refrigerator. The past two decades witnessed asignificantly increased academic activities and industrial interests in thermoelectric materials. One of themost important impetuses for this boost is the concept of “nano”, which could trace back to the pioneerworks of Mildred S. Dresselhaus at 1990s. Although the pioneer passed away, the story about the nanothermoelectric materials is still continuous. In this perspective, we will review the main mile stonesalong the concept of thermoelectric nanocomposites, and then discuss some new trends, strategies andopportunities.

© 2017 Elsevier Ltd. All rights reserved.

1. Introduction

Thermoelectric energy conversion system has great appeal interm of its silence, simplicity and reliability as compared withtraditional power generator and refrigerator. Furthermore, itscapability of easily scaling down to the small size but withoutsignificantly sacrificing its efficiency makes it unique to convert thewidely distributed waste heat into the electric energy and also todisperse the heat in the field of microelectronics and telecoms.Since the thermoelectric energy conversion system could beconsidered as a heat engine by using the electrons as energy car-riers [1], the efficiency of the thermoelectric power generator orrefrigerator is mainly determined by the Carnot efficiency andthermoelectric figure-of-merit ZT (ZT ¼ TS2s/k, where T, S, s, k arethe average temperature, Seebeck coefficient, electrical conduc-tivity and thermal conductivity, respectively [2]) due to the irre-versible heat loss. Intuitively, a good thermoelectric materialshould have low thermal conductivity, high electrical conductivityand Seebeck coefficient. Pursing a high ZT value, therefore, becomesthe main target in the thermoelectric community. However, it is ahuge challenge to get a high ZT value due to the internal linksamong these three transport parameters. To quickly be away from

the dilemma, we decompose the ZT definition equation, andrewrite it into ZT ¼ (S2n)(m/k)eT by applying the relationship ofs ¼ nem, where e, n and m are the free charge, carrier concentrationand carrier mobility, respectively [3]. The interconnection betweenS and n, called as the “Pisarenko relation”, demonstrates that thehigher n, the lower S. For the material with simple parabolic bandedge, the optimized (S2n) is mainly determined by the carriereffective mass, i.e. m*. The aim of band structure engineering stra-tegies is to increase them* by applying additional carrier pocket [4],resonant doping [5] and band convergence [6]. In the other hand,the compromise between carrier mobility and lattice thermalconductivity (klat) is an important scale for weighting the effec-tiveness of nano approaches [7,8]. Frustratingly, the in-terconnections among them are not limited to these two cases.There are still other dilemmas such as m* vs. m and band gap Eg vs.klat. For more details, the readers are encouraged to further reachthe recent deep review papers [3,9].

The past two decades witnessed a significant increase in aca-demic activities and industrial interests in thermoelectric mate-rials. A simple scale for this continuous boost is the annualpublication growth from 500 papers in 1996 to 2800 papers in 2016(based on the Web of Science Database [10]). Fig. 1 shows theannual publications on the topic of “thermoelectric” from 1996 to2016, which corresponds to near four times increase in publica-tions. One of the most important impetuses for this boost is theconcept of “nano”, which could trace back to the pioneer works of

Fig. 1. Annual publications from 1996 to 2016 on the topic of “thermoelectric” and“thermoelectric”þ“nano” based on the database of Web of Science (Mar. 2017).

W. Liu et al. / Materials Today Physics 1 (2017) 50e60 51

Mildred S. Dresselhaus at 1990s [11,12]. Prof. Dresselhaus is alsowell known as the “thermoelectric grandma” in the Chinese ther-moelectric academic community, and just passed away fewmonthsago at her age of 86. As MIT President L. Rafael Reif wrote that welost a giant, an exceptionally creative scientist and engineer whowas also a delightful human being. Although the pioneer passedaway, the story about the thermoelectric nanocomposites is stillcontinuous. In this perspective, we will review the main milestones along the concept of thermoelectric nanocomposites, andthen discuss some new trends, strategies and opportunities.

2. Direction of going nano

The original idea of going nano suggested by Prof. Dresselhausand her student Hicks was to use the size quantization effect toadjust the electronic density of state for boosting the term of S2n[11e14]. The dimensional variable was suggested as new approachto change the thermoelectric nature of the known materials.Simultaneously, they also made unique contribution to explain thedifferent scattering effect of confined interfaces to the electrons andphonons, which could be used to decompose the connection be-tween m and klat, which ignited a hug enthusiasm in the thermo-electric community to apply new nano strategies in thermoelectricmaterials.

Along the direction of going nano, the two-dimensional super-lattice was firstly used to exam the beneficial effect of the quantum[15,16]. Besides the quantum-confinement effect, a significantlyreduced lattice thermal conductivity was observed in the super-lattice structures as compared with its bulk counterpart [17]. Theeffect of various nano structures was intensely investigated to tunethe transport of phonons. Amongst the various nanostructures, thenanoinclusions and nanograins received most attentions indepen-dently at the very beginning. Nanoinclusion is a metastable pre-cipitation, or “Guinier-Preston zone”, due to the phase-separationin the melting-quench process. It is not a fresh feature in metalmaterials, but truly one exciting breakthrough in thermoelectricmaterial field when M. G. Kanatzidis's group showed that thepresence of Ag/Sb-rich nanoinclusions resulted in the high ZT valuein the materials AgPb19SbTe20 (ZT ¼ 2.2 at 800 K [7], later the au-thors confirmed a more repeatable ZT value around 1.7 at 700 K[18]). J. F. Li's group independently confirmed the advantage of thenanoinclusions in the AgPbSbTe system with a peak ZT of 1.5 at700 K by using a power metallurgy route [19] (mechanical alloying,spark plasma sintering and annealing). The idea of the nano-inclusions for the phonon engineering threw a stone in the pool ofthermoelectric community. Later, various technique routes wereexplored to form nanoinclusions embedded in grains and/or at the

grain boundary. To avoid the repetition, the readers are encouragedto reach the excellent review papers written by Kanatzidis and hisco-authors [20,21].

In the other hand, the competition to achieve nanograinedthermoelectric materials started much earlier [22,23]. The nanoparticles could be easily obtained by employing high energy ballmilling process, which is known as mechanical alloying via directlyusing the elements as starting materials. However, nano powdersalone were not enough to guarantee the final formation of nano-grained or nanocrystalline bulk materials. The nano features couldbe smeared out due to the fast grain growth in the conventional hotpress process that usually takes hours to get a dense bulk. Thistechnique challenge brought the fast current assistant hot pressingon the stage, which is also known as the spark plasmas sintering(SPS) or pulse current activated sintering (PAS), or field activatedsintering (FAS). The fabrication process of combination with highenergy ball milling and fast spark plasmas sintering for the ther-moelectric nanocrystalline materials started from Japan [24,25],and quickly propagated to Korea [26] and China [27,28] at thebeginning of 2000s. A breakthrough in this powder metallurgyroute came in 2008 when Z. F. Ren's group reported a peak ZT of 1.4in p-type Bi2Te3 [8] (30% higher than the commercial ingot) and0.95 in p-type SiGe [29] (50% higher than the previous reportedrecord in p-type SiGe alloy). Since both the ball milling and fastsintering were thermodynamically nonequivalent processes, thefinal bulk materials were not strictly pure nanocrystalline mate-rials, but with many nanoinclusions embedded in grains andlocated at grain boundaries [30]. As a result, a more accurate term,i.e. nanocomposite, was adapted to describe these type thermo-electric materials made from ball milling and fast sintering jointroute [31]. Finding more detailed discussions about the thermo-electric nanocomposite, the review paper written by Dresselhauset al. [13] was well-received. In addition to the BM-HP (or MA-SPS),other routes were also used to fabricate the nanostructured ther-moelectric bulk materials, such as melt-spinning plus spark plasmasintering (MS-SPS) [32,33], spark erosion plus spark plasma sin-tering (SE-SPS) [34] and chemically metallurgy methods [35,36].The detailed introduction of these fabrication methods can befound in our reviewed book chapter [37]. It is worthy to emphasizethat even earlier X. B. Zhao's group started to use the term of“nanocomposite” to describe their Bi2Te3 bulk materials fabricatedby the hydrothermal method combined with hot pressing [38].Generally, the term of “nanocomposite” is more accurate todescribe the bulk material made from any thermodynamicallynonequivalent process. In the beginning of 21 century, one of themost important theoretical findings is that the length scale of thephononmean free path is much wider thanwhat we expected fromthe classic theory [39], which gives the firm theoretical support tothe original idea through the nanostructures decomposing theconnection between m and klat. Nanocomposite has the intrinsicallyfavorable features to minimize klat, i.e. a multi-scale phonon scat-tering centers including grain boundary, nanoinclusion and pointdefect. In 2002, Kanatzidis's group started to adapt the term of“hierarchical” to describe their nanostructured PbTe-x%SrTe fabri-cated by combining crystal growth method and powder metallurgymethod [40], which gives a size-scale perspective to nano-composite. The concept of “hierarchical” was imitated from thebiological world, which was also earlier used to describe thesolution-derived particles with complicated nano features. Now,the separately nano approaches, i.e. nanoinclusions and nanograins, merged together. As a result, a multi-scale or all-scalephonon scattering centers are widely used to minimize klat.Although the state-of-art thermoelectric nanocomposite has kindsof hierarchical features according to the size of phonon scatteringcenters, the arrangement of these hierarchical structures is still far

W. Liu et al. / Materials Today Physics 1 (2017) 50e6052

from an ordering way or a controllable way, as compared with realhierarchical structure in the biologic world, such as the bone [41],crab exoskeleton [42], feature rachis [43] etc.. Fig. 2 compares thedifference of the hierarchical structure between the thermoelectricnanocomposite and a crab exoskeleton. It is clear that the naturalhierarchical structure is more delicate and also shows higher hi-erarchical level. In 2012, the authors had explained the benefits ofan ordering hierarchical structure in the concept of “orderingnanocomposite”. One of motivations was to reconstruct the trans-port channel of electrons in traditional nanostructured thermo-electric materials with randomly distributed defects throughmodulation dopants, regular-shaped inclusions, ordering-distributed inclusions, textured grains boundary and orderingporous structures [3]. A higher-level hierarchical nanocompositewith more delicate ordering substructure would become a direc-tion for the thermoelectric nonocomposites.

3. Mater design from atoms

Besides the efforts at microscopic, mesoscopic and nanoscopicsize-scale, people also looked further down size scale, i.e. theatomic level. In our previous paper [45], we had already reviewedthe works to find the materials with intrinsically low klat. Here, weonly focus on the development of the key concepts. The classicthermoelectric materials, Bi2Te3 and PbTe, are characterized withthe heavy atoms that are benefited to the low heat capacity andhence the low lattice thermal conductivity. In 1994, Slack predi-cated that a loose-bonding atom in the crystalline cage of the

Fig. 2. The comparison of hierarchical structure of a thermoelectric nanocomposite CoSb2archical levels as compared with 3 hierarchical levels.

Skutterudite compounds would significantly reduce klat [46], whichwas quickly experimentally confirmed in the p-type CeFe4-xCoxSb12and LaFe4-xCoxSb12 [47]. This success initiated a hug enthusiasm tosearch new filled Skutterudite and newcompoundswith crystallinecages. Along this direction, the type-I Clathrate compound (e.g.Sr8Ga16Ge30) and type-II Clathrate (e.g. Cs8Na16Si136) were quicklyexcavated [48], which share similar crystalline cage and looselybound filler of Skutterudite compound. Since both the Skutteruditeand Clathrate compounds show the complex crystalline structureas compared with the classic PbTe, other compounds with complexstructures even without crystalline cage also received attentions,such as complex chalcogenides and Zintl phase [49]. Comparedwith the former, Zintl phase is even a bigger family, which ischaracterized with intermetallic compounds composed of group 1(alkali metal) or group 2 (alkaline earth) and any post transitionmetal or metalloid (i.e. from group 13, 14, 15 or 16). Strictly, thethermoelectric Clathrates also could be considered as the Group-14Zintl phase. The more well-known thermoelectric Zintl phase wasthe Group-15 Zintl phase. In 1999, Kim et al. reported a low thermalconductivity of 1.7 W m�1K�1 in a Ba4In8Sb16 Zintl phase with acomplicate crystalline structure [50], which was characterized withvery complex polyanionic framework. The most famous Group-15thermoelectric Zintl phase-Yb14MnSb11, possessed a high ZT > 1 at1200 K, indicating an promising alternate of the classic high-temperature thermoelectric material SiGe. Moreover, the complexcrystalline features of Yb14MnSb11 were also observed [51]. Anothernotable example is Zn4Sb3, whose crystalline complex is charac-terized with the random distribution of Zn at three crystalline sites

.75Te0.2Sn0.05 [44] and lobster exoskeleton [42]. The lobster exoskeleton shows 7 hier-

Fig. 3. The timeline of selected thermoelectric materials.

W. Liu et al. / Materials Today Physics 1 (2017) 50e60 53

[52]. As a result, the thermal conductivity of Zn4Sb3 is much smallerthan its simple counterpart ZnSb. Theoretically, the complex crystalstructures have more optical phonons that do not contribute muchto heat conduction and yet can scatter acoustic phonons, leading toa lower lattice thermal conductivity. The interested reader couldreach two excellent review papers listed the known complexcrystalline [53,54].

Furthermore, there are also some block-building rules proposedto construct the complex compound. Komouto et al. has proposedthe concept of nanoblock integration into a layer-structured hybridcrystal, such as (ZnO)mIn2O3 [55], (ZnS)mIn2S3 [56], SrO(SrTiO3)n

[57] etc. Recently, Komouto's group has extended this idea to anorganic-inorganic hybrid superlattice of TiS2/[HAx(H2O)y(DMSO)z][58]. Morelli et al. proposed an alternative way to construct acomplex diamond-like compound. The process could be startedwith a simple diamond-like compound, e.g. ZnSe, and then replacedtwo Zn2þwith Cuþ and In3þ to form CuInSe2, or replaced three Zn2þ

with two Cu2þ and one Sn4þ to form Cu2SnSe3 [59]. The newcompound is also expected to possess a semiconductor behaviorsince the total charge is zero. The thermoelectric Stannite (e.g.CuZnSnSe4), Tetrahedrite (e.g. Cu10Zn2Sb4S13) could be constructedin a similar way by using the Cuþ, Zn2þ, Sb3þ, S2� and Se2� [60]. The

Fig. 4. Numerically calculated efficiency as function of (a) (ZT)avg and (b) (ZT)eng [79].

W. Liu et al. / Materials Today Physics 1 (2017) 50e6054

crystalline complex could be also achieved through engineering theanionic-site, e.g. BiCuSeO [61] and Bi2SeS2 [62].

In contrast to the large unit cell, the low klat of In3Se4 [63] andMgAgSb [64] suggested that local lattice distortion could also beanother important crystalline feature. The distorted lattice leads toanisotropic chemical bonds and laminar or linear substructures.Cu2Se was another notable example for low klat compoundswithout large unit cell [65,66], in which the loosely bound Cu ionsbehaved as a “liquid” and resulted in reduced heat capacity. The“mobile” Cu ion is characterized with its uncertain location in theCuSe4-tetrahedron which is caused by the local resonant bonds.Recently, a theoretical study in IVeVI, V2eVI3 and V classic ther-moelectric materials suggested that the resonant bounds couldcause the optical phonon softening, strong anharmonic scatteringand large phase space for three-phonon scattering processes [67].

Usually, the compounds with intrinsically low klat also show lowcarrier mobility and low power factor which is important for powergeneration application. Recently, the new compound SnSe, withsimple unit cell but a distorted low symmetry lattice, showed apotential to change this common sense because it has intrinsicallylow lattice thermal conductivity and high power factor [68]. One ofthe notable features for the high power factor is the complex Fermisurface, which means that it has a high effective mass and high S2n.Similarly, a quite high power factor of 45 mWcm�1K�2 was reportedin the new half-Heusler compound NbFeSb [69], which was asso-ciated with the complex Fermi surface. The benefit of the multicarrier pockets was early noticed in the filled Sktterudites [70,71].Liu et al. clarified that the additional carrier pocket, with edgedifference <5 kBT (kB, Boltzmann constant), could significantlyenhance the power factor [4]. However, the strategy of bandstructure engineering did not receive enough attentions untilHeremans et al. [5] published the work on Tl-doped PbTe in 2008and later Pei et al. [6] showed a band convergence in the valenceband of the PbTe-PbSe system resulted in a peak ZT of 1.8 inPbTe0.85Se0.15. As a result, the multi carrier pocket or degeneratecarrier valley is widely accepted as a strategy for enhancing thepower factor.

In the quantum scenario, only the electrons near the Fermisurface contribute to electrical conductivity. A larger Fermi surfacearea corresponds to more electronic states and larger S2n. Besidesthe number of multiple carrier pockets, the anisotropy of the Fermisurface was another important factor to determine the high powerfactor [72]. Recently, an orbital engineering was used to design thecomplex Fermi surface in the CaAl2Si2 Zintl family through mini-mizing the crystal field splitting energy of orbitals to realize highorbital degeneracy [73]. Additionally, the high power factor can alsobe achieved by reducing the scattering to major carriers or selec-tively scatter the low-energy carriers. Recently, He et al. achieved arecording power factor of ~106 mW cm�1K�2 at room temperaturein the p-type half-Heusler Nb0.95Ti0.05FeSb by reducing the grainboundary scattering to the holes [74]. Mao et al. observed a highvalue of ~102 mWcm�1K�2 at 600 �C in the Se-doped Cu56Mn42Mn2alloy, which was attributed to the low-energy carries scattered bythe twin boundaries [75]. Fig. 3 shows the timeline of selectedthermoelectric materials since 1950s. It is very attractive tocontinuously search new materials with intrinsically low latticethermal conductivity together with high power factor. Somefavorable features or mater code for the next thermoelectric super-star could be: (1) heavy atom, or atom with lone-pair electrons,such as Sb used as Sb3þ cation, (2) weak, or rattling, or resonantbonds, (3) large and complex unit cell, or distorted crystallinestructure, (4) complex Fermi surface. Simply, the new promisingthermoelectric compound would be characterized with complexstructure for low lattice thermal conductivity and complex Fermisurface for high power factor.

4. Focus shift from local peak ZT

The theoretically investigation of the conversion efficiency ofthe thermoelectric devices for power generation and refrigerationcould be dated back to Raleigh's early attempt in 1885, and Alten-kirch's first satisfactory thermodynamic interpretation in 1911. Itwas early suggested that good thermoelectric materials shouldhave large Seebeck coefficient and low thermal conductivity toretain the heat at the junction and low electrical conductivity tominimize the Joule heat [76]. Abram F. Ioffe firstly defined param-eter, z (a lower case of letter z), that embodied these desirableproperties in his book “Semiconductor Thermoelements and Ther-moelectric Cooling, Infosearch, 1957” [2]. Later, it was widely knownas the thermoelectric figure-of-merit with a unit of reciprocaltemperature. For convenience, a dimensionless figure-of-merit (zT,or ZT) was adapted according to the relationship of efficiency forpower generation and refrigeration.

h ¼ TH � TCTC

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þ ZTMÞp � 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þ ZTMÞp þ TC=TH

(1)

f ¼ TCTH � TC

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þ ZTMÞp � TH=TCffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þ ZTMÞp þ 1(2)

where the TM is the average temperature between the hot side (TH)and the cold side (TC), i.e. TM ¼ (TH þ TC)/2. It is worthy to point outthat the early researchers differentiated zT and ZT by denoting zT asthe material figure-of-merit and ZT as the device figure-of-merit.However, this difference is not widely accepted and the symbol of“ZT” is more popular in the thermoelectric community. Forexample, the symbol of “ZT” was widely used by the researchers oftheworld in the book “CRC handbook of thermoelectrics, CRC Press,1995” [76]. Although the derivation of Eqs. (1) and (2) is based on arough model with temperature-independent Seebeck coefficient,electrical conductivity and thermal conductivity, it did not impedethat the composite-indicator, ZT, become the compass to guide theadvances of thermoelectric materials from a quite small field into a

Fig. 5. Physic meaning of the conventional ZT and new (ZT)eng. t(T) is the temperature-dependent Thomson coefficient.

Fig. 6. Selected examples for the wide-temperature enhanced local ZT. (a) Cl doped In4Se3-d[84].

W. Liu et al. / Materials Today Physics 1 (2017) 50e60 55

hot research topic.The experimental measurement of ZT value for a given material

was obtained from independently measuring Seebeck coefficient,thermal conductivity, electrical conductivity and temperature. Theprimary optimization of a thermoelectric material usually involvedin finding the highest measured ZT value within a two-dimensionalregion: temperature and dopants (or carrier concentration). Thisoptimized figure of merit, ZTmax, was also widely referenced as themile stone in the many review papers [9,20,21]. Generally, themilestone materials with a high recording ZTmax did present newphysics or new mechanism, such as filled Skutterudite [47],AgPbSbTe [7], nano Bi2Te3 [8]. However, a problem occurs as we tryto estimate the theoretical efficiency of the givenmaterials with themeasured thermoelectric ZT, usually temperature dependent.Goldsmid has early suggested that an average ZT could be only usedfor a rough estimated case [76]. A more accurate calculation needsnumerically solving the one-dimensional heat flux equation underthe electrical field and thermal gradient [77]. Since the numericalcalculation is not such easily accessible, a rough estimation basedon various average strategies was used. However, the error be-tween the estimated efficiency from average ZT and the numericalcalculation did not get much attention for a long time. Recently, themore general derivation by considering the temperature transportparameters gave out a new definition of thermoelectric figure-of-merit, i.e. engineering ZT in the symbol of (ZT)eng [78]. Fig. 4 com-pares the numerical calculation efficiency of state-of-art thermo-electric materials and their average ZT (in the symbol of (ZT)avg) andalso the engineering ZT (in the symbol (ZT)eng), which clearlyshowed a better linear relationship between the new figure ofmerit (ZT)eng and the leg efficiency [79].

Considering (ZT)eng and its related efficiency formula are more

[81], (b) S doped PbTe [82], (c) Ca and Pb co-doped BiCuSeO [83], (d) Ge doped Mg2Sn

W. Liu et al. / Materials Today Physics 1 (2017) 50e6056

accurate, does it hint that we need to stop using the conventional ZT? The answer is no. However, a new physical meaning shouldendow it. It is noted that, even in the real case, Eqs. (1) and (2) couldbe rigidly valid when the temperature range is infinitely small, theZTM therefore becomes a local figure-of-merit, which just corre-sponds to the measured ZT at a temperature point [80]. In contrast,the engineering (ZT)eng was defined in a wide temperature range,which was referred as global figure-of-merit, as illustrated in Fig. 5.

We also propose a symbol of ZT@T for the local ZTat temperature

point T, while a symbol of ðZTÞeng���Th

Tcfor the global (ZT)eng in the

temperature range from Th to Tc. The (ZT)eng services as a betterbridge between the materials performance and the devices per-formance [80]. Although the concept of the global (ZT)eng is justrecently proposed, the importance to have a high ZT over the wholeapplication temperature range has been early recognized. Now,there is intensely interesting to optimize the local ZTover the widertemperature regions. Fig. 6 shows some selected examples [81e84],but not limited. The strategy of synergistically tuning the transportproperties to have a favorable S2n and suppressed klat would beimportant to have largely enhanced ZT in a wider temperaturerange. In the case of Mg2Sn0.75Ge0.25, it was found that the alloyingelement Ge in Mg2Sn0.75Ge0.25 decreased the klat, increased m*, andwidened the Eg, which could be interpreted by a new thermo-

electric material parameter B*, B*fðm*Þ3=2mT3=2

klatEg [84,85]. The new B*

Fig. 7. (aec) Physic mechanics of the pyroelectric generator, (d) SEM image of the ZnO nanofor the pyroelectric generator, (f) peak output voltage and current with the dependence of

parameter was derived from two-band model which consideredthe bipolar effect, and hence showed a good indicator for the localpeak-ZT.

Furthermore, Mahan has theoretically suggested that a ther-moelectric device having inhomogeneous doping could have anincreased efficiency [86]. Although there were few attempts ofgradient-doping Bi2Te3 [87] and PbTe [88], they did not get enoughattentions which may be due to the experimental challenge. Incontrast, segmented legs were widely used in high performancethermoelectric power generation (TEG). Recently, new recordingefficiency of 11% and 12%, were reported in Bi2Te3/PbTe [89] andBi2Te3/PbTe [90] segmented thermoelectric power generators,respectively, (Bi2Te3/PbTe module: an eight-couple segmentedmodule with the overall size of 18 � 15 � ~10 mm3 with singlesegmented leg dimension 2 � 2 � 4.8 and measurement temper-ature of Tc ¼ 10 �C and Th ¼ 100 �C; Bi2Te3/PbTe module: an eight-couple segmented module with the overall size of20 � 20 � 14.5 mm3 with single segmented leg dimension of4 � 4 � 12 mm3 and measurement temperature of Tc ¼ 35 �C andTh ¼ 576 �C.). It is also noted that the new technique, i.e. 3Dprinting, could make the whole leg-length optimization withgradient doping strategy more feasible. Additionally, for a real de-vice, the efficiency is not the only concern, other requirements,such as effectiveness, reliability and flexibility, could be moreimportant in some cases [45]. For example, the TEG devices for a

wire array-based pyroelectric generator, (e) working principle of the experiment setuptemperature change [95].

W. Liu et al. / Materials Today Physics 1 (2017) 50e60 57

wearable electronics make organic thermoelectric materials a realhot-topic [91,92], not due to its high efficiency but high flexibility[93,94].

5. Go beyond the Seebeck effect

There are other strategies available for the thermal to electricenergy conversion besides the Seebeck effect. In this section,several new efforts will be reviewed for the energy-harvestingapplication from the low-grade waste heat. In the near future, itcan be expected to process an attractive prospect despite most ofthem under the budding stage.

5.1. Pyroelectric effect

Pyroelectric effect results from the spontaneous polarizationresponsewith the environmental temperature changes in dielectricmaterials [95], as shown in Fig. 7 (aec). Theoretically, the efficiencypyroelectric power generator could be higher than that of TEGbased on Seebeck effect under the same temperature difference.The pyroelectric generator is considered as a promising self-powered nanotechnology for harvesting energy from an environ-ment of time-dependent temperature fluctuation. Fig. 7 (dee)

Fig. 8. (a) Potential changes for both positive and negative electrodes. (b) Voltage-capacity cuin the inset. (c) The dependence of voltage to the specific capacity at both 20 and 60 �C. (d)whereas the black and red curves are simulated based on experimental results. hHR is heat

shows the ZnO nanowires array-based pyroelectric generator pro-posed by Yang et al. [95]. The diameter and length of the nanowiresare about 200 nm and 2 mm, respectively. A silver film wasdeposited on the top of ZnO nanowires array and served as the topelectrode, while ITO was used as the bottom electrode. The area ofthe nanogenerator was ~15 mm2. Fig. 7 (f) shows the output peakcurrent and peak voltage of the nanogenerator. An output powerdensity of ~5 � 10�11 W cm�2 is manifested at a temperaturechange of 25 K. Recently, Leng et al. [96] showed that a pyroelectricgenerator outputted the power density of 14 � 10�6 W cm�2 undera temperature difference of 80 K, indicating a significantimprovement in pyroelectric generation for waste heat harvestingapplication. Although it displays the lower output power densitythan that of thermoelectric counterpart (around 1e2 W cm�2)[89,90], it is still promising for the availability of storage and self-powered electric devices after further improving the powerdensity.

5.2. Thermogalvanic effect

Thermogalvanic effect, which is related to the temperature-dependent of electrode potential, can convert the heat into elec-tricity by charging at a high temperature and discharging at a low

rves of the full-cell in a charging-free thermal cycle. The schematic of the cell is shownAbsolute efficiency vs. Fe(CN)63�/4�/PB mass. The blue dots represent experimental datarecuperation efficiency [98].

W. Liu et al. / Materials Today Physics 1 (2017) 50e6058

temperature [97]. Based on thermogalvanic effect, thermallyregenerative electrochemical cycle (TREC) was proposed, and itemploys a reversible electrochemical reaction to build a thermo-dynamic cycle to harvest the waste heat to electricity. Yang et al.[98] demonstrated a charging-free TREC with low cost, simplesystem and no external electricity in the temperaturerange < 100 �C in comparisonwith those electrically-assisted TREC.The design of free-charging process is realized by shifting the po-tential of positive electrode to be lower than that of negativeelectrode when the temperature changes (T1/T2), indicating thatthe full-cell voltage and DG in the reverse process at T2 arebecoming negative to induce a spontaneous discharge process.Fig. 8 (a) shows the potential changes for both positive and negativeelectrodes. The potential of positive and negative electrode shouldbe equal to result in the zero value of full-cell voltage at (T1 þ T2)/2.After changing the temperature from T1 to T2, the dischargedvoltage becomes negative. Fig. 8 (b) displays the voltage-capacitycurves of the full-cell in a charging-free thermal cycle. A flat full-cell voltage curve is acceptable to maximize the energy output(jaDT j � charge capacity, where a is temperature coefficient,DT¼ T1�T2). The inset shows the schematic of the cell, consisting ofan inexpensive soluble Fe(CN)63�/4� redox pair and solid Prussianblue (PB) particles as active materials for the two electrodes sepa-rated by a Nafion membrane. In Fig. 8 (c), the same shape of curvescan be noticed at both 20 and 60 �C. The voltage is positive at 20 �Cwhile it becomes negative at high temperature 60 �C. Thischarging-free electrochemical cell delivers a discharged capacity of23 mAh g�1 based on the PB mass. From the curves of absoluteefficiency with the dependence of Fe(CN)63�/4�/PB mass shown inFig. 8 (d). The simulated efficiency of 2.0% and experimental valueof 1.5% at heat recuperation efficiency hHR¼ 70% are reached, whichare comparable with the ZT of 0.9 and 0.65 for an ideal TE device,respectively. Consequently, the charge-free system is attractive andpromising in the application of conversion from low-grade wasteheat to electricity.

Fig. 9. (a) Schematic of a water vapor-driven generator, (b) An image of generator above a cgenerator, (d) Peak output power as a function of the loading resistance as well as the circ

5.3. Water vapor-driven generator

Water evaporation, accompanied with the thermal energy dueto its large latent heat, is a ubiquitous phenomenon in the naturalworld. Recently, Xue et al. [99] demonstrated that water evapora-tion from the surface of nanostructure carbon materials cangenerate the electricity with the sustained voltage of up to 1 V,which is comparable to a standard AA battery. Gao et al. [100] re-ported a self-sustaining polymeric nanogenerator driven by hotwater vapor for recovering energy. An open-circuit voltage of 145 Vand a short-circuit current of 0.12 mA/cm2 are outputted, as well asthe peak power density of 1.47 mW/cm3 by volume and 4.12 mW/cm2 by area. The schematic figure of awater vapor-driven generatoris shown in Fig. 9 (a). A large number of water droplets are formedon the surface of the generator after the water-vapor condensation,heating up the generator by heat release. Meanwhile, the localhumidity will be decreased under the air flow, and generator isquickly cooled down after the heat-absorption of water droplets.An image of generator above a cup of hot coffee and a photograph offlashed blue LEDs powered by water vapor driven generator aredisplayed in Fig. 9 (b) and (c), respectively. Fig. 9 (d) shows the peakoutput power as a function of the loading resistance for 28 mm-thick PVDF generator and 25 mm-thick P(VDF-TrFE) generatordriven by water vapor. A first ascending and then descending trendwith increasing loading resistance is noticed for these two vapor-driven generators. The larger power density is outputted in PVDF-based generator than P(VDF-TrFE)-based generator, which can beattributed to the higher pyroelectric coefficient 26e27 mC/m2 K ofthe former than 16e19 mC/m2 K of the latter. Fig. 9 (e) presents themechanism of water vapor-driven generator. A perpetual dipole isperpendicular to the polarized PVDF and/or P(VDF-TrFE) films,inducing charges on the two metal electrodes. Under water vaporcondensation, the decreased moments of dipole and enlargedvolume is caused by the increased temperature of generator. Thus,the decreased polarization density results in a current in theexternal circuit to balance the charge density on the metal

up of hot coffee, (c) A photograph of flashed blue LEDs powered by water vapor-drivenuit diagram in the inset, (e) Mechanism of water vapor-driven generator [100].

W. Liu et al. / Materials Today Physics 1 (2017) 50e60 59

electrodes. When evaporating the water droplets in air flow, thedipole moments increase and the polymer volume contracts due tothe decreased temperature, which can increase the polarizationdensity of the polymers, thereby producing a reversed current inthe external circuit.

6. Conclusion

Nano approach has boosted the thermoelectric materials morethan 20 years, which still shows vigor. A higher-level hierarchicalnanocomposite with more delicate ordering substructure would bea direction for the thermoelectric materials. It is also noted that thenew thermoelectric compound continuously emerged togetherwith deeper understanding of the relationship between atom,chemical bond and crystalline structure and the transport of elec-trons and phonons. Recently, the theoretical effort to redefine thethermoelectric figure-of-merit, i.e. engineering ZT in the symbol(ZT)eng, gives a better interpretation why we need high ZT over thewhole temperature range rather than an only high peak-ZT. Thegradient-doping optimization along the whole leg-length based on3D printing technique is promising. Besides thermoelectric powergenerator, there are other principles that could be used for har-vesting the waste heat, such as pyroelectric and thermogalvaniceffect. Due to the new opportunity in the wearable electronics ande-skin, flexibility of thermoelectric materials and devices would beanother requirement after efficiency, effectiveness and reliability.

Acknowledgements

The authors would like to thanks the support of SUSTech startupfund (Y01256222).

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