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Materials Today Physics 1 (2017) 50e60

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Materials Today Physics journal homepage: https://www.journals.elsevier.com/ materials-today-physics

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 China b Beijing Institute of Aeronautical Materials, AECC, Beijing 100095, PR China article info abstract

Article history: Thermoelectric energy conversion system has great appeal in term of its silence, simplicity and reliability Received 30 May 2017 as compared with traditional power generator and refrigerator. The past two decades witnessed a Received in revised form significantly increased academic activities and industrial interests in thermoelectric materials. One of the 2 June 2017 most important impetuses for this boost is the concept of “nano”, which could trace back to the pioneer Accepted 2 June 2017 works of Mildred S. Dresselhaus at 1990s. Although the pioneer passed away, the story about the nano Available online 10 June 2017 thermoelectric materials is still continuous. In this perspective, we will review the main mile stones along the concept of thermoelectric , and then discuss some new trends, strategies and opportunities. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction the dilemma, we decompose the ZT definition equation, and rewrite it into ZT ¼ (S2n)(m/k)eT by applying the relationship of Thermoelectric energy conversion system has great appeal in s ¼ nem, where e, n and m are the free charge, carrier concentration term of its silence, simplicity and reliability as compared with and carrier mobility, respectively [3]. The interconnection between traditional power generator and refrigerator. Furthermore, its S and n, called as the “Pisarenko relation”, demonstrates that the capability of easily scaling down to the small size but without higher n, the lower S. For the material with simple parabolic band significantly sacrificing its efficiency makes it unique to convert the edge, the optimized (S2n) is mainly determined by the carrier widely distributed waste heat into the electric energy and also to effective mass, i.e. m*. The aim of band structure engineering stra- disperse the heat in the field of microelectronics and telecoms. tegies is to increase the m* by applying additional carrier pocket [4], Since the thermoelectric energy conversion system could be resonant doping [5] and band convergence [6]. In the other hand, considered as a heat engine by using the as energy car- the compromise between carrier mobility and lattice thermal riers [1], the efficiency of the thermoelectric power generator or conductivity (klat) is an important scale for weighting the effec- refrigerator is mainly determined by the Carnot efficiency and tiveness of nano approaches [7,8]. Frustratingly, the in- thermoelectric figure-of-merit ZT (ZT ¼ TS2s/k, where T, S, s, k are terconnections among them are not limited to these two cases. * the average , Seebeck coefficient, electrical conduc- There are still other dilemmas such as m vs. m and band gap Eg vs. tivity and , respectively [2]) due to the irre- klat. For more details, the readers are encouraged to further reach versible heat loss. Intuitively, a good thermoelectric material the recent deep review papers [3,9]. should have low thermal conductivity, high electrical conductivity The past two decades witnessed a significant increase in aca- and Seebeck coefficient. Pursing a high ZT value, therefore, becomes demic activities and industrial interests in thermoelectric mate- the main target in the thermoelectric community. However, it is a rials. A simple scale for this continuous boost is the annual huge challenge to get a high ZT value due to the internal links publication growth from 500 papers in 1996 to 2800 papers in 2016 among these three transport parameters. To quickly be away from (based on the Web of Science Database [10]). Fig. 1 shows the annual publications on the topic of “thermoelectric” from 1996 to 2016, which corresponds to near four times increase in publica-

* Corresponding author. tions. One of the most important impetuses for this boost is the E-mail address: [email protected] (W. Liu). concept of “nano”, which could trace back to the pioneer works of http://dx.doi.org/10.1016/j.mtphys.2017.06.001 2542-5293/© 2017 Elsevier Ltd. All rights reserved. W. Liu et al. / Materials Today Physics 1 (2017) 50e60 51

grain boundary. To avoid the repetition, the readers are encouraged to reach the excellent review papers written by Kanatzidis and his co-authors [20,21]. In the other hand, the competition to achieve nanograined thermoelectric materials started much earlier [22,23]. The nano particles could be easily obtained by employing high energy ball milling process, which is known as mechanical alloying via directly using the elements as starting materials. However, nano powders alone were not enough to guarantee the final formation of nano- grained or nanocrystalline bulk materials. The nano features could be smeared out due to the fast grain growth in the conventional hot press process that usually takes hours to get a dense bulk. This technique challenge brought the fast current assistant hot pressing on the stage, which is also known as the spark plasmas sintering Fig. 1. Annual publications from 1996 to 2016 on the topic of “thermoelectric” and (SPS) or pulse current activated sintering (PAS), or field activated “thermoelectric”þ“nano” based on the database of Web of Science (Mar. 2017). sintering (FAS). The fabrication process of combination with high energy ball milling and fast spark plasmas sintering for the ther- moelectric nanocrystalline materials started from Japan [24,25], Mildred S. Dresselhaus at 1990s [11,12]. Prof. Dresselhaus is also and quickly propagated to Korea [26] and China [27,28] at the well known as the “thermoelectric grandma” in the Chinese ther- beginning of 2000s. A breakthrough in this powder metallurgy moelectric academic community, and just passed away few months route came in 2008 when Z. F. Ren's group reported a peak ZT of 1.4 ago at her age of 86. As MIT President L. Rafael Reif wrote that we in p-type Bi2Te3 [8] (30% higher than the commercial ingot) and lost a giant, an exceptionally creative scientist and engineer who 0.95 in p-type SiGe [29] (50% higher than the previous reported was also a delightful human being. Although the pioneer passed record in p-type SiGe ). Since both the ball milling and fast away, the story about the thermoelectric nanocomposites is still sintering were thermodynamically nonequivalent processes, the continuous. In this perspective, we will review the main mile final bulk materials were not strictly pure nanocrystalline mate- stones along the concept of thermoelectric nanocomposites, and rials, but with many nanoinclusions embedded in grains and then discuss some new trends, strategies and opportunities. located at grain boundaries [30]. As a result, a more accurate term, i.e. , was adapted to describe these type thermo- 2. Direction of going nano electric materials made from ball milling and fast sintering joint route [31]. Finding more detailed discussions about the thermo- The original idea of going nano suggested by Prof. Dresselhaus electric nanocomposite, the review paper written by Dresselhaus and her student Hicks was to use the size quantization effect to et al. [13] was well-received. In addition to the BM-HP (or MA-SPS), adjust the electronic of state for boosting the term of S2n other routes were also used to fabricate the nanostructured ther- [11e14]. The dimensional variable was suggested as new approach moelectric bulk materials, such as melt-spinning plus spark plasma to change the thermoelectric nature of the known materials. sintering (MS-SPS) [32,33], spark erosion plus spark plasma sin- Simultaneously, they also made unique contribution to explain the tering (SE-SPS) [34] and chemically metallurgy methods [35,36]. different scattering effect of confined interfaces to the electrons and The detailed introduction of these fabrication methods can be , which could be used to decompose the connection be- found in our reviewed book chapter [37]. It is worthy to emphasize tween m and klat, which ignited a hug enthusiasm in the thermo- that even earlier X. B. Zhao's group started to use the term of electric community to apply new nano strategies in thermoelectric “nanocomposite” to describe their Bi2Te3 bulk materials fabricated materials. by the hydrothermal method combined with hot pressing [38]. Along the direction of going nano, the two-dimensional super- Generally, the term of “nanocomposite” is more accurate to lattice was firstly used to exam the beneficial effect of the quantum describe the bulk material made from any thermodynamically [15,16]. Besides the quantum-confinement effect, a significantly nonequivalent process. In the beginning of 21 century, one of the reduced lattice thermal conductivity was observed in the super- most important theoretical findings is that the length scale of the lattice structures as compared with its bulk counterpart [17]. The mean free path is much wider than what we expected from effect of various nano structures was intensely investigated to tune the classic theory [39], which gives the firm theoretical support to the transport of phonons. Amongst the various nanostructures, the the original idea through the nanostructures decomposing the nanoinclusions and nanograins received most attentions indepen- connection between m and klat. Nanocomposite has the intrinsically dently at the very beginning. Nanoinclusion is a metastable pre- favorable features to minimize klat, i.e. a multi-scale phonon scat- cipitation, or “Guinier-Preston zone”, due to the phase-separation tering centers including grain boundary, nanoinclusion and point in the melting-quench process. It is not a fresh feature in metal defect. In 2002, Kanatzidis's group started to adapt the term of materials, but truly one exciting breakthrough in thermoelectric “hierarchical” to describe their nanostructured PbTe-x%SrTe fabri- material field when M. G. Kanatzidis's group showed that the cated by combining growth method and powder metallurgy presence of Ag/Sb-rich nanoinclusions resulted in the high ZT value method [40], which gives a size-scale perspective to nano- in the materials AgPb19SbTe20 (ZT ¼ 2.2 at 800 K [7], later the au- composite. The concept of “hierarchical” was imitated from the thors confirmed a more repeatable ZT value around 1.7 at 700 K biological world, which was also earlier used to describe the [18]). J. F. Li's group independently confirmed the advantage of the solution-derived particles with complicated nano features. Now, nanoinclusions in the AgPbSbTe system with a peak ZT of 1.5 at the separately nano approaches, i.e. nanoinclusions and nano 700 K by using a power metallurgy route [19] (mechanical alloying, grains, merged together. As a result, a multi-scale or all-scale spark plasma sintering and annealing). The idea of the nano- phonon scattering centers are widely used to minimize klat. inclusions for the phonon engineering threw a stone in the pool of Although the state-of-art thermoelectric nanocomposite has kinds thermoelectric community. Later, various technique routes were of hierarchical features according to the size of phonon scattering explored to form nanoinclusions embedded in grains and/or at the centers, the arrangement of these hierarchical structures is still far 52 W. Liu et al. / Materials Today Physics 1 (2017) 50e60 from an ordering way or a controllable way, as compared with real Skutterudite compounds would significantly reduce klat [46], which hierarchical structure in the biologic world, such as the bone [41], was quickly experimentally confirmed in the p-type CeFe4-xCoxSb12 crab exoskeleton [42], feature rachis [43] etc.. Fig. 2 compares the and LaFe4-xCoxSb12 [47]. This success initiated a hug enthusiasm to difference of the hierarchical structure between the thermoelectric search new filled Skutterudite and new compounds with crystalline nanocomposite and a crab exoskeleton. It is clear that the natural cages. Along this direction, the type-I Clathrate compound (e.g. hierarchical structure is more delicate and also shows higher hi- Sr8Ga16Ge30) and type-II Clathrate (e.g.Cs8Na16Si136) were quickly erarchical level. In 2012, the authors had explained the benefits of excavated [48], which share similar crystalline cage and loosely an ordering hierarchical structure in the concept of “ordering bound filler of Skutterudite compound. Since both the Skutterudite nanocomposite”. One of motivations was to reconstruct the trans- and Clathrate compounds show the complex crystalline structure port channel of electrons in traditional nanostructured thermo- as compared with the classic PbTe, other compounds with complex electric materials with randomly distributed defects through structures even without crystalline cage also received attentions, modulation dopants, regular-shaped inclusions, ordering- such as complex chalcogenides and Zintl phase [49]. Compared distributed inclusions, textured grains boundary and ordering with the former, Zintl phase is even a bigger family, which is porous structures [3]. A higher-level hierarchical nanocomposite characterized with intermetallic compounds composed of group 1 with more delicate ordering substructure would become a direc- () or group 2 (alkaline earth) and any post transition tion for the thermoelectric nonocomposites. metal or (i.e. from group 13, 14, 15 or 16). Strictly, the thermoelectric Clathrates also could be considered as the Group-14 3. Mater design from atoms Zintl phase. The more well-known thermoelectric Zintl phase was the Group-15 Zintl phase. In 1999, Kim et al. reported a low thermal 1 1 Besides the efforts at microscopic, mesoscopic and nanoscopic conductivity of 1.7 W m K in a Ba4In8Sb16 Zintl phase with a size-scale, people also looked further down size scale, i.e. the complicate crystalline structure [50], which was characterized with atomic level. In our previous paper [45], we had already reviewed very complex polyanionic framework. The most famous Group-15 the works to find the materials with intrinsically low klat. Here, we thermoelectric Zintl phase-Yb14MnSb11, possessed a high ZT > 1at only focus on the development of the key concepts. The classic 1200 K, indicating an promising alternate of the classic high- thermoelectric materials, Bi2Te3 and PbTe, are characterized with temperature thermoelectric material SiGe. Moreover, the complex the heavy atoms that are benefited to the low heat capacity and crystalline features of Yb14MnSb11 were also observed [51]. Another hence the low lattice thermal conductivity. In 1994, Slack predi- notable example is Zn4Sb3, whose crystalline complex is charac- cated that a loose-bonding atom in the crystalline cage of the terized with the random distribution of Zn at three crystalline sites

Fig. 2. The comparison of hierarchical structure of a thermoelectric nanocomposite CoSb2.75Te0.2Sn0.05 [44] and lobster exoskeleton [42]. The lobster exoskeleton shows 7 hier- archical levels as compared with 3 hierarchical levels. W. Liu et al. / Materials Today Physics 1 (2017) 50e60 53

Fig. 3. The timeline of selected thermoelectric materials.

[52]. As a result, the thermal conductivity of Zn4Sb3 is much smaller [57] etc. Recently, Komouto's group has extended this idea to an than its simple counterpart ZnSb. Theoretically, the complex crystal organic-inorganic hybrid of TiS2/[HAx(H2O)y(DMSO)z] structures have more optical phonons that do not contribute much [58]. Morelli et al. proposed an alternative way to construct a to heat conduction and yet can scatter acoustic phonons, leading to complex diamond-like compound. The process could be started a lower lattice thermal conductivity. The interested reader could with a simple diamond-like compound, e.g. ZnSe, and then replaced 2þ þ 3þ 2þ reach two excellent review papers listed the known complex two Zn with Cu and In to form CuInSe2, or replaced three Zn 2þ 4þ crystalline [53,54]. with two Cu and one Sn to form Cu2SnSe3 [59]. The new Furthermore, there are also some block-building rules proposed compound is also expected to possess a behavior to construct the complex compound. Komouto et al. has proposed since the total charge is zero. The thermoelectric Stannite (e.g. the concept of nanoblock integration into a layer-structured hybrid CuZnSnSe4), Tetrahedrite (e.g.Cu10Zn2Sb4S13) could be constructed þ 2þ 3þ 2 2 crystal, such as (ZnO)mIn2O3 [55], (ZnS)mIn2S3 [56], SrO(SrTiO3)n in a similar way by using the Cu ,Zn ,Sb ,S and Se [60]. The 54 W. Liu et al. / Materials Today Physics 1 (2017) 50e60 crystalline complex could be also achieved through engineering the 4. Focus shift from local peak ZT anionic-site, e.g. BiCuSeO [61] and Bi2SeS2 [62]. In contrast to the large unit cell, the low klat of In3Se4 [63] and The theoretically investigation of the conversion efficiency of MgAgSb [64] suggested that local lattice distortion could also be the thermoelectric devices for power generation and another important crystalline feature. The distorted lattice to could be dated back to Raleigh's early attempt in 1885, and Alten- anisotropic chemical bonds and laminar or linear substructures. kirch's first satisfactory thermodynamic interpretation in 1911. It Cu2Se was another notable example for low klat compounds was early suggested that good thermoelectric materials should without large unit cell [65,66], in which the loosely bound Cu ions have large Seebeck coefficient and low thermal conductivity to behaved as a “liquid” and resulted in reduced heat capacity. The retain the heat at the junction and low electrical conductivity to “mobile” Cu ion is characterized with its uncertain location in the minimize the Joule heat [76]. Abram F. Ioffe firstly defined param- CuSe4-tetrahedron which is caused by the local resonant bonds. eter, z (a lower case of letter z), that embodied these desirable Recently, a theoretical study in IVeVI, V2eVI3 and V classic ther- properties in his book “Semiconductor Thermoelements and Ther- moelectric materials suggested that the resonant bounds could moelectric Cooling, Infosearch, 1957” [2]. Later, it was widely known cause the optical phonon softening, strong anharmonic scattering as the thermoelectric figure-of-merit with a unit of reciprocal and large phase space for three-phonon scattering processes [67]. temperature. For convenience, a dimensionless figure-of-merit (zT, Usually, the compounds with intrinsically low klat also show low or ZT) was adapted according to the relationship of efficiency for carrier mobility and low power factor which is important for power power generation and refrigeration. generation application. Recently, the new compound SnSe, with pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi simple unit cell but a distorted low symmetry lattice, showed a T T ð1 þ ZT Þ 1 h ¼ H C pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiM (1) potential to change this common sense because it has intrinsically TC ð1 þ ZTMÞ þ TC =TH low lattice thermal conductivity and high power factor [68]. One of the notable features for the high power factor is the complex Fermi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T ð1 þ ZT Þ T =T surface, which means that it has a high effective mass and high S2n. f ¼ C pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiM H C (2) ð þ Þ þ Similarly, a quite high power factor of 45 mWcm 1K 2 was reported TH TC 1 ZTM 1 in the new half- NbFeSb [69], which was asso- ciated with the complex Fermi surface. The benefit of the multi where the TM is the average temperature between the hot side (TH) ¼ þ carrier pockets was early noticed in the filled Sktterudites [70,71]. and the cold side (TC), i.e. TM (TH TC)/2. It is worthy to point out Liu et al. clarified that the additional carrier pocket, with edge that the early researchers differentiated zT and ZT by denoting zT as the material figure-of-merit and ZT as the device figure-of-merit. difference <5 kBT (kB, Boltzmann constant), could significantly enhance the power factor [4]. However, the strategy of band However, this difference is not widely accepted and the symbol of “ ” structure engineering did not receive enough attentions until ZT is more popular in the thermoelectric community. For “ ” Heremans et al. [5] published the work on Tl-doped PbTe in 2008 example, the symbol of ZT was widely used by the researchers of “ and later Pei et al. [6] showed a band convergence in the valence the world in the book CRC handbook of thermoelectrics, CRC Press, ” band of the PbTe-PbSe system resulted in a peak ZT of 1.8 in 1995 [76]. Although the derivation of Eqs. (1) and (2) is based on a rough model with temperature-independent Seebeck coefficient, PbTe0.85Se0.15. As a result, the multi carrier pocket or degenerate carrier valley is widely accepted as a strategy for enhancing the electrical conductivity and thermal conductivity, it did not impede power factor. that the composite-indicator, ZT, become the compass to guide the fi In the quantum scenario, only the electrons near the Fermi advances of thermoelectric materials from a quite small eld into a surface contribute to electrical conductivity. A larger Fermi surface area corresponds to more electronic states and larger S2n. Besides the number of multiple carrier pockets, the anisotropy of the Fermi surface was another important factor to determine the high power factor [72]. Recently, an orbital engineering was used to design the complex Fermi surface in the CaAl2Si2 Zintl family through mini- mizing the crystal field splitting energy of orbitals to realize high orbital degeneracy [73]. Additionally, the high power factor can also be achieved by reducing the scattering to major carriers or selec- tively scatter the low-energy carriers. Recently, He et al. achieved a recording power factor of ~106 mWcm 1K 2 at room temperature in the p-type half-Heusler Nb0.95Ti0.05FeSb by reducing the grain boundary scattering to the holes [74]. Mao et al. observed a high 1 2 value of ~102 mWcm K at 600 C in the Se-doped Cu56Mn42Mn2 alloy, which was attributed to the low-energy carries scattered by the twin boundaries [75]. Fig. 3 shows the timeline of selected thermoelectric materials since 1950s. It is very attractive to continuously search new materials with intrinsically low lattice thermal conductivity together with high power factor. Some favorable features or mater code for the next thermoelectric super- star could be: (1) heavy atom, or atom with lone-pair electrons, þ such as Sb used as Sb3 cation, (2) weak, or rattling, or resonant bonds, (3) large and complex unit cell, or distorted crystalline structure, (4) complex Fermi surface. Simply, the new promising thermoelectric compound would be characterized with complex structure for low lattice thermal conductivity and complex Fermi surface for high power factor. 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) 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. The primary optimization of a thermoelectric material usually involved in finding the highest measured ZT value within a two-dimensional region: temperature and dopants (or carrier concentration). This optimized figure of merit, ZTmax, was also widely referenced as the mile stone in the many review papers [9,20,21]. Generally, the milestone materials with a high recording ZTmax did present new physics or new mechanism, such as filled Skutterudite [47], AgPbSbTe [7], nano Bi2Te3 [8]. However, a problem occurs as we try to estimate the theoretical efficiency of the given materials with the measured thermoelectric ZT, usually temperature dependent. Goldsmid has early suggested that an average ZT could be only used for a rough estimated case [76]. A more accurate calculation needs numerically solving the one-dimensional heat flux equation under the electrical field and thermal gradient [77]. Since the numerical calculation is not such easily accessible, a rough estimation based on various average strategies was used. However, the error be- tween the estimated efficiency from average ZT and the numerical calculation did not get much attention for a long time. Recently, the more general derivation by considering the temperature transport parameters 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) and t Fig. 5. Physic meaning of the conventional ZT and new (ZT)eng. (T) is the temperature- also the engineering ZT (in the symbol (ZT)eng), which clearly fi dependent Thomson coef cient. showed a better linear relationship between the new figure of merit (ZT)eng and the leg efficiency [79]. Considering (ZT)eng and its related efficiency formula are more

Fig. 6. Selected examples for the wide-temperature enhanced local ZT. (a) Cl doped In4Se3-d [81], (b) S doped PbTe [82], (c) Ca and Pb co-doped BiCuSeO [83], (d) Ge doped Mg2Sn [84]. 56 W. Liu et al. / Materials Today Physics 1 (2017) 50e60 accurate, does it hint that we need to stop using the conventional ZT parameter was derived from two-band model which considered ? The answer is no. However, a new physical meaning should the bipolar effect, and hence showed a good indicator for the local endow it. It is noted that, even in the real case, Eqs. (1) and (2) could peak-ZT. be rigidly valid when the temperature range is infinitely small, the Furthermore, Mahan has theoretically suggested that a ther- ZTM therefore becomes a local figure-of-merit, which just corre- moelectric device having inhomogeneous doping could have an sponds to the measured ZT at a temperature point [80]. In contrast, increased efficiency [86]. Although there were few attempts of the engineering (ZT)eng was defined in a wide temperature range, gradient-doping Bi2Te3 [87] and PbTe [88], they did not get enough which was referred as global figure-of-merit, as illustrated in Fig. 5. attentions which may be due to the experimental challenge. In We also propose a symbol of ZT@T for the local ZT at temperature contrast, segmented legs were widely used in high performance

Th thermoelectric power generation (TEG). Recently, new recording ð Þ point T, while a symbol of ZT eng for the global (ZT)eng in the Tc efficiency of 11% and 12%, were reported in Bi2Te3/PbTe [89] and temperature range from Th to Tc. The (ZT)eng services as a better Bi2Te3/PbTe [90] segmented thermoelectric power generators, bridge between the materials performance and the devices per- respectively, (Bi2Te3/PbTe module: an eight-couple segmented 3 formance [80]. Although the concept of the global (ZT)eng is just module with the overall size of 18 15 ~10 mm with single recently proposed, the importance to have a high ZT over the whole segmented leg dimension 2 2 4.8 and measurement temper- application temperature range has been early recognized. Now, ature of Tc ¼ 10 C and Th ¼ 100 C; Bi2Te3/PbTe module: an eight- there is intensely interesting to optimize the local ZT over the wider couple segmented module with the overall size of temperature regions. Fig. 6 shows some selected examples [81e84], 20 20 14.5 mm3 with single segmented leg dimension of 3 but not limited. The strategy of synergistically tuning the transport 4 4 12 mm and measurement temperature of Tc ¼ 35 C and 2 k properties to have a favorable S n and suppressed lat would be Th ¼ 576 C.). It is also noted that the new technique, i.e.3D important to have largely enhanced ZT in a wider temperature printing, could make the whole leg-length optimization with range. In the case of Mg2Sn0.75Ge0.25, it was found that the alloying gradient doping strategy more feasible. Additionally, for a real de- * element Ge in Mg2Sn0.75Ge0.25 decreased the klat, increased m , and vice, the efficiency is not the only concern, other requirements, widened the Eg, which could be interpreted by a new thermo- such as effectiveness, reliability and flexibility, could be more = * * ðm*Þ3 2mT3=2 important in some cases [45]. For example, the TEG devices for a electric material parameter B , B f Eg [84,85]. The new B* klat

Fig. 7. (aec) Physic mechanics of the pyroelectric generator, (d) SEM image of the ZnO nanowire array-based pyroelectric generator, (e) working principle of the experiment setup for the pyroelectric generator, (f) peak output voltage and current with the dependence of temperature change [95]. W. Liu et al. / Materials Today Physics 1 (2017) 50e60 57 wearable electronics make organic thermoelectric materials a real shows the ZnO nanowires array-based pyroelectric generator pro- hot-topic [91,92], not due to its high efficiency but high flexibility posed by Yang et al. [95]. The diameter and length of the nanowires [93,94]. are about 200 nm and 2 mm, respectively. A silver film was deposited on the top of ZnO nanowires array and served as the top 5. Go beyond the Seebeck effect electrode, while ITO was used as the bottom electrode. The area of the nanogenerator was ~15 mm2. Fig. 7 (f) shows the output peak There are other strategies available for the thermal to electric current and peak voltage of the nanogenerator. An output power 11 2 energy conversion besides the Seebeck effect. In this section, density of ~5 10 Wcm is manifested at a temperature several new efforts will be reviewed for the energy-harvesting change of 25 K. Recently, Leng et al. [96] showed that a pyroelectric 6 2 application from the low-grade waste heat. In the near future, it generator outputted the power density of 14 10 Wcm under fi can be expected to process an attractive prospect despite most of a temperature difference of 80 K, indicating a signi cant them under the budding stage. improvement in pyroelectric generation for waste heat harvesting application. Although it displays the lower output power density than that of thermoelectric counterpart (around 1e2Wcm2) 5.1. Pyroelectric effect [89,90], it is still promising for the availability of storage and self- powered electric devices after further improving the power Pyroelectric effect results from the spontaneous polarization density. response with the environmental temperature changes in dielectric materials [95], as shown in Fig. 7 (aec). Theoretically, the efficiency pyroelectric power generator could be higher than that of TEG 5.2. Thermogalvanic effect based on Seebeck effect under the same temperature difference. The pyroelectric generator is considered as a promising self- Thermogalvanic effect, which is related to the temperature- powered nanotechnology for harvesting energy from an environ- dependent of electrode potential, can convert the heat into elec- ment of time-dependent temperature fluctuation. Fig. 7 (dee) tricity by charging at a high temperature and discharging at a low

Fig. 8. (a) Potential changes for both positive and negative electrodes. (b) Voltage-capacity curves of the full-cell in a charging-free thermal cycle. The schematic of the cell is shown 3/4 in the inset. (c) The dependence of voltage to the specific capacity at both 20 and 60 C. (d) Absolute efficiency vs. Fe(CN)6 /PB mass. The blue dots represent experimental data whereas the black and red curves are simulated based on experimental results. hHR is heat recuperation efficiency [98]. 58 W. Liu et al. / Materials Today Physics 1 (2017) 50e60 temperature [97]. Based on thermogalvanic effect, thermally 5.3. Water vapor-driven generator regenerative electrochemical cycle (TREC) was proposed, and it employs a reversible electrochemical reaction to build a thermo- Water evaporation, accompanied with the thermal energy due dynamic cycle to harvest the waste heat to electricity. Yang et al. to its large latent heat, is a ubiquitous phenomenon in the natural [98] demonstrated a charging-free TREC with low cost, simple world. Recently, Xue et al. [99] demonstrated that water evapora- system and no external electricity in the temperature tion from the surface of nanostructure carbon materials can range < 100 C in comparison with those electrically-assisted TREC. generate the electricity with the sustained voltage of up to 1 V, The design of free-charging process is realized by shifting the po- which is comparable to a standard AA battery. Gao et al. [100] re- tential of positive electrode to be lower than that of negative ported a self-sustaining polymeric nanogenerator driven by hot electrode when the temperature changes (T1/T2), indicating that water vapor for recovering energy. An open-circuit voltage of 145 V 2 the full-cell voltage and DG in the reverse process at T2 are and a short-circuit current of 0.12 mA/cm are outputted, as well as becoming negative to induce a spontaneous discharge process. the peak power density of 1.47 mW/cm3 by volume and 4.12 mW/ Fig. 8 (a) shows the potential changes for both positive and negative cm2 by area. The schematic figure of a water vapor-driven generator electrodes. The potential of positive and negative electrode should is shown in Fig. 9 (a). A large number of water droplets are formed be equal to result in the zero value of full-cell voltage at (T1 þ T2)/2. on the surface of the generator after the water-vapor condensation, After changing the temperature from T1 to T2, the discharged heating up the generator by heat release. Meanwhile, the local voltage becomes negative. Fig. 8 (b) displays the voltage-capacity humidity will be decreased under the air flow, and generator is curves of the full-cell in a charging-free thermal cycle. A flat full- quickly cooled down after the heat-absorption of water droplets. cell voltage curve is acceptable to maximize the energy output An image of generator above a cup of hot coffee and a photograph of (jaDT jcharge capacity, where a is temperature coefficient, flashed blue LEDs powered by water vapor driven generator are DT ¼ T1T2). The inset shows the schematic of the cell, consisting of displayed in Fig. 9 (b) and (c), respectively. Fig. 9 (d) shows the peak 3/4 an inexpensive soluble Fe(CN)6 redox pair and solid Prussian output power as a function of the loading resistance for 28 mm- blue (PB) particles as active materials for the two electrodes sepa- thick PVDF generator and 25 mm-thick P(VDF-TrFE) generator rated by a Nafion membrane. In Fig. 8 (c), the same shape of curves driven by water vapor. A first ascending and then descending trend can be noticed at both 20 and 60 C. The voltage is positive at 20 C with increasing loading resistance is noticed for these two vapor- while it becomes negative at high temperature 60 C. This driven generators. The larger power density is outputted in PVDF- charging-free electrochemical cell delivers a discharged capacity of based generator than P(VDF-TrFE)-based generator, which can be 23 mAh g 1 based on the PB mass. From the curves of absolute attributed to the higher pyroelectric coefficient 26e27 mC/m2 Kof 3/4 2 efficiency with the dependence of Fe(CN)6 /PB mass shown in the former than 16e19 mC/m K of the latter. Fig. 9 (e) presents the Fig. 8 (d). The simulated efficiency of 2.0% and experimental value mechanism of water vapor-driven generator. A perpetual dipole is of 1.5% at heat recuperation efficiency hHR ¼ 70% are reached, which perpendicular to the polarized PVDF and/or P(VDF-TrFE) films, are comparable with the ZT of 0.9 and 0.65 for an ideal TE device, inducing charges on the two metal electrodes. Under water vapor respectively. Consequently, the charge-free system is attractive and condensation, the decreased moments of dipole and enlarged promising in the application of conversion from low-grade waste volume is caused by the increased temperature of generator. Thus, heat to electricity. the decreased polarization density results in a current in the external circuit to balance the charge density on the metal

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