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Materials Today Physics 1 (2017) 50E60 Materials Today Physics 1 (2017) 50e60 Contents lists available at ScienceDirect 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 nanocomposites, 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 electrons 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 temperature, Seebeck coefficient, electrical conduc- There are still other dilemmas such as m vs. m and band gap Eg vs. tivity and thermal conductivity, 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 alloy). 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. nanocomposite, 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 density 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 phonons, 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 phonon 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.
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