An insight into β-Zn4Sb3 from its crystal structure, thermoelectric performance, thermal stability and graded material

Materials Today Energy 3 (2017) 72e83

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An insight into b-Zn4Sb3 from its crystal structure, thermoelectric performance, thermal stability and graded material Jian Yang a, Guiwu Liu a, *, Zhongqi Shi b, Jianping Lin c, Xiang Ma d, Ziwei Xu a, Guanjun Qiao a, b, ** a

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China d SINTEF Materials and Chemistry, P.O.Box 124, Blindern, Oslo 0314, Norway b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 November 2016 Received in revised form 19 February 2017 Accepted 19 February 2017 Available online 27 February 2017

Thermoelectric (TE) materials have emerged as promising alternative for environment-friendly applications, being used for solid state power generator and heating/cooling to address environmental issues such as the global warming and the limitations of energy resources. Excellent TE materials should possess low thermal conductivity and high power factor, simultaneously. b-Zn4Sb3 is a p-type intermetallic compound, which is endowed with “phonon glass, electron crystal” (PGEC) behavior, exhibiting high Seebeck coefficient and low thermal conductivity close to the limit of amorphous structure. Due to its high TE performance, b-Zn4Sb3 is considered to be a promising TE material in the intermediate range of 500e750 K. In particular, remarkable advances have been recently achieved in the b-Zn4Sb3 TE material by introduction of nano-structures to tune the transport of phonons and electrons. However, its poor thermal stability is the obstacle to commercial application at the operating temperature. To understand the intrinsic nature of b-Zn4Sb3, such as crystal structure and structural variations with temperature, can help to develop appropriate strategies to enhance its TE performance and thermal stability. In this review, we shed light on the b-Zn4Sb3 TE material in details from several key aspects such as crystal structure, TE performance, thermal stability and the novel in-situ graded material. Finally, we propose the strategies for improving the TE performance and thermal stability of b-Zn4Sb3. © 2017 Elsevier Ltd. All rights reserved.

Keywords: b-Zn4Sb3 Thermoelectric performance Nano-structures Thermal stability Graded material

1. Introduction Due to the pollution and limitation of fossil sources, safe, clean and sustainable energy sources have attracted great attentions over the past decades. Nowadays, thermoelectric (TE) materials provide an alternative route to directly convert waste heat into electrical energy because of their environment-friendly advantages, low maintenance cost as well as no requiring moving components [1]. The conversion efficiency of a TE device is ultimately defined by a dimensionless figure of merit, zT:

* Corresponding author. ** Corresponding author. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China. E-mail addresses: [email protected] (G. Liu), [email protected] (G. Qiao). http://dx.doi.org/10.1016/j.mtener.2017.02.005 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

zT ¼ S2 sT=ðke þ kl Þ

(1)

where S is the Seebeck coefficient, s is the electrical conductivity, T is the absolute temperature, and ke and kl are the electronic and lattice contributions to thermal conductivity k, respectively [2,3]. The combined variable, S2s, is usually called the power factor (PF). Up to now, the TE materials had been mostly confined to niche applications, such as localized spot cooling and space exploration, due to its limited conversion efficiency and instability. Therefore, there is still a long way to go to obtain stable, high-quality and highefficiency TE materials for large-scale commercial applications. In general, the TE materials used in power generation can be divided into three categories according to their temperature ranges of application, namely low, intermediate and high temperature materials. Presently, Bi2Te3 and its alloys are operated under low temperature within the optimal temperature window of ~500 K [4]. In the intermediate temperature range of 500e900 K, tellurides,

J. Yang et al. / Materials Today Energy 3 (2017) 72e83

like PbTe [5,6], GeTe [7] and SnTe [8,9], are the most efficient materials in energy harvesting. Actually, Si-based materials, involving FeSi2 [10e12], manganese silicon [13] and magnesium silicon compounds [14e16], are also a kind of TE materials at the intermediate temperature. At the high temperatures of 1000e1300 K, SieGe alloys are usually used in power generation devices for space applications [17]. Other TE materials, such as skutterudite, halfHeusler, clathrates and chalcogenides, have also received enormous attentions. The state-of-the-art TE materials are dominated by Bi2Te3 and PbTe-based materials with the zT values of ~1 at room temperature [18,19]. The high zT values in these materials are mainly derived from nano-structures, which gives very low k in combination with high PF [20,21]. It is noted that Pb is not environment-friendly and Te is rare in the earth's crust, and thus high-cost. Hence, to explore high-performance TE materials with environmental friendliness and low cost is extremely important. One candidate is Zn4Sb3, as a promising intermediate TE material for wide industrial applications, with zT of ~1.3 at temperature around 673 K [22]. Its conversion efficiency at intermediate temperatures is much higher than those of current commercial TE materials, as shown in Fig. 1. Meanwhile, this material is very cheap and its constituent elements are “non-toxic”. Although it possesses at least three crystal phases such as a, b and g, only b-Zn4Sb3, stable from 263 to 765 K, is endowed with “phonon glass, electron crystal” (PGEC) behavior combining high s with very low k. Therefore, b-Zn4Sb3 is a kind of potential TE material in the intermediate temperature range. To motivate the further development of b-Zn4Sb3, we review its crystal structure, TE performance and thermal stability, and discuss the challenges and strategies for further improving the TE performance and thermal stability. Among which, integrating energy band engineering with nano-engineering could be an effective way, based on deep understanding of nano-engineering of b-Zn4Sb3. Furthermore, our group found an interesting phenomenon in recent experiments that the migration of Zn atoms along with the

Fig. 1. TE properties of b-Zn4Sb3 compared with other materials [23,24]: (a) TE figure of merit zT and (b) thermal conductivity k, showing that b-Zn4Sb3 has the highest zT at 423e748 K and the lowest k. The data of dotted curves are selected from the literature [24].

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direction of current flow during plasma activated sintering (PAS) can facilitate the formation of Zn concentration gradient, resulting in b-Zn4Sb3 graded materials with high TE performance. 2. Crystal structure Lots of researchers have devoted to the identification of crystal structure of b-Zn4Sb3 so far. As early as 1970s, Ignatiev et al. firstly proposed that b-Zn4Sb3 was presented as a hexagonal rhombohedral structure, R3C space group, with a ¼ 12.231 Å and c ¼ 12.417 Å [25]. Subsequently, Mayer et al. revised this result and pointed out that the lattice parameters of a and c were 12.233 and 12.428 Å, respectively [26], which authenticity was confirmed by X-ray diffraction [27]. Meanwhile, they proposed a crystal model of bZn4Sb3, consisting of 66 atoms (36 Zn atoms and 30 Sb atoms) in each unit cell with respect to Zn36Sb30 (36f-Zn, 18e-Sb1 and 12eSb2) [26]. However, the theoretical results such as mass density and crystal stoichiometry calculated by the crystal model were somewhat inconsistent with many experimental data, and the crystal model also could be not used to explain the reason why b-Zn4Sb3 exhibited low k and high s. Subsequently, they also suggested the Sb/Zn disorder on one of the Sb sites to explain the discrepancy in the crystal stoichiometry [26]. Unfortunately, the resulting calculated mass density was far from the experimental value. It can be seen clearly that the above-mentioned crystal structures do not match with the actual one that need to be re-examined. Since then, some scientists were occupied in the crystal structure of b-Zn4Sb3 under different temperatures [28e30]. For instance, Mozharivskyj et al. found that the Zn4Sb3 was not existed below 767 K, and that the observed stoichiometric Zn4Sb3 mixture was actually Zn6-dSb5 (with similar structure of Zn4Sb3) rather than Zn4Sb3 [28]. In fact, there is no evidence of Zn/Sb mixture substitution on any Sb1 sites. Instead, a significant deficiency on the Zn1 site was discovered, thus giving a defect structure. Snyder et al. proposed an alternative crystal structure (Fig. 2) [23], so that both electronic and thermal properties of b-Zn4Sb3 can be understood in view of this unique structure. Each unit cell is involved in 6 Sb4 2 and 18 isolated Sb3 ions, which requires a total of 78 electrons or 39 Zn2þ to maintain charge balance, yielding a charge balanced composition of Zn3$9Sb3. However, there are only 36 Zn atoms on normal lattice positions with respect to Zn3$6Sb3, so there must be at least three interstitial Zn atoms [23]. In other words, there are six types of atoms in the Zn4Sb3 cell rather than three types, namely 36f-Zn1, 18e-Sb1, 12e-Sb2, 36f-Zn2, 36f-Zn3 and 36f-Zn4. Meanwhile, it was also recognized that the Sb1 and Sb2 sites were full, while the lattice sites of Zn atoms displayed a considerable occupation deficiency (0.89e0.9) in terms of the Zn13Sb10. Subsequently, the composition of b-Zn4Sb3 was demonstrated to be Zn13Sb10 (Zn3$9Sb3), and the rhomboid rings Zn2Sb2 was recognized as the central structural building units of b-Zn4Sb3, which can establish a clear relationship between the b-Zn4Sb3 and the a-Zn4Sb3 [29]. Actually, the composition of Zn4Sb3 can be expressed as Zn13-dSb10 (d ¼ 0.2e0.5) [30], and thus the d becomes a decisive factor to control the TE performance of Zn4Sb3 in turn. As a consequence, the composition obtained from the refined occupancies was recognized as Zn3$83Sb3 [23,31], which is in good agreement with experimental results [22,32]. The complex and substantial structural disorder arising from additional interstitial Zn atoms and Zn vacancies (10%) is recognized to be partial origin of the extremely low thermal conduction of b-Zn4Sb3. The complex crystal structure can reduce the mean free path of acoustic phonons and effectively scatter phonons, resulting in the low kl of b-Zn4Sb3. Besides, the dynamic disorder deduced by soft dumbbell modes of Sb was quantitatively consistent with the low thermal conductivity in zinc antimony [33]. It has

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J. Yang et al. / Materials Today Energy 3 (2017) 72e83

Fig. 2. The crystal structure of b-Zn4Sb3 [23].

been recently reported that a remarkable phonon anharmonic behavior of interstitial Zn atoms could also be responsible for the low kl of b-Zn4Sb3 [34]. To sum up, the cause of the low thermal conductivity of b-Zn4Sb3 material is still lacking the unified and complete explanations. One thing is clear, however, its extraordinary low thermal conductivity is closely connected with its special crystal structure. 3. Thermoelectric performance Presently, different groups reported the zT values of b-Zn4Sb3 ranging from 0.3 to 1.4. The large variation in the zT is closely related to fabrication technology, chemical composition (such as Zn/Sb ratio, interstitial Zn atoms), and microstructure (such as bsubphases and nano-inclusions), etc [35e38]. However, among them the concentration of Zn is the most fundamental factor giving rise to the large variation [39]. The experimental results and theoretical calculations indicate that there are two main strategies in terms of Eq. (1) to optimize zT as high enough for commercial applications. One strategy focuses on employing the optimization of energy band engineering, such as to increase S by utilizing the sharp change of electrical density of states (DOS), quantum confinement and energy filtering effects. The other strategy is to enhance the scattering of heat-carrying phonons for further reducing the k through nano-structure engineering, although its k is close to that of the amorphous structure. As a simple binary compound, it is quite extraordinary that bZn4Sb3 has a k of ~1 Wm1K1 at room temperature lower than those of most glass materials, and that the k decreases to 0.7 Wm1K1 at 650 K. It is noted that it is an enormous challenge for improving the s and S and reducing the k in bulk materials, since these properties are closely intertwined and interacted due to the fact that they are determined by the fundamental characteristics of the electron and phonon system [40e42].

mainly coexisted with ZnSb phase for x < 4 or with the Zn phase for x > 4 [47]. In the ZneSb binary system, ZnSb has long been a subject of interest for use as p-type TE materials. Compared with Zn4Sb3 phase, ZnSb has higher S and s [49], suggesting a wider band gap. In contrast, Zn as a typical metal exhibits a low S and a high s [50]. Actually, it cannot draw a unified conclusion whether the secondary phases can improve the TE performance of b-Zn4Sb3 or not. Several investigations indicated that an appropriate rich-Zn or eZnSb phase could increase the electric transport property [50e52]. Among which, Zhang et al. illustrated an increase of S from 170 mVK1 for the single-phase b-Zn4Sb3 to 180 mVK1 for the richZnSb sample near 700 K, without providing information regarding the dimensionless zT [50]. Moreover, a 10 at.% Sb deficiency (Zn4Sb2.9) sample was demonstrated with a higher zT value than the nominal stoichiometric b-Zn4Sb3 [51]. It should be noted that the authors did not consider the loss of Zn, so it cannot be assured that the nominal stoichiometric sample is a single-phase b-Zn4Sb3 in those experiments. On the contrary, many more investigations indicated the Zn or ZnSb phase would deteriorate the TE performance of b-Zn4Sb3 bulk materials [53e56]. For instance, Zhu et al. investigated the effect of the secondary phase ZnSb or Zn on the TE properties of b-Zn4Sb3, showing that the highest peak zT of ~1.2 was achieved at 673 K in the single-phase b-Zn4Sb3, while two kinds of samples involving the secondary phases gave the peak zT values of 0.9 and 1.0 [55]. Why do only several single-phase b-Zn4Sb3 samples show the improved TE properties? Toberer et al. argued that few samples reported in the literature could be called as single-phase ones [46]. A little difference in chemical composition might lead to an enormous change in the TE properties. In other words, the so-called single-phase samples in some literature may not be real singlephase ones owing to the evaporation of Zn. Thus, the biggest challenge is to acquire single-phase b-Zn4Sb3 bulk samples by adjusting the stoichiometric ratio of Zn/Sb, which is involved in an indeed empirical process [46,57].

3.1. Zn/Sb ratio: influence of the constituent elements The ideal stoichiometric ratio of Zn/Sb with respect to zinc antimony can be expressed as 4:3. However, the diffusivity of Zn is much higher than that of Sb, and Zn is highly mobile in b-Zn4Sb3 [23,43,44], so extra Zn should be added to compensate for the loss of Zn during the preparation process. The Zn of 56.5e57 at.% and carrier concentrations of 6e9  1019/cm3 were determined to obtain a homogeneous single-phase b-Zn4Sb3 [45,46]. Actually, the secondary phases such as ZnSb and Zn can be produced while the Zn concentration exceeding the narrow range [47,48]. Moromura et al. fabricated the melt-spun ZnxSb3 ribbons with x of 3.6, 3.9 and 4.2 by a single-wheel process, and found that b-Zn4Sb3 phase

3.2. Element doping 3.2.1. Traditional strategy Usually, the most state-of-the-art TE materials are characteristic of the heavily doped semiconductor with the carrier concentrations on the order of 10191021 cm3. Doping is one of relatively wide and effective ways to optimize the TE properties of b-Zn4Sb3 bulks. The high-performance b-Zn4Sb3 material was achieved by increasing PF and decreasing the kl due to the variations of lattice parameters and carrier concentrations (Fig. 3). Recently, many researches focused on the systems of (MxZn1-x)4Sb3 (M ¼ Hg [35], Cd

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Fig. 3. Variations of lattice parameters and carrier concentration of b-Zn4Sb3 as a function of doping content x of Cd, Nb, Te, Ge, Pb, Cu and Ag [58e63].

dimer at the 12c Sb(2) sites to form the asymmetric SbeIn bond, which can contribute to reduce the lattice thermal conductivity [79].

3.2.2. Energy band engineering In a degenerate semiconductor, S can also be expressed by the Mott expression [81]:

. S ¼ p2 k2B T 3e½d lnðsðEÞÞ=dEE¼Ef

(2)

where kB is Boltzmann's constant, e is the electronic charge, s(E) is the electric conductivity, Ef is the Fermi energy. Here, S depends on the energy-dependent s(E) taken at the Ef. The s(E) is determined as a function of the band filling or the Ef, which is just proportional to the DOS at the Ef if the carrier scattering is independent of energy [82]:

sðEÞ ¼ pðEÞemðEÞ

(3)

pðEÞ ¼ gðEÞf ðEÞ

(4)

2

gðEÞ ¼ m*d

=

[58,64,65], Ge [61], Pb [62,66], Cu [63], Ag [63,66,67], In [66,68e72], Al [67,70], Ga [70], Sn [73], Fe [74], Bi [75]) and Zn4(Sb1-xNx)3 (N¼ Te [60], Bi [76], I [77], Se [78], In Ref. [79]) for p-type. As the representatives for the two systems, Table 1 gives the maximum zT and the corresponding Seebeck coefficient S and total thermal conductivity k at intermediate and low temperatures, showing that the doping can improve more or less the zT value. For instance, the bZn3.82In0$18Sb3 compounds exhibited an extraordinarily high zT of 1.41 at 700 K due to the significantly enhanced power factor and the intrinsic low thermal conductivity [72]. However, not all of dopings can significantly improve the TE performance of b-Zn4Sb3. The zT of Zn4Sb2$92Te0.08 sample only reached 1.0 at 673 K [80]. For the doping at Zn sites, it is quite important to clear the location of doped atoms in the crystal structure. Do they occupy the Zn vacancy or substitute for the interstitial Zn or Zn in the crystal sites? Aiming at this problem, Liu et al. performed the occupation options of Ag and Cu atoms in the disordered Zn4Sb3 based on the first principles calculations, indicating that the Ag and Cu atoms preferentially occupied the Zn vacancies in the normal sites, and subsequently substituted for the interstitial atoms [63]. However, the Hg substitution took place solely on the crystal framework site of the disordered crystal structure [35], and the doped Pb and In atoms preferentially occupied at the interstitial positions [62,72]. Although the substitution mechanisms at Zn sites are uncertain, there is no denying the fact that an effective doping can enhance the TE performance of b-Zn4Sb3. For the doping at Sb sites, Tang et al. investigated the In-substitution for Sb sites of Zn4Sb3 and found that In preferentially substituted one of Sb atoms in SbeSb

3

pffiffiffiffiffiffi.  2E Z3 p2

(5)

where, p(E) is the carrier concentration, m(E) is the carrier mobility, g(E) is the DOS, f(E) is the Fermi function, m*d is the DOS effective mass of carriers, and ħ is the reduced Plank's constant. From

Table 1 The maximum zT and the corresponding S and k of the representative (MxZn1-x)4Sb3 and Zn4(Sb1-xNx)3 systems at intermediate and low temperatures. Composition

Fabrication process

Maximum zT

S (mVK1)

k (Wm1K1)

Ref.

Zn3$96Cd0$04Sb3 Zn3$98Nb0$02Sb3 Zn3.99Ge0.01Sb3 Zn3$98Pb0$02Sb3 Zn3$75In0$05Sb3 Zn3$82In0$18Sb3 Zn4Sb3$94Bi0.06 Zn4Sb2$92Te0.08 Zn3$96Hg0$04Sb3 Zn3$96Al0$04Sb3 Zn3$96Ag0$04Sb3 Zn3$92In0$08Sb3 Zn3$98Fe0$02Sb3 Zn4Sb3$985I0.015 Zn4Sb2$985Se0.015

MS þ SPS Melting þ HP Melting Melting þ SPS Bridgman method Melting þ SPS Melting þ HP Melting þ HP Melting þ SPS Melting Melting Melting þ HP Melting þ SPS Melting þ HP Melting þ HP

1.3 (600 K) 1.1 (680 K) 1.35 (680 K) 1.2 (660 K) 1.4 (700 K) 1.41 (700 K) 1.09 (673 K) 1.0 (673 K) 0.3 (300 K) 0.23 (273 K) 0.06 (273 K) 0.09 (300 K) 0.12 (300 K) 0.1 (300 K) 0.11 (300 K)

200 180 203 180 220 222 160 170 135 115 60 120 140 110.8 120

0.65 0.72 0.72 0.75 0.62 0.62 0.7 0.75 e 0.6 1.4 1.25 0.95 1.1 0.9

[58] [59] [61] [62] [69] [72] [76] [80] [35] [67] [67] [71] [74] [77] [78]

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In 1993, the theoretical analysis performed by Hicks and Dresselhaus indicated that the enhanced S for 1D quantum wires and 2D quantum wells over that of the bulks were achieved [87,88]. Reduced dimensionality can enhance the S, which is benefited from

an enlarged DOS at the Fermi level because of the quantum confinement effect, and simultaneously break the trade-off between a high s and a large S. Subsequently, this theoretical result stimulated considerable researches on preparation of lowdimensional structures, including 0D nano-dots, 1D nano-wires and 2D quantum wells, and was also verified experimentally in CuTe/SeTe nano-wires [89e91], Bi2Te3/Sb2Te3 superlattices [92] and b-Zn4Sb3 nano-wires [93]. Generally, the low-dimensional structures can provide excellent TE performance. For instance, the dimensionless zT of the b-Zn4Sb3 nano-wires can rise to 1.59 at 675 K, which is the highest value for reported b-Zn4Sb3 [94]. However, these nano-architectures are not suitable for large-scale commercial applications because the used atomic layer deposition techniques are quite expensive only for small scale or high-end applications [95]. The instability of low-dimensional structures over a wide temperature range is also one of the major problems. In addition, ZneSb thin films also have attracted much scientific attention due to the introduction of a large amount of nano-scale defects and boundaries. Usually, the electrical transport performances of ZneSb thin films are significantly superior to those of the corresponding bulk materials [96e99]. The Seebeck coefficient of mixed ZneSb thin film indeed reached a high value of 360 mV/K [96]. Especilly, the Cu doped ZneSb based thin films showed high thermoelectric performance with an estimated zT value of ~1.35 [100]. However, these thin films mainly prepared by magnetron cosputtering need high-level equipments and usually have poor uniformity. Nowadays, nano-structured TE bulk materials for energy conversion applications gradually become the mainstream [101,102], and the peak zT values of several advanced bulk materials are more than those of the low-dimensional structures and thin films [103e105]. In order to obtain the high-performance nano-structured TE bulk materials, three challenges have to be overcome [106]: (1) how to create a material with nano-scale structures; (2) how to adjust the fabrication conditions to improve the zT; (3) how to produce a thermodynamically stable material, which make the nano-scale structures maintain throughout long service life. Despite of these difficulties, several primary ideas were proposed to fabricate the nano-structured b-Zn4Sb3 bulk materials in recent years.

Fig. 4. The calculated DOS of the undoped and Ge-doped b-Zn4Sb3 using two-vacancythree-interstitial Zn atom model: A10BCDSb10, showing that the DOS near the top of valence band and the bottom of conduction band increase significantly due to the Ge substitution [61]. Here, A is the framework Zn atom, and B, C and D are the interstitial Zn atoms.

3.3.1. Nano-grain boundary engineering The phonon mean free path (MFP) typically ranges from several nanometers to a few hundred nanometers. Based on the special crystal structure of b-Zn4Sb3, the interstitial Zn atoms can significantly scatter short-wavelength phonons [107], while the mid-tolong wavelength phonons can also propagate without significant scattering. If the short-wavelength and mid-to-long wavelength phonons which are able to transport heat can be scattered simultaneously, a notable reduction of k becomes possible. The formation of nano-structures with a large characteristic length equivalent to the MPF can scatter these phonons. Moreover, nano-grains with dimensions of 1e100 nm can act as effective filter for the mid-tolong wavelength phonons. Thus, one feasible approach is to form high-density grain boundaries as phonon scattering centers. Recently, to modify the grain size of the Zn4Sb3 bulk materials, top-down technologies were mainly involved in synthesis of nanoscale powder and subsequent consolidation by hot pressing (HP) or spark plasma sintering (SPS). Among them, ball milling (BM)eHP/ SPS process is one of the widely applicable and effective methods to prepare the nano-grains. A single-phase Zn4Sb3 prepared by the BM-HP exhibited an extremely low k of 0.69 Wm1K1 at 573 K [55], which can be attributed to further phonon scattering by nanostructures besides the scattering of interstitial Zn. From statistical analysis of the sizes of nano-particles processed by the BM, the

equation (25), an increased energy-dependent p(E) caused by the enhancement of g(E) (i.e. DOS) can improve the S, and the resonant distortion of DOS induced by doping can be reflected by the increased m*d. The Fermi energy level of undoped samples are usually located at the middle of the energy gap, and would move deeply into the conduction band or the valence band due to introduction of energy states in the band gap by doping, which can cause the resonant distortion of DOS near the Ef reflected by the increased md*. The resonant level can be acted as a bound level with an energy that falls above the conduction band edge or below the valence band edge [83]. Thus, an increase of DOS near the Ef can lead to an increased S. For instance, the introduction of a slight amount of Ge, Sm, Pr or Pb at the Zn sites of b-Zn4Sb3 beneficially modified the band structures and increased the DOS near the Ef accompanied by a large increase of m*d [61,84e86] (Fig. 4). Moreover, an almost 2fold increase of m*d of b-(Zn0.998Sm0.002)4Sb3 was manifested, resulting in ~40 mV/K increase of the S, and the zT value was ~53% higher than that of the undoped sample, reaching 1.1 at 615 K [85]. To sum up, benefited from an outstanding increase in the S and/ or a remarkable drop in the k by the introduction of impurity atoms, a certain enhancement in peak zT at operating temperatures can be achieved in p-type (MxZn1-x)4Sb3 and/or Zn4(Sb1-xMx)3-based materials. The improvement of TE performance of b-Zn4Sb3 caused by traditional doping is relatively limited since it is a heavily doped semi-conductor. Increasing the carrier effective mass by energy band engineering is a feasible way to improve its TE performance. Presently, nano-engineering is commonly introduced in diversified TE materials due to breakage of coupling relationships among the S, s and k. Therefore, integrating the energy band engineering with the nano-engineering could be an effective way for enhancing TE performance, with help of deep understanding of nanoengineering of b-Zn4Sb3. 3.3. Nano-engineering

J. Yang et al. / Materials Today Energy 3 (2017) 72e83

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Fig. 5. TEM images of (a) the as-prepared nano-powers after BM and (b) the hot-pressed Zn4Sb3 sample [55].

particle sizes were distributed in the range of 5e20 nm, while the grain size processed by the BM-HP ranged from 300 to 600 nm (Fig. 5), showing a significant grain growth in the Zn4Sb3 bulks. The k can further decrease if the sizes of nano-grains are controlled in the range of 1e100 nm [55]. Melt spinning (MS) technique is to utilize high pressure to inject molten metal on a high-speed rotating copper roller to obtain ribbons with non-equilibrium nano-scale structures. Each ribbon contains two different surfaces: the free and contact ones (Fig. 6a). Subsequently, the as-prepared ribbons were pulverized and sintered by SPS/HP to obtain compact bulks. Recently, the MS technique was used to generate nano-scale powders or precursors, and the corresponding compacted bulks showed significantly improved zT [58,108,109]. Wang et al. successfully fabricated ternary nanostructured (Zn0$99Cd0.01)4Sb3 specimens with an optimal zT value of 1.3 at 700 K by the MS-SPS [58]. Fig. 6b shows the microstructures of the MS ribbons, indicating that a large amount of flowerlike clusters of nano-sheets or nano-needles (~10 nm in size) are located on the free surface, while numerous bubble-shape “islands” of 1e2 mm in size distribute evenly on the contact surface (Fig. 6c). In the MS-SPS bulk samples, numerous nano-scale dots with the

size of 10e30 nm, originated from the multi-scale nano-structures in the MS ribbons, are embedded in the surface of nano-grains (Fig. 6d). These multi-scale nano-structures contributed significantly to the low k of 0.67 Wm1K1 at 700 K. 3.3.2. Nano-phase boundary engineering Introducing nano-inclusions (i.e. second phase) into Zn4Sb3 based nano-composites is another feasible route to form the highdensity phase boundaries. Both the coherent and incoherent nanoinclusions could inhibit the heat flow through the phonon scattering in the system (Fig. 7a), resulting in considerable reduction of the k. A reduction of kl from 0.5 Wm1K1 to 0.32 Wm1K1 was achieved at 648 K in the Zn4Sb3/Cu3SbSe4 nano-composites, which allowed the zT to reach 1.37 [83]. Besides the phonon scattering, the energy filtering effect at the phase boundary plays an important role in adjusting the carrier concentration by the effective filter for low-energy carriers (Fig. 7b), which can act as an equivalent one to increase the band gap [110]. The interface potentials controlled by the energy filtering effect can improve the PF due to an enhanced S, because the S is closely related to the DOS and relaxation time (t) reflected by the

Fig. 6. (a) Sketch of the MS process; (b,c) microstructures of the free and contact surfaces of MS ribbons and (d) HRTEM images of the MS-SPS neat bulk samples [58].

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Recently, our group applied a subsequent heat treatment plus rapid cooling on the single-phase Zn4Sb3 bulk, and found out that the in-situ Zn and ZnSb nano-inclusions were precipitated after 400 K [116]. The above-mentioned results indicate that the TE properties of Zn4Sb3 may decline due to the presence of the secondary Zn and ZnSb phases. However, the in-situ nano-inclusions derived from the decomposition of single-phase b-Zn4Sb3 failed to deteriorate the TE properties. On the contrary, the nano-inclusions, chaotic structure and compositional inhomogeneity slightly reduced the k of b-Zn4Sb3 (Fig. 8), allowing good TE performance above 673 K and the maximum zT of 1.1 at 750 K [116]. As discussed above, it is feasible to tune multi-scale nano-grains by the nano-grain boundary engineering to enhance TE properties of Zn4Sb3, and introducing nano-inclusions can also improve the TE properties by the nano-phase boundary engineering. Compared to the Zn4Sb3 nano-composites, Zn4Sb3-based composites received much more attentions, such as Zn4Sb2$94In0$06/ZnO [68], Cu5Zn3/ Zn4Sb3 [117], SiC/Zn4Sb3 [118], BiSbTe/Zn4Sb3 [119,120] based alloys. Thanks to the significantly high k and/or low S, the Zn4Sb3based composites cannot exhibit the desired TE performance. The Zn4Sb3-based nano-composites, as the promising TE materials, possess much higher zT than that of the single-phase Zn4Sb3 due to the addition of nano-structures. However, the investigations regarding the effect of the nano-inclusions on the TE properties of the b-Zn4Sb3 received fewer attentions.

Fig. 7. Schematic diagrams of (a) the phonon scattering mechanisms [114] and (b) the carrier filtering effect at coherent and incoherent phase boundary in bulk alloys [115]. Here Ec and Ev are the conduction and valence band edges, respectively.

Mott relationship. The strongly energy-dependent carrier relaxation time (t), caused by band bending at the phase boundary, can enhance the S [111,112]. The experimental evidences of the S enhancement in the Zn4Sb3-based materials were reported. For instance, a significant increase of S from 170 to 230 mVK1 was obtained in the Cu3SbSe4 nano-particles embedded b-Zn4Sb3 bulks because of the energy filtering effect at the boundary, and an approximately 30% reduction of the k was observed due to the interface scattering [113]. These results suggest that nano-phase boundary engineering can improve both the electronic and thermal properties of the Zn4Sb3 based nano-composites. So, to reasonably optimize the phase boundary should be further explored to maximize the zT value.

4. Thermal stability 4.1. Thermal instability and solving measures Despite of the outstanding TE properties of b-Zn4Sb3, the poor thermal stability is the obstacle to the commercial application at the operating temperature. Actually, b-Zn4Sb3 was deemed to be stable up to 673 K and 523 K at Ar atmosphere and in dynamic vacuum, respectively [22]. Moreover, multitudinous experimental results demonstrated that b-Zn4Sb3 was able to decompose at temperature below 500 K [121e123], forming the ZnSb and elemental Zn. The mechanism study of thermal instability of bZn4Sb3 has received the widespread attentions. Recently, one group shed light on the decomposition mechanism from the point of view of its structural evolution as a function of temperature [124]. With the increase of temperature, the lattice sites (Zn1) occupancy decreased and the interstitial sites (Zn2) occupancy climbed accordingly, indicating that more and more Zn

Fig. 8. HRTEM images of single-phase b-Zn4Sb3 after heat treatment [116]. Zone 1 represents Zn4Sb3 with crystal system of rhombohedral. Zones 2 and 3 represent Zn with crystal system of hexagonal. Zones 4 and 5 represent ZnSb with crystal system of orthorhombic. Zones 6 and 7 are chaotic region.

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Fig. 9. Structural variations of b-Zn4Sb3 as a function of temperature: (a) Occupancies of lattice (Zn1) and interstitial (Zn2) sites of Zn; (b) Zn1eSb1 bond length [24].

Fig. 10. Microstructures of b-Zn4Sb3 fabricated by PAS before and after different heat treatments: holding for 5 h at different temperatures and following rapid cooling [24].

atoms migrated from the Zn1 to the Zn2 sites, resulting in an enhanced Zn2/Zn1 ratio. Meanwhile, the stoichiometry of zinc antimony transformed from the Zn3$81Sb3 to the Zn3$86Sb3 while heated from room temperature to 525 K. An increase in the Zn content of unit cells was accompanied by the formation of elemental Sb and the ZnSb, namely: 1) Zn4Sb3 (I) / Zn4þdSb3 (II) þ Sb, and 2) Zn4Sb3(III) / Zn4þdSb3 (IV) þ ZnSb. As a result, the poor-Zn subphases (I and III) transformed into the rich-Zn subphases (II and IV), respectively. Coincidentally, our group also observed the migration of Zn1 to Zn2, leading to a 0.4% decrease of Zn1 occupancy of the single-phase sample at 425e525 K, which was further demonstrated by faster growth of the bond length of Zn1eSb1 at temperatures of over ~425 K as a sign of an increased instability of Zn1eSb1 bonds, as shown in Fig. 9 [24]. Thus, it can be deduced that the variations of Zn content and Zn1/Zn2 ratio seem crucial to the thermal stability of b-Zn4Sb3. To sum up, the diffusion of Zn atoms is the main factor for the decomposition of b-Zn4Sb3. In the b-Zn4Sb3 crystal structure, the interstitial sites provide numerous channels to accelerate Zn migration [43,44,125]. Thus it is believed that the impediment of Zn migration is a practical way to enhance the thermal stability, and that the doping of big or heavy atoms for filling vacancies may act as a barrier of the diffusion of Zn. For instance, the Cd-doped b-Zn4Sb3 bulk exhibited a large improvement of thermo-dynamic stability, that is the b-Zn4Sb3 was not degenerated after 10 heat cycles at 300e700 K or 30 h annealing at 680 K in vacuum [58]. Pedersen et al. argued that the Cd atoms could stabilize the structure and inhibit the diffusion of Zn due to the substitution for the Zn sites, especially for the interstitial Zn [126]. Similarly, Pb atoms can also hamper the migration of Zn in local disordered structure of Zn4Sb3 [62]. In addition, the original Zn content can significantly influence

Fig. 11. Schematic of the timetemperaturetransformation curve for b-Zn4Sb3. The red line is the heating path [24]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the thermal stability of b-Zn4Sb3. Dasgupta reported that the richZn samples exhibited the excellent thermal stability [38]. Moreover, the optimal Zn/Sb ratio for the thermal stability of b-Zn4Sb3 ranged from 1.23 to 1.30 based on the theoretical point of view [48]. However, excess Zn inclusions in the b-Zn4Sb3-based samples may deteriorate the TE properties compared with the single-phase bZn4Sb3 [50]. Besides, its presence decreased the zT value although the metallic oxide TiO2 with chemical stability and moderate TE properties as nano-inclusions could improve the thermal stability of b-Zn4Sb3 [127].

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Fig. 12. Distributions of (a) constituent phases and (b) Seebeck coefficient of b-Zn4Sb3 TE material prepared by SPS in different areas [132].

4.2. Thermal stability at elevated temperature Interestingly, we found a peculiar phenomenon that b-Zn4Sb3 exhibited remarkable high-temperature stability across a wide temperature range, which mechanism can be interpreted by the evolution of microstructures (Fig. 10) [24]. After a heat treatment at 523 K, the Zn and ZnSb nano-structures, derived from the decomposition of b-Zn4Sb3, were preferentially located at the grain boundaries and inside the grains, respectively. The average size of nano-structures decreased from 30 to 10 nm as the heat treatment temperature rose from 523 to 623 K. After the heat treatment at 723 K, the nano-structures disappeared due to the inverse reaction of decomposition of b-Zn4Sb3. In order to further reveal the thermal stability at high temperature, we plotted the timetemperaturetransformation (TTT) curve of b-Zn4Sb3 (Fig. 11). The first transition appeared if the bZn4Sb3 sample was heated along red curve to a temperature over 425 K. The migration of Zn atoms originated from the weakening of ZneSb bonds brought about the fluctuations in composition, resulting in the decomposition of metastable b-Zn4Sb3 into the elemental Zn and ZnSb. The second transition occurred at 565 K when the Zn and ZnSb were reintegrated into the b-Zn4Sb3, and the b-Zn4Sb3 bulk became thermodynamically stable. Thus, the optimal application temperature ranges from 565 to 765 K. In particular, the unexpected high-temperature stability allows excellent TE performance of single-phase sample above 693 K and the maximum zT of 1.4 at 748 K.

For the b-Zn4Sb3 material, the behavior of Zn migration was verified, although it belonged to random diffusion. Yin et al. found that there was a distribution heterogeneity of Zn composition in the anode and cathode zones in the b-Zn4Sb3 TE material fabricated by SPS, resulting in an apparent difference of S in different areas (Fig. 12) [132], and that the graded distribution of Zn composition became more noticeable with the increase of applied electric current [133]. There was a similar case in our recent experiments. The difference was that our Zn4Sb3 graded material was based on the active design of directional migration of Zn atoms by pre-treatment and/or design of mould during PAS to avoid the electric current scatter. As shown in Fig. 13, the Zn atom exhibited obvious concentration gradient along the current direction (top to bottom). At the top, the Zn concentration was ~51.07 at.%, while it reached 59.02 at.% at the bottom of sample, indicating that the directional migration of Zn atom occurred under the effect of electrical field. The novel Zn4Sb3 graded material gave rise to high zT of ~1.3.

6. Summary and outlook

b-Zn4Sb3 is a kind of p-type semi-conductor with an exceptionally low k, which is closely related with its complex crystal structure, involving three interstitial Zn atoms and partial Zn vacancies. The TE performance of b-Zn4Sb3 was improved by adjusting the Zn/Sb ratio, element doping and nano-engineering. In particular, producing the resonant distortion of DOS by energy

5. Graded material of b-Zn4Sb3 matrix Usually, construction of functionally graded material (FGM) aims to broaden the ranges of temperature and/or frequency of peak efficiencies. Presently, the TE FGMs are involved in a carrier graded TE material prepared by controlling a gradual change of carrier concentration along the length of a TE device [128], and a segmented TE material fabricated by using a layered structure with individual layers [129e131]. However, the segmented FGMs are apt to distort and crack due to the mismatch of thermal expansion coefficients of the various layers, and the contamination of semiconductors derived from interdiffusion between components at the interfaces is unfavorable. Moreover, it is quite difficult to control the carrier concentration in different regions of the carrier graded material. Therefore, to develop novel preparation process of TE FGMs is of great significance.

Fig. 13. Microstructures of Zn4Sb3 graded material with Zn concentration gradient.

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band engineering can increase the carrier effective mass md*, and the introduction of nano-structures into the b-Zn4Sb3 bulk can effectively scatter the mid-to-long wavelength phonons and lowenergy carriers due to formation of the nano-grain and nanophase boundaries, resulting in significant improvement of TE performance. However, the thermal instability is the key factor to restrict its applications. It was demonstrated that the selective introduction of big atoms and nano-inclusions into the b-Zn4Sb3 matrix can inhibit its decomposition to a certain extent. Unexpectedly, the single phase b-Zn4Sb3 fabricated by PAS can display the high-temperature thermal stability, which is attributed to the self-repair of decomposition of Zn4Sb3 at elevated temperature. As discussed above, there are generally two main strategies for achieving high TE performance of b-Zn4Sb3: (1) optimizing energy band structure by doping and (2) tuning the microstructure by nano-engineering. Therefore, the integration of the energy band engineering with the nano-engineering could be an effective way to improve its TE performance. For the adjustment of energy band structure, the introduction of resonant distortion of DOS by doping (involving co-doping and multi-doping of cations and/or anions), such as Sm, Pr, Pb or Pt, can increase the S. From the perspective of nano-engineering, the nano-structures are generated by adjusting the grain- and phase-boundaries, and even introducing nano-scale distorted regions, atomic-scale dislocations and point defects. That is to say, constructing the all-scale hierarchical nano-architecture can lower the k by effectively scattering the phonons with different wavelengths ranging from short to long ones. The b-Zn4Sb3 material has poor thermal stability due to the presences of Zn and ZnSb products derived from the decomposition of b-Zn4Sb3. Based on the high-temperature stability of b-Zn4Sb3 reported by our group, the applied mid-temperature of b-Zn4Sb3 graded material should be in the range from 565 to 765 K. Moreover, tuning the Zn/Sb ratio and introducing big or heavy atoms by doping and nano-structures can also widen its range of application temperature. The graded distribution of Zn composition can be controllably produced due to the directional migration of Zn atoms originated from the vertical current across the sample, and the resulting bZn4Sb3 graded material shows high TE performance. Moreover, the b-Zn4Sb3 graded material combined with the energy band engineering and/or the nano-engineering, namely introducing impurity atoms and/or nano-structures into the graded material, may exhibit preferable TE performance and thermal stability. To sum up, the novel b-Zn4Sb3 graded material, possessing both high TE performance and thermal stability, is a promising candidate for midtemperature TE materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (51572111, 11404144), the Natural Science Foundation of Jiangsu Province (BK20151340), the Six Talent Peaks Project of Jiangsu Province (2014-XCL-002), the Innovation/Entrepreneurship Program of Jiangsu Province ([2015]26), and the Qing Lan Project ([2016]15. References [1] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 321 (2008) 1457e1461. [2] K. Biswas, J.Q. He, I.D. Blum, C.I. Wu, T.P. Hogan, D.N. Seidman, V.P. Dravid, M.G. Kanatzidis, High-performance bulk thermoelectrics with all-scale hierarchical architectures, Nature 489 (2012) 414e418. [3] Y.Z. Pei, X.Y. Shi, A. LaLonde, H. Wang, L.D. Chen, G.J. Snyder, Convergence of electronic bands for high performance bulk thermoelectric, Nature 473 (2011) 66e69.

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