Tunable Optimum Temperature Range of High-Performance Zone

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Tunable Optimum Temperature Range of High-performance Zone Melted Bismuth-Telluride-based Solid Solutions Renshuang Zhai, Yehao Wu, Tie-Jun Zhu, and Xin-Bing Zhao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00605 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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Crystal Growth & Design

Dependences of carrier concentration n and carrier mobility µ on the excess Te content 257x185mm (300 x 300 DPI)

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Dependences of carrier concentration and carrier mobility on the SbI3 content 252x189mm (300 x 300 DPI)


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Sb and Te content in the axial direction examined via energy dispersive X-ray (EDX) and theoretical values of Sb content at different positions of the Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingot 757x540mm (96 x 96 DPI)



Through adjusting carrier concentration, high zTs (~ 1.2 at 300 K) are obtained in both n-type and p-type Bi2Te3-based zone melted ingots, of which high homogeneity is examined and suggests the good quality. The on-axis taper is found necessary for high thermoelectric performance and the purity of starting materials or the growth rate as slow as 8 mm/h makes little difference 727x578mm (96 x 96 DPI)


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Tunable Optimum Temperature Range of High-performance Zone Melted Bismuth-Telluride-based Solid Solutions Zhai, Renshuang; Wu, Yehao; Zhu, Tie-Jun;* Zhao, Xin-Bing State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China * Correspondence: Professor T. J. Zhu, E-mail: [email protected]

KEYWORDS:

Thermoelectric

materials,

bismuth

telluride,

zone

melting,

thermoelectric properties, homogeneity.

ABSTRACT Bismuth-telluride-based solid solutions are the unique commercially available thermoelectric (TE) materials near room temperature for solid state cooling, and zone melting (ZM) technique is commonly applied to grow their commercial ingots with preferred orientation. Herein, we tailor the carrier concentration of zone-melted ingots by proper doping to adjust the optimum operating temperature for both solid-state cooling and low-temperature power generation at 300 ~ 500 K. The room temperature zT of ~ 1.2 is obtained for both p-type and n-type ingots, and a maximum zT > 1.2 at 350 K in p-type Bi0.5Sb1.5Te3 + 2 wt% Te. Moreover, zT > 1.0 between 300 K and 400 K is achieved for p-type, and zT > 1.0 between 300 K and 475 K for n-type



counterparts. It is found that the taper of quartz tube affects the TE properties of Bi2Te3-based alloys. The different combinations of the growth rate with temperature gradient, 8 mm/h + 25 K/cm and 25 mm/h + 40 K/cm, contribute to equally high zT. In addition, the good homogeneity in both axial and radial direction implies the high quality of the ZM ingots in this work, which is significant for the industrial manufacturing. These results are favorable for TE application in low-temperature power generation, and show the possibility of improving the production efficiency of the commercial bismuth-telluride-based solid solutions.

INTRODUCTION Bismuth-telluride-based solid solutions are the unique commercially available thermoelectric (TE) materials for solid-state cooling and low-temperature power generation around room temperature. The maximum conversion efficiency of a TE device strongly depends on the dimensionless figure of merit zT = α2σT/κ, where α, σ, κ, T are Seebeck coefficient, electrical conductivity, thermal conductivity (including the carrier contribution κe and the lattice contribution κl) and the operating temperature, respectively. Nanostructuring1-4 and band engineering3, 5-7 are two main strategies to enhance TE properties of materials, aiming to reduce κl and improve the power factor (PF = α2σ), respectively. Traditionally, the maximum zT of bismuth-telluride-based solid solutions grown by zone melting (ZM) is often obtained in the vicinity of room temperature, which is favorable for solid-state cooling. Powder metallurgy has recently been applied to


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fabricate

bismuth-telluride-based

polycrystalline

alloys,1,

8-10

benefiting

to

simultaneously reduce the lattice thermal conductivity κl and enhance the mechanical properties. Through nanostructuring, the enhanced TE performance has been achieved for both p-type BiSbTe and n-type BiTeSe polycrystalline alloys. “Bottom-up” nanostructuring approach, by which the nano-particles are synthesized via ball milling (BM)1，hydrothermal method8 or melt spinning (MS)9 followed with hot pressing (HP) or spark plasma sintering (SPS), has been proven effective to enhance the TE performance of bismuth-telluride-based solid solutions. For example, Poudel et al. combined high-energy BM with hot pressing to reduce the κl remarkably and enhanced the zT of p-type BiSbTe alloy around room temperature;1 Xie et al. obtained a high zT > 1.5 at 300 K in melt-spun p-type BiSbTe alloys with unique nanostructures;9 Recently, Shen et al. developed a “top-down” process, hot deformation (HD), in which nanostructures were formed in situ and contributed to a zT > 1.3 around 300 K in p-type Bi0.5Sb1.5Te3 alloy;10 For n-type counterparts, Hu et al. directly hot deformed the commercial zone-melted Bi2Te2.79Se0.21 ingot and obtained a zT ~ 1.2 near the room temperature.11 Wu et al. combined Ag doping to tailor carrier concentration with HD process and obtained a zT ~ 1.1 in the n-type Bi2Te2.7Se0.3 HD bulk near room temperature.12 In short, the TE performance around room temperature of both p-type and n-type polycrystalline bismuth-telluride-based alloys can be effectively enhanced via nanostructuring approach. To utilize the abundant low-temperature waste heat, much work has also been focused on shifting up the maximum zT of polycrystalline bismuth-telluride-based



alloys to a higher temperature.15-20 The intrinsically narrow band gap Eg of Bi2Te3-based solid solutions, along with a relatively low optimum carrier concentration n, makes the TE properties deteriorate at high temperature due to the bipolar effect.13 Some efforts have been made to suppress the bipolar conduction, including broadening the band gap Eg

14

and increasing the majority carrier

concentration n 15. Xu et al. alloyed In into the hot deformed p-type Bi0.3Sb1.7Te3 ZM ingots and the synergistic role of band broadening, point defect engineering and multi-scale microstructures contributed to a high zT ~ 1.4 at 500 K;16 Tang et al. obtained a high zT ~ 1.1 at 600 K in the n-type Bi2Te1.9Se1.1 HD alloy with a high Se content, which possesses a larger Eg;17 To increase n, Hao et al. adopted the doping of Cd, Cu or Ag in p-type Bi0.5Sb1.5Te3 SPS bulks to suppress intrinsic excitation and obtained a high zT ~ 1.4 at 425 K;15 Liu et al. doped Cu into the twice hot-pressed n-type Bi2Te2.7Se0.3 alloy and obtained a zT ~ 1.06 at 400 K.18 Hu et al. and Xu et al. investigated hot deformed p-type Bi2-xSbxTe3 HP bulks and ZM ingots, respectively. With increasing Sb content, n increases and Eg is broadened. A zT ~ 1.3 at 380 K is obtained for HP-HD Bi0.3Sb1.7Te3 14 and a zT ~ 1.36 at 400 K for ZM-HD Bi0.4Sb1.6Te3 respectively.19 Recently, some new strategies are applied to enhance the TE performance of bismuth-telluride-based alloys. Wang et al. reported a zT ~ 1 at 800 K in n-type Te-free BiSbSe3 via band engineering and phonon softening.20 All of these aforementioned approaches and various synthesis methods21 have been demonstrated effective in enhancing the TE properties both around room temperature and at higher temperature (> 350 K) in bismuth-telluride-based alloys prepared by powder


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metallurgy. However, in spite of the less attention in recent years, zone melting has long been the mainstream fabrication technique for commercial bismuth-telluride-based solid solutions due to the simple preparation flow and equipments. Tuning optimum operating temperature is significant to endow bismuth-telluride-based solid solutions with more practical applications. For solid-state cooling around room temperature, Rosi et al. obtained the zT ~ 1.0 at 300 K in both n-type and p-type Bi2Te3-based ZM ingots as early as 1999;22 As for low-temperature power generation, Jiang et al. enlarged Eg and increased n of the p-type ZM Bi2-xSbxTe3, and obtained a zT ~ 0.95 at 400 K in Bi0.32Sb1.68Te3 with excess 3 wt% Te;23 Similarly, Wang et al. obtained a zT ~ 0.86 at 600 K in n-type 0.1 wt% I doped Bi2Te1.5Se1.5 ZM ingot.24 However, the lattice thermal conductivity κl of Bi2-xSbxTe3 and Bi2Te3-ySey ZM ingots increases at x > 1.5 or y > 0.3.25 In this work, we focused on tuning the optimum operating temperature of both n-type and p-type Bi2Te3-based ZM ingots to facilitate practical applications in both solid state cooling and low-temperature power generation. The zT of 1.2 at 300 K and 0.8 - 1.2 in the range of 325 ~ 500 K were obtained in both n-type and p-type Bi2Te3-based alloys. Furthermore, we also investigated the effects of the taper of quartz tube, the purity of element chunks, temperature gradient and growth rate. The good homogeneity in both axial and radial directions of the ZM ingots was achieved, which ensured the reliability of the calculated zT.



EXPERIMENT Highly pure of Bi, Sb, Te and Se (5N) element chunks were weighed according to the nominal compositions Bi0.5Sb1.5Te3 + x wt% Te （x = 0, 2, 3 and 4）and Bi2Te2.7Se0.3 + y wt% SbI3 (y = 0.09, 0.10, 0.11, 0.12 and 0.13). The mixtures were sealed into well-cleaned quartz tube, of which one end was shaped into taper on the flame. Subsequently, the mixtures within quartz tube were placed into a rocking furnace and melted at 1073 K for 10 h and then cooled to room temperature in the furnace. The as-obtained ingots were placed into a vertical zone melting (ZM) furnace at 923 K. The growth rate can be adjusted by controlling the speed of motor equipped in the ZM furnace. The temperature gradient was measured by placing thermocouple at different positions in the ZM furnace and it could be indirectly adjusted by stuffing refractory materials. The growth rate and the temperature gradient were adopted as 8 mm/h and 25 K/cm, if not specially mentioned. The obtained 13 cm long ZM ingot, with a diameter of 12mm and a tapered tail length of around 4 cm, was cut into a 12 mm (in-plane direction) × 2.5 mm × 3 mm bar and a φ 12 mm × 2 mm (in-plane direction) disk from the very middle part, which are used for in-plane electrical properties and thermal properties measurements, respectively. X-ray diffraction (XRD) with a Rigaku D/MAX-2550P diffractometer was applied to examine the phase structures of all samples. The freshly fractured surfaces were observed via a FEI Sirion field emission scanning electron microscope (FESEM) and the axis homogeneity was investigated via energy dispersive X-ray (EDX)


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spectrometric microanalysis equipped in FESEM. The actual chemical compositions were analyzed by electron probe microanalysis (EPMA, JEOL JEA-8100) with a wave dispersive spectrometer. A Netzsch LFA-467 flash laser apparatus with a Pyroceram standard was applied to measure the thermal diffusivity (D). The densities ρ of samples were estimated by an ordinary dimension-and-weight measurement. The specific heat Cp was adopted as Dulong-Petit value and the thermal conductivity was computed according to κ = DρCp. The Seebeck coefficient and electrical conductivity were measured simultaneously on a commercial Linseis LSR-3. The Hall coefficient (RH) at 300 K was determined on a Quantum Design PPMS-9T instrument using a four-probe configuration. Then the carrier concentration (n) and in-plane carrier mobility (µ) were calculated according to n = 1/eRH and µ = σRH, respectively. It should be stressed that all the measurements were performed along the in-plane direction.

RESULTS AND DISCUSSION Tuning optimum operating temperature range P-type Bi2Te3-based solid solutions No impurity phases were traced in the XRD patterns of p-type Bi0.5Sb1.5Te3 + x wt% Te powders, as indicated in Figure 1a. The downshift of the diffraction peak around 28.2〫in Figure 1b indicates that the excess Te enters the lattice to compensate for Te loss and hence decreases the concentration of Bi′Te antisite defects26, which results in the decrease of carrier concentration n, as displayed in



Figure 1c. Carrier mobility µ increases with increasing Te content, due to the decreased n and partly the weakened scattering of point defects by antisite defects.

Figure 1. (a) XRD patterns of Bi0.5Sb1.5Te3 + x wt% Te powders; (b) Enlarged XRD patterns from 28.0 to 28.6 degrees of Bi0.5Sb1.5Te3 + x wt% Te powders. (c) Dependences of carrier concentration n and carrier mobility µ on the excess Te content.

Figure 2a shows that with increasing excess Te content x, σ decreases due to the dominant role of the decreased n; Correspondingly, α increases and the temperature of peak α downshifts to a lower temperature (Figure 2b); As a result, PFs of > 5.0×10-3


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Wm-1K-2 at room temperature were obtained in all the samples with excess Te and an extraordinary PF ~ 5.6×10-3 Wm-1K-2 at 300 K was obtained at x = 4 (Figure 2c). The thermal conductivity κ decreases with increasing x (Figure 2d) due to the reduced carrier contribution (Figure SI). The κl was calculated by κl = κ – LσT and the Lorentz number L was estimated using the single parabolic band (SPB) model.27-28 The increase of κl with increasing excess Te content x, as shown in Figure 2e, is attributed to the enhanced bipolar diffusion and the weakened phonon scattering of antisite defects. As a consequence, the temperature of maximum zT downshifts towards the room temperature (Figure 2f). A zT > 1.2 at 350 K were obtained for the ZM ingot with 2 wt% excess Te and a zT ~ 1.2 at 300 K for the ZM ingot with 4 wt% excess Te, respectively.



Figure 2. Temperature dependences of (a) the electrical conductivity, (b) the Seebeck coefficient, (c) the power factor, (d) the thermal conductivity, (e) the lattice thermal conductivity and (f) the zT of Bi0.5Sb1.5Te3 + x wt% Te ZM ingots.

N-type Bi2Te3-based solid solutions No impurity phase was found in the XRD patterns of n-type Bi2Te2.7Se0.3 + y wt% SbI3 powders as well, as indicated in Figure 3. Due to the donor role of iodine, the n increases from 1.2×1019cm-3 to 5.6×1019cm-3, and µ decreases from 387 cm2V-1s-1 to 174 cm2V-1s-1 with increasing the content of SbI3 from 0.09 wt% to 0.13 wt% (Figure 4a). Correspondingly, with increasing the SbI3 content, the σ increases (Figure 4b) and the absolute α decreases below 400 K, and the temperature of peak α upshifts to a higher temperature (Figure 4c). An extraordinary PF ~ 5.8×10-3 Wm-1K-2 is obtained at 300 K in the sample with 0.10 wt% SbI3 (Figure 4d).


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Figure 3. XRD patterns of Bi2Te2.7Se0.3 + y wt% SbI3 powders.

Figure 4. (a) Dependences of carrier concentration and carrier mobility on the SbI3 content, and the temperature dependences of the (b) electrical conductivity, (c) the Seebeck coefficient and (d) the power factor of Bi2Te2.7Se0.3 + y wt% SbI3 ZM ingots.



The κ increases with increasing the content of SbI3 (Figure 5a), mainly attributed to the increased carrier contribution κe (Figure 5b). The SbI3 doping has the weak effect on the lattice thermal conductivity near room temperature (Figure 5c). The temperature of zTmax is upshifted to a higher temperature with increasing SbI3 content y. A zT ~ 1.2 at room temperature is obtained for the sample y = 0.10 (Figure 5d). With different doping contents of SbI3, the optimum temperature range of n-type Bi2Te2.7Se0.3 ZM ingot can be tuned to ensure zT > 1.0 in 300 - 475 K, which is favorable for low-temperature power generation.

Figure 5. Temperature dependences of (a) the thermal conductivity (b) the carrier thermal conductivity (c) the lattice thermal conductivity (d) the zT of Bi2Te2.7Se0.3 + y


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wt% SbI3 ZM ingots.

Effects of growth condition on TE performance The taper of quartz tube Pointy end endows enough energy for heterogeneous nucleation according to the simplest mechanism of crystal growth, which is favorable for texturing in zone melting, and it is common to taper the quartz tube to obtain pointy ends.29-30 However, the necessity of on-axis taper is often ignored. Figure 6a shows the schematic diagrams of the on-axis taper and off-axis taper. Since Se doping in p-type Bi2Te3-based alloys is commonly applied for various purposes22, 31-34, and to take an overall consideration of the effect of tapered shape on TE properties, three ingots of Se doped Bi0.5Sb1.5Te3-zSez + 3 wt % Te (z = 0.08, 0.09, 0.10), were zone melted with on-axis taper and off-axis taper respectively. The ZM ingots with on-axis taper (right ones) were much glossier than those with off-axis taper (left ones), as shown in Figure 6b, and the stronger texture of the former can be observed in the scanning electron microscope (SEM) image (Figure 6c,d). The α remains almost the same within 5% systemic error (Figure 6e), indicating the insignificant variation of n. Meanwhile, the σ of samples with on-axis taper are larger by ~20% than those with off-axis taper (Figure 6f), due to the stronger texture. The PFs of samples with on-axis taper are higher than those of the off-axis taper (Figure 7a). The κ is almost the same due to the tradeoff between the increase of



carrier contribution and decrease of lattice contribution (Figure 7b and 7c). The decrease of κl in samples with on-axis taper may be attributed to the elimination of the “circulating effect”, which pumps the heat along the direction of the temperature gradient via Peltier effect caused by the micro-scale inhomogeneity and is common in the ZM ingots of poor quality.35 As a result, zTs are 20% higher than those with off-axis taper (Figure 7d). Therefore, one should use an on-axis taper to obtain the optimum zT for the zone melting procedure.

Figure 6. (a) Schematic diagrams of the on-axis and off-axis taper, (b) Images of the


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ZM ingots with the on-axis taper (right) and off-axis taper (left), SEM images of freshly fractured out-of-plane surfaces of (c) typical ZM ingot with off-axis taper, (d) typical ZM ingot with on-axis taper, and the temperature dependences of (e) the Seebeck coefficient and (f) the electrical conductivity of Bi0.5Sb1.5Te3-zSez + 3 wt % Te (z = 0.08, 0.09, 0.10) ZM ingots with different tapers.

Figure 7. Temperature dependences of (a) the power factor, (b) the thermal conductivity, (c) the lattice thermal conductivity and (d) the zT of Bi0.5Sb1.5Te3-zSez + 3 wt % Te (z = 0.08, 0.09, 0.10) ZM ingots with on-axis and off-axis tapers.



Growth rate, temperature gradient and the element purity To avoid the constitutional supercooling during the zone melting process, which causes the microscale inhomogeneity and hence deteriorates TE performance,36 the slow growth rate v and the large temperature gradient G at the solid-liquid surface are necessary.

35, 37

Here four different growth conditions (v + G) for Bi0.5Sb1.5Te2.91Se0.09

+ 3 wt % Te ingots are used to investigate the effects on TE performance: 4 mm/h + 25 K/cm, 8 mm/h + 25 K/cm, 25 mm/h + 25 K/cm, 25 mm/h + 40 K/cm. Under the same G, the slower growth rate is, the more time-consuming the zone melting is and more vaporized Te/Se deposited on the cold end of the quartz tube (Figure 8a). The α increases at room temperature and the σ decreases with increasing v (Figure. 8b and 8c), which is attributed to the decreased n caused by the less off-stoichiometry and hence less antisite defects Bi′Te with increasing v. The PF at 300 K is slightly increased for higher v (Figure 8d). The slowest growth rate gives rise to a relatively high κ around room temperature due to the enhanced carrier contribution and contributes to the lowest κ at > 400 K because of the suppressed intrinsic conduction, and the high κ of the sample with v = 25 mm/h is attributed to the “circulating effect”,35 as displayed in Figure 8e. As a result, with increasing v, the temperature of peak zT downshift to a lower temperature, and high zT ~ 1.2 is obtained at 400 K and 350 K with 4 mm/h + 25 K/cm and 8 mm/h + 25 K/cm, respectively, and the combination of 25 mm/h + 25 K/cm deteriorates the TE performance during 300 ~ 500 K (Figure 8f). Note that with increasing G to 40 K/cm, the 8 mm/h + 25 K/cm and 25 mm/h +


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40 K/cm yield equally excellent zT, as shown in Figure 8f. In addition, lower purity (99.99%, 4N) of element purity did not deteriorate the TE properties of Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingot (Figure 8). However, Scherrer et al. supposed the higher purity (99.9999%, 6N) contributed higher TE performance of Bi0.5Sb1.5Te2.88Se0.12 than the purity 4N (~10%).34 Thus, 4N element chunk and 25 mm/h + 40 K/cm can contribute to enough high zT, which accelerates the production and saves considerable cost in commercial use.



Figure 8. (a) The schematic diagram (left) and photo (right) of the Te/Se depositing during zone melting process and the temperature dependences of (b) the Seebeck coefficient (c) the electrical conductivity (d) the power factor (e) the thermal conductivity and (f) the zT of Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingots with different growth conditions.

Homogeneity It is important to check the quality of the sample and the compositional distribution since the composition segregation is common in the zone melting process. The photo of the typical ZM ingot (Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te grown by 8 mm/h + 25 K/cm) in this work is shown in Figure 9a. The 18 cm long ZM ingot was prepared to examine the homogeneity of the ZM ingots. The bulk samples were cut from different positions at the interval of 3 cm (Figure 9b) for XRD measurement. The similar intensity of peaks in the XRD patterns, except the sample 1# (closest to the tapered end) and sample 3#, indicates the well-orientated texture of the ZM ingot, as shown in Figure 9c. The highly oriented layered structures are also observed in the


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SEM images of the sample 4#, which is cut from the very middle part of the ingot (Figure 9d and 9e). These results suggest the good quality of the ZM ingot.

25µ µm



Figure 9. (a) Typical photo of the ZM ingot in this work. (b) Schematic of the cutting of bulk samples(1# ~ 6#) for XRD and samples (1’# ~ 5’#) EDX measurements. (c) Out-of-plane XRD patterns of bulk samples cut from different positions of the ZM ingot, SEM images of freshly fractured surfaces (d) in the out-of-plane direction and (e) in the in-plane direction of sample 4# of Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingot, (f) Sb and Te content in the axial direction examined via energy dispersive X-ray (EDX) and theoretical values of Sb content at different positions of the Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingot, (g) SEM of freshly polished radial surface of sample 4#, and (h) Radial distribution of Bi, Sb, Te, Se content examined via the line scanning of electron probe microanalysis (EPMA) in the Bi0.5Sb1.5Te2.91Se0.09 + 3 wt % Te ZM ingot along the yellow arrow in the (g).

The compositional distribution of Sb and Te in the axial direction was examined via EDX at the interval of ~ 1 cm (Figure 9f), and the samples for compositional examination were cut from the middle part into 3 cm × 1 cm × 1 cm bars with rotation of radial 90〫, as illustrated in Figure 9b. Good homogeneity in the middle part of


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the ZM ingot (3 ~ 15 cm, setting the position of the pointy end as 0 cm) was obtained and the compositional segregation occurred due to the normal solidification at the other end of the ZM ingot, which was supposed as the last zone.38 Hence, we suppose the last zone length l is 2 ~ 3 cm in this work and calculate the theoretical values of composition according to the following equations with the l = 3 cm, which fit well with the experimental data: C =1 − (1 − k )e − kg / l C0 C =(1 − g ) k −1 C0

(0 < g < X − l )

(X −l < g < X )

Where C, C0, k, g, X and l represent the actual concentration, the concentration at the balanced state (C0 = 60 at.%), effective distribution coefficient (k ~ 1.1 was adopted in this work according to the pseudo-binary phase diagram Bi2Te3-Sb2Te3 when supposing Sb2Te3 is the solute), the length of solidified part, the total length of the ZM ingot (X = 18 cm) and the zone length (l = 3 cm), respectively.38-39. In addition, the high radial homogeneity was confirmed via line scanning of electron probe microanalysis (EPMA) along the arrow in the SEM of polished radial surface (Figure

9g and 9h). Therefore, the middle parts of the typical 13 cm long ZM ingots in this work ensure the good homogeneity both in the axial and radial directions.

CONCLUSIONS Through adjusting the content of dopants, high zTs of 0.8 ~ 1.2 between 300 500 K for both p-type and n-type bismuth-telluride-based solid solutions are obtained



by zone melting growth, which is favorable both for state-solid cooling and low-temperature power generation. The on-axis taper is important for obtaining high zT due to the strong texture and lower purity (99.99%, 4N) of element chunks has no significant effect on TE performance of Se doped Bi0.5Sb1.5Te3 ingots. The slow growth rate is not necessary for better TE performance of p-type Bi0.5Sb1.5Te3 alloys, when a sufficiently large temperature gradient at solid-liquid surface is applied. The good homogeneity in both the axial and radial directions suggests the high quality of the ZM ingots in this work, and also ensures the reliability of the eventually calculated zT.

SUPPORTING IMFORMATION Temperature dependence of the carrier thermal conductivity of Bi0.5Sb1.5Te3 + x wt% Te ZM ingots (Figure SI).

ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (61534001 and 11574267) and the National Science Fund for Distinguished Young Scholars (51725102).

REFERENCES 1. Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen G.; Ren Z.


Page 26 of 33

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634-638. 2. Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. J.; Dhin, W. H.; Li, X. S.; Lee, Y. H.; Snyder, G. J; Kim, S. W. Dense Dislocation Arrays Embedded in Grain Boundaries for High-performance Bulk Thermoelectrics. Science 2015, 348, 109-114. 3. Zhu, T.; Liu, Y.; Fu, C.; Heremans, J. P.; Snyder, J. G.; Zhao, X. Compromise and Synergy in High-Efficiency Thermoelectric Materials. Adv. Mater. 2017, 29, No. 1605884. 4. Fu, C.; Liu, Y.; He, J.; Zhao X.; Zhu, T. Enhancing the Figure of Merit of Heavy-Band Thermoelectric Materials through Hierarchical Phonon Scattering. Adv. Sci. 2016, 3, No. 1600035. 5. Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of Electronic Bands for High Performance Bulk Thermoelectrics. Nature 2011, 473, 66-69. 6. Fu, C.; Zhu, T.; Pei, Y.; Xie, H.; Wang, H.; Snyder, G. J.; Liu, Y.; Liu, Y.; Zhao, X. High Band Degeneracy Contributes to High Thermoelectric Performance in p-Type Half-Heusler Compounds. Adv. Energy Mater. 2014, 4, No. 1400600. 7. Liu, W.; Tan, X.; Yin, K.; Liu, H.; Tang, X.; Shi, J.; Zhang, Q.; Uher, C. Convergence of Conduction Bands as a Means of Enhancing Thermoelectric Performance of N-type Mg2Si1-XSnX Solid Solutions. Phys. Rev. Lett. 2012, 108, No. 166601.



Page 28 of 33

8. Zhao, X. B.; Ji, X. H.; Zhang, Y. H.; Zhu, T. J.; Tu, J. P.; Zhang, X. B. Bismuth Telluride Nanotubes and the Effects on the Thermoelectric Properties of Nanotube-Containing Nanocomposites. Appl. Phys. Lett. 2005, 86, No. 062111. 9. Xie, W.; Tang, X.; Yan, Y.; Zhang, Q.; Tritt, T. M. Unique Nanostructures and Enhanced Thermoelectric Performance of Melt-Spun BiSbTe Alloys. Appl. Phys. Lett.

2009, 94, No.102111. 10. Shen, J. J.; Zhu, T. J.; Zhao, X. B.; Zhang, S. N.; Yang, S. H.; Yin, Z. Z. Recrystallization Induced In Situ Nanostructures in Bulk Bismuth Antimony Tellurides: A Simple Top Down Route and Improved Thermoelectric. Energy Environ. Sci. 2010, 3, 1519-1523. 11. Hu, L.; Wu, H.; Zhu, T.; Fu, C.; He, J.; Ying, P.; Zhao, X. Tuning Multiscale Microstructures

to

Enhance

Thermoelectric

Performance

of

N-type

Bismuth-Telluride-Based Solid Solutions. Adv. Energy Mater. 2015, 5, No. 1500411. 12. Wu, Y.; Zhai, R.; Zhu, T.; Zhao, X. Enhancing Room Temperature Thermoelectric Performance of n-Type Polycrystalline Bismuth-Telluride-Based Alloys via Ag Doping and Hot Deformation. Mater. Today Phys.2017, 2, 62-68. 13. Goldsmid, H. J. Bismuth Telluride and Its Alloys as Materials for Thermoelectric Generation. Materials 2014, 7, 2577-2592. 14. Hu, L. P.; Zhu, T. J.; Wang, Y. G.; Xie, H. H.; Xu, Z. J.; Zhao, X. B. Shifting up the Optimum Figure of Merit of P-type Bismuth Telluride-Based Thermoelectric Materials for Power Generation by Suppressing Intrinsic Conduction. Npg Asia Mater.

2014, 6, No. e88.


Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Hao, F.; Qiu, P.; Tang, Y.; Bai, S.; Xing, T.; Chu, H. S.; Zhang, Q.; Lu, P.; Zhang, T.; Ren, D.; Chen, J.; Shi, X.; Chen, L. High Efficiency Bi2Te3-Based Materials and Devices for Thermoelectric Power Generation between 100 and 300 °C. Energy Environ. Sci. 2016, 9, 3120-3127. 16. Xu, Z.; Wu, H.; Zhu, T.; Fu, C.; Liu, X.; Hu, L.; He, J.; He, J.; Zhao, X. Attaining High Mid-Temperature Performance in (Bi,Sb)2Te3 Thermoelectric Materials via Synergistic Optimization. Npg Asia Mater. 2016, 8, No. e302. 17. Tang, Z.; Hu, L.; Zhu, T.; Liu, X.; Zhao, X. High Performance N-type Bismuth Telluride Based Alloys for Mid-Temperature Power Generation. J. Mater. Chem. C

2015, 3, 10597-10603. 18. Liu, W. S.; Zhang, Q.; Lan, Y.; Chen, S.; Yan, X.; Zhang, Q.; Wang, H.; Wang, D.; Chen, G.; Ren, Z. Thermoelectric Property Studies on Cu-doped N-type CuxBi2Te2.7Se0.3 Nanocomposites. Adv. Energy Mater. 2011, 1, 577-587. 19. Xu, Z. J.; Hu, L. P.; Ying, P. J.; Zhao, X. B.; Zhu, T. J. Enhanced Thermoelectric and Mechanical Properties of Zone Melted P-type (Bi,Sb)2Te3 Thermoelectric Materials by Hot Deformation. Acta Mater. 2015, 84, 385-392. 20. Wang, S.; Sun, Y.; Yang, J.; Duan, B.; Wu, L.; Zhang, W.; Yang, J. High Thermoelectric Performance in Te-Free (Bi,Sb)2Se3 via Structural Transition Induced Band Convergence and Chemical Bond Softening. Energy Environ. Sci. 2016, 9, 3436-3447. 21. Li, J. F.; Pan, Y.; Wu, C. F.; Sun, F. H.; Wei, T. R. Processing of Advanced Thermoelectric Materials. Sci. China Technol. Sc. 2017, 60, 1347-1364.



Page 30 of 33

22. Taylor, P. J.; Maddux, J. R.; Jesser, W. A.; Rosi, F. D. Room-temperature Anisotropic,

Thermoelectric,

and

Electrical

Properties

of

n-Type

(Bi2Te3)90(Sb2Te3)5 (Sb2Se3)5 and Compensated p-Type (Sb2Te3)72 ( Bi2Te3)25 ( Sb2Se3)3 Semiconductor Alloys. J. Appl. Phys. 1999, 85, 7807-7813. 23. Jiang, J.; Chen, L.; Bai, S.; Yao, Q.; Wang, Q. Thermoelectric Properties of P-type (Bi2Te3)x (Sb2Te3)1−x Crystals Prepared via Zone Melting. J. Cryst. Growth

2005, 277, 258-263. 24. Wang, S.; Tan, G.; Xie, W.; Zheng, G.; Li, H.; Yang, J.; Tang, X. Enhanced Thermoelectric Properties of Bi2(Te1−xSex)3-based Compounds as N-type Legs for Low-Temperature Power Generation. J. Mater. Chem. 2012, 22, 20943-20951. 25. Yim, W. M.; Rosi, F. D. Compound Tellurides and Their Alloys for Peltier Cooling-A Review. Solid State Electron. 1972, 15, 1121-1140. 26. Heon, P. H.; Young, W. C.; Ji, Y. B.; Jae, D. S. The Effect of Excess Tellurium on the Thermo Electric Properties of Bi2Te3-Sb2Te3 Solid Solutions. J. Phys. Chem. Solids 1994, 55, 1233-1238. 27. Xie, H.; Wang, H.; Pei, Y.; Fu, C.; Liu, X.; Snyder, G. J.; Zhao, X.; Zhu, T. Beneficial Contribution of Alloy Disorder to Electron and Phonon Transport in Half-Heusler Thermoelectric Materials. Adv. Funct. Mater. 2013, 23, 5123-5130. 28. Pan, Y.; Li, J. F. Thermoelectric Performance Enhancement in n-Type Bi2(TeSe)3 Alloys owing to Nanoscale Inhomogeneity Combined with a Spark Plasma-Textured Microstructure. Npg Asia Mater. 2016, 8, No. e275. 29. Zhou, Y.; Li, X.; Bai, S.; Chen, L. Comparison of Space- and Ground-Grown


Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Bi2Se0.21Te2.79 Thermoelectric Crystals. J. Cryst. Growth 2010, 312, No.775-780. 30. Brown, D. M.; Heumann, F. K. Growth of Bismuth-Antimony Single-Crystal Alloys. J. Appl. Phys. 1964, 35, 1947-1952. 31. Zhai, R. S.; Wu, Y. H.; Zhu, T. J.; Zhao, X. B. Thermoelectric Performance of p-Type Zone-Melted Se-doped Bi0.5Sb1.5Te3 Alloys. Rare Met. 2018. 37, 308-315. 32. Yim, W. M.; Fitzke, E. V.; Rosi, F. D. Thermoelectric Properties of Bi2Te3-Sb2Te3-Sb2Se3 Pseudo-Ternary Alloys in the Temperature Range 77 to 300 K. J. Mater. Sci. 1966, 1, 52-65. 33. Ettenberg, M. H.; Maddux, J. R.; Taylor, J. P.; Jesser, W. A.; Rosi, F. D. Improving Yield and Performance in Pseudo-Ternary Thermoelectric Alloys (Bi2Te3)(Sb2Te3)(Sb2Se3). J. Cryst. Growth 1997, 179, 495-502. 34. Ivanova, L. D.; Granatkina, Y. V.; Scherrer, H. Effects of Growth-Charge Purity and Perfection of Bi0.5Sb1.5Te3 Single Crystals on Their Thermoelectric Properties. Inorg. Mater. 2000, 36, 678-684. 35. Cosgrove, G. J.; McHugh, J. P.; Tiller, W. A. Effect of Freezing Conditions on the Thermoelectric Properties of BiSbTe3 Crystals. J. Appl. Phys. 1961, 32, 621-623. 36. Goldsmid, H. J. In Introduction To Thermoelectricity; Goldsmid, H. J., Eds.; Springer, Berlin, Heidelberg, 2010; Chapter 7, 99-111. 37. Champness, C. H.; Chiang, P. T.; Grabowski, K.; Muir, W. B. Influence of Growth Conditions and Tellurium Phase on the Thermoelectric Properties of Bismuth Telluride Type Materials. J. Appl. Phys. 1968, 39, 4177-4183. 38. Pfann, W. G. Techniques of Zone Melting and Crystal Growing. Solid State



Phys. 1957, 4, 423-521. 39. Ghassem, K.; Kamar, A.; Ashkan, K. Ashkan Kavei, Electrical Conductivity Variation of (Bi2Te3)0.25(Sb2Te3)0.75 Crystal Grown Using the Zone Melting Method. Int. J. Mater. Res. 2013, 104, 314-318.


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For Table of Contents Use Only

Tunable Optimum Temperature Range of High-performance Zone-Melted Bismuth-Telluride-based Solid Solutions Zhai, Renshuang; Wu, Yehao; Zhu, Tie-Jun;* Zhao, Xin-Bing

Through adjusting carrier concentration, high zTs (~ 1.2 at 300 K) are obtained in both n-type and p-type Bi2Te3-based zone melted ingots, of which high homogeneity is examined and suggests the good quality. The on-axis taper is found necessary for high thermoelectric performance and the purity of starting materials or as slow as 8 mm/h growth rate makes little difference.


Tunable Optimum Temperature Range of High-Performance Zone

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