Enhanced thermoelectric figure of merit in Bi-containing Sb2Te3 bulk crystalline alloys

Journal Pre-proof Enhanced thermoelectric figure of merit in Bi-containing Sb2Te3 bulk crystalline alloys A.M. Adam, A. El-Khouly, A.P. Novitskii, E.M.M. Ibrahim, A.V. Kalugina, D.S. Pankratova, A.I. Taranova, A.A. Sakr, A. Trukhanov, M.M. Salem, V. Khovaylo PII:

S0022-3697(19)31817-7

DOI:

https://doi.org/10.1016/j.jpcs.2019.109262

Reference:

PCS 109262

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 5 August 2019 Revised Date:

2 November 2019

Accepted Date: 4 November 2019

Please cite this article as: A.M. Adam, A. El-Khouly, A.P. Novitskii, E.M.M. Ibrahim, A.V. Kalugina, D.S. Pankratova, A.I. Taranova, A.A. Sakr, A. Trukhanov, M.M. Salem, V. Khovaylo, Enhanced thermoelectric figure of merit in Bi-containing Sb2Te3 bulk crystalline alloys, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/j.jpcs.2019.109262. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Enhanced thermoelectric figure of merit in Bi-containing Sb2Te3 bulk crystalline alloys A.M. Adam1,2, A. El-Khouly1,3, A.P. Novitskii1, E.M.M. Ibrahim2, A.V. Kalugina1, D.S. Pankratova2, A.I. Taranova3, A.A. Sakr3, A. Trukhanov1, M.M. Salem1, V. Khovaylo1,4 1 National University of Science and Technology, MISiS, Moscow 119049, Russian Federation 2 Physics Department, Faculty of Science, Sohag University,82524 Sohag, Egypt 3 Physics Department, Faculty of Science, Damanhour University, Damanhour, Egypt 4 National Research South Ural State University, Chelyabinsk 454080, Russian Federation Abstract In the present work, the structural and thermoelectrical properties of Sb2-xBixTe3 samples prepared by the usual melting method were studied. The samples were milled, pressed and then annealed under vacuum for 3 h at 250 °C. Investigations of the microstructure and surface morphology were carried out using X-ray diffraction analysis, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The Seebeck coefficient and electrical and thermal conductivities were measured from 300 to 473 K. Thermopower data indicated a p-type conduction in the samples. Structural defects were present in Sb2Te3 during Bi addition, acting as scattering centers and leading to a decrease in thermal conductivity, thus increasing the thermopower and power factor. The power factor showed its maximum value of around 24.7 µW/cm K2 for the sample containing the highest Bi content. High figure of merit of 1.14 was recorded for the same sample, indicating that the Bi-containing Sb2Te3 and the related alloys are promising materials for more efficient thermoelectric devices. Keywords: Crystalline alloys; Electrical conductivity; Thermoelectric power; Figure of merit 1. Introduction Converting thermal energy into electric power voltage is an ideal solution to meet the everincreasing demand of energy. Chalcogenides have been reported as promising candidates for use in converting waste heat to usable electricity. Compounds of Sb2Te3 and Bi2Te3 are known as extraordinary thermoelectric (TE) materials at room temperature [1] and as topological insulator materials [2–8]. In addition, Sb2Te3 and Bi2Te3 compounds have become the subject of intensive investigations both theoretically and in applied research [9]. The Sb2Te3 and its chalcogenides are potential candidates for TE generators and coolers, and back contacts for photovoltaic devices because of high Seebeck coefficient (S), low thermal conductivity (k) and high electrical

conductivity (σ) [10]. Therefore, applications in numerous areas such as television cameras, infrared spectroscopy and electronic and optoelectronic devices have been reported for these compounds [11,12]. Moreover, Sb2Te3-based alloys can be employed in different applications such as solar cells, TE devices and phase-change devices [13,14]. Additionally, they are promising for applications in thermopile sensors, micro-generators and micro-coolers [15–22]. The structure of Sb2Te3 is a collection of layers, each layer consisting of five atomic planes ordered in the sequence: Te1–Sb–Te2–Sb–Te1, where Te1 and Te2 denote the positions of Te atoms. Each of the five atomic layers/planes is referred to as a quintuple layer. The layers are connected to each other by weak Van der Waals interactions [23]. Such weak Van der Waals interactions result in interesting and even extraordinary physical properties in Sb2Te3 and related systems [24]. A high-performance TE material should have high S and σ and simultaneously possess low k. Thus, performance of a TE power material can be expressed in terms of the dimensionless parameter known as figure of merit (ZT = S2σT/k), where T is the absolute temperature. It can be considered as a measurement of the material’s TE efficiency. The main challenge in TE energy conversion is to simultaneously optimize all of the three measurable transport parameters (S, σ and k) which are highly correlated. He and Tritt [25] stated that an efficient TE material has a maximum ZT value within 1–2.5 because of the interdependency of S, σ and k. Among previous attempts to increase the ZT value of the Bi–Sb–Te system is a study of the p-type Bi0.4Sb1.6Te3 by Yali et al. [26]. In their work, micro/nanostructured alloy was prepared by hot-pressing at 703 K. The ZT value was 1.15 at room temperature and they attributed the higher ZT value to the unique micro/nanostructures which effectively lowered k. Recently, the melt–spinning–spark plasma sintering process was employed for obtaining p-type Bi0.36Sb1.64Te3 TE materials [27] and this study showed the relationship between the process conditions and TE performance. Moreover, continuous attempts to enhance efficiency of TE materials in order to commercialize them have been tried during various preparation techniques such as high-energy ball milling [28,29], spark erosion [30], melt spinning [31–33] and bottomup chemical synthesis processes [34–36]. In this regard, it is important to mention that such compounds, according to band energy calculations, are very sensitive with respect to thermoelectricity due to an-harmonic phonons. The study presented on a Bi2Te3 compound [37] is an interesting example. In the present work, we conducted a systematic study on the structural and TE properties of the crystalline Sb2-xBixTe3 bulk materials. The study aims at tuning the

optimal conditions for producing efficient and high-performance TE materials prepared by a conventional simple method from naturally abundant elements. The highest ZT that we obtained was 1.14 for the Sb1.65Bi0.35Te3 sample at 420 K. An interesting comparison was ZT > 1.1 achieved for the same composition (Bi0.35Sb1.65Te3) with S = 350 µV K−1 at 395 K [38]. 2. Experimental details Samples of crystalline Sb2-xBixTe3 alloys (x = 0.0, 0.15, 0.25, 0.35) were synthesized by the conventional direct melting method. Pure elements of Sb (99.99%), Bi (99.999%) and Te (99.999%) were charged in well-cleaned silica tubes. After charging the elements, the silica tubes were evacuated to a pressure of 3 × 10−3 Torr and then sealed by oxygen flame. The tubes then placed in a programmable furnace for melting at 1000 °C for 12 h. The furnace was cooled gradually to room temperature. The solidified samples were milled for half an hour and then pressed into pellet form by a hydraulic press. Finally, the pellets were annealed at 250 °C for 3 h under vacuum of 27 mTorr to homogenize the composition. Structure identification was performed by X-ray diffraction (XRD) technique (DRON-3M-Russia) with Co-Kα radiation of wavelength λ = 0.178897 nm in the 2θ range of 10–100° with a scanning step of 0.05° and exposure time of 5 s in point. Scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDX) (VEGA 3 SB, TESCAN) was employed to investigate the morphology, microstructure and chemical composition of the samples. Thermal diffusivity was obtained by laser flash method (LFA 457 MicroFlash, NETZSCH). The samples were covered by a thin layer of graphite to minimize errors from the translucency of the material to the laser flash. The thermal diffusivity (D) data were analyzed using the Cowan model [39] with pulse correction. The k was determined from measurements of D, specific heat (Cp) and density (d), using the relationship: kelat·Cp·d. The material d was measured by the Archimedes method and Cp was calculated by the Debye model [40,41]. The pellets were cut into bars with dimensions of 10 mm × 2.5 mm and used for simultaneous measurements of electrical transport properties. The σ and S were measured in the temperature range from room temperature to 573 K by the standard four-probe and differential methods, respectively. 3. Results and discussion 3.1. Characterization The crystalline nature of the materials was clear in the XRD patterns of the prepared samples (Fig. 1). Good crystallinity with a single crystalline Sb2Te3 phase was identified. The card

number 04-003-0996 was used for identification of antimony telluride phases. The main peaks in the XRD patterns were identified as Sb2Te3 with a rhombohedral phase, in good agreement with JCPDS card No. 65-1876 (a = 4.262 Å and c = 30.45 Å) [42]. No impurity peaks were observed, indicating that the products were Sb2Te3-based phases only. The sharp and narrow peaks in the XRD patterns indicated high crystallization of these samples.

Fig. 1. The XRD patterns of Sb2-xBixTe3 alloys (x = 0.0, 0.15, 0.25, 0.35). The SEM showed that the samples generally had similar surface morphology, and images of Sb2Te3 and Sb1.65Bi0.35Te3 samples are depicted in Fig. 2 as examples. It should be noted that the samples were well alloyed and of high density. Furthermore, the SEM images showed that the samples were homogenous and in good stoichiometric condition. In fact, the grain size was quite fine, and the grains were stacked showing well-connected laminar form. The significant difference in brightness between the two samples indicated the effect of Bi addition on the structural features of the Sb2-xBixTe3 system [43].

X=0.00

X=0.35

Fig. 2. The SEM micro-images of Sb2-xBixTe3 system at x = 0.0 and 0.35. A quantitative elemental estimation was carried out using EDX analysis. The EDX spectra and elemental distribution in the studied samples are depicted in Fig. 3. In general, the actual elemental atomic percentage was in good agreement with nominal amounts of respective elements (Table 1).

Fig. 3. The EDX spectra of Sb2-xBixTe3 system (x = 0.0, 0.15, 0.25 and 0.35). Table 1 Elemental distribution in Sb2-xBixTe3 system given by EDX analysis. Sample Sb (at. %) Te (at. %) Bi (at. %) Sb2Te3 40.70 59.30 0.00 Sb1.85Bi0.15Te3 36.00 59.00 5.00 Sb1.75Bi0.25Te3 33.00 58.40 8.60 Sb1.65Bi0.35Te3 31.90 58.60 9.50 3.2. Transport properties The σ of the concerned samples was measured in the temperature range from room temperature up to 573 K by using the standard four-probe method. The undoped and Bi-doped Sb2Te3 samples showed a degenerate semiconductor behavior vs. temperature, as a significant decrease in the σ was observed with increasing temperature over the whole range of the measurement (Fig. 4a). This behavior is commonly observed for heavily doped semiconductors such as the Sb2Te3 system, for which a high hole concentration (~1020 cm−3) was previously reported [44,45]. The high concentration of charge carriers in the Sb2Te3-based systems is mainly due to the naturally existing anti-site defects. In addition, Kong et al. [46] reported that the Fermi level deeply moves inside the valence band resulting in metallic behavior in such materials. The k of the studied samples (k equal to lattice kph and electronic ke thermal conductivities) was estimated from measurements of D, Cp and d, using the relation: = .

.

(1)

The D was obtained by laser flash diffusivity method. The total k of the concerned system, as a function of temperature, is depicted in Fig. 4b. The k of all samples exhibited serious reduction

with temperature up to a certain level, indicating a maximum efficiency as a TE material at this temperature. The highly Bi-doped samples possessed the smallest k. This can be attributed to creation of more scattering centers such as grain boundaries and point defects. This is why the incorporation of Bi in the Sb2Te3 system represents a good opportunity for improving its TE performance. Electronic thermal conductivity (kel) of Sb2-xBixTe3 was estimated using the Wiedemann–Franz formula [47]: kel = L· σ·T Where L is the Lorenz number (L = 1.5 + exp [

(2) | |

]; given in 10−8 WΩK−2 and S in µV/K) [48].

All samples possessed low kel (Fig. 4c), which is required for a TE material. In agreement with the electrical conductivity behavior (σ vs T), kel significantly decreased with increasing T, confirming the metallic nature of the studied system. The heavily doped samples (x = 0.25 and 0.35) exhibited lower kel, confirming benefits for Bi addition to the Sb2Te3 lattice. Reduction in kel with substitution can be attributed to the point defects phonon scattering mechanism [49]. Phonon scattering plays a dominant role in an alloy because the random point defects, created due to alloying, act as ideal scattering centers [24]. Generally, phonon scattering in materials usually has four mechanisms: phonon–phonon, point-defect, grain boundary and electron– phonon scattering. The point-defect scattering is related to the mass and size of disorder induced by atom substitution [50]. The lattice thermal conductivity (kPh), known as phonon thermal conductivity (Fig. 4d), was obtained by subtracting kel (Fig. 4c) from the total k (Fig. 4b). As a complementary part, kPh had an opposite behavior to that of kel. The kph was less than half of kel for all samples except for the most Bi-doped sample. In polycrystalline alloys the kph can be neglected with respect to ke [51]. Moreover, it was also reported that the contribution of kPh can be ignored for most TE materials in the temperature range of 300–500 K [52]. Using compounds with complex crystal structures, existence of inclusions and/or impurities and existence of a large number of grain boundaries notably affects the phonon propagation and phononic mean free paths without altering electrical charge propagation [53], so that, as the phonon mean free path is shortened due to the complexity of alloy crystal structures, the kph is reduced and can be neglected with respect to ke [54].

(b)

(a)

(c)

(d)

Fig. 4. Temperature dependence of (a) electrical conductivity (σ), (b) total thermal conductivity (k = kPl + ke), (c) electronic thermal conductivity (ke) and (d) phonon thermal conductivity (kPh) of Sb2-xBixTe3 samples (x = 0, 0.15, 0.25 and 0.35). The temperature dependence of S, the calculated power factor (PF) and ZT are shown in Fig. 5. Generally, the Bi content and temperature had a strong effect on the TE properties of the studied Sb2Te3 compounds. The S was greatly enhanced by Bi addition to the Sb2Te3 lattice (Fig. 5a). A distinguishable increase was also observed as temperature increased. The temperature dependence of S showed a sharp increase with temperature increment; however, the highly doped sample (x = 0.35) showed mixed conduction, which was a result of the temperature dependence of the carrier mobility and carrier concentration. The S of all samples was positive, indicating hole transport in the studied Sb2-xBixTe3 system.

(b)

(a)

(c)

Fig. 5. Temperature dependence of (a) Seebeck coefficient (S), (b) power factor and (c) figure of merit (ZT) for the Sb2-xBixTe3 samples (x = 0, 0.15, 0.25 and 0.35). It is commonly known that the semiconducting Sb2Te3 compound usually has inherent Tevacancy-related point defects [24]. The over-stoichiometric Sb atoms occupy the Te sites in the Te sublattice, giving rise to anti-site defects. Therefore, the observed p-type conduction in Sb2Te3 and Sb2-xBixTe3 samples is a natural consequence of anti-site defects in these systems. This is attributed to the fact that the Sb2Te3 crystals prepared from the melt of stoichiometric composition 2Sb/3Te always exhibit an excess of Sb [55], as Te vacancies are formed due to higher saturation vapor pressure of Te. The calculated PF (PF = S2σ) was obtained from the experimental values of S and σ. The PF values of all samples decreased monotonically with increasing temperature at higher temperatures (Fig. 5b). This behavior coincided with the σ

behavior. The highest PF value was at 24.66 µ W/cmK2 for the sample containing the highest Bi content. The temperature dependence of the ZT of TE materials is shown in Fig. 5c. As a function of temperature, ZT increased with increasing temperature up to a certain point, depending on sample composition, and then decreased with further temperature increment. Reduction of k along with enhancement of S due to Bi addition to the concerned system resulted in a large ZT compared to previously reported values. For example, the Te-doped bulk Bi2Se3 system exhibited maximum ZT of 0.14 around room temperature [56]. Furthermore, in comparison to most of the many (Sb1-xBix)2Te3 published results [57,58], the ZT value recorded in this work is much higher. The largest value of ZT in the present study was calculated at 1.14 at a temperature close to 400 K. 4. Conclusion Transport properties of polycrystalline bulk alloys of Sb2-xBixTe3 samples were measured as a function of temperature over the temperature range 300–573 K. The dc σ values showed metallic-like degenerate semiconductor behavior. The S measurements confirmed the p-type conduction in the studied samples. The highest S was found at around 0.20 mV/K for Sb1.65Bi0.35Te3 near room temperature. Addition of Bi to the binary Sb2Te3 system to produce Sb1.65Bi0.35Te3 resulted in a significant increase in values of S and PF along with a notable reduction in the k nearby and in the high temperature range. Consequently, high ZT was achieved, suggesting potential applications for the studied samples as highly efficient TE materials. Acknowledgment The authors gratefully acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (support project for young research engineers), implemented by a governmental decree dated 16 March 2013, N 211. VK acknowledges Act 211 Government of the Russian Federation, Contract No. 02.A03.21.0011. References [1] L.G. Khvostantsev, A.I. Orlov, N.K. Abrikosov, T.E. Svechnikova, S.N. Chizhevskaya, Thermoelectric properties and phase transitions in Bi2Te3 under hydrostatic pressure up to 9 GPa and temperature up to 300 C, Phys. Status Solidi a 71 (1982) 49–53.

[2] C.L. Kane, E.J. Mele, Z2 topological order and the quantum spin Hall effect, Phys. Rev. Lett. 95 (2005) 146802. [3] B.A. Berneving, S.C. Zhang, Quantum spin Hall effect, Phys. Rev. Lett. 96 (2006) 106802. [4] J.E. Moore, L. Balents, Topological invariants of time-reversal-invariant band structures, Phys. Rev. B 75 (2007) 121306. [5] F. Liang, C.L. Kane, E.J. Mele, Topological insulators in three dimensions, Phys. Rev. Lett. 98 (2007) 106803. [6] F. Liang, C.L. Kane, Topological insulators with inversion symmetry, Phys. Rev. B 76 (2007) 045302. [7] W. Zhang, R. Yu, H.J. Zhang, X. Dai, Z. Fang, First-principles studies of the three-dimensional strong topological insulator Bi2Te3, Bi2Se3 and Sb2Te3, New J. Phys. 12 (2010) 065013. [8] H. Zhang, C.H. Liu, X.L. Qi, X. Dai, Z. Fang, S.C. Zhang, Topological insulators in Bi2Te3, Bi2Se3 and Sb2Te3 with a single Dirac cone on the surface, Nat. Phys. 5 (2009) 438–442. [9] R. Venkatasubramanian, T. Colpitts, E. Watko, M. Lamvik, N.E. Masry, 1997 J. Cryst. Growth 170 817–21. [10]

B. Lv, S. Hu, W. Li, X. Di, L. Feng, J. Zhang, L. Wu, Y. Cai, B. Li, Z. Lei, Int. J.

Photoenergy 10 (2010) 1–4. [11]

Q. Wang, C. Changlong, C. Yu, Q. Chen, General solution-based route to V–VI

semiconductors nanorods from hydrolysate, J. Nanopart. Res. 9 (2007) 269–74. [12]

S. Jeetendra, N.C. Shivappa, R. Patel, M.H. Matt., Adv. Mater. Lett. 5 (2014) 639–44.

[13]

I.Y. Erdogan, U. Demir, J., Electroanal. Chem. 633 (2009) 253–258.

[14]

I.-J. Yoo et al., J. Mater. Chem. A 1 (2013) 5430.

[15]

M.Y. Kim, T.S. Oh, Mater. Trans. 53 (2012) 2160–2165.

[16]

G.J. Snyder, J.R. Lim, C.-K. Huang, J.-P. Fleurial, Nat. Mater. 2 (2003) 528–531.

[17]

H. Bottner, J. Nurnus, A. Gavrikov, G. Kuhner, M. Jagle, C. Kunzel, D. Eberhard, G.

Plescher, A. Schubert, K.-H. Schlereth, J. Microelectromech. Syst. 13 (2004) 414–420. [18]

L.W. da Silva, M. Kaviany, A. DeHennis, J.S. Dyck, Proc. 22nd Int. Conf. on

Thermoelectrics (International Society of Thermoelectrics) (2003) pp 665–668. [19]

L.M. Goncalves, C. Couto, P. Alpuim, D.M. Rowe, J.H. Correia, Sensors Actuators A

130131 (2006) 346–351.

[20]

Y. Kim, A. DiVenere, G.K.L. Wong, J.B. Ketterson, S. Cho, J.R. Meyer, J. Appl. Phys.

91 (2002) 715–718. [21]

M. Takashiri, T. Shirakawa, K. Miyazaki, H. Tsukamoto, Sensors Actuators A 138

(2007) 329–34. [22]

L.M. Goncalves, J.G. Rocha, C. Couto, P. Alpuim, G. Min, D.M. Rowe, J.H. Correia, J.

Micromech. Microeng. 17 (2007) S168–173. [23]

V.A. Kulbachinskii, A.Y. Kaminsky, K. Kindo, Y. Narumi, K. Suga, S. Kawasaki, M.

Sasaki, N. Miyajima, G.R. Wu, P. Lostak, P. Hajek, Phys. Stat. Sol. (b) 229 (2002) 1467. [24]

D. Das, K. Malik, A.K. Deb, S. Dhara, S. Bandyopadhyay, A. Banerjee, J. Appl. Phys.

118 (2015) 045102. doi: 10.1063/1.4927283 [25]

J. He, T.M. Tritt, Advances in thermoelectric materials research: looking back and

moving forward, Science 357 (2017) eaak9997. [26]

Y. Li, J. Jiang, G. Xu, W. Li, L. Zhou, Y. Li, P. Cui, Journal of Alloys and Compounds

480 (2009) 954–957. [27]

W.H. Shin, J.S. Yoon, M. Jeong, J.M. Song, S Kim, J.W. Roh, S. Lee, W.S. Seo, S.W.

Kim, K.H. Lee, Crystals 7 (2017) 180. doi:10.3390/cryst7060180. [28]

Y. Lan, B. Poudel, Y. Ma, D. Wang, M.S. Dresselhaus, G. Chen, Z. Ren, Structure study

of bulk nanograined thermoelectric bismuth antimony telluride, Nano Lett. 9 (2009) 1419–1422. Crystals 2017, 7, 180 9 of 9 [29]

Y. Ma, Q. Hao, B. Poudel, Y. Lan, B. Yu, D. Wang, G. Chen, Z. Ren, Enhanced

thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks, Nano Lett. 8 (2008) 2580–2584. [30]

P.K. Nguyen, K.H. Lee, J. Moon, S.I. Kim, K.A. Ahn, L.H. Chen, S.M. Lee, R.K. Chen,

S. Jin, A.E. Berkowitz, Spark erosion: A high production rate method for producing Bi0.5Sb1.5Te3 nanoparticles with enhanced thermoelectric performance. Nanotechnology 23 (2012) 415604. [31]

X. Cai, X.A. Fan, Z. Rong, F. Yang, Z. Gan, G. Li, Improved thermoelectric properties

of Bi2Te3−xSex alloys by melt spinning and resistance pressing sintering, J. Phys. D Appl. Phys. 47 (2014) 115101.

[32]

D.G. Ebling, A. Jacquot, M. Jägle, H. Böttner, U. Kühn, L. Kirste, Structure and

thermoelectric properties of nanocomposite bismuth telluride prepared by melt spinning or by partially alloying with IV–VI compounds, Phys. Status Solidi RRL 1 (2007) 238–240. [33]

W. Xie, J. He, H.J. Kang, X. Tang, S. Zhu, M. Laver, S. Wang, J.R. Copley, C.M.

Brown, Q. Zhang, Identifying the specific nanostructures responsible for the high thermoelectric performance of (Bi,Sb)2Te3 nanocomposites, Nano Lett. 10 (2010) 3283–3289. [34]

Y. Min, G. Park, B. Kim, A. Giri, J. Zeng, J.W. Roh, S.I. Kim, K.H. Lee, U. Jeong,

Synthesis of multishell nanoplates by consecutive epitaxial growth of Bi2Se3 and Bi2Te3 nanoplates and enhanced thermoelectric properties, ACS Nano 9 (2015) 6843–6853. [35]

Y. Min, J.W. Roh, H. Yang, M. Park, S.I. Kim, S. Hwang, S.M. Lee, K.H. Lee, U.

Jeong, Surfactant-free scalable synthesis of Bi2Te3 and Bi2Se3 nanoflakes and enhanced thermoelectric properties of their nanocomposites, Adv. Mater. 25 (2013) 1425–1429. [36]

R.J. Mehta, Y. Zhang, C. Karthik, B. Singh, R.W. Siegel, T. Borca-Tasciuc, G.

Ramanath, A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly, Nat. Mater. 11 (2012) 233–240. [37]

H. Kaddouri, S. Benet, S. Charar, M. Makowska-Janusik, J. C. Tedenac, I. V. Kityk,

Simulation of thermoelectric properties of bismuth telluride single crystalline films grown on Si and SiO2 surfaces, Phys. Rev. B. V. 62B, N 24 (2000) 17108–17116. [38]

F. Serrano-Sánchez, M. Gharsallah, N.M. Nemes, N. Biskup, M. Varela, J.L. Martínez,

M.T. Fernández-Díaz, J.A. Alonso, Scientific Reports 7 (2017) 6277. DOI:10.1038/s41598-01705428-4. [39]

R.D. Cowan, Pulse method of measuring thermal diffusivity at high temperatures, J.

Appl. Phys. 34 (1963) 926–927. [40]

M.X. Gu, C.Q. Sun, Z. Chen, T.C. Au Yeung, S. Li, C.M. Tan, V. Nosik, Size,

temperature, and bond nature dependence of elasticity and its derivatives on extensibility, Debye temperature, and heat capacity of nanostructures, Phys. Rev. B - Condens. Matter Mater. Phys. 75 (2007) 125403. doi:10.1103/PhysRevB.75.125403. [41] V.B. Lazarev, A.D. Izotov, K.S. Gavrichev, O.V. Shebershneva, Fractal model of heat capacity for substances with diamond-like structures, Thermochimica Acta, 269/270 (1995) 109-l 16. https://doi.org/10.1016/0040-6031(95)02529-4.

[42] Weidong Shi, Shuai Yu, Pei Liu and Weiqiang Fan, Hydrothermal synthesis and thermoelectric transport properties of Sb2Te3–Te heterogeneous nanostructures, Cryst. Eng. Comm, (2013), 15, 2978–2985. DOI: 10.1039/c3ce27010f. E.M.M. Ibrahim, A.M. Abdel Hakeem, A.M. Adam, E. Kh. Shokr, Structural, electrical, [43] and thermoelectrical properties of (Bi1 xSbx)2Se3 alloys prepared by a conventional melting technique, Journal of Experimental and Theoretical Physics 116 (2013) 166–172. https://doi.org/10.1134/S1063776113020064 [44]

V.D. Blank, S.G. Buga, V.A. Kulbachinskii, V.G. Kytin, V.V. Medvedev, M. Yu.

Popov, P.B. Stepanov, V.F. Skok, Phys. Rev. B 86 (2012) 075426. [45]

P. Dutta, D. Bhoi, A. Midya, N. Khan, P. Mandal, S.S. Samatham, V. Ganesan, Appl.

Phys. Lett. 100 (2012) 251912. [46]

D. Kong, Y. Chen, J.J. Cha, Q. Zhang, J.G. Analytis, K. Lai, Z. Liu, S.S. Hong, K.J.

Koski, S.K. Mo, Z. Hussain, I.R. Fisher, Z.X. Shen,, Y. Cui, Nat. Nanotechnol. 6 (2011) 705. [47]

K. Delime-Codrin et al., Jpn. J. Appl. Phys. 56 (2017) 111202.

[48]

H.-S. Kim, Z.M. Gibbs, Y. Tang, H. Wang, G.J. Snyder, Apl Materials 3 (2015) 041506.

[49]

W. Lu, W. Zhang, L. Chen, J. Alloys and Compounds, 484 (2009) 812–815.

[50]

D. Zhao, M. Zuo, L. Bo, Y. Wang, Materials 11 (2018) 728; doi:10.3390/ma11050728.

[51]

T.S. Kim, T.K. Kim, S.J. Hong, B.S. Chum, Mater. Sci. Eng. B 90 (1) (2002) 42e6.

[52]

A.J. Minnish, M.S. Dresselhause, Z.F. Ren, G. Chen, Energy Environ. Sci. 2 (2009) 466.

[53]

A. Pereira Goncalves, E. Branco Lopes, O. Rouleau, C. Godart, J. Mater. Chem. 20

(2010) 1516–1521. [54]

A.M. Adam, A. El-Khouly, E. Lilov, Sh. Ebrahim, Y. Keshkh, M. Soliman, E.M. El

Maghraby, V. Kovalyo, P. Petkov, Materials Chemistry and Physics 224 (2019) 264–270. [55]

N.Kh. Abrikosov, L.V. Poretskaya, I.P. Ivanova, Zh. Neorg. Khim. 4 (1959) 2525.

[56]

A.M. Adam, A. Elshafaie, Abd El-Moez A. Mohamed, P. Petkov, E.M.M. Ibrahim,

Mater. Res. Express 5 (2018) 03551. [57]

A.M. Adam, Journal of Alloys and Compounds 765 (2018) 1072–1081.

[58]

A.M. Adam, P. Petkov, M. El-Khouly, International Journal of Advances in Science,

Engineering and Technology 6 (2018) 2.

Highlights • Polycrystalline alloys of Sb2-xBixTe3 prepared by direct melting of pure elements. • Thermoelectric properties were measured in a wide temperature range of 300–573 K. • Bi addition to the binary Bi2Te3 system notably reduced the thermal conductivity. • The Bi addition also significantly increased Seebeck coefficient and power factor. • ZT of 1.14 for Sb1.65Bi0.35Te3 at 425 K suggests potential use at range 350470 K.

Statement Conflict of Interest No conflict of interest exists. We wish to confirm that there are no known conflicts of interest associated with this publication.