Elsevier

Materials Today Physics

Volume 6, August 2018, Pages 31-37
Materials Today Physics

Improved thermoelectric performance of p-type Bi0.5Sb1.5Te3 through Mn doping at elevated temperature

https://doi.org/10.1016/j.mtphys.2018.07.002Get rights and content

Highlights

  • The power factor is enhanced in the whole temperature range for MnxBi0.5Sb1.5-xTe3 samples.

  • Both lattice and bipolar thermal conductivity are greatly reduced.

  • A peak ZT of ∼1.3 at 430 K and average ZT of ∼1.1 between 300 K and 573 K are achieved.

Abstract

Bi2Te3-based compounds are the well-manufactured thermoelectric materials working near room temperature, however, less promising to be used in the low-temperature power generation due to the early onset of the intrinsic excitation. In this work, we used manganese (Mn) to substitute antimony (Sb) in the lattice of Bi0.5Sb1.5Te3 to dramatically increase the electrical conductivity and higher temperature Seebeck coefficient and decrease the bipolar thermal conductivity, leading to enhancement in both maximum ZT ∼ 1.3 at 430 K and average ZT above 1.1 at the temperature range from 300 K to 573 K, which is vital to obtain high conversion efficiency for low-temperature power generation.

Graphical abstract

A tiny amount of Mn could sharply increase the carrier concentration without changing the electronic band structure, consequently leading to delay the appearance of intrinsic excitation and improve PF values in the whole temperature range. Meanwhile, the lattice and bipolar thermal conductivity are both greatly reduced. Therefore, a very high ZT of ∼1.3 at 430 K and it's ZTave of 1.1 between 303 and 573 K are achieved, which is vital to push the application of the Bi2Te3-based thermoelectric materials at elevated temperature.

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Introduction

More than 50% of the total energy is dissipated as waste heat within the temperature range from room temperature to 1273 K, due to the low efficiency of current energy conversion technologies [1]. Thermoelectric (TE) materials can directly achieve the transformation between electricity and heat [2], so the high-temperature waste heat can be effectively collected and reused with the help of some excellent TE materials, such as CoSb3 [3], [4], [5], half-Heusler alloys [6], [7], SnX (X = Te, Se) [8], [9], [10], [11], BiCuSeO [12], SiGe alloys [13], [14] and so on. However, it is challenging to utilize the low-temperature waste heat ranging from 373 to 573 K because of the absence of the good TE materials in this temperature range [15], [16]. For TE devices, conversion efficiency is determined by ZT value, ZT = (S2σ/κ)T, where S, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively [17]. Recently, Bi2Te3-based compounds, which dominated the TE cooling application near room temperature, have shown their potential for low-temperature heat to electricity conversion application due to the rapidly increased ZT value with nanostructuring strategies. Ren et al. [18] obtained ZT of 1.4 at 373 K in Bi0.4Sb1.6Te3 by using ball milling and hot pressing. Tang et al. achieved the ZT of 1.56 at 303 K in Bi0.52Sb1.48Te3 prepared by melt-spinning and spark plasma sintering [19]. Kim et al. [20] increased the ZT value up to 1.86 at 323 K through a simple liquid-phase compaction process in Bi0.5Sb1.5Te3. However, the intrinsic excitation degrades the TE properties above 373 K and restricts the TE conversion applications. The feasible solution is to actively suppress the intrinsic excitation via two strategies, one is increasing the concentration of major carriers, and the other is enlarging the band gap [21], [22]. Recently, Zhu et al. adjusted the ratio of Bi/Sb to increase the hole concentration and obtained a ZT value of 1.3 at 373 K for Bi0.3Sb1.7Te3 [23]. Hu et al. [24] utilized the Ag/In co-alloying to enlarge Eg and obtained a ZT value of 1.0 at 673 K in Sb2Te3. Zhao et al. obtained a high ZT value 1.4 at 600 K by alloying and hot deformation in Bi0.3Sb1.625In0.075Te3 [25]. Kim et al. [26] improved the power factor and decreased the bipolar thermal conductivity by Pb-doping in Bi0.3Sb1.7Te3. Chen et al. used a tiny amount of acceptor (Cu/Cd) to depress the intrinsic excitation and improved the TE figure of merit (ZT) to ∼ 1.0–1.4 at about 430 K in Bi2Te3-based TE materials [27]. Beside these doping elements, manganese (Mn) has been also verified as an effective acceptor by forming substitutional defects of MnSb and MnBi in Bi0.5Sb1.5Te3 single crystal [28]. Cao et al. obtained a high ZT of 1.43 at 373 K in the Bi0.5Sb1.5Te3, including the MnSb2Se4 second phase. They ascribed the improvement to the increased hole concentration and reduced thermal conductivity [29]. These results reveal the potential for improving TE performance of p-type Bi2Te3 alloys through simple Mn doping at elevated temperature.

In this study, a tiny amount of Mn was used to substitute Sb in Bi0.5Sb1.5Te3, and the influence of Mn doping on the TE properties was investigated. The intrinsic excitation was significantly suppressed because of the increased carrier concentration, leading to an increased peak ZT ∼1.3 at 430 K and a high average ZT ∼1.1 between 300 and 573 K.

Section snippets

Experimental section

Ingots with stoichiometric compositions MnxBi0.5Sb1.5-xTe3 (x = 0, 0.0025, 0.005, 0.0075, 0.01, 0.015) were prepared by melting the raw materials (Bi chunks (99.99%, Alfa Aesar), Sb particles (99.99%, Alfa Aesar), Te chunks (99.99%, Alfa Aesar), and Mn pieces (99.99%, Alfa Aesar)) in quartz tubes with the vacuum condition of 10−5 torr. The quartz tubes were heated to 1273 K and kept at this temperature for 10 h followed by furnace cooling. The ingots were smashed and subjected to ball milling

Results and discussion

Fig. 1a shows the XRD patterns of the hot-pressed MnxBi0.5Sb1.5-xTe3 bulk samples. All the peaks can be indexed to the hexagonal structure (space group R3¯m). No second phase is detected within the detection limit. The lattice parameters are calculated from the XRD data by using the Rietveld refinement method, shown as blue plots in Fig. 1b. The lattice parameters of the MnxBi0.5Sb1.5-xTe3 samples linearly decrease with increasing content of Mn, due to the different covalent radius between Mn

Conclusions

A tiny amount of Mn is used to successfully optimize the TE properties of Bi0.5Sb1.5Te3 at elevated temperature. The Mn substitution increases the carrier concentration (electrical conductivity) and delay the onset of bipolar effect, thereby leading to improvement in the power factor and reduction in the lattice thermal conductivity and bipolar thermal conductivity. Finally the ZT value of Mn-doped Bi0.5Sb1.5Te3 is remarkably improved. For Mn0.0075Bi0.5Sb1.4925Te3, the ZTmax ∼1.3 is achieved at

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51622101, 51771065, 11674078, and 51471061).

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    e

    Y.L. is presently in Fresenius Kabi USA, LLC, Melrose Park, IL 60160, USA.

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