Cite this Article
Lu Tian-Qi, Nan Peng-Fei, Song Si-Long, Zhu Xin-Yue, Zhao Huai-Zhou, Deng Yuan. Enhanced thermoelectric performance through homogenously dispersed MnTe nanoparticles in p-type Bi0.52Sb1.48Te3 nanocomposites. Chinese Physics B, 2018, 27(4): 047207  
Permissions
Enhanced thermoelectric performance through homogenously dispersed MnTe nanoparticles in p-type Bi0.52Sb1.48Te3 nanocomposites
Lu Tian-Qi1, 2, †, Nan Peng-Fei2, †, Song Si-Long1, Zhu Xin-Yue3, Zhao Huai-Zhou2, ‡, Deng Yuan1, §
School of Materials Science and Engineering, Beihang University, Beijing 100191, China
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Electronic Science and Technology, Beijing University of Posts and Telecommunications, Beijing 102101, China

 

† Corresponding author. E-mail: Hzhao@iphy.ac.cn dengyuan@buaa.edu.cn

Abstract

In this work, we report that the thermoelectric properties of Bi0.52Sb1.48Te3 alloy can be enhanced by being composited with MnTe nano particles (NPs) through a combined ball milling and spark plasma sintering (SPS) process. The addition of MnTe into the host can synergistically reduce the lattice thermal conductivity by increasing the interface phonon scattering between Bi0.52Sb1.48Te3 and MnTe NPs, and enhance the electrical transport properties by optimizing the hole concentration through partial Mn2+ acceptor doping on the Bi3+ sites of the host lattice. It is observed that the lattice thermal conductivity decreases with increasing the percentage of MnTe and milling time in a temperature range from 300 K to 500 K, which is consistent with the increasing of interfaces. Meanwhile, the bipolar effect is constrained to high temperatures, which results in the figure of merit zT peak shifting toward higher temperature and broadening the zT curves. The engineering zT is obtained to be 20% higher than that of the pristine sample for the 2-mol% MnTe-added composite at a temperature gradient of 200 K when the cold end temperature is set to be 300 K. This result indicates that the thermoelectric performance of Bi0.52Sb1.48Te3 can be considerably enhanced by being composited with MnTe NPs.

PACS: ;72.20.Pa;;73.50.Lw;
Keyword:;MnTe nano particles;;interface phonon scattering bipolar effect;;higher engineering zT ;
1. Introduction

Thermoelectric materials can find their applications in direct conversion of heat to electric power or vise versa in solid state refrigeration with their performance being gauged by the figure of merit, , where S is the Seebeck coefficient, σ is the electrical conductivity,k is the total thermal conductivity, and T is the absolute temperature.[1–4] To date, the enhancement of zT has been realized either by increasing the power factor ( , or by reducing the thermal conductivity k that consists of electrical part ke and lattice part kL, meanwhile avoiding the significant deterioration of the electrical conductivity. The approaches that can generally be used to improve the power factor include optimizing the carrier concentration, modifying the carrier mobility[5,6] or manipulating the carrier effective mass via band engineering.[7,8] On the other hand, the reduction of lattice thermal conductivity kL can be attained through increasing phonon scatterings by multiple scale crystal defects, such as point defects, grain boundaries, intrinsic anharmonic phonons, etc.[9–12]

Bismuth–tellurium-based thermoelectric materials have been studied for several decades due to their exclusively superior performances at around room temperature. Although persistent efforts have been made to improve the zT values of these materials since the 1950s, the peak zT of commercial Bi2Te3 and its alloys remain to be around 1. As of now, most of the research on this material has focused on reducing the lattice thermal conductivity through various approaches, such as the superlattices,[13] nanostructures,[14] the fabrications of composites, and so on. Attractively in the p-type Bi2Te3/Sb2Te3 superlattice, Rama et al. reported that the lattice thermal conductivity kL at room temperature is reduced enormously, leading to at 300 K.[15] With regard to the nanostructures, Bed et al. reported a peak zT of about 1.4 at 373 K achieved in p-type nanocrystalline BiSbTe bulk material,[16] Yi et al. achieved a peak zT of about 1.3 at a temperature of 373 K.[17] Furthermore, the nano-compositing approach is conducive to improving the thermoelectric performance and has been widely used in thermoelectric systems. For instance, Wu et al. improved the thermoelectric properties of ZnO by mixing poly-paraphenylene (PPP) in 2015.[18] This method is also suitable for the system of bismuth telluride. Li et al. reported a nano-composite of BiSbTe with 1-vol% Cu3SbSe4 nanoinclusions, which attains a peak zT of 1.6 at 476 K.[19] Kim et al. achieved a peak zT of 1.38 for the incorporation of 4-wt% Ta2O5 NPs at room temperature.[20] In addition, our group have also demonstrated that the zTs in Bi0.48Sb1.52Te3 can be significantly enhanced by adding MnSb2Se4 via synergistically reducing the lattice thermal conductivity and improving the electrical transport.[21] However, owing to the complex components in the BiSbTe/MnSb2Se4 composites, the mechanism of the enhanced TE performance is yet to be clarified. Particularly, how the MnSb2Se4 can be decomposed into the relevant MnSe and Sb2Se3, and what the influence is of MnSe or similar MnTe nanoinclusions on the electrical property of BiSbTe still remains to be investigated.

In this work, the Bi0.52Sb1.48Te3 based composites incorporated with MnTe nanoparticles are fabricated via ball milling (BM) and the SPS process. We attempt to optimize the total thermoelectric performance in the whole temperature range through adjusting the milling time and quantity of nanoparticles. The Bi0.52Sb1.48Te3/2-mol% MnTe composite sample which underwent 3-h BM shows a remarkable decrease in lattice thermal conductivity and enhancement of power factor (PF) at high temperature. According to the result, we propose that most of the MnTe naonparticles are dispersed uniformly and form secondary phases to affect the properties of Bi0.52Sb1.48Te3, meanwhile, partial Mn2+ ions from MnTe would substitute the Bi3+ or Sb3+ in the host Bi0.52Sb1.48Te3, leading to increased hole concentration and enhanced electrical conductivity. As a result, the zT peak shifts toward higher temperature, the zT curves broaden, and the engineering zT is 20% higher than that of the pristine sample for the 2-mol% MnTe added composite at a temperature gradient of 200 K when the cold end temperature is set to be 300 K.

2. Experimental methods
2.1. Synthesis

Firstly, Bi0.52Sb1.48Te3 was prepared by directly ball milling the pure elements Bi (chunks, 99.99%), Sb (chunks, 99.999%), and Te (chunks, 99.999%) in a stainless steel jar with a high-energy ball mill machine, SPEX 8000D, and the pure materials were sealed into the jar in an argon-filled glove box. The sample was milled for 9 h and then pressed by the SPS process under an axial pressure of 40 MPa at 673 K for 5 min with a heating rate of 50 K/min. Similarly, the pure MnTe was fabricated by using the same method, but the milling time was changed to 6 h and the SPS process parameter was transformed into an axial pressure of 45 MPa at 1173 K for 30 min with a heating rate of 50 K/min. Secondly, Bi0.52Sb1.48Te3 was incorporated independently with 0-mol%, 1-mol%, 2-mol%, and 4-mol% MnTe, for 10 min and 3 h, respectively through the ball milling process, then the mixed powders were put into a graphite die and pressed at 673 K for 5 min under 40 MPa again, respectively. Finally, these disk-shaped samples each with a diameter of 12.7 mm∼2 mm in thickness, and relative densities of 97% were polished and cut for the characterizing and measuring the TE properties.

2.2. Measurements
2.2.1. X-ray diffraction

The phase analysis was performed by x-ray diffraction (Xpert Pro PANalytical XRD) using a Cu–Kα radiation (λ =1.5418 Å) and operating under 40 kV and 40 mA. The patterns were obtained in a 2θ range of 10°–60° in steps of 0.02° at room temperature in air.

2.2.2. Microstructure analysis

The morphologies of bulk samples were characterized by scanning electron microscopy (SEM) on the S-5200 field emission SEM (Hitachi). The microstructure and second-phase particles in the bulk samples were investigated by using a transmission electron microscope (TEM, JEM-2100 Plus).

2.2.3. Hall measurements

The room-temperature Hall measurement was performed on a Nanometrics Hall Instrument. Samples were loaded with a BN substrate. Four probes were attached to the edge of the sample. The sample was placed in a vacuum with a magnetic field (up to ±0.5 T) perpendicular to its surface. The resistivity ρ and Hall coefficient RH (along the in-plane direction) were measured by the Van de Pauw method. The effective carrier concentration ( was calculated from the relationship , where e is the elemental charge and RH is the Hall coefficient. The Hall mobility ( was calculated from the relationship .

2.2.4. Electrical properties

The obtained SPS pellets were cut into bars with dimensions 10 mm×3 mm×2 mm, and Seebeck coefficient and electrical conductivity of the dimensions were simultaneously measured from room temperature to 498 K under a low-pressure helium atmosphere by using LSR-3 equipment (Linseis, Germany).

2.2.5. Thermal conductivity

The thermal conductivity was calculated from , where the thermal diffusivity coefficient (D) was measured using the laser flash diffusivity method in a Linseis LFA-1000, the samples coated with carbon were placed into the mould and measured under flowing helium atmosphere from room temperature to 498 K; the density (d) was determined by using the Archimedes drainage method.

3. Results and discussion
3.1. Structure and morphology analysis

The room temperature XRD patterns for Bi0.52Sb1.48Te3 composites added with 0, 1, 2, and 4 mol% of MnTe through ball milling (for 10 minutes and 3 hours, respectively) and SPS process are shown in Figs. 1(a) and 1(b). It can be seen that all the samples are of pure phase that can be indexed to the hexagonal crystal structure (space group, R-3mH). The secondary phases are difficult to detect because of the limit of instrument sensitivity. Figures 1(c) and 1(d) illustrate the changes of lattice constant along the a and c axes. The results for the 2-mol% MnTe sample (3 h) are abnormal because of the error in measurement. In general, it is obvious that the lattice constant enlarges slightly with MnTe content increasing, which might be due to the substitution of Mn atom (161 pm of atomic radius) on the Bi (143 pm) site.

Fig. 1. (color online) X-ray diffraction patterns for Bi0.52Sb1.48Te3 composites obtained by adding x-mol% MnTe milled (a) 10 min, (b) 3 h; (c) lattice parameters a and (d) lattice parameters c versus MnTe content, calculated by the Rietveld method for Bi0.52Sb1.48Te3 compounds.

The low-magnification SEM image of Bi0.52Sb1.48Te3 composites with 2-mol% MnTe prepared by 3-h ball milling plus SPS process is displayed in Fig. 2(a), in which we can observe homogeneous particles at around the 3 micrometer size. Figure 2(b) shows the high-magnification SEM image of the sample in which the lamellar structure of Bi0.52Sb1.48Te3 can be clearly seen. The bright-field TEM image in Fig. 2(c) exhibits that the particles are made up of substantial nanoscale grains, indicating that a large number of interfaces exist in the samples. Figure 2(d) shows the high magnification TEM image of the marked part in Fig. 2(c), and it shows the grain boundaries and grains on the order of ∼20 nanometers clearly. In the inset of Fig. 2(d), the analysis of interplanar spacing confirms that this annotated grain is of the second phase of MnTe, which is consistent with the preliminary composite

Fig. 2. (a) Low-magnification SEM images; (b) high-magnification SEM images; (c) bright-field TEM image; (d) HRTEM image of the marked part in panel (c), the inset shows the locally enlarged drawing of Bi0.52Sb1.48Te3 composites (2-mol% MnTe milled 3 h).
3.2. Thermal and electrical transport properties
3.2.1. Thermal transport properties

Figure 3(a) displays the thermal diffusion coefficients as a function of temperature for a series of Bi0.52Sb1.48Te3 composites. It shows an obvious rising trend at low temperature but a suppressed bipolar effect with the increasing of doping concentration. Meanwhile, with the increasing of ball milling time, the thermal diffusion coefficients decrease significantly, which can be ascribed to the refined grain sizes of the composites, leading to the number of interfaces increasing and the heat carrying phonon scattering strengthening. Figure 3(b) exhibits the total thermal conductivities of the composites. As can be seen, the total thermal conductivities κtot have a similar trend to the thermal diffusion coefficients. κtot is the sum of the electrical ( and lattice thermal conductivity ( . The value of is estimated through the Wiedemann–Franz law , where L is the Lorentz number,[22] and can be easily calculated in a system dominated by one type of electrical transport carrier, based on the SPB model using the following equations:[23]

Here, λ is the scattering factor and the λ =0 for acoustic phonon scattering.[24] is the Fermi integral, η is the reduced chemical potential defined as , where EV is the energy of the valence band top and is the chemical potential. However, it is difficult to obtain it with the involvement of the so-called bipolar effect, by which minority carriers begin to contribute significantly to the electrical conductivity. Thus L is roughly calculated using an SPB with acoustic phonon-dominated scattering. In this study, L is obtained from the broadly adopted approach to fitting the Seebeck coefficient to the reduced chemical potential[25,26] (Fig. 3(c)). The results of the lattice thermal conductivities and bipolar thermal conductivities are shown in Fig. 3(d). The bipolar effect leads to an extra thermal conductivity, which results in an abnormally large lattice thermal conductivity above 400 K; this phenomenon weakens with doping amount increasing. Moreover, the lattice thermal conductivities show continuous decline with the gradual increase in MnTe percentage and milling time, which is in accord with the increasing of grain boundaries and phonon scattering.

Fig. 3. (color online) Thermal properties as a function of temperature: (a) thermal diffusivities (D); (b) total thermal conductivities ( (c) Lorenz numbers (L); (d) lattice thermal conductivities ( and bipolar thermal conductivities ( for Bi0.52Sb1.48Te3 compounds.
3.2.2. Electrical transport properties

In order to understand the influence of the MnTe additives on the electrical properties of Bi0.52Sb1.48Te3 composites, Hall measurements are carried out for all the samples at room temperature. Figure 4(a) reveals that the carrier concentration goes up with increasing the MnTe content and is unrelated to the milling time. This can be explained as that the partial Mn2+ acceptor substitutes for the Bi3+ sites of the host lattice in the SPS process. At the same time, the carrier mobility decreases gradually as shown in Fig. 4(b), indicating that the scattering of electrons by nano-particles of the second phase strengthens. The experimental data of Bi0.52Sb1.48Te3 composites can be analyzed on the basis of the Boltzmann transport equation within the relaxation time approximation. In Fig. 4(c), the Pisarenko line was plotted based on a single parabolic band (SPB) model, where the relationship between Seebeck and carrier concentration can be expressed as[27,28]

Here, , e, λ, h, and are the Boltzmann constant, electron charge, the scattering factor, Planck constant, and effective mass, respectively. The acoustic phonon scattering with λ =0 is generally accepted as the dominant scattering mechanism in limiting the carrier mean free path.[24,29] Thus, the effective mass can be obtained for the Bi0.52Sb1.48Te3 sample at room temperature. On the assumption that the effective mass does not change in other samples, the value of S can be calculated according to the carrier concentration. In Fig. 4(c), it can be seen that at 300 K the experimental Seebeck coefficients rise by about 7, 5, and for those 3-h ball milling samples, and 13, 20, and for 10-min samples compared with the calculated results given by the model, which will be beneficial to the electrical power factors of the composites.

Fig. 4. (color online) (a) Room temperature carrier concentration ( and (b) room temperature carrier mobility ( as a function of MnTe contents (x); (c) Pisarenko relation given by the black solid line under the assumption of a single parabolic band model at room temperature compared with the estimated results given by the mode on compounded samples.

The temperature-dependent electrical conductivities for a series of samples with various MnTe content are shown in Fig. 5(a). It can be seen that the electrical conductivity increases significantly with MnTe content, resulting from the higher carrier concentration caused by the doping effect of Mn atoms on Bi sites. Figure 5(b) displays the temperature dependent Seebeck coefficients of the series of samples. The substantial reduction of Seebeck coefficient for each of the composite samples is also ascribed to the increase of carrier concentration. Apart from the 4-mol% samples, others display the bipolar effects above 400 K. It is worth mentioning that all the samples that have undergone 3-h ball milling have larger Seebeck coefficients than those of 100-min milling samples.

Fig. 5. (color online) Temperature dependence of thermoelectric properties. (a) Electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) zT values for Bi0.52Sb1.48Te3 compounds.

Figure 5(c) shows the temperature-dependent power factors. The 2-mol% MnTe sample undergoing 3-h ball milling has a superior power factor than the pristine Bi0.52Sb1.48Te3 sample over the whole temperature range. The power factor of the 4-mol% sample is larger than that of the pristine sample only above 373 K. The dimensionless thermoelectric figures of merit zT as a function of temperature are presented in Fig. 5(d). The zT obtained from the 1-mol% MnTe sample is the highest in zT values of all MnTe samples and its peak value reaches 1.37 at 348 K, which is 10.4% higher than that for the pristine sample. For the 2-mol% MnTe sample undergoing 3-h ball milling, though the zT is reduced slightly from room temperature to 348 K with respect to zT value the pure Bi0.52Sb1.48Te3, it possesses a better thermoelectric performance from 373 K to 498 K.

3.3. Actual thermal efficiency

To effectively assess the desirability of materials for thermoelectric applications, Kim et al. defined the engineering power factor (PF)eng and engineering dimensionless figure of merit (zT)eng to accurately predict the energy conversion efficiency and output power density of materials other than using conventional average zT values.[30] The (PF)eng and (zT)eng can be given by

Here, , , and are the hot-side temperature, the cold-side temperature, and the temperature difference between both sides, respectively. Figure 6(a) displays the temperature difference dependence of (PF)eng for all the samples, showing improvement with increasing the MnTe and milling time. Figure 6(b) shows the dependence of (zT)eng of all samples. All samples undergoing 3-h ball milling have higher (zT)eng than those with 10-min milling, and among them the (zT)eng of the 2-mol% MnTe sample reaches 0.54 when the is 200 K, which is nearly 20% higher than that of the pure Bi0.52Sb1.48Te3.

Fig. 6. (color online) Cumulative temperature dependence of (a) engineering power factor ( , (b) engineering figure of merit ( , (c) energy conversion efficiency ( , and (d) output power density .

The concept of maximum efficiency was first proposed by Altenkirch in 1909 to measure the actual power generation efficiency of a TE generator.[31] Subsequently, Kim et al.[30] put forward the output power density to better reflect the comprehensive performance of a thermoelectric device, the output power density depends on the TE leg dimension and the material property whereas the maximum efficiency is dependent only on the material characteristics. The maximum efficiency ( and output power density based on cumulative temperature are derived as

where is the Carnot efficiency and is the dimensionless intensity factor of the Thompson effect, which is defined as follows:
where is the Seebeck coefficient at the hot-side temperature ( . Figures 6(c) and 6(d) show the cumulative temperature dependence of maximum efficiency ( and output power density . The maximum efficiency of 1-mol% sample outperforms that of the pristine Bi0.52Sb1.48Te3; however, drops quickly by adding superfluous MnTe. Generally the output power densities of samples undergoing 3-h ball milling are higher than that of the pure Bi0.52Sb1.48Te3. Overall, the 2-mol% MnTe sample undergoing 3-h ball milling has better thermoelectric performance than the others.

4. Conclusions

In this work, a series of p-type Bi0.52Sb1.48Te3/MnTe (0, 1, 2, and 4 mol%) nanocomposites are successfully fabricated through combined mechanical milling and spark plasma sintering process. The influence of MnTe second phase on the thermoelectric performance of Bi0.52Sb1.48Te3 composite is investigated. It is observed that the power factor of the composite can be enhanced by optimizing the carrier concentration through partial Mn doping on the Bi sites. The lattice thermal conductivity of the composite can be significantly suppressed by the MnTe additive due to the high-density grain boundary in the composite. Overall, the peak zT of the 1-mol% sample undergoing 3-h ball milling reaches 1.37 at 348 K, which is a 10% improvement in comparison with 1.24 of Bi0.52Sb1.48Te3. In addition, the 2-mol% sample undergoing 3-h ball milling exhibits higher zT than the pure Bi0.52Sb1.48Te3 above 348 K, an enhancement of 20% on the (zT)eng was obtained when the temperature difference reaches 200 K. These results imply that the MnTe can be used as an additive in BiSbTe based materials for enhancing thermoelectric performance.

Reference
1 Liu N Luo X G Zhang M L 2014 Chin. Phys. 23 080502
2 Bell L E 2008 Science 321 1457
3 Chen G Dresselhaus M S Dresselhaus G Fleurial J P Caillat T 2003 Int. Mater. Rev. 48 45
4 Snyder G J Toberer E S 2008 Nat. Mater. 7 105
5 Shuai J Mao J Song S Zhu Q Sun J Wang Y He R Zhou J Chen G Sligh D J Ren Z 2017 Energy Environ. Sci. 10 799
6 Zhu T Liu Y Fu C Heremans J P Snyder J G Zhao X 2017 Adv. Mater. 29 1605884
7 Tan G J Zhao L D Kanatzidis M G 2016 Chem. Rev. 116 12123
8 Zhao Q Z Zhang D L 2017 Chin. Phys. Lett. 34 034207
9 Delaire O Ma J Marty K May A F McGuire M A Du M H Singh D J Podlesnyak A Ehlers G Lumsden M D Sales B C 2011 Nat. Mater. 10 614
10 Kanatzidis M G 2010 Chem. Mater. 22 648
11 Sootsman J R Chung D Y Kanatzidis M G 2009 Angew Chem. Int. Edit. 48 8616
12 Suekuni K Avila M A Umeo K Fukuoka H Yamanaka S Nakagawa T Takabatake T 2008 Phys. Rev. 77 235119
13 Venkatasubramanian R 2000 Phys. Rev. 61 3091
14 Dresselhaus M S Chen G Tang M Y Yang R G Lee H Wang D Z Ren Z F Fleurial J P Gogna P 2007 Adv. Mater. 19 1043
15 Venkatasubramanian R Siivola E Colpitts T O’Quinn B 2001 Nature 413 597
16 Poudel B Hao Q Ma Y Lan Y C Minnich A Yu B Yan X A Wang D Z Muto A Vashaee D Chen X Y Liu J M Dresselhaus M S Chen G Ren Z F 2008 Science 320 634
17 Ma Y Hao Q Poudel B Lan Y C Yu B Wang D Z Chen G Ren Z F 2008 Nano Lett. 8 2580
18 Wu Z H Xie H Q Wang Y Y Xing J J Mao J H 2015 Chin. Phys. Lett. 32 117303
19 Li Y Y Li D Qin X Y Yang X H Liu Y F Zhang J Dou Y C Song C J Xin H X 2015 J. Mater. Chem. 3 7045
20 Kim E B Dharmaiah P Shin D Lee K H Hong S J 2017 J. Alloys Compd. 703 614
21 Cao B L Jian J K Ge B H Li S M Wang H Liu J Zhao H Z 2017 Chin. Phys. 26 017202
22 Jonson M Mahan G D 1980 Phys. Rev. 21 4223
23 May A F Toberer E S Saramat A Snyder G J 2009 Phys. Rev. 80 125205
24 Fistul’ V I 1969 Heavily doped semiconductors New York Plenum Press
25 Tan G Liu W Chi H Su X Wang S Yan Y Tang X Wong-Ng W Uher C 2013 Acta Mater. 61 7693
26 Tan G Wang S Yan Y Li H Tang X 2012 J. Alloys Compd. 513 328
27 Wang S Y She X Y Zheng G Fu F Li H Tang X F 2012 J. Electron Mater. 41 1091
28 HXie H Wang H Pei Y Z Fu C G Liu X H Snyder G J Zhao X B Zhu T J 2013 Adv. Funct. Mater 23 5123
29 May A F Flage-Larsen E Snyder G J 2010 Phys. Rev. 81 125205
30 Kim H S Liu W S Chen G Chua C W Ren Z F 2015 PNAS 112 8205
31 Altenkirch E 1909 Phys. Z. 10 560