Enhancement of the thermoelectric performance of InTe via introducing Cd dopant and regulating the annealing time

Journal of Alloys and Compounds 813 (2020) 152210

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Enhancement of the thermoelectric performance of InTe via introducing Cd dopant and regulating the annealing time Shanshan Pan a, Hui Liu b, Zhili Li a, Li You a, Shengnan Dai c, Jiong Yang c, Kai Guo a, **, Jun Luo a, c, * a b c

School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China China Academy of Space Technology, China Aerospace Science and Technology Corporation, Beijing, 100094, China Materials Genome Institute, Shanghai University, 99 Shangda Road, Shanghai, 200444, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 June 2019 Received in revised form 20 August 2019 Accepted 9 September 2019 Available online 10 September 2019

Due to the lone-pair-induced anharmonic rattling, InTe exhibits intrinsically ultralow thermal conductivity, which was considered as a candidate for thermoelectric application. However, the poor electrical transport properties result in low thermoelectric conversion efficiency. In this work, Cd doping in InTe have been performed with the aim to optimize the carrier concentration and increase the power factor. Furthermore, an essential enhancement of carrier mobility can be realized via prolonging the annealing time, leading to improved thermoelectric performance. As a result, the maximum thermoelectric figure of merit zT of 0.87 at 773 K has been achieved for In0.98Cd0.02Te undergoing 7 days annealing treatment (In0.98Cd0.02Te-7days), which is 67% higher than the value of InTe after 2 days annealing (InTe-2days, zT ¼ 0.52@773 K). The average zT increases significantly from 0.26 for InTe-2days to 0.53 for In0.98Cd0.02Te-7days in the temperature range of 323e823 K. Thus, both introducing Cd dopant and regulating the annealing time have obvious effects on improving the thermoelectric performance of InTe. © 2019 Elsevier B.V. All rights reserved.

Keywords: Carrier concentration Carrier mobility Lattice thermal conductivity Annealing time

1. Introduction Thermoelectric (TE) materials enabling the interconversion of heat energy and electricity play important roles in environment issues [1,2]. As a potential sustainable energy technology, the broad applications are limited due to the low conversion efficiency, which is represented by the dimensionless figure of merit, defined as zT ¼ S2sT/k, where s, S, T, and k are the electrical conductivity, Seebeck coefficient, absolute temperature, and the total thermal conductivity, respectively [3e5]. Accordingly, high performance TE materials are supposed to have high power factor (PF ¼ S2s) and low thermal conductivity (k ¼ kl þ ke, where kl and ke are the lattice thermal conductivity and the carrier thermal conductivity, respectively) [6,7]. Advanced strategies involving the increase of the power factor cover band engineering (resonance level [8], impurity level [9] and band convergence [10,11]), carrier concentration manipulation [12], while reductions in kl can be realized via introducing point defect scattering, interface scattering, grain-

* Corresponding author. School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China. ** Corresponding author. E-mail addresses: [email protected] (K. Guo), [email protected] (J. Luo). https://doi.org/10.1016/j.jallcom.2019.152210 0925-8388/© 2019 Elsevier B.V. All rights reserved.

boundary scattering or alloy scattering and so on [13e16]. Over the years, the thermoelectric properties of binary Te-based compounds (PbTe, GeTe and SnTe) have been studied extensively. However, the toxicity of the element lead indicates the application limitation of PbTe [14,17,18]. SnTe has been considered as a possible substitute for PbTe. Unfortunately, the high thermal conductivity remains a chief barrier to be overcame in SnTe [19]. Recently, some novel materials with intrinsically low lattice thermal conductivities have also attracted great interest such as binary InTe. The roomtemperature lattice thermal conductivity of InTe [20] is as low as 0.76 W m1 K1, while PbTe [21], GeTe [22] and SnTe [19] are 3.0 W m1 K1, 2.9 W m1 K1 and 3.2 W m1 K1, respectively. However, InTe has been rarely studied because of the low carrier concentration and mobility. The optimization of carrier concentration and mobility remains a principal issue to improve the thermoelectric performance of InTe [23]. In this work, the carrier concentration can be effectively increased with the substitution of Cd for In in InTe, leading to the increase of the electrical conductivity and power factor. In addition, prolonging the annealing time could enhance the carrier mobility significantly for optimizing the thermoelectric properties. Moreover, owing to the presence of strongly anharmonic phonons originating from the rattling vibrations of Inþ cations [20,24], the

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lattice thermal conductivity (kl ¼ 0.87 W m1 K1) of pristine InTe is extremely low at room temperature. Herein, a peak zT value of ~0.87 at 773 K and an average zT of ~0.53 are achieved for the In0.98Cd0.02Te-7days sample. 2. Experimental methods 2.1. Sample synthesis The samples of In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) were synthesized from elemental starting materials (Cd 99.999%, Aladdin, China; In 99.99%, Sigma-Aldrich, USA; Te 99.999%, Aladdin, China) by melt-annealing and subsequent hot pressing sintering. Stoichiometric mixtures of elemental powders were loaded in a graphite crucible and then sealed into an evacuated fused-silica quartz tube (~0.2 Pa). The tube was heated to 1123 K at 3 K/min and held for 10 h, and then annealed at 873 K for 2 or 7 days. Subsequently, the furnace was naturally cooled to room temperature. Finally, the as-prepared samples were grounded and hotpressed at 873 K for 20 min under a pressure of 65 MPa. 2.2. Characterization The phase structures of specimens were characterized by X-ray powder diffraction (XRD) with Cu-Ka (l ¼ 1.54056 Å) radiation (Aeris Research, PANalytical, Netherlands). The measurement of electrical resistivity and Seebeck coefficient were simultaneously carried out on a commercial available instrument (ZEM-3, ULVACRIKO, Japan). The total thermal conductivity (k) was calculated by k ¼ rlCp, where r is the density of material, l is the thermal diffusivity coefficient and Cp is the specific heat of the sample [4,25]. The thermal diffusivity coefficient was measured by a laser flash

method (LFA 457, NETZSCH, Germany). The densities of the samples were measured via the Archimedes drainage method. The specific heat Cp was estimated from CP ¼ 3 nR/M, where n is the number of atoms per formula unit, R is the gas constant, and M is the molar mass [16,22,26]. The microstructure and composition of samples were investigated by a field-resolution scanning electron microscope (SEM, Zeiss GeminiSEM-300, Germany) equipped with energy-dispersive spectroscopy (EDS). The carrier concentration and mobility of the samples were collected by Hall coefficient (RH) measurements under a reversible magnetic field of 0.9 T (8400, Lake Shore, USA). The room-temperature optical bandgap was measured using Fourier-transform infrared spectrometry with the wavelength range of 2500e25000 nm (Nicolet iS50, ThermoFisher, USA). According to the Kubelka-Munk (K-M) relationship: (ahn)1/ m ¼ B(hn -Eg), where a, h, B and n are the absorption coefficients, Plank constant, scale constant, and the frequency incident light, respectively [12,27]. The theoretical calculation in this work is based on the density functional theory using the Vienna ab initio simulation package (VASP), carried out by projector augmented wave (PAW) method [28,29]. The PerdewBurkeErnzerhof (PBE) type generalized gradient approximation (GGA) is used as the exchange-correlation function. The plane-wave energy cutoff is 400 eV. The energy convergence criterion is 104 eV for the structural relaxation and 107 eV for self-consistent calculation to obtain accurate energies. 3. Results and discussion 3.1. Cd-doped In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) The electrical conductivity of the polycrystalline samples In1(x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) are shown in Fig. 1(a).

xCdxTe

Fig. 1. Temperature dependence of the electrical conductivity (a), Seebeck coefficient (b) and power factor (c) for In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) compounds. (d) The plot of (ahn)1/2 against photon energy reveals room-temperature band gaps of 0.220, 0.205, and 0.197 eV for the InTe, In0.995Cd0.005Te, and In0.98Cd0.02Te samples, respectively.

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0:03Cd þ InTe/In0:97 Cd0:03 Te þ 0:03In The DHþ3 , representing the formation energy when Cd occupies the tervalent In site, is 0.001 eV. The small formation energy suggests that the CdIn is easy to form in InTe. In contrast, the defect formation energy for Cd at another In site is slightly higher, 0.019 eV. This indicates that Cd serves as an effective acceptor dopant to increase the hole concentration. The Seebeck coefficients of In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) as a function of

the temperature are shown in Fig. 1(b). Positive Seebeck coefficients for all the samples indicate that holes are the major carrier (p-type conduction), which dominates the electrical transport properties [31]. The Seebeck coefficient increases linearly with the rising of the temperature for all the samples. In general, the temperature-dependence for electrical conductivity and Seebeck coefficient is opposite due to the temperature-induced varieties of the carrier concentration. However, in the case of InTe-2days sample, the carrier concentration increases slightly while its carrier mobility increases essentially as the temperature rising (Fig. S3), which leads to the increased electrical conductivity at high temperature. Meanwhile, higher temperature would generate lager values of Seebeck coefficients according to the
2 3 2 8p2 k Tm* p equation S ¼ 3ehB 2 . Combining the measured electrical 3n =

The pristine InTe exhibits a comparably low electrical conductivity at room temperature (s ¼ 4.0  103 S m1), and the value rises with increasing the temperature, indicating a non-degenerate semiconducting behavior [6,18,30]. The room temperature electrical conductivity increases from 4.0  103 S m1 for the undoped sample to 1.3  104 S m1 for In0.99Cd0.01Te, which can be attributed to the increase of the carrier concentration (Table S1). However, more Cd dopant (x ¼ 0.02) would cause the decrease of the electrical conductivity, which can be understood from the higher content of the secondary phase CdTe, evidenced by XRD measurements (Fig. S1), microstructure morphology and EDS results of the samples In0.995Cd0.005Te, In0.99Cd0.01Te and In0.98Cd0.02Te (Fig. 2(aec)). The sample with x ¼ 0.005 shows homogeneous distribution of elements, which is consistent with the single-phase feature confirmed by XRD. On contrary, the microstructure of the samples for x ¼ 0.01 and 0.02 seem to be more complicated. Cd-rich Phase can be identified in Fig. 2 b and 2c, suggesting a solid solution limit of Cd in InTe. Additionally, the introduction of Cd dopant would reduce the band gap of InTe, which is beneficial for the increasing of the electrical conductivity. As observed in Fig. 1(d), the band gaps were determined to be 0.220, 0.205, 0.197 eV for the In1-xCdxTe samples with x ¼ 0, 0.005, 0.02, respectively. In addition, the theoretical calculations demonstrate that Cd atoms are more likely to occupy the In3þ position, leading to the enhanced carrier concentration. The defect formation energy can be achieved according to the following equation:

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conductivity with Seebeck coefficient, the temperature dependence of power factors for In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) samples are calculated and shown Fig. 1(c). The power factor improves with the increase of Cd content, which is mainly ascribed to the increased carrier concentration. A maximum PF of 4.2 mW cm1 K2 at 623 K is achieved for In0.98Cd0.02Te sample, which is comparable to the value of 4.9 mW cm1 K2 at 623 K for the In0.997Te [20]. Fig. 3(aeb) display the temperature dependence of the total thermal conductivity (k) and lattice thermal conductivity (kl) for In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) samples, respectively. The total thermal conductivity of the pristine InTe decreases from 0.90 W m1 K1 to 0.49 W m1 K1 with increasing temperature (323e623 K) on account for the dominant contribution of Umklapp process. The lowest total thermal conductivity, k ¼ 0.46 W m1 K1, has been obtained at 623 K for In0.98Cd0.02Te sample. Such a low k value is attributed to the strongly anharmonic phonon scattering originating from rattling vibrations of Inþ cations [20]. The lattice thermal conductivity is calculated by the subtraction of the electrical thermal conductivity (ke ¼ sLT, where L is the

Fig. 2. SEM images (in back-scattered mode) and EDS mapping for polished surface of samples of hot-pressed: (a) In0.995Cd0.005Te; (b) In0.99Cd0.01Te; (c) In0.98Cd0.02Te.

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Fig. 3. Temperature dependent of total thermal conductivity (a), lattice thermal conductivity (b), the Lorenz number (c) and figure of merit zT (d) for In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) samples.

Lorenz number) from total thermal conductivity. The L value can be estimated by [32,33]:

  jSj L ¼ 1:5 þ exp 116

1

Fig. 3(c) shows the calculated L as a function of temperature. All L values of In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) samples are between 1.5  108 W U K2 for acoustic phonon scattering and 2.45  108 W U K2 for degenerate limit [33], showing the typical semiconductor behavior [33]. Considering the similar atomic masses and ionic radii of Cd and In, the substitution of Cd for In should have little effect on kl with x  0.005. In addition, at room temperature, In0.98Cd0.02Te sample exhibits kl of ~0.73 W m1 K1, which is significantly reduced in comparison with the pure InTe. The presence of CdTe nanoparticles provides extra phonon scattering centers for mid-to-long wavelength phonons, resulting in the reduction of the lattice thermal conductivity. Based on the measured electrical conductivity, Seebeck coefficient and thermal conductivity, the temperature dependence of the dimensionless figure of merit zT for all samples have been calculated. As shown in Fig. 3(d), zT increases rapidly with the rising of temperature, resulted from the decrease in thermal conductivity. The zT value increases with increasing Cd content due to the significantly enhanced power factor. The maximum thermoelectric figure of merit of 0.57 has been achieved for In0.98Cd0.02 at 623 K, which is much higher than that of pure InTe. 3.2. Prolong the annealing time In comparison with the data of literatures [20,34], the roomtemperature electrical conductivity of InTe-2days in this work is lower due to the low carrier mobility m (~5 cm2 v1 s1) (see

Table 1). The low carrier mobility of InTe-2days sample is probably attributed to the ultrafine grain due to the short-time annealing treatment (Fig. S2) and the dominant ionization scattering at the low temperature range [35]. It is well-known that m depends on how fast an electron travels through a semiconductor or metal under an electric field [36]. The carrier mobility m can be defined as m ¼ e〈t〉=m , where e is the charge of an electron, 〈t〉 is the average I relaxation time of the carriers and mI is the inertial mass along the transport direction [37]. Therefore, m and the corresponding electrical transport properties can be enhanced once the intrinsic lattice defects and grain boundaries are reduced to minimum the scattering effect, as well as the scattering mechanism can be regulated from the ionization scattering to mixed scattering or acoustic phonon scattering [35,38]. Some successful strategies have already been developed to optimize the carrier mobility in other thermoelectric materials, such as suppressing the formation of intrinsic Bi/I antisite defects in the BiTeI system [39]. In addition, since point defects are sensitive to the preparation process, significant enhancement of m can be achieved by tuning the preparation conditions (such as hot-pressing temperature and hold time), which was successfully applied in the Mg3Sb2 and

Table 1 Carrier concentration (nH) and carrier mobility (mH) for In1-xCdxTe (x ¼ 0, 0.02) compounds with different annealing time at room temperature. Sample InTe-2days In0.98Cd0.02Te-2days InTe-7days In0.98Cd0.02Te-7days InTe-Rhyee [34] InTe-Biswas [20]

nH(cm3) 4.3 6.2 4.9 4.3 6.8 5.8

19

10 1019 1019 1019 1019 1018

mH(cm2v1s1) 5.3 4.4 11 16 21 58

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Cu2FeSnSe4 systems [36,40]. Therefore, prolonging the annealing time may be an effective method to minimum the point defect, improve grain crystalline and uniformity as well as change the scattering mechanism, and thus increasing the carrier mobility. The samples In1-xCdxTe (x ¼ 0, 0.02) with different annealing time were synthesized to further optimize the thermoelectric performance. As shown in Fig. 4(a), the diffraction peak intensities of the samples after annealing for 7 days increase obviously. This indicates that the composition uniformity and crystallinity are improved with prolonged annealing time. Simultaneously, the grain size of InTe-2days sample is significantly coarsened via prolonging the annealing time (Fig. S2), leading to improved carrier mobility. On the other hand, when bipolar conduction does not occur at the whole temperature range (Fig. 5(b)), carrier concentration is almost constant (Fig. S3). In this situation, the temperature dependence of electrical conductivity and carrier mobility can be identical [35]. As shown in Fig. 4(b), the scattering mechanism in pristine InTe sample varies from ionization scattering (InTe-2days, sfT1.5) into the mixed scattering (InTe-7days, sfT0.8) at the lower temperature range, leading to the noticeably enhanced carrier mobility. According to the Hall effect measurement, the InTe-7days sample shows a room-temperature carrier mobility of 11 cm2 v1 s1, which is about 110% enhancement as compared with that of the InTe-2days sample (see Table 1). Consequently, the room temperature electrical conductivity of the pristine InTe increases significantly from 4.0  103 S m1 to 1.2  104 S m1. The Seebeck coefficient decreases from 106 to 94 m V K1 at room temperature. A remarkable improvememt of the power factor is obtained due to the increased carrier mobility, which is ascribed to the larger grain size and the weaker effect of ionized impurity scattering. The thermal conductivity, lattice thermal conductivity, zT, and average zT value of samples with different annealing

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time are shown in Fig. 5 to compare with the literature data [20,34]. It is found that prolonging the anealing time has little effect on the lattice thermal conductivity. Eventually, a zT value of 0.70 at 773 K for pristine InTe is realized through 7-days annealing treatment, and a peak zT of 0.87 at 773 K is achieved for the sample In0.98Cd0.02Te-7days. The average zT, defined as . RT zTaverage ¼ T12 zTdT ðT  T Þ [16], for the In1-xCdxTe samples (x ¼ 0, 2 1 0.02) are shown in Fig. 5(d). Obviously, the average zT of the pure InTe increases approximately linearly with the Cd content due to the linearly increased average PF. Furthermore, the average zT are further improved via prolonging the annealing time. The maximum average zT is 0.53 for the sample In0.98Cd0.02Te-7days, which increases ~104% in comparison with that of 0.26 for the sample InTe2days. 4. Conclusions In summary, a series of In1-xCdxTe (x ¼ 0, 0.002, 0.003, 0.005, 0.01, 0.02) compounds have been synthesized in order to investigate the effect of Cd doping on the thermoelectric properties. The pristine InTe has ultralow lattice thermal conductivity in comparison with other binary Te-based compounds due to the presence of strongly anharmonic phonon scattering. The band gap of InTe is narrowed and the carrier concentration increases due to the substitution of Cd for In, leading to enhanced electrical conductivity. Prolonged annealing time is beneficial for increasing the carrier mobility, resulting in the further enhancement of the zT and average zT. Eventually, owing to the increased power factor and intrinsically ultralow lattice thermal conductivity, the peak zT values of ~0.52 and 0.87 are achieved for the samples InTe-2days and In0.98Cd0.02Te-7days, respectively. The average zT has been

Fig. 4. (a) XRD patterns of In1-xCdxTe (x ¼ 0, 0.02) with different annealing time. Temperature dependent electrical conductivity (b), Seebeck coefficient (c) and power factor (d) in comparison with the literature data [20,34].

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Fig. 5. Temperature dependent total thermal conductivity (a), lattice thermal conductivity (b), figure of merit zT (c) and Average zT (d) for In1-xCdxTe (x ¼ 0, 0.02) samples with different annealing time in comparison with the literature data [20,34].

significantly improved from 0.26 for InTe-2days to 0.53 for In0.98Cd0.02Te-7days. Our work demonstrates that prolonging the annealing time is an effective way to improve the thermoelectric performance of InTe compound. Acknowledgements The work was financially supported by the National Key Research and Development Program of China (No. 2018YFB0703600), National Natural Science Foundation of China (Grant Nos. 51772186, 21771123, and 51632005), and the research grant (No. 16DZ2260601) from Science and Technology Commission of Shanghai Municipality. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152210. References [1] L. You, Y. Liu, X. Li, P. Nan, B. Ge, Y. Jiang, P. Luo, S. Pan, Y. Pei, W. Zhang, G.J. Snyder, J. Yang, J. Zhang, J. Luo, Boosting the thermoelectric performance of PbSe through dynamic doping and hierarchical phonon scattering, Energy Environ. Sci. 11 (2018) 1848e1858. [2] Z. Zhou, J. Yang, Q. Jiang, X. Lin, J. Xin, A. Basit, J. Hou, B. Sun, Enhanced thermoelectric performance of SnTe: high efficient cation - anion Co-doping, hierarchical microstructure and electro-acoustic decoupling, Nano Energy 47 (2018) 81e88. [3] S. Xu, W. Zhu, L. Zhang, Z. Zhang, Y. Deng, Enhanced thermoelectric performance of SnTe film with optimized carrier transport induced by facile postannealing process, Mater. Lett. 221 (2018) 12e14. [4] G. Xing, J. Sun, Y. Li, X. Fan, W. Zheng, D.J. Singh, Thermoelectric properties of p-type cubic and rhombohedral GeTe, J. Appl. Phys. 123 (2018) 195105.

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