Thermoelectric Properties of Cu₂Te Nanoparticle Incorporated N-Type Bi₂Te_2.7Se_0.3

Abstract

To develop highly efficient thermoelectric materials, the generation of homogeneous heterostructures in a matrix is considered to mitigate the interdependency of the thermoelectric compartments. In this study, Cu₂Te nanoparticles were introduced onto Bi₂Te_2.7Se_0.3 n-type materials and their thermoelectric properties were investigated in terms of the amount of Cu₂Te nanoparticles. A homogeneous dispersion of Cu₂Te nanoparticles was obtained up to 0.4 wt.% Cu₂Te, whereas the Cu₂Te nanoparticles tended to agglomerate with each other at greater than 0.6 wt.% Cu₂Te. The highest power factor was obtained under the optimal dispersion conditions (0.4 wt.% Cu₂Te incorporation), which was considered to originate from the potential barrier on the interface between Cu₂Te and Bi₂Te_2.7Se_0.3. The Cu₂Te incorporation also reduced the lattice thermal conductivity, and the dimensionless figure of merit ZT was increased to 0.75 at 374 K for 0.4 wt.% Cu₂Te incorporation compared with that of 0.65 at 425 K for pristine Bi₂Te_2.7Se_0.3. This approach could also be an effective means of controlling the temperature dependence of ZT, which could be modulated against target applications.

1. Introduction

Thermoelectric (TE) technology enables direct solid-state conversion without any moving parts or harmful emissions between heat and electrical energy and shows great potential for applications in waste heat recovery. The conversion efficiency of TE devices depends on the performance of TE materials as represented by the dimensionless figure of merit ZT = S²σT/κ_tot, where σ, S, T, and κ_tot are the electrical conductivity, Seebeck coefficient, temperature, and total thermal conductivity, respectively. To realize a high ZT, it is desirable to have a high power factor (S²σ) and low thermal conductivity (κ_tot) [1,2]. However, each parameter has a trade-off relation, which makes it difficult to achieve high TE performance for practical applications.

Bi₂Te₃-based TE materials are thought to be the only materials that can be considered for cooling or low-temperature energy-harvesting applications. In particular, low-temperature heat below 400 °C accounts for more than 70% of industrial waste heat, which is the case for Bi₂Te₃-related material systems [3]. Extensive theoretical and experimental studies such as electronic band structure engineering [4,5], doping [6,7], nano-structuring [8,9], and nanocomposite fabrication [10,11] have been suggested to optimize TE performance. Kim et al. reported a significant reduction in lattice thermal conductivity without any deterioration of the electrical properties by introducing dense dislocation arrays into the simple composition of bulk p-type Bi_0.5Sb_1.5Te₃ and obtained the highest ZT value of ~1.9 near room temperature [12]. Although significant progress in thermal conductivity reduction has been attained [12,13,14], it remains necessary to boost the electrical properties of TE materials to achieve efficient power generation and cooling devices. The carrier filtering mechanism alters the Seebeck coefficient by introducing interfaces between the TE matrix and secondary nanophases, and the band bending on the interfaces induces the filtering of low-energy carriers [15,16,17,18,19,20]. The secondary nanophases also strengthen the phonon scattering to reduce thermal conductivity, which could significantly enhance ZT. The effect of the nano-phase in TE materials has been reported on PbTe-based TE materials, where the SrTe nano-precipitate enhances ZT up to ~2.5 at high temperatures [18]. For Bi₂Te₃-based materials, the addition of TE to p-type Bi_0.5Sb_1.5Te₃ thin films prepared by a pulsed laser deposition technique significantly enhances the Seebeck coefficient [19]. However, the nano-precipitate and vacuum deposition approaches require delicate processes that cannot be applied practically. In this study, we introduced Cu₂Te nanoparticles (NPs) into an n-type Bi₂Te_2.7Se_0.3 (BTS) matrix to enhance TE performance. Several papers [21,22,23] have reported the existence of Cu₂Te phase when excess Cu is incorporated on Bi-Te-based TE materials. However, they do not discuss the direct effect of Cu₂Te incorporation. Cu₂Te NPs, which were synthesized by the organic-free chemical method, could be considered as efficient additives that would provide benefits such as modulating the carrier concentration and enhancing phonon scattering. In addition, the incorporation of Cu₂Te NPs only requires a simple process that can easily be employed to alter TE properties according to the target applications.

2. Materials and Methods

The BTS matrix was synthesized using a conventional melting-quenching process. Raw elements with stoichiometric ratios (Bi, 99.999%, 5 N Plus; Te, 99.999%, 5 N Plus; Se, 99.999%, 5 N Plus) were heated at 1000 °C for 6 h under a vacuum in fused silica tubes, and the melts were quenched in a water vessel and finally ground into fine powders using ball milling.

The Cu₂Te NPs were synthesized using the chemical reduction method. Under an Ar atmosphere, 1 g Te was dissolved in 0.3 M NaBH₄ aqueous solution. Further, 0.2 M CuCl₂·H₂O aqueous solution was slowly poured into the above Te solution and stirred for 30 min. After remaining for an additional 30 min, the precipitates were centrifuged and washed with ethanol several times. For the Cu₂Te NP dispersion on the BTS matrix, the obtained Cu₂Te NPs were dispersed in ethanol and mixed with BTS by wet grinding. The mixture was dried in a vacuum oven, and fine powders were collected. The obtained powders were sintered by spark plasma sintering at 773 K for 3 min under a pressure of 60 MPa.

The consolidated samples were analysed by X-ray diffraction (XRD, Cu Kα, 1.5406 Å, New D8 Advance, Bruker, Billerica, MA, USA), scanning electron microscopy (SEM, JSM-7600F, JEOL, Tokyo, Japan), electron probe microanalysis (EPMA, 30 kV, JXA-8530F PLUS, JEOL, Tokyo, Japan), and transmission electron microscopy (TEM, 200 kV, Tecnai G2-20, FEI, Hillsboro, Oregon, USA). The TE properties were measured using a ZEM-3 (ULVAC-RIKO, Methuen, MA, USA) for the electrical parts and the laser-flash method (LFA, DLF 1300, TA, New Castle, DE, USA) for the thermal parts. Hall measurements were conducted to obtain the carrier concentration (HT-Hall, ResiTest 8300, Toyo Corporation, Tokyo, Japan).

3. Results and Discussions

Figure 1a shows the XRD pattern of as-prepared Cu₂Te NPs obtained by the chemical reduction method. Our Cu₂Te NPs has a hexagonal structure with the space group of P3m1 (JCPDS # 49-1441). The XRD peaks are not strong due to the small size of Cu₂Te NPs. The size of as-prepared Cu₂Te is 20–50 nm from the SEM and TEM images (Figure 1b,c), and the high resolution TEM image shows the lattice distance of 0.36 nm, which corresponds to (006) plane of Cu₂Te phase. Figure 1e presents the powder XRD patterns of the BTS-x wt.% Cu₂Te NP (x = 0, 0.2, 0.4, 0.6, and 0.8) composite materials. All samples show typical BTS patterns (rhombohedral structure, space group of R-3m, JCPDS # 50-0594) without any additional peaks. The lattice parameters of the BTS-x wt.% Cu₂Te NP (x = 0, 0.2, 0.4, 0.6, and 0.8) are summarized in

Table 1. The lattice parameters of BTS-x wt.% Cu₂Te NPs (x = 0, 0.2, 0.4, 0.6, and 0.8).

	Lattice Constant for a (Å)	Lattice Constant for c (Å)
BTS	4.3619	30.3632
BTS-0.2 wt.% Cu2Te NPs	4.3614	30.3604
BTS-0.4 wt.% Cu2Te NPs	4.3615	30.3641
BTS-0.6 wt.% Cu2Te NPs	4.3610	30.3606
BTS-0.8 wt.% Cu2Te NPs	4.3614	30.3650

, and there are no significant changes in lattice parameters by Cu₂Te NPs incorporation. The small size and low content of Cu₂Te NPs in the composite powder were not detectable by XRD (Figure 1e). We analysed the Cu₂Te nanophases on BTS-x wt.% Cu₂Te NP (x = 0.2, 0.4, and 0.6) bulk samples by EPMA (Figure 1f–h), where the light parts of the Cu mapping were matched with the Cu₂Te nanophases. Cu₂Te NPs can be easily observed on the parent bulk materials, and as the amount of Cu₂Te NPs increases, a homogeneous distribution of Cu₂Te NPs can be found. However, for the BTS-0.6 wt.% Cu₂Te NPs sample, some Cu₂Te NPs exhibit agglomeration, which means that a large amount (more than 0.6 wt.%) of Cu₂Te NPs on BTS cannot be dispersed uniformly.

Figure 2a presents the temperature dependence of the electrical conductivity (σ) for BTS-x wt.% Cu₂Te NPs (x = 0, 0.2, 0.4, 0.6, and 0.8). The electrical conductivity gradually decreases over the entire temperature range studied here as the Cu₂Te NP incorporation increases. The electrical conductivities of BTS-x wt.% Cu₂Te NPs (x = 0, 0.2, 0.4, and 0.6) decrease with increasing temperature, indicating semi-metallic or metallic conduction behaviour. However, the sample with a high amount of Cu₂Te NPs (0.8 wt.%) incorporated shows semiconducting behaviour at a high temperature due to the carrier concentration decrease shown in Figure 2b. The carrier concentration obtained from the Hall measurements decreases as the amount of Cu₂Te NPs increases. The Cu intercalation in the van der Waals gap between Te–Te shows donor-like behaviour, while our Cu₂Te incorporation shows acceptor-like behaviour. The temperature dependence of the Seebeck coefficient (S) for the BTS-x wt.% Cu₂Te NP (x = 0, 0.2, 0.4, 0.6, and 0.8) samples is displayed in Figure 2c. All samples exhibit negative S values, indicating that electrons constitute the majority of the charge carriers, which is consistent with the signs of the respective Hall measurements. The temperature at which the S peaks (T_max) are shifted to lower temperatures with increasing Cu₂Te incorporation, and the absolute value of S maximum (S_max) increases as the amount of incorporated Cu₂Te increases. Typically, BTS show anisotropic nature of TE properties due to Te vacancies and antisite defects [24]. It is noted that the reproducibility (collecting the results for more than 5 different batches) of electrical properties is greatly enhanced through Cu₂Te introduction.

Usually, the T_max shift to lower temperatures originates from increased bipolar conduction, but the corresponding S_max should also be decreased. Improvement of the bipolar conduction cannot explain the observed increase in S_max with increasing Cu₂Te. From Pisarenko’s relation [1,25], one can estimate the density of states (DOS) effective mass with the assumption of a single parabolic band model using the following Equation (1):

$$S=\frac{8{\pi }^2{k}_B^2}{3e{h}^2}{\left(\frac{\pi }{3n}\right)}^{2/3}{m}_d^{*}T$$

(1)

where h is the Planck constant, k_B is the Boltzmann constant, $m_d^{*}$ is the DOS effective mass, e is the electronic charge, and n is the carrier concentration. As shown in Figure 2e, the pristine BTS has an $m_d^{*}$ value of 0.97 m_e (m_e is the electron rest mass), whereas Cu₂Te NP incorporation increases the $m_d^{*}$ value of 1.16 m_e in the BTS-0.4 wt.% Cu₂Te NP sample at room temperature. Therefore, the increase in $m_d^{*}$ with Cu₂Te incorporation can enhance S_max, even when the bipolar conduction becomes strong. The n reduction observed with the incorporation of Cu₂Te is also responsible for the increase in S_max. The $m_d^{*}$ change could be attributed to the engineered band structure interface between the BTS matrix and Cu₂Te NPs. In this context, we can suggest a band diagram that describes the interfacial band bending between the BTS and Cu₂Te NPs (Figure 2f). The electron energy barrier, i.e., the hetero-interface of the conduction bands between the BTS and Cu₂Te NPs, filters the low-energy carriers. The electron affinity and band gap of BTS and Cu₂Te were obtained from the literature [26,27,28,29]. A previous theoretical study of PbTe TE materials showed that ~1.5 nm nanoinclusion significantly enhanced the S value [15]. The electrostatic potential only affected the interface between the nanoinclusion and the matrix, and larger nanoinclusions were less effective than smaller nanoinclusions. Our synthesized Cu₂Te NPs had sizes of ~50 nm, so we expected that the smaller Cu₂Te NPs could have a greater effect. The BTS-0.4 wt.% Cu₂Te NP sample has the highest calculated power factor (S²σ), which is ~15% higher at room temperature than that of the pristine BTS sample.

The temperature dependence of the total thermal conductivity (κ_tot) of BTS-x wt.% Cu₂Te NPs (x = 0, 0.2, 0.4, 0.6, and 0.8) is shown in Figure 3a. All samples display very low κ_tot values over the entire temperature range studied here and also exhibit upturns. The occurrence temperatures of the upturns in the thermal conductivity data move to lower temperatures with increasing Cu₂Te incorporation amounts, which is consistent with the behaviour of S and σ. This increase in thermal conductivity is due to the bipolar diffusion of the carriers and is still present in the BTS sample at elevated temperatures, as shown in Figure 3a. This increase in κ_tot with temperature is attributable to the thermal energy transported by electron–hole pairs, which is equal to the energy of the gap between the hot and cold sides of the sample. This bipolar diffusion phenomenon results in an increase in the heat transfer beyond what is expected from the normal carrier contribution (κ_ele) to κ defined by the Wiedemann-Franz law [30], κ_ele = LσT, where L is the Lorenz number. The Lorenz number is estimated by assuming a single parabolic band and acoustic phonon scattering using the following Equations (2)–(4):

$$S=\pm \frac{k_B}{e}\left[\frac{\left(r+5/2\right)F_{r+3/2}\left(\xi \right)}{\left(r+3/2\right)F_{r+1/2}\left(\xi \right)}-\xi \right]$$

(2)

$$F_n\left(\xi \right)={{\displaystyle \int }}_0^{\infty }\frac{x^n}{1+e^{\left(x-\xi \right)}}dx$$

(3)

$$L={\left(\frac{k_B}{e}\right)}^2\left[\frac{\left(r+7/2\right)F_{r+5/2}\left(\xi \right)}{\left(r+3/2\right)F_{r+1/2}\left(\xi \right)}-{\left(\frac{\left(r+5/2\right)F_{r+3/2}\left(\xi \right)}{\left(r+3/2\right)F_{r+1/2}\left(\xi \right)}\right)}^2\right]$$

(4)

Here, ξ, F_n(ξ), and r are the reduced Fermi energy ((E_v − E_F)/k_BT), Fermi integral of order n, and scattering parameter, respectively. We set r = 0–1/2 for acoustic phonon scattering. The temperature dependence of the lattice thermal conductivity (κ_lat) can be estimated by deducting the bipolar conduction portion (κ_bp) and electronic portion (κ_ele) of thermal transport from the total thermal conductivity; κ_lat = κ_tot − κ_bp − κ_ele. κ_bp was calculated using the below Equation (5).

$${\kappa }_{bp}=({\sum }_i{S}_i{}^2{\sigma }_i-S^2\sigma )T.$$

(5)

S_i, σ_i, S, and σ are the Seebeck coefficient and electrical conductivity of an individual band, total Seebeck coefficient, and electrical conductivity from both the conduction and valence bands, respectively. S and σ are in turn defined as follows Equations (6) and (7):

$$\sigma ={{\displaystyle \sum }}_i{\sigma }_i$$

(6)

$$S=\frac{{\sum }_i{S}_i{\sigma }_i}{{\sum }_i{\sigma }_i}$$

(7)

Using the two-band (TB) model (an extension of the single parabolic band model), which includes one valence band and one conduction band, band parameters such as the density-of-states effective mass ($m_d^{*}$) and non-degenerate mobility (μ₀) of each band were obtained. Specifically, the $m_d^{*}$ and μ₀ of individual band were estimated by fitting the TB model to Hall carrier concentration (n_H)-dependent S and n_H-dependent σ measurements, respectively [31]. The band gap between the valence and conduction bands required in the TB model was adopted from the literature [32]. Once $m_{d,i}^{*}$ and μ_0,i (i = valence and conduction bands) were estimated, corresponding S_i and σ_i (i = valence and conduction bands) were calculated. Furthermore, theoretical total S and σ (which agree well with experimental S and σ) were computed with S_i and σ_i according to Equations (6) and (7). Finally, the calculated S_i, σ_i, S, and σ were substituted back into Equation (5) to estimate κ_bp.

The incorporation of Cu₂Te NPs encourages bipolar conduction and, consequently, κ_lat (shown in Figure 3b), after Cu₂Te NP incorporation is reduced over the entire temperature range due to enhanced phonon scattering. The reduction in κ_lat occurs up to 0.4 wt.% Cu₂Te NPs incorporation, whereas more Cu₂Te NPs produce a negative phonon scattering effect by agglomeration of Cu₂Te NPs. Generally, it is acceptable that the introduction of secondary phases decreases κ_lat by phonon scattering. However, the size of the secondary phase is an important factor for enhancing phonon scattering. Particles with sizes larger than hundreds of nanometres have a limited effect on κ_lat [33]. Our κ_lat results after Cu₂Te agglomeration (more than 0.6 wt.% Cu₂Te NPs) could be described similarly. The size of the Cu₂Te NPs was ~50 nm, and if several Cu₂Te NPs were agglomerated, the size was easily greater than a hundred nanometres. In addition, the previous report also shows that reduced particle size with the same volume fraction could decrease thermal conductivity more [34]. Therefore, a homogeneous dispersion of small Cu₂Te NPs is necessary for encouraging phonon scattering and enhancing the electrical properties.

Collecting the above effects, ZT is shown as a function of temperature for BTS-x wt.% Cu₂Te NPs (x = 0, 0.2, 0.4, 0.6, and 0.8) in Figure 3c. The highest ZT of 0.75 is observed at 374 K for the BTS-0.4 wt.% Cu₂Te NPs sample, which is 15% higher than that of the pristine sample. It is noteworthy that the maximum ZT temperature for each sample is modulated by the introduction of Cu₂Te NPs, decreasing as the amount of Cu₂Te NPs increases. The simple incorporation of Cu₂Te can easily modulate the temperature dependence of n-type TE materials, and we suggest that the TE properties can be precisely controlled according to the target application, such as cooling or low-temperature energy harvesting. The average ZT value (ZT_avg) in the temperature range studied here is shown in Figure 3d. ZT_avg reaches 0.70 for the BTS-0.4 wt.% Cu₂Te NPs sample, representing 19% enhancement compared to the pristine BTS matrix.

4. Conclusions

In conclusion, we investigated the TE properties of n-type BTS by incorporating Cu₂Te NPs. The incorporation of Cu₂Te NPs encourages the Seebeck coefficient and power factor, and we thought that the band bending between the interface of the BTS and Cu₂Te NPs could filter the low-energy electron carrier. The Cu₂Te NPs on the BTS matrix also affect κ_lat by encouraging phonon scattering. Together with modulating the electrical and thermal properties, the maximum ZT value reaches 0.75 at 374 K for the BTS-0.4 wt.% Cu₂Te NPs sample, 15% higher than that of pristine BTS (0.65 at 425 K). In addition, the temperature dependence of ZT can be controlled by the Cu₂Te NP dispersion just before the sintering process, which is a great advantage for target applications. Further advances could be expected by developing the synthesis of Cu₂Te NPs with sizes of several nanometres.