Thermoelectric Composite of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/Bi₂Se₃ with Enhanced Thermopower and Reduced Electrical Resistivity

Abstract

By using the solid-state reaction approach, composite polycrystalline samples of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ were created with varying amounts of Bi₂Se₃, (x = 5%, 10%, 15%, and 20%). The hexagonal crystal structure of the composite was revealed by x-ray diffraction (XRD) with a space group of R$\overline{3 }$m. The surface of the samples was seen to have secondary particles using a field emission scanning electronic microscope. Every sample displayed the typical semi-conducting behaviour across the entire temperature range. In the complex (Bi_0.98In_0.02)₂Te_2.7Se_0.3, it was found that bismuth was coordinated with six selenium atoms and there were significant selenium vacancies. With an increase in bismuth selenide concentration, the dissolution pattern shifted to a substitutional pattern. A two fold decrease in electrical resistivity for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ composition was seen compared to (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃. The granular material was produced by sintering and scattering of potential barrier, a thermal process that increases the Seebeck coefficient. A 200% increase was observed in thermopower for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ compared to (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃ compound.

Graphical Abstract

Introduction

The best thermoelectric (TE) figure of merit is found in bismuth telluride and its alloys, which are preferred materials for solid-state cooling systems. Since thermoelectric coolers do not need a compressor and are durable, dependable, and silent, they can function without any moving parts or compressors.¹

The best TE materials now available are semiconducting materials such as Bi₂Te₃, and Bi₂Se₃, which have the smallest dimensionless figure of merit (ZT) of around 1.2.² Although these materials have been in use for more than 50 years, they continue to attract interest because it is possible to structurally alter them to increase their ZT values. For instance, Kanatzidis et al.³ created cesium intercalated bismuth telluride (CsBi4Te₆) with ZT of 2.4 at room temperature for p-type Bi₂Te₃/Sb₂Te₃ superlattice thin film. Subramanian and coworkers reported the highest figure of merit ever seen for TE materials.⁴

The weak Te₍₁₎-Te₍₁₎/Se₍₁₎-Se₍₂₎ links between the quintuples and the layered structure of Bi₂Te₃/Bi₂Se₃ are the reason for the straightforward cleavage in the a and b directions. The basal planes have higher electrical conductivity than that along the c axis and across the van der Waals gap.⁵ The thermal conductivity of the basal plane in the perpendicular direction is greater than that of polycrystal. The basal planes have roughly twice the thermal conductivity in the direction perpendicular to the crystal than that of the polycrystal orientation. The Seebeck coefficient, however, is nearly isotropic for both n- and p-type systems.⁶ Meroz et al. reported creating n-type Bi₂Te_2.4Se_0.6 by combining hot pressing with a melt-spinning technique that can maintain some of the desired crystallographic orientation needed for electrical optimization. At a temperature of 320 K, a high ZT value of 1.07 was attained.⁷ The thermoelectric characteristics of the Bi₂Te_3−xSe_x alloys were improved by Cai et al. via melt-spinning and resistance-pressing sintering.⁸ The thermoelectric characteristics of bulk compounds containing Cu-doped In₂S₃ were improved by Chen et al..⁹ Bismuth telluride-based solid solutions with a configurable optimal temperature range were created by Zhang et al..¹⁰ As far as we aware, doping on Bi₂Se₃ and Bi₂Te₃ has been extensively researched, while doping of these compounds simultaneously and in composite form has received the least attention. The low-temperature thermoelectric characteristics of the samples (Bi_1−xIn_x)₂Te_2.7Se_0.3, (Bi_1−xSn_x)₂Te_2.7Se_0.3, (Bi_1−xIn_x)₂Se_2.7Se_0.3, and (Bi_1−x In_x)₂Se_2.7Te_0.3 were examined in prior publications.^{11,12,13,14,15} The (Bi_1−xIn_x)₂S_2.7Te_0.3 crystal develops lattice defects caused by the bismuth selenide composite impurities and precipitants. By incorporating additional bismuth telluride into the matrix, bismuth clusters are produced. With increasing bismuth telluride concentration, it is thought that bismuth occupancy in interstitial locations becomes less favoured and the dissolution pattern shifts to a substitutional pattern. Along with these, we recently reported the thermoelectric properties of the (Bi_1−xIn_x)₂Se_2.7Te_0.3/x%Bi₂Te₃ system. It was observed that anti-structure defects have a considerable impact on the carrier concentration in the current study’s n-type (Bi_1−xIn_x)₂Te_2.7Se_0.3 system.¹⁶ Therefore, in continuation of the previous report, the structural, surface morphological, and thermoelectric characteristics of different composite material systems such as (Bi_0.98In_0.02)₂Te_2.7Se_0.3 with varying concentrations of Bi₂Se₃ (5%, 10%, 15%, and 20%) are examined in the high-temperature range of 30°C to 450°C.

Synthesis and Characterization Technique

The conventional solid-state reaction method is used to prepare polycrystalline samples of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ of wt 5%, 10%, 15% and 20%, respectively.

• (a) Synthesis of (Bi_0.98In_0.02)₂Te_2.7Se_0.3

(Bi_0.98In_0.02)₂Te_2.7Se_0.3 was synthesized using a solid-state reaction approach; the precursors of bismuth (99.99%), indium (99.9%), selenium (99.995%), and tellurium (99.99%) were combined in a stoichiometric ratio and mixed for 2 h while vigorously grinding in an agate mortar. Pelletization of the powder was carried out using a 5-ton compression force. The pellets were placed in a quartz ampoule and vacuum-sealed for 30 h at 480°C in order to increase the uniformity and purity of compounds. The grinding process was repeated for the sintered pellets. The pellets were again sintered for 15 h at 250°C.

• (b) Synthesis of Bi₂Se₃

In a stoichiometric ratio, precursors of bismuth (99.99%) and selenium (99.995%) were mixed and processed for 2 h in an agate mortar. Pellets were created using a hydraulic press with 5-ton compression. The pellets were sintered in a quartz tube with a 12 mm diameter at 420°C for 24 h under 1.3 × 10^–7 kPa vacuum. The grinding process was repeated for 1 h with the sintered samples to achieve the proper homogeneity and purity in the compound. After pelletization, this powder was sintered for 12 h at 200°C.

• (c) Composition of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ for (x = wt 5%, wt 10%, wt 15%, wt 20%)

To obtain the desired homogeneity of the mixture, the produced Bi₂Se₃ was added individually to (Bi_0.98In_0.02)₂Te_2.7Se_0.3 in 5%, 10%, 15%, and 20% composition, respectively, and thoroughly crushed in an agate mortar for 2 h. To densify the grains, the samples were sealed and sintered for around 12 h. The 10 × 2 × 5 mm³ pellets underwent a variety of experimental characterizations.

• (d) Characterization of synthesized compounds

X-ray diffraction (XRD) was conducted using an x-ray diffractometer (Rigaku MiniFlex) and Cu K_α rays to confirm the purity, crystallinity, and phase formation of the compounds. Field emission scanning electron microscopy (FESEM) was performed using a JEOL JSM-7100F instrument at a magnification of 35 kX and a voltage of 15 kV. Energy-dispersive x-ray spectroscopy (EDS) was used to identify the sample composition.

In the temperature range of 30°C to 450°C, the steady-state DC-method was used to concurrently determine the temperature-dependent electrical resistivity and the Seebeck coefficient.

Results and Discussion

X-ray Diffraction

To identify the purity, crystalline phase, and crystallinity of the (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composite, powder x-ray diffraction analysis was conducted between 20° and 80° at a scan rate of 2°/min. Figure 1a illustrates XRD plots for the (Bi_1−xIn_x)₂Te_2.7Se_0.3/Bi₂Se₃ system. All samples exhibit XRD patterns with a pronounced aligned XRD peak plane (015) and a hexagonal structure with space group R $\overline{3 }$ m. The materials research project data sheet mp-568390 (Fig. 1b) is in good agreement with the current XRD patterns.¹⁷ Table I lists the characterization parameters including R_p, R_wp, R_ep, and $\chi$ ² values. Significantly, as demonstrated in Fig. 4, the Te concentration significantly affects the XRD spectra. The Bi₂Se₃ composite decreases the changes in constants in a random way and causes the XRD peaks to migrate to higher angles because selenium has a smaller atomic radius than tellurium.¹⁸ Additionally, as the Se concentration reduces, the lattice constant derived from the XRD spectra drops. The position of the (015) peak, which is exclusively influenced by the lattice constant c (Fig. 2), shifts to a smaller angle and eventually to a higher angle on the 2θ side. The peak obtained at an angle of 30° from the pristine to the doped sample reveals a diminishing of the XRD peak, which indicates interlayer change due to the increase in the concentration of the doped compound. In the (Bi_0.98In_0.02)₂Te_2.7Se_0.3 sample, Se is found to be deficient, indicating that there are several Se vacancies present in the lattice. (Bi_0.98In_0.02)₂Te_2.7Se_0.3 has a layered lattice structure, and Te atomic layers are missing in the lattice. This results in an irregular variation in lattice constant of (Bi _0.98In_0.02)₂Te_2.7Se_0.3/Bi₂Se₃ (80%/20%) compared to other composite compounds. In addition, there is a change in lattice parameter c of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ because of filling of Se in Te vacancies, resulting in a complicated layered structure. As evidence, the crystallite sizes are provided in Table I, which is in good agreement with lattice parameters.^19,20 The discrepancy between theoretical and computed XRD peak patterns, which is depicted in Fig. 3, has been determined using EXPO 2014.²¹

Fig. 1

(Adapted from Materials research project data sheet mp-568390).

(a) Powder x-ray diffraction patterns of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composite samples, (b) JCPDS graph of Bi₂Se₃ XRD peak patterns.

Table I Powder x-ray diffraction analysis data of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃

Fig. 2

Shift of the XRD (015) peak for polycrystalline (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composite samples.

Fig. 3

Rietveld refinement plots of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se_3, (Bi_0.98In_0.02)₂Te_2.7Se_0.3/10%Bi₂Se₃, (Bi_0.98In_0.02)₂Te_2.7Se_0.3/15%, (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ Bi₂Se₃ composite samples.

Surface Morphological Features and Elemental Analysis

Images of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/BiSe₃ composite samples captured with a field emission scanning electron microscope (FESEM) are shown in Fig. 4i. The (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ sample of 95%/5%, which has low porosity and a compact structure, is shown in Fig. 4i, a.¹⁸ It was found that the selenium particles were dispersed randomly across the surface of indium-doped Bi₂Te₃ with a rough surface (Fig. 4i, b). Secondary particles were often seen on the surface of the (Bi_0.98In_0.02)₂Te_2.7Se_0.3/BiSe₃ sample of 85%/15% by covering the tellurium or selenium atomic layer (Fig. 4i, c). The smooth surface of (Bi_0.98In_0.02)₂Se_2.7Te_0.3/BiTe₃ with 80%/20% (Fig. 4i, d) illustrates the standardized distribution of the composites in the grains.^19,20

Fig. 4

(i) FESEM images of (a) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃ (b) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/10%Bi₂Se₃ (c) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/15%Bi₂Se₃, (d) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ composites. (ii) EDS elemental mapping of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/Bi₂Se₃ composites.

EDS was used to check whether the samples had any additional components, and the results are presented in Fig. 4. (ii) Bismuth (L line), selenium (K line), indium (K line), tellurium (K line), and selenium (L line) are present in all of the (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ samples. Additionally, EDS elemental mapping (Fig. 5) demonstrates that the elements were uniformly dispersed across the sample surface. The observed atomic percentage is approximately matching with the nominal percentage of composition (Table II).

Fig. 5

Energy-dispersive x-ray analysis of spectra (EDS) of (a) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃, (b) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/10%Bi₂Se₃, (c) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/15%Bi₂Se₃, (d) (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃composites.

Table II The elemental composition data of the BIT/BS samples

Electrical Resistivity

The electrical resistivity of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composites was determined in the temperature range of 30–450°C (Fig. 6). All the samples exhibit n-type semiconducting behaviour in the entire temperature range. In the (Bi_0.98In_0.02)₂Te_2.7Se_0.3complex, it was found that bismuth coordinated with six selenium atoms and there were significant selenium vacancies.^22,23,24,25 The indium dopant in the compound disturbs this coordination. The places in the host lattice where indium and selenium are coordinated appear to have a higher volume than the host.²⁶ All the samples exhibited the usual semiconducting behaviour because of the scattering of carriers in grain boundaries and the point defects established by the random distribution of Te and Se in the crystallite positions. The (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃ and (Bi_0.98In_0.02)₂Te_2.7Se_0.3/10%Bi₂Se₃ samples are found to have higher electrical resistivity than (Bi_0.98In_0.02)₂Te_2.7Se_0.3/15%Bi₂Se₃ due to the self-interacting percolation channels. This causes the impurity atoms to be spatially redistributed.²⁷ Clusters of bismuth are generated by adding more bismuth selenide to the matrix. It is assumed that bismuth occupation in interstitial sites is less preferred with increasing bismuth selenide content and the dissolution pattern changes to a substitutional pattern. In addition, the substitution of 0.02 In for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ decreased the electrical resistivity. This suggests that In acts as a scattering centre that disturbs electron conduction. Hence, there is a twofold decrease in electrical resistivity for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ composition relative to that of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃.

Fig. 6

Temperature-dependent electrical resistivity of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composites.

In the (Bi_1−xIn_x)₂Te_2.7Se_0.3/x%Bi₂Te₃ system, at the high temperature zone, the hopping mechanism between nearest neighbouring sites is caused by thermally activated tiny polarons, as shown by the linear plot between ln(ρ/T) and 1/T in Fig. 7. Because selenium atom composition may change after sintering, E_A values are reported to fluctuate inconsistently with the doping concentration.^28,29 This causes the (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ samples to power scattering centres.³⁰

Fig. 7

Linear plot of small polaron hopping model of region in (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composites.

Seebeck Coefficient and Thermopower

The Seebeck coefficient was determined in the temperature range of 30–450°C for the synthesized polycrystalline samples and is displayed in Fig. 8. When Te/Se sites are occupied by Bi and In atoms in n-type (Bi_1−xIn)₂Te_2.7Se_0.3/Bi₂Se₃ alloys, anti-structure defects are formed. It was observed that a decrease in the electronegativity disparity between the component atoms was conducive to an increase in the formation of anti-structure defects.^31,32 Because of poor van der Waals connection between their layers, cleavage slip of mechanical deformation caused selenium vacancies to arise during the sintering process. The Bi atoms will enter Se vacancies because of the smaller difference of electronegativity between them. An anti-site defect was produced, resulting in the formation of the vacancies V_Bi.³³ Therefore, the concentration of carriers increases in (Bi_1−xIn_x)₂Te_2.7Se_0.3/Bi₂Se₃ as the content of Bi₂Te₃ increases. At higher temperatures, thermal excitation of carriers reduces bipolar transport, which also shifts the temperature dependence of the Seebeck value. Additionally, potential barrier scattering, a thermally increases the Seebeck coefficient, in the highly granular material by sintering. As a result, there is a Gaussian-shaped curve of Seebeck coefficients at 350°C for all the samples.³⁴ The theoretical carrier concentration is calculated using equation

$$ S\left( T \right) = \frac{{8k_{B}^{2} \pi^{2} m^{*} }}{{3eh^{2} }}\left( {\frac{\pi }{3n}} \right)^{2/3} T $$

(1)

where m* is the effective mass of the electron, e is the elementary charge of an electron, h is the Planck’s constant, n is the carrier concentration, and T is the absolute temperature, where S(T) is the temperature-dependent Seebeck coefficient, k_B is Boltzmann’s constant.³⁵ The mobility can be calculated using

$$ \mu_{H} = \frac{1}{ne\rho } $$

(2)

Fig. 8

Temperature-dependent Seebeck coefficient of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composites.

The theoretical values of charge density, mobility, activation energy and scattering factors of (Bi_1−xIn_x)₂Te_2.7Se_0.3/Bi₂Se₃ were calculated and are given in Table III.

Table III Theoretical charge density n_Th, mobility (µ_Th), activation energy (E_A) and scattering factor (γ_Th) of (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composites at 30°C

In addition, the temperature-dependent power factor of the investigated (Bi_1−xIn_x)₂Te_2.7Se_0.3/Bi₂Se₃ composites is shown in Fig. 9. The pristine (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃ has the lowest thermopower of 45 µW/mK² at 300°C, but the greatest thermopower of nearly 100 µW/mK² was found for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃. There is a 200% increase in thermopower for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ relative to (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃.

Fig. 9

Temperature-dependent thermopower of the (Bi_1−xIn_x)₂Te_2.7Se_0.3/Bi₂Se₃ system.

Conclusions

The thermoelectric characteristics of the (Bi_0.98In_0.02)₂Te_2.7Se_0.3/x%Bi₂Se₃ composite system with composition of 5%, 10%, 15% and 20% are investigated in this paper. The hexagonal crystal structure with a space group of $R\overline{3 }$ m is confirmed by the XRD study. When selenium or tellurium is coated in an atomic layer, secondary particles are observed under a field emission scanning electronic microscope on the sample surface. Every sample displayed semiconducting behaviour across the entire temperature range. The significant impact of grain boundaries on the electrical characteristics of the samples can be seen in the reported electrical resistivity, which results in a larger concentration of scattering centres in the composites. With an increase in bismuth selenide concentration, the bismuth occupancy in interstitial sites becomes less favoured, and the dissolution pattern switches to a substitutional pattern. Hence, there is a twofold decrease in electrical resistivity for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ composition compared to (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃. Potential barrier scattering is a thermal process that raises the Seebeck coefficient of a material because of sintering. The Seebeck coefficients have a Gaussian form at 350°C for all the samples. There is a twofold increase in thermopower for (Bi_0.98In_0.02)₂Te_2.7Se_0.3/20%Bi₂Se₃ relative to (Bi_0.98In_0.02)₂Te_2.7Se_0.3/5%Bi₂Se₃.