Microstructure and thermoelectric properties of Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ alloys fabricated by vacuum melting and hot pressing

Abstract

The Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ alloys (x = 0.02–0.20) were synthesized by vacuum melting and hot pressing methods at 753 K, 60 MPa for 30 min. Effects of Na and In dual partial substitutions for Sb on the thermoelectric properties were investigated from 300 to 500 K. Substituting Sb with Na and In can enhance the Seebeck coefficient effectively near room temperature. The electrical resistivity of the Na and In dual-doping samples is higher within the whole test temperature range. The Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ samples (x = 0.02, 0.06) play a great role in optimizing the thermal conductivity. As for the Bi_0.5 Na_0.02Sb_1.46In_0.02Te₃ alloy, the minimum value of thermal conductivity reaches 0.53 W·m⁻¹·K⁻¹ at 320 K. The thermoelectric performance of the Na and In dual-doped samples is greatly improved, and a figure of merit ZT of 1.26 is achieved at 300 K for the Bi_0.5Na_0.02Sb_1.42In_0.06Te₃, representing 26 % enhancement with respect to ZT = 1.0 of the undoped sample.

1 Introduction

Thermoelectric devices attracted a considerable amount of attention due to their ability of quietly converting waste heat from different sources into electricity or electrical power directly into cooling and heating [1–3]. The performance of thermoelectric materials can be defined by the dimensionless figure of merit ZT = (S ²/ρκ)T, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity and T is the absolute temperature [4]. A good thermoelectric material requires a large Seebeck coefficient, low electrical resistivity and low thermal conductivity. Bi₂Te₃-based alloys are the most important commercial thermoelectric devices near room temperature. In recent years, the fabrication and thermoelectric properties of Bi₂Te₃-based alloys have been investigated by many researchers. For instance, utilizing hydrothermal synthesis, ball milling, rapid solidification and melt spinning, combining hot pressing (HP) [5–7], spark plasma sintering (SPS) [8–10], evacuated- and-encapsulated sintering [11], high-pressure sintering [12], hot extrusion [13], and nanocomposites [14–17] successfully introduced into bulk materials and relatively high thermoelectric performances of Bi₂Te₃-based alloys have been achieved.

Enhancement of the figure of merit is challenging because of the interdependence of physical parameters that define it. It is well known that doping can alter thermoelectric properties of the materials. It is certainly possible that more improvement is achieved based on the appropriately doped Bi₂Te₃-based alloys [18–21]. The In with substitution of three valence electrons for Sb with five valence electrons in the Bi_0.5Sb_1.5Te₃ alloy can be expected to be beneficial to the optimization of the carrier concentration, which results in an improvement of electrical conductivity. Moreover, alkali metal doping can effectively improve TE performance [22]. Up to now, few efforts were made to investigate the dual-doped Bi₂Te₃-based alloys. To study the feasibility of the dual doping in improving thermoelectric properties of the Bi₂Te₃-based bulk materials, a preliminary work on Na and In dual partial substitutions for Sb in the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ alloys was carried out. In this paper, the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02–0.20) and the Bi_0.5Sb_1.5Te₃ samples were fabricated by vacuum melting combining with hot-pressing method. The influences of Na and In dual doping on thermoelectric properties were investigated.

2 Experimental

Elemental powders of Bi (99.99 %), Sb (99.99 %), Te (99.99 %), Na (99.5 %) and In (99.99 %) were weighed in the atomic ratios of Bi_0.5Sb_1.5Te₃ and Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02, 0.04, 0.06, 0.10, 0.20), respectively, They were charged into a quartz tube at 1 × 10⁻³ Pa. The elemental mixtures were melted at 1,073 K for 8 h using a rocking furnace to ensure the composition homogeneity, and then they were cooled to room temperature in the furnace. The obtained ingots were pulverized using the agate mortar in atmospheric environment. The sizes of obtained powders were controlled using the 300-mesh standard sieve. The powders were hot-pressed in the graphite dies under 60 MPa pressure at 753 K in a vacuum of 1 × 10⁻³ Pa for 30 min. Bulk disk-shaped pellets of Φ19.4 mm × 4 mm were fabricated by vacuum hot-pressing method.

The phase structure and cross-section morphology were characterized by X-ray diffraction (XRD, BRUKER, D8ADVANCE with Cu Kα radiation, λ = 0.15406 nm) and scanning electron microscopy (SEM, TESCAN, VEGA II LSU), respectively. The Seebeck coefficient S and the electrical resistivity ρ were measured simultaneously from 300 to 500 K via temperature differential and four-point probe methods in a custom-designed apparatus under vacuum circumstance. The electrical properties were measured perpendicular to the hot-pressing direction. The thermal conductivity κ was calculated from the thermal diffusivity (λ) parallel to the hot-pressing direction; the heat capacity (C _p) was obtained from LFA 457 laser flash apparatus. The thermal conductivity was calculated according to the equation: λ = κ/(DC _p), where D is the density, which is calculated using the sample dimensions and mass. The figure of merit ZT was evaluated according to the equation: ZT = S ² T/ρκ. All measurements were performed in the temperature range of 300–500 K.

3 Results and discussion

3.1 Microstructure

The XRD patterns of the Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ bulk samples are shown in Fig. 1. The XRD patterns of the Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ bulk samples prepared by HP are also consistent with rhombohedral Bi_0.5Sb_1.5Te₃ (JCPDS 49-1713) phase structure. The (00l) peaks including (006), (009), (0015) and (0018) become more intense in the diffraction spectrums. The diffraction spectrums of the bulk samples show the (00l) plane sharp peak with a preferentially oriented perpendicular to hot-pressing direction. The (00l)-oriented corresponds to the rhombohedral crystal structure of Bi₂Te₃ and Sb₂Te₃. We calculated the lattice constants of the samples according to the (006) and (015) peaks, using the following equation:

Fig. 1

XRD patterns of bulk Bi_0.5Sb_1.5Te₃ sample and Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ samples

$$ \frac{1}{{d^{2}_{hkl} }} = \frac{4}{3}\frac{{h^{2} + hk + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }} $$

(1)

The lattice parameters a of the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02–0.20) gradually increase in comparison to those of the Bi_0.5Sb_1.5Te₃ in Table 1. Such a change corresponds to the proposed formation of substitutional defects by Na and In atoms, replacing Sb atoms in their lattice sites. The formation of structural defects by Na and In atoms in the cation sublattice should also be reflected in changes for the crystal lattice parameters. As the bonding radiuses of the Na and In atoms (r _Na = 0.154 nm, r _In = 0.144 nm) are bigger than that of the Sb atom (r _Sb = 0.140 nm), it is possible to expect a bigger volume of unit cell in the crystals containing substitutional defects of Na″_Sb and In″_Sb. It is evident that the volume of unit cell increases with the increase of In concentration. Based on the atomic radius of In and Na, one would expect the increase in the volume of unit cell.

Table 1 Lattice parameters of the Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ alloys

Figure 2a, c indicates the SEM images of the bulk Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.42In_0.06Te₃ alloys from sections parallel to hot-pressing direction. Compared with the SEM images of the Bi_0.5Sb_1.5Te₃, Fig. 2c shows a typical laminated structure composed of micro-nanolayers with the layer thicknesses of about 0.5 μm. Figure 2b and d indicates the SEM images of the bulk Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.42In_0.06Te₃ alloys from sections perpendicular to hot-pressing direction. The polyhedral sheet-like grains are randomly arranged. The Bi_0.5Sb_1.5Te₃ sample is found to have few randomly spaced pores in Fig. 2b

Fig. 2

SEM images of Bi_0.5Sb_1.5Te₃: a from sections parallel to hot-pressing direction and b from sections perpendicular to hot-pressing direction, SEM images of Bi₀₅Na_0.02Sb_1.42In_0.06Te₃: c from sections parallel to hot-pressing direction and d from sections perpendicular to hot-pressing direction

3.2 Thermoelectric properties

The temperature dependence of the Seebeck coefficient, the electrical resistivity, the thermal conductivity and the ZT value for the bulk Bi_0.5Sb_1.5Te₃ and the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02–0.20) alloys are shown in Fig. 3. As shown in Fig. 3a, all the samples exhibit the p-type semiconductor behavior. The Na and In dual doping results in substantial increase in Seebeck coefficient from 300 to 360 K. Compared with the undoped sample, Fig. 3b shows that the electrical resistivity of the Na and In dual-doping samples is larger ranging from 300 to 500 K. From the analysis of Table 1, it is also obvious that Na and In dual substitutions for Sb enlarge the volume of unit cell, which is a further proof that Na and In atoms enter the lattice and occupy the Sb sites. As shown in Fig. 3c, the thermal conductivity of the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02, 0.06) samples is lower than that of the undoped sample within the whole test temperature range. The alkali atoms tend to have soft rattling-type phonon modes, which results in low thermal conductivity in these materials [23]. With the increase in substitution fraction (x = 0.04, 0.10, 0.20), the thermal conductivity of the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ samples is larger than that of the undoped sample. For the Bi_0.5Na_0.02Sb_1.46In_0.02Te₃ specimen, a minimal thermal conductivity of 0.53 W·m⁻¹·K⁻¹ reaches at 320 K. Figure 3d shows temperature dependence of the ZT value. Expect from the Bi_0.5 Na_0.02Sb_1.42In_0.06Te₃ specimen, the ZT values of the other samples are lower than that of the Bi_0.5Sb_1.5Te₃ alloy from 300 to 500 K. Compared with the undoped sample, the Bi_0.5Na_0.02Sb_1.42In_0.06Te₃ sample shows higher ZT values from 300 to 380 K. A ZT of 1.26 is achieved at 300 K for the Bi_0.5Na_0.02Sb_1.42In_0.06Te₃, representing 26 % enhancement with respect to ZT = 1.0 of the undoped sample. The results confirm that appropriate Na and In dual partial substitutions for Sb are effective in enhancing thermoelectric figure of merit of Bi_0.5Sb_1.5Te₃ alloy.

Fig. 3

Temperature dependence of a Seebeck coefficient, b electrical resistivity, c thermal conductivity and d ZT value of bulk Bi_0.5Sb_1.5Te₃ and Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ (x = 0.02–0.20) alloys

To confirm effects of Na and In co-doping on thermoelectric properties of Bi_0.5Sb_1.5Te₃ alloy, thermoelectric properties of Bi_0.5Sb_1.5Te_3, Na-doped Bi_0.5Sb_1.5Te_3, In-doped Bi_0.5Sb_1.5Te₃ and Na–In co-doped Bi_0.5Sb_1.5Te₃ samples were investigated. Figure 4a shows that the Na and In co-doping results in an increase in Seebeck coefficient at near room temperature. Compared with the undoped sample, Fig. 4b shows that the electrical resistivity of the In-doping Bi_0.5Sb_1.5Te₃ has the lowest values from 300 to 500 K. However, the electrical resistivity of the Na-doped Bi_0.5Sb_1.5Te_{3 -}samples is the highest within the whole test temperature range. As shown in Fig. 4c, the thermal conductivity of the co-doped Bi_0.5Sb_1.5Te₃ sample reduces effectively within the whole test temperature range. Figure 4d shows temperature dependence of the ZT value of the four samples. Compared with the undoped sample, the ZT values of the Na-doped Bi_0.5Sb_1.5Te₃ samples have an obvious decrease from 300 to 500 K. The ZT values of In-doped Bi_0.5Sb_1.5Te₃-sample are still lower than that of the Bi_0.5Sb_1.5Te₃ samples at near room temperature. The Na–In co-doped Bi_0.5Sb_1.5Te₃ samples have an increase in the ZT values from 300 to 375 K. It can be concluded that Na and In co-doping are effective in enhancing thermoelectric figure of merit of Bi_0.5Sb_1.5Te₃ alloy.

Fig. 4

Temperature dependence of a Seebeck coefficient, b electrical conductivity, c thermal conductivity and d ZT value of Bi_0.5Sb_1.5Te₃, Bi_0.5Na_0.02Sb_1.48Te_3, Bi_0.5Sb_1.44In_0.06Te₃ and Bi_0.5Na_0.02Sb_1.42In_0.06Te₃ samples

4 Conclusion

Thermoelectric properties of Na and In dual partial substitutions for Sb in the Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ alloys were investigated. The Bi_0.5Na_0.02Sb_1.48−x In _x Te₃ samples result in substantial increase in Seebeck coefficient from 300 to 360 K. The electrical resistivity of the Na and In dual-doped samples is larger within the whole test temperature range. For the Bi_0.5Na_0.02Sb_1.46In_0.02Te₃ specimen, a minimal thermal conductivity of 0.53 W·m⁻¹·K⁻¹ is reached at 320 K. A ZT of 1.26 is achieved at 300 K for the Bi_0.5Na_0.02Sb_1.42In_0.06Te. Thermoelectric properties of Bi_0.5Sb_1.5Te_3, Na-doped Bi_0.5Sb_1.5Te_3, In-doped Bi_0.5Sb_1.5Te₃ and Na–In dual-doped Bi_0.5Sb_1.5Te₃ samples were also investigated. The results further confirm that Na and In dual partial substitutions for Sb are effective in enhancing thermoelectric figure of merit of Bi_0.5Sb_1.5Te₃ alloy.