稀有金属(英文版) 2015,34(11),770-775

收稿日期：22 March 2013

基金：financially supported by the National Natural Science Foundation of China (No. 51161009);the Research Project of Jiangxi Provincial Education Department (No. GJJ13722 and GJJ11615);

Microstructure and thermoelectric properties of Bi_0.5Na_0.02Sb_1.48-xIn_xTe₃ alloys fabricated by vacuum melting and hot pressing

Xing-Kai Duan Kong-Gang Hu Da-Hu Ma Wang-Nian Zhang Yue-Zhen Jiang Shu-Chao Guo

Center for New Energy Materials Research, School of Mechanical and Materials Engineering, Jiujiang University

Abstract：

The Bi_0.5Na_0.02Sb_1.48-xIn_xTe₃ 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-xIn_xTe₃ samples(x = 0.02,0.06) play a great role in optimizing the thermal conductivity.As for the Bi_0.5Na_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 dualdoped 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.

Keyword：

Microstructure; Dual doping; Hot pressing; Thermal conductivity; Thermoelectric properties;

Author： Xing-Kai Duan e-mail: duanxingkai@163.com;

Received： 22 March 2013

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 per- formance of thermoelectric materials can be defined by the dimensionless figure of merit ZT = (S²/qj)T, where S is the Seebeck coefficient, q is the electrical resistivity, j is the thermal conductivity and T is the absolute temperature [4]. A good thermoelectric material requires a large See- beck 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 proper- ties of Bi₂Te₃-based alloys have been investigated by many researchers. For instance, utilizing hydrothermal synthesis, ball milling, rapid solidification and melt spinning, com- bining 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 thermo- electric properties of the materials. It is certainly possible that more improvement is achieved based on the appro- priately 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_1.5Te₃alloy can be expected to be beneficial to the optimization of the carrier concen- tration, 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 ther- moelectric properties of the Bi₂Te₃-based bulk materials, a preliminary work on Na and In dual partial substitutions for Sb in the Bi0.5Na0.02Sb1.48-xInxTe3alloys was carried out. In this paper, the Bi0.5Na0.02Sb1.48-xInxTe3(x = 0.02–0.20) and the Bi_0.5Sb_1.5Te₃samples were fabricated by vacuum melting combining with hot-pressing method. The influ- ences 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 Bi0.5Sb1.5Te3and Bi0.5Na0.02 Sb_1.48-xIn_xTe₃(x = 0.02, 0.04, 0.06, 0.10, 0.20), respec- tively, They were charged into a quartz tube at 1 9 10^-3Pa. 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 environ- ment. The sizes of obtained powders were controlled using the 300-mesh standard sieve. The powders were hot-pres- sed in the graphite dies under 60 MPa pressure at 753 K in a vacuum of 1 9 10^-3Pa for 30 min. Bulk disk-shaped pellets of U19.4 mm 9 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 Ka radiation, k = 0.15406 nm) and scanning electron microscopy (SEM, TESCAN, VEGA II LSU), respectively. The Seebeck coefficient S and the electrical resistivity q were measured simulta- neously 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 j was calculated from the thermal diffusivity (k) 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: k = j/(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 = S²T/qj. All measurements were performed in the temperature range of 300–500 K.

3 Results and discussion

3.1 Microstructure

The XRD patterns of the Bi0.5Sb1.5Te3and the Bi0.5Na0.02 Sb_1.48-xIn_xTe₃bulk samples are shown in Fig. 1. The XRD patterns of the Bi0.5Sb1.5Te3and the Bi0.5Na0.02Sb1.48-x In_xTe₃bulk samples prepared by HP are also consistent with rhombohedral Bi_0.5Sb_1.5Te₃(JCPDS 49-1713) phase struc- ture. 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 Bi₂Te₃and Sb₂Te₃. We calculated the lattice constants of the samples according to the (006) and (015) peaks, using the following equation:

The lattice parameters a of the Bi0.5Na0.02Sb1.48-xInxTe3 (x = 0.02–0.20) gradually increase in comparison to those of the Bi_0.5Sb_1.5Te₃in 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 Na00_Sband In00_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.

Figure 2a, c indicates the SEM images of the bulk Bi0.5Sb1.5Te3and the Bi0.5Na0.02Sb1.42In0.06Te3alloys 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-nanolayerswith the layer thicknesses of about 0.5 lm. Figure 2b and d indicates the SEM images of the bulk Bi_0.5Sb_1.5Te₃and the Bi0.5Na0.02Sb1.42In0.06Te3alloys from sections perpendic- ular to hot-pressing direction. The polyhedral sheet-like grains are randomly arranged. The Bi_0.5Sb_1.5Te₃sample is found to have few randomly spaced pores in Fig. 2b

Fig. 1 XRD patterns of bulk Bi0.5Sb1.5Te3sample and Bi0.5Na0.02Sb1.48-xInxTe3samples

Table 1 Lattice parameters of the Bi0.5Sb1.5Te3and the Bi0.5Na0.02Sb1.48-xInxTe3alloys  下载原图

Table 1 Lattice parameters of the Bi0.5Sb1.5Te3and the Bi0.5Na0.02Sb1.48-xInxTe3alloys

3.2 Thermoelectric properties

The temperature dependence of the Seebeck coefficient, the electrical resistivity, the thermal conductivity and the ZT value for the bulk Bi0.5Sb1.5Te3and the Bi0.5Na0.02 Sb_1.48-xIn_xTe₃(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 Bi0.5Na0.02Sb1.48-xInxTe3(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 Bi0.5Na0.02Sb1.48-xInxTe3 samples is larger than that of the undoped sample. For the Bi0.5Na0.02Sb1.46In0.02Te3specimen, a minimal thermal conductivity of 0.53 W m^-1K^-1reaches at 320 K. Figure 3d shows temperature dependence of the ZT value. Expect from the Bi0.5Na0.02Sb1.42In0.06Te3specimen, the ZT val- ues of the other samples are lower than that of the Bi_0.5Sb_1.5Te₃alloy from 300 to 500 K. Compared with the undoped sample, the Bi0.5Na0.02Sb1.42In0.06Te3sample shows higher ZT values from 300 to 380 K. A ZT of 1.26 is achieved at 300 K for the Bi0.5Na0.02Sb1.42In0.06Te3, repre- senting 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_1.5Te₃alloy.

Fig. 2 SEM images of Bi0.5Sb1.5Te3: a from sections parallel to hot-pressing direction and b from sections perpendicular to hot-pressing direction, SEM images of Bi05Na0.02Sb1.42In0.06Te3: c from sections parallel to hot-pressing direction and d from sections perpendicular to hot- pressing direction

To confirm effects of Na and In co-doping on thermo- electric properties of Bi_0.5Sb_1.5Te₃alloy, thermoelectric properties of Bi0.5Sb1.5Te3,Na-doped Bi0.5Sb1.5Te3,In- doped Bi0.5Sb1.5Te3and Na–In co-doped Bi0.5Sb1.5Te3 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_1.5Te₃has the lowest values from 300 to 500 K. However, the electrical resistivity of the Na-doped Bi0.5Sb1.5Te3 -samples is the highest within the whole test temperature range. As shown in Fig. 4c, the thermal con- ductivity of the co-doped Bi_0.5Sb_1.5Te₃sample reduces effectively within the whole test temperature range. Fig- ure 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_1.5Te₃samples 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_1.5Te₃samples at near room temperature. The Na–In co-doped Bi_0.5Sb_1.5Te₃samples 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 Bi0.5Sb1.5Te3alloy.

Fig. 3 Temperature dependence of a Seebeck coefficient, b electrical resistivity, c thermal conductivity and d ZT value of bulk Bi0.5Sb1.5Te3and Bi0.5Na0.02Sb1.48-xInxTe3(x = 0.02–0.20) alloys

Fig. 4 Temperature dependence of a Seebeck coefficient, b electrical conductivity, c thermal conductivity and d ZT value of Bi0.5Sb1.5Te3, Bi0.5Na0.02Sb1.48Te3,Bi0.5Sb1.44In0.06Te3and Bi0.5Na0.02Sb1.42In0.06Te3samples

4 Conclusion

Thermoelectric properties of Na and In dual partial sub- stitutions for Sb in the Bi0.5Na0.02Sb1.48-xInxTe3alloys were investigated. The Bi0.5Na0.02Sb1.48-xInxTe3samples 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 tem- perature range. For the Bi0.5Na0.02Sb1.46In0.02Te3speci- men, a minimal thermal conductivity of 0.53 W m^-1K^-1is reached at 320 K. A ZT of 1.26 is achieved at 300 K for the Bi0.5Na0.02Sb1.42In0.06Te. Thermoelectric properties of Bi0.5Sb1.5Te3,Na-dopedBi0.5Sb1.5Te3,In-doped Bi0.5Sb1.5Te3and Na–In dual-doped Bi0.5Sb1.5Te3samples 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 Bi0.5Sb1.5Te3 alloy.