Abstract
Thermoelectric (TE) materials have attracted more and more attention as they can convert heat into electricity directly[
Among thermoelectric materials, the Pb-Te based[
1 Experimental procedure
Bi(NO3)3·5H2O and thiourea were used as the raw materials to synthesize the Bi2S3 powders. In the typical hydrothermal synthesis, 1.5 g Bi(NO3)3·5H2O and 1.5 g thiourea were dissolved in 40 mL deionized water with continuous stirring, and then transferred to a Teflon-lined stainless-steel autoclave with a capacity of 50 mL. After that the hybrid solution was maintained at 180 ℃ for 24 h, then it was air-cooled to room temperature. The synthesized precipitates of Bi2S3 were filtered and washed with distilled water and ethanol, and dried in air at 60 ℃. Afterwards, appropriate amounts of commercial BiCl3 (99.99%, Alfa Aesar) and Bi2S3 powders with different molar ratios were mixed in 40 mL ethylalcohol with stirring for 2 h, and the molar ratios of BiCl3 were controlled to be 0, 0.25%, 0.5% and 1.0%, respectively. They were sonicated for 30 min, and then the sample was dried in vacuum oven at 60 ℃. Different composite powders were loaded into a graphite die with an inner diameter of 12.6 mm and then sintered at 723 K for 10 min at a heating rate of 100 K/min under an axial compressive stress of 45 MPa in vacuum by using a spark plasma sintering (SPS) system (SPS-211LX). The SPSed specimens were disk-shaped with dimensions of 12.6 mm× 4 mm.
The phase structure was analyzed by X-ray diffraction (XRD) with graphite monochromatized CuKα radiation (λ=0.15418 nm) (D/max-rA). The morphologies of the powders and the bulk samples after SPS were observed by field-emission scanning electron microscopy (FESEM, LEO-1525), and transmission electron microscopy (TEM, JEM-2100).
Thermoelectric properties were measured with specimen surface perpendicular to the pressing direction of SPS. Seebeck coefficient and electrical resistivity were measured by Seebeck coefficient/electric resistance measuring system (ZEM-3). The thermal conductivity was calculated from the product of measured thermal diffusivity, specific heat and density. The thermal diffusivity was measured by the flash method (LFA427, NETZSCH). Before measurement, the sample was coated with a thin layer of graphite by graphite spray (Graphite33) to improve the thermal homogeneity of the sample. The specific heat capacity Cp was determined by differential scanning calorimetry (DSC 404F3, NETZSCH), which is in the range of 0.24-0.29 J·g-1·K-1 between room temperature and 750 K. The density was measured by Archimedes method. The thermal conductivity was calculated via the equation κ=ρDCP (κis the thermal conductivity, ρ is the density, D is the thermal diffusivity, and Cp is the specific heat capacity). Hall coefficients were measured on a home-built system with magnetic fields in the range of 0-1.25 T, utilizing a simple four-contact Hall-bar geometry in both negative and positive polarity to eliminate Joule resistive errors.
2 Results and discussion
Figure 1(a) shows the XRD patterns of BiCl3/Bi2S3 composite powders with different xmol% (x= 0, 0.25, 0.5, and 1.0) BiCl3. The results show that all the patterns correspond to the orthorhombic Bi2S3, indicating that the main phase of all the samples is Bi2S3 without preferred orientation, and no obvious impurity phases are observed as the content of BiCl3 no more than 1.0mol%. The enlarged patterns of 2θ in the range of 24°-32.5° are shown in Fig. 1(b). The ionic radius of Cl- is 0.181 nm, which is slightly smaller than that of S2-(0.184 nm), so the substitution of S2- with Cl- inducesa slight shrinkage of the unit cell, which results in the refined right shift of the major diffraction peaks with BiCl3 doping[
Figure 1.XRD patterns (a) and the enlarged patterns of 2
The FESEM images of Bi2S3 powders and fractured surfaces of sintered samples of Bi2S3 doped with xmol% BiCl3 (x=0, 0.25, 0.5, 1.0) and TEM images of Bi2S3 doped with xmol% (x= 0, 1.0) BiCl3after SPS are shown in Fig. 2. It is observed in Fig. 2(a) that the major morphology of Bi2S3 powders synthesized by the hydrothermal method are spherical with diameter around 3-4 μm and the spherical particles are hierarchical, consisting of fine Bi2S3 nanorods with diameter in the range of 100-200 nm and length in the range of 1-3 μm[
Figure 2.SEM images of Bi2S3 powders (a), fractured surfaces of SPSed samples of Bi2S3 doped with
Figure 3.SEM image of the fractured surfaces of Bi2S3 doped with 1.0mol% BiCl3 bulk after SPS (a), corresponding elemental mappings of Bi, S and Cl (b-d)
Table Infomation Is Not EnableFigure 4 shows the temperature dependence of thermoelectric performances for BiCl3/Bi2S3 composite samples. The negative Seebeck coefficients in Fig. 4(a) indicates that the composites are n-type semiconductors and the major carriers are electrons. The Seebeck coefficient of pure Bi2S3 sample is about -442.0 μV·K-1 at 336 K and the corresponding values of BiCl3/Bi2S3 composites are in the range of -322.0 - -247.3 μV·K-1. The values of carrier concentration at room temperature are displayed in Table 2, which shows that the carrier concentration increases monotonically with increasing BiCl3 content. It is obvious that near room temperature Seebeck coefficient de-creases with increasing amount of BiCl3 due to the increased carrier concentration with addition of BiCl3[
Figure 4.Temperature dependence of thermoelectric performances for BiCl3/Bi2S3 composite samples
(a) Seebeck coefficient; (b) Electrical conductivity; (c) Power factor; (d) Total thermal conductivity; (e) Lattice thermal conductivity; (f) Figure of merit (
The thermal conductivity (κ) of the samples decreases with increasing temperatures, as shown in Fig. 4(d), which is attributed to the strong phonon scattering at high temperatures. The composite sample has a lower κ than that of the pure Bi2S3 as the second phase BiCl3 enhances the phonon scattering[
3 Conclusions
n-type BiCl3/Bi2S3 composite samples were fabricated by hydrothermal method combined with SPS technique. The addition of BiCl3 effectively increased the electrical conductivity and decreased the thermal conductivity of Bi2S3. Bi2S3 doped with 0.5mol% BiCl3 shows a maximum electrical conductivity of 45.1 S·cm-1 at 762 K, which is more than twice higher than that of pure Bi2S3(12.9 S·cm-1), and Bi2S3 doped with 0.25mol% BiCl3 achieves the minimum thermal conductivity of 0.31 W·m-1·K-1 at 762 K, more than 30% decrease as compared with pure Bi2S3. Due to the higher electrical conductivity and lower thermal conductivity, a maximum ZT value of 0.63 is achieved at 762 K for Bi2S3 doped with 0.25mol% BiCl3, a significant enhancement compared to that of pure Bi2S3 (0.22).
References
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