SnSe + Ag2Se composite engineering with ball milling for enhanced thermoelectric performance
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Earth-abundant IV–VI semiconductor SnSe is regarded as a promising thermoelectric material due to its intrinsic low thermal conductivity. In this report, the highly textured SnSe/Ag2Se composites were first designed by solid solution method followed by spark plasma sintering (SPS) and their thermoelectric properties in two directions were investigated, and then, the performance of composites was further optimized with an additional ball milling. The coexistence of SnSe and Ag2Se phases is clearly confirmed by energy-dispersive X-ray spectroscopy (EDX) in transmission electron microscopy (TEM). After ball milling, the size of SnSe grains as well as the incorporated Ag2Se particles reduces effectively, which synergistically optimizes the electrical and thermal transport properties at high temperature range. As a result, a maximum ZT of ~ 0.74 at 773 K for SnSe + 1.0%Ag2Se in the direction vertical to the pressing direction is achieved. Composite engineering with additional ball milling is thus proved to be an efficient way to improve the thermoelectric properties of SnSe, and this strategy could be applicable to other thermoelectric systems.
KeywordsThermoelectrics SnSe Composite engineering Ball milling
Thermoelectric (TE) materials, which could directly convert heat into electrical energy and vice versa, are deemed as promising materials for applications in thermoelectric power generators and refrigerators [1, 2, 3]. Generally, the TE conversion efficiency is depended on the figure of merit (ZT) defined as ZT = S 2 σT/κ, where S, σ, T, κ and S 2 σ are the Seebeck coefficient, electrical conductivity, absolute temperature, thermal conductivity and power factor, respectively. The most common approaches for pursuing high ZT values include maximizing high power factor by band engineering, such as resonant doping , modulation doping  and band convergence/degeneracy [3, 6], or minimizing the thermal conductivity through all-scale hierarchical structuring , nano-structuring , etc. The widely investigated IV–VI semiconductors lead chalcogenides (PbTe, PbSe and PbS) performed the best TE properties at medium temperature range from 573 to 773 K [9, 10, 11, 12, 13, 14, 15]. Besides, SnTe compound, which possesses similar crystal structure and valence band characteristics with PbTe and owns narrow band gap (E g ~ 0.3 eV), also shows potential to be a good thermoelectric material [16, 17, 18, 19, 20]. However, the use of Pb and Te elements limits their further development due to the toxicity and high cost.
Tin selenide (SnSe), another IV–VI compound, crystalizes with a layered structure with orthorhombic symmetry (Pnma space group) and undergoes a phase transition at ~ 800 K to a higher symmetry orthorhombic symmetry (Cmcm space group). SnSe could be a topological crystalline insulator at low temperature , and devices based on this materials have been widely applied, such as photovoltaic applications , memory switching devices  and anode materials for lithium batteries . In recent years, SnSe is regarded as a promising thermoelectric material since a record high ZT of ~ 2.6 was achieved in its single crystal along b-axis at 923 K , and also the highest ZT dev ~ 1.34 was realized in hole-doped single-crystalline SnSe from 300 to 773 K . However, the time-consuming and poor mechanical properties seriously prevented the practical use of SnSe single crystals. The study on polycrystalline SnSe thus attracted lots of attentions [27, 28, 29, 30, 31, 32], although the TE performance of pristine polycrystalline SnSe is not competitive, which is mainly due to the poor electrical transport properties caused by the intrinsic low carrier concentration of ~ 1 × 1017 cm−3 . In order to optimize the carrier concentrations and improve the TE properties of polycrystalline SnSe, trials of doping and alloying were conducted hitherto. For instance, substituting Sn with Ag is proved to be an effective way to introduce p-type carriers, and the ZT value of Ag0.01Sn0.99Se is found to be 0.6 at 750 K ; alkali metal such as Na or K has been introduced in SnSe to lift up the hole carrier concentrations, leading to a peak ZT ~ 0.8 in Na0.015Sn0.985Se and ~ 1.1 in K0.01Sn0.99Se at 773 K, respectively [28, 32]. In addition, Na and Te co-doped p-type Sn0.99Na0.01Se0.84Te0.16 obtained a maximum ZT of 0.72 at 773 K due to the reduced thermal conductivity by alloy scattering of phonons and increased carrier concentration by Na-doping . For S and I co-doped n-type SnSe0.87S0.1I0.03 sample, a maximum ZT ~ 1.0 at 773 K is obtained benefiting from the lower thermal conductivity and enhanced Seebeck coefficient .
Moreover, composite engineering is also considered as an effective method to optimize TE materials [36, 37]. SnSe composite with SiC particles formed phonon-scattering center, leading to a decreased thermal conductivity and a maximum ZT of 0.125 at 300 K . On the other hand, SnSe composites with 2D MoSe2 improved electrical properties and a ZT of 0.5 at 773 K was obtained . Considering the superior performance of Ag doping and huge potential profit of composite engineering in SnSe, in this contribution, we investigated the TE properties of SnSe/Ag2Se composites. The prepared anisotropic polycrystalline samples were measured in two different directions, and the incorporation of Ag2Se is proved to be an effective method to enhance the TE properties of SnSe, especially in the direction vertical to the pressing direction. The two samples with a better performance were further optimized by adding a ball milling processing after solid solution, resulting in higher carrier concentration and lower thermal conductivity simultaneously in high temperature range, leading to a maximum ZT of ~ 0.74 for SnSe + 1%Ag2Se at 773 K. This value is higher than any other Ag-doped polycrystalline SnSe at the same temperature [27, 40, 41].
The reagents used are Sn powder (99.5%, Aladdin), Se powder (99+%, Alfa Aesar), Ag2Se powder (Aldrich).
Samples with nominal compositions of SnSe + x mol%Ag2Se (x = 0, 0.5, 1.0 and 3.0) were synthesized by the solid solution method with raw materials loaded and sealed into evacuated quartz ampoules (~ 1 × 10−4 Pa), then slowly heated to 950 °C in 9.5 h, soaked at this temperature for 24 h and slowly cooled down to room temperature. The ingots were ground into powders by hand milling at first (Samples labeled by HM). Then, SnSe + x%Ag2Se (x = 0.5, 1.0) were refined by a planetary ball mill (QM-3SP2, Nanjing University, China) in a nitrogen atmosphere for further optimization (Samples labeled by BM). In the ball milling process, stainless steel vessels and balls were used, and the weight ratio of ball to powder was 20:1. Then the obtained powders were consolidated using spark plasma sintering (SPS-211Lx, Japan) at 723 K for 5 min with a 50 MPa uniaxial pressure.
2.3 X-ray diffraction
The phase structures were investigated by X-ray diffractometer (XRD, Rigaku, Tokyo, Japan) at a scanning rate of 4 (°)·min−1.
2.4 Thermoelectric properties
The Seebeck coefficient and the electrical resistance were simultaneously measured using an Ulvac Riko ZEM-3 instrument under a rare helium atmosphere. The thermal conductivity was calculated by κ = DC p ρ, where the thermal diffusivity coefficient (D) was measured using the laser flash diffusivity method in a Netzsch LFA457 (NETZSCH, LFA457, Germany), the density (ρ) was determined by the Archimedes method and C p is the specific heat capacity obtained from previous research .
2.5 Electron microscopy
The microstructure was investigated by field emission scanning electron microscope (FESEM, Zeiss Merlin, Germany) with operation voltage of 5 kV, and transmission electron microscope (TEM) investigations were carried out using a FEI Tecnai F30 microscope operated at 300 kV in South University of Science and Technology of China. The thin TEM specimens were prepared by conventional standard methods. The procedures include cutting, grinding, dimpling, polishing and Ar-ion milling on a liquid nitrogen cooling state subsequently.
2.6 Hall measurement
The Hall coefficients (R H) were measured by the van der Pauw method on a commercial Hall effect measurement system (Lake Shore 8400 Series). Hall carrier concentration (n H) was then estimated to be equal to 1/eR H, and Hall carrier mobility (μ H) was calculated according to the equation: μ H = R H σ.
3 Results and discussion
Carrier concentration and Hall mobility of SnSe + x%Ag2Se (x = 0, 0.5, 1.0 and 3.0) conducted by hand milling and measured parallel and vertical to pressing direction
Carrier concentration/(1019 cm−3)
SnSe + 0.5%Ag2Se
SnSe + 1%Ag2Se
SnSe + 3%Ag2Se
The temperature dependence on Seebeck coefficient is shown in Fig. 2b. The positive Seebeck coefficient values in the entire temperature range for all the samples, indict p-type semiconductor behaviors (the dominant charge carrier is hole), which are originated from easily produced Sn vacancies. It is noticed that the Seebeck coefficient of composited samples first increases from 323 to ~ 600 K, but slightly decreases in high temperature, which is possibly due to the bipolar effect. The rising temperature promotes minority carrier jumps across the band gap and offsets the majority ones, leading to the increase in carrier concentration. Moreover, Ag2Se achieves higher carrier concentration compared with SnSe ; thus, the Seebeck coefficient of composited samples is basically lower than pristine SnSe.
Attributed to the remarkable enhancement in electrical conductivity and comparable Seebeck coefficient, the composite samples achieve higher power factor in vertical direction than that in parallel direction as shown in Fig. 2c. The tendency of the temperature-dependent power factor is similar to the electrical conductivity, and all the composite samples show better performance in moderate temperature (around 423 K). In this system, SnSe + 0.5%Ag2Se exhibits the highest power factor over the entire temperature range, and the peak value of ~ 573 μW·m−1·K−2 is obtained at 423 K.
Figure 2d shows the total thermal conductivity of SnSe + x%Ag2Se (x = 0, 0.5, 1.0 and 3.0) in two directions. All the samples exhibit a decreasing trend from 323 to 723 K, and due to the higher mobility, samples measured in vertical direction achieve higher thermal conductivity. As known, the total thermal conductivity (κ tot) consists of electronic thermal conductivity (κ ele) and lattice thermal conductivity (κ lat). The electronic thermal conductivity can be calculated by Wiedemann–Franz law, κ ele = LσT, where L is the Lorenz number, σ is the electrical conductivity and T is the operating temperature; then, the lattice thermal conductivity is obtained by subtracting electronic thermal conductivity from total thermal conductivity, κ lat = κ tot − κ ele. The lattice thermal conductivity is plotted in Fig. 2e. As shown in Fig. 2a, although compositing with Ag2Se effectively enhances the electrical conductivity, the value is still relatively low, leading to negligible electrical thermal conductivities. Thus, the lattice thermal conductivity is dominant in total thermal conductivity.
Benefiting from the better electrical transport properties, samples in vertical direction exhibit higher ZT values as shown in Fig. 2f, and SnSe + x%Ag2Se (x = 0.5, 1.0) samples express the highest TE properties in vertical direction. However, the maximum values are only about 0.45 at 723 K. Thus, an additional ball milling process was performed to further optimize the TE properties of SnSe + x%Ag2Se (x = 0.5, 1.0) composites.
As a result, BM samples achieve higher power factor and lower thermal conductivity at the same time at high temperature (673–773 K) than HM samples, leading to much higher ZT value as plotted in Fig. 4f. In particular, ZT value of SnSe + 1.0%Ag2Se sample reaches a maximum of ~ 0.74 at 773 K, which is much higher than 0.45 for pristine SnSe at same temperature and 23% higher than Ag-doped SnSe (~ 0.6 at 750 K) in previous report .
To better understand the transport properties of SnSe + x%Ag2Se (x = 0.5, 1.0) grounded by HM and BM, the carrier concentration and mobility of these four samples were measured in the vertical direction using the van der Pauw method. As shown in Fig. 5a, the carrier concentration of both HM and BM samples are significantly enhanced at high temperature, possibly because some of the composited Ag2Se dissolves in the SnSe matrix at high temperature, which effectively increases the carrier concentration. Compared with hand milling, ball milling creates more boundaries and facilitates Ag2Se dissolving into the matrix, as shown in Fig. 6. Thus, BM samples achieve higher carrier concentration at high temperature. In addition, ball milling effectively decreases the grain size as shown in SEM images in Fig. 3, leading to denser boundaries and lower texture degree; thus, BM samples show lower mobility in the vertical direction from 323 to ~ 600 K.
In summary, SnSe/x%Ag2Se (x = 0, 0.5, 1.0 and 3.0) composites with highly textured features were first prepared by solid solution and SPS to investigate their thermoelectric properties from 323 to 723 K. SnSe + x%Ag2Se (x = 0.5, 1.0) expressed better TE performance were further optimized with an additional ball milling proceeded after solid solution. As a result, the grain size significantly decreases, and the electrical conductivity and thermal conductivity are optimized simultaneously in the high temperature range, leading to a maximum ZT of ~ 0.74 at 773 K for SnSe + 1.0%Ag2Se in the vertical direction. This value is much higher than that of HM sample of ~ 0.45 for pristine SnSe and 23% higher than that of Ag-doped SnSe (0.6 for Ag0.01Sn0.99Se at 750 K) . Thus, compositing Ag2Se coupled with ball milling is deemed as an effective method to improve thermoelectric performance of SnSe, and this approach could be applicable to other thermoelectric systems for further optimization.
This work was financially supported by the National Science Foundation (No. DMR-1410636), the Natural Science Foundation of Guangdong Province (No. 2015A030308001), the Leading Talents of Guangdong Province Program (No. 00201517), the Science, Technology and Innovation Commission of Shenzhen Municipality (Nos. JCYJ20150831142508365, KQTD2016022619565991 and KQCX2015033110182370) and the National Natural Science Foundation of China (No. 51632005). This work was also supported by Project funded by China Postdoctoral Science Foundation.
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