Thermoelectric Properties of Hot-Pressed Bi-Doped n-Type Polycrystalline SnSe
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We report on the successful preparation of Bi-doped n-type polycrystalline SnSe by hot-press method. We observed anisotropic transport properties due to the (h00) preferred orientation of grains along the pressing direction. The electrical conductivity perpendicular to the pressing direction is higher than that parallel to the pressing direction, 12.85 and 6.46 S cm−1 at 773 K for SnSe:Bi 8% sample, respectively, while thermal conductivity perpendicular to the pressing direction is higher than that parallel to the pressing direction, 0.81 and 0.60 W m−1 K−1 at 773 K for SnSe:Bi 8% sample, respectively. We observed a bipolar conducting mechanism in our samples leading to n- to p-type transition, whose transition temperature increases with Bi concentration. Our work addressed a possibility to dope polycrystalline SnSe by a hot-pressing process, which may be applied to module applications.
We have successfully achieved Bi-doped n-type polycrystalline SnSe by the hot-press method.
We observed anisotropic transport properties due to the [h00] preferred orientation of grains along pressing direction.
We observed a bipolar conducting mechanism in our samples leading to n- to p-type transition.
KeywordsThermoelectricity 2D materials SnSe Hot press Bipolar transport
Field emission scanning electron microscopy
Thermoelectric figure of merit
Thermoelectric materials can directly convert waste heat into electricity, which is one of the most important global sustainable energy solutions, or can be used as solid-state Peltier coolers. These thermoelectric devices have exhibited many advantages such as no involvement of moving part, small size, light weight, no noise, no pollution, and long life service. However, their applications are still limited by the economical reasons and low energy conversion efficiency, which is evaluated by the dimensionless thermoelectric figure of merit, ZT = S2σT/κ, where S is the Seebeck coefficient, T is absolute temperature, σ is electrical conductivity, and κ is thermal conductivity. The good thermoelectric material should have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, these three transport coefficients are inter-dependent . There are two main ways to enhance ZT, enhancing power factor (PF, S2σ) or lowering total thermal conductivity. Electrical conductivity and Seebeck coefficient are inversely related each other in most materials, which limit the thermoelectric power factor. Lower thermal conductivity can be achieved by increasing the phonon scattering center or adding a number of interfaces in materials such as superlattices, alloys, nanowires, and nanotubes. Bi2Te3 and PbTe are two traditional thermoelectric materials, whose ZTs are much improved, 1.8 at 320 K for Bi0.5Sb1.5Te3  and 2.2 at 915 K for PbTe + 2%Na + 4%SrTe . However, there are many disadvantages for the systems because Bi and Te elements are rare on the earth, resulting in increase of costs with the development of the LED industry , and lead is a toxic element. Therefore, it is necessary to explore economical and non-toxic (lead-free) alternative materials for thermoelectric applications.
IV–VI compound semiconductor SnSe is a robust candidate for thermoelectric conversion applications, which has been recently reported with high thermoelectric performance, ZT = 2.6 at 923 K in un-doped p-type and ZT = 2.0 at 773 K in intentionally hole-doped SnSe single crystal [5, 6]. Recently, we achieved ZT = 2.2 in n-type Bi-doped SnSe single crystal . These high ZT values are attributed to the ultralow intrinsic thermal conductivity due to the long-range interaction along the <100> direction caused by resonant bonding, leading to optical phonon softening, strong anharmonic scattering and large phase space for three-phonon scattering processes . Bulk SnSe belongs to orthorhombic Pnma space group (a = 11.49 Å, b = 4.44 Å, c = 4.14 Å) with an indirect band gap energy of Eg = 0.829 eV at 300 K. When temperature is increased, it changes to orthorhombic Cmcm space group (a = 11.71, b = 4.31, and c = 4.32 Å) with a direct band gap of Eg = 0.464 eV around 807 K . SnSe exhibits a two-dimensional (2D) layered structure, where each Sn atom is surrounded by a highly distorted octahedron of Se atoms to form a zigzag structure. Along the b-c plane, there is a strong Sn–Se covalent bonding, and along the a-axis, there is a weak van der Waals force, which gives a strong anisotropic transport and very weak mechanical properties. The most common technique to fabricate single-crystal SnSe is the Bridgman technique which is quite specific and hard to produce in industry scale-up . Considering the large-scale applications and the poor mechanical properties in layered material, polycrystalline SnSe is a possible solution.
Recently, un-doped p-type polycrystalline SnSe has been reported with ZT = 0.5 at 823 K and ZT = 1.3 at 850 K for rock-salt SnSe, and doped p-type SnSe has been reported with the highest ZT = 0.6 at 750 K for Ag dopant [1, 10, 11]. Polycrystalline n-type SnSe has been reported with the ZT range from 0.6 to 1.2 for Te, I, BiCl3, and Br dopants [4, 12, 13, 14]. Hot pressing and spark plasma sintering (SPS) are the most general techniques used to fabricate a polycrystalline of un-doped and doped SnSe.
Here we report on the successful preparation of Bi-doped n-type polycrystalline SnSe by hot-press method. We observed anisotropic transport properties due to the (h00) preferred orientation of grains along pressing direction. We also observed a bipolar conducting mechanism in our samples leading to n- to p-type transition, whose transition temperature increases with Bi concentration.
The aim of this paper is fabricating and investigating thermoelectric properties of n-type Bi-doped SnSe polycrystalline with various Bi concentrations (0, 2, 4, 6, and 8%). The doping process is fulfilled by mixing and hot-pressing SnSe with Bi powders. The details of fabrications and characterizations of the samples are as below.
Fabrication of SnSe Compound by Temperature Gradient Technique
We fabricated the SnSe compound using the temperature gradient technique. The high-purity (99.999%) Sn and Se powders were weighed in an atomic ratio of 1:1 using a balance with a resolution of 10−4 g. The powders were mixed and sealed in an evacuated (< 10−4 Torr) quartz ampoule. The ampoule was then sealed in another evacuated bigger quartz ampoule in order to prevent the sample from oxidation by air in the case when the inner ampoule is broken owing to the difference of thermal expansion between the crystal and quartz. The ampoules were slowly heated up to 600 °C for 30 h. It was maintained at this temperature for 1 h and then continuously heated up to 950 °C for 35 h. To complete the reaction between Sn and Se, we maintained the ampoules at this temperature for 16 h and then slowly cooled down to room temperature. An excellent SnSe compound with dimensions of 13 mm diameter × 25 mm length was obtained.
Fabrication of n-Type Bi-Doped SnSe Polycrystalline Samples by Hot-Press Technique
The obtained ingots above were ground into powders and mixed with various Bi (0, 2, 4, 6, and 8%) amounts for 1 h using a mixing machine. The mixed powder was loaded into a 13-mm diameter mold and then hot-pressed at 800 °C using 30 MPa pressure in Ar environment for 30 min to form a dense pellet with a 13-mm diameter and 15-mm length.
The samples were analyzed by X-ray diffraction (XRD) both parallel and perpendicular to the pressing direction. Field emission scanning electron microscopy (FE-SEM) was used to observe the microscopic image in the fractured surface of the samples. To probe the anisotropic transport and thermoelectric properties, the samples were cut into 2 × 1.5 × 8 mm bars for transport and 13 × 13 × 1.5 mm for thermal diffusivity measurements along both parallel (//) and perpendicular (⊥) directions using a diamond saw. Electrical conductivity and the Seebeck coefficient were simultaneously collected from room temperature to 773 K with a collinear four-probe configuration under an Ar atmosphere to prevent oxidation and evaporation of sample. The laser flash diffusivity method (model: LFA-457, NETZSCH, Germany) was used to determine thermal diffusivity from room temperature to 773 K. Mass density was determined by measuring the sample’s dimensions and mass. Heat capacity was taken from Sassi’s work for polycrystalline SnSe . Thermal conductivity was calculated by the relationship κ = DCpρ, where D, Cp, and ρ are the thermal diffusivity, the heat capacity, and the mass density, respectively.
Results and Discussion
In conclusion, polycrystalline SnSe has been doped with various Bi concentrations by hot-press method (Additional file 1). The samples exhibited the layered structure with a preferential (h00) orientation. An anisotropic transport and thermoelectric properties have been observed. The electrical conductivities perpendicular to the pressing direction (12.85 S cm−1) are higher than those parallel to the pressing direction (6.46 S cm−1) at 773 K for SnSe:Bi 8% sample, while thermal conductivities perpendicular to the pressing direction (0.81 W m− 1 K−1) are higher than those parallel to the pressing direction (0.60 W m−1 K−1) at 773 K for SnSe:Bi 8% sample. We observed a bipolar conducting mechanism in our samples leading to n- to p-type transition, whose temperature increases with Bi concentration. The optimum Bi doping concentration was 6% with the highest ZT value of 0.025 at 723 K. This ZT value is quite low due to the low electrical conductivity and Seebeck coefficient. Our work addressed a possibility to dope polycrystalline SnSe by a hot-pressing process, which may be applied to module applications.
There is no acknowledgment.
This work was supported by the National Research Foundation of Korea [NRF-2009-093818 and NRF-2014R1A4A1071686] and by the Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry and Energy (MOTIE) (Project No. 10050296, Large scale (Over 8″) synthesis and evaluation technology of two-dimensional chalcogenides for next-generation electronic devices). This work was partially supported by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2016.09.
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article (and its Additional file 1).
VQN, THN, VTD, and ATD synthesized the samples and performed the thermoelectric transport measurements. JEL and SDP measured the thermal conductivity, SC initiated the study and edited the manuscript, and JYS and HMP characterized the crystal structure of samples. VQN wrote the paper with discussion and comments from all the authors. SC supervised the project. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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