Method for optimizing polycrystalline P-type tin selenide-based thermoelectric material by using grain boundary regulation and defect engineering

Abstract

The invention belongs to the technical field of preparation of high-performance thermoelectric materials, and particularly relates to a method for optimizing a polycrystalline P-type tin selenide-based thermoelectric material by using grain boundary regulation and defect engineering. In order to improve the thermoelectric performance of the tin selenide-based thermoelectric material, the polycrystalline SnSe material is synthesized by using a molten salt medium assisted melting method. According to the method, the growth of the grain size is realized, the carrier concentration is improved by constructing a high-density small-angle grain boundary and introducing Sn vacancy defects with positive charges, and the average electric transport performance of the polycrystalline tin selenide-based thermoelectric material is greatly improved in the two aspects. And meanwhile, the carrier mobility is further improved by introducing an Ag element, so that the electrical property of the SnSe material in a medium-low temperature region is further improved. Moreover, the introduction of the low Ag content can inhibit the high-temperature thermal conductivity of the material, thereby achieving the collaborative optimization of electric heating, and further improving the overall thermoelectric performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-performance thermoelectric material preparation, and specifically relates to a method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by utilizing grain boundary control and defect engineering. Background Art

[0002] With the acceleration of global industrialization and the continued growth of the population, the demand for energy for industrial production and daily life is rapidly increasing. Currently, traditional fossil fuels, such as coal, oil, and natural gas, not only face the severe challenge of resource depletion but also contribute to increasingly severe environmental degradation. Furthermore, the energy utilization rate of burning fossil fuels is relatively low, typically reaching only 30%-40%, with the remainder largely dissipated as heat. Thermoelectric materials, on the other hand, generate electricity through temperature differences, converting natural heat sources and various low-grade thermal energy sources into electricity over a wide temperature range, meeting today's demand for improved energy efficiency. Leveraging the Seebeck, Peltier, and Thomson effects, thermoelectric materials can achieve bidirectional heat-to-electricity conversion—converting heat into electricity while also providing electro-cooling capabilities. They are currently being applied in waste heat power generation, intelligent temperature control, and chip cooling. Among them, the emerging tin selenide-based thermoelectric materials show significant advantages: not only do they have high thermoelectric conversion performance, but their constituent elements (tin and selenium) are abundant in the earth's crust and inexpensive, and they have both environmentally friendly properties and low-cost preparation advantages.

[0003] However, polycrystalline tin selenide materials have a highly random grain orientation, forming a high-density grain boundary structure, which exacerbates the carrier scattering effect and ultimately leads to low average thermoelectric performance. Common methods for improving carrier mobility include symmetry regulation, band engineering, and lattice sizing. These methods require the precise introduction of multiple doping elements, which can easily lead to ionized impurity scattering or the introduction of impurity phases, which is not conducive to carrier transport; at the same time, the process is complex, resulting in a long time consumption and not conducive to large-scale preparation. Therefore, there is an urgent need to develop a new synthesis process to achieve improved thermoelectric performance of tin selenide-based thermoelectric materials. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a method for preparing polycrystalline P-type tin selenide-based thermoelectric materials using a molten salt medium-assisted vacuum melting method, which can improve the thermoelectric properties of tin selenide by regulating the grain boundaries, thereby solving the dilemma of existing polycrystalline tin selenide-based thermoelectric materials in improving the mobility.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by utilizing grain boundary control and defect engineering, the method comprising the following steps:

[0007] S1. Place tin particles (Sn), selenium particles (Se) and inert salt powder in a vacuum container and make ingots through melting reaction.

[0008] S2. Grinding the ingot into powder, and then performing spark plasma sintering on the powder to obtain an optimized P-type polycrystalline SnSe thermoelectric material.

[0009] During the implementation of steps S1 and S2, oxygen must be isolated to prevent the formation of harmful SnO phases that deteriorate thermoelectric performance.

[0010] Preferably, S1 is to place tin particles, selenium particles, silver wires and inert salt powder in a vacuum container and make an ingot through a melting reaction.

[0011] Preferably, the molar ratio of tin particles to selenium particles is 1-2:1-2, and the mass ratio of inert salt to the total amount of tin particles and selenium particles is 1-2:1-2.

[0012] Preferably, the tin particles are ultrapure tin particles with a purity of ≥99.99%, the selenium particles are ultrapure selenium particles with a purity of ≥99.999%, and the inert salt is ultrapure powder with a purity of ≥99.99%.

[0013] Preferably, the inert salt is sodium chloride.

[0014] Preferably, the molar ratio of the tin particles, selenium particles and silver wire is 0.99-0.9975:1:0.0025-0.01, and the silver wire is a high-purity silver wire with a purity of ≥99.99%.

[0015] The raw materials (tin particles, selenium particles, silver wire and inert salt powder) were weighed in a glove box filled with inert gas (the gas pressure was not less than 1 atm).

[0016] Preferably, the vacuum degree in the vacuum container is less than 10 -4 Pa.

[0017] Preferably, the melt reaction is carried out in a double-layer quartz tube.

[0018] Preferably, the melting reaction is to heat up to 950±50°C at a rate of 1-2°C / min, keep the temperature for 720±60mins, then cool down to 800°C at a rate of 0.1-0.2°C / min, and then cool to room temperature in the furnace.

[0019] Preferably, after obtaining the ingot, it is placed in water for ultrasonic cleaning to remove NaCl attached to the surface, and then wiped dry with dust-free paper and surface polished with 2000-grit sandpaper.

[0020] Preferably, the spark plasma sintering is pulse power pressure sintering performed at a temperature of 500±50° C. and a pressure of 60±5 MPa for 10±3 min.

[0021] The present invention also provides a polycrystalline P-type tin selenide-based thermoelectric material prepared by the above method.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] To enhance the thermoelectric performance of tin selenide-based thermoelectric materials, the present invention synthesized polycrystalline SnSe materials using a molten salt medium-assisted melting method. This method achieves grain size growth, increases carrier concentration by constructing high-density small-angle grain boundaries, and introduces positively charged Sn vacancy defects. These two aspects significantly improve the average electrical transport performance of polycrystalline tin selenide-based thermoelectric materials. Furthermore, by further introducing the Ag element to increase carrier mobility, the electrical performance of the SnSe material in the medium and low temperature regions is further improved, with an average power factor of 7.2 μW m -1 ·K -2 This also demonstrates the high scalability of the molten salt method in the preparation of thermoelectric materials and its potential for further promotion and application. Furthermore, the present invention also discovered that a low Ag content can suppress the material's high-temperature thermal conductivity, achieving synergistic optimization of electricity and heat, thereby improving the overall thermoelectric performance, achieving a thermoelectric figure of merit zT of 1.4 and an average zT of 0.7.

[0024] Specifically, the present invention has the following advantages:

[0025] (1) Through the molten salt medium-assisted vacuum melting synthesis method, the ions in the liquid molten salt environment can accelerate their movement by virtue of more intense convection and diffusion processes. Therefore, compared with the solid-phase reaction, the SnSe grains obtain a larger grain size, thereby reducing the large-angle grain boundaries. The increase in small-angle grain boundaries can reduce the grain boundary scattering effect, and ultimately improve the carrier mobility in the near-room temperature region.

[0026] (2) The ratio of Sn and Se components in the matrix can be adjusted through the melting reaction. At the same time, Ag partially replaces the Sn lattice position, which improves the carrier mobility and thus the electrical conductivity. In addition, because the molten salt environment enhances the convection and diffusion of ions, the high thermal conductivity Ag-rich phase, which is harmful to thermoelectric performance, is transformed from a micron-sized block distribution to a banded distribution along the grain boundaries, ultimately optimizing the thermal conductivity. At 823K, the zT value reaches 1.4 and the average zT value reaches 0.7.

[0027] (3) Weighing and grinding the raw materials in an argon atmosphere glove box can effectively prevent the possibility of raw materials being oxidized. In addition, the double-layer quartz tube vacuum melting can also prevent the raw materials from being oxidized during the reaction and avoid the risk of the single-layer quartz tube exploding and oxidizing the raw materials, making the prepared tin selenide more economical and environmentally friendly than Bi2Te3. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 These are test data of the temperature-dependent conductivity of the thermoelectric materials based on P-type polycrystalline SnSe prepared in Examples 1 to 5 and the comparative example. The conductivity is obtained by testing the conductivity Seebeck coefficient test system.

[0029] Figure 2 These are test data of Seebeck coefficient variations with temperature for thermoelectric materials based on P-type polycrystalline SnSe prepared in Examples 1 to 5 and the comparative example. The Seebeck coefficients were obtained by testing using a conductivity Seebeck coefficient test system.

[0030] Figure 3 The power factor of the thermoelectric materials based on P-type polycrystalline SnSe prepared in Examples 1 to 5 and the comparative example varies with temperature. The power factor (PF) is calculated by the conductivity (σ) and the Seebeck coefficient (S): PF = σ × S 2 .

[0031] Figure 4 The thermal conductivity of the P-type polycrystalline SnSe-based thermoelectric materials prepared in Examples 1 to 5 and the comparative example varies with temperature. The thermal conductivity (κtot) is calculated by combining the thermal diffusivity (D) obtained by a laser thermal conductivity meter, the density (r) obtained by the Archimedean drainage method, and the Cp calculated by the Dulong-Pétier law: κtot = D × Cp × r.

[0032] Figure 5 This figure shows the temperature-dependent lattice thermal conductivity of the P-type polycrystalline SnSe thermoelectric materials prepared in Examples 1-5 and the comparative example. Lattice thermal conductivity (κlat) is calculated by subtracting electronic thermal conductivity (κele) from thermal conductivity (κtot). Electronic thermal conductivity is estimated using the formula κele = L × σ × T, where L is the Lorentz constant, T is the absolute temperature, and σ is the electrical conductivity. Lattice thermal conductivity κlat = κtot - L × σ × T.

[0033] Figure 6 The test data of the thermoelectric figure of merit of the thermoelectric materials based on P-type polycrystalline SnSe prepared in Examples 1 to 5 and the comparative example are as follows: The thermoelectric figure of merit (ZT) is calculated by the power factor (S 2 It is calculated by multiplying the ratio of thermal conductivity (σ) to absolute temperature (T).

[0034] Figure 7 These are the X-ray diffraction results of thermoelectric materials based on P-type polycrystalline SnSe prepared in Examples 1 to 5 and the comparative example; all the results contain diffraction peaks corresponding to the composite SnSe phase standard PDF card #48-1224, indicating that the molten salt-assisted melt synthesis method can prepare pure-phase SnSe materials without interfering with the melting method.

[0035] Figure 8 Fresh cross-sectional SEM test images of thermoelectric materials based on P-type polycrystalline SnSe prepared in Comparative Example (a) and Example 1 (b).

[0036] Figure 9 The distribution diagram and proportion of EBSD small-angle grain boundary analysis of thermoelectric materials based on P-type polycrystalline SnSe prepared in comparative example and embodiment 1.

[0037] Figure 10 As the Ag doping content increases, the carrier concentration and carrier mobility of SnSe materials prepared by the molten salt method change.

[0038] Figure 11 The scanning electron microscope backscattered electron image of the sample in Example 3 and the energy spectrum scanning diagram of each main element are shown. DETAILED DESCRIPTION

[0039] The following is a further description of specific embodiments of the present invention. It should be noted that the description of these embodiments is intended to facilitate understanding of the present invention and does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0040] The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples are commercially available unless otherwise specified.

[0041] In the following examples, a high-precision electronic scale was used to weigh the raw materials, and the sample was weighed with an accuracy of ±0.3 mg. Example 1: A P-type polycrystalline SnSe thermoelectric material and a molten salt-assisted melting method for its preparation

[0042] (1) In a glove box filled with 100% argon (at a pressure of 1 atm), the two raw materials were weighed at a molar ratio of Sn:Se = 1:1. Subsequently, NaCl was weighed at a mass ratio of NaCl:SnSe = 1:1. Sn was ultrapure tin particles with a purity of ≥99.99%, Se was ultrapure selenium particles with a purity of ≥99.999%, and NaCl was ultrapure powder with a purity of ≥99.99%.

[0043] (2) The raw materials were placed in a quartz tube in the order of Se and Sn, and NaCl was added. The inner diameter of the quartz tube used was 17±0.2 mm, the wall thickness was 1.5±0.2 mm (cleaned with alcohol and dried for 24 hours before use), and the tube was necked at a position of about 100 to 130 mm from the bottom. Finally, glass beads with a diameter of 16±0.2 mm were added.

[0044] (3) The quartz tube is transferred to a tube sealing machine for vacuum treatment until the vacuum degree is less than 10-4Pa and then packaged.

[0045] (4) The packaged quartz tube is cut off from the necking position to the tube mouth, and then placed in a quartz tube with a diameter of 30±0.2mm and a wall thickness of 1.5±0.2mm (cleaned with alcohol and dried for 24 hours) for vacuuming and packaging again.

[0046] (5) The encapsulated quartz tube was heated to 950°C at a rate of 1°C / min, held at that temperature for 720 min to allow for a melt reaction, then cooled to 800°C at a rate of 0.1°C / min and cooled to room temperature in the furnace to obtain an ingot. The ingot was ultrasonically treated in deionized water for 5 min to remove NaCl attached to the surface, then wiped dry with dust-free paper and polished with 2000-grit sandpaper.

[0047] (6) The obtained ingot was hand-ground in a glove box using a mortar for 20 min, and the obtained powder was then packed into a graphite mold with an inner diameter of 12.7 ± 0.3 mm for later use.

[0048] (7) Pulse current sintering was performed at 500°C and 60 MPa for 10 min to obtain a P-type polycrystalline SnSe thermoelectric material. The obtained P-type polycrystalline SnSe thermoelectric material was wire-cut and polished into a 3 mm × 3 mm × 10 mm rectangular parallelepiped for performance testing.

[0049] Examples 2-5: A P-type polycrystalline Ag alloyed SnSe thermoelectric material and a molten salt assisted melting preparation method

[0050] The difference from Example 1 is that in Example 2, in addition to Se, Sn, and NaCl, Ag is added to the raw materials. The difference between Examples 2 to 5 is the different molar ratios of Sn, Se, and Ag. In Example 2, the molar ratio of Sn, Se, and Ag is 0.9975:1:0.0025; in Example 3, the molar ratio of Sn, Se, and Ag is 0.995:1:0.005; in Example 4, the molar ratio of Sn, Se, and Ag is 0.9925:1:0.0075; and in Example 5, the molar ratio of Sn, Se, and Ag is 0.99:1:0.01. The Ag is high-purity silver wire with a purity of ≥99.99%.

[0051] Comparative Example: A P-type polycrystalline SnSe thermoelectric material and its melting preparation method

[0052] The difference from Example 1 is that no NaCl is added to the raw materials of Comparative Example 1. Other aspects are the same as those of Example 1.

[0053] Experimental Example: Performance Test Based on P-Type Polycrystalline SnSe Thermoelectric Material

[0054] Figure 7 The following are the powder X-ray diffraction patterns of each group of SnSe samples and a standard SnSe phase PDF card. Each group of SnSe samples is confirmed to be SnSe, with no impurity peaks present, demonstrating that the molten salt-assisted synthesis method can successfully synthesize SnSe without introducing impurity phases.

[0055] like Figures 1 to 6 As shown in Table 1, Example 1 introduces Sn vacancies through molten salt assisted melting preparation, which increases the carrier concentration. Due to the inverse relationship between carrier concentration and mobility, an increase in carrier concentration will inevitably lead to a decrease in mobility. Figure 8 It can be seen that the SnSe prepared by molten salt assisted melting has a relatively large grain size, which leads to Figure 9 The proportion of small-angle grain boundaries of 2 to 15° shown in the figure increases, thus retaining high carrier mobility and improving the conductivity in the entire temperature range. In addition, due to the additional Sn vacancy defects introduced into the SnSe material by the molten salt assisted melting preparation, Figure 4 、 Figure 5 The corresponding material thermal conductivity and lattice thermal conductivity indicate that phonon scattering is enhanced, so the lattice thermal conductivity of Example 1 decreases in the entire temperature range.

[0056] Subsequently, under this process condition, different concentrations of Ag elements were introduced into the SnSe-based thermoelectric material to partially replace the Sn element, as shown in Examples 2 to 5. The carrier concentration and mobility change with the Ag doping ratio as shown in Figure 10 As shown in the figure, the carrier mobility increases with the increase of Ag doping amount, while the carrier concentration increases first and then decreases, reaching the highest value in Example 2, that is, when Ag = 0.25%. Among them, the solution of Example 3 is the best. After replacing 0.5% molar ratio of Sn element with equal amount of Ag element in the raw material, its electrical transmission performance is further optimized. Figure 1 、 2 It can be seen that the conductivity of the Ag-doped P-type polycrystalline SnSe thermoelectric material in the low-temperature region of 300K to 723K is much higher than that of Example 1, and has no adverse effect on the Seebeck coefficient, indicating that the doping of Ag ions greatly improves the mobility in the near-room temperature region without increasing the carrier concentration. Figure 2The Seebeck coefficient (S) can also confirm that the SnSe thermoelectric materials prepared in Examples 1-5 are all P-type materials. Figure 4 、 Figure 5 、 Figure 11 In the case of Ag ions, the high thermal conductivity Ag-rich phase is distributed along the grain boundaries, which increases the thermal conductivity and lattice thermal conductivity of the material near room temperature. However, the introduction of Ag ions forms Ag point defects and introduces lattice disorder, which reduces the lattice thermal conductivity of the material at the highest temperature.

[0057] It can be seen that the method of the present invention optimizes both the electrical and thermal transport properties of the P-type polycrystalline SnSe thermoelectric material. Thanks to the optimization of electrical transport, the maximum power factor at 823K reaches 7.6μW m -1 ·K -2 , the average power factor reached 7.2μW m -1 ·K -2 ( Figure 3 ); the thermoelectric figure of merit zT reached 1.4, and the average zT reached 0.7 ( Figure 6 ).

[0058] Table 1 EPMA composition analysis of thermoelectric materials based on P-type polycrystalline SnSe prepared in comparative example and example 1

[0059]

[0060] The embodiments of the present invention are described in detail above, but the present invention is not limited to the described embodiments. It is apparent to those skilled in the art that various changes, modifications, substitutions, and variations of these embodiments may be made without departing from the principles and spirit of the present invention, and the changes still fall within the scope of protection of the present invention.

Claims

1. A method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials using grain boundary control and defect engineering, characterized in that: The following steps are involved: S1. Place tin particles, selenium particles and inert salt powder in a vacuum container and make ingots through melting reaction. S2. Grinding the ingot into powder, and then performing spark plasma sintering on the powder to obtain an optimized P-type polycrystalline SnSe thermoelectric material.

2. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 1, characterized in that: S1 is to place tin particles, selenium particles, silver wires and inert salt powder in a vacuum container and make ingots through melting reaction.

3. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 1, characterized in that: The molar ratio of tin particles to selenium particles is 1-2:1-2, and the mass ratio of inert salt to the total amount of tin particles and selenium particles is 1-2:1-2.

4. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 1, characterized in that: The tin particles are ultra-pure tin particles with a purity of ≥99.99%, the selenium particles are ultra-pure selenium particles with a purity of ≥99.999%, and the inert salt is ultra-pure powder with a purity of ≥99.99%.

5. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 1, characterized in that: The inert salt is sodium chloride.

6. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 2, characterized in that: The molar ratio of the tin particles, selenium particles and silver wire is 0.99-0.9975:1:0.0025-0.01, and the silver wire is a high-purity silver wire with a purity of ≥99.99%.

7. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 2, characterized in that: The vacuum degree in the vacuum container is less than 10 -4 Pa.

8. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 2, characterized in that: The melting reaction is to heat up to 950±50°C at a rate of 1-2°C / min, keep the temperature for 720±60mins, then cool to 800°C at a rate of 0.1-0.2°C / min, and then cool to room temperature in the furnace.

9. The method for optimizing polycrystalline P-type tin selenide-based thermoelectric materials by using grain boundary control and defect engineering according to claim 2, characterized in that: The spark plasma sintering is performed by pulse power pressurization sintering at a temperature of 500±50° C. and a pressure of 60±5 MPa for 10±3 min.

10. A polycrystalline P-type tin selenide-based thermoelectric material prepared by the method according to any one of claims 1 to 9.

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