Preparation method of CsBi4Te6 thermoelectric material

Abstract

The invention discloses a preparation method of a CsBi4Te6 thermoelectric material, which comprises the following steps: smelting Bi and Te elementary substance raw materials and Ba and CsCl powder into a cast ingot, and dissociating the cast ingot into CsBi4Te6 polycrystals through water washing; carrying out single crystal growth on the CsBi4Te6 polycrystal through a descent method, and then carrying out annealing treatment to obtain a CsBi4Te6 single crystal; and carrying out hot extrusion on the CsBi4Te6 single crystal. According to the preparation method provided by the invention, a single crystal growth method and a directional hot extrusion method are combined, and the problems of disordered polycrystal orientation and poor single crystal mechanical property in the preparation process of the high-brittleness cesium-bismuth-tellurium thermoelectric material are solved.

Description

Technical Field

[0001] This invention belongs to the field of thermoelectric functional materials technology, and particularly relates to a method for preparing CsBi4Te6 thermoelectric material. Background Technology

[0002] CsBi4Te6 is a narrow-bandgap thermoelectric semiconductor with a layered structure. Due to its high Seebeck coefficient, low thermal conductivity, and high ZT value (0.8–1.0) in the 150–250 K temperature range, it is considered a representative of next-generation low-temperature thermoelectric materials. However, the interlayer connections in a typical CsBi4Te6 structure rely on weak van der Waals forces, resulting in high overall material brittleness and poor processability, severely limiting its widespread application in micro-devices and modular refrigeration systems.

[0003] Previous researchers have prepared CsBi4Te6 using methods including sintering, hydrothermal methods, and redox methods. Specifically: Duck-Young Chung et al. used Cs and Bi2Te3 as raw materials to sinter and synthesize CsBi4Te6; others used Cs2Te and Bi2Te3 as raw materials to sinter and synthesize CsBi4Te6 (J.Am.Chem.Soc.2004,126,6414-6428); Anuja Datta et al. used a hydrothermal method to synthesize CsBi4Te6 using CsCH3COO, Bi(NO3)3·5H2O, and Na2TeO3 (ACSAppl.Mater.Interfaces 2012,4,772-776); Hua Lin et al. prepared CsBi4Te6 using a redox method and explored the influence of various rare earth elements on the yield (Dalton Trans.2016,45,11931–11934). The above synthesis methods have certain drawbacks: in the sintering method, Cs metal is reactive and easily oxidized in air, posing certain safety hazards; the uniformity and crystal quality of the samples are poor, making them unsuitable for large-scale production; moreover, they need to be carried out in a special H-shaped quartz tube, making the experimental operation complex; the hydrothermal method is difficult to operate; each batch can only prepare about 0.3g of CsBi4Te6, which is not suitable for large-scale production; the yield is limited by the volume of the hydrothermal reactor and the amount of reactants fed.

[0004] Traditional polycrystalline sintering methods, such as cold pressing and hot pressing, while relatively simple in process and suitable for large-size sample fabrication, struggle to achieve good crystal orientation, resulting in low material texture and discontinuous carrier transport paths. This leads to underutilization of thermoelectric properties and insufficient mechanical properties, making the materials prone to cracking or failure during use. In contrast, single-crystal materials, due to their highly ordered crystal structure, help improve carrier mobility and reduce grain boundary scattering, thus achieving superior thermoelectric performance. However, CsBi4Te6 single-crystal materials suffer from drawbacks such as high brittleness, poor mechanical strength, high processing difficulty, and size limitations, particularly hindering subsequent structural fabrication and shaping during device integration. Furthermore, there are very few research reports on CsBi4Te6 single crystal growth methods at home and abroad. The main reason is that the Cs element in this material has high activity and high volatility, and it is very easy to react with conventional quartz or tapered tubes, leading to tapered tube corrosion, system contamination, and ultimately the formation of impurity phases mainly composed of Bi2Te3, as well as defects such as polycrystalline, twinned, and dislocations, which seriously affect the crystallization quality and purity of CsBi4Te6 single crystals.

[0005] Therefore, there is an urgent need to develop a preparation process that can effectively suppress Cs volatilization and tube wall reactions while achieving high-quality single crystal growth to ensure the integrity of the material structure and the uniformity of its composition. On the other hand, how to improve the mechanical strength and dimensional machinability of the material while maintaining the thermoelectric properties of the single crystal is also a key technical challenge in the current preparation of CsBi4Te6 materials.

[0006] Therefore, some studies have proposed using hot extrusion technology to improve its density and mechanical properties. However, for CsBi4Te6, the synergistic optimization between hot extrusion process and single crystal controlled growth is not yet mature, and there are few related reports. The selection of hot extrusion parameters, crystal structure regulation, and microtexture evolution mechanisms are still unclear. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a method for preparing CsBi4Te6 thermoelectric materials, solving key issues such as the volatility of Cs, poor crystal quality, and weak mechanical properties. This invention employs a single-crystal growth combined with directional hot extrusion to obtain CsBi4Te6 thermoelectric materials. This method optimizes the growth environment to obtain high-quality single crystals, followed by hot extrusion to achieve texture enhancement and mechanical strengthening, significantly improving the overall thermoelectric performance and structural reliability of the material.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] On one hand, the present invention provides a method for preparing CsBi4Te6 thermoelectric material, comprising the following steps:

[0010] (1) The elemental raw materials of Bi and Te and Ba and CsCl powders are melted into ingots and then dissociated into CsBi4Te6 polycrystalline by water washing;

[0011] (2) The CsBi4Te6 polycrystalline obtained in step (1) is grown into a single crystal by the descent method, and then CsBi4Te6 single crystal is obtained by annealing.

[0012] (3) The CsBi4Te6 single crystal obtained in step (2) is hot-extruded to obtain the CsBi4Te6 thermoelectric material.

[0013] In a preferred embodiment, in step (1), the molar ratio of the elemental raw material Bi, the elemental raw material Te Ba, and the CsCl powder is 8-10:8-13:1-3:2-5.

[0014] In a preferred embodiment, in step (1), the heating rate of the melting process is 25-45°C / h; the melting temperature is 900-950°C; and the melting time is 85-160h.

[0015] In a preferred embodiment, in step (1), the water washing and dissociation process is as follows: the ingot is ultrasonically washed in water; in some specific embodiments, the ultrasonic time is 1 to 20 minutes; the water washing and dissociation also includes ethanol washing and drying.

[0016] In a preferred embodiment, in step (2), during the single crystal growth process of the CsBi4Te6 polycrystalline material, the following are added: ① elemental Bi; or ② elemental Bi and a dopant; the dopant is selected from at least one of Rb, Pb, Se, K, Na, Sn, In, Zn, Cd, Sb, S, Ge, SbI3, La, Pr and Nd; the mass ratio of the CsBi4Te6 polycrystalline material, elemental Bi and the dopant is 1:0.01~0.2:0~0.1; in some specific embodiments, the mass ratio of the CsBi4Te6 polycrystalline material, elemental Bi and the dopant is 1:0.01~0.1:0.03~0.1.

[0017] In the technical solution of this invention, the addition of the aforementioned dopant can regulate the carrier concentration through elemental doping, thereby altering the band structure of the crystal. This invention can precisely control the type and content of doped elements according to different application requirements, enabling precise control of the electrical properties of single-crystal materials and meeting diverse application scenarios.

[0018] In a preferred embodiment, in step (2), the single crystal growth is carried out using a container with a conical head;

[0019] Preferably, the bottom angle of the conical head is 30-70°; in the technical solution of the present invention, the bottom angle of the conical head can make the melt flow more smoothly during crystal growth, reduce the melt flow resistance, facilitate the uniform growth of crystal, avoid crystal growth defects caused by poor melt flow, and thus improve the yield and quality of single crystal.

[0020] In some specific embodiments, in step (2), the container used for single crystal growth is made of any one of quartz, graphite and boron nitride.

[0021] In a preferred embodiment, in step (2), the inner wall of the container used for single crystal growth is carbon-plated; the carbon plating process includes the following steps:

[0022] Ethanol is sprayed onto the inner wall of the container and heated to form a carbon film.

[0023] In the technical solution of the present invention, carbon plating the inner wall of the container can prevent Cs from reacting with the tube wall at high temperature, thereby improving the nucleation purity and growth quality of single crystals.

[0024] Preferably, the heating temperature is 950–1100°C;

[0025] Preferably, the heating time is 10 to 30 minutes;

[0026] In the technical solution of the present invention, forming a carbon layer on the inner wall of the container can significantly inhibit the corrosion and reaction of Cs elements on the pipe wall, effectively improving the purity and crystal integrity of single crystals.

[0027] In a preferred embodiment, step (2) includes the following descent method:

[0028] The material is melted in a high-temperature zone, lowered to a gradient zone, and then continues to descend in the gradient zone; wherein the high-temperature zone and the gradient zone are adjacent in the vertical direction; the temperature gradient of the gradient zone decreases; and the temperature of the high-temperature zone is greater than or equal to the initial temperature of the gradient zone.

[0029] Preferably, the temperature of the high-temperature zone is 550–800°C; the temperature range of the gradient zone is 300–550°C.

[0030] Preferably, the temperature gradient in the gradient region is 1–5 °C / mm, more preferably 1.5–3 °C / mm;

[0031] Preferably, the rate of descent to the gradient region is 0.5–10 mm / h, more preferably 0.8–3.6 mm / h;

[0032] Preferably, the continued descent is a rotational descent; the rotational rate during the continued descent is 0–20 r / min, preferably 2–20 r / min; the descent rate during the continued descent is 0.5–10 mm / h.

[0033] In the technical solution of this invention, after the material melts in the high-temperature zone, crystals begin to grow as the temperature decreases during the descent. During the descent, due to the effect of the temperature gradient, the crystals begin to crystallize from bottom to top along the b-axis. By controlling the descent rate and the cooling rate, the growth rate of the crystals and the stability of the growth interface can be effectively controlled, promoting the directional growth of the crystals along the b-axis and reducing the generation of crystal defects. This invention uses a lower growth temperature, further improving the purity and crystal integrity of the single crystal.

[0034] In a preferred embodiment, in step (2), the annealing temperature is 300–450°C;

[0035] Preferably, the annealing time is 10 to 50 hours;

[0036] Preferably, the annealing further includes a cooling process; the cooling rate is 45–90 °C / h.

[0037] In the technical solution of the present invention, annealing can reduce dislocation density and thermal stress, thereby improving the integrity of single crystal.

[0038] In a preferred embodiment, during step (3), the force direction of the CsBi4Te6 single crystal is parallel to the b-axis direction of the crystal during the hot extrusion process. In the technical solution of the present invention, the b-axis direction of the crystal has specific crystallographic characteristics. Setting the hot extrusion direction parallel to the b-axis direction of the crystal can enhance the thermoelectric orientation of the crystal, enabling electrons to be transported more efficiently in the crystal along a specific direction, significantly improving the thermoelectric conversion efficiency of the material, and enhancing the thermoelectric performance of the material.

[0039] Preferably, the hot extrusion is performed by holding the CsBi4Te6 single crystal at 350-450°C for 0.5-2 hours.

[0040] Preferably, the heating rate of the hot extrusion is 10–30 °C / min;

[0041] Preferably, the extrusion ratio of the hot extrusion is 3 to 10, more preferably 2 to 4; in the technical solution of the present invention, the extrusion ratio is the ratio of the initial cross-sectional area of ​​the material to the cross-sectional area after hot extrusion.

[0042] Preferably, the deformation rate during hot extrusion is 0.5 to 5 mm / min.

[0043] Preferably, the hot extrusion is carried out in an inert atmosphere; in some specific embodiments, the inert atmosphere is selected from any one of argon, nitrogen, and a hydrogen-argon mixture.

[0044] In some specific embodiments, the hot extrusion involves placing the CsBi4Te6 single crystal in an extrusion die; the extrusion die is a steel die or a steel die with a SiC bushing; the die steel has good strength, hardness, and wear resistance, and can withstand the high temperature and high pressure during the hot extrusion process, ensuring the service life of the die; the SiC bushing has excellent high temperature resistance and corrosion resistance, and has little interfacial reaction with the melt, which can effectively reduce the interfacial reaction between the die and the melt during the hot extrusion process, reduce the introduction of impurities, and improve the purity and quality of the single crystal material.

[0045] In another aspect, the present invention provides a CsBi4Te6 thermoelectric material obtained by the above preparation method.

[0046] The present invention has the following beneficial effects:

[0047] This invention achieves the following technological breakthroughs through a "highly oriented single crystal growth + single crystal-strengthened hot extrusion" process: highly consistent crystal orientation and hot extrusion-induced alignment of the Bi-Te layers along the b-axis, significantly improving carrier mobility; scalable fabrication and compatibility with powder metallurgy processes, suitable for mass production of rods and sheets, possessing engineering application potential. The preparation method provided by this invention combines the performance advantages of CsBi4Te6 single crystals with the successful application experience of hot extrusion technology in thermoelectric materials, not only solving its fragility and poor mechanical properties, but also achieving a "dual improvement" in thermoelectric performance and structural reliability, promoting its widespread application in practical refrigeration devices.

[0048] This invention combines single-crystal growth and hot extrusion processes to prepare CsBi4Te6 material, improving its crystal orientation, mechanical strength, and thermoelectric properties, making it suitable for low-temperature thermoelectric refrigeration devices. Furthermore, this invention significantly improves the material's processability, providing technical support and a new method for the reliable application of CsBi4Te6 material in thermoelectric devices. Attached Figure Description

[0049] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a schematic diagram of the tapered tube structure with a base angle of 30° used in Embodiment 1 of the present invention;

[0051] Figure 2 This is a diagram showing the result of carbon coating on the inner wall of the quartz tapered tube in Embodiment 1 of the present invention;

[0052] Figure 3 This is a diagram showing the three temperature zones of the single crystal growth furnace used in Embodiment 1 of the present invention.

[0053] Figure 4 This is a diagram illustrating the single crystal growth effect in Example 1 of the present invention;

[0054] Figure 5 This is a single-crystal X-ray powder diffraction pattern of the CsBi4Te6 rod thermoelectric material in Example 1 of the present invention;

[0055] Figure 6 The energy dispersive spectrum of the CsBi4Te6 rod thermoelectric material in Embodiment 1 of the present invention is shown.

[0056] Figure 7 The graph shows the thermoelectric figure of merit (ZT) test results of the CsBi4Te6 rod thermoelectric material in Embodiment 2 of the present invention.

[0057] Figure 8 Here is a physical image of the CsBi4Te6 rod thermoelectric material in Embodiment 2 of the present invention:

[0058] Figure 9 The following is a diagram showing the three-point bending resistance test results of the CsBi4Te6 rod thermoelectric material in Embodiment 2 of the present invention:

[0059] Figure 10 The image shows the Vickers hardness test results of the CsBi4Te6 rod thermoelectric material and CsBi4Te6 in Embodiment 2 of the present invention. Detailed Implementation

[0060] The following embodiments are merely some, not all, of the embodiments of the present invention. Therefore, the detailed descriptions of the embodiments provided below are not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0061] In this invention, unless otherwise specified, all equipment and raw materials are commercially available or commonly used in the industry. The methods described in the following embodiments are conventional methods in the art, unless otherwise specified.

[0062] Example 1

[0063] This embodiment provides a CsBi4Te6 thermoelectric material, and the preparation process is as follows:

[0064] (1) A quartz tapered tube with a base angle of 30° ( Figure 1 The inner surface is thoroughly cleaned, and then anhydrous ethanol is sprayed onto the inner surface of the conical tube. After the anhydrous ethanol has fully impregnated the surface, it is preheated and sintered at 950°C for 15 minutes to form a carbon film. Excess carbon film at the edges is removed using an edge-trimming process. In this embodiment, a schematic diagram of the carbon film coating on the inner wall of the quartz conical tube is shown below. Figure 2 ;

[0065] (2) 30g Bi, 27.48g Te, 2.46g Ba, and 6.05g CsCl powder were melted at 920℃ for 120h at a heating rate of 25℃ / h to obtain an ingot; the ingot was ultrasonicated in pure water for 5min and then washed in ethanol to decompose it into needle-like CsBi4Te6 polycrystalline material.

[0066] (3) Place the 60g CsBi4Te6 polycrystalline material and 6.667g Bi (flux) obtained in step (2) into a quartz conical tube with a carbon film coated on the inner wall; according to Figure 3 The growth furnace temperature zone settings are shown. A conical tube is placed inside the growth furnace for single crystal growth. The specific process is as follows: The conical tube is adjusted to the high-temperature zone (590℃), and the material inside the tube begins to melt. After all the material has melted, the conical tube is lowered to a temperature-decreasing gradient zone (temperature gradient of 2℃ / mm, descent rate of 3.2mm / h, temperature range of 477~527℃). The conical tube is rotated and lowered in the gradient zone at a descent rate of 3.2mm / h and a rotation rate of 2r / min. During this process, the cooling rate of the conical tube in the gradient zone is 6.4mm / h. As the temperature decreases during descent, crystal growth begins. After all the material has crystallized, the conical tube is lowered to the low-temperature zone (480℃) and held for 12 hours for annealing. Finally, it is cooled to room temperature at a cooling rate of 90℃ / h to obtain a CsBi4Te6 single crystal.

[0067] (4) The CsBi4Te6 single crystal was loaded into the extrusion mold and heated to 350℃ for 30 min at a heating rate of 10℃ / min under an argon atmosphere. Then, hot extrusion was carried out along the b-axis of the crystal with an extrusion ratio of 2 and a deformation rate of 1 mm / min during the extrusion process to obtain CsBi4Te6 rod thermoelectric material.

[0068] Figure 4 The image shown is a physical photograph of the CsBi4Te6 single crystal obtained in this embodiment.

[0069] Figure 5 The image shows the X-ray powder diffraction pattern of CsBi4Te6 rod thermoelectric material. The test was performed using a MiniFlux600 (Rigaku, Japan) at 300 K with Cu K. α Radiation was conducted. The graph shows that all diffraction peaks perfectly match the standard diffraction peaks, indicating that the obtained material is pure-phase CsBi4Te6.

[0070] Figure 6 The image shows the energy dispersive spectrum of CsBi4Te6 single crystal obtained by energy dispersive spectroscopy (EDS). It can be seen from the figure that the elemental distribution of CsBi4Te6 single crystal is uniform and the proportion is close to the stoichiometry, indicating that the obtained material has good crystal quality.

[0071] Figure 7 The figure shown is the thermoelectric figure of merit (ZT) test results of CsBi4Te6 rod thermoelectric material. It can be seen from the figure that the ZT of the material reaches 0.75 at 250K.

[0072] Figure 8 The image shown is a physical picture of CsBi4Te6 rod thermoelectric material.

[0073] Figure 9 The image shows the results of a three-point bending test on CsBi4Te6 thermoelectric rods. The test was performed on a universal testing machine (model AGX-10kNVD) at a loading rate of 0.5 mm / min. Before the bending strength test, the sample was cut into 3×3×8 mm pieces. 3 The rectangular parallelepiped is polished to ensure it is free of cracks. The figure shows a three-point flexural strength >45MPa, which is greater than the zone melting Bi. 0.5 Sb 1.5 Te3(18MPa, Y.Zheng, Q.Zhang,XLSu,HYXie,SCShu,TLChen,GJTan,YGYan,XFTang,C.Uher,GJSnyder,Adv.EnergyMater.5(2015)1401391).

[0074] Figure 10 The image shows the Vickers hardness test results for CsBi4Te6 rod thermoelectric material and CsBi4Te6 single crystal. The test used the CSM (Continuous Stiffness) method, and the instrument used was a Bruker TI-980 nanoindenter. The sample surface should be smooth and flat before hardness testing to ensure the accuracy of the results. The image shows that the hardness of the sample increased after hot extrusion, with the average hardness of the single crystal reaching 77.3 N / mm². 2 Increased to 104.4 N / mm 2 .

[0075] In summary, the preparation method provided by this invention combines crystal growth with hot extrusion, achieving orientation adjustment and performance improvement of highly brittle cesium bismuth tellurium thermoelectric materials. It produces large-size cesium bismuth tellurium rods with high strength and high thermoelectric figure of merit, providing material assistance for subsequent multi-stage devices to achieve maximum temperature difference.

[0076] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a CsBi4Te6 thermoelectric material, characterized in that, Includes the following steps: (1) The elemental raw materials of Bi and Te and Ba and CsCl powders are melted into ingots and then dissociated into CsBi4Te6 polycrystalline by water washing; (2) The CsBi4Te6 polycrystalline obtained in step (1) is grown into a single crystal by the descent method, and then CsBi4Te6 single crystal is obtained by annealing. (3) The CsBi4Te6 single crystal obtained in step (2) is hot-extruded to obtain the CsBi4Te6 thermoelectric material.

2. The method according to claim 1, characterized in that, In step (1), the molar ratio of the elemental raw material Bi, the elemental raw material Te Ba, and the CsCl powder is 8-10:8-13:1-3:2-5; Preferably, in step (1), the heating rate of the melting process is 25-45℃ / h; the melting temperature is 900-950℃; and the melting time is 85-160h. Preferably, in step (1), the water washing and dissociation process is: ultrasonically washing the ingot in water.

3. The method according to claim 1, characterized in that, In step (2), during the single crystal growth process of the CsBi4Te6 polycrystalline material, the following are added: ① elemental Bi; or ② elemental Bi and a dopant. The dopant is selected from at least one of Rb, Pb, Se, K, Na, Sn, In, Zn, Cd, Sb, S, Ge, SbI3, La, Pr and Nd. The mass ratio of the CsBi4Te6 polycrystalline material, elemental Bi and the dopant is 1:0.01~0.2:0~0.

1.

4. The method according to claim 1, characterized in that, In step (2), the single crystal growth is carried out using a container with a conical head; Preferably, the base angle of the conical head is 30 to 70°.

5. The method according to claim 1, characterized in that, In step (2), the inner wall of the container used for single crystal growth is carbon-plated; the carbon plating treatment Includes the following steps: Ethanol is sprayed onto the inner wall of the container and heated to form a carbon film. Preferably, the heating temperature is 950–1100°C; Preferably, the heating time is 10 to 30 minutes.

6. The method according to claim 1, characterized in that, In step (2), the descent method includes: The material is melted in a high-temperature zone, lowered to a gradient zone, and then continues to descend in the gradient zone; wherein the high-temperature zone and the gradient zone are adjacent in the vertical direction; the temperature gradient of the gradient zone decreases; and the temperature of the high-temperature zone is greater than or equal to the initial temperature of the gradient zone.

7. The method according to claim 6, characterized in that, The temperature of the high-temperature zone is 550–800°C; the temperature range of the gradient zone is 300–550°C. Preferably, the temperature gradient in the gradient region is 1–5 °C / mm, more preferably 1.5–3 °C / mm; Preferably, the rate of descent to the gradient region is 0.5–10 mm / h, more preferably 0.8–3.6 mm / h; Preferably, the continued descent is a rotational descent; the rotational rate during the continued descent is 0–20 r / min, preferably 2–20 r / min; the descent rate during the continued descent is 0.5–10 mm / h.

8. The method according to claim 1, characterized in that, In step (2), the annealing temperature is 300–450°C; Preferably, the annealing time is 10 to 50 hours; Preferably, the annealing further includes a cooling process; the cooling rate is 45–90 °C / h.

9. The method according to claim 1, characterized in that, In step (3), during the hot extrusion process, the force direction of the CsBi4Te6 single crystal is parallel to the b-axis direction of the crystal; Preferably, the hot extrusion is performed by holding the CsBi4Te6 single crystal at 350-450°C for 0.5-2 hours. Preferably, the heating rate of the hot extrusion is 10–30 °C / min; Preferably, the extrusion ratio of the hot extrusion is 3 to 10, more preferably 2 to 4; Preferably, the deformation rate during hot extrusion is 0.5–5 mm / min; Preferably, the hot extrusion is carried out in an inert atmosphere.

10. The CsBi4Te6 thermoelectric material obtained by any of the preparation methods described in claims 1-9.

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