Bismuth telluride-based alloys and their processing methods

Abstract

This invention discloses a bismuth telluride-based alloy and its processing method. The processing method includes the following steps: (1) providing a bismuth telluride-based master alloy prepared by powder metallurgy; (2) annealing the bismuth telluride-based master alloy in a vacuum environment or under an inert gas atmosphere; and (3) hot forging the annealed bismuth telluride-based master alloy. This invention utilizes annealing and hot forging processes to simultaneously improve the mechanical strength and thermoelectric properties of the bismuth telluride-based alloy.

Description

Technical Field

[0001] This invention relates to the field of energy materials, specifically to a bismuth telluride-based alloy and its processing method. Background Technology

[0002] Thermoelectric materials possess the ability to convert heat energy into electrical energy and are highly anticipated in alleviating the energy crisis. In recent years, with the rapid development of 5G communication and next-generation communication technologies, the temperature control of optical modules used for signal transmission has become a significant factor restricting the miniaturization and high integration of this technology. Thermoelectric cooling devices based on thermoelectric materials have become the optimal strategy for solving the precise temperature control of optical modules due to their advantages of being all-solid-state, vibration-free, and easily modular. While thermoelectric cooling devices have already achieved some applications in this field, to meet the cooling requirements of smaller, higher-power communication chips, thermoelectric devices need to achieve better cooling performance within a smaller size. Manufacturing thermoelectric devices requires cutting thermoelectric materials into thermoelectric arms; therefore, thermoelectric materials need to have smaller processable dimensions, i.e., higher mechanical properties. Furthermore, thermoelectric materials also need to possess higher thermoelectric performance, i.e., a higher dimensionless thermoelectric figure of merit ZT, expressed as: ZT = σ·S 2 ·T / κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, κ is the thermal conductivity, and σ·S 2 This is the power factor.

[0003] Bismuth telluride-based thermoelectric materials are currently the only commercially viable room-temperature thermoelectric materials and are also the raw materials for thermoelectric cooling devices used in the communications field, attracting significant attention from researchers over the past few decades. Solid solution treatment of antimony in bismuth telluride to form a bismuth-antimony-telluride alloy ((Bi,Sb)₂Te₃) is a common method for transforming it into a p-type thermoelectric material and obtaining high room-temperature thermoelectric performance. The crystal structure of bismuth telluride is space group 1. Bismuth telluride has a rhombohedral crystal system and can be viewed as a layered structure, where every five atomic layers form a periodic unit, and adjacent periodic units are connected by van der Waals forces. Early research primarily used zone melting or the Bridgman process to prepare bulk or single crystals of bismuth telluride, and currently, most commercially available bismuth telluride is still prepared using zone melting. Samples obtained by these two methods exhibit high orientation and anisotropy, displaying good thermoelectric properties in the direction parallel to the atomic layers. However, poor mechanical properties and high thermal conductivity along the direction parallel to the atomic layers limit further improvements in its thermoelectric properties. Therefore, nanocrystalline polycrystalline bismuth telluride can be prepared to enhance mechanical properties while reducing lattice conductivity.

[0004] Currently, powder metallurgy is the main method for preparing polycrystalline bismuth telluride with nanostructures, involving two key steps: obtaining a nano-precursor and sintering the nano-precursor into a bulk. Methods for obtaining the nano-precursor mainly include mechanical alloying, melting and pulverizing, and melt spinning. Subsequent sintering to obtain the bulk is mainly done through hot pressing or spark plasma sintering. The bulk obtained by sintering nanoscale powder has a small grain size and can retain some of the nanostructures from the precursor, resulting in a high-strength bulk due to strengthening mechanisms such as grain refinement and nano-second phase reinforcement. Furthermore, the scattering effect of grain boundaries and nanostructures on phonons gives the powder metallurgy-prepared bismuth telluride bulk a low lattice thermal conductivity and excellent thermoelectric properties. Many designs have been developed around the powder metallurgy process. For example, in CN1974079A, high-energy ball milling is first used to mechanically alloy elemental powders, and the resulting alloyed powders are then sintered using spark plasma sintering to obtain bismuth telluride-based bulk thermoelectric materials. Another example is CN101786162A, which first uses high-energy ball milling to synthesize bismuth telluride nanocrystals from elemental powders, and then uses microwave irradiation to sinter the powders into bismuth telluride. However, currently, the mechanical properties of bismuth telluride-based alloys prepared by powder metallurgy are still insufficient, and their mechanical strength cannot meet the requirements of microdevice manufacturing. Summary of the Invention

[0005] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the objective of this invention is to provide a bismuth telluride-based alloy and its processing method, to address the problem that the mechanical properties of bismuth telluride-based alloys prepared by powder metallurgy cannot meet the requirements of microdevice manufacturing.

[0006] In a first aspect of the invention, a method for processing bismuth telluride-based alloys is provided, comprising the following steps:

[0007] (1) Provides a bismuth telluride-based intermediate alloy prepared by powder metallurgy process;

[0008] (2) The bismuth telluride-based master alloy is annealed in a vacuum environment or under an inert gas.

[0009] (3) The annealed bismuth telluride-based master alloy is hot-forged in a vacuum environment or under an inert gas.

[0010] According to a first aspect of the invention, annealing and hot forging processes simultaneously improve the mechanical strength and thermoelectric properties of bismuth telluride-based master alloys. Specifically, annealing the bismuth telluride-based alloy obtained by powder alloying promotes uniform element distribution, facilitates dislocation movement to absorb tellurium vacancies within the alloy, and promotes grain boundary movement, causing the alloy to expand and transforming micropores into interconnected macropores. Subsequently, hot forging compresses the alloy that has expanded due to annealing, increasing its density and simultaneously generating a large number of dislocations within the alloy. The increased density ensures good carrier transport properties, while the presence of numerous dislocations enhances phonon scattering, thereby jointly improving thermoelectric properties. Simultaneously, the high density and dislocation density also improve mechanical properties.

[0011] According to an embodiment of the present invention, in step (1), the chemical formula of the bismuth telluride-based master alloy is Bi. x Sb 2- x Te 3+y Where 0.3≤x≤0.5, 0≤y≤0.3. Thus, by introducing solid-solution Sb into bismuth telluride to form a bismuth-antimony-tellurium alloy ((Bi,Sb)₂Te₃), it transforms into the p-type to obtain high room-temperature thermoelectric properties. Excess tellurium (y>0) can fill tellurium vacancies within the alloy.

[0012] Preferably, 0 ≤ y ≤ 0.05. This allows excess tellurium that did not fill the tellurium vacancies during the annealing process to evaporate as much as possible, reducing the residue of tellurium.

[0013] According to an embodiment of the present invention, in step (1), the bismuth telluride-based master alloy further includes a second phase, which comprises nano-SiC. Thus, the introduction of nano-SiC as a second phase for strengthening further increases the alloy strength.

[0014] According to an embodiment of the present invention, based on the mass of the matrix phase of the bismuth telluride-based master alloy, the mass fraction of nano-SiC is less than 0.5%. This increases the alloy strength while avoiding the adverse effects of excessively high thermal conductivity and decreased electrical conductivity caused by too much nano-SiC.

[0015] According to an embodiment of the present invention, the Dv50 of the nano-SiC is 3nm to 200nm. This further enhances the alloy strength.

[0016] According to an embodiment of the present invention, in step (2), the annealing temperature is 300℃~600℃. As a result, the alloy expands during the annealing process, causing the internal pores to expand and connect, while simultaneously promoting dislocation movement to absorb tellurium vacancies.

[0017] According to an embodiment of the present invention, the annealing time is 3h to 72h.

[0018] According to an embodiment of the present invention, the hot forging treatment in step (3) is performed in a discharge plasma sintering apparatus, and the hot forging conditions include:

[0019] After evacuating the furnace cavity, the axial pressure is increased to 10MPa to 80MPa at an alloy temperature of 300℃ to 500℃ to complete the hot forging. As a result, the alloy is compressed due to hot forging, leading to an increase in density and the generation of a large number of dislocations inside.

[0020] According to an embodiment of the present invention, the pressurization time is 5 min to 20 min.

[0021] According to an embodiment of the present invention, the vacuum degree of the furnace cavity is 0.1 Pa to 20 Pa.

[0022] According to an embodiment of the present invention, the processing technology of the bismuth telluride-based alloy in step (1) is as follows: (101) according to the chemical formula Bi x Sb 2-x Te 3+y Bi, Sb, and Te sources are weighed in stoichiometric proportions and ball-milled to achieve mechanical alloying, thereby obtaining a precursor; wherein, 0.3≤x≤0.5, 0≤y≤0.3; (102) the precursor is sintered to obtain a bismuth telluride-based alloy. Thus, a bismuth telluride-based alloy with Te element solid solution is obtained.

[0023] According to an embodiment of the present invention, step (101) further includes:

[0024] Based on the mass of Bi, Sb, and Te sources, no more than 0.5% of nano-SiC was weighed and ball-milled together with the Bi, Sb, and Te sources. This process achieved mechanical alloying through ball milling, yielding the nano-precursor.

[0025] According to an embodiment of the present invention, the Bi source includes at least one of elemental Bi, Bi2O3, BiCl3, Bi(NO3)3, and Bi2Te3.

[0026] According to an embodiment of the present invention, the Sb source includes at least one of elemental Sb, Sb₂O₃, SbCl₃, Sb₂S₃, and Sb₂Te₃.

[0027] According to an embodiment of the present invention, the Te source includes at least one of elemental Te, TeO2, and tellurate.

[0028] According to an embodiment of the present invention, step (101) ball milling is carried out under a protective atmosphere, thereby avoiding oxidation of the raw materials.

[0029] According to an embodiment of the present invention, the protective atmosphere includes at least one of an inert gas and an argon-hydrogen mixture.

[0030] According to an embodiment of the present invention, the volume fraction of hydrogen in the argon-hydrogen mixture is less than 6%.

[0031] According to an embodiment of the present invention, the sintering in step (102) is carried out in a discharge plasma device, and the sintering conditions include: evacuating the furnace cavity, sintering for 3 min to 20 min under an axial pressure of 10 MPa to 80 MPa and a precursor temperature of 300°C to 500°C. Thus, a bismuth telluride-based alloy with a main phase structure of (Bi,Sb)₂Te₃, good crystallinity, and relatively high density can be obtained.

[0032] According to an embodiment of the present invention, the vacuum degree of the furnace cavity is 0.1 Pa to 20 Pa.

[0033] In a second aspect of the invention, a bismuth telluride-based alloy obtained in the first aspect is used.

[0034] The bismuth telluride-based alloy of the second aspect of the present invention has both high mechanical strength and high thermoelectric properties, with a bending strength of up to 140 MPa, a compressive strength of up to 224 MPa, and a ZT value of up to 1.50 at 348 K.

[0035] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0036] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0037] Figure 1 These are the X-ray diffraction patterns of bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2;

[0038] Figure 2 These are scanning electron microscope (SEM) images of the cross-sections of bismuth telluride-based alloy samples from Examples 1-3 and Comparative Examples 1-3, wherein... Figure 2 (a)~ Figure 2 (f) Corresponding to Examples 1-3 and Comparative Examples 1-3 in sequence;

[0039] Figure 3 These are scanning electron microscope images of the polished surfaces of the bismuth telluride-based alloys in Example 1 and Comparative Example 1, wherein... Figure 3 (a) Corresponding to Example 1, Figure 3 (b) Response ratio 1;

[0040] Figure 4 These are morphological images of the bismuth telluride-based alloy prepared in Example 3 under a transmission electron microscope. Figure 4 (a) Figure 4 (b) Figure 4 (c) Topographic images corresponding to different locations;

[0041] Figure 5 The graphs show the electrical conductivity of bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 as a function of temperature.

[0042] Figure 6 The Seebeck coefficients of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 vary with temperature.

[0043] Figure 7 The graphs show the power factor of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 as a function of temperature.

[0044] Figure 8 The graphs show the thermal conductivity of bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 as a function of temperature.

[0045] Figure 9 The graphs show the lattice thermal conductivity plus bipolar thermal conductivity of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 as a function of temperature.

[0046] Figure 10 The graphs show the thermoelectric figure of merit (ZT) values ​​of the bismuth telluride-based alloy samples in Examples 1-3 and Comparative Examples 1-2 as a function of temperature. Detailed Implementation

[0047] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the invention, and should not be construed as limiting the invention.

[0048] In a first aspect of the invention, a method for processing bismuth telluride-based alloys is provided. In some embodiments, the method for processing the bismuth telluride-based alloy includes the following steps:

[0049] (1) Provides a bismuth telluride-based intermediate alloy prepared by powder metallurgy process;

[0050] In some specific embodiments, in step (1), the chemical formula of the bismuth telluride-based master alloy is Bi. x Sb 2- x Te 3+yWhere 0.3≤x≤0.5, for example, x=0.3, x=0.4, x=0.5, etc., and 0≤y≤0.3, for example, y=0, y=0.01, y=0.05, y=0.1, y=0.21, y=0.3, etc., a bismuth-antimony-tellurium alloy ((Bi,Sb)2Te3) is formed by introducing solid-solution antimony into bismuth telluride, which transforms into the P-type to obtain high room temperature thermoelectric properties. Excess Te (y>0) and excess tellurium can fill tellurium vacancies in the alloy. Preferably, 0≤y≤0.05. This allows excess tellurium that does not fill tellurium vacancies during annealing to volatilize as much as possible, reducing tellurium residue and avoiding the occurrence of a large amount of eutectic liquid phase-induced grain growth at grain boundaries during annealing. Furthermore, it avoids the adverse effects of tellurium-rich phases on the thermoelectric and mechanical properties of the alloy.

[0051] In some specific embodiments, in step (1), the bismuth telluride-based master alloy further includes a second phase, which comprises nano-SiC. Thus, nano-SiC is introduced as a second phase for strengthening, thereby further increasing the alloy strength. Optionally, based on the mass of the matrix phase (bismuth-antimony-telluride alloy) of the bismuth telluride-based master alloy, the mass fraction of the nano-SiC is below 0.5%, for example, 0.1%, 0.1915%, 0.3%, 0.4%, 0.5%, etc. This increases the alloy strength while avoiding the adverse effects of excessively high thermal conductivity and decreased electrical conductivity due to enhanced carrier scattering caused by excessive nano-SiC. Optionally, the Dv50 of the nano-SiC is 3nm to 200nm, for example, Dv50 is 3nm, 10nm, 80nm, 150nm, 200nm, etc. This further enhances the alloy strength.

[0052] In some specific embodiments, the preparation process of the bismuth telluride-based alloy in step (1) is as follows:

[0053] (101) According to the chemical formula Bi x Sb 2-x Te 3+yBi, Sb, and Te sources are weighed according to their stoichiometric ratio and ball-milled to achieve mechanical alloying, thereby obtaining a precursor; wherein 0.3≤x≤0.5, 0≤y≤0.3; optionally, step (101) further includes: weighing nano-SiC with a mass fraction not exceeding 0.5% based on the mass of Bi, Sb, and Te sources, and ball-milling it together with the Bi, Sb, and Te sources. Thus, mechanical alloying is completed by ball milling to obtain a precursor containing a second phase. Optionally, the Dv50 of the nano-SiC is 3nm~200nm. Optionally, the ball milling conditions include: a ball-to-raw material ratio of (10-20):1, for example, 10:1, 15:1, 20:1, etc.; a ball mill speed of 400 r / min to 500 r / min, for example, 400 r / min, 450 r / min, 500 r / min, etc.; and a ball milling time of 3 h to 12 h, for example, 3 h, 5 h, 8 h, 10 h, 12 h, etc. Optionally, the Bi source includes at least one of elemental Bi, Bi₂O₃, BiCl₃, Bi(NO₃)₃, and Bi₂Te₃. Optionally, the Sb source includes at least one of elemental Sb, Sb₂O₃, SbCl₃, Sb₂S₃, and Sb₂Te₃. Optionally, the Te source includes at least one of elemental Te, TeO₂, and tellurate (such as sodium tellurate Na₂TeO₃). In some specific embodiments, step (101) of ball milling and mixing is carried out under a protective atmosphere, thereby preventing oxidation of the raw materials. Optionally, the protective atmosphere includes at least one of an inert gas and an argon-hydrogen mixture. Further, in the argon-hydrogen mixture, the volume fraction of hydrogen is less than 6%.

[0054] (102) The precursor is sintered to obtain a bismuth telluride-based alloy. This yields a bismuth telluride-based alloy with Te element solid solution. Further, the sintering in step (102) is performed in a discharge plasma apparatus, and the sintering conditions include: evacuating the furnace cavity, sintering for 3 min to 20 min at an axial pressure of 10 MPa to 80 MPa and a precursor temperature of 300°C to 500°C; for example, sintering for 3 min, 5 min, 10 min, or 20 min at axial pressures of 10 MPa, 30 MPa, 50 MPa, 80 MPa, etc., and sintering temperatures of 300°C, 350°C, 400°C, 450°C, 500°C, etc. Thus, a bismuth telluride-based alloy with a main phase structure of (Bi,Sb)₂Te₃, good crystallinity, and relatively high density can be obtained. Optionally, the vacuum degree of the furnace cavity is 0.1 Pa to 20 Pa, for example, 0.1 Pa, 1 Pa, 5 Pa, 15 Pa, 20 Pa, etc.

[0055] (2) The bismuth telluride-based master alloy is annealed in a vacuum environment or under an inert gas atmosphere. In some specific embodiments, the annealing temperature in step (2) is 300℃ to 600℃, for example, 300℃, 400℃, 500℃, 600℃, etc. As a result, the alloy expands during annealing, causing the internal pores to expand and connect, while simultaneously promoting dislocation movement to absorb tellurium vacancies. In some specific embodiments, the annealing time is 3h to 72h, for example, 3h, 10h, 30h, 48h, 72h, etc.

[0056] (3) The annealed bismuth telluride-based master alloy is hot-forged.

[0057] In some specific embodiments, the hot forging process in step (3) is carried out in a discharge plasma sintering apparatus. The hot forging conditions include: after evacuating the furnace cavity, applying axial pressure to 10MPa to 80MPa within 5min to 20min at an alloy temperature of 300℃ to 500℃ to complete the hot forging. For example, at alloy temperatures of 300℃, 350℃, 400℃, 500℃, etc., applying axial pressure to 10MPa, 20MPa, 30MPa, 50MPa, 80MPa, etc., within 5min, 10min, 15min, 20min, etc. As a result, the alloy is compressed due to hot forging, resulting in increased density and the generation of a large number of dislocations inside. Optionally, the vacuum degree of the furnace cavity is 0.1Pa to 20Pa, for example, 0.1Pa, 1Pa, 3Pa, 5Pa, 15Pa, 20Pa, etc.

[0058] According to a first aspect of the invention, annealing and hot forging processes simultaneously improve the mechanical strength and thermoelectric properties of bismuth telluride-based master alloys. Specifically, annealing the bismuth telluride-based alloy obtained by powder alloying promotes uniform element distribution, facilitates dislocation movement to absorb tellurium vacancies within the alloy, and promotes grain boundary movement, causing the alloy to expand and transforming micropores into interconnected macropores. Subsequently, hot forging compresses the alloy that has expanded due to annealing, increasing its density and simultaneously generating a large number of dislocations within the alloy. The increased density ensures good carrier transport properties, while the presence of numerous dislocations enhances phonon scattering, thereby jointly improving thermoelectric properties. Simultaneously, the high density and dislocation density also improve mechanical properties.

[0059] In a second aspect of the invention, a bismuth telluride-based alloy obtained in the first aspect is used.

[0060] The bismuth telluride-based alloy of the second aspect of the present invention has both high mechanical strength and high thermoelectric properties, with a bending strength of up to 140 MPa, a compressive strength of up to 224 MPa, and a ZT value of up to 1.50 at 348 K.

[0061] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0062] The Bi elemental powder (purity 99.99%) and Sb elemental particles (purity 99.99%) used in the embodiments and comparative examples of this invention were produced by Shanghai Aladdin Biochemical Technology Co., Ltd., the Te elemental powder (purity 99.999%) was produced by Zhongnuo New Materials (Beijing) Technology Co., Ltd., and the nano SiC (purity 95%) was produced by Alfa Aesar.

[0063] Example 1

[0064] S1, according to the chemical formula Bi 0.4 Sb 1.6 Te3 (i.e., chemical formula Bi) x Sb 2-x Te 3+y In the stoichiometric ratio of x=0.4, y=0), 2.5285g of Bi elemental powder, 5.8930g of Sb elemental particles, and 11.5784g of Te elemental powder were weighed.

[0065] Weigh the mixture and put it into a stainless steel ball mill jar. The mass ratio of stainless steel grinding balls to raw materials is 20:1. High-purity argon gas is filled into the ball mill jar as a protective gas. The mixture is ball milled in a planetary ball mill at a speed of 480 r / min for 6 hours to complete the mixing.

[0066] S2, Load the ball-milled powder into a container with a diameter of... The graphite mold is compacted and then placed in a spark plasma sintering furnace (SPS). The vacuum degree is evacuated to less than 5 Pa, and the sintered block is obtained by holding it at 400℃ for 5 min under an axial pressure of 50 MPa. This is the bismuth telluride-based master alloy.

[0067] S3. Place the bismuth telluride-based intermediate alloy body inside a quartz tube and evacuate to a vacuum level of 5 × 10⁻⁶. -3 Pa and seal the tube, then place it in a muffle furnace and heat it to 400℃ at a rate of 5℃ / min, and anneal it at 400℃ for 48h.

[0068] S4. Place the annealed bismuth telluride-based master alloy in a container with a diameter of [missing information]. The graphite mold is placed in an SPS, the vacuum is evacuated to less than 5 Pa, the temperature is raised to 400 °C, and the pressure is increased to 50 MPa within 15 min to obtain the bismuth telluride-based alloy.

[0069] Example 2

[0070] The other operations are the same as in Example 1, except that in S1, 0.0383g of nano-SiC with a Dv50 of 30nm is also weighed and mixed.

[0071] Example 3

[0072] The other operations are the same as in Example 1, except that in S1 and S3, x = 0.4, y = 0.01, 2.5189g of Bi elemental powder, 5.8704g of Sb elemental particles, 11.5724g of Te elemental powder, and 0.0383g of nano-SiC (Dv50 is 30nm) are weighed and mixed; in S3, the temperature is raised to 550℃ at a rate of 5℃ / min and held for annealing for 48h in a muffle furnace.

[0073] Comparative Example 1

[0074] The other operations are the same as in Example 1, except that S3 and S4 are not present.

[0075] Comparative Example 2

[0076] The other operations are the same as in Example 1, except that S4 is performed directly without going through S3.

[0077] Comparative Example 3

[0078] The other operations are the same as in Example 1, except that S4 is not performed.

[0079] The following performance tests and structural characterizations were performed on Examples 1-3 and Comparative Examples 1-3.

[0080] I. Characterization of Phase and Microstructure

[0081] 1. The absolute density was obtained by Archimedes' displacement method. The theoretical density, according to the standard PDF card No. 072-1836, is 6.7932 g·cm³. -3 The relative density is obtained by dividing the absolute density by the theoretical density, and the data are shown in Table 1.

[0082] 2. X-ray diffraction analysis

[0083] The bismuth telluride-based alloys of Examples 1-3 and Comparative Examples 1-2 were tested using X-ray diffraction. The test results are shown in [Figure number missing]. Figure 1 .

[0084] from Figure 1 It can be seen that the phase structures of the products of Examples 1-3 and Comparative Examples 1-2 are similar to those of pure phase Bi. 0.5 Sb 1.5 The fact that Te3 is the same indicates that the processing method of the present invention can stably obtain bismuth telluride-based alloys.

[0085] 3. Scanning electron microscopy analysis

[0086] The fracture morphology of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 was characterized using field emission scanning electron microscopy. The results are shown in the figure. Figure 2 The morphology of the polished surfaces of the bismuth telluride-based alloys in Example 1 and Comparative Example 1 was characterized using field emission scanning electron microscopy. The results are shown in [Figure number missing]. Figure 3 .

[0087] Figure 2 This indicates that the grain size of the sample annealed at 400℃ for 48 hours and then hot-forged did not show significant growth compared to before annealing and hot forging, while the grain size of the sample annealed at 550℃ for 48 hours and then hot-forged showed slight growth. Figure 3 It can be seen that, compared with the polished surface of Comparative Example 1, the polished surface of Example 1 has fewer internal pores, while Comparative Example 1 has a large number of pores. This indicates that the sintered block obtained in S2, after annealing in S3 and hot forging in S4, has significantly reduced internal pores. This may be the main reason for the increased density of the sample after annealing and subsequent hot forging. Figure 2 As can be seen in (f), there are a large number of interconnected pores inside Comparative Example 3. These pores closed after hot forging in S4. Therefore, it can be inferred that the micropores in the sintered block obtained in S2 were transformed into interconnected macropores after annealing in S3. These macropores closed after hot forging in S4, thereby improving the density of the sample.

[0088] 4. Transmission electron microscopy analysis

[0089] The microstructure of the bismuth telluride-based alloy in Example 3 was characterized using transmission electron microscopy (TEM), and the results are shown in the figure. Figure 4 . Figure 4 As seen in (a) and (b), the sample contains a large number of dislocations of different morphologies. This high density of dislocations hinders the movement of dislocations, thereby improving the mechanical strength of the sample. Furthermore, Figure 4 (c) A large number of nano-SiCs embedded in the grain boundaries can be observed. The size of the nano-SiCs ranges from 30μm to 100μm. These nano-SiCs, as a second phase, can also hinder dislocation movement, produce a dispersion strengthening effect, and improve the mechanical strength of the sample.

[0090] II. Mechanical Property Testing

[0091] The flexural and compressive strengths of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 were tested using a universal testing machine. The flexural strength was tested using the three-point bending method, and the sample size was 15 × 2 × 2 mm. 3 The span is fixed at 10mm, and the loading rate is 0.2mm·min. -1 The sample size used for the compressive strength test was 6×3×3mm. 3The loading rate is 0.3 mm / min. -1 The results are shown in Table 1.

[0092] Table 1

[0093]

[0094] Table 1 shows the absolute and relative densities of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2. Comparing Example 1 and Comparative Example 2, it can be seen that the bismuth telluride-based alloy of Example 1, annealed at 400°C for 48 hours in S3 and then hot-forged, has a higher density than the bismuth telluride-based alloy of Comparative Example 2, which was only hot-forged without annealing. This indicates that annealing in S3 before hot forging can increase the density of the bismuth telluride-based alloy. Comparing the data of Comparative Example 1 and Comparative Example 2, it can be seen that directly performing hot forging in S3 after S2 actually leads to a decrease in the density of the bismuth telluride-based alloy. Comparing Example 2 and Example 1, it can be seen that the density decreases slightly after adding nano-SiC. This is because SiC has a lower density than bismuth telluride, and the presence of SiC as hard particles may also affect the ball milling in S1 and the sintering in S2. Therefore, the density decreases after adding SiC. By comparing Example 3 and Example 2, it can be seen that adding excess Te and increasing the annealing temperature can further increase the density of bismuth telluride-based alloys.

[0095] Table 1 also shows that the flexural and compressive strengths of Examples 1-3 are higher than those of Comparative Examples 1 and 2. The higher flexural and compressive strengths of Example 1 compared to Comparative Examples 1 and 2 indicate that the bismuth telluride-based alloy obtained by S2 undergoing S2 annealing followed by S3 hot forging significantly improves its flexural and compressive strength, while the hot forging treatment alone in Comparative Example 2 does not significantly improve its strength. Furthermore, the addition of nano-SiC in Example 2 further enhances the flexural and compressive strength of the sample. In Example 3, the addition of nano-SiC followed by excess Te slightly reduces the sample strength, but the bismuth telluride-based alloy exhibits higher thermoelectric properties. Therefore, by treating the bismuth telluride-based alloy using the method of annealing followed by hot forging, ideal mechanical strength and thermoelectric properties can be obtained by controlling the amount of nano-SiC and excess Te added.

[0096] III. Thermoelectric Performance Testing

[0097] 1. Conductivity σ

[0098] The electrical conductivity of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 was tested using a thermoelectric performance testing system (ZEM-3, manufactured by Ulvac-Riko, Japan). For testing, rectangular strip-shaped specimens were cut from the alloys and their surfaces were polished smooth. The length of the specimens was perpendicular to the pressure direction of sintering and hot forging. The testing temperature range was from room temperature to 200°C (473 K), and the tests were conducted in a high-purity helium atmosphere. The data obtained are shown below. Figure 5 .

[0099] from Figure 5 It can be seen that the electrical conductivity of the bismuth telluride-based alloys in Examples 1-3 is higher than that in Comparative Examples 1-2 at the same temperature. The electrical conductivity of both examples and comparative examples decreases with increasing temperature. Compared to Comparative Example 1, Comparative Example 2 shows that the electrical conductivity of the bismuth telluride-based alloy obtained by S2 sintering does not change significantly when directly subjected to S4 hot forging. Compared to Comparative Example 2, Examples 1-3 show that adding S3 annealing treatment before S4 hot forging significantly improves the electrical conductivity of the bismuth telluride-based alloy. Compared to Example 1, Example 2 shows that adding nano-SiC does not significantly improve the electrical conductivity. Compared to Example 2, Example 3 further improves the electrical conductivity of the bismuth telluride-based alloy by adding excess Te and increasing the annealing temperature.

[0100] 2. Seebeck coefficient S

[0101] The Seebeck coefficients of the samples in Examples 1-3 and Comparative Examples 1-2 were tested using a thermoelectric performance testing system (ZEM-3, manufactured by Ulvac-Riko, Japan). For testing, rectangular strip-shaped specimens were cut from the alloy and their surfaces were polished smooth. The length of the specimen was perpendicular to the pressure direction of sintering and hot forging. The testing temperature range was from room temperature to 200°C (473 K), and the tests were conducted in a high-purity helium atmosphere. The data obtained are shown in [reference needed]. Figure 6 .

[0102] from Figure 6 It can be seen that the Seebeck coefficients of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 all first increase and then decrease with increasing temperature. Comparative Examples 1 and 2 show that the Seebeck coefficient of the sintered sample of S2 does not change significantly after hot forging with S4. Comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the Seebeck coefficient of the bismuth telluride-based alloy decreases after adding S3 annealing treatment before hot forging with S4. Adding nano-SiC to Example 1 increases the Seebeck coefficient of Example 2. Further adding excess Te and increasing the annealing temperature to Example 2 decreases the Seebeck coefficient of the bismuth telluride-based alloy of Example 3.

[0103] 3. Power factor σ·S 2

[0104] The power factor of the samples in Examples 1-3 and Comparative Examples 1-2 can be calculated from the measured conductivity and Seebeck coefficient, where power factor = σ·S 2 Where σ is the conductivity and S is the Seebeck coefficient, the data obtained are shown in [reference needed]. Figure 7 .

[0105] Depend on Figure 7 It can be seen that the power factors of the bismuth telluride-based alloys in Examples 1-3 are higher than those in Comparative Examples 1-2 at the same temperature. The power factors of Examples 1-3 and Comparative Examples 1-2 all decrease with increasing temperature. Specifically, the power factor of the sintered samples does not show significant change after hot forging. When annealing is added before hot forging, the power factor of the samples increases. Furthermore, adding nano-SiC, and further adding excess Te and increasing the annealing temperature, further improves the conductivity of the samples. Finally, the chemical formula of the nano-SiC with a volume fraction of 0.4% is Bi. 0.4 Sb 1.6 Te 3.01 The sample, after sintering, underwent vacuum annealing at 550℃ for 48 hours, followed by hot forging, achieving a power factor of 5.17 mW·m. -1 ·K -2 .

[0106] 4. Thermal conductivity κ

[0107] The thermal diffusivity of the samples in Examples 1-3 and Comparative Examples 1-2 was tested using a laser thermal conductivity meter (LFA457, manufactured by Netzsch, Germany). The thermal conductivity of the samples was calculated based on the thermal diffusivity, and the data are shown below. Figure 8 For the thermal diffusivity test, a circular piece with a diameter of 6 mm and a thickness of approximately 1 mm was cut from the alloy, and the surface was polished smooth. The thickness direction was perpendicular to the pressure direction of sintering and hot forging. High-purity argon was introduced as the purge gas during the test, and the test temperature ranged from 30℃ (303K) to 200℃ (473K). The formula for calculating thermal conductivity is κ=λC. p d. Where λ is the thermal diffusivity of the sample, d is the density of the sample, and C p The isobaric heat capacity of the sample can be estimated using the Dulong-Petit equation.

[0108] Figure 8 It can be seen that the thermal conductivity of Examples 1-3 and Comparative Examples 1-2 first decreases and then increases with increasing temperature. Specifically, the thermal conductivity of the sintered samples does not show a significant change after hot forging. When annealing is added before hot forging, the thermal conductivity of the samples increases slightly before 75°C. On this basis, the addition of nano-SiC, and further addition of excess Te and increasing the annealing temperature, all increase the thermal conductivity of the samples.

[0109] 5. Lattice thermal conductivity

[0110] The sum of the lattice thermal conductivity and bipolar thermal conductivity of the samples in Examples 1-3 and Comparative Examples 1-2 can be obtained by subtracting the electronic thermal conductivity, calculated from the electrical conductivity, from the thermal conductivity. The electronic thermal conductivity is calculated using the Wiedeman-Franz law. The difference between the thermal conductivity and the electronic thermal conductivity can be considered the lattice thermal conductivity of the sample in the near-room temperature range.

[0111] Figure 9 It can be seen that the lattice thermal conductivity of the bismuth telluride-based alloys in Examples 1-3 is lower than that in Comparative Examples 1-2 at the same temperature. The lattice thermal conductivity of the bismuth telluride-based alloys in Examples 1-3 and Comparative Examples 1-2 all show an increasing trend with increasing temperature. Specifically, the lattice thermal conductivity of the sintered samples does not show a significant change after hot forging. When annealing is added before hot forging, the lattice thermal conductivity of the bismuth telluride-based alloy decreases. Adding nano-SiC slightly increases the lattice thermal conductivity. Further adding excess Te and increasing the annealing temperature further increases the electrical conductivity of the bismuth telluride-based alloy.

[0112] 6. ZT value

[0113] The ZT value measures the thermoelectric properties of a sample. The ZT value was calculated from the power factor and thermal conductivity of the samples in Examples 1-3 and Comparative Examples 1-2. The results are shown in [Figure number missing]. Figure 10 Where ZT = σ·S 2 ·T / κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, κ is the thermal conductivity, and σ·S 2 This is the power factor.

[0114] Figure 10 It can be seen that the ZT values ​​of the bismuth telluride-based alloys in Examples 1-3 are all higher than those in Comparative Examples 1-2 at the same temperature. The ZT values ​​of Examples 1-3 and Comparative Examples 1-2 first increase and then decrease with increasing annealing temperature. Specifically, the ZT values ​​of the sintered samples do not show significant changes after hot forging. When annealing is added before hot forging, the ZT value of the bismuth telluride-based alloy increases. Adding nano-SiC further increases the ZT value. Further addition of excess Te and increasing the annealing temperature further increases the ZT value. Finally, adding 0.4% by volume of nano-SiC with the chemical formula Bi... 0.4 Sb 1.6 Te 3.01 The sample, after sintering, underwent vacuum annealing at 550℃ for 48 hours, followed by hot forging. At 340K, the ZT value reached as high as 1.50. In contrast, the ZT values ​​of the bismuth telluride-based alloys in Comparative Examples 1 and 2 at the same temperature were less than 1.20.

[0115] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0116] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for processing bismuth telluride-based alloys, characterized in that, Includes the following steps: (1) Provide a bismuth telluride-based master alloy prepared by powder metallurgy process; (2) The bismuth telluride-based master alloy is annealed in a vacuum environment or under an inert gas. (3) The annealed bismuth telluride-based master alloy is hot-forged in a vacuum environment or in an inert gas to obtain a bismuth telluride-based alloy. In step (2), the annealing temperature is 300℃~600℃; Annealing time is 3 h to 72 h; The hot forging process in step (3) is carried out in a discharge plasma sintering apparatus, and the hot forging conditions include: After evacuating the furnace cavity, hot forging is performed by applying axial pressure to 10 MPa to 80 MPa at a temperature of 300℃ to 500℃ for the annealed bismuth telluride-based master alloy. The pressurization time is 5 min to 20 min; The vacuum level of the furnace chamber is 0.1 Pa to 20 Pa.

2. The processing method for bismuth telluride-based alloys according to claim 1, characterized in that, In step (1), the chemical formula of the bismuth telluride-based master alloy is Bi. x Sb 2-x Te 3+y ， in, 0.3≤ x ≤0.5，0≤ y ≤0.3。 3. The processing method for bismuth telluride-based alloys according to claim 2, characterized in that, 0≤ y ≤0.05。 4. The processing method for bismuth telluride-based alloys according to claim 2, characterized in that, In step (1), the bismuth telluride-based master alloy further includes a second phase, which includes nano-SiC.

5. The processing method for bismuth telluride-based alloys according to claim 4, characterized in that, Based on the mass of the matrix phase of the bismuth telluride-based master alloy, the mass fraction of nano-SiC does not exceed 0.5%.

6. The processing method for the bismuth telluride-based alloy according to claim 4, characterized in that, The Dv50 of the nano-SiC is 3 nm to 200 nm.

7. The processing method of the bismuth telluride-based alloy according to any one of claims 1-2 and 4-6, characterized in that, The preparation method of the bismuth telluride-based master alloy in step (1) is as follows: (101) According to the chemical formula Bi x Sb 2-x Te 3+y Bi source, Sb source and Te source were weighed in stoichiometric proportions and mixed and ball-milled to obtain the precursor; Where, 0.3≤ x ≤0.5, 0≤ y ≤0.3; (102) The precursor is sintered to obtain the bismuth telluride-based intermediate alloy.

8. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, Step (101) also includes: Based on the mass of Bi, Sb, and Te sources, nano-SiC with a mass fraction not exceeding 0.5% was weighed and mixed with the Bi, Sb, and Te sources and ball-milled.

9. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, The Bi source includes at least one of elemental Bi, Bi2O3, BiCl3, Bi(NO3)3, and Bi2Te3.

10. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, The Sb source includes at least one of elemental Sb, Sb₂O₃, SbCl₃, Sb₂S₃, and Sb₂Te₃.

11. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, The Te source includes at least one of elemental Te, TeO2, or tellurate.

12. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, Step (101), the mixing ball milling is carried out under a protective atmosphere.

13. The processing method of the bismuth telluride-based alloy according to claim 12, characterized in that, The protective atmosphere includes at least one of an inert gas and an argon-hydrogen mixture.

14. The processing method of the bismuth telluride-based alloy according to claim 13, characterized in that, In the argon-hydrogen mixture, the volume fraction of hydrogen is less than 6%.

15. The processing method of the bismuth telluride-based alloy according to claim 7, characterized in that, The sintering in step (102) is performed in a discharge plasma device, and the sintering conditions include: The furnace cavity is evacuated, and sintering is carried out for 3 to 20 minutes under the conditions of axial pressure of 10 MPa to 80 MPa and precursor temperature of 300℃ to 500℃.

16. The processing method of the bismuth telluride-based alloy according to claim 15, characterized in that, The vacuum level of the furnace chamber is 0.1 Pa to 20 Pa.

17. A bismuth telluride-based alloy, characterized in that, It is processed using the processing method of bismuth telluride-based alloy as described in any one of claims 1 to 16.

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