Preparation method of isotropic polycrystalline bismuth telluride

Abstract

The invention discloses a preparation method of isotropic polycrystalline bismuth telluride, which comprises the following steps: firstly, preparing a bismuth telluride alloyed powder raw material, ultrasonically vibrating, screening powder with specific granularity, putting into a drying oven for drying treatment, loading into a graphite mold, and transferring into a vacuum hot pressing furnace; and successively carrying out slow heating and pressurizing treatment, heat preservation and pressure maintaining treatment and heat preservation and pressure relief treatment, and carrying out furnace cooling to obtain the high-performance isotropic polycrystalline bismuth telluride block. The method provided by the invention makes up the defects of the existing bismuth telluride process and method for preparation, meets the urgent demand on the high-performance isotropic polycrystalline bismuth telluride thermoelectric material in fundamental research and application production, and avoids the brittleness problem caused by mechanical property difference in different directions after the bismuth telluride material is deformed; and the optimization of the thermoelectric performance and the uniform stability in all directions are realized, the manufacturing cost is greatly saved, and a good material preparation basis is provided for subsequent processes of scribing, pelletizing, packaging and the like of bismuth telluride.

Description

Technical Field

[0001] The invention relates to the field of preparation technology and performance optimization of bismuth telluride thermoelectric semiconductor materials, and in particular provides a method for preparing isotropic polycrystalline bismuth telluride. Background Art

[0002] Bismuth telluride-based thermoelectric materials are currently the most widely used commercial thermoelectric materials. They have good electrical conductivity and low thermal conductivity and are mainly used in the fields of thermoelectric refrigeration and temperature difference power generation. Bismuth telluride materials have the obvious characteristics of topological insulators, which allow electrons to flow freely on their surface without losing any energy. This will undoubtedly bring a qualitative leap in the improvement of chip working efficiency and greatly improve the operating speed of computer chips. Bismuth telluride materials are usually prepared by zone melting, hot extrusion, plasma spark sintering (SPS), hot pressing and other process methods. The Cohen coefficient of the prepared material reaches a maximum of 0.5 to 0.8 (0.333 when uniform in all directions), indicating that there is severe anisotropy in the crystal orientation or texture distribution.

[0003] The crystal structure of bismuth telluride is rhombohedral and belongs to the R-3m space group. When plastic deformation occurs, the deformation modes of bismuth telluride material include basal plane Slip, cone<a+c> Slip and {0001} twin deformation, among which displacement slip is considered to be the most important deformation mechanism in thermoelectric materials. In particular, the Te-Te bonds in bismuth telluride crystals are combined by van der Waals forces, and the atomic layer where they are located is very easy to break. In addition, the existence of van der Waals forces between Te-Te bonds is also the essential reason for the anisotropy of the thermoelectric transport properties of bismuth telluride materials. In terms of thermoelectric properties, the electron mobility perpendicular to the c-axis in bismuth telluride is about 4 times that of the c-axis, the hole mobility is about 3 times, and the lattice thermal conductivity is about 2 times. Therefore, along the C-axis direction of the bismuth telluride crystal, the thermoelectric and mechanical properties of bismuth telluride are much smaller than those perpendicular to the C-axis direction, so people often face the problem of easy processing and brittle fracture along the C-axis and can only be used along the direction perpendicular to the C-axis.

[0004] For the above problems, no good solution has been found so far. Strongly textured thermoelectric materials will cause cracking and breakage of materials during material preparation and subsequent processing, resulting in great waste and increased manufacturing costs. It is also impossible to guarantee the uniformity and stability of the thermoelectric transmission performance of the material in all directions. This method can regulate the composition, control the grain size, introduce point defects and surface defects, etc., which can effectively affect the antisite defect concentration, carrier concentration, carrier mobility, and scattered high-frequency short-wavelength phonons. It is a very promising technology for the preparation and performance optimization of bismuth telluride thermoelectric materials. At present, scholars are also looking for some effective methods to improve the performance and stability of bismuth telluride. However, according to existing patents, there is currently no method for preparing high-performance isotropic polycrystalline bismuth telluride. Summary of the invention

[0005] In view of the shortcomings of the prior art and the urgent need for applied basic research, the present invention provides a method for preparing high-performance isotropic polycrystalline bismuth telluride with simple process, strong operability and excellent technical effect.

[0006] The present invention adopts the following technical solution: a method for preparing isotropic polycrystalline bismuth telluride, the preparation method specifically comprising the following steps:

[0007] S1) using bismuth telluride as a raw material, performing an alloying treatment on the bismuth telluride, and then performing ultrasonic vibration and screening to obtain a bismuth telluride alloy powder with a certain particle size;

[0008] S2) drying the bismuth telluride alloy powder obtained in S1);

[0009] S3) placing the bismuth telluride alloyed powder processed in S2) in a mold for charging and combining the powder as a whole;

[0010] S4) transferring the mold containing the alloyed powder in S3) to a vacuum hot pressing furnace, and performing a slowly heating and pressurizing treatment to obtain a bismuth telluride material;

[0011] S5) performing heat preservation and pressure preservation treatment on the bismuth telluride material of S4);

[0012] S6) After unloading the pressure, the bismuth telluride material obtained in S5) is subjected to a heat preservation and pressure relief treatment, and is cooled along with the furnace to obtain an isotropic polycrystalline bismuth telluride block.

[0013] Furthermore, the chemical formula of the bismuth telluride in S1) satisfies (Bi, Sb)2(Te, X)3, wherein X is a doping element with a content of 0 to 1%.

[0014] Further, the alloying treatment in S1) is any one of gas atomization treatment, ball milling mechanical alloying, vacuum melting, swing melting, zone melting melting and subsequent crushing;

[0015] The particle size after the screening treatment is no greater than 75 μm.

[0016] Further, the drying process in S2) is carried out in an inert atmosphere or in a vacuum;

[0017] The inert atmosphere is argon, nitrogen or helium; the vacuum degree is: -0.1MPa.

[0018] Further, the mold in S3) is a graphite mold made of isostatic pressing material, and its shape is square or round;

[0019] The mold comprises a base, a sleeve, a pressing head and a backing plate;

[0020] In addition, during the loading process, there is a graphite paper transition between the base, the sleeve, the bismuth telluride alloy powder, the pressure head and the pad.

[0021] Furthermore, the heating rate in S4) is 4-12°C / min, and the pressurization rate is 0.2-0.8MPa / min.

[0022] Furthermore, the insulation temperature in S5) is 360-480° C., the insulation pressure is 15-25 MPa, and the insulation time is 30-90 min.

[0023] Furthermore, the pressure relief and heat preservation temperature in S6) is 420-450° C., and the heat preservation time is 20-60 min.

[0024] Furthermore, the Cohen coefficient of the isotropic polycrystalline bismuth telluride block in S6) is between 0.283 and 0.383 in all directions, and the ZT value is not less than 1.0.

[0025] An isotropic polycrystalline bismuth telluride is prepared by the above-mentioned preparation method.

[0026] The performance index of thermoelectric materials can be measured by the dimensionless parameter thermoelectric figure of merit ZT value, and its calculation formula is shown in formula (1):

[0027]

[0028] Among them, the parameters S, σ, κ, and T represent the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature of the thermoelectric material, respectively. As can be seen from the above formula, the thermoelectric performance mainly depends on the characteristics of the material itself, and has nothing to do with the appearance size and morphology of the material.

[0029] Generally speaking, temperature difference materials with good thermoelectric performance require a large power factor P to ensure a significant thermoelectric effect. The calculation formula of the power factor is shown in formula (2):

[0030] P=S 2 σ (2)

[0031] Combining formulas (1) and (2), it can be seen that in order to obtain a ZT value as high as possible, the material needs to have both high electrical conductivity and Seebeck coefficient, and relatively low thermal conductivity.

[0032] Electrons and holes are two common types of carriers in semiconductor materials. For (Bi, Sb)2(Te, X)3 materials, the majority of carriers (i.e., majority carriers) are holes. The conductivity σ depends on the concentration and mobility of holes. Its calculation is shown in formula (3):

[0033] σ=peμ p (3)

[0034] Among them, the parameters P, e, μ p They represent the hole concentration, charge, and hole mobility of thermoelectric materials. The hole concentration and mobility are related to the material type, doping amount, crystal structure, defects, and temperature.

[0035] The Seebeck coefficient is an intrinsic physical quantity of a material, and its physical meaning is the magnitude of the potential difference generated within a certain temperature difference range. For P-type semiconductors, holes are carriers, and the Seebeck coefficient is a positive value. For semiconductors with degenerate characteristics, the relationship between the Seebeck coefficient and the carrier concentration can be expressed by formula (4):

[0036]

[0037] Among them, the parameter k B ,h,m * , γ represent the Boltzmann constant, Planck constant, effective carrier mass, and scattering factor, respectively. From the above formula, we can see that the Seebeck coefficient is positively correlated with the effective carrier mass and scattering factor, and negatively correlated with the carrier concentration.

[0038] The thermal conductivity κ is determined by the electronic thermal conductivity κ e and lattice thermal conductivity κ l Contribution, where the electronic thermal conductivity is the energy carried by holes during lattice transport, and the lattice thermal conductivity is the energy generated by phonons during lattice conduction. Therefore, thermal conductivity can be expressed as formula (5):

[0039]

[0040] Among them, parameter C V , ν a , l P They represent the isotropic heat capacity, the speed of sound, and the mean free path of phonons, respectively. From the above formula, we can see that the electronic thermal conductivity is positively correlated with the electrical conductivity and is more closely related to the scattering mechanism of the carriers, while the lattice thermal conductivity has nothing to do with the electronic structure of the material.

[0041] The beneficial effects of the present invention are as follows: due to the adoption of the above technical scheme, the material undergoes macroscopic plastic deformation under a certain temperature and external force, and its microscopic scale is accompanied by grain crushing, grain boundary migration, orientation adjustment and dislocation slip, etc., and an isotropic bismuth telluride sample with an average grain size of 1 to 10 μm and similar Cohen coefficients in all directions is prepared. Thermal deformation controls the grain size, which not only brings about changes in the lattice level, but also causes the recombination of electron-hole pairs, produces a donor-like effect, and then adjusts the carrier concentration and mobility. At the same time, the submicron structured grain boundaries, twin boundaries, dislocations, and nanoscale second phase defects introduced by the thermal deformation process can effectively scatter phonons of specific wavelengths, strengthen the thermal vibration of the lattice, introduce multiple scattering effects, optimize carrier mobility and thermal conductivity, and improve the power factor. In addition, by introducing an appropriate amount of pores into the bulk material prepared by the present invention, the specific heat capacity, sound velocity, and phonon mean free path of the material are adjusted, promoting enhanced phonon scattering and reduced thermal conduction, which helps to maintain a higher electrical conductivity while reducing thermal conductivity, thereby achieving improved thermoelectric performance.

[0042] The method of the present invention makes up for the shortcomings of the existing bismuth telluride preparation process and method, meets the urgent demand for high-performance isotropic polycrystalline bismuth telluride thermoelectric materials in basic research and application production, avoids the brittleness problem caused by the difference in mechanical properties of bismuth telluride materials in different directions after deformation, and achieves the optimization of thermoelectric performance and uniform stability in all directions, greatly saves manufacturing costs, and provides a good material preparation basis for subsequent bismuth telluride slicing, dicing and packaging processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The process flow chart of the method for preparing high-performance isotropic polycrystalline bismuth telluride of the present invention;

[0044] Figure 2 SEM morphology of bismuth telluride alloyed powder in Example 1;

[0045] Figure 3 Pole figures and Cohen coefficients of high performance isotropic bismuth telluride prepared in Example 1;

[0046] Figure 4 ZT value-temperature distribution diagram of high-performance isotropic bismuth telluride prepared in Example 1;

[0047] Figure 5 Metallographic microstructure of high performance isotropic bismuth telluride prepared in Example 2;

[0048] Figure 6 Force-displacement curve of high-performance isotropic bismuth telluride prepared in Example 2. DETAILED DESCRIPTION

[0049] The present invention is explained in detail below in conjunction with a number of specific embodiments. The process flow chart of the method for preparing high-performance isotropic polycrystalline bismuth telluride is as follows: Figure 1 shown.

[0050] The present invention provides a method for preparing high-performance isotropic polycrystalline bismuth telluride, the method comprising the following steps:

[0051] (1) preparing bismuth telluride alloy powder raw material, and performing ultrasonic vibration and screening treatment;

[0052] (2) placing the alloyed powder after the treatment in step (1) in an oven for drying;

[0053] (3) placing the alloyed powder processed in step (2) in a specific mold for loading, and assembling the base, sleeve, alloy powder, pressure head and backing plate together;

[0054] (4) transferring the mold containing the alloyed powder in step (3) to a vacuum hot pressing furnace for slowly heating and pressurizing;

[0055] (5) subjecting the bismuth telluride material of step (4) to a heat preservation and pressure preservation treatment;

[0056] (6) Unloading the pressure, performing a heat preservation and pressure relief treatment on the bismuth telluride material of step (5), and cooling it along with the furnace temperature drop, so as to obtain a high-performance isotropic polycrystalline bismuth telluride block.

[0057] As the preferred technical solution:

[0058] The main elements of the bismuth telluride described in step (1) are Bi, Sb, and Te, and its chemical formula satisfies (Bi, Sb)2(Te, X)3, and the content of the doping element X is 0-1%.

[0059] The purity of each element in the bismuth telluride in step (1) is not less than 99.99%.

[0060] The bismuth telluride alloy powder is prepared by gas atomization, ball milling mechanical alloying, vacuum melting, swing melting, zone melting and then crushing.

[0061] The particle size of the bismuth telluride alloyed powder after screening treatment in step (1) is no more than 75 μm.

[0062] The drying treatment in step (2) is carried out in an inert atmosphere such as argon, nitrogen, helium or in a vacuum.

[0063] The specific mold in step (3) is a graphite mold made of isostatic pressing material and has a square or circular shape.

[0064] During the loading process described in step (3), there is a graphite paper transition between the base, sleeve, alloy powder, pressure head and pad.

[0065] The heating rate in step (4) is 4-12°C / min, and the pressurization rate is 0.2-0.8MPa / min.

[0066] The insulation temperature in step (5) is 360-480° C., the insulation pressure is 15-25 MPa, and the insulation time is 30-90 min.

[0067] The pressure relief and heat preservation temperature in step (6) is 420-450° C., and the heat preservation time is 20-60 min.

[0068] The high-performance isotropic polycrystalline bismuth telluride block described in step (6) has a Cohen coefficient in all directions between 0.283 and 0.383 and a ZT value not less than 1.0.

[0069] Example 1

[0070] This embodiment specifically includes the following steps:

[0071] A method for preparing high-performance isotropic polycrystalline bismuth telluride, the method comprising the following steps:

[0072] (1) Bi, Te, Sb, and Se with a purity of 4N were weighed in the ratio of (Bi, Sb)2(Te, Se)3. After swing melting and disc grinding, the powder was sieved through 200 meshes for ultrasonic vibration and sieving to obtain a bismuth telluride alloy powder raw material with a particle size of no more than 75 μm. Its SEM morphology is as follows: Figure 2 As shown;

[0073] (2) adding anhydrous ethanol to the alloyed powder treated in step (1), placing the powder in an oven, and drying the powder under an argon environment;

[0074] (3) placing the alloyed powder processed in step (2) in a specific square graphite mold for loading, with graphite paper transitions between the base, sleeve, alloy powder, pressure head and backing plate, and combining these parts as a whole;

[0075] (4) transferring the mold containing the alloyed powder in step (3) to a vacuum hot pressing furnace, and then slowly heating and pressing at a heating rate of 6° C. / min and a pressure rate of 0.3 MPa / min;

[0076] (5) subjecting the bismuth telluride material of step (4) to a heat preservation and pressure preservation treatment, wherein the heat preservation temperature is 460° C., the heat preservation pressure is 21 MPa, and the heat preservation time is 60 min;

[0077] (6) raising the hot pressing furnace pressure head and unloading the pressure, subjecting the bismuth telluride material of step (5) to a heat preservation and pressure relief treatment at 430° C. for 30 min, and cooling the material along with the furnace temperature drop, thereby obtaining a high-performance isotropic polycrystalline bismuth telluride block, wherein the Cohen coefficients of the prepared bismuth telluride blocks in all directions are between 0.32 and 0.34, and the ZT values ​​are all greater than 1.04, such as Figures 3-4 shown.

[0078] Example 2

[0079] This embodiment specifically includes the following steps:

[0080] A method for preparing high-performance isotropic polycrystalline bismuth telluride, the method comprising the following steps:

[0081] (1) Bi, Te, Sb, and Se with a purity of 4N are weighed in the ratio of (Bi, Sb)2(Te, Se)3, and after zone melting and jaw crushing, the powder is sieved through 270 meshes for ultrasonic vibration and sieving to obtain a bismuth telluride alloy powder raw material with a particle size of no more than 53 μm;

[0082] (2) adding anhydrous ethanol to the alloyed powder treated in step (1), placing the powder in an oven, and drying the powder under vacuum;

[0083] (3) placing the alloyed powder processed in step (2) in a specific square graphite mold for loading, with graphite paper transitions between the base, sleeve, alloy powder, pressure head and backing plate, and combining these parts as a whole;

[0084] (4) transferring the mold containing the alloyed powder in step (3) to a vacuum hot pressing furnace, and then slowly heating and pressing at a heating rate of 8° C. / min and a pressure rate of 0.43 MPa / min;

[0085] (5) The bismuth telluride material of step (4) is subjected to heat preservation and pressure preservation treatment, with the heat preservation temperature being 430° C., the heat preservation pressure being 23 MPa, and the heat preservation time being 70 min. ;

[0086] (6) raising the hot pressing furnace pressure head and unloading the pressure, subjecting the bismuth telluride material of step (5) to a heat preservation and pressure relief treatment at 430° C. for 45 minutes, and cooling the material along with the furnace temperature drop, thereby obtaining a high-performance isotropic polycrystalline bismuth telluride block. Figure 5 It can be observed from the metallographic microstructure pictures that the prepared bismuth telluride block has no obvious preferred orientation, the Cohen coefficient in each direction is between 0.29 and 0.37, and the compressive strength is 92.8, 89.7, and 93.7 MPa. Figure 6 shown.

[0087] The above is a detailed introduction to a method for preparing isotropic polycrystalline bismuth telluride provided in the embodiment of the present application. The description of the above embodiment is only used to help understand the method and its core idea of ​​the present application; at the same time, for those skilled in the art, according to the idea of ​​the present application, there will be changes in the specific implementation method and application scope. In summary, the content of this specification should not be understood as limiting the present application.

[0088] For example, certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that hardware manufacturers may use different nouns to refer to the same component. This specification and claims do not use differences in names as a way to distinguish components, but use differences in the functions of components as the criteria for distinction. As mentioned throughout the specification and claims, "including" and "comprising" are open-ended terms, so they should be interpreted as "including / including but not limited to". "Approximately" means that within an acceptable error range, those skilled in the art can solve the technical problem within a certain error range and basically achieve the technical effect. The subsequent description of the specification is a preferred embodiment of the present application, but the description is for the purpose of illustrating the general principles of the present application, and is not used to limit the scope of the present application. The scope of protection of the present application shall be determined by the definition of the attached claims.

[0089] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a product or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such a product or system. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the product or system including the elements.

[0090] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

Claims

1. A method for preparing isotropic polycrystalline bismuth telluride, characterized in that: The preparation method specifically comprises the following steps: S1) using bismuth telluride as a raw material, performing an alloying treatment on the bismuth telluride, and then performing ultrasonic vibration and screening to obtain a bismuth telluride alloy powder with a certain particle size; S2) drying the bismuth telluride alloy powder obtained in S1); S3) placing the bismuth telluride alloyed powder processed in S2) in a mold for charging and combining the powder as a whole; S4) transferring the mold containing the alloyed powder in S3) to a vacuum hot pressing furnace, and performing a slowly heating and pressurizing treatment to obtain a bismuth telluride material; S5) performing heat preservation and pressure preservation treatment on the bismuth telluride material of S4); S6) After unloading the pressure, the bismuth telluride material obtained in S5) is subjected to a heat preservation and pressure relief treatment, and is cooled along with the furnace to obtain an isotropic polycrystalline bismuth telluride block.

2. The preparation method according to claim 1, characterized in that: The chemical formula of the bismuth telluride in S1) satisfies (Bi, Sb)2(Te, X)3, wherein X is a doping element with a content of 0-1%.

3. The preparation method according to claim 1, characterized in that: The alloying treatment in S1) is any one of gas atomization treatment, ball milling mechanical alloying, vacuum melting, swing melting, and zone melting followed by crushing; The particle size after the screening treatment is no greater than 75 μm.

4. The preparation method according to claim 1, characterized in that: The drying process in S2) is carried out in an inert atmosphere or in a vacuum; The inert atmosphere is argon, nitrogen or helium; the vacuum degree is: -0.1MPa.

5. The preparation method according to claim 1, characterized in that: The mold in S3) is a graphite mold made of isostatic pressing material, and its shape is square or round; The mold comprises a base, a sleeve, a pressing head and a backing plate; In addition, during the loading process, there is a graphite paper transition between the base, the sleeve, the bismuth telluride alloy powder, the pressure head and the pad.

6. The preparation method according to claim 1, characterized in that: The heating rate in S4) is 4-12°C / min, and the pressurization rate is 0.2-0.8MPa / min.

7. The preparation method according to claim 1, characterized in that: The insulation temperature in S5) is 360-480°C, the insulation pressure is 15-25MPa, and the insulation time is 30-90min.

8. The preparation method according to claim 1, characterized in that: The pressure relief and heat preservation temperature in S6) is 420-450° C., and the heat preservation time is 20-60 min.

9. The preparation method according to claim 1, characterized in that: The isotropic polycrystalline bismuth telluride block in S6) has a Cohen coefficient in all directions between 0.283 and 0.383 and a ZT value not less than 1.

0.

10. An isotropic polycrystalline bismuth telluride, characterized in that: The isotropic polycrystalline bismuth telluride is prepared by the preparation method according to any one of claims 1 to 9.

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