N-type PbTe-based polycrystalline thermoelectric material and preparation method thereof

Abstract

The present invention discloses an N-type PbTe-based polycrystalline thermoelectric material and a preparation method thereof. The thermoelectric material comprises: a PbTe matrix doped with sulfur and iodine; and elemental lead, wherein the molar ratio of the elemental lead to the lead in the PbTe matrix is (0.01-0.05):1. The N-type PbTe-based polycrystalline thermoelectric material has good stability and excellent thermoelectric performance within a temperature range of 400-850K. For example, the power factor of the thermoelectric material can be as high as 29μW / (cm K 2 ), the thermoelectric figure of merit can be as high as 1.6 at 800K, and the average thermoelectric figure of merit between 400-800K can be as high as 1.2, which is consistent with the high thermoelectric figure of merit range of P-type PbTe-based polycrystalline thermoelectric materials, thereby making the thermoelectric module formed by combining N-type PbTe-based polycrystalline thermoelectric materials with P-type PbTe thermoelectric materials have better performance.

Description

Technical Field

[0001] The present invention relates to the technical field of thermoelectric materials, and in particular to an N-type PbTe-based polycrystalline thermoelectric material and a preparation method thereof. Background Art

[0002] Thermoelectric materials are functional materials that utilize the Seebeck and Peltier effects to convert electrical energy into thermal energy. Thermoelectric devices fabricated from these materials offer advantages such as being noise-free, pollution-free, and lacking moving parts. They can be used to achieve electrical cooling or generate electricity using temperature differences in various operating environments, offering broad application prospects.

[0003] The performance of thermoelectric materials is measured by the dimensionless thermoelectric figure of merit ZT, which is: ZT = (σS 2 )T / κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, T is the absolute temperature, and PF = σS 2 is the power factor, κ=κ e +κ l , κ e Thermal conductivity contributed by carriers, κ l Thermal conductivity contributed to the lattice. To achieve a high thermoelectric figure of merit, thermoelectric materials must possess high electrical conductivity and Seebeck coefficient, as well as low thermal conductivity. However, these three parameters are coupled and influenced by the material's electronic band structure, internal carrier concentration, and phonon scattering mechanisms. Currently, achieving thermoelectric materials with high thermoelectric figures of merit remains a hot topic in related fields. Summary of the Invention

[0004] This application is mainly based on the following questions and findings:

[0005] Lead telluride (PbTe) is one of the most competitive thermoelectric materials in the medium-temperature range. Its large atomic mass and low thermal conductivity contribute to its high thermal conductivity. Due to its double valence band structure, P-type PbTe thermoelectric materials can effectively increase the Seebeck coefficient and thus enhance the material's electrical performance through band convergence strategies, without compromising electrical conductivity. The highest thermoelectric figure of merit is achieved by doping with Eu to form dislocations that soften the lattice and reduce thermal conductivity, reaching a peak value of 2.6. Furthermore, by combining it with SrTe, the figure of merit can also reach 2.5. Compared to P-type PbTe thermoelectric materials, N-type PbTe has a lower figure of merit. To combine P-type and N-type thermoelectric materials to form a thermoelectric module with improved performance, it is necessary to improve the thermoelectric figure of merit of N-type PbTe.

[0006] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. One object of the present invention is to provide an N-type PbTe-based polycrystalline thermoelectric material with good stability, high power factor and high thermoelectric figure of merit.

[0007] In one aspect of the present invention, an N-type PbTe-based polycrystalline thermoelectric material is proposed, comprising: a PbTe matrix doped with sulfur and iodine; and elemental lead, wherein the molar ratio of the elemental lead to the lead in the PbTe matrix is (0.01-0.05):1. In the thermoelectric material, sulfur and iodine replace part of Te, and the elemental lead is located in the lead vacancies of the PbTe matrix. The doped sulfur element can reduce the lattice thermal conductivity of the thermoelectric material, and the doped iodine element can balance the balance between the electrical conductivity and the Seebeck coefficient to obtain a higher power factor. Controlling the lead element to the above content can achieve the effect of reducing the lattice thermal conductivity while improving the power factor. The N-type PbTe-based polycrystalline thermoelectric material has good stability and excellent thermoelectric performance in the temperature range of 400-850K. For example, the power factor of the thermoelectric material can be as high as 29μW / (cmK 2 ), the thermoelectric figure of merit can be as high as 1.6 at 800K, and the average thermoelectric figure of merit between 400-800K can be as high as 1.2, which is consistent with the high thermoelectric figure of merit range of P-type PbTe-based polycrystalline thermoelectric materials, thereby making the performance of the thermoelectric module formed by the combination of N-type PbTe-based polycrystalline thermoelectric materials and P-type PbTe thermoelectric materials better.

[0008] In addition, the N-type PbTe-based polycrystalline thermoelectric material according to the above embodiment of the present invention may also have the following additional technical features:

[0009] According to an embodiment of the present invention, the general formula of the thermoelectric material is: PbTe 0.998-x S x I 0.002 -y%Pb, where 0.02≤x≤0.08, 1≤y≤5.

[0010] According to an embodiment of the present invention, the thermoelectric figure of merit of the thermoelectric material at 800K is not less than 1.6, and the average value of the thermoelectric figure of merit between 400 and 800K is not less than 1.2.

[0011] In another aspect of the present invention, a method for preparing an N-type PbTe-based polycrystalline thermoelectric material is proposed. According to an embodiment of the present invention, the method comprises: (1) mixing lead, tellurium, sulfur and lead iodide and smelting the mixture to obtain an ingot; (2) grinding the ingot to obtain a precursor powder; (3) sintering the precursor powder to obtain a bulk N-type PbTe-based polycrystalline thermoelectric material, wherein the ratio of the total molar number of tellurium, sulfur and iodine to the total molar number of lead in the mixture of the lead, tellurium, sulfur and lead iodide is 1:(1.01-1.05). The preparation process of the method is simple, not only simple and convenient to operate, but also repeatable, easy to implement and easy to mass-produce; in addition, the N-type PbTe-based polycrystalline thermoelectric material prepared by the method has good stability and excellent thermoelectric performance in the temperature range of 400-850K. For example, the power factor of the thermoelectric material can be as high as 29μW / (cm K 2 ), the thermoelectric figure of merit can be as high as 1.6 at 800K, and the average thermoelectric figure of merit between 400-800K can be as high as 1.2, which is consistent with the high thermoelectric figure of merit range of P-type PbTe-based polycrystalline thermoelectric materials, thereby making the performance of the thermoelectric module formed by the combination of N-type PbTe-based polycrystalline thermoelectric materials and P-type PbTe thermoelectric materials better.

[0012] According to an embodiment of the present invention, in step (1), the lead, the tellurium, the sulfur and the lead iodide are prepared according to the general formula PbTe 0.998-x S x I 0.002 -y% Pb in a stoichiometric ratio, wherein 0.02≤x≤0.08, 1≤y≤5; optionally, the purity of the lead, the tellurium, the sulfur and the lead iodide is independently not less than 99.9%.

[0013] According to an embodiment of the present invention, in step (1), the temperature of the smelting treatment is 900-1100° C. and the time is 5-10 h; optionally, the heating rate of the smelting treatment is 1-2° C. / min; optionally, the lead, the tellurium, the sulfur and the lead iodide are placed in a quartz tube for the smelting treatment, and the inner surface of the quartz tube is provided with a carbon coating layer.

[0014] According to an embodiment of the present invention, step (1) further includes: sequentially performing quenching and annealing on the material obtained by the smelting process.

[0015] According to an embodiment of the present invention, the temperature of the annealing treatment is 550-620° C. and the time is 2-3 days; optionally, the heating rate of the annealing treatment is 1-2° C. / min.

[0016] According to an embodiment of the present invention, the sintering process adopts a spark plasma sintering method.

[0017] According to an embodiment of the present invention, the sintering temperature of the sintering treatment is 500-550°C and the time is 5-10 min; optionally, the heating rate of the sintering treatment is 50-100°C / min, the vacuum degree is 5-10 Pa, and the pressure is 40-60 MPa.

[0018] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0020] Figure 1 1 is a flow chart of a method for preparing an N-type PbTe-based polycrystalline thermoelectric material according to an embodiment of the present invention.

[0021] Figure 2 3 is a curve comparison diagram of the power factor of the thermoelectric materials prepared in Example 1 of the present invention and Comparative Examples 1 to 5 changing with temperature.

[0022] Figure 3 1 is a comparison graph of the thermal conductivity of the thermoelectric materials prepared in Example 1 of the present invention and Comparative Examples 1 to 5 as a function of temperature.

[0023] Figure 4 3 is a curve comparison of the thermoelectric figure of merit of the thermoelectric materials prepared in Example 1 of the present invention and Comparative Examples 1 to 5 as a function of temperature. DETAILED DESCRIPTION

[0024] The following embodiments of the present invention are described in detail. The embodiments described below are exemplary and are only used to explain the present invention, and are not to be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this field or the product specifications are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be obtained commercially.

[0025] In one aspect of the present invention, the present invention provides an N-type PbTe-based polycrystalline thermoelectric material, comprising: a PbTe matrix doped with sulfur and iodine; and elemental lead, wherein the molar ratio of the elemental lead to the lead in the PbTe matrix is (0.01-0.05):1. For example, the molar ratio of the elemental lead to the lead in the PbTe matrix can be 0.01 / 1, 0.02 / 1, 0.03 / 1, 0.04 / 1 or 0.05 / 1, etc., and the elemental lead is dispersed in the lead vacancies in the PbTe matrix to improve the mobility and carrier concentration of the material.

[0026] According to the embodiments of the present invention, the inventors found that the key factor determining the thermoelectric figure of merit is the correlation between the Seebeck coefficient, electrical conductivity, and thermal conductivity, wherein the electrical conductivity is determined by the carrier concentration n and the mobility μ; the Seebeck coefficient of the material is expressed as Among them, k B is the Boltzmann constant, e is the electron constant, n is the carrier concentration, m* is the effective mass of the state density, and the Seebeck coefficient S is inversely proportional to the 2 / 3 power of the carrier concentration n, that is, the higher the carrier concentration, the lower the Seebeck coefficient.

[0027] According to an embodiment of the present invention, the inventors found that, on the one hand, in an N-type PbTe-based polycrystalline thermoelectric material, the addition of elemental lead can improve the mobility μ and the carrier concentration n of the material, improve the electrical properties of the material, thereby improving the electrical conductivity σ of the material, and thus improving the thermoelectric figure of merit ZT of the N-type PbTe-based thermoelectric material. However, at the same time, the amount of elemental lead added will also affect the carrier concentration and thus affect the thermoelectric figure of merit of the material. If the amount of elemental lead added is too much, the carrier concentration is too high. Although it will increase the electrical conductivity of the material and make the metallic property of the thermoelectric material more obvious, it will also reduce the Seebeck coefficient S and thus cause a significant decrease in the power factor PF. This is because the Seebeck coefficient S is a square relationship in the power factor. The reduction in the Seebeck coefficient S has a greater impact on the power factor, and a high carrier concentration will also increase the thermal conductivity κ contributed by the carriers. e , thereby increasing thermal conductivity and reducing the thermoelectric figure of merit; if the amount of elemental lead added is too little, the carrier concentration of the material will be too low, which will make the electrical conductivity of the material low, thereby weakening the thermoelectric figure of merit of the N-type PbTe-based thermoelectric material; the inventors have discovered and verified through a large number of experiments that when the amount of elemental lead added is 0.01-0.05% of the molar amount of lead in the PbTe matrix, the carrier concentration of the material can be kept in an optimal range, which can better balance the balance between electrical conductivity and Seebeck coefficient, thereby improving the thermoelectric figure of merit of the N-type PbTe-based thermoelectric material. On the other hand, in N-type PbTe-based polycrystalline thermoelectric materials, the doping of sulfur elements can increase the lattice distortion inside the material and reduce the thermal conductivity κ contributed by the material lattice. l , thereby reducing the thermal conductivity κ of the material, and thus improving the thermoelectric figure of merit of the N-type PbTe-based thermoelectric material; on the other hand, in the N-type PbTe-based polycrystalline thermoelectric material, the doping of iodine can better regulate the carrier concentration, thereby regulating the balance between the Seebeck coefficient, electrical conductivity and thermal conductivity of the material, and thus further improving the thermoelectric figure of merit of the N-type PbTe-based thermoelectric material.

[0028] In summary, the N-type PbTe-based polycrystalline thermoelectric material of the above embodiment of the present invention has at least the following advantages: the doped sulfur element can reduce the lattice thermal conductivity of the thermoelectric material, the doped iodine element can balance the balance between electrical conductivity and Seebeck coefficient to obtain a higher power factor, and controlling the lead element to the above content can achieve the effect of reducing the lattice thermal conductivity while improving the power factor. The N-type PbTe-based polycrystalline thermoelectric material has good stability and excellent thermoelectric performance in the temperature range of 400-850K. For example, the power factor of the thermoelectric material can be as high as 29μW / (cm K 2 ), the thermoelectric figure of merit can be as high as 1.6 at 800K, and the average thermoelectric figure of merit between 400-800K can be as high as 1.2, which is consistent with the high thermoelectric figure of merit range of P-type PbTe-based polycrystalline thermoelectric materials, thereby making the performance of the thermoelectric module formed by the combination of N-type PbTe-based polycrystalline thermoelectric materials and P-type PbTe thermoelectric materials better.

[0029] The N-type PbTe-based polycrystalline thermoelectric material according to the above embodiment of the present invention is described in detail below.

[0030] According to an embodiment of the present invention, the general formula of the thermoelectric material may be: PbTe 0.998-x S x I 0.002 -y% Pb, wherein 0.02≤x≤0.08, 1≤y≤5, it can be seen from the above general formula that in the polycrystalline thermoelectric material, PbTe 0.998-x S x I 0.002 Based on the molar number of Pb in the PbTe, the molar doping amount of elemental Pb can be 1 to 5 mol%. 0.998-x S x I 0.002 Taking the molar number of Pb in the thermoelectric material as 1 as an example, the total molar number of sulfur, tellurium and iodine is 1, the molar doping amount of sulfur is 0.02-0.08, specifically 0.02, 0.03, 0.04, 0.05, 0.07, 0.08, etc., the molar doping amount of iodine is 0.002, and the ratio of the total molar number of tellurium, sulfur and iodine to the total molar number of lead in the thermoelectric material is 1: (1.01-1.05). The inventors found that controlling the iodine element to the above doping amount can better balance the balance between conductivity and Seebeck coefficient, improve the power factor and thus improve the thermoelectric figure of merit; if the doping amount of sulfur is too little, the action ability of sulfur is weakened, the lattice distortion inside the material is less, and the thermal conductivity κ contributed by the lattice is reduced. lThe thermal conductivity κ of the material is large, and it is difficult to effectively improve the thermoelectric figure of merit of the thermoelectric material. As the amount of sulfur doping increases, the effect of sulfur on improving the thermoelectric figure of merit of the PbTe-based thermoelectric material first increases and then decreases. When the amount of sulfur doping is too much, the carrier mobility will decrease, the electrical properties of the material will decrease, and the electrical conductivity will decrease, which has no promoting effect on improving the thermoelectric figure of merit of the material. In the present invention, by controlling the doping amount of sulfur to be within the above range, the lattice thermal conductivity of the PbTe-based thermoelectric material can be effectively reduced and its thermoelectric figure of merit can be improved.

[0031] According to some specific embodiments of the present invention, the thermoelectric material may include PbTe 0.978 S 0.02 I 0.002 -1% Pb, PbTe 0.968 S 0.03 I 0.002 -2% Pb, PbTe 0.958 S 0.04 I 0.002 -2% Pb, PbTe 0.948 S 0.05 I 0.002 -3% Pb, PbTe 0.938 S 0.06 I 0.002 -4% Pb and PbTe 0.918 S 0.08 I 0.002 -5% Pb, etc., thereby making the N-type PbTe-based polycrystalline thermoelectric material have better stability, higher power factor and higher thermoelectric figure of merit in the temperature range of 400-850K. Preferably, the thermoelectric material can be PbTe 0.958 S 0.04 I 0.002 -2% Pb, its thermoelectric figure of merit at 800K is 1.6, and the average thermoelectric figure of merit between 400 and 800K is 1.2. This thermoelectric material is consistent with the temperature range of high thermoelectric figure of merit of P-type PbTe-based thermoelectric materials. In addition, this N-type PbTe-based thermoelectric material also has a high power factor, which can reach 29μW / (cm K 2 ), which, combined with its low thermal conductivity, can make the thermoelectric figure of merit of N-type PbTe-based thermoelectric materials higher, thereby further improving the performance of the thermoelectric module formed by the combination of N-type PbTe-based polycrystalline thermoelectric materials and P-type PbTe thermoelectric materials.

[0032] According to some specific embodiments of the present invention, the purity of various raw materials for preparing N-type PbTe-based polycrystalline thermoelectric materials can be no less than 99.9%, preferably no less than 99.99%, thereby further ensuring the purity of the polycrystalline thermoelectric material and thus ensuring that it has good stability and thermoelectric performance.

[0033] In another aspect of the present invention, the present invention provides a method for preparing an N-type PbTe-based polycrystalline thermoelectric material. Figure 1 , the method comprising:

[0034] (1) Mixing lead, tellurium, sulfur and lead iodide and smelting them to obtain an ingot

[0035] According to an embodiment of the present invention, in the mixture of the lead, the tellurium, the sulfur and the lead iodide, the ratio of the total molar number of tellurium, sulfur and iodine to the total molar number of lead can be 1:(1.01~1.05), specifically 1 / 1.01, 1 / 1.02, 1 / 1.03, 1 / 1.04 or 1 / 1.05, etc., thereby making the thermoelectric figure of merit of the finally obtained N-type PbTe-based polycrystalline thermoelectric material higher.

[0036] According to some specific embodiments of the present invention, lead, tellurium, sulfur and lead iodide can be prepared according to the general formula PbTe 0.998- x S x I 0.002 -y% Pb are weighed and mixed in a stoichiometric ratio, wherein 0.02≤x≤0.08, 1≤y≤5, thereby preparing an N-type PbTe-based polycrystalline thermoelectric material having the general formula composition, so that the prepared polycrystalline thermoelectric material has better stability, higher power factor and higher thermoelectric figure of merit in the temperature range of 400-850K.

[0037] According to an embodiment of the present invention, the lead, tellurium, sulfur, and lead iodide can be placed in a quartz tube for the smelting process, and the inner surface of the quartz tube needs to be provided with a carbon coating. This can prevent the lead from reacting with the quartz tube during the smelting process, thereby ensuring both the smooth progress of the smelting and the quality and performance of the polycrystalline thermoelectric material. According to some specific examples of the present invention, a quartz tube carrying a mixture of lead, tellurium, sulfur, and lead iodide can be placed in a heating device such as a muffle furnace for heating and smelting, and the inner surface of the quartz tube is provided with a carbon coating. It is understood that the specific thickness of the carbon coating is not particularly limited, as long as it can prevent the quartz tube from reacting with the lead. Those skilled in the art can flexibly select according to actual conditions, and no further details will be given here.

[0038] According to an embodiment of the present invention, the purity of the lead, the tellurium, the sulfur and the lead iodide can be independently not less than 99.9%. Preferably, the purity of the lead, the tellurium, the sulfur and the lead iodide can be independently not less than 99.99%. Thus, the concentration of PbTe can be further reduced. 0.998-x S x I 0.002The introduction of impurities in -y% Pb makes the final thermoelectric material have higher purity, better stability and thermoelectric performance.

[0039] According to some specific embodiments of the present invention, the temperature of the smelting treatment can be 900-1100°C, for example, 1000-1100°C, specifically 1000°C, 1010°C, 1020°C, 1030°C, 1050°C, 1070°C, 1090°C, 1100°C, etc. The inventors found that if the temperature of the smelting treatment is too high, on the one hand, the quartz tube will be deformed when used at too high a temperature, affecting the smooth progress of the smelting. On the other hand, due to the inconsistent expansion coefficients of the quartz tube and the carbon coating, it is easy to cause the carbon coating to fall off, which in turn causes the lead to react with the quartz tube, affecting the grade and performance of the obtained thermoelectric material. On the other hand, when the smelting temperature is too high, the volatilization of Pb, I, etc. in the thermoelectric material is more serious, so that the actual content of each element in the final obtained N-type PbTe-based polycrystalline thermoelectric material is different from the expected thermoelectric material composition (such as PbTe 0.998-x S x I 0.002-y% Pb) differs greatly, thereby affecting the grade of the thermoelectric material and further affecting the thermoelectric performance of the thermoelectric material; if the smelting temperature is too low, lead, tellurium, sulfur and lead iodide cannot be completely smelted, affecting the formation effect of the ingot; however, in the present invention, by controlling the smelting temperature within the above range, it is not only more conducive to the smooth progress of the smelting process, but also to obtain a thermoelectric material with the expected composition, thereby making it more conducive to the final N-type PbTe-based polycrystalline thermoelectric material having better stability and thermoelectric performance. Furthermore, the smelting treatment time can be 5 to 10 hours, specifically 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, etc. The inventors have found that if the smelting treatment time is too short, the lead, tellurium, sulfur and lead iodide cannot be completely melted into a liquid after mixing, making it difficult to mix the lead, tellurium, sulfur and iodine elements in the thermoelectric material evenly, the smelting effect is poor, and thus the performance of the final thermoelectric material is poor; and if the smelting treatment time is too long, the efficiency of the smelting treatment is affected; in the present invention, by controlling the smelting treatment to the above time range, not only can the lead, tellurium, sulfur and iodine elements in the thermoelectric material be mixed more evenly, thereby better improving the performance of the thermoelectric material, but also the smelting efficiency can be further improved while ensuring the smelting effect. In addition, the heating rate of the smelting process can be 1 to 2°C / min, specifically 1°C / min, 1.5°C / min, 1.6°C / min, 2°C / min, etc. The inventors have found that if the heating rate of the smelting process is too fast, the volatilization rates of the various materials are different, which will also affect the mixing uniformity of the final thermoelectric material, and thus affect the grade and thermoelectric properties of the thermoelectric material. In the present invention, by controlling the heating rate of the smelting process to be within the above range, not only can the smelting effect be further improved, but it is also more conducive to improving the stability and thermoelectric properties of the final N-type PbTe-based polycrystalline thermoelectric material.

[0040] According to some specific examples of the present invention, the smelting process can also be carried out in a vacuum environment, wherein the vacuum degree of the vacuum environment can be 5×10 -4 ~9×10 -4 Pa, the inventors found that oxidation of the material can be avoided by performing the smelting process in the above vacuum environment.

[0041] According to an embodiment of the present invention, after lead, tellurium, sulfur and lead iodide are mixed and smelted, the material obtained by the smelting treatment can be further quenched and annealed in sequence to obtain an ingot. The inventors have found that by quenching the material obtained by the smelting treatment, the grain size can be effectively reduced, and the grain size can be prevented from being too large, thereby increasing the grain boundary area and further reducing the lattice thermal conductivity; by performing annealing treatment, the components inside the material can be diffused more evenly, the grains can be further refined, the structural defects can be reduced, and the residual internal stress can be removed, which can further help to reduce the lattice thermal conductivity, thereby improving the thermoelectric figure of merit of the thermally conductive material and improving the mechanical properties of the thermoelectric material.

[0042] According to some specific examples of the present invention, the temperature of the annealing treatment can be 550-620°C, specifically 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, etc. Preferably, the temperature of the annealing treatment can be 600°C. Insulation within this temperature range can make the elements inside the material diffuse more evenly, refine the grains and further reduce the generation of tissue defects such as pores and defects, which is more conducive to reducing lattice thermal conductivity and improving the thermoelectric figure of merit. Furthermore, the annealing time can be 2 to 3 days, for example, 48 hours, 54 hours, 60 hours, 66 hours or 72 hours. The inventors found that if the annealing time is too short, the elements inside the material do not have enough time to diffuse, which can easily lead to uneven distribution of the components, poor grain size uniformity of the material, and more structural defects, which seriously affect the stability and thermoelectric performance of the thermoelectric material; and if the annealing time is too long, it will affect the annealing efficiency of the material. In the present invention, by controlling the annealing time to the above time range, the components inside the material can have enough time to diffuse evenly, so that the final thermoelectric material has better uniformity and lower lattice thermal conductivity, thereby improving the thermoelectric figure of merit of the thermoelectric material. Preferably, the annealing time can be 2 days. In addition, the heating rate of the annealing treatment can be 1 to 2°C / min, specifically 1°C / min, 1.5°C / min, 1.6°C / min, 2°C / min, etc. The inventors found that it is difficult to maintain complete consistency in the temperature control accuracy and degree of temperature control of each device. By controlling the heating rate of the annealing treatment within the above range, the temperature rise during the annealing treatment can be more balanced, preventing overshoot caused by excessive heating, which in turn affects the effect of the annealing treatment.

[0043] (2) Grinding the ingot to obtain precursor powder

[0044] According to an embodiment of the present invention, although quenching and annealing the material obtained by smelting can significantly reduce the grain size of the material, reduce structural defects, and make the interior of the material denser, there may still be some problems of excessive grain growth; by grinding and crushing the ingot, the grain size can be further reduced, the number of grain boundaries can be increased, and the grain size can be avoided from being too large, thereby further reducing the thermal conductivity of the material and thus improving the thermoelectric figure of merit of the material. It is understandable that the particle size of the precursor powder obtained by grinding is not particularly limited, as long as the precursor powder has no metallic luster. Those skilled in the art can flexibly select it according to actual conditions, and will not go into details here. It is understandable that the grinding mortar is not particularly limited, for example, it can be an agate mortar and an agate pestle, etc. Those skilled in the art can flexibly select it according to actual conditions, and will not go into details here.

[0045] (3) Sintering the precursor powder to obtain a bulk N-type PbTe-based polycrystalline thermoelectric material.

[0046] According to an embodiment of the present invention, the ingot obtained after the smelting process is not dense inside during the cooling process, and may have structural defects such as pores, and its mechanical properties are poor. In addition, after cooling, some grains may still be too large. In the present invention, by grinding and re-sintering the ingot, on the one hand, the mechanical properties of the final block N-type PbTe-based polycrystalline thermoelectric material can be improved, and on the other hand, the material can be further controlled to have a lower grain size, which can be more conducive to improving the stability and thermoelectric performance of the thermoelectric material.

[0047] According to an embodiment of the present invention, the sintering process can adopt a spark plasma sintering method. When sintering is carried out by this method, the sintering is faster, and the growth of grains during the sintering process can be better reduced, thereby further ensuring a lower grain size and a larger number of grain boundaries, thereby reducing the thermal conductivity contributed by the material lattice and improving the thermoelectric figure of merit of the thermoelectric material. It can effectively solve the problems of high time cost of the existing commonly used hot pressing sintering method and excessive growth of grains during the sintering process, which causes excessive increase in the thermal conductivity contributed by the lattice.

[0048] According to some specific examples of the present invention, the sintering temperature of the sintering treatment can be 500-550°C, specifically 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, etc., and the sintering time of the sintering treatment can be 5-10min, specifically 5min, 6min, 7min, 8min, 9min, 10min, etc. The inventors found that if the sintering time of the sintering treatment is too long, it will cause excessive grain growth, increase grain size, and reduce the number of grain boundaries, thereby increasing the thermal conductivity contributed by the lattice of the material, thereby reducing the thermoelectric figure of merit of the thermoelectric material; and if the sintering time of the sintering treatment is too short and the sintering treatment is incomplete, the performance of the bulk N-type PbTe-based polycrystalline thermoelectric material will be poor; in the present invention, by controlling the above-mentioned sintering conditions, the growth of grains during the sintering process can be reduced, thereby effectively avoiding the problem of a significant increase in lattice thermal conductivity caused by an increase in grain size, which is more conducive to improving the thermoelectric figure of merit of the thermoelectric material. Furthermore, the heating rate of the sintering treatment can be 50-100°C / min, specifically 50°C / min, 55°C / min, 60°C / min, 70°C / min, 80°C / min, 90°C / min, 100°C / min, etc., thereby further shortening the total time required for the sintering treatment, thereby further reducing the growth of grains during the sintering process, and thus being more conducive to improving the thermoelectric figure of merit of the thermoelectric material.

[0049] According to some specific examples of the present invention, the vacuum degree of the sintering process can be 5-10Pa, specifically 5Pa, 6Pa, 7Pa, 8Pa, 9Pa, 10Pa, etc. Preferably, the vacuum degree of the sintering process is 2-6Pa, thereby further reducing the introduction of impurities and reducing the oxidation of the material, so that the actual content of Pb, I, and S in the final obtained N-type PbTe-based polycrystalline thermoelectric material is consistent with the expected thermoelectric material composition (such as PbTe 0.998-x S x I 0.002 -y% Pb) have a small difference in the standard contents of Pb, I, and S, which makes the grade and thermoelectric performance of N-type PbTe-based polycrystalline thermoelectric materials better.

[0050] According to some specific examples of the present invention, the pressure of the sintering treatment can be 40~60MPa, specifically 40MPa, 45MPa, 50MPa, 55MPa, 60MPa, etc. The inventors found that in order to make the grains react with each other when pressed into blocks and realize diffusion connection, a certain pressure needs to be applied. If no pressure is applied or the pressure applied is too small, it is difficult for the precursor powder to form a block, and a higher sintering temperature is required to melt the material again before it can form a block. In the present invention, by controlling the pressure of the sintering treatment to be within the above-mentioned range, sintering can be assisted, and the rapid reaction of the material in the sintering treatment in the solid state can be promoted. There is no need for an excessively high sintering temperature. The precursor powder can be formed into a block in a shorter time at 500~550°C, and the formation efficiency is high.

[0051] It is understandable that the material of the mold used in the sintering process is not particularly limited, as long as the precursor powder does not react with the mold. For example, it can be a graphite mold. When a graphite mold is used in the sintering process, it and the precursor powder can be separated by carbon paper. Those skilled in the art can flexibly choose according to actual conditions, and will not be elaborated here.

[0052] According to a specific embodiment of the present invention, the method for preparing N-type PbTe-based polycrystalline thermoelectric material may include: firstly, lead, tellurium, sulfur and lead iodide are mixed according to the general formula PbTe 0.958 S 0.04 I 0.002 The stoichiometric ratio of -2% Pb is weighed and placed in a quartz tube for mixing. The material obtained after mixing is placed in a muffle furnace for smelting treatment, and the temperature is increased to 1050°C at a heating rate of 1°C / min and kept warm for 6 hours; the material obtained by the smelting treatment is quenched in ice water, and the material obtained after quenching is annealed in a muffle furnace, and the temperature is increased to 600°C at a heating rate of 1°C / min and kept warm for 2 days to obtain an ingot; the ingot is ground using an agate mortar and an agate pestle to obtain a precursor powder; the precursor powder is sintered, and a discharge plasma sintering process is used to sinter for 5 minutes under the conditions of a sintering vacuum degree of 5Pa, a pressure of 50MPa and a sintering temperature of 550°C to obtain an N-type PbTe-based polycrystalline thermoelectric material.

[0053] In summary, the method for preparing N-type PbTe-based polycrystalline thermoelectric materials described above in the present invention has a simple preparation process, is not only simple and convenient to operate, but also repeatable, easy to implement, and easy to mass produce. In addition, the N-type PbTe-based polycrystalline thermoelectric materials prepared by this method have good stability and excellent thermoelectric performance in the temperature range of 400-850K. For example, the power factor of the thermoelectric material can be as high as 29μW / (cm K 2), the thermoelectric figure of merit can reach as high as 1.6 at 800K, and the average thermoelectric figure of merit between 400-800K can reach as high as 1.2, which is consistent with the high thermoelectric figure of merit range of P-type PbTe-based polycrystalline thermoelectric materials. As a result, the performance of the thermoelectric module formed by combining the N-type PbTe-based polycrystalline thermoelectric material with the P-type PbTe thermoelectric material is improved. It should be noted that the characteristics and effects described for the above-mentioned N-type PbTe-based polycrystalline thermoelectric material are also applicable to the method for preparing the N-type PbTe-based polycrystalline thermoelectric material, and will not be detailed here.

[0054] The following examples are intended to illustrate the present invention but are not intended to limit the scope of the invention. Where specific techniques or conditions are not specified in the examples, the techniques or conditions described in the literature in the art or in the product instructions were used. Unless otherwise specified, the reagents or instruments used in the examples are commercially available.

[0055] The inner surface of the quartz tube involved in the following embodiments is provided with a carbon coating layer, and the graphite mold and the precursor powder involved are separated by carbon paper.

[0056] Example 1

[0057] Lead, tellurium, sulfur and lead iodide (purity not less than 99.9%) are prepared according to the general formula PbTe 0.958 S 0.04 I 0.002 The invention discloses a novel N-type PbTe-based polycrystalline thermoelectric material, which is a stoichiometric ratio of 1% to 2% Pb, and is vacuum-smelted at 1050°C, quenched in ice water, and then annealed to obtain an ingot, wherein the annealing temperature is 600°C at a heating rate of 1-2°C / min and the annealing time is 48 hours. The ingot is then ground to obtain a precursor powder. The precursor powder is placed in a graphite mold with a diameter of 10 mm and sintered at 550°C for 5 minutes using a spark plasma sintering process (sintering vacuum degree of 5-10 Pa and sintering pressure of 50 MPa) to obtain a N-type PbTe-based polycrystalline thermoelectric material with a maximum ZT value of 1.62.

[0058] Comparative Example 1

[0059] Lead, tellurium and lead iodide (purity not less than 99.9%) are prepared according to the general formula PbTe 0.998 I 0.002 The stoichiometric ratio was determined, and the materials were vacuum melted at 1050°C and then cooled to room temperature in the furnace to obtain an ingot. The ingot was ground to obtain a precursor powder. The precursor powder was placed in a graphite mold with a diameter of 10 mm and sintered at 550°C for 5 minutes using a spark plasma sintering process to obtain a bulk thermoelectric material with a maximum ZT value of 1.05.

[0060] Comparative Example 2

[0061] The difference from Example 1 is that lead, tellurium and lead iodide (with a purity of not less than 99.9%) are prepared according to the general formula PbTe 0.998 I 0.002 The ingot is prepared by vacuum melting at 1050° C., quenching in ice water, and then annealing.

[0062] The maximum ZT value of the obtained N-type PbTe-based polycrystalline thermoelectric material is 1.5.

[0063] Comparative Example 3

[0064] The difference from Example 1 is that lead, tellurium, sulfur and lead iodide (purity not less than 99.9%) are prepared according to the general formula PbTe 0.958 S 0.04 I 0.002 -0.5% Pb in a stoichiometric ratio, after vacuum melting at 1050°C, quenching in ice water and then annealing to obtain an ingot.

[0065] The maximum ZT value of the obtained N-type PbTe-based polycrystalline thermoelectric material is 1.37.

[0066] Comparative Example 4

[0067] The difference from Example 1 is that lead, tellurium, sulfur and lead iodide (purity not less than 99.9%) are prepared according to the general formula PbTe 0.958 S 0.04 I 0.002 -8% Pb in a stoichiometric ratio, after vacuum melting at 1050°C, quenching in ice water and then annealing to obtain an ingot.

[0068] The maximum ZT value of the obtained N-type PbTe-based polycrystalline thermoelectric material is 1.28.

[0069] Comparative Example 5

[0070] The difference from Example 1 is that lead, tellurium, sulfur and lead iodide (purity not less than 99.9%) are prepared according to the general formula PbTe 0.898 S 0.1 I 0.002 -2% Pb in a stoichiometric ratio, after vacuum melting at 1050°C, quenching in ice water and then annealing to obtain an ingot.

[0071] The maximum ZT value of the obtained N-type PbTe-based polycrystalline thermoelectric material is 1.29.

[0072] Figure 2 The figure is a comparison of the power factor of the thermoelectric materials prepared in Example 1 and Comparative Examples 1 to 5 as a function of temperature. Figure 2It can be seen that the power factors of the materials in Example 1 and Comparative Example 2 are greatly improved. In the temperature range below 800K, the power factors of the materials are all greater than 20μW / (cm K 2 ), with excellent electrical properties.

[0073] Figure 3 The figure is a comparison of the thermal conductivity of the thermoelectric materials prepared in Example 1 and Comparative Examples 1 to 5 as a function of temperature. Figure 3 It can be seen that compared with Comparative Example 2, the thermal conductivity of Example 1 is reduced in the entire temperature range, especially in the high temperature region greater than 700K, which shows that an appropriate amount of sulfur doping can significantly reduce the thermal conductivity of the material.

[0074] Figure 4 The figure is a comparison chart of the thermoelectric figure of merit of the materials in Example 1 and Comparative Examples 1 to 5 as a function of temperature. Figure 4 It can be seen that the thermoelectric figure of merit (ZT) of the thermoelectric material prepared in Example 1 has the highest average and maximum values in the temperature range of 400-850K, with the maximum ZT value being 1.62. The highest thermoelectric figure of merit of the thermoelectric material prepared in Comparative Example 1 is 1.05, and its thermoelectric figure of merit in the high temperature zone is the lowest, and its thermoelectric performance is poor. The average and maximum values of the thermoelectric figure of merit of the thermoelectric material prepared in Comparative Example 2 are both smaller than those of the thermoelectric material prepared in Example 1, and the thermoelectric figure of merit of the thermoelectric material prepared in Comparative Example 3 is smaller than that of Comparative Example 2 and larger than that of Comparative Examples 4 and 5. In combination with Example 1 and Comparative Examples 2-5, it can be illustrated that for the N-type PbTe-based polycrystalline thermoelectric material in the present invention, too little or too much addition of sulfur, as well as too little or too much addition of elemental lead, will have an adverse effect on the thermoelectric figure of merit of the thermoelectric material and affect the thermoelectric performance of the thermoelectric material. In addition, in combination with Figures 2-4 It can be seen that although the power factor of Comparative Example 2 is slightly higher than that of Example 1, its thermal conductivity is also relatively high, and the thermoelectric figure of merit of the thermoelectric material having the composition of the embodiment of the present application is higher, which further illustrates that the thermoelectric material having the composition of the embodiment of the present application can better balance the relationship between the power factor and thermal conductivity and obtain a higher thermoelectric figure of merit; in addition, Example 1 not only has the highest thermoelectric figure of merit, but also maintains a high thermoelectric figure of merit throughout the entire temperature range, and its average thermoelectric figure of merit is also higher.

[0075] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0076] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. An N-type PbTe-based polycrystalline thermoelectric material, characterized in that: include: a PbTe matrix doped with sulfur and iodine; and Elemental lead, wherein the molar ratio of the elemental lead to the lead in the PbTe matrix is (0.01-0.05):

1.

2. The N-type PbTe-based polycrystalline thermoelectric material according to claim 1, characterized in that: The general formula of the thermoelectric material is: PbTe 0.998-x S x I 0.002 -y%Pb, where 0.02≤x≤0.08, 1≤y≤5.

3. The N-type PbTe-based polycrystalline thermoelectric material according to claim 2, characterized in that: The thermoelectric figure of merit of the thermoelectric material at 800K is not less than 1.6, and the average value of the thermoelectric figure of merit between 400 and 800K is not less than 1.

2.

4. A method for preparing an N-type PbTe-based polycrystalline thermoelectric material, characterized in that: include: (1) mixing lead, tellurium, sulfur and lead iodide and smelting them to obtain an ingot; (2) grinding the ingot to obtain a precursor powder; (3) sintering the precursor powder to obtain a bulk N-type PbTe-based polycrystalline thermoelectric material. Wherein, in the mixture of the lead, the tellurium, the sulfur and the lead iodide, the ratio of the total molar number of tellurium, sulfur and iodine to the total molar number of lead is 1:(1.01-1.05).

5. The method according to claim 4, characterized in that In step (1), the lead, tellurium, sulfur and lead iodide are prepared according to the general formula PbTe 0.998-x S x I 0.002 -y% Pb stoichiometric ratio, wherein 0.02≤x≤0.08, 1≤y≤5.

6. The method according to claim 4, characterized in that In step (1), the purity of the lead, the tellurium, the sulfur and the lead iodide is independently not less than 99.9%.

7. The method according to claim 4, characterized in that In step (1), the smelting treatment temperature is 900-1100° C. and the time is 5-10 hours.

8. The method according to claim 4, characterized in that In step (1), the heating rate of the smelting treatment is 1 to 2°C / min.

9. The method according to claim 4, characterized in that In step (1), the lead, the tellurium, the sulfur and the lead iodide are placed in a quartz tube for the smelting treatment, and the inner surface of the quartz tube is provided with a carbon coating layer.

10. The method according to any one of claims 4 to 9, characterized in that Step (1) also includes: sequentially performing quenching and annealing on the material obtained by the smelting treatment.

11. The method according to claim 10, characterized in that The annealing treatment is performed at a temperature of 550-620° C. and for a time of 2-3 days.

12. The method according to claim 10, characterized in that The heating rate of the annealing treatment is 1-2°C / min.

13. The method according to any one of claims 4, 11 and 12, characterized in that: The sintering process adopts spark plasma sintering method.

14. The method according to claim 4, characterized in that The sintering temperature of the sintering treatment is 500-550° C. and the time is 5-10 minutes.

15. The method according to claim 4, characterized in that The heating rate of the sintering process is 50-100°C / min, the vacuum degree is 5-10Pa, and the pressure is 40-60MPa.

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