Preparation method of N-type bismuth telluride-based thermoelectric material

Abstract

According to the preparation method of the N-type bismuth telluride-based thermoelectric material, a small amount of Cu is doped in a bismuth telluride matrix, the anion defect density in the matrix is reduced, the donor-like effect is inhibited, the lattice planarization effect in the material is achieved, and then the thermoelectric performance of the material is effectively optimized through a hot extrusion process. The preparation method comprises the following steps: uniformly mixing four elementary substance raw materials of Bi, Te, Se and Cu by using a ball mill, preparing a composite material by using a vacuum sintering method, processing by hot extrusion, and annealing to prepare the composite material crystal bar.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermoelectric materials, and in particular relates to a method for preparing an N-type bismuth telluride-based thermoelectric material. Background Art

[0002] With the progress of society, environmental pollution and energy crisis are becoming increasingly serious. People are paying more and more attention to the sustainable development of society and nature, and are also paying more and more attention to the development and utilization of new environmentally friendly energy replacement materials. Thermoelectric effect is a general term for the electric effect caused by temperature difference and the reversible thermal effect caused by current, including Seebeck effect, Peltier effect and Thomson effect. Thermoelectric materials are a kind of green energy material that uses thermoelectric effect to realize the direct conversion between thermal energy and electrical energy. They have many advantages, such as small size, light weight, compact structure, no mechanical rotating parts, no pollution, no leakage, no noise, precise temperature control, fast response speed, etc. These advantages make thermoelectric materials an important type of functional materials, with broad application space and prospects in both temperature difference power generation and thermoelectric refrigeration, such as the recycling of industrial waste heat, infrared sensors, portable thermoelectric refrigerators, special space power sources, etc.

[0003] At present, the bottleneck restricting the practical application of thermoelectric materials is their relatively low thermoelectric conversion efficiency. The performance figure of merit ZT of thermoelectric materials is an important indicator to measure the thermoelectric conversion efficiency. The ZT value can be expressed as ZT = α 2 σT / κ=α 2 σT / (κ L +κ e ), where α is the Seebeck coefficient of the material, σ is the electrical conductivity, and κ is the total thermal conductivity, including the lattice thermal conductivity κ L and the electronic thermal conductivity κ e , T is the absolute temperature. A good thermoelectric material should have a large Seebeck coefficient (α) and electrical conductivity (σ) and a small thermal conductivity (κ). These three parameters are mutually coupled and interrelated, and are all related to carrier transport and scattering. The Seebeck coefficient and electrical conductivity depend on the electronic structure of the material and the scattering of carriers, and show opposite trends with the increase of carrier concentration; while thermal conductivity is not only related to the scattering of carriers, but also affected by the chemical composition and crystal structure of the material. Therefore, how to optimize these three parameters of thermoelectric materials to achieve coordinated regulation of electrical and thermal transport properties is the key to improving the thermoelectric performance of materials.

[0004] N-type bismuth telluride alloy is a thermoelectric and magnetoelectric functional material with excellent performance. It is an important material for preparing solid-state electric refrigeration devices, thermoelectric power generation devices and magnetoelectric devices. The special 4f electron layer structure of rare earth elements enables them to exhibit excellent performance in optoelectronics, magnetoelectricity and chemical properties. When the temperature drops, the conduction of 4f electrons is suppressed and its resistance decreases, which just meets the requirements of being a thermoelectric conversion material. However, due to the low thermoelectric performance of commercial N-type bismuth telluride, this limits its use in some demanding applications. Therefore, the development of high-performance N-type Bi2Te3 polycrystals is imminent. Summary of the invention

[0005] The present invention is carried out to solve the above-mentioned problems. It targets the thermoelectric performance limitations that restrict the larger-scale application and promotion of the current commercial N-type bismuth telluride thermoelectric materials. By doping a small amount of Cu into the bismuth telluride matrix, the anion defect density in the matrix is ​​reduced, the donor-like effect is suppressed, and the lattice flattening effect in the material is achieved. The thermoelectric performance of the material is effectively optimized through a hot extrusion process.

[0006] The present invention provides an improved N-type bismuth telluride-based thermoelectric material and a preparation method thereof. Cu powder is doped into bismuth telluride material, four single-substance raw materials of Bi, Te, Se and Cu are uniformly mixed by a ball mill, a composite material is prepared by a vacuum sintering method, and then hot extrusion is performed, and a composite material crystal rod is prepared after annealing.

[0007] The specific technical solutions of the present invention are as follows:

[0008] In a first aspect of the present invention, a method for preparing an N-type bismuth telluride-based thermoelectric material is provided, comprising the following steps:

[0009] 1) mixing four elemental raw materials of Bi, Te, Se and Cu, and then melting them in vacuum to obtain an ingot, which is then crushed to obtain a bismuth telluride-based powder;

[0010] 2) cold pressing the mixed powder by a hydraulic press, and then vacuum sintering to obtain a bismuth telluride-based bulk material;

[0011] 3) The bismuth telluride-based block material is placed in a hot extrusion die to obtain a composite material crystal rod, which is then annealed.

[0012] The optimal conditions for the three steps are as follows:

[0013] In step (1), the molar ratio of the four single-substance raw materials Bi, Te, Se and Cu is 2:2.7:0.3:0.003; the purity of the four single-substance raw materials Bi, Te, Se and Cu is 99.999%.

[0014] The operation of vacuum melting to obtain an ingot is as follows: the four elements of Bi, Te, Se and Cu are mixed and placed in a graphite crucible, the graphite crucible is placed in a quartz tube, the quartz tube is evacuated and sealed, and then vacuum melted. The vacuum melting conditions are as follows: under a vacuum degree of 0.01Pa, the temperature is raised to 700-800℃, kept at this temperature for at least 8h, and then cooled to room temperature.

[0015] In step (2), the cold pressing conditions are: 800-1100 kN axial pressure, holding pressure for 3 minutes; vacuum sintering conditions: vacuum degree less than 1 MPa, holding at a temperature of 500-560° C. for 9-10 hours.

[0016] In step (3), the hot extrusion temperature is set to 400-490° C., a pressure of 100-200 MPa is applied in the extrusion direction, and the annealing temperature is set to 300-380° C. for 10 h.

[0017] In a second aspect, the present invention provides an N-type bismuth telluride-based thermoelectric material prepared by the above method.

[0018] In a specific embodiment of the present invention, the performance of the copper-doped N-type bismuth telluride thermoelectric material was tested. The results show that the electrical conductivity σ of the Cu-doped mixed material crystal rod gradually decreases with increasing temperature; the thermal conductivity κ does not change significantly with temperature; the Seebeck coefficient α decreases first and then increases with increasing temperature, and is the lowest near 347K; the ZT value increases first and then decreases with increasing temperature, and is the highest near 347K.

[0019] Functions and Effects of the Invention

[0020] The present invention aims at the thermoelectric performance limitation that restricts the current commercial N-type bismuth telluride thermoelectric material from being applied and promoted on a larger scale. The method of doping copper elements is used to reduce the anion defect density in the matrix, suppress the donor-like effect, and then effectively optimize the thermoelectric performance of the material through a hot extrusion process method, so that the thermoelectric performance relative to commercial bismuth telluride materials is improved. The present application scheme has a great role in promoting the expansion of the current application scope of N-type bismuth telluride materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a process flow chart for preparing the N-type bismuth telluride-based thermoelectric material of the present invention.

[0022] Figure 2 A plot of the electrical properties of the samples as a function of temperature is shown.

[0023] Figure 3 A plot of the Seebeck coefficient of the samples as a function of temperature is shown.

[0024] Figure 4 A plot of the thermal conductivity of the samples as a function of temperature is shown.

[0025] Figure 5 The graph showing the variation of the thermoelectric figure of merit ZT of the samples with temperature. DETAILED DESCRIPTION

[0026] The present invention is described in detail below in conjunction with the embodiments and the accompanying drawings. However, the following embodiments should not be considered as limiting the scope of the present invention.

[0027] 1. Preparation method of N-type bismuth telluride-based thermoelectric material

[0028] See the process Figure 1 , the specific steps are as follows:

[0029] (1) According to the molar ratio of Bi, Te, Se and Cu of 2:2.7:0.3:0.003, 3000g of pure Bi, Te, Se and Cu particles were weighed, mixed and placed in a graphite crucible, and the graphite crucible was placed in a quartz tube and vacuumed until the pressure in the tube was less than 2Pa and sealed. Then the quartz tube was placed in a tubular heating furnace, the temperature was set to 800℃, and the temperature was increased to 700-800℃ at a rate of 10℃ / min under a vacuum of 0.01Pa, kept warm for at least 8h, and cooled outside the furnace. The high-temperature synthesized material was mechanically crushed for 3-5min, and sifted to obtain bismuth telluride powder.

[0030] (2) The obtained composite powder material is pressed by a hydraulic press under an axial pressure of 800 to 1100 kN for 3 minutes, cold pressed into a blank, and then vacuum sintered. Vacuum sintering parameters: temperature 500 to 560°C, vacuum degree less than 1 MPa, and heat preservation for 9 to 10 hours.

[0031] (3) The sintered billet is placed in a hot extrusion die, and the die is placed in a heater together with the die, and the heater temperature is set to 400-490°C; a pressure of 100-200 MPa is applied in the extrusion direction, and the extruded rod is annealed after being kept at 300-380°C for 10 hours.

[0032] Figure 2 to Figure 5 The graphs showing the changes of the electrical properties, Seebeck coefficient, thermal conductivity and thermoelectric figure of merit ZT of the rod with temperature are shown respectively. The results show that for the Cu-doped mixed material crystal rod, the electrical conductivity σ gradually decreases with the increase of temperature; the thermal conductivity κ does not change significantly with temperature; the Seebeck coefficient α decreases first and then increases with the increase of temperature, and is the lowest near 347K; the ZT value increases first and then decreases with the increase of temperature, and is the highest near 347K.

[0033] Compared with commercial N-type bismuth telluride thermoelectric materials, a small amount of Cu is doped into the bismuth telluride matrix to reduce the anion defect density in the matrix, inhibit the donor-like effect, achieve the lattice flattening effect in the material, and then effectively optimize the thermoelectric properties of the material through hot extrusion process.

[0034] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. The present invention may be subject to various changes and improvements without departing from the spirit and scope of the present invention. These changes and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for preparing an N-type bismuth telluride-based thermoelectric material, characterized in that: The steps include: 1) mixing four elemental raw materials of Bi, Te, Se and Cu, and then melting them in vacuum to obtain an ingot, which is then crushed to obtain a bismuth telluride-based powder; 2) cold pressing the mixed powder by a hydraulic press, and then vacuum sintering to obtain a bismuth telluride-based bulk material; 3) The bismuth telluride-based block material is placed in a hot extrusion die to obtain a composite material crystal rod, which is then annealed.

2. The method for preparing the N-type bismuth telluride-based thermoelectric material according to claim 1, characterized in that: in, In step (1), the molar ratio of the four elemental raw materials of Bi, Te, Se and Cu is 2:2.7:0.3:0.

003.

3. The method for preparing the N-type bismuth telluride-based thermoelectric material according to claim 1, characterized in that: in, The purity of the four single-element raw materials, Bi, Te, Se and Cu, is 99.999%.

4. The method for preparing the N-type bismuth telluride-based thermoelectric material according to claim 1, characterized in that: in, The operation of vacuum melting to obtain an ingot is as follows: the four elements of Bi, Te, Se and Cu are mixed and placed in a graphite crucible, the graphite crucible is placed in a quartz tube, the quartz tube is evacuated and sealed, and then vacuum melted. The vacuum melting conditions are as follows: under a vacuum degree of 0.01Pa, the temperature is raised to 700-800℃, kept at this temperature for at least 8h, and then cooled to room temperature.

5. The method for preparing the N-type bismuth telluride-based thermoelectric material according to claim 1, characterized in that: in, In step (2), the cold pressing conditions are: 800-1100 kN axial pressure, and the pressure is maintained for 3 minutes; Vacuum sintering conditions: vacuum degree is less than 1MPa, and the temperature is kept at 500-560℃ for 9-10h.

6. The method for preparing the N-type bismuth telluride-based thermoelectric material according to claim 1, characterized in that: in, In step (3), the hot extrusion temperature is set to 400-490° C., a pressure of 100-200 MPa is applied in the extrusion direction, and the annealing temperature is set to 300-380° C. for 10 h.

7. An N-type bismuth telluride-based thermoelectric material, characterized in that: The method is prepared by any one of claims 1 to 6.

Images