CN108242500B - Copper-selenium-based nano composite thermoelectric material and preparation method thereof - Google Patents

Abstract

The invention relates to a copper-selenium-based nano-composite thermoelectric material and a preparation method thereof. The p-type nano-composite thermoelectric material comprises Cu _2-x Se _{1-y-z S y} T ez and distributed in the Cu _2-x Se _{1‑y‑ z} S _y T _z carbon nanotubes, wherein 0≤x≤0.15, 0≤y≤1, 0≤z≤1, and y+z≤1, the mass percentage of the carbon nanotubes ≤2%. The preparation method of the present invention has rich sources of raw materials, low cost, simple production process and production equipment, and good controllability and repeatability.

Description

A kind of copper-selenium-based nanocomposite thermoelectric material and preparation method thereof

technical field

The invention relates to a copper-selenium-based nanocomposite thermoelectric material and a preparation method thereof, in particular to a novel p-type nanocomposite thermoelectric material and a preparation method thereof, belonging to the field of thermoelectric materials.

Background technique

Thermoelectric conversion materials are a class of clean energy materials that utilize the Seebeck effect and Peltier effect of the material itself to directly realize the mutual conversion between thermal energy and electrical energy. It can use natural temperature difference and industrial waste heat and waste heat to generate electricity, and can also be made into a refrigerator with no noise, no transmission device and high reliability. The conversion efficiency of thermoelectric materials is determined by the high and low temperature of the material and the intrinsic properties of the material. For a certain use environment, the high and low end temperatures are usually determined, so in order to improve the conversion efficiency, we can only start from optimizing the material itself. The dimensionless thermoelectric figure of merit ZT is usually used to evaluate the energy conversion efficiency of thermoelectric materials, which is defined as: ZT=S ² Tσ/κ, where S is the thermoelectric potential (Seekbeck coefficient), and T is the absolute temperature , σ is the electrical conductivity, κ is the thermal conductivity. In order to obtain high thermoelectric conversion efficiency, the material must have a high thermoelectric figure of merit. The thermoelectric materials that have been used at present are mostly metal compounds and their solid solutions, such as Bi ₂ Te ₃ , SiGe, PbTe, etc., but the preparation conditions of these thermoelectric materials are relatively high, and they need to be carried out under a certain protective gas, and contain harmful effects on the human body. Harmful heavy metals, and because the ZT value is about 1.0, so the energy conversion efficiency is not high. In recent decades, researchers have achieved a substantial improvement in the thermoelectric properties of thermoelectric materials through various means, such as doping, compounding, nano-scale and finding new compounds.

The copper-selenide-based compound Cu _2-x M (M=S, Se or Te) is a new type of thermoelectric material with simple composition, low cost of raw materials, high Seebeck coefficient and low thermal conductivity, thermoelectric Excellent performance. Cu _2-x M (M=S, Se or Te) exhibits p-type conductivity due to Cu vacancies, and the conductivity increases with the value of x. Because of its moderate band gap (Cu ₂ Se and Cu ₂ S are about 1.2eV, Cu ₂ Te is about 1.1eV), it is an ideal material for solar cells, so the research on such materials is mostly focused on batteries, only A few literatures have reported that such materials have large thermoelectric potential and very low thermal conductivity, but little research has been done on the thermoelectric properties of solid solutions formed between these compounds.

In recent years, processes such as vacuum melting, high-temperature self-propagating synthesis, hydrothermal synthesis, and spark plasma sintering have been successively used to prepare copper-selenium-based bulk thermoelectric materials. Among them, vacuum fusion combined with spark plasma sintering is the manufacturing process of most copper-selenium-based bulk thermoelectric materials. However, Cu ₂ Se and Cu ₂ Te are easily precipitated during this preparation process, resulting in an increase in the carrier concentration, which greatly increases their thermal conductivity, thereby reducing the thermoelectric properties of the material. On the other hand, the materials prepared by this method have larger grains and weaker scattering of short-wave phonons.

SUMMARY OF THE INVENTION

To this end, the present invention provides a copper-selenium-based nanocomposite thermoelectric material and a preparation method thereof.

In one aspect, the present invention provides a p-type nanocomposite thermoelectric material (copper selenium-based nanocomposite thermoelectric material), wherein the p-type nanocomposite thermoelectric material comprises Cu _2-x Se _1-yz S _y T _z , and distributed in The carbon nanotubes in the Cu _2-x Se _1-yz S _y Te _z , wherein 0≤x≤0.15, 0≤y≤1, 0≤z≤1, and y+z≤1, the carbon nanotubes The mass percentage of the tube is less than or equal to 2%.

Preferably, the diameter of the carbon nanotubes is 10-20 nanometers, and the length is >3 μm.

On the other hand, the present invention also provides a preparation method of a p-type nanocomposite thermoelectric material, comprising:

Using the elemental powders and carbon nanotubes of the general formula Cu _2-x Se _1-yz S _y T _z as the initial raw materials, according to the stoichiometric ratio of Cu _{2- x} Se _1-yz S _y T _z , and Cu ₂ The mass ratio of _-x Se _1-yz S _y T _z to carbon nanotubes is weighed and then ball-milled at a ball milling speed of 450-500 rpm for 5-48 hours;

After the ball-milled mixed powder is prepared into a shape, pressure-sintered at 450-600° C. and 60-65 MPa to obtain the p-type nanocomposite thermoelectric material.

In the present invention, the elemental powders and carbon nanotubes in the general formula Cu _2-x Se _1-yz S _y T _z are used as initial raw materials, and after mixing, ball milling (high-energy ball milling: at a ball milling speed of 450-500 rpm) is used. The mechanical energy generated by ball milling (5-48 hours) induces changes and chemical reactions in the structure, structure and properties of the material, induces lattice defects and dislocations in the material particles, plastic deformation and cold welding, so as to obtain carbon nanotubes with uniform distribution. Nanocomposites. The reaction process is roughly as follows: (2-x)Cu+(1- _yz )Se+yS+ _zTe +n%CNT→Cu _2- xSe 1- _yzSyTez /n%CNT). Then combined with pressure sintering to obtain a bulk material. On the one hand, the method of the invention can adjust the carrier concentration of the copper selenide-based compound, and on the other hand, the crystal grains can be nanosized, the phonon scattering can be enhanced, the thermal conductivity can be reduced, and the thermoelectric performance can be improved.

Preferably, the carbon nanotubes have a purity of >96% and an electrical conductivity of >10 ⁴ S/m.

Preferably, the particle size of the elemental powders of each element is less than or equal to 100 meshes.

Also, preferably, the purity of the elemental powder of each element is more than 99.9%.

Preferably, the initial raw materials and the tungsten carbide balls are put into a ball milling jar made of tungsten carbide in an argon atmosphere glove box for ball milling.

Also, preferably, the mass ratio of the tungsten carbide balls to the initial raw material is 5:1 to 20:1.

Preferably, the pressure sintering is discharge plasma sintering, and the time of the discharge plasma sintering is 5-10 minutes.

In yet another aspect, the present invention provides the application of the p-type nanocomposite thermoelectric material of the present invention in a thermoelectric device. The thermoelectric devices include thermoelectric power generation or thermoelectric refrigeration devices in medium and high temperature regions, such as in automobile exhaust and industrial production, especially in the metallurgical industry, thermoelectric power generation or thermoelectric refrigeration devices in medium and high temperature regions.

In the present invention, the p-type nanocomposite thermoelectric material is suitable for thermoelectric power generation or thermoelectric refrigeration in medium and high temperature regions. It can realize the effective use of low-density heat sources, and achieve the purpose of energy saving and emission reduction to a certain extent.

The Seebeck coefficient of the material of the invention increases gradually with the increase of temperature, and the electrical conductivity changes non-monotonically with the increase of temperature, and the change trend of the electrical conductivity changes near the solid phase transition temperature. At the same time, the carrier concentration decreases with the increase of carbon nanotube content, and the thermal conductivity decreases with the increase of carbon nanotube content. The thermoelectric figure of merit can reach about 1.0 at 750K, and the thermoelectric performance is better.

In addition, the preparation method of the present invention has abundant raw material sources, low cost, simple production process and production equipment, and good controllability and repeatability. In the present invention, the carbon nanotubes in the p-type nanocomposite thermoelectric material are uniformly distributed and have better thermoelectric performance.

Description of drawings

Fig. 1 is the schematic flow sheet of the preparation method of the present invention;

Figure 2 is the thermoelectric properties of the nanocomposite thermoelectric material (Cu ₂ Se/0.25%CNTs) in one embodiment of the present invention, wherein (a) is the resistivity of the nanocomposite material, (b) is the Seebeck coefficient of the nanocomposite material, (c) is the thermal conductivity of the nanocomposite, and (d) is the thermoelectric figure of merit ZT of the nanocomposite;

3 is the thermoelectric properties of the nanocomposite thermoelectric material (Cu ₂ Se/0.5%CNTs) in one embodiment of the present invention, wherein (a) is the resistivity of the nanocomposite material, (b) is the Seebeck coefficient of the nanocomposite material, (c) is the thermal conductivity of the nanocomposite, and (d) is the thermoelectric figure of merit ZT of the nanocomposite;

4 is the thermoelectric properties of the nanocomposite thermoelectric material (Cu ₂ Se _0.5 Te _0.5 /0.5%CNTs) in one embodiment of the present invention, wherein the upper left graph is the resistivity of the nano composite material, and the upper right graph is the Seebeck coefficient of the nano composite material , the lower left figure is the thermal conductivity of the nanocomposite, and the lower right figure is the thermoelectric figure of merit ZT of the nanocomposite;

5 is a scanning electron microscope image of a nanocomposite thermoelectric material (Cu ₂ Se/0.5% CNTs) in an embodiment of the present invention.

Detailed ways

The present invention is further described below through the following embodiments, and it should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention.

The general formula of the material synthesized in the present invention is Cu _2-x Se _1-yz S _y Te _z /m% CNTs (wherein CNTs are carbon nanotubes, m% is mass percentage), wherein the value of x is 0-0.15, y The value of z is 0-1, the value of z is 0-1, y+z≤1, and the mass percentage of carbon nanotubes is 0<m%≤2%.

The preparation process of the present invention is realized through the processes of batching, ball milling (high-energy ball milling) and spark plasma sintering. Figure 1 shows the process flow diagram of the preparation of this material, which specifically includes: with the general formula Cu _2-x Se _1-yz The stoichiometric ratio of S _y T _z /m% CNTs was weighed as the initial raw material of the high-purity powder of each element element and the carbon nanotubes with a mass percentage of m %. After weighing and charging, high-energy ball milling is performed to obtain nanocomposite thermoelectric material powder. The obtained nanocomposite thermoelectric material powder is then sintered by SPS to obtain a dense disc, and finally the thermal conductivity, electrical conductivity, Seebeck and other performance tests are performed. The following exemplifies the preparation method of the novel p-type nanocomposite thermoelectric material provided by the present invention.

Using the elemental powders and carbon nanotubes of the general formula Cu _2-x Se _1-yz S _y T _z as the initial raw materials, according to the stoichiometric ratio of Cu _{2- x} Se _1-yz S _y T _z , and Cu ₂ The mass ratio of _-x Se _1-yz S _y Te _z to carbon nanotubes is weighed. The diameter of the carbon nanotubes may be 10-20 nanometers, and the length is greater than 3 μm. The particle size of the elementary powder of each element is less than or equal to 100 meshes, and the purity of the elementary powder of each element is greater than 99.9%. The present invention uses elemental high-purity powder and carbon nanotubes as initial raw materials, such as copper powder (about 100 meshes) with a purity of 99.95%, selenium powder (about 100 meshes) with a purity of 99.9%, and sulfur powder with a purity of 99.9%, respectively. (about 100 mesh) and 99.9% pure tellurium powder (about 100 mesh). Carbon nanotubes are multi-walled carbon nanotubes with a purity of >96%, a diameter of 10-20 nanometers, a length of >3 mm, and a conductivity of >10 ⁴ S/m. The starting materials were weighed in the nominal stoichiometric ratio in Cu _2-x Se _1-yz S _y Te _z /m% CNTs.

After weighing the initial raw materials, high-energy ball milling is performed at a ball milling speed of 450-500 rpm for 5-48 hours. The starting material was charged into a tungsten carbide ball mill jar under an inert atmosphere. The ball to material ratio of high energy ball mill can be 5:1-20:1. As an example, the charging process is to load the initial raw material powder (elemental high-purity powder and carbon nanotubes) and tungsten carbide balls into a glove box with an inert atmosphere (such as an argon atmosphere, etc.), which is made of tungsten carbide. ball milling jar, and then high-energy ball milling is started, and the mass ratio of the tungsten carbide balls to the initial raw material may be 5:1 to 20:1.

The nanocomposite obtained by ball milling is sintered and formed. As an example, the p-type nanocomposite thermoelectric material is obtained by sintering the mixed powder after high-energy ball milling under pressure at 450-600° C. and 60-65 MPa. The sintering method may be spark plasma sintering.

A sintered mold (such as a graphite mold) can be used for the above-mentioned molding, and boron nitride (BN) is sprayed inside the mold and at the upper and lower indenters for insulation. Among them, the sintering temperature is 450-600 DEG C, the pressure is 60-65MPa, and the sintering time is 5-10 minutes.

In the present invention, high-purity copper powder, selenium powder (sulfur powder or tellurium powder) and carbon nanotubes are subjected to high-energy ball milling in an inert atmosphere to obtain nano-composite material powder, which is then sintered by discharge plasma (SPS) to obtain bulk material. The method is simple and reliable, the grain size of the obtained material is nanoscale, and the carbon nanotubes are uniformly distributed, which can significantly reduce the thermal conductivity of the lattice and improve the thermoelectric performance.

In the present invention, the thermoelectric figure of merit ZT of the nanocomposite Cu _2-x Se _1-yz S _y Te _z /m% CNTs partially controlled by the carrier concentration can reach 1.0 and above at 750K, which is suitable for Applications in medium and high temperature regions. Moreover, the Cu _{2- x} Se _1-yz S _y Te _z /m%CNTs nanocomposite thermoelectric material has a high Seebeck coefficient and a low thermal conductivity.

The following further examples are given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention, and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present invention belong to the present invention. scope of protection. The specific process parameters and the like in the following examples are only an example of a suitable range, that is, those skilled in the art can make selections within the suitable range through the description herein, and are not intended to be limited to the specific numerical values exemplified below.

Example 1: _Cu2Se /0.25%CNTs (x=0, y=0, z=0, m=0.25)

The high-purity powder raw materials Cu powder and Se powder are weighed in a molar ratio of 2:1, and carbon nanotubes with a mass fraction of 0.25% are added. Then, the powder raw materials, carbon nanotubes and tungsten carbide balls were put into a ball milling jar made of tungsten carbide in an argon atmosphere glove box. The ratio of ball to material is 10:1. Then perform high-energy ball milling at 500 rpm, and the ball milling time is 48 hours;

The nanocomposite obtained by ball milling was subjected to spark plasma sintering (SPS sintering), the sintering temperature was 480 °C, the pressure was 65 MPa, and the sintering time was 5 minutes, and finally a dense bulk material was obtained.

As shown in Fig. 2, the thermoelectric performance measurement of the obtained Cu ₂ Se/0.25%CNTs bulk material shows that the material has a high Seebeck coefficient and a low resistivity in the measured temperature range (300-750 K). At the same time, it has very low thermal conductivity: in the temperature range of 300-750K, its value is <0.8Wm ^-1 K ^-1 . The ZT value of this material can reach 1.05 at 750K, calculated from the performance measurements.

Example ₂ : Cu2Se/0.5%CNTs (x=0, y=0, z=0, m=0.5)

The high-purity powder raw materials Cu powder and Se powder are weighed in a molar ratio of 2:1, and then carbon nanotubes with a mass fraction of 0.5% are added. Then, the powder raw materials, carbon nanotubes and tungsten carbide balls were put into a ball milling jar made of tungsten carbide in an argon atmosphere glove box. The ratio of ball to material is 10:1. Then perform high-energy ball milling at 500 rpm, and the ball milling time is 48 hours;

The nanocomposite obtained by ball milling was subjected to spark plasma sintering (SPS sintering), the sintering temperature was 480 °C, the pressure was 65 MPa, and the sintering time was 5 minutes, and finally a dense bulk material was obtained.

As shown in Fig. 3, the thermoelectric performance measurement of the obtained Cu ₂ Se/0.5%CNTs bulk material shows that the material has a high Seebeck coefficient and a low resistivity in the measured temperature range (300-750 K). At the same time, it has very low thermal conductivity: in the temperature range of 300-750K, its value is <0.7Wm ^-1 K ^-1 . The ZT value of the material was calculated to reach 1.25 at 750K based on the performance measurements.

Example 3: Cu ₂ Se _0.5 Te _0.5 /0.5%CNTs (x=0, y=0, z=0.5, m=0.5)

The high-purity powder raw materials Cu powder, Se powder and Te powder are weighed in a molar ratio of 2:0.5:0.5, and carbon nanotubes with a mass fraction of 0.5% are added. Then, the powder raw materials, carbon nanotubes and tungsten carbide balls were put into a ball milling jar made of tungsten carbide in an argon atmosphere glove box. The ratio of ball to material is 10:1. Then perform high-energy ball milling at 500 rpm, and the ball milling time is 24 hours;

The nanocomposite obtained by ball milling was subjected to spark plasma sintering (SPS sintering), the sintering temperature was 600° C., the pressure was 65 MPa, and the sintering time was 5 minutes, and finally a dense bulk material was obtained.

As shown in Fig. 4, the thermoelectric performance measurement of the obtained Cu ₂ Se _0.5 Te _0.5 /0.5%CNTs bulk material shows that in the measured temperature range (300-750K), the material has a high Seebeck coefficient, a low resistivity and lower thermal conductivity. The ZT value of this material can reach 0.7 at 750K, calculated from the performance measurements.

5 is a scanning electron microscope image of the nanocomposite thermoelectric material (Cu ₂ Se/0.5% CNTs) in an embodiment of the present invention, and it can be seen that the grain size is about several tens of nanometers.

Claims (8)

1. a p-type nanocomposite thermoelectric material, characterized in that the p-type nanocomposite thermoelectric material comprises Cu _2-x Se _{1- z} Te _z and distributed in the Cu _2-x Se _1-z Te _z The carbon nanotubes in the carbon nanotubes, wherein 0≤x≤0.15, 0≤z≤1, the mass percentage of the carbon nanotubes≤2%; The preparation method of the p-type nanocomposite thermoelectric material includes: Using the elemental powders and carbon nanotubes in the general formula Cu _2-x Se _1-z Te _z as the initial raw materials, according to the stoichiometric ratio of Cu _2-x Se _1-z Te _z and Cu _2-x Se ₁ After weighing the mass ratio of _-z Te _z to carbon nanotubes, ball mill in an inert atmosphere at a ball milling speed of 450 to 500 rpm for 5 to 48 hours to obtain Cu _2-x Se _1-z Te _z / CNT nanotubes. composite material; After the ball-milled Cu _2-x Se _1-z Te _z /CNT nanocomposite material is prepared and molded, the p-type nanocomposite thermoelectric material is obtained by sintering under pressure at 450-600° C. and 60-65MPa. 2 . The p-type nanocomposite thermoelectric material according to claim 1 , wherein the carbon nanotubes have a diameter of 10-20 nanometers and a length of >3 μm. 3 . 3 . The p-type nanocomposite thermoelectric material according to claim 1 , wherein the carbon nanotubes have a purity of >96% and an electrical conductivity of >10 ⁴ S/m. 4 . 4 . The p-type nanocomposite thermoelectric material according to claim 1 , wherein the particle size of the elemental powder of each element is less than or equal to 100 mesh. 5 . 5 . The p-type nanocomposite thermoelectric material according to claim 4 , wherein the purity of the elemental powder of each element is greater than 99.9%. 6 . 6 . The p-type nanocomposite thermoelectric material according to claim 1 , wherein the initial raw materials and the tungsten carbide balls are loaded into a ball milling jar made of tungsten carbide in an argon atmosphere glove box to perform ball milling. 7 . 7 . The p-type nanocomposite thermoelectric material according to claim 6 , wherein the mass ratio of the tungsten carbide balls to the initial raw material is 5:1 to 20:1. 8 . 8 . The p-type nanocomposite thermoelectric material according to claim 1 , wherein the pressure sintering is discharge plasma sintering, and the discharge plasma sintering time is 5-10 minutes. 9 .

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