CN108198934B

CN108198934B - Composite thermoelectric material and preparation method thereof

Info

Publication number: CN108198934B
Application number: CN201711461912.9A
Authority: CN
Inventors: 不公告发明人
Original assignee: Longnan Xinlongye New Material Co ltd
Current assignee: Longnan xinlongye New Material Co.,Ltd.
Priority date: 2017-12-28
Filing date: 2017-12-28
Publication date: 2021-03-09
Anticipated expiration: 2037-12-28
Also published as: CN108198934A

Abstract

The invention provides a composite thermoelectric material and a preparation method thereof, belonging to the technical field of thermoelectric materials, wherein the composite thermoelectric material comprises Cu, La, Se and N-doped SiC, wherein the grain diameter of the N-doped SiC is less than 100nm, the content of the N-doped SiC is 0.1-1 mass%, and the ratio of Cu to Se is 1.5-1.9: 1. The thermoelectric property and the mechanical property of the composite thermoelectric material prepared by the invention are greatly improved.

Description

Composite thermoelectric material and preparation method thereof

Technical Field

The invention belongs to the technical field of thermoelectric materials, and particularly relates to a nano-composite thermoelectric material and a preparation method thereof.

Background

A compound semiconductor is a compound that is composed of at least two types of elements other than one type of element (e.g., silicon or germanium) and functions as a semiconductor. Various types of compound semiconductors have been developed and are currently being used in various industrial fields. In general, a compound semiconductor can be used for a thermoelectric conversion element utilizing a Peltier Effect (Peltier Effect), a light-emitting device utilizing a photoelectric conversion Effect (such as a light-emitting diode or a laser diode), a fuel cell, and the like.

In particular, thermoelectric conversion elements are used for thermoelectric conversion power generation or thermoelectric conversion cooling applications, and generally include an N-type thermoelectric semiconductor and a P-type thermoelectric semiconductor electrically connected in series and thermally connected in parallel. Thermoelectric conversion power generation is a method of generating power by converting thermal energy into electric energy using a thermoelectromotive force generated by generating a temperature difference in a thermoelectric conversion element. Further, thermoelectric conversion cooling is a method of generating cooling by converting electric energy into thermal energy using the effect of generating a temperature difference between both ends of a thermoelectric conversion element when a direct current flows through both ends of the thermoelectric conversion element. The thermoelectric conversion efficiency of the thermoelectric conversion element generally depends on the performance index value or ZT of the thermoelectric conversion material. Here, ZT (thermoelectric figure of merit) may be determined according to Seebeck coefficient (Seebeck coefficient), electric conductivity, and thermal conductivity, and as the ZT value increases, the performance of the thermoelectric conversion material is better. Many thermoelectric materials that can be used for thermoelectric conversion elements have been proposed and developed, and among them, CuxSe (x.ltoreq.2) is proposed as a Cu-Se-based thermoelectric material and is being developed.

In particular, it has recently been reported that lower thermal conductivity and high thermoelectric figure of merit are achieved in CuxSe (1.98. ltoreq. x.ltoreq.2). However, reasonably good thermoelectric figures of merit were observed at 600 ℃ to 727 ℃, but thermoelectric figures of merit were found to be very low, on average less than 1.3, at temperatures less than or equal to 600 ℃. Although a thermoelectric material has a high thermoelectric figure of merit at a high temperature, if the thermoelectric material has a low thermoelectric figure of merit at a low temperature, such a thermoelectric material is not preferable, and in particular, is not suitable for a thermoelectric material for power generation. Even if such a thermoelectric material is applied to a high-temperature heat source, a certain region of the material may experience a much lower temperature than desired due to the temperature gradient of the material itself. Therefore, the following thermoelectric materials need to be developed: the thermoelectric material still has a high thermoelectric figure of merit of >1.6 in a temperature range of less than 600 ℃ (e.g., 100 ℃ to 600 ℃).

Disclosure of Invention

In view of the above technical problems, the present invention provides a composite thermoelectric material comprising Cu, La, Se, N-doped SiC, wherein the particle size of the N-doped SiC is less than 100nm, the content of the N-doped SiC is 0.1 to 1 mass%, the ratio of Cu to Se is 1.5 to 1.9:1, and the content of La is 1 to 5 mass%.

Further wherein the ratio of Cu to Se is 1.8: 1.

Further, the content of N-doping in SiC was 0.5 mass%.

Further, the grain size of the N-doped SiC is 50 nm.

Further, wherein the La content is 2%.

The invention also provides a method for preparing the composite thermoelectric material, wherein the method comprises the following steps

1) Ball-milling and mixing the N-doped SiC powder until the particle size is less than 100 nm;

2) taking Cu powder, La powder and Se powder as raw materials according to a certain proportion, grinding the raw materials to form particles, smelting the particles in a vacuum sealing environment, adding the powder prepared in the step (1), uniformly stirring the mixture, and cooling the mixture;

3) and (3) grinding the solid solution prepared in the step (2) into powder, filling the ground powder into a graphite die, and placing the die into a discharge plasma sintering furnace cavity to be sintered to obtain a product.

Further, the sintering conditions of the spark plasma sintering furnace cavity in the step 4 are as follows: applying axial pressure of 30-2GPa, sintering under vacuum condition, heating at the heating rate of 150 ℃/min of 100-.

According to the method provided by the invention, the ball milling and mixing in the step (1) are carried out at 7500-10000 r/min, for example, at 8000 r/min for 3-5 hours.

The composite thermoelectric material prepared by the invention has an amorphous/nanocrystalline nano composite structure in the matrix, and the crystal grains are obviously refined. The N-doped SiC is used as an amorphous state, nanoparticles form a composite structure in the melting process of forming nanocrystals by metal, Cu is positioned at the grain boundary of the amorphous/nanocrystals, the composite structure reduces the thermoelectric conversion energy barrier, and the N-doped SiC can effectively improve the thermoelectric performance and the electric conductivity. The addition of the rare earth metal element can further achieve the microalloying effect due to the special electronic structure of the rare earth metal element, and La is easy to segregate at grain boundaries, so that the grain boundaries are strengthened, the thermoelectric conversion energy barrier and the lattice heat energy are further reduced, and the thermoelectric performance is improved. The thermoelectric material of the present invention can have a low thermal diffusivity (thermal diffusivity), a low thermal conductivity, a high seebeck coefficient, and a high thermoelectric figure of merit in a wide temperature range between 100 ℃ and 600 ℃.

In the present invention, the particle size of the SiC powder is required to be 100nm or less, and a smaller particle size is advantageous for uniform mixing of the thermoelectric material and for lowering the lattice thermal conductivity, and a smaller particle size is better, and is preferably 50nm in consideration of the balance between cost and performance.

Meanwhile, the invention also discovers that the mechanical property of the prepared product can be effectively improved by adding a certain proportion of N-doped SiC and La, the La element has the effects of refining the alloy structure and reducing the crystal spacing, and simultaneously can react with impurity element P and the like, so that the solidification temperature range of the alloy can be narrowed, the casting property of the alloy can be improved, the cracking can be reduced, the compactness can be improved, and the nano-phase N-doped SiC has a synergistic promotion effect.

Compared with the prior art, the invention has the following beneficial effects:

(1) by adding a certain proportion of La and N doped SiC, the mechanical property of the prepared thermoelectric material is improved to a certain extent;

(2) the thermoelectric property, especially the low-temperature thermoelectric property of the amorphous/crystalline thermoelectric material with the nano composite structure prepared by the invention is greatly improved.

Detailed Description

The invention provides a composite thermoelectric material, which comprises Cu, La, Se and N-doped SiC, wherein the grain diameter of the N-doped SiC is less than 100nm, the content of the N-doped SiC is 0.1-1 mass%, the ratio of Cu to Se is 1.5-1.9:1, and the content of La is 1-5 mass%.

Examples 1-5 and comparative examples 1-8 were prepared according to the formulations of Table 1 as follows

1) SiC was weighed in a glove box in Ar atmosphere according to the formulation of Table 1 and placed in a steel ball mill jar for intermittent ball milling for 3 hours, wherein the rotation speed of the ball mill jar was 8000 rpm.

2) A high-frequency induction suspension smelting device is adopted, a Cu block (with the purity of 99.99 percent), an La block (with the purity of 99.999 percent) and an Se block (with the purity of 99.99 percent) are used as raw materials, the raw materials are proportioned and weighed in an Ar atmosphere according to a formula shown in a table 1, the prepared raw materials are put into a red copper crucible for smelting, the powder in the step 1 is added after being smelted, the mixture is stirred uniformly, argon after deoxidation is used as a protective atmosphere in the smelting process, and the highest power is 14 kw.

3) And (2) adopting induction smelting rapid quenching furnace equipment, loading the cast ingot obtained in the step (2) into a quartz tube with an opening at the lower end, vertically placing the quartz tube into an induction smelting coil of a cavity of the rapid quenching equipment, vacuumizing the cavity, filling protective argon into the cavity through the rapid quenching equipment to reach-0.03 MPa, adjusting the injection pressure to be 0.02MPa, spraying the melt to a copper roller with the rotation speed of 40m/s to obtain a molten mass, throwing the molten mass out to form a strip, and collecting the strip.

4) Placing the collected strip in a glove box with an argon atmosphere with the oxygen content lower than 0.5ppm to be ground into powder, placing the ground powder in a graphite mold, placing the mold in an SPS sintering cavity, applying axial pressure of 30MPa, sintering under the vacuum condition that the total air pressure is lower than 5Pa, heating at the heating rate of 100-150 ℃/min, keeping the temperature for 10-20min at the sintering temperature of 800-900 ℃, and cooling to room temperature along with the furnace to obtain the amorphous/nanocrystalline nano composite structure solid solution.

TABLE 1 formulation of each example and comparative example

And (3) performance detection: the thermal conductivity of the material is calculated according to the thermal diffusion coefficient, the specific heat and the density of the material measured by a TC-1200RH type laser pulse thermal analyzer. The seebeck coefficient and conductivity of the material were measured using ZEM-2 electrical property tester 2. Thermoelectric figure of merit of the material is determined from the above measurement according to the formula Z ═ alpha²σ/κ was obtained.

Using the 0.1mm thick sheets obtained by pressing the samples of examples and comparative examples, after brazing between ceramic sheets of 10 mm. times.10 mm. times.20 mm at 400 ℃ in a vacuum atmosphere, test pieces of 3 mm. times.4 mm. times.40 mm were cut out, and the breaking strength at each 10 points was measured by four-point bending test in accordance with JIS R1601. (test method was carried out according to JIS R1601).

Comparing examples with comparative examples 1 and 2, it was found that controlling the content of La element in a suitable range helps to improve various properties of the product, and excessive or insufficient La causes the properties of the product to be lowered. Comparing the examples with comparative examples 3 and 4, it is understood that controlling the molar ratio of Cu to Se can effectively improve the thermoelectric performance of the product, and it is presumed that the energy barrier of the crystal lattice formed by Cu and Se in the range of 1.5-1.9:1 is lower. Comparing the examples with comparative examples 5 and 6, it is found that SiC with a certain N doping amount can effectively improve the thermoelectric property and the breaking strength of the product, and ensure that the conductivity is not reduced as a whole. Comparing examples with comparative examples 7 and 8, the thermoelectric performance can be effectively improved by adding N-doped SiC, but it is also found that the particle size of N-doped SiC needs to be controlled within a certain range, and the thermoelectric performance and the breaking strength can be enhanced while the electrical conductivity is ensured. In conclusion, under the formula of the invention, various components are mutually matched and act synergistically, so that the technical performance of the product is greatly improved.

The above description should not be taken as limiting the invention to the embodiments, but rather, as will be apparent to those skilled in the art to which the invention pertains, numerous simplifications or substitutions may be made without departing from the spirit of the invention, which shall be deemed to fall within the scope of the invention as defined by the claims appended hereto.

Claims

1. A composite thermoelectric material is characterized by comprising Cu, La, Se and N-doped SiC, wherein the grain diameter of the N-doped SiC is less than 100nm, the content of the N-doped SiC is 0.1-1 mass%, the ratio of Cu to Se is 1.5-1.9:1, and the content of La is 1-5 mass%.

2. The composite thermoelectric material of claim 1, wherein the ratio of Cu to Se is 1.8: 1.

3. The composite thermoelectric material according to claim 1, wherein the content of N-doping in SiC is 0.5 mass%.

4. The composite thermoelectric material according to claim 1, wherein the particle size of SiC is 50 nm.

5. The composite thermoelectric material according to claim 1, wherein the La content is 2%.

6. A method of making a composite thermoelectric material according to any one of claims 1 to 5, comprising the steps of:

3) and (3) grinding the solid solution prepared in the step (2) into powder, filling the ground powder into a graphite die, and placing the die into a cavity of a spark plasma sintering furnace for sintering to obtain the product.

7. The method for preparing a composite thermoelectric material according to claim 6, wherein the sintering conditions of the spark plasma sintering furnace chamber in the step 3 are as follows: applying axial pressure of 30MPa, sintering under vacuum condition, heating at a heating rate of 150 ℃/min of 100-.