WO2008149910A1

WO2008149910A1 - Method for production of thermoelectric conversion element

Info

Publication number: WO2008149910A1
Application number: PCT/JP2008/060320
Authority: WO
Inventors: Junya Murai; Takuji Kita
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2007-06-06
Filing date: 2008-05-29
Publication date: 2008-12-11
Also published as: JP2008305918A

Abstract

Disclosed is a method for producing a thermoelectric conversion element, which comprises the steps of: preparing a solution containing a salt of an element constituting a thermoelectric conversion material; adding the solution drop-wise to a solution containing a reducing agent to cause the precipitation of a raw material particle for the thermoelectric conversion material; heating the solution; and firing the resulting product.

Description

Technical data Manufacturing method of thermoelectric conversion elements Technical field

The present invention relates to a method for manufacturing a thermoelectric conversion element that converts heat into electricity or electricity into heat. Background art

The thermoelectric conversion material is a material that can mutually convert heat energy and electric energy, and is a material constituting a thermoelectric conversion element used as a thermoelectric cooling element or a thermoelectric power generation element. This thermoelectric conversion material uses the Zeebeck effect to perform thermoelectric conversion, and its thermoelectric conversion performance is expressed by the following equation (1) called the figure of merit Z T.

Ζ Τ = α ² σ Τ eight (1)

(Where α is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and Τ is the measured temperature)

As is clear from the above equation (1), in order to improve the thermoelectric conversion performance of the thermoelectric conversion material, it is necessary to increase the Seebeck coefficient α and the electric conductivity σ of the material to be used, and to decrease the thermal conductivity κ. Recognize. Here, in order to reduce the thermal conductivity κ of the material, Journal of Applied Physics, 97, 044317 (2005) proposes miniaturizing the thermoelectric conversion material. That is, by miniaturizing the thermoelectric conversion material particles, the phonon, which is the main factor of heat conduction in the thermoelectric conversion material, is scattered at the interface of the fine particles, and the thermal conductivity / can be reduced.

In the disclosed technique, the oxide of the metal constituting the thermoelectric conversion material is 2 5 Since heat treatment was performed at 0 to 3500 and further alloyed at 3500 to 4500, the grain size of the crystal grains in the final thermoelectric conversion element was coarsened to 1 δ0 to 2500 nm. If the particle size is so coarse, phonon scattering at the grain boundary is insufficient, the effect of reducing thermal conductivity is considered insufficient, and the performance improvement is also insufficient. Accordingly, an object of the present invention is to solve the above-mentioned conventional problems and provide a method for manufacturing a thermoelectric conversion element having an excellent figure of merit. Disclosure of the invention

In order to solve the above problems, according to the present invention, after preparing a solution containing a salt of an element constituting a thermoelectric conversion material, the solution is dropped into a solution containing a reducing agent to obtain raw material particles of the thermoelectric conversion material. A method for producing a thermoelectric conversion thermoelectric element is provided, which includes the steps of precipitation, heat treatment, and sintering.

According to the present invention, by adding a solution containing a salt of an element constituting the thermoelectric conversion material to a solution containing a reducing agent, the raw material particles of the thermoelectric conversion material having an average particle size of 10 to 100 nm The raw material particles are heat-treated and sintered to obtain a thermoelectric conversion element composed of crystal particles of a thermoelectric conversion material having an average particle diameter of 10 to 100 nm. It shows phonon scattering at the grain boundaries, reduces thermal conductivity reduction, and improves the figure of merit ZT. Brief Description of Drawings

Fig. 1 is a graph showing the relationship between the structural dimensions of the thermoelectric conversion material, the Seebeck coefficient, the electrical conductivity σ, or the thermal conductivity /.

FIG. 2 is an image of the thermoelectric conversion element of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

First, the relationship between the figure of merit ZT and the structure of the thermoelectric material will be described in detail with reference to FIG. As shown in Fig. 1, the thermal conductivity κ of the thermoelectric conversion material gradually decreases as the microstructure size of the thermoelectric conversion material becomes smaller starting from the length of the mean free path of phonon. Therefore, the figure of merit Z T is improved by designing the structure size to be smaller than the phonon mean free path.

On the other hand, even if the microstructure size of the thermoelectric conversion material becomes smaller than the mean free path of phonon, the electrical conductivity of the thermoelectric conversion material does not decrease, and the particle size is generally less than the mean free path of the carrier. When it becomes, it decreases. In this way, by utilizing the fact that the structural dimension of the thermoelectric conversion material where the thermal conductivity K begins to decrease and the structural dimension of the thermoelectric conversion material where the electrical conductivity σ begins to decrease, the rate of decrease in electrical conductivity is determined. At least some of the microstructure dimensions of the thermoelectric conversion material should be greater than the mean free path of the carrier and less than the mean free path of the phonon so that the structure size of the thermoelectric conversion material has a larger reduction rate of the thermal conductivity κ than Thus, the figure of merit Ζ Τ represented by the above formula (1) can be further increased.

Here, it is the particle size of the particles constituting the thermoelectric conversion material that defines the tissue size of the thermoelectric conversion material. Therefore, according to the method of the present invention, the particle size of at least a part of the particles constituting the thermoelectric conversion material is made equal to or less than the mean free path of the phonon of the thermoelectric conversion material. By making the particle size of the thermoelectric conversion material less than the mean free path of the phonon of the thermoelectric conversion material, phonon scattering at the grain boundary is shown, and the reduction in thermal conductivity is reduced, and the figure of merit Ζ Τ Can be improved.

The mean free path (M F P) is calculated using the following formula.

Carrier MFP = (Mobility X Effective mass X Carrier speed) Z charge Elementary quantity Phonon MFP = 3 X lattice thermal conductivity Z specific heat Z sonic speed In the above equation, each value is converted from the literature value and approximate equation of temperature characteristics, and only the specific heat is measured.

Where Co. ₉₄ N i. . Shows Q ₆ S b ₃ and C o S b ₃ a calibration Li A MFP and results Fuono emissions MFP calculated for below.

table 1

Calculation results for carrier MFP and phonon MFP (Yuyuki)

Thus, the carrier MFP and phonon MFP are determined by the material and temperature. In the thermoelectric conversion element obtained by the present invention, at least a part of the structure size is the power factor of the thermoelectric conversion material.

⁽² σ) is not good equal to or less than the mean free path of Fuono down at the maximum output. Since the C o S b ₃ system 4 0 0 Power factor (alpha ² flight) is the highest output, but not more than the mean free path of the phono down when at 4 0 0.

In order to manufacture such a thermoelectric conversion element, in the present invention, first, a solution containing a salt of an element constituting the thermoelectric conversion material is prepared.

The thermoelectric conversion material to be formed may be saddle type or saddle type. There is not any specific restriction on the material of Ρ type thermoelectric conversion material, for example, Β i ₂ T e ₃ based, P b T e system, Z n ₄ S b ₃ system, C o S b ₃ system, Hafuho Chrysler system, full-Heusler System, SiGe system, etc. can be used. As the material of the N-type thermoelectric conversion material, known materials can be applied without any particular limitation. For example, B i ₂ Te ₃ system, P b Te system, Z n ₄ S b ₃ system, Co S b ₃ system, half-Heusler system, full-Heusler system, S i G e system, Mg ₂ S i system, Mg ₂ Sn system, Co S i system, etc. it can.

The thermoelectric conversion material formed in the present invention preferably has an output factor larger than 1 mWZK ^2, more preferably 2 mWZK ² or more, and further preferably 3 mWZK ² or more. If the output factor is 1 mWZK ² or less, a significant performance improvement cannot be expected. In addition, the thermal conductivity K of the thermoelectric conversion material is preferably larger than 5 WZmK, more preferably 7 W / mK or more, and further preferably 10 W mK or more. When the thermal conductivity κ is larger than 5 WZ m K, the effect of the present invention is particularly remarkable. In other words, the effect of controlling the microstructure dimensions of the thermoelectric conversion material with the nano-order specified in the present invention is that the lower the thermal conductivity κ, the more the thermoelectric conversion material with higher thermal conductivity c is used. In particular, when a thermoelectric conversion material with a thermal conductivity / c greater than 5 WZmK is used, the effect of reducing the thermal conductivity κ is significant.

For example, when the thermoelectric conversion material is Co S b ₃ , the salt of the element constituting such a thermoelectric conversion material is cobalt chloride hydrate and antimony chloride, and in the case of Co ^ N ix S bs Means cobalt hydrate, nickel chloride and antimony chloride. Then, considering the composition of the thermoelectric conversion material to be formed, the salt of the element constituting the thermoelectric conversion material to be used and the amount thereof are selected.

Water or alcohol can be used as the solvent of the salt solution of the elements constituting the thermoelectric conversion material, and ethanol is preferred.

Thus, after preparing the solution of the salt of the element which comprises the said thermoelectric conversion material, this dispersion liquid is dripped at the solution containing a reducing agent. As a reducing agent, Any element that can reduce the ions of the elements constituting the thermoelectric conversion material, such as Na BH hydrazine, can be used.

In the dispersion containing the salt of the element constituting the thermoelectric conversion material, raw material ions of the thermoelectric conversion material, such as Co ions and Sb ions, are present. Therefore, when mixed with a solution containing a reducing agent, these ions are reduced, and raw material particles of the thermoelectric conversion material, such as Co particles and Sb particles, are precipitated. In this reduction, in addition to the C o particles and S b particles, by-product thereof, for example N a C 1 and N a BO ₃ generates. In order to remove this by-product, it is preferable to perform filtration. Furthermore, after filtration, it is preferable to add alcohol to the water to wash away by-products.

The dispersion is then heat-treated, preferably hydrothermally treated, and thermoelectric conversion material particles are synthesized from the raw material particles of the thermoelectric conversion material, washed and dried as necessary, and then subjected to a general sintering method. For example, the thermoelectric conversion element of the present invention can be obtained by performing SPS sintering at 580. The method for producing a thermoelectric conversion material of the present invention makes it possible to control the structure size (particle diameter of thermoelectric conversion material particles) in the nano-order. That is, by reducing the salt of the element constituting the thermoelectric conversion material, raw material particles of the thermoelectric conversion material having a particle size of 10 to 100 nm are formed, and the thermoelectric conversion material particles are formed therefrom. By preparing, the dimension of the thermoelectric conversion element (particle diameter of thermoelectric conversion material particles) force is less than the mean free path of the phonon, preferably more than the mean free path of the carrier and less than the mean free path of the phonon, Scattering of phonons in the thermoelectric conversion element occurs sufficiently, and the thermal conductivity / c can be reduced. As a result, a thermoelectric conversion element having a large figure of merit ZT represented by the equation (1) is obtained. Thus, according to the method of manufacturing a thermoelectric conversion element of the present invention, an excellent thermoelectric conversion element exhibiting a high figure of merit ZT, and the figure of merit ZT, which has been difficult to manufacture in the past, exceeds 2. A thermoelectric conversion element can also be obtained. Example

Cobalt chloride (1.0 g) and antimony chloride (3.06 g) were added to ethanol (10 O mL) and dissolved, and then nickel chloride (0.064 g) was added to the solution and mixed uniformly. This solution was added dropwise to a reducing agent solution prepared by dissolving 2.0 g of sodium borohydride in 10 mL of ethanol. Next, after removing impurities by washing with a mixed solution of ethanol and water, hydrothermal synthesis was performed at 20 00 for 48 hours, and the thermoelectric conversion material Co _e . ₉₄ Ni _Q. _{Q 6} S b ₃ compounds formed.

Thereafter, ethanol was evaporated in an inert gas atmosphere, dried, and the obtained particles were filled, and SPS sintering was performed at 60 ° C. for 30 minutes to obtain the thermoelectric conversion element of the present invention. A T EM image of this device is shown in FIG. 2. The crystal grain size was 10 to 100 nm. The thermal conductivity of this thermoelectric conversion element was measured by the flash method. It was 1. SWZmZK, which was the conventional product (crystal grain size: 1550 to 2500 nm, thermal conductivity: 3.5 W / m / K) was reduced by 60%.

Claims

The scope of the claims

1. After preparing the solution containing the salt of the element constituting the thermoelectric conversion material, this solution is dropped into the solution containing the reducing agent to precipitate the raw material particles of the thermoelectric conversion material, heat treatment, and then sintering. The manufacturing method of the thermoelectric conversion thermoelectric element including a process.

2. The method for producing a thermoelectric conversion element according to claim 1, wherein the salt of the element constituting the thermoelectric conversion material is selected from cobalt chloride, antimony chloride, bismuth chloride, and tellurium chloride.

3. The method for producing a thermoelectric conversion element according to claim 1, wherein the reducing agent is sodium borohydride.