JP2011129635A - Nano-composite thermoelectric conversion material and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nano-composite thermoelectric conversion material that has nano-particles of a dispersed material dispersed in the mother phase of the thermoelectric conversion material, and has low thermal conductivity κ, and a method of manufacturing the same. <P>SOLUTION: The present invention relates to the nano-composite thermoelectric conversion material that has the nano-particles of the dispersed material dispersed in the mother phase of the thermoelectric conversion material, wherein the dispersed material is porous nano-particles each having a hole opened on the external surface of the particle, and the thermoelectric material of the mother phase enters the inside of the hole; and the method of manufacturing the same employing liquid phase synthesis. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nanocomposite thermoelectric conversion material and a method for producing the same, and more particularly, a nanocomposite thermoelectric conversion material capable of providing low thermal conductivity by a specific configuration of the interface between the matrix of the thermoelectric conversion material and the nanoparticles of the dispersion material And its production method by liquid phase synthesis.

In recent years, in order to reduce carbon dioxide emissions due to the global warming problem, there is an increasing interest in technologies that reduce the proportion of energy obtained from fossil fuels. The thermoelectric conversion material which can be directly converted into energy is mentioned.
A thermoelectric conversion material is a material that enables direct conversion from heat to electrical energy without the need for a two-step process of converting heat to kinetic energy and then to electrical energy, as in thermal power generation. is there.

And conversion from a heat | fever to an electrical energy is performed using the temperature difference of the both ends of the bulk body shape | molded from the thermoelectric conversion material. The phenomenon in which a voltage is generated due to this temperature difference was discovered by Seepec and is called the Seepek effect.
The performance of the thermoelectric conversion material is represented by a figure of merit Z obtained by the following equation.
Z = α ² σ / κ (= Pf / κ)

Here, α is the Seebeck coefficient of the thermoelectric conversion material, σ is the conductivity of the thermoelectric conversion material, and κ is the heat conductivity of the thermoelectric conversion material. The terms α ² σ are collectively referred to as an output factor Pf. Z has a dimension of the reciprocal of temperature, and ZT obtained by multiplying the figure of merit Z by the absolute temperature T is a dimensionless value. This ZT is called a dimensionless figure of merit and is used as an index representing the performance of the thermoelectric conversion material.
In order to use the thermoelectric conversion material widely, it is required to further improve its performance. As is apparent from the above formula, higher Seebeck coefficient α, higher conductivity σ, and lower thermal conductivity κ are required to improve the performance of the thermoelectric conversion material.
However, it is difficult to improve all these items at the same time, and many attempts have been made to improve any of the above items of thermoelectric conversion materials.

  For example, Patent Document 1 includes a matrix microparticle in which a first substance is at least a low thermal conductivity material such as a ceramic, and a second substance includes a high thermal conductivity material such as bismuth, antimony, and tellurium alloy, A thermoelectric nanocomposite material in which a second substance forms an independent ultrathin layer, substantially covers the periphery of the matrix microparticles, and substantially isolates the matrix microparticles, and has a porosity of 5% or less; and A production method by solid mixing is described.

Patent Document 2 discloses a ceramic powder as a dispersion material in a base material of a thermoelectric conversion material represented by CoSb _X (2.7 <X <3.4), specifically, a thermoelectric conversion material of CoSb _3. A method for producing a semiconductor material for thermoelectric conversion is described in which the dispersed semiconductor material for thermoelectric conversion and the raw material powder represented by CoSb _X and ceramic powder as a dispersing material are mixed, molded, and fired. However, the publication does not mention the interface between the matrix of the thermoelectric conversion material and the dispersion particles, and the thermal conductivity of the semiconductor material for thermoelectric conversion specifically described does not mix ceramic powder. Although it is low compared with about 5 W / m / K of the thermoelectric conversion material, it is 1.8 to 3 W / m / K.

JP-A-3-148879 Japanese Unexamined Patent Publication No. 2000-261047

However, even with these conventional techniques, the reduction of the thermal conductivity κ is insufficient, and the performance of the obtained thermoelectric conversion material is low.
Accordingly, an object of the present invention is to provide a thermoelectric conversion material in which nanoparticles of a dispersion material are dispersed in a matrix (matrix) of the thermoelectric conversion material, and which enables a low thermal conductivity κ. It is.
Another object of the present invention is to produce a thermoelectric conversion material in which nanoparticles of a dispersion material are dispersed in a matrix (matrix) of the thermoelectric conversion material, which enables a low thermal conductivity κ. Is to provide a method.

The present invention is a nanocomposite thermoelectric conversion material in which nanoparticles of a dispersion material are dispersed in a matrix of a thermoelectric material, wherein the dispersion material is a porous nanoparticle having pores opened on the outer surface of the particle, Further, the present invention relates to a nanocomposite thermoelectric conversion material in which the matrix thermoelectric material has entered the pores.
Further, the present invention provides a thermoelectric material and a dispersion material by dropping a reducing agent into a slurry containing a salt of a precursor material of the thermoelectric material and porous nanoparticles having pores opened on the outer surface of the particle. The present invention relates to a method for producing a nanocomposite thermoelectric conversion material in which a matrix thermoelectric material has entered the pores, the method including a step of obtaining a mixture with porous nanoparticles.
In the present invention, the fact that the thermoelectric material of the parent phase has entered the pores means that the thermoelectric material of the parent phase has entered the vicinity of the opening of the pores.

According to the present invention, a nanocomposite thermoelectric conversion material having a low thermal conductivity κ can be provided.
Further, according to the present invention, a nanocomposite thermoelectric conversion material having a low thermal conductivity κ can be easily obtained.

FIG. 1 is an enlarged schematic side view showing the state of mesoporous silica dispersed in the nanocomposite thermoelectric conversion material of one embodiment of the present invention. FIG. 2 is a schematic front view of mesoporous silica dispersed in the nanocomposite thermoelectric conversion material thermoelectric conversion material of one embodiment of the present invention. FIG. 3 is a partially enlarged schematic front view of mesoporous silica in the nanocomposite thermoelectric conversion material of one embodiment of the present invention. FIG. 4 is a partially enlarged side schematic view of mesoporous silica in the nanocomposite thermoelectric conversion material of one embodiment of the present invention. FIG. 5: shows the flowchart of the main processes of the manufacturing method of the nanocomposite thermoelectric conversion material of one embodiment of this invention. FIG. 6 is a copy of the appearance STEM image of the nanocomposite thermoelectric conversion material obtained in Example 1. FIG. 7 is a copy of a cross-sectional image of mesoporous silica in the nanocomposite thermoelectric conversion material obtained in Example 1 as viewed from the front. FIG. 8 is a copy of a cross-sectional image of mesoporous silica in the nanocomposite thermoelectric conversion material obtained in Example 1 as viewed from the side.

  According to an embodiment of the present invention, a nanocomposite thermoelectric conversion material in which mesoporous silica as a dispersion material is dispersed in a matrix of the thermoelectric conversion material, the dispersion material having pores open to the outer surface of the particles. The nanocomposite thermoelectric conversion material is obtained because the thermoelectric material is a porous nanoparticle and the matrix phase thermoelectric material has entered the pores, resulting in large irregularities at the interface between the dispersion material and the matrix phase and a decrease in thermal conductivity. It is done.

  According to an embodiment of the present invention, the thermoelectric material is obtained by dropping a reducing agent into a slurry containing a salt of a precursor material of the thermoelectric material and nanoparticles having pores opened on the outer surface of the particle. By including the step of obtaining a mixture of the dispersion material with the porous nanoparticles, the thermoelectric material is deposited in the pores of the porous dispersion material, and the thermoelectric material of the parent phase enters the pores. A nanocomposite thermoelectric conversion material can be produced.

Hereinafter, the present invention will be described with reference to FIGS.
Referring to FIG. 1, porous nano particles of a dispersion material made of mesoporous silica are dispersed in the matrix of the thermoelectric material, and the thermoelectric material enters into the pores of the mesoporous silica, so that the interface roughening occurs in the vicinity of the pore openings. May occur and the thermal conductivity of the nanocomposite thermoelectric conversion material may decrease. The mesoporous silica which is an example of the dispersion material in the present invention is a porous nanoparticle having pores opened on the outer surface of the particle as shown in FIGS.
It is considered that the decrease in the thermal conductivity of the nanocomposite thermoelectric material of the present invention is due to activation of phonon scattering due to the interface roughness in the vicinity of the opening of the hole.

Referring to FIG. 5, by adding dropwise an ethanol solution of NaBH ₄ as a reducing agent to a slurry containing bismuth chloride, tellurium chloride and antimony chloride, which are examples of a precursor substance salt of a thermoelectric material, and mesoporous silica. The salt of the precursor material that provides the phase thermoelectric material can be entrained in the pores of the mesoporous silica. In the method of the present invention, the nanocomposite thermoelectric conversion material can then be obtained by continuously performing the solidification separation from the solvent and the alloying and drying steps by hydrothermal treatment to obtain the thermoelectric conversion material.

  In the present invention, in the above method, a reducing agent is dropped into a slurry containing a salt solution of a precursor material of a thermoelectric material and porous nanoparticles having pores opened on the outer surface of the particle to perform a reduction reaction. Porous with pores open to the outer surface of the nanoparticle in the parent phase of the thermoelectric material through subsequent solid separation, alloying by hydrothermal treatment and drying steps Nanoparticles, for example, mesoporous nanoparticles, for example, nanoparticles of a dispersion material made of mesoporous silica, are dispersed, and the thermoelectric material enters into the pores including the vicinity of the opening of the pore, thereby causing the interface roughness in the vicinity of the opening of the pore. It is thought that it will be possible to generate

The porous nanoparticle having pores opened on the outer surface of the particle in the present invention is not limited as long as it is a porous nanoparticle having pores on the surface. For example, mesoporous silica, mesoporous zirconia, mesoporous titania, etc. And mesoporous nanoparticles, preferably mesoporous silica. The mesoporous nanoparticles are considered to be aggregates having a substantially cylindrical shape, for example, hexagonal hexagonal units in the number of 10 to several hundred thousand. And the average pore diameter of the hole which is substantially cylindrical is 2 to 10 nm, for example, 2 to 6 nm among them, and the length in the direction perpendicular to the hexagonal cross section is usually about 50 to 1000 nm. The mesoporous nanoparticles usually have a specific surface area of about 300 to 1500 m ² / g. The mesoporous nanoparticles have an average particle size of about 50 to 1000 nm.

There is no restriction | limiting in particular as a thermoelectric material in this invention, For example, at least 2 or more types selected from Bi, Sb, Ag, Pb, Ge, Cu, Sn, As, Se, Te, Fe, Mn, Co, Si A material containing an element, for example, a BiTe-based material or a crystal of a CoSb ₃ compound containing Co and Sb as a main component contains an element other than Co and Sb, for example, a transition metal. Examples of the transition metal include Cr, Mn, Fe, Ru, Ni, Pt, and Cu. Thermoelectric conversion material containing Ni among these transition metals, in particular (in the formula, 0.03 <X <0.09,2.7 <X <3.4) chemical composition _Co 1-x _Ni x Sb _Y at Some provide N-type thermoelectric conversion materials, and the composition contains Fe, Sn, Ge, for example, a chemical composition of CoSb _p Sn _q or CoSb _p Ge _q (where 2.7 <p <3. 4, 0 <q <0.4, p + q> 3) can give a P-type thermoelectric conversion material.

  Examples of the salt of the thermoelectric material include a salt of at least one element selected from Bi, Sb, Ag, Pb, Ge, Cu, Sn, As, Se, Te, Fe, Mn, Co, and Si, For example, a salt of Bi, Co, Ni, Sn or Ge, for example, a halide of the element, for example, chloride, fluoride, bromide, preferably chloride, sulfate, nitrate, etc. Examples of other salts include elements other than the above-mentioned elements, for example, Sb salts, such as halides of the above-mentioned elements, such as chlorides, fluorides, bromides, preferably chlorides, sulfates, nitrates, and the like.

  Further, the solvent that gives the slurry is not particularly limited as long as it can uniformly disperse the thermoelectric material, and particularly can be dissolved. For example, methanol, ethanol, isopropanol, dimethylacetamide, N-methylpyrrolidone, Suitable examples include alcohols such as methanol and ethanol.

  The reducing agent is not particularly limited as long as it can reduce the salt of the thermoelectric material. For example, tertiary phosphine, secondary phosphine and primary phosphine, hydrazine, hydroxyphenyl compound, hydrogen, hydrogen One or more kinds of substances such as alkali borohydride, such as sodium borohydride, potassium borohydride, lithium borohydride, etc. Is mentioned.

  Since the composite nanoparticle of thermoelectric material / dispersant is obtained as a slurry of a solvent such as ethanol by the above-described method, the composite nanoparticle is usually mixed with a solvent such as ethanol or a mixed solvent of a large amount of water and a small amount of solvent (for example, And a volume ratio of water: solvent = 100: 25 to 75), and then washed, in a sealed pressurized container, for example, in a sealed autoclave, at a temperature of 200 to 400 ° C. for 10 hours or more, for example, 10 to 100 It can be alloyed by hydrothermal treatment for about 24 to 100 hours. Next, it is usually dried under a non-oxidizing atmosphere, for example, an inert atmosphere, to obtain a powdery nanocomposite thermoelectric conversion material.

When it is necessary to obtain a bulk body, the powdery nanocomposite thermoelectric conversion material is subjected to SPS sintering (discharge plasma sintering: Spark Plasma Sintering) at a temperature of 400 to 600 ° C. A bulk conversion material can be obtained.
The SPS sintering can be performed using an SPS sintering machine equipped with a punch (upper part, lower part), an electrode (upper part, lower part), a die and a pressure device.
Further, at the time of sintering, only the sintering chamber of the sintering machine may be isolated from the outside air to be an inert sintering atmosphere, or the entire system may be surrounded by a housing to be an inert atmosphere.

  By the above method, the porous nano particles of the dispersing material are dispersed in the matrix of the thermoelectric material as described above, and the thermoelectric material of the matrix has entered the pores, and the thermal conductivity is 0.7 W / m / K. The following powdery or bulk nanocomposite thermoelectric conversion material can be obtained.

  Using the nanocomposite thermoelectric conversion material of the present invention, an N-type nanocomposite thermoelectric conversion material, a P-type nanocomposite thermoelectric conversion material, an electrode, and an insulating substrate can be assembled by a method known per se.

  In the present specification, the embodiment is specifically described based on a combination of a specific thermoelectric material and a dispersion material, but the present invention is not limited to the combination of the thermoelectric material and the dispersion material having the specific chemical composition, As long as the characteristics of the present invention are satisfied, the present invention can be applied to a combination of a matrix of any thermoelectric material and porous nanoparticles of a dispersion material.

Examples of the present invention will be described below.
In each of the following examples, the obtained thermoelectric conversion material was evaluated by the following method. In addition, the following measuring methods are illustrations, and can be similarly measured using an equivalent measuring method.
1. STEM Sample Preparation A sintered body having a diameter of 10 mm × 1 to 2 mm was cut into 1 to 2 mm × 1 to 2 mm by isomerit. Thereafter, mechanical polishing was performed until the thickness became 100 μm or less to prepare a sample. Thereafter, the sample was bonded to a Cu mesh for STEM with an adhesive (trade name: Araldite) and dried. Next, it was mechanically ground with a dimple grinder (manufactured by GATAN) until a part of the film was 20 μm or less in thickness. Thereafter, using Ar ion milling (manufactured by GATAN), the thinned portion was thinned until the thickness of the thinned portion became 10 to 100 nm.

2. STEM Observation STEM observation was performed on the portion where the thickness became 100 nm or less in the sample preparation process. The conditions for STEM observation are as follows.
Equipment: Technai (manufactured by FEI)
Acceleration voltage: 300kV
3. Analysis of interface roughness High resolution STEM images of each sample were taken and directly observed. The captured high-resolution image was subjected to FFT transform and IFFT (inverse Fourier transform) to extract only the lattice information, and image analysis was performed to obtain the average interface roughness.

4). Measurement of Interface Density The particle size of about 50 particles was measured by STEM, and the interface density was calculated from the average.
5). Measurement of thermal conductivity According to a steady-state thermal conductivity evaluation method (measurement temperature: 27 ° C.).

Example 1
According to the flowchart shown in FIG. 5, liquid phase synthesis was performed in the following steps.
Preparation of raw material slurry The following raw material was mixed with 100 mL of ethanol to prepare a slurry.
Mother phase raw material Bismuth chloride (BiCl ₃ ) 0.4 g
Antimony chloride (SbCl ₃ ) 1.34 g
Matrix raw material and dispersed particle raw material
Tellurium chloride (TeCl ₄ ) 2.56 g
Mesoporous silica 0.19g
(Manufactured by Admatechs, trade name: Admaporous, used as ethanol slurry, average pore size: about 4 nm, average particle size 400 nm)

Reduction A solution obtained by dissolving 2.8 g of NaBH ₄ as a reducing agent in 100 ml of ethanol was added dropwise to the raw material slurry.
The ethanol slurry containing mesoporous silica precipitated by reduction was filtered and washed with a solution of 500 ml of water and 300 ml of ethanol, and further filtered and washed with 300 ml of ethanol.

Heat treatment After that, it was placed in a closed autoclave and hydrothermally treated at 240 ° C. for 48 hours to alloy the base material.
Subsequently, it was dried in an N ₂ gas flow atmosphere, and the powder was recovered. At this time, about 2.9 g of powder was recovered.
Sintering The collected powder is subjected to spark plasma sintering (SPS) at 360 ° C., and a base material (in which 20 vol% mesoporous silica is dispersed as a dispersion material) is made of a thermoelectric material (Bi, Sb) ₂ Te _3. A composite thermoelectric conversion material was obtained.

Observation of constituent phase The obtained powder was observed by STEM. FIG. 6 shows an appearance STEM image, FIG. 7 shows a cross-sectional image of the mesoporous silica viewed from the front, and FIG. 8 shows a cross-sectional image of the mesoporous viewed from the side.
From FIG. 7, since the thermoelectric material (Bi, Sb) ₂ Te ₃ is buried in the pores of the mesoporous silica, black spots are seen in an island shape in the cross section.
Further, from FIG. 8, very large (10 nm) interface roughness is generated in the nanocomposite thermoelectric conversion material.
Performance The following are the structural characteristics of average particle diameter, interface density, interface roughness and thermal conductivity, and thermal conductivity reduction rate.

Evaluation result of nanocomposite thermoelectric conversion material Dispersant: Mesoporous silica interface density (1 / nm) having an average particle diameter of 400 nm: 0.005
Interface roughness (nm): 10
Thermal conductivity (W / m / K): 0.7
Thermal conduction reduction rate (%): 53
The thermal conductivity decrease rate indicates the decrease rate with respect to the thermal conductivity of the thermoelectric conversion material obtained in Reference Example 1 described later described in the literature considered to be the best as a conventional example for comparison.
It can be seen that the nanocomposite thermoelectric conversion material of Example 1 has a markedly improved thermal conductivity of about ½ of the conventional value.

Comparative Example 1
Instead of mesoporous silica having an average particle size of 400 nm, spherical silica (solid silica having no holes on the surface, used as PGM slurry, PGM: propylene glycol 1-monomethyl ether) was used as a dispersion material. Others were carried out in the same manner as in Example 1 to obtain a nanocomposite thermoelectric conversion material in which spherical silica silica particles were dispersed.
The evaluation result about the obtained nanocomposite thermoelectric conversion material is shown below.
Evaluation results of nanocomposite thermoelectric conversion material Dispersant: Spherical silica interface density with average particle size of 400 nm (1 / nm): 0.005
Interface roughness (nm): 0.1
Thermal conductivity (W / m / K): 1.3
Thermal conductivity reduction rate (%): 13
The nanocomposite thermoelectric conversion material of Comparative Example 1 has a slightly improved thermal conductivity with respect to the conventional value.

Reference example 1
Literature: According to the method described in J. Jiang et al. Journal of Crystal Growth 277 (2005) 258-263, Bi, Sb, Te powder with a purity of 99.9% was enclosed in a carbon-coated quartz tube, and a rocking furnace The alloy was produced by heating to 700 ° C. and changing _X of (Bi ₂ Te ₃ ) _X (Sb ₂ Te ₃ ) _1-X . Thereafter, a crystal material was produced at a growth rate of 0.6 mm / h in a zone melting furnace (an apparatus for producing a single crystal).
Among the above, those having the closest composition to the thermoelectric conversion material obtained in Example 1 are (Bi ₂ Te ₃ ) _X (Sb ₂ Te ₃ ) _1-X where X = 0.24, that is, Bi _{0. 48} Sb _1.52 Te ₃ . The thermal conductivity was 1.5 W / m / K.

  According to the present invention, a thermoelectric conversion material in which nanoparticles of a dispersion material are dispersed in a parent phase (matrix) of a thermoelectric material, the thermoelectric conversion material enabling low thermal conductivity κ, and such a thermoelectric conversion material A manufacturing method is provided.

Claims (2)

  A nanocomposite thermoelectric conversion material in which nanoparticles of a dispersion material are dispersed in a matrix of a thermoelectric conversion material, wherein the dispersion material is a porous nanoparticle having pores opened on the outer surface of the particle, and A nanocomposite thermoelectric conversion material in which a thermoelectric material of a phase enters the pores.   By adding a reducing agent to a slurry containing a salt of a precursor material of a thermoelectric material in a solvent and porous nanoparticles having pores opened on the outer surface of the particle, the porous nanoparticle of the thermoelectric material and the dispersion material is dropped. The manufacturing method of the nanocomposite thermoelectric conversion material in which the thermoelectric material of a parent phase has penetrated in the said void | hole including the process of obtaining a mixture with particle | grains.