JP2013219116A - Nanocomposite thermoelectric conversion material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nanocomposite thermoelectric conversion material having enhanced thermoelectric conversion performance, by optimizing the shape of a phonon scattering particle dispersed in a thermoelectric conversion material matrix to further reduce thermal conductivity.SOLUTION: In a nanocomposite thermoelectric conversion material including phonon scattering particles dispersed in a thermoelectric conversion material matrix, the phonon scattering particles have a recessed part. An absolute value of a curvature of the recessed part is preferably 31.6/nm or less.

Description

  The present invention relates to a nanocomposite thermoelectric conversion material in which nano-sized phonon scattering particles are dispersed in a matrix made of a thermoelectric conversion material.

  The thermoelectric conversion material is an energy material that performs direct conversion between thermal energy and electric energy based on two basic thermoelectric effects, the Seebeck effect and the Peltier effect.

  Thermoelectric power generation devices using thermoelectric conversion materials have a simple structure, robustness, high durability, no moving parts, easy microfabrication, no maintenance, and reliability compared to conventional power generation technology There are many advantages such as high life, long life, no noise, no pollution and low temperature waste heat can be used.

  Compared to conventional compression cooling technology, thermoelectric cooling devices using thermoelectric conversion materials do not require chlorofluorocarbon, do not cause contamination, are easily downsized, have no moving parts, and do not generate noise. is there.

  Therefore, especially in recent years, energy and environmental issues have become more serious, such as aviation / space, national defense construction, geological and meteorological observation, medical hygiene, microelectronics, etc. Is expected to be put to practical use for a wide range of applications.

As an index for evaluating the performance of the thermoelectric conversion material, a power factor P = S ² σ and a dimensionless performance index ZT = (S ² σ / κ) T are used. Here, S: Seebeck coefficient, σ: conductivity, κ: thermal conductivity, T: absolute temperature. That is, in order to obtain good thermoelectric properties, it is necessary that the Seebeck coefficient S and the electrical conductivity σ are high and the thermal conductivity κ is low.

  In order to reduce the thermal conductivity κ, it is effective to scatter phonons, one of the players in heat conduction, and a composite thermoelectric conversion material in which phonon scattering particles are dispersed in a thermoelectric conversion material matrix is proposed. Has been.

Patent Document 1 proposes a nanocomposite thermoelectric conversion material in which ceramic particles are dispersed in a thermoelectric conversion material matrix having a composition of CoSb _x (2.7 <x <3.4). Thus, CoSb against _x thermoelectric conversion material matrix alone thermal conductivity in the range of 5W / Km, reported that dropped to 1.8 -3 W ^{/ Km} in the nanocomposite thermoelectric conversion material obtained by ceramic particles dispersed Has been.
However, since the size of the dispersed ceramic particles is submicron to several hundred microns, the phonon scattering effect is small and the effect of lowering the thermal conductivity is insufficient.

In Patent Document 2, the interval between dispersed particles is preferably not less than the mean free path of the carrier of the thermoelectric conversion material and not more than the mean free path of the phonon of the thermoelectric conversion material, preferably 1 nm or more and 100 nm or less, more preferably 10 nm or more. It is disclosed that it is 100 nm or less.
However, in the above prior art, no particular consideration is given to the shape of the dispersed particles, and it is assumed that the particles are simple spheres or ellipsoids.

Japanese Unexamined Patent Publication No. 2000-261047 JP 2008-305919 A

  It is an object of the present invention to provide a nanocomposite thermoelectric conversion material with improved thermoelectric conversion performance by further reducing the thermal conductivity by optimizing the shape of the phonon scattering particles dispersed in the thermoelectric conversion material matrix. To do.

  In order to achieve the above object, according to the present invention, a nanocomposite thermoelectric conversion material in which phonon scattering particles are dispersed in a thermoelectric conversion material matrix, wherein the phonon scattering particles have a recess. A thermoelectric conversion material is provided.

  The present inventor has found that the thermal conductivity is remarkably lowered when the phonon scattering particles have concave portions, and has completed the present invention.

1 (1) to (5) show typical examples of TEM images (all having the same field of view). (1) shows the recessed part of the particle | grains which have a recessed part with a broken line. (2) shows the outline of the particle by subjecting the image of (1) to image processing. (3) and (4) show (3) an example in which one particle forms one recess and (4) an example in which one recess is formed by a plurality of particles. (5) shows the definition of the curvature (negative value) of the recess and the maximum absolute value of the observed curvature (= 31.6 [1 / nm]). FIG. 2 shows the distribution of the aspect ratio (short side / long side) measured in the TEM field for the recessed SbO ₃ phonon scattering particles of the example. The measurement was performed on particles having a diameter of 5 nm or more. 3, examples _{(Bi 0.2 Sb 0.8) 2 Te} 3 / recess with SbO ₃ nanocomposite thermoelectric conversion material and the comparative examples of _{(Bi 0.2 Sb 0.8) 2 Te} 3 / spherical SiO _The result of having measured thermal conductivity about ₂ nanocomposite thermoelectric conversion material is compared and shown.

Until now, the influence of the shape of the phonon scattering particles on the thermoelectric conversion performance of the nanocomposite thermoelectric conversion material has not been considered at all.
The present inventor paid attention to the relationship between the shape of the phonon scattering particles and the thermoelectric conversion performance as a new problem, and by changing the particle shape, which has been a simple sphere or ellipsoid, into a shape having a recess, It was newly found that the conductivity can be remarkably lowered, and thereby the thermoelectric conversion performance can be greatly improved.

In the present invention, the composition of the thermoelectric conversion material matrix of the nanocomposite thermoelectric conversion material and the composition of the phonon scattering particles need not be particularly limited.
EXAMPLES The present invention will be specifically described below with reference to examples, but the matrix and particle composition of the present invention are not limited to these examples.

〔Example〕
According to the present invention, a nanocomposite thermoelectric conversion material was produced in which SbO ₃ nanoparticles having recesses as phonon scattering particles were dispersed in a matrix of a thermoelectric conversion material (Bi _0.2 Sb _0.8 ) ₂ Te ₃ .

<Raw material solution preparation>
Bismuth chloride (BiCl ₃ ) 0.4 g, antimony chloride (SbCl ₃ ) 1.34 g, and tellurium chloride (TeCl ₄ ) 2.56 g were dissolved in 100 mL of ethanol to obtain a raw material solution. Here, in the above blending, antimony chloride (SbCl ₃ ) was blended in such an amount that Sb was excessive with respect to the stoichiometric composition of the matrix thermoelectric conversion material (Bi _0.2 Sb _0.8 ) ₂ Te ₃ .

<Reducing agent solution preparation>
12 g of sodium borohydride was dissolved in 100 mL of ethanol to obtain a reducing agent solution.

<Synthesis of nanoparticles>
A reducing agent solution was added dropwise to the raw material solution, and Bi, Sb, and Te nanoparticles were synthesized by reducing and precipitating the respective chlorides.
The obtained ethanol slurry containing nanoparticles was filtered and washed with 40 L of ion exchange water, and further filtered and washed with 10 L of ethanol.

<Alloying>
Hydrothermal treatment was performed at 240 ° C. for 48 hours in the air. As a result, the nanoparticles of the thermoelectric conversion material alloyed with the stoichiometric composition (Bi _0.2 Sb _0.8 ) ₂ Te ₃ and the Sb oxide SbO ₃ compounded excessively with respect to the stoichiometric composition. A mixed powder consisting of nanoparticles was obtained.

<Sintering>
After forming the mixed powder, discharge plasma sintering is performed at 360 ° C., and an SbO ₃ nanoparticle having recesses as phonon scattering particles in an alloy (Bi _0.2 Sb _0.8 ) ₂ Te ₃ thermoelectric conversion material matrix Of (Bi _0.2 Sb _0.8 ) ₂ Te ₃ / recessed SbO ₃ nanocomposite thermoelectric conversion material according to the present invention was obtained.

[Comparative Example]
For comparison, a nanocomposite thermoelectric conversion material in which spherical SiO ₂ nanoparticles were dispersed as phonon scattering particles in a matrix of thermoelectric conversion material (Bi _0.2 Sb _0.8 ) ₂ Te ₃ was prepared.

<Raw material solution preparation>
Bismuth chloride (BiCl ₃ ) 0.4 g, antimony chloride (SbCl ₃ ) 0.96 g, tellurium chloride (TeCl ₄ ) 2.56 g, SiO ₂ nanoparticles (average particle size 5 nm) 20 vol% were dissolved in 100 mL ethanol. A raw material solution was obtained.

<Reducing agent solution preparation>
12 g of sodium borohydride was dissolved in 100 mL of ethanol to obtain a reducing agent solution.

<Synthesis of nanoparticles>
A reducing agent solution was added dropwise to the raw material solution, and Bi, Sb, and Te nanoparticles were synthesized by reducing and precipitating the respective chlorides. The SiO ₂ nanoparticles are maintained as they are blended.
The ethanol slurry containing each nanoparticle was filtered and washed with 40 L of ion exchange water, and further filtered and washed with 10 L of ethanol.

<Alloying>
Hydrothermal treatment was performed at 240 ° C. for 48 hours in the air. Thus, a mixture comprising nanoparticles of thermoelectric conversion material alloyed with stoichiometric composition (Bi _0.2 Sb _0.8 ) ₂ Te ₃ and SiO ₂ nanoparticles (5 nm) initially blended in the raw material solution. A powder was obtained.

<Sintering>
After forming the mixed powder, discharge plasma sintering is performed at 360 ° C., and spherical SiO ₂ nanoparticles as phonon scattering particles are formed in an alloy (Bi _0.2 Sb _0.8 ) ₂ Te ₃ thermoelectric conversion material matrix. A comparative (Bi _0.2 Sb _0.8 ) ₂ Te ₃ / spherical SiO ₂ nanocomposite thermoelectric conversion material dispersed at a concentration of 20 vol% was obtained.

≪Confirmation of dispersed particle shape≫
TEM observation of the (Bi _0.2 Sb _0.8 ) ₂ Te ₃ / recessed SbO ₃ nanocomposite thermoelectric conversion material of the present invention produced in the examples was performed.

1 (1) to 1 (5) show typical examples of TEM images (all having the same field of view).
(1) shows the recessed part of the particle | grains which have a recessed part with a broken line.
(2) shows the outline of the particle by subjecting the image of (1) to image processing.
(3) and (4) show (3) an example in which one particle forms one recess and (4) an example in which one recess is formed by a plurality of particles.
(5) shows the definition of the curvature (negative value) of the recess and the maximum absolute value of the observed curvature (= 31.6 [1 / nm]).

For _{(Bi 0.2 Sb 0.8) 2 Te} 3 / spherical SiO ₂ nanocomposite thermoelectric conversion material of the comparative example, SiO ₂ nanoparticles are phonon scattering particles had been maintained at the first spherical blended.

<< Aspect ratio of dispersed particles >>
FIG. 2 shows the distribution of the aspect ratio (short side / long side) measured in the TEM field for the recessed SbO ₃ phonon scattering particles of the example. The measurement was performed on particles having a diameter of 5 nm or more. As shown in FIG. 2, all the particles to be measured had an aspect ratio of less than 1, more specifically, 0.8 or less, and a circular shape (aspect ratio = 1) was not recognized. The average aspect ratio was 0.472.

The above result is a measurement result for an arbitrary cross-sectional shape (planar shape) of the particle. However, if the three-dimensional shape is spherical, the arbitrary cross-sectional shape (planar shape) must be circular. Therefore, it can be concluded that all the SbO ₃ phonon scattering particles with recesses in the examples are not spherical particles in the range of at least 5 nm in diameter that can be measured.

≪Evaluation of thermal conductivity≫
Example (Bi _0.2 Sb _0.8 ) ₂ Te ₃ / SbO ₃ nanocomposite thermoelectric conversion material with recess and comparative example (Bi _0.2 Sb _0.8 ) ₂ Te ₃ / spherical SiO ₂ nanocomposite thermoelectric The thermal conductivity was measured for the conversion material. The measurement results are shown in FIG.

From FIG. 3, the thermal conductivity of the comparative example in which the phonon scattering particles are spherical is 0.47 [W / Km], whereas the thermal conductivity of the example in which the phonon scattering particles have a concave shape is 0. .30 [W / Km], which is reduced to 0.64% in the case of spherical particles. Since the thermoelectric conversion performance represented by the dimensionless figure of merit ZT = (S ² σ / κ) T is proportional to the reciprocal of κ, changing the phonon scattering particles from the conventional spherical shape to the concave shape of the present invention gives 1 /0.64=1.57 times, that is, 57% improvement.

  According to the present invention, by limiting the shape of the phonon scattering particles dispersed in the thermoelectric conversion material matrix to a form having a concave portion, the nanocomposite thermoelectric conversion material further reducing the thermal conductivity and improving the thermoelectric conversion performance. Is provided.

Claims (3)

  A nanocomposite thermoelectric conversion material in which phonon scattering particles are dispersed in a thermoelectric conversion material matrix, wherein the phonon scattering particles have a recess.   2. The nanocomposite thermoelectric conversion material according to claim 1, wherein an absolute value of the curvature of the concave portion is 31.6 / nm or less.   3. The nanocomposite thermoelectric conversion material according to claim 1, wherein the aspect ratio of phonon scattering particles having a diameter of 5 nm or more is 0.8 or less.