JP2001135865A - Thermoelectric conversion material and manufacturing method for it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology for improving the thermoelectric characteristics of a CoSb3 material. SOLUTION: The void of a unit lattice of skutterudite compound CoSb3 is filled with Yb which is a rare earth element, to cause a phonon scatter due to rattling effect independent of surrounding lattice oscillation, providing a relatively low lattice heat conductivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric conversion element having a function of converting heat to electricity and a function of converting electricity to heat, and more particularly to a function of converting heat to electricity utilizing the Seebeck effect. The present invention relates to a technology that is effective when applied to a thermoelectric conversion material.

[0002]

2. Description of the Related Art A skutterudite compound is considered to be promising as a thermoelectric material because it has a strong covalent bond and a complicated crystal structure. Above all, cobalt antimony (CoSb ₃ ) exhibiting n-type conductivity has attracted attention as a thermoelectric material because it has a high Seebeck coefficient and a large output factor even at a high carrier density (electric conductivity).

[0003] The efficiency of a thermoelectric conversion system is determined by a material parameter called a figure of merit. The figure of merit Z is represented by the following equation: Z = α ² σ / κ. Here, α is the Seebeck coefficient (V / K) indicating the magnitude of the thermoelectromotive force generated per unit temperature, σ is the electric conductivity (Ω ⁻¹ cm ⁻¹ ), and κ is the heat conductivity (W / cmK).
And α ² σ is called a power factor. This figure of merit is calculated using the material factor (Ma
It is known that terial Factor) increases as β increases.

Β = Nk _B Tμ / (eκ _L ) ∝m ^* 3/2 (μ / κ _L ) where N is the effective state density, k _B is Boltzmann's constant, T
Is the absolute temperature, e is the electron charge, μ is the mobility (cm ² /
Vs), κ _L is a component of thermal conductivity κ due to lattice vibration, m ^*
Is the effective mass of carriers (electrons or holes).

Accordingly, the performance of a thermoelectric material is determined by how efficiently electricity can be extracted from heat by the Seebeck effect, and the efficiency generally increases as the effective mass and mobility of the carrier increases and the lattice thermal conductivity decreases. I do.

The thermoelectric properties of the CoSb ₃ material are described, for example, in Journal of Applied Physics, 80 (8), p4442, 1996.
T. Caillat, A. Borshchevsky and JP Fleurial).

[0007]

However, since heat is carried by the carrier in addition to the lattice vibration, the heat conductivity generally tends to increase as the electric conductivity increases.

CoSb ₃ also has a lattice thermal conductivity that is about three to four times higher than that of other thermoelectric materials, for example, bismuth tellurium (Bi ₂ Te ₃ ) -based materials.
_In the improvement of the thermoelectric performance of _3, the reduction of the lattice thermal conductivity remains as an issue.

An object of the present invention is to provide a technique capable of improving the thermoelectric properties of a CoSb ₃ -based material.

The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0011]

SUMMARY OF THE INVENTION Among the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows.

(1) The thermoelectric conversion material of the present invention is a material in which the voids of CoSb ₃ constituting the skutterudite unit cell are filled with ytterbium (Yb) acting as a donor, which is 99% of the theoretical density. %.

(2) In the thermoelectric conversion material of the present invention, a part of the antimony (Sb) of CoSb ₃ constituting the skutterudite unit cell is converted from a group VIb element such as tellurium (Te) or selenium (Se), ) Is partially replaced by nickel (Ni), palladium (Pd) or platinum (P
The site is replaced by t), and the voids of the skutterudite unit cell are filled with Yb acting as a donor.

(3) The method for producing a thermoelectric conversion material of the present invention comprises the steps of polishing Yb in an argon (Ar) atmosphere, weighing Yb, Co and Sb, and placing the weighed material in a graphite crucible. Melting in an Ar atmosphere and then cooling to produce an ingot, crushing the molten material using a stainless steel bowl in an Ar atmosphere, and crushing the crushed material in an Ar atmosphere by a discharge plasma sintering method. And a step of sintering.

According to the above-mentioned means, by filling the voids of the unit lattice of the skutterudite compound CoSb ₃ with Yb, the lattice thermal conductivity of the lattice is smaller than the lattice thermal conductivity of CoSb ₃ without reducing the output factor. Thermal conductivity is obtained, resulting in a relatively high dimensionless figure of merit (dimens
ionless figure of merit (ZT) is obtained.

The skutterudite compound CoSb ₃
By replacing some of the constituent elements of the unit lattice with Te, Pd or Ni, and filling the voids of the unit lattice with Yb, the lattice thermal conductivity relatively smaller than the lattice thermal conductivity of CoSb ₃ is obtained. Is obtained, the output factor is improved by optimizing the carrier density, and the dimensionless figure of merit can be improved.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings.

The thermoelectric material according to one embodiment of the present invention comprises:
A skutterudite compound, CoSb ₃ , in which the voids of the unit cell are filled with the rare earth element Yb, and the Yb filled in the voids rattles like a rattle independently of the surrounding lattice vibration (rattling effect). The lattice thermal conductivity can be reduced by phonon scattering.

Next, Yb of the present embodiment is replaced with CoSb.
An example of a manufacturing method of Yb _y Co ₄ Sb ₁₂ filled in ₃ will be described with reference to the process chart shown in FIG.

First, after Yb is polished in an Ar atmosphere (step 100), Yb, Co and Sb are each weighed (step 101). The purity of Yb, Co and Sb was 99.9%, 99.998% and 99.99999, respectively.
%. These mixing ratios are, for example, Yb: Co: Sb
= 0 to 0.25: 4: 13, and Yb and Sb are excessively weighed in anticipation of a loss in the spark plasma sintering method described later.

Next, the weighed material was placed in a graphite crucible and melted for about 1 hour in an Ar atmosphere, and then cooled to produce an ingot (Step 102). The melting temperature is, for example, about 1000 ° C. Next, the produced ingot is pulverized in an Ar atmosphere using a stainless steel bowl to reduce the particle size to 106 μm or less (step 103), and the obtained pulverized material is sintered in an Ar atmosphere using a discharge plasma sintering method. by sintering, to produce a dense Yb _y Co ₄ Sb ₁₂ having a sintered density of 99% or more of the theoretical density (step 104). The sintering temperature in spark plasma sintering is
The sintering pressure is, for example, about 40 MPa, and the sintering time is, for example, about 60 to 90 minutes.

Yb _y Co ₄ Sb produced by the above method
₁₂ evaluates the crystal structure and lattice constant by, for example, X-ray diffraction measurement, and evaluates the filling factor of Yb by Rietveld analysis, and measures electrical conductivity, Seebeck coefficient, and thermal conductivity as main thermoelectric characteristics. Further, a Hall measurement was performed at room temperature. Electric conductivity and Seebeck coefficient are 3
The temperature was measured in the temperature range of 00 to 900K, and the thermal conductivity was measured by a laser flash method.

Next, various properties possessed by the Yb _y Co ₄ Sb _12, will be described in detail below with reference to figures 2-11. FIG.
Is a graph showing the relationship between the charge of the lattice constants and Yb of Yb _y Co ₄ Sb _12, FIG. 3, Yb _y C at room temperature
o ₄ graph showing the relationship between the carrier density and Yb filling factor of Sb _12, FIG. 4 is a graph showing the relationship between hole mobility and carrier density of Yb _y Co ₄ Sb ₁₂ at room temperature,
Figure 5 is a graph showing the relationship between the Seebeck coefficient and the carrier density of Yb _y Co ₄ Sb ₁₂ at room temperature, 6,
Graph showing the temperature dependence of the Seebeck coefficient of Yb _y Co ₄ Sb _12, FIG. 7 is a graph showing the temperature dependence of the electrical conductivity of Yb _y Co ₄ Sb _12, 8, Yb _y Co ₄ S
graph showing the temperature dependency of the output factor of b _12, 9,
Graph showing the Yb filling rate dependence of the thermal resistance of the Yb _y Co ₄ Sb ₁₂ at room temperature, 10, Yb _y Co ₄ Sb
₁₂ is a graph showing the relationship between the lattice thermal conductivity and the carrier density of FIG. 12, and FIG. 11 is a graph showing the temperature dependence of the dimensionless figure of merit of Yb _0.075 Co ₄ Sb ₁₂ .

FIG. 2 shows the relationship between the lattice constant of Yb _y Co ₄ Sb ₁₂ and the charged amount of Yb. As shown in FIG.
_The lattice constant of yCo ₄ Sb ₁₂ monotonically increases with an increase in the charged amount of Yb. However, when the charged amount of Yb reaches about 0.2, the lattice constant is almost saturated at about 9.042 °.

Further, the Yb filling rate of Yb _y Co ₄ Sb ₁₂ also increases with an increase in the charged amount of Yb as in the case of the lattice constant. When the charged amount of Yb is 0.25, the Yb filling rate becomes about 0.075. . Also, from the X-ray diffraction pattern,
Although Yb _y Co ₄ Sb ₁₂ includes a minute secondary phase, it was confirmed that a single phase of approximately skutterudites.

[0026] FIG. 3 shows the relationship between the carrier density and Yb filling factor of Yb _y Co ₄ Sb ₁₂ kick to room temperature. In the figure, the solid line indicates the carrier density when Yb is a divalent cation in the void and the activity rate of the carrier is assumed to be 0.3, and the dotted line indicates the Yb is a divalent cation in the void. , The carrier density when the activity rate of the carrier is assumed to be 1.

The carrier density increases almost monotonously as the Yb filling rate increases, and the carrier density at a Yb filling rate of 0.075 is about 1.4 × 10 ²⁰ cm ⁻³ . Further, since the measured carrier density substantially coincides with the carrier density when the carrier activity rate is set to 0.3, the CoS
It can be seen that the activation rate of b ₃ is about 0.3 regardless of the filling of Yb. Incidentally, the conductivity type of the Yb _y Co ₄ Sb ₁₂ was produced are all n-type.

[0028] Figure _4, Yb _y Co 4 Sb ₁₂ at room temperature
FIG. 5 shows the relationship between hole mobility and carrier density in FIG.
_FIG . 5 shows the relationship between the Seebeck coefficient of Yby Co ₄ Sb ₁₂ and the carrier density at room temperature. In the figure, the solid line indicates Comparative Example 1.
N-type CoSb ₃ with site substitution of Te or Pd
Shows the hole mobility or Seebeck coefficient of a single crystal.

The hole mobility of Yb _y Co ₄ Sb ₁₂ is larger than the hole mobility of Comparative Example 1 sites substituted, Y
Seebeck coefficient b _y Co ₄ Sb ₁₂ is smaller than the site substituted Seebeck coefficient of Comparative Example 1. This is presumably because the effective mass of electrons was reduced by the filling of Yb.

[0030] FIG. 6 shows the temperature dependence of the Seebeck coefficient of Yb _y Co ₄ Sb _12, in FIG. 7, Yb _y Co ₄ Sb ₁₂
3 shows the temperature dependence of the electrical conductivity of the sample. The Seebeck coefficient gradually increases as the temperature increases, and tends to decrease with a maximum value. Further, the electric conductivity gradually decreases as the temperature increases. Further, as the Yb filling rate increases, the Seebeck coefficient decreases, but the electrical conductivity increases.

Next, FIG. 8 shows the temperature dependence of the output factor of the Yb _y Co ₄ Sb ₁₂ calculated from the electric conductivity shown in Seebeck coefficient and FIG 7 shown in FIG. 6.
The output factor at room temperature increases with an increase in the Yb filling rate, and the Yb _y at a filling rate of 0.075 (a charged amount of 0.25) is increased.
In Co ₄ Sb _12, the power factor becomes approximately 40μWcm ^-1 K ^-2, showing a relatively large value compared with other Yb _y Co ₄ Sb _12.

[0032] Figure 9, Yb _y Co ₄ Sb ₁₂ at room temperature
Shows the Yb filling rate dependence of the thermal resistivity of Yb. Here, the thermal resistivity is the reciprocal of the lattice thermal conductivity obtained by subtracting the electronic thermal conductivity calculated by the Wiedemann-Franz rule from the measured thermal conductivity. In the figure, the thermal resistivity of La _y Co ₄ Sb ₁₂ filled with lanthanum (La) is shown as Comparative Example 2, and the thermal resistivity obtained from theoretical calculation of phonon scattering by synthetic scattering is shown as Comparative Example 3.

It can be seen that phonon scattering due to the motion of the filled Yb rattling like rattle (rattling effect) has a greater effect of increasing the thermal resistivity than combined scattering.
Moreover, La _y Co ₄ than the Sb ₁₂ thermal resistance of the Yb _y Co _{4 Sb! 2} thermal resistivity greater shown as Comparative Example 2, which in air gap towards the Yb ions than La ions This is considered to be due to the large atomic displacement parameter and strong rattling effect (rattling effect).

[0034] Figure 10, Yb at room temperature _y Co ₄ Sb
₁₂ shows lattice thermal conductivity. In the figure, as Comparative Example 4, Co was used.
_{(Sb 1-x Te x)} La 0.2 C as ₃ and Comparative Example 5
₄ shows lattice thermal conductivity of o ₄ Sb ₁₂ . Yb _y Co ₄ Sb ₁₂
Has a relatively small lattice thermal conductivity as compared with the lattice thermal conductivity of other thermoelectric materials, and is 1/1 / Co (Sb _1-x Te _x ) _3.
2 or less are obtained.

FIG. 11 shows that the Yb filling rate is 0.075.
The temperature dependence of the dimensionless figure of merit of _0.075 Co ₄ Sb ₁₂ is shown. In the figure, the dimensionless figure of merit of CoSb ₃ is shown as Comparative Example 6. At a temperature of 750K, Yb _0.075 Co ₄ Sb
The dimensionless figure of merit of ₁₂ becomes the maximum value and shows a relatively high value of about 0.8.

As described above, according to the present embodiment, Yb is formed in the space of the unit cell of the skutterudite compound CoSb _3.
Filling causes phonon scattering due to the rattling effect (rattling effect) of Yb irrespective of the surrounding lattice vibration, and the CoS is reduced without reducing the output factor.
than the lattice thermal conductivity of b ₃ obtained relatively small lattice thermal conductivity can be obtained 0. of about 8 relatively high dimensionless figure of merit.

The material factor β of Yb _0.075 Co ₄ Sb ₁₂ at room temperature was 0.065, and a sample in which a part of Sb was site-substituted with Te in order to improve the performance of CoSb ₃ (approximately 0. as compared to the material factor [beta] 0. 028 8), about 2 times or more greater, in the present embodiment, the dimensionless performance index of both samples of the above is of the same order, Yb _{0.07 5}
By optimizing the carrier density of Co ₄ Sb ₁₂ , a potential improvement in thermoelectric performance can be expected.

In the present embodiment, Yb was filled into the voids of the unit cell of the skutterudite compound CoSb ₃ , but a part of Sb was replaced by Te of a VIb group element or a part of Se and Co was replaced by a group VIII element. Yb may be filled into the voids of the unit cell of CoSb ₃ substituted with Ni, Pd or Pt. As described above, by filling with Yb, a lattice thermal conductivity relatively smaller than the lattice thermal conductivity of CoSb ₃ is obtained, and at the same time, the carrier density is increased by the addition of the above-mentioned site substitution element, so that the output is increased. It is possible to improve the factor and obtain a relatively high dimensionless figure of merit.

Although the invention made by the inventor has been specifically described based on the embodiments of the present invention, the present invention is not limited to the above embodiments, and various modifications may be made without departing from the gist of the invention. Needless to say, it can be changed.

[0040]

Advantageous effects obtained by typical ones of the inventions disclosed by the present application will be briefly described as follows.
It is as follows.

According to the present invention, a relatively small lattice thermal conductivity can be obtained by filling the gaps of the unit lattice of the skutterudite compound CoSb ₃ with Yb, and the thermoelectric characteristics can be improved. it can.

The skutterudite compound CoSb ₃
By filling the voids of the unit lattice with Yb and further substituting a part of the unit lattice for sites, a relatively small lattice thermal conductivity and an optimal carrier density can be obtained,
It is possible to improve thermoelectric characteristics.

[Brief description of the drawings]

1 is an embodiment of the present invention Yb _y Co ₄ Sb
FIG. ₁₃ is a process drawing illustrating a manufacturing method of No. ₁₂ .

2 is an embodiment of the present invention Yb _y Co ₄ Sb
_{FIG. 13} is a graph showing a relationship between a lattice constant of ₁₂ and a charged amount of Yb.

Figure 3 is an embodiment of the present invention Yb _y Co ₄ Sb
_{FIG. 12} is a graph showing the relationship between the carrier density and the Yb filling rate at room temperature in FIG.

According to an embodiment of the present invention; FIG Yb _y Co ₄ Sb
_{FIG. 12} is a graph showing the relationship between hole movement and carrier density at room temperature in FIG.

5 is an embodiment of the present invention Yb _y Co ₄ Sb
₁₂ is a graph showing the relationship between the Seebeck coefficient and the carrier density at room temperature of FIG.

6 is an embodiment of the present invention Yb _y Co ₄ Sb
FIG. ₁₃ is a graph showing the temperature dependence of a Seebeck coefficient of No. ₁₂ .

7 is an embodiment of the present invention Yb _y Co ₄ Sb
FIG. ₁₂ is a graph showing the temperature dependence of the electrical conductivity of FIG.

8 is an embodiment of the present invention Yb _y Co ₄ Sb
FIG. ₁₄ is a graph showing the temperature dependence of ₁₂ output factors.

9 is an embodiment of the present invention Yb _y Co ₄ Sb
_{FIG. 12} is a graph showing the dependence of the thermal resistivity at room temperature on the Yb filling rate of No. ₁₂ .

According to an embodiment of the invention; FIG Yb _y Co ₄ S
It is a graph showing the lattice thermal conductivity at room temperature of b _12.

FIG. 11 shows Yb _0.075 Co according to an embodiment of the present invention.
₄ is a graph showing the temperature dependence of the dimensionless performance index of Sb _12.

Claims (9)

[Claims] 1. A method for forming a skutterudite unit cell
A thermoelectric conversion material characterized in that the voids of the original compound are filled with ytterbium. 2. The thermoelectric conversion material according to claim 1,
The thermoelectric conversion material, wherein the binary compound is cobalt antimony. 3. The thermoelectric conversion material according to claim 2,
The thermoelectric conversion material, wherein the cobalt antimony has a sintered density of 99% or more with respect to a theoretical density. 4. A skutterudite unit cell 2
Part of the source compound is a group VIb element tellurium, selenium or
Group VIII element nickel, palladium, replaced by platinum,
A thermoelectric conversion material, wherein voids of the skutterudite unit cell are filled with ytterbium. 5. The thermoelectric conversion material according to claim 4,
The thermoelectric conversion material, wherein the binary compound is cobalt antimony. 6. The thermoelectric conversion material according to claim 1, wherein the ytterbium is an n-type dopant. 7. A polishing step of polishing ytterbium;
A weighing step of weighing the ytterbium, cobalt and antimony obtained in the polishing step, a melting step of putting the material obtained in the weighing step into a graphite crucible and melting, and then cooling to produce an ingot, A method for producing a thermoelectric conversion material, comprising: a crushing step of crushing a material obtained in a step; and a sintering step of sintering the material obtained in the crushing step. 8. The method for producing a thermoelectric conversion material according to claim 7, wherein the polishing step, the melting step, the pulverizing step, and the sintering step are performed in an inert gas atmosphere. Material manufacturing method. 9. The method for producing a thermoelectric conversion material according to claim 7, wherein the material obtained in the pulverizing step is sintered by a discharge plasma method.