JP3069701B1 - Composite oxide with high Seebeck coefficient and high electrical conductivity - Google Patents

Abstract

An object of the present invention is to provide a material which is composed of elements having low toxicity and abundance, has excellent heat resistance, chemical durability and the like, and has high thermoelectric conversion efficiency. A general _{formula: Ca 3-x Bi x Co} 4 O y (0 ≦ x ≦
1, 8.5 ≦ y ≦ 10) and a Seebeck coefficient of 90
A composite oxide having a μV / K or more and an electric conductivity of 5.0 × 10 ³ S / m or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a composite oxide having a high Seebeck coefficient and a high electric conductivity, and a thermoelectric conversion material using the oxide.

[0002]

2. Description of the Related Art In Japan, the yield of effective energy from primary supply energy is only about 30%, and is about 70%.
% Of energy is ultimately discarded into the atmosphere as heat. Also, heat generated by combustion in factories, refuse incineration plants, and the like is discarded into the atmosphere without being converted into other energy. In this way, human beings waste a great deal of heat energy wastefully, and have obtained only a small amount of energy from actions such as burning fossil energy.

In order to improve the energy yield,
It is effective to make available thermal energy that is discarded in the atmosphere. For that purpose, thermoelectric conversion, which directly converts heat energy into electric energy, is an effective means. The thermoelectric conversion utilizes the Seebeck effect, and is an energy conversion method in which a temperature difference is applied between both ends of a thermoelectric conversion material to generate a potential difference and generate power.
In this thermoelectric power generation, electricity is obtained simply by placing one end of a thermoelectric conversion material in a high-temperature portion generated by waste heat, placing the other end in the atmosphere (room temperature), and connecting conductors to both ends, There is no need for a moving device such as a motor or a turbine necessary for general power generation. For this reason, the cost is low, the gas is not discharged due to combustion or the like, and the power generation can be continuously performed until the thermoelectric conversion material is deteriorated.

As described above, thermoelectric power generation is expected to play a part in solving the energy problem of concern in the future, but in order to realize thermoelectric power generation, it has high thermoelectric conversion efficiency, heat resistance, A thermoelectric conversion material having excellent chemical durability and the like is required. At present, a substance known as a substance having high thermoelectric conversion efficiency is an intermetallic compound, and among them, a material having the highest conversion efficiency is Bi ₂ Te ₃ . However, Bi ₂ Te _{3 has} a thermoelectric conversion efficiency of at most about 10%, and can be used only at a temperature of 200 ° C. or less. Moreover, considering that Te is a rare element and has toxicity, there is a limit to its application as a practical material. Therefore, development of a material having low toxicity, a large amount of constituent elements, excellent heat resistance, chemical durability and the like, and high thermoelectric conversion efficiency is expected.

As a material having excellent heat resistance and chemical durability, a metal oxide can be considered, but the thermoelectric conversion efficiency of the metal oxide is one digit lower than that of Bi ₂ Te ₃ at present.
This is because the oxide having an electric conductivity of 1 × 10 ³ S / m or more has a low Seebeck coefficient of only several tens μV / K.

[0006]

SUMMARY OF THE INVENTION The main object of the present invention is to:
An object of the present invention is to provide a material which is formed of an element having a low toxicity and a large amount, has excellent heat resistance, chemical durability and the like, and has high thermoelectric conversion efficiency.

[0007]

The present inventor has conducted various studies in view of the current state of the above-mentioned thermoelectric conversion materials.
A composite oxide having a specific composition containing i, Ca, Co and O as constituent elements has high Seebeck coefficient and electric conductivity, and is found to be useful as a thermoelectric conversion material in a thermoelectric conversion element. The present invention has been completed.

That is, the present invention provides the following composite oxide and thermoelectric conversion material. 1. General _{formula: Ca 3-x Bi x Co} 4 O y (0 ≦ x ≦ 1,8.
5 ≦ y ≦ 10), and a Seebeck coefficient of 90 μV / K
As described above, a composite oxide having an electric conductivity of 5 × 10 ³ S / m or more. 2. In the general formula described in the above item 1, 0 <x ≦ 1
Item 2. The composite oxide according to the above item 1, wherein 3. In the general formula described in the above item 1, 0.2 ≦ x
Item 2. The composite oxide according to item 1, wherein ≦ 0.8. 4. Item 4. A P-type thermoelectric conversion material comprising the composite oxide according to any one of Items 1 to 3.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The composite oxide of the present invention has a general formula:
_{Ca 3-x Bi x Co 4} O y (0 ≦ x ≦ 1,8.5 ≦ y ≦ 1
0).

Such a composite oxide has a perovskite-type crystal structure similar to Ca ₂ Co ₂ O ₅ which is an oxygen-deficient perovskite. To clarify this point, FIG. 1 shows a powder X-ray diffraction pattern of the composite oxide obtained in Example 1 described later. According to this X-ray diffraction pattern, since no peak due to the impurity phase is observed, it is understood that the complex oxide has a defect at the Ca site of Ca ₂ Co ₂ O ₅ . FIG. 2 shows a schematic diagram of the crystal structure of the composite oxide of the present invention.

The composite oxide having the above specific composition is 9
It has a Seebeck coefficient of 0 μV / K or more and an electric conductivity of 5 × 10 ³ S / m or more. By having such a high Seebeck coefficient and high electric conductivity at the same time, high thermoelectric conversion efficiency can be exhibited when the composite oxide of the present invention is used as a thermoelectric conversion material of a thermoelectric conversion element. Further, the composite oxide has good heat resistance, good chemical durability and the like, and the constituent elements have low toxicity,
Due to its large abundance, it is highly practical as a thermoelectric conversion material.

In the composite oxide of the present invention, the value of x in the above general formula is 0 or more and 1 or less.
If Bi is included as an essential component in the composite oxide, a high Seebeck coefficient is exhibited. Further, when 0.2 ≦ x ≦ 0.8, the Seebeck coefficient is extremely high, and the material is particularly useful as a thermoelectric conversion material.

In the above general formula, when y = 9, the amount of oxygen becomes a stoichiometric amount, but the desired complex oxide can be obtained even in the range of about 8.5 ≦ y ≦ 10. .

The composite oxide of the present invention can be obtained by mixing raw materials at a predetermined mixing ratio and firing in an oxidizing atmosphere.

The raw material is not particularly limited as long as it can form a target composite oxide by firing, and may be a simple metal, an oxide, various compounds (such as carbonates) and the like.
For example, as a Bi source, bismuth oxide (Bi ₂ O ₃ , Bi
₂ O ₅ ), bismuth nitrate (Bi (NO ₃ ) ₃ ), bismuth chloride (BiCl ₃ ), bismuth hydroxide (Bi (OH) ₃ ),
Tripropoxybismuth (Bi (OC ₃ H ₇ ) ₃ ) or the like can be used. As a Ca source, calcium oxide (CaO),
Calcium chloride (CaCl ₂ ), calcium carbonate (Ca
CO ₃ ), calcium nitrate (Ca (NO ₃ ) ₂ ), calcium hydroxide (Ca (OH) ₂ ), dimethoxycalcium (Ca (OCH ₃ ) ₂ ), diethoxycalcium (Ca
(OC ₂ H ₅ ) ₂ ), dipropoxy calcium (Ca (O
C ₃ H ₇ ) ₂ ) and the like can be used, and as a Co source, cobalt oxide (CoO, Co ₂ O ₃ , Co ₃ O ₄ ), cobalt chloride (C
oCl ₂ ), cobalt carbonate (CoCO ₃ ), cobalt nitrate (Co (NO ₃ ) ₂ ), cobalt hydroxide (Co (O
H) ₂ ), dipropoxy cobalt (Co (OC ₃ H ₇ ) ₂ )
Etc. can be used.

The firing means is not particularly limited, and an electric heating furnace,
Any means such as a gas heating furnace can be adopted, and firing may be performed in an oxidizing atmosphere such as an oxygen stream or air.

The firing temperature and the firing time are not particularly limited, as long as the desired complex oxide is formed. Usually, the firing may be performed at about 920 to 1100 ° C. for about 24 to 48 hours. . When a carbonate, an organic compound, or the like is used as the raw material, it is preferable that the raw material be decomposed by calcining before firing, and then fired to form a target composite oxide. For example, when a carbonate is used as a raw material, 800 to 900 ° C.
After firing for about 24 hours, firing may be performed under the above conditions.

The amount of oxygen in the resulting composite oxide can be controlled by the oxygen partial pressure during firing, the firing temperature, the firing time, and the like. The higher the oxygen partial pressure, the higher the value of y in the above general formula can do.

The composite oxide of the present invention thus obtained has a high Seebeck coefficient and a high electric conductivity at the same time, and can be effectively used as a thermoelectric conversion material of a thermoelectric conversion element.

FIG. 3 is a schematic view showing an example of a thermoelectric conversion element using the composite oxide of the present invention as a thermoelectric conversion material. The structure of the thermoelectric conversion element is the same as a known thermoelectric conversion element,
In the thermoelectric conversion element composed of the substrate 1 for the high temperature part, the substrate 2 for the low temperature part, the P-type thermoelectric conversion material 3, the N-type thermoelectric conversion material 4, the electrode 5, the conductive wire 6, etc., the composite oxide of the present invention is P
It may be used as a thermoelectric conversion material.

[0021]

The composite oxide of the present invention has a high Seebeck coefficient and a high electric conductivity and is excellent in heat resistance, chemical durability and the like.

The composite oxide can be applied as a high-temperature thermoelectric conversion material, which is impossible with a conventional intermetallic compound material. Therefore, it is expected that by incorporating the oxide material of the present invention into a thermoelectric power generation system, it will be possible to effectively utilize the heat energy that has been discarded in the atmosphere.

[0023]

The present invention will be described below with reference to examples.
Make the rollers even clearer. Example 1 As a Bi source, bismuth oxide (Bi_TwoO_Three), As a Ca source
Calcium carbonate (CaCO_Three) And oxidation as Co source
Cobalt (Co_ThreeO_Four) Using Ca: Bi: Co (mol
Ratio) = 2.50: 0.50: 4.00
After thoroughly mixing the substances, place them in an alumina crucible and
Calcination was performed in an air furnace at 900 ° C. for 24 hours. Powder this calcined product
After crushing and pressing, in an oxygen stream at 960 ° C. for 24 hours
By firing, a composite oxide was synthesized. The resulting composite oxide
Has the chemical formula: Ca_2.5Bi_0.5Co _FourO_9.3Represented by
Met.

100 ° C. to 700 ° C. of the obtained composite oxide
FIG. 4 is a graph showing the temperature dependence of the Seebeck coefficient (S) in the above. FIG. 4 shows that the composite oxide was 100
It can be seen that the polymer exhibits a Seebeck coefficient of 100 μV / K or more in a temperature range of from 0 ° C. to 700 ° C.

FIG. 5 is a graph showing the temperature dependence of the electrical conductivity (σ) of the composite oxide measured by the DC four-terminal method. From FIG. 5, the electric conductivity of the composite oxide shows a high value exceeding 7.0 × 10 ³ S / m,
It increases with increasing temperature, and 1 × 10 ^{4 at} 700 ° C.
It can be seen that the value is as high as S / m.

The composite oxide has a performance index Z calculated from the Seebeck coefficient (S), the electric conductivity (σ), and the thermal conductivity (κ) measured by a laser flash method.
FIG. 6 is a graph showing the temperature dependency of ZT obtained by multiplying (= S ² σ / κ) by the temperature (T). ZT is an index generally adopted as a standard for evaluating the conversion efficiency of a thermoelectric conversion material. FIG. 6 shows that the composite oxide of the present invention has high thermoelectric conversion efficiency. Example 2-10 Formula: in _{Ca 3-x Bi x Co 4} O y, the raw materials are mixed so as the x value shown in Table 1, as the calcining temperature and sintering conditions shown in Table 1 Example 1 except for changing to
In the same manner as in the above, a composite oxide was produced.

For each of the obtained composite oxides, Example 1
Table 1 shows the results of measuring the Seebeck coefficient (S), the electric conductivity (σ), and the ZT value at 700 ° C. in the same manner as described above.

[0028]

[Table 1]

From the above results, the composite oxide of the present invention is:
It turns out that it has a high Seebeck coefficient and a high electrical conductivity, and has good heat conversion efficiency.

[Brief description of the drawings]

FIG. 1 is a view showing a powder X-ray diffraction pattern of a composite oxide obtained in Example 1.

FIG. 2 is a schematic view showing a crystal structure of a composite oxide of the present invention.

FIG. 3 is a schematic diagram of a thermoelectric conversion element using the composite oxide of the present invention as a thermoelectric conversion material.

FIG. 4 is a graph showing the temperature dependence of the Seebeck coefficient of the composite oxide obtained in Example 1.

FIG. 5 is a graph showing the temperature dependence of the electrical conductivity of the composite oxide obtained in Example 1.

FIG. 6 is a graph showing the temperature dependence of ZT of the composite oxide obtained in Example 1.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate for high temperature part 1, 2 ... Substrate for low temperature part, 3 ... P-type thermoelectric conversion material, 4 ... N-type thermoelectric conversion material, 5 ... Electrode, 6 ... Conductor wire

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-163080 (JP, A) JP-A-4-292904 (JP, A) JP-A-4-111369 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) C01G 51/00 H01L 35/22

Claims (3)

(57) [Claims] 1. A general _{formula: Ca 3-x Bi x Co} 4 O y (0 <x ≦
1 , 8.5 ≦ y ≦ 10) and a Seebeck coefficient of 90
A composite oxide having a μV / K or more and an electric conductivity of 5 × 10 ³ S / m or more. 2. A method according to claim 1, wherein:
2. The composite oxide according to claim 1, wherein 0.2 ≦ x ≦ 0.8. 3. A P-type thermoelectric conversion material composed of a composite oxide according to claim 1 or 2.