JP2002141562A - Composite oxide excellent for thermoelectric conversion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new material which is composed of elements abundantly existing but having respectively low toxicities and is excellent in its heat resistance and its chemical durability, etc., and further, has a high Seebeck coefficient and good electric conductivity. SOLUTION: By melting and cooling rapidly the raw material obtained by mixing each other in a specific ratio a Bi compound, a Pb compound, an Sr compound, a Ca compound, and a Co compound, a solid material is obtained. Then, by subjecting the solid material to a heat treatment in an oxygen atmosphere, there is grown as a fiber-form single crystal a composite oxide having an excellent thermoelectric conversion performance which is represented by the chemical formula of Bi1.6-2.2Pb0-0.25Sr1.1-2.2Ca0-0.8Co2O9-x (0<=x<=1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a novel composite oxide having excellent thermoelectric conversion performance and a method for producing the same.

[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 for generating power by generating a potential difference by applying a temperature difference between both ends of a thermoelectric conversion material.
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, No moving devices such as motors and turbines required for general power generation are required. 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 having a high thermoelectric conversion efficiency is an intermetallic compound. Among them, a material having a high conversion efficiency in a temperature range of about 600 to 1000 K which is a temperature range of waste heat is TeAgSb. It is a base metal compound. However, the application of TeAgSb-based metal compounds as practical materials is limited in view of the fact that Te and Sb are toxic rare elements and cannot be used in air because they are easily oxidized. Because of this, it is composed of elements with low toxicity and high abundance,
Development of a material having 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. At present, the thermoelectric conversion efficiency of the metal oxide is one digit lower than that of a TeAgSb-based metal compound. This is the conventional 10 mΩcm
This is because an oxide having good electrical conductivity having an electric resistivity of about or less has a Seebeck coefficient of only a low value of about several tens of μV / K or less.

[0006]

SUMMARY OF THE INVENTION The main object of the present invention is to:
It is composed of elements with low toxicity and high abundance, heat resistance,
An object of the present invention is to provide a novel material having excellent chemical durability and the like, and having a high Seebeck coefficient and good electric conductivity.

[0007]

The present inventor has conducted various studies in view of the current state of the above-mentioned thermoelectric conversion materials.
i compound, Pb compound, Sr compound, Ca compound and C
The solid obtained by melting and quenching the raw material obtained by mixing the o compound in a specific ratio is heat-treated in an oxygen atmosphere, whereby the composite oxide having excellent thermoelectric conversion performance grows as a fibrous single crystal. This has led to the completion of the present invention.

That is, the present invention provides the following composite oxide and a method for producing the same. 1. Chemical _formula: Bi 1.6~2.2 Pb 0~0.25 Sr 1.1~2.2
A composite oxide represented by Ca 0 to _0.8 Co ₂ O _9-x (0 ≦ x ≦ 1). 2. MO-M'O-M'O-MO (M is Ca or Sr
The composite oxide according to claim 1, wherein the composite oxide has a structure in which layers having a rock salt type structure and Mn are CoO ₂ layers stacked alternately in this order. 3. Item 3. The composite oxide according to Item 1 or 2, which is a fibrous single crystal. 4. 150 μV / K at 400 K (absolute temperature)
Item 4. The composite oxide according to any one of Items 1 to 3, having the above Seebeck coefficient and an electric resistivity of 10 mΩcm or less. 5. Item 5. The method for producing a composite oxide according to any one of Items 1 to 4, wherein a solidified product obtained by melting the raw material mixture and rapidly cooling the melt is heat-treated in an oxygen-containing atmosphere. 6. Bi-containing material, Pb-containing material, Sr-containing material, Ca-containing material and Co-containing material are converted into Bi: Pb: Sr: Ca: Co
(Atomic composition ratio) = 0.8 to 1.2: 0 to 1.2: 0.8
Item 6. The method for producing a composite oxide according to Item 5, wherein a mixture containing the mixture in a ratio of from 1.2 to 0: 1.2: 2 is used as a raw material. 7. Item 7. A composite oxide obtained by the method according to item 5 or 6.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The composite oxide of the present invention has a chemical formula:
Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂
O _9-x (0 ≦ x ≦ 1).

Such a composite oxide has a high Seebeck coefficient of 150 μV / K or more at 400 K (absolute temperature) and has a good electric conductivity of an electric resistivity of 10 mΩcm or less. Although there are fluctuations, the Seebeck coefficient generally increases with increasing temperature, and the electrical resistivity tends to decrease. The composite oxide of the present invention can exhibit excellent thermoelectric conversion performance when used as a thermoelectric conversion material by having such a high Seebeck coefficient and a low electric resistivity at the same time.

The composite oxide of the present invention represented by the above chemical formula can be obtained by melting a raw material mixture and then rapidly cooling the melt to obtain a heat treatment in an oxygen-containing atmosphere.

The raw materials include Bi-containing substances and Pb-containing substances.
Material, Sr-containing material, Ca-containing material, and Co-containing material.
These raw materials can form oxides by firing.
It can be used without particular limitation if it is, simple metal, oxide,
Various compounds (such as carbonates) can be used. example
For example, as a Bi source, bismuth oxide (Bi_TwoO_Three), Nitric acid
Smas (Bi (NO_Three)_Three), Bismuth chloride (BiC
l_Three), Bismuth hydroxide (Bi (OH)_Three), Alkoxy
Compound (Bi (OCH_Three)_Three, Bi (OC_TwoH_Five)_Three, B
i (OC_ThreeH₇)_Three) Can be used as the Pb source
Lead oxide (PbO), lead nitrate (Pb (NO_Three)_Two),salt
Lead (PbCl_Two), Lead hydroxide (Pb (OH)_Two), Al
Coxide compound (Pb (OCH_Three)_Two, Pb (OC_TwoH_Five)
_Two, Pb (OC_ThreeH₇)_Three) Can be used, and the Sr source
Strontium oxide (SrO), strontium chloride
Titanium (SrCl_Two), Strontium carbonate (SrC
O_Three), Strontium nitrate (Sr (NO_Three)_Two), Hydroxyl
Strontium fluoride (Sr (OH) _Two), Alkoxidation
Compound (Sr (OCH_Three)_Two, Sr (OC_TwoH_Five)_Two, Sr
(OC _ThreeH₇)_Two) Can be used as a Ca source
Are calcium oxide (CaO), calcium chloride (CaC)
l_Two), Calcium carbonate (CaCO_Three), Calcium nitrate
(Ca (NO_Three)_Two), Calcium hydroxide (Ca (OH)
_Two), Alkoxide compounds (Ca (OCH_Three)_Two, Ca
(OC_TwoH_Five)_Two, Ca (OC_ThreeH₇)_Two) Etc.
Co source can be cobalt oxide (CoO, Co
_TwoO_Three, Co_ThreeO_Four), Cobalt chloride (CoCl_Two), Carbonated
Cobalt (CoCO_Three), Cobalt nitrate (Co (NO_Three)
_Two), Cobalt hydroxide (Co (OH)_Two), Alkoxide
Compound (Co (OC_ThreeH₇)_Two) Etc. can be used
You. Further, the composite oxide of the present invention contains two or more constituent atoms.
Raw material may be used.

The melting conditions of the raw material mixture may be any conditions under which the raw material can be uniformly melted. In order to prevent contamination from the melting vessel and evaporation of the raw material components, for example, when an alumina crucible is used. Is preferably heated to about 1200 to 1400 ° C. and melted. There is no particular limitation on the heating time, and heating may be performed until the raw material is uniformly melted, and usually, the heating time may be about 30 minutes to 1 hour. The heating means is not particularly limited, and any means such as an electric heating furnace and a gas heating furnace can be adopted. The atmosphere at the time of melting may be an oxygen-containing atmosphere such as in air or an oxygen stream. However, when the raw material contains a sufficient amount of oxygen, the melting may be performed in an inert atmosphere.

The quenching condition is not particularly limited, but the quenching may be performed under such a condition that at least the surface portion of the solidified product becomes a glassy amorphous layer. For example, the melt may be poured on a metal plate and rapidly cooled by a means such as compression from above. Usually, the cooling rate may be about 500 ° C./sec or more, and preferably 10 ³ ° C./sec or more.

Next, the solidified material formed by rapid cooling is subjected to a heat treatment in an oxygen-containing atmosphere, whereby a composite oxide grows from the surface of the solidified material.

The heat treatment temperature may be about 880-930 ° C., and the heating may be performed in an oxygen-containing atmosphere such as air or an oxygen stream. When heating in an oxygen stream, for example, heating may be performed in an oxygen stream at a flow rate of about 300 ml / min or less. There is no particular limitation on the heat treatment time,
The heating time may be determined depending on the degree of growth of the target composite oxide, but is usually set to about 60 to 1000 hours.

The mixing ratio of the raw materials can be determined according to the desired composition of the composite oxide. In particular,
When the composite oxide is formed from the amorphous layer portion on the surface of the solidified product, the composition of the melt in the amorphous portion is defined as a liquid phase composition, and an oxide having a solid phase composition in phase equilibrium with the liquid phase composition Since the crystal grows, the composition of the starting material can be determined by the relationship between the composition of the melt phase and the composition of the solid phase (single crystal) which are in equilibrium with each other.

[0018] The chemical _formula: Bi 1.6~2.2 Pb 0~0.25 Sr
_{In order} to obtain the composite oxide of the present invention represented by _{1.1 to 2.2} Ca 0 to _0.8 Co ₂ O _9-x (0 ≦ x ≦ 1), for example, Bi-containing material, Pb-containing material, Sr-containing material, Ca The content and the Co content are represented by Bi: Pb: Sr: Ca: Co (atomic composition ratio) =
0.8 to 1.2: 0 to 1.2: 0.8 to 1.2: 0
A mixture containing the mixture in a ratio of 1.2: 2 may be used as a raw material.

According to the above-described method, the composite oxide of the present invention grows as a fibrous single crystal having a ribbon shape.

The size of the oxide single crystal can vary depending on the kind of the raw material, the composition ratio, the heat treatment conditions, etc., for example, about 10 to 1000 μm in length and 20 to 200 μm in width.
A fibrous oxide single crystal having a thickness of about 1 to 5 μm is formed.

FIG. 1 shows a scanning electron microscope (SEM) photograph of the oxide single crystal obtained in Example 1 described later. FIG.
As can be seen from the composite oxide obtained by the method of the present invention,
It is a fibrous single crystal having a ribbon shape. The well-grown surface is the ab plane, and the thickness direction corresponds to the c-axis.

FIG. 2 shows a transmission electron microscope (TEM) photograph of the oxide single crystal obtained in Example 1, and FIG. 3 shows a schematic diagram of such a crystal structure. As can be seen from these figures, the composite oxide obtained by the above-described method has a CoO ₂ layer in which a unit cell in which six oxygen atoms are octahedrally coordinated around Co is layered such that one side thereof is shared. And MO-
Rock salt (NaC) stacked in the order of M'O-M'O-MO
1) It has a structure in which layers having a type structure are alternately stacked in the c-axis direction. Here, M is Ca or Sr, and M ′ is Bi or Pb.

The composite oxide of the present invention having the above specific composition has a high Seebeck coefficient and a low electric resistivity at the same time, so that when used as a thermoelectric conversion material of a thermoelectric conversion element, it exhibits excellent thermoelectric conversion performance. Can demonstrate.

Further, the composite oxide of the present invention is composed of elements having low toxicity and high abundance, and has heat resistance,
It has good chemical durability and the like, and is a highly practical substance as a thermoelectric conversion material.

When the composite oxide of the present invention is used as a thermoelectric conversion material, the structure of the thermoelectric conversion element may be the same as that of a known thermoelectric conversion element. It may be used as a conversion material.

[0026]

The composite oxide of the present invention is a substance having both high Seebeck coefficient and low electric resistivity and excellent thermoelectric conversion performance, and also has good heat resistance and chemical durability.

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

[0028]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1 Bismuth oxide (Bi ₂ O ₃ ), lead oxide (PbO), strontium carbonate (SrCO ₃ ), calcium carbonate (CaC)
O ₃ ) and cobalt oxide (Co ₃ O ₄ ) as starting materials, Bi: Pb: Sr: Ca: Co (atomic ratio) = 1:
After sufficiently mixing the starting materials in a ratio of 1: 1: 1: 2, the mixture was placed in an alumina crucible and placed in an air using an electric furnace in air.
The raw material powder was melted at 300 ° C. for 30 minutes. Next, this molten material was poured onto a copper plate, sandwiched between another copper plate, and rapidly cooled to obtain a glass precursor. This glass precursor was heat-treated at 930 ° C. for 600 hours in an oxygen stream (150 ml / min) to grow a single crystal of a composite oxide on the surface of the glass precursor.

FIG. 1 shows a scanning electron micrograph of the obtained composite oxide, and FIG. 2 shows a transmission electron micrograph thereof.
The single crystal of this composite oxide has the chemical formula: Bi _2.05 P
It was represented by _{b 0. 11 Sr 1.90 Ca 0.42 Co} 2 O 8.9.

100 to 973K of the obtained composite oxide
FIG. 4 is a graph showing the temperature dependence of the Seebeck coefficient (S) in (absolute temperature). From FIG. 4, it can be seen that the Seebeck coefficient of this composite oxide sharply decreases at 500 to 600 K, but increases with increasing temperature in other temperature ranges. The same temperature dependence was observed in all the examples described later, and the Seebeck coefficient exceeded 150 μV / K at a temperature of 400 K or more.

Further, 100 to 973 K of the composite oxide
FIG. 5 is a graph showing the temperature dependence of the electrical resistivity (ρ) at (absolute temperature). From FIG. 5, the composite oxide is
The semiconductor exhibited a semiconductor behavior in which the electrical resistivity decreased with increasing temperature, and the electrical resistivity was less than 10 mΩcm at a temperature of 400 K or more. In all examples described later, the same temperature dependence of the electrical resistivity (ρ) was observed.

Examples 2 to 17 The starting materials were mixed so as to have the mixing ratios shown in the starting composition in Tables 1 and 2, and a glass precursor was produced in the same manner as in Example 1. Next, a single crystal of a composite oxide was grown on the surface of the glass precursor by heat-treating the glass precursor under the conditions of the heat treatment temperature, the heat treatment time, and the oxygen flow rate shown in Tables 1 and 2.

The composition of the obtained composite oxide, all the chemical _{formula: Bi 1.6~2.2 Pb 0~0.25 Sr 1 .1~2.2} Ca 0~0.8
It was within the range of the composition represented by Co ₂ O _9-x (0 ≦ x ≦ 1). The single crystal composition section in Tables 1 and 2 shows the average composition of the composite oxide obtained in each example. Tables 1 and 2 also show the measurement results of the Seebeck coefficient and the electrical resistivity at 973 K of the composite oxide obtained in each example.

[0035]

[Table 1]

[0036]

[Table 2]

[Brief description of the drawings]

FIG. 1 is a scanning electron micrograph showing the crystal structure of the composite oxide obtained in Example 1.

FIG. 2 is an output drawing of data obtained by digitizing a transmission electron micrograph showing the crystal structure of the composite oxide obtained in Example 1.

FIG. 3 is a schematic view showing a crystal structure of a composite oxide of the present invention.

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 resistivity of the composite oxide obtained in Example 1.

Claims (7)

[Claims] 1. A chemical _formula: Bi 1.6~2.2 Pb 0~0.25 Sr
_1.1-2.2 Ca _0-0.8 A composite oxide represented by Co ₂ O _9-x (0 ≦ x ≦ 1). 2. A layer having a rock-salt type structure stacked in the order of MO-M'OM-M'O-MO (M is Ca or Sr, M 'is Bi or Pb), and a CoO ₂ layer. The composite oxide according to claim 1, which has a structure in which are alternately stacked. 3. The composite oxide according to claim 1, which is a fibrous single crystal. 4. At 150 K (absolute temperature), 150 μm
The composite oxide according to any one of claims 1 to 3, which has a Seebeck coefficient of V / K or more and an electric resistivity of 10 mΩcm or less. 5. The composite oxide according to claim 1, wherein a solidified product obtained by melting the raw material mixture and rapidly cooling the melt is heat-treated in an oxygen-containing atmosphere. Production method. 6. Bi-containing material, Pb-containing material, Sr-containing material, C
The content of a and the content of Co are represented by Bi: Pb: Sr: Ca:
Co (atomic composition ratio) = 0.8 to 1.2: 0 to 1.2:
The method for producing a composite oxide according to claim 5, wherein a mixture containing 0.8 to 1.2: 0 to 1.2: 2 in a ratio of 0.8 to 1.2 is used as a raw material. 7. A composite oxide obtained by the method according to claim 5.