JP3950956B2 - Method for producing complex oxide single crystal - Google Patents

Description

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【図面の簡単な説明】
【図１】本発明の複合酸化物単結晶を熱電変換材料として用いた熱電変換材料として用いた熱電発電モジュールの模式図。
【図２】実施例１で得られた複合酸化物単結晶のＸ線回折図。
【図３】実施例１で得られた複合酸化物単結晶のゼーベック係数の温度依存性を示すグラフ。
【図４】実施例１で得られた複合酸化物の電気伝導度の温度依存性を示すグラフ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a complex oxide single crystal.
[0002]
[Prior art]
In Japan, the effective energy yield from primary supply energy is only about 30%, and about 70% of energy is finally discarded as heat into the atmosphere. In addition, heat generated by combustion in factories and garbage incinerators is discarded into the atmosphere without being converted into other energy. In this way, we humans are wasting a great deal of thermal energy, and have gained little energy from actions such as burning fossil energy.
[0003]
In order to improve the energy yield, it is effective to be able to use the thermal energy discarded in the atmosphere. For this purpose, thermoelectric conversion that directly converts thermal energy into electrical energy is an effective means. This thermoelectric conversion uses the Seebeck effect and is an energy conversion method in which a potential difference is generated by generating a temperature difference at both ends of the thermoelectric conversion material to generate power. In this thermoelectric power generation, one end of the thermoelectric conversion material is placed in a high temperature part generated by waste heat, the other end is placed in the atmosphere (room temperature), and electricity is obtained simply by connecting a conductive wire to each end. No movable devices such as motors and turbines necessary for general power generation are required. For this reason, the cost is low, there is no discharge of gas due to combustion or the like, and power generation can be continuously performed until the thermoelectric conversion material deteriorates.
[0004]
In this way, thermoelectric power generation is expected as a technology that will play a part in solving energy problems that are a concern in the future, but in order to realize thermoelectric power generation, it has high thermoelectric conversion efficiency, heat resistance, chemical durability. It is necessary to supply a large amount of thermoelectric conversion materials excellent in properties and the like.
[0005]
At present, intermetallic compounds are known as substances having high thermoelectric conversion efficiency. However, the thermoelectric conversion efficiency of intermetallic compounds is about 10% at the maximum, and it can be used only in air at a temperature of about 500K or less. Some types of intermetallic compounds include toxic elements and rare elements as constituent elements.
[0006]
For this reason, thermoelectric power generation using waste heat has not yet been put into practical use, is composed of elements that are less toxic, have abundant amounts, have excellent heat resistance, chemical durability, etc., and have high thermoelectric conversion efficiency. Development of materials is expected.
[0007]
In recent years, Co-based composite oxides containing Ca, Bi, Sr, Na, and the like have been reported as materials having excellent durability and high thermoelectric conversion efficiency, and their practical application is considered promising. However, although these composite oxides exhibit high performance in single crystals, the performance decreases to about 1/3 or less in a polycrystal that is easily synthesized.
[0008]
For this reason, a method capable of producing a large amount of the above-described complex oxide single crystal in a large amount by a simple method and a method capable of producing a large complex oxide single crystal according to the application are desired.
[0009]
[Problems to be solved by the invention]
The present invention has been made to solve the above-described problems of the prior art, and mainly provides a method capable of producing a complex oxide single crystal having excellent thermoelectric conversion performance by a simple method. Objective.
[0010]
[Means for Solving the Problems]
As a result of intensive studies in view of the problems as described above, the present inventor has obtained a mixture obtained by further adding at least one compound selected from Sr compound and Ba compound to a raw material containing Ca compound and Co compound. According to the method of cooling after heating and melting, a single crystal of a complex oxide represented by the general formula: Ca _{2.8 to 4} Co ₄ O _{8.8 to 12} is produced with high yield by a relatively simple method. In addition, the present inventors have found that a large single crystal can be easily manufactured, and the present invention has been completed here.
[0011]
That is, the present invention provides the following method for producing a complex oxide single crystal and a complex oxide single crystal.
1. (1) heating and melting a raw material mixture containing Ca-containing material, (2) Co-containing material, and (3) at least one component selected from Sr-containing material and Ba-containing material, and then cooling. A method for producing a complex oxide single crystal characterized by the following.
2. The method for producing a complex oxide single crystal according to claim 1, wherein the raw material mixture further contains at least one compound selected from an alkali metal compound and boric acid.
3. The method for producing a composite oxide single crystal according to claim 1 or 2, wherein the elemental ratio of the metal contained in the raw material mixture is within the range of the following formula:
Ca: Co: A: B = 2.6-22: 4: 1-32: 0-20
(In the formula, A is the total amount of Sr and Ba, and B is the total amount of metals other than Ca, Co, Sr and Ba).
4). Item 4. The method for producing a complex oxide single crystal according to any one of Items 1 to 3, wherein the cooling rate of the raw material mixture is 20 ° C or less per hour. The resulting complex oxide single crystal is
General formula: Ca _{2.8 to 4} Co ₄ O _{8.8 to 12}
The method for producing a complex oxide single crystal according to any one of Items 1 to 4, which is a complex oxide represented by the formula:
6). General formula obtained by any one of the above items 1 to 5: Ca _{2.8 to 4} Co ₄ O _{8.8 to 12}
A complex oxide single crystal represented by:
7). The composite oxide single crystal according to Item 6, having a Seebeck coefficient of 90 μV / K or more at 500 K (absolute temperature) and an electric conductivity of 10 ³ S / m or more at 300 K (absolute temperature).
8). 8. A thermoelectric conversion material comprising the composite oxide single crystal according to item 6 or 7.
9. The thermoelectric power generation module containing the thermoelectric conversion material as described in said item 8.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the method of the present invention, first, a raw material mixture containing (1) a Ca-containing material, (2) a Co-containing material, and (3) at least one component selected from an Sr-containing material and a Ba-containing material is prepared.
[0013]
The type of the compound used as the raw material is not particularly limited as long as it can form a uniform melt when the raw material mixture is heated. For example, simple metals, oxides, various compounds (such as carbonates) ) Etc. can be used.
[0014]
Specific examples of Ca-containing materials that can be used as raw materials include calcium oxide (CaO), calcium peroxide (CaO ₂ ), calcium carbonate (CaCO ₃ ), calcium nitrate (Ca (NO ₃ ) ₂ ), calcium hydroxide ( Ca (OH) ₂ ), calcium chloride (CaCl ₂ ), alkoxide compound (dimethoxy calcium (Ca (OCH ₃ ) ₂ ), diethoxy calcium (Ca (OC ₂ H ₅ ) ₂ ), dipropoxy calcium (Ca (OC ₃₎ H ₇ ) ₂ ), etc.). Co-containing materials include cobalt oxide (CoO, Co ₂ O ₃ , Co ₃ O ₄ ), cobalt carbonate (CoCO ₃ ), cobalt nitrate (Co (NO ₃ ) ₂ ), and cobalt hydroxide (Co (OH) ₂ ). , Cobalt chloride (CoCl ₂ ), alkoxide compounds (dipropoxycobalt (Co (OC ₃ H ₇ ) _2, etc.), etc. Further, without using different compounds as Ca-containing materials and Co-containing materials, A compound containing both of Co and Co may be used.
[0015]
Specific examples of the Sr-containing material include strontium oxide (SrO), strontium peroxide (SrO ₂ ), strontium carbonate (SrCO ₃ ), strontium nitrate (Sr (NO ₃ ) ₂ ), and strontium hydroxide (Sr (OH) ₂ ), alkoxide compounds (dimethoxystrontium (Sr (OCH ₃ ) ₂ ), diethoxystrontium (Sr (OC ₂ H ₅ ) ₂ ), dipropoxystrontium (Sr (OC ₃ H ₇ ) ₂ ), etc.) Specific examples of the Ba-containing material include barium oxide (BaO), barium peroxide (BaO ₂ ), barium carbonate (BaCO ₃ ), barium nitrate (Ba (NO ₃ ) ₂ ), and barium hydroxide (Ba). (OH) _2), alkoxide compounds (dimethoxy barium _{(Ba (OCH 3) 2)} , diethoxy barium (B _{(OC 2 H 5) 2)} , and di propoxy barium _{(Ba (OC 3 H 7)} 2) , etc.). Sr-containing materials and Ba-containing materials can be used singly or in combination of two or more.
[0016]
In the present invention, the raw material mixture containing (1) Ca-containing material, (2) Co-containing material, and (3) at least one component selected from Sr-containing material and Ba-containing material is further necessary. Accordingly, other components may be added for the purpose of adjusting the melting point of the melt.
[0017]
The kind of the component that can be added as necessary is not particularly limited, and for example, an alkali metal compound, a boron-containing compound, or the like can be used. Specific examples of the alkali metal compound include alkali metal chlorides such as lithium chloride (LiCl), sodium chloride (NaCl), and potassium chloride (KCl), hydrates thereof, lithium carbonate (Li ₂ CO ₃ ), sodium carbonate ( Examples thereof include alkali metal carbonates such as Na ₂ CO ₃ ) and potassium carbonate (K ₂ CO ₃ ), and specific examples of boron-containing compounds include boric acid (B ₂ O ₃ ). . These components can also be used singly or in combination of two or more.
[0018]
The mixing ratio of each component in the raw material mixture is preferably such that the elemental ratio of the metal contained in the raw material mixture falls within the range of the following formula.
[0019]
Ca: Co: A: B = 2.6-22: 4: 1-32: 0-20
In the above formula, A is the total amount of Sr and Ba, and B is the total amount of metal components other than the Ca-containing material, Co-containing material, Sr-containing material and Ba-containing material contained in the raw material mixture.
[0020]
In particular, the element ratio of each component in the raw material mixture is more preferably within the range of the following formula.
[0021]
Ca: Co: A: B = 2.8 to 8: 4: 3 to 30: 0 to 15
There is no particular limitation on the method for preparing the raw material mixture, and any method can be used as long as the above-described components can be sufficiently mixed. However, if necessary, the efficiency of the melt reaction at the time of heating described later can be appropriately pulverized. Can be improved.
In the method of the present invention, first, the raw material mixture is heated and melted. As for the heating conditions, any material may be used as long as the raw material mixture is melted into a uniform solution state. However, in order to prevent contamination from the melting container and evaporation of the raw material components, for example, when an alumina crucible is used. Is preferably heated to about 650 to 1000 ° C. and melted. There is no particular limitation on the heating time, and the heating may be performed until the raw material mixture becomes a uniform solution, and the heating time is usually about 1 to 3 hours.
[0022]
It does not specifically limit as a heating means, Arbitrary means, such as an electric heating furnace and a gas heating furnace, are employable. The atmosphere at the time of melting may be an oxygen-containing atmosphere such as in air or an oxygen stream. However, when the source material contains a sufficient amount of oxygen, the atmosphere may be melted in an inert atmosphere.
[0023]
The above-described method is a so-called flux method, in which a part of the components contained in the raw material mixture is melted by heating, and the entire raw material is in a solution state due to its chemical change, dissolving action, etc. By cooling the raw material substance in a solution state, a target single crystal can be grown using a supersaturated state accompanying cooling. At this time, since a single crystal of a complex oxide containing Ca and Co having a solid phase composition that is in phase equilibrium with a solution formed by melting the raw material grows, the melt phase and the solid phase ( The ratio of the Ca-containing material and the Co-containing material in the raw material can be determined according to the composition of the target composite oxide single crystal.
[0024]
Next, the molten mixture is cooled and solidified to form a complex oxide single crystal. The size and yield of the formed complex oxide single crystal can vary depending on the type and composition ratio of the raw material, the composition of the molten component, the cooling rate, etc., for example, the sample solidifies at a cooling rate of 20 ° C. or less per hour. Can be obtained, a single crystal having a plate-like shape with a length and width of about 2 mm or more and a thickness of about 1 to 50 μm can be obtained, and the yield is expected from the charged composition in the Co ion ratio. It becomes a high value of about 40% or more of the maximum value. Furthermore, if the cooling rate is slowed, the single crystal can be made larger.
[0025]
Next, the composite oxide single crystal represented by the general formula: Ca _{2.8 to 4} Co ₄ O _{8.8 to 12} is removed from the solidified product formed by cooling by removing components other than the target composite oxide single crystal. Can be obtained.
[0026]
As a method for removing components other than the target product, for example, water-soluble components adhering to the composite oxide single crystal, such as chlorides, are repeatedly washed with distilled water and filtered, and if necessary washed with ethanol. By using in combination, it can be removed from the object. Since the water-insoluble residue usually exists in the form of powder or granules, it can be removed from the target composite oxide single crystal by, for example, separation using a sieve or the like.
[0027]
The obtained complex oxide single crystal becomes a single crystal having a layered structure, and the Seebeck coefficient (S) is equivalent to that of a polycrystal having the same composition, and becomes 90 μV / K or more at a temperature of 500 K or more.
[0028]
Further, the composite oxide single crystal has a high electrical conductivity due to being a single crystal, and exhibits a non-metallic behavior in which the electrical conductivity increases as the temperature rises. At a temperature of 300 K or higher, High electrical conductivity of 10 ³ S / m or more is exhibited.
[0029]
Thus, the complex oxide single crystal obtained by the method of the present invention has a high Seebeck coefficient and a high electric conductivity at the same time, and can exhibit excellent thermoelectric conversion performance.
[0030]
The composite oxide single crystal obtained by the method of the present invention utilizes the above-described characteristics, for example, a thermoelectric conversion material used at high temperature in the air, for example, p-type, which is impossible with a conventional intermetallic compound material. It can be used effectively as a thermoelectric conversion material. Therefore, by incorporating the complex oxide single crystal into the system as the p-type material of the thermoelectric power generation module, it is possible to effectively use the thermal energy that has been discarded up to now. Moreover, application to a thermoelectric module using the Peltier effect is also possible.
[0031]
FIG. 1 shows a schematic diagram of an example of a thermoelectric power generation module using a thermoelectric conversion material made of a complex oxide single crystal of the present invention as a p-type thermoelectric conversion element. The structure of the thermoelectric power generation module is the same as that of a known thermoelectric power generation module, and is composed of a high-temperature part substrate, a low-temperature part substrate, a p-type thermoelectric conversion material, an n-type thermoelectric conversion material, an electrode, a conductor, and the like. This is a module, and the composite oxide of the present invention is used as a p-type thermoelectric conversion material.
[0032]
【The invention's effect】
According to the method of the present invention, a complex oxide single crystal having a high Seebeck coefficient (S) and a high electric conductivity and having an excellent thermoelectric conversion performance can be obtained in a high yield, and a thermoelectric conversion having an excellent performance. Mass production of materials is possible.
[0033]
Moreover, a large single crystal can be produced by slowing the cooling rate, and a large thermoelectric conversion material according to the application can be easily obtained.
[0034]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0035]
Example 1
Calcium carbonate (CaCO ₃ ), cobalt oxide (Co ₃ O ₄ ) and strontium chloride (SrCl ₂ ) were sufficiently mixed so that the element ratio of the metal component was Ca: Co: Sr = 3: 4: 18. After that, it was put in an alumina crucible and melted by heating in air at 900 ° C. for 3 hours in an electric furnace.
[0036]
Thereafter, it was cooled at a rate of 6 ° C. per hour, and the solidified product was taken out at room temperature. The obtained solidified product was repeatedly washed with distilled water and filtered three times, and finally washed with ethanol and filtered to obtain a simple compound oxide represented by the formula: Ca _3.4 Co ₄ O _9. Crystals were obtained.
[0037]
The X-ray diffraction pattern of the obtained complex oxide single crystal is shown in FIG. FIG. 2 a) is an X-ray diffraction pattern of the above complex oxide single crystal, and b) is a known substance (S. Li et al., J. Mater. Chem., Vol. 9 pp. 1659-1660 (1999)). is a powder X-ray diffraction pattern of Ca ₃ Co ₄ O ₉ polycrystal is. As a result, it was found that the complex oxide single crystal had a layered crystal structure similar to Ca ₃ Co ₄ O ₉ .
[0038]
About the obtained complex oxide single crystal, the graph which shows the temperature dependence of Seebeck coefficient (S) in 300-1000K (absolute temperature) is shown in FIG. FIG. 3 shows that this composite oxide has a Seebeck coefficient exceeding 90 μV / K at a temperature of 500 K or higher, and the Seebeck coefficient increases with increasing temperature. It should be noted that the same temperature dependence was observed in all examples described later, and the Seebeck coefficient exceeded 90 μV / K at a temperature of 500 K or higher.
[0039]
Furthermore, the graph which shows the temperature dependence of the electrical conductivity ((sigma)) in 300-1000K (absolute temperature) of this complex oxide single crystal is shown in FIG. The same temperature dependence is observed in all the examples described later, showing a semiconductor behavior in which the electrical conductivity increases as the temperature rises, and a high electrical conductivity exceeding 10 ³ S / m at a temperature of 300 K or higher. Met.
[0040]
Examples 2-83
The heat treatment was performed in the same manner as in Example 1 with the raw material composition, heating temperature, and treatment time shown in Tables 1 to 7.
[0041]
Then, it cools by the cooling rate shown in Table 1-Table 7, takes out the solidified material at room temperature, and is combined with the washing | cleaning and filtration by distilled water, and washing | cleaning and filtration by ethanol similarly to Example 1. An oxide single crystal was obtained.
[0042]
Tables 1 to 7 show the average composition, yield, and measured value of the thermoelectric conversion index (ZT) at 973 K of the composite oxide single crystal obtained in each example. Here, ZT is a value defined by the following equation, and indicates the thermoelectric conversion efficiency of the material. The higher this value, the higher the conversion efficiency. In the present invention, in all Examples, ZT was 973K and a value exceeding 1.0.
[0043]
ZT = S ² Tσ / κ
S: Seebeck coefficient, T: absolute temperature, σ: electrical conductivity, κ: thermal conductivity
[Table 1]

[0045]
[Table 2]

[0046]
[Table 3]

[0047]
[Table 4]

[0048]
[Table 5]

[0049]
[Table 6]

[0050]
[Table 7]

[Brief description of the drawings]
FIG. 1 is a schematic diagram of a thermoelectric power generation module using a composite oxide single crystal of the present invention as a thermoelectric conversion material.
2 is an X-ray diffraction pattern of a complex oxide single crystal obtained in Example 1. FIG.
3 is a graph showing the temperature dependence of the Seebeck coefficient of the complex oxide single crystal obtained in Example 1. FIG.
4 is a graph showing the temperature dependence of the electrical conductivity of the composite oxide obtained in Example 1. FIG.

Claims (8)

(1) Ca-containing compound, (2) Co-containing compound, and (3) after at least one component selected from Sr-containing compound and Ba-containing material, heating the raw material mixture comprising melted and cooled A method for producing a complex oxide single crystal, comprising growing a complex oxide single crystal represented by a composition formula: Ca _{2.8 to 4} Co ₄ O _{8.8 to 12} . The method for producing a complex oxide single crystal according to claim 1, wherein the raw material mixture further contains at least one compound selected from an alkali metal compound and boric acid. The method for producing a complex oxide single crystal according to claim 1 or 2, wherein the elemental ratio of the metal contained in the raw material mixture is within the range of the following formula:
Ca: Co: A: B = 2.6-22: 4: 1-32: 0-20
(In the formula, A is the total amount of Sr and Ba, and B is the total amount of metals other than Ca, Co, Sr and Ba). The method for producing a complex oxide single crystal according to any one of claims 1 to 3, wherein a cooling rate of the raw material mixture is 20 ° C or less per hour. Obtained by the method according to any one of claims 1 to 4.
Composition formula : Ca _{2.8 to 4} Co ₄ O _{8.8 to 12}
A complex oxide single crystal represented by: The composite oxide single crystal according to claim 5 , which has a Seebeck coefficient of 90 µV / K or more at 500 K (absolute temperature) and an electric conductivity of 10 ³ S / m or more at 300 K (absolute temperature). A thermoelectric conversion material comprising the complex oxide single crystal according to claim 5 . A thermoelectric power generation module comprising the thermoelectric conversion material according to claim 7 .

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