JP4164652B2 - Thermoelectric conversion material and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a novel thermoelectric conversion material and a method for producing the same.
[0002]
[Prior art]
In recent years, environmental issues and the problem of alternative energy for fossil fuels have attracted much social attention. Under such circumstances, for example, a) thermoelectric conversion energy technology that effectively converts unused heat energy such as exhaust heat into electrical energy without discharging harmful gases such as carbon dioxide and nitrogen oxides; b) harmful Expectations are growing for thermoelectric cooling technology that does not use chlorofluorocarbon-based gases. In order to meet these expectations, development of high-performance thermoelectric conversion materials is essential.
[0003]
A thermoelectric conversion material is a material that can convert thermal energy and electrical energy to each other, and its figure of merit (Z) is given by:
Z = S²/ (Ρκ)
In the formula, S represents the thermoelectromotive force (Seebeck coefficient), ρ represents the electrical resistivity, and κ represents the thermal conductivity. In order to increase the thermoelectric conversion efficiency, it is necessary that the absolute value of S is large and both ρ and κ are small.
[0004]
Various thermoelectric conversion materials have been proposed so far. For example, Bi₂Te_ThreeThe system thermoelectric conversion material exhibits a high performance index in a temperature range from room temperature to 200 ° C., and is used as a Peltier element for thermoelectric cooling or the like. The PbTe-based thermoelectric conversion material exhibits a high performance index in a temperature range of 200 ° C. to 500 ° C., and is mainly used as a power generator.
[0005]
However, since these are non-oxide materials, there is a problem that the synthesis process is complicated. Moreover, since Te which comprises these thermoelectric conversion materials is scarce as a resource and is toxic, there exists a problem when the influence on an environment and use in a general household are considered. For these reasons, a search for a new thermoelectric conversion material that does not contain harmful elements in an oxide material that is relatively easy to synthesize is underway.
[0006]
Under such circumstances, in particular, α-NaFeO₂Na, an oxide having a type crystal structure_0.5CoO₂Is in the limelight as a thermoelectric conversion material.
[0007]
Non-Patent Document 1 includes Na._0.5CoO₂Has been reported to exhibit a large electromotive force of 100 μV / K at room temperature and a low electrical resistivity of 200 μΩm. Na_0.5CoO₂Output factor (S²/ Ρ) is the Bi₂Te_ThreeThis is expected to be a new thermoelectric conversion material.
[0008]
Non-Patent Document 2 includes Na._0.5CoO₂Has been reported as the reason why the above characteristics are exhibited.
[0009]
Na_0.5CoO₂As shown in FIG.₂Layered compound in which layers and Na layers are alternately laminated, and P6_ThreeIt has a crystal structure with 22 symmetry. The symmetry of the crystal can be easily confirmed by, for example, X-ray diffraction. P6_ThreeAn example of an X-ray diffraction pattern representing 22 symmetry is shown by a dotted line in FIG.
[0010]
CoO₂In the layer, CoO₆The octahedron shares a ridge. In the Na layer, Na randomly occupies 1/2 of Na sites, and atomic vacancy type lattice defects exist in the remaining 1/2 sites. Of these layers, electrical conduction is CoO.₂The Na layer is thought to be responsible for the heat conduction. As a result, Na_0.5CoO₂Exhibits excellent thermoelectric conversion characteristics in a wide temperature range from liquid nitrogen temperature to 400 ° C. or higher.
[0011]
However, P6_ThreeNa with 22 symmetry_0.5CoO₂Then, CoO₂The inter-layer distance of the layers is relatively large at 0.54 nm, and CoO contained per unit volume₂Since the number of layers and Na layers is small, CoO₂The electric conduction efficiency by the layer and the heat conduction inhibition efficiency by the Na layer are not sufficient.
[0012]
Therefore, Na_0.5CoO₂The Seebeck coefficient is still inadequate, and further improvement is necessary to achieve a more practical thermoelectric conversion material. In this regard, Patent Document 1 discloses Na._0.5CoO₂As a means for improving the thermoelectric conversion performance, a method of partially replacing Na sites or Co sites with other elements is disclosed. However, Na of the system containing such impurities_0.5CoO₂But the thermoelectric conversion performance is still not enough. Moreover, in these conventional techniques, there is a point to be improved in the manufacturing method.
[0013]
Na_0.5CoO₂And a crystal in which Na or Co in the crystal is partially substituted with another element have anisotropy in thermoelectric conversion performance. For example, Na_0.5CoO₂Then, CoO₂The in-plane of the layer (ie, the direction perpendicular to the c-axis) shows good electrical conduction and CoO₂Since the conductivity between layers (ie, c-axis direction) is poor, CoO₂A large thermoelectric conversion performance is exhibited only in the plane of the layer (that is, in a direction perpendicular to the c-axis).
[0014]
When collecting crystals having such anisotropy to produce a polycrystal, Na_0.5CoO₂In the polycrystalline body in which the orientations of the crystals shown in FIG. 2 are randomly assembled, there is a problem that the thermoelectric conversion performance is greatly deteriorated and becomes disadvantageous as compared with the single crystal.
[0015]
In Patent Document 2, as a method for producing a polycrystalline body in which crystals having such anisotropy are collected and the orientations of the crystals are aligned, a laminate in which a plurality of crystals are formed so that the c-axis directions of the crystals are aligned A method of manufacturing a sintered body by forming a body and heat-treating the laminated body in the c-axis direction of the crystal is disclosed.
[0016]
However, such a method for producing a polycrystalline body has a problem in that thermoelectric conversion performance is deteriorated because the crystal is thermally decomposed during heat treatment to produce cobalt oxide as an insulator as an impurity.
[0017]
Furthermore, the Na_0.5CoO₂In this production method, carbonates and oxides are used as the source of alkali metal elements, but since heat treatment at high temperatures is required, alkali metals are likely to evaporate and it is difficult to control the amount of alkali metals. . In addition, it is difficult to produce a cobalt-containing layered oxide that is stable in the atmosphere containing an alkali metal other than Li and Na (for example, K, Rb, Cs, etc.).
[0018]
[Non-Patent Document 1]
Ichiro Terasaki, Yoshitaka Saeko, Kunimitsu Uchinokura, “Physical Review B”, USA, No. 56, 1997, p. R12585
[0019]
[Non-Patent Document 2]
M. M.Von Jansen, Earl. Hope, "Zeitschrift fur Anorganisch und Allegorisch Chemie", Germany, No. 408, 1974, p. 104
[0020]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-266038
[0021]
[Patent Document 2]
JP 2000-269560 A
[0022]
[Problems to be solved by the invention]
The main object of the present invention is to efficiently provide a material having superior thermoelectric conversion performance than conventional products.
[0023]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventor has found that a material produced by a specific method can achieve the above object, and has completed the present invention.
[0024]
  That is, this invention relates to the following thermoelectric conversion material and its manufacturing method.
1. General formula Na _x CoO ₂ A thermoelectric conversion material composed of an oxide represented by (where 0 <x ≦ 0.25) and having a cadmium chloride type crystal structure.
2. Item 2. The thermoelectric conversion material according to Item 1, wherein the interlayer distance L has an interlayer structure in a range of 0.3 nm <L <0.54 nm.
3. Item 3. The thermoelectric conversion material according to Item 1 or 2, comprising a single crystal or a single crystal aggregate with high orientation.
4). Item 4. The thermoelectric conversion material according to any one of Items 1 to 3, wherein the crystal has fine pores having an average pore diameter of 5 to 100 nm.
5.1) heat-treating a mixture containing at least one hydroxide containing an alkali metal element and 2) at least one compound containing cobalt and at least one metal cobalt. _x CoO ₂ (However, 0 <x ≦ 0.25) and a method for producing a thermoelectric conversion material comprising an oxide having a cadmium chloride type crystal structure.
6). Item 6. The method for producing a thermoelectric conversion material according to Item 5, wherein at least one of the hydroxides containing an alkali metal element is sodium hydroxide.
7). Furthermore, the manufacturing method of the thermoelectric conversion material of said claim | item 5 or 6 which mix | blends the halide containing an alkali metal element.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Thermoelectric conversion material
The thermoelectric conversion material of the present invention has a general formula A_xCoO₂(Where 0 <x <0.5, A is an alkali metal element).
[0026]
As the alkali metal element, at least one of Li, Na, K, Rb and Cs is preferable. Among these, Na is more preferably used in the present invention.
[0027]
The above x represents the content of an alkali metal element, and the value is 0 <x <0.5, preferably 0.01 ≦ x ≦ 0.45, more preferably 0.02 ≦ x ≦ 0.4, and most preferably Is 0.02 ≦ x ≦ 0.3. By limiting to this content, conventional products (Na_0.5CoO₂) And superior thermoelectric conversion performance.
[0028]
In particular, when Na is used as the alkali metal element, the value of x is preferably 0 <x ≦ 0.25, more preferably 0.01 ≦ x ≦ 0.2, and 0.02 ≦ x ≦ 0.15. Most preferred. An oxide using Na as an alkali metal element and having the value of x limited to a range of 0 <x ≦ 0.25 has a cadmium chloride crystal structure, and thus can exhibit particularly excellent thermoelectric conversion performance.
[0029]
That is, the general formula A_xCoO₂(However, among the thermoelectric conversion materials of the present invention comprising an oxide represented by 0 <x <0.5, A is an alkali metal element), the general formula Na_xCoO₂(However, a thermoelectric conversion material composed of an oxide represented by 0 <x ≦ 0.25) and having a cadmium chloride type crystal structure is particularly preferable.
[0030]
In the thermoelectric conversion material of the present invention, the general formula Na_xCoO₂(However, the oxide structure is not constant except for the oxides represented by 0 <x ≦ 0.25) and having a cadmium chloride type crystal structure, but generally the more the content of alkali metal elements, Cadmium chloride type crystal structure and α-NaFeO₂There is a tendency to be a mixed crystal with a type crystal structure.
[0031]
The cadmium chloride type crystal structure has crystal symmetry represented by R-3m, for example, as shown in the schematic diagram of FIG. FIG. 3 shows Na having a cadmium chloride type crystal structure (R-3m)._xCoO₂(However, the layered structure of 0 <x ≦ 0.25) is specifically illustrated. As shown in FIG. 3, the cadmium chloride type crystal structure is α-NaFeO.₂Type crystal structure (P6_Three22) Na_0.5CoO₂Similarly to the above, a stacked structure having [A-O-Co-O (A is an alkali metal)] as a minimum unit is formed in the c-axis direction.
[0032]
On the other hand, in the cadmium chloride type crystal structure (R-3m), the laminated structure of the oxygen layer with respect to the c-axis direction is α-NaFeO.₂Type crystal structure (P6_Three22).
[0033]
For example, a) an oxygen layer stacking method is defined as α, b) an oxygen layer stacking method different from α is defined as β, and c) an oxygen layer stacking method different from both α and β is defined as γ. The laminated structure of the oxygen layer is expressed as shown in FIG. 1 and FIG.
[0034]
FIG. 1 shows α-NaFeO.₂1 shows a stacked structure of a type crystal structure, and the stacked structure of an oxygen layer has (αββα) as a minimum unit. On the other hand, FIG. 2 shows a laminated structure of a cadmium chloride type crystal structure, and the laminated form of the oxygen layer has (αβγ) as a minimum unit.
[0035]
Note that the two crystal structures can be clearly distinguished by an X-ray diffraction (XRD) pattern, an electron beam diffraction (ED) pattern, or the like.
[0036]
Thus, in the cadmium chloride type crystal structure, since the oxygen layer is stacked with (αβγ) as the minimum unit, the conventional α-NaFeO having (αββα) as the minimum unit.₂Compared to the type crystal structure, the oxygen layer can be stacked more densely. That is, CoO contained per unit volume₂Since the number of layers and Na layers increases, CoO₂The electric conduction efficiency by the layer and the heat conduction inhibition efficiency by the Na layer are increased, and as a result, the heat transfer performance can be increased synergistically.
[0037]
As shown in FIG. 3, the interlayer distance L is determined as CoO.₂In a crystal structure in which layers and alkali metal layers are alternately stacked, one CoO₂Co face (2) contained in layer (1) and the CoO₂CoO closest to layer (1)₂It refers to the spacing between the Co plane (4) contained in the layer (3). The interlayer distance L can be calculated by X-ray diffraction (XRD) analysis and high-resolution transmission electron microscope (TEM).
[0038]
In the thermoelectric conversion material of the present invention, CoO₂The interlayer distance L between the layers is preferably about 0.3 nm <L <0.54 nm, and more preferably 0.45 nm ≦ L ≦ 0.47 nm. By limiting the interlayer distance to such a range, the thermoelectric conversion performance can be further enhanced.
[0039]
The thermoelectric conversion material of the present invention may be composed of either single crystal or polycrystal (including a single crystal aggregate). In the present invention, it is preferable that the thermoelectric conversion performance of the present invention is made of a single crystal or a single crystal aggregate having a substantially high orientation, particularly from the viewpoint of more reliably obtaining excellent thermoelectric conversion performance.
[0040]
As the degree of orientation, in the XRD pattern, a crystal plane perpendicular to the c-axis (lamination direction of the layered structure) when the crystal structure is displayed in a hexagonal crystal plane, that is, a crystal plane other than the (00L) (L is an integer) plane. All of the thermoelectric conversion materials of the present invention include those in which the derived XRD peak is not substantially observed. Note that in a highly oriented polycrystal, crystals having other compositions (for example, CoO) can be used as long as a predetermined thermoelectric conversion performance is obtained.₂) Etc. may be included.
[0041]
The thermoelectric conversion material of the present invention is preferably porous. Specifically, it is preferable to have fine pores in the crystal having an average pore diameter of 100 nm or less, preferably 50 nm or less, particularly 5 to 10 nm. If it is a porous material, since thermal conductivity can be reduced, the thermoelectric conversion performance can be more effectively enhanced.
[0042]
Such a thermoelectric conversion material of the present invention can achieve an excellent value such that the Seebeck coefficient at 300K is usually 80 μV / K or more, particularly 100 μV / K or more. Among the thermoelectric conversion materials of the present invention, in particular, the general formula Na_xCoO₂(Provided that 0 <x ≦ 0.25) and an oxide having a cadmium chloride type crystal structure has an excellent value such that the Seebeck coefficient at 300K is usually 100 μV or more, particularly 150 μV / K or more. Can also be achieved.
[0043]
The thermoelectric conversion material of the present invention can exhibit excellent thermoelectric conversion performance in a wide temperature range from liquid nitrogen temperature to 300 ° C. and does not contain toxic Te, and can be used widely including home use. For example, it is useful as a thermoelectric conversion element, a power generation device that converts heat energy such as exhaust heat into electrical energy, a thermoelectric cooling device, a temperature sensor, and the like.
[0044]
Moreover, the oxide which comprises the thermoelectric conversion material of this invention is useful as an electrode material, especially a negative electrode material of a lithium battery other than a thermoelectric conversion material, for example.
[0045]
Method for producing thermoelectric conversion material
The method for producing a thermoelectric conversion material of the present invention comprises 1) at least one hydroxide containing an alkali metal element (hereinafter also referred to as “alkali metal hydroxide”), 2) a compound containing cobalt, and metal cobalt. A mixture containing at least one (hereinafter also referred to as “cobalt component”) is heat-treated.
[0046]
It does not specifically limit as an alkali metal hydroxide, What contains lithium ion, sodium ion, potassium ion, rubidium ion, a cesium ion etc. as a cation is mentioned. Specific examples include sodium hydroxide, lithium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide and the like. These alkali metal hydroxides can be used alone or in admixture of two or more.
[0047]
In the production method of the present invention, as the alkali metal hydroxide, those containing sodium ions are particularly preferable, and sodium hydroxide is particularly preferable among those listed above. As the sodium hydroxide, a commercially available product can be used, and those having a purity of 90% or more are particularly preferred.
[0048]
It does not specifically limit as a cobalt component, Metal cobalt, cobalt oxide, basic cobalt carbonate, cobalt hydroxide, cobalt chloride (hexahydrate) etc. are mentioned. Among these, metal cobalt is preferable because unreacted residues can be easily removed by magnetic force and a highly oriented polycrystalline body can be easily produced.
[0049]
In the production method of the present invention, other components can be blended in the raw materials of 1) alkali metal hydroxide and 2) cobalt component as necessary. For example, a halide containing an alkali metal element (hereinafter also referred to as “alkali metal halide”) can be suitably blended. By blending while adjusting the amount of alkali metal halide, the content of alkali metal in the resulting thermoelectric conversion material can be controlled. The blending amount of the alkali metal halide is not limited, but is preferably adjusted within a range of 10 to 50 parts by weight with respect to 100 parts by weight of the alkali metal hydroxide.
[0050]
The alkali metal halide is not particularly limited. For example, the anion includes fluoride ion, chloride ion, bromide ion, iodide ion and the like. Moreover, what contains lithium ion, sodium ion, potassium ion, rubidium ion, cesium ion etc. as a cation is mentioned.
[0051]
Specifically, lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, cesium fluoride, lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, lithium oxalate, sodium oxalate, oxalate Examples include potassium, rubidium oxalate, cesium oxalate, lithium iodide, sodium iodide, potassium iodide, rubidium iodide, cesium iodide, and the like. These can be used individually or in mixture of 2 or more types.
[0052]
In the production method of the present invention, alkali metal halides containing chloride ions as anions are preferable because they are chemically stable. Specifically, lithium chloride, sodium chloride, potassium chloride, rubidium chloride and cesium chloride are preferable.
[0053]
In the production method of the present invention, the blending ratio of the raw materials described above can be appropriately set according to the combination of the raw materials. Usually the reaction product is A_xCoO₂(However, 0 <x <0.5, preferably 0.01 ≦ x ≦ 0.45, more preferably 0.02 ≦ x ≦ 0.4, most preferably 0.02 ≦ x ≦ 0.3) What is necessary is just to set.
[0054]
However, when Na is used as the alkali metal element, the reaction product is Na_xCoO₂(However, it is preferable to set so that 0 <x ≦ 0.25, more preferably 0.01 ≦ x ≦ 0.2, and most preferably 0.02 ≦ x ≦ 0.15).
[0055]
Usually, it can adjust suitably in the range of alkali metal hydroxide: alkali metal halide: cobalt component converted into the amount of metallic cobalt = 20: (0-20): 0.1-10 (weight ratio). However, it is not necessarily limited to this range.
[0056]
Next, the raw materials are mixed. The mixing method is not limited. For example, a mixer, a ball mill, a mortar or the like can be used. You may shape | mold a mixture by well-known methods, such as press molding and a granulation method, before heat processing.
[0057]
Next, the mixture or molded body is heat-treated. Although heat processing temperature is not specifically limited, Usually, 400 degreeC or more, Preferably it is 500 to 900 degreeC, More preferably, it is 550 to 800 degreeC, Most preferably, it is 550 to 700 degreeC. The heat treatment time may be appropriately adjusted according to the heat treatment temperature and the like. The heat treatment atmosphere may be an oxidizing atmosphere or air.
[0058]
In addition, when using a container at the time of heat processing, it is preferable to use the container which does not contaminate a reaction product. For example, an alumina container having a purity of 95% or more can be suitably used.
[0059]
In the production method of the present invention, heat treatment is performed at a lower temperature than in the conventional method, so that it is easy to control the amount of alkali metal in the material obtained by suppressing transpiration of alkali metal, and higher quality oxidation as a thermoelectric conversion material. Things are obtained.
[0060]
In the production method of the present invention, the reaction product after the heat treatment is preferably gradually cooled (30 ° C./hr or less) in order to promote crystal grain growth. By slowly cooling, it becomes easy to obtain a polycrystalline body containing crystals with large grain growth. For example, a flaky polycrystalline body having a particle size of 5 mm or more, particularly 10 mm or more is obtained.
[0061]
The obtained reaction product may be washed with water and dried as necessary. By washing with water, unreacted raw materials (alkali metal hydroxide, alkali metal halide, etc.) remaining in the reaction product can be removed. Drying may be either natural drying or heat drying. Although the drying temperature in the case of heat drying is not specifically limited, Usually, it is 300 degrees C or less, Preferably it is 110-250 degreeC. If it is this temperature range, it can dry efficiently.
[0062]
Such a production method of the present invention has the general formula A_xCoO₂(However, 0 <x <0.5, A is an alkali metal element)_xCoO₂(However, it is useful for the production of a thermoelectric conversion material composed of an oxide represented by 0 <x ≦ 0.25) and having a cadmium chloride type crystal structure.
[0063]
【The invention's effect】
(1) The thermoelectric conversion material of the present invention has a general formula A_xCoO₂(However, 0 <x <0.5, A is an alkali metal element)_0.5CoO₂) Higher Seebeck coefficient, thermoelectric conversion performance, etc., and exhibits excellent performance as a thermoelectric conversion material. Specifically, an excellent value can be achieved in which the Seebeck coefficient at 300K is usually 80 μV / K or more, particularly 100 μV / K or more. The thermoelectric conversion material of the present invention is preferable in that it can exhibit excellent thermoelectric conversion characteristics in a wide temperature range from liquid nitrogen temperature to 300 ° C., does not contain toxic Te, and has a wide range of uses including use at home. Applicable to.
(2) Among the thermoelectric conversion materials of the present invention, the general formula Na_xCoO₂(However, 0 <x ≦ 0.25) and a thermoelectric conversion material made of an oxide having a cadmium chloride type crystal structure has a particularly excellent Seebeck coefficient, thermoelectric conversion performance, etc. Excellent performance can be exhibited as a material. Specifically, a very excellent value such that the Seebeck coefficient at 300 K is usually 100 μV or more, particularly 150 μV / K or more can be achieved.
(3) The thermoelectric conversion material of the present invention having such characteristics is useful as, for example, a thermoelectric conversion element, a power generation device that converts heat energy such as exhaust heat into electric energy, a thermoelectric cooling device, a temperature sensor, and the like. In addition to the thermoelectric conversion material, the oxide constituting the thermoelectric conversion element of the present invention is useful as, for example, an electrode material, particularly a negative electrode material for a lithium battery.
(4) The production method of the present invention is useful in that the thermoelectric conversion material according to the above (1), particularly (2) can be efficiently produced. Advantages of using an alkali metal hydroxide in the production method of the present invention include the following (1) to (5) as compared with the conventional method using sodium carbonate as a sodium supply source. When sodium hydroxide is used, these advantages can be obtained particularly effectively.
[0064]
(1) Since the heat treatment can be performed in a relatively low temperature range, the manufacturing cost can be reduced. Since heat treatment in a low temperature range is possible, it is possible to suppress or prevent transpiration of alkali metal during the production process, thereby reducing heat equipment wear and increasing the accuracy of the cobalt: alkali metal element ratio in the reaction product. .
[0065]
{Circle around (2)} By the heat treatment in the low temperature region, a hexagonal plate-like crystal having the same or larger size can be obtained as compared with the conventional method of heat treatment up to 1050 ° C.
[0066]
(3) It is possible to control the content of the alkali metal element in the crystal by mixing a halide containing 10 to 50% by weight of the alkali metal with respect to the hydroxide containing the alkali metal element during the heat treatment. it can.
[0067]
(4) Since the thermal conductivity can be reduced by forming micropores with an average pore diameter of about 100 nm or less in the produced crystal, Na produced by the conventional method is used._0.5CoO₂Better thermoelectric conversion performance.
[0068]
(5) A polycrystal having large grains can be produced by heat treatment at a relatively low temperature without using a specific flux.
[0069]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the examples.
[0070]
Example 1
(1) Manufacturing method
NaOH: 20 g, NaCl: 5 g and metal Co: 1 g were weighed and mixed, and placed in an alumina crucible. In order to prevent evaporation of NaOH, NaCl, etc. during heat treatment, the crucible was covered with an alumina lid. Next, after heat treatment in air at 650 ° C. for 12 hours, it was gradually cooled in an electric furnace at a cooling rate of 27 ° C./hr. The reaction product is washed with distilled water to remove residual NaOH, NaCl, etc., and a highly oriented polycrystal having a size of about 5 mm (a highly oriented hexagonal plate-like single crystal aggregate Many bodies were obtained. The obtained polycrystal was dried at 150 ° C. for 2 hours.
(2) Evaluation method of reaction product (polycrystal)
The physical properties of the reaction product obtained in (1) above were examined.
[0071]
As an evaluation method, first, an X-ray diffraction (XRD) pattern and an electron beam diffraction (ED) pattern were measured to confirm whether a desired substance was obtained. Moreover, the chemical composition analysis of the reaction product was performed by observation using the elemental analysis (EDS) apparatus attached to an ultra high resolution transmission electron microscope (TEM). Furthermore, the interlayer distance was determined from the XRD pattern and the ED pattern. Next, a fine pore structure in the crystal was observed by TEM image observation. Finally, the Seebeck coefficient was measured at a temperature of 300K.
(3) Evaluation results of reaction products
[XRD pattern]
It was confirmed that the reaction product had a cadmium chloride type (R-3m) crystal structure. Only a diffraction peak substantially derived from the 00L (L is an integer) plane was observed, and it was confirmed that the film had high orientation. The XRD pattern is changed to the conventional Na_0.5CoO₂(P6_ThreeComparison with the XRD pattern of 22) is shown in FIG.
[Chemical composition analysis]
The chemical composition of the reaction product is Na_0.063CoO₂Met.
[Distance between layers]
The interlayer distance of the reaction product was 0.44 nm.
[TEM observation image]
As a result of TEM observation, it was confirmed that a large number of micropores (white portions) having a pore diameter of 5 to 10 nm exist in the crystal.
[0072]
One of the reasons why the thermoelectric conversion material of the present invention has a thermoelectric conversion performance superior to that obtained by the conventional method may be that the micropores reduce the heat conduction in the crystal.
[0073]
Reaction product: Na_0.063CoO₂TEM image observation results are shown in FIGS. FIG. 5 shows the result of observing (observing from above) the flat portion of the hexagonal plate crystal. FIG. 6 shows the result of observing the side surface of the hexagonal plate crystal (observed from the side surface). The relationship between the hexagonal plate crystal and the observation direction is shown in FIG.
[Seebeck coefficient]
The Seebeck coefficient at 300K was 230 μV / K. Compared with the material obtained by the conventional method (about 100 μV / K), the Seebeck coefficient was twice or more.
[0074]
Example 2
A polycrystal was produced in the same manner as in Example 1 except that NaCl was not added as a raw material. The composition of the product is Na_0.33CoO₂The Seebeck coefficient at 300K was 105 μV / K. The interlayer distance was 0.466 nm.
[Brief description of the drawings]
FIG. 1 Na_0.5CoO₂Layered structure (P6_Three22).
FIG. 2 is a diagram showing a layered structure (R-3m) of an oxide having a cadmium chloride type crystal structure.
FIG. 3 is a diagram showing a crystal structure (layered structure) of a thermoelectric conversion material of the present invention and an interlayer distance L thereof.
4 shows Na obtained in Example 1. FIG._0.063CoO₂It is a figure which shows the X-ray-diffraction pattern of.
FIG. 5 shows Na obtained in Example 1._0.063CoO₂TEM image (observed from above: × 490,000 times).
6 shows Na obtained in Example 1. FIG._0.063CoO₂TEM image (observed from the side: × 490,000 times).
7 shows Na obtained in Example 1. FIG._0.063CoO₂It is a figure which shows the relationship between the hexagonal plate-shaped crystal | crystallization and observation direction in TEM observation of.

Claims (7)

Formula _Na x CoO ₂ (where, 0 <x ≦ 0.25) is indicated by, and the thermoelectric conversion material ing oxide having a cadmium chlorinated crystal structure. The thermoelectric conversion material according to claim 1, wherein the interlayer distance L has an interlayer structure in a range of 0.3 nm <L <0.54 nm. The thermoelectric conversion material according to claim 1 or 2 , comprising a single crystal or a single crystal aggregate with high orientation. The thermoelectric conversion material according to any one of claims 1 to 3 , wherein the crystal has fine pores having an average pore diameter of 5 to 100 nm . 1) at least one hydroxide containing an alkali metal element, 2) characterized by heat-treating the compound containing cobalt, and mixtures containing at least one metal of cobalt, the general formula Na _x CoO ₂ ( However, the manufacturing method of the thermoelectric conversion material which consists of an oxide which is shown by 0 <x <= 0.25) and has a cadmium chloride type crystal structure . The method for producing a thermoelectric conversion material according to claim 5 , wherein at least one of the hydroxides containing an alkali metal element is sodium hydroxide. Furthermore, the manufacturing method of the thermoelectric conversion material of Claim 5 or 6 which mix | blends the halide containing an alkali metal element.

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