JP2002280623A - Method for manufacturing oxide thermoelectric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide thermoelectric material having a larger output factor. SOLUTION: A method for manufacturing an oxide thermoelectric material represented by a compositional formula Nax-y Ay CoO2 +d [0.6<x<=1.0, 0<=y<0.28, -0.4<d<=0], where A in the formula is at least one kind selected from Mg, Ca, Sr, Li or K and one kind of rare-earth elements. The method comprises at least a mixing step of uniformly mixing a complex of the A ion with a complex of Na ion and a complex of Co ion so that the ratio of the A ion, an Na ion and a Co ion in a solution has a prescribed value within the above range, a gelling step of adding a polyhydric alcohol to the solution obtained by the mixing step and heating the solution to gel it, a thermal decomposition step of heating the obtained gel to decompose an unrequired organic compound in the gel, and a baking step of baking the material after the thermal decomposition step in an oxidized atmosphere at a prescribed temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermoelectric conversion material capable of directly converting heat into electric energy, and more particularly to a method for producing an oxide thermoelectric material having higher thermoelectric conversion characteristics in a practical temperature range.

[0002]

2. Description of the Related Art Thermoelectric conversion is a technique for directly converting heat energy and electric energy. Since there is no sliding part in the middle, there is no conversion loss due to friction, so that high conversion efficiency can be obtained and quiet. Also excellent in nature.

The basic physical phenomenon of this technology is based on the Seebeck effect, that is, a phenomenon in which when different materials are brought into contact with each other and a temperature difference is applied to a junction thereof, a thermoelectromotive force is generated at both ends. Therefore, by connecting a load to the outside of the junction, a current flows through the circuit, and it is possible to extract the electric power. This technology has been put to practical use as a generator using waste heat from a furnace or as a power source for space and military use.

On the other hand, as a reverse phenomenon of the Seebeck effect, there is a Peltier effect in which when an electric current is applied to a junction of different materials, one end is heated and the other end is cooled. If this effect is used, it can be applied as a temperature controller for heating or cooling a substance to a constant temperature by adjusting the power supplied to the element, and has already been put to practical use as an element for a CPU of a computer. The thermoelectric conversion element is an element utilizing these reversible Seebeck effect and Peltier effect.

The most practical thermoelectric conversion element at present is based on a Bi ₂ Te ₃ -based compound semiconductor, and Sb or the like is partially dissolved in this compound to form p-type and n-type elements. It consists of a "pn junction" formed by joining. Bi ₂ Te ₃ -based compound semiconductors have excellent thermoelectric conversion characteristics near room temperature, which is a practical temperature range. However, as the temperature rises, the composition changes due to the volatilization of Bi having a high vapor pressure, and the like. In the region, its characteristics deteriorate rapidly. In general, in order to extract a large amount of electric power, it is required to use a material at a high temperature. Therefore, the emergence of a material capable of maintaining excellent thermoelectric conversion characteristics even at a high temperature has been desired.

[0006] Under these circumstances, a compound containing Co and having excellent thermoelectric conversion characteristics up to a high temperature has recently been discovered. Its chemical formula is represented by Na _0.5 CoO ₂ , and has a high Seebeck coefficient of about 100 μV / K near room temperature and a high conductivity of about 5000 Scm ⁻¹ , and its thermoelectric property is a conventional Bi ₂ Te ₃ type compound. It is reported to be comparable to that of semiconductors.

The thermoelectric conversion characteristics of these thermoelectric materials are generally expressed by a performance index Z expressed by the following equation (1) using three parameters of Seebeck coefficient α, conductivity σ, and thermal conductivity κ.
Defined by Z = α ² σ / κ (1) As can be seen from the above equation (1), in order to increase the figure of merit Z, the Seebeck coefficient α and the conductivity σ are increased and the output factor P = It is desired to increase α ² σ and reduce the thermal conductivity κ. Among them, the Seebeck coefficient α is the square of its magnitude, which contributes to the magnitude of the figure of merit Z,
It is the most important of the three parameters. However, since these parameters are a function of the carrier concentration in the semiconductor and are interrelated, it is not possible to have the contradictory properties of high Seebeck coefficient and high conductivity in the conventional band theory framework, It was believed that the expected Z value had an upper limit.

However, on the other hand, the aforementioned Na
_{Since 0.5} CoO ₂ has both a high Seebeck coefficient and a high metallic conductivity, it has attracted attention as a next-generation thermoelectric material having a new electronic structure that cannot be explained by the conventional band theory.

[0009]

SUMMARY OF THE INVENTION These Na _0.5 Co
As a method for producing oxide thermoelectric materials represented by O _2, the
Sodium peroxide (Na) which is an oxide powder of each constituent element
_{2 O 2)} and weighed powder cobalt oxide (Co 3 _{O 4),} have been prepared by a solid phase reaction method is fired after mixing, are produced by these solid phase reaction method Na
_{Although the 0.5} CoO ₂ thermoelectric material has superior characteristics as compared with the conventionally used Bi ₂ Te ₃ as described above, its performance index Z cannot be said to be sufficient for practical use. There has been a long-felt need for a manufacturing method capable of obtaining a higher figure of merit Z, that is, a higher power factor P.

Therefore, the present invention has been made in view of the above-mentioned problems, and has the same composition and crystal structure.
It is an object of the present invention to provide a method for producing an oxide thermoelectric material that can produce an oxide thermoelectric material having a higher output factor P.

[0011]

In order to solve the above-mentioned problems, a method for producing an oxide thermoelectric material according to the present invention uses a composition formula of Na _xy A _y CoO _{2 + d} [0.6 <x ≦ 1. 0.0, 0
≦ y <0.28, −0.4 <d ≦ 0], wherein A in the above formula is at least one of Mg, Ca, Sr, Li, K, and a rare earth element. In the production method, the A ion complex is mixed with the Na ion complex and the Co ion complex such that the ratio of the A ion to the Na ion and the Co ion present in the solution is a predetermined ratio within the above range. A mixing step of homogenizing, a gelling step of adding a polyhydric alcohol to the aqueous solution obtained in the mixing step, and heating the aqueous solution to form a gel, and heating the obtained gel to form a gel. And a firing step of firing the material after the thermal decomposition step in an oxidizing atmosphere at a predetermined temperature. According to this feature, compared to the conventional solid-phase reaction method, mixing of each metal atom in the crystal is performed by mixing of the complex in the solution, so that a crystal with excellent uniformity can be obtained. As a result, the electrical conductivity of the crystal is improved, so that an oxide thermoelectric material having a higher power factor P can be obtained.

In the method for producing an oxide thermoelectric material according to the present invention, it is preferable that the ligand compound serving as the complex ligand is citric acid. With this configuration, the use of citric acid having a simple structure and constituent elements of hydrogen, oxygen, and carbon makes it possible to reduce the amount of gas and the like generated in the thermal decomposition step, and to compare the processing time. It is economical because it can be made short, and there is no element remaining as an impurity in the material after the thermal decomposition step.

In the method for producing an oxide thermoelectric material according to the present invention, it is preferable that citric acid is mixed with the aqueous solution so that the molar ratio of citric acid to each metal ion in the aqueous solution is 4 to 10. . In this case, the molar ratio is 4 to 10
By mixing an excess amount of citric acid so as to obtain, the formation of each complex compound is promoted, and the generation of a precipitate is suppressed, so that a more uniform mixed state can be obtained.

In the method for producing an oxide thermoelectric material according to the present invention, the polyhydric alcohol is preferably ethylene glycol. In this way, by using ethylene glycol having a simple structure and constituting elements of hydrogen, oxygen and carbon, the amount of gas and the like generated in the thermal decomposition step is relatively small, and the processing time can be compared. It is economical because it can be made short, and there is no element remaining as an impurity in the material after the thermal decomposition step.

According to the method for producing an oxide thermoelectric material of the present invention, the produced oxide thermoelectric material has a composition formula of Na _x CoO ₂ [0.1.
It is preferable that the oxide thermoelectric material is represented by the following formula: 6 <x ≦ 1.0]. In this way, a relatively high power factor P
, The output factor P of Na _x CoO ₂ [0.6 <x ≦ 1.0] can be further improved.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart showing a method for manufacturing an oxide thermoelectric material of the present invention.

FIG. 1 is a flow chart showing the process of the present invention.
Formula of NaCl Thermoelectric Material_xyA _yCoO_{2 + d} [0.
6 <x ≦ 1.0, 0 ≦ y <0.28, −0.4 <d
 ≦ 0] Production method using Ca as element A
Is an example.

First, as raw materials, sodium acetate, calcium acetate, and cobalt acetate, which are acetates of the respective metal elements constituting the oxide thermoelectric material to be obtained, were weighed, and these acetates were purified. The citric acid, which is a complex ligand that forms a complex with the metal ions in the solution while being poured into water, is dissolved in a solution having a number of moles corresponding to about 5 times the total number of moles of each metal ion in the solution. To form a complex and mix to form a uniform sol. (Mixing process)

Then, ethylene glycol, which is a polyhydric alcohol, is added to the sol homogenized in the mixing step, and the sol is stirred and mixed.
The gelation is performed by heating to 00 ° C. (Gelling process)

As described above, in this embodiment, a solid acetate is used as a raw material. However, the present invention is not limited to this, and citric acid and pure water are added to each acetate in advance to prepare a complex solution. And the respective complex solutions may be weighed and mixed.

In this embodiment, acetate is used as each of the raw material powders.
The acetic acid group has the simplest structure as an acid, and its constituent elements are composed only of hydrogen, oxygen, and carbon. Although it is preferable because the time required for separation can be shortened, the present invention is not limited to this, and a metal salt serving as a supply source of these metal elements is
What is necessary is just to select suitably from the said viewpoint, cost, etc.

In the embodiment, citric acid as a complex ligand is added to pure water as a solvent as described above. When the molar ratio of the citric acid to the respective metal ions in the solvent is 4 times or less, the solution becomes turbid and complete dissolution (complex formation) is not performed, and a good homogenized state of each metal ion is obtained. On the other hand, when the electrical resistance of the obtained crystal increases and the output factor P decreases, and when the molar ratio becomes 10 times or more, the heating required for forming the gel in the aforementioned gelation step is performed. Since the time becomes long and it becomes uneconomical, the molar ratio may be in the range of 4 to 10 times, more preferably about 5 times, for each metal ion.

Further, in this embodiment, citric acid is used as a ligand as described above, and the use of citric acid as described above is because these citric acids have a relatively simple structure as a ligand. The constituent elements are composed only of hydrogen, oxygen, and carbon, and in the thermal decomposition described below,
Although it is preferable because it does not leave an element serving as an impurity, the present invention is not limited thereto, and these ligands may be appropriately selected from those described above in view of the above-described viewpoints. I just want to choose.

In the present embodiment, the sol (aqueous solution)
Ethylene glycol is added for the purpose of accelerating the polymerization of the complex (ligand) of the above. The amount of the ethylene glycol and the like may be appropriately present in excess relative to the amount of the complex in the solution. If the amount is too large, the heating time in the later-described thermal decomposition increases, which is uneconomical. Therefore, it is preferable to use the minimum necessary amount.

Next, an acetic acid group having hydrogen and carbon, which are unnecessary elements remaining in the gel formed in the gelling step, and citric acid and ethylene glycol are reacted at 350 ° C.
To separate from the gel (heat decomposition step). The temperature and time for these thermal decompositions may be appropriately selected depending on the ligands and raw materials to be used, and the treatment time may be determined by measuring the weight loss of the treated gel.

These thermally decomposed gels are calcined at 600 to 675 ° C. for 12 hours in an oxidizing atmosphere to be crystallized, and then formed into a predetermined shape by a press or the like, and are then oxidized. Main firing is performed at 600 to 800 ° C. for 12 hours to obtain an oxide thermoelectric material.

On the other hand, as a comparative example, a flow chart of FIG. 2 shows a conventional method of producing Na _0.7 CoO ₂ by a solid-phase reaction method. As a raw material, Na ₂ O ₂ powder was used as a Na supply source, and Co ₃ O ₄ powder was used as a cobalt supply source.

The Na ₂ O ₂ powder and the Co ₃ O ₄ powder were weighed so that the composition ratio of Na and Co in the oxide thermoelectric material obtained by firing was 0.70: 1.00. Thereafter, the mixture is put into a mortar and stirred and mixed so as to be uniform.

The mixture homogenized by the mixing is transferred to a firing vessel, and then put into a firing apparatus together with the firing vessel.
After baking in air at 800 ° C. for 12 hours, the baked product is pulverized into a powder, and the pulverized product is mixed to make the composition uniform, and then baked again at 850 ° C. for 12 hours. This was performed to homogenize the crystallinity, and after the completion of firing, was gradually cooled to obtain an oxide thermoelectric material.

According to the production method using the complex shown in FIG. 1 of the present embodiment, the composition ratio x of the Na atom is set to 0.7, and the substitution ratio y to Ca is set to 0.0, 0.07, 0.14. FIG. 3 shows the results of a comparison of the Seebeck coefficient at each temperature between the changed oxide thermoelectric material and the Na _0.7 CoO ₂ oxide thermoelectric material obtained by the conventional solid-state reaction method shown in FIG. FIG. 4 shows the result of comparing the resistivity ρ = 1 / σ.
FIG. 5 shows the result of comparing the output factor P from these data.

First, when the Seebeck coefficient of this embodiment and that of the conventional solid-phase reaction method are compared, as shown in FIG. Although the Seebeck coefficient is higher than that of the above, there is no significant difference between the two Seebeck coefficients in a high temperature region higher than room temperature. On the other hand, according to the production method of the present embodiment, when a part of sodium is replaced by y = 0.07 or y = 0.14 calcium, calcium replacement in which y = 0 in almost the entire temperature range. As a result, a higher Seebeck coefficient is obtained as compared with those obtained by both of the production methods which do not carry out the above.

In addition, even if calcium substitution was not performed, which had no difference in the Seebeck coefficient, FIG.
As shown in (1), in this example, the electrical resistivity is low and the electrical conductivity is high in the entire temperature range.

As for the relationship between the electrical resistivity and the above-mentioned calcium substitution, when the amount of the calcium substitution becomes large, the electric resistivity increases particularly in a low temperature region.

When the output factor P is compared based on the Seebeck coefficient and the electrical resistivity, as shown in FIG. 5, the thermoelectric material manufactured using the complex of the present embodiment has the above-described decrease in the electrical resistivity. It can be seen that the contribution of (improvement of electric conductivity) can greatly improve the output factor P particularly in a practical high temperature region.

Further, the calcium-substituted thermoelectric material produced by the production method of the present embodiment has y =
As shown in the data of 0.07, it was found that it was possible to obtain a higher output factor P, and the production method of this example was effective for producing an oxide thermoelectric material by replacing sodium. You can see that.

The reason why the oxide thermoelectric material produced by using the complex of this embodiment has a lower electric resistivity than that obtained by the conventional solid-state reaction method is mainly that of the ceramic sample. Since the conductivity is greatly affected by the composition deviation in the crystal grains and the impurities precipitated between the crystal grains, the crystal grains produced from the homogenous gel as in the present embodiment have the composition deviation and the composition deviation of these compositions. Since the generation of impurity phases due to shifts etc. is less than in the conventional solid phase reaction method,
It is considered that the electrical conductivity improved (the electrical resistivity decreased). In addition, as for the amount of sodium to be replaced by calcium, when calcium is used as these replacing elements as in this example, these calcium do not show sublimation like Na even at a high temperature, so that stable production can be achieved. Is preferable, and a decrease in performance at a high temperature can be suppressed. However, when these substitution amounts are y = 0.28,
Above, the amount of impurities in the X-ray diffraction pattern increases remarkably.
The range may be in the range of 0 to 0.28.

In this embodiment, calcium is used as the A atom for substituting sodium. However, the present invention is not limited to this, and these A atoms may be close to the ionic radius of sodium. Alternatively, elements having an ionic radius smaller than the ionic radius of sodium can be used, and these elements include Mg, Ca, S
A single substance of r, Li, K, and a rare earth element or a mixture of two or more of them can be used.

When the ratio x of sodium to cobalt is 0.6 or less, Co ₂ O ₃
Is mixed into the crystal as an impurity, which lowers both the Seebeck coefficient and the electric conductivity. When x is 1.0 or more, Na ₄ CoO ₄ is mixed as an impurity and the Seebeck coefficient and the electric conductivity are reduced. Since both of the degrees are lowered, as the ratio x of the sodium,
It is preferable that 0.6 <x ≦ 1.0.

As described above, the oxide thermoelectric material produced by the production method using the complex of the present embodiment has a small Seebeck coefficient α, but has a low electric conductivity, as compared with the conventional solid-state reaction method. It is improved over the entire practical temperature range, so that a high figure of merit can be obtained, and thus it has high practicality.

[0040]

The present invention has the following effects. (A) According to the first aspect of the invention, compared with the conventional solid-phase reaction method, mixing of each metal atom in a crystal is performed by mixing of a complex in a solution, so that excellent uniformity is achieved. A crystal can be obtained, and as a result, the electrical conductivity of the crystal is improved, so that an oxide thermoelectric material having a higher output factor P can be obtained.

(B) According to the second aspect of the present invention, the amount of gas and the like generated in the thermal decomposition step is reduced by using citric acid whose structure is simple and whose constituent elements are hydrogen, oxygen and carbon. Relatively few, the processing time can be made relatively short, which is economical, and there is no element remaining as an impurity in the material after the thermal decomposition step.

(C) According to the third aspect of the present invention, the formation of each complex compound is promoted by mixing an excessive amount of citric acid so that the molar ratio becomes 4 to 10, thereby suppressing generation of a precipitate. A more uniform mixing state can be obtained.

(D) According to the invention of claim 4, the amount of gas and the like generated in the thermal decomposition step is reduced by using ethylene glycol whose structure is simple and whose constituent elements are hydrogen, oxygen and carbon. Relatively few, the processing time can be made relatively short, which is economical, and there is no element remaining as an impurity in the material after the thermal decomposition step.

(E) According to the fifth aspect of the present invention, the relatively high
Na with power factor P_xCoO ₂[0.6 <x ≦ 1.
0] can be further improved.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a method for manufacturing an oxide thermoelectric material according to an example of the present invention.

FIG. 2 is a flowchart showing a conventional method for producing an oxide thermoelectric material.

FIG. 3 is a graph showing the Seebeck coefficient at each temperature of an oxide thermoelectric material according to an example of the present invention and an oxide thermoelectric material according to a conventional manufacturing method.

FIG. 4 is a graph showing electrical resistivity at various temperatures of an oxide thermoelectric material according to an example of the present invention and an oxide thermoelectric material manufactured by a conventional method.

FIG. 5 is a graph showing output factors at various temperatures of the oxide thermoelectric material according to the example of the present invention and the oxide thermoelectric material according to the conventional manufacturing method.

[Explanation of symbols]

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G030 AA02 AA03 AA04 AA07 AA08 AA09 AA11 AA28 BA01 BA21 GA10 4G048 AA04 AA05 AB01 AB02 AB05 AC08 AE05 AE08

Claims (5)

[Claims] 1. The composition formula: Na _xy A _y CoO _{2 + d} [0.1.
6 <x ≦ 1.0, 0 ≦ y <0.28, −0.4 <d
≦ 0], wherein A in the above formula is Mg, Ca, Sr,
A method for producing an oxide thermoelectric material that is at least one of Li, K, and a rare earth element, wherein the ratio of the A ion to Na ion and Co ion present in a solution is a predetermined ratio within the above range. Thus, the mixing step of mixing and homogenizing the A ion complex with the Na ion complex and the Co ion complex, and after adding the polyhydric alcohol to the aqueous solution obtained in the mixing step, heating the aqueous solution A gelling step of gelling, a heating decomposition step of heating the obtained gel to decompose unnecessary organic compounds in the gel, and a firing step of firing the material after the thermal decomposition step in an oxidizing atmosphere at a predetermined temperature. And a method for producing an oxide thermoelectric material. 2. The ligand compound serving as the complex ligand,
The method for producing an oxide thermoelectric material according to claim 1, which is citric acid. 3. The method for producing an oxide thermoelectric material according to claim 2, wherein the aqueous solution is mixed with citric acid so that the molar ratio of citric acid to each metal ion in the aqueous solution is 4 to 10. . 4. The method for producing an oxide thermoelectric material according to claim 1, wherein said polyhydric alcohol is ethylene glycol. 5. The oxide thermoelectric material to be produced has a composition formula of Na
_x CoO ₂ method of manufacturing an oxide thermoelectric material according to claim 1 is an oxide thermoelectric material represented by [0.6 <x ≦ 1.0].