JP2006339378A - Manufacturing method of oxide thermoelectric conversion material, and thermoelectric conversion device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of an oxide thermoelectric material, and a thermoelectric conversion device which is a strong correlated thermoelectric material of a long life which allows efficient thermoelectric conversion by crystal generation processing by a complex polymerization method of the material. <P>SOLUTION: The manufacturing method of the thermoelectric conversion material composed of an oxide represented by a general formula Na<SB>x</SB>CoO<SB>y</SB>(0.5≤x≤1, 1≤y≤2) includes a process for forming a metal complex by a complex polymerization method, a thermal decomposition process for removing organics, and a process for baking thereafter. The manufacturing method of an oxide thermoelectric material is characterized by forming of fine particles on the particle surfaces before the baking process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a novel method for producing a thermoelectric conversion material, and particularly relates to a novel method for producing a strongly correlated thermoelectric material Na _x CoO _y -based material capable of thermoelectric conversion with high efficiency, which has recently attracted attention.

  In recent years, environmental issues and the problem of alternative energy for fossil fuels have attracted much social attention. Under such circumstances, for example, 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, and harmful CFCs Expectations for heat transfer technology that does not use gas are increasing. In order to meet these expectations, development of high-performance thermoelectric conversion materials and the like is indispensable.

The thermoelectric conversion material is a material that can mutually convert heat energy and electric energy, and the figure of merit (Z) is given by the following equation.
Z = S ² / (ρκ)
In the formula, S represents a thermoelectromotive force (Seebeck coefficient), ρ represents an electrical resistivity, and κ represents a 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.

Various thermoelectric conversion materials have been proposed so far. For example, Bi ₂ Te ₃ -based thermoelectric conversion materials exhibit a high performance index in the temperature range from room temperature to 200 ° C., and are used for thermoelectric cooling and the like as Peltier elements. The PbTe-based thermoelectric conversion material exhibits a high performance index in a temperature range of 200 to 500 ° C., and is mainly used as a power generator.

  However, since these are non-oxide materials, there is a problem that the synthesis process is complicated. Moreover, since Te is scarce as a resource and is toxic, there is a problem when considering the influence on the environment and use in a general household. For these reasons, a search for a new thermoelectric material that does not contain harmful elements in an oxide material that is relatively easy to synthesize is underway.

Under such circumstances, Na _0.5 Co ₂ O ₄ , which is an oxide having a NaFeO ₂ type crystal structure, has attracted attention as a thermoelectric conversion material. That is,
Na _xy A _y CoO _{2 + d} (0.6 <x ≦ 1.0, 0 ≦ y <0.28, −0.4 <d ≦ 0), and A is Mg, Ca, Sr, Li, A precursor of an oxide material that is at least one of K and rare earth elements is synthesized by a complex polymerization method, then heated in an oxidizing atmosphere, crystallized (preliminary firing), and press-molded into a predetermined shape. There has been proposed a method in which main firing is performed in an oxidizing atmosphere to obtain high thermoelectric conversion characteristics (see, for example, Patent Document 1).

Further, it has been reported that Na _0.5 CoO ₄ exhibits a large thermoelectromotive force of 100 μV / K at room temperature and a low electric resistance of 200 μΩm (for example, see Non-Patent Document 1). The output factor of Na _0.5 CoO ₄ exceeds that of Bi ₂ Te ₃ and is expected as a new thermoelectric conversion material. However, since this material easily reacts with moisture in the atmosphere, there is a problem that the thermoelectric characteristics are likely to change with time. A solution is desired.
JP 2002-280623 A Ichiro Terasaki, Yoshitaka Saeko, Kunimitsu Uchinokura, Phys. Rev. B56 (1997) 12585

  The present invention has been made in consideration of the above-described circumstances, and is an oxide that is a long-lived strongly correlated thermoelectric material that can be thermoelectrically converted with high efficiency by crystal formation processing by a complex polymerization method of an oxide thermoelectric conversion material. It aims at providing the manufacturing method and thermoelectric conversion apparatus of a thermoelectric material.

In order to solve the above problems, the invention described in claim 1 is a thermoelectric conversion comprising an oxide represented by the general formula Na _x CoO _y (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2). A method for producing a material, including a step of forming a metal complex by a complex polymerization method, a thermal decomposition step for removing organic substances, a step of firing after thermal decomposition, and an oxide thermoelectric that forms fine particles on the particle surface before the firing step. The manufacturing method of the conversion material is the main feature.

  The invention according to claim 2 is the invention according to claim 1, wherein the firing step comprises at least a first firing step and a plurality of firing steps of the second firing step. The main feature is a method for producing an oxide thermoelectric conversion material in which the fine particles are formed on the particle surface after the firing step.

  The invention according to claim 3 is the invention according to claim 1 or 2, wherein the oxide thermoelectric conversion material in which the fine particles formed on the particle surface before the firing step are Na or a compound made of Na. The manufacturing method is the main feature.

  The invention according to claim 4 is the invention according to claim 2, wherein the first firing temperature is mainly characterized by a method for producing an oxide thermoelectric conversion material lower than the second firing temperature. .

  The invention according to claim 5 is the invention according to claim 2, wherein Na fine particles formed on the surface of the particles after the first firing are compound fine particles made of Na after the first firing. The main feature is a method for producing an oxide thermoelectric conversion material smaller than particles.

  The invention according to claim 6 is the invention according to claim 2, wherein the oxide thermoelectric element in which the fine particles of Na or Na formed on the particle surface after the first firing step are water-soluble. The manufacturing method of the conversion material is the main feature.

The invention according to claim 7 is the invention according to claim 3, wherein Na or Co compound fine particles on the surface of the metal oxide are Na _x CoO _y (however, 0.00. The main feature is a method for producing a thermoelectric conversion material which is a supply source of oxide Na, represented by 5 ≦ x ≦ 1, 1 ≦ y ≦ 2.

The invention according to claim 8 is the invention according to claim 3, wherein the presence or absence of Na or Na compound fine particles on the surface of the metal oxide and Na _x CoO _y after the second firing step (however, , 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2), the main feature is a method for producing a thermoelectric conversion material having the following relationship with an increase in the crystal grain size of the oxide.
Δ1 <Δ2
Δ1: Increase in crystal grain size after the second firing step in the absence of compound fine particles Δ2: Increase in the crystal grain size after the second firing step in the presence of compound fine particles

  The invention according to claim 9 is the invention according to claim 7 or 8, wherein the oxide thermoelectric conversion material has a step of removing excess Na or a compound comprising Na after the second baking step. The manufacturing method is the main feature.

  The invention described in claim 10 is characterized in that a thermoelectric conversion device using the thermoelectric conversion element manufactured in the firing step for the thermoelectric conversion element according to any one of claims 1 to 9 is a main feature. To do.

According to the present invention, there is provided a method for producing a thermoelectric conversion material comprising an oxide represented by the general formula Na _x CoO _y (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2), A process for forming a metal complex, a pyrolysis process for removing organic matter, a process for firing after pyrolysis, and a method for producing an oxide thermoelectric material characterized by forming fine particles on the particle surface before the firing process, A step of forming a metal complex by a complex polymerization method using a thermoelectric conversion material composed of an oxide represented by the formula Na _x Co ₂ O _y (where 1 ≦ x ≦ 2, 2 ≦ y ≦ 4), and organic matter by thermal decomposition A first baking step for crystallizing grains, and a second baking step for increasing the crystal grain size, and by forming fine particles on the particle surface before the second baking step, the crystals are enlarged High-efficiency and thermoelectric conversion characteristics over time It becomes possible to obtain a thermoelectric conversion capable of reducing dynamic.

  In addition, the fine particles formed on the particle surface before the second baking step are made of Na or a compound made of Na, so that Na can be supplied and crystals can be increased in the second baking step.

  Furthermore, Na or Na compound fine particles composed on the surface of the particles before the second firing are water-soluble, thereby increasing the degree of freedom in setting the removal process and reducing the cost of removing excess Na. became.

  Further, the crystal grain size can be controlled by controlling the amount of Na or Na compound fine particles on the surface of the metal oxide.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
As a manufacturing method of the oxide thermoelectric material represented by these Na _x CoO _y (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2), there is a complex polymerization method. The details of the complex polymerization method are described in “Powder and Powder Metallurgy”, Vol. 40, No. 20, February 1993, p137, and the details are omitted here. And a stable chelate complex is dissolved in and dispersed in ethylene glycol and subjected to thermal polymerization esterification to finally form a polymer gel (complex polymer) having a three-dimensional network structure. The complex polymer is heated to remove unnecessary elements remaining in the gel.

  The present invention heats the gel obtained by this thermal decomposition, performs the first firing to crystallize the particles, forms fine particles on the particle surface, and then increases the crystal grain size at a higher temperature. The second baking is characterized in that an oxide thermoelectric material having a target composition is obtained. A specific embodiment will be described.

  FIG. 1 is a flowchart of the manufacturing process of the thermoelectric conversion material of the present invention. As a starting material, 4.8 g of sodium acetate monohydrate and 8.98 g of cobalt acetate tetrahydrate were weighed into a Pyrex (registered trademark) glass vessel, respectively, and then poured into 60 ml of pure water. Dissolve with stirring (S1). Thereafter, 66.1 g of citric acid is added to the solution (S2), and stirred and dissolved with a magnetic stirrer to form a complex and a sol.

  Next, 4.85 g of ethylene glycol was mixed with the sol (S3) and stirred, and then the solution was heated on a hot plate (HP) for up to 80 minutes at 200 ° C., then at 250 ° C. for 40 minutes, and then at 300 ° C. for 100 minutes. It heated and gelatinized (S4). In addition, temperature shows the surface temperature of HP. Then, it heated at 350 degreeC for 1 hour, the gel was thermally decomposed (S5), and the unnecessary citric acid, ethylene glycol, and the acetic acid group were burned. Next, it grind | pulverized with the agate mortar and mixed so that the whole might become uniform (S6). Thereafter, the crushed and mixed gel particles were transferred to an alumina ceramic vessel, and heated in air or in an oxygen atmosphere at 800 ° C. for 5 hours to perform first firing (S7).

  Here, after forming smaller particles (referred to as fine particles) on the surface of the base particles (referred to as mother particles), the mother particles baked earlier using an agate mortar (S8) and homogenized. It was. As a result of observing the surface of the mother particle with a scanning electron microscope, the shape is scaly, the average size of one mother particle is 2 to 5 μm, and one fine particle formed on the mother particle surface. The size is smaller than that of the mother particle and is formed dispersed on the surface.

Table 1 shows the results of EPMA analysis of the amount of Na when the mother particles after the first firing are not washed and when washed. From this result, it is apparent that Na or Na compound is formed on the surface of the mother particle by the first firing. In the table, the amount of Na in the case of washing is shown by setting the amount of Na in the case of not washing as 1. The reason why the Na value after washing is not 0 is considered that the mother particles are already made of a compound of Na, Co and O in the first firing, and the Na signal from this was detected. Even in the XRD measurement result (not shown), the diffraction pattern of Na, Co, O compound and Co ₃ O ₄ is confirmed on the mother particles in the first firing.

  The fine particles are water-soluble, and the dissolved liquid is alkaline. The surface photograph of the particles is shown in FIG. 2, and the particle surface photographs before and after washing with water are shown in FIG.

  Next, after the powder was pressure-molded to 2.5 × 2.5 × 10 mm with a pressure-molding die, it was transferred to an alumina ceramic vessel and oxygen was introduced at 20 to 200 ml / min in an electric furnace at 900 ° C. The second heating and baking (S9) was performed for 20 hours to produce a p-type thermoelectric element having a schematic size of 2.5 × 2.5 × 10 mm. As described above, the second baking temperature is higher than the first baking temperature, so that the crystal grains can be enlarged.

Table 2 shows the results of investigating the relationship between the increase in crystal grains, the first firing temperature, and the second firing temperature.
In the table, Δ indicates little change, ○ indicates a slight increase, and ◎ indicates a large increase by each symbol.

In this embodiment, an example of an amount necessary for manufacturing a small element is shown, but the amount is not limited to this example, and the same effect can be expected in a large amount of manufacturing.
Thus performed first firing, the surface of the mother particle to form particles smaller than (fine particles) which carried out the second fired to obtain a Na _0.7 CoO ₂ single phase. Moreover, the crystal grain could be enlarged. On the other hand, when the second baking is performed after removing the fine particles with water or the like after the first baking, the crystal grains are not enlarged, and a plurality of layers of Na _0.7 CoO ₂ and Co ₃ O ₄ are formed. If the fine particles are washed and removed after the first firing and the second firing is performed, the intended composition is not obtained.

FIG. 4 is an explanatory diagram of the relationship between the presence or absence of fine particles and the crystal grain size of the conversion material of the present invention.
By performing the first firing, forming smaller particles (fine particles) on the surface of the mother particles and performing the second firing, the crystal grains can be enlarged. On the other hand, when the first baking is performed and then the fine particles are removed with water or the like and the second baking is performed, it is understood that the crystal grains do not increase.

  Thus, by performing the first firing and forming smaller particles (fine particles) on the surface of the mother particles, it is possible to increase the crystals in the second firing, and also function as a Na supply source. The desired composition could be obtained.

  FIG. 5 is a flowchart of the crystal production process of the thermoelectric conversion material of the present invention. As in Example 1, 3.8 g of sodium acetate monohydrate and 7.2 g of cobalt acetate tetrahydrate were weighed into a Pyrex (registered trademark) glass vessel as a starting material, respectively, and then poured into 48 ml of pure water. Then, after stirring and dissolving with a magnetic stirrer (S1), 52.9 g of citric acid is added to the solution (S2), and stirring and dissolving with a magnetic stirrer to form a complex and a sol.

  Next, 4.85 g of ethylene glycol was mixed with the sol (S3) and stirred, and then the solution was heated on a hot plate (HP) for 70 minutes at 200 ° C., then at 250 ° C. for 30 minutes, and then at 300 ° C. for 100 minutes. It heated and gelatinized (S4). Thereafter, the gel was heated at 350 ° C. for 60 minutes to thermally decompose (S5), and unnecessary citric acid, ethylene glycol, and acetic acid groups were burned. Next, it grind | pulverized with the agate mortar and mixed so that the whole might become uniform (S6). Thereafter, the crushed and mixed gel particles are transferred to an alumina ceramic vessel, heated in air or in an oxygen atmosphere at 800 ° C. for 5 hours, and subjected to a first firing (S7). After forming the particles, the mother particles baked using an agate mortar were pulverized (S8) and made uniform. At this time, the size of one mother particle is 5 μm or less, and the size of one fine particle on the surface of the mother particle is 0.5 μm or less, which is smaller than the particles serving as a base material, and is dispersed on the surface. .

  Next, after press-molding to 5 × 5 × 10 mm using a press-molding mold (S9), it is transferred to an alumina ceramic vessel, and oxygen is introduced at 20 to 200 ml / min in an electric furnace, and 20 at 900 ° C. After heat baking for a second time and second baking (S10), pulverization and pulverization (S11) are performed by a wet bead mill method using water as a dispersing agent, and the crystal grain size is made uniform and surplus Na and Na Removal of compound components was performed.

  After that, only the powder is taken out and pressure-molded again to 2.5 × 2 × 10 mm (S12), oxygen is introduced into the electric furnace at 20 to 200 ml / min, and the third baking is performed at 900 ° C. for 20 hours (S13). Went. Thus, the removal process of Na which is a surplus component element, and a Na compound was shortened by implementing as a grinding | pulverization and a crushing process.

  As in Example 1, 2.88 g of sodium acetate monohydrate and 5.38 g of cobalt acetate tetrahydrate were weighed into a Pyrex (registered trademark) glass vessel as starting materials, respectively, and then poured into 36 ml of pure water. After stirring and dissolving with a magnetic stirrer (S1), 39.7 g of citric acid is added to the solution (S2), and stirring and dissolving with a magnetic stirrer to form a complex and a sol. Next, 2.91 g of ethylene glycol was mixed with the sol (S3) and stirred, and then the solution was heated on a hot plate (HP) at 200 ° C. for 80 minutes, then at 250 ° C. for 40 minutes, and then at 300 ° C. for 100 minutes. It heated and gelatinized (S4). Then, it heated at 350 degreeC for 1 hour, the gel was thermally decomposed (S5), and the unnecessary citric acid, ethylene glycol, and the acetic acid group were burned.

  Next, it grind | pulverized with the agate mortar and mixed so that the whole might become uniform (S6). Thereafter, the gel particles are transferred to an alumina ceramic vessel, and the crushed and mixed gel particles are heated in air or in an oxygen atmosphere at 800 ° C. for 5 hours, and subjected to first firing (S7). Then, the particles fired previously using a zirconia ball of φ5 mm and a zirconia pot of φ50 mm × 120 mm were pulverized and mixed by a dry method (S8) to make uniform. Next, the particles pulverized and mixed from the zirconia pot are collected, pressure-molded to 2.5 × 2 × 10 mm using a pressure mold, transferred to an alumina ceramic vessel, and oxygen is 20-200 ml in an electric furnace. / Min. And second baking (S9) was performed at 900 ° C. for 20 hours to produce a p-type thermoelectric element.

  FIG. 6 is a configuration diagram of a thermoelectric conversion device constructed with the thermoelectric conversion material of the present invention. As shown in FIG. 6, an n-type thermoelectric element 1 such as a La—Bi—Ni—O system was used, and a p-type thermoelectric element 2 and a pair produced by this method were combined to construct a thermoelectric conversion device 3. Specifically, an electrode 5 is formed on an alumina ceramic insulating substrate 4 having a thickness of 0.5 mm, the aforementioned p-type and n-type thermoelectric element end faces are joined with a conductive joining material, and one side is similarly conductive. An alumina ceramic insulating substrate 7 having a thickness of 0.5 mm, on which electrodes were formed with a bonding material, was bonded. Thereafter, the power lead-out line 8 is connected to the electrodes of the p and n thermoelectric elements.

  Electric power can be generated by providing a temperature difference between the upper and lower alumina ceramic insulating substrates. Furthermore, it is possible to apply a temperature in a temperature range higher than that used in the Bi-Te system to the thermoelectric conversion device, thereby obtaining a thermoelectromotive force. In addition, it is possible to obtain a large electric power by constituting a plurality of sets of this set of p and n type thermoelectric elements on an alumina ceramic insulating substrate. Moreover, it is also possible to apply a current to this element to cool or heat it, and use it as a so-called Peltier element for equipment that requires various temperature controls. Moreover, since excess Na or Na compound is removed, it became possible to reduce the characteristic fluctuation due to aging.

It is a flowchart of the manufacturing process of the thermoelectric conversion material of this invention. It is a scanning electron micrograph of the surface of the mother particle after pulverization of the thermoelectric conversion material of the present invention. It is the surface photograph of the particle | grains after the water washing process of the thermoelectric conversion material of this invention. It is explanatory drawing of the relationship between the presence or absence of the fine particle of the conversion material of this invention, and a crystal grain diameter. It is a flowchart of the crystal production process of the thermoelectric conversion material of this invention. It is a block diagram of the thermoelectric conversion apparatus constructed | assembled with the thermoelectric conversion material of this invention.

Explanation of symbols

1 n-type thermoelectric element 2 p-type thermoelectric element 3 thermoelectric conversion device 4 alumina ceramic insulating substrate 5 electrode (upper side)
6 Electrode (lower side)
7 Alumina ceramic insulating substrate 8 Power extraction line

Claims (10)

A method for producing a thermoelectric conversion material comprising an oxide represented by the general formula Na _x CoO _y (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2),
Forming a metal complex by a complex polymerization method;
A pyrolysis process to remove organic matter;
A step of firing after pyrolysis,
A method for producing an oxide thermoelectric material, wherein fine particles are formed on a particle surface before a firing step.   The calcination step comprises at least a first calcination step and a plurality of calcination steps of a second calcination step, wherein the fine particles are formed on the particle surface after the first calcination step. A method for producing the oxide thermoelectric conversion material according to 1.   The method for producing an oxide thermoelectric conversion material according to claim 1 or 2, wherein the fine particles formed on the particle surface before the firing step are Na or a compound comprising Na.   The temperature in said 1st baking process is lower than the temperature in a 2nd baking process, The manufacturing method of the oxide thermoelectric conversion material of Claim 2 characterized by the above-mentioned.   The method for producing an oxide thermoelectric conversion material according to claim 2, wherein the fine particles of Na or Na formed on the particle surface after the first firing step are smaller than the particles after the first firing.   3. The method for producing an oxide thermoelectric conversion material according to claim 2, wherein the fine particles of Na or Na composed on the particle surface after the first firing step are water-soluble. Na or Na compound fine particles on the surface of the metal oxide are supplied in the second firing step as Na _x CoO _y (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2). The method for producing an oxide thermoelectric conversion material according to claim 3, wherein the oxide thermoelectric conversion material is a source. The presence or absence of Na or Na compound fine particles on the surface of the metal oxide and Na _x CoO _y after the second firing step (where 0.5 ≦ x ≦ 1, 1 ≦ y ≦ 2) 4. The method for producing an oxide thermoelectric conversion material according to claim 3, wherein the increase in crystal grain size is in the following relationship.
Δ1 <Δ2
Δ1: Increase in crystal grain size after the second firing step in the absence of compound fine particles Δ2: Increase in the crystal grain size after the second firing step in the presence of compound fine particles   The method for producing an oxide thermoelectric conversion material according to claim 7 or 8, further comprising a step of removing surplus Na or a compound comprising Na after the second baking step.   A thermoelectric conversion device using the oxide thermoelectric material manufactured in the firing step for the thermoelectric conversion element according to claim 1.

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