JP2016115896A - Manufacturing method of thermoelectric transducer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a thermoelectric transducer capable of easily manufacturing a thermoelectric transducer with high power generation performance from a spray coated film manufactured by spraying a calcium cobaltate powder onto a substrate.SOLUTION: In a method of manufacturing a thermoelectric transducer from a spray coated film manufactured by spraying a calcium cobaltate powder onto a substrate, the spray coated film is sintered by maintaining a temperature of exceeding 926°C and less than or equal to 950°C under an ambient atmosphere for longer than or equal to 1 hour and shorter than or equal to 10 hours. The sintering processing is preferably applied to the spray coated film twice or more.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for producing a thermoelectric conversion element from a sprayed coating formed by spraying calcium cobaltate (Ca ₉ Co ₁₂ O ₂₈ ) powder on a substrate.

A thermoelectric conversion module including a thermoelectric conversion element using an oxide-based thermoelectric conversion material excellent in heat resistance and oxidation resistance is a heat source in an oxidizing atmosphere having a high temperature exceeding 600 ° C. like waste heat from an industrial furnace. Therefore, it is expected as a power generator that efficiently extracts power. Therefore, as a method for manufacturing a thermoelectric conversion element having an arbitrary shape and area using an oxide thermoelectric conversion material in a short time, easily and at low cost, for example, Patent Document 1 discloses an oxide thermoelectric conversion element. It is disclosed that a thermoelectric conversion element is formed using a sprayed coating produced by spraying a conversion material powder on a substrate.

JP 2014-107443 A

For example, when an oxide-based thermoelectric conversion material powder having the property of being decomposed when heated to a certain temperature or more, such as calcium cobaltate (Ca ₉ Co ₁₂ O ₂₈ ), the oxide-based thermoelectric conversion material powder is sprayed. Is exposed to a thermal spray flame of 2100 to 2600 ° C., the oxide thermoelectric conversion material is decomposed, resulting in a problem that the composition of the thermal spray coating changes from the oxide thermoelectric conversion material composition. Further, since the sprayed coating formed on the substrate is immediately cooled, there is a problem that minute cracks are likely to occur in the sprayed coating. For this reason, in the thermoelectric conversion element manufactured using the sprayed coating, the value of a thermoelectromotive force and electric conductivity becomes small, and there exists a problem that electric power generation performance falls.

The present invention has been made in view of such circumstances, and a thermoelectric conversion element having a high thermoelectromotive force and electric conductivity from a sprayed coating produced by spraying calcium cobaltate powder (Ca ₉ Co ₁₂ O ₂₈ powder) on a substrate. It is an object of the present invention to provide a method for manufacturing a thermoelectric conversion element that can be easily manufactured.

The method for producing a thermoelectric conversion element according to the present invention in accordance with the above object is a method for producing a thermoelectric conversion element from a thermal spray coating produced by thermal spraying calcium cobaltate powder on a substrate.
A baking treatment is performed in which the thermal spray coating is held for 1 hour to less than 10 hours in an air atmosphere of more than 926 ° C. and less than 950 ° C.

In the method for manufacturing a thermoelectric conversion element according to the present invention, it is preferable to repeat the baking treatment twice or more on the sprayed coating.
Thereby, the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element formed from the thermal spray coating can be further improved.

In the method for manufacturing a thermoelectric conversion element according to the present invention, when the thermal spray coating baking treatment, during spraying produced by the decomposition of calcium cobaltate _{(Ca 9 Co 12 O 28)} , constitutes the inside of the thermal spray coating Ca ₉ Co Ca ₃ Co ₂ O ₆ dispersed between ₁₂ O ₂₈ particles can be moved to the surface of the thermal spray coating to reduce the Ca ₃ Co ₂ O ₆ content in the thermal spray coating. Thereby, the Ca ₉ Co ₁₂ O ₂₈ content in the thermal spray coating is improved, and the thermoelectromotive force of the thermoelectric conversion element formed from the thermal spray coating can be improved. In addition, when the sprayed coating is fired, sintering between Ca ₉ Co ₁₂ O ₂₈ particles constituting the sprayed coating is promoted, and minute cracks generated in the sprayed coating can be eliminated. Thereby, it becomes possible to improve the electrical conductivity of the thermoelectric conversion element formed from a sprayed coating.
As a result, the power generation performance of the thermoelectric conversion element formed from the sprayed coating can be improved.

It is a graph which shows the influence of the baking time which each exerts on the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element manufactured by baking the thermal spray coating of a calcium cobaltate at 930 degreeC. It is explanatory drawing which shows the compositional analysis result by the X ray diffraction of the thermal spray coating of a calcium cobaltate, Comprising: (A) was after thermal spraying, (B) performed the baking process for 2 hours and 5 hours by 930 degreeC air | atmosphere atmosphere. Thereafter, (C) was subjected to a baking treatment for 10 hours and 40 hours in an air atmosphere at 930 ° C., and (D) was an X-ray of the sprayed coating after being subjected to a baking treatment for 60 hours in an air atmosphere at 930 ° C. It is a diffraction pattern. It is a graph which shows the influence of the repetition frequency of a baking process which each has on the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element manufactured using the thermal spray coating of the calcium cobaltate which repeated the baking process for 5 hours at 930 degreeC. It is explanatory drawing which shows the compositional analysis result by the X-ray diffraction of the thermal spray coating of a calcium cobaltate, Comprising: (A) repeats baking processing for 5 hours at 930 degreeC twice, (B) is 10 hours at 930 degreeC. It is an X-ray-diffraction pattern of the sprayed coating after performing this baking processing.

Next, embodiments of the present invention will be described with reference to the accompanying drawings for understanding of the present invention.
In the method for manufacturing a thermoelectric conversion element according to an embodiment of the present invention, calcium oxide powder (Ca ₉ Co ₁₂ O ₂₈ powder) that is an oxide-based thermoelectric conversion material is used as an alumina substrate (heat resistance, insulation, low heat). An example of a conductive substrate) is subjected to a firing process in which a thermal spray coating produced by high-speed flame spraying is held in an air atmosphere of more than 926 ° C. and less than 950 ° C. for 1 hour or more and less than 10 hours. Manufactures conversion elements. Details will be described below.

Calcium cobaltate (Ca ₉ Co ₁₂ O ₂₈ ) is a solid phase obtained by baking a mixture of calcium carbonate (CaCo ₃ ) and cobalt oxide (Co ₃ O ₄ ) in an air atmosphere at 900 to 930 ° C. for 5 to 30 hours. Synthesized by reaction. After the synthesized calcium cobaltate was pulverized, it was granulated using a spray dryer, and a granulated product having a particle size of 106 μm or less was used as a calcium cobaltate powder for thermal spraying. A spray gun used for high-speed flame spraying was supplied in the form of a slurry prepared by dispersing calcium cobaltate powder in a solvent. The high-speed flame spraying was performed by setting the spray flame temperature in the range of 2100 to 2600 ° C. and the spraying speed (the speed of the powder delivered from the spray gun) in the range of 1800 to 2000 m / sec. Further, the spraying distance (distance between the spraying gun and the alumina substrate) can be arbitrarily set within the range of 100 to 300 mm so that the alumina substrate and the sprayed coating are not damaged.

The thermal treatment of the sprayed coating is constant up to the firing temperature set in the range of more than 926 ° C and less than 950 ° C in an air atmosphere by setting the alumina substrate on which the sprayed coating is formed in an electric furnace (an example of a firing furnace) This was performed by heating at a heating rate of 1 mm and holding for a firing time set in a range of 1 hour or more and less than 10 hours, and then cooling at a constant cooling rate. The rate of temperature rise and the rate of temperature fall can be arbitrarily set within a range in which the sprayed coating is not peeled off from the alumina substrate or cracks are generated.

Since the calcium cobaltate powder supplied to the thermal spray gun is blown into a thermal spray frame at 2100 ° C. or higher, a part of the calcium cobaltate powder is decomposed to produce Ca ₃ Co ₂ O ₆ . For this reason, Ca ₃ Co ₂ O ₆ is dispersed in the formed sprayed coating (between Ca ₉ Co ₁₂ O ₂₈ particles constituting the sprayed coating). Therefore, when the sprayed coating is fired, Ca ₃ Co ₂ O ₆ dispersed in the sprayed coating can be moved to the surface of the sprayed coating, and the content of Ca ₃ Co ₂ O ₆ in the sprayed coating is reduced. (The content of Ca ₉ Co ₁₂ O ₂₈ constituting the thermal spray coating can be relatively improved), and the thermoelectromotive force of the thermoelectric conversion element formed from the thermal spray coating can be improved. For this reason, the electric power generation performance of the thermoelectric conversion element formed from a sprayed coating can be improved.

Here, the calcination temperature was over 926 ° C. because when Ca ₃ Co ₂ O ₆ moves in the thermal spray coating, the Ca ₃ Co ₂ O ₆ moves markedly in the temperature range exceeding 926 ° C. It was decided because there was. On the other hand, when the firing temperature exceeds 950 ° C., the formation of Ca ₃ Co ₂ O ₆ due to the decomposition of Ca ₉ Co ₁₂ O ₂₈ forming the sprayed coating during the firing treatment becomes significant, and the surface of the sprayed coating is Ca. ₃ Co ₂ O ₆ are integrated to cover the surface of the sprayed coating. Here, since the thermoelectromotive force of Ca ₃ Co ₂ O ₆ has anisotropy that varies greatly depending on the crystal direction, in the case of a sprayed coating in which it is difficult to align the crystal direction, the surface of the sprayed coating is Ca ₃ Co _When covered with ₂ O ₆ , it leads to a decrease in thermoelectromotive force. For this reason, the upper limit of the firing temperature was set to 950 ° C.

The reason why the firing time is set to 1 hour or longer is that when the firing time is less than 1 hour, the Ca ₃ Co ₂ O ₆ is not sufficiently transferred to the surface of the sprayed coating, and the Ca ₃ Co ₂ in the sprayed coating is not sufficient. This is because the O ₆ content cannot be reduced. On the other hand, if the firing time is 10 hours or longer, the production of Ca ₃ Co ₂ O ₆ due to the decomposition of Ca ₉ Co ₁₂ O ₂₈ cannot be ignored. For this reason, baking time was made into less than 10 hours.

The thermal spray coating is rapidly cooled because it is formed on the alumina substrate and the heating by the thermal spray frame is interrupted. For this reason, there is a high possibility that many fine cracks are introduced into the sprayed coating. When a large number of minute cracks are introduced into the thermal spray coating, the electrical conductivity of the thermal spray coating decreases. Here, when the sprayed coating is fired, sintering between the Ca ₉ Co ₁₂ O ₂₈ particles constituting the sprayed coating proceeds and fine cracks introduced into the sprayed coating disappear. It becomes possible to improve the degree. For this reason, the electric power generation performance of the thermoelectric conversion element formed from a sprayed coating can be improved.

As described above, when a firing treatment is performed in which the thermal spray coating is held in an air atmosphere of more than 926 ° C. and lower than 950 ° C. for 1 hour to less than 10 hours, the thermal spray coating is suppressed while suppressing decomposition of Ca ₉ Co ₁₂ O ₂₈ Ca ₃ Co ₂ O ₆ can be moved to the surface of the sprayed coating. When this baking treatment is repeated twice or more for the sprayed coating, the Ca ₉ Co ₁₂ O ₂₈ forming the sprayed coating is reduced while reducing the Ca ₃ Co ₂ O ₆ content in the sprayed coating. As a result, it is possible to further promote the crystallization, and it is possible to further improve the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element formed from the sprayed coating.
Then, by repeating the firing treatment, thermal stability of the thermal spray coating (stabilization of thermoelectromotive force and Ca ₉ Co ₁₂ O _{28 as} the movement of Ca ₃ Co ₂ O ₆ to the surface of the thermal spray coating converges). Therefore, even if heating and cooling are repeated, deterioration of the power generation performance of the thermoelectric conversion element and durability can be achieved.

Example 1
A mixture of calcium carbonate and cobalt oxide was baked for 10 hours in an air atmosphere at 930 ° C., and calcium cobaltate (Ca ₉ Co ₁₂ O ₂₈ ) was synthesized by a solid phase reaction. The synthesized calcium cobaltate was pulverized and then granulated using a spray dryer, and the granulated product that passed through a 106 μm sieve was used as a sprayed calcium cobaltate powder (Ca ₉ Co ₁₂ O ₂₈ powder).
A calcium cobalt powder (Ca ₉ Co ₁₂ O ₂₈ powder) is dispersed in methanol to prepare a slurry, and while supplying this slurry to a spray gun, an alumina substrate (length 50 mm, width 60 mm, thickness 0. 1 mm), a sprayed coating (vertical 15 mm, horizontal 50 mm, thickness 0.05 mm) was formed. The high-speed flame spraying was performed under the conditions of a spray flame temperature of 2100 ° C., a spray velocity of 1800 m / sec, and a spray distance of 250 mm.
Next, an alumina substrate on which a sprayed coating is formed is set in an electric furnace, heated to a firing temperature of 930 ° C. in an air atmosphere at a heating rate of 200 ° C./hr, and 2, 5, 10, 40 at 930 ° C. And after hold | maintaining only for each baking time of 60 hours, the thermoelectric conversion element was manufactured by performing the baking process which cools with the temperature-fall rate of 200 degrees C / hr.

One side of the obtained thermoelectric conversion element is heated to 800 ° C. in an air atmosphere using an electric heater, and the other side is air-cooled to generate a temperature difference, so that a thermoelectromotive force (TEMF) and electrical conductivity (EC) are obtained. Was measured. Moreover, the thermoelectromotive force and electrical conductivity were measured also about the thermoelectric conversion element produced using the thermal spray coating (the baking process is not performed, ie, baking time is 0 hour) after thermal spraying. The result is shown in FIG. Moreover, the composition analysis by X-ray diffraction was performed about the sprayed coating after thermal spraying, and the thermal sprayed coating after baking processing (baking temperature is 930 degreeC, baking time is 2, 5, 10, 40, and 60 hours). The results are shown in FIGS.

As shown in FIG. 2 (A), a diffraction peak indicating Ca ₉ Co ₁₂ O ₂₈ was detected from the thermoelectric conversion element manufactured using the sprayed coating after spraying. _{Incidentally, Ca} 3 _Co 2 _{O 6} produced by the decomposition of Ca _{9 Co} 12 _{O 28} because that exists at the grain boundaries of _Ca 9 _Co 12 _{O 28,} is believed to have not been detected as a diffraction peak. Moreover, when the surface of the thermoelectric conversion element produced using the sprayed coating after thermal spraying was observed with a scanning electron microscope, it was confirmed that minute cracks were generated in the sprayed coating. As shown in FIG. 1, the small electrical conductivity of the thermoelectric conversion element produced using the sprayed coating immediately after spraying is considered to be due to the influence of minute cracks.

As shown in FIG. 2 (B), a diffraction peak indicating Ca ₉ Co ₁₂ O ₂₈ was detected from the sprayed coating obtained by performing the baking treatment for 2 to 5 hours, and the surface of the sprayed coating was observed. When observed with a scanning electron microscope, that the fine cracks were present after spraying has been _solved, the surface of Ca _{9 Co} 12 _{O 28} particles _{that Ca} 3 _Co 2 _{O 6} fine particles are precipitated, It was confirmed that Ca ₃ Co ₂ O ₆ fine particles increased as the firing time increased. As shown in FIG. 1, the reason why the electrical conductivity of the thermoelectric conversion element produced from the sprayed coating subjected to the firing treatment was increased is because the minute cracks were eliminated.

In addition, the electric conductivity of the thermoelectric conversion element produced from the thermal spray coating which performed the baking process for 5 hours is the electric conductivity of the thermoelectric conversion element produced from the thermal spray film which performed the baking process for 2 hours. The decrease compared to the conductivity is thought to be due to an increase in Ca ₃ Co ₂ O ₆ fine particles precipitated on the surface of the sprayed coating. Moreover, the thermoelectromotive force of the thermoelectric conversion element produced from the thermal spray coating which performed the baking process for 5 hours is the heat | fever of the thermoelectric conversion element produced from the thermal spray film which performed the baking process for 2 hours. The improvement compared to the electromotive force is thought to be due to a decrease in the Ca ₃ Co ₂ O ₆ content in the sprayed coating.

As shown in FIG. 2C, Ca ₉ Co ₁₂ O ₂₈ and Ca ₃ Co ₂ are produced from a thermoelectric conversion element manufactured using a sprayed coating obtained by performing a baking treatment for 10 to 40 hours. A diffraction peak indicating O ₆ was detected. Further, when observing the surface of the thermoelectric conversion element manufactured firing time from the thermal spray coating which has undergone the baking treatment at 10, 40 hours with a scanning electron microscope, it was deposited on the surface of the thermal spray coating Ca ₃ Co ₂ O ₆ microparticles It was confirmed that they were fused together and coated with a sprayed coating. The fact that a diffraction peak indicating Ca ₃ Co ₂ O ₆ was detected corresponds to the fact that the surface of the sprayed coating was coated with Ca ₃ Co ₂ O ₆ .

As shown in FIG. 1, the thermoelectric conversion element produced from the thermal spray coating obtained by performing the firing process for 10 hours was produced from the thermal spray coating obtained by performing the firing process for 5 hours. Although there was no change in electrical conductivity as compared with the thermoelectric conversion element, a decrease in thermoelectromotive force was observed. This is presumably because Ca ₃ Co ₂ O ₆ having anisotropy in the crystal direction in the thermoelectromotive force covers the surface of the sprayed coating.
On the other hand, when a thermoelectric conversion element produced from a sprayed coating that has been fired for 40 hours is observed with a scanning electron microscope, the gap between Ca ₉ Co ₁₂ O ₂₈ particles is reduced, and the sintered density is improved. It was seen. As shown in FIG. 1, the electric conductivity of the thermoelectric conversion element produced from the thermal spray coating subjected to the firing treatment for 40 hours is the thermoelectric conversion produced from the thermal spray coating subjected to the firing treatment for 10 hours. The improvement in the electrical conductivity of the device is considered to be due to an improvement in the sintered density of the sprayed coating.

As shown in FIG. 2D, a diffraction peak indicating the presence of Ca ₉ Co ₁₂ O ₂₈ and AlCo ₂ O ₄ was detected from the thermoelectric conversion element obtained by performing the baking treatment with a baking time of 60 hours. And no diffraction peak indicating Ca ₃ Co ₂ O ₆ was detected. The reason why the diffraction peak of AlCo ₂ O ₄ was detected was that the alumina substrate and Ca ₃ Co ₂ O ₆ reacted. As shown in FIG. 1, since AlCo ₂ O ₄ was generated, the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element were lowered.

(Example 2)
A thermal spray coating was formed in the same manner as in Example 1, and a thermoelectric conversion element was produced using the thermal spray coating in which the baking treatment at 930 ° C. for 5 hours was repeated 2 to 4 times.
And for each thermoelectric conversion element, the thermoelectromotive force and electric conductivity were measured by the same method as in Example 1. The result is shown in FIG. FIG. 3 also shows the measurement results of the thermoelectromotive force and electrical conductivity of a thermoelectric conversion element manufactured using a thermal spray coating produced by performing a baking process at 930 ° C. for 5 hours once. Moreover, the composition analysis by X-ray diffraction was performed about the sprayed coating which repeated the baking process twice. The result is shown in FIG.

As shown in FIG. 3, when the baking treatment at 930 ° C. for 5 hours was repeated, the thermoelectromotive force and the electrical conductivity were improved according to the increase in the number of treatments. Further, as shown in FIG. 4 (A), in the X-ray diffraction pattern of the sprayed coating when the baking treatment at 930 ° C. for 5 hours is repeated twice and the heating for a total of 10 hours is performed, FIG. ), The diffraction peak of Ca ₃ Co ₂ O ₆ is not detected as compared with the X-ray diffraction pattern of the sprayed coating when the baking treatment at 930 ° C. for 10 hours is performed once. Therefore, the thermal spray coating is formed while reducing the Ca ₃ Co ₂ O ₆ content in the thermal spray coating by repeatedly performing the firing process under the firing conditions that suppress thermal decomposition of Ca ₉ Co ₁₂ O ₂₈ . It was confirmed that the sintering of Ca ₉ Co ₁₂ O ₂₈ can proceed and the thermoelectromotive force and electrical conductivity of the thermoelectric conversion element can be further improved.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above-described embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included.

Claims (2)

In a method of manufacturing a thermoelectric conversion element from a sprayed coating prepared by spraying calcium cobaltate powder on a substrate,
A method for producing a thermoelectric conversion element, comprising performing a baking treatment for holding the sprayed coating in an air atmosphere of more than 926 ° C. and not more than 950 ° C. for 1 hour or more and less than 10 hours. The method for manufacturing a thermoelectric conversion element according to claim 1, wherein the firing treatment is repeatedly performed twice or more on the sprayed coating.

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