JP4051441B2 - Thin film thermoelectric conversion material and method for forming the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film thermoelectric conversion material and a method for forming the same.
[0002]
[Prior art]
In Japan, the effective energy yield from primary supply energy is only about 30%, and about 70% of energy is finally discarded as heat into the atmosphere. In addition, heat generated by combustion in a factory or a waste incineration plant is discarded in the atmosphere without being converted into other energy. In this way, we humans are wasting a great deal of thermal energy, and have gained little energy from actions such as burning fossil energy.
[0003]
In order to improve the energy yield, it is effective to be able to use the thermal energy discarded in the atmosphere. For this purpose, thermoelectric conversion that directly converts thermal energy into electrical energy is an effective means. Thermoelectric conversion uses the Seebeck effect and is an energy conversion method in which a potential difference is generated by generating a temperature difference at both ends of a thermoelectric conversion material to generate power. In this thermoelectric power generation, one end of the thermoelectric conversion material is placed in a high temperature part generated by waste heat, the other end is placed in the atmosphere (room temperature), and electricity is obtained simply by connecting a conductive wire to each end. No movable devices such as motors and turbines necessary for general power generation are required. Therefore, the cost is low, gas is not discharged due to combustion, and power generation can be continuously performed until the thermoelectric conversion material deteriorates.
[0004]
In this way, thermoelectric power generation is expected as a technology that will play a part in solving energy problems that are a concern in the future, but in order to realize thermoelectric power generation, it has high thermoelectric conversion efficiency, heat resistance, chemical durability. It is necessary to supply a large amount of thermoelectric conversion materials excellent in properties and the like.
[0005]
So far, various composite oxides have been studied as substances exhibiting excellent thermoelectric conversion performance in high-temperature air. For example, CoO ₂ layered oxides such as Ca ₃ Co ₄ O ₉ , Bi , Pb, Sr, Ca, Co, and the like have been reported (see Patent Document 1 below). As these complex oxides, polycrystals, single crystals, and the like have been reported, and attempts have been made to use them as thermoelectric conversion elements by shaping them into shapes according to the purpose of use.
[0006]
In order to further expand the range of use of such a complex oxide having excellent thermoelectric conversion performance, it is desired to use it in various shapes. For example, if these complex oxides can be formed in a thin film on various substrates, new applications such as incorporation into an electronic circuit and use in a fine portion become possible.
[0007]
As a method for forming a thin film thermoelectric conversion material, for example, a single crystal substrate such as MgO, SrTiO ₃ , or LSAT is heated to a high temperature of 700 ° C. or higher, and a complex oxide thin film is formed on the substrate by a pulse laser method. A method has been reported (for example, see Non-Patent Document 1 below). This method is a method of growing a complex oxide thin film epitaxially on a single crystal substrate. Since it is necessary to use a single crystal substrate, the substrate is expensive and the usable substrates are very limited. The In addition, since the substrate is heated to a high temperature when the thin film is formed, the composition of the deposited complex oxide thin film is likely to vary, and it is difficult to form a complex oxide thin film having the desired composition. For this reason, the thin film formed by the above-described method tends to be unstable in thermoelectric conversion performance and cannot exhibit satisfactory performance.
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-141562
[Non-Patent Document 1]
48th Joint Conference on Applied Physics Lecture Proceedings, Title: 30p-P13-12, March 2001, p223
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described prior art, and the main object thereof is to form a thin film of a thermoelectric conversion material on various substrates, and the formed composite oxide has a good thermoelectric conversion. It is an object of the present invention to provide a novel method for forming a thin film thermoelectric conversion material having performance.
[0011]
[Means for Solving the Problems]
The present inventor has intensively studied to achieve the above-described object. As a result, according to a method in which a Bi-based composite oxide having a specific composition is deposited on a substrate by using a vapor deposition method such as a pulsed laser deposition method, and then heat treatment is performed at a specific temperature range, It has been found that a composite oxide thin film having excellent thermoelectric conversion performance can be formed. According to this method, since a complex oxide thin film can be formed on a substrate at room temperature, a composition fluctuation hardly occurs during the formation of the thin film, and a thermoelectric conversion film with stable performance can be formed, and the type of the substrate is not limited. Therefore, it has been found that an inexpensive substrate can be used, and when a substrate having low thermal conductivity is used, it is possible to suppress the influence of the substrate and exhibit good thermoelectric conversion performance. The present invention has been made based on these findings.
[0012]
That is, the present invention provides the following thin film thermoelectric conversion material and a method for forming the same.
1. A compound oxide thin film represented by a general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1) is formed on a substrate (however, a single crystal substrate) A thin film thermoelectric conversion material formed on top.
2. Item 2. The thin film thermoelectric conversion material according to Item 1, wherein the substrate is a substrate having low thermal conductivity.
3. 3. The thin film thermoelectric conversion material according to item 2, wherein the substrate has a thermal conductivity of 10 W / m · K or less at 25 ° C.
4). Item 4. The thin film thermoelectric conversion material according to any one of Items 1 to 3, wherein the substrate is a glass substrate, a ceramic substrate, or a plastic substrate.
5. Using a Bi-containing material, a Pb-containing material, a Sr-containing material, a Ca-containing material, a Co-containing material mixture or a fired product of the mixture as a raw material, a general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr A thin film characterized by depositing a composite oxide represented by _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1) on a substrate and then performing heat treatment at 600 to 740 ° C. Of forming a thermoelectric conversion material.
6). Item 6. The method according to Item 5, wherein the vapor deposition method is a pulsed laser deposition method.
7). Item 7. The method according to Item 5 or 6, wherein the substrate temperature when depositing the composite oxide is room temperature.
8). Item 8. The method according to any one of Items 5 to 7, wherein the substrate is a glass substrate, a ceramic substrate, or a plastic substrate.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The thin film thermoelectric conversion material of the present invention comprises a composite oxide represented by a general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1). It will be.
[0014]
Such a composite oxide has a high Seebeck coefficient and a low electrical resistivity and good electrical conductivity. Although there are some fluctuations, the Seebeck coefficient increases as the temperature increases as a whole. The electric resistivity tends to decrease. The composite oxide of the present invention can exhibit excellent thermoelectric conversion performance when used as a thermoelectric conversion material by having such a high Seebeck coefficient and a low electrical resistivity at the same time.
[0015]
The thin film thermoelectric conversion material composed of the complex oxide is formed by using a raw material having a predetermined composition and depositing it on a substrate by vapor deposition, followed by heat treatment at 600 to 740 ° C. Can do.
[0016]
As a raw material, a Bi-containing material, a Pb-containing material, a Sr-containing material, a Ca-containing material, and a Co-containing material can be mixed and used, or these mixtures can be fired and used as a fired product.
[0017]
Bi-containing materials, Pb-containing materials, Sr-containing materials, Ca-containing materials, and Co-containing materials are particularly limited as long as they can form oxides by being vaporized by vapor deposition and deposited on a substrate. Can be used without For example, a metal simple substance containing the above-mentioned metal component, an oxide, various compounds (carbonate etc.), etc. can be used. For example, Bi-containing materials include bismuth oxide (Bi ₂ O ₃ ), bismuth nitrate (Bi (NO ₃ ) ₃ ), bismuth chloride (BiCl ₃ ), bismuth hydroxide (Bi (OH) ₃ ), alkoxide compound (Bi (OCH ₃ ) ₃ , Bi (OC ₂ H ₅ ) ₃ , Bi (OC ₃ H ₇ ) _3, etc.) can be used, and Pb-containing materials include lead oxide (PbO), lead nitrate (Pb (NO _{3) 2} ), lead chloride (PbCl ₂ ), lead hydroxide (Pb (OH) ₂ ), alkoxide compound (Pb (OCH ₃ ) ₂ , Pb (OC ₂ H ₅ ) ₂ , Pb (OC ₃ H7) ₃ etc.) or the like can be used, strontium oxide as Sr-containing compound (SrO), strontium chloride (SrCl _2), strontium carbonate (SrCO _3), strontium nitrate (Sr (NO _{3 2),} 2 strontium hydroxide (Sr _(OH)), alkoxide compounds _{(Sr (OCH 3) 2,} Sr (OC 2 H 5) 2, Sr (OC 3 H 7) 2 , etc.) or the like can be used. Examples of Ca-containing materials include calcium oxide (CaO), calcium chloride (CaCl ₂ ), calcium carbonate (CaCO ₃ ), calcium nitrate (Ca (NO ₃ ) ₂ ), calcium hydroxide (Ca (OH) ₂ ), alkoxide compounds ( Ca (OCH ₃ ) ₂ , Ca (OC ₂ H ₅ ) ₂ , Ca (OC ₃ H ₇ ) _2, etc. can be used, and Co-containing materials include cobalt oxide (CoO, Co ₂ O ₃ , Co _3). O _4, etc.), cobalt chloride (CoCl _2), cobalt carbonate (CoCO _3), cobalt nitrate _{(Co (NO 3) 2)} , cobalt hydroxide (Co (OH) ), Alkoxide compounds _{(Co (OC 3 H 7)} 2) or the like can be used. Moreover, you may use the raw material which contains 2 or more types of structural atoms of the complex oxide of this invention.
[0018]
In the present invention, the Bi-containing material, the Pb-containing material, the Sr-containing material, the Ca-containing material, and the Co-containing material are mixed so that the metal ratio is the same as the metal component ratio of the target composite oxide. Although it is possible to use these, it is particularly preferable to mix these raw materials and to fire them. By using a fired product, handling of the raw material is facilitated during vapor phase vapor deposition described later.
[0019]
There are no particular limitations on the firing conditions of the raw material, and it may be fired at a high temperature at which the complex oxide crystal represented by the above general formula is formed, or there is no occurrence of the complex oxide crystal. The calcination may be performed at a relatively low temperature so that a calcined body is formed. The firing means is not particularly limited, and any means such as an electric heating furnace or a gas heating furnace can be adopted. The firing atmosphere is usually an oxidizing atmosphere such as an oxygen stream or air, but it is also possible to fire in an inert atmosphere.
[0020]
In the present invention, the above-mentioned Bi-containing material, Pb-containing material, Sr-containing material, Ca-containing material and Co-containing material mixture or a fired product thereof is used as a raw material, and the target composite oxide is obtained by a vapor deposition method. A thin film is formed on the substrate.
[0021]
The vapor deposition method is not particularly limited as long as it is a method capable of forming an oxide thin film on a substrate using the above-described raw materials. For example, a pulse laser deposition method, a sputtering method, a vacuum evaporation method, an ion A physical vapor deposition method such as a plating method, a plasma assist vapor deposition method, an ion assist vapor deposition method, or a reactive vapor deposition method can be suitably employed. Among these methods, the pulsed laser deposition method is preferable in that the composition does not easily change when a composite oxide containing multiple elements is vapor-deposited.
[0022]
In the method of the present invention, when depositing the composite oxide, it is not necessary to heat the substrate, and the substrate temperature may remain at room temperature. In the state where the composite oxide is deposited on the substrate at room temperature, the composite oxide has a very low degree of crystallization and cannot exhibit good thermoelectric conversion performance. The crystallization of the oxide proceeds and good thermoelectric conversion performance can be exhibited.
[0023]
In the present invention, the substrate that can be used conventionally is not limited to a single crystal substrate. For this reason, any material can be used without limitation as long as it is a material that does not change in quality at the heat treatment temperature to be described later, and there are many types of substrates that can be used, and inexpensive substrates can be used. Further, since a substrate having a low thermal conductivity such as a glass substrate or a ceramic substrate can be used, the influence of the substrate temperature on the thermoelectric conversion performance of the formed complex oxide film can be greatly reduced by using such a substrate. In addition, a plastic substrate can be used as long as the material does not change at a heat treatment temperature described later.
[0024]
In the present invention, it is particularly preferable to use a low thermal conductivity substrate having a thermal conductivity of about 10 W / m · K or less at 25 ° C., more preferably a thermal conductivity of about 5 W / m · K or less, still more preferably thermal conductivity. A substrate with a rate of about 2 W / m · K or less is preferably used.
[0025]
When forming a complex oxide thin film on a substrate, vapor deposition may be performed under reduced pressure according to a conventional method. The specific decompression conditions may be determined as appropriate so that the desired composite oxide film is formed according to the vapor deposition method employed. For example, the pressure may be about 10 ² Pa or less in the pulse laser deposition, the pressure may be about 1 Pa or less in the sputtering method, and the pressure may be about 10 ⁻³ Pa or less in the vacuum evaporation method.
[0026]
The thickness of the complex oxide thin film to be formed is not particularly limited, and may be appropriately set within a range in which good thermoelectric conversion performance can be exhibited according to the use mode of the thin film, for example, about 100 nm or more, Preferably, good performance can be exhibited at a thickness of about 300 nm or more. Further, the upper limit of the film thickness is not particularly limited, but when considering use as a thin film, it is usually about 10 μm or less, preferably about 5 μm or less, more preferably about 2 μm or less.
[0027]
After the complex oxide thin film is formed on the substrate by the method described above, heat treatment is performed at 600 to 740 ° C. By performing the heat treatment in this temperature range, the crystallization of the composite oxide thin film proceeds to have good thermoelectric conversion performance. If the heat treatment temperature is too low, crystallization does not proceed sufficiently, and the thermoelectric conversion performance becomes inferior. On the other hand, if the heat treatment temperature is too high, another phase appears and the thermoelectric conversion performance is lowered, which is not preferable.
[0028]
About the atmosphere at the time of heat processing, what is necessary is just to usually set it as oxidizing atmospheres, such as the atmosphere which contains about 5% or more in air | atmosphere. The pressure at this time is not particularly limited, and may be any of reduced pressure, atmospheric pressure, and increased pressure, and can be, for example, in the range of about 10 ⁻³ Pa to 2 MPa.
[0029]
The heat treatment time varies depending on the size of the object to be treated and the thickness of the complex oxide thin film, but the heat treatment may be performed until the crystallization of the complex oxide thin film is sufficiently advanced, and is usually about 3 minutes to 10 hours. The heat treatment time is preferably about 1 to 3 hours.
[0030]
By the above method, thin films of complex oxides represented by the general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1) are formed on various substrates. Can be formed on top.
[0031]
The formed composite oxide has a structure in which insulating Bi—Sr—O layers and conductive Co—O layers are alternately stacked, and has a high Seebeck coefficient and a low electrical resistivity. The temperature dependence of the electrical resistivity has semiconductor characteristics. A complex oxide thin film having such characteristics has excellent thermoelectric conversion performance as a P-type thermoelectric material, and it can be incorporated into an electronic circuit or used in a fine part by utilizing the thin film form. Etc. are possible.
[0032]
【The invention's effect】
According to the present invention, a composite oxide thin film having excellent thermoelectric conversion performance can be formed on various types of substrates without being limited to the types of substrates. Therefore, an inexpensive substrate can be used, and a thin film of thermoelectric conversion material can be formed at low cost. Moreover, it is possible to use a board | substrate with low heat conductivity, By using such a board | substrate, the influence of the board | substrate with respect to thermoelectric conversion performance can be suppressed.
[0033]
Furthermore, since a complex oxide thin film can be deposited on a substrate at room temperature, a composition fluctuation hardly occurs during the formation of the thin film, and a thermoelectric conversion film with stable performance can be formed.
[0034]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0035]
Example 1
Bi ₂ O ₃ , SrCO _3, and Co ₃ O ₄ were used as raw materials, and these were mixed so that Bi: Sr: Co (atomic ratio) = 2: 2: 2 and temporarily mixed in the atmosphere at 800 ° C. for 10 hours. After firing, firing was performed at 850 ° C. for 20 hours to obtain a pellet-shaped sintered body having a diameter of 2 cm and a thickness of 3 mm.
[0036]
A composite oxide was deposited on the substrate by a pulse laser deposition method using an ArF excimer laser using the obtained sintered body as a target and a fused silica glass as a substrate. At this time, the substrate was brought to room temperature without heating.
[0037]
Specific film forming conditions are as follows.
・ Laser: ArF excimer laser ・ Laser output: 150 mJ
・ Repetition frequency: 5Hz
・ Pressure: 5 × 10 ⁻⁵ Torr
・ Distance between target and substrate: 3cm
-Substrate: quartz glass-Substrate temperature: room temperature The composite oxide thin film having a thickness of 1700 nm obtained by the method described above was heat-treated at various temperatures in the range of 450 to 750 ° C for 10 hours in the air atmosphere. The XRD pattern of each thin film after heat treatment is shown in FIG. From FIG. 1, when the heat treatment is performed at a temperature of 600 ° C. or higher, the composite oxide thin film is crystallized to have a crystal structure similar to that of the sintered body (bulk), and a heterogeneous phase appears at a heat treatment temperature of 750 ° C. or higher. I understand.
[0038]
Further, the composite oxide thin film deposited by the above-described method was subjected to heat treatment at 700 ° C. for 2 hours, and then subjected to electrical measurement. This composite oxide thin film was represented by a composition formula: Bi ₂ Sr ₂ Co ₂ O ₉ and had a structure in which Bi—Sr—O layers and Co—O layers were alternately laminated. A graph showing the temperature dependence of the electrical resistivity of the complex oxide thin film after the heat treatment is shown in FIG. As is clear from FIG. 2, the temperature dependence of the electrical resistivity of the composite oxide showed a semiconductor behavior, and the electrical resistivity at room temperature was 30 mΩcm. Moreover, the Seebeck coefficient at room temperature was 88 μV / K, indicating the thermoelectric performance as a P-type thermoelectric conversion material.
[Brief description of the drawings]
1 is a graph showing an X-ray diffraction pattern of a complex oxide thin film that has been heat-treated at various temperatures in Example 1. FIG.
2 is a graph showing the temperature dependence of the electrical resistivity of the complex oxide thin film obtained in Example 1. FIG.

Claims (8)

A compound oxide thin film represented by a general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1) is formed on a substrate (however, a single crystal substrate) A thin film thermoelectric conversion material formed on top. The thin film thermoelectric conversion material according to claim 1, wherein the substrate is a substrate having low thermal conductivity. The thin film thermoelectric conversion material according to claim 2, wherein the substrate has a thermal conductivity of 10 W / m · K or less at 25 ° C. The thin film thermoelectric conversion material according to claim 1, wherein the substrate is a glass substrate, a ceramic substrate, or a plastic substrate. Using a Bi-containing material, a Pb-containing material, a Sr-containing material, a Ca-containing material, a Co-containing material mixture or a fired product of the mixture as a raw material, a general formula: Bi _{1.6 to 2.2} Pb _{0 to 0.25} Sr A thin film characterized by depositing a composite oxide represented by _{1.1 to 2.2} Ca _{0 to 0.8} Co ₂ O _9-x (0 ≦ x ≦ 1) on a substrate and then performing heat treatment at 600 to 740 ° C. Of forming a thermoelectric conversion material. The method according to claim 5, wherein the vapor deposition method is a pulsed laser deposition method. The method according to claim 5 or 6, wherein the substrate temperature when depositing the composite oxide is room temperature. The method according to claim 5, wherein the substrate is a glass substrate, a ceramic substrate, or a plastic substrate.

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