JP7559428B2 - Thermoelectric conversion materials and thermoelectric conversion elements - Google Patents

Description

The present invention relates to a thermoelectric conversion material and a thermoelectric conversion element.

Thermoelectric conversion technology, which converts heat into electricity using thermoelectric conversion materials, has been attracting attention as a clean energy source that can effectively convert minute amounts of thermal energy, such as body heat and exhaust heat from factories, cars, and homes, as well as various types of heat in nature, into electricity. There are various thermoelectric effects that can be used in thermoelectric conversion technology, but the most common system uses the Seebeck effect, in which an electromotive force is generated by an electron gradient that occurs within a material in response to the temperature difference when two different temperatures are applied to both ends of the material, which is made up of a combination of semiconductors and metals.

Thermoelectric conversion materials that can convert between thermal energy and electrical energy are used in thermoelectric conversion elements such as thermoelectric power generation elements and Peltier elements. Thermoelectric conversion elements are elements that convert heat into electricity and are made up of a combination of semiconductors and metals. Representative thermoelectric conversion elements are classified into p-type semiconductors alone, n-type semiconductors alone, and combinations of p-type and n-type semiconductors. To obtain a larger potential difference, thermoelectric conversion elements generally use a combination of p-type and n-type semiconductors as materials.

Thermoelectric conversion elements, such as Peltier elements, are used as thermoelectric modules that combine a number of elements into a plate or cylinder shape. Because they can directly convert thermal energy into electricity, they can be used, for example, as a power source for wristwatches that operate with body heat, and for terrestrial and satellite power generation. The performance of a thermoelectric conversion element depends on the performance of the thermoelectric conversion material, etc.

As described in Non-Patent Document 1, a dimensionless thermoelectric figure of merit ZT is used as an index representing the performance of a thermoelectric conversion material. In addition, a power factor PF (= ^S2 ·σ) is sometimes used as an index representing the performance of a thermoelectric conversion material.
The dimensionless thermoelectric figure of merit ZT is expressed by the following formula (1).
ZT=((S ²・σ)/к)・T Formula (1)
Here, S is the Seebeck coefficient (V/K), σ is the electrical conductivity (S/m), T is the absolute temperature (K), and κ is the thermal conductivity (W/(m·K)). The thermal conductivity κ is expressed by the following formula (2).
к=α・ρ・C Formula (2)
Here, α is the thermal diffusivity (m ² /s), ρ is the density (kg/m ³ ), and C is the specific heat capacity (J/(kg·K)).
That is, in order to improve the thermoelectric conversion performance (hereinafter also referred to as thermoelectric properties), it is important to increase the Seebeck coefficient or electrical conductivity while decreasing the thermal conductivity.

Typical thermoelectric conversion materials include inorganic materials such as bismuth-tellurium (Bi-Te) for temperatures from room temperature to 500 K, lead-tellurium (Pb-Te) for temperatures from room temperature to 800 K, and silicon-germanium (Si-Ge) for temperatures from room temperature to 1000 K.

However, thermoelectric conversion materials containing these inorganic materials often contain rare elements and are therefore expensive, or may contain harmful substances. In addition, inorganic materials are difficult to process, which makes the manufacturing process complicated. As a result, thermoelectric conversion materials containing inorganic materials require high manufacturing energy and costs, making them difficult to generalize. Furthermore, since materials containing inorganic materials are rigid, it is difficult to process them into shapes that are not flat, or to manufacture thermoelectric conversion elements that are flexible. In addition, thermoelectric conversion materials containing inorganic materials generally have a high specific gravity, which presents an issue in that they tend to be heavy.

In response to this, research is underway into thermoelectric conversion elements that use organic materials instead of conventional inorganic materials. Organic materials are lightweight, have excellent moldability, and are more flexible than inorganic materials, making them versatile within the temperature range in which they do not decompose. In addition, because the elements can be manufactured using printing technology, etc., they are also more advantageous than inorganic materials in terms of manufacturing energy and costs.

For example, Patent Document 1 discloses a thermoelectric conversion material containing a polymer dispersant having an organic dye skeleton and carbon nanotubes (CNTs), and a thermoelectric conversion element using the same. However, the thermoelectric conversion element disclosed in the invention of Patent Document 1 does not have sufficient thermoelectric conversion performance, and has a problem of lacking stability under high temperature and high humidity conditions.

International Publication No. 2015/050113

Takenobu Kajikawa, "Thermoelectric Conversion Technology Handbook (First Edition)", NTS Publishing, p. 19

The problem that this invention aims to solve is to provide a thermoelectric conversion material that can achieve both thermoelectric conversion performance and stability under high temperature and high humidity conditions.

The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention.
That is, the present invention relates to a thermoelectric conversion material comprising a conductive material (A) and a tackifier (B) having a cyclic aliphatic hydrocarbon structure.

The present invention also relates to the above thermoelectric conversion material, in which the conductive material (A) contains a carbon material.

The present invention also relates to the above thermoelectric conversion material, in which the content of the tackifier (B) having a cyclic aliphatic hydrocarbon structure is 1 part by mass or more and 200 parts by mass or less per 100 parts by mass of the conductive material (A).

The present invention also relates to a thermoelectric conversion element having a thermoelectric conversion film containing the above-mentioned thermoelectric conversion material and electrodes, in which the thermoelectric conversion film and the electrodes are electrically connected to each other.

The present invention makes it possible to provide a thermoelectric conversion material that combines thermoelectric conversion performance with stability under high temperature and high humidity conditions.

<Conductive Material (A)>
The conductive material (A) refers to a material that conducts electricity.
The conductive material (A) is not particularly limited as long as it is a material having electrical conductivity (carbon material, metal material, conductive polymer, etc.), and examples of the carbon material include graphite, carbon nanotubes, carbon black, graphene (including graphene nanoplates), etc. Examples of the metal material include gold, silver, copper, nickel, chromium, palladium, rhodium, ruthenium, indium, silicon, aluminum, tungsten, molybdenum, germanium, gallium, platinum, etc. Examples of the metal material include alloys of these metals and fine particles containing metals in which a core substance is coated with a substance different from the core substance, for example, fine particles such as silver-coated copper powder in which a core substance is copper and its surface is coated with silver. Other examples include metal oxides such as silver oxide, indium oxide, tin oxide, zinc oxide, ruthenium oxide, ITO (tin-doped indium oxide), AZO (aluminum-doped zinc oxide), and GZO (gallium-doped zinc oxide), as well as metal compounds other than metal oxides such as ZnSe, CdS, InP, GaN, SiC, and SiGe. Examples of conductive polymers include PEDOT/PSS (a composite of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid), polyaniline, polyacetylene, polypyrrole, polythiophene, and polyparaphenylene.

Graphite includes flake graphite such as CMX, UP-5, UP-10, UP-20, UP-35N, CSSP, CSPE, CSP, CP, CB-150, CB-100, ACP, ACP-1000, ACB-50, ACB-100, ACB-150, SP-10, SP-20, J-SP, SP-270, HOP, GR-60, LEP, F#1, F#2, manufactured by Nippon Graphite Industries Co., Ltd. Examples of the spherical natural graphite include BF-3AK, FBF, BF-15AK, CBR, CPB-6S, CPB-3, 96L, 96L-3, K-3, SC-120, SC-60, HLP, CP-150, and SB-1 manufactured by Chuetsu Graphite Co., Ltd., EC1500, EC1000, EC500, EC300, EC100, and EC50 manufactured by Ito Graphite Co., Ltd., and 10099M and PB-99 manufactured by Nishimura Graphite Co., Ltd. Examples of the spherical natural graphite include CGC-20, CGC-50, CGB-20, and CGB-50 manufactured by Nippon Graphite Co., Ltd. Examples of amorphous graphite include Blue P, AP, AOP, and P#1 manufactured by Nippon Graphite Industries Co., Ltd., and APR, K-5, AP-2000, AP-6, 300F, and 150F manufactured by Chuetsu Graphite Co., Ltd. Examples of artificial graphite include PAG-60, PAG-80, PAG-120, PAG-5, HAG-10W, and HAG-150 manufactured by Nippon Graphite Industries Co., Ltd., and G-4AK, G-6S, G-3G-150, G-30, G-80, G-50, SMF, EMF, SFF, SFF-80B, SS-100, BSP-15AK, and BSP-100AK manufactured by Chuetsu Graphite Co., Ltd. , WF-15C, and SEC Carbon's SGP-100, SGP-50, SGP-25, SGP-15, SGP-5, SGP-1, SGO-100, SGO-50, SGO-25, SGO-15, SGO-5, SGO-1, SGX-100, SGX-50, SGX-25, SGX-15, SGX-5, and SGX-1.

Examples of conductive carbon fibers and carbon nanotubes include vapor grown carbon fibers such as VGCF manufactured by Showa Denko K.K., single-walled carbon nanotubes such as EC1.5 and EC1.5-P manufactured by Meijo Nano Carbon Co., Ltd., TUBALL manufactured by OCSiAl Corporation, and ZEONANO manufactured by Zeon Nano Technology Co., Ltd., FloTube 9000, FloTube 7000, and FloTube 2000 manufactured by CNano Corporation, NC7000 manufactured by Nanocyl Corporation, and 100T and 200P manufactured by Knano Corporation.

Carbon black includes Tokai Carbon's Toka Black #4300, #4400, #4500, and #5500, Degussa's Printex L, and Colombian's Raven 7000, 5750, 5250, 5000 ULTRA III, 5000 ULTRA, Conductex SC ULTRA, and Conductex 975. ULTRA, PUERBLACK 100, 115, 205, Mitsubishi Chemical Corporation's #2350, #2400B, #2600B, #3050B, #3030B, #3230B, #3350B, #3400B, #5400B, Cabot Corporation's MONARCH 1400, 1300, 900, Vulcan XC-72R, Black Pearls 2000, Examples include furnace blacks such as Ensaco 250G, Ensaco 260G, Ensaco 350G, and Super P-Li manufactured by Timcal Corporation; ketjen blacks such as EC-300J and EC-600JD manufactured by Lion Corporation; and acetylene blacks such as Denka Black, Denka Black HS-100, and FX-35 manufactured by Denki Kagaku Kogyo Co., Ltd.

Of the above conductive materials (A), from the viewpoint of achieving both a high Seebeck coefficient and high electrical conductivity, it is preferable that the conductive material (A) contains a carbon material. The carbon material preferably contains carbon nanotubes, carbon black, and graphene, and more preferably contains carbon nanotubes. The carbon nanotubes preferably contain single-walled carbon nanotubes.

The shape of the conductive material (A) is not particularly limited, and amorphous, aggregated, scaly, microcrystalline, spherical, flake, wire, etc. may be used as appropriate. In addition, the conductive material (A) may contain only one type, or two or more types.

<Tackifier (B) Having a Cyclic Aliphatic Hydrocarbon Structure>
The tackifier (B) having a cyclic aliphatic hydrocarbon structure (hereinafter sometimes abbreviated as tackifier (B)) is not particularly limited as long as it is a tackifier containing a cyclic aliphatic hydrocarbon structure, but examples thereof include petroleum resins such as aromatic modified rosin resins, aromatic modified terpene resins, C5 petroleum resins, C9 petroleum resins, and C5C9 petroleum resins. Further examples include hydrogenated petroleum resins in which the unsaturated bonds and aromatic rings of these petroleum resins are hydrogenated. Examples of hydrogenated petroleum resins include partially hydrogenated petroleum resins in which unsaturated bonds and aromatic rings of petroleum resins are partially hydrogenated, and fully hydrogenated petroleum resins in which the unsaturated bonds and aromatic rings of petroleum resins are all hydrogenated and have no unsaturated bonds. Among the above tackifiers (B), C5 petroleum resins, C9 petroleum resins, and C5C9 petroleum resins, as well as these hydrogenated petroleum resins, are preferred.

Tackifiers are also often called "tackifiers," and therefore tackifiers having a cyclic aliphatic hydrocarbon structure are also included in tackifier (B).

C5 petroleum resin refers to petroleum resin made from the C5 fraction of petroleum (naphtha), C9 petroleum resin refers to petroleum resin made from the C9 fraction of petroleum, and C5C9 petroleum resin refers to petroleum resin made from the C5 and C9 fractions of petroleum. Examples of C5 fractions include isoprene, cyclopentadiene, and pentane. Examples of C9 fractions include styrene, vinyltoluene, and indene. Examples of C5C9 fractions include dicyclopentadiene (DCPD), which is a dimerization of cyclopentadiene, a type of C5 fraction. For example, C5 petroleum resin may be a copolymer of an olefin and a diolefin from a C5 fraction. The same applies to C9 petroleum resin and C5C9 petroleum resin.

Examples of commercially available tackifiers (B) include Clearon K110 and YS Polystar UH115 manufactured by Yasuhara Chemical Co., Ltd., Escorez 5300 and Escorez 5600 manufactured by Exxon Corp., Quintone 1325 and Quintone 1345 manufactured by Zeon Corp., Arcon P-90, Arcon P-100, Arcon P-125, Arcon P-140, Arcon M-90, Arcon M-100, Arcon M-115 and Arcon M135 manufactured by Arakawa Chemical Industries Co., Ltd., and SUKOREZ SU-100, SUKOREZ SU-120, SUKOREZ SU-130, SUKOREZ SU-210 and SUKOREZ SU-220 manufactured by KOLON Corp. Examples include SU-420, SUKOREZ SU-500, SUKOREZ SU-640, and Idemitsu Kosan's Imav S-100, Imav S-110, Imav P-100, and Imav P125.

From the viewpoint of weather resistance, the hydrogenation rate of the tackifier (B) is preferably 20% or more, more preferably 35% or more, and even more preferably 50% or more.

The softening point of the tackifier (B) is preferably 80°C or higher, more preferably 90°C or higher, and particularly preferably 100°C or higher, in terms of even better thermal stability. Also, in terms of maintaining the flexibility of the thermoelectric conversion film, it is preferably 200°C or lower. In this specification, the softening point of the tackifier resin is a value measured by the ring and ball method in accordance with JIS K2207.

The weight average molecular weight (Mw) of the tackifier (B) having a cyclic aliphatic hydrocarbon structure is preferably 100 to 200,000, more preferably 200 to 20,000, and even more preferably 300 to 5,000.

The content of the tackifier (B) having a cyclic aliphatic hydrocarbon structure is preferably 1 part by mass or more and 200 parts by mass or less, more preferably 10 parts by mass or more and 200 parts by mass or less, and particularly preferably 20 parts by mass or more and 200 parts by mass or less, per 100 parts by mass of the conductive material (A).

(solvent)
The solvent can be used as a medium when mixing the conductive material (A) and the tackifier (B). The solvent that can be used is not particularly limited as long as it can dissolve or disperse the conductive material (A) and the tackifier (B), and examples of the solvent that can be used include organic solvents and water, and two or more of them may be used in combination. Examples of the organic solvent include alcohols such as methanol, ethanol, propanol, butanol, ethylene glycol methyl ether, diethylene glycol methyl ether, terpineol, dihydroterpineol, 2,4-diethyl-1,5-pentanediol, 1,3-butylene glycol, isobornylcyclohexanol, ethylene glycol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, triethylene glycol, glycerin, polyethylene glycol, polypropylene glycol, trifluoroethanol, m-cresol, and thiodiglycol, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone, ethers such as tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, and diethylene glycol dimethyl ether, hydrocarbons such as hexane, heptane, and octane, aromatics such as benzene, toluene, xylene, and cumene, esters such as ethyl acetate and butyl acetate, and N-methylpyrrolidone. These can be appropriately selected as needed.
The solvent is preferably N-methylpyrrolidone.

The thermoelectric conversion material of the present invention preferably contains an inorganic base, an inorganic metal salt, or an organic base. This improves dispersibility when the conductive material (A) and the tackifier (B) are mixed. The inorganic base and inorganic metal salt preferably contain an alkali metal or an alkaline earth metal. Examples of the inorganic base and inorganic metal salt include chlorides, hydroxides, carbonates, nitrates, sulfates, phosphates, tungstates, vanadates, molybdates, niobates, borates, and the like of alkali metals or alkaline earth metals. Among these, chlorides, hydroxides, and carbonates of alkali metals and alkaline earth metals are preferred in terms of the ease with which cations can be supplied.

Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include calcium hydroxide, magnesium hydroxide, etc. Examples of alkali metal carbonates include lithium carbonate, lithium hydrogen carbonate, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, etc. Examples of alkaline earth metal carbonates include calcium carbonate, magnesium carbonate, etc. Among these, lithium hydroxide, sodium hydroxide, lithium carbonate, and sodium carbonate are preferred. The metal in the inorganic base or inorganic metal salt may be a transition metal.

Examples of the organic base include primary, secondary or tertiary alkylamines and compounds containing a basic nitrogen atom, as shown below.
Examples of primary alkylamines include propylamine, butylamine, isobutylamine, octylamine, 2-ethylhexylamine, laurylamine, stearylamine, oleylamine, 2-aminoethanol, 3-aminopropanol, 3-ethoxypropylamine, and 3-lauryloxypropylamine.
Examples of secondary alkylamines include dibutylamine, diisobutylamine, N-methylhexylamine, dioctylamine, distearylamine, and 2-methylaminoethanol.

Tertiary alkylamines include triethylamine, tributylamine, N,N-dimethylbutylamine, N,N-diisopropylethylamine, dimethyloctylamine, trioctylamine, dimethyldecylamine, dimethyllaurylamine, dimethylmyristylamine, dimethylpalmitylamine, dimethylstearylamine, dilaurylmonomethylamine, triethanolamine, 2-(dimethylamino)ethanol, etc. The number of carbon atoms in the alkylamines is preferably 1 to 30, more preferably 1 to 20.

Other examples of organic bases include compounds containing a basic nitrogen atom, such as 1,8-diazabicyclo[5.4.0]undecene-7 (DBU), 1,5-diazabicyclo[4.3.0]nonene-5 (DBN), 1,4-diazabicyclo[2.2.2]octane (DABCO), tri-n-butylamine, dimethylbenzylamine, monoethanolamine, imidazole, 1-methylimidazole, tetramethylammonium hydroxide, and tetraethylammonium hydroxide.

The amount of inorganic base, inorganic metal salt, and organic base to be mixed is preferably 1 to 50 parts by mass per 100 parts by mass of conductive material (A). By using such amounts, dispersibility is further improved.

When producing a dispersion containing a thermoelectric conversion material, for example, the thermoelectric conversion material, the solvent, and other components as necessary are mixed, and then dispersed using a disperser or ultrasonic waves. There are no particular limitations on the disperser, and for example, a kneader, an attritor, a ball mill, a sand mill using glass beads or zirconia beads, a media disperser such as a Scandex, an Eiger mill, a paint conditioner, or a paint shaker, or a colloid mill can be used.

<Thermoelectric conversion element>
The thermoelectric conversion element of the present invention has a thermoelectric conversion film containing the above-mentioned thermoelectric conversion material and electrodes, and the thermoelectric conversion film and the electrodes are electrically connected to each other. The thermoelectric conversion film is excellent in heat resistance and flexibility in addition to electrical conductivity and thermoelectric properties. Therefore, a high-quality thermoelectric conversion element can be easily produced.

The thermoelectric conversion film may be a film obtained by applying a thermoelectric conversion material onto a substrate. Since the thermoelectric conversion material has excellent formability, it is easy to obtain a good film by the application method. A wet film formation method is mainly used to form the thermoelectric conversion film. Specifically, there are methods using various means such as spin coating, spraying, roller coating, gravure coating, die coating, comma coating, roll coating, curtain coating, bar coating, inkjet, dispenser, silk screen printing, and flexographic printing. An appropriate method can be selected from the above methods depending on the thickness to be applied and the viscosity of the material.

The thickness of the thermoelectric conversion film is not particularly limited, but as described below, it is preferable that the film has a certain thickness or more so that a temperature difference can be generated and transmitted in the thickness direction or surface direction of the thermoelectric conversion film. From the viewpoint of thermoelectric properties, the film thickness of the thermoelectric conversion film is preferably in the range of 0.1 to 500 μm, more preferably in the range of 1 to 300 μm, and even more preferably in the range of 1 to 200 μm.

The material of the substrate is not particularly limited, but examples include nonwoven fabric, paper, plastic films such as polyethylene, polyethylene terephthalate (PET), polyethylene naphthalate, polyethersulfone, polypropylene, polyimide, polycarbonate, and cellulose triacetate, and glass. These substrates may have an aluminum deposition layer or a barrier layer on the substrate surface to prevent deterioration of the thermoelectric conversion material due to the effects of water and oxygen.

In order to improve the adhesion between the substrate and the thermoelectric conversion film, various treatments can be performed on the substrate surface. Specifically, prior to application of the thermoelectric conversion material, UV ozone treatment, corona treatment, plasma treatment, or easy-adhesion treatment may be performed.

The thermoelectric conversion film may be laminated with a substrate, or may be a free-standing film without a substrate. There are no particular limitations on how a free-standing film can be produced, but it can be obtained, for example, by forming a thermoelectric conversion film on a release sheet and then removing the release coat.

Examples of release sheets include plastic films such as polyester film, polyethylene film, polypropylene film, and polyimide film that have been subjected to a release treatment.

The thermoelectric conversion element can be constructed using techniques well known in the art, except that it is constructed using the above-mentioned thermoelectric conversion material. A more specific configuration of the thermoelectric conversion element and its manufacturing method will be described below.

In a thermoelectric conversion element, the thermoelectric conversion film and the electrodes are electrically connected. Here, "electrically connected" means that they are joined together or can conduct electricity through other components such as wires.

The material of the electrode is not particularly limited as long as it functions as an electrode, and can be selected from metals, alloys, semiconductors, etc. In one embodiment, a metal or alloy is preferred because it has high electrical conductivity and low contact resistance of the thermoelectric conversion film. For example, it is preferred that the material contains at least one selected from the group consisting of gold, silver, copper, platinum, nickel, and aluminum, and it is even more preferred that the material contains silver.

The method for forming the electrodes is not particularly limited, and they can be formed by methods such as vacuum deposition, thermocompression bonding of an electrode material foil or a film having an electrode material film, or application of a paste in which fine particles of the electrode material are dispersed. From the viewpoint of process simplicity, methods using thermocompression bonding of an electrode material foil or a film having an electrode material film, or application of a paste in which the electrode material is dispersed are preferred.

Typical examples of the structure of a thermoelectric conversion element can be broadly classified into (1) a structure in which electrodes are formed on both ends of the thermoelectric conversion film of the present invention, and (2) a structure in which the thermoelectric conversion film of the present invention is sandwiched between two electrodes, based on the positional relationship between the thermoelectric conversion film and a pair of electrodes.
The thermoelectric conversion element having the structure (1) above can be obtained, for example, by forming a thermoelectric conversion film on a substrate, and then applying silver paste to both ends of the film to form the first and second electrodes. In this way, a thermoelectric conversion element having electrodes formed on both ends of the thermoelectric conversion film can easily increase the distance between the two electrodes. Therefore, it is easy to generate a large temperature difference between the two electrodes and perform efficient thermoelectric conversion.

A thermoelectric conversion element having the structure of (2) above can be obtained, for example, by applying silver paste onto a substrate to form a first electrode, forming the thermoelectric conversion film of the present invention on top of that, and then applying silver paste on top of that to form a second electrode. In this way, a thermoelectric conversion element in which the thermoelectric conversion film of the present invention is sandwiched between two electrodes can utilize the temperature difference in the film thickness direction of the thermoelectric conversion film, i.e., in the direction perpendicular to the substrate, and can be used in a form attached to a heat source. This is preferable because it has the advantage of being able to extract heat from a wide range from the heat source. In a thermoelectric conversion element having the structure of (2) above, it is also possible to ensure a temperature difference by increasing the film thickness to increase the distance between the two electrodes.

Thermoelectric conversion elements can generate a high voltage when connected in series, and can generate a large current when connected in parallel. The thermoelectric conversion element may be a combination of two or more thermoelectric conversion elements. According to the present invention, the thermoelectric conversion element has excellent flexibility, so that it can be well installed even on a heat source that is not flat.

Thermoelectric conversion element may have a heat absorption layer or heat storage layer to efficiently transfer heat from the heat source, and may also have a heat insulation layer or heat dissipation layer to ensure a temperature difference. Furthermore, depending on the application and the amount of power required, the extracted electricity can be boosted using a boost circuit, or the extracted electrical energy can be temporarily stored in a condenser, capacitor, or secondary battery for use.

The present invention will be explained in more detail below with reference to experimental examples, but the present invention is not limited thereto. In the examples, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." In addition, "NMP" refers to N-methylpyrrolidone.

<Production of Dispersion Containing Thermoelectric Conversion Material>
[Example 1]
(Dispersion 1)
0.3 parts of SU-100 (tackifier resin "SUKOREZ SU-100" manufactured by KOLON Corporation) as the tackifier (B), 0.4 parts of SWCNT (single-walled carbon nanotubes "TUBALL" manufactured by OCSiAl Corporation) as the conductive material (A), 0.03 parts of sodium hydroxide as the inorganic base, and 79.3 parts of NMP as the solvent were weighed and mixed. Beads were further added, and the mixture was shaken with a Scandex for 4 hours, and then the beads were removed by filtration, thereby obtaining a dispersion liquid 1 of a thermoelectric conversion material.

[Examples 2 to 17, Comparative Example 1]
(Dispersions 2-17, 101)
Dispersions 2 to 17 of thermoelectric conversion material were obtained in the same manner as in Dispersion 1, except that the types and amounts of materials were changed as shown in Table 1. Dispersion 101 was obtained by weighing and mixing 0.4 parts of SWCNT (single-walled carbon nanotubes "TUBALL" manufactured by OCSiAl Corporation) as the conductive material (A), 0.03 parts of sodium hydroxide as the inorganic base, and 79.3 parts of NMP as the solvent, and further adding beads. After shaking with a Scandex for 4 hours, the mixture was filtered to remove the beads, and thus dispersion 101 of thermoelectric conversion material was obtained.

The abbreviations for the materials listed in Table 1 are as follows.
(Conductive Material (A))
A1: OCSiAl single-walled carbon nanotube "TUBALL"
A2: Multi-walled carbon nanotube "Knanos100P" manufactured by Kumho Petrochemical Co., Ltd.
A3: Graphene nanoplatelets "xGNP M5" manufactured by XG Sciences

(Tackifier (B) Having a Cycloaliphatic Hydrocarbon Structure)
B1: Tackifier resin "SUKOREZ SU-100" manufactured by KOLON (softening point 105°C, hydrogenated petroleum resin containing C5 petroleum resin, Mw 400)
B2: Tackifying resin "SUKOREZ SU-210" manufactured by KOLON (softening point 110°C, hydrogenated petroleum resin containing C5 petroleum resin, Mw 620)
B3: Tackifier resin "SUKOREZ SU-420" manufactured by KOLON (softening point 120°C, hydrogenated petroleum resin containing C5 petroleum resin and C9 petroleum resin, Mw 680)
B4: Tackifier resin "SUKOREZ SU-640" manufactured by KOLON (softening point 136°C, hydrogenated petroleum resin containing C5 petroleum resin, Mw 550)
B5: Arakawa Chemical Industries Co., Ltd. tackifier resin "Arcon P-125" (softening point 125°C, hydrogenated petroleum resin including C9 petroleum resin, Mw 700-900)
B6: Arakawa Chemical Industries Co., Ltd. tackifier resin "Arcon P-140" (softening point 140°C, hydrogenated petroleum resin containing C9 petroleum resin, Mw 900)
B7: Arakawa Chemical Industries Co., Ltd. tackifier resin "Arcon M-135" (softening point 135°C, partially hydrogenated petroleum resin including C9 petroleum resin, Mw 700-900)

<Evaluation of Thermoelectric Conversion Materials>
The obtained dispersions 1 to 17 and 101 were each applied to a 75 μm thick PET film, which was a sheet-like substrate, using an applicator, and then dried at 120° C. for 30 minutes to obtain a laminate having a thermoelectric conversion film with a thickness of 2 μm. The Seebeck coefficient, which is one of the characteristics that indicate thermoelectric conversion performance, and the stability under high temperature and high humidity were evaluated for the obtained laminates having thermoelectric conversion films (hereinafter also referred to as coating films) according to the following method. The results are shown in Table 1.

(Seebeck coefficient)
The obtained laminate was cut into a size of 25 mm × 50 mm, and the Seebeck coefficient (μV/K) at 80° C. was measured using a ZEM-3LW manufactured by Advance Riko Co., Ltd. Table 1 shows the relative absolute values of the Seebeck coefficient of each laminate to Comparative Example 1, where the absolute value of the Seebeck coefficient of Comparative Example 1 was set to 1.0.

(Stability)
The laminate immediately after preparation was left to stand for one day under conditions of 85°C and 85% humidity, and then the Seebeck coefficient was measured. The rate of change in the Seebeck coefficient relative to the Seebeck coefficient of the laminate immediately after preparation (the absolute value of the Seebeck coefficient listed in Table 1) was determined and evaluated based on the following criteria.
A: The rate of change in the Seebeck coefficient is within 10%. (Very good)
B: The rate of change in the Seebeck coefficient is greater than 10% and less than 20% (good).
C: The rate of change in the Seebeck coefficient is greater than 20% and less than 50% (usable).
D: The rate of change in the Seebeck coefficient is more than 50% (poor).

As shown in Table 1, the thermoelectric conversion material of the present invention exhibited a high Seebeck coefficient. Furthermore, it was confirmed that the material had high stability under high temperature and high humidity conditions (85°C, 85%). This is presumably because the inclusion of tackifier (B) prevents deterioration due to moisture in the air, etc.

<Manufacture of Thermoelectric Conversion Element>
The above dispersions were applied onto a PET film having a thickness of 50 μm, and five thermoelectric conversion layers having a thickness of 20 μm and a shape of 5 mm × 30 mm were prepared at intervals of 10 mm. Next, four silver circuits (electrodes) having a thickness of 10 μm and a shape of 5 mm × 33 mm were prepared using silver paste so that each thermoelectric conversion layer was connected in series, and thermoelectric conversion elements were manufactured. As the above silver paste, REXALPHA RA-FS 074 manufactured by Toyo Ink Co., Ltd. was used. When the electromotive force of each thermoelectric conversion element was measured, it was confirmed that the thermoelectric conversion elements manufactured using the dispersions manufactured in Examples 1 to 17 had a superior electromotive force to the thermoelectric conversion element manufactured using the dispersion manufactured in Comparative Example 101.

Claims (4)

A thermoelectric conversion material comprising a conductive material (A) and a tackifier (B) having a cyclic aliphatic hydrocarbon structure, wherein the conductive material (A) comprises carbon nanotubes, carbon black or graphene, and the tackifier (B) having a cyclic aliphatic hydrocarbon structure comprises a C5 petroleum resin, a C9 petroleum resin, a C5C9 petroleum resin, or a hydrogenated petroleum resin thereof . The thermoelectric conversion material according to claim 1 , wherein the softening point of the tackifier (B) is 80° C. or higher and 200° C. or lower . The thermoelectric conversion material according to claim 1 or 2, wherein the content of the tackifier (B) having a cyclic aliphatic hydrocarbon structure is 1 part by mass or more and 200 parts by mass or less per 100 parts by mass of the conductive material (A). A thermoelectric conversion element comprising a thermoelectric conversion film comprising the thermoelectric conversion material according to any one of claims 1 to 3 and electrodes, the thermoelectric conversion film and the electrodes being electrically connected to each other.