P-type material for thermoelectric conversion, its manufacturing method, and thermoelectric conversion element

Abstract

Disclosed is a production method for a p-type material for thermoelectric conversion that includes a step for doping a thermoelectric conversion material that contains carbon nanotubes and a conductive resin with a dopant that includes at least one type of compound selected from the group that consists of sulfinates and dithionites. Said step includes an impregnation step in which at least a portion of the thermoelectric conversion material is impregnated with a dopant solution that contains the dopant and a solvent and a solvent removal step in which the solvent is removed. The dopant concentration of the dopant solution is 0.3–8.0 mM.

Description

[Technical Field]

[0001] The present invention relates to a p-type material for thermoelectric conversion, a method for producing the same, and a thermoelectric conversion element. [Background technology]

[0002] Thermoelectric conversion is a technology that uses the Seebeck effect to directly convert heat into electricity, and is attracting attention as an energy recovery technology that converts waste heat generated when using fossil fuels into electricity.

[0003] Conventionally, inorganic materials have been mainly studied as thermoelectric conversion materials. However, due to problems such as difficulty in installation on curved surfaces, the use of rare or toxic elements, and unsuitability for installation over large areas, organic materials have attracted attention as thermoelectric conversion materials (e.g., Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-098299 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-332638 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-323758 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to drive electronic devices such as IoT devices, it is important to increase the open-circuit voltage of the energy harvester. However, organic materials used as thermoelectric conversion materials have a low Seebeck coefficient, making it difficult to ensure a sufficient open-circuit voltage.

[0006] Therefore, a main object of the present invention is to provide a method for producing a p-type material for thermoelectric conversion that can increase the Seebeck coefficient of the thermoelectric conversion material in the positive direction. [Means for solving the problem]

[0007] The present inventors conducted extensive research to solve the above problems and discovered that doping a specific thermoelectric conversion material with a specific dopant increases the Seebeck coefficient in the positive direction, thereby further improving thermoelectric conversion performance, and thus completed the present invention.

[0008] That is, the present invention provides a method for producing a p-type material for thermoelectric conversion according to [1] and [2], a p-type material for thermoelectric conversion according to [3] and [4], and a thermoelectric conversion element according to [5]. [1] A method for producing a p-type material for thermoelectric conversion, comprising a step of doping a thermoelectric conversion material containing carbon nanotubes and a conductive resin with a dopant, wherein the dopant contains at least one compound selected from the group consisting of sulfinates and dithionites, the step comprising: an impregnation step of impregnating at least a portion of the thermoelectric conversion material with a dopant solution containing the dopant and a solvent; and a solvent removal step of removing the solvent, wherein the concentration of the dopant in the dopant solution is 0.3 to 8.0 mM. [2] The method according to [1], wherein the dopant comprises at least one compound selected from the group consisting of sodium methanesulfinate, sodium hydroxymethanesulfinate, and sodium dithionite. [3] A p-type material for thermoelectric conversion obtained by doping a thermoelectric conversion material containing carbon nanotubes and a conductive resin with a dopant, wherein the dopant contains at least one compound selected from the group consisting of sulfinates and dithionites. [4] The p-type material for thermoelectric conversion according to [3], wherein the dopant contains at least one compound selected from the group consisting of sodium methanesulfinate, sodium hydroxymethanesulfinate, and sodium dithionite. [5] A thermoelectric conversion element comprising the p-type material for thermoelectric conversion according to [3] or [4]. [Effects of the Invention]

[0009] The present invention provides a method for producing a p-type material for thermoelectric conversion that can increase the Seebeck coefficient of the thermoelectric conversion material in a positive direction. The present invention also provides a p-type material for thermoelectric conversion obtained by such a production method. Furthermore, the present invention also provides a thermoelectric conversion element using such a p-type material for thermoelectric conversion. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0011] In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this specification, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In numerical ranges described in this specification, the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0012] In this specification, unless otherwise specified, the materials exemplified below may be used alone or in combination of two or more within the range that meets the conditions. When multiple substances corresponding to each component are present, the content of each component means the total amount of the multiple substances unless otherwise specified.

[0013] In this specification, whether a thermoelectric conversion material exhibits p-type conductivity or n-type conductivity can be determined by the positive or negative value of the Seebeck coefficient. When the Seebeck coefficient is positive, the thermoelectric conversion material exhibits p-type conductivity, and a material having such properties is called a p-type material for thermoelectric conversion. When the Seebeck coefficient is negative, the thermoelectric conversion material exhibits n-type conductivity, and a material having such properties is called an n-type material for thermoelectric conversion. The Seebeck coefficient can be measured, for example, by the measurement method in the Examples described below, and the polarity of the thermoelectric conversion material can be determined from the positive or negative value of the measured value.

[0014] <P-type materials for thermoelectric conversion> In one embodiment, the p-type material for thermoelectric conversion is obtained by doping a thermoelectric conversion material with a dopant. Here, the thermoelectric conversion material before doping may be either a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion. In one embodiment, the thermoelectric conversion material before doping may be a p-type material for thermoelectric conversion. The p-type material for thermoelectric conversion of this embodiment (the p-type material for thermoelectric conversion after doping) may have a Seebeck coefficient that is greater in the positive direction than the thermoelectric conversion material before doping with the dopant (the p-type material for thermoelectric conversion or the n-type material for thermoelectric conversion).

[0015] (Thermoelectric conversion materials) The thermoelectric conversion material contains carbon nanotubes and a conductive resin.

[0016] The carbon nanotubes may be single-walled, double-walled, or multi-walled (three or more walls), and are preferably single-walled from the viewpoint of further improving the Seebeck coefficient and electrical conductivity of the thermoelectric conversion material (p-type material for thermoelectric conversion or n-type material for thermoelectric conversion).

[0017] The carbon nanotubes preferably include single-walled carbon nanotubes. The content of single-walled carbon nanotubes relative to the total amount of carbon nanotubes is preferably 25% by mass or more, more preferably 50% by mass or more, and may be, for example, 100% by mass.

[0018] The diameter of the single-walled carbon nanotube is not particularly limited, but may be, for example, 20 nm or less, preferably 10 nm or less, more preferably 3 nm or less. The lower limit of the diameter of the single-walled carbon nanotube may be, for example, 0.4 nm or more, or 0.5 nm or more.

[0019] In this specification, the diameter of a single-walled carbon nanotube is determined by Raman spectroscopy to be 100 to 300 cm -1 The wave number of the peak that appears in -1 )) can be calculated using the formula diameter (nm) = 248 / ω.

[0020] The G / D ratio in laser Raman spectroscopy is known as a method for evaluating single-walled carbon nanotubes. In this embodiment, the G / D ratio of single-walled carbon nanotubes in laser Raman spectroscopy at a wavelength of 532 nm is preferably 10 or more, more preferably 20 or more. By using such single-walled carbon nanotubes, a thermoelectric conversion material with even better electrical conductivity tends to be obtained. The upper limit of the G / D ratio is not particularly limited, and may be, for example, 500 or less, or 300 or less.

[0021] The carbon nanotube content may be, for example, 20% by mass or more, preferably 30% by mass or more, and more preferably 40% by mass or more, based on the total amount of solids in the thermoelectric conversion material. The carbon nanotube content may be, for example, 99% by mass or less, preferably 95% by mass or less, and more preferably 90% by mass or less, based on the total amount of solids in the thermoelectric conversion material.

[0022] The conductive resin is not particularly limited, and known conductive resins used in thermoelectric conversion materials (especially p-type materials for thermoelectric conversion) can be used. Examples of conductive resins include polyaniline-based conductive polymers, polythiophene-based conductive polymers, polypyrrole-based conductive polymers, polyacetylene-based conductive polymers, polyphenylene-based conductive polymers, and polyphenylene vinylene-based conductive polymers. An example of a polythiophene-based conductive polymer is poly(3,4-ethylenedioxythiophene).

[0023] The conductive resin may be a conductive resin composed of poly(3,4-ethylenedioxythiophene) (hereinafter sometimes referred to as "PEDOT") and an electron acceptor. Use of such a conductive resin tends to increase the electrical conductivity of the thermoelectric conversion material.

[0024] Examples of the electron acceptor include polystyrene sulfonic acid, polyvinyl sulfonic acid, poly(meth)acrylic acid, polyvinyl sulfonic acid, toluenesulfonic acid, dodecylbenzenesulfonic acid, camphorsulfonic acid, bis(2-ethylhexyl) sulfosuccinate, chlorine, bromine, iodine, phosphorus pentafluoride, arsenic pentafluoride, boron trifluoride, hydrogen chloride, sulfuric acid, nitric acid, tetrafluoroboric acid, perchloric acid, iron(III) chloride, tetracyanoquinodimethane, etc. From the viewpoint of further improving the electrical conductivity of the thermoelectric conversion material, the electron acceptor may be polystyrene sulfonic acid (hereinafter sometimes referred to as "PSS").

[0025] The content of the conductive resin may be, for example, 1% by mass or more, preferably 5% by mass or more, and more preferably 10% by mass or more, based on the total amount of solids in the thermoelectric conversion material. The content of the conductive resin may be, for example, 80% by mass or less, preferably 70% by mass or less, and more preferably 60% by mass or less, based on the total amount of solids in the thermoelectric conversion material.

[0026] The Seebeck coefficient of the thermoelectric conversion material (coating film) may be, for example, less than 28 μV / K. From the viewpoint of improving the output of the thermoelectric conversion module, the Seebeck coefficient of the thermoelectric conversion film may be, for example, 10 μV / K or more, preferably 12 μV / K or more, and more preferably 14 μV / K or more. The Seebeck coefficient refers to, for example, the magnitude of the electromotive force per 1°C of temperature difference when a temperature difference is applied to the thermoelectric conversion film.

[0027] (dopant) As used herein, a dopant is intended to mean a substance that changes the Seebeck coefficient of the material into which it is doped.

[0028] In this specification, "changing the Seebeck coefficient" means increasing the value of the Seebeck coefficient in a positive direction, and more specifically, means increasing the value of the Seebeck coefficient in a positive direction of a thermoelectric conversion material having p-type conductivity and a positive Seebeck coefficient (p-type material for thermoelectric conversion) or a thermoelectric conversion material having n-type conductivity and a negative Seebeck coefficient (n-type material for thermoelectric conversion).

[0029] The dopant includes at least one compound selected from the group consisting of sulfinates and dithionites. The compound may be a component that acts as a reducing agent for the thermoelectric conversion material (particularly, an electron acceptor). By doping such a dopant into a thermoelectric conversion material (a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion), it is possible to increase the Seebeck coefficient of the material in the positive direction.

[0030] The reason why such an effect is achieved is not entirely clear, but the present inventors believe as follows: Doping a dopant into a thermoelectric conversion material (a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion) tends to decrease the number of electron acceptor holes (positive holes), thereby decreasing the carrier concentration. The increase in the Seebeck coefficient value in the positive direction is thought to be due to a change in the Fermi energy level of the material caused by a decrease in the carrier concentration.

[0031] Sulfinates are salts of monovalent or divalent cations and sulfinate anions. Examples of sulfinates include compounds represented by the following formula (1) and compounds represented by the following formula (2).

[0032] [ka]

[0033] In formula (1), R 1 represents a monovalent organic group, and M + indicates a monovalent cation.

[0034] R 1The monovalent organic group represented by may be, for example, an alkyl group, a fluoroalkyl group, a hydroxyalkyl group, an aryl group, a monovalent heterocyclic group, or an aralkyl group.

[0035] The number of carbon atoms in the alkyl group may be, for example, 1 to 20, 1 to 10, 1 to 6, or 1 to 3. The alkyl group may be any of a linear alkyl group, a branched alkyl group, and a cyclic alkyl group. Examples of linear alkyl groups include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, and an n-hexyl group. Examples of branched alkyl groups include an isopropyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group. Examples of cyclic alkyl groups include a cyclopentyl group and a cyclohexyl group. The alkyl group may be, for example, a methyl group.

[0036] The fluoroalkyl group refers to an alkyl group in which at least a portion of the hydrogen atoms of the alkyl groups exemplified above are substituted with fluorine atoms. Specific examples of the fluoroalkyl group include a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a 2-fluoroethyl group, a 1,2-difluoroethyl group, a 2,2-difluoroethyl group, and a perfluoroethyl group. The fluoroalkyl group may be, for example, a trifluoromethyl group.

[0037] The hydroxyalkyl group refers to an alkyl group in which at least a portion of the hydrogen atoms of the alkyl groups exemplified above have been substituted with hydroxy groups. Specific examples of the hydroxyalkyl group include a hydroxymethyl group and a hydroxyethyl group. The hydroxyalkyl group may be, for example, a hydroxymethyl group.

[0038] An aryl group refers to a group obtained by removing one hydrogen atom directly bonded to a carbon atom constituting the ring from an aromatic hydrocarbon. Aromatic hydrocarbons include benzene, benzene-containing fused-ring hydrocarbon compounds, and compounds in which two or more directly bonded hydrocarbons selected from the group consisting of benzene and benzene-containing fused-ring hydrocarbon compounds are selected. Specific examples of aryl groups include phenyl, 1-naphthyl, 2-naphthyl, 1-anthracenyl, 2-anthracenyl, and 9-anthracenyl. Some of the hydrogen atoms in the aryl group may be substituted with alkyl groups, fluorine atoms, etc.

[0039] A monovalent heterocyclic group refers to a group obtained by removing one hydrogen atom directly bonded to a carbon atom constituting the ring from a heterocyclic compound such as furan, thiophene, pyrrole, tetrahydrofuran, tetrahydrothiophene, or pyrrolidine. Specific examples of monovalent heterocyclic groups include a furyl group, a thienyl group, a pyrrolyl group, a tetrahydrofuryl group, a tetrahydrothienyl group, and a pyrrolidinyl group. Some of the hydrogen atoms in the monovalent heterocyclic group may be substituted with an alkyl group, a fluorine atom, or the like.

[0040] The aralkyl group refers to an alkyl group in which at least a portion of the hydrogen atoms of the alkyl group exemplified above has been substituted with an aryl group exemplified above. Examples of the aralkyl group include a benzyl group, a (2-methylphenyl)methyl group, a (3-methylphenyl)methyl group, a (4-methylphenyl)methyl group, a (2,4-dimethylphenyl)methyl group, an (ethylphenyl)methyl group, and a naphthylmethyl group.

[0041] Examples of the monovalent cation include alkali metal ions such as lithium ion, sodium ion, potassium ion, etc. The monovalent cation may be, for example, a sodium ion.

[0042] Specific examples of the compound represented by formula (1) include sodium methanesulfinate, sodium hydroxymethanesulfinate (Rongalite), sodium triflate, sodium 1,1-difluoroethanesulfinate, sodium cyclohexanesulfinate, and sodium tetrahydrofuransulfinate.

[0043] [ka]

[0044] In formula (2), R 2 and R 3 each independently represents a monovalent organic group; M 2+ indicates a divalent cation.

[0045] R 2 and R 3 The monovalent organic group represented by may be, for example, an alkyl group, a fluoroalkyl group, a hydroxyalkyl group, an aryl group, a monovalent heterocyclic group, or an aralkyl group.

[0046] R 2 and R 3 The alkyl group, fluoroalkyl group, hydroxyalkyl group, aryl group, monovalent heterocyclic group, or aralkyl group represented by the formula: 1 Examples of the alkyl group include the alkyl group, fluoroalkyl group, hydroxyalkyl group, aryl group, monovalent heterocyclic group, and aralkyl group represented by the formula (I). The alkyl group may be, for example, a methyl group. The hydroxyalkyl group may be, for example, a hydroxymethyl group. The fluoroalkyl group may be, for example, a trifluoromethyl group. R 2 and R 3 may be the same or different, but are preferably the same.

[0047] Examples of divalent cations include alkaline earth metal ions such as calcium ion and magnesium ion; and ions of Group 12 elements such as zinc ion.

[0048] Specific examples of the compound represented by formula (2) include calcium bis(hydroxymethanesulfinate), zinc bis(hydroxymethanesulfinate) (safolin), zinc bis(trifluoromethanesulfinate), zinc bis(difluoromethanesulfinate), and zinc bis(benzenesulfinate).

[0049] Dithionites are salts of monovalent or divalent cations and dithionite anions. Examples of monovalent cations include alkali metal ions. Examples of divalent cations include alkaline earth metal ions and ions of Group 12 elements.

[0050] Specific examples of dithionites include sodium dithionite and zinc dithionite.

[0051] From the viewpoint of more sufficiently increasing the Seebeck coefficient of the thermoelectric conversion material in the positive direction, the dopant may contain at least one compound selected from the group consisting of sodium methanesulfinate, sodium hydroxymethanesulfinate, and sodium dithionite.

[0052] The dopant may contain, as necessary, a component other than at least one compound selected from the group consisting of sulfinates and dithionites. Such a component is not particularly limited as long as it does not inhibit the function of the dopant, and examples thereof include solvents such as water and organic solvents.

[0053] There are no particular limitations on the method for producing a p-type material for thermoelectric conversion obtained by doping a thermoelectric conversion material with a predetermined dopant, but it can be produced, for example, by the following method.

[0054] <Method of manufacturing p-type materials for thermoelectric conversion> The manufacturing method of one embodiment includes a step of doping a thermoelectric conversion material (a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion) containing carbon nanotubes and a conductive resin with a dopant (hereinafter, sometimes referred to as a "doping step"). The doping step includes an impregnation step and a solvent removal step.

[0055] (Impregnation process) This step is a step of impregnating at least a portion of the thermoelectric conversion material with a dopant solution containing a dopant and a solvent (first solvent).

[0056] The dopant solution can be obtained by mixing the dopant and a first solvent, such as water, acetonitrile, acetone, methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, dimethyl sulfoxide (DMSO), N-methylpyrrolidone, N,N-dimethylformamide, or N,N-dimethylacetamide.

[0057] The first solvent may contain dimethyl sulfoxide (DMSO). When the first solvent contains dimethyl sulfoxide (DMSO), insulating components present in small amounts in the thermoelectric conversion material are easily removed, which tends to further improve the carrier mobility of the thermoelectric conversion material. Furthermore, the first solvent may contain water in addition to dimethyl sulfoxide (DMSO).

[0058] The dopant concentration in the dopant solution is 0.3 to 8.0 mM. When the dopant concentration in the dopant solution is 0.3 mM or more, it is possible to sufficiently increase the Seebeck coefficient of the thermoelectric conversion material in the positive direction. On the other hand, according to the studies of the present inventors, it has been found that if the dopant concentration in the dopant solution is too high, the thermoelectric conversion material (p-type material for thermoelectric conversion) becomes n-type. When the dopant concentration in the dopant solution is 8.0 mM or less, it is possible to sufficiently increase the Seebeck coefficient of the thermoelectric conversion material in the positive direction while suppressing the conversion of the thermoelectric conversion material (p-type material for thermoelectric conversion) to n-type. The concentration of the dopant in the dopant solution may be 0.5 mM or more, 0.7 mM or more, 1.0 mM or more, 1.5 mM or more, 2.0 mM or more, or 2.5 mM or more, and may be 7.8 mM or less, 7.6 mM or less, or 7.5 mM or less.

[0059] The dopant solution may contain, in addition to the dopant and the first solvent, other components, such as a binder resin, an antioxidant, a thickener, a surfactant, etc., as long as the effects of the present invention are not impaired.

[0060] The method for impregnating a thermoelectric conversion material with a dopant solution is not particularly limited, and examples thereof include a method of immersing a thermoelectric conversion material in a dopant solution, a method of adding a dopant solution to a thermoelectric conversion material, etc. Among these, the method for impregnating a thermoelectric conversion material with a dopant solution may be a method of immersing a thermoelectric conversion material in a dopant solution, since this method allows the thermoelectric conversion material to be easily impregnated with a dopant solution while maintaining the shape of the thermoelectric conversion material.

[0061] In one embodiment, the method for immersing a thermoelectric conversion material in a dopant solution may be a method comprising the steps of: applying a mixed liquid containing carbon nanotubes, a conductive resin, a dispersant, and a second solvent onto a support; and removing the second solvent from the applied mixed liquid to form a coating film containing a thermoelectric conversion material (a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion); and immersing the obtained coating film in a dopant solution.

[0062] The mixed solution can be obtained by mixing carbon nanotubes, a conductive resin, a dispersant, and a second solvent. Examples of the dispersant include a resin and a surfactant. Examples of the second solvent include the same solvent as the first solvent of the dopant solution. The second solvent may include, for example, water. The mixed solution may contain components other than the carbon nanotubes, the conductive resin, the dispersant, and the second solvent.

[0063] Examples of the support include resin substrates such as polyimide, polyethylene terephthalate, polyethylene naphthalate, polyethylene isophthalate, polybutylene terephthalate, polycarbonate, polyether ether ketone, polyphenyl sulfide, and polysulfone; and inorganic substrates such as glass, copper, silver, gold, and aluminum. Among these, the support may be a resin substrate selected from the group consisting of polyimide, polyethylene terephthalate, and polyethylene naphthalate, because the resulting thermoelectric conversion material exhibits good flexibility.

[0064] The method for applying the mixed liquid is not particularly limited, and can be appropriately selected from, for example, a doctor blade method, a casting method, a dip coating method, a spray coating method, a spin coating method, and the like.

[0065] The method for removing the second solvent from the applied mixed liquid is not particularly limited, and can be appropriately selected from known methods such as heating and reducing pressure depending on the type of the second solvent, and multiple methods may be combined.

[0066] In this way, a coating film containing a thermoelectric conversion material (a p-type material for thermoelectric conversion or an n-type material for thermoelectric conversion) can be obtained. A portion of the second solvent may remain in the coating film.

[0067] The resulting coating film is then immersed in a dopant solution. The time for immersing the coating film in the dopant solution may be, for example, 30 seconds to 72 hours, or may be 1 minute to 24 hours. When the time for impregnation with the dopant solution is within this range, productivity of the p-type material for thermoelectric conversion tends to be excellent.

[0068] In the impregnation step, a material (coating) impregnated with the dopant solution is obtained, and the material (coating) is subjected to a solvent removal step. Note that, of the first solvent used in the impregnation step, any solvent other than that impregnated or attached to the material may be removed at the end of the impregnation step. For example, when the material is immersed in the dopant solution in the impregnation step, the material may be removed from the dopant solution and subjected to the solvent removal step.

[0069] (Solvent removal process) This step is a step of removing the first solvent from the thermoelectric conversion material impregnated with the dopant solution. In this step, it is not necessary to remove all of the first solvent, and the first solvent may remain to an extent that the material sufficiently functions as a p-type material for thermoelectric conversion.

[0070] The solvent removal step may be, for example, a step of removing the first solvent by natural drying, or a step of removing the first solvent by carrying out a heat treatment, a reduced pressure treatment, or the like.

[0071] The solvent removal step may include a step of heat-treating the thermoelectric conversion material impregnated with the first solvent. In the step of heat-treating the thermoelectric conversion material, the first solvent, whose compatibility with the conductive resin has been improved by heating, is thought to cause the conductive resin in the material to flow, thereby filling the voids between the carbon nanotubes and forming a denser structure. Therefore, by including such a step in the solvent removal step, a more significant improvement in the thermoelectric conversion properties of the thermoelectric conversion material can be expected.

[0072] The heat treatment temperature is not particularly limited and may be, for example, 40°C or higher, preferably 50°C or higher, and more preferably 60°C or higher. Increasing the heat treatment temperature tends to increase the Seebeck coefficient of the thermoelectric conversion material in a positive direction. The heat treatment temperature may be, for example, 250°C or lower, preferably 225°C or lower, and more preferably 200°C or lower. Decreasing the heat treatment temperature tends to improve the electrical conductivity of the thermoelectric conversion material. The Seebeck coefficient and electrical conductivity of thermoelectric conversion materials tend to vary depending on the heat treatment temperature. Therefore, the heat treatment temperature can be appropriately selected, for example, within the above ranges, taking into account the balance between the Seebeck coefficient and electrical conductivity values.

[0073] The heat treatment time is not particularly limited and may be, for example, 1 minute or more, preferably 10 minutes or more, and may be 12 hours or less, preferably 6 hours or less.

[0074] The purpose of the heat treatment is not necessarily to remove the first solvent, and the solvent removal step may be carried out after the heat treatment.

[0075] In this way, a thermoelectric conversion film containing a p-type material for thermoelectric conversion can be obtained, which has a Seebeck coefficient value that is larger in the positive direction than the thermoelectric conversion material (p-type material for thermoelectric conversion or n-type material for thermoelectric conversion) before being doped with a dopant.

[0076] From the viewpoint of improving the output of the thermoelectric conversion module, the Seebeck coefficient of the p-type material for thermoelectric conversion (thermoelectric conversion film) may be, for example, 28 μV / K or more, preferably 30 μV / K or more, and more preferably 32 μV / K or more. The upper limit of the Seebeck coefficient of the p-type material for thermoelectric conversion is not particularly limited, but may be, for example, 50 μV / K or less.

[0077] The shape of the p-type material for thermoelectric conversion is not particularly limited. For example, as described above, a thermoelectric conversion film containing the p-type material for thermoelectric conversion may be obtained as a film supported on a support by doping a predetermined dopant into a coating film formed by forming the thermoelectric conversion material on a support.

[0078] The thickness of the thermoelectric conversion film containing the p-type material for thermoelectric conversion is preferably 100 nm to 1 mm, more preferably 200 nm to 800 μm, and even more preferably 300 nm to 600 μm, from the viewpoint of obtaining appropriate electrical resistance and excellent flexibility.

[0079] The p-type material for thermoelectric conversion of this embodiment can be suitably used as a p-type material for thermoelectric conversion elements, and can also be suitably used for applications such as Peltier elements and temperature sensors.

[0080] <Thermoelectric conversion element> A thermoelectric conversion element of one embodiment includes the above-described p-type material for thermoelectric conversion.

[0081] The thermoelectric conversion element of this embodiment may include, for example, two conductive substrates and a thermoelectric conversion film disposed between the conductive substrates and containing the p-type material for thermoelectric conversion.

[0082] The conductive substrate may be any of various known conductive substrates, and may contain at least one conductive material selected from the group consisting of tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), and stainless steel, since these materials are resistant to corrosion and elution.

[0083] The two conductive substrates can also be referred to as a first electrode and a second electrode, respectively.

[0084] The thermoelectric conversion element of this embodiment can be manufactured, for example, by a method including a lamination step of arranging a thermoelectric conversion film containing a p-type material for thermoelectric conversion on a conductive substrate.

[0085] The thermoelectric conversion element of this embodiment can be manufactured by a method including, for example, a first lamination step of arranging a resin layer containing a thermoelectric conversion material (p-type material for thermoelectric conversion) on one conductive substrate; an impregnation step of impregnating the resin layer with a dopant solution containing a dopant and a first solvent; a solvent removal step of removing the first solvent to obtain a thermoelectric conversion film containing the p-type material for thermoelectric conversion; and a second lamination step of laminating the other conductive substrate on the thermoelectric conversion film.

[0086] The thermoelectric conversion element may further include components other than those described above. For example, the thermoelectric conversion element may further include a sealant for sealing the thermoelectric conversion film, wiring for electrically connecting the thermoelectric conversion elements to each other or for extracting power to an external circuit, a heat insulating material or a thermally conductive material for controlling the thermal conductivity of the thermoelectric conversion element, etc.

[0087] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. [Example]

[0088] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, the following experiments were carried out at room temperature (23°C).

[0089] Example 1 <Preparation of mixed solution> A mixture was prepared so that the carbon nanotube content and PEDOT / PSS content were 1 (50% by mass) to 1 (50% by mass) based on the total amount of the coating (solid content of the thermoelectric conversion material). Specifically, 50 g of a carbon nanotube dispersion (concentration: 0.2% by mass, aqueous dispersion, single-walled carbon nanotubes (diameter: 0.9 to 1.7 nm, G / D ratio: 42.1)) was concentrated by vacuum drawing until the carbon nanotube concentration was 0.4% by mass. Next, 8.3 g of a PEDOT / PSS aqueous dispersion ("Clevious® PH1000" manufactured by Heraeus, solid content concentration: 1.2% by mass) and the concentrated carbon nanotube dispersion were thoroughly stirred with a Three-One Motor ("PM203" manufactured by AS ONE Corporation) for 30 minutes. Subsequently, the mixture was thoroughly degassed (treatment time: 3 minutes) using a planetary centrifugal mixer (Thinky Corporation's "Awatori Rentaro ARE-310") to prepare a mixed liquid.

[0090] <Coating film formation> 5 g of the mixed solution was placed on a polyimide film (25 μm thick) that had been washed with acetone, coated using a doctor blade with a gap of 2.7 mm, and then placed in a fan dryer set at 60°C and dried for 2 hours. This formed a coating film on the polyimide film. A high-precision Digimatic Micrometer (MDH-25MB, manufactured by Mitutoyo Corporation) was used to measure the film thickness. The film thickness of the area where the coating film was formed and the area where only the polyimide film was present were measured, and the difference between the two was calculated as the coating film thickness. The coating film thickness was 46.2 μm.

[0091] <Preparation of dopant solution> An aqueous solution of sodium hydroxymethanesulfinate was prepared by dissolving 0.154 g of sodium hydroxymethanesulfinate dihydrate in 10 mL of water. A dopant solution was prepared by mixing 0.015 mL of the aqueous solution of sodium hydroxymethanesulfinate with 2.985 mL of dimethyl sulfoxide (DMSO). The concentration of sodium hydroxymethanesulfinate in the dopant solution was 0.5 mM (mmol / L).

[0092] <Immersion treatment in dopant solution> The coating film on the polyimide film obtained above was cut to a size of 1.0 cm x 1.0 cm. The cut coating film was immersed in the dopant solution obtained above at room temperature for 120 minutes. Thereafter, the solvent was removed by placing it on a hot plate set at 60°C for 120 minutes, thereby obtaining a thermoelectric conversion film of Example 1. The thickness of the obtained thermoelectric conversion film was measured in the same manner as the thickness of the above coating film. The thickness of the thermoelectric conversion film of Example 1 was 15.4 μm.

[0093] <Measurement of thermoelectric conversion properties> The Seebeck coefficient (S), electrical conductivity (σ), and power factor (PF) of the thermoelectric conversion film of Example 1 were measured by the following methods. The Seebeck coefficient of the thermoelectric conversion film was 32.0 μV / K, the electrical conductivity was 2720 S / cm, and the power factor was 279 μW / (mK 2 ) was.

[0094] Seebeck coefficient The Seebeck coefficient of the thermoelectric conversion film was measured by the following method. A test specimen was prepared by cutting the thermoelectric conversion film placed on the polyimide film into a 10mm x 10mm piece. One end of the long side of the test specimen was cooled (initial temperature of the test specimen -5°C = 18°C), and the other end of the long side of the test specimen was heated (initial temperature of the test specimen +5°C = 28°C). The temperature difference and voltage generated at both ends were measured using an alumel-chromel thermocouple, and the Seebeck coefficient was calculated from the slope of the temperature difference and voltage. Electrical conductivity The electrical conductivity was measured by the four-probe method (using a resistivity meter Loresta GP MCP-T610 manufactured by Nitto Seiko Analytech Co., Ltd. (formerly Mitsubishi Chemical Analytech Co., Ltd.), probe: QPP). Power Factor The power factor (PF) was calculated using the following formula: PF=S 2 σ (S: Seebeck coefficient (V / K), σ: electrical conductivity (S / m))

[0095] Example 2 Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 1.0 mM, a thermoelectric conversion film of Example 2 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 2 are shown in Table 1.

[0096] Example 3 Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 2.5 mM, a thermoelectric conversion film of Example 3 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 3 are shown in Table 1.

[0097] Example 4 Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 5.0 mM, the thermoelectric conversion film of Example 4 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 4 are shown in Table 1.

[0098] Example 5 Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 7.5 mM, the thermoelectric conversion film of Example 5 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 5 are shown in Table 1.

[0099] Example 6 Except for changing the dopant solution containing sodium methanesulfinate at a concentration of 1.0 mM from the dopant solution containing sodium hydroxymethanesulfinate at a concentration of 0.5 mM, a thermoelectric conversion film of Example 6 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 6 are shown in Table 2.

[0100] Example 7 Except for changing the dopant solution containing sodium hydroxymethanesulfinate at a concentration of 0.5 mM to a dopant solution containing sodium dithionite at a concentration of 1.0 mM, a thermoelectric conversion film of Example 7 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Example 7 are shown in Table 2.

[0101] (Comparative Example 1) Except for changing the dopant solution to DMSO (100% by mass) containing no dopant, a thermoelectric conversion film of Comparative Example 1 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Comparative Example 1 are shown in Table 1.

[0102] (Comparative Example 2) Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 0.1 mM, a thermoelectric conversion film of Comparative Example 2 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Comparative Example 2 are shown in Table 1.

[0103] (Comparative Example 3) Except for changing the concentration of sodium hydroxymethanesulfinate in the dopant solution from 0.5 mM to 10.0 mM, a thermoelectric conversion film of Comparative Example 3 was obtained in the same manner as in Example 1. The film thickness and thermoelectric conversion characteristics of the thermoelectric conversion film of Comparative Example 3 are shown in Table 1.

[0104] [Table 1]

[0105] [Table 2]

[0106] As shown in Table 1, the thermoelectric conversion films of Examples 1 to 5, which used sodium hydroxymethanesulfinate as a dopant and were immersed in dopant solutions of predetermined concentrations, exhibited positive Seebeck coefficients and demonstrated performance as p-type materials for thermoelectric conversion. Furthermore, the thermoelectric conversion films of Examples 1 to 5 had higher Seebeck coefficients than the thermoelectric conversion film of Comparative Example 1, which was not doped with a dopant, and the thermoelectric conversion film of Comparative Example 2, which was immersed in a dopant solution with a lower concentration than the predetermined one. The thermoelectric conversion film of Comparative Example 3, which was immersed in a dopant solution with a higher concentration than the predetermined one, exhibited a negative Seebeck coefficient and did not demonstrate performance as a p-type material for thermoelectric conversion.

[0107] Furthermore, as shown in Table 2, the Seebeck coefficients of the thermoelectric conversion film of Example 6, which used sodium methanesulfinate as the dopant, and the thermoelectric conversion film of Example 7, which used sodium dithionite as the dopant, were comparable to or higher than the Seebeck coefficient of the thermoelectric conversion film of Example 2, which used sodium hydroxymethanesulfinate as the dopant.

[0108] From the above, it was confirmed that the method for producing a p-type material for thermoelectric conversion of the present invention can increase the Seebeck coefficient of the thermoelectric conversion material in the positive direction.

Claims

1. doping a thermoelectric conversion material containing carbon nanotubes and a conductive resin with a dopant; A method for producing a p-type material for thermoelectric conversion, wherein the dopant contains at least one compound selected from the group consisting of sulfinates and dithionites, The process comprises: an impregnation step of impregnating at least a portion of the thermoelectric conversion material with a dopant solution containing the dopant and a solvent; a solvent removal step of removing the solvent, The manufacturing method, wherein the concentration of the dopant in the dopant solution is 0.3 to 8.0 mM.

2. 2. The method according to claim 1, wherein the dopant comprises at least one compound selected from the group consisting of sodium methanesulfinate, sodium hydroxymethanesulfinate, and sodium dithionite.

3. A p-type material for thermoelectric conversion obtained by doping a thermoelectric conversion material containing carbon nanotubes and a conductive resin with a dopant, The p-type material for thermoelectric conversion, wherein the dopant comprises at least one compound selected from the group consisting of sulfinates and dithionites.

4. 4. The p-type material for thermoelectric conversion according to claim 3, wherein the dopant comprises at least one compound selected from the group consisting of sodium methanesulfinate, sodium hydroxymethanesulfinate, and sodium dithionite.

5. A thermoelectric conversion element comprising the p-type thermoelectric conversion material according to claim 3 or 4.

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