Thermoelectric element

Abstract

To provide a thermoelectric conversion element with excellent thermoelectric conversion characteristics. [Solution] The present invention provides a thermoelectric conversion element comprising a conductive thin film and a pair of electrodes connected via the conductive thin film and facing each other in the planar direction of the conductive thin film, wherein the conductive thin film contains a rod-shaped conductive material, and when the direction in which the pair of electrodes face each other is defined as the X direction and the direction perpendicular to the X direction within the plane of the conductive thin film is defined as the Y direction, the conductive thin film has a density distribution of the rod-shaped conductive material in the Y direction, and the density distribution includes a high-density region having a density 1.5 times or more than the average density.

Description

[Technical Field]

[0001] This invention relates to a thermoelectric conversion element. [Background technology]

[0002] Thermoelectric conversion elements, which can directly convert thermal energy into electrical energy, have attracted attention for some time. In these thermoelectric conversion elements, inorganic conductive materials have been used for the thermoelectric conversion material layer, such as a conductive thin film, which is responsible for the energy conversion.

[0003] In recent years, it has been proposed to use a specific inorganic conductive material in the thermoelectric material layer of a thermoelectric element in order to improve its thermoelectric conversion characteristics. For example, Patent Document 1 proposes using carbon nanotubes supported with metal nanoparticles to fabricate the thermoelectric material layer of a thermoelectric element, with the aim of enabling the thermoelectric element to exhibit sufficiently excellent thermoelectric conversion characteristics. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2017 / 122805 [Overview of the project] [Problems that the invention aims to solve]

[0005] In addition to using the aforementioned carbon nanotubes to fabricate the thermoelectric conversion material layer, there is a desire to develop thermoelectric conversion elements with superior thermoelectric conversion characteristics.

[0006] Therefore, the present invention aims to provide a thermoelectric conversion element with excellent thermoelectric conversion characteristics. [Means for solving the problem]

[0007] The inventors diligently conducted research to achieve the above objectives. They then discovered that the above problems could be solved by a thermoelectric conversion element comprising a conductive thin film containing a rod-shaped conductive material and having a predetermined density distribution, and a pair of electrodes connected via the conductive thin film and facing each other in the planar direction of the conductive thin film. Thus, the inventors completed the present invention.

[0008] In other words, the present invention aims to advantageously solve the above problems, [1] The present invention is a thermoelectric conversion element comprising a conductive thin film and a pair of electrodes connected via the conductive thin film and facing each other in the planar direction of the conductive thin film, wherein the conductive thin film contains a rod-shaped conductive material, and when the direction in which the pair of electrodes face each other is the X direction and the direction perpendicular to the X direction in the plane of the conductive thin film is the Y direction, the conductive thin film has a density distribution of the rod-shaped conductive material in the Y direction, and the density distribution includes a high-density region having a density of 1.5 times or more than the average density. The thermoelectric conversion elements described above have excellent thermoelectric conversion characteristics. In this specification, the density distribution of a rod-shaped conductive material in the Y direction of a conductive thin film can be determined by observing the conductive thin film at 10,000x magnification using a scanning electron microscope (SEM) and calculating the surface density by image analysis.

[0009] Here, we will explain in more detail the density distribution of the rod-shaped conductive material in the Y direction of the conductive thin film using Figure 1. Figure 1 is a schematic plan view showing an example of the relationship between the conductive thin film and the density distribution of the rod-shaped conductive material in the Y direction of the conductive thin film. In Figure 1, X is the direction in which a pair of electrodes (not shown) face each other, and Y is the direction perpendicular to the X direction in the plane of the conductive thin film. The conductive thin film 10 shown in Figure 1 has a density distribution 20 of the rod-shaped conductive material 11 in the Y direction. In the density distribution 20, the dotted line on the left represents the average density, and the dotted line on the right represents 1.5 times the average density. The density distribution 20 of the conductive thin film 10 shown in Figure 1 includes high-density regions 21a and 21b that have a density of 1.5 times or more the average density.

[0010] [2] In the thermoelectric conversion element described in [1] above, the average aspect ratio of the rod-shaped conductive material is 350 or more, the average orientation angle of the rod-shaped conductive material with respect to the X direction is -5° or more and +5° or less, and the rod-shaped conductive material includes rod-shaped conductive material A, whose orientation angle with respect to the X direction is in the range of -50° or more and +50° or less, and rod-shaped conductive material B, whose orientation angle with respect to the X direction is in the range of -90° or more and less than -50° and greater than +50° and less than or equal to 90°, and it is preferable that the quantity ratio of rod-shaped conductive material A to rod-shaped conductive material B is 2 / 1 or more and 4 / 1 or less. With thermoelectric conversion elements like those described above, the thermoelectric conversion characteristics can be effectively improved. In this specification, the average aspect ratio of a rod-shaped conductive material can be determined by dropping a dispersion of the rod-shaped conductive material, sufficiently diluted with a solvent, onto a silicon (Si) substrate, observing it with a scanning electron microscope (SEM), measuring the diameter and length of 100 rod-shaped conductive materials, and calculating the average value of the ratio of diameter to length (length / diameter). Furthermore, the average diameter and average length of the rod-shaped conductive material can be determined by calculating the average values ​​of the diameter and length of 100 rod-shaped conductive materials measured by observation with a scanning electron microscope (SEM). In this specification, the orientation angle of a rod-shaped conductive material with respect to the X direction can be determined by observing the thermoelectric conversion element with a scanning electron microscope (SEM) and measuring the orientation angle of each rod-shaped conductive material. Furthermore, the average orientation angle of the rod-shaped conductive material with respect to the X direction can be determined by calculating the average of the orientation angles of 100 rod-shaped conductive materials with respect to the X direction.

[0011] [3] In the thermoelectric conversion element described in [1] or [2] above, it is preferable that the ratio of the width of the high-density region is less than 30%. If the proportion of the width of the high-density region is less than the above upper limit, the thermoelectric conversion characteristics can be improved. Here, "the ratio of the width of the high-density region" refers to the ratio of the total width of the high-density region on the vertical axis (corresponding to L2a + L2b in Figure 1) to the total length of the vertical axis (corresponding to L1 in Figure 1) in the density distribution of the conductive thin film (in Figure 1, L2a + L2b / L1 × 100%).

[0012] [4] In any of the thermoelectric conversion elements [1] to [3] above, it is preferable that the density distribution includes a plurality of the high-density regions, and two adjacent high-density regions are connected by the rod-shaped conductive material. If two adjacent high-density regions are connected by a rod-shaped conductive material, the thermoelectric conversion characteristics can be improved.

[0013] [5] In any of the thermoelectric conversion elements [4] above, it is preferable that the distance between the two adjacent high-density regions is not more than twice the average length of the rod-shaped conductive material. If the distance between two adjacent high-density regions is not more than twice the average length of the rod-shaped conductive material, the thermoelectric conversion characteristics can be further improved.

[0014] [6] In any of the thermoelectric conversion elements [1] to [5] above, it is preferable that the rod-shaped conductive material is at least one selected from the group consisting of carbon fiber, transition metal compound nanofiber, and metal nanofiber. If the rod-shaped conductive material is at least one selected from the group consisting of carbon fiber, transition metal compound nanofiber, and metal nanofiber, the thermoelectric conversion characteristics can be improved. [Effect of the Invention]

[0015] According to the present invention, a thermoelectric conversion element excellent in thermoelectric conversion characteristics can be provided. [Brief Description of the Drawings]

[0016] [Figure 1] It is a schematic plan view showing an example of the relationship between a rod-shaped conductive material and the density distribution of the rod-shaped conductive material in the Y direction of the conductive thin film. [Figure 2] It is a schematic perspective view showing an example of the thermoelectric conversion element of the present invention. [Embodiments for Carrying Out the Invention]

[0017] Hereinafter, embodiments of the present invention will be described in detail. Here, the thermoelectric conversion element of the present invention can be used in a thermoelectric conversion module including a plurality of thermoelectric conversion elements. As a specific thermoelectric conversion module, for example, a thermoelectric conversion module formed by combining a plurality of thermoelectric conversion elements in a plate shape, and at least one of the plurality of thermoelectric conversion elements is the thermoelectric conversion element of the present invention.

[0018] (Thermoelectric conversion element) The thermoelectric conversion element of the present invention includes a conductive thin film and a pair of electrodes connected through the conductive thin film and facing each other in the plane direction of the conductive thin film. Further, in the thermoelectric conversion element of the present invention, the conductive thin film contains a rod-shaped conductive material. And when the direction in which the pair of electrodes face each other is the X direction and the direction orthogonal to the X direction in the plane of the conductive thin film is the Y direction, the conductive thin film has a density distribution of the rod-shaped conductive material in the Y direction, and the density distribution includes a high-density region having a density of 1.5 times or more with respect to the average density. The thermoelectric conversion element as described above has excellent thermoelectric conversion characteristics. The reason is presumed as follows.

[0019] First, a thermoelectric conversion element with excellent thermoelectric conversion characteristics usually has a high value of the power factor (PF). Here, the power factor PF is an index indicating the generated power per unit temperature change (unit: μW·m -1 ·K -2 ), and using the Seebeck coefficient S (μV·K -1 ) and the conductivity σ (S·cm -1 ), the following formula (1): PF = S 2 ×σ / 10000 ··· (1) is calculated. As can be understood from the above formula (1), when both the values of the Seebeck coefficient S and the conductivity σ increase, the value of the power factor PF of the thermoelectric conversion element increases. However, since the Seebeck coefficient S and the conductivity σ of the conductive material (thermoelectric conversion material) that can be used for forming the conductive thin film of the thermoelectric conversion element are generally in an inverse relationship, there is a limit to increasing the power factor of the thermoelectric conversion element only by selecting the conductive material. In the thermoelectric conversion element of the present invention, the conductive thin film has a density distribution of rod-shaped conductive material in the Y direction, and the density distribution includes a high-density region having a density 1.5 times or more than the average density. As a result, a good conductive path is formed in the X direction within the conductive thin film, and it is presumed that the thermoelectric conversion element has excellent thermoelectric conversion characteristics.

[0020] Here, we will illustrate the specific configuration of the thermoelectric conversion element of the present invention with reference to Figure 2, but the thermoelectric conversion element of the present invention is not limited to this. Figure 2 is a schematic perspective view showing an example of a thermoelectric conversion element of the present invention. The thermoelectric conversion element 100 in Figure 2 consists of a conductive thin film 10 and a pair of electrodes 30. The pair of electrodes 30 face each other in the planar direction of the conductive thin film 10 (a direction perpendicular to the thickness direction of the conductive thin film) and are both connected to the conductive thin film 30. The following describes conductive thin films and electrodes.

[0021] <Conductive thin film> The conductive thin film includes a rod-shaped conductive material and may optionally include a binder. Here, the conductive thin film is not particularly limited as long as it satisfies the predetermined requirements described above, and may be a single layer film made of conductive thin films or a laminated film made of two or more conductive thin films. Furthermore, the conductive thin film may optionally contain components other than the rod-shaped conductive material and binder (hereinafter sometimes referred to as "other components").

[0022] <<Rod-shaped conductive material>> A rod-shaped conductive material is a rod-shaped material (particle) that possesses conductivity, and typically has an average aspect ratio of 10 or more. Here, the average aspect ratio of the rod-shaped conductive material is preferably 350 or more, and more preferably 1000 or more. If the average aspect ratio of the rod-shaped conductive material is above the lower limit mentioned above, the thermoelectric conversion characteristics can be improved. The average aspect ratio of the rod-shaped conductive material is, for example, 100,000 or less, or it may be 10,000 or less.

[0023] The rod-shaped conductive material preferably has an average diameter of 0.5 nm or more, more preferably 1 nm or more, more preferably 15 nm or less, and more preferably 5 nm or less. If the average diameter of the rod-shaped conductive material is above the lower limit mentioned above, the breakage of the rod-shaped conductive material within the conductive thin film can be effectively suppressed. On the other hand, if the average diameter of the rod-shaped conductive material is below the above upper limit, the thermoelectric conversion characteristics can be improved.

[0024] The rod-shaped conductive material preferably has an average length of 100 nm or more, more preferably 500 nm or more, even more preferably 10 μm or more, and preferably 500 μm or less. If the average length of the rod-shaped conductive material is above the lower limit mentioned above, the thermoelectric conversion characteristics can be improved.

[0025] The content of the rod-shaped conductive material in the conductive thin film is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 95% by mass or more, and particularly preferably 100% by mass. In other words, it is particularly preferable that the conductive thin film consists only of the rod-shaped conductive material. If the proportion of rod-shaped conductive material is within the lower limit of the above range, the thermoelectric conversion characteristics can be improved.

[0026] Here, the rod-shaped conductive material is not particularly limited, but it is preferable that it be at least one selected from the group consisting of carbon fibers, transition metal compound nanofibers, and metal nanofibers, as these materials have relatively high electrical conductivity or Seebeck coefficients. Examples of carbon fibers include carbon nanotubes, PAN-based carbon fibers, pitch-based carbon fibers, graphite fibers, and conductive polymer nanofibers. Examples of transition metal compound nanofibers include MoS2, and examples of metal nanofibers include Ag and Cu. Among these, carbon fibers are preferred as the rod-shaped conductive material, and carbon nanotubes are more preferred. The following section will provide a detailed explanation of carbon nanotubes, but it should be noted that the rod-shaped conductive materials included in conductive thin films are not limited to carbon nanotubes.

[0027] Carbon nanotubes (hereinafter sometimes referred to as CNTs) are nanocarbon materials having a structure in which graphene sheets are wound into a cylindrical shape, and are broadly classified into single-walled CNTs and multi-walled CNTs based on the number of components in their outer wall. The carbon nanotubes (CNTs) used in conductive thin films are not particularly limited, and single-walled carbon nanotubes (WYNs) and / or multi-walled carbon nanotubes (WYNs) can be used, but it is preferable that the CNTs include single-walled carbon nanotubes. This is because single-walled carbon nanotubes tend to have superior thermoelectric properties (Seebeck coefficient) compared to multi-walled carbon nanotubes, thus improving thermoelectric conversion characteristics. Furthermore, as single-walled carbon nanotubes (WYNTs), CNTs can be produced in accordance with a method (SuperGrowth method; see International Publication No. 2006 / 011655) in which raw material compounds and a carrier gas are supplied to a substrate having a catalyst layer for CNT production on its surface, and when CNTs are synthesized by chemical vapor deposition (CVD), a small amount of oxidizing agent (catalyst activator) is present in the system to dramatically improve the catalytic activity of the catalyst layer (hereinafter, CNTs produced in accordance with this method may be referred to as "SGCNTs"). SGCNTs tend to have a higher Seebeck coefficient compared to other CNTs. Therefore, if CNTs include SGCNTs, the thermoelectric conversion characteristics can be improved.

[0028] Here, depending on their geometric structure (how the carbon atoms are bonded), CNTs can be of two types: metallic type (hereinafter sometimes referred to as "metallic CNTs") which have high conductivity like a metal, and semiconductor type (hereinafter sometimes referred to as "semiconductor CNTs") which have semiconductor properties. When a conductive thin film contains CNTs, the proportion of semiconductor CNTs in the total CNTs in the conductive thin film (sum of semiconductor CNTs and metallic CNTs) is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 85% by mass or more. If the ratio of the semiconducting CNTs in the total CNTs in the conductive thin film is at least the above lower limit, the thermoelectric conversion characteristics can be improved. The ratio of the semiconducting CNTs in the total CNTs in the conductive thin film is, for example, 99% by mass or less, and may be 95% by mass or less. In this specification, the ratio of the semiconducting CNTs in the total CNTs can be determined by measuring the optical absorption spectrum of the conductive thin film or CNT dispersion liquid composed of CNTs, and calculating the ratio of the semiconducting CNTs and metallic CNTs from the integrated intensity ratio of the absorption peaks respectively derived from the semiconducting CNTs and metallic CNTs.

[0029] The BET specific surface area of the CNTs is preferably 600 m 2 / g or more, more preferably 800 m 2 / g or more, and still more preferably 1000 m 2 / g or more. If the BET specific surface area of the CNTs is at least the above lower limit, the thermoelectric conversion characteristics can be improved. On the other hand, the BET specific surface area of the CNTs is, for example, 2600 m 2 / g or less, and may be 2000 m 2 / g or less. In this specification, the BET specific surface area of the CNTs is the nitrogen adsorption specific surface area measured using the BET method.

[0030] For the CNTs, the ratio of the G-band peak intensity to the D-band peak intensity (G / D ratio) in the Raman spectrum is preferably 0.5 or more, and more preferably 3.0 or more. The G / D ratio is, for example, 50 or less, and may be 30 or less.

[0031] The rod-shaped conductive materials such as CNTs contained in the conductive thin film may be those subjected to chemical treatment such as doping or those not subjected to chemical treatment. However, from the viewpoints of the manufacturing cost and durability of the rod-shaped conductive materials and the conductive thin film containing the same, those not subjected to chemical treatment are preferable.

[0032] <<Binder>> The conductive thin film may contain any binder, but piezoelectric polymers are preferred. Examples of piezoelectric polymers include ferroelectric polymers such as fluororesins and nylon resins (hereinafter sometimes referred to as "ferroelectric polymers"); and non-ferroelectric piezoelectric polymers such as electrets (polymer materials that do not conduct electricity well) made from polyimide, polyacrylonitrile, polycarbonate, polypropylene, etc., and optically active polymers (chiral materials) such as polylactic acid and polybenzyl glutamate. Specific examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), perfluoroalkoxyalkanes (PFA), copolymers of hexafluoropropylene and tetrafluoroethylene (FEP), copolymers of tetrafluoroethylene and ethylene (ETFE), polychlorotrifluoroethylene (PCTFE), and copolymers of vinylidene fluoride and trifluoroethylene (VDF-TrFE). Specific examples of nylon resins include odd-numbered nylons (for example, nylon 5, nylon 7, nylon 9, nylon 11, nylon 13).

[0033] The binder content in the conductive thin film is preferably 50% by mass or less, more preferably 20% by mass or less, even more preferably 5% by mass or less, and particularly preferably 0% by mass. In other words, it is particularly preferable that the conductive thin film does not contain a binder.

[0034] <<Other ingredients>> Other components that the conductive thin film may optionally contain include, for example, cellulose nanofibers, metal particles such as Pd and Mo, and so on.

[0035] The content of other components in the conductive thin film is preferably 5% by mass or less, preferably 1% by mass or less, more preferably 0.1% by mass or less, and particularly preferably 0% by mass. In other words, it is particularly preferable that the conductive thin film does not contain other components.

[0036] <<Properties of conductive thin films>> In a conductive thin film, the average orientation angle of the rod-shaped conductive material with respect to the X direction is preferably -5° or greater, more preferably -2° or greater, preferably +5° or less, and more preferably +2° or less. If the average orientation angle of the rod-shaped conductive material with respect to the X direction is within the above range, the thermoelectric conversion characteristics can be improved.

[0037] In a conductive thin film, if the rod-shaped conductive material includes rod-shaped conductive material A having an orientation angle with respect to the X direction in the range of -50° or more and +50° or less, and rod-shaped conductive material B having an orientation angle with respect to the X direction in the range of -90° or more and less than -50° and greater than +50° and less than or equal to 90°, the quantity ratio of rod-shaped conductive material A to rod-shaped conductive material B (rod-shaped conductive material A / rod-shaped conductive material B) is preferably 2 / 1 or more, more preferably 2.5 / 1 or more, preferably 4 / 1 or less, and more preferably 3.5 / 1 or less. If the quantity ratio of rod-shaped conductive material A to rod-shaped conductive material B is within the above range, the thermoelectric conversion characteristics can be improved.

[0038] In one embodiment of the present invention, the average aspect ratio of the rod-shaped conductive material is 350 or more, the average orientation angle of the rod-shaped conductive material with respect to the X direction is -5° or more and +5° or less, and it is preferable that the quantity ratio of rod-shaped conductive material A to rod-shaped conductive material B is 2 / 1 or more and 4 / 1 or less. With thermoelectric conversion elements like those described above, the thermoelectric conversion characteristics can be effectively improved.

[0039] In the density distribution of a conductive thin film, the proportion of the width of the high-density region is preferably less than 30%, and more preferably 25% or less. If the proportion of the width of the high-density region is less than the above upper limit, the thermoelectric conversion characteristics can be improved. In the density distribution of the conductive thin film, the proportion of the width of the high-density region is, for example, 1% or more, and may also be 10% or more.

[0040] Here, it is preferable that the density distribution of the conductive thin film includes multiple high-density regions. If the density distribution of the conductive thin film includes multiple high-density regions, multiple good conductive paths are formed in the X direction of the conductive thin film, which can further improve the thermoelectric conversion characteristics.

[0041] When the density distribution of a conductive thin film includes multiple high-density regions, it is preferable that two adjacent high-density regions are connected by a rod-shaped conductive material. If two adjacent high-density regions are connected by a rod-shaped conductive material, multiple conductive paths are connected in the X direction of the conductive thin film, which can improve the thermoelectric conversion characteristics.

[0042] When the density distribution of a conductive thin film includes multiple high-density regions, and two adjacent high-density regions are connected by a rod-shaped conductive material, the distance between the two adjacent high-density regions is preferably twice or less the average length of the rod-shaped conductive material, and more preferably 1.5 times or less. If the distance of the high-density region is less than or equal to the above upper limit of the average length of the rod-shaped conductive material, multiple conductive paths can be easily connected in the X direction of the conductive thin film, and as a result, the thermoelectric conversion characteristics can be improved. On the other hand, the distance between the two high-density regions is, for example, 0.5 times or more the average length of the rod-shaped conductive material, and may also be 1 time or more. Furthermore, if the density distribution of a conductive thin film includes two or more locations where two adjacent high-density regions are connected by a rod-shaped conductive material, then "the distance between two adjacent high-density regions" refers to the distance between the shortest high-density regions.

[0043] In the entire conductive thin film, the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction is preferably 30° or more, more preferably 40° or more, preferably 60° or less, and more preferably 50° or less. If the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction in the entire conductive thin film is within the above range, the thermoelectric conversion characteristics can be improved. In this specification, the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction in the entire conductive thin film can be calculated from the orientation angle of each rod-shaped conductive material measured by observing the thermoelectric conversion element with a scanning electron microscope (SEM).

[0044] In the vicinity of the electrodes of the conductive thin film, the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction is preferably 40° or less. If the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction near the electrode of the conductive thin film is above the above lower limit, the thermoelectric conversion characteristics can be improved. In this specification, "the vicinity of the electrode of the conductive thin film" means the region in which a rod-shaped conductive material is in contact with the electrode. In this specification, the standard deviation of the orientation angle of the rod-shaped conductive material with respect to the X direction near the electrode of the conductive thin film can be calculated from the orientation angle of each rod-shaped conductive material measured by observing the thermoelectric conversion element with a scanning electron microscope (SEM).

[0045] In the vicinity of the electrode of the conductive thin film, the proportion of semiconductor-type CNTs to the total CNTs is preferably 90% by mass or more, and more preferably 95% by mass or more. If the proportion of semiconductor-type CNTs in the vicinity of the electrodes of a conductive thin film is above the above lower limit, the thermoelectric conversion characteristics can be improved.

[0046] <<Fabrication of conductive thin films>> The conductive thin film of the thermoelectric conversion element of the present invention can be obtained, for example, by the following first to third methods for fabricating conductive thin films. In the following first to third methods for fabricating conductive thin films, the case in which CNTs are used as the rod-shaped conductive material is described, but the rod-shaped conductive material is not limited to CNTs.

[0047] [Method for fabricating the first conductive thin film] In the first method for producing a conductive thin film, a composite material of CNTs and rubber (such as silicone rubber) is first obtained. This composite material can be obtained, for example, by (1) dissolving the rubber in an organic solvent, dispersing the CNTs and mixing in an optional component, and then removing the organic solvent; (2) dispersing the rubber in a solvent in which the rubber does not dissolve, dispersing the CNTs and mixing in an optional component, and then removing the solvent; or (3) kneading the rubber with CNTs and the optional component without using a solvent. Next, by stretching the composite material in a uniaxial direction, the CNTs become weakly oriented in the stretching direction, and bundled portions (high-density portions) and unbundled portions (low-density portions) of the CNTs are randomly formed within the composite material. By fixing the rubber while maintaining this state, a desired conductive thin film can be obtained.

[0048] [Method for fabricating the second conductive thin film] In the second method for producing a conductive thin film, first, powdered CNTs are mixed with a solvent (ethanol, NMP, etc.) and stirred with an ultrasonic homogenizer or the like to obtain a CNT dispersion. Next, the obtained CNT dispersion is dropped onto a substrate with low thermal conductivity, and the dispersion is made to flow in a certain direction by methods such as air blowing or spin coating, which allows the bundles (high-density parts) of CNTs present in the dispersion to be weakly oriented. Then, by repeatedly coating the dispersion multiple times, the desired conductive thin film can be obtained.

[0049] [Method for fabricating a third conductive thin film] In the third method for fabricating a conductive thin film, first, a CNT forest (oriented aggregate of CNTs) is obtained by vertically growing CNTs on a substrate. Next, some of the CNTs in the CNT forest are picked up and pulled out in one direction to obtain an oriented sheet in which bundled and unbundled portions are arranged alternately. Then, the desired conductive thin film can be obtained by appropriately fixing this oriented sheet on a substrate. Furthermore, the density distribution of the conductive thin film can be adjusted by further etching some of the CNTs in the obtained conductive thin film using laser ablation or the like.

[0050] <Electrode> For electrodes, for example, metals, alloys, semiconductors, etc., can be used, but metals and alloys are preferred due to their high conductivity, and gold, silver, copper, aluminum, and their alloys are preferred. The two electrodes may have the same composition or different compositions.

[0051] Electrodes can be formed by methods such as vacuum deposition; thermocompression bonding of electrode material foils or films having electrode material films; and coating of a paste containing dispersed electrode material particles. Of these, from the viewpoint of process simplicity, electrode formation by thermocompression bonding of electrode material foils or films having electrode material films, or by coating of a paste containing dispersed electrode material, is preferred. [Examples]

[0052] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" used to express quantities refer to mass unless otherwise specified.

[0053] In this example, the thermoelectric conversion characteristics of the thermoelectric conversion element were confirmed by simulation using the thermoelectric random stick network (TE-RSN) method. The TE-RSN method is a simulation technique that combines the electrical network and thermal network of a random stick network (a model that calculates electrical resistance by replacing fibers such as CNTs with sticks, scattering them in a circuit network, and then replacing it with an equivalent circuit). Assuming the rod-shaped conductive material is CNT (average diameter 1.4 nm, average length 500 nm, average aspect ratio 357), the simulation method can be specifically explained as follows: First, a potential difference is applied to both ends of the conductive thin film of the RSN model, and it is converted into an electrical network composed of two types of electrical resistances: the electrical resistance within the CNTs and the electrical resistance at the contact points between the CNTs, and the electrical conductivity of the thin film is calculated. Next, a temperature difference is applied to both ends of the conductive thin film of the RSN model, and it is converted into a thermal network composed of two types of thermal resistances: the thermal resistance within the CNTs and the thermal resistance at the contact points between the CNTs, and the temperature difference at all nodes is calculated. After that, an electrical network is constructed by adding the thermoelectric voltage, which is the product of the temperature difference and the Seebeck coefficient (both the Seebeck coefficient of the material itself and the contact Seebeck coefficient between conductive materials), and by solving these, the Seebeck coefficient of the entire conductive thin film can be determined (J. Kobayashi and T. Yamamoto Jpn. J. Appl. Phys. 61 095001 (2022).).

[0054] Furthermore, by combining the TE-RSN method with the optimization technique, the genetic algorithm (NSGA-II), we obtained conductive thin films of the RSN model with specific properties. Specifically, we obtained conductive thin films of the RSN model with specific properties by the following methods (1) to (5). (1) An initial population consisting of N random CNT thin film structures is created using the TE-RSN method. (2) From the N individuals, N / 2 individuals with high performance (higher electrical conductivity and Seebeck coefficient compared to the other individuals) are selected. This selected group is designated as the parent population. (3) Two individuals are selected from the parent population, and half of the CNTs are copied to each to create a new CNT thin film (this operation is called crossover). (4) Each time the CNT thin film is copied, there is a 0.1% chance of replacing it with a random CNT (this operation is called mutation). By performing the mutation operation, the bias of the population can be reduced and the model can be prevented from moving towards a local minimum. (5) Repeat steps (2) to (4) for the desired number of times to optimize.

[0055] Table 1 below shows the results of the thermoelectric conversion characteristics obtained from the simulation. Note that in Table 1, "inf" indicates that the rod-shaped conductive material is oriented completely randomly and the orientation angle standard deviation is infinite. "Rod-shaped conductive material A" refers to a rod-shaped conductive material whose orientation angle with respect to the X direction is within the range of -50° to +50°. "Rod-shaped conductive material B" refers to a rod-shaped conductive material whose orientation angle with respect to the X direction is within the range of -90° or more and less than -50°, and greater than +50° and less than or equal to 90°.

[0056] [Table 1]

[0057] As is clear from Table 1, Examples 1, 3-7 that satisfy the requirements of the present invention exhibit excellent thermoelectric conversion characteristics (especially power factor PF). [Industrial applicability]

[0058] According to the present invention, a thermoelectric conversion element with excellent thermoelectric conversion characteristics can be provided. [Explanation of Symbols]

[0059] 10 Conductive Thin Film 11. Rod-shaped conductive material 20 Density distribution 21 High density area 30 electrodes 100 Thermoelectric conversion elements

Claims

1. Conductive thin film and A pair of electrodes facing each other in the planar direction of the conductive thin film, connected via the conductive thin film, A thermoelectric conversion element comprising, The conductive thin film includes a rod-shaped conductive material, When the direction in which the pair of electrodes face each other is defined as the X direction, and the direction perpendicular to the X direction within the plane of the conductive thin film is defined as the Y direction, The conductive thin film has a density distribution of the rod-shaped conductive material in the Y direction, A thermoelectric conversion element in which the density distribution includes a high-density region having a density 1.5 times or more than the average density.

2. The average aspect ratio of the aforementioned rod-shaped conductive material is 350 or more. The average orientation angle of the rod-shaped conductive material with respect to the X direction is -5° or more and +5° or less. The aforementioned rod-shaped conductive material, The material includes a rod-shaped conductive material A whose orientation angle with respect to the X direction is within the range of -50° to +50°, and a rod-shaped conductive material B whose orientation angle with respect to the X direction is within the range of -90° to less than -50° and greater than +50° and less than or equal to 90°. The thermoelectric conversion element according to claim 1, wherein the quantity ratio of the rod-shaped conductive material A to the rod-shaped conductive material B is 2 / 1 or more and 4 / 1 or less.

3. The thermoelectric conversion element according to claim 1, wherein the ratio of the width of the high-density region is less than 30%.

4. The density distribution includes a plurality of the high-density regions, The thermoelectric conversion element according to claim 1, wherein two adjacent high-density regions are connected by the rod-shaped conductive material.

5. The thermoelectric conversion element according to claim 4, wherein the distance between the two adjacent high-density regions is less than or equal to twice the average length of the rod-shaped conductive material.

6. The thermoelectric element according to any one of claims 1 to 5, wherein the rod-shaped conductive material is at least one selected from the group consisting of carbon fibers, transition metal compound nanofibers, and metal nanofibers.

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