A thermally stable n-type doping agent, thermoelectric conversion element, thermoelectric module, and n-type doping method for nanocarbon materials.

Abstract

To provide a nanocarbon material that is of a controlled n-type and has thermostability at high temperatures, and an n-type doping agent and an n-type doping method therefor.SOLUTION: The present invention discloses an n-type doping method that uses an n-type doping agent containing at least one compound selected from amidine and guanidine compounds to make the n-type doping agent contact and permeate the surface of a nanocarbon material. The method can provide a nanocarbon material that can retain n-type properties for a long time even in the air and at high temperatures, allowing stable operation of molecular elements.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an n-type nanocarbon material used in electronic devices and energy devices.

Background Art

[0002] Electronic devices and energy devices using molecular compounds are expected because they have advantages such as low cost, light weight, flexibility, large area, and low resource constraints. Among them, nanocarbon materials such as carbon nanotubes (CNT) and graphene have high carrier mobility and mechanical strength, so there is high expectation for practical application. On the other hand, in order to operate devices such as transistors, solar cells, thermoelectric conversion elements, temperature sensors, and infrared sensors with high efficiency, both p-type and n-type materials are required. Generally, a material in which the majority carriers are holes is used as a p-type material, and a material in which the majority carriers are electrons is used as an n-type material. Nanocarbon materials are usually known to exhibit p-type in the atmosphere, and in order to construct high-performance devices, it is necessary to convert the polarity to n-type that forms a pair with p-type. That is, an operation of changing the majority carriers from holes to electrons is essential. Thus, the technology of controlling the carriers of a material is called doping, and the substance used for this purpose is called a doping agent or a dopant. In recent years, a search for a doping agent for converting the polarity of nanocarbon materials from p-type to n-type, that is, an n-type doping agent, has been conducted.

[0003] As an n-type doping agent for carbon nanotubes, it has been reported that by using nicotinamide or reduced nicotinamide mononucleotide (NMN), nicotinamide adenine dinucleotide (NAD), or nicotinamide adenine dinucleotide phosphate (NADP) and bringing them into contact with carbon nanotubes for n-type doping, long-term doping stability in air can be ensured (Patent Document 1). It has also been reported that ferrocene is used as an n-type doping agent (Non-Patent Document 1).

[0004] The aforementioned devices inevitably generate heat due to current application and light reception during use. Furthermore, in thermoelectric conversion applications, they are installed on a heat source to convert waste heat into energy and are exposed to high temperatures during device operation. Similarly, temperature sensors are also expected to be used to measure high temperatures. Therefore, for n-type nanocarbon materials obtained by applying n-type doping agents, stability in air, i.e., mere storage stability, is insufficient; stability against heat that may be experienced during device operation must also be considered. However, n-type nanocarbon materials have the challenge of being less stable with respect to heat compared to p-type materials. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-292714 [Non-patent literature]

[0006] [Non-Patent Document 1] Chem. Asian J. (2016) vol.11, p.2423 [Overview of the project] [Problems that the invention aims to solve]

[0007] Known n-type doping agents include alkali metals, polyethyleneimines, viologens, and nicotinamides, but they have had problems with stability at high temperatures. In order to make n-type controlled nanocarbon materials practical, the present invention aims to ensure not only storage stability but also thermal stability under device operation.

[0008] Furthermore, while nanocarbon materials used in devices may be used in single forms such as a single carbon nanotube or a single graphene sheet, they are often used in aggregates of several nanocarbon materials, or in assemblies such as membranes or threads. The object of this invention is to provide a doping method that uniformly disperses these n-type doping agents even inside nanocarbon material assemblies when they are used. [Means for solving the problem]

[0009] Molecules containing atoms with lone pairs of electrons have the potential to behave as Lewis bases. In this invention, molecules containing nitrogen, particularly cyclic molecules, are used as atoms with lone pairs of electrons. By adopting a cyclic structure, the steric position of the lone pair of electrons within the molecule can be defined, allowing it to be used as an effective base (n-type doping agent) for nanocarbons. Furthermore, by using a substance that contains both π and σ bonds within the molecule, the structure of the conjugate acid after electron donation to the nanocarbons can be resonance-stabilized, thereby stabilizing the negative charge of the n-type nanocarbon material, which is the conjugate base. Furthermore, by employing a doping method that uniformly disperses these n-type doping agents, adsorption or binding of the n-type doping agents can occur throughout the entire nanocarbon material, effectively enhancing its heat resistance. As a result of diligent research from this perspective, we confirmed remarkably high n-type stability in air and at high temperatures for nanocarbon materials doped with amidine and guanidine compounds, particularly guanidine compounds having a cyclic structure, thus completing the present invention.

[0010] The present invention relates to the following n-type composite materials (1) or (6) to (13), and n-type doping agents (2) to (5). (1) An n-type composite material comprising at least one compound selected from amidine compounds and guanidine compounds, and a nanocarbon material or a nanocarbon material aggregate, characterized in that it is stable in air or heat resistant. (2) At least one compound selected from amidine compounds and guanidine compounds, which is an n-type doping agent for nanocarbon materials used in the composite material described in (1) above. (3) The n-type doping agent according to (2) above, wherein the amidine compound and the guanidine compound are cyclic compounds. (4) The n-type doping agent according to (2) or (3) above, wherein the amidine compound is diazabicycloundecene (1,8-diazabicyclo[5.4.0]undec-7-ene) or diazabicyclononene (1,5-diazabicyclo[4.3.0]non-5-ene). (5) The n-type doping agent according to (2) or (3) above, wherein the guanidine compound is guanidine, 1,1,3,3-tetramethylguanidine, triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene), or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene). (6) The n-type composite material described in (1) above, wherein the nanocarbon material is selected from carbon nanotubes, graphene, or graphite. (7) The n-type composite material according to (1) above, wherein the aggregate of nanocarbon materials is carbon nanotubes, graphene, graphite, or aggregates of two or more of these. (8) The n-type composite material according to (1) or (7) above, wherein the aggregate of nanocarbon materials is in the form of fibers, membranes, threads, or cloth. (9) An n-type composite material according to (1) or any of (6) to (8), wherein at least one compound selected from the amidine compound and the guanidine compound is physically in contact with or chemically bonded to the surface of the nanocarbon material. (10) An n-type composite material according to any one of (1) or (6) to (9) above, comprising a carbon nanotube in which at least one compound selected from the amidine compound and the guanidine compound is encapsulated in its internal space. (11) The n-type composite material according to any one of (1) or (7) to (10), wherein at least one compound selected from the amidine compound and the guanidine compound is dispersed inside an aggregate of nanocarbon material. (12) The n-type composite material according to any one of (1) or (7) to (10), wherein at least one compound selected from the amidine compound and the guanidine compound is deposited on the outer surface of the aggregate of nanocarbon material. (13) An n-type composite material according to any one of (1) or (6) to (12) above, wherein at least one compound selected from the amidine compound and the guanidine compound is present in an amount of 0.1% or more relative to the weight of the nanocarbon material or the aggregate of nanocarbon materials.

[0011] Furthermore, the present invention relates to a method for forming n-type composite materials as described in (14) to (19) below, or a method for n-type doping of nanocarbon materials and nanocarbon material assemblies, and to a thermoelectric conversion element, temperature sensor, infrared sensor, field-effect transistor, PN junction diode, CMOS, or solar cell as described in (20). (14) A method for forming an n-type composite material according to (1) or any of (6) to (13) above, or a method for n-type doping of a nanocarbon material and a nanocarbon material aggregate. (15) A method for forming an n-type composite material according to (14), or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein at least one compound selected from the amidine compound and the guanidine compound is dispersed by contacting the surface of the nanocarbon material, depositing it on the outer surface of the nanocarbon material assembly, or penetrating or embedding it inside. (16) A method for forming an n-type composite material according to (14) or (15) above, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, comprising immersing the nanocarbon material or an aggregate of nanocarbon materials in a solution of at least one compound selected from the amidine compound and the guanidine compound dissolved in a solvent, or dropping the solution onto these materials, thereby causing contact, deposition, or penetration and dispersion into the interior of the material aggregate. (17) A method for forming an n-type composite material according to (14) or (15) above, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein at least one compound selected from the amidine compound and the guanidine compound is sublimated or evaporated to deposit on the surface of the nanocarbon material or nanocarbon material assembly, or penetrated into the interior of the material assembly and dispersed. (18) A method for forming an n-type composite material according to any one of claims 14 to 16, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein the n-type properties of the n-type composite material according to (1) above are controlled by the type of solvent used to dissolve at least one compound selected from the amidine compound and the guanidine compound. (19) A method for forming an n-type composite material according to (14) above, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein at least one compound selected from the amidine compound and the guanidine compound is composited by encapsulating it in the internal space of a carbon nanotube. (20) Thermoelectric elements, temperature sensors, infrared sensors, field-effect transistors, PN junction diodes, CMOSs, or solar cells manufactured using the n-type composite material described in (1) or any of (6) through (13) above. [Effects of the Invention]

[0012] The n-type polarity of the n-type nanocarbon material obtained in the present invention persists over a long period even at a high temperature of 100 °C, and thus an effect of enabling long-term stable operation of a device can be expected. In addition, since a doping agent using only elements such as carbon and nitrogen that are ubiquitously present can be used, there are few resource constraints and an n-type material can be manufactured at low cost. The n-type nanocarbon material of the present invention can maintain n-type characteristics over a long period even under high temperature conditions in the atmosphere, enabling stable operation of molecular devices.

Brief Description of the Drawings

[0013] [Figure 1] Measurement data of the Seebeck coefficient (EC1.5 CNT) (a)(b) undoped film, (c)(d) TBD-doped film. The dashed lines in (a)(c) indicate the temperature difference applied to the EC1.5 CNT film, and the solid lines indicate the generated voltage. The points in (b)(d) are the measured values, and the straight lines indicate the approximate straight lines thereof. [Figure 2] Measurement data of the Seebeck coefficient (CNT fiber) (a)(b) undoped fiber, (c)(d) TBD-doped fiber. The dashed lines in (a)(c) indicate the temperature difference applied to the CNT fiber, and the solid lines indicate the change over time of the generated voltage. (b)(d) show the relationship between the generated voltage and the temperature difference applied to the CNT fiber. [Figure 3] (a) Evaluation of thermal stability ■: TMG, □: DBU, ●: TBD, ◇: Me-TBD-doped EC1.5 CNT film, change over time of the Seebeck coefficient (heating at 100 °C, measurement at 25 °C), (b) Example of evaluation of thermal stability (comparative example) Thin ●: ferrocene, ●: NADH-doped EC1.5 CNT film, change over time of the Seebeck coefficient (heating at 100 °C, measurement at 25 °C) [Figure 4] Evaluation result of the thermal stability of TBD doping for CNT fiber. Change over time of the Seebeck coefficient (heating at 100 °C, measurement at 25 °C) [Figure 5] Solvent dependence of the Seebeck coefficient and conductivity of an EC1.5 CNT film doped with a TBD doping solution. [Figure 6](a) Prototype example of a multilayer thermoelectric module (b) Example of evaluation of the thermoelectric power of a multilayer thermoelectric module; solid line represents voltage, dashed line represents temperature difference. (c) Example of evaluation of the output power of a multilayer thermoelectric module [Figure 7] (a) Method for fabricating a fiber thermoelectric module (b) Example of a prototype fiber thermoelectric module (c) Example of evaluation of the thermoelectric power of a fiber thermoelectric module, showing the relationship between the applied temperature difference and the generated voltage. (d) Example of evaluation of the output power of a fiber thermoelectric module [Modes for carrying out the invention]

[0014] The present invention uses at least one compound selected from amidine compounds and guanidine compounds as an n-type doping agent. The nanocarbon material to be n-type doped with these compounds is carbon nanotubes, graphene, or graphite, and the aggregate of the nanocarbon material is carbon nanotubes, graphene, graphite, or aggregates of two or more of these. In this invention, n-type doping refers to converting the polarity of a nanocarbon material from p-type to n-type, and the compound used for this purpose is called an n-type doping agent.

[0015] Examples of the amidine compounds of the present invention include diazabicycloundecene (1,8-diazabicyclo[5.4.0]undec-7-ene) (DBU) or diazabicyclononene (1,5-diazabicyclo[4.3.0]non-5-ene) (DBN), which have the structure shown in (1) below. [ka]

[0016] Examples of guanidine compounds of the present invention include guanidine, 1,1,3,3-tetramethylguanidine (TMG) having the structure shown in (2) below, triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) (TBD) having the structure shown in (3) below, or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene) (Me-TBD) having the structure shown in (4) below. [ka] [ka] [ka]

[0017] Amidine and guanidine compounds containing nitrogen as an atom with a lone pair of electrons, especially when cyclic, can define the steric position of the lone pair in the compound and can be used as effective bases (n-type doping reagents) for nanocarbons. Furthermore, by using materials that contain both π and σ bonds in their molecules, the structure of the conjugate acid after electron donation to the nanocarbon can be resonance-stabilized, thereby stabilizing the negative charge of the n-type nanocarbon material that is the conjugate base.

[0018] By doping nanocarbon materials with the n-type doping agent of the present invention, the heat resistance of the doping function of the nanocarbon materials can be improved. The n-type doping method of the present invention is not particularly limited as long as it uses the n-type doping agent of the present invention to perform n-type doping.

[0019] Furthermore, when bringing an n-type doping agent into contact with a nanocarbon material or a nanocarbon material aggregate, ensuring uniform dispersion of the n-type doping agent allows for the adsorption or binding of the n-type doping agent throughout the entire nanocarbon material, effectively enhancing its heat resistance. To ensure uniform dispersion of the n-type doping agent, it is possible to sublimate or evaporate the n-type doping agent and coat the nanocarbon material at a molecular level before contact. Alternatively, a solution of the n-type doping agent dissolved in a solvent can be used. Polar solvents such as water, alcohol, acetone, and N,N-dimethylformamide can be used as solvents, and the n-type properties of the nanocarbon material or nanocarbon material aggregate can be controlled by the type of solvent used to dissolve the n-type doping agent.

[0020] When using an n-type doping agent solution, the nanocarbon material or nanocarbon material aggregate is immersed in the solution, or the solution is dropped onto these materials, thereby bringing the nanocarbon material into contact with the surface of the nanocarbon material by adsorption, bonding, or fixation, or by penetrating or embedding it between the layers of the nanocarbon material aggregate to achieve uniform dispersion. Furthermore, by sublimating or evaporating the n-type doping agent and bringing it into contact with the nanocarbon material or nanocarbon material aggregate, the n-type doping agent can be coated onto the nanocarbon material at the molecular level, adsorbing, bonding, or fixing to the surface of the nanocarbon material, and penetrating between the layers of the nanocarbon material aggregate. In either case, the amount of n-type doping agent used affects the n-type doping state of the nanocarbon material, and the more n-type doping agent is used, the more advanced the n-type doping state of the nanocarbon material becomes.

[0021] Nanocarbon materials or nanocarbon material aggregates doped using the above-described n-type doping method for nanocarbon materials can maintain n-type properties over long periods of time, even in the atmosphere and under high-temperature conditions, as shown in the following examples. There are no particular restrictions on the form of the n-type doped nanocarbon material. For example, it may be a single CNT or a single graphene sheet, an aggregate of two or more CNTs or two or more graphene sheets stacked together, graphite, or a composite or laminate of two or more of these nanocarbon materials, or an assembly of films or threads. There are also no restrictions on the area or thickness of these nanocarbon material structures or assemblies. They may be self-supporting or supported by a substrate or the like.

[0022] These n-type doped nanocarbon materials or nanocarbon material assemblies can be applied to a variety of fields, such as field-effect transistors, PN junction diodes, solar cells, CMOS devices, thermoelectric conversion elements, temperature sensors, and infrared sensors. [Examples]

[0023] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. Unless otherwise specified, room temperature is 25°C. [Examples]

[0024] [Manufacturing of n-type doped nanocarbon materials] (1) Reagents The following type n doping agents were used: Amidines • 1,8-Diazabicyclo[5.4.0]-7-Undecene (Compound (1): Hereinafter referred to as "DBU") Guanidines • 1,1,3,3-tetramethylguanidine (compound (2): hereinafter referred to as "TMG") • 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (Compound (3): Hereinafter referred to as "TBD") 7-methyl-1,5,7-triazabicyclo[4.4.0]deca-5-ene (compound (4): hereafter referred to as "Me-TBD")

[0025] Furthermore, the nanocarbon material used for n-type doping primarily consisted of carbon nanotubes (CNTs) (manufactured by Meijo Nanocarbon Co., Ltd., catalog number EC1.5, synthesis method: enhanced Direct Injection Pyrolytic Synthesis (eDIPS)). In addition, to demonstrate the broad applicability of n-type doping agents, we also used CNTs (NanoIntegris: HiPCO CNTs) and graphite, which are synthesized using different methods.

[0026] (2) CNT film fabrication method and doping method The nanocarbon material film was fabricated as follows. 1.5-2.0 mg of nanocarbon material, 20 μL of polyoxyethylene lauryl ether (surfactant), and 15 mL of deionized water were all placed in a 50 mL screw-top bottle, and the nanocarbon material was dispersed using an ultrasonic homogenizer. The resulting dispersion was filtered by suction onto a polytetrafluoroethylene membrane filter with a pore size of 0.2 μm, depositing the nanocarbon material onto the filter surface to form a film. This film was peeled from the membrane filter with tweezers, washed with acetone and water, and dried in air at room temperature. The resulting film had a diameter of 17 mm and a thickness of approximately 10 μm.

[0027] The resulting film was used in the next doping step. The above-mentioned n-type doping agents were dissolved in a solvent at room temperature in 30 mL screw-cap bottles. The concentration of the doping agent is not particularly limited, but here, the concentration of the doping agent was 71 mmol / L, dissolved in the solvent N,N-dimethylformamide (DMF). The nanocarbon material film was immersed in this solution at room temperature to allow the n-type doping agent to permeate and uniformly disperse within the nanocarbon material film, and to effectively adsorb the n-type doping agent throughout the entire nanocarbon material assembly. After 5 minutes, it was removed and dried in a vacuum for 30 minutes to complete the doping process.

[0028] (3) Method for producing CNT fibers (spinning method) and doping method An aqueous dispersion containing 0.2 wt% single-walled carbon nanotube (Meijo eDIPS), a nanocarbon material, and 1.8 wt% sodium cholate (surfactant) was obtained. This aqueous dispersion was injected into isopropyl alcohol at a rate of 0.2 mL / min through a 1.24 mm inner diameter nozzle using a syringe pump, and solidified. The fibers obtained in this process were washed by immersing them in water twice for one day each time, and then dried in the air at 25°C. The resulting fibers had a cross-sectional area of ​​approximately 0.0009–0.002 mm². 2 The fibers were 20-200 cm long. Furthermore, the conductivity of the fibers measured using the DC four-terminal method was 3000-6000 S / cm.

[0029] The resulting fibers were used in the next doping process. The above-mentioned n-type doping agents were dissolved in a solvent at room temperature in 30 mL screw-cap bottles. The concentration of the doping agents was not particularly limited, but here, the doping agent TBD was dissolved in DMF, the solvent, at a concentration of 700 mmol / L. To permeate and uniformly disperse the n-type doping agent into the nanocarbon material fibers and to effectively adsorb the n-type doping agent throughout the entire nanocarbon material assembly, the nanocarbon material fibers were immersed in this solution at room temperature for more than one hour. After that, the fibers were removed from the solution and dried in a vacuum for 30 minutes to complete the doping process. [Examples]

[0030] [Evaluation of n-type doping] We determined whether doped nanocarbon materials were p-type or n-type by using measurements based on the Seebeck effect. The Seebeck effect is a phenomenon in which a temperature difference applied to a substance is converted into a voltage. The voltage generated per unit temperature difference applied to a sample is called the Seebeck coefficient and is defined as follows. Seebeck coefficient (S) = -generated voltage (ΔV) / temperature difference (ΔT) The polarity of a material can be determined by the sign of its Seebeck coefficient; a positive sign indicates p-type, while a negative sign indicates n-type.

[0031] The Seebeck coefficient was measured in air at a temperature of approximately 25°C. Measurements were taken not only for the doped film but also for the undoped film (hereinafter referred to as the "undoped film") for comparison. The measurement results are shown in Figure 1. Figure 1(a) is a graph plotting the temperature difference (ΔT: right vertical axis, dashed line) applied to the EC1.5 CNT undoped film against the voltage generated from the film (-ΔV: left vertical axis, solid line) against time. It can be seen that the voltage increases immediately as the temperature difference increases. Figure 1(b) shows the result of plotting the voltage against the temperature difference observed in Figure 1(a), and the straight line indicates that the Seebeck effect is occurring. Furthermore, since the slope of the line has a positive sign, it can be determined that the undoped film is p-type. Figures 1(c) and (d) show the results of similar measurements performed on EC1.5 CNT films doped with TBD. The slope of the line has turned negative, indicating that n-type CNTs were obtained by doping with TBD.

[0032] Table 1 shows representative Seebeck coefficients obtained by doping for doping agents other than TBD. For all substances, the Seebeck coefficient of doped EC1.5 CNTs turned negative, indicating successful control to n-type. The conductivity values ​​listed in Table 1 showed a significant increase with doping compared to undoped films. Generally, conductivity is known to be proportional to carrier concentration. Furthermore, the absolute value of the Seebeck coefficient is reduced compared to the undoped film. Generally, the Seebeck coefficient is known to decrease with carrier concentration. From these findings, it can be seen that the doping agent in this invention is a compound effective for n-type doping, capable of injecting electrons at a concentration higher than the hole concentration of the original p-type nanocarbon material. Moreover, since the immersion time in the n-type doping agent solution is only 5 minutes, it also has the effect of shortening the process time.

[0033] Furthermore, when an EC1.5 CNT film was immersed in DMF alone for the same amount of time and then vacuum-dried, the Seebeck coefficient did not turn negative, and it showed almost the same Seebeck coefficient and conductivity as the undoped sample. Therefore, the effect of converting CNTs to n-type is due to the doping agent, and not to the reaction of solvent molecules with CNTs. [Table 1]

[0034] The nanocarbon materials to which these doping agents can be applied are not limited to the EC1.5 CNTs described above, but may also be CNTs obtained by other synthesis methods, or they may be graphite or graphene. Table 2 shows, as an example, the Seebeck coefficient and conductivity results when HiPCO CNTs and graphite were film-formed in the same manner as EC1.5 CNTs and doped with TBD in the same manner. Note that only the graphite was measured in its deposited state on the filter. It was confirmed that in all nanocarbon materials, the sign of the Seebeck coefficient turned negative, resulting in n-type properties. [Table 2]

[0035] Next, the Seebeck coefficients of the fibers before and after TBD doping were measured in air at a temperature of approximately 25°C. The measurement results are shown in Figure 2. The CNT fibers used in the experiment in Figure 2 are the CNT yarns produced in Example 1. Figure 2(a) is a graph plotting the temperature difference (ΔT: right vertical axis, dashed line) applied to the undoped CNT fiber against the voltage generated from the film (-ΔV: left vertical axis, solid line) against time. As the temperature difference increases, the voltage increases. Figure 2(b) is the result of plotting the voltage against the temperature difference observed in Figure 2(a), and since a straight line is obtained, it can be seen that the Seebeck effect is also exhibited in the fiber. Furthermore, since the slope of the line has a positive sign, the undoped fiber is p-type. Figures 2(c) and (d) show the results of similar measurements performed on TBD-doped CNT fibers. The slope of the line has turned negative, indicating that TBD doping yielded n-type CNTs in the fibers as well.

[0036] The Seebeck coefficients of CNT fibers before and after doping were measured and compared. As shown in Table 3, and similar to the results explained in Figure 2 above, the sign changed from +30 μV / K before doping to -20 μV / K after doping, indicating that the fibers were n-type CNTs. [Table 3] [Examples]

[0037] [Stability evaluation under high temperature conditions in the atmosphere] n-type carbon nanotube (CNT) films obtained through doping were heated at 100°C in an incubator. To evaluate stability in air, heating was performed without reducing the pressure in the incubator or introducing an inert gas. After heating for a predetermined time, the CNT films were removed from the incubator, and the Seebeck coefficient was measured at 25°C. After measurement, the films were returned to the incubator and heating at 100°C was resumed. This process was repeated to evaluate thermal stability. The results are shown in Figure 3(a). The horizontal axis represents the net heating time in the incubator, and the vertical axis represents the Seebeck coefficient after heating. For this measurement, EC1.5 CNT membranes doped with DMF solutions of TBD (71 mmol / L), TMG (760 mmol / L), and DBU (540 mmol / L) were used.

[0038] The duration of the n-type properties of doped nanocarbon materials was determined by the time the Seebeck coefficient remained negative. In the case of TMG doping, the negative Seebeck coefficient was maintained for approximately 128 hours, and for DBU doping, for approximately 696 hours. Further heating beyond this time would cause the material to revert to the p-type form. In other words, at 100°C, TMG has the effect of maintaining n-type properties for approximately 128 hours, and DBU for approximately 696 hours. Furthermore, in the case of TBD doping, the material remained remarkably stable as an n-type substance for over 3000 hours (approximately 4 months or more) without reverting to the p-type form. Thus, the n-type doping agent in the present invention not only controls the properties of nanocarbon materials to be n-type, but also has the effect of maintaining n-type polarity for hundreds of hours to thousands of hours or more, even at high temperatures of 100°C. In particular, it can exhibit remarkably high stability in doping agents with cyclic structures such as DBU and TBD.

[0039] As described above, the n-type nanocarbon material produced by the present invention has extremely good heat resistance and can therefore be usefully used as a component in devices that require operation under high-temperature conditions (200°C or lower, more preferably 150°C or lower, and even more preferably 120°C or lower), such as thermoelectric conversion elements and temperature sensors. Furthermore, Figure 3(b) shows comparative examples. These are the results of evaluating the heat resistance of EC1.5 CNTs doped with ferrocene (Non-Patent Literature 1) and β-nicotinamide adenine dinucleotide disodium salt hydrate (NADH) (Patent Literature 1) using the same method. In the case of doping with NADH, heating at 150°C for 3 minutes was performed to react the CNTs with NADH before proceeding to the evaluation of the Seebeck coefficient and heat resistance. With ferrocene and NADH, heating for 15 minutes and 180 minutes, respectively, caused the sign of the Seebeck coefficient to turn positive and return to the p-type, indicating that it is difficult to apply these methods to continuous device operation under high-temperature conditions.

[0040] The thermal stability of n-type CNT fibers obtained by doping was evaluated using the same method as the stability evaluation of n-type CNT films described above, based on the Seebeck coefficient. CNT fibers doped with n-type CNTs using a TBD (700 mmol / L) DMF solution as described in Example 1 were heated at 100°C in an incubator. During this heating, no depressurization or introduction of inert gas was performed in order to evaluate stability in air. After heating for a predetermined time, the CNT fibers were removed from the incubator and the Seebeck coefficient was measured at 25°C. After measurement, the fibers were returned to the incubator and heating at 100°C was resumed. This process was repeated to evaluate thermal stability. Figure 4 shows the results. The horizontal axis represents the net heating time in the incubator, and the vertical axis represents the Seebeck coefficient after heating. The duration of the n-type properties was determined from the time the Seebeck coefficient remained negative. In the case of TBD-doped CNT fibers, they remained remarkably stable as n-type materials for more than 1000 hours without reverting to p-type. [Examples]

[0041] [Control of n-type properties using solvents that dissolve doping agents] In Examples 1-3, DMF was used as the solvent for dissolving the n-type doping agent, but other solvents can also be used. Here, we demonstrate that the adsorption and doping efficiency of the n-type doping agent can be controlled by the type of solvent, and that the performance of the n-type nanocarbon material can be controlled. Note that the type of solvent, the concentration of the n-type doping agent, and the immersion time are not limited to these examples.

[0042] Polar solvents are known for their ability to dissolve a wide variety of substances and are used as a medium for liquid-phase chemical reactions. Polar solvents are broadly classified into protic polar solvents and aprotic polar solvents based on their molecular structure and properties. Qualitatively, protic polar solvents are polar solvents that contain acidic hydrogen, while aprotic polar solvents are polar solvents that do not contain acidic hydrogen. This example shows the results when water or ethanol is used as the protic polar solvent and DMF or acetone is used as the aprotic polar solvent in TBD doping. TBD was dissolved in each solvent at a concentration of 71 mmol / L. The EC1.5 CNTs, which had been film-formed under the same conditions as in Example 1, were immersed in the solution for 5 minutes, then vacuum-dried, and the Seebeck coefficient and conductivity were measured at approximately 25°C.

[0043] Figure 5 shows the average values ​​of the results. Parent represents the undoped film. In all cases where the polar solvent was water, ethanol, DMF, and acetone, the Seebeck coefficient of the TBD-doped CNT turned negative, indicating a change to n-type. When comparing protic polar solvents and aprotic polar solvents, significantly higher conductivity is observed when using aprotic polar solvents. As described in Example 2, since conductivity is proportional to carrier concentration, nanocarbon materials can be more effectively n-type doped when using aprotic polar solvents. In protic polar solvents, acidic hydrogen in the solvent molecules solvates the lone pairs of electrons in the doping agent, suppressing electron donation to the nanocarbon material. On the other hand, aprotic polar solvents do not contain such acidic hydrogen, and therefore electron donation is not suppressed. In this way, by changing the type of solvent used for doping, the electrical properties of nanocarbon materials can be controlled according to the intended application.

[0044] [Mechanisms of n-type doping and stabilization] In the examples, a significant difference in stability was observed between TMG, which does not have a ring structure, and DBU and TBD, which do have a ring structure. When a ring structure is present, the σ bond fixes the stereochemistry of the nitrogen atom and, consequently, the lone electron pair on the nitrogen atom, allowing for effective doping of the nanocarbon material. Furthermore, after donating electrons to the nanocarbon material, these doping agents become relatively positively charged conjugate acids, but this positive charge is stabilized by resonance structures mediated by σ and π bonds. Furthermore, even in nanocarbon materials that have been negatively charged by electron donation, the negative charge is delocalized, resulting in good electrical compatibility with conjugate acids that form resonance structures. This allows for strong mutual adsorption, and the doped state can be stably maintained for the entire material. In particular, when a ring structure is present, the steric position of the nitrogen atom is fixed, which allows for strong interactions with the nanocarbon material and greatly contributes to improved stability. [Examples]

[0045] [Examples of prototype thermoelectric conversion elements] The n-type nanocarbon material produced by this invention, which possesses both atmospheric stability and heat resistance, can be applied to various devices by combining it with p-type materials. Here, we present a prototype example of a thermoelectric conversion element by connecting an undoped p-type CNT film with an n-type CNT film obtained by doping. Note that the type of nanocarbon material, the form, size, configuration method, and number of modules when incorporated into the element are not limited to this embodiment.

[0046] Thermoelectric elements are devices that convert waste heat into electricity and are attracting attention as a clean power generation technology. The materials that make up thermoelectric elements are called thermoelectric materials. Conventional thermoelectric materials have mainly been inorganic substances such as bismuth, tellurium, and lead, but due to problems such as brittleness, toxicity, and limited reserves, there is growing expectation for thermoelectric materials using organic compounds. In this example, we will show prototype examples of thermoelectric elements using CNT films and CNT fibers.

[0047] (1) Fabrication of thermoelectric elements using CNT films The thermoelectric conversion element was fabricated as follows (hereinafter referred to as a stacked thermoelectric module). As shown in Figure 6(a), a film of EC1.5 CNT was fabricated with a diameter of approximately 73 mm. The film fabrication method was a scaled-up version of the method used in Examples 1 to 4, and the film thickness was approximately 10 μm. Multiple similar films were fabricated. As shown in Figure 6(a), from here a 23 × 44 mm film was fabricated. 2Twelve rectangular films were cut out to the specified size. A polyimide film of the same area was also prepared as an insulator. Of the twelve cut CNT films, six were used undoped as p-type material. The remaining six were doped with TBD as n-type material using the method described in Example 1.

[0048] Next, as shown in Figure 6(a), the CNT films and polyimide films were alternately laminated in the order of p-type CNT film, polyimide film, n-type CNT film, polyimide film, p-type CNT film, and so on. In this process, as shown in the cross-sectional view in Figure 6(a), the p-type CNT films and n-type CNT films were laminated with polyimide films in between so that both ends were in contact with a width of 3 mm, thereby completing the laminated thermoelectric module.

[0049] In a multilayer thermoelectric module, the p-type CNT film and the n-type CNT film are electrically connected in series using this connection method. When a temperature difference is applied to this multilayer thermoelectric conversion element in the direction shown in the cross-sectional view of Figure 6(a), the thermoelectric powers of the p-type and n-type materials are added together, effectively increasing the voltage that can be generated from the temperature difference. This is because the p-type material and the n-type material each generate voltages with opposite signs under a temperature difference.

[0050] Figure 6(b) is a graph plotting the temperature difference applied to a multilayer thermoelectric module (ΔT: right vertical axis, dashed line) against the voltage generated from the module (-ΔV: left vertical axis, solid line) against time. It can be seen that a voltage of mV or more is generated even when the temperature difference is around 10°C or less. The thermoelectric power calculated from this measurement data is 415 μV K -1 This indicates that the thermoelectric forces of the p-type CNT film and the n-type CNT film can be effectively added together.

[0051] Figure 6(c) shows an example of evaluating the output power of the multilayer thermoelectric module, with output power plotted on the vertical axis and the value of the connected load resistance on the horizontal axis. The measurement was performed by measuring the voltage while maintaining a temperature difference of approximately 40°C and connecting the load resistance in parallel with the multilayer thermoelectric module. The output power was calculated by dividing the square of the voltage by the load resistance value. The output power was maximized when a load resistance of 14Ω was connected, yielding a value of approximately 4.7μW.

[0052] (2) Fabrication of thermoelectric elements using CNT fibers A thermoelectric element using CNT fibers was fabricated as follows. In a 30 mL screw-top bottle, the doping agent TBD was dissolved in acetone to a concentration of 700 mmol / L. Next, undoped CNT fibers obtained in the same manner as in Example 1 were wound around a glass plate as shown in Figure 7(a). The wound glass plate was placed on a 60°C hot plate, and approximately 10 μL per drop of the above TBD solution was added to only the CNT fibers on the back side using a pipette. By using acetone as the solvent, the solvent evaporated immediately after addition, preventing the dopant from penetrating the CNT fibers on the front side via the threads, and allowing the n-type doping agent to be rapidly adsorbed onto the surface and interior of the CNT fibers. The addition of the TBD solution to the same location on the fibers was repeated about 5 times. After confirming that the solvent had completely evaporated, the glass plate was removed from the hot plate and cooled to room temperature.

[0053] When the doped CNT fibers were removed from the glass plate, as shown in Figure 7(a), fibers were produced in which p-type and n-type regions were repeatedly and continuously doped on the same fiber, alternating between p-type and n-type regions. One combination of p-type and n-type regions will be called a pair. When these CNT fibers were placed and fixed on a glass plate as shown in Figure 7(a), and a temperature difference was applied in a direction parallel to the pair of p-type and n-type fibers, the thermoelectric power of the p-type and n-type fibers were added together by one pair, effectively increasing the voltage that could be generated from the temperature difference at both ends of the fiber. In this embodiment, as shown in Figure 7(b), long, narrow strips of double-sided tape were attached to both ends of the glass plate, and the doped fibers, consisting of eight pairs, were fixed to the double-sided tape at the boundary between the p-type and n-type regions. After that, polyimide tape was applied to the boundary area to fix it in place. Silver paste was applied to both ends of the fiber and dried to create electrical contacts for voltage measurement.

[0054] Figure 7(c) is a graph plotting the temperature difference (ΔT) applied to a multilayer thermoelectric module against the voltage (ΔV) generated by the module against time. Similar to the CNT film element, a voltage of the order of mV or higher is generated even when the temperature difference is around 10°C or less. The thermoelectric power calculated from this measurement data is 421 μV K -1 This shows that the thermoelectric power of eight pairs of p-type CNT fibers and n-type CNT fibers can be effectively added together.

[0055] Figure 7(d) shows an example of evaluating the output power of the multilayer thermoelectric module, with the output power plotted on the vertical axis and the value of the connected load resistance on the horizontal axis. The measurement was performed by measuring the voltage while maintaining a temperature difference of approximately 40°C and connecting the load resistance in parallel with the multilayer thermoelectric module. The output power was calculated by dividing the square of the voltage by the load resistance value. The output power was maximized when a load resistance of 500Ω was connected, yielding a value of approximately 0.11μW.

[0056] In recent years, applications of thermoelectric conversion elements for energy harvesting have attracted attention. Furthermore, advancements are underway in ultra-low power consumption technology for wireless devices aimed at the Internet of Things (IoT), reducing power consumption to below μW. Thermoelectric conversion elements are particularly promising as a power supply technology for wireless devices used in applications such as monitoring factories, offices, infrastructure, livestock, pets, and human health. The thermoelectric conversion elements that can be manufactured according to this invention are expected to have new applications, such as energy harvesting technology and power supply technology for IoT, due to the high stability of their n-type nanocarbon material components. [Industrial applicability]

[0057] The n-type polarity of the n-type nanocarbon material obtained by this invention persists for several months or more even at high temperatures of 100°C, thus enabling long-term stable operation of devices. Furthermore, since doping agents consisting only of universally present elements such as carbon and nitrogen can be used, there are fewer resource constraints, and it is possible to manufacture n-type materials at low cost. The n-type nanocarbon material or aggregate of n-type nanocarbon material, whose heat resistance has been improved by the doping method of the present invention, can be widely used in applications such as field-effect transistors, PN junction diodes, solar cells, CMOS devices, thermoelectric conversion elements, temperature sensors, and infrared sensors.

Claims

1. A thermoelectric element comprising a guanidine compound, namely triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene), and a nanocarbon material or an aggregate of nanocarbon materials, manufactured using an n-type composite material that is stable in air or heat resistant.

2. The thermoelectric conversion element according to Claim 1, wherein the guanidine compound is triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene).

3. At least one compound selected from triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene), which is the guanidine compound, and is an n-type doping agent for nanocarbon materials used in the n-type composite material according to Claim 1 or Claim 2.

4. The thermoelectric element according to claim 1 or 2, wherein the nanocarbon material is manufactured using an n-type composite material selected from carbon nanotubes, graphene, or graphite.

5. The thermoelectric element according to claim 1 or 2, wherein the aggregate of nanocarbon materials is manufactured using an n-type composite material, which is a collection of carbon nanotubes, graphene, graphite, or aggregates of two or more of these.

6. The thermoelectric element according to claim 1, claim 2, or claim 5, wherein the aggregate of nanocarbon materials is manufactured using an n-type composite material in the form of fibers, membranes, threads, or cloth.

7. A thermoelectric element according to claim 1 or claim 2, or any one of claims 4 to 6, manufactured using an n-type composite material in which the guanidine compound is physically in contact with or chemically bonded to the surface of a nanocarbon material.

8. A thermoelectric element according to claim 1 or claim 2, or any one of claims 4 to 7, manufactured using an n-type composite material containing a carbon nanotube in which the guanidine compound is encapsulated in its internal space.

9. A thermoelectric element according to claim 1 or claim 2, or any one of claims 5 to 8, manufactured using an n-type composite material in which the guanidine compound is dispersed inside an aggregate of nanocarbon materials.

10. A thermoelectric element according to claim 1 or claim 2, or any one of claims 5 to 8, manufactured using an n-type composite material in which the guanidine compound is deposited on the outer surface of an aggregate of nanocarbon materials.

11. A thermoelectric element according to claim 1 or claim 2, or any of claims 4 to 10, manufactured using an n-type composite material obtained by compounding the guanidine compound with 0.1% or more of the weight of a nanocarbon material or an aggregate of nanocarbon materials.

12. A method for forming an n-type composite material according to claim 1 or claim 2, or any of claims 4 to 11, or a method for n-type doping of a nanocarbon material and a nanocarbon material assembly, The process includes a step of using the guanidine compound, triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene), as an n-type doping agent and dispersing the n-type doping agent on the surface of the nanocarbon material, A method for forming an n-type composite material, or a method for n-type doping a nanocarbon material and a nanocarbon material aggregate, wherein the dispersion step is a step of dispersing the compound by contacting it with the surface of the nanocarbon material, depositing it on the outer surface of the aggregate of nanocarbon materials, or penetrating or embedding it inside.

13. The method for forming an n-type composite material according to claim 12, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) is used as the guanidine compound.

14. The method for forming an n-type composite material according to claim 12 or claim 13, or a method for n-type doping a nanocarbon material and a nanocarbon material aggregate, wherein the dispersion step is a step of immersing the nanocarbon material or an aggregate of nanocarbon materials in a solution in which the compound is dissolved in a solvent, or dropping the solution onto these materials, thereby causing contact with the surface, deposition, or penetration into the interior of the material aggregate and dispersion.

15. The method for forming an n-type composite material according to claim 12 or 13, or a method for n-type doping a nanocarbon material and a nanocarbon material assembly, wherein the dispersion step is a step of sublimating or evaporating the compound and depositing it on the surface of the nanocarbon material or nanocarbon material assembly, or penetrating it into the interior of the material assembly and dispersing it.

16. A method for forming an n-type composite material according to any one of claims 12 to 14, or a method for n-type doping a nanocarbon material and a nanocarbon material aggregate, wherein the dispersion step controls the n-type properties of the n-type composite material according to claim 1 by using a protic polar solvent and a non-protic polar solvent as the type of solvent for dissolving the compound.

17. A method for forming an n-type composite material according to claim 1 or claim 2, or any of claims 4 to 11, or a method for n-type doping of a nanocarbon material and a nanocarbon material assembly, The aforementioned nanocarbon material is a carbon nanotube. The process includes using the guanidine compound triazabicyclodecene (1,5,7-Triazabicyclo[4.4.0]dec-5-ene) or 7-methyltriazabicyclodecene (7-Methyl-1,5,7-Triazabicyclo[4.4.0]dec-5-ene) as an n-type doping agent, and encapsulating the n-type doping agent in the internal space of a carbon nanotube. The step of encapsulating the compound in the internal space of a carbon nanotube is a step of sublimating or evaporating an n-type doping agent and bringing it into contact with a nanocarbon material or nanocarbon material aggregate to penetrate between the layers of the nanocarbon material or nanocarbon material aggregate, thereby forming an n-type composite material, or a method of n-type doping carbon nanotube nanocarbon material and nanocarbon material aggregate.

18. A method for forming an n-type composite material comprising at least one compound selected from amidine compounds and guanidine compounds, and a nanocarbon material or nanocarbon material aggregate, which is stable in air or heat resistant, or a method for n-type doping of nanocarbon materials and nanocarbon material aggregates, The process includes dispersing at least one compound selected from the amidine and guanidine compounds as an n-type doping agent on the surface of the nanocarbon material, The dispersion step involves controlling the n-type properties of the n-type composite material by using a protic polar solvent and an aprotic polar solvent as the type of solvent for dissolving the compound, thereby providing a method for forming an n-type composite material, or a method for n-type doping a nanocarbon material and a nanocarbon material aggregate.

19. The method for forming an n-type composite material according to claim 18, or a method for n-type doping a nanocarbon material and a nanocarbon material aggregate, wherein the dispersion step is a step of immersing the nanocarbon material or an aggregate of nanocarbon materials in a solution in which the compound is dissolved in a solvent, or dropping the solution onto these materials, thereby causing contact with the surface, deposition, or penetration into the interior of the material aggregate and dispersion.

20. A thermoelectric module manufactured using the thermoelectric element described in Claim 2, The n-type composite material is composed of the guanidine compound and a carbon nanotube film which is an aggregate of the nanocarbon material. The thermoelectric conversion element is composed of an n-type carbon nanotube film doped with the guanidine compound, a p-type carbon nanotube film not doped with the guanidine compound, and an insulator. A thermoelectric module having a module configuration in which the p-type carbon nanotube film, the insulator, the n-type carbon nanotube film, and the insulator are alternately stacked, and the p-type carbon nanotube film and the n-type carbon nanotube film are stacked via the insulator so that both ends of the stacked films are in contact.

21. A thermoelectric module manufactured using the thermoelectric conversion element described in Claim 2, The n-type composite material is composed of the guanidine compound and carbon nanotube fibers, which are aggregates of the nanocarbon material. The thermoelectric conversion element is composed of n-type carbon nanotube fibers doped with the guanidine compound and p-type carbon nanotube fibers not doped with the guanidine compound. A thermoelectric module having a module configuration in which the n-type carbon nanotube fibers and the p-type carbon nanotube fibers are arranged alternately and continuously to form the same single fiber.

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