JP2017220469A - Thermoelectric conversion material and method of producing thermoelectric conversion material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermoelectric conversion material suitable for an improvement in thermoelectric conversion performance.SOLUTION: A thermoelectric conversion material has a porous structure including a conductive polymer compound, the ratio of gaps in the porous structure being 60 vol.% or more.SELECTED DRAWING: None

Description

  The present invention relates to a thermoelectric conversion material and a method for producing the thermoelectric conversion material.

In research on thermoelectric conversion materials using inorganic materials, a search for increasing thermoelectric conversion efficiency by using rare metals such as bismuth and tellurium and changing the type of material and the crystal structure of the alloy is conducted (for example, non-patented). Reference 1), inorganic thermoelectric conversion materials that do not use rare metals (for example, Non-Patent Document 2) have also been reported.
In addition, for thermoelectric conversion materials using inorganic materials, the Seebeck coefficient is increased by making the material nanoscale fine wires (for example, Non-Patent Document 3), and the thermoelectric conversion efficiency is improved by the rattling effect (for example, Non-Patent Documents). 4 etc.) is also reported.
Furthermore, Patent Document 1 discloses a thermoelectric conversion element in which a thermoelectric conversion layer containing an inorganic oxide semiconductor as a main component and having a void structure is laminated on a substrate having an aluminum porous anodic oxide film. ing.

On the other hand, in the research of thermoelectric conversion materials using organic materials, mainly in thin films of conductive polymer compounds, search for trying to increase thermoelectric conversion efficiency by changing the type of polymer compound has been performed (for example, Non-patent document 5).
Patent Document 2 discloses a thermoelectric conversion element including a thermoelectric conversion layer containing hollow particles and a conductive polymer, and Patent Document 3 discloses a mixed solution containing a π-conjugated conductive polymer and water. A method for manufacturing a thermoelectric conversion material is disclosed in which the fine particles of a mixed solution are attached to a substrate and dried by atomization.

International Publication No. 2013/058327 Japanese Patent Laying-Open No. 2015-38961 Japanese Patent Laying-Open No. 2006-1711

Science, 320, 634-638 (2008). Nature, 508, 373-377 (2014). Adv. Mater., 19, 1043-1053 (2007). Appl. Phys. Lett., 73, 178-180 (1998). Energy Environ. Sci. 6, 788-792 (2013).

  As described above, research on thermoelectric conversion materials using organic materials is mainly performed on thin films of conductive polymer compounds. However, thin films of conductive polymer compounds are generally thin and have high thermal conductivity, so that heat can be transferred easily even if there is a temperature difference on both sides of the thin film, so high thermoelectric conversion performance must be realized. Is difficult.

The present invention has been made in view of the above circumstances.
That is, the subject of this invention is providing the manufacturing method of the thermoelectric conversion material suitable for improvement of the thermoelectric conversion performance, and the thermoelectric conversion material.

As a result of intensive studies, the present inventors have found that the above problem can be solved by setting the void ratio in the porous structure of the thermoelectric conversion material to 60% by volume or more, and have completed the present invention.
That is, specific means for solving the above problems include the following aspects.

<1> It has a porous structure containing a conductive polymer compound,
A thermoelectric conversion material having a void ratio in the porous structure of 60% by volume or more.
<2> The thermoelectric conversion material according to <1>, wherein the number average diameter of the voids is 300 μm or less.

<3> The thermoelectric conversion material according to <1> or <2>, which is a lyophilized product of the composition containing the conductive polymer compound.
<4> The thermoelectric conversion material according to any one of <1> to <3>, wherein the conductive polymer compound is fibrous.
<5> The thermoelectric conversion material according to <3> or <4>, wherein the void ratio is 90% by volume or more.
<6> The thermoelectric conversion material according to any one of <3> to <5>, wherein the number average diameter of the voids is 100 μm or more and 220 μm or less.

<7> The thermoelectric conversion material according to <1> or <2>, which is a foam of a composition containing the conductive polymer compound.
<8> The thermoelectric conversion material according to <7>, wherein the number average diameter of the voids is 15 μm or more and 100 μm or less.

<9> A freezing step of freezing a composition for freeze-drying comprising a conductive polymer compound and a solvent;
Drying the frozen composition for lyophilization to form a porous structure having a void ratio of 60% by volume or more;
The manufacturing method of the thermoelectric conversion material which has this.
<10> The method for producing a thermoelectric conversion material according to <9>, wherein the freezing step is a step of freezing the freeze-dried composition with liquid nitrogen.
<11> A foaming step of foaming the foaming agent in the foaming composition containing a conductive polymer compound and a foaming agent to form bubbles and forming a porous structure having a void ratio of 60% by volume or more is provided. A method for producing a thermoelectric conversion material.
<12> The method for producing a thermoelectric conversion material according to <11>, wherein the foaming agent is supercritical carbon dioxide.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the thermoelectric conversion material suitable for improvement of the thermoelectric conversion performance and the thermoelectric conversion material is provided.

It is a SEM image obtained by observing a section about an example of a freeze-dried body in an embodiment of the present invention. It is a SEM image obtained by observing a section about other examples of a freeze-drying object in an embodiment of the present invention. It is a SEM image obtained by observing a section about an example of a foam in an embodiment of the present invention. It is a SEM image obtained by observing a section about other examples of a foam in an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

[Thermoelectric conversion material]
The thermoelectric conversion material of this embodiment has a porous structure containing a conductive polymer compound, and the ratio of voids in the porous structure (hereinafter also referred to as “void ratio”) is 60% by volume or more.
In this embodiment, the thermoelectric conversion material suitable for the improvement of thermoelectric conversion performance is obtained by setting it as the said structure.
In a material using a conductive polymer compound, for example, when the purpose is to improve conductivity, the conductivity tends to decrease if the number of voids increases too much, so try to improve the performance of the material by reducing the voids Is common. However, the present inventors have found that a thermoelectric conversion material suitable for improving the thermoelectric conversion performance can be obtained by setting the porosity to a value as high as 60% by volume or more.

Here, the “void” refers to a region where there is no solid or liquid substance at room temperature (25 ° C.). A gas may be present in the gap.
In other words, since the void is a vacuum region or a region filled with gas, the thermal conductivity is lower than that of a solid portion containing a conductive polymer compound (that is, a region other than the void). And since the thermoelectric conversion material of this embodiment contains many space | gap with low heat conductivity compared with the thing with a low porosity, the heat conductivity as the whole thermoelectric conversion material also becomes low.
In the thermoelectric conversion material, the higher the Seebeck coefficient, the higher the electrical conductivity, and the lower the thermal conductivity, the higher the thermoelectric conversion performance. And the thermoelectric conversion material of this embodiment has low heat conductivity compared with the thermoelectric conversion material which does not have a space | gap, for example like a cast film, and the thermoelectric conversion material with a small porosity. Therefore, it is considered that the thermoelectric conversion material of the present embodiment is suitable for improving the thermoelectric conversion performance as compared with the thermoelectric conversion material having low thermal conductivity.

  In addition, since the thermoelectric conversion material of this embodiment includes many voids, it can be made thicker than a thin film (for example, cast film) of a conductive polymer compound having the same mass and not including voids. Therefore, the thermoelectric conversion material of the present embodiment uses the same mass of the conductive polymer compound, and has a low thermal conductivity and a high temperature compared to the thermoelectric conversion material with a low porosity, and thus the temperature on both sides. Even if it makes a difference, it is difficult for heat to be transmitted (that is, heat insulation is high). And since the temperature difference is easy to be maintained when the heat insulation property of the thermoelectric conversion material is high, it is considered that it is more suitable as a thermoelectric conversion material.

In addition, as a space | gap, the bubble trace in the foam mentioned later other than the solvent removal trace in the freeze-dried body mentioned later is mentioned, for example.
The method for measuring and calculating the porosity is as follows. Specifically, first, the sample was cut with a microtome, and the cross section was photographed at 100 to 1500 times with a scanning electron microscope (SEM, manufactured by JEOL, model number: JSM-6510) to obtain an SEM image. It confirms that it has a porous structure from an image. Next, the obtained SEM image is binarized by image analysis software (pickmap 32-2.4), the void area and the solid portion area are obtained, and the void ratio is calculated from the ratio.
In addition, as a porosity, 60 volume% or more and 99.7 volume% or less are preferable, for example.

  The number average diameter of the voids is preferably, for example, 300 μm or less. When the porosity is increased, the Seebeck coefficient and conductivity of the thermoelectric conversion material as a whole may be lower than those with a low porosity, but by reducing the number average diameter of the voids, the Seebeck coefficient is decreased and the conductivity is decreased. Reduction is suppressed. The reason for this is not clear, but voids with a large diameter tend to obstruct the conductive path of the conductive polymer compound, but reducing the number average diameter of the voids increases the number of conductive paths of the conductive polymer compound. The decrease in Seebeck coefficient and the decrease in conductivity are suppressed, and it is presumed that the Seebeck coefficient is more suitable for improving the thermoelectric conversion performance.

The method for measuring and calculating the number average diameter of the voids is as follows. Specifically, first, similarly to the measurement of the porosity, an SEM image of a cross section obtained by cutting the sample with a microtome is obtained. Next, the obtained SEM image is subjected to particle analysis using image analysis software (Image J), and the number average diameter of voids (specifically, the number average of diameters in 500 to 1000 voids) is calculated.
When the gap is not circular, the diameter of a circle having the same area as the gap is calculated, and the value is defined as “diameter”.
The number average diameter of the voids is preferably 10 μm or more and 250 μm or less, more preferably 15 μm or more and 220 μm or less, further preferably 15 μm or more and 200 μm or less, and more preferably 18 μm or more and 150 μm, from the viewpoint of suppressing the decrease in Seebeck coefficient and the decrease in conductivity. The following are particularly preferred:

In this specification, the “conductive polymer compound” refers to a polymer compound that exhibits conductivity by doping with a dopant (that is, addition of a small amount of an electron-accepting substance or an electron-donating substance).
Examples of the conductive polymer compound include conjugated polymer compounds. Specifically, for example, aliphatic conjugated polymer compounds such as polyacetylene, and aromatic conjugated polymers such as poly (p-phenylene). Compounds, mixed conjugated polymer compounds such as poly (p-phenylene vinylene), heterocyclic conjugated polymer compounds such as polypyrrole and polythiophene, heteroatom-conjugated polymer compounds such as polyaniline, and derivatives thereof Can be mentioned. These conductive polymer compounds may be used alone or in combination of two or more.

Among these, the conductive polymer compound is preferably a polymer compound having a hetero atom such as a heterocyclic conjugated polymer compound or a heteroatom-containing conjugated polymer compound from the viewpoint of thermoelectric conversion performance.
The polymer compound having a hetero atom is preferably a polythiophene polymer compound, a polyaniline polymer compound, or a polypyrrole polymer compound.
Specific examples of the polythiophene polymer compound include polythiophene, poly (3-alkylthiophene), poly (3-thiophene-β-ethanesulfonic acid), polyalkylenedioxythiophene, and the like. Examples of the polyalkylene dioxythiophene include polyethylene dioxythiophene (hereinafter also referred to as “PEDOT”), polypropylene dioxythiophene, poly (ethylene / propylene) dioxythiophene, and the like.
Specific examples of the polyaniline polymer compound include polyaniline, polymethylaniline, polymethoxyaniline and the like.
Specific examples of the polypyrrole-based polymer compound include polypyrrole, poly-3-methylpyrrole, poly-3-octylpyrrole and the like.

  Among these, as the conductive polymer compound, a polymer compound having a sulfur atom is preferable, a polythiophene-based polymer compound is more preferable, polyalkylenedioxythiophene is further preferable, and PEDOT is particularly preferable.

The weight average molecular weight of the conductive polymer compound varies depending on the type of the conductive polymer compound and is not particularly limited, and examples thereof include 1000 or more and 200,000 or less. Moreover, as a weight average molecular weight of PEDOT, 1000 or more and 2500 or less are mentioned, for example.
The weight average molecular weight is a value measured by gel permeation chromatography (GPC).

The thermoelectric conversion material may include a dopant that imparts conductivity to the conductive polymer compound.
Examples of the dopant include polyanions. Specific examples of the polyanion include, for example, polyvinyl sulfonate ion, polystyrene sulfonate ion (hereinafter also referred to as “PSS”), polyallyl sulfonate ion, polyacrylate ethyl sulfonate ion, polybutyl acrylate ion, In addition to polyanions of polymers having sulfo groups such as 2-acrylamido-2-methylpropane sulfonate ion and polyisoprene sulfonate ion, polyvinyl carboxylate ion, polystyrene carboxylate ion, polyallyl carboxylate ion, polyacryl carboxyl Acid ions, polymethacrylic acid ions, poly-2-acrylamido-2-methylpropanecarboxylic acid ions, polyisoprenecarboxylic acid ions, polyacrylic acid ions and the like can be mentioned.
The polyanion may be a homopolymer ion or a copolymer ion, and the weight average molecular weight of the polyanion is, for example, from 5000 to 150,000.

Further, as a dopant, Besides, 4- tetrafluoroborate ion (BF ₄ ^over), (hereinafter also referred to as "TOS") p-toluenesulfonic acid ion, an anthraquinone-2-sulfonic acid ion, triisopropyl naphthalene sulfonic acid Ions, polyvinyl sulfonate ions, alkylbenzene sulfonate ions (for example, dodecylbenzene sulfonate ions, etc.), sulfonate ions such as alkyl sulfonate ions, camphor sulfonate ions, n-propyl phosphate ions, perchlorate ions ( ClO ₄ ^over), hexafluorophosphate ion (PF ₆ ^{chromatography),} bis (also referred to as a trifluoromethanesulfonyl) imide ion (hereinafter "BTFMSI"), iron chloride ions (FeCl ₄ ^over), gold chloride ions (AuCl ₄ ^over), chloride ion (Cl ^{chromatography),} sulfides Lee ON (SO ₄ ^2- ), iodine ion, phosphate ion, and the like.

  Although the addition amount of the dopant with respect to 100 mass parts of conductive polymer compounds changes with kinds of a conductive polymer compound and a dopant, 100 mass parts or more and 1000 mass parts or less are mentioned, for example. In particular, when the conductive polymer compound is PEDOT and the dopant is PSS, the amount of dopant added to 100 parts by mass of the conductive polymer compound is 250 parts by mass from the viewpoint of improving the conductivity of the thermoelectric conversion material. The amount is preferably 600 parts by mass or less.

Examples of the combination of the conductive polymer compound and the dopant include, for example, polythiophene-ClO ₄ , polythiophene-BF ₄ , polythiophene-PF ₆ , polythiophene-FeCl ₄ , polythiophene-AuCl ₄ , polythiophene-iodine, poly (3-alkylthiophene). ) -ClO ₄ , poly (3-alkylthiophene) -BF ₄ , poly (3-alkylthiophene) -PF ₆ , poly (3-alkylthiophene) -FeCl ₄ , poly (3-alkylthiophene) -AuCl ₄ , poly (3-alkylthiophene) - _{iodine, PEDOT-PSS, PEDOT-TOS} , PEDOT-BTFMSI, PEDOT-ClO 4, PEDOT-BF 4, PEDOT-PF 6, PEDOT-Cl, PEDOT-SO 4, polyaniline - alkylbenzene Sulfonic acid, polyaniline-camphorsulfonic acid, polyaniline-Cl, polyaniline-SO ₄ , polyaniline-phosphoric acid, polypyrrole-ClO ₄ , polypyrrole-BF ₄ , polypyrrole-PF ₆ , polypyrrole-FeCl ₄ , polypyrrole-AuCl ₄ , polypyrrole— Examples thereof include alkylbenzenesulfonic acid and polypyrrole-iodine.

Among the above combinations, as a preferable combination of the conductive polymer compound and the dopant, from the viewpoint of thermoelectric conversion performance, the conjugated polymer compound is a polythiophene polymer compound, and the dopant has a sulfo group-containing heavy compound. A combination which is at least one of a combined polyanion and sulfonate ion is exemplified. Among these, more preferable combinations of the conductive polymer compound and the dopant include PEDOT-PSS, PEDOT-TOS, PEDOT-BTFMSI, PEDOT-ClO ₄ , PEDOT-BF ₄ , and PEDOT-PF _6. More preferred are PEDOT-PSS and PEDOT-TOS, and particularly preferred is PEDOT-PSS.

The thermoelectric conversion material may contain other components as additives in addition to the conductive polymer compound and the dopant.
Examples of other components include secondary dopants of conductive polymer compounds, fillers (inorganic particles, simple metal particles, organic particles, etc.), and the like.

Examples of the secondary dopant of the conductive polymer compound include ethylene glycol and dimethyl sulfoxide.
When the thermoelectric conversion material contains the secondary dopant, the addition amount of the secondary dopant with respect to 100 parts by mass of the conductive polymer compound varies depending on the type and combination of the conductive polymer compound and the secondary dopant. A range of not less than 200 parts and not more than 20000 parts by mass is exemplified.

  The shape of the thermoelectric conversion material is appropriately selected according to the purpose of use, and is not particularly limited, and examples thereof include a film-like material. When the thermoelectric conversion material is in the form of a film, the thickness is, for example, from 1 μm to 3000 μm, and from the viewpoint of heat insulation, it is preferably from 200 μm to 3000 μm, and more preferably from 1000 μm to 3000 μm.

As the thermoelectric conversion material of the present embodiment, for example, a freeze-dried composition (hereinafter also simply referred to as “lyophilized product”) containing the conductive polymer compound or foaming of the composition containing the conductive polymer compound is used. Body (hereinafter also simply referred to as “foam”).
It is a lyophilized product or a foamed product, and by setting the porosity to the above range, the heat insulating property is enhanced while maintaining the Seebeck coefficient and the electrical conductivity, so that it is suitable for improving the thermoelectric conversion performance.

  For example, in a porous thin film obtained by mixing a conductive polymer compound and hollow particles, only the hollow portions of the hollow particles are used as “voids”, and it is difficult to achieve a high porosity. In particular, when a hollow particle made of a material other than the conductive polymer compound is used, the higher the porosity, the lower the total content of the conductive polymer compound and the dopant in the solid part. In addition, the conductivity tends to decrease. Compared to that, freeze-dried products and foams can easily achieve a high porosity and also maintain the Seebeck coefficient and conductivity because the total content of the conductive polymer compound and dopant in the solid part is suppressed. However, the heat insulation can be improved.

In addition, the content of the conductive polymer compound in the solid part of the porous structure includes, for example, 20% by mass or more. From the viewpoint of suppressing the decrease in Seebeck coefficient and suppressing the decrease in conductivity, 25% by mass or more is preferable. 30 mass% or more is more preferable.
The total content of the conductive polymer compound and the dopant in the solid part of the porous structure is, for example, 70% by mass or more, and 90% by mass from the viewpoint of suppressing the decrease in Seebeck coefficient and the conductivity. The above is preferable, and 95% by mass or more is more preferable.
Hereinafter, a freeze-dried body and a foamed body will be described as examples of the thermoelectric conversion material in the present embodiment.

<Freeze dried product>
The freeze-dried product is obtained by freeze-drying a composition containing a conductive polymer compound (a composition containing a conductive polymer compound and a dopant when the thermoelectric conversion material contains a dopant). An example of a freeze-dried product is shown in FIGS. FIG. 1 is a cross-sectional SEM image of a lyophilized product obtained by freezing with liquid nitrogen, and FIG. 2 is a cross-sectional SEM image of a lyophilized product obtained by freezing at −20 ° C.
As shown in FIGS. 1 and 2, examples of the form of the lyophilized body include a cotton-like structure in which fibrous conductive polymer compounds are collected.

The porosity in the freeze-dried body is preferably 80% by volume or more and 99.7% by volume or less, more preferably 90% by volume or more and 99.7% by volume or less, and 95% by volume or more and 99.5% by volume from the viewpoint of heat insulation. The following is more preferable.
Control of the porosity in the lyophilized body is performed, for example, by adjusting the content of the conductive polymer compound, the freezing temperature, the temperature lowering rate in the freezing step, etc. with respect to the entire composition for lyophilization described later. The porosity is determined by the method described above.

The number average diameter of the voids in the lyophilized product is not particularly limited, and examples thereof include 300 μm or less. From the viewpoint of suppressing Seebeck coefficient decrease and conductivity decrease, 80 μm or more and 250 μm or less are preferable, and 100 μm or more and 220 μm or less are preferable. More preferably, it is 100 μm or more and 200 μm or less, and particularly preferably 110 μm or more and 150 μm or less.
Control of the number average diameter of the voids in the lyophilized body is performed, for example, by adjusting the content of the conductive polymer compound in the entire composition for lyophilization described later, the freezing temperature, the temperature-decreasing rate in the freezing step, and the like. The number average diameter is obtained by the method described above.

Examples of the conductive polymer compound contained in the lyophilized product include the above-described conductive polymer compounds, and preferred types of conductive polymer compounds are also as described above.
The total content of the conductive polymer compound and the dopant in the solid part of the porous structure of the lyophilized body is preferably 90% by mass or more, and more preferably 95% by mass or more.
In particular, a freeze-dried product can be obtained without adding an additive by undergoing freezing and drying steps, so the content of components (additives) other than the conductive polymer compound and the dopant in the solid part can be reduced. Easy to reduce. And even if it raises the porosity and raises heat insulation by raising content of the conductive polymer compound in a solid part, it is thought that the fall of Seebeck coefficient and the fall of conductivity are controlled.
The freeze-dried product is considered to be easily oriented in the process in which the composition containing the conductive polymer compound is lyophilized, and the porosity is reduced by the orientation of the conductive polymer compound. Even if it raises and heat insulation is improved, it is estimated that the fall of electrical conductivity is suppressed.

-Method for producing freeze-dried product-
The thermoelectric conversion material that is a freeze-dried product includes, for example, a freezing step of freezing a freeze-drying composition containing a conductive polymer compound and a solvent, and a drying step of drying the frozen freeze-drying composition. It is obtained by going through.

(Composition for lyophilization)
The composition for lyophilization contains at least a conductive polymer compound and a solvent, and may contain other additives in addition to a dopant as necessary. However, from the viewpoint of increasing the total content of the conductive polymer compound and the dopant in the solid part of the obtained lyophilized product, the smaller the other additives, the more preferable, and the addition of no other additives is more preferable. .

Examples of the conductive polymer compound and the dopant used for the freeze-drying composition include the aforementioned conductive polymer compound and dopant.
Examples of the solvent used for the freeze-drying composition include water, dimethyl sulfoxide and the like, and among them, water is preferable.
Additives used in the lyophilization composition as necessary include, for example, secondary dopants (for example, ethylene glycol, dimethyl sulfoxide, etc.) of conductive polymer compounds, fillers (inorganic particles, single metal particles, organic systems). Particles) and the like.

Examples of the total content of the conductive polymer compound and the dopant with respect to the entire composition for lyophilization including the solvent include 0.2% by mass or more and 1.3% by mass or less, and the viewpoint of reducing the number average diameter of the voids. Therefore, 0.2 mass% or more and 0.7 mass% or less are preferable, and 0.2 mass% or more and 0.5 mass% or less are more preferable.
Moreover, when using the said secondary dopant as an additive, as an addition amount of the secondary dopant with respect to a total of 100 mass parts of an electroconductive polymer compound and a dopant, the range of 100 mass parts or more and 500 mass parts or less is mentioned, for example.

(Freezing process)
In the freezing step, the lyophilized composition is frozen to obtain a solid lyophilized composition. Specifically, for example, the freeze-dried composition is frozen in a state where it is put in a container (for example, a screw tube, a petri dish or the like). The size and shape of the container used in the freezing step are appropriately selected according to the shape of the intended lyophilized body.

The freezing temperature is not limited as long as it is lower than the freezing point of the composition for lyophilization, and examples thereof include −10 ° C. or lower, preferably −20 ° C. or lower, more preferably −40 ° C. or lower, and −100 ° C. or lower. Is more preferable, and −150 ° C. or lower is particularly preferable. Although the lower limit of freezing temperature is not specifically limited, For example, -270 degreeC or more is mentioned, -200 degreeC or more is preferable.
The freezing temperature is the environmental temperature of the freeze-drying composition in the freezing step, and the temperature of the freeze-drying composition itself does not have to reach the freezing temperature.

Examples of the freezing method include a method of freezing the freeze-dried composition in a freezer set at the freezing temperature, a method of immersing the freeze-dried composition in liquid nitrogen, and the like. Among these, it is preferable to perform freezing by a method of immersing the composition for freeze-drying in liquid nitrogen, which makes it easy to obtain a freeze-dried product having high conductivity. The reason for this is not clear, but because the freezing temperature is low, the temperature-decreasing rate of the composition for lyophilization is increased, and the fibrous solid part forms a fine and dense network. This is probably because many paths are formed.
As a temperature fall rate of the composition for freeze-drying in a freezing process, 0.025 degreeC / s or more and 25 degrees C / s or less are mentioned, for example, 20 degrees C / s or more and 25 degrees C / s or less are preferable.

  The freezing time (that is, the time during which the composition for freeze-drying is placed in the environment of the freezing temperature) is not particularly limited, and varies depending on the freezing temperature. Examples of the freezing time include a range of 5 seconds to 30 seconds in freezing using liquid nitrogen, and a range of 1200 seconds to 6000 seconds in freezing in a freezer whose freezing temperature is set to −20 ° C. Is mentioned.

(Drying process)
In the drying step, the solid lyophilized composition obtained in the freezing step (that is, the frozen lyophilized composition) is dried to remove the solvent from the lyophilized composition. Specifically, for example, a solid lyophilization composition is placed under reduced pressure, and the solvent contained in the lyophilization composition is removed by sublimation.
In the drying step, drying may be performed under reduced pressure, or drying may be performed under atmospheric pressure without reducing pressure.
Examples of the gauge pressure (difference from atmospheric pressure) in the drying step include −0.1 MPa (G) or more and 0 MPa (G) or less.
Moreover, as temperature in a drying process, 20 degreeC or more and 50 degrees C or less are mentioned, for example.
The drying process may be a process of drying the freeze-drying composition through a plurality of stages. As the drying process that passes through multiple stages, for example, a primary drying process that condenses and collects the solvent evaporated from the freeze-drying composition, and a solvent that remains in the freeze-drying composition that has undergone the primary drying process is removed. Secondary drying process to be performed, and a drying process passing through.

<Foam>
The foam is obtained by solidifying a composition containing a conductive polymer compound (in the case where the thermoelectric conversion material contains a dopant, a composition containing the conductive polymer compound and the dopant) containing bubbles. An example of the foam is shown in FIGS. FIG. 3 is a cross-sectional SEM image of a foam foamed by heating to 160 ° C. using sodium bicarbonate as a foaming agent, and FIG. 4 is foamed by heating to 160 ° C. using azodicarbonamide as the foaming agent. 2 is a cross-sectional SEM image of a foam.
As a form of a foam, the form containing an open cell other than the form containing a closed cell is mentioned, for example. As a thermoelectric conversion material which is a foam, it is preferable that it is a form containing a closed cell from a viewpoint of making heat insulation and electroconductivity compatible.

The porosity in the porous structure of the foam is 60% by volume or more, and preferably 60% by volume or more and 90% by volume or less, and preferably 60% by volume or more and 70% by volume or less from the viewpoint of heat insulation, electrical conductivity, and film strength. More preferably, it is 60 volume% or more and 65 volume% or less.
The porosity of the foam is controlled, for example, by adjusting the type of foaming agent added to the foaming composition to be described later, the foaming agent content relative to the entire foaming composition, the foaming temperature, the foaming time, and the like. The porosity is determined by the method described above.

Although the number average diameter of the voids in the foam is not particularly limited, for example, 300 μm or less can be mentioned. From the viewpoint of suppressing the decrease in Seebeck coefficient and the decrease in conductivity, it is preferably 10 μm or more and 150 μm or less, and more preferably 15 μm or more and 100 μm or less. Preferably, it is 15 μm or more and 30 μm or less.
The number average diameter of the voids in the foam is controlled by adjusting, for example, the type of foaming agent added to the foaming composition to be described later, the foaming agent content relative to the entire foaming composition, the foaming temperature, the foaming time, and the like. Is called. The number average diameter is a value determined by the method described above.

Examples of the conductive polymer compound contained in the foam include the above-described conductive polymer compounds, and preferred types of conductive polymer compounds are also as described above.
Moreover, when a foam contains a dopant, as a dopant, the above-mentioned dopant is mentioned, for example, The preferable combination of a conductive polymer compound and a dopant is also as above-mentioned.
The total content of the conductive polymer compound and the dopant in the solid part of the foam is preferably 70% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more.
In addition, the foam is a material in which the conductive polymer compound is oriented by stretching the conductive polymer compound in a specific direction in the process of generating bubbles in the composition containing the conductive polymer compound. It is thought that it is easy to obtain. Therefore, even if a foam raises the porosity and improves heat insulation, it is estimated that the fall of electrical conductivity is suppressed.

-Method for producing foam-
The thermoelectric conversion material that is a foam is obtained, for example, by passing through a foaming step in which a foaming agent is foamed to generate bubbles in a foaming composition containing a conductive polymer compound and a foaming agent.
In addition, when the thermoelectric conversion material which is the target foam contains a dopant, the foaming composition contains at least a conductive polymer compound, a dopant, and a foaming agent.

(Foaming agent)
The foaming agent may be a chemical foaming agent or a physical foaming agent.
Examples of the chemical foaming agent include organic pyrolytic foaming agents, organic reactive foaming agents, inorganic pyrolytic foaming agents, inorganic reactive foaming agents, and the like. From the viewpoint of suppressing a decrease in Seebeck coefficient and electrical conductivity of the molecular compound, an organic pyrolytic foaming agent and an inorganic pyrolytic foaming agent are preferable.
Examples of the organic pyrolytic foaming agent include azodicarbonamide (hereinafter also referred to as “ADCA”), N, N′-dinitropentamethylenetetramine, benzenesulfonylhydrazide, toluenesulfonylhydrazide, 4,4 ′. -Oxybisbenzenesulfonyl hydrazide etc. are mentioned.
Examples of the inorganic pyrolytic foaming agent include hydrogen carbonates such as sodium hydrogen carbonate and ammonium hydrogen carbonate; carbonates such as ammonium carbonate; nitrites such as sodium nitrite; and the like.
Examples of the physical foaming agent include liquefied gases such as hydrocarbons; supercritical fluids such as supercritical carbon dioxide and supercritical nitrogen; and the like.

From the viewpoint of increasing the total content of the conductive polymer compound and the dopant in the solid part of the foam, the foaming agent is preferably a physical foaming agent, more preferably a supercritical fluid, and more preferably supercritical carbon dioxide. preferable.
From the viewpoint of increasing the porosity in the foam, the foaming agent is preferably a supercritical fluid, and more preferably supercritical carbon dioxide. When a supercritical fluid is used as a foaming agent, a foam with a high porosity can be obtained by dissolving a foaming agent in a conductive polymer compound under high pressure so that a large amount of the foaming agent can easily penetrate into the conductive polymer compound. It is thought that it is easy to obtain.
From the viewpoint of easily controlling the porosity and the number average diameter of the voids in the foam, the foaming agent is preferably sodium hydrogen carbonate, ADCA, or supercritical carbon dioxide.
Hereinafter, as an example of the foam production method, a case where a chemical foaming agent such as sodium hydrogen carbonate is used as the foaming agent and a case where supercritical carbon dioxide is used as the foaming agent will be described.

(Method for producing foam using chemical foaming agent)
When a chemical foaming agent is used as the foaming agent, the foam production method includes preparing a foaming composition containing a conductive polymer compound and a chemical foaming agent (and a dopant if necessary), for example, heating. The method of foaming the chemical foaming agent contained in the foaming composition to obtain a foam is mentioned.
As the foaming composition, for example, a coating solution is formed by applying a mixed liquid containing a conductive polymer compound, a dopant (if necessary), a chemical foaming agent, and a solvent to a base material, and then drying the coating layer. And the solvent obtained by removing the solvent.

  In addition, the foaming composition used for a foaming process may contain the solvent. Specifically, for example, a coating liquid is formed by applying a mixed liquid containing a conductive polymer compound, (optionally a dopant) and a chemical foaming agent and a solvent to a substrate, and then removing the solvent from the coating layer. During the process, the foaming agent may be foamed by heating the coating layer as a foaming composition.

Although the solvent used for the said liquid mixture is suitably selected according to the kind etc. of a conductive polymer compound and a foaming agent, Organic solvents, such as water and a dimethylsulfoxide, these mixtures etc. are mentioned, for example.
The mixed solution may contain other additives as necessary. Other additives include, for example, secondary dopants for conductive polymer compounds (for example, ethylene glycol, dimethyl sulfoxide, etc.), foaming aids (urea, zinc acetate, etc.), fillers (inorganic particles, single metal particles, organic materials) System particles, etc.).

Examples of the total content of the conductive polymer compound and the dopant with respect to the entire mixed solution include 0.5% by mass or more and 1.7% by mass or less, and preferably 1.0% by mass or more and 1.7% by mass or less. 1.0 mass% or more and 1.3 mass% or less are more preferable.
Although the amount of the foaming agent to be added relative to 100 parts by mass of the conductive polymer compound and the dopant varies depending on the type of the foaming agent, a range of 25 parts by mass to 50 parts by mass is exemplified.
The heating temperature, heating time, and pressure when foaming the foaming agent are appropriately selected depending on the type of foaming agent.

(Method for producing foam using supercritical carbon dioxide)
When supercritical carbon dioxide is used as the foaming agent, examples of the method for producing the foam include the following methods.
First, by preparing a composition containing a conductive polymer compound (and optionally a dopant), placing the composition in a supercritical carbon dioxide environment and allowing the supercritical carbon dioxide to penetrate into the composition, A foaming composition comprising a conductive polymer compound and (optionally a dopant) and supercritical carbon dioxide is obtained. Then, the supercritical carbon dioxide contained in the foaming composition is foamed by pressure reduction or heating, and foamed.

As a composition containing the said conductive polymer compound (and dopant as needed), for example, the liquid mixture containing a conductive polymer compound and (optionally a dopant) solvent is apply | coated to a base material. What was obtained by forming a coating layer, drying a coating layer, and removing a solvent is mentioned.
Although the solvent used for the said liquid mixture is suitably selected according to the kind etc. of a conductive polymer compound and a foaming agent, Organic solvents, such as water and a dimethylsulfoxide, these mixtures etc. are mentioned, for example.
The mixed solution may contain other additives as necessary. Examples of other additives include secondary dopants of conductive polymer compounds (for example, ethylene glycol and dimethyl sulfoxide), entrainers (for example, polyethylene glycol and the like) that increase the solubility of supercritical carbon dioxide, fillers ( Inorganic particles, simple metal particles, organic particles, etc.).
Examples of the total content of the conductive polymer compound and the dopant with respect to the entire mixture include 1.0% by mass to 1.7% by mass, preferably 1.0% by mass to 1.5% by mass, 1.0 mass% or more and 1.3 mass% or less are more preferable.

Examples of the pressure in the supercritical carbon dioxide environment include 5 MPa or more and 50 MPa or less. Moreover, as temperature in a supercritical carbon dioxide environment, the range of 10 to 200 degreeC is mentioned, for example. Examples of the time for placing the composition in a supercritical carbon dioxide environment include 1 minute to 10 hours.
When foaming of supercritical carbon dioxide is performed by reducing the pressure, examples of the pressure decreasing rate include a range of 0.1 MPa / min to 20 MPa / min. In addition, when foaming of supercritical carbon dioxide by pressure reduction, you may pass through the process of reducing temperature before or after reducing pressure. Examples of the step of lowering the temperature include a step of quenching under isobaric conditions, and examples of the temperature reduction rate include 1 ° C./min to 100 ° C./min.

<Uses of thermoelectric conversion materials>
As a use of the thermoelectric conversion material of this embodiment, the thermoelectric conversion element used for the generator, power supply, etc. which generate electric power using a temperature difference is mentioned, for example.
Examples of the thermoelectric conversion element include a substrate, a thermoelectric conversion layer formed on the substrate and made of the thermoelectric conversion material of the present embodiment, an electrode, and the like.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof.

The method for measuring and calculating the porosity and the number average diameter of the voids in this example are as described above.
In addition, the method for measuring the Seebeck coefficient, thermal conductivity, and conductivity and the calculation method thereof in this embodiment are as follows.

-Seebeck coefficient and conductivity-
Thermoelectric force measurement and current-voltage measurement using a sample (distance between electrodes: 5 mm) in which two electrodes are formed on the obtained thermoelectric conversion material using a conductive adhesive (manufactured by Fujikura Kasei Co., Ltd., Dotite). (IV measurement) was performed.
Specifically, a vacuum apparatus TSH071E (combination of a diaphragm pump and a turbo pump) manufactured by PFEIFFER was used as an experimental chamber, and the degree of vacuum in the chamber was reached to 10 ⁻⁵ Pa. Temperature control on the stage on which the sample was placed was performed using a cryomini compressor CA112 manufactured by Iwatani Corporation and a temperature controller Model 331 manufactured by Lakeshore. Further, the electrical properties were measured while heating the inside of the chamber using a temperature control device DB1000 manufactured by CHINO as a heater for controlling the temperature on the high temperature side of the sample. A vibration isolation table was used so that the vibration of the refrigerator would not be transmitted to the stage on which the sample was placed.

As the voltmeter, 2182A manufactured by KEITHLEY (measurable voltage range: 1 nV to 100 V) is used to measure the value of the thermoelectromotive force when the temperature difference between the two electrodes is ΔT = 2K, 4K, 6K, and 8K. And the Seebeck coefficient was calculated from the linearity.
Further, 6430 (measurable voltage range: 1 μV to 110 V, measurable current range: 10 aA to 100 mA) manufactured by KEITHLEY was used as an IV measuring apparatus in the two-terminal method. The control of the IV measurement was performed through a GP-IB cable using LabVIEW (National Instruments) on a PC. The conductivity was calculated using the resistance value obtained from the IV measurement, the distance between the electrodes, and the cross-sectional area of the sample. Note that the cross-sectional area of the sample is the fraction of the solid part calculated using the thickness of the sample measured using a micrometer, the sample width, and the area ratio of the solid part obtained by analysis of the SEM image. The area.

-Thermal conductivity-
The thermal diffusivity, the constant pressure specific heat capacity, and the density were measured, and the thermal conductivity was obtained as the product of them (ie, thermal conductivity = thermal diffusivity × constant pressure specific heat capacity × density).
Specifically, the thermal diffusivity was measured by a periodic heating method using ai-Phase (manufactured by Hitachi High-Tech) as a measuring device.
The constant pressure specific heat capacity was measured using a differential scanning calorimeter (thermo plus DSC 8230, manufactured by Rigaku). As the standard sample, the standard sample alumina for DSC (manufactured by Rigaku) was used.
The density was measured by the volume method, and the literature values of the PEDOT: PSS cast film and the air density were used for the density of the solid part and the fluid part, respectively.

[Example A1]
<Production of freeze-dried product A1>
Using a freeze dryer (manufactured by EYELA, model number: FDU-1200), freeze drying was performed as follows.
Specifically, PEDOT: PSS aqueous dispersion (manufactured by Clevios, PH500, solid content concentration: 1.0 to 1.3% by mass, mass ratio (PEDOT (conductive polymer compound): PSS = 1: 2.5) )) Was diluted 5 times with distilled water into a screw tube and immersed in liquid nitrogen for 30 seconds to freeze (freezing step) and dried to form a porous structure (drying step).
The temperature lowering rate in the freezing step was 20 ° C./s, the gauge pressure in the drying step was −0.1 MPa (G), and the temperature in the drying step was 15 ° C.
The obtained freeze-dried product was a film and had a thickness of 1900 μm.
The SEM image of the cross section which cut | disconnected the obtained freeze-dried body with a microtome is shown in FIG.
In addition, Table 1 shows the presence / absence of the porous structure and the porosity, the number average diameter of the voids, the Seebeck coefficient, the conductivity, and the thermal conductivity of the obtained lyophilized product, which were measured by the method described above.

[Example A2]
<Production of freeze-dried product A2>
In the freezing step, a lyophilized product was obtained in the same manner as in Example A1, except that the dispersion was placed in a screw tube and immersed in liquid nitrogen, except that the dispersion was placed in a petri dish and frozen in a freezer. .
The freezing temperature in the freezing step is −20 ° C., the freezing time in the freezing step is 30 minutes, the cooling rate in the freezing step is 0.022 ° C./s, the gauge pressure in the drying step is −0.1 MPa (G), and the drying step The temperature at 15 ° C. was 15 ° C., and the obtained lyophilized product was a film and had a thickness of 1046 μm.
In addition, Table 1 shows the presence / absence of the porous structure and the porosity, the number average diameter of the voids, the Seebeck coefficient, the conductivity, and the thermal conductivity of the obtained lyophilized product, which were measured by the method described above.

[Example A3]
<Production of freeze-dried product A3>
A lyophilized product was obtained in the same manner as in Example A2, except that the PEDOT: PSS aqueous dispersion (manufactured by Clevios, PH500) was diluted 2-fold with distilled water.
The freezing temperature in the freezing step is −20 ° C., the freezing time in the freezing step is 30 minutes, the cooling rate in the freezing step is 0.022 ° C./s, the gauge pressure in the drying step is −0.1 MPa (G), and the drying step The temperature at was 15 ° C.
The obtained freeze-dried product was a film and had a thickness of 1059 μm. The SEM image of the cross section which cut | disconnected the obtained freeze-dried body with a microtome is shown in FIG.
In addition, Table 1 shows the presence / absence of the porous structure and the porosity, the number average diameter of the voids, the Seebeck coefficient, the conductivity, and the thermal conductivity of the obtained lyophilized product, which were measured by the method described above.

[Example A4]
<Production of freeze-dried product A4>
A lyophilized product was obtained in the same manner as in Example A1, except that the PEDOT: PSS aqueous dispersion (manufactured by Clevios, PH500) was diluted 2-fold with distilled water.
In addition, the temperature-fall rate in the freezing process was 20 ° C./s.
About the obtained freeze-dried product, the presence or absence of the porous structure and the porosity, and the number average diameter of the voids were measured by the method described above. As a result, there was a porous structure, the porosity was 90.0% by volume, and the number average diameter of the voids was It was 23.5 μm.

[Example B1]
<Manufacture of foam B1>
A solution obtained by mixing 0.048 g of sodium hydrogen carbonate (manufactured by Kokusan Chemical Co., Ltd., reagent special grade) in 0.5 ml of distilled water was mixed with 1 ml of PEDOT: PSS aqueous dispersion (PH500, manufactured by Clevios). 400 μl of the obtained mixed solution was cast on a polytetrafluoroethylene substrate and vacuum-dried to form a film.
The obtained film was heated from 25 ° C. to 160 ° C. at a rate of temperature increase of 5 ° C./min under a gauge pressure of −0.1 MPa (G), and maintained at 160 ° C. for 5 minutes. went.
The obtained foam was a film-like foam having a thickness of 100 μm and containing closed cells. The SEM image of the cross section which cut | disconnected the obtained foam with the microtome is shown in FIG.
In addition, Table 1 shows the presence / absence of the porous structure and the porosity, the number average diameter of the voids, the Seebeck coefficient, the electrical conductivity, and the thermal conductivity of the obtained foam, as measured by the method described above.

[Example B2]
<Manufacture of foam B2>
PEDOT: PSS aqueous dispersion (PH500 made by Clevios) 1 ml mixed with 0.01 g azodicarbonamide (ADCA, Tokyo Chemical Industry Co., Ltd.) dissolved in 0.25 ml dimethyl sulfoxide (Sigma-Aldrich), Furthermore, 0.005 g of urea (manufactured by Wako Pure Chemical Industries, Ltd., special grade reagent) as a foaming aid was dissolved to obtain a mixed solution. 400 μl of the obtained mixed solution was cast on a polytetrafluoroethylene substrate and vacuum-dried to form a film.
The obtained film was heated from 25 ° C. to 160 ° C. at a rate of temperature increase of 5 ° C./min under a gauge pressure of −0.1 MPa (G), and maintained at 160 ° C. for 5 minutes. went.
The obtained foam was a film-like foam having a thickness of 100 μm and containing closed cells. The SEM image of the cross section which cut | disconnected the obtained foam with the microtome is shown in FIG.
In addition, Table 1 shows the presence / absence of the porous structure and the porosity, the number average diameter of the voids, the Seebeck coefficient, the electrical conductivity, and the thermal conductivity of the obtained foam, as measured by the method described above.

[Example B3]
<Manufacture of foam B3>
Polyethylene glycol 10000 (manufactured by Sigma-Aldrich) was added to 1 ml of PEDOT: PSS aqueous dispersion (Clevios PH500) so as to be 1% by mass, and polyethylene glycol 10000 was dissolved to obtain a mixed solution. The step of casting 300 μl of the obtained mixed solution on a polytetrafluoroethylene substrate and drying, and casting 200 μl of the mixed solution on the dried film and drying was repeated twice. The obtained sample was peeled from the substrate and placed in a sample container of a supercritical apparatus (manufactured by Jasco, model number: SCF-Get, SCF-Bpg). The sample container containing the sample was placed in the heater of the supercritical apparatus, the heater temperature was set to 50 to 100 ° C., and the sample was warmed for 1 hour. Carbon dioxide gas was introduced and held at 15-25 MPa for 2 hours. Then, it rapidly cooled on 0 degreeC and 15-25 MPa on the isobaric conditions. Then, foaming was performed by releasing the pressure at a rate of 0.5 MPa per minute.
The obtained foam was a film-like foam having a thickness of 200 μm and containing closed cells.
Moreover, the obtained foam had a porous structure and the porosity was 60 volume% or more.

[Comparative Example 1]
<Manufacture of comparative thin film C1>
The PEDOT: PSS aqueous dispersion (Clevios PH500) 300 μl is cast on a polytetrafluoroethylene substrate and dried, and the process of casting 200 μl of the mixture on the dried film and drying is repeated twice. A thin film was obtained.
The obtained thin film (cast film) had a thickness of 200 μm.
In addition, Table 1 shows the presence or absence of the porous structure, the porosity, the number average diameter of the voids, the Seebeck coefficient, the conductivity, and the thermal conductivity of the obtained thin film measured by the method described above.

  From the above results, in the example, a thermoelectric conversion material having a lower thermal conductivity than that of the comparative example and suitable for improving the thermoelectric conversion performance is obtained. In particular, in Example A1 and Example B2, the decrease in Seebeck coefficient and the decrease in conductivity are suppressed while lowering the thermal conductivity compared to the other examples, which is more suitable for improving the thermoelectric conversion performance. It can be seen that a thermoelectric conversion material is obtained.

Claims (12)

Having a porous structure containing a conductive polymer compound;
A thermoelectric conversion material having a void ratio in the porous structure of 60% by volume or more.   The thermoelectric conversion material according to claim 1, wherein the number average diameter of the voids is 300 μm or less.   The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is a lyophilized product of the composition containing the conductive polymer compound.   The thermoelectric conversion material according to any one of claims 1 to 3, wherein the conductive polymer compound is fibrous.   The thermoelectric conversion material according to claim 3 or 4, wherein a ratio of the voids is 90% by volume or more.   The thermoelectric conversion material according to any one of claims 3 to 5, wherein the number average diameter of the voids is 100 µm or more and 220 µm or less.   The thermoelectric conversion material according to claim 1, wherein the thermoelectric conversion material is a foam of a composition containing the conductive polymer compound.   The thermoelectric conversion material according to claim 7, wherein the number average diameter of the voids is 15 μm or more and 100 μm or less. A freezing step of freezing a composition for lyophilization comprising a conductive polymer compound and a solvent;
Drying the frozen composition for lyophilization to form a porous structure having a void ratio of 60% by volume or more;
The manufacturing method of the thermoelectric conversion material which has this.   The method for producing a thermoelectric conversion material according to claim 9, wherein the freezing step is a step of freezing the composition for freeze-drying with liquid nitrogen.   A thermoelectric conversion material having a foaming step of forming bubbles by foaming the foaming agent in a foaming composition comprising a conductive polymer compound and a foaming agent to form a porous structure having a void ratio of 60% by volume or more Manufacturing method.   The method for producing a thermoelectric conversion material according to claim 11, wherein the foaming agent is supercritical carbon dioxide.