JP6749831B2 - Thermoelectric conversion material - Google Patents

Description

The present invention relates to thermoelectric conversion materials.

In recent years, sulfides have attracted attention as thermoelectric conversion materials. The thermoelectric conversion material is a material that generates a voltage by a temperature difference between both ends of the material by the Seebeck effect to convert heat energy into electric energy, or a temperature difference by an electric energy by the Peltier effect. As the thermoelectric conversion material, there are an n-type thermoelectric conversion material in which an electric current is generated by the movement of electrons from a higher thermal energy to a lower thermal energy and a p-type thermoelectric conversion material in which a current is generated by the movement of holes. As thermoelectric conversion materials, alloy-based thermoelectric conversion materials, silicide-based thermoelectric conversion materials, tellurium-based thermoelectric conversion materials, and oxide-based thermoelectric conversion materials are known.

In recent years, sulfides that have been attracting attention particularly as thermoelectric conversion materials are sulfides containing copper and other metals. For example, in Non-Patent Documents 1 to 3, Cu ₂₆ V ₂ M ₆ S ₃₂ (M=Ge, Sn) having a corusite structure and Cu _26-X Zn _X V ₂ M ₆ S having a corusite structure, respectively. ₃₂ , and the thermoelectric properties of Cu _12-x Ni _x Sb ₄ S ₁₃ tetrahedrite have been reported. Further, Patent Document 1 is made of a composite metal sulfide having a predetermined crystal structure and containing copper and at least one metal other than copper, which is a first transition element or a p-block element, as main components. , Thermoelectric conversion materials are described.

JP, 2016-48730, A

Applied Physics Letters, (US), 2014, Vol. 105, 132107 Journal of Applied Physics, (US), 2014, Vol. 116, 063706 Journal of Applied Physics, (US), 2013, Vol. 113, 043712

According to the description of Patent Document 1, there is room for devising a new thermoelectric conversion material using a sulfide. Further, the materials described in Non-Patent Documents 1 to 3 need to include a metal having toxicity such as vanadium (V) or antimony (Sb).

At present, more than 60% of the energy used is discarded as heat, and 70% or more of the discarded heat is said to be low-temperature waste heat. Therefore, from an economical point of view, a thermoelectric conversion material having high thermoelectric properties in a medium to low temperature range (for example, room temperature to 400°C) is desired. The tellurium-based thermoelectric conversion material has high thermoelectric properties in the medium to low temperature range. However, tellurium simple substance and tellurium compound have high toxicity, and tellurium is a rare element, so that the tellurium-based thermoelectric conversion material is difficult to be widely used in general applications.

Therefore, the present invention provides a thermoelectric conversion material that can exhibit high thermoelectric conversion characteristics in a medium to low temperature range by containing a specific sulfide without containing vanadium, antimony, and tellurium.

The present invention is
It contains a sulfide having a crystal structure containing copper, tin and/or germanium, and a Group 6 metal element, and the crystal structure has a 2θ=14.x in an X-ray diffraction pattern obtained using CuKα rays. 14°±0.1°, 2θ=16.18°±0.1°, and 2θ=23.04°±0.1° diffraction peaks or 2θ=16.32°±0.1°, 2θ= Provided is a thermoelectric conversion material having a diffraction peak of 23.15°±0.1°, and 2θ=28.46°±0.1°.

Further, the present invention is
Provided is a thermoelectric conversion material containing copper, tin and/or germanium, and a Group 6 metal element, and containing a sulfide having a hemusite type or kiddcreekite type crystal structure.

Even if the thermoelectric conversion material does not contain vanadium, antimony, and tellurium, it can exhibit high thermoelectric conversion characteristics in the medium to low temperature range.

The graph which shows the X-ray diffraction pattern of the thermoelectric conversion material which concerns on Example 1. A graph showing an X-ray diffraction pattern of a sulfide having a composition of Cu ₆ MoSnS _8.

Hereinafter, embodiments of the present invention will be described. In addition, the following description relates to an example of the present invention, and the present invention is not limited thereto.

The thermoelectric conversion material according to the present invention contains a sulfide having a specific crystal structure containing copper, tin and/or germanium, and a Group 6 metal element. Its crystal structure is 2θ=14.14°±0.1°, 2θ=16.18°±0.1°, and 2θ=23.04° in an X-ray diffraction pattern obtained using CuKα ray. ±0.1° diffraction peak or 2θ=16.32°±0.1°, 2θ=23.15°±0.1°, and 2θ=28.46°±0.1° diffraction peak ..

From another viewpoint, the thermoelectric conversion material according to the present invention contains a specific sulfide. The sulfide contains copper, tin and/or germanium, and a Group 6 metal element, and has a hemusite-type or kiddcreekite-type crystal structure. This thermoelectric conversion material can exhibit high thermoelectric conversion characteristics in a medium to low temperature range without containing vanadium, antimony, and tellurium. The crystal structure of the sulfide contained in the thermoelectric conversion material can be determined, for example, by obtaining the X-ray diffraction pattern of the thermoelectric conversion material. In this case, for example, CuKα rays are used as the X-rays. The hemesite type crystal structure is, for example, 2θ=14.14°±0.1°, 2θ=16.18°±0.1°, and 2θ in an X-ray diffraction pattern obtained using CuKα rays. =23.04°±0.1°. The kiddcreekite type crystal structure is, for example, 2θ=16.32°±0.1°, 2θ=23.15°±0.1°, and 2θ= in an X-ray diffraction pattern obtained using CuKα rays. It has a diffraction peak of 28.46°±0.1°.

The Group 6 metal element contained in the sulfide is preferably molybdenum or tungsten. This allows the thermoelectric conversion material to more reliably exhibit high thermoelectric conversion characteristics in the medium to low temperature range.

The sulfide crystal structure desirably does not include lead, antimony, selenium, tellurium, and vanadium. This makes it possible to produce a sulfide without using a simple substance of lead, antimony, selenium, tellurium, and vanadium or a compound containing these elements, and thus it is easy to produce a sulfide safely. Moreover, since sulfides can be produced without using toxic substances, environmental pollution due to materials can be prevented.

The dimensionless figure of merit ZT and the power factor PF are known as indexes showing the performance of thermoelectric conversion materials. Here, Z means a figure of merit and has a dimension of K ^-1 . Moreover, T means an absolute temperature. The larger the dimensionless figure of merit ZT, the higher the performance of the thermoelectric conversion material. The dimensionless figure of merit ZT and the power factor PF of the thermoelectric conversion material are defined by the following equations (1) and (2), respectively. α, σ, and κ mean the Seebeck coefficient, electrical conductivity, and thermal conductivity of the thermoelectric conversion material, respectively.
ZT=α ² σT/κ (1)
PF=α ² σ (2)

The thermoelectric conversion material has a dimensionless figure of merit ZT of 0.04 or more at 40 to 50°C, for example. As described above, the sulfide-containing thermoelectric conversion material can exhibit high thermoelectric conversion characteristics in the medium to low temperature range.

The crystal structure of the sulfide may include elements other than copper, tin and/or germanium, a Group 6 metal element, and sulfur. For example, the number of atoms of elements other than copper, tin and/or germanium, a Group 6 metal element, and sulfur which may be included in the crystal structure of sulfide includes all atoms included in the crystal structure of sulfide. It is 10% or less of the number of atoms of the element. The crystal structure of the sulfide desirably contains only copper, tin and/or germanium, a Group 6 metal element, and sulfur.

Next, an example of the method for producing a thermoelectric conversion material according to the present invention will be described. The method for producing a thermoelectric conversion material includes, for example, a mixing step of mixing raw materials containing copper, tin and/or germanium, a Group 6 metal element, and sulfur, and a sintering step of sintering the mixture obtained by the mixing step And In the mixing step, sulfur, copper, tin and/or germanium, and a simple substance or a compound as a raw material of the Group 6 metal element are mixed in a predetermined mass ratio. The shape and size of the raw material are not particularly limited, and may be a lump having a certain size, or a granule or a powder. The method of mixing the raw materials is not particularly limited, and for example, a method of pulverizing and mixing using a planetary ball mill, a bead mill, a powder mixer, or the like can be considered. This allows the raw materials to be crushed and uniformly mixed. As a method for mixing the raw materials, a known method known as dry mixing or wet mixing can be used in addition to the method by pulverizing and mixing.

Moreover, the method for producing the thermoelectric conversion material may include a step of synthesizing a sulfide and a step of sintering the sulfide. For example, in the step of synthesizing a sulfide, by adding a sulfiding agent to an aqueous solution of a metal salt such as a metal nitrate, a chloride of a metal, or an acetate of a metal or an aqueous solution of a metal complex such as a metal acetylacetonate, Can synthesize things. In this case, examples of the sulfiding agent include sulfides of alkali metals such as sodium sulfide and potassium sulfide, sulfides of alkaline earth metals, sulfides of ammonium ions such as ammonium sulfide, and thiourea or thioacetamide. Examples include compounds such as sulfur organic compounds. The sulfide can also be obtained by blowing hydrogen sulfide gas into an aqueous solution in which metal ions are dissolved under an appropriate hydrogen ion concentration to generate a precipitate of a composite metal sulfide.

In the step of synthesizing sulfide, sulfide may be synthesized in an organic solvent. In this case, the particle size of the sulfide primary particles can be reduced to several nanometers. For example, in an amine solvent such as triethylenetetramine or octylamine, or in a thiol solvent such as octanethiol or decanethiol, a metal salt such as metal nitrate, metal chloride, or metal carboxylate or metal acetylacetate. A sulfide can be obtained by mixing a metal complex such as a nato and a sulfur-containing organic compound such as thiourea, thioacetamide, or dithiocarbamic acid or a sulfur powder in a predetermined amount and refluxing the mixture.

In the step of synthesizing sulfide, sulfide may be synthesized by hydrothermal synthesis. In this case, for example, a copper compound, a tin and/or germanium compound, a Group 6 metal element compound, and a sulfur compound or elemental sulfur are added to water and mixed to prepare a mixed solution. Hydrothermal synthesis is performed by placing the mixed solution in an environment of a predetermined temperature and a predetermined pressure for a predetermined period. Thereby, sulfide can be synthesized. The copper compound used in this method includes, for example, chlorides such as CuCl and CuCl _2, copper nitrate such as Cu(NO ₃ ) ₂ or carvone such as Cu(CH ₃ COO) and Cu(CH ₃ COO) _2. It is acid copper. The tin or germanium compound used in this method is, for example, a chloride such as SnCl ₂ , SnCl ₄ , GeCl ₂ , or GeCl ₄ , a nitric acid compound of tin or germanium, or a carboxylic acid compound of tin or germanium. The compound of the Group 6 metal element used in this method is, for example, a chloride, a nitric acid compound, or a carboxylic acid compound of the Group 6 metal element. The sulfur compounds used in this method are, for example, organic sulfur compounds such as thiourea and thioacetamide. The mixed solution is prepared, for example, in an environment of normal temperature (20° C.±15° C.: Japanese Industrial Standard JIS Z 8703) and normal pressure.

The temperature of the environment in which the hydrothermal synthesis is performed in the above method is, for example, 120 to 300°C, and preferably 150 to 250°C. The pressure of this environment is, for example, a gauge pressure of 0.1 to 5 MPa (megapascal), and preferably 0.2 to 3 MPa.

The sulfide obtained in the aqueous solution or in the organic solvent is separated from the aqueous solution or the organic solvent by a solid-liquid separation method such as filtration or centrifugation. In this case, since the molecules of the impurities derived from the aqueous solution, the organic solvent, or the raw material are attached to the surface of the sulfide particles, it is necessary to perform washing or calcination before the sintering step described below. It is preferable to remove the impurities from the sulfide particles by the method (1).

In the sintering step, the mixture obtained by mixing the raw materials or the synthesized sulfide is filled in a mold having a predetermined shape and sintered while being pressurized. As a method of sintering the mixture or the synthesized sulfide while pressurizing, the spark plasma sintering can be used, for example. The sintering temperature is not particularly limited, but is, for example, 150°C to 1500°C, preferably 200°C to 1000°C. The sintering time is, for example, 0 minutes to 10 minutes, and preferably 0 to 5 minutes. Further, the temperature rising time required from the start of the sintering process to the maximum temperature during the sintering process is, for example, 2 minutes to 10 minutes. For example, the temperature inside the mold filled with the mixture or the synthesized sulfide is raised to the maximum temperature at the above-mentioned heating rate, and the temperature inside the mold is kept at the maximum temperature for a predetermined time (sintering time) After that, the heating is stopped and the sintered body is naturally cooled. Thus, by shortening the sintering time, it becomes possible to sinter while maintaining the primary particle size. Thereby, the thermal conductivity can be lowered and the figure of merit can be improved. The pressure for pressurizing the mixture or the synthesized sulfide during the sintering step is, for example, 0.5 MPa to 100 MPa, and preferably 5 MPa to 50 MPa. This sintering process can be performed in an inert gas atmosphere or a vacuum atmosphere. This sintering process is preferably performed in a vacuum atmosphere. Thus, the thermoelectric conversion material can be manufactured. As described above, the mixture of the raw materials or the synthesized sulfide is sintered while being pressurized, so that the thermoelectric conversion material having high mechanical strength can be manufactured.

Hereinafter, the present invention will be described in detail with reference to examples. The following examples are examples of the present invention, and the present invention is not limited to the following examples.

<Example 1>
Copper powder 1.34 g, tin powder 0.42 g, molybdenum powder 0.34 g, and sulfur powder 0.90 g were charged into a Lecce planetary ball mill, and the planetary ball mill was rotated at 300 rpm (rotations per minute) for 9 minutes, and then for 1 minute. The operation of stopping the ball mill was repeated for 3 hours to obtain a mixed powder. The obtained mixed powder was sintered using a discharge plasma sintering device (manufactured by Sinterland Co., model number: LABOX-125). 0.75 g of the mixed powder was charged into a cylindrical die having an inner diameter of 10 mm, the temperature of the die was raised to 450° C. at 100° C./min, and the temperature of the die was raised to 500° C. at 50° C./min. The mixed powder was sintered by maintaining the die temperature at 500° C. for 2 minutes. Both sides of the pellet obtained by sintering were polished by a rotary polishing machine having a particle size of #2000 based on JIS R 6001:1998 to obtain a thermoelectric conversion material according to Example 1 having a thickness of about 2 mm.

The dimensionless figure of merit (ZT) at 42.9° C. of the obtained thermoelectric conversion material was 4.32×10 ⁻² . Here, with respect to the thermoelectric conversion material of Example 1, the electrical resistivity at 42.9° C., 113° C., 208° C., 305° C., and 404° C. was measured by the four-terminal measurement method, and the electrical resistivity at each temperature was measured. The electric conductivity σ at each temperature was obtained as the reciprocal. Further, with respect to the thermoelectric conversion material of Example 1, the thermal conductivity κ at room temperature (about 27° C.) was measured by the laser flash method. The thermal conductivity κ was 1.47 W/(m·K). For this measurement, a laser flash method thermophysical property measuring device LFA-502 manufactured by Kyoto Electronics Manufacturing Co., Ltd. was used. Furthermore, by heating one part so that the average temperature of the thermoelectric conversion material of Example 1 is 42.9°C, 113°C, 208°C, 305°C, and 404°C, and cooling another part, A predetermined temperature difference was generated on the upper surface of the thermoelectric conversion material of No. 1, and the electromotive force was measured in this state. A graph showing the relationship between the temperature difference and the electromotive force in the thermoelectric conversion material of Example 1, in which the vertical axis represents the electromotive force and the horizontal axis represents the temperature difference, was created. The Seebeck coefficient of the thermoelectric conversion material of Example 1 at each temperature was calculated from the slope of this graph. The dimensionless performance coefficient was calculated based on the equation (1) based on the electric conductivity, the thermal conductivity, and the Seebeck coefficient thus obtained. The power factor PF was calculated based on the equation (2) based on the electric conductivity and the Seebeck coefficient. The results are shown in Table 1.

<X-ray diffraction pattern>
An X-ray diffraction pattern of the thermoelectric conversion material according to Example 1 was obtained using an X-ray diffractometer (Rigaku Corporation, product name: MiniFlex600). CuKα ray was used as the X-ray. The X-ray diffraction pattern of the thermoelectric conversion material according to Example 1 is shown in FIG.

FIG. 2 shows an X-ray diffraction pattern of Cu ₆ MoSnS ₈ having a hemesite type crystal structure, which is included in PDF-4 of the International Center for Diffraction Data (ICDD). The result of the XRD pattern shown in FIG. 1 is 2θ=14.14°±0.1°, 2θ=16.18°±0.1°, and 2θ=23.04°±0. It had a diffraction peak of 1°, and it was suggested that the sulfide contained in the thermoelectric conversion material according to Example 1 had a hemesite-type crystal structure.

Claims (4)

It contains a sulfide having a crystal structure containing copper, tin and/or germanium, and a Group 6 metal element, and the crystal structure has an angle of 2θ of 14.30 in an X-ray diffraction pattern obtained using CuKα rays. 14°±0.1°, 2θ=16.18°±0.1°, and 2θ=23.04°±0.1° diffraction peaks or 2θ=16.32°±0.1°, 2θ= A thermoelectric conversion material having a diffraction peak of 23.15°±0.1°, and 2θ=28.46°±0.1°. A thermoelectric conversion material containing copper, tin and/or germanium, and a Group 6 metal element, and a sulfide having a hemusite-type or kiddcreekite-type crystal structure. The thermoelectric conversion material according to claim 1, wherein the Group 6 metal element is molybdenum or tungsten. The thermoelectric conversion material according to any one of claims 1 to 3, which has a dimensionless figure of merit ZT of 0.04 or more at 40 to 50°C.

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