JP7548796B2 - Thermoelectric materials and modules - Google Patents

Description

This disclosure relates to thermoelectric materials and thermoelectric modules.

Conventionally, thermoelectric modules capable of converting thermal energy and electrical energy are known. Thermoelectric modules are typically constructed using two types of thermoelectric materials, p-type and n-type. For example, Patent Document 1 discloses a bismuth telluride (Bi ₂ Te ₃ ) thin film as a thermoelectric material. However, Te contained in Bi ₂ Te ₃ is highly toxic and expensive. Conventionally known thermoelectric materials may also contain highly toxic and expensive elements such as lead (Pb) and selenium (Se), and in particular, thermoelectric materials having high thermoelectric properties at 500° C. or less often contain the above elements.

Non-Patent Document 1 describes the use of CZTS as a thermoelectric material. Although the constituent elements of CZTS (Cu, Zn, Sn, S) are relatively low in toxicity, CZTS has poor thermoelectric properties. In addition, Patent Document 2 discloses a method for manufacturing a CZTS thin film.

JP 2016-143758 A JP 2013-512311 A

Haoran Yang et al., “Nontoxic and Abundant Copper Zinc Tin Sulfide Nanocrystals for Potential High-Temperature Thermoelectric Energy Harvesting”, Nano Lett. 2012, 12, 540-545

As described above, although the constituent elements of CZTS (Cu, Zn, Sn, S) have relatively low toxicity, conventional CZTS (typically Cu ₂ ZnSnS ₄ ) has poor thermoelectric properties. The present disclosure has been made in view of the above-mentioned circumstances, and has as its main object to provide a CZTS-based thermoelectric material having good thermoelectric properties.

The present disclosure provides a thermoelectric material having a CZTS crystal phase containing Cu, Zn, Sn, and S, the CZTS crystal phase having Cu vacancies.

According to the present disclosure, since the CZTS crystal phase has Cu vacancies, it is possible to obtain a thermoelectric material with good thermoelectric properties.

In the above disclosure, the thermoelectric material may be doped with an alkali metal.

In the above disclosure, the alkali metal may include at least one of Li, Na, and K.

In the above disclosure, the alkali metal may include at least Na.

In the above disclosure, the thermoelectric material may have a composition represented by (Cu _1-α ) ₂ ZnSnS ₄ A _X , where α satisfies 0.01≦α≦0.10, A is an alkali metal, and X satisfies 0≦X≦0.5.

The present disclosure also provides a thermoelectric module having an element having the above-mentioned thermoelectric material.

According to the present disclosure, a thermoelectric module having good thermoelectric properties can be obtained by including elements having the above-mentioned thermoelectric materials.

In the above disclosure, the a-axis direction of the CZTS crystal phase in the element may be oriented to coincide with the direction in which current flows.

The thermoelectric material disclosed herein has the advantage of having good thermoelectric properties.

FIG. 2 is a schematic diagram showing a crystal structure of a CZTS crystal phase in the present disclosure. FIG. 2 is a schematic diagram showing a crystal structure of a CZTS crystal phase in the present disclosure. 1 is a schematic cross-sectional view illustrating a thermoelectric module according to the present disclosure. FIG. 2 is a schematic cross-sectional view showing a method for synthesizing a polycrystal in Comparative Example 1. 1 is a graph showing the heat treatment conditions of a polycrystal in Comparative Example 1. FIG. 2 is a schematic cross-sectional view showing a method for synthesizing a single crystal in Comparative Example 1. 1 is a graph showing the results of Examples 1 to 3 and Comparative Example 1.

The thermoelectric materials and thermoelectric modules disclosed herein are described in detail below.

A. Thermoelectric Material The thermoelectric material in the present disclosure comprises a CZTS crystal phase containing Cu, Zn, Sn and S, and the CZTS crystal phase has Cu vacancies. FIG. 1 is a schematic diagram showing the crystal structure of the CZTS crystal phase in the present disclosure. As shown in FIG. 1, the thermoelectric material in the present disclosure comprises a CZTS crystal phase containing Cu, Zn, Sn and S. In the CZTS crystal phase, the cation layers of Cu-Sn, Cu-Zn, Cu-Sn and Cu-Zn are arranged in order along the c-axis. The CZTS crystal phase in the present disclosure usually has a kesterite structure. In addition, as shown in FIG. 1, the CZTS crystal phase has Cu vacancies V. A Cu vacancy refers to a site where a Cu atom does not exist at a site where a Cu atom should be present in an ideal CZTS single crystal.

According to the present disclosure, since the CZTS crystal phase has Cu vacancies, a thermoelectric material with good thermoelectric properties can be obtained. Here, as described above, the constituent elements of CZTS (Cu, Zn, Sn, S) have relatively low toxicity. In other words, the thermoelectric material of the present disclosure has the advantage of being less toxic and cheaper to obtain than the constituent elements of conventionally known thermoelectric materials (e.g., Te, Pb, Se).

On the other hand, conventional Cu ₂ ZnSnS ₄ has low thermoelectric properties, for example, a ZT value of about _{0.1 at 700 K. In contrast, the thermoelectric material of the present disclosure has a ZT value of 0.5 or more at 700 K, for example, as shown in Examples 1 to 3 described later, and the thermoelectric properties are significantly improved compared to conventional Cu 2} ZnSnS _4. Although the mechanism by which the thermoelectric properties are improved is not completely clear, it is speculated that the impartation of Cu vacancies to the CZTS crystal phase increases the carrier concentration, thereby increasing the electrical conductivity, or that the power factor (the square of the Seebeck coefficient × the conductivity) increases due to the improved electrical conductivity.

The thermoelectric material in this disclosure has a CZTS crystal phase containing Cu, Zn, Sn, and S. The thermoelectric material usually has the CZTS crystal phase as the main phase, and is preferably a single-phase material of the CZTS crystal phase. The presence of a CZTS crystal phase in a thermoelectric material can be confirmed by observing peaks of the tetragonal kesterite phase in XRD and Raman spectroscopy.

The CZTS crystal phase in the present disclosure has Cu vacancies. The proportion of Cu vacancies in the CZTS crystal phase is, for example, 1% or more, and may be 3% or more, or 5% or more. If the proportion of Cu vacancies is too small, the effect of improving the thermoelectric properties may not be obtained. The proportion of Cu vacancies in the CZTS crystal phase refers to the proportion of Cu vacancies to all Cu sites in an ideal CZTS single crystal. On the other hand, the proportion of Cu vacancies in the CZTS crystal phase is, for example, 10% or less, and may be 8% or less. If the proportion of Cu vacancies is too large, the crystallinity of the CZTS crystal phase may decrease. The proportion of Cu vacancies can be measured, for example, by SEM-EDX (scanning electron microscope-energy dispersive X-ray spectroscopy).

The thermoelectric material in the present disclosure is preferably doped with an alkali metal. Since the ionic radius of the alkali metal is small, a distortion that does not adversely affect the thermoelectric properties can be introduced into the CZTS crystal phase, and the thermoelectric properties can be further improved. For example, as shown in FIG. 2, the CZTS crystal phase preferably has Cu vacancies V and doped alkali metal A. Examples of the alkali metal include Li, K, and Na. The thermoelectric material may be doped with one type of alkali metal, or may be doped with two or more types of alkali metals. The alkali metal preferably has at least Na. This is because the thermoelectric properties can be improved. In addition, at least a part of the doped alkali metal may be located in the Cu vacancy. The doping concentration of the alkali metal is, for example, 0.01 mol% or more, may be 0.05 mol% or more, or may be 0.1 mol% or more. On the other hand, the doping concentration of the alkali metal is, for example, 50 mol% or less, may be 30 mol% or less, or may be 10 mol% or less. The doping concentration of the alkali metal can be measured, for example, by ICP-AES (inductively coupled plasma atomic emission spectroscopy).

The composition of the thermoelectric material is not particularly limited, and may be, for example, (Cu _1-α ) ₂ ZnSnS ₄ A _X. α is, for example, 0.01 or more, may be 0.03 or more, or may be 0.05 or more. If α is too small, the thermoelectric properties may not be sufficiently improved. On the other hand, α is, for example, 0.10 or less, may be 0.08 or less. If α is too large, the crystallinity of the CZTS crystal phase may be reduced. A is an alkali metal. The alkali metal may be one type or two or more types. The alkali metal is as described above. X may be 0 or may be greater than 0. In the latter case, a strain that does not adversely affect the thermoelectric properties can be introduced into the CZTS crystal phase, and the thermoelectric properties can be further improved. X is, for example, 0.0001 or more, may be 0.0005 or more, or may be 0.001 or more. On the other hand, X is, for example, 0.5 or less, may be 0.3 or less, or may be 0.1 or less. X may satisfy X<2α. That is, the doping amount of the alkali metal may be less than the amount of Cu vacancies.

The thermoelectric material in this disclosure may be a p-type material or an n-type material. The shape of the thermoelectric material is not particularly limited. The thermoelectric material is preferably a single crystal material, since good thermoelectric properties can be obtained. The ZT value of the thermoelectric material is, for example, 0.4 or more at 700 K, and may be 0.6 or more, or may be 0.8 or more. The method of calculating the ZT value will be described in detail in the examples described later.

The method for producing the thermoelectric material in the present disclosure is not particularly limited, but may include, for example, a production method having a step of subjecting a raw material containing Cu, Zn, Sn, and S to heat treatment to obtain a polycrystalline material having a CZTS crystal phase, and a step of obtaining a single crystal material having a CZTS crystal phase from the polycrystalline material by the Travelling Heater Method (THM).

B. Thermoelectric Module The thermoelectric module in the present disclosure includes an element having the above-mentioned thermoelectric material. Fig. 3 is a schematic cross-sectional view illustrating a thermoelectric module in the present disclosure. The thermoelectric module 10 shown in Fig. 3 includes a first electrode substrate 3 having a first substrate 1 and a first electrode 2, a second electrode substrate 6 having a second substrate 4 and a second electrode 5, and a p-type element 7 and an n-type element 8 disposed between the opposing first electrode 2 and second electrode 5.

The second electrode substrate 6 has a plurality of second electrodes 5, and each second electrode 5 is electrically connected to a pair of p-type element 7 and n-type element 8. Furthermore, in two adjacent second electrodes 5, the p-type element 7 in one second electrode 5 and the n-type element 8 in the other second electrode 5 are electrically connected to the same first electrode 2. The thermoelectric module 10 shown in FIG. 3 corresponds to a thermoelectric module in which a pair of pn elements having a π-type structure are connected in series. In this disclosure, at least one of the p-type element 7 and the n-type element 8 has the thermoelectric material described in "A. Thermoelectric element" above.

According to the present disclosure, a thermoelectric module having good thermoelectric properties can be obtained by including elements having the above-mentioned thermoelectric materials.

The above element is, for example, a thin-film thermoelectric material. It is also preferable to orient the a-axis direction of the CZTS crystal phase in the above element so that it coincides with the direction of current flow. This is because better thermoelectric properties can be obtained. As shown in the examples described later, the ZT value in the a-axis direction tends to be greater than the ZT value in the c-axis direction. The direction of current flow refers to the direction from the high-temperature electrode substrate to the low-temperature electrode substrate in p-type elements, and refers to the direction from the low-temperature electrode substrate to the high-temperature electrode substrate in n-type elements.

The thermoelectric module may have only p-type elements or only n-type elements, but it is preferable to have both p-type and n-type elements, because a larger potential difference can be obtained. In addition, the pn element pair may have the above-mentioned π-type structure or a U-type structure.

Thermoelectric modules usually have an electrode substrate having a substrate and electrodes arranged on the substrate. There are no particular limitations on the electrode substrate, and any known substrate can be used. Examples of substrates include ceramics such as aluminum nitride. On the other hand, examples of electrodes include copper.

The thermoelectric module in the present disclosure is preferably a thermoelectric power generation module, for example because it can directly recover electrical energy from waste heat. In addition, the thermoelectric module is preferably a module used to convert thermal energy, such as exhaust heat from vehicles, factory exhaust heat, geothermal energy, body heat, etc., into electrical energy.

This disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and anything that has substantially the same configuration as the technical ideas described in the claims of this disclosure and has similar effects is included within the technical scope of this disclosure.

[Comparative Example 1]
_A single crystal of _Cu2ZnSnS4 was synthesized. First, the quartz tube for synthesis was cleaned and carbon-coated. Next, Cu, Zn, and Sn (all high-purity materials) were prepared as raw materials other than S, and washed with hydrochloric acid (HCl). Next, as shown in FIG. 4(a), Cu, Zn, Sn, and S were added to the quartz tube and vacuum-sealed. At this time, the amount of each raw material added was Cu:Zn:Sn:S = 2:1:1:4 molar ratio.

After that, as shown in Fig. 4(b), the vacuum-sealed quartz tube was placed in an electric furnace and heat-treated. As shown in Fig. 5, the heat treatment conditions were as follows: heating from room temperature to 650°C over 4 hours, holding at 650°C for 20 hours, heating from 650°C to 1100°C over 6 hours, holding at 1100°C for ₂₀ hours, and cooling from 1100°C to room temperature over 10 hours. As a result, polycrystals of _Cu2ZnSnS4 were obtained as shown in Fig. 4(c).

Then, as shown in Fig. 6(a), Sn, which is a solvent metal, was placed at the bottom of the quartz tube, and polycrystals of Cu ₂ ZnSnS ₄ were placed on top of it and vacuum-sealed. The vacuum-sealed quartz tube was placed in an electric furnace, and the tip of the quartz tube was heated to 850°C by a heater. Meanwhile, the temperature gradient of the cooling part in the electric furnace was set to 30-40°C/cm, and the quartz tube was moved at a speed of 4-5 mm/day, as shown in Fig. 6(b). After a certain number of days had passed, the temperature in the electric furnace was cooled to room temperature, and a thermoelectric material (single crystal of Cu ₂ ZnSnS ₄ ) was obtained.

[Example 1]
_A single crystal of _Cu1.9ZnSnS4 (a single crystal having Cu vacancies) was synthesized. Specifically, a thermoelectric material (a _single crystal of Cu1.9ZnSnS4) was obtained in the same manner as in Comparative Example 1, except that the amount of each raw material added was changed to a molar ratio of Cu:Zn:Sn:S = 1.9:1:1: ₄ .

[Example 2]
_A single crystal of Na-doped _Cu1.9ZnSnS4 (single crystal having Cu vacancies) was synthesized. Specifically, the amount of each raw material added was changed to a molar ratio of Cu:Zn:Sn:S = 1.9:1:1:4, and _Na2S was used as a dopant and added so that the Na doping concentration was 0.1 mol%, but otherwise the same procedure as in Comparative Example 1 was followed to obtain a thermoelectric material (single crystal of _Na -doped _Cu1.9ZnSnS4 ).

[Example 3]
A thermoelectric material (a single crystal of Na-doped _Cu1.9ZnSnS4 ) was obtained in the same manner as in Example 2, except that the Na-doping concentration was changed to 0.04 _mol %.

[evaluation]
The ZT values of the thermoelectric materials obtained in Examples 1 to 3 and Comparative Example 1 were determined.
ZT = σS ² T/κ
where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity.

The electrical conductivity and Seebeck coefficient were measured by cutting the thermoelectric material to a size of 2-5 mm x 2-5 mm x 10-15 mm to prepare samples, and measuring them using a thermoelectric property evaluation device (Advance Riko's ZEM-3) at a temperature of 300-800 K in a He atmosphere. The samples were cut along the a-axis and along the c-axis.

On the other hand, the thermal conductivity κ was calculated from the following formula.
κ= _λC
where λ is the thermal diffusivity, _Cp is the heat capacity, and D is the density.

Thermal diffusivity was measured by cutting the thermoelectric material to a size of 8 mm x 8 mm x 1 mm to prepare samples, using a laser flash analyzer (Netzsch LFA457) at a measurement temperature of 300-800 K in an Ar atmosphere. On the other hand, heat capacity was measured by cutting the thermoelectric material to a size of 3 mm x 3 mm x 3 mm to prepare samples, using a differential scanning calorimeter (Rigaku Thermo plus EVO2 DSCvesta) at a measurement temperature of 300-800 K in a vacuum atmosphere. Density was measured at room temperature using the Archimedes method.

The results of the obtained ZT value are shown in FIG. 7. As shown in FIG. 7, the thermoelectric material (Cu ₂ ZnSnS ₄ ) obtained in Comparative Example 1 had a low ZT value, for example, about 0.1 at 700K. In contrast, the thermoelectric material (Cu _1.9 ZnSnS ₄ ) obtained in Example 1 had a higher ZT value than Comparative Example 1. In this way, it was confirmed that the thermoelectric properties were improved by providing Cu vacancies. In addition, the ZT value of the thermoelectric materials (Cu _1.9 ZnSnS ₄ doped with Na) obtained in Examples 2 and 3 was significantly higher than that of Example 1. In this way, it was confirmed that the thermoelectric properties were significantly improved by doping with an alkali metal such as Na. It is speculated that the reason for the improvement in the thermoelectric properties is that the ionic radius of alkali metals such as Na is small, and therefore a distortion that does not adversely affect the thermoelectric properties can be introduced into the CZTS crystal phase. 7, the results in the a-axis direction showed better thermoelectric properties than the results in the c-axis direction in all of Examples 1 to 3. Therefore, it was confirmed that better thermoelectric properties can be obtained by orienting the a-axis direction of the CZTS crystal phase in the thermoelectric module so that it coincides with the direction of current flow.

REFERENCE SIGNS LIST 1 first substrate 2 first electrode 3 first electrode substrate 4 second substrate 5 second electrode 6 second electrode substrate 7 p-type element 8 n-type element 10 thermoelectric module

Claims (5)

1. A thermoelectric module comprising an element having a thermoelectric material,
The thermoelectric material has a CZTS crystalline phase containing Cu, Zn, Sn and S;
The CZTS crystal phase has Cu vacancies,
A thermoelectric module, wherein the a-axis direction of the CZTS crystal phase in the element is oriented to coincide with the direction in which a current flows. The thermoelectric module of claim 1 , wherein the thermoelectric material is doped with an alkali metal. The thermoelectric module of claim 2 , wherein the alkali metal comprises at least one of Li, Na, and K. The thermoelectric module according to claim 2 or 3, wherein the alkali metal comprises at least Na. The thermoelectric module according to any one of claims 1 to 4, wherein the thermoelectric material has a composition represented by (Cu _1-α ) ₂ ZnSnS ₄ A _X , where α satisfies 0.01≦α≦0.10, A is an alkali metal, and X satisfies 0≦X≦0.5.

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