KR101464699B1 - Dichalcogenide thermoelectric materials - Google Patents

Abstract

A decalcogenide thermoelectric material having a very low thermal conductivity compared to a conventional metal or semiconductor is provided.

The thermoelectric material has a structure represented by the following formula (1)

≪ Formula 1 >

RX _2-a Y _a

In the formula,

Wherein R represents a rare earth or transition metal magnetic element,

X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, ,

And a has a range of 0? A <2.

Description

Dichalcogenide thermoelectric materials &lt; RTI ID = 0.0 &gt;

The present invention relates to a decalcogenide thermoelectric material having a very low thermal conductivity as compared with conventional metals and semiconductors.

In general, thermoelectric materials are materials that can be applied to active cooling and cogeneration using Peltier effect and Seebeck effect. As shown in FIG. 1, the Peltier effect is a phenomenon in which electrons in the holes of the p-type material and electrons of the n-type material move when a DC voltage is applied from the outside, thereby generating heat and endothermic heat at both ends of the material. As shown in FIG. 2, the Seebeck effect refers to a phenomenon in which electrons and holes move when a heat is supplied from an external heat source, and a current flows in the material to generate electric power.

The active cooling using the thermoelectric material improves the thermal stability of the device, eliminates vibration and noise, and does not use a separate condenser and a refrigerant. Thus, the active cooling is recognized as a small-volume, environmentally friendly method. Application of active cooling using such thermoelectric materials can be applied to a non-refrigerated refrigerator, an air conditioner, and various micro cooling systems. Particularly, when thermoelectric elements are attached to various memory devices, The temperature can be maintained at a uniform and stable temperature, and the performance of the device can be improved.

On the other hand, if the thermoelectric material is used for thermoelectric power generation by using the Seebeck effect, waste heat can be utilized as an energy source, and it can be used as an energy source for automobile engines and exhaust devices, waste incinerators, waste heat of steelworks, Such as the power of the power source, or to collect waste heat and use it in various fields.

As a factor for measuring the performance of such a thermoelectric material, the dimensionless figure of merit ZT, which is defined by the following Equation 1, is used.

&Quot; (1) &quot;

(Where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity).

In order to increase the dimensionless figure of merit ZT, a material having high Seebeck coefficient, high electrical conductivity and low thermal conductivity should be sought.

Though many kinds of thermoelectric materials have been developed so far, many materials exhibiting properties at room temperature or higher temperature are known to exhibit excellent performance at room temperature (300 to 400 K), which is known as Bi ₂ Te ₃ and its solid solution compounds.

However, the development of various thermoelectric materials capable of improving the performance of the Bi ₂ Te ₃ has been demanded. For this purpose, interest in thermoelectric materials having a decalcogenide structure is increasing.

For example, US Pat. Nos. US2003 / 0056819 and P2002-270907 filed by NEC in Japan are known as patents relating to the development of a dichalcogenide thermoelectric material having a two-dimensional layer structure. The thermoelectric material is represented by a chemical formula of A _x BC _2-y (0? _{X? 2} , and 0? _Y <1), and Li, Na, K, Rb, Cs, Mg, , Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, And at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Ir, and Sn. S, Se, and Te. In the examples of this document, the thermoelectric properties of A _x TiS ₂ materials are summarized and reported ZT values are very high at 2.9 at room temperature and 3.9 at 700 K. However, And the ZT value in the case of A _x TiS ₂ has not been reported to exceed 0.2 at room temperature (see Phys. Rev. B vol. 64, 241104, 2001 / J. Appl. Phys. vol. 102, 073703, 2007). Therefore, there is a problem that it is difficult to commercialize the thermoelectric material described in the above document.

Catalin Chiritescu et al. (2007) produced WSe ₂ thin films with low thermal conductivity (see Science vol 315, p. 351, 2007). When WSe ₂ having a two-dimensional layer structure is irregular in the in-plane direction and the thin film is stacked on the c-axis, the thermal conductivity is very small, about 0.05 W / mK. This means that materials with a well-ordered two-dimensional layered structure on the c-axis do not have directionality for in-plane, but thermal conductivity is very low. However, such a material is a nonconductor and its electrical conductivity is very low, so its application to thermoelectric materials is unsuitable. Also, there is a problem that it is difficult to bulk-form materials that are randomly aligned in the in-plane direction.

A problem to be solved by the present invention is to provide a thermoelectric material having a very small thermal conductivity and a large power factor as compared with conventional metals and semiconductors.

According to an aspect of the present invention,

There is provided a thermoelectric material having a structure represented by the following formula

&Lt; Formula 1 >

RX _2-a Y _a

In the formula,

Wherein R represents a rare earth or transition metal magnetic element,

X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, ,

And a has a range of 0? A <2.

According to one embodiment of the present invention, R represents at least one element selected from the group consisting of lanthanide rare-earth elements, Mn, Fe, Co, Ni, Cu, Zn and Ag.

According to one embodiment of the present invention, X represents S, Se or Te.

According to one embodiment of the present invention, the thermoelectric material exhibits a thermal conductivity of 2 W / mK or less at room temperature.

According to another aspect of the present invention,

dimensional layered structure having an in-plane random arrangement,

There is provided a thermoelectric material having a structure represented by the following formula (1).

&Lt; Formula 1 >

RX _2-a Y _a

In the formula,

Wherein R represents a rare earth or transition metal magnetic element,

X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, ,

And a has a range of 0? A <2.

According to an embodiment of the present invention, the lamellar structure of the thermoelectric material is characterized in that X and R are alternately arranged between the layers of X having a single layer structure, and at least a part of X is substituted with Y if necessary Structure.

According to an embodiment of the present invention, the thermoelectric material forms a covalent bond in the in-plane direction, and the interlayer bond forms an ionic bond and / or a Van der Waals bond.

The present invention also provides compounds of formula 1:

&Lt; Formula 1 >

RX _2-a Y _a

In the formula,

Wherein R represents a rare earth or transition metal magnetic element,

X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, ,

And a has a range of 0? A <2.

Since the present invention provides a thermoelectric material having a large power factor and a very small thermal conductivity, the thermoelectric material can be usefully used in a non-refrigerated refrigerator, air conditioner, waste heat power generation, thermoelectric nuclear power generation for military aerospace, micro cooling system,

The thermoelectric material according to the present invention has a structure represented by the following formula (1)

&Lt; Formula 1 >

RX _2-a Y _a

In the formula,

Wherein R represents a rare earth or transition metal magnetic element,

X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, ,

And a has a range of 0? A <2.

The thermoelectric material of Formula 1 according to the present invention has a two-dimensional layered structure, has an irregular crystal structure in the in-plane direction, has crystallinity in the c-axis, and consequently exhibits low thermal conductivity, The Y component as a doping element is selectively added to RX _{[a] 2} as _a basic component to improve the electric conductivity, thereby increasing the ZT value of the following formula (1).

&Quot; (1) &quot;

(Where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity).

In the thermoelectric material of Formula 1, R represents a rare earth or transition metal magnetic element. For example, at least one element selected from the group consisting of lanthanide rare earth elements, Mn, Fe, Co, Ni, Cu, As the lanthanide rare earth element, Ce is preferable.

In the thermoelectric material of Formula 1, X, which forms a two-dimensional layer structure, which is a basic structure together with R, includes S, Se, Te, P, As, Sb, Bi, C, si, Ge, Sn, In, and S, Se or Te is particularly preferable.

Wherein R and X form a covalent bond with each other to form _a basic structure of RX _2-a Y _a , wherein a is a real number having a range of 0 to less than 2, and Y is selectively added as a doping element, Thereby optimizing the current density of the material.

The compound having the basic structure of RX _2-a Y _a may exhibit a two-dimensional layered structure. As shown in FIG. 3, the two-dimensional layered structure is characterized in that R and X are alternately And has an ordered structure. In the above structure, the crystal structure is irregular in the in-plane direction, and the c-axis has crystallinity.

Also, in this structure, the layer consisting of X in the in-plane direction forms a strong bond by forming a covalent bond and forms weak bond by ionic bonding or van der Waals bonding in the c-axis direction, The transfer of phonons is difficult, and the thermal conductivity decreases in the c-axis direction. In particular, it has an irregular arrangement with respect to the direction perpendicular to the in-plane, and thus has an optimal condition that the thermal conductivity is low.

In general, the thermal conductivity (k _tot ) can be distinguished by the lattice thermal conductivity (k _Latt ) and the electron thermal conductivity (k _el ) as k _tot = k _Latt + k _el , It is not an artificially small factor because it is determined according to the Wiedemann-Frantz law, Therefore, a good thermoelectric material should have a low lattice thermal conductivity, which can be obtained by controlling the lattice structure.

&Quot; (2) &quot;

_{K el = LT / ρ (L} = 2.44 X 10 -8 ΩW / K 2)

A method of synthesizing a thermoelectric material having the chemical formula of RX _{[a] 2} is divided into a polycrystalline synthesis method and a single crystal growth method.

1. Polycrystalline synthesis method

(1) Method using Ampoule: The raw material element is placed in an ampoule made of a quartz tube or a metal such as tungsten or tantalum, sealed by vacuum, and heat-treated for several hours to several hours at or above the melting point Methods of inclusion.

(2) Arc melting method: a step of placing a raw material element in a chamber and discharging an arc in an inert gas atmosphere to prepare a sample by melting the raw material element.

(3) Solid state reaction: Solid-state reaction: the powder is well mixed and hardened and then heat-treated at a temperature of 70 to 90% of the melting point for several hours to several hours, or the mixed powder is heated for several hours to several tens of times And then sintering at a temperature of 70 to 90% of the melting point for several hours to several tens of hours.

2. Single crystal growth method

(1) Metal flux method: A raw material element which provides an atmosphere so that a raw material element and a raw material element can grow well at high temperature, for example, congruent melting, The low metal element is placed in a crucible and the crystal is grown by heat treatment at a rate of, for example, 1 to 10 ^° C / hour while slow cooling from a temperature at which the metal element coalesces and melts to a temperature at which crystals are produced Lt; / RTI &gt;

(2) Bridgeman method: The raw material element is placed in a crucible and heated at a high temperature, for example, at a temperature higher than the melting point of the raw material element for several hours to several tens of hours until the raw material element is dissolved, To 10 &lt; ⁰ &gt; C, so that the sample is slowly dissolved in the sample at a rate of 1 to 10 ^o C / hour while allowing the entire sample to pass through the crystal growth region to grow crystals Methods of inclusion.

(3) Optical floating zone: The raw material element is made into a rod-shaped seed rod and a feed rod. Then, the feed rod is locally fused by collecting the light of the lamp in a focal point, &Lt; / RTI &gt; and slowly pulling up the portion upward to grow crystals.

(4) Vapor transport method: The raw material element is placed under the quartz tube and the raw element part is heated at the vaporization temperature for several hours to several hours, and the upper part of the quartz tube is kept at a low temperature, And a step of growing crystals causing a solid-phase reaction.

The present invention can produce thermoelectric materials using any of the various methods described above without limitation, and there is no particular limitation.

The thermoelectric material obtained by such a process optimizes the current density through additional element doping so that when two band conduction in which electrons and holes coexist occurs, only one of the electrons or holes conducts the conduction characteristic, Thermoelectric material with a high thermal conductivity and a high thermal conductivity.

When the element doping is performed as described above, the thermoelectric material essentially contains Y, which is a doping element, and thereby the current density is optimized to have improved electrical conductivity. This increases the power factor (S ² ?) In the above equation (1) and increases the ZT value as a result.

In other words, by replacing the doping element Y with the X-position, the current density of either the hole or the electron becomes large, and as a result, the effect of canceling electrons and holes can be suppressed, . Due to such improved conduction characteristics, the power factor (S ² σ) increases and the Seebeck coefficient increases.

As the Y component to be doped, at least one of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga, At least one of Bi, C, Si, Ge, Sn, B, Al, Ga and In is preferable. The content of the Y component may be less than 2 moles per 1 mole of the R component of the formula (1).

The Y component, which is a doping element, may be added in the form of a one-component system, a two-component system or a three-component system. In the case of the two-component system, the molar ratio may be added in a ratio of 1: 9 to 9: 1: 0.1 to 9.0: 0.1 to 9.0, but are not limited thereto.

In the doping process, the Y component replaces a portion of the X component of the RX _2-a Y _a , which is the basic structure, and as a result, the current density is optimized. Such a doping process may be performed by adding the source material as a part of the polycrystalline growth method or the single crystal growth method.

The thermoelectric material of the present invention as described above exhibits a low thermal conductivity and additionally dopes electrons and holes by doping to improve the anti-bekking factor of the electron-hole to increase the anti-coercive force and to optimize the current density, The conductivity can be improved, so that a high thermoelectric performance can be expected.

Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited thereto.

Example 1

The raw material elements Ce, Te and Sn were quantitatively determined at predetermined ratios according to a method using an ample, which is a polycrystalline growth method, and placed in an ampoule made of quartz tube, sealed with a vacuum, and heat-treated at 850 ° C for 24 hours to obtain a rare- CeTe ₂ , CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.7 Sn _0.3 , CeTe _1.5 Sn _0.5, and CeTeSn were synthesized. The molar ratio of the compounds was confirmed by inductively coupled plasma spectroscopy.

The CeTe ₂ has a two-dimensional layer structure, and a weak ionic bond is formed between the Te layer and the Ce-Te block. CeTe _2-x Sn _x is doped with Sn in the CeTe ₂ , And has a substituted structure.

Example 2

Ce, Se and Sn as raw materials were quantitatively determined in accordance with a method using an ample, which is a polycrystalline growth method, into a ampoule made of quartz tube, sealed with a vacuum, and heat-treated at 850 ° C for 24 hours to obtain a rare- CeSe ₂ , CeSe _1.9 Sn _0.1 , CeSe _1.8 Sn _0.2 and CeSe _1.5 Sn _0.5 were synthesized. The molar ratio of the compounds was confirmed by inductively coupled plasma spectroscopy.

As shown in FIG. 4, the CeSe ₂ has an orthorhombic structure flat in the b direction, has a two-dimensional layer structure, forms a weak ionic bond between the Se layer and the Ce-Se block, and CeSe _2-x Sn _x is the CeSe ₂ has a structure in which Sn is doped to replace a part of Se.

Experimental Example 1: Measurement of thermal conductivity

The thermal conductivity of the CeTe ₂ , CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.5 Sn _0.5 and CeTeSn compounds obtained in Example 1 is shown in FIG. 5, and the thermal conductivity is measured by a laser flash method using thermal relaxation Thermal relaxation was measured and calculated. As can be seen from FIG. 5, they exhibited very low thermal conductivities, particularly CeTe ₂ and CeTe _1.95 Sn _0.05 having very low thermal conductivities of about 1.50 to 1.58 W / mK at 300 K, which is comparable to the commercialized Sb-doped Compared to Bi ₂ Te ₃ , it is about 55% lower than that of Bi ₂ Te ₃ , which is much lower than other thermoelectric materials. When the molar ratio _x of Sn in CeTe _2-x Sn _x is 1.0 or more, the thermal conductivity is increased and the performance is somewhat lowered. The thermal conductivity of CeTe ₂ and CeTe _1.5 Sn _0.5 is compared with that of commercially available thermoelectric materials.

[Table 1]

division K _tot (W / mK) _el k (W / mK) k _latt (W / mK) Bi ₂ Te ₃ 2.9 1.6 1.3 Bi ₂ Te ₃
(33.3 mol% of Sb ₂ Te ₃ ) 2.5 1.6 0.9 Bi ₂ Te ₃
(66.7 mol% of Sb ₂ Te ₃ ) 2.8 2.2 0.6 CeTe ₂ 1.40 0.09 1.31 CeTe _1.5 Sn _0.5 1.36 0.37 0.99

As shown in Table 1, the lattice constants of the CeTe ₂ and CeTe _1.5 Sn _0.5 of the present invention were similar to those of the commercialized thermoelectric materials, but the low thermal conductivity due to electrons resulted in a very low value of the resulting thermal conductivity of 2.0 or less . &Lt; / RTI &gt;

The thermal conductivity of the CeSe ₂ , CeSe _1.9 Sn _0.1 , CeSe _1.8 Sn _0.2, and CeSe _1.5 Sn _0.5 compounds obtained in Example 2 is shown in FIG. 6, and the thermal conductivity was measured by measuring the thermal relaxation by a laser flash method. As can be seen from FIG. 6, they exhibited very low thermal conductivity, and as the content of Sn, which is a doping component, increases, the thermal conductivity is further lowered to reach 0.2 W / mK for CeSe _1.5 Sn _0.5 .

Experimental Example 2: Measurement of Seebeck coefficient

FIG. 7 shows the Seebeck coefficients of CeTe ₂ , CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.5 Sn _0.5 and CeTeSn compounds obtained in Example 1 above. The 4-terminal method was used to measure the Seebeck coefficient. As can be seen from FIG. 7, the absolute value of the Seebeck coefficient increases when Ce is substituted for Te in CeTe ₂ . The CeTe ₂ has a small-gap semiconductor structure in which electrons and holes coexist. Since holes are formed in the Te layer and electrons are moved in the Ce-Te block, the conduction characteristics to the c- Which may cause the degradation of the Seebeck coefficient. Therefore, it is necessary to reinforce the Seebeck coefficient to compensate for this, so that in the case of CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.5 Sn _0.5 and CeTeSn compounds substituted with Sn in the Te layer, The coefficient is further improved. This is because by substituting Sn for Te, it is possible to control the current density of electrons and holes, which increases the Seebeck coefficient.

The Seebeck coefficient of the CeSe _1.9 Sn _0.1 and CeSe _1.8 Sn _0.2 compounds obtained in Example 2 is shown in FIG. As can be seen from FIG. 8, when the Se is replaced with Sn, the Seebeck carrier is controlled to improve the Seebeck coefficient, so that they have a relatively high Seebeck coefficient.

Experimental Example 3: Measurement of electric resistance value

The electrical resistance of CeTe ₂ , CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.7 Sn _0.3 , CeTe _1.5 Sn _0.5 and CeTe Sn compound obtained in Example 1 was measured and shown in FIG. Electrical resistance was measured by 4-terminal method. As shown in FIG. 9, CeTe ₂ having an electric resistance of about 10 mΩW-cm has an electric resistance value of CeTe _1.5 Sn _0.5 of ₂ mΩW-cm by the doping treatment.

This is because the electric resistance depending on the temperature may be changed by the doping treatment. This is because the number of holes is increased by substituting the doping element Sn for the Te site, so that the electric resistance is lowered.

The electrical resistance of the CeSe ₂ , CeSe _1.9 Sn _0.1 and CeSe _1.8 Sn _0.2 compounds obtained in Example 2 was measured and shown in FIG. Electrical resistance was measured by 4-terminal method. As can be seen from FIG. 10, the CeSe ₂ , CeSe _1.9 Sn _0.1 and CeSe _1.8 Sn _0.2 compounds have a very high electrical resistance.

Therefore, it is possible to improve the electrical conductivity by lowering the electric resistance by doping depending on the material, and as a result, it is possible to improve the value of the Seebeck coefficient by increasing the power factor.

1 is a schematic view showing thermoelectric cooling by the Peltier effect.

Fig. 2 is a schematic diagram showing the thermoelectric generation by the Seebeck effect.

Figure 3 shows a two-dimensional layered structure of the RX _{2 - a} Y _a compound according to the present invention.

4 shows a two-dimensional layered structure of the CeSe ₂ compound according to the present invention.

5 is a graph showing the relationship between CeTe _{2 - x} Sn _x ( _x ? 1.0) obtained in Example 1 of the present invention, Is a graph showing the thermal conductivity of the substrate.

FIG. 6 is a graph showing the relationship between CeSe _{2 - x} Sn _x ( _x ? 0.5) obtained in Example 2 of the present invention, Is a graph showing the thermal conductivity of the substrate.

7 is a graph showing the relationship between CeTe _{2 - x} Sn _x ( _x ? 1.0) obtained in Example 1 of the present invention, In the graph of FIG.

8 is a graph showing the relationship between CeSe _{2 - x} Sn _x ( _x ? 0.5) obtained in Example 2 of the present invention, In the graph of FIG.

9 is a graph showing the relationship between CeTe _{2 - x} Sn _x ( _x ? 1.0) obtained in Example 1 of the present invention, Of Fig.

10 is a graph showing the relationship between CeSe _{2 - x} Sn _x ( _x ? 0.5) obtained in Example 2 of the present invention, Of Fig.

Claims (9)

A thermoelectric material having a structure represented by the following formula &Lt; Formula 1 > RX _{2 - a} Y _a In the formula, Wherein R represents a rare earth or transition metal magnetic element, X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga, The above a has 0? A < 2. The method according to claim 1, Wherein the R is at least one element selected from the group consisting of lanthanide rare earth elements, Mn, Fe, Co, Ni, Cu, Zn and Ag. The method according to claim 1, Wherein X is S, Se or Te. The method according to claim 1, Wherein the thermoelectric material exhibits a thermal conductivity of 0.2 to 2 W / mK at room temperature. dimensional layered structure having an in-plane random arrangement, A thermoelectric material having a structure represented by the following formula &Lt; Formula 1 > RX _2-a Y _a In the formula, Wherein R represents a rare earth or transition metal magnetic element, X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga, And a has a range of 0? A <2. The method according to claim 1, Wherein X and R are alternately arranged between the layers of X in which the layered structure of the thermoelectric material has a structure of a single layer. The method according to claim 5 or 6, And at least a part of X is substituted with Y. 6. The method of claim 5, Wherein the thermoelectric material forms a covalent bond in an in-plane direction, and the interlayer bond forms an ionic bond or a Van der Waals bond. A compound of formula &Lt; Formula 1 > RX _2-a Y _a In the formula, Wherein R represents a rare earth or transition metal magnetic element, X and Y are different from each other and represent at least one element selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga, And a has a range of 0? A <2.

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