KR20090106320A - Dichalcogenide thermoelectric materials - Google Patents

Abstract

PURPOSE: A dichalcogenide thermoelectric material is provided to ensure a very low thermal conductivity in comparison with a conventional metal or semiconductor and to be applied in refrigerant-free refrigerators, air conditioners, microcooling systems, and the like. CONSTITUTION: A composition comprising a dichalcogenide thermoelectric material of formula 1: RX2-aYa, wherein R is a rare earth or transition metal magnetic element, X and Y are each independently 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 In, and 0<=a<2.

Description

Dichalcogenide thermoelectric materials

The present invention relates to a dichalcogenide thermoelectric material having a very low thermal conductivity compared to conventional metals or semiconductors.

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

Active cooling using such thermoelectric materials improves the thermal stability of the device, has no vibration and noise, and is recognized as a small and environmentally friendly method because no separate condenser and refrigerant are used. Applications such as active cooling using thermoelectric materials can be used in refrigerant-free refrigerators, air conditioners, and various micro cooling systems. In particular, by attaching thermoelectric elements to various memory devices, the devices can be reduced in volume compared to conventional cooling methods. It can be maintained at a uniform and stable temperature, thereby improving the performance of the device.

On the other hand, if the thermoelectric material is used for thermoelectric power generation using the Seebeck effect, waste heat can be used as an energy source, and thus, an automobile engine and an exhaust system, a waste incinerator, waste steel waste, and human body medical equipment using human heat. It can be applied to various fields to improve energy efficiency, such as power supply, or to collect and use waste heat.

As a factor for measuring the performance of such a thermoelectric material, a dimensionless performance index ZT value defined by Equation 1 below is used.

<Equation 1>

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 performance index ZT, a material having high Seebeck coefficient and high electrical conductivity and low thermal conductivity should be found.

Until now, many kinds of thermoelectric materials have been developed, but most of them are materials exhibiting characteristics at high temperatures above room temperature, and materials exhibiting excellent performance at room temperature (300 to 400 K) are known as Bi ₂ Te ₃ and solid solution compounds thereof.

However, the development of various thermoelectric materials capable of improving the performance of the Bi ₂ Te ₃ is required, and for this purpose, there is increasing interest in thermoelectric materials having a decalcogenide structure.

For example, US Patent Publication No. 2003/0056819 and Japanese Patent Application Publication P2002-270907 filed by Japan NEC are known as patents related to the development of dichalcogenide thermoelectric materials having a two-dimensional layer structure. The thermoelectric material is represented by the chemical formula A _x BC _2-y (0≤x≤2, and 0≤y <1), in place of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba , Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Sc , Y, rare earth elements, and at least one element of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W, Ir, Sn, and at the place of C, It consists of one element of S, Se, and Te. In the examples of this document, the thermoelectric properties of A _x TiS ₂ materials are summarized, and the reported ZT values reported very high ZT values, such as 2.9 at room temperature and 3.9 at 700 K. No value has been reported, and in fact, in the case of A _x TiS ₂ , it is reported that the ZT value does not exceed 0.2 at room temperature (see Phys. Rev. B vol. 64, 241104, 2001 / J. Appl. Phys. vol. 102, 073703, 2007). Therefore, there exists a problem that the thermoelectric material described in the said document is difficult to commercialize.

In 2007, Catalin Chiritescu et al. Fabricated a very low thermal conductivity WSe ₂ thin film (see Science vol. 315, p. 351, 2007). Two-dimensional layered WSe ₂ is stacked in an in-plane direction irregularly on the c-axis and has a very small thermal conductivity of 0.05 W / mK. This means that the material has no directivity with respect to the in-plane, but has a very low thermal conductivity for a material having a well-aligned two-dimensional layer structure on the c-axis. However, such a material is a nonconductor, so the electrical conductivity is very low, the application of the thermoelectric material is unsuitable. It also has the problem that it is difficult to bulkify the material with random alignment in the in-plane direction.

The problem to be solved by the present invention is to provide a thermoelectric material having a very low thermal conductivity and a large power factor compared to a conventional metal or semiconductor.

In order to achieve the above object, the present invention,

It provides a thermoelectric material having a structure of formula (I):

<Formula 1>

RX _2-a Y _a

Food,

R represents a rare earth or transition metal magnetic element,

X and Y are elements 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 In. ,

A has a range of 0 ≦ a <2.

According to one embodiment of the 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 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.

As another method for achieving the above object, the present invention,

represents a two-dimensional layered structure with an in-plane irregular array,

It provides a thermoelectric material having a structure of formula (1).

<Formula 1>

RX _2-a Y _a

Food,

R represents a rare earth or transition metal magnetic element,

X and Y are elements 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 In. ,

A has a range of 0 ≦ a <2.

According to one embodiment of the invention, the layered structure of the thermoelectric material is X and R are alternately arranged between the layers consisting of X having a single layer structure, if necessary, at least a portion of the X is substituted with Y Has a structure.

According to one embodiment of the present invention, the thermoelectric material forms covalent bonds in the in-plane direction, and the interlayer bonds form ionic bonds and / or van der Waals bonds.

The invention also provides a compound of formula

<Formula 1>

RX _2-a Y _a

Food,

R represents a rare earth or transition metal magnetic element,

X and Y are elements 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 In. ,

A has a range of 0 ≦ a <2.

Since the present invention provides a thermoelectric material having a large power factor and a very low thermal conductivity, the thermoelectric material may be usefully used in a refrigerantless refrigerator, an air conditioner, waste heat generation, thermoelectric nuclear power generation for military aerospace, and micro cooling system.

The thermoelectric material according to the present invention has the structure of Formula 1 below:

<Formula 1>

RX _2-a Y _a

Food,

R represents a rare earth or transition metal magnetic element,

X and Y are elements 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 In. ,

A has a range of 0 ≦ a <2.

The thermoelectric material of Chemical Formula 1 according to the present invention has a two-dimensional layered structure, irregular crystal structure in the in-plane direction, crystallinity in the c-axis, as a result shows a low thermal conductivity, in particular The Y component, which is a doping element, is selectively added to RX _{[a] 2, which} is _a basic component, to improve electrical conductivity, thereby increasing the ZT value of Equation 1 below.

<Equation 1>

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 Chemical Formula 1, R represents a rare earth or transition metal magnetic element, and for example, one or more elements selected from the group consisting of lanthanide rare earth elements, Mn, Fe, Co, Ni, Cu, Zn and Ag may be used. Ce is preferred as the lanthanide rare earth element.

X forming the two-dimensional layered structure which is a basic structure with R in the thermoelectric material of Formula 1 is S, Se, Te, P, As, Sb, Bi, C, si, Ge, Sn, B, Al, Ga and One or more elements selected from the group consisting of In can be used, with S, Se or Te being particularly preferred.

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

The compound having a basic structure of RX _2-a Y _a may exhibit a two-dimensional layered structure, such that the two-dimensional layered structure alternates between R and X between layers of X as shown in FIG. 3. It has an arranged structure. In the above structure, the crystal structure is irregular in the in-plane direction, and the crystallinity is obtained in the c-axis.

In this structure, the X-layer in the in-plane direction is covalently bonded to form strong bonds, and in the c-axis direction, weak bonds are formed by ionic bonds or van der Waals bonds. Since the transfer of phonons is difficult, thermal conductivity is lowered in the c-axis direction. In particular, since the array is irregular in the direction perpendicular to the in-plane, it has an optimal condition of low thermal conductivity.

In general, thermal conductivity (k _tot ) can be _classified into thermal conductivity (k _Latt ) by lattice and thermal conductivity (k _el ) by electron, such as k _tot = k _Latt + k _el . Since it is determined by the Wiedemann-Frantz law as in Equation 2, it is not an artificially small factor. Therefore, good thermoelectric materials should have low lattice thermal conductivity, which can be obtained through control of the lattice structure.

<Equation 2>

K _el = LT / ρ (L = 2.44 X 10 ^-8 ΩW / K ² )

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

1. Polycrystalline Synthesis Method

(1) Method using Ampoule: Inserting raw material into quartz tube or ampoule made of metal such as tungsten or tantalum, sealing with vacuum and heat-treating for several hours to several tens of hours at or near melting point How to include.

(2) Arc melting method: A method comprising the step of putting a raw material element into the chamber to discharge the arc in an inert gas atmosphere to melt the raw material element to make a sample.

(3) Solid state reaction (Solid state reaction): The powder is mixed well and processed hardly and then heat treated for several hours to several tens of hours at the temperature of 70 to 90% of the melting point, or the mixed powder for several hours to several tens at the temperature of the melting point or more. Heat-treating for an hour and then grinding at room temperature, and then sintering it for several hours to several tens of hours at a temperature of 70 to 90% of the melting point.

2. Single Crystal Growth Method

(1) Metal flux method: A raw material element, such as congruent melting, that provides an atmosphere for the raw material and the raw material to grow well into crystals at a high temperature, and has a melting point above the melting point of the desired crystal. The low metal element is placed in a crucible, and is slowly cooled from the temperature at which the metal element is roughly melted to the temperature at which the crystal is formed, followed by heat treatment at a rate of, for example, 1 to 10 ^o C / hour to grow the crystal. Method comprising the steps.

(2) Bridgeman method: The raw material is placed in a crucible and heated at a high temperature, for example, at a temperature above the melting point of the raw material for several hours to several tens of hours until the raw material is dissolved, and then the temperature difference at both ends of the crucible To slowly move the high temperature region to generate tens of tens of ^o C, for example, to locally dissolve the sample at a rate of 1 to 10 ^o C / hour to pass the entire sample into the crystal growth region to grow crystals. How to include.

(3) Optical floating zone: The raw material is made into seed rods and feed rods in the form of rods, and then the feed rods are melted by locally dissolving the light of the lamp in one focal point. Slowly growing the portion upwards to grow the crystals.

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

The present invention can be used to produce a thermoelectric material using any of the various methods described above without limitation, there is no particular limitation.

In the thermoelectric material obtained by the above process, when the two-band conduction of electrons and holes coexists by optimizing the current density through additional element doping, only one of the electrons or holes causes the power factor to occur. It is made of a thermoelectric material having a large and very low thermal conductivity.

When elemental doping is performed as described above, the thermoelectric material essentially includes Y, a doping element, and thus the current density is optimized to have improved electrical conductivity. This increases the power factor S ² σ in Equation 1, resulting in an increase in the ZT value.

That is, by replacing the doping element Y with the X site, the current density of either the hole or the electron is increased, and as a result, the canceling effect caused by the electron and the hole can be suppressed, thereby further improving the conduction characteristics on the c-axis. It becomes possible. Due to this improved conduction characteristic, the power factor S ² σ is increased to increase the Seebeck coefficient.

As the Y component doped as described above, one or more of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga, and In may be used, and P, As, Sb, At least one of Bi, C, Si, Ge, Sn, B, Al, Ga and In is preferred. The content of the Y component is preferably less than 2 moles with respect to 1 mole of the R component of Formula 1.

The doping element Y component may be added in the form of a one-component, two-component or three-component system, in the case of the two-component system may be added in a ratio of 1: 9 to 9: 1, in the case of a three-component system, 1: 0.1-9.0: It may be added in the ratio of 0.1-9.0, but it is not specifically limited to these.

Such a Y component is to replace a part of the X component of the components of the basic structure RX _2-a Y _a during the doping process, thereby optimizing the current density. Such a doping process may be performed by adding as part of the raw material element of the polycrystalline growth method or the single crystal growth method.

As described above, the thermoelectric material of the present invention exhibits low thermal conductivity and additionally injects electrons and holes by doping treatment to improve the Seebeck coefficient cancellation phenomenon of electron-holes, thereby increasing the Seebeck coefficient and optimizing the current density. Since the conductivity can be improved, high thermoelectric performance can be expected.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1

According to the method using the ampoule, a polycrystalline growth method, the raw material elements Ce, Te, and Sn are quantified in a predetermined ratio, placed in an ampoule made of quartz tube, sealed with vacuum, and heat-treated at 850 ° C for 24 hours for rare earth calkenoids. 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, respectively. The molar ratio of the compound was confirmed through inductively coupled plasma spectroscopy.

The CeTe ₂ has a two-dimensional layer structure, and forms a weak ionic bond between the Te layer and the Ce-Te block, CeTe _2-x Sn _x is doped with Sn in the CeTe ₂ to position a portion of Te It has a substituted structure.

Example 2

According to the method using the polycrystalline growth method Ampoule, the rare earth calkenonide was quantitatively determined in a proportion of the raw material elements Ce, Se and Sn in a quartz tube, sealed in vacuum and heat-treated at 850 ° C for 24 hours. CeSe ₂ , CeSe _1.9 Sn _0.1 , CeSe _1.8 Sn _0.2 and CeSe _1.5 Sn _0.5 were synthesized, respectively. The molar ratio of the compound was confirmed through inductively coupled plasma spectroscopy.

The CeSe ₂ is a flat orthorhombic structure in the b direction as shown in FIG. 4, 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 Sn is doped at ₂ to replace a part of Se.

Experimental Example 1 Measurement of Thermal Conductivity

The thermal conductivity 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 is shown in FIG. 5, and the thermal conductivity measurement method is thermal relaxation using a laser flash method. The thermal relaxation was measured and calculated. As can be seen from FIG. 5 they exhibited very low thermal conductivity, in particular CeTe ₂ and CeTe _1.95 Sn _0.05 have very low thermal conductivity on the order of 1.50 to 1.58 W / mK at 300K, which is commercially available Sb-doped The value corresponds to about 55% lower than that of Bi ₂ Te ₃ and significantly lower than that of other thermoelectric materials. When the molar ratio _x of Sn in the CeTe _2-x Sn _x is 1.0 or more, the thermal conductivity becomes large, resulting in a phenomenon of deterioration. Table 1 shows the results of comparing the thermal conductivity of CeTe ₂ and CeTe _1.5 Sn _{0.5 with those} of commercially available thermoelectric materials.

TABLE 1

division K _tot (W / mK) k _el (W / mK) k _latt (W / mK) Bi ₂ Te ₃ 2.9 1.6 1.3 Bi ₂ Te ₃ (Sb ₂ Te ₃ 33.3 mol%) 2.5 1.6 0.9 Bi ₂ Te ₃ (Sb ₂ Te ₃ 66.7 mol%) 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 thermal conductivity due to the lattice of CeTe ₂ and CeTe _1.5 Sn _0.5 of the present invention is similar to the commercialized thermoelectric material, but the resulting thermal conductivity is very low value of 2.0 or less due to low thermal conductivity due to electrons. It can be seen that it has.

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 compound obtained in Example 2 is shown in Figure 6 and the thermal conductivity measurement method was calculated by measuring the thermal relaxation by the laser flash method. As can be seen from FIG. 6, they exhibited very low thermal conductivity. In particular, as the content of Sn as the doping component is increased, the thermal conductivity is further lowered, resulting in CeSe _1.5 Sn _0.5 reaching 0.2 W / mK.

Experimental Example 2: Seebeck coefficient measurement

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 are shown in FIG. 7. The Seebeck coefficient was measured using the 4-terminal method. As can be seen from FIG. 7, it can be seen that when Te is replaced with Sn in CeTe ₂ , the absolute value of the Seebeck coefficient increases. The CeTe ₂ has a small gap semiconductor structure in which electrons and holes coexist, and holes move in the Te layer and electrons move in the Ce-Te block, thereby deteriorating conduction characteristics in the c-axis, thereby canceling effects caused by the electrons and holes. This can cause a decrease in Seebeck coefficient. Therefore, it is necessary to reinforce the Seebeck coefficient by supplementing it. Accordingly, in case of CeTe _1.95 Sn _0.05 , CeTe _1.9 Sn _0.1 , CeTe _1.5 Sn _0.5, and CeTeSn compounds in which Sn is substituted for Te, the Seebeck coefficient is controlled by controlling the carrier of the Te layer. The coefficients are further improved. It is possible to control the current density of electrons and holes by substituting Sn for Te, thereby increasing the Seebeck coefficient.

Seebeck coefficients of the CeSe _1.9 Sn _0.1 and CeSe _1.8 Sn _0.2 compounds obtained in Example 2 are shown in FIG. 8. As can be seen from FIG. 8, when Se is replaced with Sn, it is understood that the Seebeck coefficient is improved by controlling the carrier of the Se layer so that they have a relatively high amount of Seebeck coefficient.

Experimental Example 3: Measurement of Electrical Resistance

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 compounds obtained in Example 1 were measured and shown in FIG. 9. Electrical resistance was measured by the 4-terminal method. As shown in FIG. 9, it can be seen that CeTe ₂ having an electrical resistance of about 10 mΩW-cm has an electrical resistance value of CeTe _1.5 Sn _0.5 of ₂ mΩW-cm by the doping treatment.

This may also change the electrical resistance value according to the temperature by the doping treatment, which is lowered because the number of holes is increased by replacing the doping element Sn in the Te site.

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

Therefore, it is possible to improve the electrical conductivity by lowering the electrical resistance by doping treatment depending on the material, and as a result, it can be seen that the power factor is increased to improve the value of the Seebeck coefficient.

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

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

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 CeTe _{2 - x} Sn _x (x≤1.0) obtained in Example 1 of the present invention. It is a graph showing thermal conductivity of.

6 is CeSe _{2 - x} Sn _x (x≤0.5) obtained in Example 2 of the present invention. It is a graph showing thermal conductivity of.

7 is CeTe _{2 - x} Sn _x (x≤1.0) obtained in Example 1 of the present invention. This graph shows the Seebeck coefficient of.

8 is CeSe _{2 - x} Sn _x (x≤0.5) obtained in Example 2 of the present invention. This graph shows the Seebeck coefficient of.

9 is CeTe _{2 - x} Sn _x (x≤1.0) obtained in Example 1 of the present invention. It is a graph showing the electrical resistance value of.

10 is CeSe _{2 - x} Sn _x (x≤0.5) obtained in Example 2 of the present invention. It is a graph showing the electrical resistance value of.

Claims (9)

A thermoelectric material having a structure of Formula 1 <Formula 1> RX _{2 - a} Y _a Food, R represents a rare earth or transition metal magnetic element, X and Y are different from each other, and represent one or more elements selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga and In, A has 0 ≦ a <2. The method of claim 1, And wherein 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 of claim 1, Thermoelectric material, characterized in that X is S, Se or Te. The method of claim 1, And the thermoelectric material exhibits a thermal conductivity of 2 W / mK or less at room temperature. represents a two-dimensional layered structure with an in-plane irregular array, A thermoelectric material having a structure of Formula 1 <Formula 1> RX _a Y _b Food, R represents a rare earth or transition metal magnetic element, X and Y are different from each other, and represent one or more elements selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga and In, A has a range of 0 ≦ a <2. The method of claim 1, The thermoelectric material according to claim 1, wherein X and R are alternately arranged between layers of X having a single layer structure. The method according to claim 5 or 6, A thermoelectric material, characterized in that at least part of X has a structure substituted with Y. The method of claim 5, The thermoelectric material forms a covalent bond in an in-plane direction, and the interlayer bonds form ionic bonds and / or van der Waals bonds. A compound of formula <Formula 1> RX _2-a Y _a Food, R represents a rare earth or transition metal magnetic element, X and Y are different from each other, and represent one or more elements selected from the group consisting of S, Se, Te, P, As, Sb, Bi, C, Si, Ge, Sn, B, Al, Ga and In, A has a range of 0 ≦ a <2.

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