KR102339632B1 - Half-heusler type thermoelectric material, method for manufacturing the same, thermoelectric element comprising the same - Google Patents

Abstract

The present invention relates to a half-Heusler-based thermoelectric material, a method for manufacturing the same, and a thermoelectric device including the same, and more particularly, to a half-Heusler-based thermoelectric material exhibiting high thermoelectric performance due to improved Seebeck coefficient and electrical conductivity, and the like It relates to a manufacturing method and a thermoelectric device including the same.

Description

HALF-HEUSLER TYPE THERMOELECTRIC MATERIAL, METHOD FOR MANUFACTURING THE SAME, THERMOELECTRIC ELEMENT COMPRISING THE SAME

The present invention relates to a half-Heusler-based thermoelectric material, a method for manufacturing the same, and a thermoelectric device including the same, and more particularly, to a half-Heusler-based thermoelectric material exhibiting high thermoelectric performance due to improved Seebeck coefficient and electrical conductivity, and the like It relates to a manufacturing method and a thermoelectric device including the same.

Recently, due to environmental problems caused by resource depletion and combustion, research on thermoelectric materials using waste heat as one of alternative energy is accelerating.

The energy conversion efficiency of such a thermoelectric material depends on ZT, which is a figure of merit value of the thermoelectric material. Here, ZT is determined according to a Seebeck coefficient, electrical conductivity, thermal conductivity, and the like. More specifically, ZT is proportional to the square of the Seebeck coefficient and electrical conductivity, and is inversely proportional to thermal conductivity. Therefore, in order to increase the energy conversion efficiency of the thermoelectric conversion element, it is necessary to develop a thermoelectric conversion material having a high Seebeck coefficient or electrical conductivity or a low thermal conductivity.

Recently, as Half-Heusler-based compounds are known to have excellent thermal stability even at high temperatures of 500° C. or higher, interest as one of the candidates for thermoelectric materials is increasing. 1 is a schematic diagram showing a general crystal lattice structure of such a Haf-Heusler-based compound. The Haf-Heusler-based compound has the general formula of ABC (A is a typical metal element, a transition metal element or a rare earth metal element, B is a typical metal element, a transition metal element or a rare earth metal element, and C is a typical metal element. ), and each element of A, B, and C (red, blue, and green elements in FIG. 1) may form a crystal lattice structure as shown in FIG. These Haf-Heusler-based compounds exhibit relatively high electrical conductivity, Seebeck coefficient, and the like, and thus are spotlighted as one of the strong candidates for thermoelectric materials.

However, even when a previously known Haf-Heusler-based compound is used, its energy conversion efficiency is not sufficient due to relatively high thermal conductivity, insufficient electrical conductivity, Seebeck coefficient, and the like. For this reason, various attempts have been made to further increase its ZT value, that is, energy conversion efficiency, but it is not yet sufficient.

Accordingly, the present invention provides a novel half-Heusler-based thermoelectric material exhibiting high thermoelectric performance due to improved Seebeck coefficient and electrical conductivity, and a method for manufacturing the same.

Another object of the present invention is to provide a thermoelectric device including the thermoelectric material.

The present invention provides a half-Heusler-based thermoelectric material represented by the following formula (1):

[Formula 1]

MgCuM _x Sb

In Formula 1, x is 0.001 to 0.1 as an addition molar ratio of M,

M is a monovalent metal element selected from the group consisting of Ag, Au, Na, K, Rb and Cs.

The present invention also comprises the steps of heat-treating a mixture comprising a raw material including Mg, Cu and Sb and a raw material including a monovalent metal selected from the group consisting of Ag, Au, Na, K, Rb and Cs; and

It provides a method of manufacturing the thermoelectric material comprising the step of sintering the heat-treated mixture.

In addition, the present invention provides a thermoelectric device including the thermoelectric material.

Hereinafter, a half-Heusler-based thermoelectric material according to a specific embodiment of the present invention, a method for manufacturing the same, and a thermoelectric device including the same will be described in more detail.

According to one embodiment of the invention, there is provided a half-Heusler-based thermoelectric material represented by the following Chemical Formula 1:

[Formula 1]

MgCuM _x Sb

In Formula 1, x is 0.001 to 0.1 as an addition molar ratio of M,

M is a monovalent metal element selected from the group consisting of Ag, Au, Na, K, Rb and Cs.

As a result of the experiment of the present invention, by adding a monovalent metal element such as Ag to a known Haf-Heusler-based compound of MgCuSb, the thermoelectric material of one embodiment in which the monovalent metal element is included in the crystal lattice structure has a significantly improved output factor (PF=σS ² ) and the figure of merit ZT value, that is, it was confirmed that very good thermoelectric performance could be exhibited. This is expected to be due to the following technical principles.

As demonstrated in Examples to be described later, it was confirmed that the specific peak on the XRD pattern of the known compound of MgCuSb is slightly shifted depending on whether or not the monovalent metal element such as Ag is added and the content thereof. From this fact, in the crystal lattice structure of the thermoelectric material of the embodiment, it is predicted that the monovalent metal element is added to some vacancies in the crystal lattice structure of the known compound of MgCuSb. More specifically, in the crystal lattice structure of a known compound of MgCuSb, three elements of Mg, Cu and Sb occupy positions of each element in the crystal lattice as shown in FIG. 1 , and the monovalent metal element is added In this case, it is predicted that these monovalent metal elements are added to and occupy the vacant positions of the crystal lattice remaining after the three types of elements are occupied (that is, have the structure of Chemical Formula 1).

Due to the occupation of the crystal lattice of the monovalent metal element, the density of states and entropy around the Fermi level are increased, and it is predicted that the thermoelectric material according to an embodiment may exhibit a more improved Seebeck coefficient. In addition, as the monovalent metal element forming monovalent cations differently from the three elements of Mg, Cu, and Sb occupies some crystal lattice sites, it is predicted that hole or electron mobility will increase. As a result, it is predicted that the thermoelectric material of one embodiment may exhibit more improved electrical conductivity and the like.

In addition, the monovalent metal element acts as a kind of point defect in the crystal lattice structure of a known compound of MgCuSb, thereby reducing the thermal conductivity of the compound of MgCuSb, or at least the electrical conductivity and the rise of the Seebeck coefficient are smaller than It is expected that width will only maintain or slightly increase thermal conductivity.

As a result, the thermoelectric material of one embodiment can exhibit excellent electrical conductivity, Seebeck coefficient and output factor, etc., compared to the conventionally known half-Heusler-based thermoelectric material such as MgCuSb, and on the other hand, it does not exhibit a large increase in thermal conductivity. Therefore, it seems that a significantly improved figure of merit ZT value, that is, very good thermoelectric performance can be exhibited. Therefore, the thermoelectric material of one embodiment can be very preferably applied as a thermoelectric material for various thermoelectric devices.

On the other hand, the thermoelectric material of one embodiment is a monovalent metal added to the known compound of MgCuSb, more specifically, at least one monovalent metal (monovalent cation) selected from the group consisting of Ag, Au, Na, K, Rb and Cs. metals that can be Among them, since the monovalent metal of Ag is included, the thermoelectric material of an embodiment may exhibit superior electrical conductivity and Seebeck coefficient, and thus may exhibit superior thermoelectric performance.

And, in the thermoelectric material of the embodiment, the monovalent metal may be added in a ratio of 0.001 to 0.1 moles with respect to 1 mole of the MgCuSb compound (or each of Mg, Cu, or Sb contained therein). By adding such a ratio, the degree of improvement of thermoelectric performance by the addition of the monovalent metal (the degree of improvement of the output factor according to the increase of electrical conductivity/Seebeck coefficient) can be further increased, and the crystal structure of the basic Haf-Heusler-based compound is characteristics may not be compromised.

In addition, as already described above, in the thermoelectric material of one embodiment, the monovalent metal is added to the vacancy of the crystal lattice remaining after three elements of Mg, Cu, and Sb in the known compound structure of MgCuSb. By occupying it, it is possible to occupy the crystal lattice site. Accordingly, the thermoelectric material of the embodiment may have the structure of Formula 1 above.

Meanwhile, according to another embodiment of the present invention, a mixture comprising a raw material including Mg, Cu and Sb and a raw material including a monovalent metal selected from the group consisting of Ag, Au, Na, K, Rb and Cs is heat-treated to do; and sintering the heat-treated mixture is provided.

It was confirmed that a thermoelectric material according to an exemplary embodiment exhibiting excellent thermoelectric performance can be appropriately manufactured by such a manufacturing method. Hereinafter, the manufacturing method of these other embodiments will be looked at at each step.

First, in the manufacturing method, a mixture including a raw material including Mg, Cu and Sb and a raw material including a monovalent metal selected from the group consisting of Ag, Au, Na, K, Rb and Cs may be formed. . In forming such a raw material mixture, a raw material (metal itself or a metal compound, etc.) containing each target metal element is added in a desired composition ratio, and then, grinding or milling, and optionally pelletizing may be performed. The raw material mixture thus formed may be in a powder state, a pellet state, an ingot state, or the like depending on the formation process thereof.

Such a raw material mixture may be formed and a heat treatment/reaction step may be performed thereon. This heat treatment step may be performed by a method similar to various intermetallic reaction processes known before, and as an example, the above raw material mixture formed in a solid state may be heat-treated to proceed with the reaction.

And, this heat treatment step, for example, proceeds at a temperature of 500 ℃ to 1100 ℃, it is possible to properly react the metals in the raw material mixture. This heat treatment step may be performed in an electric furnace or the like, and may be performed in a vacuum or an inert gas atmosphere.

In addition, the heat treatment step may be performed in a single step, or may be performed in two or more steps.

In addition, the step of pulverizing the mixture may be further performed after the heat treatment step or during the heat treatment step (eg, between the plurality of steps when the heat treatment step is performed in a plurality of steps).

On the other hand, after the above-described heat treatment step and optional grinding step, the step of sintering the heat-treated mixture may be performed. By the progress of this sintering step, the thermoelectric material of the above-described embodiment may be manufactured.

This pressure sintering step may be performed by spark plasma sintering or hot pressing well known to those skilled in the art, for example, may be performed at a temperature of 400° C. or higher and under a pressure of 5 MPa or higher.

And, after the above sintering, a cooling step may be optionally further performed.

However, each of the above-described reaction steps may be carried out according to conventional reaction conditions and methods for forming a metal compound such as a Haf-Heusler-based compound, and specific reaction conditions and methods are described in Examples to be described later, so that Further description will be omitted.

Meanwhile, according to another embodiment of the present invention, there is provided a thermoelectric device including the thermoelectric material of the embodiment described above. Such a thermoelectric element may include the thermoelectric material of the embodiment as a p-type or n-type thermoelectric material, and for this purpose, it may include an additional p-type element or an n-type element in the thermoelectric material of the embodiment in a state in which an additional p-type element or an n-type element is additionally doped. have. However, at this time, the type or doping method of the usable p-type element or n-type element is not particularly limited, and the element and doping method that have been generally used for applying the p-type or n-type thermoelectric material before may be applied. .

The thermoelectric element may include a thermoelectric element formed by processing and molding the p-type or n-type thermoelectric material in a sintered state, and may include an insulating substrate and an electrode together with the p-type or n-type thermoelectric material. The combination structure of the thermoelectric element, the insulating substrate, and the electrode may follow the structure of a conventional thermoelectric element.

In addition, as the insulating substrate, a sapphire substrate, a silicon substrate, a pyrex substrate, a quartz substrate, etc. may be used, and an electrode including any metal or a conductive metal compound may be used as the electrode.

As the above-described thermoelectric element includes the thermoelectric material of one embodiment, it may exhibit excellent thermoelectric performance, and may be preferably applied as a thermoelectric cooling system or a thermoelectric power generation system in various fields and uses.

According to the present invention, a half-Heusler-based thermoelectric material exhibiting high thermoelectric performance due to improved Seebeck coefficient and electrical conductivity, a method for manufacturing the same, and a thermoelectric device including the same can be provided.

1 is a schematic diagram showing a general crystal lattice structure of a general Haf-Heusler-based compound.
2 is an XRD pattern of the thermoelectric materials of Comparative Examples and Examples 1 to 3, and shows that certain peaks of the XRD pattern are partially shifted as Ag is added in the thermoelectric materials of Examples 1 to 3;
3 is a graph showing the comparison of electrical conductivity according to temperature of the thermoelectric materials of Comparative Examples and Examples 1 to 3;
4 is a graph showing the comparison of Seebeck coefficients for each temperature of the thermoelectric materials of Comparative Examples and Examples 1 to 3;
5 is a graph showing the comparison of output factors for each temperature of the thermoelectric materials of Comparative Examples and Examples 1 to 3;
6 is a graph showing the comparison of thermal conductivity according to temperature of the thermoelectric materials of Comparative Examples and Examples 1 to 3;

The invention is described in more detail in the following examples. However, the following examples only illustrate the present invention, and the content of the present invention is not limited by the following examples.

Comparative Example: Preparation of thermoelectric material (MgCuSb)

Cu, Mg and Sb powders were prepared, and they were mixed with a hand mill to form a mixture of MgCuSb composition. This mixture was placed in a quartz tube and vacuum sealed to make an ampoule, and then the ampoule was placed in a tube furnace and heat-treated at a temperature of 630°C. The powder thus synthesized was pressurized at 50 MPa and sintered under pressure at a temperature of 600° C. for 7 minutes using a discharge plasma sintering method.

Example 1: Preparation of thermoelectric material (MgCuAg _0.005 Sb; 0.5 mol% of Ag added)

A compound of MgCuSb was synthesized by the method of Comparative Example. Then, Ag powder was added and mixed with a hand mill _{to form a mixture of MgCuAg 0.005} Sb composition (0.005 mol (0.5 mol %) of Ag based on 1 mol of Cu, Mg, and Sb was used). This mixture was placed in a quartz tube and vacuum-sealed to make an ampoule, and then the ampoule was placed in a tube furnace and first heat treated at a temperature of 630°C. After the first heat treatment, the resultant was pulverized, and then the second heat treatment was performed again at the same temperature. The powder synthesized in this way was pulverized and classified through a sieve to obtain a powder having a particle size of 70 μm or less.

This powder was pressurized to 50 MPa and sintered under pressure at a temperature of 600° C. for 7 minutes by a discharge plasma sintering method.

Example 2: Preparation of thermoelectric material (MgCuAg _0.01 Sb; Ag 1.0 mol% added)

A thermoelectric material was prepared in the same manner as in Example 1, except that 0.01 mol (1.0 mol%) of Ag was used based on 1 mol of Cu, Mg, and Sb (based on 1 mol of the compound of MgCuSb).

Example 3: Preparation of thermoelectric material (MgCuAg _0.015 Sb; Add 1.5 mol% of Ag)

A thermoelectric material was prepared in the same manner as in Example 1, except that 0.015 mol (1.5 mol%) of Ag was used based on 1 mol of Cu, Mg, and Sb (based on 1 mol of the MgCuSb compound).

Experimental example

1. Phase analysis according to XRD pattern

Phase analysis of the thermoelectric materials prepared in Examples 1 to 3 and Comparative Examples using an X-ray diffraction analyzer (XRD) is shown in FIG. 2 .

Referring to FIG. 2 , as compared to the comparative example in which Ag was not added, a shift of a specific peak at a low angle was confirmed in the XRD patterns of the thermoelectric materials of Examples 1 to 3 . This indicates an increase in the lattice parameter, confirming the addition of Ag to the crystal lattice structure.

In addition, in Examples 1 to 3, as the amount of Ag added increased, the shift of the peak appeared to be large. This also confirmed the addition of Ag to the crystal lattice structure.

2. Temperature dependence of electrical conductivity

The electrical conductivity of the thermoelectric material specimens prepared in Examples 1 to 3 and Comparative Examples was measured according to temperature change, and is shown in FIG. 3 . Such electrical conductivity was measured in a temperature range of 50 to 500° C. by using LSR-3 manufactured by Linseis, which is a resistivity measuring device, and using a direct current probe method.

As shown in FIG. 3 , it was confirmed that the Example showed a large increase in electrical conductivity compared to the Comparative Example according to the addition of Ag, and this increase in electrical conductivity was confirmed in all temperature bands to be measured.

3. Seebeck coefficient measurement and temperature dependence of Seebeck coefficient

For the thermoelectric material specimens prepared in Examples 1 to 3 and Comparative Examples, the Seebeck coefficient (S) was measured according to temperature change, and is shown in FIG. 4 . The Seebeck coefficient was measured in a temperature range of 50 to 500° C. using a measuring device, LSR-3 manufactured by Linseis, and applying a differential voltage/temperature technique.

As shown in FIG. 4 , it was confirmed that the Seebeck coefficient in the Example was significantly increased compared to the Comparative Example according to the addition of Ag, and this increase in the Seebeck coefficient was confirmed in all temperature bands to be measured.

4. Temperature dependence on output factor

For the thermoelectric material specimens prepared in Examples 1 to 3 and Comparative Examples, the output factors were calculated according to temperature changes and shown in FIG. 5 .

The output factor is defined as Power factor (PF) = σS ² , and was calculated using the values of σ (electrical conductivity) and S (Seebeck coefficient) shown in FIGS. 3 and 4 .

As shown in FIG. 5 , it was confirmed that the output factor of the Example was significantly increased compared to the Comparative Example according to the addition of Ag (average 5 μW/cm·K ² to 10 μW/cm·K ² , about 100% improved), this increase in the output factor was confirmed in all temperature bands to be measured.

5. Temperature dependence of thermal conductivity

The thermal conductivity of the thermoelectric material specimens prepared in Examples 1 to 3 and Comparative Examples was measured according to temperature change and is shown in FIG. 6 . In the measurement of such thermal conductivity, first, a thermal conductivity measuring equipment, Netzsch's LFA457 equipment, was used and a laser scintillation method was applied to measure _{thermal diffusivity (D) and heat capacity (C p ).} These measured values were applied to the formula "thermal conductivity (k) = DρC _p (ρ is the sample density measured by the Archimedes method)" to calculate the thermal conductivity (k).

In addition, the total thermal conductivity (κ= κ _L + κ _E ) is divided into lattice thermal conductivity (κ _{L ) and thermal conductivity (κ E} ) calculated according to the Wiedemann-Franz law (κ _E = L σT). (L) uses a value calculated from the Seebeck coefficient according to temperature.

As shown in FIG. 6 , although an increase in thermal conductivity was observed in the Example compared to the Comparative Example according to the addition of Ag, the increase in the thermal conductivity decreased as the measurement temperature increased. It was confirmed that there was almost no difference between the figures.

Claims (9)

A half-Heusler-based thermoelectric material represented by the following formula (1):
[Formula 1]
MgCuM _x Sb
In Formula 1, x is 0.001 to 0.015 as an addition molar ratio of M,
M is a monovalent metal element selected from the group consisting of Ag, Au, Na, K, Rb and Cs.
The half-Heusler-based thermoelectric material according to claim 1, wherein the monovalent metal is added in a ratio of 0.001 to 0.015 moles based on 1 mole of Mg.
heat-treating a mixture including a raw material including Mg, Cu and Sb and a raw material including a monovalent metal selected from the group consisting of Ag, Au, Na, K, Rb and Cs; and
The method for manufacturing the thermoelectric material of claim 1, comprising the step of sintering the heat-treated mixture.
The method of claim 3 , wherein the heat treatment is performed by a solid phase reaction.
The method of claim 3 , wherein the heat treatment is performed at a temperature of 500° C. or higher.
The method of claim 3 , wherein the sintering step is performed by a discharge plasma sintering method or hot pressing.
The method of claim 3 , wherein the sintering step is performed at a temperature of 400° C. or higher and a pressure of 5 MPa or higher.
The method of claim 3, further comprising pulverizing the mixture after the heat treatment step or during the heat treatment step.
A thermoelectric element comprising the thermoelectric material of claim 1 or 2.

Images