KR102399079B1 - Half-heusler type thermoelectric material, method for manufacturing the same, thermoelectric element comprising the same - Google Patents

Abstract

The present invention provides a semi-Heusler-based thermoelectric material capable of exhibiting excellent thermoelectric properties by increasing the Seebeck coefficient and decreasing thermal conductivity while minimizing the decrease in electrical conductivity, a method for manufacturing the same, and a thermoelectric element including the same.

Description

Semi-Heusler-based thermoelectric material, method for manufacturing same, and thermoelectric element including same

The present invention relates to a semi-Heusler-based thermoelectric material capable of exhibiting excellent thermoelectric properties by increasing a Seebeck coefficient and decreasing thermal conductivity while minimizing a decrease in electrical conductivity, a method for manufacturing the same, and a thermoelectric device including the same.

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

The energy conversion efficiency of such a thermoelectric conversion material depends on ZT, which is a thermoelectric figure of merit value of the thermoelectric conversion material. Here, ZT is determined according to a Seebeck coefficient, electrical conductivity, and thermal conductivity as in Equation 1 below. More specifically, ZT is proportional to the square of the Seebeck coefficient and electrical conductivity, and is inversely proportional to thermal conductivity.

[Equation 1]

ZT=σS ² T/κ

(In Equation 1, σ is electrical conductivity, S is Seebeck coefficient, κ is total thermal conductivity, and T is absolute temperature.)

Therefore, in order to increase the energy conversion efficiency of the thermoelectric element, the Seebeck coefficient (S) or the electrical conductivity (σ) is high to exhibit a high output factor (PF=σS ² ), or a thermoelectric conversion material having a low total thermal conductivity (κ) development is needed. However, the Seebeck coefficient and the electrical conductivity have an exchange-cancellation relationship, and as one value increases according to a change in the concentration of electrons or holes as carriers, the other value decreases. For example, a metal having high electrical conductivity has a low Seebeck coefficient, and an insulating material having low electrical conductivity has a high Seebeck coefficient. The trade-off relationship between the Seebeck coefficient and the electrical conductivity is a great constraint in increasing the output factor PF.

Recently, as it is known that a Half-Heusler-based compound has excellent thermal stability even at a high temperature of 500° C. or higher, interest as one of the candidate materials for thermoelectric materials is increasing. The semi-Heusler-based compound exhibits a high output factor (PF) due to high electrical conductivity along with high mechanical strength, but has high thermal conductivity compared to other materials due to the interdependence of electrical/thermal properties, which is the ZT value acts as a lowering factor.

Recently, a composition in which a portion of Fe is substituted with Co in an NbFeSb-based material has been developed, but since Co substitution imparts an n-type property, there is a problem in that electrical conductivity is reduced and an output factor is reduced.

Since the electrical conductivity, Seebeck coefficient, and thermal conductivity constituting the thermoelectric figure of merit are related to each other, it is difficult to simultaneously improve the output factor and reduce the thermal conductivity with the conventional substitution composition.

US Patent Publication No. 2015-0270465

Accordingly, the present invention is to provide a novel semi-Heusler-based thermoelectric material capable of exhibiting excellent thermoelectric properties by increasing the Seebeck coefficient and decreasing thermal conductivity while minimizing the decrease in 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.

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

[Formula 1]

(Nb _1-a M _a FeSb _1-b ) _1-x (AgSbTe ₂ ) _x

In Formula 1,

M is one or more transition metals selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr,

0≤a<1, 0<b≤0.1, and 0<x<0.04.

According to another embodiment of the present invention, a raw material including Nb, Fe, Ag, Sb, and Te, respectively, and Ti, Zr, Hf, V, Ta, Mo, W, Co, and Cr selected from the group consisting of heat-treating a mixture containing a raw material including one or more transition metals (M) to be used; And it provides a method for producing the semi-Heusler-based thermoelectric material comprising the step of pressure sintering the heat-treated mixture.

Furthermore, according to another embodiment of the present invention, there is provided a thermoelectric device including the above-described semi-Heusler-based thermoelectric material.

Hereinafter, a semi-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.

A semi-Heusler-based thermoelectric material according to an embodiment of the present invention is represented by the following Chemical Formula 1:

[Formula 1]

(Nb _1-a M _a FeSb _1-b ) _1-x (AgSbTe ₂ ) _x

In Formula 1,

M is one or more transition metals selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr,

a is the molar ratio of addition or substitution of M, 0≤a<1,

b is the defect molar ratio of Sb, 0<b≤0.1,

x is the introduction molar ratio of (AgSbTe ₂ ) in the thermoelectric material, 0<x<0.04.

Specifically, in the thermoelectric material according to an embodiment of the present invention, a part of Nb is substituted with a specific transition metal (M) or an unsubstituted (Nb _{1 - a} M _a FeSb ₁ - _b )-based compound forms a matrix. and the compound of (AgSbTe ₂ ) is dispersed. Since the compound of (AgSbTe ₂ ) dispersed in the matrix scatters only low-energy charge carriers, the high energy charge carriers can transport charges without being affected.

The (AgSbTe ₂ ) compound may be present as a nano-sized structure dispersed in the matrix or may be present as an alloy. When the (AgSbTe ₂ ) compound is present as a nano-sized structure on the grain boundary between the (Nb _1-a M _a FeSb _1-b )-based compound particles, it seems to cause a lot of phonon scattering due to this.

In addition, the (AgSbTe ₂ ) compound has a very low thermal conductivity of 0.6 W/mK level. Accordingly, by controlling the content of the (AgSbTe ₂ ) compound introduced into the (Nb _{1 -a} M _{a FeSb 1 -b} )-based compound-containing matrix, the movement of the charge carrier (hole) is not restricted and occurs during the movement of heat. It is possible to maximize phonon-scattering. As a result, excellent thermoelectric properties can be exhibited by increasing the Seebeck coefficient and decreasing the thermal conductivity while minimizing the decrease in electrical conductivity.

Specifically, the (AgSbTe ₂ ) compound is an amount corresponding to x in the semi-Heusler-based thermoelectric material represented by Chemical Formula 1, that is, a molar ratio of 0<x<0.04 or 100 mol% of the semi-Heusler thermoelectric material. It may be included in an amount corresponding to more than 0 mol% and less than 4 mol%. When included in the above content range, it may be included homogeneously dispersed in the main matrix without forming a second phase, and as a result, multiple scattering effects are exhibited to improve the Seebeck coefficient and reduce thermal conductivity. can Since the secondary phase is not formed, only the half-Heusler (HH) phase is observed in the phase analysis using X-ray diffraction analysis (XRD) for the thermoelectric material. However, when the AgSbTe ₂ is included in an excess of 4 mol% or more outside the above content range, a Fe-Sb-based secondary phase or a Ti-Te-based secondary phase is formed while the raw material is decomposed, thereby reducing electrical conductivity. . (AgSbTe 2 ) In terms of significant improvement effect by controlling the content of the compound (AgSbTe ₂ ) contained in the thermoelectric material, the (AgSbTe ₂ ) compound is preferably 0.01≤x≤ in the semi-Heusler-based thermoelectric material represented by Formula 1 It may be included in a molar ratio of 0.03.

Meanwhile, in the present specification, the secondary phase means phases different from the phase having a desired composition. The existence of the secondary phase can be confirmed through XRD phase analysis, and the content of the secondary phase can be confirmed through analysis by Rietveld refinement of the XRD pattern. R _wp is one of the reliability factors of the Rietveld method analysis. Roughly, if R _wp is less than 5, the analysis result is considered reliable.

The (AgSbTe ₂ ) compound is an Ag-rich composition, has a crystal structure or has an amorphous strain, and may be a nanostructure such as nanoparticles, nanowires, nanorods, nanotubes or fragments. there is. In the present invention, nano means a size of 1 nm to 1,000 nm as a size in nanometers (nm), and if it has a size at the nano level, it can induce phonon scattering at the grain boundary, so it is better to lower the thermal conductivity. effective. More specifically, it may be nanoparticles having an average particle size (D50) of 1 nm to 100 nm.

On the other hand, the (Nb _{1 - a} M _a FeSb ₁ - _b )-based compound constituting the matrix in the semi-Heusler thermoelectric material according to an embodiment of the present invention is a semi-Heusler material based on p-type NbFeSb In a semiconductor compound in which a part of Nb is substituted or unsubstituted with the transition metal M, when the transition metal M is substituted, the electrical conductivity is further increased, thereby increasing the output factor and reducing the thermal conductivity.

The (Nb _{1 -a} M _a FeSb _{1 -b} )-based compound has _a cubic structure, and the space group is F-43m. The crystal structure of the (Nb _{1 -a} M _{a FeSb 1 -b} )-based compound in the thermoelectric material can be confirmed through XRD Rietveld refinement analysis.

In general, when an element is changed to another element at the same site of the crystal structure, cell parameters, crystallinity, intensity, etc. may change due to differences in size between elements, differences in chemical properties of elements, differences in bonding strength between elements, etc. In the present invention, even if the transition metal M selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr replaces a part of Nb, there is no change in the crystal structure.

More specifically, in the crystal lattice structure of a known compound of NbFeSb, three elements of Nb, Fe, and Sb occupy positions of each element in the crystal lattice. When the transition metal M is included, among the three elements A part of the crystal lattice site of Nb is substituted and the crystal lattice site is occupied. Due to the occupancy or substitution of the crystal lattice of the transition metal element, the density of states and entropy around the Fermi level increase, so that the thermoelectric material of an embodiment may exhibit a more improved Seebeck coefficient. In addition, as the transition metal element occupies some of the crystal lattice sites, hole or electron mobility increases, and as a result, the thermoelectric material of an embodiment may exhibit more improved electrical conductivity. Accordingly, it is possible to exhibit superior electrical conductivity, Seebeck coefficient and output factor, etc., compared to conventional semi-Heusler-based thermoelectric materials such as NbFeSb, and also to lower the lattice thermal conductivity value due to the solid solution alloying effect, greatly An improved figure of merit ZT value, that is, a very good thermoelectric performance may be exhibited.

The thermoelectric material according to an embodiment of the present invention is a transition metal M added to the known compound of NbFeSb, and specifically, one selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr. It may include the above metals, and among them, by including Ti, the thermoelectric material of one embodiment may exhibit superior electrical conductivity and Seebeck coefficient, and thus may exhibit superior thermoelectric performance.

In this case, the transition metal M may be added or substituted in an amount corresponding to a, that is, in a molar ratio of 0<a<1 to 1 mole of the Nb _{1 - a} M _a FeSb _{1 - b} -based compound (transition metal M is not provided). a=0). By addition within the above content range, the degree of improvement in thermoelectric performance by the addition of the transition metal without impairing the crystal structure or characteristics of the Van-Heusler-based compound (improving output factor due to increase in electrical conductivity/Seebeck coefficient) degree) can be further improved. Considering the significant improvement effect of the transition metal addition amount control, the addition amount may preferably be 0.05≤a≤0.4.

Further, in the Nb _{1 - a} M _a FeSb _{1 - b} -based compound, b is the defect molar ratio of Sb, and is 0<b≤0.1, preferably 0<b≤0.05, more preferably 0<b≤0.04 or b=0.04. When b=0, a FeSb-based secondary phase is formed, and when it exceeds 0.1, the thermoelectric figure of merit is greatly reduced.

More specifically, in consideration of the remarkable improvement effect of optimizing the content of constituent elements, the semi-Heusler-based thermoelectric material according to an embodiment of the present invention may be represented by the following Chemical Formula 2:

[Formula 2]

(Nb _0.85 M _0.15 FeSb _0.96 ) _1-x (AgSbTe ₂ ) _x

In Formula 2, M is as defined above, and 0<x<0.04, more preferably 0.005≤x≤0.03.

Considering that the thermoelectric performance index (ZT) of the thermoelectric material is improved by inducing phonon scattering and lowering the thermal conductivity as the particle size of the thermoelectric material decreases, the thermoelectric material according to an embodiment of the present invention is nanometer It is desirable to have a level of particle size.

On the other hand, according to another embodiment of the invention, from the group consisting of a raw material containing Nb, Fe, Ag, Sb, and Te, respectively, Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr heat-treating a mixture including a raw material including one or more selected transition metals (M); and sintering the heat-treated mixture is provided.

It was confirmed that a thermoelectric material according to an exemplary embodiment exhibiting excellent thermoelectric performance may 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 raw material including Nb, Fe, Ag, Sb, and Te, respectively, and at least one selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr A mixture comprising a raw material including a transition metal (M) may be formed. In forming such a raw material mixture, a raw material (metal itself or a metal compound, etc.) including each target metal element is added in a desired composition ratio, and then mixed, grinded or milled, and optionally pelletized. 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, by proceeding at a temperature of 500 ℃ to 1200 ℃, more specifically 1000 ℃ to 1200 ℃, 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 the optional pulverization step, the step of pressure 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, at a temperature of 500° C. or more, more specifically 500° C. to 1100° C., and 5 MPa or more, More specifically, it may proceed under a pressure of 10 MPa to 70 MPa.

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

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

The thermoelectric material manufactured by the above-described manufacturing method is represented by Chemical Formula 1, and does not include an Fe-Sb-based secondary phase and a Ti-Te-based secondary phase, or a Fe-Sb-based secondary phase and a Ti-Te-based secondary phase. 5 wt% or less, more specifically 4 wt% or less, and even more specifically 2 wt% or less of at least one of the thermoelectric materials based on the total weight of the thermoelectric material. In the present invention, the inclusion and content of the secondary phase in the thermoelectric material can be confirmed through XRD Rietveld refinement analysis.

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. there is. 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 generally used for applying the thermoelectric material to the p-type or n-type from the past 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 semi-Heusler-based thermoelectric material capable of exhibiting excellent thermoelectric properties by increasing the Seebeck coefficient and decreasing thermal conductivity while minimizing the decrease in electrical conductivity, a method for manufacturing the same, and a thermoelectric element including the same can be provided.

1 is a graph showing the results of X-ray diffraction (XRD) analysis of powders and sintered bodies obtained by pulverizing synthetic ingots prepared through solid-phase reaction in the thermoelectric material manufacturing process of Example 1 and Comparative Examples 1 and 2;
2 is a graph showing the comparison of electrical conductivity according to temperature of the thermoelectric materials of Examples 1 and 2 and Comparative Examples 1 and 2;
3 is a graph showing the comparison of Seebeck coefficients for each temperature of the thermoelectric materials of Examples 1 and 2 and Comparative Examples 1 and 2;
4 is a graph showing the comparison of output factors for each temperature of the thermoelectric materials of Examples 1 and 2 and Comparative Examples 1 and 2;
5 is a graph showing a comparison of the total thermal conductivity (κ _tot ) of the thermoelectric materials of Examples 1 and 2 and Comparative Examples 1 and 2 for each temperature.
6 is a graph showing the comparison of thermoelectric performance indexes for each temperature of the thermoelectric materials of Examples 1 and 2 and Comparative Examples 1 and 2;
7 is _a graph showing the thermoelectric figure of merit (ZT) according to the Sb composition ratio in the (Nb _{1 -a} M _a FeSb _{1 -b} ) composition in the compound semiconductor according to an embodiment of the present invention.

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.

Example 1

Nb _{0 . 8415} Ti _{0 . 1485} Fe _{0 . 99} Ag _{0 . 01} Sb _{0 .} The powder of each element was weighed and mixed according to a mol ratio of _{9604 Te 0.02 ,} followed by cold press. The resulting mixture was placed in an alumina crucible, placed in a silica tube, vacuum sealed, and heated at 1050° C. for 12 hours. After pulverizing the resultant ingot, it is charged in a graphite mold having a diameter of 12.7 mm, pressurized to 50 MPa, and sintered in a spark plasma sintering (SPS) method at 850 to 950 ° C. for 10 minutes, ( A thermoelectric material having a composition of Nb _0.85 Ti _0.15 FeSb _0.96 ) _0.99 (AgSbTe ₂ ) _0.01 was prepared

Example 2

Nb _{0 . 833} Ti _{0 . 147} Fe _{0 . 98} Ag _{0 . 02} Sb _{0 .} The powder of each element was weighed and mixed according to a mol ratio of _{9608 Te 0.04 ,} followed by cold press. The resulting mixture was placed in an alumina crucible, placed in a silica tube, vacuum sealed, and heated at 1050° C. for 12 hours. After pulverizing the resultant ingot, it is charged in a graphite mold having a diameter of 12.7 mm, pressurized to 50 MPa, and sintered in a spark plasma sintering (SPS) method at 850 to 950 ° C. for 10 minutes, ( A thermoelectric material having a composition of Nb _0.85 Ti _0.15 FeSb _0.96 ) _0.98 (AgSbTe ₂ ) _0.02 was prepared

Comparative Example 1

Nb ₀ except for AgSbTe ₂ based composition _{. 85} Ti _{0 . 15} A thermoelectric material having a composition of Nb _0.85 Ti _{0.15 FeSb 0.96} was prepared in the same manner as in Example 1, except that each element powder was mixed according to a mol ratio of 15 _FeSb 0.96.

Comparative Example 2

The composition ratio of Nb _{0 . 816} Ti _{0 . 144} Fe _{0 . 96} Ag _{0 . 04} Sb _{0 . 9616} Te was carried out in the same manner as in Example 1, except that the powder of each element was mixed according to a mol ratio of _0.08 (Nb _0.85 Ti _{0.15 FeSb 0.96} ) _0.96 (AgSbTe ₂ ) _0.04 composition of thermoelectric material was prepared.

Experimental example

1. Phase Analysis

In the preparation of the thermoelectric material according to Example 1 and Comparative Examples 1 and 2, the mixture in which the raw material powders of each element were mixed and the sintered body obtained after sintering the electric discharge plasma were imaged using an X-ray diffraction analyzer (XRD), respectively. analysis, and the results are shown in FIG. 1 .

Referring to FIG. 1 , in the case of Example 1, in which AgSbTe ₂ was added in a small amount at 1 mol% to the matrix, only the Van-Heusler (HH) phase was observed during XRD phase analysis, and the secondary phase was not observed.

However, in the case of Comparative Example 2 in which AgSbTe ₂ was added in an excess of 4 mol%, it was observed that Fe-Sb secondary phase and Ti-Te secondary phase were formed during XRD phase analysis, and the raw material was decomposed. From this, in the case of Comparative Example 2, it can be expected that the electrical conductivity is reduced.

In addition, secondary phase content analysis was performed using X-ray diffraction Rietveld refinement for the thermoelectric materials (sintered body) prepared in Examples 1 and 2 and Comparative Examples 1 and 2, and , the results are shown in Table 1 below. R _wp is one of the reliability factors of the Rietveld method analysis. Roughly, if R _wp is less than 5, the analysis result is considered reliable.

Lattice parameter (Å) Rwp (%) (Nb _1-a M _a FeSb _1-b ) _1-x (AgSbTe ₂ ) _x content (wt%) FeSb content (wt%) TiTe content (wt%) Comparative Example 1 5.951 (5.952) 3.826 (4.365) 97.82(100) 2.18(-) -(-) Comparative Example 2 5.950 (5.951) 3.812 (5.196) 92.87 (99.64) 5.78 (0.11) 1.35 (0.26) Example 1 5.950 (5.950) 3.605 (5.566) 97.77 (99.95) 2.12 (0.05) 0.11 (-) Example 2 5.950 (5.951) 3.993 (4.719) 96.27 (99.81) 3.43 (0.19) 0.30 (-)

In Table 1, the numerical values in parentheses are the results of the powder obtained by pulverizing the synthetic ingot obtained after the solid-phase reaction in the manufacture of thermoelectric materials according to Example 1 and Comparative Examples 1 and 2, and the numerical values outside the parentheses are the sintered body obtained after discharge plasma sintering ( thermoelectric material).

As a result of the analysis, the content of the Fe-Sb-based secondary phase and the Ti-Te-based secondary phase in the thermoelectric materials prepared in Examples 1 and 2 was less than 4% by weight based on the total weight of the thermoelectric material.

2. Thermoelectric property evaluation

Thermoelectric properties of the thermoelectric material specimens prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated in the following manner, and the results are shown in Tables 2 to 3 and FIGS. 2 to 6 .

Electrical conductivity (σ): LSR-3 manufactured by Linseis, a specific resistance measuring device, was used, and electrical conductivity was measured in a temperature range of 50° C. to 500° C. through direct current probe method. The results are shown in FIG. 2 .

Seebeck coefficient (S): using the measuring equipment Linseis LSR-3, by applying a differential voltage / temperature technique was measured in a temperature range of 50 ℃ to 500 ℃. The results are shown in FIG. 3 .

Output factor (PF): The output factor was calculated according to Equation 2 below using the electrical conductivity and Seebeck coefficient values measured above. The results are shown in FIG. 4 .

[Equation 2]

Power factor (PF) = σS ²

In Equation 2, σ is electrical conductivity, and S is Seebeck coefficient.

Total thermal conductivity (κ _tot ) and lattice thermal conductivity (κ _lat ): The total thermal conductivity (κ _tot ) was calculated using the thermal diffusivity (D) obtained using an LFA457 from Netzsch to determine the apparent density (d) and heat capacity ( C _p ) was calculated from (κ _tot =DdC _p ). lattice thermal conductivity (κ _lat or κ _L ) was calculated from the total thermal conductivity value (κ _{tot =} κ _L + κ _E ) using the electron thermal conductivity (κ _E ) value calculated according to the Wiedemann-Franz law (κ _E =LσT), Lorenz For the number (L), a value calculated from the Seebeck coefficient according to temperature was used. The total thermal conductivity (κ _tot ) is shown in FIG. 5, and the lattice thermal conductivity (κ _L ) for the thermoelectric materials of Example 1 and Comparative Example 1 are shown in Tables 2 and 3, respectively.

Thermoelectric figure of merit (ZT): It was calculated according to Equation 1 below using the values of S (Seebeck coefficient), σ (electrical conductivity), T (absolute temperature), and κ (total thermal conductivity) obtained above. The results are shown in FIG. 6 .

[Equation 1]

ZT= σS ² T/κ

Thermoelectric performance evaluation of Example 1 T (℃) σ(S/cm) S (μV/K) PF (μW/cmK ² ) κ _lat (W/mK) κ _tot (W/mK) ZT 52.0 5,048 110.1 61.16 7.71 10.41 0.19 101.1 4,265 121.4 62.83 6.98 9.55 0.25 199.5 3,096 137.2 58.32 5.89 8.19 0.34 298.6 2,288 154.6 54.68 5.21 7.20 0.43 399.2 1,758 173.8 53.11 4.74 6.49 0.55

Thermoelectric performance evaluation of Comparative Example 1 T(℃) σ(S/cm) S (μV/K) PF (μW/cmK ² ) κ _lat (W/mK) κ _tot (W/mK) ZT 52.0 5,143 103.4 54.94 8.30 11.51 0.16 100.8 4,359 113.2 55.90 7.38 10.45 0.20 199.2 3,165 132.2 55.32 6.17 8.88 0.29 298.3 2,344 151.8 54.04 5.44 7.78 0.40 398.9 1,792 169.9 51.71 4.87 6.92 0.50

Experimental results, compared with the thermoelectric material of Comparative Example 1 (AgSbTe ₂ ) In the thermoelectric materials of Examples 1 and 2, in which the composition was further added, compared with Comparative Example 1, the electrical conductivity was reduced to an average of about 2% and about 8% in a temperature range of 50°C to 400°C, but compared to the Seebeck coefficient Compared to Example 1, it was improved to an average of about 4% and about 7%, respectively, and as a result, the output factor was also increased to an average of about 7% and about 6%.

On the other hand, (AgSbTe ₂ ) In the case of the thermoelectric material of Comparative Example 2 in which the composition was added in excess, a decrease in electrical conductivity was observed due to the formation of Fe-Sb secondary phase and Ti-Te secondary phase, and although the Seebeck coefficient slightly increased, the output due to the decrease in electrical conductivity factor decreased. On the other hand, the thermoelectric material of Comparative Example 2 exhibited increased thermal conductivity due to the formation of the Fe-Sb secondary phase and Ti-Te secondary phase favorable for heat transfer, but the thermoelectric figure of merit ZT was higher than that of the thermoelectric material of Comparative Example 1. lowered

3. Thermoelectric performance evaluation according to Sb composition ratio

In the thermoelectric material according to the present invention, the effect of the Sb content in the (Nb _{1 -a} M _{a FeSb 1 -b} ) composition on the generation of the secondary phase and the thermoelectric performance was evaluated.

Specifically, Nb _{0 . 85} Ti _{0 . 15} In FeSb ₁ - _b , substances corresponding to b=0, 0.02, 0.04, 0.06, 0.08, and 0.1 were synthesized, and the FeSb-based secondary phase was generated and its content was confirmed by performing the same method as in 1. phase analysis. did The results are shown in Table 4 below.

Lattice parameter
(Å) R _wp (%) Nb _{0 . 85} Ti _{0 . 15} FeSb _{1 -b}
Content (wt%) FeSb content
(wt%) b=0.1 5.951 4.178 100 - b=0.08 5.951 3.849 100 - b=0.06 5.951 4.600 100 - b=0.04 5.950 3.323 100 - b=0.02 5.951 4.155 100 - b=0 5.950 3.512 94.17 5.03

As a result of the experiment, it was confirmed that the FeSb-based secondary phase was observed at b=0, and that all were synthesized as a single phase from b=0.02 to b=0.1.

In addition, thermoelectric figure of merit (ZT) values were observed for the thermoelectric material of the (Nb _{1 -a} M _{a FeSb 1 -b} ) composition prepared above, and the results are shown in FIG. 7 . At this time, the thermoelectric figure of merit was measured in the same manner as in 2. Thermoelectric property evaluation.

In FIG. 7 , Sb90 represents b=0.1, Sb92 represents b=0.08, Sb94 represents b=0.06, Sb96 represents b=0.04, and Sb98 represents b=0.02, respectively.

Referring to FIG. 7 , when b=0.04, the thermoelectric figure of merit value is the best, and as b increases to 0.06, 0.08, and 0.1, the thermoelectric figure of merit value tends to gradually decrease. Therefore, when b is greater than 0.1, it can be seen that since the thermoelectric figure of merit value is further reduced and the performance of the thermoelectric element may be degraded, b is preferably greater than 0 and less than or equal to 0.1 in the thermoelectric material according to an embodiment of the present invention. there is.

Claims (14)

A semi-Heusler-based thermoelectric material represented by the following Chemical Formula 1:
[Formula 1]
(Nb _1-a M _a FeSb _1-b ) _1-x (AgSbTe ₂ ) _x
In Formula 1,
M is one or more transition metals selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr,
0≤a<1, 0<b≤0.1, 0<x<0.04.
According to claim 1,
The (Nb _{1 -a} M _a FeSb _{1 -b} ) forms _a matrix, and (AgSbTe ₂ ) is dispersed and included in the matrix, a semi-Heusler-based thermoelectric material.
In claim 1,
The (Nb _1-a M _a FeSb _1-b ) has a cubic structure and a space group of F-43m, a semi-Heusler-based thermoelectric material.
According to claim 1,
Wherein M is Ti, 0.05≤a≤0.4, 0<b≤0.04, semi-Heusler-based thermoelectric material.
According to claim 1,
A semi-Heusler based thermoelectric material with 0.01≤x≤0.03.
The method of claim 1,
A semi-Heusler-based thermoelectric material represented by the following Chemical Formula 2:
[Formula 2]
(Nb _0.85 M _0.15 FeSb _0.96 ) _1-x (AgSbTe ₂ ) _x
In Formula 2,
M is one or more transition metals selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr,
0<x<0.04.
According to claim 1,
It does not contain a Fe-Sb-based secondary phase and a Ti-Te-based secondary phase, or contains at least one of a Fe-Sb-based secondary phase and a Ti-Te-based secondary phase in an amount of 5 wt% or less based on the total weight of the thermoelectric material, Semi-Heusler based thermoelectric material.
A raw material containing each of Nb, Fe, Ag, Sb, and Te, and one or more transition metals (M) selected from the group consisting of Ti, Zr, Hf, V, Ta, Mo, W, Co and Cr heat-treating the mixture including the raw material comprising; and
The method of claim 1, comprising the step of pressure sintering the heat-treated mixture.
9. The method of claim 8,
The heat treatment step is performed by a solid-phase reaction, semi-Heusler-based method of manufacturing a thermoelectric material.
9. The method of claim 8,
The heat treatment step is performed at a temperature of 500 °C to 1100 °C, a semi-Heusler-based method for producing a thermoelectric material.
9. The method of claim 8,
The pressure sintering step is performed by a discharge plasma sintering method or hot pressing, a semi-Heusler-based method of manufacturing a thermoelectric material.
9. The method of claim 8,
The pressure sintering step is performed under a temperature of 500° C. or more and a pressure of 5 MPa or more, a semi-Heusler-based method for producing a thermoelectric material.
9. The method of claim 8,
The method for producing a semi-Heusler-based thermoelectric material, further comprising the step of pulverizing the mixture after the heat treatment step or during the heat treatment step.
A thermoelectric element comprising the semi-Heusler-based thermoelectric material according to any one of claims 1 to 7.

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