KR101514233B1 - Manufacturing method for thermoelectric material of solid solution and thermoelectric material - Google Patents

Abstract

The present invention relates to a method for manufacturing a thermoelectric material. The method includes the steps of: preparing powder of Mg_2Si and Mg_2Ge raw materials; mixing the powder of the raw materials at a nonstoicheiometric composition ratio; and synthesizing Mg_2Si and Mg_2Ge in a solid phase by making the mixed power of the raw material react in a solid phase by heat treatment. Mg_2Si and Mg_2Ge, synthesized in the step of solid-phase synthesis, are mixed in a solid solution phase. The thermoelectric material is obtained by mixing Mg_2Si and Mg_2Ge in a solid solution phase and thus can increase thermoelectric performance. The thermoelectric material is produced by making the power of the raw materials in a solid phase react. According to the method of the present invention, the thermoelectric material in which Mg_2Si and Mg_2Ge are mixed in a solid solution phase can be manufactured by only a process of making the powder of the raw materials react in a solid phase. Therefore, the thermoelectric material with increased performance can be manufactured at low costs.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for manufacturing a solid solution thermoelectric material,

The present invention relates to a method of manufacturing a thermoelectric material and a thermoelectric material, and more particularly to a Mg ₂ X thermoelectric material with improved thermoelectric performance and a method of manufacturing the same.

In general, Mg ₂ X (X = Si, Ge, and Sn), which exhibits excellent thermoelectric properties in the mid-temperature range from 500K to 800K, is not toxic to the elements and has a large amount of reserves, and is attracting attention as an eco-friendly and economical thermoelectric material.

In order to improve the figure of merit (Z = α2σ / κ) of thermoelectric materials, high Seebeck coefficient, α, electrical conductivity, and low thermal conductivity required, and, a material factor _{^{β = (m * / m e}} ) 3/2 μκ L -1 (m * is the density-of-states effective mass, m e is the mass of electron, μ is the carrier mobility, κ _L is the lattice thermal conductivity). Consequently, it is necessary to have low thermal conductivity and high carrier mobility in order to obtain a high performance index. Comparing β of thermoelectric materials, SiGe has a value of 1.2 to 2.6 and FeSi ₂ has a value of 0.05 to 0.8, whereas Mg ₂ X has a high value of 3.7 to 14. In addition, it has phonon scattering with decreasing electrical resistivity The thermal conductivity can be reduced.

As a result, studies for improving the thermoelectric performance of Mg ₂ X based thermoelectric materials have been actively conducted.

On the other hand, Bi ₂ Te ₃ and its solid solution materials are currently used among materials having a high thermoelectric performance index. Particularly, in the case of solid-state thermoelectric materials, the thermal conductivity decreases due to the influence of the phonon scattering due to the point defect, thereby improving the figure of merit. However, when segregation occurs, There is a problem that a uniform solid solution is formed. (Registration No. 10-0440268)

In addition, although efforts have been made to produce solid solution thermoelectric materials for Mg ₂ X based thermoelectric materials, solid solution thermoelectric materials having satisfactory characteristics due to the oxidation characteristics of Mg and the unstable melting points of raw materials have not been manufactured In fact.

Registration No. 10-0440268

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and it is an object of the present invention to provide a Mg ₂ X thermoelectric material having improved thermoelectric performance and a method of manufacturing the same.

According to another aspect of the present invention, there is provided a method of manufacturing a thermoelectric material, comprising: preparing a raw material powder of Mg ₂ Si and Mg ₂ Ge; Mixing the raw material powders according to a stoichiometric composition ratio; And a step of solid-phase synthesis of Mg ₂ Si and Mg ₂ Ge by reacting the mixed starting material powder in a solid state through heat treatment, wherein Mg ₂ Si and Mg ₂ Ge synthesized in the solid- And the like.

A solid solution is a state in which atoms or molecules of other elements are uniformly distributed in a crystal of a solid and are in the same state as a solution having the same composition in any part.

Mg ₂ Si and Mg ₂ Ge are both compounds of Mg and exhibit thermoelectric performance. However, it is difficult to control the composition, structure and performance of Mg, Si, and Ge. It is difficult to produce a solid solution due to the problem that segregation due to gravity and unreacted elements appear due to a constant melting point, and therefore, a thermoelectric material composed of a solid solution of Mg ₂ Si and Mg ₂ Ge is manufactured Technology has not been used.

The inventors of the present invention invented a method for producing a thermoelectric material in a solid solution state of Mg ₂ Si and Mg ₂ Ge by a solid phase reaction method. Of course, the solid-phase reaction method of reacting a solid material is a process that has been used in the past, but Mg ₂ Si and Mg ₂ Ge are formed when the solid phase reaction is performed after mixing the raw material powders of Mg, Si and Ge And that the thermoelectric material performance is improved by constructing a solid solution of these materials naturally is unpredictable in the past.

At this time, it is preferable that the solid-phase synthesis step is performed for 1 to 10 hours in a temperature range of 773 to 973K. When the solid phase synthesis is carried out at a temperature lower than the above range for a short time, Mg ₂ Si and Mg ₂ Ge are not completely synthesized and solid solution can not be formed. When the solid phase synthesis is carried out at a temperature higher than this range for a long time, There is a problem that the second phase is formed or Mg is oxidized or volatilized.

The solid solution thermoelectric material prepared by solid phase synthesis may be formed by hot compression in a vacuum state. The hot compression molding is preferably performed at a pressure of 10 to 100 MPa and a temperature range of 973 to 1173K. In the case of hot compression at a pressure and a temperature lower than this range, it can not be molded at a desired density. In the case of hot compression at a pressure and temperature higher than this range, manufacturing cost increases and a second phase is formed.

Also, it is preferable to perform cold compression of the mixed raw material powder prior to performing the solid phase synthesis step, and the cold compression is preferably performed in a pressure range of 10 to 1000 MPa. The cold compression process is intended to promote the diffusion of the raw material powders in the subsequent solid phase reaction by widening the contact area of the raw material powders. When the cold compression process is not performed, the solid phase reaction partially progresses, A synthesis result can not be obtained and there is a problem that unreacted raw material remains. When cold compression is performed at a pressure lower than this pressure, solid phase reaction and diffusion promotion effect by cold compression can not be obtained, and in case of performing cold compression at a higher pressure, the cost is increased.

Further, in order to optimize the electrical conductivity and the Seebeck coefficient of the solid solution thermoelectric material prepared, the doping element can be doped. For this, doping element or solid phase synthesis step is performed before the solid phase synthesis step is performed, Lt; / RTI >

The thermoelectric material for achieving the above object is characterized in that Mg ₂ Si and Mg ₂ Ge are mixed in a solid solution state. Such a thermoelectric material may be one produced by reacting a raw powder in a solid state.

In this case, a thermoelectric material in which Mg ₂ Si and Mg ₂ Ge are mixed in a solid solution state can be doped with a dopant, and the thermoelectric performance can be further improved by doping a dopant that optimizes electric conductivity and a Seebeck coefficient.

When the molar ratio of Mg ₂ Si to Mg ₂ Ge is 8: 2 to 4: 6, the thermoelectric performance of the solid solution thermoelectric material is good.

The thermoelectric material constituted as described above has an effect of improving thermoelectric performance by mixing Mg ₂ Si and Mg ₂ Ge in solid solution state.

In addition, since the above-described method for producing a thermoelectric material can produce a thermoelectric material in which Mg ₂ Si and Mg ₂ Ge are mixed in a solid solution state only by a step of mixing raw material powders and reacting in a solid state, And thus the improved thermoelectric material can be produced.

FIG. 1 is a flowchart showing a manufacturing process of a thermoelectric material according to an embodiment of the present invention.
FIG. 2 shows X-ray diffraction results of the powder after the solid-phase reaction process.
FIG. 3 shows the results of X-ray diffraction analysis of specimens subjected to vacuum hot compression molding.
4 is a graph showing the change in lattice constant of the prepared specimen.
5 to 9 are electron micrographs of the microstructure of the prepared specimen.
10 shows the result of measuring the electrical conductivity according to the temperature of the prepared specimen.
11 shows the result of measuring the whiteness coefficient according to the temperature of the produced specimen.
FIG. 12 shows the result of measuring the thermal conductivity according to the temperature of the produced specimen.
13 shows the results of calculating the dimensionless thermoelectric performance index according to the temperature of the produced specimen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawings, embodiments of the present invention will be described in detail.

FIG. 1 is a flowchart showing a manufacturing process of a thermoelectric material according to an embodiment of the present invention.

First, powders of Mg (99.99%, <149 μm), Si (99.99%, <45 μm) and Ge (99.99%, <45 μm) as raw materials were prepared.

The raw material powders were mixed so that the molar ratios of Mg ₂ Si and Mg ₂ Ge were 3: 7, 5: 5 and 7: 3 respectively according to the stoichiometric composition ratio. For comparison, only Mg ₂ Si and Mg ₂ Ge A prepared raw material powder was also prepared.

The raw material powder mixed at the above ratio was subjected to cold compression at a pressure of 600 MPa. By performing such a cold compression process, the contact area of the raw material powders can be widened, thereby promoting the diffusion between the raw material powders in a subsequent solid phase reaction process. In the case where the cold compression process is not carried out, the solid-state reaction partially progresses, and as a result, a homogeneous synthesis result as a whole can not be obtained, and unreacted raw material may remain.

Next, after filling the alumina crucible, a solid phase reaction process was performed at 773K for 6 hours to synthesize Mg ₂ Si and Mg ₂ Ge.

The synthesized powder was charged into a high strength graphite die having an inner diameter of 10 mm and vacuum hot compression molding was performed at a pressure of 70 MPa at a temperature of 1073 K for 2 hours to prepare a specimen.

Various physical properties of the specimens prepared by the above process were measured.

FIG. 2 shows the results of X-ray diffraction analysis of powders after the solid-phase reaction process, and FIG. 3 shows X-ray diffraction analysis results of the specimens subjected to vacuum hot compression molding. The compositions of the thermoelectric materials prepared according to convenience were expressed as Mg ₂ Si _1-x Ge _x (x = 0, 0.3, 0.5, 0.7, 1) according to the molar ratio.

In FIG. 2, only peaks corresponding to Mg ₂ Si and / or Mg ₂ Ge were observed in the powder subjected to the solid phase reaction as compared with the standard diffraction peaks of Mg ₂ Si and Mg ₂ Ge shown at the bottom, It can be confirmed that the synthesis is successfully performed. In FIG. 3, it can be seen that no other secondary phase is formed in the process of vacuum hot compression molding. Thus, in the present embodiment, the complete synthesis of Mg ₂ Si and Mg ₂ Ge only by the solid-phase reaction process by heat treatment is performed by performing the cold compression process prior to the solid-phase reaction by the heat treatment, .

However, the larger the amount of Ge added, the smaller the diffraction peak moved to the lower side, and the lattice constant increased.

4 is a graph showing the change in lattice constant of the prepared specimen.

FIG. 4 shows the fraction of _x in the Mg ₂ Si _1-x Ge _x , that is, the fraction of Ge. As the Ge content increases, the lattice constant linearly increases from 0.635 nm to 0.639 nm. This is because it belongs to the lattice constant range of Mg ₂ Si (0.6352 nm) and Mg ₂ Ge (0.6392 nm) confirmed in the preceding studies, and consequently meets the Begard's law, Mg ₂ Si and Mg ₂ Ge It can be confirmed that they are well-structured.

After all the powder and a specimen subjected to a vacuum heat press molding step synthesis after the solid-phase reaction process is both to check that employ chain of Mg ₂ Si and Mg ₂ Ge, a lattice constant increases as the Mg ₂ Ge is employed in the Mg ₂ Si Respectively.

5 to 9 are electron micrographs of the microstructure of the prepared specimen. 5 is a specimen consisting of Mg ₂ Si alone, or 6 to 8 Mg ₂ Si and Mg ₂ Ge ratio is respectively 7: 3, 5: 5 and 3: and the 7 specimens, Figure 9 is Mg ₂ Ge It is a single psalm.

Through vacuum hot compression molding, compacts with a theoretical density of 98% or more were obtained in all specimens. The Mg ₂ Si of FIG. 5 showed a dark gray, the Mg ₂ Ge of FIG. 9 showed a light gray, and FIGS. 6 to 8 show a structure divided into two regions, .

Table 1 shows the result of elemental analysis by EDS analysis for the bright portion indicated by 1 and the dark portion indicated by 2 in the specimen shown in FIG.

Region Element Composition (at%) One Mg 62.2 Si 3.7 Ge 34.1 2 Mg 67.7 Si 28.1 Ge 4.2

As shown in the above table, it can be confirmed that the bright portion indicated by 1 is a portion containing a large amount of Ge and the dark portion indicated by 2 is a portion containing a large amount of Si.

From the above results, it can be confirmed that the specimen prepared according to this embodiment exhibits an antifluorite structure and two phases coexist, and that each phase is a solid solution solid of Mg ₂ Si and Mg ₂ Ge, and the X-ray diffraction analysis It is the same as the result confirmed in the result.

Table 2 shows the results of measurement of the electron mobility at room temperature for the specimens produced in this example.

Specimen Hall coefficient
(cm ³ / C) Mobility
(cm ² / Vs) Carrier concentration
(cm ^-3 ) Mg ₂ Si -210.6 104 3.0 ㅧ 10 ¹⁶ Mg ₂ Si _0.7 Ge _0.3 -15.4 113 4.0 ㅧ 10 ¹⁷ Mg ₂ Si _0.5 Ge _0.5 -8.4 28 7.3 ㅧ 10 ¹⁷ Mg ₂ Si _0.3 Ge _0.7 5.5 4 1.1 ㅧ 10 ¹⁸ Mg ₂ Ge 6.8 14 9.1 ㅧ 10 ¹⁷

For the Mg ₂ Si concentration is 3.0 ㅧ carrier 10 was ¹⁶ cm ^-3, depending on the case of employing the Sikkim Mg ₂ Ge carrier concentration is increased by Mg ₂ Si _0.3 Ge _0.7 1.1 ㅧ of 10 ¹⁸ cm ^-3, Mg ₂ Ge cases showed a value of 9.1 ㅧ 10 ¹⁷ cm ^-3, showed the mobility is decreased. In the solid solution represented _by Mg ₂ Si _1-x Ge _x , n = 0 to 0.5 showed n-type conductivity, but transitioned to p-type when x≥0.7. It is considered that this is because the characteristic of Mg ₂ Ge is stronger as the content of Ge increases.

10 shows the result of measuring the electrical conductivity according to the temperature of the prepared specimen.

As the temperature increased in all compositions, the electrical conductivity increased and the non - depletion semiconductor characteristics were observed. Mg ₂ Si _0.7 Ge _0.3 specimen showed the highest electrical conductivity at room temperature and Mg ₂ Si showed the lowest value.

All specimens showed different electrical conductivities depending on the composition. This is because the Ge atoms occupy the positions of Si atoms in the crystal structure and the electric conduction form changes according to the change of the high capacity of Ge and bipolar conduction occurs in which electrons migration and hole migration are mixed to be. It is considered that the carrier concentration, the mobility and the effective mass differ as Mg ₂ Si and Mg ₂ Ge form solid solution.

11 shows the result of measuring the whiteness coefficient according to the temperature of the produced specimen.

As the content of Ge increased, the Seebeck coefficient at room temperature changed greatly from -537 ㎶ / K to 565 ㎶ / K, and the sign of Seebeck coefficient coincided with the Hall coefficient of Table 2. That is, the main carriers contributing to electrical conduction are changed from electrons (n-type) to holes (p-type). As a result, the Mg ₂ Si _1-x Ge _x solid solution is a thermoelectric material which is considerably sensitive to the element composition.

(

: 제벡계수 절댓값, r : 스캐터링 파라미터, c : 상수, n : 캐리어 농도) 식에 따라 Ge 함량이 증가할수록 n이 증가하게 되고, 결국

는 감소하게 된다. 모든 조성에서 온도가 증가할수록 캐리어 농도가 증가하여

는 감소하고 포화되는 경향을 보인다. 또한 x ≥ 0.7인 경우 상온에서는 p-형 전도특성을 나타내지만, 고온에서는 n-형 전도특성으로 전환되었다. 이것은 Ge의 첨가에 따라 페르미 에너지가 변하게 되고 이에 따라 전자구조의 변화가 발생한 것에 기인한 것으로 보인다.As the Ge content increases and the temperature increases, the absolute value of the Seebeck coefficient decreases due to the increase of the thermally activated carrier.

(

(N = carrier concentration), where n is the number of particles, and n is the number of particles.

. As the temperature increases in all compositions, the carrier concentration increases

Is decreased and saturated. In the case of x ≥ 0.7, p-type conduction characteristics are exhibited at room temperature, but n-type conduction characteristics are converted at high temperature. This seems to be due to the change of the Fermi energy with the addition of Ge, resulting in the change of the electronic structure.

FIG. 12 shows the result of measuring the thermal conductivity according to the temperature of the produced specimen.

The thermal conductivity of Mg ₂ Si _1-x Ge _x solid solution is much lower than the thermal conductivity of Mg ₂ Si and Mg ₂ Ge at room temperature. The solubilized atoms act as a center of phonon scattering and are the result of a typical alloying effect in which thermal conductivity is reduced. Thermal conductivity reason lower than the solid solution of Mg ₂ Si in the 623K or more are judged as due to the small amount of contribution of the electronic effects on the thermal conductivity due to the carrier density of the Mg ₂ Si 10 times to 100 times smaller. The thermal conductivity decreased with increasing temperature in all compositions, but the thermal conductivity increased by intrinsic conduction at over 723K. Mg ₂ Si _0.7 Ge _0.3 and Mg ₂ Si _0.5 Ge _0.5 exhibited a very low thermal conductivity of 2.2 ~ 3.5 W / mK at all measured temperature ranges.

13 shows the results of calculating the dimensionless thermoelectric performance index according to the temperature of the produced specimen.

In the case of the solid solution samples prepared according to the present embodiment, the figure of merit tended to increase with increasing temperature. In particular, Mg ₂ Si _0.7 Ge _0.3 was found to have the highest applicability in the middle temperature range.

As described above, it can be confirmed that a thermoelectric material composed of a solid solution of Mg ₂ Si and Mg ₂ Ge can be easily manufactured by the solid-phase reaction method according to the present embodiment. In addition, the thermoelectric material prepared according to this embodiment has a reduced thermal conductivity due to the formation of solid solution of Mg ₂ Si and Mg ₂ Ge, thus confirming the improvement of the thermoelectric performance due to solid solution formation.

In this embodiment, doping is not performed in order to confirm the improvement of the thermoelectric performance due to the solid solution formation of Mg ₂ Si and Mg ₂ Ge. However, when doping is performed to optimize the electric conductivity and the Seebeck coefficient, the thermoelectric performance is further improved Will be.

Meanwhile, the doping for improving the thermoelectric performance may be performed by mixing the doping elements together in the mixing step of the raw material in the manufacturing process according to the present embodiment, or by forming the solid solution through solid phase reaction and then doping the dopant It can also be applied. Various studies have been conducted on dopants used for optimizing the electric conductivity and the Seebeck coefficient of the Mg ₂ X based thermoelectric material. If the essence of the Mg ₂ X based thermoelectric material is not adversely affected, the dopant can be used without any limitation, so a detailed description of the dopant is omitted .

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Those skilled in the art will understand. Therefore, the scope of protection of the present invention should be construed not only in the specific embodiments but also in the scope of claims, and all technical ideas within the scope of the same shall be construed as being included in the scope of the present invention.

Claims (13)

Preparing a raw material powder of Mg ₂ Si and Mg ₂ Ge;
Mixing the raw material powders according to a stoichiometric composition ratio; And
Reacting the mixed raw material powder in a solid state through heat treatment to solid-phase synthesize Mg ₂ Si and Mg ₂ Ge,
Wherein the Mg ₂ Si and Mg ₂ Ge synthesized in the solid-phase synthesis step are mixed in a solid solution state.
The method according to claim 1,
Wherein the solid-phase synthesis step is performed by heat treatment in a temperature range of 773 to 973K.
The method of claim 2,
Wherein the solid phase synthesis step is performed for 1 to 10 hours.
The method according to claim 1,
Further comprising performing hot compression molding in a vacuum state after the solid-phase synthesis step.
The method of claim 4,
Wherein the step of performing the hot compression molding is performed at a pressure of 10 to 100 MPa and a temperature range of 973 to 1173K.
The method according to claim 1,
Wherein the mixed raw material powder is cold-pressed before performing the solid-phase synthesis step.
The method of claim 6,
Wherein the cold pressing step is performed at a pressure of 10 to 1000 MPa.
The method according to claim 1,
Further comprising the step of adding a doping element before performing the solid-phase synthesis step.
The method according to claim 1,
Further comprising the step of doping the doping element after performing the solid-phase synthesis step.
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