KR101427194B1 - Cr-DOPED Mn-Si THERMOELECTRIC MATERIAL AND MANUFACTURING METHOD FOR THE SAME - Google Patents

Abstract

The present invention relates to a Cr-doped Mn-Si thermoelectric material. The Mn-Si thermoelectric material with a composition of MnSi1.75-δ(0<=δ<=0.03) is doped with Cr. Also, The present invention relates to a method for manufacturing the Cr-doped Mn-Si thermoelectric material which includes a first step of mixing Mn powder and Si powder prepared with the composition of MnSi1.75-δ(0<=δ<=0.03) and Cr powder prepared as a dopant as material powder; a second step of forming higher manganese silicides phase with regard to the mixed raw material powder; and a third step of sintering thermoelectric material powder with the higher manganese silicides phase. The present invention provides Mn-Si thermoelectric materials with improved thermoelectric performance by using Cr as a dopant replaced by Mn. Also, the described method for manufacturing the Cr-doped Mn-Si thermoelectric material manufactures a Cr-added Mn-Si thermoelectric material with high thermoelectric performance by controlling MnSi phase.

Description

TECHNICAL FIELD The present invention relates to a chromium-added manganese-silicon thermoelectric material and a method of manufacturing the same. More particularly, the present invention relates to a Cr-doped Mn-Si thermoelectric material,

TECHNICAL FIELD The present invention relates to a thermoelectric material and a method of manufacturing the same, and more particularly, to a thermoelectric material having improved thermoelectric performance by adding chromium to a manganese-silicon-based material and a method of manufacturing the same.

In recent years, interest in the development and conservation of alternative energy has been increasing, and researches and researches on efficient energy conversion materials are being actively carried out. In particular, researches on thermoelectric materials, which are materials for converting heat into electric energy, are being accelerated.

Such a thermoelectric material is a metal or ceramic material having a function of directly converting heat into electricity or electricity into heat, and it is advantageous that power generation is possible without moving parts if only a temperature difference is given.

Research on potential thermal suicide was first reported by Nikitin1, the _{MnSi, MnSi 2, CrSi 2,} CoSi has been reported as a thermal suicide candidates.

Among them, manganese silicide has attracted attention as a thermoelectric material which operates at a high temperature due to its low cost and resource-rich components and high oxidation resistance at high temperature, and the composition having excellent thermoelectric performance is not MnSi ₂ but high manganese silicide (HMS, higher manganese silicides) called MnSi _{1.72 ~ 1.75} range (Si content: a composition of 63.0 ~ 63.6%), and manganese silicide exhibit the p- type semiconductor having a narrow bandgap energy of 0.4 ~ 0.7eV behavior.

The high manganese silicide has _four phases of Mn ₄ Si ₇ (MnSi _1.75 ), Mn ₁₁ Si ₁₉ (MnSi _1.72 ), Mn ₁₅ Si ₂₆ (MnSi _1.73 ) and Mn ₂₇ Si ₄₇ (MnSi _1.74 ) Known as a tetragonal structure, and each phase has a c-axis which is considerably longer than the a-axis due to the action of the constituent material. These awards are called Nowotny chimney-ladder awards.

High manganese silicide is generally produced by a method such as dissolution method, crystal growth method, chemical reaction method and thin film method, but MnSi secondary phase which vertically separates c-axis remains and becomes a factor to deteriorate thermoelectric properties.

These MnSi secondary phases are difficult to remove from the base of the high manganese silicide due to the slow diffusion rate of Si atoms and the entanglement of the high manganese silicide, but the thermoelectric performance is improved by controlling the size and distribution pattern of the MnSi phase.

However, efforts to improve the thermoelectric performance of such a high-manganese silicide are limited to the modification of the manufacturing method, and the results of the doping materials and the like for improving the thermoelectric performance are insufficient.

1. T. Itoh and M. Yamada, J. Elec. Mater., 38 (7), (2009), 925-929. 2. W. Luo, H. Li, Y. Yang, Z. Lin, X. Tang, Q. Zhang, and C. Uher, Intermetallics, 19, (2011), 404-408

It is an object of the present invention to provide a manganese-silicon thermoelectric material having improved thermoelectric performance and a method of manufacturing the same.

The chromium-doped manganese-silicon based thermoelectric material to attain the above object is characterized in that Cr-doped manganese-silicon thermoelectric material having a composition of MnSi _1.75-delta (0??? 0.03) is doped.

The inventors of the present invention invented the present invention using Cr as a dopant to further improve the thermoelectric performance of the high manganese silicide. Since Cr is a p-type dopant, it provides less balance electrons than Mn and does not change the crystal structure of the high manganese silicide, so partial substitution of Mn by Cr effectively increases the hole concentration, Thereby improving the thermoelectric performance.

In particular, it has been conventionally known that the addition of Cr increases the thermal conductivity and hinders the thermoelectric performance. However, in the embodiment of the present invention, there is almost no change in the thermal conductivity due to the addition of Cr, and the thermoelectric performance is improved by the improvement of the electric conductivity Respectively.

At this time, it is preferable that the composition of the thermoelectric material is MnSi _1.75-delta : Cr _x (0??? 0.03, 0 < _{x ?} 0.1). The effect of Cr addition is also shown in the case of adding a very small amount, but when it is added at 0.1 or more, there arises a problem that the thermoelectric performance is decreased.

The method for producing a manganese-silicon based thermoelectric material to which the chromium is added is characterized in that a manganese powder prepared in accordance with the composition of MnSi _{1.75 -?} (0??? 0.03), a silicon powder and a chromium powder prepared by dopant are mixed A first step of mixing as a powder; A second step of forming a high manganese silicide phase on the mixed raw material powder; And a third step of sintering the thermoelectric material powder having the high manganese silicide phase formed thereon.

At this time, it is preferable that the raw material powder to be mixed in the first step is mixed according to the composition of MnSi _1.75-δ : Cr _x (0 ≦ _δ 0.03, 0 < _x ≦ 0.1).

The second step may be performed by one of the mechanical alloying method and the solid state reaction method. When the solid phase reaction method is employed, the second step may be performed in a vacuum state at a temperature ranging from 1100K to 1400K.

The third step is preferably performed by vacuum hot pressing, and the vacuum hot pressing is preferably performed at a temperature of 1073 K or more and a pressure of 60 MPa or more for 1 hour or more.

The manganese-silicon thermoelectric material to which the chromium is added as described above has the effect of providing a manganese-silicon thermoelectric material having improved thermoelectric performance by using chromium as a dopant to be substituted with manganese.

Further, the above-mentioned method for producing a manganese-silicon thermoelectric material to which chromium is added has the effect of producing a manganese-silicon thermoelectric material to which chromium is added with excellent passive performance by controlling the MnSi phase.

FIG. 1 is a result of XRD analysis of a thermoelectric material specimen manufactured according to an embodiment of the present invention.
2 shows the results of electrical conductivity measurement of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.
3 is a graph showing the results of measurement of the Seebeck coefficient for the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.
4 is a graph showing the results of evaluating the output factors of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.
5 is a graph showing the results of measurement of thermal conductivity of a thermoelectric material according to an embodiment of the present invention and a thermoelectric material of a comparative example.
6 is a graph showing a result of evaluating the thermoelectric performance index of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.

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

First, the chromium-added manganese-silicon based thermoelectric material manufacturing method of this embodiment mixes raw material powders in accordance with the stoichiometric ratio. (First step)

In this example, a Mn powder having a purity of less than 45 탆 and a purity of 99.9% and a purity of less than 45 탆, having a purity of 99.99%, were prepared in accordance with the composition formula of MnSi _1.75-δ : Cr _x (δ = 0, 0.01, 0.02, 0.03, % Si powder and 99.9% pure Cr powder with a purity of less than 45 [mu] m were mixed in a stoichiometric ratio.

The mixed powder is then subjected to a solid phase reaction process to form a high manganese silicide. (Second step)

The raw material powder mixed in the above composition was placed in an alumina crucible and subjected to a solid-phase reaction process in a vacuum state at a temperature of 1273K for 6 hours.

Finally, the chromium-added manganese-silicon thermoelectric material prepared by the solid-phase reaction process is placed in a cylindrical high-strength graphite mold and subjected to vacuum hot pressing. (Third step)

In this embodiment, a graphite mold having a diameter of 10 mm was used. In order to prevent the phase change during the sintering process, a vacuum state was maintained and hot pressing was performed for 1 hour at a pressure of 70 MPa at a temperature of 1173 K, .

FIG. 1 is a result of XRD analysis of a thermoelectric material specimen manufactured according to an embodiment of the present invention. (a) is MnSi _1.75: If the Cr _0.01 composition, (b) is MnSi _1.74: If the Cr _0.01 composition, (c) is MnSi _1.73: If the Cr _0.01 composition and (d) is MnSi _1.72: the Cr _0.01 Composition XRD analysis results.

The phase diagram for each composition is shown in the ICDD standard diffraction data of Mn ₁₁ Si ₁₉ (PDF # 03-065-2862), Mn ₁₅ Si ₂₆ (PDF # 00-020-0724), Mn ₂₇ Si ₄₇ -1251) and Mn ₄ Si ₇ (PDF # 01-072-2069), no phase change was observed due to the addition of Cr, and metallic MnSi (0-0.8%) was analyzed as the second phase, Analysis is within detection limit. All the specimens had a tetragonal crystal structure and were very similar to the lattice constants reported in the literature as shown in Table 1, indicating that the high-manganese silicide phase was successfully synthesized by the solid- Can be confirmed.

Table 1 shows the result of measuring the lattice constant of the thermoelectric material of Comparative Example not containing Cr for comparison with the thermoelectric material doped with Cr according to the present embodiment. The theoretical values of the lattice constants for the thermoelectric materials of the Comparative Examples without addition of Cr were selected from the research data conducted in the past.

Furtherance The lattice constants of Examples and Comparative Examples Theoretical lattice constant a (nm) c (nm) a (nm) c (nm) Mn ₁₁ Si ₁₉ (MnSi _1.72 ) 0.5517 4.8090 0.5518 4.8136 Mn ₁₁ Si ₁₉ (MnSi _1.72 ): Cr _0.01 0.5535 4.8145 - - Mn ₁₅ Si ₂₆ (MnSi _1.73 ) 0.5517 6.5415 0.5531 6.5311 Mn ₁₅ Si ₂₆ (MnSi _1.73 ): Cr _0.01 0.5524 6.5439 0.5525 6.5504 Mn ₂₇ Si ₄₇ (MnSi _1.74 ) 0.5524 11.758 0.5530 11.790 Mn ₂₇ Si ₄₇ (MnSi _1.74 ): Cr _0.01 0.5529 11.789 - - Mn ₄ Si ₇ (MnSi _1.75 ) 0.5526 1.7467 0.5525 1.7436 Mn ₄ Si ₇ (MnSi _1.75 ): Cr _0.01 0.5532 1.7478 - -

As shown in Table 1, the lattice constants were increased by Cr doping. Especially, the c-axis was more increased than the a-axis, so that a high-manganese silicide thermoelectric material in which Cr was substituted with Mn was prepared .

2 shows the results of electrical conductivity measurement of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.

As shown in the figure, the thermoelectric material of this embodiment and the thermoelectric material of the comparative example exhibit a degeneracy semiconductor characteristic in which the electric conductivity is slightly reduced with an increase in temperature in all the compositions.

It has been confirmed that the electric conductivity of the thermoelectric material of this embodiment doped with Cr is higher than that of the comparative example because Cr provides more balanced electrons than Mn, and therefore, the partial substitution of Mn by Cr causes the hole concentration Of the total population.

3 is a graph showing the results of measurement of the Seebeck coefficient for the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.

As shown in the figure, the thermoelectric material of this embodiment and the thermoelectric material of the comparative example exhibit p-type conductivity having a positive white count value in all compositions. And the increase of the temperature increases the Seebeck coefficient.

The Cr-doped thermoelectric material of the present example exhibited a decrease in the Seebeck coefficient as a whole compared to the thermoelectric material of the comparative example. In the case of MnSi _1.75 , the whitening coefficient was 234 kJ / K at 823 K. On the other hand, in the case of MnSi _1.75 : Cr _0.01 Exhibited a Seebeck coefficient of 224 kJ / K at 823K. The maximum Seebeck coefficient of MnSi _1.75 : Cr _x composition appears to be due to the band cap size of each composition.

4 is a graph showing the results of evaluating the output factors of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.

As shown, the output factor was increased by the doping of Cr and the maximum output factor of 1.01 mW / mK ² at 723K for MnSi _1.73 : Cr _0.01 composition. The tendency of the change of the output factor by Cr doping is similar to the tendency of change of the electric conductivity, and it seems that the output factor is increased by the increase of the electric conductivity.

5 is a graph showing the results of measurement of thermal conductivity of a thermoelectric material according to an embodiment of the present invention and a thermoelectric material of a comparative example. The graph inserted inside is the result of measuring the lattice thermal conductivity.

Regardless of the composition of the high manganese silicide and the doping of Cr. The thermoelectric materials of all compositions showed a minimum value at 623 K, and the thermoelectric materials of this embodiment doped with Cr showed 2.1 to 2.6 W / mK.

In the conventional research, it has been reported that when Cr is doped into the high manganese silicide, the thermal conductivity at high temperature is greatly increased due to the bipolar effect due to electron-hole pairs. However, the thermoelectric material of this embodiment showed a slightly lower or similar thermal conductivity in the measured temperature range than the thermoelectric material of the comparative example, and this is because the lattice thermal conductivity decreased as shown in the inset.

6 is a graph showing a result of evaluating the thermoelectric performance index of the thermoelectric material according to the embodiment of the present invention and the thermoelectric material of the comparative example.

The performance index of the thermoelectric material of the present embodiment and the thermoelectric material of the comparative example increased with increasing temperature in all the compositions and it was confirmed that the thermoelectric material doped with Cr improved the performance index compared with the thermoelectric material of the comparative example.

As described above, the increase of the output factor due to the increase of the electric conductivity rather than the change of the thermal conductivity through the Cr doping led to the increase of the thermoelectric performance index. The maximum performance index showed a performance index of 0.36 at 823 K for a thermoelectric material of MnSi _1.73 : Cr _0.01 composition.

As described above, it was confirmed that the thermoelectric material doped with Cr of the present embodiment had improved electrical conductivity and improved thermoelectric performance.

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 (9)

A chromium _- doped manganese-silicon thermoelectric material characterized in that Cr-doped manganese-silicon thermoelectric material having a composition of MnSi _{1.75 -?} (0??? 0.03) is doped.
The method according to claim 1,
The composition of the thermoelectric material MnSi _{1 .75-δ:} Cr _x (0??? 0.03, 0 <x? 0.1).
A first step of mixing manganese powder prepared according to the composition of MnSi _{1.75 -?} (0??? 0.03), silicon powder and chromium powder prepared as a dopant as raw material powders;
A second step of forming a high manganese silicide phase on the mixed raw material powder; And
And a third step of sintering the thermoelectric material powder having the high-manganese silicide phase formed thereon, and a third step of sintering the thermoelectric material powder having the high-manganese silicide phase.
The method of claim 3,
The raw material powder to be mixed in the first step _{1 MnSi 1 .75-δ: Cr} x (0??? 0.03, 0? X? 0.1).
The method of claim 3,
Wherein the second step is carried out by one of a mechanical alloying method or a solid state reaction method.
The method of claim 5,
Wherein the second step is performed in a solid state reaction method and the solid state reaction method is performed in a vacuum state in a temperature range of 1100K to 1400K.
The method of claim 3,
Wherein the third step is performed by vacuum hot pressing. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
The method of claim 7,
Wherein the vacuum hot pressing is performed at a temperature of 1073K or higher. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
The method of claim 8,
Wherein the vacuum hot pressing is performed at a pressure of 60 MPa or more for 1 hour or more.

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