KR20170017214A - Thermoelectric material and a method of manufacturing the zinc oxide is mixed - Google Patents

Abstract

The present invention relates to a thermoelectric material mixed with oxide and a method of manufacturing the same. The thermoelectric material mixed with oxide is manufactured through a step of forming a Bi_2Te_3_-_xSe_x-ZnO nanocomposite thermoelectric material by mixing and sintering a thermoelectric material powder made of bismuth (Bi), tellurium (Te) and selenium (Se), and zinc oxide (ZnO) nano powder. By 0 <= x <= 3, it is possible to improve the thermoelectric properties of nanocomposite thermoelectric materials by mixing zinc oxide with n-type Bi_2Te_3_-xSe_x thermoelectric materials. In addition, by mixing zinc oxide, the anisotropy of the nanocomposite thermoelectric material is reduced, and a nanocomposite thermoelectric material having homogeneous thermoelectric properties regardless of any regions may be obtained.

Description

TECHNICAL FIELD The present invention relates to a thermoelectric material and a method of manufacturing the same,

The present invention relates to a thermoelectric material in which zinc oxide is mixed and a method of producing the same, and more particularly, to a thermoelectric material in which zinc oxide is mixed with an n-type Bi ₂ Te _{3 - x} Se _x thermoelectric material, And a method of manufacturing the same.

The thermolelectric effect is a phenomenon in which, when there is a temperature difference between thermoelectric materials, the electromotive force is generated, or a voltage difference is applied to both ends of the thermoelectric material to cause a current to flow. On the other hand, Peltier effect phenomenon. The thermoelectric material refers to a substance in which such a thermal transfer phenomenon occurs strongly. The thermoelectric properties of the thermoelectric material improve the efficiency of a thermoelectric device manufactured using the thermoelectric material.

Thermoelectric properties that determine thermoelectric performance are thermoelectric power (V), Seebeck coefficient (S), Peltier coefficient (π), Thomson coefficient (τ), Nernst coefficient (Q), Etingshausen coefficient (P), electrical conductivity ), An output factor (PF), a figure of merit (Z), a dimensionless figure of merit (ZT), thermal conductivity (κ), Lorentz number (L), electrical resistivity (ρ) Among them, the dimensionless figure of merit (ZT) is an important index to determine the thermoelectric conversion energy efficiency.

ZT = S ² / ρκ

Where T is the absolute temperature [K], S is the Seebeck coefficient [V / K], ρ is the resistivity of the thermoelectric material [Ω × cm], and κ is the thermal conductivity of the thermoelectric material [W / mK ² ]. In this equation, the part excluding κ is a measure for evaluating the thermoelectric conversion characteristics as a power factor. The larger the ZT value, the better the thermoelectric conversion characteristics. The larger the Zebeck coefficient, the lower the specific resistance and the lower the thermal conductivity. However, all of these variables are closely related to the behavior of the electrons in the thermoelectric materials, making it difficult to control them independently of each other.

On the other hand, the various thermoelectric materials studied to date have a unique temperature range in which the ZT value shows a maximum value as shown in FIG. 1 and FIG. FIG. 1 is a graph showing an n-type material and a dimensionless performance index showing the best thermoelectric conversion characteristics by temperature region, and FIG. 2 is a graph showing a p-type material and a dimensionless performance index. As can be seen from Figs. 1 and 2, Bi ₂ Te ₃ is the n-type at room temperature and Sb ₂ Te ₃ is the highest ZT at the p-type. Therefore, in order to maximize ZT based on Bi ₂ Te ₃ and Sb ₂ Te ₃ , the thermoelectric material to be used at room temperature is Bi _x Sb _{2 - x} Te ₃ (p-type), which is a solid solution of these two materials by the addition of _{Se Bi 2 Te 3 - y Se} y (n-type) , such as a multi-element material has been developed and is mainly used, such iodine in the material (I), chlorine (Cl), bromine (Br), copper (Cu ), Silver (Ag), zinc (Zn), cadmium (Cd), and the like.

In the case of the p-type material Bi _x Sb _{2 - x} Te _3, the nano structuring technique was introduced by the conventional bulk process based synthesis method to reduce the thermal conductivity by maximizing the phonon scattering The ZT value has been greatly improved in recent years. As a representative example, a thermoelectric material of Japanese Patent Publication No. 2014-22731, and B. Poudel et al., Science p. 320, p.634 "High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys" in 2008, BiSbTe The powder was made into nano powder by ball milling technique and then the nano powder was sintered by hot pressing method to synthesize Bi _x Sb _{2 - x} Te ₃ thermoelectric material with ZT = 1.4. This thermoelectric material is much higher than ZT = 1.0 which was recognized as a conventional limit.

On the other hand, the n-type material based on Bi ₂ Te ₃ has a large gap between the p-type material and the p-type material because most of the research results are in ZT <0.8. It is not easy to obtain. In addition, X.Yan et al., A recent representative study of ZT ~ 1, published Nano Letters No. 10, p.3373 "Experimental studies on anisotropic thermoelectric properties and structures of n-type Bi ₂ Te _{2 .7} Se _{0 .3} " As a result, there are few studies on the possibility of practical use, such as two hot-pressing operations to improve the ZT value.

Therefore, there is a desperate need to improve the thermoelectric properties of n-type Bi ₂ Te ₃ series material, which can be easily implemented without significantly altering the existing conventional bulk process based synthesis method.

Accordingly, an object of the present invention is to provide a thermoelectric material in which zinc oxide is mixed with an n-type Bi ₂ Te _{3 - x} Se _x thermoelectric material to improve the thermoelectric properties and a method of manufacturing the thermoelectric material.

It is another object of the present invention to provide a thermoelectric material in which anisotropy of a nanocomposite thermoelectric material is reduced by mixing zinc oxide and mixed with zinc oxide having homogeneous thermoelectric properties irrespective of the portion of the nanocomposite thermoelectric material through the same.

It is therefore an object of the present invention, bismuth (Bi), tellurium (Te), selenium (Se) and the thermal material raw material powder and zinc oxide (ZnO) nano-oxide powder consisting of a mixture and sintering _{Bi 2 Te 3 - x Se x} - ZnO nanocomposite thermoelectric material to form a ZnO nanocomposite thermoelectric material. (0? X? 3)

Preparing a thermoelectric material powder and the zinc oxide nano powder; Milling the thermoelectric material powder and the zinc oxide nano powder to form a mixed powder; And sintering the mixed powder to form the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material.

Also, in order to reduce anisotropy of the thermoelectric properties, the zinc oxide nano powder is mixed at 0.1 to 5 vol.% With respect to the Bi ₂ Te _{3 - x} Se _x thermoelectric material formed through the thermoelectric material powder, and the Bi ₂ Te _{3 -x} Se _x -ZnO nanocomposite thermoelectric material and the zinc oxide nanopowder is controlled so that the grain growth of the Bi ₂ Te _3-x Se _x -ZnO nanocomposite thermoelectric material can be controlled, Preferably, the thermoelectric raw material powder is pulverized to a diameter of 200 nm or less, and the zinc oxide nano powder is dispersed in the range of 10 to 100 nm so that the microstructure of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material can be dense. It is preferable to have a diameter of 50 nm.

The above-mentioned object is also achieved by a method for producing a Bi ₂ Te _{3- x} Se _{x -type single} crystal obtained by mixing and sintering a raw material for thermoelectric material consisting of bismuth (Bi), tellurium (Te) and selenium (Se) ZnO nanocomposite thermoelectric material, and the ZnO nanocomposite thermoelectric material. (0? X? 3)

According to the structure of the present invention described above, zinc oxide can be mixed with an n-type Bi ₂ Te _{3 - x} Se _x thermoelectric material to improve the thermoelectric properties of the nanocomposite thermoelectric material.

Also, by mixing zinc oxide, the anisotropy of the nanocomposite thermoelectric material is reduced, and thus, a nanocomposite thermoelectric material having homogeneous thermoelectric properties regardless of the region can be obtained.

FIG. 1 is a graph showing an n-type material and a dimensionless performance index showing the best thermoelectric conversion characteristics by temperature region,
FIG. 2 is a graph showing a p-type material and a dimensionless performance index showing the best thermoelectric conversion characteristics in each temperature region,
3 is a view for defining the direction of compression and sintering of the thermoelectric material,
FIG. 4 is a graph showing the bending coefficient in the horizontal direction with respect to the c-axis of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material according to the embodiment of the present invention,
5 is a graph showing the Seebeck coefficient in the direction perpendicular to the c axis of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material,
6 is a graph showing the thermal conductivity of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material in the horizontal direction with respect to the c axis,
7 is a graph showing the thermal conductivity of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material in a direction perpendicular to the c axis,
8 is a graph showing the dimensionless figure of merit in the horizontal direction with respect to the c axis of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material,
9 is a graph showing the dimensionless figure of merit in a direction perpendicular to the c axis of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material,
10 is a graph showing anisotropy of the dimensionless figure of merit of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material,
11 is a photograph of a section of Bi ₂ Te _{3 - x} Se _x thermoelectric material taken by a scanning electron microscope,
12 is a photograph of a cross section of a Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material taken by a scanning electron microscope,
FIG. 13 is a graph comparing compressive fracture strengths of the Bi ₂ Te _{3 - x} Se _x thermoelectric material and the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material.

Hereinafter, a thermoelectric material in which zinc oxide is mixed according to an embodiment of the present invention and a method of manufacturing the same will be described in detail with reference to the drawings.

First, Bi ₂ Te _{3 - x} Se _x thermoelectric material powder and ZnO nano powder are prepared.

Bi ₂ Te _{3 - x} Se _x thermoelectric material is used in the present invention. Bismuth (Bi), tellurium (Te), and selenium (Se) thermoelectric material powders are prepared in order to produce such thermoelectric materials. The purity of the raw material powder is generally 99.99% or more, and the higher the purity, the better the thermoelectric conversion characteristics of the final thermoelectric material. The Bi ₂ Te _3-x Se _x thermoelectric material can be in the form of a powder, a plate, or the like formed by a powder method or a melt spinning method, It is not limited. In order to increase the properties of Bi ₂ Te _{3 - x} Se _x thermoelectric materials, antimony (Sb), iodine (I), chlorine (Cl), bromine (Br), copper (Cu) (Au), zinc (Zn), cadmium (Cd), and a mixture thereof.

The zinc oxide (ZnO) nanopowder preferably has a purity of 99% or more and a diameter of 10 to 50 nm. When the diameter of the zinc oxide nano powder is less than 10 nm, the zinc oxide nano powder tends to aggregate. When the diameter exceeds 50 nm, the size of the powder may be too large to be smoothly mixed with the Bi ₂ Te _{3 - x} Se _x thermoelectric material. In addition, the size of the powder is large, and the microstructure of the thermoelectric material is hard to be dense. The most preferable average diameter of the zinc oxide nanopowder is 14 nm.

The raw material powder for thermoelectric material and the zinc oxide nano powder are mixed and pulverized to form a mixed powder.

Bi ₂ Te _{3 - x} Se _x thermoelectric material After mixing raw material powders Bi, Te, Se and ZnO nano powder, the raw material powders are pulverized to have a desired diameter. In the present invention, Bi, Te, Se, and ZnO nano powders were each quantitatively mixed with a planetary ball miller (PM-100, Retsch Inc.) in accordance with the stoichiometry, mm of stainless steel balls are put together and rotated at 300 rpm for 2 hours to be pulverized so as to have an average diameter of 200 nm or less. In this case, when the average diameter of the raw material powder is 200 nm or less, the effect of decreasing the thermal conductivity by phonon scattering can be maximized, and the zinc oxide nano powder can be added to the grain boundary of the Bi ₂ Te _{3 - x} Se _x thermoelectric material And it is easy to control the grain growth in the subsequent sintering process.

In the case of the Bi ₂ Te _{3 - x} Se _x thermoelectric material, it is preferable that x is 0.10 to 0.30, but the mixing ratio is not particularly limited even in the other ranges. The most preferable molar fraction is x to 0.15, and in the present invention, a thermoelectric material is synthesized so that x = 0.15.

It is preferable that the ZnO nano powder is mixed at 0.1 to 5 vol.% With respect to the Bi ₂ Te _{3 - x} Se _x thermoelectric material. When the ZnO nanopowder is mixed with less than 0.1 vol.%, The effect of the addition of the nano powder is hardly exhibited. If the ZnO nanopowder exceeds 5 vol.%, The thermoelectric efficiency is lowered beyond the doping level. In addition, anisotropy is not lowered even if it exceeds 5 vol.%. The most preferable mixing ratio is 0.5% by volume of ZnO nanopowder mixed.

The mixed powder is sintered to form a Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material.

Synthesis of Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric materials is finally achieved by sintering mixed powder of Bi ₂ Te _{3 - x} Se _x thermoelectric material and ZnO nano powder. In the present invention, the mixed powder is sintered by various methods such as spark plasma sintering or hot pressing while injecting the mixed powder in an inert gas atmosphere and a pressure of 30 to 300 MPa at 400 to 500 ° C. Here, sintering is performed for 5 to 20 minutes. When the sintering time is less than 5 minutes, it is difficult to obtain a thermoelectric material having a uniform thermoelectric property. If the sintering time is more than 20 minutes, deformation may occur in the thermoelectric material. Here, the most preferable sintering time is 10 minutes. When the sintering pressure is less than 30 MPa, compression is difficult due to insufficient compression, and when the pressure exceeds 300 MPa, cracks may occur due to a large force applied to the material.

The inert gas is argon (Ar), nitrogen (N ₂ ), helium (He), neon (Ne), krypton (Kr) , Xenon (Xe), radon (Rn), and mixtures thereof.

In general, as shown in FIG. 3, if the direction of the pressure applied during sintering is the direction c, the anisotropic property is not the same in the c direction and the direction perpendicular thereto. However, in the present invention, the ZnO nano powder is mixed with the thermoelectric material powder, thereby reducing the anisotropy.

When the Bi ₂ Te _{3 - x} Se _{x -} ZnO nanocomposite thermoelectric material is synthesized through these steps, the ZnO nano powder is mainly located at the grain boundaries and the thermal conductivity is decreased. At the same time, the Seebeck coefficient is increased and the thermoelectric conversion characteristic is improved as a result. FIGS. 4 to 7 show the results of Seebeck coefficient of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material synthesized in the present invention according to the compression direction during thermal sintering.

FIG. 4 shows the values of the Seebeck coefficient when the volume ratio of ZnO mixed in the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material is 0%, 0.2%, 0.5% and 1% The results are shown in Fig. As can be seen from the graph, when the ZnO is mixed with the thermoelectric material at 0.5%, the value of the Seebeck coefficient is the highest.

5 is a graph showing the result of measuring the Seebeck coefficient in a direction perpendicular to the c axis which is the compression direction. A vertical direction similarly as the direction parallel to the ZnO nano-powder was a _{Bi 2 Te 3-x Se x} -ZnO was the highest Seebeck coefficient of the thermoelectric nanocomposite material mixed with 0.5%, and the overall pattern parallel to the c-axis graphs and It can be seen that they are similar.

FIG. 6 shows the thermal conductivity of the nanocomposite thermoelectric material according to the mixing ratio of ZnO nano powder. When the thermal conductivity is measured in the direction parallel to the c-axis, 1.0% of the ZnO nanopowder mixture shows the lowest thermal conductivity . It can be seen that the thermal conductivity decreases as the mixing ratio of ZnO nano powder increases.

FIG. 7 is a graph showing the thermal conductivity measured in a direction perpendicular to the c-axis. As shown in FIG. 6, 1.0% of ZnO nanopowder was mixed with the lowest thermal conductivity. Also, the overall graphical appearance is similar to Fig.

Figs. 8 and 9 show the dimensionless figure of merit (ZT) with respect to the direction perpendicular to the c-axis and the parallel direction, and the dimensionless figure of merit is calculated through the Seebeck coefficient and the thermal conductivity. In the direction perpendicular to the c-axis, the dimensionless figure of merit increased by 4.8%, while the direction parallel to the c-axis increased by 21.9%.

10 is a graph showing anisotropy of the nanocomposite thermoelectric material through the dimensionless figure of merit, which is a result obtained by subtracting the dimensionless figure of merit parallel to the c-axis from the dimensionless figure of merit in the direction perpendicular to the c-axis . As can be seen from the graph, the anisotropy of the nanocomposite thermoelectric material containing ZnO nanopowder is very low, whereas the anisotropy of the nanocomposite thermoelectric material containing ZnO nano powder is very low. This demonstrates that mixing of ZnO nanoparticles reduces the anisotropy of nanocomposite thermoelectric materials.

Fig. 11 is a photograph of a section of a Bi ₂ Te _{2 .7} Se _{0 .3} thermoelectric material without ZnO nano powder added thereto, observed by an electron microscope, showing that the structure of the thermoelectric material is not dense. In contrast, FIG. 12 is a photograph of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material with ZnO nanoparticles added thereto, which is observed by an electron microscope. It can be seen that the microstructure becomes considerably dense as compared with FIG. This is because ZnO nanoparticles are mainly located in the grain boundaries and inhibit grain growth which occurs during the sintering process. As crystal grain growth is inhibited and consequently grain grains are reduced, phonon scattering is increased and the thermal conductivity is decreased, thereby improving the thermoelectric conversion characteristics of the thermoelectric material. 13 shows that the Bi ₂ Te _{2 .7} Se _{0 .3} thermoelectric material containing no ZnO nano-powder and the ZnO nano-powder have a mechanical strength of 1.0 vol% .% Bi ₂ Te _{2 .7} Se _{0 .3} Compressive strength of thermoelectric material was compared. As can be seen from FIG. 13, when the ZnO nano powder is mixed, the compressive fracture strength is increased by about 2.2%.

In the prior art, thermoelectric materials and manufacturing methods with increased non-dimensional performance index and thermoelectric properties of p-type have been well known, but a technique of increasing thermoelectric properties of n-type thermoelectric materials having properties different from those of p- Little was found. Therefore, the present invention is to produce a Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material by mixing zinc oxide nano powder together with an n-type thermoelectric material Bi ₂ Te _{3 - x} Se _x thermoelectric material. The thermoelectric properties of the nanocomposite thermoelectric materials are improved and the anisotropy of the nanocomposite thermoelectric materials is reduced. When the anisotropy is reduced, the nanocomposite thermoelectric material has homogeneous thermoelectric properties regardless of the region. If the anisotropy is high, the portion where the desired thermoelectric property can not be obtained can not be used and the cost efficiency is poor. In contrast, the present invention has a low anisotropy to prevent the waste of the nanocomposite thermoelectric material.

Claims (9)

A method for manufacturing a thermoelectric material in which zinc oxide is mixed,
Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material is formed by mixing and sintering a thermoelectric material powder and a zinc oxide (ZnO) nano powder composed of bismuth (Bi), tellurium (Te), selenium Wherein the zinc oxide is mixed with the zinc oxide. (0? X? 3) The method according to claim 1,
Preparing the thermoelectric material powder and the zinc oxide nano powder;
Milling the thermoelectric material powder and the zinc oxide nano powder to form a mixed powder;
And sintering the mixed powder to form the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material. The method according to claim 1,
Wherein the zinc oxide nano powder is mixed at 0.1 to 5 vol.% With respect to the Bi ₂ Te _{3 - x} Se _x thermoelectric material formed through the thermoelectric material powder to reduce anisotropy of thermoelectric properties. Gt; The method according to claim 1,
The Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material has improved thermoelectric properties while the zinc oxide nanopowder has a grain growth of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material Wherein the thermoelectric material powder is pulverized to a diameter of 200 nm or less so that the thermoelectric material powder can be controlled. The method according to claim 1,
Wherein the zinc oxide nanopowder has a diameter of 10 to 50 nm so that the microstructure of the Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material can be dense. Gt; The method according to claim 1,
In order to increase the thermoelectric properties, antimony (Sb), iodine (I), chlorine (Cl), bromine (Br), copper (Cu), silver (Ag), gold (Au), zinc (Zn) And an element made of a mixture thereof is added to the thermoelectric material. The method according to claim 1,
Wherein the sintering is performed in an inert gas atmosphere at 400 to 500 ° C. The method according to claim 1,
Wherein the sintering is performed by spark plasma sintering or hot pressing. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt; In the thermoelectric material in which zinc oxide is mixed,
Bi ₂ Te _{3 - x} Se _x -ZnO nanocomposite thermoelectric material obtained by mixing and sintering a thermoelectric material powder and a zinc oxide (ZnO) nano powder made of bismuth (Bi), tellurium (Te) Wherein the thermoelectric material is a mixture of zinc oxide. (0? X? 3)

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