KR20110092762A - MANUFACTURING METHOD OF Mg2Si THERMOELECTRIC MATERIAL USING MECHANICAL ALLOYING AND Mg2Si THERMOELECTRIC MATERIAL - Google Patents

Abstract

PURPOSE: An Mg2Si thermoelectric material manufacturing method and a Mg2Si thermoelectric material manufactured by the same are provided to reduce the costs for manufacturing a Mg2Si thermoelectric material by employing a mechanical alloying process. CONSTITUTION: An Mg2Si thermoelectric material manufacturing method comprises steps of: preparing Mg and Si powder of element state, mixing the powder, mechanically alloying the mixed powder, and hot-forming the alloyed powder.

Description

Method for manufacturing Mg 2 Si thermoelectric material by mechanical alloying and Mg 2 Si thermoelectric material manufactured by the present invention.

The present invention relates to a method of producing a thermoelectric material having a high thermoelectric performance index by alloying Mg and Si through a mechanical alloying process, and a high efficiency thermoelectric material Mg ₂ Si obtained thereby.

Recently, as interest in development and saving of alternative energy is increasing, researches and studies on efficient energy conversion materials are actively conducted. In particular, research on thermoelectric materials, which are thermo-electric energy conversion materials, has been accelerated.

Such a thermoelectric material is a metal or ceramic material having a function of directly changing heat to electricity or electricity directly to heat, and there is an advantage in that power generation is possible without a moving part by providing only a temperature difference.

This thermoelectric material was developed in the late 1930s after the discovery of the thermoelectric phenomena, Seeback effect, Peltier effect, and Thomson effect. It is developed as a material.

Recently, thermoelectric materials have been used as special power supply devices such as mountain wallpaper, space, military, etc., using thermoelectric power generation characteristics. It is used for small cooling columns.

As such, as the field of using thermoelectric materials expands, a need for thermoelectric materials having excellent efficiency is emerging.

The efficiency of a thermoelectric material is evaluated by the dimensionless figure of merit, which is defined as ZT = α ² Tρ ⁻¹ λ ^{− 1} . α is the Seebeck coefficient, T is the maximum temperature, ρ is the electrical resistivity, and λ is the thermal conductivity.

Basic conditions for excellent thermoelectric properties include large unit lattice, complex crystal structure, heavy atomic mass, strong covalent bonds, large effective carrier mass, and high carrier mobility. 10 ³ cm ² / Vs), narrow energy bandgap (~ K _B T), and small differences in electronegativity between constituent atoms are required.

Among them, Mg ₂ Si, which has recently been spotlighted as a thermoelectric material, can be used as a high-performance thermoelectric material because of its high Seebeck coefficient, high electrical conductivity, and low thermal conductivity, compared to conventional thermoelectric materials PbTe and CoSb ₃ . Mg ₂ Si is also a non-toxic material and is of greater interest in that its constituents are abundant in the earth's crust.

On the other hand, conventional thermoelectric material manufacturing methods include single crystal growth by band melting, powder sintering by grinding, long annealing, etc. Is concerned about the change. In addition, many problems have been raised, such as lengthening of powder production time.

In particular, although Mg ₂ Si (a component of Mg ₂ B ^IV , (B ^IV = Si, Ge, Sn)) is spotlighted as a thermoelectric material, elements Mg and Si are both elements due to large vapor pressure difference and insolubility of the components. There was a problem that it is difficult to combine.

In other words, obtaining a homogeneous composition of Mg ₂ Si is difficult to overcome by the high temperature synthesis method by the conventional dissolution or crystalline phase method due to the difference in the properties of both volatile and reactive Mg and the relatively high melting point both elements of Si. There is a problem.

An object of the present invention is to provide a high-efficiency thermoelectric material Mg ₂ Si manufacturing method having a high thermoelectric performance index by hot pressing and doping homogeneous Mg ₂ Si powder obtained through a mechanical alloy process.

Still another object of the present invention is to provide a high efficiency thermoelectric material, Mg ₂ Si, having a high thermoelectric performance index.

In order to achieve the above object, the present invention provides a high efficiency thermoelectric material manufacturing method consisting of the following steps.

Preparing Mg and Si powder in an elemental state;

Mixing the powder;

Mechanically alloying the mixed powder; And

Hot forming the mechanically alloyed powder.

The present inventors have been studying a method for producing Mg ₂ Si more efficiently as a thermoelectric material, and have found that a uniform alloy powder can be obtained when the Mg and Si element powders in a solid state are subjected to a mechanical alloy process.

In addition, the present inventors have found that the manufacturing cost can be lowered compared to the melting and pulverizing method, which is a conventional thermoelectric material manufacturing method, to complete the present invention.

The mechanical alloying process relates to a process for obtaining an alloy powder by stirring or ball milling an elemental powder with a solid ball (or steel ball) at high mechanical energy in a solid state reaction.

In this mechanical alloying process, all materials such as metals, ceramics, and polymers are targeted, and various combinations of solid phase, liquid phase, and gas phase are possible as phases.

In general, the mechanical alloying process is divided into three stages according to the change in powder shape. First, in the initial stage, the constituent powder is predominantly cold pressure welding due to repeated collisions between the balls and the powder becomes coarse.

In this case, the powder cross-sectional structure shows that the constituent powders have a layered structure, but the atomic scale mixing, that is, alloying is not performed.

In the next intermediate step, the coarse particles are subjected to continuous work hardening, in which powder is not alloyed but the powder components are mixed very finely. The final stage is a steady-state stage in which only tens of micrometer-sized microparticles with a uniform particle size distribution appear, and mechanical alloying is completed. In other words, the element powder is changed into an alloy powder by repeatedly breaking high energy and pressing.

On the other hand, it is preferable to perform a doping step after hot forming the mechanical alloying powder.

This invention process has the advantage of lowering the manufacturing cost compared to the melting and grinding method of the conventional thermoelectric material manufacturing method, through which the thermoelectric performance index can be produced Mg ₂ Si that can be spotlighted as a thermoelectric material.

According to the present invention described above, Mg ₂ Si, which has been spotlighted as a thermoelectric material through a mechanical alloying process, can be manufactured at a lower cost than the melting and grinding method of a conventional thermoelectric material manufacturing method.

In addition, it is possible to provide a thermoelectric material Mg ₂ Si having an excellent thermoelectric performance index.

1 is a photograph of Mg ₂ Si powder synthesized by a mechanical alloy for 24 hours according to a preferred embodiment of the method of manufacturing a thermoelectric material of the present invention.
Figure 2 Figure 2 is a graph showing the thermoelectric material of the present invention the X- ray diffraction patterns of the mechanical alloying and hot pressing coarse Mg ₂ Si in accordance with the manufacturing method, (a) Mg ₂ Si alloy subjected to the mechanical process, for 12 hours X-ray diffraction pattern, (b) X-ray diffraction pattern of Mg ₂ Si subjected to mechanical alloying process for 24 hours, (c) X-ray diffraction pattern of Mg ₂ Si pressurized at 1073K.
Figure 3 is a photograph showing the microstructure of the hot pressurized Mg ₂ Si, (a) fine structure of Mg ₂ Si pressed at 773K, (b) the microstructure of the Mg ₂ Si pressed at 873K, (c) at 973K a photograph showing the microstructure of a pressure on the microstructure of a pressurized _{Mg 2 Si, (d) 1073K} Mg 2 Si.
4 is a graph showing the bulk density change according to the hot press temperature.
5 is a graph showing various thermoelectric properties of hot-pressed Mg ₂ Si, (a) electron conductivity, (b) Seebeck coefficient, (c) thermal conductivity, and (d) performance index values.

Hereinafter, the configuration of the present invention will be described in more detail with reference to specific examples, but the scope of the present invention is not limited to the following examples.

In order to manufacture the thermoelectric material according to this embodiment, first, a high purity raw material of Mg powder (<149 μm, purity 99.99%) and silicon powder (<45 μm, purity 99.99%) was prepared. The two high purity materials and hard steel balls were placed in an alloy vial.

In order to suppress the oxidation of the raw material Mg, Si and the alloy Mg ₂ Si, an alloying vessel equipped with a vacuum valve (vacuum) was made in a vacuum state and filled with argon gas 2-3 times in an inert atmosphere. .

The vessel was then placed in a planetary grinder (Pulverisette 5 model of Fritsch) and subjected to mechanical alloy processing for 12 to 24 hours. The synthesized powder was hot formed in a cylindrical tungsten carbide container having an inner diameter of 10 mm at a pressure of 70 MPa at a temperature of 773 to 1073 K for 2 hours in a vacuum state.

The material obtained in this way was subjected to phase analysis through X-ray diffraction results. X-ray analysis was performed with an X-ray diffractometer (Dig2500VPC model from Rigaku) using CuK _α light (40 kV, 200 mA). Hot pressed pellets are 3 x 3 x 10 mm ³ rectangles for Seebeck coefficient and electrical conductivity measurements, and 10 mm (diameter) x 1 mm (thickness) disks for thermal conductivity and Hall effect measurements. Cut into disc shape. The Hall effect was measured using a Keithley 7065 system instrument under constant magnetic field conditions of temperature 300K, current 50mA and size 1T.

Seebeck coefficient (α) and electrical conductivity (σ) were measured by temperature differential and 4-probe methods, respectively, on a Ulvac-Riko SEM-M8 instrument in a helium atmosphere.

Thermal conductivity (κ) was measured from thermal diffusivity (D), specific heat (C _p ) and density (d) using a laser flash Ulvac-Riko TC7000 system in vacuum.

Figure 1 is a SEM photograph of the Mg ₂ Si powder synthesized by a mechanical alloying process for 24 hours, the average particle size of several μm was obtained and it can be seen that the shape of the particles are irregular.

The element Mg, which is a raw material, has ductility as particles having a size of 149 µm or less, and the element Si has brittleness as particles of 45 µm or less.

Since the mechanical alloying process is a unique solid-state alloying process in which cold welding and crushing occur continuously and as a result fine grain size and phase homogenization can be obtained, the mechanical alloying process has been successfully performed in Mg-Si (ductile-brittle) system. It can be seen that Mg ₂ Si particles can be synthesized.

FIG. 2 is an X-ray diffraction pattern of Mg ₂ Si powder and hot pressed Mg ₂ Si molded product by a mechanical alloy. FIG. It can be seen that as the milling time increases, the peak intensities of Mg and Si are lowered and Mg ₂ Si reflection occurs. Moreover, the diffraction peaks widened due to the increase in lattice strain and decrease in particle size.

In FIG. 2, (a) shows Mg ₂ Si at a milling time of 12 hours, and the peaks of Si and Mg appeared somewhat, indicating that alloying did not occur completely. Therefore, it is preferable to carry out mechanical alloy process treatment for 12 hours or more.

As shown in FIG. 2 (b), when the milling time was 24 hours, a single phase of Mg ₂ Si appeared and none of the other second phases appeared. These particles could be strengthened by hot forming at 1073K to maintain the Mg ₂ Si phase. This means that the Mg ₂ Si compound, which has undergone mechanical alloying and hot forming, is thermodynamically stable.

However, some diffraction peaks for MgO can be seen in Figure 2 (c). It can be seen that even if done in a vacuum, magnesium was oxidized at high temperature during the hot forming process. This is because magnesium is chemically responsive, especially for oxygen and water vapor.

Table 1 shows the electrical transfer properties of intrinsic Mg ₂ Si at a temperature of 300K compared to the values shown in other literature.

The Hall coefficient in Table 1 is (-), which means that the electrical conduction is mostly due to electrons. As a result, the carrier concentration of hot-formed Mg ₂ Si was 2.4 X 10 ¹⁷ cm ^{- 3} and Hall mobility was 48 cm ² V ^-1 s ^-1 , which was discharge plasma sintered It can be seen that it is lower than spark-plasma-sintered Mg ₂ Si (Mg ₂ Si) or crystal-grown Mg ₂ Si.

This may be due to differences in chemical purity, particle size and the presence of a secondary phase (eg MgO). Indeed, an electron concentration of 10 ¹⁷ cm ^-3 is too low for thermoelectric applications. Good thermoelectric materials should have an electron concentration of 10 ¹⁹ to 10 ²⁰ . Therefore, an appropriate atom should be doped with intrinsic Mg ₂ Si, which will be further studied.

Electron Transport Characteristics of Mg ₂ Si at Temperature 300K How to prepare Hall coefficient
[ cm ³ C ^-One ] Electron concentration
[ cm ^-3 ] Hall mobility
[ cm ² V ^-One s ^-One ] Reference Hot forming -26.3 2.4 X 10 ¹⁷ 48 Embodiment according to the present invention Spark plasma  Sintered -14.5 4.3 X 10 ¹⁷ 204 More than Tani (H.J. Goldsmid, Electronic Refrigeration, Pion Limited, Lodon, p42 (1985)) Perpendicular Bridgeman  Growth Law -7.4 8.4 X 10 ¹⁷ 176 Akasaka et al. (M. Akasaka, T. Iida, A. Matsumoto, K. Yamanaka, Y. Takanashi, T. Imai and N. Hamada, J. Appl. Phys. 104,013703 (2008))

3 and 4 show the change in density and microstructure of Mg ₂ Si hot formed at different temperatures. As the molding temperature increases, it can be seen that the fine structure becomes more dense. Samples molded at 1073 K exhibit nearly complete density, which is equivalent to 2.001 gcm ^-3 , the theoretical density of Mg ₂ Si. It can be seen that the value is close.

FIG. 5 shows the change in thermoelectric properties with temperature of Mg ₂ Si compared with data reported by Tani et al. (J. Tani and H. Kido, Physica B364,218).

The electron conductivity (σ) of the highly doped n-type semiconductor can be expressed by the following equation (1).

(1)

(One)

Where μ is electron mobility, m * is the effective mass of electrons, and τ is the relaxation time of electrons.

FIG. 5A shows the temperature dependence of electrical conductivity in Mg ₂ Si. Electrical conductivity rapidly increases with increasing temperature, which means that intrinsic Mg ₂ Si behaves like a non-degenerate semiconductor. That is, at high temperatures, electrical conductivity is increased by intrinsic conduction due to a bandgap of 0.77 eV. The temperature dependence of electrical conductivity is shown because the increase in carrier concentration due to intrinsic conductivity overcomes the reduction of electron transfer due to electron-phonon scattering.

In this regard, the Seebeck coefficient α of the n-type semiconductor is as shown in Equation (2) below.

(2)

(2)

κ is Boltzmann constant, e is charge amount, r is power function exponent of energy dependent relaxation time, Ec is ground state energy of conduction band, E _f is Fermi energy, T is absolute temperature, Nc is effective density in conduction band, n is electron Concentration.

As shown in (b) of FIG. 5, as the temperature is increased, the absolute value of the Seebeck coefficient decreases due to the increase in the electron concentration due to intrinsic conduction. The Seebeck coefficient shows a negative value, which is in good agreement with the sign of the Hall coefficient values in Table 1, indicating that the intrinsic Mg ₂ Si is an n-type semiconductor.

On the other hand, the thermal conductivity κ is the sum of lattice thermal conductivity κ _L due to phonon and electron thermal conductivity κ _E due to carrier, and is represented by the following formula (3).

(3)

(3)

d is density, Cp is specific heat, and D is thermal diffusivity. The two elements can be separated by the Wiedemann-Franz law (κ _E = LσT), for which the Lorentz number is assumed to be a constant (L = 2.45 X 10 ^-8 V ² K ^-2 ). .

5 (c) shows the temperature dependence of the Mg ₂ Si thermal conductivity. The thermal conductivity decreases with increasing temperature, and it can be seen that it is almost constant beyond 650K.

By separating the lattice element from the electronic element, it can be confirmed from this calculation that the lattice contribution to the thermal conductivity in intrinsic Mg ₂ Si is dominant, and that 99% of the thermal conductivity is attributable to the lattice element at all temperatures investigated. This is because the carrier concentration (10 ¹⁷ cm ^-3 ) is low, even if the temperature increases to increase the electrical conductivity and hence the electron's thermal conductivity.

The efficiency of the thermoelectric figure-of-merit (Z) is determined by equation (4) below (CB Vining, CRC Handbook of Thermoelectrics, Ed. By DM Rowe, CRC Press, New York, p. 277 ( 1995))

(4)

(4)

m _e is the mass of the electron.

5 (d) shows the temperature dependency of the Mg ₂ Si thermoelectric performance index (Z). It can be seen from FIG. 5 (d) that the Z value increases with increasing temperature in the section from 300K to 500K and is maintained up to 850K. However, the Z value of intrinsic Mg ₂ Si is low due to the low electrical conductivity. This can be improved by increasing the carrier concentration with doping.

From the above, it was confirmed that mechanical alloying and hot pressing were excellent methods for producing thermoelectric materials of ductile Mg and highly brittle Si.

In other words, the difficulty of obtaining a homogeneous composition of Mg ₂ Si due to the difference in properties between the high volatility and reactivity Mg and the relatively high melting point of both elements of Si, which is a problem of high temperature synthesis method by the conventional melting or crystal growth method, It can be seen that the low-temperature synthesis method as a solid phase reaction according to the mechanical alloying of the present invention was overcome.

Simple modifications and variations of the present invention can be easily made by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

Claims (11)

Preparing Mg and Si powder in an elemental state;
Mixing the powder;
Mechanically alloying the mixed powder; And
Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying comprising the step of hot forming the mechanical alloying powder. The method according to claim 1,
After the hot forming step, characterized in that the addition of the doping step, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 1,
The mechanical alloying treatment is performed by milling, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 3,
The milling is carried out for more than 12 hours, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 4,
The milling is performed for 24 hours or more, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 1,
The hot forming temperature is 773-1073K, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 1,
The size of the Mg powder is smaller than 149μm, the size of the Si powder is smaller than 45μm, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 1,
The mechanical alloying treatment is performed in an inert atmosphere, Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying. The method according to claim 1,
A method of producing Mg ₂ Si thermoelectric material by mechanical alloying, characterized in that an alloying vessel equipped with a vacuum valve is used to suppress oxidation of the raw materials Mg, Si and Mg ₂ Si. The method according to claim 9,
Mg ₂ Si thermoelectric material manufacturing method by mechanical alloying, characterized in that the alloying vessel is vacuumed and then repeatedly filled with argon gas 2-3 times. Mg ₂ Si thermoelectric material produced by the method of any one of claims 1 to 9.

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