KR20090026667A - Sn-filled and te-doped skutterudite thermoelectric material and method for manufacturing the same - Google Patents

Abstract

A Sn-filled and Te-doped skutterudite thermoelectric material and method for manufacturing the same is provided to increase thermal conductivity by optimizing the concentration of a carrier. An Sn-filled and Te-doped skutterudite thermoelectric material is comprised of the steps: inserting cobalt, antimony, indium, and tellurium into the quartz tube; heating and melting the inserted material with high frequency induction power; rapidly freezing in the water in order to prevent formation of the second phase after melting; perform vacuum heating treatment to fill indium into the aperture and activate Te.

Description

SN-FILLED AND Te-DOPED SKUTTERUDITE THERMOELECTRIC MATERIAL AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high efficiency thermoelectric material, and a method for manufacturing the same, and more particularly, Sn _z Co ₄ Sb ₁₂ filled with Sn and substituted Te with Sb in the pores of CoSb ₃ scouterite. _{- y} Te _y hibiscus Teruel filled with Sn using a die teugye thermoelectric material and sealed induction melting method and heat treatment, and relates to a manufacturing method for doped with Te.

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

The efficiency of thermoelectric materials is evaluated by the dimensionless thermoelectric figure of merit (ZT), which is defined as ZT = α ² Tρ ^-1 λ ^-1 . α is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and λ is the thermal conductivity.

Basic conditions for having 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. This notion of a maximum value of ZT of 1 was considered a theoretical limit, and the concept has been maintained for decades. It is known that ZT values can be greater than 1 for custom-made materials using a specific method employing the PGEC (Phonon Glass and Electron Crystal) concept in the superlattice.

Crystallographically, binary skutterudite structures with unit grids belonging to the cubic Im3 space group have been investigated as the most potent materials that meet the conditions for high ZT values.

The scrutherite structure includes not only 32 atoms in 8 TX ₃ groups in a unit grid, but also a lattice structure in which the unit grid is relatively large and the thermoelectric properties can be improved by reducing the lattice thermal conductivity. Here, T is occupied by elements such as Co, Rh and Ir as transition elements, and X is occupied by P, As, and Sb elements as niconic elements.

However, binary scutterudite alone exhibits low efficiency thermoelectric properties due to relatively high lattice thermal conductivity. In order to improve this problem, a method of reducing lattice thermal conductivity by filling a filler element in two voids in a unit of sterudite lattice and causing a rattling effect, A method of improving the thermoelectric performance index by controlling the concentration of hole carriers and inducing lattice scattering by replacing some of them with doping elements has been proposed.

CoSb ₃ is the most promising scuderudite thermoelectric material, but it also exhibits low efficiency thermoelectric properties due to relatively high lattice thermal conductivity and exhibits the properties of p-type semiconductors at room temperature.

The present invention has been invented to solve the above problems, an object of the present invention is to provide a scutterrudite-based thermoelectric material excellent in the thermoelectric properties of Sn is filled in the pores in the lattice, Te is substituted with Sb. Another object of the present invention is to provide a method for producing the thermoelectric material.

In order to achieve the above object, in the scutterrudite thermoelectric material according to the present invention, in the scutterrudite thermoelectric material using CoSb ₃ , the pores in the unit lattice are filled with Sn and doped with Te to give Sn _z Co ₄ Sb. _It has a composition of _12-y Te _y , characterized in that z and y are in the range 0 <z ≤ 0.25 and 0 <y ≤ 0.375.

In addition, the method for producing a skutterudite-based thermoelectric material according to the present invention is a method for producing a skutterrudite thermoelectric material having a composition of Sn _z Co ₄ Sb _12-y Te _y , wherein Co, Sb, Sn, and Te as raw materials are used. Charging to a quartz tube and sealing under vacuum, heating and dissolving the mixture of charged raw materials in a hermetic induction furnace by high frequency induction power, and dissolving the dissolved material in water to prevent the formation of a second phase. Quenching, vacuum-heat-treating the material for filling voids of Sn and activating Te.

According to the present invention, Sn is filled and Te is doped to provide a Sn _z Co ₄ Sb _12-y Te _y scerrudite-based thermoelectric material having an excellent thermoelectric index, and the thermoelectric material is manufactured without a problem of forming a second phase. can do.

The inventors have CoSb ₃ of (Co ₈ Sb ₂₄₎ in so reason indicating the low ZT value as compared to when the case packed with Sn filled with other atoms is low Seebeck coefficient and a high concentration carriers, Sn _z Co ₈ Sb ₂₄ In order to improve the thermoelectric properties, it was determined that the carrier concentration was optimized by doping. Accordingly, Te was doped with Sn _z Co ₈ Sb ₂₄ to improve thermoelectric properties.

Hereinafter, the present invention will be described in more detail with reference to Examples.

To prepare a raw material according to this embodiment, first, a raw material was prepared. The raw material was prepared with high purity elemental materials of Co (purity 99.95%), Sb (purity 99.999%), Sn (purity 99.99%) and Te (purity 99.99%), and these were charged into a quartz tube and vacuum-sealed.

And these raw materials were dissolved using an encapsulated induction melting (EIM) furnace. Dissolution conditions were dissolved for 1 hour at a power of 7kW and a frequency of 40kHz, and after the dissolution was quenched in water to prevent the formation of a second phase.

The ingot formed through the above process was heat-treated at a temperature of 823K for 144 hours (6 days). This not only gives Sn the time to fill the pores of the scrudentite structure, but also for the activation of the Te dopant.

In this manner, Sn-filled and Te-doped Sn _z Co ₄ Sb _12-y Te _y (z = 0, 0.25, y = 0, 0.375) compounds were synthesized.

In order to confirm the physical properties of the thus prepared Sn _z Co ₄ Sb _12-y Te _y , in particular the thermoelectric properties, various measurements and analysis were performed.

First, X-ray diffraction pattern was measured under scanning interval of 0.004 ° and scanning speed of 1 ° / min using high-resolution X-ray diffractometer (HRXRD, Rigaku DMAX2500VPC) using CuK (40kV, 200mA) radiation to check the phase. It was.

Seebeck coefficient, electrical resistivity and thermal conductivity were measured from 300K to 700K. Seebeck coefficient and electrical resistivity were measured by temperature differential method and DC 4-terminal method (Ulvac-Riko ZEM2-M8), respectively, and thermal conductivity was determined from the measured values of thermal diffusivity, specific heat and density by laser flash method (Ulvac-Riko TC7000). Calculated. Hall effect was measured at magnetic field 1T, current 50mA and 300K.

Heated ingots were cut into 3 × 3 × 10 mm size to measure Seebeck coefficient and electrical resistivity, and were cut into 10φ × 1 mm to measure thermal conductivity and Hall effect.

The results of analyzing the measured physical properties as described above are as follows.

1 is an X-ray diffraction pattern of Sn _z Co ₄ Sb _12-y Te _y (z = 0, 0.25, y = 0, 0.375) prepared by EIM and post-heat treated at 144 hours (6 days) at a temperature of 823K. to be. This suggests that this material is a polycrystalline structure with only a single δ-phase. The pore radius of CoSb ₃ is 1.892 Å and the atomic radius of Sn is 1.72 Å. In the present embodiment, as shown in FIG. 1, no second phase was found, which means that Sn is positioned in the pore and the Sn _z Co ₄ Sb _12-y Te _y (z = 0, 0.25, y = 0, 0.375) compound is thermodynamically modified. Reassure stability.

FIG. 2 shows the temperature dependence of the Seebeck coefficient for Sn _z Co ₄ Sb _12-y Te _y (z = 0, 0.25, y = 0, 0.375) skuterudite. Intrinsic CoSb ₃ exhibited n-type conductivity between 300K and 400K and transitioned to p-type conductivity at about 400K. Sn _0.25 Co ₄ Sb ₁₂ from 300K to 700K shows p-type conductivity, while Co ₄ Sb _11.625 Te _0.375 and Sn _0.25 Co ₄ Sb _11.625 Te _0.375 show n-type conductivity. The absolute Seebeck coefficient of Sn-filled and / or Te-doped CoSb ₃ increases with increasing temperature. The increase in Seebeck coefficient (thermoelectric capacity) in n-type material is due to the Te doping effect which increases carrier concentration and decreases carrier mobility, as shown in Table 1.

Figure 3 shows the temperature dependence of the electrical resistivity for Sn _z Co ₄ Sb _12-y Te _y prepared by EIM method and post-treated at 823 K for 144 hours. Intrinsic Co ₈ Sb ₂₄ has a higher resistivity than Sn filled and Te doped materials. In the case of CoSb ₃ , the resistivity decreases with increasing temperature. However, the electrical resistivity increases with increasing temperature for Sn filling and Te doping, and the electrical resistivity of Te-doped specimens (Co ₄ Sb _11.625 Te _0.375 , Sn _0.25 Co ₄ Sb _11.625 Te _0.375 ) is almost independent of temperature. not.

As shown in Table 1, because the carrier concentrations of Sn-filled and Te-doped specimens range from 10 ¹⁹ to 10 ²⁰ cm ^-3 , the material is highly degenerate and its temperature dependence of electrical resistivity is comparable to metal behavior. .

Figure 4 shows the temperature dependence of the thermal conductivity for Sn _z Co ₄ Sb _12-y Te _y prepared by EIM method and post-treated at 823 K for 144 hours. Intrinsic CoSb ₃ has high thermal conductivity, which is reduced to 0.11 W / cmK at 300K and 0.07 W / cmK at 700K. Thermal conductivity is significantly reduced with Sn filling and / or Te doping. Sn _0.25 Co ₄ Sb _11.625 Te _0.375 has higher thermal conductivity than Co ₄ Sb _11.625 Te _0.375 and Sn _0.25 Co ₄ Sb ₁₂ , but has lower thermal conductivity than CoSb ₃ . From this, the Sn filler acts as a rattler, reducing the average free path of phonons, resulting in lower thermal conductivity, and further increasing impurity-phonon scattering by the dopant Te. It is confirmed. However, it can be seen that Sn is not an effective filler when Te is doped.

Figure 5 shows the temperature dependence of the thermoelectric performance index (ZT) for Sn _z Co ₄ Sb _12-y Te _y prepared by EIM method and post-treated at 823 K for 144 hours. Sn filling and Te doping significantly improve ZT. ZT increases with temperature, mainly due to the increase of Seebeck coefficient and the maintenance of low thermal conductivity. Sn _0.25 Co ₄ Sb _11.625 Te _0.375 has a lower ZT value than Co ₄ Sb _11.625 Te _0.375 , but has an improved ZT value than Sn _0.25 Co ₄ Sb ₁₂ , which is considered to be the result of optimizing the carrier concentration by Te doping.

From the above, it is possible to synthesize Sn _z Co ₄ Sb _12-y Te _y filled with Sn and Te-doped by sealed induction melting and subsequent vacuum heat treatment, and the compound thus prepared is composed of a single δ phase. . In addition, it is confirmed as n-type conductivity from the Seebeck coefficient measurement, and as a degenerate semiconductor from the temperature dependence investigation of the electrical resistivity, further reducing the electrical resistivity and thermal conductivity. As a result, the thermoelectric performance is remarkably improved, and in particular, the thermoelectric performance is improved as compared to Sn-filled screuterite, which is more useful as a thermoelectric material.

FIG. 1 is a graph showing an X-ray diffraction pattern of Sn _z Co ₄ Sb _12-y Te _y skaterrudite prepared by EIM and post-treated at 823 K for 6 days.

2 is a graph showing the Seebeck coefficient of Sn _z Co ₄ Sb _12-y Te _y as a function of temperature.

3 is a graph showing the electrical resistivity of Sn _z Co ₄ Sb _12-y Te _y as a function of temperature.

4 is a graph showing the thermal conductivity of Sn _z Co ₄ Sb _12-y Te _y as a function of temperature.

5 is a graph showing the thermoelectric performance index of Sn _z Co ₄ Sb _12-y Te _y as a function of temperature.

Claims (6)

Sn-filled and Te-doped scouterite-based thermoelectric materials, The pores in the unit grid are filled with Sn and doped with Te to have a composition of Sn _z Co ₄ Sb _12-y Te _y , And z and y are in the range of 0 <z ≦ 0.25 and 0 <y ≦ 0.375. The method of claim 1, Scutterrudite-based thermoelectric material, characterized in that z = 0.25 and y = 0.375. As a method of manufacturing Sn-filled and Te-doped skateriteite-based thermoelectric materials, Charging the raw materials Co, Sb, Sn and Te into a quartz tube and then sealing them under vacuum; Heating and dissolving the charged raw material mixture in a closed induction furnace by high frequency induction power; Method of producing a scutter ludite-based thermoelectric material, characterized in that to form Sn _z Co ₄ Sb _12-y Te _y comprising the step of vacuum heat-treating the solidified material after the melting to fill the voids and activate the Te. The method of claim 3, And quenching the dissolved material in water to prevent formation of a second phase. The method according to claim 3 or 4, The vacuum heat treatment step is a method for producing a skutterrudite-based thermoelectric material, characterized in that the constant temperature heat treatment for 8 days at 823K. The method of claim 5, And z = 0.25 and y = 0.375.

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