KR100645596B1 - Method for producing skutterudite cosb3 thermoelectric materials - Google Patents

Abstract

A skutterudite CoSb3 thermoelectric material producing method is provided to improve the thermoelectric characteristic like a seebeck coefficient, electric conductivity, and thermoelectric power factor by producing skutterudite CoSb3 by an arc melting method and changing a microstructure by annealing the skutterudite CoSb3 under an optimal condition. The skutterudite CoSb3 thermoelectric material producing method comprises the steps of: arc-melting Co(Cobalt) and Sb(Stibium) elements under the atmosphere of Ar(Argon); executing re-melting more than one time to prevent the homogenization and segregation of a composition; and annealing the CoSb3 produced by an arc melting method. The CoSb3 is annealed at 300-500 deg.C. The CoSb3 has a microstructure without pores or cracks and good thermoelectric characteristic if the CoSb3 is annealed at 400 deg.C for 24 hours.

Description

SCHERTUDITE COTSB 3 Thermoelectric material manufacturing method {METHOD FOR PRODUCING SKUTTERUDITE CoSb3 THERMOELECTRIC MATERIALS}

1 shows the microstructures of arc melted and vacuum annealed CoSb ₃ in (a) solidified state, (b) 400 ° C./24hrs, (c) 500 ° C./24hrs, (d) 600 ° C./24hrs, (e) It is a graph shown about the conditions of 700 degreeC / 24hrs, (f) 800 degreeC / 24hrs.

2 shows the results of X-ray diffraction analysis of the phase change according to the annealing conditions, (a) as it is solidified, (b) 300 ° C / 24hrs, (c) 400 ° C / 24hrs, (d) 500 ° C / 24hrs, ( e) It is a graph shown about the conditions of 600 degreeC / 24hrs, (f) 700 degreeC / 24hrs, and (g) 800 degreeC / 24hrs.

3 is a graph showing the temperature dependence of the Seebeck coefficient.

Figure 4 is a graph showing the change in electrical conductivity with temperature changes.

5 is a graph illustrating a thermoelectric power factor analysis from Seebeck coefficient and electrical conductivity measured from room temperature to 600K.

Recently, studies on scutterudite compounds as a next-generation material having thermal-electric energy conversion characteristics have been conducted.

Scuterrudite compounds are expected to improve thermoelectric energy conversion characteristics by reducing lattice thermal conductivity and are rich in potential. Hibiscus Teruel die teuneun as hibiscus Teruel de natural minerals from the (Skutterud) of Norway (Fe, Co, Ni) have the basic formula of As _3. Scudrudite structure is crystallographically belong to the space group of Im3 (T _h ⁵ ) and its prototype is CoAs ₃ , which contains 32 atoms in 8 TX ₃ groups in the unit grid. Because of the relatively large unit lattice, it is possible to improve the thermoelectric properties by reducing the lattice thermal conductivity. Here, T is occupied by Co, Rh, Ir, Fe, Ru, Os elements as transition elements, and X is occupied by P, As, Sb elements as niconic (pnicogen) elements. Also, the melting point, composition, and bandgap energy vary depending on which element occupies the T and X positions, which means that the composition and doping concentration can be optimized to meet the requirements of the specific operating temperature of the thermoelectric material. do.

In general, basic conditions for excellent thermoelectric performance indexes require complex crystal structures, heavy atomic weights and effective masses, strong covalent bonds, high carrier mobility, small electronegativity differences between constituent atoms, and complex energy band structures. Scutterrudite is in the spotlight as a next-generation thermoelectric material because it is a material satisfying all the above conditions.

CoSb ₃ is expected to be the most excellent thermoelectric material among binary scutterudite materials, and is an active research material. The lattice constant of CoSb ₃ is 9.0385 Å, the pore radius is 1.892 Å and the cell decomposition temperature is 876 ° C. CoSb ₃ exhibits semiconductor characteristics and has a bandgap energy of about 0.5 eV. Undoped intrinsic CoSb ₃ is a p-type semiconductor, and Ni, Pd, Pt, Te, and the like are used as n-type dopants of CoSb ₃ . CoSb ₃ is generally produced by crystal growth, high frequency induction melting, powder metallurgy, melting / sintering mixing, and the like.

The present invention is to provide a method for producing Scutterrudite CoSb ₃ which can obtain a material having particularly excellent thermoelectric properties by improving the physical properties of CoSb ₃ described above.

In order to achieve the above object, the present invention provides a manufacturing method of CoSb _{3 having the} following configuration.

Arc dissolving elemental states of Co and Sb in an Ar atmosphere;

Conducting one or more redissolutions to prevent homogenization and segregation of the composition; And

Hibiscus Teruel die bit CoSb ₃ The method comprises the step of annealing the CoSb ₃ prepared by the arc melting method.

In the present invention, it has been noted that the crystal structure of CoSb ₃ varies depending on the annealing conditions, and thus the thermoelectric properties are remarkably changed. Thus, annealing conditions showing excellent thermoelectric properties were found, and the annealing is preferably performed at 300-500 ° C, particularly preferably at 400 ° C.

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

Preparation of CoSb ₃ according to the present embodiment is as follows.

First, elemental Co and Sb were arc-dissolved at a current of 300 A for 60 seconds using a 100 kW electron beam generator in an Ar atmosphere. In order to homogenize the composition and prevent segregation, redissolved five times. Ti getters were used to remove Ar gas and oxygen in the chamber. At the time of dissolution, the vacuum degree in the chamber was evacuated to 1.0 × 10 ⁻⁵ mbar or less, and then, the high purity Ar gas was injected to adjust the vacuum degree to 2.0 × 10 ⁻⁴ mbar. The deflection mode of the electron beam was an elliptic method. The CoSb ₃ thus prepared was subjected to vacuum annealing up to 300-800 ° C. for 24 hours.

CoSb ₃ thus prepared was observed in a microstructure using a scanning electron microscope (SEM), and the composition was analyzed using an energy dispersive spectrometer (EDS). In addition, phase analysis was performed using an X-ray diffraction analyzer to investigate the phase change occurring during the dissolution and annealing process. In addition, the thermoelectric properties (seebeck coefficient, electrical conductivity, thermoelectric factor) of CoSb ₃ were measured, and the temperature dependence and annealing effects were also analyzed. The specimen size for measuring thermoelectric properties was 3x3x8㎣ and the properties in the square direction were measured. The Seebeck coefficient was measured by a constant temperature gradient method of measuring a thermal electric motive force generated by applying a temperature difference of about 5 ° C. at both ends of the specimen, and using a conventional DC 4-probe method. The electrical conductivity was measured by. The thermoelectric power factor was calculated from the measured Seebeck coefficient and the electrical conductivity to comprehensively evaluate the thermoelectric properties of the material. At this time, the measurement was performed in vacuum for temperature stabilization and heat transfer in a steady state. In addition, the thermoelectric properties were measured by changing the temperature of the specimen to 300-600K to investigate the temperature dependence of the thermoelectric properties due to the phase transformation according to the annealing conditions.

The structure and measured physical properties of CoSb ₃ thus prepared are shown in FIGS. 1 to 5.

1 is a graph showing the microstructure according to vacuum annealing conditions for arc-dissolved CoSb ₃ . After arc melting, the solidified specimens which were not annealed could obtain a healthy microstructure without pores and cracks, and maintained a healthy microstructure even after annealing at 400 ° C. for 24 hours. However, when the annealing temperature was raised above 500 DEG C, significant pores and cracks were found. This may be due to phase change (lattice change) due to annealing and cooling, thermal expansion / contraction, phase decomposition, and evaporation of Sb. Co-Sb compounds include β-CoSb (hexagonal, a = 3.8800 Hz, c = 5.1850 Hz), γ-CoSb ₂ (orthogonal, a = 5.5960 Hz, b = 6.3730 Hz, c = 3.3700 Hz), γ ' -CoSb ₂ (single monoclinic, a = 6.5081 b, b = 6.3883Å, c = 6.5434Å) and δ-CoSb ₃ (cubic system, a = 9.0385Å), and their crystal structure and lattice constant differ greatly Therefore, cracks (expansion / contraction) may occur due to lattice transformation when the mechanical strength is relatively low, such as melting / solidification specimens. In fact, when the annealing at 500 ℃ or more and cooled to room temperature, the phenomenon that the specimen is destroyed. In addition, when the CoSb ₃ phase was exposed to high temperature, it was found that the Sb evaporated due to phase decomposition and many pores were observed.

2 is a result of analyzing the phase change according to the annealing conditions by X-ray diffraction. For specimens not annealed, β-CoSb, γ-CoSb ₂ as well as δ-CoSb ₃ And Sb coexisted. This means that the Co and Sb elements are not fully alloyed due to complex phase transitions during dissolution / solidification. After annealing at 300 ° C. for 24 hours as shown in FIG. 2B, the diffraction intensities of β, γ, and Sb were decreased and the diffraction intensity of the δ phase was relatively increased, but a complete phase change to δ-CoSb ₃ did not proceed. However, δ-CoSb ₃ by annealing at 400 ° C. for 24 hours as shown in FIG. Single phase was successfully synthesized. It turns out that when annealing temperature rises to 500 degreeC or more, a delta phase will decompose | disassemble into a (beta) phase and a (gamma) phase again. In addition, when the annealing temperature was raised to 700 ° C. or higher, the δ phase and the γ phase were completely decomposed to confirm that only the β phase was present. The phase decomposition process is summarized as follows.

δ-CoSb ₃ → γ-CoSb ₂ + Sb ↑ (500 ℃ <T <600 ℃ at 10 ^-6 torr)

γ-CoSb ₂ → β-CoSb + Sb ↑ (700 ℃ <T <800 ℃ at 10 ^-6 torr)

Where ↑ means evaporation. As shown in (d)-(g) of FIG. 2, no diffraction peaks of Sb decomposed from the δ and γ phases were observed in the specimens annealed at 500 ° C. or higher. This is because the melting point of Sb is 630.8 ° C. and the vapor pressure is relatively high, so that Sb produced after the phase decomposition is melt evaporated.

Since γ-CoSb ₂ and β-CoSb are metal phases, Sb is a semimetallic phase, and δ-CoSb ₃ is a semiconductor phase, the thermoelectric properties change depending on the type of the composition. Seebeck coefficient, electrical conductivity and thermoelectric parameters were measured from room temperature to 600K to investigate the change of thermoelectric properties and the temperature dependence according to the phase change before and after annealing.

3 shows the temperature dependence of the Seebeck coefficient. In the case of solidified specimen before annealing, it showed very low value of several tens of ㎶ / K at all measurement temperature ranges, and it increased slightly to about 90㎶ / K at 550-600K even after annealing at 300 ℃ for 24 hours, at 300 ℃. No significant change in Seebeck coefficient by annealing was seen. This is because there are β-CoSb, γ-CoSb ₂ and Sb, which are unwanted metal phases or semimetal phases. However, when the annealing temperature was raised to 400 ℃, all transformed into δ-CoSb ₃ phase as shown in the X-ray diffraction analysis shown in (c) of FIG. 2, and the Seebeck coefficient rapidly increased as the measured temperature was increased, up to 229 ㎶. / K was shown.

Figure 4 shows the change in electrical conductivity with temperature changes. The specimen not annealed showed a high value of 488 Ω ^-1 cm ^-1 at room temperature and a strong metallic property with decreasing electrical conductivity with increasing temperature. In addition, compared with the annealed specimens, high electrical conductivity was maintained at the measured temperature range. However, the electrical conductivity of the annealed specimens decreased, especially when annealed at 400 ° C, which showed a low value of 209Ω ^-1 cm ^-1 at room temperature. Appeared. This is the result of completely transforming into the desired thermoelectric semiconductor δ-CoSb ₃ by annealing at 400 ° C. as well as the temperature dependence of the Seebeck coefficient of FIG. 3. On the other hand, as the temperature of the specimen increases for all specimens, the electrical conductivity tended to saturate to 250-300Ω ^-1 cm ^-1 , which is considered to be because the lattice scattering of the carrier predominantly increases with temperature.

The thermoelectric factor was analyzed from the Seebeck coefficient and the electrical conductivity measured from room temperature to 600K, and the results are shown in FIG. 5. All specimens showed very low values of about 1㎼ / cmK ² from room temperature to 450K. In the case of specimens annealed at 400 ° C, the temperature increased rapidly at temperatures above 450K, and the maximum thermal power of 13㎼ / cmK ² at 575K. The factors are shown. CoSb ₃ prepared by the arc dissolving method according to the present embodiment showed all p-type conductivity.

Summarizing the above results, the unannealed solidified specimen was δ-CoSb ₃ In addition to β-CoSb, γ-CoSb ₂ And the Sb element mixed phase showed metallic thermoelectric properties. However, when vacuum annealing at 400 ° C. for 24 hours, phase transformation to δ-CoSb ₃ , which has a healthy microstructure without pores and cracks, proceeded and showed excellent thermoelectric properties. When annealed at a temperature above 500 ° C, micropores and cracks were observed due to phase decomposition, evaporation of Sb, and thermal expansion / contraction, and all specimens were destroyed during cooling after annealing. All of the undoped CoSb ₃ exhibited intrinsic semiconductor properties with p-type conductivity.

According to the present invention described above, by preparing the Scutterrudite CoSb ₃ by the arc dissolution method and annealing it under optimum conditions, the microstructure can be changed to improve thermoelectric properties such as Seebeck coefficient, electrical conductivity, and thermoelectric factor. There is.

Claims (3)

Arc dissolving elemental states of Co and Sb in an Ar atmosphere; Conducting one or more redissolutions to prevent homogenization and segregation of the composition; An annealing method of CoSb ₃ prepared by the arc dissolving method, characterized in that it comprises a Scutterrudite CoSb ₃ manufacturing method. According to claim 1, hibiscus root CoSb ₃ Teruel die method, it characterized in that the annealing takes place in the 300-500 ℃. The method of claim 2 wherein the annealing is performed at about 400 ° C. ₃ .

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